InGaN material characterization literature review: Difference between revisions

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This page describes selected literature available on InGaN Material Characterization.

Effects of Substrate Temperature on Indium Gallium Nitride Nanocolumn Crystal Growth[1]

Abstract: Indium gallium nitride films with nanocolumnar microstructure were deposited with varying indium content and substrate temperatures using plasma-enhanced evaporation on amorphous SiO2 substrates. Field emission scanning electron microscopy and X-ray diffraction results are presented, showing that more crystalline nanocolumnar microstructures can be engineered at lower indium compositions. Nanocolumn diameter and packing factor (void fraction) was found to be highly dependent on substrate temperature, with thinner and more closely packed nanocolumns observed at lower substrate temperatures.

• PV device performance can be improved by engineering the microstructure of the material to increase the optical path length and provide light trapping.
• Nanocolunms are ideal microstructure as it provides increasing optical path length, reduces in strain and defect states, improve flexibility and wear characteristics on the macro scale.
• Breakthrough: using scalable plasma-enhanced evaporation deposition of In_xGa_1-xN on an amorphour SiO₂ substrate, instead of an expensive sapphire substrate.
• Microstructure was significantly influenced by the indium content of the samples. As the nominal indium content increases, the crystallinity decreases and a less ordered microstructure is observed.
• Higher temperature, less indium incorporation and less uniformity.

Analytical Model for the Optical Functions of Indium Gallium Nitride with Application to Thin Film Solar Photovoltaic Cells^[2]

Abstract:This paper presents the preliminary results of optical characterization using spectroscopic ellipsometry of wurtzite indium gallium nitride (InxGa1-xN) thin films with medium indium content (0.38<x<0.68) that were deposited on silicon dioxide using plasma-enhanced evaporation. A Kramers-Kronig consistent parametric analytical model using Gaussian oscillators to describe the absorption spectra has been developed to extract the real and imaginary components of the dielectric function (ε1, ε2) of InxGa1-xN films. Scanning electron microscope (SEM) images are presented to examine film microstructure and verify film thicknesses determined from ellipsometry modelling. This fitting procedure, model, and parameters can be employed in the future to extract physical parameters from ellipsometric data from other InxGa1-xN films.

• SEM image shows that the surface microstructure was influenced by the indium content of the films. Generally, the InxGa1-xN films display a clear relationship of larger grain or nanocolumn sizes with increasing indium content.

• Film thickness, surface roughness, optical constants and absorption coefficients can be acquired via ellipsometry raw data modeling
• The difference between the real and imaginary parts of the dielectric function indicates the films presented in the experiments are not perfectly crystalline
• The increased broadening of the dielectric functions and index of refraction curves can be largely attributed to the greater lattice mismatch the In-rich InxGa1-xN films presented in this paper
• unknown reason for additional absorption peaks around 2.5-2.8eV found in In-rich films. Further examination is required to determine the exact source of the absorption

Optical characterization of InxGa1-xN alloys[3]

Abstract: InGaN layers were grown by molecular beam epitaxy (MBE) either directly on (0 0 0 1) sapphire substrates or on GaN-template layers deposited by metal-organic vapor-phase epitaxy (MOVPE). Author(s) combined spectroscopic ellipsometry (SE), Raman spectroscopy (RS), photoluminescence (PL) and atomic force microscopy (AFM) measurements to investigate optical properties, microstructure, vibrational and mechanical properties of the InGaN/GaN/sapphire layers.

The analysis of SE data was done using a parametric dielectric function model, established by in situ and ex situ measurements. A dielectric function database, optical band gap, the microstructure and the alloy composition of the layers were derived. The variation of the InGaN band gap with the In content (x) in the 0 < x ≤ 0.14 range was found to follow the linear law Eg = 3.44–4.5x.

The purity and the stability of the GaN and InGaN crystalline phase were investigated by RS.

• it is possible that the thickness of the MOVPE GaN-template layer influences the optical properties of InGaN layers grown on it.
• AFM micrograph indicates an improvement of the quality of the InGaN film grown over GaN/Al2O3, the grains are more uniformly distributed and of a better crystallinity. It has clear and regular grains with the average size estimated to be approximately 0.5 μm, the films grown directly on sapphire exfoliate easier.
• Raman spectra indicates wurtzite GaN and InGaN crystals found in samples and no phonon modes related to the cubic phase are observed.
• band gap of InxGa1−xN layers obtained by SE decreases with the In content as expected, but the values available in the literature are very different from author to author. The reason of this discrepancy could be either the use of inappropriate values of reference data or the existence of a rough interface between the GaN and InGaN layer, which may have an effect upon the estimation of dielectric constants.
• The optical gap of InGaN layer determined by SE was in good agreement with that found by PL.
• New parametric reference file set for InxGa1−xN helps to obtain not only the main fit parameters, but also the indium content. The x-values estimated in this way were in good agreement with those found by HRXRD.
• A linear optical gap dependence on In content, by fitting in situ and ex situ SE measurements on InGaN at room temperature, was established and data were fitted with equation: Eg = 3.44–4.5x for 0<x<0.14.

Optical band gap in Ga1-xInxN (0<x<0.2) on GaN by photoreflection spectroscopy[4]

Abstract: The optical band gap in 40 nm Ga1−xInxN/GaN single heterostructures is investigated in the composition range 0<x<0.2 by photoreflection spectroscopy (PR) at room temperature and compared with photoluminescence (PL) data. Clear PR oscillations at the GaInN band gap are observed as originating in the large piezoelectric field. Effective band gap bowing parameters b are derived for pseudomorphically stressed GaInN on GaN: b = 2.6 eV (PR) and b = 3.2 eV (PL in localized states). Using experimental deformation potentials of GaN, b = 3.8 eV is extrapolated for the optical band gap in relaxed GaInN material. Previously reported smaller values are discussed.

• gave different values of bowing parameter for strained and relaxed GaInN, and considered bandgap of InGaN to be 1.89eV (but most of them do not match to the newly available results)
• Biaxial strain therefore affects the determination of the composition and induces a band gap shift.

Small band gap bowing in In1-xGaxN alloys[5]

Abstract: High-quality wurtzite-structured In-rich In1−xGaxN films (0 < x < 0.5) have been grown on sapphire substrates by molecular beam epitaxy. Their optical properties were characterized by optical absorption and photoluminescence spectroscopy. The investigation reveals that the narrow fundamental band gap for InN is near 0.8 eV and that the band gap increases with increasing Ga content. Combined with previously reported results on the Ga-rich side, the band gap versus composition plot for In1−xGaxN alloys is well fit with a bowing parameter of ~1.4 eV. The direct band gap of the In1−xGaxN system covers a very broad spectral region ranging from near-infrared to near-ultraviolet.

• the PL peak energy shows a strong blueshift from the band gap of InN (0.77 eV at room temperature) with increasing Ga content. The linewidth of PL peak is significantly broadened as x increases.
• In all cases, absorption coefficient reaches ~10⁵ cm–1 for a photon energy of ~0.5 eV above the absorption edge, which is typical of direct semiconductors. curves of absorption coefficient squared are essentially linear in the range of photon energy investigated, which also implies a direct fundamental band gap.
• absorption edge shifts rapidly to higher energy as x increases.
• the composition dependence of the room-temperature band gap in the entire composition range can be well fit by the following standard equation: E_G(x) 3.42x + 0.77(1-x) - 1.43x(1-x)
• this pseudolinear composition dependence on Ga-rich side is just direct evidence of small bowing in the entire composition range. An additional significance of above equation is that it demonstrates that the fundamental band gap of this ternary alloy system alone covers a wide spectral region ranging from near-infrared at ~1.6 µm to near-ultraviolet at ~0.36 µm.
• At higher Ga concentrations, PL peak energy is shifted towards lower energy as compared with the absorption edge. observed Stokes shift increases with increasing Ga content and is as large as 0.56 eV for x = 0.5.
• Stokes shift tends to reach the maximum near the middle of composition, indicating inhomogeneous distribution of In and Ga atoms. The large, composition dependent, Stokes shift indicates that PL measurement is not a reliable technique to determine the bowing parameter.
• emission spectrum measured by PL spectroscopy reflects distribution of localized states in smaller-gap regions that have larger-than-average In compositions while absorption transition largely reflects onset of density of delocalized states. Therefore, the fact that Stokes shift reaches maximum around the middle of composition implies that largest degree of composition fluctuation and/ or structural disorder occurs near the middle.
• Increasing linewidth of the PL signal with increasing Ga concentration is corresponded to large degree of composition fluctuation and/or structural disorder occurs near the middle and thus cannot be simple explained by pure statistical randomness in the alloy composition

Spectroscopic ellipsometry characterization of (InGa)N on GaN[6]

Abstract: Pseudodielectric function spectra of hexagonal (InGa)N epitaxial layers on GaN were obtained by spectroscopic ellipsometry and compared with photoreflection spectra. Composition and thickness of the InxGa1−xN layers grown by metalorganic chemical vapor deposition, were varied between 0.04 < x < 0.10 and 15–60 nm, respectively. The pseudodielectric function exhibits a clear maximum at the fundamental gap energy of the (InGa)N, which allows a determination of the In content via the composition dependence of that gap energy. The pseudodielectric function spectrum of a complete GaN/(InGa)N/(AlGa)N/GaN light-emitting diode structure shows maxima arising from fundamental gap interband transitions of all constituent layers including the (InGa)N active region.

• spectroscopic ellipsometry can in principle be used, similar to PR spectroscopy, for a determination of In content of InGaN layers via the composition dependence of the E0 gap energy. In contrast to PR spectroscopy, however, the applicability of spectroscopic ellipsometry is not limited by the requirement of a certain amount of band bending, which has to be flattened by the pump beam in order to produce a detectable PR signal.
• Using already known composition dependence of fundamental gap energy, the composition of InGaN layer can thus be deduced.
• Applying spectroscopic ellipsometry to analysis of a complete GaN/(InGa)N/(AlGa)N/GaN LED structure, E0 gap interband transitions from all constituent layers were resolved.

Structural and optical properties of an InxGa1-xN/GaN nanostructure[7]

Abstract: The structural and optical properties of an InxGa1−xN/GaN multi-quantum well (MQW) were investigated by using X-ray diffraction (XRD), atomic force microscopy (AFM), spectroscopic ellipsometry (SE) and photoluminescence (PL). The MQW structure was grown on c-plane (0 0 0 1)-faced sapphire substrates in a low pressure metalorganic chemical vapor deposition (MOCVD) reactor. The room temperature photoluminescence spectrum exhibited a blue emission at 2.84 eV and a much weaker and broader yellow emission band with a maximum at about 2.30 eV. In addition, the optical gaps and the In concentration of the structure were estimated by direct interpretation of the pseudo-dielectric function spectrum. It was found that the crystal quality of the InGaN epilayer is strongly related with the Si doped GaN layer grown at a high temperature of 1090 °C. The experimental results show that the growth MQW on the high-temperature (HT) GaN buffer layer on the GaN nucleation layer (NL) can be designated as a method that provides a high performance InGaN blue light-emitting diode (LED) structure.

