Study of Electrical Properties of Microcrystalline Silicon Films Using AC Measurements
- PDF / 204,889 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 100 Downloads / 190 Views
Study of Electrical Properties of Microcrystalline Silicon Films Using AC Measurements Ely A. T. Dirani1,2, Alexandre M. Nardes1, Adnei M. Andrade1, Fernando J. Fonseca1 and Reginaldo Muccillo3 1 LME, Dept. Eng. de Sistemas Eletrônicos, Escola Politécnica da USP, São Paulo,SP, BRAZIL 2 Dept. de Engenharia Elétrica – PUC, São Paulo, SP, BRAZIL. 3 CMDMC, CCTM, IPEN, São Paulo, SP, BRAZIL ABSTRACT Hydrogenated amorphous (a-Si:H) and microcrystalline (µc-Si:H) silicon films are indispensable materials for large area electronic devices like solar cells, image sensors and thin film transistors (TFTs). The interest of the µc-Si:H films arise from the fact that they combine the high optical absorption of a-Si:H and the electrical transport properties close to those of crystalline silicon. In this work we show the correlation between substrate deposition temperature, crystallinity and electrical properties of a-Si:H and µc-Si:H films. The films were prepared by a conventional PECVD (13.56 MHz) RF system from PH3/SiH4/H2 gas mixtures in the temperature range of 100 to 250oC. While phosphorus doped (n) a-Si:H are deposited yielding conductivity values no better than 10-2 S/cm, (n) µc-Si:H layers deposited at substrate temperature of 250oC show conductivity values higher than 101 S/cm, crystalline fraction up to 80% and Hall mobility of about 0.9 cm2. V-1.s-1. It was observed that a change in the dark conductivity behavior occurs around 140oC, with a large increase in the conductivity values. A corresponding increase is not seen in the average grain size and in the crystalline volume fraction, which shows an almost linear increase with the deposition temperature. This stronger influence of the temperature in the electrical characteristics of the µc-Si:H films may be related to the phosphorus activation, which occurs in higher degree at higher deposition temperatures. The correlation among Raman spectroscopy, Hall effect and AC conductivity measurements (frequency range 6 Hz to 13 MHz) shows that the crystalline phase dominates the electrical transport mechanism in µc-Si:H films. Preliminary results of AC measurements indicate that the electrical resistivity of each phase of this material can be determined. 1. INTRODUCTION It has been reported [1-3] that a-Si:H and µc-Si:H films have been deposited at temperatures well below the conventional CVD temperature processes. In order to maintain the properties of certain substrates, Plasma Enhanced Chemical Vapor Deposition process can be tailored. In most cases the existing processes make use of high temperature that may damage polymeric substrates. To develop a low temperature process it is necessary to understand the role played by the deposition temperature in the growth mechanism. This commences with the nucleation of “islands” of deposited material followed by the columnar growth and coalescence, which makes the film more compact and continuous [1,2]. This can be observed by the behavior of the deposition rate that, at first, diminishes with the deposition time until it reaches a
Data Loading...
