Optoelectronically probing the density of nanowire surface trap states to the single state limit

Abstract: Due to the large surface-to-volume ratio, surface trap states play a dominant role in the optoelectronic properties of nanoscale devices(1-6). Understanding the surface trap states allows us to properly engineer the device surfaces for better performance. But characterization of surface trap states at nanoscale has been a formidable challenge using the traditional capacitive techniques based on metal-insulator-semiconductor (MIS) structures(7) and deep level transient spectroscopy (DLTS)(8-11). Here, we demonstrate a simple but powerful optoelectronic method to probe the density of nanowire surface trap states to the limit of a single trap state. Unlike traditional capacitive techniques (Fig1a), in this method we choose to tune the quasi-Fermi level across the bandgap of a silicon nanowire photoconductor, allowing for capture and emission of photogenerated charge carriers by surface trap states (Fig1b). The experimental data show that the energy density of nanowire surface trap states is in a range from 10^9cm^-2/eV at deep levels to 10^12cm^-2/eV in the middle of the upper half bandgap. This optoelectronic method allows us to conveniently probe trap states of ultra-scaled nano/quantum devices at extremely high precision.