• When a GaN epilayer is desired to be grown on sapphire substrates, it is necessary to grow a thin GaN nucleation layer at low temperature of about 500C due to a large lattice-mismatch between GaN and sapphire
• X-ray reflectivity scanning demonstrate an uniform sample surface. Satellite peaks up to SL+5th order were observed, suggesting the influenced interface roughness and thickness of the individual layers building up the multilayer period.
• High order satellites indicate very good InGaN/GaN interfaces. Finer structures are seen between the satellite peaks showing good crystallinity, however, fringes between satellites are not clearly resolved, thus no information about the layer roughnesses is obtained. Program simulation, the well width, barrier width and indium composition were estimated to be approximately as 2.2nm, 15.7nm and 9.6%, respectively
• XRD symmetric(002) incorporated with asymmetric(102)scan and reciprocal space map of the (004) were used to characterize the crystal quality of films
• AFM image of the surface layer shows no nano pipes and GaN droplets formation. The parallel and straight terraces shown on AFM image indicate a typical step-flow morphology or growth mode.
• SE measurement displays optical gap in InxGa1-xN layers as 2.86eV. In concentration was estimated as x=0.12+-0.02 using bowing parameter of 3.8eV.
• PL spectrum shows a strong blue emission at 2.86eV with a much weaker emission at about 2.3eV, this shoulder emission peak would be partially explained by formation of Ga-impurity complex, which are trapped at the side faces.

Photoluminescence measurements on cubic InGaN layers deposited on a SiC substrate[8]

Abstract: In this work Author(s) report optical experiments on pseudomorphic cubic InxGa1−xN epilayers grown on cubic GaN/3C-SiC templates. We make a detailed study of photoluminescence (PL) and photoluminescence excitation spectroscopy on these samples, with spectra taken at various temperatures (between 2 K and 300 K) and using variable wavelength sources to excite the PL spectra. The combined use of these techniques suggests the existence of indium-rich clusters, constituting a negligibly small fraction of the volume of the total layer. Our results reinforce the notion that the large Stokes-like shift (a difference of approximately 300 meV between emission and absorption) observed in these samples is due to the fact that light absorption occurs in the bulk alloy of average composition while recombination occurs within the indium-rich clusters.

• Differences between photoluminescence excitation spectroscopy(PLE) and absorption spectrum: the intensities of the different features do not represent actual absorption strengths. Rather, the reflect the effect that such an absorption has on the recombination efficiency of the PL feature on which the analysing spectrometer is centred. Absorption in regions where recombination is very efficient could, in principle, produce features in the PLE spectrum that have considerable intensity, even in the cases where the volume fraction occupied by these regions is so small as to render them undetectable by other techniques
• The remaining main feature in the main PL spectrum when photon energy of the exciting radiation is below the absorption edge of the InGaN layer indicates that recombination occurs because of absorption occurring within the alloy layer, but not by the 'bulk' part of the layer having average composition. Thus, this PL feature support the idea that In segregates into clusters of small dimensions, constituting a negligible fraction of the volume of the InGaN layer and remaining undetected by x-ray diffraction or optical absorption
• 'When excitation photon energy falls below GaN energy gap, in PL spectrum, lines at 2.75 and 2.85 disappear. The only feature in the PL spectra can be attributed to InGaN layer.'
• 'The persistence of main PL feature at E=2.5eV from T=2.5K up to room temperature favours the interpretation of phase segregation in the form of In-rich clusters.'

Dielectric function and Van Hove singularities for In-rich InxGa1-xN alloys: Comparison of N- and metal-face materials[9]

Abstract: Spectroscopic ellipsometry is applied in order to determine the complex dielectric function (DF) for In-rich InxGa1−xN alloys with N-face polarity from near-infrared into the vacuum ultraviolet spectral region. The results are compared to corresponding data for metal-face films. The optical properties of both types of hexagonal films agree in the essential features which emphasizes that the extracted DFs do not depend on the polarity but represent therefore bulk characteristics. Besides the band gap, five critical points of the band structure are clearly resolved within the composition range of 1>x>0.67. Their transition energies are determined by a fit of the third derivative of the DF. With increasing Ga content, all transitions undergo a continuous shift to higher energies characterized by small bowing parameters. Model calculations of the imaginary part of the DF close to the band gap that take the influence of band filling and conduction-band nonparabolicity into account are presented. A comparison to the experimental data yields the position of the Fermi energy. With the calculated values for the carrier-induced band-gap renormalization and the Burstein-Moss shift, the zero-density values for the fundamental band gaps are obtained. Their dependence on the alloy composition is described by a bowing parameter of b=1.72 eV.

Indium incorporation into InGaN and InAlN layers grown by metalorganic vapor phase epitaxy[10]

Abstract: Experimental data on indium incorporation in InGaN and InAlN layers grown by metalorganic chemical vapor epitaxy (MOVPE) on bulk GaN substrates are presented and discussed. For the step-flow growth mode, realized for InGaN layers grown at relatively high temperatures (around 800 °C), incorporation of indium increases with growth rate, and similarly, with a decrease in GaN substrate misorientation. Both dependences are explained by a higher velocity of flowing steps incorporating the indium atoms. For InAlN layers, three-dimensional nucleation takes place, and thus no significant changes of indium incorporation versus either growth rate or GaN substrate misorientation were observed.

Band gaps and lattice parameters of 0.9 µm thick InxGa1-xN films for 0<x<0.140[11]

Abstract: The c0 lattice parameter, band gap, and photoluminescence spectra of n-type 0.9 μm thick InxGa1−xN films with x=0, 0.045, 0.085, and 0.140 were examined. The c0 lattice parameter followed Vegard’s law using c0=0.5185 nm for GaN and c0=0.569 nm for InN. Band gap measurements by photocurrent spectroscopy fit well with data published by one other research group, with the combined set being described by the equation Eg=3.41−7.31x+14.99x2 for 0⩽x⩽0.15. Photoluminescence measurements with a pulsed nitrogen laser showed a peak well below the measured band gap, as well as significant luminescence above the measured band gap. The above-gap luminescence appears to be due to band filling by the high intensity laser pulses.

• Strain can both alter band gap for a given composition and result in incorrect composition measurements if it is not taken into account when using Vegard's law with x-ray diffraction measurements.
• band gap bowing model may not be adequate to describe the band gap of InxGa1–xN
• segregation has been predicted by a theoretical model.
• band filling in PL spectra is mainly caused by intense laser pulses, and suggests that excitation intensity may be an important consideration when using photoluminescence to estimate the band gap of InxGa1–xN material.

Determination of the critical layer thickness in the InGaN/GaN heterostructures[12]

Abstract: Author(s) present an approach to determine the critical layer thickness in the InxGa1−xN/GaN heterostructure based on the observed change in the photoluminescence emission as the InxGa1−xN film thickness increases. From the photoluminescence data, Author(s) identify the critical layer thickness as the thickness where a transition occurs from the strained to unstrained condition, which is accompanied by the appearance of deep level emission and a drop in band edge photoluminescence intensity. The optical data that indicate the onset of critical layer thickness, was also confirmed by the changes in InxGa1−xN surface morphology with thickness, and is consistent with x-ray diffraction measurements.

• To obtain best electrical and optical properties of QW structures, thickness of strained InxGa1–xN must be kept below critical layer thickness (CLT). Layers thinner than the CLT have all their mismatch accommodated by elastic strain, whereas thicker layers accommodate lattice mismatch by combinations of strain, dislocations, and three dimensional growth.
• CLT can be measured by transmission electron microscopy (TEM), x-ray diffraction (XRD), and optical measurements.
• In PL spectra, it can be assumed that deep level emission is defect related and high-energy peak is band edge related. For thicker fully relaxed films, PL emission was attributed to band edge emission and was found to be independent of film thickness.
• CLT was defined as the thickness where the band gap of InxGa1–xN in the strained region equals the band gap of the relaxed thick films.
• Author(s) provided PL spectra analysis on the basis CLT.
• It is possible that when strained InxGa1–xN films undergo relaxation as the thickness nears CLT, structural defects or three dimensional nucleation at InGaN/GaN interface takes place. These structural defects can give rise to deep levels that dominate the PL emissions of these films.
• strained films with t<CLT have smooth surfaces, whereas for relaxed films with t>CLT there is a sudden deterioration in quality of surface morphology. thickness at which surface morphology deteriorates corresponds to thickness at which deep level intensities arise in PL spectra. Author(s) conclude that transition to a three dimensional growth mode corresponds to the relaxation of film.

The critical thickness of InGaN on (0 0 0 1)GaN[13]

Abstract: The critical thickness for the relaxation of InGaN layers grown on (0 0 0 1) GaN on sapphire for an indium content between 10% and 20% has been determined experimentally. The layers were grown by metal-organic vapour phase epitaxy (MOVPE). The indium content was varied by changing growth temperature between 700 and 750 °C. In-situ ellipsometry could identify a growth mode transition during layer growth, from relatively smooth InGaN layer to a rougher layer with higher indium content. X-ray diffraction found a completely strained layer with lower indium content and a completely relaxed layer with higher indium content. These findings were consistent with absorption and photoluminescence measurements.

• growth of InGaN is very challenging because of problems like spinodal and binodal decomposition, relaxation, or segregation.
• Author(S) suggest two mechanism leading to higher indium incorporation. a) roughness of the relaxed layer offer many stable incorporation sites, thus indium is fast incorporated compared to indium diffusing on a flat surface b) Gallium on strained surface is more loosely bound, since additional strain lower the binding energy, thus it has a higher chance of desorption or segregation.
• growth of thick InGaN layers was observed by a transition of growth mode from 2D fully strained to 3D rough relaxed layers by in-situ spectral ellipsometry as well as by ex-situ X-ray diffraction.
• Author(s) propose that layer relaxation occurs after passing a critical thickness. Using in-situ ellipsometric transients and ex-situ reciprocal space maps, CLT for several compositions can be measured. The data could be described quite well using Fischer's model for the critical thickness.

Strain relaxation and quantum confinement in InGaN/GaN nanoposts[14]

Abstract: Nanoposts of 10–40 nm top diameter on an InGaN/GaN quantum well structure were fabricated using electron-beam lithography and inductively coupled plasma reactive ion etching. Significant blue shifts up to 130 meV in the photoluminescence (PL) spectrum were observed. The blue-shift range increases with decreasing post diameter. For nanoposts with significant strain relaxation, the PL spectral peak position becomes less sensitive to carrier screening. On the basis of the temperature-dependent PL and time-resolved PL measurements and a numerical calculation of the effect of quantum confinement, Author(s) conclude that the optical behaviours of the nanoposts are mainly controlled by the combined effect of 3D quantum confinement and strain relaxation.

• Higher excitation intensity of laser is expected to generate stronger effects of carrier screening and band filling, leading to a more significant blue shift in particular samples.
• strain field originally in the QW is more relaxed in a nanopost of smaller diameter.
• nanopost of smaller diameter has a smaller red shift with increasing temperature. This result could be due to the fact that temperature dependence becomes weaker when 3D quantum confinement is more significant.
• with quantum confinement, recombination efficiency is increased and hence the PL decay time is reduced.
• In thinner nanopost, strain relaxation is more significant and hence range of its PL spectral blue shift is larger. In such a sample, carrier screening effect becomes less effective in causing spectral shift.
• effect of strain relaxation plays an important role in the blue shifts of PL spectra. However, the effect of quantum confinement is also important.
• the reduction of spectral-width in the nanopost is a combined effect of reduction of QW potential tilt, decrease of indium composition fluctuation, and size variation of the quantum disc.
• formation of In-rich clusters is usually accompanied with the existence of stacking faults near the clusters. In other words, defect density becomes higher on increasing the indium content.

