Energy band tilt in ultra-thin InGaN film affected by the surface adsorption and desorption

The luminescence characteristics of InGaN ultra-thin films with and without GaN cap layer are investigated by temperature-dependent photoluminescence (TDPL) measurements. It is experimentally observed that, during the TDPL measurements at some fixed temperatures, the PL spectra of bare InGaN film show anomalous attenuation and redshift with time passing. This may be attributable to the enhancement of quantum-confined Stark effect in InGaN layer due to the desorption of the negatively charged particles on the surface of bare InGaN film, because the adsorbed negative charges on InGaN surface partially counteract the piezoelectric field in the ultra-thin InGaN film. However, for the GaN-capped InGaN ultra-thin film, those anomalous spectral variations with time vanish, as the adsorption and desorption process on the InGaN surface are screened by the GaN cap layer.