Studies of the Gap States Density in Undoped and Doped Amorphous Hydrogenated Silicon
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Density of states
Dangling bond
Photocurrent
Photoconductivity
We report on the spectral response and intensity dependence of photoconductivity (PC) and persistent photoconductivity (PPC) in plasma-enhanced chemical vapour deposition grown sulphur-doped n-type a-Si:H films. From the intensity dependence of PC it is found that the addition of sulphur changes the recombination mechanism from monomolecular for intrinsic and low-doped films to bimolecular at a high sulphur doping level. The photo-induced metastable increase of dark conductivity in these films is found to be quite similar to that for compensated and doping-modulated a-Si:H films. The PPC effect is detectable up to an illumination temperature of at least 380 K the highest temperature used in this study. At 300 K the conduction persists at a level of one order higher than the equilibrium dark conductivity for over 103 s after removing the excitation. The PPC in a-Si, S:H is explained in terms of the valence alternation pair model.
Photoconductivity
Metastability
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Exposure of intrinsic glow-discharge hydrogenated amorphous silicon films to white light at an intensity of 100 mW cm−2 for several hours induces changes in the temperature dependence of photoconductivity. These changes, which are reversed by heating to 200 °C, can be explained by an increase in the density of states in the lower half of the gap. The new states are different from the two types of states previously inferred from the temperature dependence of photoconductivity.
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Glow discharge
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Studies of transient photoconductivity show that the primary effect of light soaking in hydrogenated amorphous silicon is to introduce dangling bonds of bulk density 1016–1017 cm−3. Light soaking reduces the μτ product of both electrons and holes to values which are quantitatively related to the observed increase in the dangling bond electron spin resonance (ESR) spin density. In addition, no change in the carrier drift mobilities is detected after light soaking.
Photoconductivity
Dangling bond
Transient (computer programming)
Electron Mobility
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The light-induced decrease of the photoconductivity in deuterated amorphous silicon is a factor of 3 less even though the defect density increase is greater than in hydrogenated material having equivalent as-deposited properties. Consequent changes in the average recombination cross section of the defects is illustrated. Since the differences in the light soaking behavior upon isotopic substitution has been found to disappear in films deposited at low temperatures, the changes are thought to arise from differences in the silicon network occurring during growth.
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Intrinsic hydrogenated amorphous silicon exhibiting unchanged room-temperature photoconductivity of 1.5×10−5 (Ω cm)−1 at 100 mW cm−2 after prolonged illumination and a photo-to-dark conductivity ratio over 106 is reported for the first time. Photoconductivity and CPM data in the as-deposited and light-soaked states, supported by dark conductivity and analytical measurements, suggest that the metastable midgap states in this material are significantly lower than levels previously reported.
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Metastability
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The recombination processes in undoped hydrogenated amorphous silicon under steady-state illumination have been studied in order to explain the temperature dependences of photoconductivity. The recombinations of free carriers through the exponential tail states and through the Gaussian distributed dangling bond states were calculated in terms of the Shockley–Read theory and the occupation statistics of correlated defects. An additional recombination of trapped electrons and holes in the tail states through the dangling bonds by tunneling should be included in order to explain the experimental data of the activated photoconductivity at low temperatures.
Photoconductivity
Dangling bond
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Dangling bond
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Abstract Studies of spin-dependent photoconductivity in a-Si: H are reported. It is shown that, with increasing E.S.R. spin density, the photoconductivity decreases, but its spin dependence increases. The results demonstrate that dangling bonds are important trapping or recombination centres at room temperature. Prolonged exposure to above-gap illumination increases the spin dependence of photoconductivity and provides further evidence that light induces dangling bonds. The spin dependence of photoconductivity is compared with that of luminescence.
Photoconductivity
Dangling bond
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Photoconductivity
Intensity
Amorphous semiconductors
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