Properties of Nano-crystalline Silicon-Carbide Films Prepared Using Modulated RF-PECVD
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Properties of Nano-crystalline Silicon-Carbide Films Prepared Using Modulated RFPECVD Feng Zhu, Jian Hu, Ilvydas Matulionis, Augusto Kunrath, and Arun Madan MVSystems, Inc., 500 Corporate Circle, Suite L, Golden, CO, 80401，USA ABSTRACT We report on the fabrication of nano-crystalline silicon-carbide (nc-SiC) using pulse modulated RF-PECVD technique, from silane (SiH4) and methane (CH4) gas mixtures which is highly diluted in hydrogen (H2). The microstructure of nc-SiC material is nanometer-size silicon crystallites embedded in amorphous silicon-carbide (a-SiC) matrix. As carbon incorporation in nc-Si film increases, the bandgap is enlarged from 1.1eV to 1.55eV as measured by Photothermal Deflection Spectroscopy (PDS) while the crystalline volume fraction decreases from 70% to about 20%. It is found that the crystalline volume fraction, grain size and dark conductivity of nc-SiC films can be enhanced with applying a negative DC bias to substrate during deposition. INTRODUCTION It is well-known that the conversion efficiency of amorphous silicon (a-Si) solar cell is limited by the inherent light-induced degradation, which leads to an increase in the density of defect with prolonged illumination [1, 2]. As a result, the depletion width of the a-Si cell decreases and the recombination of the photo-generated carriers in the intrinsic layer increases, leading to a smaller collection width and hence a reduction in the open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF). To reduce the instability, devices are generally made in a tandem junction configuration where the a-Si as the top cell is thin (typically 200-300nm). This limits the Jsc of the tandem cell. For instance, due to the requirement of current matching in the tandem configuration, Jsc is limited to 11-12 mA/cm2 for the dual cells comprising of a-Si/a-Si or a-Si/nc-Si [3-5]; for a triple junction stack comprising of a-Si /a-SiGe/nc-Si or a-Si/a-SiGe/a-SiGe, Jsc is further reduced to ~9 mA/cm2 [6-8]. In order to increase the conversion efficiency of a-Si based solar cell, it is evident that either the problem of light-induced degradation in a-Si needs to be solved or the unstable a-Si component needs to be removed from the entire structure [2, 9]. Over the last three decades, extensive studies have been performed to understand and solve the light-induced degradation effect but with limited success. In contrast, the use of nc-Si materials with an appropriate crystalline volume fraction (Xc) of 60-70% ， nc-Si solar cells have exhibited less light-induced degradation [9-11]. These types of films contain fine grains whose size is in the range of 10-20 nm, embedded in an a-Si matrix. However, due to their lower bandgap (~1.1eV) the open circuit voltage of these devices is lower than that obtained in a-Si solar cell. Incorporating carbon in nc-Si film to form nanometer sized silicon crystallites embedded
in amorphous silicon-carbide matrix could lead to a promising material with higher bandgap (Eg) for application in thin
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