Balakrishnam Jampana

Design and Realization of Wide-Band-Gap (

The design of coherently strained InGaN epilayers for use in InGaN p-n junction solar cells is presented in this letter. The X-ray diffraction of the epitaxially grown device structure indicates two InGaN epilayers with indium compositions of 14.8% and 16.8%, which are confirmed by photoluminescence peaks observed at 2.72 and 2.67 eV, respectively. An open-circuit voltage of 1.73 V and a short-circuit current density of 0.91 mA/cm2 are observed under concentrated AM 0 illumination from the fabricated solar cell. The photovoltaic response from the InGaN p-n junction is confirmed by using an ultraviolet filter. The solar cell performance is shown to be related to the crystalline defects in the device structure.

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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.

2.67 eV) InGaN p-n Junction Solar Cell

In this article several kinetic effects are proposed that induce compositional instabilities in thick InGaN heteroepitaxial layers on GaN templates grown by metalorganic chemical vapor deposition. It was found that by reducing the growth temperature, or increasing the growth rate, or introducing Mg doping, the epitaxial layer changes from a pseudomorphic InGaN with a low indium mole fraction to a relaxed InGaN with a high indium mole fraction. In certain circumstances, both phases can be present in a single layer. The composition and strain inhomogeneity was correlated to the surface morphology and crystalline quality, governed by the growth conditions. It is believed that the compositional instability in InGaN originates from the coupled effects of compressive strain and surface morphology. A smooth surface allows for the growth of pseudomorphic low-indium InGaN, whereas a rough surface promotes the formation of a relaxed high-indium InGaN layer.

Correlation of crystalline defects with photoluminescence of InGaN layers

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-InGaN 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 VOC and FF compared to those using p-GaN window layers. The VOCs 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/Al, which also verifies the tunneling principle.

Compositional instability in strained InGaN epitaxial layers induced by kinetic effects

We report on the structural, morphological, and optical qualities of thick InxGa1−xN heteroepitaxial layers grown by metalorganic chemical vapor deposition with various growth conditions for applications in wide-band gap solar cells. The indium incorporation depending on the growth temperature and indium precursor flow rate and the crystalline and optical qualities of InGaN layers depending on indium mole fraction were investigated. The InGaN layers with high structural and optical qualities were obtained for indium mole fractions, xIn < 0.18, whereas significant degradation of material qualities was observed for xIn < 0.18, which is associateWe report on the structural, morphological, and optical qualities of thick InxGa1−xN heteroepitaxial layers grown by metalorganic chemical vapor deposition with various growth conditions for applications in wide-bandgap solar cells. The indium incorporation depending on the growth temperature and indium precursor flow rate and the crystalline and optical qualities of InGaN layers depending on indium mole fraction were investigated. The InGaN layers with high structural and optical qualities were obtained for indium mole fractions, xIn < 0.18, whereas significant degradation of material qualities was observed for xIn < 0.18, which is associated with the change of growth mode induced by reduced growth temperature. Stokes shift and microscopic and macroscopic phase separations were also studied. Two types of additional phases besides InGaN matrix, i.e., indium-rich InGaN microstructures and macroscopic InGaN domains, were demonstrated to be suppressed by controlling surface adatom mobility and growth rates.d with the change of growth mode induced by reduced growth temperature. Stokes shift and microscopic and macroscopic phase separations were also studied. Two types of additional phases besides InGaN matrix, i.e., indium-rich InGaN microstructures and macroscopic InGaN domains, were demonstrated to be suppressed by controlling surface adatom mobility and growth rates.

