Crystal Surface Stoichiometry and the Fermi Level Effects on Outdiffusion of Si in GaAs
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CRYSTAL SURFACE STOICHIOMETRY AND THE FERMI LEVEL EFFECTS ON OUTDIFFUSION OF Si IN GaAs HORNG-MING YOU, ULRICH M. GOSELE, AND TEH YU TAN Department of Mechanical Engineering and Materials Science Duke University, Durham, NC 27708-0300
ABSTRACT GaAs samples doped with Si to a concentration of -2.7x101 8 cm" 3 were annealed at temperatures between 800 and 10000C for 3 to 20 hours under As-rich and Aspoor conditions for Si outdiffusion which were then measured using the capacitance-voltage method employing an electrochemical profiler. The deduced Si diffusivity showed strong dependencies on the As 4 vapor phase pressure, PAs 4' and on the electron concentration, n. When reduced to that under intrinsic conditions, activation enthalpies of 3.91 eV and 4.19 eV were obtained for As-rich and As-poor annealing cases, respectively. From these results, it is concluded that Si outdiffusion in GaAs is governed by the triply-negatively-charged Ga vacancies, 3-
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I.
INTRODUCTION
Silicon is the main n-type dopant used for the fabrication of devices based on GaAs and related Ill-V materials. The indiffusion behavior of Si in GaAs has been characterized by different authors [1-7]. Yu et al. [61 have proposed a Fermi-level effect model which fitted the data of many authors [1,2,5,7]. Kavanagh et al. [3] proposed a vacancy concentration gradient model in which a very important point has been pointed out. That is, in all Si indiffusion experiments [1-3,7] using RTA and polysilicon source materials, a vacancy injection transient process is involved. Therefore, to further clarify the Si diffusion mechanism, experiments under GaAs point defect thermal equilibrium conditions need to be conducted. It has been thought that outdiffusion of Si from predoped substrate will approximate the situation far better than any Si indiffusion experiments. In this paper, we report the results of such an experiment. The Si or electron concentrations, n, as measured by the capacitance-voltage (CV) method, is -2.7x101 8 cm" 3 in the starting GaAs wafers. This n value is not very high so that Si self-compensation due to its amphoteric nature [1,2,6] should have not been significant, and the results obtained are therefore that due to the diffusion of the donor species Si(a occupying the Ga sublattice sites.
II.
EXPERIMENTS
The samples used in this study were prepared from (001)-oriented GaAs wafers cut from crystals grown by the liquid-encapsulated Czochralski method, doped with Si to a concentration of -2.7x10 1 8 cm" 3 during growth. After cleaning, the samples were sealed in cleaned quartz ampules pumped down to a vacuum of-10-7 torr. At each annealing temperature, an appropriate amount of metallic As was Mat. Res. Soc. Symp. Proc. Vol. 282. @1993 Materials Research Society
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enclosed in some of the ampoules to ensure an As 4 vapor pressure of -1 atm during annealing (the As-rich condition), while in other ampoules no As was included (the As-poor condition). Diffusion annealing was carried out at 800, 850, 900, 950 and 1000°C for 20, 12, 10,
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