Structure And Its Physical Effects In Hydrogenated Amorphous Silicon Thin Films

Hydrogenated amorphous silicon(a-Si:H) thin film is a technologically important semiconductor material used in many applications due to its good optoelectronic properties. However, the structure and properties of a-Si:H thin film are very difficult to be completely understood and accurately predicted or controlled for its inherent structural complexity. The difficulty comes not only from the random arrangement of atoms and the various and flexible Si-H bonds, but from the sensitivity to deposition process and the different microstructures formed under certain preparation conditions. This dissertation focus on the structure of a-Si:H thin film and its physical effects. Based on the combination of theory and experiement, the structural characteristics and dynamics, as well as the structural origin of the related optoelectronic properties in a-Si:H thin film, have been investigated in detail using different deposition technologies and multiple characterization methods. The major works can be summarized as follows:(1) The in-situ structural evolution, including the order of amorphous network, the Si-H bond configuration and the weak Si-Si bond, in a-Si:H thin film has been studied by multiple characterization methods. Meanwhile, the electronic state density near band edge, as well as its structural origin, has been thoroughly analyzed by constructing a model consistent with the electronic properties of a-Si:H. Results show that the increase of substrate temperature, on the one hand, improves the order of amorphous network and the distribution of weak Si-Si bonds, and on the other hand, changes the hydrogen content and the Si-H bond modes. The variation in optical absorption reveals that for the two competing mechanisms, namely, the hydrogenation effect and the order of amorphous network, the former is dominant.(2) The effects of unconventional process parameters(gas pre-heat and microwave annealing) on the structure and properties of a-Si:H thin film have been investigated using different deposition technologies(the plasma enhanced chemical vapor deposition and the magnetron sputtering) and characterization methods. The dynamics related to the unconventional process has been illustrated from the standpoint of the properties of plasma and the nonthermal effect of microwave, respectively. Results suggest that when other process parameters are fixed at the optimum value, the structure and properties of a-Si:H thin film can be further improved by a gas pre-heat before glow discharge due to its possible impact on the vibration energy state of gas molecules and the thermophoretic behavior of particles. There is no crystallization in the a-Si:H thin film exposed to microwave field without auxiliary materials, but the atomic arrangement(the order of amorphous network and the Si-H bonds) changes dramatically on account of the lowest-energy transport of hydrogen atoms. According to the obtained experiemental results, a dynamics model has been proposed to describe the nonthermal effect of the microwave field and its interaction with the amorphous structure.(3) The dispersion model, the optical properties, the electronic state density and its structural origin have been analyzed based on spectroscopic ellipsometry. Results show that there are discrepancies between the data derived from Forouhi-Bloomer(FB) model and Tauc-Lorentz(TL) model because of the different physical assumptions and mathematical calculations. In order to decrease the physical imperfection of the FB model, we have proposed a modified FB model, which is able to better fit the optical constants of the samples with different microstructures. The electronically structural imformation is extracted from the ellipsometry spectrum via the parameterization of dielectric function using the empirical density of states(DOS) model and the modified minimum value search algorithm. Results show that the variation in electronic structure is directly related to the thin film structure. The increase of disorder in amorphous network has a strong impact on the DOS in band tails, especially in the valence band tail. The optical absorption exhibits a contrary trend in the energy region below and above ~1.7 e V. This is because the films with higher deposition temperature have larger DOS in band tails and lower optical gap. In addition, the different definition of surface roughness leads to the inconsistent root mean square roughness values between the spectroscopic ellipsometry and the atomic force microscopy.(4) Aiming at the structural dynamics during the doping and alloying of a-Si:H thin film, the P-doped(Phosphorus) and Ru-doped(Ruthenium) films have been studied using many modern characterization technologies. The increase of substrate temperature decreases the four-coordinated P and hence the doping efficiency. The metallic element Ru is introduced into a-Si:H thin film by rf co-sputtering technique for the first time. Raman spectra reveal that the addition of Ru further disarranges the intrinsically disordered amorphous network and generates more coordinated defects. Meanwhile, a new paramagnetic signal with g=2.02, associated with the holes localized in valence band tail, has been observed. Moreover, the conductivity increases by about nine orders of magnitude with the increase of doping concentration. The temperature coefficient of resistance(TCR) shows that this material may have a potential application in the infrared detector. It is inferred from the related physics and the experiemental data that Ru could bond to Si in the amorphous network in the form of sd3 hybridization, generating the acceptor-like state above valence band and increasing the conductivity of samples, as well as transforming the charge of dangling bonds.