LARGE GRAIN POLY-SI THIN FILMS BY METAL INDUCED CRYSTALLIZATION OF a-Si:H Marwan A Albarghouti' , Husam H. Abu-Safe' , Harneed A. Naseem' ,William D. Brown'. Mowafak M. Al-Jassirn' , and Kim M. Jones' Photovoltiaics Research2Center,Department of Electrical Engineering, University of Arkansas, fayetteville, AR 72701 National Renewable Energy Laboratory (NREL). Golden, CO 80401 1 ABSTRAC Large grain poly-Si thin fiIms on glass substrates have been successfully fabricated. The film thickness was 400 nm and the grains sizes were in excess of several microns. The films were fabricated using aluminum-induced crystallization of a-Si:H in the presence of a silicon oxide layer at the Alla-Si:H interface. The aSi:H film was deposited on glass substrates and the annealing temperatures and annealing times were kept below 450°C and 30 minutes, respectively. The resulting poly-Si was heavily Al-doped. INTRODUCTION Poly-Si has been a subject of considerable research for the past decade. It is a material that is promising to enhance the performance of devices such as thin film transistors (TFTs) and thin-film solar cells. However, most of the conventional methods currently used to fabricate polySi thin films suffer from serious problems. For instance, poly-Si thin films by solid phase crystallization of a-Si:H and low pressure chemical vapor deposition (LPCVD) of poly-Si. suffer from high processing temperatures (> 60OoC) [I], which are not suited for low cost consumer products. Also, at these temperatures, the grain size of the poly-Si produced is usually less than lpm, Laser crystallization of a-Si has also been used to fabricate ply-Si thin films. This method usually yields good quality poly-Si with large grains [2]. however it is a low throughput method since it is limited by the size of the laser beam used, which doesn't exceed few millimeters in diameter. Low temperature poly-Si by hydrogen dilution of silane or by post-deposition hydrogen treatment has also been reported (3-51. However, the poly-Si produced by this method is known to be smallgrain poly-Si. One method that has been attracting a lot of research lately is metal-induced crystallization(MIC) of a- Si. This phenomenon was first observed by Herd et a/ in 1972 IS] and was later confirmed by several other experiments [q. Metal such as AI, Ag, Pd, and Ni have since been used to induce the crystallization in amorphous Si f8-1 I]. The processing temperatures and 0-7803-8707-4105/$20.00 Q2005 IEEE. 1070 the annealing times in this method are significantly lower than in other methods. A record temperature of 150'C for initiating the crystallization in *Si:H using AI was reported by Kishore et a\ [9]. The grain size of the polysi fabricated using the MIC method was also observed to be significantly large for such low annealing temperatures or short annealing times. The grain size was observed to further increase when a dielectric layer is introduced between the metal layer and the a-Si layer. Choi et a/ [Ill reported grain sizes in excess of 20 microns using Ni-mediated crystaIlization of a-Si through SiN cap layer. Nast et a/ [I21 also reported the formation of poly-Si thin films with farge grains (1 5-10 pm) using AI-induced crystallization of a-Si through an aluminum oxide layer separating the a-Si and the AI layer. In this communication, we report the formation of largegrain poly-Si thin films using aluminum induced crystallization of a-Si:H on glass substrate at temperatures below 450°C and for annealing times below 30 minutes. EXPERIMENT We deposited a 400 nm thick hydrogenated amorphous silicon (a-Si:H) layer on a glass substrate using plasma enhanced chemical vapor deposition (PECVD) at 17OoC.The a-Si:H layer was then exposed to several ambient to allow for a thin Si02 layer of various thicknesses to grow. The environments included room ambient, steam, and boiling water. After that, a 200 nm thick AI layer was depositedat room temperature using rf magnetron sputtering. Following fabrication, the samples were annealed at temperatures between 38O45O0C for various periods of time. After crystallization, the aluminum was etched away from the films using a standard etch solution and the poly-Si was examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscope (TEM). RESULTS'AND DISSCUTION Fig.1 shows a SEM micrograph of the poly-Si surface after AI removal. The a-Si:H of this sample was