Controlled Grain Size and Location in Ge Thin Films on Silicon Dioxide by Low Temperature Selective Solid Phase Crystall
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C.M. YANG AND HARRY A. ATWATER Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, [email protected]
ABSTRACT Selective solid phase crystallization for control of grain size and location in polycrystalline thin Ge films on amorphous silicon dioxide substrates is described. The approach consists of selective solid phase crystal nucleation via an alloy reaction at predefined nucleation sites, which consist of metal islands deposited on top of the amorphous Ge film, followed by lateral solid phase epitaxial growth. Grain sizes as large as 30 ym have been achieved in 50 nm thick Ge films at temperatures less than 475 0 C.
INTRODUCTION Selective solid phase crystallization is an approach for fabrication of thin polycrystalline Ge and Si films on amorphous substrates with potentially very large grain sizes and controlled grain boundary locations. Achievement of very large grain sizes in thin Ge and Si films on amorphous insulating substrates is of interest to future low-cost thin film electronic device technologies such as displays and thin film polycrystalline solar cells. Particularly important for low-cost thin film solar cell applications is the use of low cost substrates, such as glass. Since grain size enhancement and film growth must be accomplished at temperatures below the softening point of the glass substrate, solid phase crystallization methods are of particular interest. While approaches that employ film melting and solidification can lead to large grain size[l], the high temperatures required are most likely incompatible with low-cost substrates. In this letter, we describe results of selective solid phase crystallization for producing large grained (20 pm) polycrystalline Ge films at low temperatures (< 500°C), which is the first step in a process for fabrication of low-cost polycrystalline GaAs solar cell on glass substrates. For GaAs thin film cells, achievement of a large grain sizes is a concern as grain boundaries act as traps for minority carriers that lower cell efficiency[2]. Since GaAs is lattice-matched to Ge, a Ge film with large grain size could be used to seed epitaxial growth of a GaAs overlayer to fabricate solar cells with large GaAs grain size[3]. The approach to selective solid phase crystallization taken here is based on a thin film reaction between a deposited metal layer and an amorphous semiconductor film. This reaction causes selective nucleation to occur at a much earlier time than random homogeneous nucleation. Thus crystals which are selectively nucleated can grow to very large sizes via lateral solid phase epitaxy before their growth is impeded by
113 Mat. Res. Soc. Symp. Proc. Vol. 403 0 1996 Materials Research Society
Figure 1: In (a), bright-field transmission electron micrograph illustrating selective nucleation and growth of Ge grains in a 50 nm thick amorphous Ge film on Si0 2 following a two-step vacuum anneal of T = 350°C for 20 min to induce selective nucleation, followed by a T = 425°C anneal for 2 hours to
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