| Summary: | 博士 === 國立中央大學 === 化學工程與材料工程學系 === 105 === In this study, we report a photocurrent generation mechanism in the SnO2 thin film
by the charged chemisorption O ions on the SnO2 thin film surface induced by O2-
annealing. Both XPS and PL results indicate that the amount of the surface
chemisorption O ions of the SnO2 thin film increases with being annealed in O2 ambient.
The surface chemisorption O ions would form the surface space charge region and the
build-in electric field in the SnO2 thin film, which would separate the photo-excited
electron-hole pairs by UV-laser irradiation (266 nm) in the SnO2 surface layer. This
phenomenon can prolong the lifetime and reduce the recombination probability of the
photo-excited electron-hole pairs. That is the key for the photocurrent generation in
the SnO2 thin film by the charged chemisorption O ions. We also investigate the
phenomenon of the separation of photo-excited electron-hole pairs by the theoretical
calculation. We find that the width of space charge region from the results of poisson’s
equation do not conform with the common width of the space charge region.
Therefore, we assume that the oxygen vacancy concentration is not constant, but a
profile distributing from surface into thin films. The calculated width of the space
charge region is 4.32 nm. Since the time of the separation of electron-hole pairs is
shorter than the time of the recombination, we can study the time of separation of
electron-hole pairs by built-in electric field to verify the enhancement of lifetime of
photo-carriers. The time of electron drift across the space charge region is calculated
to be about 3.31 10-10 s. This results can prove that the space charge region of the
surface oxygen adsorptions can effectively separate the electron-hole pairs and enhance
the lifetime of photo-carriers.
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