Schottky behavior at InN–GaN interface[15]

Abstract: In this work, GaN Schottky diodes were fabricated by depositing InN on GaN surfaces. The junction between these two materials exhibits strong rectifying behavior. The barrier heights were determined to be 1.25 eV, 1.06 eV, and 1.41 eV by current-voltage, current-voltage-temperature, and capacitance-voltage methods, respectively. These values exceed those of any other metal/GaN Schottky barriers. Therefore, the conduction-band offset between InN and GaN should not be smaller than the barrier heights obtained here.

• junction between InN and GaN exhibited strong rectifying behavior.
• Si doping in the active layer initiates a new conducting path, other than thermionic emission.
• C-V profile is determined by the Si diffusion at interface between the doped and the undoped GaN layers
• difference in the barrier height measured by C-V and I-V methods, and the deviation of the ideality factor from unity, indicate that spatial inhomogeneities exist at the interface.
• These inhomogeneities can result from high dislocation density in III-nitrides or from the huge lattice mismatch between GaN and InN.

Efficient rainbow color luminescence from InxGa1−xN single quantum wells fabricated on {11-22} microfacets[16]

Abstract: Rainbow color luminescence from InxGa1−xN single quantum wells (SQWs) is achieved and almost covers the entire visible range when the layers are fabricated on {11math2} facets with a few micron-width using a regrowth technique on striped GaN templates. These facets are tilted 56° with respect to the (0001) facets and border the (0001) and {11math0} facets. The emission wavelength on the {11math2} facets is redshifted from the {11math0} side to (0001) side due to the variations of the In composition, which leads to the color contrast with the rainbow geometry. The temperature dependence of the photoluminescence intensity shows that the internal quantum efficiency at room temperature is 33% due to the very small internal electric fields and a small threading dislocation density compared to that in conventional (0001) InxGa1−xN SQWs. Since the emission efficiency does not show a noticeable emission wavelength dependence, this type of structure has potential as light-emitting devices with multiwavelengths that perform numerous color controllability such as pastel and white colors.

Single phase InxGa1−xN 0.25 ≤ x ≤ 0.63 alloys synthesized by metal organic chemical vapor deposition[17]

Abstract: Author(s) present the results of single phase InxGa1−xN alloys for the In composition ranging from 25% to 63% synthesized by metal organic chemical vapor deposition. Single peak of x-ray diffraction θ-2θ scans of the (002) plane in InGaN alloys confirms that there is no phase separation. It was found both electron mobility and concentration increase with an increase of In content. Atomic force microscopy measurements revealed that the grown films have a surface roughness that varies between 1.5 and 4.0 nm and are free from In droplets. The results suggest that it is possible to synthesize single phase InGaN alloys inside the theoretically predicted miscibility gap.

InGaN quantum dots grown by metalorganic vapor phase epitaxy employing a post-growth nitrogen anneal[18]

Abstract: Author(s) describe the growth of InGaN quantum dots (QDs) by metalorganic vapor phase epitaxy. A thin InGaN epilayer is grown on a GaN buffer layer and then annealed at the growth temperature in molecular nitrogen inducing quantum dot formation. Microphotoluminescence studies of these QDs reveal sharp peaks with typical linewidths of ~ 700 μeV at 4.2 K, the linewidth being limited by the spectral resolution. Time-resolved photoluminescence suggests that the excitons in these structures have lifetimes in excess of 2 ns at 4.2 K.

Growth temperature effects on InxGa1−xN films studied by X-ray and photoluminescence[19]

Abstract: The InGaN films were grown between 850°C and 600°C by the metalorganic chemical vapor deposition method and characterized by X-ray diffraction and photoluminescence (PL). The incorporation of In into the ternary films was found to increase from x=0.01 to 0.28 as the temperature decreases. In films grown at 750°C and higher, both the X-ray and PL results show gradual changes and indicate 5% In molar fraction difference that may be due to the alloy composition fluctuation. However, in films grown at 700°C and lower, the near band edge emission disappears and the impurity transitions (IT) become dominant in the PL spectra, in contrast to X-ray diffraction where the line width broadens sharply from less than 300 arcsec to larger than 500 arcsec. Author(s) also found that IT is relatively insensitive to the sample temperature. Besides, the correlation between enhancing PL intensity and patterned micro-structure is observed.

Nitrogen supply rate dependence of InGaN growth properties, by RF-MBE[20]

Abstract: Nitrogen flow rate dependence of InGaN growth mechanism on sapphire (0 0 0 1) was studied. The indium content of the InGaN layer can be controlled by changing the nitrogen supply rate by structural and optical investigations. In case of lower nitrogen flow rate, the formation of phase separation due to dissociation of InN and condensation of indium on the growing surface were observed. In case of higher nitrogen flow rate (under N-rich condition), on the other hand, the indium content became uniform and the photoluminescence (PL) emission showed single peak. It is considered that the causes of phase separation formation were suppressed and nonequilibrium condition was enhanced by the increase of the nitrogen supply rate and the growth rate. Judging from the estimated thermal activation energies and degree of fluctuation in the indium content by PL line-width, however, the non-radiative recombination centers within the spontaneously formed potential wells as well as in the extended states were formed in case of excess nitrogen supply rate. The structural defects that were caused by the suppression of the surface migration of group-III atoms with excessive activated nitrogen might have worked as non-radiative recombination centers. Thus, a slightly N-rich condition is desirable for the realization of the uniform InGaN alloy composition and the high optical quality, especially at relatively high growth temperature.

• indium content in the InGaN layer became uniform at N-rich condition. It is considered that the phase separation has been suppressed at the expense of the surface flatness due to the reduction of the surface migration of group-III adatoms by the excess nitrogen.
• With increasing nitrogen flow rate, growth rate monotonically increased due to indium incorporation. This result shows that the dissociation of InN was prevented due to an increase of the activated nitrogen on the growing surface.
• phase separation has been suppressed at high nitrogen flow rate is because of suppression of dissociation of InN due to excess activated nitrogen on the growing surface, and the enhancement of the non-equilibrium condition due to increase in growth rate. Also, it seems that they prevent indium condensation by its limited diffusion length on the growing surface.
• It is considered that non-radiative recombination center is attributed to structural defects caused by the difference of bond length between In–N and Ga–N, and point defects caused by growths under the strongly off-stoichiometric conditions. impurities such as oxygen may be easily incorporated into defective high-indium-content region and they might work as non-radiative recombination centers.
• slightly N-rich condition is desirable for realization of InGaN growth in terms of realization of a uniform alloy composition and high optical quality.

Growth of In-rich InGaN on InN template by radio-frequency plasma assisted molecular beam epitaxy[21]

Abstract: High-quality epitaxial InN films have been employed as underlying templates for the growth of In-rich InxGa1−xN (0.71<x<0.90) films by radio-frequency plasma assisted molecular beam epitaxy. The epitaxial InN films (InN templates) with the thickness of 500 nm were grown on (0 0 0 1) sapphire substrates with a low-temperature deposited 10 nm thick InN (LT-InN) buffer layer, and then In-rich InxGa1−xN films with the thickness of 250 nm were grown on these InN templates. As compared with In-rich InxGa1−xN films grown directly on the LT-InN buffer layers, the In-rich InxGa1−xN films grown on the InN templates showed a decrease in full-width at half-maximum of the (0 0 0 2) X-ray rocking curves. It was found that the insertion of the InN template was very effective in improving the crystalline quality of In-rich InxGa1−xN. Optical properties were also investigated by photoluminescence. Lattice distortion of the In-rich InxGa1−xN films induced by the underlying InN templates is also discussed.

Epitaxial growth and optical properties of semipolar (11-22) GaN and InGaN/GaN quantum wells on GaN bulk substrates[22]

Abstract: GaN and InGaN/GaN multiple quantum well (MQW) were grown on semipolar (11-22) GaN bulk substrates by metal organic vapor phase epitaxy. The GaN homoepitaxial layer has an atomically flat surface. Optical reflection measurements reveal polarization anisotropy for the A, B, and C excitons. Free A excitons dominate the photoluminescence (PL) spectrum at 10 K and are accompanied by a weaker, sharp doublet emission due to neutral donor-bound excitons. The InGaN/GaN MQW grown on a GaN homoepitaxial layer involves fast radiative recombination processes. The PL decay monitored at 428 nm can be fitted with a double exponential curve, which has lifetimes of 46 and 142 ps at 10 K. These values are two orders of magnitude shorter than those in conventional c-oriented QWs and are attributed to the weakened internal electric field. The emissions from GaN and MQW polarize along the [1-100] direction with polarization degrees of 0.46 and 0.69, respectively, due to the low crystal symmetry.

Efficient radiative recombination from 〈11-22〉 -oriented InxGa1−xN multiple quantum wells fabricated by the regrowth technique[23]

Abstract: InxGa1−xN multiple quantum wells (QWs) with [0001], <11-22>, and <11-20> orientations have been fabricated by means of the regrowth technique on patterned GaN template with striped geometry, normal planes of which are (0001) and {11-20}, on sapphire substrates. It was found that photoluminescence intensity of the {11-22} QW is the strongest among the three QWs, and the internal quantum efficiency of the {11-22} QW was estimated to be as large as about 40% at room temperature. The radiative recombination lifetime of the {11-22} QW was about 0.38 ns at low temperature, which was 3.8 times shorter than that of conventional [0001]-oriented InxGa1−xN QWs emitting at a similar wavelength of about 400 nm. These findings strongly suggest the achievement of stronger oscillator strength owing to the suppression of piezoelectric fields.

Spatial and temporal luminescence dynamics in an InxGa1−xN single quantum well probed by near-field optical microscopy[24]

Abstract: Spatial distribution of photoluminescence (PL) with spectral, spatial, and/or time resolution has been assessed in an InxGa1−xN single-quantum-well (SQW) structure using scanning near-field optical microscope (SNOM) under illumination-collection mode at 18 K. The near-field PL images revealed the variation of both intensity and peak energy in PL spectra according to the probing location with the scale less than a few hundredths of a nanometer. PL linewidth, the value of which was about 60 meV in macroscopic PL, was as small as 11.6 meV if the aperture size was reduced to 30 nm. Clear spatial correlation was observed between PL intensity and peak wavelength, where the regions of strong PL intensity correspond to those of long wavelength. Time-resolved SNOM–PL study showed the critical evidence that supports the model of diffusion of carriers to potential minima.

Electrical and optical properties of p-type InGaN[25]

Abstract: Mg-doped InxGa1−xN alloys were grown by metal organic chemical vapor deposition on semi-insulating c-GaN/sapphire templates. Hall effect measurements showed that Mg-doped InxGa1−xN epilayers are p-type for x up to 0.35. Mg-acceptor levels (EA) as a function of x, (x up to 0.35), were experimentally evaluated from the temperature dependent hole concentration. The observed EA in Mg-doped In0.35Ga0.65N alloys was about 43 meV, which is roughly four times smaller than that in Mg doped GaN. A room temperature resistivity as low as 0.4 Ω cm (with a hole concentration ~ 5×1018 cm−3 and hole mobility ~ 3 cm2/V s) was obtained in Mg-doped In0.22Ga0.78N. It was observed that the photoluminescence (PL) intensity associated with the Mg related emission line decreases exponentially with x. The Mg energy levels in InGaN alloys obtained from PL measurements are consistent with those obtained from Hall-effect measurements.

