Wide gap p-type NiO-Ga₂O₃ alloy via electronic band engineering

Gallium oxide (Ga₂O₃) has gained significant interest in recent years due to its wide bandgap and related unique properties, making it suitable for many high power and deep ultra-violet optoelectronic and photodetection devices. Nevertheless, Ga₂O₃ can only be doped effectively n-type, and its full potential in device applications is severely limited by the lack of a reliable p-type material. Here, we report on modifying electronic bands of Ga₂O₃ through alloying with NiO to achieve p-type conducting Ga₂O₃-NiO alloy (Ni_xGa_1−xO) thin films. We find that room temperature sputter-deposited stoichiometric and O-rich alloys with low Ni content (x < 0.22) have an amorphous structure. In contrast, films with higher Ni content (x ≳ 0.22) are polycrystalline with the rocksalt (RS) NiO structure. O-rich RS-alloys are p-type with resistivity ~20 Ω-cm (for x~0.6) and decreases to< 10 Ω-cm with increasing x. Optically, p-type O-rich films with x ≥ 0.46 have strong sub-gap absorption in the low energy region (˂ 3.5 eV) due to a high concentration of Ni vacancies V_Ni and this results in a low transmittance which also decreases with x from ~70 % (x = 0.3) to ~ 40 % (x = 1). The band gap E_g of the alloy films exhibits a wide tunability with a monotonic decrease with increasing x from 4.98 (x = 0) to 3.53 (x = 1). The E_g for the RS alloys follows the virtual crystal approximation with a small bandgap bowing of 0.36 eV and an extrapolated E_g of 4.6 eV for RS Ga₂O₃. Furthermore, we find that the amorphous and RS crystalline alloys have a type II (staggered) band offset (ΔE_V~1.8 eV and ΔE_C~1.4 e_V) with a stepwise upshift of the valence band maximum (VBM) position from ~8–6.2 eV below the vacuum level at the amorphous to RS transition (x~0.2). This significant uplift of the VBM is believed to be responsible for the measured p-type conductivity of the O-rich RS alloys. It also suggests that deep acceptors in Ga₂O₃ would become shallow in these alloys and hence effective acceptor doping to further improve their p-type conductivity is possible.