Size-dependent persistent photocurrent and its origin in dc sputtered indium oxide films under UV and sub-band gap illuminations

The size and spectral dependence of the persistent photocurrent (PPC) of dc sputtered indium oxide (IO) films has been studied under UV and sub-band gap illuminations. PPC follows bi-exponential decay with a fast and a slow process having time constants (denoted by τ_f and τ_s, respectively) that differ by about two orders of magnitude. τ_s is associated with carrier scattering from an initial surface state to a surface or bulk state with the former dominating below a characteristic length scale of ∼60 nm. On the other hand, τ_f is characterized by the process where both the initial and final states are surface related. Treating the IO film surface with tetramethyl tetraphenyl trisiloxane (TTTS) decreases τ_s by a factor of 5 with τ_f remaining almost unaffected, which is a clear indication of reduction of defects specific to the slow relaxation process. Based on the molecular structure and chemical activity of TTTS, it is suggested that TTTS may passivate mainly the dangling oxygen-bonds at the film surface. The spectral dependence of τ_s indicates that the associated surface states exhibit a maximum around 2.5 eV above the level from where strong optical transitions are allowed.