• Achieving highly conductive p-type GaN and AlGaN is very difficult due to high activation energies (EA) of the Mg-acceptor, which is a universally accepted p-type dopant for GaN and related alloys.
• Since EA decreases with a decrease in band gap energy, Mg-doped InGaN (InGaN:Mg) is expected to have a higher hole concentration (p) than Mg-doped GaN.
• P-type doping in relatively high In content InGaN alloys is highly challenging due to the presence of high background electron concentrations, which is believed to originate from defects such as oxygen and hydrogen impurities or nitrogen vacancies.
• presence of high background electron concentration is main hindrance for obtaining p-type conductivity and p-type InxGa1−xN alloys with In content x>0.35.
• It was found that EA continuously decreases with an increase in x. Lower values of EA are the main physical reason for higher values of p in InxGa1−xN:Mg alloys of higher x.
• InxGa1−xN materials with x>0.35 are generally highly n-type and conversion of these materials to p-type by Mg doping is still very difficult. Author(s) results indicate that p-type conductivity in InGaN:Mg could be further improved if a better control of the background electron concentration could be achieved.
• In PL spectra, deep donors appeared in Mg doped GaN but disappeared in all InGaN:Mg alloys. Author(s) suggest that the lower growth temperatures employed for InGaN alloys somehow suppresses the formation of these deep donors.
• Mg impurity related PL emission intensity is found to decrease exponentially with In-content, reduction in PL intensity may be related with the incorporation of impurities, which are also responsible for the high background electron concentrations in high In content InGaN alloys.

Investigation on the Correlation Between the Crystalline and Optical Properties of InGaN Using Near-Field Scanning Optical Microscopy[26]

Author(s) have performed the polarization-modulation near-field scanning optical microscopy (PM-NSOM) and photoluminescence NSOM (PL-NSOM) measurements on the InGaN alloy epitaxial layer. Spatial variations in the crystalline quality of nanoscale domains in InGaN film were found by PM-NSOM. It was found that the luminescent property of InGaN correlates closely with the local crystalline quality. Regions with better crystallinity have higher luminescence intensity and longer emission wavelength, while regions with poorer crystallinity exhibit a luminescence of lower intensity and shorter emission wavelength. Author(s) show that the combination of PM-NSOM and PL-NSOM is a useful diagnostic tool to the correlation between crystalline and optical properties of the nanostructures.

Compositional dependence of the strain-free optical band gap in InxGa1−xN layers

Abstract: The effect of strain on the compositional and optical properties of a set of epitaxial single layers of InxGa1−xN was studied. Indium content was measured free from the effects of strain by Rutherford backscattering spectrometry. Accurate knowledge of the In mole fraction, combined with x-ray diffraction measurements, allows perpendicular strain (ϵzz) to be evaluated. Optical band gaps were determined by absorption spectroscopy and corrected for strain. Following this approach, the strain free dependence of the optical band gap in InxGa1−xN alloys was determined for x < 0.25. Our results indicate an “anomalous,” linear, dependence of the energy gap on the In content, at room temperature: Eg(x) = 3.39–3.57x eV. Extension of this behavior to higher concentrations is discussed on the basis of reported results.

Luminescences from localized states in InGaN epilayers

Abstract: Optical spectra of the bulk three-dimensional InGaN alloys were measured using the commercially available light-emitting diode devices and their wafers. The emission from undoped InxGa1−xN(x<0.1) was assigned to the recombination of excitons localized at the potential minima originating from the large compositional fluctuation. The emission from heavily impurity-doped InGaN was also pointed out related to the localized states.

Photoluminescence from quantum dots in cubic GaN/InGaN/GaN double heterostructures[27]

Abstract: Author(s) have measured photoluminescence spectra of molecular-beam-epitaxy-grown cubic GaN/InxGa1−xN/GaN double heterostructures with x between 0.09 and 0.33. Author(s) observe a luminescence peak at about 2.3–2.4 eV which is almost independent of the InGaN layer composition. High-resolution x-ray diffraction measurements revealed a pseudomorphic In-rich phase with x = 0.56±0.02 embedded in the InGaN layers. Including strain effects Author(s) calculate a gap energy Eg = 2.13 eV of this phase. In cubic InGaN, spontaneous polarization and strain-induced piezoelectric fields are negligible. Therefore, the observed difference between the luminescence energy and the gap of the In-rich phase is assumed to be due to the localization of excitons at quantum-dot-like structures with a size of about 15 nm.

• gap energy is different for strained and strain free InGaN.
• In InGaN with cubic crystal structure (c-InGaN) spontaneous polarization does not exist due to the higher crystal symmetry, and due to the (001) growth direction, strain-induced piezoelectric fields are negligible.
• Author(s) also considered the effect of a biaxial compression on the gap energy by taking into account the variation Eg of gap energy due to an biaxial in-plane strain.
• Since in c-III-nitrides no spontaneous polarization- or piezoinduced electric fields exist, the difference between the InGaN gap and the luminescence peak energy is equal to the localization energy Eloc of the excitons. (need to check the validity of this argument)
• In-rich inclusions have also been found in thick c-InGaN layers. Since micro-Raman experiments revealed clear evidence that these inclusions form nanometer sized QD-like structures, author(S) assume that PL from their DH structures is due to the recombination of excitons localized at In-rich QD-like structures which are embedded in the InGaN layers. The existence of QD-like structures in c-GaN/InGaN/GaN DH structures is further supported by the fact that the PL and EL from c-InGaN/GaN DH and QW structures grown by MOCVD is also observed close to the energy gap.
• The shift of PL peak energy is mainly due to the decrease of the QD size with decreasing In content of the layers.

Photoluminescence associated with quantum dots in cubic GaN/InGaN/GaN double heterostructures

Abstract: Author(s) report on investigations of the photoluminescence of cubic GaN/ InxGa1−xN/ GaN double heterostructures with x between 0.09 and 0.33. The room temperature emission of all samples is found at about 2.3-2.4 eV. High resolution X-ray diffraction measurements reveal an In-rich phase with x=0.56. Luminescence line narrowing in resonant excitation experiments indicate that the photoluminescence stems from quantum-dot-like structures of the In-rich phase. Postgrowth annealing at temperatures up to 700°C demonstrates an obvious stability of the quantum dots.

InxGa1−xN refractive index calculations

Abstract: The growth of InxGa1−xN Wurtzite structure is a well established fact. It permits to design optoelectronic devices such as laser diodes or LEDs, from the near ultraviolet to the infrared light spectrum. This sweeps indeed, the whole of the visible spectrum and, hence, appears to be very useful to the recent development of liquid crystal display screens, or designing photodiodes and perhaps solar cells (after studying their energetical efficiencies). Nevertheless, refractive indices of InxGa1−xN structure have not been studied. The refractive index of such structures is increasing from the GaN refractive index to the InN one, with therefore, a bowing of the curve due to the lattice mismatch between these two constituting binary alloys. The index is, in a certain range of the “n(x)” characteristic, less than the GaN one. This seems to be particularly interesting in the integrated optics domain or optical waveguides realization, because the growth of GaN is easier than the growth of InxGa1−xN.

Refractive index and gap energy of cubic InxGa1−xN

Abstract: Spectroscopic ellipsometry studies have been carried out in the energy range from 1.5 to 4.0 eV in order to determine the complex refractive indices for cubic InGaN layers with various In contents. The films were grown by molecular-beam epitaxy on GaAs(001) substrates. By studying GaN films, we prove that for the analysis of optical data, a parametric dielectric function model can be used. Its application to the InGaN layers yields, in addition, the composition dependence of the average fundamental absorption edge at room temperature. From the latter, a bowing parameter of 1.4 eV is deduced.

InxGa1−xN refractive index calculations

Abstract: The growth of InxGa1−xN Wurtzite structure is a well established fact. It permits to design optoelectronic devices such as laser diodes or LEDs, from the near ultraviolet to the infrared light spectrum. This sweeps indeed, the whole of the visible spectrum and, hence, appears to be very useful to the recent development of liquid crystal display screens, or designing photodiodes and perhaps solar cells (after studying their energetical efficiencies). Nevertheless, refractive indices of InxGa1−xN structure have not been studied. The refractive index of such structures is increasing from the GaN refractive index to the InN one, with therefore, a bowing of the curve due to the lattice mismatch between these two constituting binary alloys. The index is, in a certain range of the “n(x)” characteristic, less than the GaN one. This seems to be particularly interesting in the integrated optics domain or optical waveguides realization, because the growth of GaN is easier than the growth of InxGa1−xN.

Optical Properties of Strained AlGaN and GaInN on GaN

Abstract: The composition of alloys in strained ternary alloy layers, Al xGa1- xN (0<x<0.25) and Ga1- xIn xN (0<x<0.20), on thick GaN was precisely determined using the high-resolution X-ray diffraction profile. The band gap of strained AlGaN is found to increase almost linearly according to the AlN molar fraction, while that of strained GaInN has a large bowing parameter of 3.2 eV.

Optical and microstructural properties versus indium content in InxGa1−xN films grown by metal organic chemical vapor deposition

Abstract: Author(s) present comparative investigations of single phase InxGa1−xN alloys for a varying In content (x = 0.07 to 0.14) grown by metal organic chemical vapor deposition (MOCVD) technique. While the composition was determined using secondary ion mass spectroscopy, Author(S) have investigated the microstructures in InxGa1−xN/GaN films by using transmission electron microscopy and correlated these with the refractive index of the material. Based on ellipsometric analysis of the films, the dispersion of optical indices for InxGa1−xN films is determined by using Tauc–Lorentz dispersion equations.

Time-Resolved Photoluminescence Studies of Indium-Rich InGaN Alloys

Abstract: Time-resolved photoluminescence (PL) spectroscopy has be used to investigate indium-rich InGaN alloys grown on sapphire substrates by metal organic chemical vapor deposition. Photoluminescence measurement indicates two dominant emission lines originating from phase-separated high- and low-indium-content regions. Temperature and excitation intensity dependence of the two main emission lines in these InGaN alloys have been measured. Temperature and energy dependence of PL decay lifetime show clearly different decay behaviour for the two main lines. Author(s) results show that photo-excited carriers are deeply localized in the high indium regions while photo-excited carriers can be transferred within the low-indium-content regions as well as to high-content regions.

Correlation of crystalline defects with photoluminescence of InGaN layers

Abstract: Author(s) report structural studies of InGaN epilayers of various thicknesses by x-ray diffraction, showing a strong dependence of the type and spatial distribution of extended crystalline defects on layer thickness. The photoluminescence intensity for the samples was observed to increase with thickness up to 200 nm and decrease for higher thicknesses, a result attributed to creation of dislocation loops within the epilayer. Correlation of physical properties with crystalline perfection open the way for optimized designs of InGaN solar cells, with controlled types and dislocation densities in the InGaN epilayers, a key requirement for realizing high photocurrent generation in InGaN.

MOVPE growth and Mg doping of InxGa1−xN (x∼0.4) for solar cell

Abstract: MOVPE growth and Mg doping of InGaN films are studied to develop technologies for the InGaN-based solar cell. By optimizing growth temperature and the TMI/(TMI+TEG) molar ratio, InGaN films with an In content up to 0.37 are successfully grown without phase separation and metallic In incorporation. It is found that the In composition in the InGaN films is governed by growth temperature, and the TMI/(TMI+TEG) molar ratio has very small effect on the composition change. InGaN films doped with Mg using CP2Mg show the compensation effect of carriers and those with an In content up to 0.2 show p-type conduction. The film with an In content of 0.37 shows phase separation when the CP2Mg/(TMI+TEG) molar ratio exceeds 0.05, indicating that Mg atoms incorporated have a significant effect on the crystal growth of InGaN.

