InGaN solar cells literature review
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|This is in a series of literature reviews on InGaN solar cells, which supported the 2013 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 coming 2/23/2014) Others:|
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.
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
- 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.
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.
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
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%.
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.
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
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.
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.
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
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.
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
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.
 Photoluminescence associated with quantum dots in cubic GaN=InGaN=GaN double heterostructures
Abstract: We 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 di9raction 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.
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.
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.
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 In(x)Ga(1−x)N films grown by metal organic chemical vapor deposition
Abstract: We 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, we 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.
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. Our 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.
Abstract: We 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.
Correlation of crystalline defects with photoluminescence of InGaN layers. N. Faleev, B. Jampana, O. Jani, H. Yu, R. Opila, I. Ferguson and C. Honsberg. APPLIED PHYSICS LETTERS 95, 051915 (2009)
Abstract: MOVPE growth and Mgdoping 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.
MOVPE growth and Mg doping of InxGa1xN (x~0.4) for solar cell. M. Horie, K. Sugita, A. Hashimoto and A. Yamamoto. Solar Energy Materials & Solar Cells 93(2009)1013–1015
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.
 High-quality In 0.47 Ga 0.53 N/GaN heterostructure on Si(111) and its application to MSM detector
Abstract: Purpose – This paper aims to report on the use of radio frequency nitrogen plasma-assisted molecular beam epitaxy (RF-MBE) to grow high-quality n-type In0.47Ga0.53N/GaN on Si(111) substrate using AlN as a buffer layer.
Design/methodology/approach – Structural analyses of the InGaN films were performed by using X-ray diffraction, atomic force microscopy, and Hall measurement. Metal-semiconductor-metal (MSM) photodiode was fabricated on the In0.47Ga0.53N/Si(111) films. Electrical analysis of the MSM photodiodes was carried out by using current-voltage (I-V) measurements. Ideality factors and Schottky barrier heights for Ni/In0.47Ga0.53N, was deduced to be 1.01 and 0.60 eV, respectively.
Findings – The In0.47Ga0.53N MSM photodiode shows a sharp cut-off wavelength at 840 nm. A maximum responsivity of 0.28 A/W was achieved at 839 nm. The detector shows a little decrease in responsivity from 840 to 200 nm. The responsivity of the MSM drops by nearly two orders of magnitude across the cut-off wavelength.
Originality/value – Focuses on III-nitride semiconductors, which are of interest for applications in high temperature/power electronic devices.
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 x 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.
- InGaN films with different In contents were grown on (0001) sapphire substrates by metal organic vapor phase epitaxy (MOVPE).
- The purpose of designing two sets of growth conditions was to prevent the preferential evaporation of InN during the growth of an alloy of InN and GaN at elevated temperatures. It also allowed a correlation between the film properties and growth temperatures for the individual compositions.
- p-InGaN was achieved only for the second set of samples grown at elevated temperatures.
- Except for the samples intended for an In content of 80%, all the InGaN samples grown at temperatures higher than 550 °C showed a lower In content than those grown at 550 °C. This is attributed to the high evaporation rate of InN at elevated temperatures.
- All the InGaN layers grown at 550 °C showed a narrower full-width at half-maximum (FWHM) than those grown at temperatures higher than 550 °C.
- The PL measurement of the InGaN films showed the progressive shifts of emission wavelength from about 1.7 mm to 361 nm for both sets of samples.
- The FWHM varied from sample to sample and was also affected by the growth temperatures used. The AFM analysis of the surface morphology indicated a progressively smoother surface toward the Ga-rich end of the InGaN films grown. This was observed for both sets of samples.
- Using elevated growth temperatures, p-type InGaN was achieved after annealing up to an In content of 56%. This was likely the highest In content reported for p-InGaN.
- The increasing deviation noted for the second set of samples toward the Ga-rich end was consistent with the preferential evaporation of InN at high temperatures. The annealing at increasing temperatures caused a large reduction in both the electron concentration and mobility for both InN and In-rich InGaN films.
- By the Hall measurement of the p-InGaN grown, an activation rate of about 10% was estimated for Mg in these InGaN films.
- Only those grown at elevated temperatures showed p-behavior after annealing up to an In content above 50%. Apparently, the growth at a higher temperature benefited the incorporation of Mg into a favorable site for the subsequent activation to occur.
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.We 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.
Abstract: We 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. Our 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. We suggest that pulse laser assisted technique is effective for low-temperature growth of InGaN with high indium content.
Abstract: Nanocolumn InGaN/GaN crystals were deposited on micropillared Si substrate by molecular beam epitaxy. Low-temperature InN was used as interlayer. With enough free space, the column crystals grew around all the surface plane of the Si pillars and formed InGaN/GaN quantum-well flower structure. The QW crystals are about 100 nm in diameter and 1.1-1.4 μm in length. Raman spectra measurement of the fower structure shows that E2 mode peak line observed at 567.28 cm–1. Photoluminescence measurement indicates a room temperature PL peak position of 620 nm and two peak positions of 404 nm and 519 nm at temperature 15 K. Hg lamp excited photoluminescence demonstrated a clear fluorescence distribution in the flower structure and much stronger emission compared with the quantum-well crystals on the flat Si substrate.
Abstract: This work demonstrates the formation of InGaN nanorod arrays with indium mole fractions by hydride vapor phase epitaxy. The nanorods grown on (0001) sapphire substrates are preferentially oriented in the c-axis direction. We found that the In mole fractions in the nanorods were linearly increased at x < 0.1. However, In mole fractions were slightly increased at x ≥ 0.1 and then were gradually saturated at x = 0.2. CL spectra show strong emissions from 380 nm (x = 0.04, 3.26 eV) to 470 nm (x = 0.2, 2.64 eV) at room temperature.
Abstract: In this paper, the chemical vapor deposition (CVD) growth of InGaN nanowires was systematically studied. The catalyst was Au and the starting materials were Ga, In and NH3. The samples were characterized with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron spectroscopy (TEM), and X-ray diffraction (XRD), etc. The influence of the growth temperatures, Au thicknesses, gas flowrates and Ga and In amount on the morphology and properties of InGaN nanowires was investigated. It is found that 600 ◦C is a suitable growth temperature. On the substrate with Au thickness of 150A° , helical InGaN nanowires are obtained. The change of NH3 partial pressure and Au thickness will result in the morphology change of the samples. An increase of Ga results in shorter InGaN nanowires while an increase of In amount will lead to longer InGaN nanowires. The morphology will also change when both the amount of In and Ga were increased or reduced without changing the ratio of Ga to In.
Abstract: Single crystalline and single phase InxGa1−xN nanopillars were grown spontaneously on (111) silicon substrate by plasma assisted molecular beam epitaxy. The surface morphology, structural quality, and optoelectronic properties of InGaN nanopillars were analyzed using scanning electron microscopy (SEM), energy dispersive x-ray (EDXA) analysis, high resolution x-ray diffraction (HR-XRD), and both room and low temperature photoluminescence spectra. The EDXA results showed that these nanopillars were composed of InGaN and the amount of indium incorporation in InxGa1−xN NPs could be controlled by changing the growth temperature. The room temperature and low temperature PL spectra revealed that the emission wavelength could be tuned from a blue to green luminescent region depending on the growth temperature. The wavelength tuning was attributed to a higher amount of In incorporation at a lower growth temperature which was consistent with the EDXA and HR-XRD results.
