Optical Absorption at Digitally and Continuously Graded Indium Gallium Nitride Schottky Barriers
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1012-Y12-34
Optical Absorption at Digitally and Continuously Graded Indium Gallium Nitride Schottky Barriers Choudhury Jayant Praharaj formerly with Cornell University, Ithaca, NY, 14853
ABSTRACT We present numerical calculations of the optical absorption characteristics of graded indium gallium nitride Schottky Barriers, and study their implications for photovoltaic and photodetector applications. We consider the two cases of digital and continuous grading because of their different effects on the collection of photo-generated carriers due to band discontinuities. Composition grading can achieve desired spectral response between the ranges of 0.7 eV and 3.43 eV afforded by the indium gallium nitride alloy system. The presence of spontaneous and piezoelectric polarizations in this material system adds bulk and/or interface bound charges in graded layers. This has a non-trivial effect on the band profile seen by the photo-generated carriers. The layer thicknesses needed for optimal absorption characteristics are well above the theoretical critical thickness limits reported in the literature for abrupt heterojunctions. However, experimental data about critical thicknesses is scarce, especially for graded compositions. Therefore, we calculate the characteristics of the Schottky barrier for the case of spontaneous polarization only and also for the case of both spontaneous and piezoelectric polarization assuming no relaxation. The low or even negative Schottky barrier heights at low gallium composition necessitates the use of high gallium composition layers next to the metal, in order to suppress the excessive dark currents.
INTRODUCTION The group III nitride semiconductors have a wide range of band-gaps from 0.7eV up to 6.2 eV. This offers the possibility of realizing optical sources and detectors from the infrared to the ultraviolet. The group III nitrides offer the possibility of solar-cell photovoltaic applications because of their absorption characteristics [1-2]. Group III nitride quantum wells have been used for blue lasers and light-emitting diodes. GaN and AlGaN have been used to build photodetectors for ultraviolet radiation [3-6]. In particular, Schottky photodetectors have been reported for the ultraviolet range of the spectrum [7-10]. Schottky barriers offer simplicity of processing over p-n junctions as no in-situ or diffused doping of n and p type are required. Further, the activation energy of ptype dopants is very high in nitride semiconductors, which would make it difficult to obtain solar cells with multiple p-n junctions. A Schottky barrier offers the possibility of using the band-gap range of the nitrides to absorb wide-spectrum radiation. We present theoretical calculations of light absorption at Schottky barriers to indium gallium nitride, and explore the implications of digitally and continuously grading the barrier.
ABSORPTION COEFFICIENTS The absorption spectrum of direct band-gap semiconductors has an onset at roughly the frequency corresponding to the bandgap and then increases to higher
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