A Consistent Model for Disordering of GaAs/AlAs-Superlattices During Zinc Diffusion
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A CONSISTENT MODEL FOR DISORDERING OF GaAs/AlAs- SUPERLAlTICES DURING ZINC DIFFUSION H. ZIMMERMANN, T.Y. TAN, AND U. GOESELE Dept. of Mechanical Engineering and Materials Science, School of Engineering, Duke University, Durham, N.C. 27706, USA ABSTRACT A model for the disordering of GaAs/AlAs-superlattices during zinc diffusion, which is consistent with recently established models for gallium self-diffusion and zinc diffusion in GaAs, is presented. Four coupled partial differential equations resulting from the model are solved numerically. In accordance with measured data in the literature, no disordering without zinc can result for temperatures around 6000C. Zinc diffusion, however, produces a large amount of gallium selfinterstitials, which leads to a complete disordering of superlattices with a period thickness of 32 nm to a depth of about 0.8 pim within one hour. The used values for the diffusion coefficient and the equilibrium concentration of gallium selfinterstitials are a consistent splitting of the gallium interstitial dominated selfdiffusion coefficient. INTRODUCTION GaAs/AlxGalxAs superlattices (SL) are being used in electronic and optoelectronic devices. Such devices contain regions produced by dopant induced SL-disordering. The electrical barrier between the ordered and disordered portions acts to confine the carriers. For instance in multi-quantum-well (MQW) lasers, the optically active region is the region not diffused with zinc [1]. In the regions diffused with zinc, the MOW layers are distroyed completely by zinc indiffusion induced disordering. Negligible Al-Ga inter-diffusion in the active MQW, i.e. outside of the zinc diffused region, is, however, essential for the performance of the device. Associated with applications, issues such as the effect of capping [2] and the zinc diffusion mechanism [3], are obviously important. In studying the zinc diffusion mechanism, the role of the native point defects, i.e. of self-interstitials and of vacancies, in GaAs crystals needs to be incorporated. Furthermore, there is a need to determine the role of intrinsic point defects in diffusion enhanced disordering phenomena. The purpose of this article is to introduce a model for the quantitative simulation of superlattice disordering, which is induced by the diffusion of p-type dopants in GaAs/AlAs-superlattices. It will be shown that the high concentration of gallium self-interstitials, which is present during zinc diffusion, allows the explanation of available experimental disordering results. A complete set of parameters for the modeling of zinc diffusion induced SLdisordering will be presented. THEORY Zinc and beryllium diffusion proceeds via the kick-out mechanism [4]. The corresponding reaction for an interstitial impurity atom (Ai), a substitutional impurity atom (As) and a gallium self-interstitial 'Ga can be written as:
A +2+ I Ga(1
Al :,.A-
Mat. Res. Soc. Symp. Proc. Vol. 262. 01992 Materials Research Society
862
With the assumption that the diffusion of As- via the normal vacancy and interstiti
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