Diffusion and Interdiffusion in Multilayered Semiconductor Systems
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DIFFUSION AND INTERDIFFUSION IN MULTILAYERED SEMICONDUCTOR SYSTEMS A. OURMAZD, Y. KIM, and M. BODE AT&T Bell Laboratories, Holmdel, NJ 07733, USA. ABSTRACT We apply quantitative chemical mapping techniques to study thermal interdiffusion and ion-implantation induced intermixing at single heterointerfaces at the atomic level. Our results show thermal interdiffusion to be strongly depth dependent. This is related to the need for the presence of native point defects (interstitials and vacancies) to bring about interdiffusion. Since their initial concentration in the bulk is negligible, the point defects must be injected at the surface and transported to the interface for interdiffusion to occur. In the case of ion-implanted samples, we find the passage of a single energetic ion through a sample at 77 K causes significant intermixing, even when the sample receives no subsequent thermal treatment. INTRODUCTION The formation of chemically different (doped) layers has long been the basis of solid state technology. Of more recent importance is the ability to tailor the properties of materials by the growth of heterostructure multilayers, with individual layer thicknesses approaching the lattice constant. Such structures are far from equilibrium; the sophistication required for their growth bears testimony to this fact. It is thus important to investigate the stability of modern multilayers and the ways in which they can relax. Here, we are concerned with the relaxation of lattice matched, pseudomorphic systems such as GaAs/AIGaAs, which relax by interdiffusion. Since these materials are isostructural, interdiffusion causes only chemical changes. We use modern chemical lattice imaging and pattern recognition techniques 11,2,3] to investigate interdiffusion in GaAs/A1GaAs multilayers after annealing, or ion-implantation. Since we can measure interdiffusion coefficients as small as 10-21cm 2 1s in volumes as small as 10"19 cm 3 , we are able to investigate the chemical relaxation of solids at the atomic level. Our results show thermally-induced interdiffusion to depend strongly on the depth of the interface from the surface. This is a general effect, which is related to the injection and arrival of native point defects (interstitials and vacancies) from the surface during the anneal. Several important consequences follow. (a) The layer and hence device stability depend critically on the depth of the layer. Thus the layer depth must be regarded as an important design parameter in device technology. (b) Measurements of interdiffusion are meaningful only if they refer to a known and welldefined depth. (c) Since chemical interdiffusion is assisted by the native point defects injected from the surface, it is a sensitive means of detecting the arrival and passage of such defects. In the case of ion-implantation, we find the passage of a single energetic ion to cause intermixing, which can be detected and quantified. This allows us to investigate the fundamental atomic processes involved in the transfer of energy from an energetic i
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