Application and analysis of the constant photocurrent method in studies on amorphous silicon and other thin-film semiconductors
The opto-electronic properties of amorphous hydrogenated silicon films (a-Si:H) produced by plasma enhanced chemical vapour deposition (PECVD) in a commercial reactor and hot-wire chemical vapour deposition (HWCVD) in a laboratory setup, are subject to light-soaking and annealing and investigated using transient photoconductivity (TPC) and constant photocurrent (CPM) measurements. As a result it is to be found thatthe HWCVD produced material is competitive with the commercially produced PECVD material and shows a tendency to be more resistant against light-soaking as long as significant contamination does not occur in the deposition process.Depth profiling by chemical etching and correlation of infrared spectroscopy (FTIR) and CPM results are used to analyse the effect of oxygen and carbon contamination on the photoelectrical properties of a-Si:H material deposited using HWCVD method. It is shown how changing dissociation processes at the heated wire surface, surrounded by silane gas, affect the electronic and structural properties of the a-Si:H material produced. The optimum growth of a-Si:H material by HWCVD depends very much on the ‘substrate to filament’ distance, investigated by steady-state conductivity and CPM measurements. It is found that this has a big influence on the optoelectronic properties of the film and on the uniformity of film thickness, which is important for designing large area industrial HWCVD deposition units.The influence of the silane content (SC) (silane concentration in hydrogen) during the deposition process, in HWCVD and PECVD technique, on the film properties were investigated using CPM and FTIR measurement methods. From correlation between CPM and FTIR results it is shown that for low silane content (< 10%) the structure of the resulting material is predominately microcrystalline (pc-Si:H) and amorphous for highsilane content.Proton irradiation effects on undoped a-Si:H and pc-Si:H samples from PECVD and HWCVD deposition method were investigated by using steady-state photoconductivity and CPM experimental methods before and after 100 MeV proton irradiation. In contrast to 1 MeV electron irradiation, where major changes in the density of states were reported, the proton irradiation did not have any observable effect on the optoelectronic properties of the samples investigated.As a major part of this thesis the validity and precision of the constant photocurrent method - CPM, as a means to determine the density of states in thin films, is subject to investigation. In the past clear differences have been observed in the absorption coefficient spectrum of a-Si:H between DC - and AC - CPM measurements. In this thesis an explanation for these differences will be proposed and elaborated. DC-CPM measurement gives a consistently higher value for the absorption coefficient a at low photon energies. A small-signal analysis of the photoconductive response to modulated sub-gap illuminationreveals low frequency poles associated with thermal emission processes, which explains this discrepancy. Computer simulation demonstrates that while DC - CPM, which includes these transitions, gives a more accurate value for absorption, AC - CPM provides a more accurate means of determining the distribution of occupied gap-states. Further it is shown that combining DC and AC methods allows determination of the distribution of deep unoccupied gap-states. These concepts are applied to experimental results for severalundoped a-Si:H films.The CPM method has mainly been applied to measurements on thin film silicon materials. However, in a study on ‘non-silicon’ materials (i.e. CdTe, GaAs, CdSe & CdS2 ) the applicability of the CPM method to these photoconductive materials is demonstrated in this work.
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