|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.
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.
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.
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.
- EG(n) as a function of charge carrier concentrations for GaAs, InN, and GaN crystals shows a nearly linear dependence on n1/3 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.
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.
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[edit | edit source]
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.
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.
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[edit | edit source]
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[edit | edit source]
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[edit | edit source]
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[edit | edit source]
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.
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[edit | edit source]
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.
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[edit | edit source]
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.
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.
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.
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 GR 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 GR.
- results suggest that GR 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 GR. 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 GR 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.
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.
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