Spectral Response of GaN P-N Junction Photovoltaic Structures
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ABSTRACT GaN ultraviolet photovoltaic and photoconductive detectors were grown on sapphire substrates by metalorganic chemical vapor deposition. The spectral response was analyzed considering the detector structure of a p-n junction connected back-to-back with a Schottkty barrier. Based on the one-dimensional model of abrupt p-n junctions, the diffusion length of minority carriers was derived to be about 0.1 p'm in n-GaN. To further characterize the n-GaN material, photoconductivity experiments have also been realized. The majority carrier lifetime of about 0. 1 ms was obtained by analyzing the voltage-dependent responsivity of GaN photoconductors. The current-responsivity under a bias of 8 V was about 1 A/W. INTRODUCTION The 111-V nitrides, with wide, direct bandgap, are attractive materials for developing high performance solar blind photodetectors in the UV region. These detectors have numerous applications such as the detection of solar UV rays reflected from orbiting craft, engine monitoring and flame detection, whenever there is a visible or infrared background.[1] In this paper we report the growth and characterization of GaN photovoltaic and photoconductive detectors. EXPERIMENT Material Growth and Characterization The GaN epitaxial layers were grown in a metalorganic chemical vapor deposition reactor (MOCVD). Trimethylgallium, trimethylaluminum and ammonia were used as Ga, Al and N source materials, respectively. The Mg source, bis-cyclopentadienylmagnesium, was used as an acceptor dopant in GaN. The growth temperature was about 1000°C, and the growth rate was 0.5 - 1 rim/h. The carrier concentration for autodoped n-GaN was 1017_1011 cm'. The as-grown GaN:Mg layer was semi-insulating. After annealing in N 2-ambient at the Mg in the layer was effectively "activated" to produce p-type GaN, with a typical carrier concentration of 1016 - 1017 cm3. Further details and characterization results have already been reported.[2-4] Optical evidence of the transformation to p-type GaN is seen in the photoluminescence data for the pGaN film, as shown in Figure 1. The emission from p-type GaN was broadband and red-shifted with respect to the position of the peak for n-type GaN, because of the transition from the donor level to the acceptor level rather than from the donor level to the valence band.
955 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
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A schematic cross-section of the GaN photovoltaic structure with photoluminescence of the n- and p-type layers.
Photodetector Devices Both a photodiode and a photoconductor were realized using GaN. The photodiode structure is shown in Figure 1. The substrate used was sapphire (00. 1). The GaN homo p-n junction had a 0.5 pm thick n- GaN layer and a 2 gtm thick p-GaN layer. The geometry of the contacts employed circumvents the difficulties in etching the top p-type GaN layer. Instead of the classical mesa structure,
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