CW Argon-ion laser initiated aluminum induced crystallization of amorphous silicon thin films

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A4.5.1

CW Argon-ion laser initiated aluminum induced crystallization of amorphous silicon thin films

Sampath K. Paduru*, Husam H. Abu-safe, Hameed A. Naseem, Adnan Al-Shariah, and William D. Brown Arkansas Advanced Photovoltaic Research Center, Electrical Engineering Department, University of Arkansas, Fayetteville, AR, 72701

ABSTRACT

CW Argon-ion laser initiated aluminum induced crystallization (AIC) of RF magnetron sputtered amorphous silicon (a-Si) thin films has been investigated. It was found that lasers could be effectively used to initiate AIC process at very low threshold power densities. An argon-ion laser (λ=514.5 nm) was used to anneal Al/a-Si/glass structures with varying power densities ranging between 55 and 125 W/cm2 and exposure times ranging from 10 to 120 s. X-ray diffraction analysis showed the resulting films to be polycrystalline. The crystallization rate increased both with power density and exposure time. Environmental scanning electron microscopy (ESEM) analysis showed that the surface features change with increasing power density and irradiation time. A dendritic growth pattern was observed in the initial stages of interaction between the films. A strong crystalline Raman peak at around 520 cm-1 was observed in the Raman spectra of the crystallized samples.

INTRODUCTION

The formation of polycrystalline silicon (poly-Si) by annealing its amorphous precursor has been receiving a great deal of attention due to the immense potential it has in large area microelectronic applications [1]. Fabricating poly-Si thin films by metal induced crystallization (MIC) is attractive due to the low temperatures, simple processing steps, and very little processing times involved. It is a well-known fact that amorphous silicon in contact with certain metals and subjected to thermal annealing, crystallizes at very low temperatures [2]. Various metals such as Al, Ag, Ni, and Au have been used to crystallize amorphous silicon films using this method [3-6]. In the case of Al, temperatures as low as 150°C have been reported [7]. In general, the excitation forces for the interaction between metal-Si interfaces in all the MIC studies were thermal sources like optical and microwave ovens. Besides MIC, laser crystallization (LC) technique has been widely investigated in the creation of poly-Si thin films for more than a decade. Lasers have been known to be helpful in reducing the intra-grain defects and increasing the carrier mobilities. Due to the reduced processing times, lasers allow a wide range of selection of the substrates. The purpose of this study is to investigate if lasers can be combined with MIC to induce crystallization process at lower temperatures and laser powers and at much reduced exposure times.

A4.5.2

EXPERIMENTAL

Amorphous silicon and Al films were deposited on Corning 7059 glass substrates by RF magnetron sputtering process. During the deposition of the films, the base pressure was 10-8 torr and the RF power was 150 W for both films. The deposition temperature was 250 °C for a-Si and 40 º

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