Femto-second transient absorption spectroscopy for probing near-surface carrier-photon dynamics in gallium nitride

Periodicity of a crystal breaks at the surface leading to a large number of surface dangling bonds. These bonds create additional states in the bandgap of a semiconductor, which induces a static built-in electric field near the surface. This field modifies the spatial distribution of carriers. With increasing surface to volume ratio, it is indispensable to understand carrier and photon dynamics at the surface. In this work, two distinct surface conditions have been created by annealing GaN surface at high temperature in oxygen and nitrogen ambients and the characteristics are compared with a pristine GaN sample. UV-visible spectroscopy and XPS measurements are carried out to explore bandgap and surface stoichiometry under these surface conditions, which correlate with the pump-probe spectroscopy observations. Pristine GaN surface tends to get oxidized with time and behaves similarly to that of oxygen treated samples in longer time. The non-destructive femto-second pump-probe spectroscopy is employed to probe the carrier and photon evolution at the surface. The surface potential is measured by KPFM, which corroborates our observations that bands favor the accumulation of holes for some cases, while accumulation of electrons is facilitated in other cases. Photoluminescence analysis also suggest modification of the transition energy with these treatments.