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

Epitaxial layers of NiO and Ni 0.5 Co 0.5 O 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 Ni 0.5 Co 0.5 O layers with respect to GaN is [ 111 ] NiO ( Ni 0.5 Co 0.5 O ) ∥ [ 0001 ] GaN and [ 1 ̅ 10 ] NiO ( Ni 0.5 Co 0.5 O ) ∥ [ 1 ̅ 2 1 ̅ 0 ] 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 (∆E V ) and the band alignment at the heterojunctions (HJs). The ∆E V at NiO/GaN and Ni 0.5 Co 0.5 O/GaN HJs are 1.2 ± 0.2 eV and 1.8 ± 0.2 eV, respectively. The conduction band offsets (∆E C ) have also been estimated using the determined values of optical band gap (NiO: Eg = ∼ 3.5 eV; Ni 0.5 Co 0.5 O: Eg = ∼ 3.3 eV) from optical transmission measurements. The values of ∆E C are found to be 1.5 ± 0.2 eV and 1.9 ± 0.2 eV at NiO/GaN and Ni 0.5 Co 0.5 O/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 Ni 0.5 Co 0.5 O/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 Ni 0.5 Co 0.5 O/GaN HJ can be utilized to confine the carriers.