Temperature Dependence of The Hall Mobility in Polycrystalline Silicon
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TEMPERATURE DEPENDENCE OF THE HALL MOBILITY IN POLYCRYSTALLINE SILICON S. E. READY, J. B. BOYCE, D. K. FORK, P. MEI, G. B. ANDERSON, AND R. I. JOHNSON XEROX Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304.
ABSTRACT Crystallization of amorphous silicon thin films by various methods has fostered enhancements in the electrical characteristics over their amorphous counterparts. For example, carrier mobilities ranging from 10 to >100 cm2/V.sec have been reported for laser crystallized films. The rather large variability of the transport characteristics with crystallization processing conditions is not well understood and, as a result, greatly complicates device process debugging. In addition, while it is generally believed that defects inherent in the grain boundaries provide the primary barriers degrading transport properties relative to single crystal silicon, the specific nature of these defects is not known. In this paper, we present data on the temperature dependence of the Hall mobility of thin silicon films crystallized by thermal and excimer laser processing. Hall data for the lasercrystallized phosphorus-doped material show a temperature dependence which differs dramatically from that for thermally crystallized materials, while the effects of hydrogenation are similar, reducing the barriers at the grain boundaries. INTRODUCTION We have previously reported on the results of Hall measurements[1-41 that characterize the electrical quality of excimer laser crystallized (LC) silicon thin
films. However, due to practical limitations imposed by equipment capabilities and the resistance of our 1000 A films, only highly doped material can be measured. While Hall transport measurements are a reliable and quick process monitor, the detailed transport processes in highly doped laser crystallized films are unclear. Others have shown that thermally crystallized (TC) films display mobility minima[5,61 attributed to maxima in the barriers developed at the grain boundaries as the doping density is increased from intrinsic to degenerate. The minima arise as a result of the formation of a depletion region due to the trapping of carriers at defect sites in the boundary region. As the number of available carriers increases with doping concentration, the traps saturate and any further increase in doping concentration tends to decrease the depletion region, allowing conduction to take place across the grain boundaries by thermally activated processes as well as tunneling. Grain size effects on the mobility minimum have been reported[61 to be important for grain sizes of less than 1000 A. For grains larger than 1000 A, the depletion region begins to be less than the grain size. Since the grain size of laser crystallized thin films is generally 1000 A or larger, our films should be beyond the regime where grain size effects are important. In an effort to understand transport in polysilicon films, we have performed Hall mobility experiments on laser processed films as well as thermally crystallized films. We have
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