Metal Induced Growth of Poly-Si Solar Cells and Silicide Nanowires by use of Multiple Catalyst Layers
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Metal Induced Growth of Poly-Si Solar Cells and Silicide Nanowires by use of Multiple Catalyst Layers
Joondong Kim, Chunhai Ji and Wayne A. Anderson Department of Electrical Engineering University at Buffalo, State University of New York, Buffalo NY 14260
ABSTRACT Polycrystalline Si thin films and single NiSi crystalline nanowires were made by the metal induced growth method. The different growth mechanisms for poly-Si and nanowires lie in the metal silicide formation. Poly-Si growth is based on metal disilicide formation and the growth of nanowires depends on metal monosilicide formation. The metal silicide formation depends on catalyst species and sputtering power. Following catalyst coating, Si was deposited in a DC magnetron sputtering system to grow poly-Si or nanowires. Several metals (Ni, Co, Co/Ni and Pd) were used as catalysts to confirm the nanowire growth mechanism as well as to fabricate solar cells. Poly-Si thin films were about 5 µm thick with up to 1 µm crystallite size. Nanowires were up to 10 µm long with about 50 nm diameter. INTRODUCTION Metal-induced crystallization has been popular in Si due to a reduced processing temperature to form polycrystalline Si. Ni implanted amorphous Si followed by annealing produces an epitaxial Si crystal based on the Ni disilicide (NiSi2) formation [1]. Metal Induced Growth (MIG) is a promising method to make polycrystalline Si (poly-Si) as well as single crystalline nanowires [2]. In poly-Si, the metal catalyst (Ni and Co) formed a metal disilicide to grow a Si film with low lattice mismatch of 0.4% and 1.2% for Ni and Co, respectively. Good quality poly-Si can be used to make cost-efficient solar cells. The straight line growth property of MIG nanowires can be used to fabricate nanobridges and solar cell absorbers. In single crystalline growth, metal monosilicide formation is a significant step to grow NWs.
EXPERIMENTAL DETAILS A 200~300 nm SiO2 layer was first deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) acting as a buffer layer against metal diffusion into the Si substrate. Metal catalyst (Co or Ni) was thermally evaporated onto the SiO2 coated Si wafer. Then, Si was sputtered from a Si target on the metal layer by DC magnetron. The sputtering process was performed in 5 % H2/Ar, where H2 passivated the dangling bonds. A field emission
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scanning electron microscope (FESEM, Hitachi S-4000 accelerated at 20 ~ 30 kV) was used to observe the Si nanowire growth morphology. A thin tungsten (W) substrate was also used for both poly-Si and NW growth. A tungsten substrate does not require a SiO2 coating and provides a spontaneous back Ohmic contact. In the sputtering process, the power was 30~200 W for poly-Si and 5 ~30 W for NW growth. Au was deposited on the poly-Si to make Schottky solar cells in metal-insulator-semiconductor (MIS) structures. The fabricated solar cells were tested with AM1.5 (100 mW/cm2) illumination.
RESULTS and DISCUSSION Morphological changes The deposited Ni layer on a SiO2 coated Si wafer was grooved and a
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