Interface Study Of Nanocrystalline Silicon and Crystalline Silicon Using Microwave Photoconductivity Decay
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0910-A04-04
Interface Study Of Nanocrystalline Silicon and Crystalline Silicon Using Microwave Photoconductivity Decay Mahdi Farrokh Baroughi, and Siva Sivoththaman Electrical and Computer Engineering, Universty of Waterloo, 200 University Ave. W., Waterloo, Ontario, N2L 3G1, Canada ABSTRACT This paper presents a measurement technique for studying of the interface between a nanocrystalline silicon (nc-Si) film and a crystalline silicon (c-Si) substrate using microwave photoconductivity decay (MWPCD). The nc-Si films were deposited using plasma enhanced chemical vapor deposition of highly hydrogen-diluted silane. The films were deposited on both sides of the high purity float-zone (FZ) Si wafers. The high resolution transmission electron microscope (HRTEM) analysis of the interface and the characterization of the effective excess carrier lifetime of the samples using MWPCD revealed the following results: (i) The crystallinity of the deposited nc-Si films is very high. The nc-Si film follows the crystal orientation of the substrate such that not a well-defined boundary between nc-Si film and the c-Si substrate is observed. (ii) A surface recombination velocity of less than 10 cm/s was measured for the interface region of the nc-Si/c-Si junctions. (iii) A small discontinuity in the band-energy diagram of the interface region was observed. INTRODUCTION The excess carrier recombination lifetime is a very important parameter of the c-Si material and it directly reflects the quality of the material in terms of the defect density. MWPCD technique uses a pulse laser for exciting excess electron-hole pairs and hence changing the conductivity of the material. The conductivity decay of the sample is monitored by measuring the quality factor of a microwave resonator including a microwave antenna, an air gap and the sample. The carrier lifetime is calculated from the measured conductivity decay [1,2]. In this work, MWPCD technique is employed for the characterization of the nc-Si interface to a c-Si substrate. The deposition of the nc-Si films directly on top of a single c-Si substrates using plasma enhanced chemical vapor deposition (PECVD) results in a quasi-epitaxial growth of silicon [3,4]. Highly doped nc-Si emitters have been successfully employed for the fabrication of solar cells [5]. The Study of the electronic quality of the interface and the film itself by conventional electrical measurements is not straight forward because the detected signal from thin film interferes with the detected signal from the crystalline substrate especially in a case where properties of the thin film and the substrate are close. In this work, we present a contact-less measurement method based on recombination of excess carriers using MWPCD that is sensitive solely to the electronic quality of the interface and the deposited film. In this paper, we first present the experiments and measurement results and then introduce a new model to interpret the measurement results. EXPERIMENTAL RESULTS Interface of the nc-Si/c-Si junction was first studi
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