Invited Paper SS2-TuA3
Surface Photovoltage and Band Bending in GaN

Tuesday, November 10, 2009, 2:40 pm, Room M

Although GaN is a wide-band gap semiconductor being successfully utilized in high-temperature and optoelectronic devices, the influence of surface effects such as band bending on the electrical and optical properties of GaN is not yet well understood. It is known that negative surface charge on air-exposed, undoped n-type GaN causes a substantial upward band bending of 0.4 – 1.5 eV. This band bending can be reduced by 0.3 to 0.9 eV during UV illumination, known as surface photovoltage (SPV), due to the accumulation of photo-generated holes at the surface. We have investigated the dependence of the SPV on illumination intensity and wavelength, as well as its transients after turning on and off illumination, using a Kelvin probe mounted in an optical cryostat. As expected, the initial SPV rise is slow at low illumination intensities and fast at higher intensities, where its initial slope is a linear function of illumination intensity. The SPV signal in air ambient under UV illumination increases within seconds to a maximum (0.4 to 0.7 V), but then gradually decreases during illumination. A similar behavior occurs in an oxygen environment (but not nitrogen), indicating that surface oxygen species are the primary component responsible for the observed SPV decrease. In vacuum, the SPV signal quickly increases but then slowly increases under UV illumination. This SPV behavior is consistent with the photo-induced adsorption of negatively charged oxygen species in air and their desorption in vacuum. After ceasing illumination, the SPV decays nearly logarithmically regardless of environment, but requires substantially longer to reach its dark value after extended illumination in vacuum.

We explain the transient and steady-state SPV under UV illumination using both internal and external mechanisms. The internal mechanism involves photogenerated holes that accumulate at surface states and recombine with free electrons from the bulk, whereas the external mechanism involves electrons that tunnel through a thin oxide layer and are captured by surface species, resulting in chemisorption. Our proposed phenomenological model quantitatively explains SPV data for low intensity UV light using only the internal mechanism; however, illumination at high intensities requires the inclusion of photo-induced chemisorption or desorption processes. Our studies to date have focused on n-type GaN, but we have preliminary data for p-type GaN indicating an initial downward band bending of ~1 eV and an unexpected increase in band bending under UV illumination. Studies of the band bending and SPV transients in different ambients are in progress and will be reported as well.