Resistivity and Carrier Mobilities in Heavily Doped Polycrystalline Silicon Thin Films
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RESISTIVITY AND CARRIER MOBILITIES IN HEAVILY DOPED POLYCRYSTALLINE SILICON THIN FILMS Dae M. Kim*, Feng Qian*, Charles U. Bickford**, and Simon Yu** Department of Applied Physics and Electrical Engineering, Oregon Graduate Center, 19600 N.W. Von Neumann Drive, Beaverton, OR 97006 ** Tektronix Laboratories, Tektronix Inc., P.O. Box 500, Beaverton, OR 97077 *
ABSTIACT Electrical conduction data from heavily n and p-doped polysilicon thin films are presented. The sheet resistance in the range from 1 kf1/O to 100 f1/0J is characterized over temperatures from 20°K to 450°K. It is shown that the polysilicon resistivity, larger than the corresponding crystalline value by a factor - 10 in the same doping range, is temperature insensitive. This larger resistivity is correlated to the degree of dopant activation and the mobility. The measured mobility varying from 8 to 20 cm 2/V.s is smaller than the corresponding crystalline value by a factor 10 - 3.
INTRODUCTION The use of polycrystalline silicon (polysilicon) in silicon bipolar technology provided super self-aligned device structures and higher current gain[1,2J. These devices appear to hold up the potential for various high-frequency circuit applications. The large current gain attained in these bipolar transistors was extensively examined from different points of view. One of the approaches for modeling the device I-V characteristics is based on invoking the inherent electronic properties of polysilicon thin films used for device fabrication[3,4]. Specifically, the higher current gain was quantitatively described in terms of the reduced minority carrier mobility in polysilicon emitter contact layers[4]. We present in this paper the electrical properties of heavily doped polysilicon thin films. The resistivities and mobilities of both electrons and holes are presented as a function of doping concentration and temperature.
EXPERIMENT Resistors and Van der Pauw structures were fabricated in LPCVD polysilicon thin films with the thickness of 0.2 and 0.4 pRm. These films were deposited at 600'C. The samples were implanted with B+, As+ or P + at several different doses. Some of the samples were passivated with the use of nitride overcoating, followed by 400'C annealing for 30 min.
RESULTS Figure 1 presents the sheet resistance, Rs of p-type polysilicon as a function of doping concentration for both passivated and as-deposited samples. The resistivity, p remains flat at a near intrinsic level over a wide doping range. This is due to the fact that the mobile holes donated by B+ ions are captured in the grain boundary trap sites. Near the critical doping concentration corresponding to the value of effective volume density of grain boundary trap sites, the sheet resistance is drastically reduced. This can be understood, since the trap sites are saturated and mobile holes become available for conduction. With the grain boundary passivation, the behavior of sheet Mat. Res. Soc. Symp. Proc. Vol. 106. V1988 Materials Research Society
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