Modeling of Aluminum Induced Lateral Crystallization of Hydrogenated Amorphous Silicon

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Modeling of Aluminum Induced Lateral Crystallization of Hydrogenated Amorphous Silicon Mohammad Saad Abbasi, Husam Abu-Safe, Hameed Naseem, and W. D. Brown Arkansas Photovoltaic Research Center, Department of Electrical Engineering, University of Arkansas, Fayetteville, AR, 72701 ABSTRACT Metal induced lateral crystallization (MILC) of hydrogenated amorphous silicon (a-Si:H) was studied and a model was developed based on the resistance measurement of the films. Hydrogenated amorphous silicon films of 300 nm thickness were deposited using plasma enhanced chemical vapor deposition (PECVD) on oxidized p-type (100) silicon wafers. Thermally evaporated 200 nm thick aluminum layer was deposited over amorphous silicon and patterned using photolithography. The samples were annealed at different temperatures for different time periods. After annealing the resistance of amorphous silicon between aluminum pads was measured. Based on these measurements, a model was developed to predict the lateral crystallization velocity. In this model, the resistance change due to loss of hydrogen from the film was also taken into account. For this purpose, another set of experiments was conducted. In this set, hydrogenated amorphous silicon films of 300 nm thickness were deposited on Corning 7059 glass. The samples were annealed for different period of time at different temperatures. After annealing, parallel bars of silver paint were formed on the samples and the resistance of each sample was measured. The theoretically determined lateral crystallization velocity was verified using optical microscope observations and X-ray diffraction analysis and was found to be in close agreement. INTRODUCTION Recently, polycrystalline silicon has received much attention for fabrication of certain devices like solar cells and thin film transistors [1-5]. This is mainly due to its characteristics such as high stability, good performance at high ambient temperature, and mobility sufficient enough for high speed (>5 MHZ) applications. In particular, polysilicon thin film transistors have attracted much attention because of their use in the driver circuitry of active matrix liquid crystal displays [1-3, 5]. The major challenge is to reduce the deposition temperature of polysilicon so that the devices can be fabricated on low cost glass or plastic substrate. This would lower the cost of displays considerably and would be a necessary step towards the development of roll up displays. Numerous techniques are being investigated to lower the deposition temperature from the solid phase crystallization temperature of about 600oC. Important among them are rapid thermal annealing (RTA) and excimer laser annealing [6, 7]. RTA is a high temperature process and the resulting film contains lots of defects. While excimer laser annealing can produce a good quality polysilicon at low temperature, it suffers from the drawback of high initial cost and process complexity. A technique called metal induced crystallization is now being widely investigated. In this techniqu