This is in a series of literature reviews on InGaN solar cells, which supported the comprehensive review by D.V.P. McLaughlin & J.M. Pearce, "Progress in Indium Gallium Nitride Materials for Solar Photovoltaic Energy Conversion"Metallurgical and Materials Transactions A 44(4) pp. 1947-1954 (2013). open access
Others: InGaN solar cells| InGaN PV| InGaN materials| InGan LEDs| Nanocolumns and nanowires| Optical modeling of thin film microstructure| Misc.

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Type Literature review
Authors User:Ankitvora
Chenlong Zhang
Published 2011
License CC-BY-SA-4.0
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InGaN Material Characterization.[edit | edit source]

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

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 InxGa1-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][edit | edit source]

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][edit | edit source]

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][edit | edit source]

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][edit | edit source]

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: EG(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][edit | edit source]

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][edit | edit source]

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][edit | edit source]

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
  • The secondary maximum at lower photon energies can also be observed in some of the samples, which signifies that the emssion peak is composed of a superposition of peaks certred at different photon energies.
  • When excitation photon energy falls below GaN energy gap, in PL spectrum, lines at 2.75 and 2.85 disappear, but the main feature of the spectrum still appears. The only feature in the PL spectra can be attributed to InGaN layer, but it is not the build layer of average composition that responsible for this absorption. Rather, the light seems to be absorbed by an In-rich phase occupying a negligible fraction of the layer's volume.
  • 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 with dimensions small enough to produce a substantial confinement of the carriers trapped in them.
  • The rather large energy difference between the PL maximum and the absorption edge in a given sample is due to the difference between the energy gaps of the two phases.

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

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][edit | edit source]

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.

  • Due to the significant different evaporation temperature between indium and gallium, more than 95% of indium atoms evaporated from the terraces and step edges before being incorported into the solid state.
  • Indium incorporation increases with growth rate for InGaN layers indicating that high growth rate may to some extend keep indium from escaping away from the solid layer.
  • XRD scans for InGaN grown on GaN substrates with different misorientation indicates that InGaN tends to have smaller indium content if grown on large misorientation substrate, but the growth rate in vertial direction was independent of substrate misorientation.
  • AFM morphology of InGaN and InAlN layers shows that for the smallest GaN substarte misorientation, spiral steps around dislocations can be seen,these atomic steps are jagged, which is typical for InGaN if grown in N2 atmosphere without using hydrogen as a carrier gas. For larger misorientation the structure of steps very similar but are direct, indicating a step-flow growth mode. InAlN layers grown on the same GaN substrates all show three-dimensional growth.
  • Assuptions: different indium incorporation for different substrate misorientations indicate that only for perfectly identical atomic steps we would get non-segregated InGaN layers.The morphology of InGaN layer and In-segregation are closely interrelated.

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

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][edit | edit source]

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.
  • For the thick films, PL emission is independent of thickness and is characterized by band edge emiision indicative of relaxed InGaN films. For films with intermediate layer thickness, PL emission FWHM is broad due to the domination of deep levels emitting accompanied with a lower intensity emission. For thin films the PL emission spectra revealed a deep edge emission due to strained films.
  • 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 but needed other techniques and approaches to confirm the values obtained for 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][edit | edit source]

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.In this paper the author combine in-situ ellipsometric thickness measurements with ex-situ X-ray reciprocal space mapping to clearly distinguish relaxation from decomposition.
  • 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.
  • The author concludes that values only obtained from PL data is not suitable for determine CLT of high indium composition layer.
  • 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][edit | edit source]

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.

  • Usingeletron-beam lithography and ICP RIE the author(s) fabricate InGaN/GaN nanoposts and conclude that the effects of strain relaxation and quantum confinement are both important in controlling the optical properties of such a nanostructure.
  • The author attributes that blue shift in PL spectrum to two reasons:the strain relaxation and quantum confinement. The trend of decreasing spectral width can be due to the reductions of the QW petential tilt and the range of indium composition fluctuation in the nanoposts.
  • 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. However the strain is not completely relax even in a quantum dics diameter is as small as 11-18nm.
  • 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][edit | edit source]

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][edit | edit source]

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.

  • Quantum confinement Stark effect(QCSE) caused by strong peizoelectric polarization in strained InGaN/GaN QWs oriented in [0001] direction is the primary reason accounts for poor external quantum efficiency of InGaN/GaN QW.
  • To use planes tilted with respect to the [0001] direction is an approach to decrease the polarity of the plane, and that leads to optimized external quantum efficiency of InGaN/GaN QWs
  • The author choose {11-22} facet as a ideal facet for high emission efficiency due to 1) its proposed angle of 56 degree with respect to (0001) lower the polarity; 2)it lacks of dislocations since threading dislocations propagated toward the [0001] direction and some were bent by 90 degree toward the <11-20> direction.
  • In composition monotonously increased from 25% on the (11-20) side to 40% on the (0001) side. This is caused by growth characteristics of InGaN on GaN microfacets.growth rate became faster in the order (0001)>{11-22)>{11-20},suggesting that the atoms migrated from {11-20} toward (0001) through the {11-22} facet.
  • Since the In distribution in a plane is controlled by the growth conditions, various apparent colors including white can be realized using only the structure proposed in this study.
  • High internal efficiency was confirm by temperature dependence of the integrated PL intensity, which is twice as high in the {11-22} as in the (0001) QW.

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

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.

  • XRD(002) theta-2theta spectra of InGaN alloys grown on GaN/Al2O3 templates shows no multiple peaks even at InN peak position, impies that InGaN alloys are not phase separated.
  • Previously thought miscibility gap by MOCVD may be attributed to 1) the presence of strain between the InGaN thin film and the epitemplate. 2) nonequilibrium growth processes taking place in epitaxial growth techniques like MOCVD and 3) relatively low growth temperatures.
  • FWHM of theta-2theta curves depends on many factors, including the homogeneity of the solid solution.
  • Both the growth rate and In content increase linearly as the growth temperature decreases. In content can also be increased by increasing growth rate, growth pressure, and flow rate of TMIn but these factors are all less pronounced as compared to the growth temperature.
  • Root-mean-square, the surface roughness quantification, was found to be increased from 1.5 to 4.0 as x increases from 0.25 to 0.59. No indium droplets on the surface were observed in any sample.

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

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][edit | edit source]

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.

  • PL spectra of InGaN samples grown at different temperature showing strong BE emissions for >750C and dominant IT(impurity transition) for <750C.
  • SEM image revealed that the samples have sub-micron size and a hexagonal hillock patterned surface when deposited at 750¡C and higher temperatures.
  • Microcavity can enhance the PL intensity.
  • High growth temperature plays an essential role in surface roughening and pattern formation in Low content samples that consequently helps the PL signal increase.

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