Abstract: We have studied Si-doping profiles of InN films grown by plasma-assisted molecular-beam epitaxy and their photoluminescence (PL) properties. We confirmed experimentally that Si acts as a donor in InN. Undoped and Si-doped InN films with electron densities (n) of 1.6E18 - 1.4E19 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 = 1:6E18 - 4.7E18 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. 10.1063/1.119013
 InGaN Photovoltaics
Abstract: A heterojunction structure photodetector was fabricated by evaporating a semitransparent Ni/Au metal film on the InGaN/GaN structure. The photocurrent (PC) spectra show that both the Schottky junction (NiAu/InGaN) and the InGaN/GaN isotype heterojunction contribute to the PC signal which suggests that two junctions are connected in series and result in a broader spectral response of the device. Secondary electron, cathodoluminescence and electron-beam-induced current images measured from the same area of the edge surface clearly reveal the profile of the layer structure and distribution of the built-in electric field around the two junctions. A band diagram of the device is drawn based on the consideration of the polarization effect at the InGaN/GaN interface. The analysis is consistent with the physical mechanism of a tandem structure of two junctions connected in series.
Abstract: In this work we present the growth and characterization of InxGa1-xN-based materials and solar cells with x up to 0.54. Growth of single phase InxGa1-xN is achieved using Plasma Assisted Molecular Beam Epitaxy (PAMBE) with flux modulation for active species. The material is characterized by x-ray diffraction, electrochemical capacitance-voltage, time-resolved photo-luminescence, and contactless electroreflectance. Fabricated devices are then studied for photo-response under simulated AM0 spectral conditions to evaluate solar cell characteristics. The dark and illuminated J-V results indicate the existence of significant shunt and series resistances arising from material defects and non-optimized device design.
Abstract: High internal and external quantum efficiency GaN/InGaN solar cells are demonstrated. The internal quantum efficiency was assessed through the combination of absorption and external quantum efficiency measurements. The measured internal quantum efficiency, as high as 97%, revealed an efficient conversion of absorbed photons into electrons and holes and an efficient transport of these carriers outside the device. Improved light incoupling into the solar cells was achieved by texturing the surface. A peak external quantum efficiency of 72%, a fill factor of 79%, a short-circuit current density of 1.06 mA/cm2, and an open circuit voltage of 1.89 V were achieved under 1 sun air-mass 1.5 global spectrum illumination conditions.
Abstract: We investigate theoretically the characteristics of monolithic InGaN/Si two-junction series-connected solar cells using the air mass 1.5 global irradiance spectrum. The addition of an InGaN junction is found to produce significant increases in the energy conversion efficiency of the solar cell over that of one-junction Si cells. Even when Si is not of high quality, such two-junction cells could achieve efficiencies high enough to be practically feasible. We also show that further, though smaller, improvements of the efficiency can be achieved by adding another junction to form an InGaN/InGaN/Si three-junction cell.
Abstract: This work addresses the required properties and device structures for an InGaN solar cell. Homojunction InGaN solar cells with a bandgap greater than 2.0 eV are specifically targeted due to material limitations. These devices are attractive because over half the available power in the solar spectrum is above 2.0 eV. Using high growth rates, InGaN films with indium compositions ranging from 1 to 32% have been grown by Molecular Beam Epitaxy with negligible phase separation according to X-ray diffraction analysis, and better than 190 arc-sec ω-2θ FWHM for ~0.6 μm thick In0.32Ga0.68N film. Using measured transmission data, the adsorption coefficient of InGaN at 2.4 eV was calculated as α ≅ 2×105 cm–1 near the band edge. This results in the optimal solar cell thickness of less than a micron and may lead to high open circuit voltage while reducing the constraints on limited minority carrier lifetimes.
Characteristics of InGaN designed for photovoltaic applications. E. Trybus, O. Jani, S. Burnham, I. Ferguson, C. Honsberg, M. Steiner and W.A. Doolittle. p hys. stat. sol. (c) 5, No. 6, 1843–1845 (2008) / DOI 10.1002/pssc.200778693
Abstract: The InGaN alloy system offers a unique opportunity to develop high efficiency multi-junction solar cells. In this study, single junction solar cells made of InxGa1–xN are successfully developed, with x = 0, 0.2, and 0.3. The materials are grown on sapphire substrates by MBE, consisting of a Si-doped InGaN layer, an intrinsic layer and an Mg-doped InGaN layer on the top. The I–V curves indicate that the cell made of all-GaN has low series resistance (0.12 Ω cm2) and insignificant parasitic leakage. Contact resistances of p and n contacts are 2.9 × 10–2 Ω cm2 and 2.0 × 10–3 Ω cm2, respectively. Upon illumination by a 200 mW/cm2, 325 nm laser, Voc is measured at 2.5 V with a fill factor of 61%. Clear photo-responses are also observed in both InGaN cells with 0.2 and 0.3 Indium content when illuminated by outdoor sunlight. But it is difficult to determine the solar performance due to the large leakage current, which may be caused by the material defects. A thicker buffer layer or GaN template can be applied to the future growth process to reduce the defect density of InGaN films
Growth, fabrication, and characterization of InGaN solar cells. X. Chen, K.D. Mattews, D. Hao, W. J. Schaff and L.F. Eastman. p hys. stat. sol. (a) 205, No. 5, 1103–1105 (2008) / DOI 10.1002/pssa.200778695
Abstract: We report on the fabrication and photovoltaic characteristics of InGaN solar cells by exploiting InGaN/GaN multiple quantum wells (MQWs) with In contents exceeding 0.3, attempting to alleviate to a certain degree the phase separation issue and demonstrate solar cell operation at wavelengths longer than previous attainments (>420 nm). The fabricated solar cells based on In0.3Ga0.7N/GaN MQWs exhibit an open circuit voltage of about 2 V, fill factor of about 60%, and an external efficiency of 40% (10%) at 420 nm (450 nm).
InGaN-GaN multiple quantum well solar cells with long operating wavelengths. R. Dahal, B. Pantha, J. Li, J.Y. Lin and H.X. Jiang. APPLIED PHYSICS LETTERS 94, 063505 (2009)
Abstract: The III-nitride material system offers substantial potential to develop high-efficiency solar cells. Theoretical modeling of InGaN solar cells indicate strong band bending at the top surface of p-lnGaN junction caused due to piezoelectric polarization-induced charge at the strained p-GaN window interface. A counterintuitive strained n-GaN window layer is proposed, modeled and experimentally verified to improve performance of InGaN solar cells. InGaN solar cells with band gap of 2.9 eV are grown using MOCVD with p-type and n-type strained GaN window layers, and fabricated using variable metallization schemes. Fabricated solar cells using n-GaN window layers yield superior Voe and FF compared to those using p-GaN window layers. The Voe's of InGaN solar cells with n-GaN window layers are further enhanced from 1.5 V to 2 V by replacing the conventional NiOx top contact metal with Ti/AI, which also verifies the tunneling principle.
Optimization of GaN window layer for InGaN solar cells using polarization effect. Omkar Jani , Balakrishnam Jampana , Mohit Mehta , Hongbo Yu , Ian Ferguson , Robert Opila and Christiana Honsberg. 978-1-4244-1641-7/08/ ©2008 IEEE
Abstract: In this letter, we report on the fabrication and photovoltaic characteristics of p-i-n GaN/InGaN thin-film solar cells. The thin-film solar cells were fabricated by removing sapphire using a laser lift-off technique and, then, transferring the remaining p-i-n structure onto a Ti/Ag mirror-coated Si substrate via wafer bonding. The mirror structure is helpful to enhance light absorption for a solar cell with a thin absorption layer. After the thin-film process for a conventional sapphire-based p-i-n solar cell, the device exhibits an enhancement factor of 57.6% in current density and an increment in conversion efficiency from 0.55% to 0.80%. The physical origin for the photocurrent enhancement in the thin-film solar cell is related to multireflection of light by the mirror structure.
