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J. PALMER, J. YI, R. WALLACE, AND W. ANDERSON State University of New York at Buffalo, Department of Electrical and Computer Engineering, Center for Electronic and Electro-Optic Materials, 215 Bonner Hall, Buffalo, NY 14260

ABSTRACT Hydrogenated amorphous silicon films deposited on molybdenum sheet metal substrates have been crystallized by thermal annealing at 850 0 C for 4 hours in a nitrogen atmosphere. Xray diffraction and scanning electron microscopy results indicated that the average grain size in the crystallized films was approximately 200A. Palladium contacts were fabricated and the resulting Pd/Si/Mo structures were electrically characterized. Current-voltage-temperature2 measurements for phosphorus doped and undoped annealed samples resulted in a J -c V characteristic consistent with space-charge-limited current. Using this data, mobility as a function of temperature from IOOK-300K was obtained. In phosphorus doped samples, the mobility appeared to be limited by energy barriers at the grain boundaries. In undoped samples, a a • T" 2 exp(-b/T 114 ) temperature dependence consistent with variable-range hopping conduction was observed. INTRODUCTION For solar energy to become practical in terrestrial applications, a method of fabricating inexpensive large-area solar cells needs to be developed. Conversion efficiencies of approximately 23% are possible using single crystal silicon cells [1], but such cells are too expensive to be made into large area panels. Thin-film cells made of hydrogenated amorphous silicon (a-Si:H) can be fabricated inexpensively with efficiencies over 12% [2]. However, aSi:H exhibits the Staebler-Wronski Effect [3] whereby the photovoltaic response of the material degrades as a function of light exposure. An alternate method for producing solar cells would involve crystallizing an a-Si:H layer deposited on a sheet metal substrate into a polycrystalline film [4]. Such a procedure would combine the cost-effectiveness of a-Si:H cells with the stability of single crystal cells. This paper presents the conduction mechanisms found in silicon films prepared in this manner.

EXPERIMENT The starting material consisted of a-Si:H films deposited on molybdenum sheet metal substrates using a dc glow discharge in silane gas. Two different types of samples were compared in this study. One used undoped a-Si:H, 5pm thick, and the other used phosphorus doped (ND -101 7 cm-3 ) a-Si:H, 4pAm thick. Both a-Si:H films had a thin n+ layer adjacent to the molybdenum to insure that the back contacts were ohmic. The samples were annealed at 850 0 C for 4 hours in a tube furnace using a nitrogen gas flow of 2 liters/minute to crystallize the silicon films. 695 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society

Molybdenum was chosen as the substrate metal because of its relatively low thermal expansion coefficient compared to other metals. Also, an Auger electron spectroscopy depth profile on annealed samples was unable to detect any Mo in the silicon, indicating that almost none of the metal