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摘要
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 pho-todetection 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 mod-ifying 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 re-sistivity-20 Omega-cm (for x-0.6) and decreases to < 10 Omega-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 (Delta EV-1.8 eV and Delta EC-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 ac-ceptors in Ga2O3 would become shallow in these alloys and hence effective acceptor doping to further improve their p-type conductivity is possible.
