Photoreflectance surface Fermi level measurements of GaAs subjected to various chemical treatments

Abstract

Electroreflectance (ER), and its contactless form photoreflectance (PR), are optical characterization techniques that can be used to directly measure the surface electric field, and hence, the surface Fermi level, of GaAs films via the Franz–Keldysh effect and the Schottky relation. Photoreflectance was used to measure the surface electric field of an as-grown Si-doped GaAs film before and after exposure to a photochemical wash of low-conductivity water or an Na2S ⋅ 9H2O treatment at 295 K. As reported recently by various authors, these treatments are claimed to reduce the GaAs surface state density and unpin the surface Fermi level. Calibration ER experiments were performed on a separate n-GaAs Ni Schottky barrier and found barrier heights of about 0.80 eV at 82 and 295 K. For the calibration sample, the surface Fermi level of an air exposed surface was determined to be 0.73 eV below the conduction band at 295 K based on the PR measurements. For the as-grown films, after treatment, the surface Fermi levels were found to be about 0.63 eV. The measurements imply that the treatments affect a minority of the surface states and leave the surface Fermi level essentially pinned. Furthermore, these states have a large influence on the photoluminescence intensity and surface recombination velocity of the GaAs surface. The results of the treatments are not inconsistent with the unified defect model.