Polycrystalline silicon obtained by vacuum-enhanced nickel induced crystallization of amorphous silicon

N. BUDINI; P. A. RINALDI; F. M. OCHOA; J. A. SCHMIDT; R. D. ARCE; R. H. BUITRAGO

Cancún

Congreso; XXI International Materials Research Congress; 2012

In this work, we have investigated the nickel induced crystallization (NIC) of intrinsic hydrogenated amorphous silicon (a-Si:H) thin films under vacuum conditions. We deposited 350 nm-thick a-Si:H films in a plasma-enhanced chemical vapor deposition (PECVD) reactor operated at 50 MHz and a power density of 120 mW/cm2. Nickel atomic densities of ~1014?1015 at./cm2 were dc-sputtered onto the a-Si:H films in order to allow for NIC by conventional furnace annealing under vacuum (P ≈ 10?6 Torr) at 580 °C. Crystallization and crystalline quality were checked by optical microscopy and reflectance in the ultraviolet region. To compare these results with those of our previous works on NIC at atmospheric pressure (AP) [1?3], we have conditioned the furnace in order to achieve a simultaneous annealing at AP (under a N2 ambient) and under vacuum. In this way, after a 24 h annealing period the samples under vacuum crystallized completely while those at AP were in an intermediate crystallization state, with a crystalline fraction of 70%. The grain size obtained under vacuum was around 30 μm, which resulted lower than the final grain size of about 100 μm of the AP samples. In order to analyze the annealing time required to achieve full crystallization under vacuum, we proceeded with shorter vacuum annealing periods. After a 6 h annealing, the crystalline fraction was almost 90% and the grain size again around 30 μm. Extending further the annealing period for 6 h, the samples fully crystallized. Thus, a faster crystallization and a smaller grain size were observed for vacuum annealing, with a reduction greater than 50% in the time required for full crystallization. Large grains were obtained in both cases, with sizes of 30 and 100 μm under vacuum and AP, respectively. We propose a thermodynamical interpretation of the enhanced crystallization phenomenon under vacuum, based on experimental evidence acquired from infrared transmittance and photoacoustic spectroscopy measurements.