Abstract: The p-GaN/In0.06Ga0.94N/n-GaN double heterojunctional solar cells with solely formed nanorod arrays of p-GaN have been fabricated on sapphire (0001). The p-GaN nanorod arrays are demonstrated to significantly reduce the reflectance loss of light incidence. A stress relief of the intrinsic InGaN region is observed from high-resolution X-ray diffraction analyses. The electroluminescence emission peak is blue shifted compared with the conventional solar cells. These results are reflected by the spectral dependences of the external quantum efficiency (EQE) that show a shorter cutoff wavelength response. The maximum EQE value is 55.5%, which is an enhancement of 10% as compared with the conventional devices.
Abstract: Nanometer-scale InGaN self-assembled quantum dots (QDs) have been prepared by growth stop during the metalorganic chemical vapor deposition growth. With a 12 s growth interruption, we successfully formed InGaN QDs with a typical lateral size of 25 nm and an average height of 4.1 nm. The QDs density was about 2 · 1010 cm-2. Nitride-based QD metal–semiconductor–metal (MSM) photodetectors were also fabricated. It was found that we could significantly enhance the photocurrent to dark current contrast ratio of the MSM photodetectors by the use of QD structure.
 Analytical model for the optical functions of amorphous semiconductors from the near-infrared to the ultraviolet: Applications in thin film photovoltaics.
To be expanded.
Abstract: Two dispersion models of disordered solids, one parameterizing density of states (PDOS) and the other parameterizing joint density of states (PJDOS), are presented. Using these models, not only the complex dielectric function of the materials, but also some information about their electronic structure can be obtained. The numerical integration is necessary in the PDOS model. If analytical expressions are required the presented PJDOS model is, for some materials, a suitable option still providing information about the electronic structure of the material. It is demonstrated that the PDOS model can be successfully applied to a wide variety of materials. In this paper, its application to diamond-like carbon (DLC), a-Si and SiO2-like materials are discussed in detail. Unlike the PDOS model, the presented PJDOS model represents a special case of parameterization that can be applied to limited types of materials, for example DLC, ultrananocrystalline diamond (UNCD) and SiO2-like.
Abstract: The effect of grain boundaries on electrical properties of thin GaN layers is studied by photoconductivity and its functional dependence on surface photovoltage, and by resistance changes produced by ion implantation damage. These two independent experiments provide strong evidence that the conductivity in GaN can be described by the Grain Boundary Controlled Transport model. According to this model, charged interface states at the grain boundaries form potential barriers for inter-grain conduction.
Abstract: In this work we present a study of the optoelectronic properties of nanocrystalline GaN (nc-GaN) and amorphous GaON (a-GaON) grown by ion-assisted deposition. The two classes of film show very distinct photoconductive responses; the nc-GaN has a fast small response while the a-GaON films have a much larger response which is persistent. To describe the observed intensity, wavelength, and temperature dependence of the photoconductivity in each class of film, we build a model which takes into account the role of a large density of localized states in the gap. The photoconductivity measurements are supplemented by thermally stimulated conductivity, measurement of the absorption coefficient, and determination of the Fermi level. Using the model to aid our interpretation of this data set, we are able to characterize the density of states in the gap for the two materials.
 InGaN nanorod arrays grown by molecular beam epitaxy: Growth mechanism structural and optical properties
Abstract: Vertically c-axis-aligned InGaN nanorod arrays were synthesized on c-plane sapphire substrates by radio-frequency molecular beam epitaxy. In situ reflection high-energy electron diffraction was used to monitor the growth process. X-ray diffraction, transmission electron microscopy, field-emission scanning electron microscope, and photoluminescence were used to investigate the structural and optical properties of the nanorods. The growth mechanism was studied and a growth model was proposed based on the experimental data. A red shift of photoluminescence spectrum of InGaN nanorods with increasing growth time was found and attributed to the partial release of stress in the InGaN nanorods.
 Gallium nitride nanorod arrays as low-refractive-indextransparent media in the entire visiblespectral region
Abstract: A photovoltaic device comprised of an array of 20 nm wide, 32 nm pitch array of silicon nanowires is modeled as an optical material. The nanowire array (NWA) has characteristic device features that are deep in the subwavelength regime for light, which permits a number of simplifying approximations. Using photocurrent measurements as a probe of the absorptance, we show that the NWA optical properties can be accurately modeled with rigorous coupled-wave analysis. The densely structured NWAs behave as homogeneous birefringent materials into the ultraviolet with effective optical properties that are accurately modeled using the dielectric functions of bulk Si and SiO2, coupled with a physical model for the NWA derived from ellipsometry and transmission electron microscopy.
Abstract: This work demonstrates the formation of InGaN nanorod arrays with indium mole fractions by hydride vapor phase epitaxy. The nanorods grown on (0001) sapphire substrates are preferentially oriented in the c-axis direction. We found that the In mole fractions in the nanorods were linearly increased at x < 0.1. However, In mole fractions were slightly increased at x ≥ 0.1 and then were gradually saturated at x = 0.2. CL spectra show strong emissions from 380 nm (x = 0.04, 3.26 eV) to 470 nm (x = 0.2, 2.64 eV) at room temperature.
Abstract: We demonstrate the realization of the high-brightness and high-efficiency light emitting diodes (LEDs) using dislocation-free indium gallium nitride (InGaN)/gallium nitride (GaN) multi-quantum-well (MQW) nanorod (NR) arrays by metal organic-hydride vapor phase epitaxy (MO-HVPE). MQW NR arrays (NRAs) on sapphire substrate are buried in silicon dioxide (SiO2) to isolating individual NRs and to bring p-type NRs in contact with p-type electrodes. The MQW NRA LEDs have similar electrical characteristics to conventional broad area (BA) LEDs. However, due to the lack of dislocations and the large surface areas provided by the sidewalls of NRs, both internal and extraction efficiencies are significantly enhanced. At 20 mA dc current, the MQW NRA LEDs emit about 4.3 times more light than the conventional BA LEDs, even though overall active volume of the MQW NRA LEDs is much smaller than conventional LEDs.
 Rapid growth and characterization of InN nanocolumns on InGaN buffer layers at a low ratio of N/In
• Grew InN nanocolumns on InGaN substrate • Good info on InN and nanocolumns o InN – smallest band gap of III-nitrides at 0.65eV, large lattice mismatch with common substrates, impurity-prone surface, tight growth range of 460C to 490C o Nanocolumns – quantum confinement effect may yield novel functions and improve performance
 Selective area metalorganic molecular-beam epitaxy of GaN and the growth of luminescent microcolumns on Si/SiO[sub 2]
Abstract: We demonstrate the selective area growth of gallium nitride on patterned Si~111!/GaN/SiO2 wafers by metalorganic molecular beam epitaxy using triethyl gallium as a Ga source. We show that such selective area deposition may be used to grow isolated microcolumns of GaN with lateral dimensions of tens of nanometers on Si/SiO2 wafers. Via high resolution cathodoluminescence imaging we show that such microcolumn structures are highly luminescent inspite of a large surface to volume ratio, indicating that nonradiative recombination at free surfaces is not a significant issue in this system.
