GaAs Solar Cell Using an Alternate Arsenic Source
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GaAs SOLAR CELL USING AN ALTERNATE ARSENIC SOURCE V. S. Sundaram, J. E. Avery, G. R. Girard, H. E. Hager, A. G. Thompson and L. M. Fraas High Technology Center, Boeing Electronics, P.O. Box 24969 MS 9Z-80 Seattle, WA 98124-6269 ABSTRACT Using an alternate arsenic source, namely, Tertiary Butyl Arsine, a concentrator GaAs solar cell has been grown in a low pressure metal organic chemical vapor deposition reactor. Under 72 sun, air mass 1.5 illumination, the cell had an open circuit voltage of 1.1 V, a fill factor of 83% and an overall efficiency of 21%. INTRODUCTION Alternate arsenic sources for use in the Metal Organic Chemical Vapor Deposition (MOCVD) of GaAs is an active area of research because of the high toxicity of arsine which is currently used in many laboratories [1]. Attempts in the past using alkyl sources have ended with mixed results. Both ethyl and methyl based alkylarsine compounds with Tri Methyl Gallium (TMG) were used for growing GaAs in an atmospheric pressure reactor by Lum et al. [2]. They find that with these alkyl sources the films are n type but highly compensated with large carbon impurities (as high as 1*101M cm- 3 ). Stringfellow et al. first reported that with a new source, namely, Tertiary Butyl Arsine (tBAs), and TMG at a V to III ratio of 5 they were able to grow morphologically good ýgAs epi layers with background doping levels as low as 5*I010cm- 3 (n type) and room temperature mobilities of 4000 cm2 V- S-1 in an atmospheric reactor [3]. The vendor of tBAs, American Cyanamid, has since then synthesized much purer tBAs and has grown GaAs epi layers with room temperature mobilities of 8000 cm2 V-1s- 1 [4]. Herein, we report the first successful device fabrication using an alternate arsenic source, tBAs. EPI GROWTH In our present study, GaAs and A1GaAs epitaxial layers were grown in a low pressure EMCORE GS-3200 reactor at a pressure of 60 torr. The growth temperature was varied between 620°C and 690°C. Both Cr doped and Si doped substrates oriented 20 off the (100) surface towards the (110) were used. Hydrogen was used as the carrier gas and the total gas flow through the reactor during the run was 13 slm. The tBAs bubbler was maintained at 0°C and it is reported [4] that the tBAs is liquid at this temperature with a vapor pressure of 52 torr. The gallium source, TMG, was maintained at -5°C. The mole fractions of TMG and tBAs during these experiments were kept at 1.2*10-4 and 1.3*10-3 respectively, corresponding to a V to III ratio of 11. The surface morphology of the GaAs layers grown with tBAs Mat. Res. Soc. Symp. Proc. Vol. 145. @1989 Materials Research Society
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was found to be excellent at the growth temperatures used in The undoped lmers were found to be p type with a this study. hole concentration of 2*10 cm-3 (sample #179) and 77K mobility of 5200 cm 2 V-ls-I for growth at 655°C. For growth at 690°C the corresponding numbers were 5*10 1 5 cm- 3 and 4800 cm2 V-IS-ýsample #180) respectively. Typical photoluminescence (PL) spectra from sample #179 taken at 14 K using an argo
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