|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.
Photovoltaic (PV) cells convert the energy from the sun into useful electrical energy. Indium gallium nitride (InGaN) is a III-N type semiconductor material, meaning elements from group III are combined with nitrogen to produce a semiconductor, that is gaining ground in the PV market as a viable and tunable device. By varying the composition of the material, the band gap of the material (the energy level at which the material responds most efficiently to incoming light) can be shifted. Typically, the composition of such alloys is written as InxGa1-xN, with x indicating the atomic percent portion of In in the alloy. The following provides a comprehensive background on the current literature available for the material, with links provided to the original documents where possible. This article is a work-in-progress, and will be updated continuously.
To quickly calculate the bandgap of InGaN as a function of In fraction, x use: File:InGaN bandgap calc.ods
Construction[edit | edit source]
The typical construction for an InxGa1-xN cell as grown on a silicon, sapphire or glass substrate. The amorphous GaN (a-GaN) layer is deposited to match the crystal lattices between the InGaN and SiO2 layers, as the mis-match introduces residual stresses into the crystal lattice, leading to distorted optical and electrical properties. The top layer of InGaN is the layer in which solar energy is converted to electrical energy.
Overview Articles[edit | edit source]
Progress in Indium Gallium Nitride Materials for Solar Photovoltaic Energy Conversion[edit | edit source]
- Comprehensive 2013 review
Abstract: The world requires inexpensive, reliable, and sustainable energy sources. Solar photovoltaic (PV) technology, which converts sunlight directly into electricity, is an enormously promising solution to our energy challenges. This promise increases as the efficiencies are improved. One straightforward method of increasing PV device efficiency is to utilize multi-junction cells, each of which is responsible for absorbing a different range of wavelengths in the solar spectrum. Indium gallium nitride (In x Ga1−x N) has a variable band gap from 0.7 to 3.4 eV that covers nearly the whole solar spectrum. In addition, In x Ga1−x N can be viewed as an ideal candidate PV material for both this potential band gap engineering and microstructural engineering in nanocolumns that offer optical enhancement. It is clear that In x Ga1−x N is an extremely versatile potential PV material that enables several known photovoltaic device configurations and multi-junctions with theoretic efficiencies over 50 pct. This potential is driving immense scientific interest in the material system. This paper reviews the solar PV technology field and the basic properties of In x Ga1−x N materials and PV devices. The challenges that remain in realizing a high-efficiency In x Ga1−x N PV device are summarized along with paths for future work. Finally, conclusions are drawn about the potential for In x Ga1−x N photovoltaic technology in the future.
InGaN: An overview of the growth kinetics, physical properties and emission mechanisms.[edit | edit source]
An excellent overview of the efforts to characterize the properties of InGaN. This article is a great resource to those seeking general information about the properties of InGaN and it summarizes the findings from many experiments carried out over the past decade. Optical, electrical, and growth properties are reviewed and discussed. As well, this overview discusses some of the uncertainities surrounding InGaN and the various theories that have been proposed to account for observed optical properties.
Complete compositional tunability of InGaN nanowires using a combinatorial approach.[edit | edit source]
This article covers the first experiment in which the composition of InxGa1-xN nanowires were varied in composition over the entire range of x = 0 to x = 1, that is, from pure GaN to pure InN. It demonstrates the tunability of the band gap of InGaN from the near infrared region to the near ultraviolet region. The article also discusses some of the challenges of growing these nanowires which include:
- Threading dislocations in both a-GaN and InGaN layers which lead to recombination centres, significantly reducing efficiencies within the cell.
- An improved technique over hydride vapour-phase epitaxy, which is only a viable method up to x = 0.2, due to the elevated levels of hydrogen which are known to interfere with the incorporation of In into the crystal lattice.
- Reduced effects of carbon contamination as observed in metal-organic chemical vapour deposition (MOCVD).
The procedure used involved a horizontal single-zone furnace divided into four temperature zones, which was used to evaporate the raw materials onto Si or sapphire substrates at unique compositions determined by the precursor mixing gradients of temperature and evaporation rates.
Several tests were then performed on the resulting material, including X-ray diffraction (XRD) to verify composition, scanning electron microscopy (SEM) images were captured to determine crystal structure and transmission electron microscopy (TEM) to verify ordered crystal structure within nanowires. Optical analysis demonstrated the PL spectrum over which the cell responded (varied from 1.0 eV to 4.0 eV, or approximately 325 nm to 850 nm).
InGaN Material Characterization[edit | edit source]
Effects of Substrate Temperature on Indium Gallium Nitride Nanocolumn Crystal Growth[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.
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). We 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.
- The growth of the GaN and InGaN films of different thickness (Table 1) was performed in a standard MBE system equipped with a radio frequency nitrogen plasma source from EPI.
- They were either grown directly on sapphire without any nitridation or on 2 μm thick MOVPE-GaN/sapphire template layers.
- Hexagonal GaN grown in the wurtzite structure belongs to the point group C6ν having four atoms per unit cell. The group theory predicts eight sets of phonon modes: 2A1 + 2B + 2E1 + 2E2 from which the E2 modes are Raman active, the A1 and E1 modes are both Raman and infrared active, one A1 and one E1 are acoustic modes. The B modes are silent. (read once more)
- The lines of InGaN/GaN/Al2O3 are very faint and are superposed on GaN structure. No phonon modes related to the cubic phase are observed in Raman spectra of the samples.
- The optical band gap (Eg) of GaN/Al2O3 samples can be determined directly from the peak energy of the interband transition structure in the real part of pseudo-dielectric function 〈 ɛ1 〉 spectrum. For the InxGa1−xN/GaN/Al2O3 samples the strong oscillatory behavior near Eg leads to difficulties in determining the Eg energies of the alloys directly from the pseudo-dielectric function, 〈 ɛ1 〉 , spectra of the samples (see Fig. 3 top).
