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


Photoluminescence associated with quantum dots in cubic GaN InGaN=GaN double heterostructures[edit | edit source]

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.

Refractive index and gap energy of cubic InxGa1-xN[edit | edit source]

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

InxGa1-xN refractive index calculations[edit | edit source]

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

Optical Properties of Strained AlGaN and GaInN on GaN[edit | edit source]

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[edit | edit source]

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.

Time-Resolved Photoluminescence Studies of Indium-Rich InGaN Alloys[edit | edit source]

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.

Correlation of crystalline defects with photoluminescence of InGaN layers[edit | edit source]

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)

MOVPE growth and Mg doping of InxGa1xN (x~0.4) for solar cell[edit | edit source]

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

Cathodoluminescent investigations of InxGa1-xN layers[edit | edit source]

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[edit | edit source]

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.

Magnesium Doping of In-rich InGaN[edit | edit source]

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.

Optical studies on a coherent InGaN/GaN layer[edit | edit source]

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.

[/03701972/v234i0003/787_bgohiaia.xml Band Gap of Hexagonal InN and InGaN Alloys][edit | edit source]

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.

Thermal Annealing of Cubic-InGaN/GaN Double Heterostructures[edit | edit source]

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.

Pulse laser assisted MOVPE for InGaN with high indium content[edit | edit source]

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.

InGaN/GaN quantum-well nanocolumn crystals on pillared Si substrate with InN as interlayer[edit | edit source]

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.

Formation of InGaN nanorods with indium mole fractions by hydride vapor phase epitaxy[edit | edit source]

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.

CVD growth of InGaN nanowires[edit | edit source]

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.

InGaN nanopillars grown on silicon substrate using plasma assisted molecular beam epitaxy[edit | edit source]

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.

Control of electron density in InN by Si doping and optical properties of Si-doped InN[edit | edit source]

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.

Optical properties of InN-the bandgap question[edit | edit source]

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

GaN: from fundamentals to applications[edit | edit source]

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[edit | edit source]

Photoelectric characteristics of metal/InGaN/GaN heterojunction structure[edit | edit source]

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.

Growth and characterization of ingan for photovoltaic devices[edit | edit source]

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.

High internal and external quantum efficiency InGaN/GaN solar cells[edit | edit source]

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.

Modeling of InGaN/Si tandem solar cells[edit | edit source]

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.

Characteristics of InGaN designed for photovoltaic applications[edit | edit source]

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

Growth, fabrication, and characterization of InGaN solar cells[edit | edit source]

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

InGaN-GaN multiple quantum well solar cells with long operating wavelengths[edit | edit source]

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)

Optimization of GaN window layer for InGaN solar cells using polarization effect[edit | edit source]

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

Improved Conversion Efficiency of GaN/InGaN Thin-Film Solar Cells[edit | edit source]

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.

Photovoltaic Effects of InGaN/GaN Double Heterojunctions With p-GaN Nanorod Arrays[edit | edit source]

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.

InGaN quantum dot photodetectors[edit | edit source]

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.[1][edit | edit source]

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.

  1. 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).
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