Investigation of crystalline and band alignment properties of NiO/GaN and Ni0.5Co0.5O/GaN heterojunctions using synchrotron radiation based techniques

Abstract

Epitaxial layers of NiO and Ni0.5Co0.5O have been deposited on GaN templates using RF magnetron sputtering deposition technique. The epitaxial relationship in out-of-plane and in-plane directions for NiO and Ni0.5Co0.5O layers with respect to GaN is [111]NiO(Ni0.5Co0.5O)∥[0001]GaN and [1̅10]NiO(Ni0.5Co0.5O)∥[1̅21̅0]GaN, respectively. The epi-layers are found to have two triangular domain structures oriented along [111] direction with an in-plane rotation of 60° with respect to each other. Photoelectron spectroscopy (PES) has been used to determine the valence band offset (∆EV) and the band alignment at the heterojunctions (HJs). The ∆EV at NiO/GaN and Ni0.5Co0.5O/GaN HJs are 1.2 ± 0.2 eV and 1.8 ± 0.2 eV, respectively. The conduction band offsets (∆EC) have also been estimated using the determined values of optical band gap (NiO: Eg = ∼ 3.5 eV; Ni0.5Co0.5O: Eg = ∼ 3.3 eV) from optical transmission measurements. The values of ∆EC are found to be 1.5 ± 0.2 eV and 1.9 ± 0.2 eV at NiO/GaN and Ni0.5Co0.5O/GaN HJs, respectively. A type-II band alignment is observed at both the HJs. Higher values of valence band and conduction band offsets are observed at Ni0.5Co0.5O/GaN HJ in comparison to NiO/GaN. These experimentally determined values of band offsets can be used to design wide band gap HJ based devices like photodetectors, where higher values of band off-set at the Ni0.5Co0.5O/GaN HJ can be utilized to confine the carriers.