- The In0.14Ga0.86N samples were modeled with a four layer system (from top to bottom): the surface roughness layer, the InGaN layer, GaN template layer and sapphire. The roughness layer was modeled as 50% voids + 50% InGaN using the Bruggeman effective medium approximation.
- The 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. In order to obtain appropriate reference data for InGaN we performed our own in situ SE measurements. We found a linear dependence of the Eg on In content (x), for x ≤ 14%.
Optical band gap in Ga(sub 1−x)In(sub x)N (0<x<0.2) on GaN by photoreflection spectroscopy[edit | edit source]
Abstract: The optical band gap in 40 nm Ga12xInxN/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: b52.6 eV ~PR! and b53.2 eV ~PL in localized states!. Using experimental deformation potentials of GaN, b53.8 eV is extrapolated for the optical band gap in relaxed GaInN material. Previously reported smaller values are discussed.
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 In12xGaxN 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.
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 InxGa12xN 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.
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 (0001)-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 1090C. 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.
Photoluminescence measurements on cubic InGaN layers deposited on a SiC substrate.[edit | edit source]
Abstract: This article looks at InGaN thin films deposited on SiC substrate with an intermediate GaN layer. PL spectroscopy at temperatures from 2.5 K to 200 K show the temperature dependance of PL peaks generated through defects. As well, the broadening of the PL peaks supports phase segregation of the InGaN into small clusters of indium-rich regions. The authors also found a large Stokes-like shift between absorption and emission measurements, which they attribute to the indium-rich clusters. Even though the indium-rich clusters occupied a tiny fraction of the total volume, the sites have a high recombination efficiency. Coupled with the fact that most of the absorption is occurring in the bulk, the Stokes-like shift observed supports this theory.
Abstract: InxGa1−xN thin films mainly having large InN molar fractions are grown on α-Al2O3 (0001) and GaAs (111) B substrates by reactive evaporation, and some properties of them are investigated. C-axis oriented InxGa1−xN films are similarly obtained on each substrate; however, their crystallinity deteriorates with increasing GaN molar fractions. Band gap energies of these films are also measured and the bowing parameter is estimated.
Dielectric function and Van Hove singularities for In-rich InxGa1−xN alloys: Comparison of N- and metal-face materials[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 zerodensity 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 vapour phase epitaxy[edit | edit source]
Abstract: Experimental data on indium incorporation into 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 800oC), incorporation of indium increases with the growth rate, and similarly, with a decrease of 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 neither the growth rate, nor GaN substrate misorientation, were observed.
Abstract: The c0 lattice parameter, band gap, and photoluminescence spectra of n-type 0.9 mm thick InxGa12xN films with x=0, 0.045, 0.085, and 0.140 were examined. The c0 lattice parameter followed Vegard's law using c=0.5185 nm for GaN and c=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.
Abstract: We present an approach to determine the critical layer thickness in the InxGa12xN/GaN heterostructure based on the observed change in the photoluminescence emission as the InxGa12xN film thickness increases. From the photoluminescence data, we 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 InxGa12xN surface morphology with thickness, and is consistent with x-ray diffraction measurements.
Abstract: The critical thickness for the relaxation of InGaN layers grown on (0001)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 temperature effects on InxGa1−xN films studied by X-ray and photoluminescence[edit | edit source]
Abstract: The InGaN films were grown between 850C and 600C by the metalorganic chemical vapor deposition method and characterized by X-ray di¤raction 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 di¤raction where the line width broadens sharply from less than 300 arcsec to larger than 500 arcsec. We also found that IT is relatively insensitive to the sample temperature. Besides, the correlation between enhancing PL intensity and patterned micro-structure is observed.
Investigation on the Correlation Between the Crystalline and Optical Properties of InGaN Using Near-Field Scanning Optical Microscopy[edit | edit source]
Abstract: We 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. We 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 In(sub x)Ga(sub 1−x)N layers[edit | edit source]
Abstract: The effect of strain on the compositional and optical properties of a set of epitaxial single layers of InxGa12xN 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 (e zz) 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 InxGa12xN 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)53.39– 3.57x eV. Extension of this behavior to higher concentrations is discussed on the basis of reported results.
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[edit | edit source]
Abstract: We have measured photoluminescence spectra of molecular-beam-epitaxy-grown cubic GaN/InxGa12xN/GaN double heterostructures with x between 0.09 and 0.33. We 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 x50.5660.02 embedded in the InGaN layers. Including strain effects we calculate a gap energy Eg52.13 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.
- 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).
- InGaN: An overview of the growth kinetics, physical properties and emission mechanisms. F.K. Yam, Z. Hassan. Superlattices and Microstructures, Volume 43, Issue 1. (Jan 2008)
- Complete compositional tunability of InGaN nanowires using a combinatorial approach.T. Kuykendall, Philipp Ulrich, Shaul Aloni, Peidong Yang. Nature Vol. 6 (Oct 2007)
- Effects of Substrate Temperature on Indium Gallium Nitride Nanocolumn Crystal Growth. S. Keating, M.G. Urquhart, D.V.P. McLaughlin and J.M. Pearce, "Effects of Substrate Temperature on Indium Gallium Nitride Nanocolumn Crystal Growth", Crystal Growth & Design, 11 (2), pp 565–568, 2011
- Photoluminescence measurements on cubic InGaN layers deposited on a SiC substrate. D. G. Pacheco-Salazar, J. R. Leite, F. Cerdeira, E. A. Meneses, S. F. Li, D. J. As, and K Lischka. Semicond. Sci. Technol. 21 No 7 (July 2006) 846-851.