Optimization of the metal/silicon ratio on nickel assisted crystallization of amorphous silicon
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Optimization of the metal/silicon ratio on nickel assisted crystallization of amorphous silicon L. Pereira1, M. Beckers2, R.M.S. Martins2, E. Fortunato1, R. Martins1 1 Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa and CEMOP, Campus da Caparica, 2829-516 Caparica, Portugal 2 Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, P.O.B. 510119, 01314 Dresden, Germany
ABSTRACT The aim of this work is to optimize the metal/silicon ratio on nickel metal induced crystallization of silicon. For this purpose amorphous silicon layers with 80, 125 and 220 nm thick were used on the top of which 0.5 nm of Ni was deposited and annealed during the required time to full crystallize the a-Si. The data show that the 80 nm a-Si layer reaches a crystalline fraction of 95.7% (as detected by spectroscopic ellipsometry) after annealed for only 2 hours. No significant structural improvement is detected by ellipsometry neither by XRD when annealing the films for longer times. However, on 125 nm thick samples, after annealing for 2 hours the crystalline fraction is only 59.7%, reaching a similar value to the one with 80 nm only after 5 hours, with a crystalline fraction of 92.2%. Here again no significant improvements were achieved by using longer annealing times. Finally, the 220 nm thick a-Si sample is completely crystallized only after 10 hours annealing. These data clear suggest that the crystallization of thicker a-Si layers requires thicker Ni films to be effective for short annealing times. A direct dependence of the crystallization time on the metal/silicon ratio was observed and estimated.
INTRODUCTION For several years, polycrystalline silicon (poly-Si) received special attention aiming the use on large area devices, such as solar cells and thin film transistors for active matrix displays. One of the first techniques used to obtain poly-Si was by low-pressure chemical vapour deposition (LPCVD) that requires high process temperatures on the order of 600-650ºC. However the demanding for low cost devices and consequent use of low cost substrates such as commercial glass, forced to a decrease on the poly-Si processing temperature [1,2]. Besides that, the crystallinity is not high enough for as deposited poly-Si and the material has much intra grain defects and high roughness [3]. So, lately, the production of poly-Si for device’s application has been done by crystallization of amorphous silicon (a-Si), using either laser or thermal annealing. Solid Phase Crystallization (SPC) was the first technique employed to crystallize a-Si [4]. However, for this process the crystallization temperature is still too high and the time required for full crystallization is too long. Nowadays, excimer laser annealing (ELA) is widely adopted as crystallization method for low temperature poly-Si [5], but it has some problems such as nonuniform crystallization [1] and high production cost [6]. So, metal induced crystallization (MIC) emerged as an alternative cr
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