Low-temperature crystallization of amorphous silicon (a-Si) films by adsorbing metal on the films was investigated. Amorphous Si films were deposited by low-pressure chemical vapor deposition, spin-coated by metal solutions and subsequently annealed in Ar atmosphere. Polycrystalline silicon (poly-Si) thin-film transistors (TFT) were fabricated using the annealed Si films with Cu adsorption. And the thermodynamic parameters were extracted by observing crystallization behaviors.
The a-Si thin films, deposited on $SiO_2$ at 545℃ with the deposition rate of 2.8 and 4.0 nm/min, were crystallized at $600\,^\circ\!C$ in Ar ambient and the crystallization behavior was observed. The crystallization time, temperature and final grain size of the Si film were increased when the deposition rate was 4.0 nm/min. These results were caused by the crystallites observed in the as-deposited Si films with 2.8 nm/min deposition rate. The lower deposition rate, the more crystallites formed at the Si/$SiO_2$ interface and could be easily nucleated and crystallized. The incubation time activation energies of the film deposited with the rate of 4.0 and 2.8 nm/min were 5.05 and 3.62 eV, respectively.
We proposed a simple new process which lowered the solid phase crystallization temperature and shortened crystallization time. The a-Si films adsorbed from 1000 ppm of Cu and Au solutions were almost completely crystallized with (111) preferred orientation after annealing for 20 h at 530℃, which was well below the commonly employed solid phase crystallization temperature, 600℃. Other metal solutions, such as Ag, Zn, Fe, Cr and Al did not clearly enhance crystallization of a-Si films. The factors that enhanced clearly low-temperature crystallization by Cu and Au adsorption were the electronegativity, eutectic temperature and diffusion length. The adsorbed-Cu clusters acted as surface nucleation sites and caused fractal growth with the shape of tree branches. The fractal size of the Cu-adsorbed Si films was 30-50 ㎛ at 530℃ within 10 h. The fractal was composed by thin and long needlike crystallites which propagated fast in direction of [112] by capturing Si atoms in a-Si matrix. About 99\% of the adsorbed copper particles $(4 ×10^14 atoms/㎠)$ were evaporated in the early stage of annealing at 530℃. And about 1% of the adsorbed Cu particles migrated and formed clusters on the surface of a-Si. After 2 h annealing, the overall Cu concentration reduced below 0.1%. According to various pre-cleaning processes of a-Si films, the existence of hydrocarbon and native oxide on the surface of a-Si films suppressed the adsorption of Cu and Au. This result is a direct evidence that metal ions in solution are adsorbed on a-Si film as metal atoms by accepting electrons from Si surface. Using this property, a-Si film could be selectively crystallized by $SiO_2$ masking.
The nucleation and growth behavior of the Cu-adsorbed a-Si films was easily controlled by changing the solution concentration. Secondary ion mass spectroscopy analysis indicated that the crystallization enhancement was generally promoted by increasing Cu concentration in solution. By the film adsorbed from 10 ppm Cu solution was more enhanced than the film adsorbed from 100 ppm. It turned out that the adsorbed Cu concentration was minimum value of $5×10^{13} atoms/㎠$ with the 100 ppm Cu solution. The adsorbed Cu concentration was controlled by both th pH value and the Cu concentration in solution. The crystallization fraction after 20 h annealing at 530℃ was proportional to the as-adsorbed Cu concentration. The grain size of the film adsorbed from 1000 ppm Cu solutions and annealed at 530℃ for 20 h was about 0.3 ㎛, which was 40% larger than the intrinsic a-Si film annealed at 600℃ for 20 h. The grain growth rate of Cu-adsorbed a-Si film was much higher than that of intrinsic a-Si, probably due to high diffusivity of Cu atoms. The incubation time activation energy decreased with increasing Cu concentration in solution.
The characteristics of TFTs, fabricated using the films adsorbed from Cu solution, and annealed at 530℃ for 20 h, were similar to those of TFTs fabricated without Cu-adsorbed films. The threshold voltage was 11.8 V and $I_on/I_off$ was $ ×10^5$ before hydrogen passivation, regardless Cu concentration. The field-effect mobility was highest in TFT without Cu adsorption and degraded from 10 to 6 ㎠/V ㆍs with increasing Cu concentration in solution. It is assumed that impurity scattering is increased by Cu atoms.
The crystallization time and crystallization temperature decreased with increasing Au concentration in solution up to 100 ppm. Above 100 ppm, the crystallization behavior was same. The fractal growth was not observed in the film adsorbed from Au solution and annealed. The electrical resistivity was decreased to 5×10^3 Ω㎝ as the Au concentration in solution increased and also as the annealing time increased. The crystallization temperature increased when the a-Si film was deposited with higher deposition rate and was treated with Au adsorption. Two-step annealing was carried out after adsorbing from 1 and 10 ppm on the a-Si film deposited by the rate of 4.0 nm/min to enlarge grain size. The grain size did not increase remarkably. But we suggested the possibility to make large grains using the control of distance between nucleation sites and the crystallization temperature difference between intrinsic and Au-adsorbed a-Si films.
