P-Type Doping Control of Magnetron Sputtered NiO for High Voltage UWBG Device Structures

A major challenge in the design and fabrication of power devices from ultra-wide bandgap (UWBG) materials is the lack of a native shallow acceptor dopant for most materials in that class. P-type regions in UWBG devices may alternatively be formed by the deposition of p-type wide bandgap materials such as nickel oxide (NiO) to form heterojunctions. This work examines the effectiveness of the modulation of the acceptor concentration $\left(N_{\mathrm{A}}\right)$ of magnetron sputtered NiO via the control of O 2 partial pressure during sputtering. $\mathrm{NiO} / \mathbf{n}^{+}-\mathrm{Ga}_{2} \mathrm{O}_{3} \mathrm{PN}$ junctions are fabricated via sputtering using $\mathrm{O}_{2} / \mathrm{Ar}$ percentage ratios ranging from $0 \%$ to $12.5 \%$. The acceptor doping and acceptor level characteristics are studied via the dependence of PN junction capacitance upon voltage, temperature and frequency. It is found that the $N_{\mathrm{A}}$ can be controlled between $9 \times 10^{17} \mathrm{~cm}^{-3}$ at $0 \% \mathrm{O}_{2} / \mathrm{Ar}$ to $2 \times 10^{18} \mathrm{~cm}^{-3}$ at $12.5 \% \mathrm{O}_{2} / \mathrm{Ar}$ partial pressure. Studies of the capacitance of the diodes shows that the associated first ionization level of the $\mathrm{Ni}^{3+}$ vacancy thought to be responsible for the p-type doping of the NiO is 0.35eV, with suggestion of a second ionization level at 0.54eV seen in the $0 \% \mathrm{O}_{2} / \mathrm{Ar}$ sample. To demonstrate the feasibility of utilizing magnetron sputtered NiO in power devices, a lateral RESURF terminated Ga 2 O 3 Schottky diode is fabricated. Charge balance to maximize breakdown voltage is demonstrated in the Schottky diode as a function of NiO RESURF thickness, showing the viability of NiO in high voltage power devices for field spreading regions such as RESURF layer's and JTE structures.