 Periodic Si Nanopillar Arrays Fabricated by Colloidal Lithography and Catalytic Etching for Broadband and Omnidirectional Elimination of Fresnel Reflection
Abstract: Periodic Si nanopillar arrays (NPAs) were fabricated by the colloidal lithography combined with catalytic etching. By varying the size of colloidal crystals using oxygen plasma etching, Si NPAs with desirable diameter and fill factor could be obtained. The Fresnel reflection can be eliminated effectively over broadband regions by NPAs; i.e., the wavelength-averaged specular reflectance is decreased to 0.70% at wavelengths of 200-1900 nm. The reflectance is reduced greatly for the incident angles up to 0.70% for both s- and p-polarized light. These excellent antireflection performances are attributed to light trapping effect and very low effective refractive indices, which can be modified by the fill factor of Si in the NPA layers.
Abstract: We report real-time high temperature transmission electron microscopy observations of the growth of GaN nanowires via a self-catalytic vapor-liquid-solid (VLS) mechanism. High temperature thermal decomposition of GaN in a vacuum yields nanoscale Ga liquid droplets and gallium/nitrogen vapor species for the subsequent GaN nanowire nucleation and growth. This is the first direct observation of self-catalytic growth of nanowires via the VLS mechanism and suggests new strategies for synthesizing electronically pure single-crystalline semiconductor nanowires.
Conclusion: The direct observation of nanowire growth unambiguously confirms the validity of vapor-liquid-solid crystal growth mechanism at the nanometer scale and should allow us to rationally control the nanowire growth which is critical for their potential implementation into the nanoscale electronic and optoelectronic devices.
Abstract: Solar energy represents one of the most abundant and yet least harvested sources of renewable energy. In recent years, tremendous progress has been made in developing photovoltaics that can be potentially mass deployed1, 2, 3. Of particular interest to cost-effective solar cells is to use novel device structures and materials processing for enabling acceptable efficiencies4, 5, 6. In this regard, here, we report the direct growth of highly regular, single-crystalline nanopillar arrays of optically active semiconductors on aluminium substrates that are then configured as solar-cell modules. As an example, we demonstrate a photovoltaic structure that incorporates three-dimensional, single-crystalline n-CdS nanopillars, embedded in polycrystalline thin films of p-CdTe, to enable high absorption of light and efficient collection of the carriers. Through experiments and modelling, we demonstrate the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates with enhanced carrier collection efficiency arising from the geometric configuration of the nanopillars.
Abstract: Self-assembled GaAs nanowires have been grown on Si by molecular beam epitaxy without the use of any outside metal catalyst. The growth occurs on Si facets obtained by the cleavage of Si(100) substrates. The growth has been obtained with or without Ga pre-deposition. In both cases two kinds of nanowires have been obtained. The wires of the first type clearly present a Ga droplet at their free end and have a lattice structure that is wurtzite for wide regions beneath the Ga droplet. The second type, in contrast, ends with pyramidally shaped GaAs and has a crystal lattice that is mainly zincblende with only a few and small wurtzite regions, if any. The Ga-ended nanowires are longer than the others and thinner on average. The experimental findings suggest that the two types of nanowires grow after different growth processes.
Abstract: Single-crystalline,one-dimensional semiconductor nanostructures are considered to be one of the critical building blocks for nanoscale optoelectronics1. Elucidation of the vapour–liquid–solid growth mechanism2 has already enabled precise control over nanowire position and size1,3,4–8, yet to date, no reports have demonstrated the ability to choose from different crystallographic growth directions of a nanowire array. Control over the nanowire growth direction is extremely desirable, in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization,and bandgap may be used to tune the physical properties of nanowires made from a given material.Here we demonstrate the use of metal–organic chemical vapour deposition (MOCVD) and appropriate substrate selection to control the crystallographic growth directions of high-density arrays of gallium nitride nanowires with distinct geometric and physical properties. Epitaxial growth of wurtzite gallium nitride on (100) γ-LiAlO2 and (111) MgO singlecrystal substrates resulted in the selective growth of nanowires in the orthogonal  and  directions, exhibiting triangular and hexagonal cross-sections and drastically different optical emission. The MOCVD process is entirely compatible with the current GaN thin-film technology, which would lead to easy scale-up and device integration.
Abstract: This tutorial review focuses on recent work addressing the properties and potential of semiconductor nanowires as building blocks for photovoltaic devices based on investigations at the single nanowire level. Two central nanowire motifs involving p-i-n dopant modulation in axial and coaxial geometries serve as platforms for fundamental studies. Research illustrating the synthesis of these structural motifs will be reviewed first, followed by an examination of recent studies of single axial and coaxial p-i-n silicon nanowire solar cells. Finally, challenges and opportunities for improving efficiency enabled by controlled synthesis of more complex nanowire structures will be discussed, as will their potential applications as power sources for emerging nanoelectronic devices.
Abstract: We examine light trapping in thin silicon nanostructures for solar cell applications. Using group theory, we design surface nanostructures with an absorptance exceeding the Lambertian limit over a broad band at normal incidence. Further, we demonstrate that the absorptance of nanorod arrays closely follows the Lambertian limit for isotropic incident radiation. These effects correspond to a reduction in silicon mass by 2 orders of magnitude, pointing to the promising future of thin crystalline silicon solar cells.
Abstract: InN nanocolumns were grown on (0 0 0 1) sapphire substrates by radio-frequency plasma-assisted molecular-beam epitaxy, simply with substrate nitridation, InN nucleation, and high V/III growth conditions. Here, the InN nucleation was conducted by forming In droplets on the nitridated substrates and subsequently nitriding these In droplets. We investigated the growth evolution of InN nanocolumns and the growth-temperature dependence of the morphology. X-ray rocking curves (XRC) for the (0 0 0 2) reflection of the samples basically consisted of two components: a broad peak and a very sharp one. We propose that the broad peak came from the lower part of relatively short nanocolumns and the near-interface region of relatively long nanocolumns. In contrast, the sharp peak came from the (strain-free) upper part of relatively short nanocolumns and the high-crystal-quality region apart from the interface in relatively long InN nanocolumns. The relative intensity of the latter to the former increased with growth time. The shape of the nanocolumns varied with growth temperature: nanocolumns grown at 380 and 420 1C had a taper-like appearance, but the top broadened with increasing growth temperature, becoming broader at the top than the base at 470 and 500 1C. When we grew InN nanocolumns under the same conditions, but without surface nitridation, the subsequent columns were not all along a c-axis. If In droplets were not used, then columns did not form. Thus, both the surface nitridation and In droplets were needed to form c-axis aligned nanocolumns.
Abstract: Vertically aligned gallium nitride (GaN) nanorod arrays grown by the catalyst-free, self-organized method based on plasma-assisted molecular-beam epitaxy are shown to behave as subwavelength optical media with low effective refractive indices. In the reflection spectra measured in the entire visible spectral region, strong reflectivity modulations are observed for all nanorod arrays, which are attributed to the effects of Fabry-Pérot microcavities formed within the nanorod arrays by the optically flat air/nanorods and nanorods/substrate interfaces. By analyzing the reflectivity interference fringes, we can quantitatively determine the refractive indices of GaN nanorod arrays as functions of light wavelength. We also propose a model for understanding the optical properties of GaN nanorod arrays in the transparent region. Using this model, good numerical fitting can be achieved for the reflectivity spectra.
