Persistent Photoconductivity in SnO<sub>2</sub> Thin Films Grown by Molecular Beam Epitaxy: The Dominant Roles of Water Vapor and Carrier Concentration
Rodrigo M. Gazoni, Martin Allen, Roger J. Reeves
Abstract
The persistent photoconductivity (PPC) of high-quality SnO2(101) films grown by molecular beam epitaxy (MBE) was investigated as a function of atmosphere, carrier concentration (n) and temperature (T = 300–10 K). The decay of the persistent photocurrent induced by sub-band-gap (405 nm) illumination was well described by the Kohlrausch stretched-exponential function, with the largest characteristic recovery times (τ) achieved in high-vacuum conditions, and a steady decrease in τ observed at increasing pressures above 1 Torr. This is consistent with the accepted PPC model involving the light-induced desorption and postillumination readsorption of an acceptor-like surface adsorbate, and the associated removal and reformation of a near-surface electron depletion layer. However, no difference in τ was found in 100% N2 and 100% O2 atmospheres, which is surprising given the dominant acceptor role usually assigned to adsorbed oxygen species, in particular O2– superoxide ions. In contrast, a significant decrease in τ was observed in humid N2, with a smaller decrease found in air, suggesting that water vapor plays a dominant role in the PPC of high-quality MBE SnO2 films. An almost 100-fold increase in τ with increasing n from ∼1016 to ∼1019 cm–3 was observed, with the photocurrent consistent with a simple parallel conduction model involving the decrease in the thickness of the electron depletion layer from ∼80 to ∼1 nm with n. Temperature-dependent measurements from 300 to 10 K revealed a significant decrease in τ with decreasing T that matched the temperature dependence of n, while the presence of two different recovery mechanisms was observed at T below 100 K.