Conduction in Polycrystalline Silicon: Generalized Thermionic Emission-Diffusion Theory and Extended State Mobility Mode
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CONDUCTION IN POLYCRYSTALLINE SILICON: GENERALIZED THERMIONIC EMISSION-DIFFUSION THEORY AND EXTENDED STATE MOBILITY MODEL A.N. KHONDKER, * D.M. KIM,* AND R.R. SHAH** *Electrical Engineering Department, Rice University, Houston, Texas 77251 "**Texas Instruments, P.O. Box 225012, Dallas, Texas 75265 ABSTRACT We present a general theory of conduction in polysilicon. The theoretical framework reconciles two apparently divergent approaches for modeling conduction processes in polysilicon and provides a physical basis to correctly interpret and to point out the deficiencies of previously reported thermionic and thermionic field emission theory. This model is based on an extended state mobility in the disordered grain boundary and the thermionic emission-diffusion theory for conduction of current. The attractive features of our theory are (a) it can explain the experimental data without the use of an artificial factor, f, (b) the conduction process is characterized explicitly by the inherent material properties of the grain and the grain boundary. Our model is particularly suited for describing the electrical properties of laser restructured polysilicon, where because of large grain size the diffusion process is expected to be dominant. INTRODUCTION Recent advances in laser and electron beam processing of polysilicon films hold out an attractive promise for 3-d integration in integrated circuit technology. The ultimate goal of localized transient radiation processing is to generate device quality material from polysilicon deposited at appropriate locations on insulating or conducting layers during the course of device fabrication. However, due to the complex nature of recrystallization phenomena and their dependence on a variety of factors, the generation of single crystal regions may not always be possible without the use of special techniques such as seeding, graphoepitaxy, encapsulation, etc. Even so, the recrystallized, near single crystal regions may contain defects and low angle grain boundaries. Extensive effort has been made in recent years to fabricate basic unipolar and bipolar active device elements in radiation processed polysilicon of different grain size. It is, therefore, important to have a quantitative understanding of conduction mechanisms in Polysilicon. Until recently there has been only one predominant school of thought to describe conduction in polysilicon: (1) charge carriers generated by the dopant atoms are trapped by the dangling bonds in the grain boundaries, (2) the resulting space-charge barrier potential is modeled as symmetric back-to-back Schottky diodes, and (3) conduction across the barrier is described by invoking thermionic and field emission [1-4]. This approach, although reasonably successful in computationally modelling electrical properties of polysilicon, can not adequately provide a physical basis for the conduction mechanism. This approach neglects the physics of conduction within the grain boundary -- an amorphous material, and, then mathematically Mat. Res. Soc. Symp. Proc. Vol
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