Effects of the Second Step Annealing Time and Temperature in Two Steps Annealing on Low Temperature Poly-Silicon Film Fabrication

This study fabricates large grain low temperature poly-crystalline silicon film which can be applied to solar cell. The first annealed poly-crystalline silicon film is used as the seeding layer in the crystallization of the next thicker amorphous silicon film. The thickness of the silicon film is over 1 mum. The annealing temperature is set at 500degC and last for 1 hour in the first annealing stage. The crystallinity of the first annealed silicon film is discussed in this paper. In the second annealing stage, two different annealing temperatures (450 and 500 degC) and five different annealing time periods (15, 30, 60, 120 and 240 minutes) are chosen to see the effects of annealing temperature and time period on the crystallization of poly silicon film. Leakage current and surface topography are also being studied. XRD and Raman spectra analysis are used to identify the crystallinity of specimens made under different annealing time periods and temperatures. The surface and cross-section are observed and discussed via scanning electron micrographs. The I-V characteristic is tested to see the magnitude of leakage current of poly silicon film in our study. The leakage current of the poly-silicon film fabricated in this study falls in about 10^-7 A/cm². The resistivity of the poly-silicon film falls between 10³~10⁶ H-cm. The lateral grain size observed from SEM graph is about 3~5 mum The maximum carrier mobility obtained in this study is about 23.4 cm²/V.s.

Introduction

Solar cell is a clean and ever-lasting energy source that attracts more and more attention in the world as the crude oil and coal resources are getting less and less in the future. The materials of solar cell commonly used are bulk single or poly crystal silicon material. However, as for the need for cost reduction, the development of silicon thin film solar cells is a matter of great urgency for further reducing the cost. Since the carrier mobility of a poly-Si thin film is 10 to 100 times larger than that of an a-Si:H film [1], which reaches the order of about , poly silicon film is the most promising way to be applied to the solar cell. There are three categories for manufacturing poly-Si film on glass: catalytic-CVD method [2], excimer laser annealing (ELA) method [3], [4] and metal induced crystallization (MIC) method [5]–[9].