Zinc Diffusion in GaAs-AlGaAs Heterojunction Bipolar Transistor Structures
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ZINC DIFFUSION IN GaAs-AIGaAs HETEROJUNCTION BIPOLAR TRANSISTOR STRUCTURES W. S. HOBSON, S. J. PEARTON, and A. S. JORDAN AT&T Bell Laboratories, 600 Mountain Avenue Murray Hill, NJ 07974
ABSTRACT We have examined the diffusion of Zn from the base of GaAs-AIGaAs heterojunction bipolar transistor (HBT) structures during growth by organometallic vapor phase epitaxy. The role of Si doping in the emitter-contact, emitter, and collector/subcollector in enhancing the Zn diffusion has been determined by separately doping each layer. For a growth temperature of 675°C Zn shows no observable redistribution up to concentrations of 3x1019 cm- 3 without Si doping. The addition of Si to the adjacent AIGaAs emitter and GaAs collector/subcollector layers causes significant diffusion from the base, while Si doping of the GaAs emitter-contact results in even greater Zn redistribution. Silicon counter-doping in the base region retards the Zn diffusion. These results are consistent with a recent model which shows that the n-type surface layer enhances the formation of gallium interstitials which diffuse into the structure and displace the Zn in the base via a kick-out mechanism. INTRODUCTION Heterojunction bipolar transistors (HBTs) are currently undergoing intense investigation for possible high speed applications [1-4]. For GaAs-AIGaAs HBTs the most common dopant for the p+ base region is Zn for material grown by organometallic vapor phase epitaxy (OMVPE). Redistribution of the base dopant causes severe degradation of the HBT characteristics by increasing both the transit time of carriers through the base and recombination in the emitter region [5]. Here, we report an examination of the redistribution of Zn in OMVPE - grown HBT structures. The results of Enquist et al. [6,7] suggest that in GaAs-AIGaAs HBT structures the Zn diffusivity can be accelerated by the n' doping levels in the emitter and emitter-contact layers. One proposed explanation advanced by Enquist [6] was strain-enhanced diffusion due to the large Te dopant atom. Recent detailed experiments by Nordell et al. [8] confirmed the general idea of Zn redistribution. In this publication we report our results on the diffusion of Zn in GaAs-AIGaAs HBTs. We have systematically varied the n type doping in the emitter-contact, emitter, and collector/subcollector layers. Si doping was used to eliminate strain as a factor. We have also deliberately introduced strain through the formation of an InGaAs cap and the addition of In to the base. Si counterdoping in the base of same samples was utilized to suppress the n' dopant gradient and observe its effect upon Zn redistribution. Post-growth annealing of HBT structures containing only Zn in the base was also examined. Annealing in the growth chamber with an overpressure of arsine was compared to rapid thermal annealing. EXPERIMENTAL All of the HBT structures were grown in an atmospheric pressure, barrel geometry OMVPE reactor using ultra-high purity He as the carrier gas. The source chemicals were trimethylgallium (TMGa), trimethyla
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