Zone-Melting Recrystallization of Semiconductor Films
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M. W. GELS, HENRY I. SMITH, B-Y. TSAUR, JOHN C. C. FAN, D. J. SILVERSMITH, R. W. MOUNTAIN AND R. L. CHAPMAN Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02173
ABSTRACT
The use of zone melting recrystallization (ZMR) to prepare large-grain (and in some cases single-crystal) semiconductor films is reviewed, with emphasis on recent work on Si on Si0 2 . Encapsulants are generally required to minimize contamination and decomposition, induce a crystalline texture, improve surface morphology and prevent agglomeration. In the case of Si, the solid-liquid interface is faceted, which gives rise to subboundaries. These can be entrained by laterally modulating the temperature through the use of an optical absorber on top of the encapsulant. Control of thermal gradients and in-plane crystallographic orientation are important for reliable entrainment.
Preparation of high-quality semiconductor films on insulating substrates has been the subject of numerous investigations. One of the more promising techniques for producing such films is zone.melting recrystallization (ZHR) which is accomplished by scanning a molten zone through the semiconductor film. The resulting film usually consists of large grains with a specific crystallographic texture, and has electrical properties approaching those of bulk single crystal material. In this article we review briefly the development of ZMR and discuss its application to Si on Si0 2 . The concept of ZMR was first discussed by Leitzi as a possible method for 2 obtaining single-crystal films of ZnS. Mitchell et al. used a zone melting technique to produce single-crystal films of AgCl and AgBr. Later, ZUR was used to produce crystalline films of Ge, InSb and, in recent years, Si on SiO2 . Various methods have been used to produce the molten zone. For relatively low-melting-temperature materials, such as AgCl, AgBr and certain organics, a small hot plate, which was moved with respect to the substrate, was sufficient. Materials with higher melting temperatures, such as Si and Ge, require higher power densities, which can be obtained with electron or laser beams. If the substrate is heated to near the melting temperature of the semiconductor by some auxiliary means, a resistively heated wire or carbon rod, or a focused lamp, can be used to supply the additional heat necessary to melt a narrow zone in the semiconductor.J11 Most investigators have found it necessary to use an encapsulant layer on top of the film to be recrystallized by zone melting. These encapsulants serve the obvious purposes of minimizing contamination and decomposition of the film during ZMR. However, they also induce a crystalline texture, improve the surface morphology of the film, and reduce or prevent agglomeration. For example, an Si0 2 encapsulant was found necessary to obtain a (100) texture in 9 ZMR of Si films on Si0 2 . As shown in Fig. 1, the surface morphology of a Si film improves with the Si0 2 encapsulant thickness. 1 9 In 6the case of ZMR of InSb, the natural oxide, In 2
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