Cathodoluminescent investigations of In x Ga1− x N layers

Abstract: The aim of this work was the investigation of the InGaN epilayers of various contents and various thickness; namely the influence of these two factors upon the cathodoluminescent (CL) properties. The studied epilayers were grown by plasma assisted molecular beam epitaxy. The samples were studied by electron probe microanalysis, CL, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Some interesting peculiarities of CL spectra were obtained; the nature of the CL bands is discussed.

Magnesium Doping of In-rich InGaN

Abstract: InN and In-rich InGaN were grown by metal organic vapor phase epitaxy with magnesium doping. A set of samples were grown at 550 °C, whereas a second set of samples were grown at increasing temperature with Ga content. Upon annealing, p-InGaN was obtained from the second set up to an In content above 50%, with an acceptor concentration of ~1×1019 cm-3 and a mobility of 1–2 cm2 V-1 s-1. None of the samples grown at a constant temperature of 550 °C showed a p-behavior after heat treatment. The electrical, optical, structural and morphological characteristics of the films grown were analyzed, and the leveling off of hole concentration beyond an In content of 30% was consistent with the reported decreasing activation energy of Mg with increasing In content.

Optical studies on a coherent InGaN/GaN layer

Abstract: Photoluminescence (PL), photoluminescence excitation (PLE) and selective excitation (SE-PL) studies were performed in an attempt to identify the origin of the emission bands in a pseudomorphic In0.05Ga0.95N/GaN film. Besides the InGaN near-band-edge PL emission centred at 3.25 eV an additional blue band centred at 2.74 eV was observed. The lower energy PL peak is characterized by an energy separation between absorption and emission–the Stokes’ shift–(~500 meV) much larger than expected. A systematic PLE and selective excitation analysis has shown that the PL peak at 2.74 eV is related to an absorption band observed below the InGaN band gap. Author(s) propose the blue emission and its related absorption band are associated to defect levels, which can be formed inside either the InGaN or GaN band gap.

Band Gap of Hexagonal InN and InGaN Alloys

Abstract: A survey of most recent studies of optical absorption, photoluminescence, photoluminescence excitation, and photomodulated reflectance spectra of single-crystalline hexagonal InN layers is presented. The samples studied were undoped n-type InN with electron concentrations between 6 × 1018 and 4 × 1019 cm—3. It has been found that hexagonal InN is a narrow-gap semiconductor with a band gap of about 0.7 eV, which is much lower than the band gap cited in the literature. We also describe optical investigations of In-rich InxGa1—xN alloy layers (0.36 < x < 1) which have shown that the bowing parameter of b ~ 2.5 eV allows one to reconcile our results and the literature data for the band gap of InxGa1—xN alloys over the entire composition region. Special attention is paid to the effects of post-growth treatment of InN crystals. It is shown that annealing in vacuum leads to a decrease in electron concentration and considerable homogenization of the optical characteristics of InN samples. At the same time, annealing in an oxygen atmosphere leads to formation of optically transparent alloys of InN–In2O3 type, the band gap of which reaches approximately 2 eV at an oxygen concentration of about 20%. It is evident from photoluminescence spectra that the samples saturated partially by oxygen still contain fragments of InN of mesoscopic size.

• High doping levels lead to changes in the interband absorption coefficient due to the Burstein-Moss effect. (equation is given in the paper)
• absorption coefficient a(w) for InN samples measured at 300 K rapidly reaches values of the order of a(w) > 4 � 10⁴ cm–^–1 at photon energy close to 1 eV. This high value of absorption coefficient is typical of an interband absorption in direct bandgap semiconductors.
• optical absorption and luminescence bands of InN crystals clearly have characteristics typical of interband transitions. but at large electron concentrations, absorption edge is well above the fundamental band gap (approximately by the value equal to the Fermi energy).
• Comparison of Fermi energies of GaAs, InN, and GaN crystals leads to conclusion that electron effective mass in InN lies between those in GaAs and GaN.
• E_G(n) as a function of charge carrier concentrations for GaAs, InN, and GaN crystals shows a nearly linear dependence on n^1/3 where the expression is given is the paper.
• chemical composition analysis by Auger spectroscopy and Rutherford backscattering technique revealed very high oxygen contents in widegap samples, up to 20 %, much higher than in author(s) samples. It can be assumed that it is oxygen that is responsible for a high concentration of defects. In this case an increase of band gap in wide-gap samples can be caused by formation of oxynitrides, which have much larger band gaps than that of InN.
• Burstein-Moss effect in polycrystalline samples with high charge carrier concentrations may also be responsible for sizable changes in the band gap.

Thermal Annealing of Cubic-InGaN/GaN Double Heterostructures

Abstract: Author(s) have performed annealing experiments with c-InGaN/GaN double heterostructures in order to obtain information on the thermal stability and the formation process of In-rich clusters in the InGaN layers. While the as grown samples showed a dominating luminescence at about 2.3 eV, the annealed samples showed a new luminescence peak at 2.8–3.0 eV which may be due to a band gap emission of a regenerated layer with an In-content of about x = 0.20. These results are corroborated by micro Raman spectroscopy. Author(s) annealing experiments show that at elevated temperatures In-atoms can diffuse in c-InGaN layers while In-rich aggregates are stable at growth temperature.

Pulse laser assisted MOVPE for InGaN with high indium content

Abstract: In0.53Ga0.47N film was grown at 600 °C by Nd:YAG pulse laser assisted MOVPE. The optical and structural properties of the film were compared with that grown without laser assistance at the same condition. The results of XRD measurements showed that the crystallinity of the film grown with laser was better than that of the one grown without laser. The surface morphology and cross-sectional SEM image of the film grown with laser revealed that there were no In droplets on the film. The band-edge emission of the film grown with laser at room temperature and 77 K was observed at 840 nm. The results of micro-Raman measurement showed that the film grown with laser had better crystalline structure than that of the film grown without laser and the radiative recombination which contributed to photoluminescence mainly occurred at In0.53Ga0.47N region. Those results imply that pulse laser enhances the surface migration and reaction of elements in spite of low-growth temperature. Author(s) suggest that pulse laser assisted technique is effective for low-temperature growth of InGaN with high indium content.

Control of electron density in InN by Si doping and optical properties of Si-doped InN

Abstract: Author(s) have studied Si-doping profiles of InN films grown by plasma-assisted molecular-beam epitaxy and their photoluminescence (PL) properties. Author(s) confirmed experimentally that Si acts as a donor in InN. Undoped and Si-doped InN films with electron densities (n) of 1.6 × 1018 − 1.4 × 1019 cm−3 showed clear n dependences of PL properties. The PL peak shifted to the higher energy side with increasing n, and the PL intensity decreased with increasing n. These were characteristics of degenerated semiconductors with a large density of defects and/or dislocations. The band-gap energy of degenerated InN films with n = 1.6 × 1018 − 4.7 × 1018 cm−3 was estimated to be about 0.6 eV by assuming a nonparabolic conduction band and a constant band-renormalization effect. By taking the band-gap shrinkage of about 20 meV due to the conduction-band renormalization into account, we suggest that the band-gap energy of intrinsic InN is 0.6–0.65 eV.

Optical properties of InN—the bandgap question

Abstract: The recent controversy on the bandgap of InN is addressed, with reference to optical data on single crystalline thin film samples grown on sapphire. The optical absorption spectra deduced from transmission data or spectroscopic ellipsometry are consistent with a lowest bandgap around 0.7 eV in the low doping limit. Further, these data from a number of different independent authors and samples give values for the absorption coefficient within a factor 2 well above the absorption edge, supporting an intrinsic direct bandgap process. The presence of Mie resonances due to In inclusions in the InN matrix affects the shape of the absorption above the edge, but is less relevant for the discussion of the bandgap for pure InN. The alternative model of a deep level to conduction band transition requires the presence of a deep donor at a concentration close to 1020 cm−3; in addition this concentration has to be the same within a factor 2 in all samples studied so far. This appears implausible, and no such deep donor could so far be identified from SIMS data in the highest quality samples studied. The line shape of the photoluminescence spectra can be quite well reproduced in a model for the optical transitions from the conduction band states to localized states above the valence band, including the Coulomb effects of the impurity potentials. A value of 0.69 eV for the bandgap of pure InN is deduced at 2 K. For samples that appear to be only weakly degenerate n-type two narrow peaks are observed in the photoluminescence at low temperature, assigned to conduction band—acceptor transitions. These peaks can hardly be explained in the deep level model. Recent cathodoluminescence data on highly n-doped InN films showing that the emission appears to be concentrated around In inclusions can also be explained as near bandgap recombination, considering the plausible enhancement due to interface plasmons. Finally, recent photoluminescence data on quantum structures based on InN and InGaN with a high In content appear to be consistent with moderate upshifts of the emission from a 0.7 eV value due to electron confinement.

GaN: from fundamentals to applications

Abstract: The fundamental differences between GaN and SiC are reviewed, then the problems of doping GaN are explored. The range of energy band gaps obtainable with alloys of all the III-Nitrides extends from 1.9 to 6.2 eV. Finally, various applications of the III-Nitrides are described with emphasis on solar blind UV detectors, light-emitting and modulating devices, cold cathodes and, in more detail, a heterojunction bipolar transistor that uses a SiC base layer and operates above 500°C.

InGaN: An overview of the growth kinetics, physical properties and emission mechanisms[28]

Abstract: This article reviews the fundamental properties of InGaN materials. The growth kinetics associated with the growth parameters, such as growth temperatures, V/III ratios, and growth rates which influence the quality of the InGaN epilayers, are briefly described. An overview of the properties of the InGaN alloys, such as the optical, structural and electrical characteristics, is presented. The design and fabrication of novel optoelectronic device structures require an accurate knowledge of the band gap as a function of alloy composition; therefore, attention is paid to Vegard’s law and the bowing parameter; in addition, the major factors leading to the uncertainties of the bowing parameter of InGaN are addressed. Apart from that, the determination of indium composition by X-ray diffraction (XRD) using different assumptions and various equations are summarized. The erroneous measurements of the indium composition by using this technique are also described. Finally, different emission mechanisms of the strained InGaN quantum wells proposed by different groups of researchers are also discussed.

Complete composition tunability of InGaN nanowires using a combinatorial approach[29]

Abstract: The III nitrides have been intensely studied in recent years because of their huge potential for everything from high-efficiency solid-state lighting and photovoltaics to high-power and temperature electronics. In particular, the InGaN ternary alloy is of interest for solid-state lighting and photovoltaics because of the ability to tune the direct bandgap of this material from the near-ultraviolet to the near-infrared region. In an effort to synthesize InGaN nitride, researchers have tried many growth techniques. Nonetheless, there remains considerable difficulty in making high-quality InGaN films and/or freestanding nanowires with tunability across the entire range of compositions. Here Author(s) report for the first time the growth of single-crystalline InxGa1-xN nanowires across the entire compositional range from x=0 to 1; the nanowires were synthesized by low-temperature halide chemical vapour deposition9 and were shown to have tunable emission from the near-ultraviolet to the near-infrared region. We propose that the exceptional composition tunability is due to the low process temperature and the ability of the nanowire morphology to accommodate strain-relaxed growth, which suppresses the tendency toward phase separation that plagues the thin-film community.

Growth and properties of InAlN nanocolumns emitting in optical communication wavelengths[30]

Abstract: InxAl1-xN nanocolumns (0.71lesxInles1.00) were fabricated on Si (111) substrates by RF-MBE. The room temperature photoluminescence (RT-PL) in optical communication wavelengths from 0.95 to 1.94 mum with changing xIn was observed. InN/InAlN heterostructures were also fabricated.

