The Polysilicon-Silicon Interface
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THE POLYSILICON-SILICON INTERFACE ROBERT W. KEYES IBM T.J. Watson Research Center, P.O. Heights, NY 10598
Box 218,
Yorktown
ABSTRACT Polycrystalline silicon is frequently deposited on single crystal silicon as a part of devices. The poly is used as both a dopant source and a low resistance contact in a modern method of fabriof The randomness cating high gain bipolar transistors. polysilicon and its interface with the single crystal is an inherent source of differences among nominally identical transistors, however, and is a marked departure from the trend towards more and more perfect crystals that characterizes the rest of silicon technology. INTRODUCTION Polysilicon is grown on a crystalline silicon surface in various device structures, notably bipolar transistors. The deposition of polysilicon on a single crystal can serve several purposes. The poly can be heavily doped and used to make a low resistance electrical contact to part of a device in the crystal. The doped poly can serve as a source for diffusion of the dopant into the crystal. It can also be used as a cover to create a buried region with composition different from the region adjacent to the surface. The polycrystalline-silicon interface is not a major chemical discontinuity and passivates the crystal surface. Of course, it may simultaneously perform more than one of these services. A schematic representation of the result of such growth is shown in Fig. 1. The interface is a plane made up of areas of grain boundary with varying orientational relationships. The deposit is clearly different from the substrate from a macroscopic point of view. However, from a more microscopic viewpoint the poly-single interface is not obviously different from other grain boundaries. Indeed, it shares most of their properties. A large body of scientific literature concerned with polysilicon, its properties, anO 4ts interfaces with other materials has grown up. This short paper will not be able to cover this literature. Selected references to reviews that will lead the reader further on or to work that seems particularly appropriate to an observation will be given, but many important papers must be omitted. Much significant new information is also contained in the other papers presented at this symposium. GRAIN BOUNDARIES The properties of grain boundaries within a layer of polysilicon are very important in devices fabricated in such layers. Common examples are p-n junctions for solar cells and for transistors for large area displays [1]. Any effect of a grain boundary on a p-n junction would affect the performance of such devices. Consequently, such boundaries have been intensively investigated and reviewed [2]. The knowledge gained from these Mat. Res. Soc. Symp. Proc. Vol. 106. 41988 Materials Research Society
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Monocrystal Figure 1. A drawing of a layer of polysilicon deposited on a single crystal, showing grain boundaries and the poly-single interface.
studies also applies to the interfacial grain boundaries shown in Fig. 1. The most prominent electronic eff
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