Diffusion in Gallium Arsenide and GaAs-Based Layered Structures
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DIFFUSION IN GALLIUM ARSENIDE AND GaAs-BASED LAYERED STRUCTURES School
U. Gosele, T. Y. Tan and Shaofeng Yu of Engineering, Duke University, Durham, NC
27706,
USA
ABSTRACT The mechanisms of Ga self-diffusion can be derived from interdiffusion experiments in intrinsic and doped GaAs-based superlattices. These experiments allow to conclude that Ga self-diffusion in intrinsic and n-doped GaAs is carried by triply negatively charged gallium vacancies whereas in p-doped GaAs positively charged gallium self-interstitials dominate Ga self-diffusion. The diffusion mechanisms of Zn and Be are discussed with special emphasis on the difference between their in- and out-diffusion behavior which is due to diffusion-induced non-equilibrium point defects. INTRODUCTION Gallium arsenide and silicon are presently the most important semiconductors in the microelectronics and optoelectronics industry. Whereas diffusion processes and mechanisms are fairly well understood in silicon [1,2] such an understanding is just emerging in the case of GaAs. Interdiffusion experiments in intrinsic as well as in doped GaAs/AlAs superlattices have proven to be essential in determining self- and dopant diffusion mechanisms in GaAs [3,4]. This paper will mainly deal with developments over the last few years concerning gallium self-diffusion in GaAs and the diffusion of dopants predominantly dissolved on the gallium sublattice such as Si, Zn and Be. More general overviews covering the diffusion in GaAs and other Ill-V compound have been given by Kendall [5], Casey [6] and Tuck [7]. Before discussing superlattice interdiffusion results and their implications for diffusion mechanisms in GaAs we will first summarize some essential concepts underlying our present understanding of diffusion phenomena in silicon. Similar concepts also appear to govern diffusion processes in GaAs. DIFFUSION IN Si AND GaAs - SIMILARITIES AND DIFFERENCES Although it is certainly risky to extrapolate from the simple case of the elemental semiconductor silicon to the more complex case of the compound semiconductor GaAs with two sublattices, some basic concepts which play a role in diffusion processes in silicon can be applied in modified form to GaAs. We summarize these concepts and basic results in the following: i) Both vacancies 1an self-interstiticles are involved in self- and dopant diffusion processes in silicon [1,2,8,9]. Similarly, both gallium vacancies and gallium self-interstistials have to be taken into account to understand gallium selfdiffusion and dopant diffusion in GaAs [3,4,10]. ii) Vacancies (V) and self-interstitials (I) may occur in various charge states. Their equilibrium concentrations depend on the position of the Fermi level, i.e., on the concentration of electrons (n) or holes (p) and are given for a charged species Xk (X = V or I) with charge k (0, ± 1,±2. ... )by eq
eq
Cx'k(nCxk(ni) = (n/n)-k.
()
In eq. (1) ni is the intrinsic electron concentration. In a different context this Fermi level effect had already been discussed by Shockley and Moll
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