Abstract: Infrared (IR) transmission spectra of GaN nanocolumns were analyzed. In addition to the bulk GaN optical phonon signal, a broad absorption peak was observed from undoped and Mg-doped nanocolumns. The central position and width of the broad peak changed with the growth condition and Mg concentration. Based on the Lorentz–Drude model composed of phonon and plasmon modes associated with depolarization fields in GaN nanocolumns, IR transmission spectra were fitted by adjusting the free-electron concentration and scattering rate. Dependence of these values on the column size and impurity concentration is discussed.
Abstract: Wurtzite GaN nanocolumns are reproducibly grown by plasma-assisted molecular beam epitaxy on Si(111) and c-sapphire substrates. The nanocolumns density and diameter (600–1500 Å) are effectively controlled by means of the III/V ratio. The nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narrow (2 meV) low-temperature photoluminescence excitonic lines at 3.472–3.478 eV. In addition, the spectra reveal a doublet at 3.452–3.458 eV and a broad line centered at 3.41 eV. This broad emission shows a sample-dependent spectral energy dispersion, from 3.40 to 3.42 eV, explained as due to the effect of strain and/or electric fields associated with extended structural defects located at the nanocolumns bottom interface. From cathodoluminescence data, it is concluded that the doublet emission lines originate at the nanocolumns volume, most probably related to GaI defects, given the column growth mode (Ga balling).
Abstract: Researchers from Sophia University in Tokyo, Japan compared the optical properties of a GaN film grown by an RF-plasma assisted molecular beam epitaxy process resulting in nanocolumns with an MOCVD-grown amorphous GaN film. The results of stimulated emission from the two films were compared with the following notable results:
- The wavelength at which peak emission was observed shift higher for increasing intensity of incident light. This increase was nonlinear.
- Higher MBE growth temperatures resulted in narrowing of the nanocolumns at their base, while lower temperatures caused the nanocolumns to coalesce at their base. The transition temperature for this transformation was approximately 850 C.
- The effects of residual strains from differences in thermal expansion between substrate and active layers were observed.
- The increased optical performance of the nanocolumns was explained by a reduction in dislocation density and non-radiative recombination, compared to that of the amorphous film.
Abstract: We have investigated the behavior of excitons in GaN nanocolumns using time-integrated and time-resolved micro-photoluminescence measurements. In the weak confinement limit, the model of fractional-dimensional space gives an intermediate dimensionality of 2.14 for GaN nanocolumns, with an average diameter of 80 nm. Enhanced exciton and donor binding energies are deduced from a fractional-dimensional model and a phenomenological description. Time-integrated photoluminescence spectra as a function of temperature show a curved emission shift. Recombination dynamics are deduced from the temperature dependence of the PL efficiency and decay times.
Abstract: Spanish researchers used photoluminescence, Raman scatteringW, SEM imaging and cathodeluminescenceW (CL) to characterize the optical and microstructural properties of GaN nanocolumns grown by MBE. Outputs from CL were used to link the emissions at various peaks with their corresponding locations in the nanocolumns; the lower-energy peaks in PL observed came from defects at the nanocolumn-substrate interface, while the higher-energy peaks were emitted along the length of the nanocolumns. Another correlation was made between stoichiometry and resulting microstucture. Under nitrogen-rich conditions, the flux of Ga could be varied to produce different nanocolumn populations.
Abstract: This paper reports a scalable process for the growth of high-quality GaN nanowires and uniform nanowire arrays in which the position and diameter of each nanowire is precisely controlled. The approach is based on conventional metalorganic chemical vapor deposition using regular precursors and requires no additional metal catalyst. The location, orientation, and diameter of each GaN nanowire are controlled using a thin, selective growth mask that is patterned by interferometric lithography. It was found that use of a pulsed MOCVD process allowed the nanowire diameter to remain constant after the nanowires had emerged from the selective growth mask. Vertical GaN nanowire growth rates in excess of 2 ím/h were measured, while remarkably the diameter of each nanowire remained constant over the entire (micrometer) length of the nanowires. The paper reports transmission electron microscopy and photoluminescence data.
- Reports a scalable process for the growth of high-quality GaN nanowires and uniform nanowire arrays in which the position and diameter of each nanowire is precisely controlled based on conventional metal-organic chemical vapor deposition (MOCVD) using trimethygallium (TMGa) and ammonia (NH3) and requires no additional metal catalyst.
- The location, orientation, and diameter of each GaN nanowire are controlled using a thin, selective-growth mask that is patterned by interferometric lithography.
- This method enables to fabricate high quality symmetrical hexagonal sidewall facets over the entire micrometer length of nanowires.
- This method was carried out in two phases, first phase used continous MOCVD till nanowires emerged out of mask and second phase was pulse MOCVD to fabricate nanowires emerged out of mask at the end of phase one, using pulsed MOCVD in second phase maintained the quality of nanowires and were highly alligned and ordered as desired with respect to the mask.
- The diameter of the nanowire remains constant as it emerges from the growth mask, confirming from XTEM images that the nanowire diameter is indeed controlled by the diameter of the growth mask aperture.
- The XTEM images show no threading dislocations (TDs) in the GaN nanowires, even though TDs were observed in the planar GaN film beneath the growth mask. This was found to be the case for GaN nanowire growth on all substrates, including growth on silicon (111).
- The recorded band-edge PL peak intensity for the nanowire sample was 100 times greater than that measured for a 5 micron planar GaN film and more than 200 times greater than that measured for a 0.6 micron planar GaN film. Much of this intensity increase is undoubtedly due to the geometry of a nonplanar nanowire sample, where the input coupling of the PL pump beam and the out-coupling of the resulting PL will both be increased significantly.
 Structural and optical properties of GaN nanocolumns grown on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy. 
Abstract: GaN nanocolumns were grown with AlN buffer layers on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy. The AlN buffer layers underneath the nanocolumns were used to nucleate the nanocrystals. The thickness of the AlN buffer layer affected the column configuration (size, shape), the density and the optical properties of the nanocolumns; when the thickness increased from 1.8 to 8.2 nm, the average column diameter gradually decreased from 150 to 52 nm with a small kink, but the column density peaked at a thickness of 3.2 nm at 5×109 cm−2 and finally decreased to 2×108 cm−2. Based on TEM observations, it is suggested that GaN nanocolumns were not grown just on AlN grain but on the edge of AlN grain. Further, the growth behavior of a nanocolumn as a function of AlN buffer layer thickness is suggested. The room-temperature photoluminescence intensity of the nanocolumns was maximized at a buffer thickness of 4.6 nm, where the intensity was 4 times stronger than that of high-quality bulk GaN crystals grown by HVPE with a threading dislocation density of 8×106 cm−2.
Abstract: The large-scale assembly of nanowires with controlled orientation on surfaces remains one challenge preventing their integration into practical devices. We report the vapor-liquid-solid growth of aligned, millimeter-long, horizontal GaN nanowires with controlled crystallographic orientations on different planes of sapphire. The growth directions, crystallographic orientation, and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. This paves the way to highly controlled nanowire structures with potential applications not available by other means.
 InGaN and Light Emitting Diodes (LED)
InGaN is a relatively new material to photovoltaic technology, but similar materials have been used in LEDWs for some time now. In operation, an LED is to a solar cell as a fan is to a turbine. In the LED, electricity comes in and light is emitted, while in a solar cell light comes in and electricity comes out. The following articles cover details on some advancements in the field of LED technologies closely related to photovoltaics.
