Charge Transfer and Fermi Level Position at the NiO/GaN Heterointerface Studied by Contactless Electroreflectance—Implications for Heterojunction Diode Engineering

Nickel oxide (NiO) is known as a p-type semiconductor that is increasingly used in various semiconductor devices, including GaN-based devices, but the charge transfer at the NiO/GaN heterointerface and the Fermi level position at this heterointerface have not been thoroughly studied. In this work, NiO with different electrical properties (nominally n- and p-type) was used to deposit onto GaN-cap/GaN:Si structures and fabricate NiO/GaN heterojunction diodes. The valence band discontinuity between NiO and GaN was determined using ultraviolet photoelectron spectroscopy to be ∼1.9 eV. The Fermi level position at the NiO/GaN heterojunction and carrier transfer after junction formation were investigated using contactless electroreflectance (CER). It was observed that at the nominally n-NiO/GaN heterointerface, electrons are transferred from the NiO layer to the GaN layer and the Fermi level in GaN is located near the conduction band, while at the p-NiO/GaN heterointerface, electrons from the surface states in GaN are transferred to the NiO layer and the Fermi level at the p-NiO/GaN heterointerface shifts toward the valence band and is located ∼0.84 eV below the conduction band in GaN. NiO/GaN heterojunction diodes were obtained for both nominally n-type and p-type NiO layers. The potential barrier for p-NiO/GaN heterojunction diodes reached a value of ∼1.0 eV, which corresponds to the Fermi position at the p-NiO/GaN heterointerface determined by the CER method (∼0.84 eV), and the breakdown voltage exceeds 600 V. For nominally n-NiO/GaN heterojunction diodes, the potential barrier is much lower (∼0.5 eV), and the breakdown voltage is also lower (∼400 V). This is fully understood since the NiO/GaN heterojunction is type II with a barrier for electrons in the NiO layer, and this layer is depleted due to formation of the n-NiO/GaN heterointerface in contrast to the p-NiO/GaN heterointerface.