Growth of Buried CoSi 2 Layers in Si(100) by Molecular Beam Allotaxy
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ABSTRACT Buried single crystalline CoSi2 layers in Si(100) have been grown using molecular beam allotaxy. In this paper, we investigated the diffusive interaction of two buried silicide precipitate layers, one with a small Co peak concentration of 10 at% and another one with 26 at%. Annealing causes first local coarsening in each layer, and then dissolution of the thinner precipitate layer. The accumulation of the Co atoms at the thicker layer is described by a simple model for the diffusional redistribution. INTRODUCTION Molecular beam allotaxy [1-5] is an alternative method to ion beam synthesis (IBS) [6-8] for the fabrication of buried, epitaxial silicide layers in Si(100). Allotaxy consists of two processes. First silicide precipitates are embedded in an epitaxial Si(100) matrix by coevaporating Si and a metal, e.g. Co, with an understoichiometric proportion on a heated Si substrate [1]. The formation of the buried uniform silicide layer is achieved by high temperature annealing, the second processing step. This latter step is similar to the formation of buried silicides by ion beam synthesis, except for differencies in the initial precipitate size and depth distributions [2]. The basic mechanisms leading to uniform layer formation are precipitate coarsening (Ostwald ripening), diffusive redistribution and layer planarization. The driving force for these processes is the tendency of the system to minimize the interfacial energy. As found for IBS, the peak concentration of the approximately trapezoidal shaped concentration depth profile of the metallic component in the Si single crystal has to exceed a certain 'critical peak concentration', in order to obtain a continuous buried layer during annealing. For CoSi 2 the critical Co concentration amounts to a 20 at% i.e. _ 60% of the stoichiometric concentration [1-3]. In spite of numerous experimental investigations [8-10] and a theoretical approach [11] the process is not fully understood. Best layer quality in terms of uniformity and single crystallinity have been achieved for CoSi2, a metallic silicide with CaF 2 structure and excellent electrical properties (14 1.trcm) and a small lattice mismatch relative to Si of -1.2% [1-3]. In the present paper we present cross-section transmission electron microscopy (XTEM) and Rutherford backscattering spectroscopy (RBS) investigations of the diffusive interaction of two buried CoSi, precipitate layers, one with a subcritical Co concentration of 10 at% and another one with ; supercritical concentration of 26 at% Co in the peak of the precipitate distribution. EXPERIMENTAL A sample with two separated layers of CoSi 2 precipitates was grown. The substrate was a ntype, 1000 crcm, 3" Si(100) wafer, which was cleaned wet-chemically prior to its introduction
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Mat. Res. Soc. Symp. Proc. Vol. 320. '•1994 Materials Research Society
into the UHV growth chamber. During the coevaporation of Si and Co the Si deposition rate was kept constant at 0.2 nm/s, whereas the Co deposition rate was varied in order to p~oduce the two prec
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