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Low Temperature Photoconductivity of Hydrogenated Amorphous Silicon (a-Si:h) Thin Flims
(Izmir Institute of Technology, 2003) Erdoğan, Gökhan; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
In this study low temperature photoconductivity of undoped hydrogenated amorphous silicon(a-Si:H) thin films have been studied to investigate the effect of native and Staebler-Wronski defects. The study covers undoped a-Si:H films prepared by various deposition techniques such as DC glow discharge, RF-PECVD with and without H-dilution, RF magnetron sputtering and hot-wire(HW) CVD.In the annealed state, the samples were characterized using temperature dependence of dark conductivity, steady-state photoconductivity, .ph, versus light intensity at room temperature and steady-state photoconductivity versus temperature down to 90 0K at three different intensities. Activation energy ,EF, of the samples changes from 0.60 eV to 1.0 eV. .ph shows a few orders of magnitude higher values from the dark conductivity and its magnitude is sample dependent due to differences in deposition conditions. The intensity dependence of .ph ,., (.ph . F.) is close to unity and varies between 0.70 to 0.90, indicating recombination kinetics through the midgap defect states in the bandgap of a-Si:H. Low temperature photoconductivity versus 1000/T spectrum shows three distinctly different regions. In Region I, .ph decreases with temperature until a transition temperature. Then Region II begins, where .ph begins to increase resulting a peak in spectrum or remains to be unchanged until a second transition temperature to Region III, where .ph continuously decreases with T. Transition temperatures and the degree of increase in .ph in Region II is sample dependent. These results indicate the presence of at least two different types of midgap defect states in the bandgap and exponential tail state present in the annealed state.In the light soaked state, Staebler-Wronski effect (SWE) was investigated after exposing the samples to white light illumination of a few suns intensity. The characterization involves dark conductivity and steady-state photoconductivity at room temperature and .ph versus temperature down to 90 0K for different intensities. Dark conductivity values decreased a certain factor indicating a slight shift in EF through midgap. .ph values decreased substantially from its annealed values due to creation of Steabler-Wronski defects in the bandgap. The intensity dependence of .ph become almost equal and close to unity for all the films even it shows slight variation in the annealed state. The shape of low temperature photoconductivity spectra becomes almost the same for all samples even drastic differences were observed in the annealed state. The spectrum is mainly dominated by only two regions.Region I dominates from room temperature down to 170 0K, where .ph decreases with a constant slope as T decreases. After that temperature, Region II sets in. .ph remains to be constant until temperature used in this study. Region III can only be detected at higher intensity and temperatures lower than 90 0K. Results indicate that more defects around the midgap are created by light, which decrease .ph and relatively less defects are created away from midgap and closer to band edge, which improve .ph instead of decreasing it as temperature decreases. The defect states in Region I responsible for decreasing .ph are more likely that they are neutral silicon dangling bond defects ,D0, and those in Region II responsible for increasing .ph are non-D0 defect states. They act as photosensitising defects with a very low capture cross-sections for electrons. They could be charged silicon dangling bonds ,D+ and D-, or floating bonds results in defect models proposed for a-Si:H.
Subgap Absorption Spectroscopy in Microcrystalline Silicon Thin Films
(Izmir Institute of Technology, 2004) Göktaş, Oktay; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
Intrinsic hydrogenated microcrystalline silicon thin films prepared by VHF-PECVD and HW-CVD methods under different deposition conditions have been investigated using steady state photoconductivity method (SSPC), photothermal deflection spectroscopy (PDS) and dual beam photoconductivity (DBP) method, and transmission spectroscopy. Absorption spectra of the investigated thin films were measured in a wide energy region using PDS and DBP. A procedure, for the firs time, was used to calculate fringe free absolute absorption coefficient of thin films from DBP yield spectrum and simultaneously measured transmission signal. The results were found to be in agreement with those of PDS above the bandgap energy. However, there are differences between below the bandgap energy in the spectra of both methods. The differences are discussed to be consistent with the underlying physics of these methods. For some of investigated thin films there are remaining fringes in the .(h.) spectra measured using both methods. This is a strong indication of inhomogeneity present in the films in growth direction. DBP measurements were also performed for ac monochromatic light incident from substrate side in order to investigate the effect of inhomogeneous microstructure of the material on the absorption spectrum. It is found that some films have remaining fringes on their spectra for back ac measurements both for VHF-PECVD and HW-CVD grown thin films, whereas there is no remaining fringes observed for front ac measurements or vice versa. These findings are discussed to be an indication of inhomogeneity in growth direction which is already reported from TEM and Raman study. Sub-bandgap absorption coefficients .(0.8 eV) were correlated with the silane concentration, which is main parameter to change the microstructure of these films. It is found that the thin films that deposited in the transition region, where a transition from a fully amorphous growth to full microcrystalline growth occurs, have smaller absorption coefficients indicating that the thin films deposited at transition region have less defect density. However, thin films deposited at the highly crystalline region have the highest defect density due to etching effect of H during the deposition. These results are also consistent with reported ESR studies.

Master Degree / Yüksek Lisans Tezleri

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