Wide-Gap Zn1−xNixO Alloy: A Transparent p -Type Oxide

Abstract

The development of transparent bipolar oxide devices is largely hampered by the lack of oxides with reliable p-type conductivity. Recent calculations show that rock salt (RS) structured $\mathrm{Zn}\mathrm{O}$ alloys are promising candidates for transparent p-type oxides. Here, we synthesize wide-gap ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ alloy thin films over the entire composition range on glass substrates by means of radiofrequency magnetron sputtering. We find that the ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ alloy thin films undergo a phase transition from wurtzite (WZ) to RS structure as the $\mathrm{Ni}$ content x increases to x \ensuremath{\sim} 0.27. Interestingly, the band gap of RS-${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ is about 4.6 eV at the WZ to RS transition composition (x \ensuremath{\sim} 0.27) and decreases with x to the value of RS-$\mathrm{Ni}\mathrm{O}$ (\ensuremath{\sim}3.8 eV). Nominally undoped alloy thin films sputtered in pure $\mathrm{Ar}$ are semi-insulating, while $\mathrm{O}$-rich RS-${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ thin films with relatively high x (e.g., x \ensuremath{\ge} 0.5) sputtered in ${\mathrm{Ar}}^{+}$ 1.4% ${\mathrm{O}}_{2}$ and/or with $\mathrm{Cu}$ doping exhibit good p-type conductivity at room temperature. The hole transport in these RS-${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ alloy thin films follows a small polaron hopping process, with activation energy ranges from 0.19 to 0.32 eV. Drastic differences in the electronic band structure of the WZ and RS ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ alloys are also observed with a type II band offset for alloys at the WZ to RS transition composition. The valence band maximum (VBM) of the RS phase is g1 eV above that of the WZ phase, making them energetically more favorable for the formation of native acceptor defects. The much higher VBM position of RS alloys also favors their extrinsic p-type doping efficiency.