Research On The Preparation, Microstructure And Optoelectronic Properties Of Hydrogenated Amorphous Silicon Thin Films

Hydrogenated amorphous silicon (a-Si:H) thin films have attracted much more attention for use in uncooled microbolometers, solar cells and medical apparatus, etc., due to its enhanced optical absorption, high temperature of resistance and compatibility with semiconductor technology. However, the problems on the poor conductivity and stability of a-Si:H films are yet not ultimately resolved. Thus, the studies at present or in the future are to be carried out on preparing device-quality and high stability a-Si:H films.In this dissertation, Phosphor-doped (P-doped) a-Si:H films were deposited by plasma-enhanced chemical vapor deposition (PECVD). The effects of substrate temperature, silane temperature (before glow-discharge) and nitrogen doping, on the microstructure and photoelectronic properties of a-Si:H films, have been investigated by means of many modern characterization methods. Furthermore, electron irradiation effects on the properties of heavily P-doped a-Si:H films have been studied by prolonging irradiation time or irradiating with electrons of different energies. Then the relationship between the microstructure and properties of a-Si:H films are revealed. The main results in the dissertation are shown as follows:(1) There is an improvement in amorphous network order and an increase in dark conductivity of P-doped a-Si:H films with increasing silane temperature (T_g) before glow discharge. However, the spin density of unpaired electrons and the temperature coefficient of resistance (TCR) of the films decrease. For the films deposited at T_g=160℃, the isolated silicon-hydrogen (SiH) bonding configuration is predominant, and the dark conductivity increases by two orders of magnitude compared with those deposited at T_g=RT (room temperature). Although the TCR decreases by about 1.6 %/℃at T_g=160℃, it can be obtained above 2.0 %/℃. These results indicate that a-Si:H films with better quality can be prepared at higher silane temperatures. After holding at 130℃for some time, the resistance variation,ΔR/R, of the films deposited at higher temperature is much lower than that of the films deposited at T_g=RT, indicating that a-Si:H films deposited at higher silane temperatures behavior better in electronic stability.(2) The ordering evolution of the amorphous network in a-Si:H thin films was investigated by Raman spectroscopy. There exists a gradual ordering of the amorphous network on the short and intermediate scales towards the surface of a-Si:H thin films. Annealing taken on the films leads to an improved ordering of amorphous network on the short and intermediate scales in the interior region, but the network in the surface region becomes more ordered only on the intermediate scale.(3) Hydrogen bonding configurations and their evolution in a-Si:H films were intensively studied by Fourier transform infrared (FTIR) spectroscopy. The relationship between hydrogen bonding configurations and the properties of the films were discussed. It is found that the SiH dominates in a-Si:H films for hydrogen content c_h<16 at.%, but the polysilanes dominate in the films for c_H>16 at.%. With the increase of polysilanes amount in a-Si:H films, the amorphous network becomes more disordered and the dark conductivity decreases. Since the dihydrides (SiH₂) and the polysilanes (SiH_n) are more easily dissociated than SiH, a-Si:H films dominated by SiH behavior better in thermal stability than those films containing more SiH₂ or SiH_n.(4) When nitrogen (N) concentration in a-Si:H films is relatively low, the microstructure and electronic properties of heavily P-doped films change little. With the continuous increase of N concentration in a-Si:H films, hydrogen content decreases and amorphous network becomes more disordered. Meanwhile, the surface morphology of the films gets worse and the optical bandgap widens. Furthermore, the electronic properties of the films deteriorate significantly for N content c_N>1.0 at.%.(5) The microstructure and optical constants of a-Si:H films have been intensively studied by Ellipsometry. The results obtained from the transmittance spectra agree well with those measured by the reflecttance spectra, indicating that the microstructure and optical properties of a-Si:H films can also be accurately determined from transmittance spectra.(6) Electron irradiation induces the breaking of Si-H bonds and structural damage in heavily P-doped a-Si:H films. However, the structural damage and the degradation in dark conductivity of the films come to saturations after irradiation for some hours. This is because there is an irradiation-induced annealing effect during electron irradiation. Depth profile studies on irradiated P-doped a-Si:H films reveal that, the degradation in dark conductivity is much distinct in the near surface, and the film surfaces become more disordered as compared with their interior regions. These indicate that the created defects and structural damage are concentrated in the near surface of a-Si:H films. When heavily P-doped a-Si:H films are irradiated with lower electron energies, the degradation in dark conductivity of the films is greater and the amorphous network becomes more disordered.