Plasma ehnancement of metalorganic chemical vapor deposition and properties of Er2O3 nanostructured thin films[31]

Abstract: An O2 remote plasma metal organic chemical vapor deposition (RP-MOCVD) route is presented for tailoring the structural, morphological, and optical properties of Er2O3 thin films grown on Si(100) using the tris(isopropylcyclopentadienyl)erbium precursor. The RP-MOCVD approach produced highly (100)-oriented, dense, and mechanically stable Er2O3 films with columnar structure.

Temperature induced shape change of highly aligned ZnO nanocolumns[32]

Abstract: Vertically well-aligned ZnO nanocolumns were grown on Al2O3 (0 0 0 1) substrates via metalorganic chemical vapor deposition without using any metal catalyst. Their morphology was investigated as a function of the growth temperature (Tg), which was found to be a key processing parameter to control their shape. At View the Tg> 450 C, vertically well-aligned ZnO nanocolumns started to grow. It was found that a higher Tg yielded slimmer, needle shaped nanocolumns, whereas a lower Tg yielded thicker nanocolumns.

AlGaN Nanocolumns Grown by Molecular Beam Epitaxy: Optical and Structural Characterization[33]

Abstract: High quality AlGaN nanocolumns have been grown by molecular beam epitaxy on Si(111) substrates. Scanning Electron Microscopy micrographs show hexagonal, single crystal columns with diameters in the range of 30 to 60 nm. The nominal Al content of the nanocolumns was changed from 16% to 40% by selecting the flux ratio between the Al and the total III-element, while keeping the growth temperature and the active nitrogen constant. The nominal values of the Al content are consistently lower than the experimental ones, most likely due to the high Ga desorption rates at the growth temperature. The Al composition trend versus the Al flux is consistent with the E2 phonon energy values measured by inelastic light scattering. These results open the possibility to grow high quality low dimensional structures based on AlGaN/GaN/AlGaN heterocolumns for basic studies and device applications.

Solid phase immiscibility in GaInN

Abstract: The large difference in interatomic spacing between GaN and InN is found to give rise to a solid phase miscibility gap. The temperature dependence of the binodal and spinodal lines in the Ga1−xInxN system was calculated using a modified valence‐force‐field model where the lattice is allowed to relax beyond the first nearest neighbor. The strain energy is found to decrease until approximately the sixth nearest neighbor, but this approximation is suitable only in the dilute limit. Assuming a symmetric, regular‐solutionlike composition dependence of the enthalpy of mixing yields an interaction parameter of 5.98 kcal/mole. At a typical growth temperature of 800 °C, the solubility of In in GaN is calculated to be less than 6%. The miscibility gap is expected to represent a significant problem for the epitaxial growth of these alloys.

Measurement of polarization charge and conduction-band offset at InxGa1−xN/GaN heterojunction interfaces

Abstract: The spontaneous and piezoelectric polarization fields in group-III nitride semiconductors lead to the presence of large electrostatic sheet charge densities at nitride semiconductor heterojunction interfaces. Precise quantitative knowledge of these polarization-induced charge densities and of the band-edge discontinuities at nitride heterojunction interfaces is therefore essential in nitride semiconductor device design and analysis. Author(s) have used capacitance–voltage profiling to measure the conduction-band offset and polarization charge density at InxGa1−xN/GaN heterojunction interfaces with x = 0.054 and x = 0.09. Author(s) obtain conduction-band offsets ΔEC = 0.09±0.07 eV for x = 0.054 and ΔEC = 0.22±0.05 eV for x = 0.09, corresponding to an averaged conduction-to-valence-band offset ratio ΔEC:ΔEV of 58:42. Our measurements yield polarization charge densities of (1.80±0.32)×1012 e/cm2 for x = 0.054 and (4.38±0.36)×1012 e/cm2 for x = 0.09. These values are smaller than those predicted by recent theoretical calculations, but in good agreement with values inferred from a number of optical experiments.

Photoluminescence from In0.3Ga0.7N/GaN multiple-quantum-well nanorods[34]

Abstract: The fabrication of In0.3Ga0.7N/GaN multiple-quantum-well nanorods with diameters of 60–100 nm and their optical characteristics performed by micro-photoluminescence measurements are presented. The nanorods were fabricated by inductively coupled plasma dry etching from a light-emitting diode wafer. The structure and surface properties of fabricated nanorods were verified by the field emission scanning electron microscopy and the transmission electron microscopy. The photoluminescence (PL) spectra with sharp linewidths of typically 1.5 nm were observed at 4 K. The excitation-power-dependent spectra show that no energy shift was observed for these sharp peaks. Moreover, increasing the excitation power instead leads to an occurrence of new, sharp PL peaks at the higher energy tail of the PL spectra, which suggest that excitons are strongly confined in quantum-dot-like regions or localization centres.

Interpretation of double x-ray diffraction peaks from InGaN layers

Abstract: The presence of two, or more, x-ray diffraction (XRD) peaks from an InGaN epilayer is sometimes regarded as an indicator of phase segregation. Nevertheless, detailed characterization of an InGaN/GaN bilayer by a combination of XRD and Rutherford backscattering spectrometry (RBS) shows that splitting of the XRD peak may be completely unrelated to phase decomposition. Wurtzite InGaN/GaN layers were grown in a commercial reactor. An XRD reciprocal space map performed on the (105) plane shows that one component of the partially resolved InGaN double peak is practically aligned with that of the GaN buffer, indicating that part of the layer is pseudomorphic to the GaN template. The other XRD component is shown to have the same indium content as the pseudomorphic component, from a consideration of the effect of strain on the c- and a-lattice constants. The composition deduced from XRD measurements is confirmed by RBS. Depth-resolving RBS channeling angular scans also show that the region closer to the GaN/InGaN interface is nearly pseudomorphic to the GaN substrate, whereas the surface region is almost fully relaxed.

Evolution of phase separation in In-rich InGaN alloys[35]

Abstract: Evolution of phase separation in InxGa1−xN alloys (x ∼ 0.65) grown on AlN/sapphire templates by metal organic chemical vapor deposition has been probed. It was found that growth rate, GR, is a key parameter and must be high enough (>0.5 μm/h) in order to grow homogeneous and single phase InGaN alloys. Our results implied that conditions far from thermodynamic equilibrium are needed to suppress phase separation. Both structural and electrical properties were found to improve significantly with increasing GR. The improvement in material quality is attributed to the suppression of phase separation with higher GR. The maximum thickness of the single phase epilayer tmax (i.e., maximum thickness that can be grown without phase separation) was determined via in situ interference pattern monitoring and found to be a function of GR. As GR increases, tmax also increases. The maximum value of tmax for In0.65Ga0.35N alloy was found to be ~ 1.1 μm at GR>1.8 μm/h.

• As G_R increases, phase separation and inhomogeneity are gradually suppressed, as evidenced by the emergence of a single peak from the top InxGa1−xN (x ∼ 0.65) layer.
• once single phase and homogeneous alloy are attained, structural properties remain almost independent of G_R.
• results suggest that G_R needs to be greater than 1.0 μm/h to obtain single phase In0.65Ga0.35N with reasonable homogeneity and crystalline quality.
• It was found that μe increases with GR.
• Author(s) have observed that both XRD and Hall results are improved with increasing G_R. Electron mobility has increased by more than a factor of 2 when GR was increased from 0.5 to 1.4 μm/h while n remained the same, although very high.
• The maximum thickness, tmax, of In0.65Ga0.35N alloy that can be grown without phase separation were found to be strongly correlated with GR.
• Further increase in G_R did not further increase tmax. Author(s) believe that as GR increases, thermodynamic conditions shifted toward more nonequilibriumlike, which promoted the growth of single phase thick layers.
• This and previous studies indicate that growing InGaN alloys far away from the thermodynamic equilibrium conditions (e.g., with higher growth rate) promotes growth of single phase and improves material quality of In-rich InGaN epilayers inside the theoretically predicted miscibility gap region.

X-ray diffraction of III-nitrides

Abstract: The III-nitrides include the semiconductors AlN, GaN and InN, which have band gaps spanning the entire UV and visible ranges. Thin films of III-nitrides are used to make UV, violet, blue and green light-emitting diodes and lasers, as well as solar cells, high-electron mobility transistors (HEMTs) and other devices. However, the film growth process gives rise to unusually high strain and high defect densities, which can affect the device performance. X-ray diffraction is a popular, non-destructive technique used to characterize films and device structures, allowing improvements in device efficiencies to be made. It provides information on crystalline lattice parameters (from which strain and composition are determined), misorientation (from which defect types and densities may be deduced), crystallite size and microstrain, wafer bowing, residual stress, alloy ordering, phase separation (if present) along with film thicknesses and superlattice (quantum well) thicknesses, compositions and non-uniformities. These topics are reviewed, along with the basic principles of x-ray diffraction of thin films and areas of special current interest, such as analysis of non-polar, semipolar and cubic III-nitrides. A summary of useful values needed in calculations, including elastic constants and lattice parameters, is also given. Such topics are also likely to be relevant to other highly lattice-mismatched wurtzite-structure materials such as heteroepitaxial ZnO and ZnSe.

III–nitrides: Growth, characterization, and properties

Abstract: During the last few years the developments in the field of III–nitrides have been spectacular. High quality epitaxial layers can now be grown by MOVPE. Recently good quality epilayers have also been grown by MBE. Considerable work has been done on dislocations, strain, and critical thickness of GaN grown on different substrates. Splitting of valence band by crystal field and by spin-orbit interaction has been calculated and measured. The measured values agree with the calculated values. Effects of strain on the splitting of the valence band and on the optical properties have been studied in detail. Values of band offsets at the heterointerface between several pairs of different nitrides have been determined. Extensive work has been done on the optical and electrical properties. Near band-edge spectra have been measured over a wide range of temperatures. Free and bound exciton peaks have been resolved. Valence band structure has been determined using the PL spectra and compared with the theoretically calculated spectra. Strain and its effect on the optical properties of the III–nitride layers have been studied both theoretically and experimentally. Both n and p conductivity have been achieved. InGaN quantum wells with GaN and AlGaN barriers and cladding layers have been investigated. PL of the quantum wells is affected by confinement effects, band filling, quantum confined Stark effect, and strain. This work has led to the fabrication of advanced optoelectronic and electronic devices. The light-emitting decodes emitting in the blue and green regions of the spectrum have been commercialized. The work leading to these developments is reviewed in this article. The device processing methods and actual devices are not discussed.

Giant electric fields in unstrained GaN single quantum wells

Abstract: Author(s) demonstrate that, even in unstrained GaN quantum wells with AlGaN barriers, there exist giant electric fields as high as 1.5 MV/cm. These fields, resulting from the interplay of the piezoelectric and spontaneous polarizations in the well and barrier layers due to Fermi level alignment, induce large redshifts of the photoluminescence energy position and dramatically increase the carrier lifetime as the quantum well thickness increases.

Thermodynamic and kinetic processes involved in the growth of epitaxial GaN thin films

Abstract: Our experimental results using reactive magnetron sputtering, combined with earlier literature, are used to understand the thermodynamic and kinetic processes involved in GaN film growth and the limiting factors involved in the incorporation of nitrogen during the growth process. Author(s) show that GaN films fabricated with low pressure growth techniques (<0.1 Torr) such as sputtering and molecular beam epitaxy are formed under metastable conditions with a nonequilibrium kinetically limited reaction. For these methods, the growth process is controlled by a competition between the forward reaction, which depends on the arrival of activated nitrogen species at the growing surface, and the reverse reaction whose rate is limited by the unusually large kinetic barrier of decomposition of GaN. In practice, the thermally activated rate of decomposition sets an upper bound to the growth temperature.