Abstract: Optical characterization has been performed on an InGaN/GaN nanocolumn structure grown by nitrogen plasma assisted molecular beam epitaxy not only in macroscopic configuration but also in a microscopic one that can be assessed to a single nanocolumn. The photoluminescence (PL) decay monitored at 500 nm is fitted with a double exponential curve, which has lifetimes of 0.67 and 4.33 ns at 13 K. These values are two orders of magnitude smaller than those taken at the same wavelength in conventional InGaN/GaN quantum wells (QWs) grown toward the C orientation. PL detection of each single nanocolumn was achieved using a mechanical lift-off technique. The results indicate that the very broad, macroscopically observed PL spectrum is due to the sum of the sharp PL spectrum from each nanocolumn, the peak energy of which fluctuates. Moreover, unlike conventional QWs, the blueshift of a single nanocolumn is negligibly small under higher photoexcitation. These findings suggest that carrier localization as well as the piezoelectric polarization field is suppressed in InGaN/GaN nanocolumns.
Abstract: Indium–gallium nitride (InGaN) multiple-quantum-well (MQW) light-emitting diode (LED) membranes, prefabricated on sapphire growth substrates, were created using pulsed-excimer laser processing. The thin-film InGaN MQW LED structures, grown on sapphire substrates, were first bonded onto a Si support substrate with an ethyl cyanoacrylate-based adhesive. A single 600 mJ/cm2, 38 ns KrF (248 nm) excimer laser pulse was directed through the transparent sapphire, followed by a low-temperature heat treatment to remove the substrate. Free-standing InGaN LED membranes were then fabricated by immersing the InGaN LED/adhesive/Si structure in acetone to release the device from the supporting Si substrate. The current–voltage characteristics and room-temperature emission spectrum of the LEDs before and after laser lift-off were unchanged.
 InGaN/GaN quantum-well heterostructure light-emitting diodes employing photonic crystal structures
Abstract: Electrical operation of InGaN/GaN quantum-well heterostructure photonic crystal light-emitting diodes (PXLEDs) is demonstrated. A triangular lattice photonic crystal is formed by dry etching into the top GaN layer. Light absorption from the metal contact is minimized because the top GaN layers are engineered to provide lateral current spreading, allowing carrier recombination proximal to the photonic crystal yet displaced from the metal contact. The chosen lattice spacing for the photonic crystal causes Bragg scattering of guided modes out of the LED, increasing the extraction efficiency. The far-field radiation patterns of the PXLEDs are heavily modified and display increased radiance, up to ~1.5 times brighter compared to similar LEDs without the photonic crystal.
 Spontaneous emission of localized excitons in InGaN single and multiquantum well structures.
Abstract: Emission mechanisms of InGaN single quantum well blue and green light emitting diodes and multiquantum well structures were investigated by means of modulation spectroscopy. Their static electroluminescence (EL) peak was assigned to the recombination of excitons localized at certain potential minima in the quantum well. The blueshift of the EL peak caused by the increase of the driving current was explained by combined effects of the quantum-confinement Stark effect and band filling of the localized states by excitons.
 Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-GaN surface
Abstract: We report on an InGaN-based light-emitting diode (LED) with a top p-GaN surface microroughened using the metal clusters as a wet etching mask. The light-output power for a LED chip with microroughening was increased compared to that for a LED chip without one. This indicates that the scattering of photons emitted in the active layer was much enhanced at the microroughened top p-GaN surface of a LED due to the angular randomization of photons inside the LED structure, resulting in an increase in the probability of escaping from the LED structure. By employing the top surface microroughened in a LED structure, the power conversion efficiency was increased by 62%.
 Growth Kinetics and Microstructure
 Growth and properties of InAlN nanocolumns emitting in optical communication wavelengths. 
To be expanded.
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 
To be expanded.
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.
To be expanded.
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 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
To be expanded.
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.
 Optical Modelling of Thin Film Microstructures
Interference trends are commonly observed whenever optical phenomenon are observed. By modeling the material in question as a Fabry-Perot microcavity, the analysis can continue. This paper describes a method in which the peaks observed in the PL (photoluminescence) data are extracted and 2*n/λ is plotted against peak index (i.e. peak index = 1, 2, 3…), the inverse of the slope on the linear fit to the data yields the film thickness. Note that n, the refractive index, is dependent on the material through which the photon is travelling (i.e. its composition x) and the wavelength (energy) of light (photon) propagating. The refractive index is evaluated at the wavelength at which the peak in PL is noted.
The group of III-nitrides has many uses as semiconducting materials beyond PV cells, including LEDs and laser diodes. Due to the heavy reliance on optical properties to determine and improve the performance of these devices, it is crucial to have a well defined set of optical properties for the material used. This paper examines the methods for determining the index of refraction for both materials using two different models.
- The first model was originally developed by Bergmann and Casey which shifts the refractive index of GaN to produce that for InGaN, based on the composition of In and the band gap energies of the constituents.
- A Sellmeier dispersion formula relates the refractive index to the wavelength or energy associated with the photon passing through the material.
An important observation that arises from this paper from the multitude of figures included is that as the photon energy approaches 3.4 eV (approximately 365 nm), the refractive index increases dramatically. This must be taken into account when designing for optical properties.
 Optical properties of wurtzite structure GaN on sapphire around fundamental absorption edge (0.78-4.77 eV) by spectroscopic ellipsometry and the optical transmission method.
A more rigorous SellmeierW dispersion model is fit to refractive index data as a function of wavelength, with well defined fitting parameters determined and reported with 90% confidence limits. The measurements were achieved using spectroscopic ellipsometry performed at an angle of incidence of 60°over 260-830 nm with optical transmission measurement over the 370-1600 nm wavelength range. Film thickness was on the order of 1.25 μm. Note that one key assumption made in the Sellmeier model was that the extinction coefficient, the complex portion of the refractive index, was zero. This translates to no loss in intensity of the photon moving through the material, and does not significantly impact the resulting model.
The following excerpt from this paper summarizes its goal succinctly. “For calculations of nitride based LDs [laser diodes], refractive indicies are also needed for the solid solutions of AlxGa1-xN and InxGa1-xN. To our knowledge, the only refractive index data for the solid solutions is for Al0.1Ga0.9N .
In Sec. II, we approximate the refractive index for the solid solutions by shifting the GaN data according to the difference in band gap energy between the solid solution and GaN.”
 The measurement of absorption edge and band gap properties of novel nanocomposite materials. 
Demonstration that band gap energy can be determined for a material by interpolating from the absorption spectra.
The affect of annealing ZnO nanoparticles in a hydrogen atmosphere was examined, and its affect on the control over optical properties was reported. The result of annealing ZnO nanorods in a H-rich atmosphere was an increase in free charge carrier density equating to a greater conductivity. A more generalizable result of the paper was the derived model of the dielectric function. Using a combination of a Lorentz-Drude model with a Bruggeman effective medium approximation, the effective dielectric function was determined.
Abstract: We review both the theoretical electromagnetic response and the spectroscopic measurements of metamaterials. To critically examine published results for metamaterial structures operating in the range from terahertz to optical frequencies, we focus on protocols allowing one to extract the optical constants from experimental observables. We discuss the complexity of this task when applied to metamaterials exhibiting electric, magnetic, and magneto-optical response. The general theory of the electromagnetic response of such systems is presented and methods are described. Finally, we briefly overview possible solutions for implementing metamaterials with tunable resonant behavior.
 Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant 
Using a Maxwell-Garnett effective medium approximation, a model of optical absorption was developed which related absorption to aspect ratio (R) and the dielectric function of the medium supporting the nanoparticles. An important conclusion made was that the relationship between aspect ratio and both the absorption coefficient and the medium dielectric constant is linear. Also, two distinct peaks in the absorption spectra were noted; one for absoprtion in the longitudinal direction (along the length of the nanorods) and the other in the transverse direction (perpendicular to the length of the nanorods).
This article explores the interaction between incident light and an array of silver nanocolumns in a gelatin matrix. Very few numbers are provided relating the results to the material, which means it is very simple to extract the information relavent to any material. Using a Maxwell-Garnett effective medium approximation, the effective dielectric function of the nanorods constructed can be determined based on the known volume fraction of nanorods and the dielectric functions of the matrix and the nanorod materials. The analysis is further extended to investigate the effects of size (diameter) of the nanorods (varried between 10 nm and 60 nm), as well as the order/periodicity of the array.
 Gallium nitride nanorod arrays as low refractive index transparent media in the entire visible spectral region. 
Researchers in Taiwan successfully modeled GaN nanorods grown by plasma-assisted MBE (PA-MBE). It was demonstrated by comparing the results of modelling with Fresnel equations and a Bruggemann effective medium model that an effective medium approximation was valid for thin films (valid up to thickness of approximately 1.2 um). For thicker films (on the order of 2 um), this model broke down as the columns tended to coalesce (merge to form larger, abnormally shaped columns) at their tips. The model produced was valid in the transparent region of the material in near-UV light, or at wavelengths < 365 nm.
Abstract: Optical properties of GaN nanocolumnar films with and without Mg doping are characterized in the visible and ultraviolet regions. Strong uniaxial anisotropy of dielectric constants is observed by ellipsometry. The complex dielectric functions determined from the reflectance and transmittance spectra showed that the 2 value is found to be reduced by approximately 50% of that of the epitaxial-GaN film in the energy range above the band gap regardless of Mg doping. This anisotropy and reduction in dielectric constants are due to polarization fields of nanocolumnar crystallites and their interactions. The absorption in undoped GaN nanocolumnar film extends below the band gap of epitaxial GaN, probably due to defects in the nanocolumnar film. Further extension of the absorption tail by Mg doping can be attributed to the transition from a Mg-acceptor level detected in the cathodoluminescence spectra from Mg-doped samples.
 Broadband and omnidirectional antireflection from conductive indium-tin-oxide nanocolumns prepared by glancing-angle deposition with nitrogen. 
Abstract: Characteristic formation of highly oriented indium-tin-oxide (ITO) nanocolumns is demonstrated using electron-beam evaporation with an obliquely incident nitrogen flux. The nanocolumn material exhibits broadband and omnidirectional antireflective characteristics up to an incidence angle of 70° for the 350–900 nm wavelength range for both s- and p-polarizations. Calculations based on a rigorous coupled-wave analysis indicate that the superior antireflection arises from the tapered column profiles which collectively function as a gradient-index layer. Since the nanocolumns have a preferential growth direction which follows the incident vapor flux, the azimuthal and polarization dependence of reflectivities are also investigated. The single ITO nanocolumn layer can function as antireflection contacts for light emitting diodes and solar cells.
 Raman scattering by longitudinal optical phonons in InN nanocolumns grown on Si(1 1 1) and Si(0 0 1) substrates. 
Abstract: Raman measurements in high-quality InN nanocolumns and thin films grown on both Si(1 1 1) and Si(1 0 0) substrates display a low-energy coupled LO phonon–plasmon mode together with uncoupled longitudinal optical (LO) phonons. The coupled mode is attributed to the spontaneous accumulation of electrons on the lateral surfaces of the nanocolumns, while the uncoupled ones originates from the inner part of the nanocolumns. The LO mode in the columnar samples appears close to the E1(LO) frequency. This indicates that most of the incident light is entering through the lateral surfaces of the nanocolumns, resulting in pure longitudinal–optical mode with quasi-E1 symmetry. For increasing growth temperature, the electron density decreases as the growth rate increases. The present results indicate that electron accumulation layers do not only form on polar surfaces of InN, but also occur on non-polar ones. According to recent calculations, we attribute the electron surface accumulation to the temperature dependent In-rich surface reconstruction on the nanocolumns sidewalls.
 Optimization of Open-Circuit Voltage in Amorphous Silicon Solar Cells with Mixed Phase (Amorphous + Nanocrystalline) p-Type Contacts of Low Nanocrystalline Content.
This article details the use of real time spectroscopic ellipsometery (RTSE) to find the surface roughness layer thickness and the bulk layer thickness of depositions of thin film a-Si. Using the relative changes between surface roughness layer thickness and bulk layer thickness, it can be determined when the transition from a-Si to (a-Si + u-Si) and the transition from (a-Si +u-Si) to single phase u-Si occurs. It is seen that a protocrystalline silicon layer with a low volume fraction of silicon nanocrystals results in the highest open circuit voltage.
 Monoclinic optical constants, birefringence, and dichroism of slanted titanium nanocolumns determined by generalized ellipsometry.
Abstract: Generalized spectroscopic ellipsometry determines the principal monoclinic optical constants of thin films consisting of slanted titanium nanocolumns deposited by glancing angle deposition under 85° incidence and tilted from the surface normal by 47°. Form birefringence measured for wavelengths from 500 to 1000 nm renders the Ti nanocolumns monoclinic absorbing crystals with c-axis along the nanocolumns, b-axis parallel to the film interface, and 67.5° monoclinic angle between the a- and c-axes. The columnar thin film reveals anomalous optical dispersion, extreme birefringence, strong dichroism, and differs completely from bulk titanium. Characteristic bulk interband transitions are absent in the spectral range investigated.
 Growth of vacuum evaporated ultraporous silicon studied with spectroscopic ellipsometry and scanning electron microscopy
Abstract:Using a combination of variable-angle spectroscopic ellipsometry and scanning electron microscopy, we investigated the scaling behavior of uniaxially anisotropic, ultraporous silicon manufactured with glancing angle deposition. We found that both the diameter of the nanocolumns and the spacing between them increase with film thickness according to a power-law relationship consistent with self-affine fractal growth. An ellipsometric model is proposed to fit the optical properties of the anisotropic silicon films employing an effective medium approximation mixture of Tauc-Lorentz oscillator and void. This study shows that the optical response of silicon films made at glancing incidence differs significantly from that of amorphous silicon prepared by other methods due to highly oriented nanocolumn formation and power-law scaling.
 Characterisation of nanostructured GaSb: Comparison between large-area optical and local direct microscopic techniques. 
This paper outlines a comprehensive analysis of GaSb nanowires grown in bulk GaSb, with heights of approximately < 55nm to 300nm. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), spectroscopic ellipsometry (SE) and photo-elastic modulated spectroscopic ellipsometry (PMSE) were used to determine the physical and optical properties of the materials. An optical model was constructed using a effective medium approximationW in which the conical nanocolums were modeled as stacks of cylinders with constant diameter to a good first approximation of optical properties resulting in estimations of physical properties. All characteristic parameters of the nanocolums were nondimensionalized where possible for generatlity. The results of SE and PMSE measurments were a completed Mueller matrix, which permitted the determination of the degree of polarizationW and depolarization indexW. It was noted that the degree of polarization decreased with increasing column height, which is thought to be the result of mutliple scattering effects. These effects pointed to inaccuracies in the effective medium approximation for these larger columns.
