Defect Formation During Zn Diffusion into GaAs
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DEFECT FORMATION DURING Zn DIFFUSION INTO GaAs
MARTINA LUYSBERG, W. JAGER, K. URBAN, M. PERRET* N.A. STOLWIJK and H. MEHRER Institut fUr Festk6rperforschung, KFA J~lich, D-5170 JMlich, F.R. Germany * Institut fOr Metallforschung, Universit~t MUnster, F.R. Germany ABSTRACT The microstructure induced by the Zn diffusion at 1170 K into doped and undoped semi-insulating GaAs single crystals was characterized for various diffusion times t < 1740 min by analytical electron microscopy. The results were compared with Zn concentration profiles obtained by spreading resistance measurements (SRM) on the same samples. At the diffusion front the formation of prismatic interstitial dislocation loops, dislocation networks, and of cavities partly filled with Ga was observed. Closer to the surface facetted voids and, for the undoped samples, vacancy-type dislocation loops formed. The near surface region of highest Zn-concentration showed a high density of Zn-rich precipitates. A model is presented which accounts.for these observations. It is based on fast interstitial Zn diffusion and the kick-out mechanism for interstitial-substituional exchange. INTRODUCTION Galliumarsenide as compound semiconductor is besides Si the most important material in solid-state microelectronics and optoelectronics. The fabrication of p-n junctions and heterojunctions requires the diffusion of dopants at elevated temperatures. An example is the production of laser diodes by diffusion of Zn from the vapor phase into n-type GaAs. The anomalously fast diffusion and the large solubility of Zn in GaAs lead to step like diffusion profiles with high Zn concentrations at the surface and a anomalously steep decrease towards smaller concentrations at the diffusion front [1-5]. It is generally accepted that diffusion of Zn into GaAs is governed by an interstitial-substitutional exchange mechanism. Two mechanisms are suggested to account for the interchange between interstitial Zni and substitutional Zns: (i) The dissociative mechanism [6,7] involves Gallium vacancies VGa. Interstitial Zni is incorporated as substitutional Zns on vacant Ga lattice sites. Interstitial Zn is generally considered to be a donor whereas Zn atoms occupy sites on the Ga sublattice as shallow acceptors. (ii) In the " kick-out" mechanism [8] the interstitial Zn-atoms take substitutional Ga sites by pushing Ga-atoms into the interstitial lattice. Experiments in which the concentration dependence of the Zn diffusion coefficient and its variation with As vapour pressure was measured could not differentiate between the two models. Also it was found that the dislocation density of the starting material has essentially no influence on Zn diffusion [4]. From recent experiments of diffusion-induced superlattice disordering in GaAs/AlAs it has been concluded that Zn-diffusion is governed by the kick-out mechanism [9]. On the other hand, Zn diffusion itself induces a high density of extended crystal defects into GaAs. This has been concluded first from the Mat. Res. Soc. Symp. Proc. Vol. 163. 11990
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