Optical bandgap energy of wurtzite InN

Abstract: Wurtzite InN films were grown on a thick GaN layer by metalorganic vapor phase epitaxy. Growth of a (0001)-oriented single crystalline layer was confirmed by Raman scattering, x-ray diffraction, and reflection high energy electron diffraction. Author(s) observed at room temperature strong photoluminescence (PL) at 0.76 eV as well as a clear absorption edge at 0.7–1.0 eV. In contrast, no PL was observed, even by high power excitation, at ∼1.9 eV, which had been reported as the band gap in absorption experiments on polycrystalline films. Careful inspection strongly suggests that a wurtzite InN single crystal has a true bandgap of 0.7–1.0 eV, and the discrepancy could be attributed to the difference in crystallinity.

Activation Energy for the Sublimation of Gallium Nitride

Abstract: Gallium nitride was found to sublime congruently from a torsion—effusion cell when the ratio of orifice area to sample area was about 1/30 and incongruently to yield nitrogen gas and liquid gallium when this ratio was about 1/100 or less. A mass‐spectrometer investigation revealed no measurable concentrations of gallium nitride vapor molecules. The heat of activation for the reaction 2GaN(s)=2Ga(1)+N2(g) was calculated to be 39 kcal at 1300°K from the temperature dependence of the effusion data. The rate of the reaction 2GaN(s)=2Ga(g)+N2(g) was measured by a torsion—Langmuir method. From the temperature dependence of sublimation the heat of activation for this reaction was calculated to be ΔH1300‡=218.6 kcal compared to 173 kcal for the equilibrium reaction, and the entropy of activation was calculated to be 74.3 cal/deg.

Unusual properties of the fundamental band gap of InN

Abstract: The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular-beam epitaxy have been characterized by optical absorption, photoluminescence, and photomodulated reflectance techniques. These three characterization techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The photoluminescence peak energy is found to be sensitive to the free-electron concentration of the sample. The peak energy exhibits very weak hydrostatic pressure dependence, and a small, anomalous blueshift with increasing temperature.

Relaxation Process of the Thermal Strain in the GaN/α-Al2O3 Heterostructure and Determination of the Intrinsic Lattice Constants of GaN Free from the Strain

Abstract: The relaxation process of the thermal strain in a GaN film due to the thermal expansion coefficient difference in the GaN(0001)/α-Al2O3(0001) heterostructure is studied by varying the film thickness of GaN in a wide range from 1 to 1200 µm. The lattice constant c has a large value of 5.191 Å at a film thickness less than a few microns, while it decreases to about 150 µm, and becomes constant above 150 µm, indicating that the strain is almost completely relaxed. The intrinsic lattice constants of wurtzite GaN free from the strain, a0 and c0, are determined to be 3.1892±0.0009 and 5.1850±0.0005 Å, respectively.

Variation of band bending at the surface of Mg-doped InGaN: Evidence of p-type conductivity across the composition range

Abstract: The variation of band bending as a function of composition at oxidized (0001) surfaces of Mg-doped InxGa1−xN is investigated using x-ray photoelectron spectroscopy. Distinctly different trends in barrier height are seen for the Mg-doped compared to undoped alloys, which is explained in terms of Fermi-level pinning at the surface and virtual gap states. Solutions of Poisson’s equation within the modified Thomas-Fermi approximation are used to model the band bending and corresponding variation of carrier concentration with depth below the surface. A transition from a surface inversion layer for In-rich alloys to a surface hole depletion layer for Ga-rich alloys occurs at x≈0.49. The trend in barrier height, calculated space-charge profiles, and difference of barrier height for undoped and Mg-doped InN indicate that Mg doping induces bulk p-type conductivity across the entire composition range.

High internal electric field in a graded-width InGaN/GaN quantum well: Accurate determination by time-resolved photoluminescence spectroscopy

Abstract: Time-resolved photoluminescence (PL), at T = 8 K, is used to study a graded-width InGaN/GaN quantum well. Across the sample, the well width continuously varies from ∼5.5 to 2.0 nm corresponding to PL peak energies varying between 2.0 and 2.9 eV and to PL decay rates covering four orders of magnitude. The plot of decay times versus PL energies is very well fitted by a calculation of the electron–hole recombination probability versus well width. The only fitting parameter is the electric field in the well, which Author(s) find equal to 2.45±0.25 MV/cm, in excellent agreement with experimental Stokes shifts for this type of samples.

Discrimination of local radiative and nonradiative recombination processes in an InGaN/GaN single-quantum-well structure by a time-resolved multimode scanning near-field optical microscopy

Abstract: Precise identification of recombination dynamics based on local, radiative, and nonradiative recombination has been achieved at room temperature in a blue-light-emitting InxGa1−xN/GaN single-quantum-well structure by comparing the photoluminescence (PL) spectra taken by illumination-collection mode (I-C mode) and those by illumination mode (I-mode) in scanning near-field microscopy. The PL data mapped with PL lifetimes, as well as with PL spectra, revealed that the probed area could be classified into four different regions whose dominating processes are (1) radiative recombination within a probing aperture, (2) nonradiative recombination within an aperture, (3) diffusion of photogenerated excitons/carriers out of an aperture resulting in localized luminescence, and (4) the same diffusion process as (3), but resulting in nonradiative recombination.

Determination of the chemical composition of distorted InGaN/GaN heterostructures from x-ray diffraction data

Abstract: An evaluation algorithm for the determination of the chemical composition of strained hexagonal epitaxial films is presented. This algorithm is able to separate the influence of strain and composition on the lattice parameters measured by x-ray diffraction. The measurement of symmetric and asymmetric reflections delivers the strained lattice parameters a and c of hexagonal epitaxial films. These lattice parameters are used to calculate the relaxed lattice parameters employing the theory of elasticity. From the relaxed parameters, the chemical composition of the epitaxial film can be determined by Vegard's rule. The algorithm has been applied to InGaN/GaN/Al2O3(00.1) heterostructures.

Heteroepitaxial growth of In-face InN on GaN (0001) by plasma-assisted molecular-beam epitaxy

Abstract: The thermodynamic aspects of indium-face InN growth by radio frequency plasma-assisted molecular-beam epitaxy (rf-MBE) and the nucleation of InN on gallium-face GaN (0001) surface were investigated. The rates of InN decomposition and indium desorption from the surface were measured in situ using reflected high-energy electron diffraction and the rf-MBE “growth window” of In-face InN (0001) was identified. It is shown that sustainable growth can be achieved only when the arrival rate of active nitrogen species on the surface is higher than the arrival rate of indium atoms. The maximum substrate temperature permitting InN growth as a function of the active nitrogen flux was determined. The growth mode of InN on Ga-face GaN (0001) surface was investigated by reflected high-energy electron diffraction and atomic force microscopy. It was found to be of the Volmer–Weber-type for substrate temperatures less than 350 °C and of the Stranski–Krastanov for substrate temperatures between 350 and 520 °C. The number of monolayers of initial two-dimensional growth, in the case of Stranski–Krastanov mode, varies monotonically with substrate temperature, from 2 ML at 400 °C to about 12 ML at 500 °C. The evolution and coalescence of nucleated islands were also investigated as a function of substrate temperature. It was found that at higher temperature their coalescence is inhibited leading to porous-columnar InN thin films, which exhibit growth rates higher than the nominal value. Therefore, in order to achieve continuous InN layers on GaN (0001) a two-step growth approach is introduced. In that approach, InN is nucleated at low temperatures on GaN and the growth continues until full coalescence of the nucleated islands. Subsequently, this nucleation layer is overgrown at higher substrate temperature in order to achieve high-quality continuous films. The InN films grown by the two-step method were investigated by x-ray diffraction, Hall-effect measurements, and transmission electron microscopy. It was found that the lattice mismatch between InN and GaN is almost completely accommodated by the development of a misfit dislocation network at the interface. Optimum group-III to active nitrogen flux ratios and substrate temperature conditions were identified for the two-step growth process. Films, grown under those conditions, exhibited full width at half maximum of x-ray rocking curves at (0004) and (105) diffractions equal to 360 and 435 arc sec, respectively. Room-temperature Hall mobility was found to depend sensitively on the group-III to active nitrogen flux ratio during growth of the main step and to be independent of the structural properties of the films. Mobilities up to 860 cm2/V s at carrier concentration of 1.6×1019 cm−3 were measured.

Compositional modulation in InxGa1−xN: TEM and X-ray studies

Abstract: Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD) have been used to study compositional modulation in InxGa1−x N layers grown with compositions close to miscibility gap. The samples (0.34 < x < 0.8) were deposited by molecular beam epitaxy using either a 200 nm thick AlN or GaN buffer layer grown on a sapphire substrate. Periodic compositional modulation leads to extra electron diffraction spots and satellite reflections in XRD in the theta–2theta coupled geometry. The ordering period Δ measured along c-axis was about Δ = 45 Å for x = 0.5 and Δ = 66 Å for x = 0.78 for samples grown on AlN buffer layer. TEM and XRD determinations of Δ were in good agreement. Compositional modulation was not observed for the sample with x = 0.34 grown on a GaN buffer layer. Larger values of Δ were observed for layers with higher In content and for those having larger mismatch with the underlying AlN buffer layer. The possibility that the roughness of the AlN growth surface promotes strong In segregation on particular crystallographic planes leading to compositional modulation is considered.

Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures

Abstract: Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×1012 to 2×1013 cm−2 in the GaN channel with increasing Al-concentration from x = 0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by piezoelectric and spontaneous polarization effects.

Minority carrier diffusion length and lifetime in GaN

Abstract: Electron beam induced current measurements on planar Schottky diodes on undoped GaN grown by metalorganic chemical vapor deposition are reported. The minority carrier diffusion length of 0.28 μm has been measured, indicating minority carrier lifetime of 6.5 ns. The tapping mode atomic force microscopy imaging of the surfaces and scanning electron microscopy of the cross sections have been used to characterize the linear dislocations and columnar structure of the GaN. The possible influence of recombination on the extended defects in GaN on the minority carrier diffusion length and lifetime is discussed, and contrasted to other recombination mechanisms.

Absorption coefficient, energy gap, exciton binding energy, and recombination lifetime of GaN obtained from transmission measurements

Abstract: The absorption coefficient for a 0.4-μm-thick GaN layer grown on a polished sapphire substrate was determined from transmission measurements at room temperature. A strong, well defined exciton peak for the A and B excitons was obtained. The A, B, and C excitonic features are clearly defined at 77 K. At room temperature, an energy gap Eg = 3.452±0.001 eV and an exciton binding energy ExA,B = 20.4±0.5 meV for the A and B excitons and ExC = 23.5±0.5 meV for the C exciton were determined by analysis of the absorption coefficient. From this measured absorption coefficient, together with the detailed balance approach of van Roosbroek and Shockley, the radiative constant B = 1.1×10−8 cm3/s was obtained.

Microcavity effects in GaN epitaxial films and in Ag/GaN/sapphire structures

Abstract: Luminescence spectra of GaN epitaxial layers grown on sapphire display a strong intensity modulation of the below-band gap transitions and on the low-energy side of the near-band gap transition. The intensity modulation is attributed to a microcavity formed by the semiconductor–air and semiconductor–substrate interface. The microcavity effect is enhanced by using metallic reflectors which increase the cavity finesse. It is shown that microcavity effects can be used to determine the refractive index of the microcavity active material. Using this method, the GaN refractive index is determined and expressed analytically by a Sellmeir fit.