 Optical-model analysis of elastic scattering and polarization of 49.5 MeV protons on Sm. 
Researchers at the Wheatstone Physics Laboratory in London, England, developed an optical model to estimate the effect of protons on samarium (Sm) isotopes. This analysis is fairly unrelated to solar technologies, however several of the same assumptions in the model may transfer. One of particular interest was the thought of how to handle difficulties in the model. The following quote from the article outlines the procedure quite well.
Smearing of the angular distributions due to the finite beam spot size, beam divergence and angular acceptance of the spectrometer were included in the theoretical predictions rather than subtracted from experimental results. The errors in the elastic cross sections are statistical, to which should be added absolute errors of...
It is anticipated that in the development of optical models for InGaN, a similar procedure will be followed.
This paper discovers trends in transmittance as well as the real portion of the refractive index for materials. To relate the refractive index to the wavelength of light, a Cauchy relation (up to fourth-order) was fit to the data. Again, the assumption of a ‘real portion only’ fit to the refractive index data significantly simplifies analysis with little loss in relevance to the actual data, as this assumption assumes no losses in intensity, but no other distortion of the photon’s pathways.
A comprehensive development of the Fabry-Perot microcavity model is completed for organic light emitting devices, which have similar structure and optical properties to inorganic materials (such as InGaN), but have greater manufacturing flexibility, require less power, and are produced at lower cost. This paper also describes the Airy function used to model interference in the PL data, which is a common way of representing interference in a Fabry-Perot type system. The Airy function is proportional to 1/sin2(d) where d is the path (phase) difference between two waves exiting a thin film.
A rigorous model for fitting an entire spectrum of PL data is developed in this paper. A function which includes the sum of two Gaussian peaks which is then multiplied by the Airy function is developed to model the data with approximately 12 fitting parameters to be adjusted. A significant amount of time can be spent on this model improving the fit, and automated curve fitting by a computer must be completed with caution as the large number of fitting parameters make it exceedingly likely that the function will fall into local minima when optimizing. Software such as Origin can be used with relative ease to develop an appropriate model.
Abstract: Nanocrystalline tin sulfide (SnS) was prepared by chemical bath deposition, and the photovoltaic behavior of SnS/TiO2 was studied. The X-ray diffraction pattern and transmission electron microscopy revealed an ∼6 nm SnS polycrystalline orthorhombic structure. The SnS film exhibited a band gap of 1.3 eV, and its absorption coefficient was more than 1 × 104 cm−1 in the visible light range. The electrical conductivity activation energy of the SnS film was 0.22 eV, determined when the sample was heated in the temperature range of 111−144 °C. Although the sample was insulating at room temperature, photovoltaic behavior was found in a SnS/TiO2 structure, with an open-circuit voltage (Voc) of 471 mV, a short-circuit current density (Jsc) of 0.3 mA/cm2, and the conversion efficiency (η) of 0.1% under 1 sun illumination. The properties of SnS and the reasons behind the photovoltaic phenomenon of SnS/TiO2 are discussed.
 PHOTOVOLTAICS LITERATURE SURVEY
- Provides a list of recently published journals on photovoltaics - great resource
Abstract: In order to help keep readers up-to-date in the field each issue of Progress in Photovoltaics will contain a list of recently published journal articles most relevant to its aims and scope. This list is drawn from an extremely wide range of journals, including IEEE Transactions on Electron Devices, Journal of Applied Physics, Applied Physics Letters, Progress in Photovoltaics and Solar Energy Materials and Solar Cells. To assist the reader, the list is separated into broad categories, but please note that these classifications are by no means strict. Also note that inclusion in the list is not an endorsement of a paper’s quality. If you have any suggestions please email Dr. Avi Shalav at firstname.lastname@example.org
Abstract: The nanowire geometry provides potential advantages over planar waferbased or thin-film solar cells in every step of the photoconversion process. These advantages include reduced reflection, extreme light trapping, improved band gap tuning, facile strain relaxation, and increased defect tolerance. These benefits are not expected to increase the maximum efficiency above standard limits; instead, they reduce the quantity and quality of material necessary to approach those limits, allowing for substantial cost reductions. Additionally, nanowires provide opportunities to fabricate complex single-crystalline semiconductor devices directly on low-cost substrates and electrodes such as aluminum foil, stainless steel, and conductive glass, addressing anothermajor cost in current photovoltaic technology. This review describes nanowire solar cell synthesis and fabrication, important characterization techniques unique to nanowire systems, and advantages of the nanowire geometry.
- 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.
- Analytical model for the optical functions of amorphous semiconductors from the near-infrared to the ultraviolet: Applications in thin film photovoltaics. A.S. Ferlauto, G.M. Ferfeira, J.M. Pearce, C.R. Wronski, R.W. Collins, X. Deng, G. Ganguly. Journal of Applied Physics. Vol 2 Num 5 (2002).
- Plasmon effects on infrared spectra of GaN nanocolumns. T. Iwanaga, T. Suzuki, S. Yagi, T. Motooka. Applied Physics Letters, Vol 86 (2005).
- Luminescence properties and defects in GaN nanocolumns grown by molecular beam epitaxy. E. Callega, M.A. Sanchez-Garcia, F. J. Sanchez, F. Calle, F.B. Naranjo, E. Munoz. Physical Review, Vol 62 (2000).
- Stimulated emission from GaN nanocolumns. A. Kikuchi, K. Yamano, M. Tada, K. Kishino. Physical Status Solidi, Vol 241, Issue 12 (2004).
- Two-dimensional exciton behavior in GaN nanocolumns grown by molecular-beam epitaxy. J.H. Na, R.A. Taylor, J.H. Rice, J.W. Robinson, K.H. Lee, Y.S. Park, C.M. Park, T.W. Kang. Applied Physics Letters, Vol 86 (2005).
- Structural and optical characterization of intrinsic GaN nanocolumns. J. Sanchez-Paramo, J.M. Calleja, M.A. Sanchez-Garcia, E. Calleja, U. Jhan. Physica E: Low-dimensional systems and nanostructures. Vol 13, Issue 2-4 (2002).
- The controlled growth of GaN nanowires. S.D.Hersee, X. Sun, X. Wang. Nano Letters. Vol 6, No 8 (2006).
- Structural and optical properties of GaN nanocolumns grown on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy. H. Sekiguchi, T. Nakazato, A. Kikuchi, K. Kishino. Journal of Crystal Growth, Vol 300 issue 1 (2006).
- Origin of high oscillator strength in green-emitting InGaN/GaN nanocolumns. Y. kawakami, S. Suzuki, M. Funato, A. Kikuchi, K Kishino. Applied Physics Letters, Vol 89 (2006).
- Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off. W.S. Wong, T. Sands, N.W. Cheung, M. Kneissl, D.P. Bour, P. Mei, L.T. Romano, N.M. Johnson. Applied Physics Letters, Vol 75 (1999).
- InGaN/GaN quantum-well heterostructure light-emitting diodes employing photonic crystal structures. J.J. Wierer, M.R. Krames, J.E. Epler, N.F. Gardner, M.G. Craford, J.R. Wendt, J.A. Simmons, M.M. Sigalas. Applied Physics Letters, Vol 84 (2004).
- Spontaneous emission of localized excitons in InGaN single and multiquantum well structures. S. Chichibu, T. Azuhata, T. Sota, S. Nakamura. Applied Physics Letters, Vol 69 (1996).
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