Enhanced Conductivity in n-Type Microcrystalline Silicon
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ENHANCED CONDUCTIVITY IN n-TYPE MICROCRYSTALLINE SILICON E.A.T.Dirani, Lab. de Microeletr6nica Cx.P.8174, CEP E-mail :
A.M. de Andrade, 1. Pereyra, Escola Polit~cnica - Univ. de Sdo Paulo 05508-900, S~o Paulo, Brazil. [email protected]
ABSTRACT Phosphorus doped pc-Si:H films have been prepared in a PECVD system from SiH4 + PH3 + H2 gaseous mixtures. The films are obtained as a result of the competition between two simultaneously occuring processes: a deposition one and an etching one due to the high concentration of hydrogen in the plasma. This competition process favours the growth of films with crystalline structure. Highly conductive pc-Si:H layers, in the range of 20 - 30 S.cm-, were obtained for samples in the range of 70 to 85 nm thick. From Raman Dispersion Spectroscopy and X-Ray Diffraction measurements, evidences of the dominant role of the crystalline phase in the charge transport mechanisms are shown. Through Infrared Spectroscopy measurements it was possible to obtain the concentration of the SiHn groups on the pc-Si:H n+ structure and to infer their role on the electrical properties of the films. INTRODUCTION Hydrogenated microcrystalline silicon films ( pc-Si:H ) deposited in a PECVD system have attracted much attention in the past few years. Since the works of Matsuda et al. [1] in 1980 and Veprek et al. [21 in 1981, who systematically studied the deposition conditions for the pc-Si:H films, the interest for this material is continuously increasing. Various growth models of pc-Si have been proposed. In particular, the formation of pc-Si has been described by a chemical equilibrium between growth and etching induced by H atoms [5). Since energetically unfavorable configurations are preferentially eliminated, H2 etching reduces the temperature required to remove weak and strained bonds and produces pc-Si:H [6]. The possible applications in large area electronic devices has encouraged many researchers to develop deposition techniques for this important material. Several utilizations of the pc-Si:H material have been reported in solar cells, pressure sensors and other devices [2-4]. In this work the electrical properties of phosphorus doped pc-Si:H layers were investigated as a function of the films structure. The characterization of the deposited layers was done through the electrical conductivity, Raman dispersion spectroscopy, X-ray diffraction and IR spectroscopy measurements. EXPERIMENTAL PROCEDURES
The samples were prepared onto Corning Glass 7059 substrates for electrical and structural measurements. Optically flat glass slides were used for thickness determination. High resistivity silicon wafers polished in both sides were used for IR mdasurements. The samples were deposited on a PECVD system already described [7]. To get an optimized deposition condition for the phosphorus doped films, a bias voltage of +75 V was applied to the grid electrode of the system which yields a maximum value for the electrical conductivity [8]. To confine the plasma region to the substrate holder, a magnetic
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