Efficient Acceptor Doping of Sputter-Deposited p-Type NixGa1–xO Rocksalt Alloy with Li

Ga2O3 as an ultrawide band gap semiconductor has attracted much attention in high-power electronic applications, but its full potential and functionalities have been hampered by its inability to obtain reliable p-type materials. Previously, we demonstrated p-type Ga2O3 by alloying with NiO in an O-rich environment (NixGa1–xO1+δ). This is attributed to the >1.8 eV upward movement of the valence band maximum (VBM) when the alloy forms a rocksalt structure for x ≳ 0.2, making the Ni vacancy (VNi) acceptors to become shallow. Here, we improve the p-type conductivity of NixGa1–xO1+δ by extrinsic p-type doping with Li by magnetron sputtering. While stoichiometric NixGa1–xO alloys are highly resistive throughout the whole composition range, Li-doped NixGa1–xO exhibits p-type conductivity with x > 0.4, confirming that despite the lack of a high VNi concentration, Li is an effective acceptor in RS-NixGa1–xO. The doping efficiency of Li is further improved in O-rich alloys NixGa1–xO1+δ due to the enhanced incorporation of Li, so that p-type conducting NixGa1–xO1+δ:Li alloys with Ni composition as low as x ∼ 0.2 is achieved. With Li doping, the ρ of Ni-rich alloys with x > 0.5 is <10 Ω·cm, which is over an order of magnitude lower than that in undoped alloys. The enhanced p-type conductivity of NixGa1–xO1+δ:Li is in good agreement with the position of their Fermi level with respect to the VBM, as revealed by VB spectra from X-ray photoelectron spectroscopy measurements. With further optimization of the doping concentration, these p-type NixGa1–xO1+δ:Li films can be exploited to form p–n junction structures on n-type Ga2O3.