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 (NixGa1−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 VNi and this results in a low transmittance which also decreases with x from ~70 % (x = 0.3) to ~ 40 % (x = 1). The band gap Eg 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 Eg for the RS alloys follows the virtual crystal approximation with a small bandgap bowing of 0.36 eV and an extrapolated Eg of 4.6 eV for RS Ga2O3. Furthermore, we find that the amorphous and RS crystalline alloys have a type II (staggered) band offset (ΔEV~1.8 eV and ΔEC~1.4 eV) 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.