Lateral Reactions of GaAs with Ni Studied by Transmission Electron Microscopy
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S. H. CHENt, C. B. CARTER, C. J. PALMSTROMtt AND T. OHASHI* Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, NY 14853 *School of Electrical Engineering, Phillips Hall, Cornell University, Ithaca, NY 14853
ABSTRACT A new method has been developed for making self-supporting, thin films which can be used for the in situ study, by hot-stage, transmission electron microscopy, of the reaction between Ni and GaAs. The thin-film,
lateral diffusion-couples have been used to study both the kinetics and the formation of new phases. The growth rate of the ternary compound, Ni 2 GaAs showed a parabolic time dependence. At an annealing temperature of 300"C, the present experimental results show that Ni is the diffusing species and that the Ga and As remain essentially immobile. Diffusion coefficients obtained by this method are in very good agreement with those which have been obtained using conventional thin-film techniques. The results of this new technique are particularly important in view of the difficulty in identifying the composition of the product phase by methods which do not have the same lateral resolution. INTRODUCTION Compounds consisting of metal and Ga and/or As formed by solid state reaction may be used as Ohmic or Schottky contacts to GaAs devices [1]. The reaction between Ni and GaAs has received much attention since it is commonly used as an ohmic contact in combination with Au and Ge. The interaction between a thin film of Ni and a GaAs substrate has been studied by several investigators using various analytical techniques, including transmission electron microscopy (TEM) [2-91. However, the reaction between Ni and GaAs proceeds so quickly that observations of growth rates of compounds are experimentally difficult. The Ni thin film reacts readily with GaAs at temperatures as low as 200"C, and can be consumed completely to form Ni2 GaAs in several minutes (see, for example, ref. 3), which is usually less than the time needed for reaching the desired temperatures for furnace annealing. Difficulties in kinetics studies therefore arise, as pointed out by Zheng et al. [10], since measurements of diffusion coefficients for thin-film couples are limited to values of less than about 10"1 cm2/s due to available film thickness, which is usually less than 1 gm, and the minimum time required for heating and cooling in furnace annealing. It has been shown [10,11] in metal-Si systems that lateral diffusion couples, where typical diffusion lengths are between 1pm and 100pm, provide an important bridge between the regions which can be studied by using bulk diffusion couples and those which can be studied by conventional thin-film couples. A new specimen preparation for TEM investigation has been successfully developed for metal-Si systems [12-15]. However, similar specimen preparation techniques cannot be applied to metal-GaAs systems since NaCI, which was used as a substrate for the depositions of Si and metal, is unsuitable for the growth of GaAs in a molecular-beam, epitaxy system
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