Kang-Cheng Qi

Structure and electronic states in a-Si:H thin films

The hydrogenated amorphous silicon (a-Si:H) thin films were prepared by plasma enhanced chemical vapor deposition at various substrate temperatures. This paper examined the relationship between structural evolution and electronic states of the tested thin films. Raman spectroscopy was used to evaluate the structural evolution in amorphous network. Meanwhile, Fourier transform infrared spectroscopy was applied to explore the change of hydrogen in thin films. Results show that the order of network on short and intermediate scales, the content and bonding mode of bonded hydrogen, as well as the intrinsic stress and silicon coordination defects, and closed rings in the thin films, vary with the deposition temperature. The dielectric spectra of samples were measured using SE850 spectra ellipsometer. The density of electronic band states (DOS) in both valence band and conduction band for a-Si:H thin films was obtained by fitting the measured dielectric spectra. The results, verified by optical measurement, reveal that the effect of hydrogenation on band edge DOS is predominant in comparison with that of network relaxation.

Effects of gas temperature on optical and transport properties of a-Si:H films deposited by PECVD

Effects of silane temperature (T g) before glow-discharge on the optical and transport properties of hydrogenated amorphous silicon (a-Si:H) thin films were investigated. The optical measurements show that the refractive index increases with increasing T g. The transport characterizations show that when T g increases, the dark conductivity increases. However, the temperature coefficient of resistance decreases. In addition, after holding at 130°C for 20 h, the resistance variation, ΔR/R, of the films deposited at T g = room temperature (10.8%) is much larger than those deposited at silane temperatures of 80°C (3%) and 160°C (2%). This can be attributed to different rates of defect creation in a-Si:H films caused by various T g.

Structural evolution and electronic properties of n-type doped hydrogenated amorphous silicon thin films

The relationship between structure and electronic properties of n-type doped hydrogenated amorphous silicon (a-Si:H) thin films was investigated. Samples with different features were prepared by plasma enhanced chemical vapor deposition (PECVD) at various substrate temperatures. Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy were used to evaluate the structural evolution, meanwhile, electronic-spin resonance (ESR) and optical measurement were applied to explore the electronic properties of P-doped a-Si:H thin films. Results reveal that the changes in materials structure affect directly the electronic properties and the doping efficiency of dopant.

Structural and photoelectronic properties of a-SiGe:H thin films with varied Ge prepared by PECVD

Hydrogenated amorphous silicon-germanium (a-SiGe:H) alloy thin films were fabricated by conventional radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) with a gas mixture of silane (SiH4) and germane (GeH4). The structural, optical and electrical properties of the films with different gas volume fraction of germane were investigated by Raman and Fourier transform infrared (FTIR) spectroscopy, ultraviolet and visible (UV-vis) spectroscopy and I–V curves, respectively. The amorphous network and structural disorder in the a-SiGe:H thin films were evaluated by Raman spectroscopy. Meanwhile, the Si-H and Ge-H configurations of the films were investigated by FTIR spectroscopy. From UV-vis spectroscopy and I–V curves, the optical and electrical properties of the testing films could be deduced with varied germanium. It can be concluded that the structural and photoelectronic properties of a-SiGe:H thin films can be influenced apparently by varing of GeH4/(SiH4+ GeH4) ratio in PECVD process.

Manufacturing and photoelectrical properties of P-doped a-Si:H thin films deposited by PECVD

The effect of gas temperature (Tg) on surface morphology, surface roughness, photoelectrical performances of a-Si:H thin films deposited by PECVD at 250°C substrate temperature has been investigated by atomic force microscopy, spectrometric ellipsometry and semiconductor characterization system, respectively. It is found that the surface morphology and density (ρ) as well as the photoelectrical properties such as refractive index (n), dark conductivity (σ), temperature coefficient of resistance (TCR) and activation energy (Ea) remarkably depend on Tg of SiH4 fed in reaction chamber. The higher the Tg, the larger the clusters of a-Si:H thin films deposited. Also, refractive index of a-Si:H thin films increase as Tg rises and the relationship between Tg enhancement of n and the densification of the films is observed. It is indicated that σ varies by two orders of magnitude but TCR decreases by 1.6 %/°C, and Ea gradually decreases linearly from 289.0 to 138.1 meV with Tg varying from room temperature to 160°C. The results of present study suggest that Tg in PECVD chamber plays an important role in the deposition of a-Si:H thin films and directly affects the surface morphology and photoelectrical properties of films. Control of surface morphology, photoelectrical properties of a-Si:H thin films through changing Tg can be usefully applied to the manufacturing of photoelectrical devices.