Nonalloyed ohmic contacts on GaN using InN/GaN short‐period superlattices

Abstract: It is well known that ohmic contacts on GaN, a highly promising material for electronic and optoelectronic devices with a wide band gap of about 3.4 eV, constitute a major obstacle to further development of devices based on this material. Author(s) demonstrated a novel scheme of nonalloyed ohmic contacts on GaN using a short‐period superlattice (SPS), composed of GaN and narrow band‐gap InN, sandwiched between the GaN channel and an InN cap layer. Comparison with a similar layer without the SPS structure indicates that quantum tunneling through the SPS conduction band effectively reduces the potential barrier formed by the InN/GaN heterostructure leading to low contact resistivities. From the transmission‐line‐method measurements, specific contact resistances as low as 6×10−5 Ω cm2 with GaN doped at about 5×1018 cm−3 have been obtained without any post‐annealing. Theoretical estimation based on the SPS tunneling model is consistent with the experiment.

RF-Molecular Beam Epitaxy Growth and Properties of InN and Related Alloys

Abstract: The fundamental band gap of InN has been thought to be about 1.9 eV for a long time. Recent developments of metalorganic vapor phase epitaxy (MOVPE) and RF-molecular beam epitaxy (RF-MBE) growth technologies have made it possible to obtain high-quality InN films. A lot of experimental results have been presented very recently, suggesting that the true band-gap energy of InN should be less than 1.0 eV. In this paper, Author(s) review the results of the detailed study of RF-MBE growth conditions for obtaining high-quality InN films. The full widths at half maximum (FWHMs) of ω-mode X-ray diffraction (XRD), ω–2θ mode XRD and E2 (high-frequency)-phonon-mode peaks in the Raman scattering spectrum of the grown layer were 236.7 arcsec, 28.9 arcsec and 3.7 cm-1, respectively. The carrier concentration and room temperature electron mobility were 4.9×1018 cm-3 and 1130 cm2/Vs, respectively. Photoluminescence and optical absorption measurements of these high-quality InN films have clearly demonstrated that the fundamental band gap of InN is about 0.8 eV. Studies on the growth and characterization of InGaN alloys over the entire alloy composition further supported that the fundamental band gap of InN is about 0.8 eV.

Intrinsic Electron Accumulation at Clean InN Surfaces

Abstract: The electronic structure of clean InN(0001) surfaces has been investigated by high-resolution electron-energy-loss spectroscopy of the conduction band electron plasmon excitations. An intrinsic surface electron accumulation layer is found to exist and is explained in terms of a particularly low Γ-point conduction band minimum in wurtzite InN. As a result, surface Fermi level pinning high in the conduction band in the vicinity of the Γ point, but near the average midgap energy, produces charged donor-type surface states with associated downward band bending. Semiclassical dielectric theory simulations of the energy-loss spectra and charge-profile calculations indicate a surface state density of 2.5   (±0.2)×1013   cm-2 and a surface Fermi level of 1.64±0.10   eV above the valence band maximum.

Origin of defect-insensitive emission probability in In-containing (Al,In,Ga)N alloy semiconductors

Abstract: Group-III-nitride semiconductors have shown enormous potential as light sources for full-colour displays, optical storage and solid-state lighting. Remarkably, InGaN blue- and green-light-emitting diodes (LEDs) emit brilliant light although the threading dislocation density generated due to lattice mismatch is six orders of magnitude higher than that in conventional LEDs. Here Author(s) explain why In-containing (Al,In,Ga)N bulk films exhibit a defect-insensitive emission probability. From the extremely short positron diffusion lengths (<4 nm) and short radiative lifetimes of excitonic emissions, Author(s) conclude that localizing valence states associated with atomic condensates of In–N preferentially capture holes, which have a positive charge similar to positrons. The holes form localized excitons to emit the light, although some of the excitons recombine at non-radiative centres. The enterprising use of atomically inhomogeneous crystals is proposed for future innovation in light emitters even when using defective crystals.

Calculation of critical layer thickness versus lattice mismatch for GexSi1−x/Si strained‐layer heterostructures

Abstract: A calculation of the critical layer thickness hc for growth of GexSi1−x strained layers on Si substrates is presented for 0≤x≤1.0. The present results are obtained assuming misfit dislocation generation is determined solely by energy balance. This approach differs therefore from previous theories (e.g., Matthews et al.), in which the absence of mechanical equilibrium for grown‐in threading dislocations determines the onset of the generation of interfacial misfit dislocations. It is assumed that interfacial misfit dislocations will be generated when the areal strain energy density of the film exceeds the energy density associated with the formation of a screw dislocation at a distance from the free surface equal to the film thickness h. For films thicker than this critical value, screw (and edge) dislocations will be generated at the film/substrate interface. Values obtained for the critical thickness versus lattice mismatch are in excellent agreement with measurements of hc for GexSi1−x strained layers on Si substrates.

248 nm cathodoluminescence in Al1−xInxN(0001) thin films grown on lattice-matched Ti1−yZryN(111) seed layers by low temperature magnetron sputter epitaxy

Abstract: Single-crystal Al0.8In0.2N(0001) thin films were grown epitaxially onto lattice-matched Ti0.2Zr0.8N(111) seed layers on MgO(111) substrates at 300 °C by magnetron sputter epitaxy. Low-energy ion-assisted epitaxial growth conditions were achieved by applying a substrate potential of −15 V. Cross-sectional high-resolution electron microscopy verified the epitaxy and high-resolution x-ray diffraction ω-rocking scans of the Al0.8In0.2N 0002 peak (full width at half maximum ∼ 2400 arc sec) indicated a high structural quality of the films. Cathodoluminescence measurements performed in a scanning electron microscope at 5 K revealed Al0.8In0.2N luminescence at 248 nm, or equivalently 5.0 eV, showing that Al0.8In0.2N is a promising material for deep-ultraviolet optoelectronic devices.

Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV

Abstract: Author(s) report values of pseudodielectric functions 〈ε〉=〈ε1〉+i〈ε2〉 measured by spectroscopic ellipsometry and refractive indices ñ=n+ik, reflectivities R, and absorption coefficients α calculated from these data. Rather than correct ellipsometric results for the presence of overlayers, Author(s) have removed these layers as far as possible using the real-time capability of the spectroscopic ellipsometer to assess surface quality during cleaning. Author(s) results are compared with previous data. In general, there is good agreement among optical parameters measured on smooth, clean, and undamaged samples maintained in an inert atmosphere regardless of the technique used to obtain the data. Differences among Author(s) data and previous results can generally be understood in terms of inadequate sample preparation, although results obtained by Kramers-Kronig analysis of reflectance measurements often show effects due to improper extrapolations. The present results illustrate the importance of proper sample preparation and of the capability of separately determining both ε1 and ε2 in optical measurements.

Hydrogen generation by solar water splitting using p-InGaN photoelectrochemical cells

Abstract: Photoelectrochemical effects in p-InxGa1−xN (0 ≤ x ≤ 0.22) alloys have been investigated. Hydrogen generation was observed in p-InGaN semiconducting electrodes under white light illumination with additional bias. It was found that p-InGaN alloys possess much higher conversion efficiencies than p-GaN. Time dependent photocurrent density characteristics showed that the stability of p-InGaN in aqueous HBr is excellent. The photocurrent density was found to increase almost linearly with hole mobility and excitation light intensity.

Direct hydrogen gas generation by using InGaN epilayers as working electrodes

Abstract: Author(s) report on the growth and exploitation of InGaN epilayers as a photoelectrochemical cell (PEC) material for direct generation of hydrogen by splitting water using photoelectrochemical hydrolysis. Under white light illumination, a drastic dependence of the photocurrent density on the In content was observed. Direct hydrogen gas generation by splitting water was accomplished using an n-type InxGa1−xN epilayer with a relatively high In content (x ∼ 0.4) as a working electrode. This demonstration of hydrogen generation by water splitting accomplished using InGaN based PEC is highly encouraging.

Activation of acceptors in Mg‐doped GaN grown by metalorganic chemical vapor deposition

Abstract: The activation kinetics of acceptors was investigated for heteroepitaxial layers of GaN, doped with Mg. After growth, the samples were exposed to isochronal rapid thermal anneals in the temperature range from 500 to 775 °C. The samples were studied by variable temperature Hall effect measurements and photoluminescence (PL) spectroscopy in the as‐grown condition and after each temperature step. The thermal treatment reduced the resistivity by six orders of magnitude and the p‐type conductivity was found to be dominated by an acceptor with an activation energy of ∼170 meV. This acceptor is attributed to Mg atoms substituting for Ga in the GaN lattice and the activation process is consistent with dissociation of electrically inactive Mg–H complexes. It is shown that the appearance of a blue emission band in the PL spectrum of Mg‐doped GaN does not directly correlate with the increase in p‐type conductivity.

Phase separation in InGaN thick films and formation of InGaN/GaN double heterostructures in the entire alloy composition

Abstract: Author(s) report the growth of InGaN thick films and InGaN/GaN double heterostructures by molecular beam epitaxy at the substrate temperatures 700–800 °C, which is optimal for the growth of GaN. X-ray diffraction and optical absorption studies show phase separation of InN for InxGal−xN thick films with x>0.3. On the other hand, InxGal−xN/GaN double heterostructures show no evidence of phase separation within the detection capabilities of Author(s) methods. These observations were accounted for using Stringfellow’s model on phase separation, which gives a critical temperature for miscibility of the GaN–InN system equal to 2457 K.

Low-resistance nonalloyed ohmic contact to p-type GaN using strained InGaN contact layer

Abstract: A strained InGaN contact layer inserted between Pd/Au and p-type GaN resulted in low ohmic contact resistance without any special treatments. The thickness and In mole fraction of the p-type InGaN varied from 2 nm to 15 nm and from 0.14 to 0.23, respectively. Strained InGaN layers are effective in reducing the contact resistance. A contact layer of 2 nm thick strained In0.19Ga0.81N showed the lowest specific contact resistance of 1.1×10−6 Ω cm2. The mechanism for the lower contact resistance is ascribed to enhanced tunneling transport due to large polarization-induced band bending at the surface as well as to the high hole concentration in p-type InGaN.

Universality of electron accumulation at wurtzite c- and a-plane and zinc-blende InN surfaces

Abstract: Electron accumulation is found to occur at the surface of wurtzite (11-20), (0001), and (000-1) and zinc-blende (001) InN using x-ray photoemission spectroscopy. The accumulation is shown to be a universal feature of InN surfaces. This is due to the low Γ-point conduction band minimum lying significantly below the charge neutrality level.

When group-III nitrides go infrared: New properties and perspectives

Abstract: Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼ 1.9 μm (0.64 eV for InN) to the ultraviolet at ∼ 0.36 μm (3.4 eV for GaN) or 0.2 μm (6.2 eV for AlN). The continuous range of bandgap energies now spans the near infrared, raising the possibility of new applications for group-III nitrides. In this article author(s) present a detailed review of the physical properties of InN and related group III-nitride semiconductors. The electronic structure, carrier dynamics, optical transitions, defect physics, doping disparity, surface effects, and phonon structure will be discussed in the context of the InN bandgap re-evaluation. Author(s) will then describe the progress, perspectives, and challenges in the developments of new electronic and optoelectronic devices based on InGaN alloys. Advances in characterization and understanding of InN and InGaN nanostructures will also be reviewed in comparison to their thin film counterparts.

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