Optimization of GaN window layer for InGaN solar cells using polarization effect

The III-nitride material system with band gap ranging from 0.7eV to 6.2eV has substantial potential to develop high-efficiency solar cells. The III-nitride materials are grown by MOCVD on a lattice mismatched sapphire substrate (0001). This paper presents the generation of extended crystalline defects and their spatial distribution in the GaN and In0.12Ga0.88N layers as a function of In0.12Ga0.88N thickness. The material is characterized by photoluminescence, and the primary peak intensity is observed to increase with thickness, up to 200 nm, but the intensity diminishes with further increase in thickness. Additional photoluminescence peaks are observed for In0.12Ga0.88N thicknesses greater than 100 nm. These observations are attributed to extended crystalline defects and are characterized by high resolution x-ray diffraction. A detailed analysis of these extended crystalline defects is presented based on rocking curves, symmetric and asymmetric reciprocal space maps. The crystalline defects are unavoidable during epitaxial growth, but knowledge of their generation process yields better control over them.

Growth and characterization of InxGa1−xN alloys by metalorganic chemical vapor deposition for solar cell applications

In this report we present recent results for MOCVD growth of high indium content InGaN films on ZnO substrates. Growth was attempted on both bulk ZnO as well as ZnO epilayers grown on sapphire by MOCVD. ZnO is an attractive alternative substrate for III-Nitrides because of its superior lattice match: specifically ZnO is perfectly matched with In0.18Ga0.82N and low cost of substrates. Stable InGaN films with ≫18% indium were achieved on the bulk substrates and were characterized by HRXRD, PL, and optical transmission. Varying the growth parameters - primarily growth temperature and In/(In + Ga) flow ratio - was found to affect the optical and structural properties of the films. By growing on a better matched substrate the high indium composition InGaN epitaxial films experience less strain and can therefore be grown thicker without creating relaxation-induced extended crystal defects. This is important, as high indium content InGaN films cannot be grown on GaN thick enough for full above-bandgap absorption without introducing detrimental extended crystal defects. This limitation is thought to be a limiting factor in the achievable ISC in InGaN solar cells.

High quality InGaN for photovoltaic applications: Type and spatial distribution of crystalline defects and “phase” separation

The III-nitride material system offers substantial potential to develop high-efficiency solar cells. Currently InGaN based solar cells have been demonstrated on sapphire substrate. This substrate expense adds up significantly to the cost of solar cells realization and further issues like sapphire substrate removal are of concern. Alternatively, InGaN epitaxial layers have been successfully grown on silicon substrate. An InGaN based quantum well solar cell structure is grown simultaneously by MOCVD on both GaN/sapphire and GaN/silicon substrates. The fabricated solar cells have comparable photo-response. The Voc of InGaN solar cell on sapphire is higher while the FF of InGaN solar cell on silicon is higher.

High indium composition (≫20%) InGaN epi-layers on ZnO substrates for very high efficiency solar cells

The III-nitride material system offers substantial potential to develop high-efficiency solar cells. The solar cell operation requires the formation of a depletion region. Conventionally, this is achieved by a p-n junction. The piezoelectric polarization introduces a strong band bending at the hetero-junction interface and hence creating a depletion region. The growth of a thin AlN or GaN epi-layer on InGaN introduces the required piezoelectric polarization to create a depletion region. This paper presents the polarization-incorporated simulations in “Silense” showing the depletion region formation by GaN or AlN epilayers on p-InGaN. Three structures are then MOCVD grown and characterized for crystal quality and electrical properties. The fabricated devices demonstrated the diode characteristics with an open-circuit voltages ≫ 2.0 V.

Realization of InGaN solar cells on (111) silicon substrate

InGaN alloys are widely researched in diverse optoelectronic applications. This material has also been demonstrated as a photovoltaic material. This paper presents the study to achieve optimum electrically active p-type InGaN epi-layers. Mg doped InGaN films with 20% In composition are grown on GaN templates/sapphire substrates by MOCVD. It is found that the hole concentration of p-type InGaN depends strongly on the Mg flow rate and V/III molar ratio and hole concentration greater than 2×10 19 cm −3 has been achieved at room temperature. The optimum activation temperature of Mg-doped InGaN layer has been found to be 550-600°C, which is lower than that of Mg-doped GaN. A solar cell was realized successfully using the InGaN epi-layers presented here.