Efficient $p$-type doping of sputter-deposited NiO thin films with Li, Ag, and Cu acceptors

Nickel oxide, NiO, is an important $p$-type oxide semiconductor that has been studied for applications in solar cells, junction diodes, and other optoelectronic devices. In a nominally undoped NiO, depending on its oxygen stoichiometry, it only has a modest $p$-type conductivity of ∼0.1 S/cm due to Ni vacancy acceptors. However, the overall transport can be improved by extrinsic doping. In this study, we carry out a combined experiment and computational study of the effects of acceptor dopants, including Li, Ag, and Cu on the properties of NiO. Our ab initio calculations show that among all the acceptors studied, substitutional Li (${\mathrm{Li}}_{\mathrm{Ni}}$) acceptor species has the lowest formation and ionization energies. Measured electrical properties of the undoped and doped oxygen-rich NiO ($\mathrm{Ni}{\mathrm{O}}_{1+\delta }$) show an increase in conductivity and hole concentration for the doped samples. In particular, Li is an efficient acceptor to achieve highly conducting $p$-type NiO with >40% transmittance in the visible range for a 100-nm-thick film. The improvement in the electrical properties with different dopant species studied is in good agreement with the calculated defect formation and ionization energies. A remarkable increase in the temperature-dependent Hall mobility is also observed in the doped samples. Based on the small-polaron hoping model, we analyzed the conduction mechanism in the doped samples, which revealed a hopping dominated activation with energies in the range of 172–208 meV.

Figure 1

Measured (a) real ${\varepsilon }_1$ and (c) imaginary ${\varepsilon }_2$ parts and calculated (b) real ${\varepsilon }_1$ and (d) imaginary ${\varepsilon }_2$ parts of the complex dielectric function of NiO and doped NiO systems.

Figure 2

(a) Calculated absorption spectra of stoichiometric NiO, NiO with Ni vacancies, and doped NiO. (b) Absorption spectra extracted from fittings of spectroscopic ellipsometry data for undoped and doped NiO (solid lines) and undoped and doped $\mathrm{Ni}{\mathrm{O}}_{1+\delta }$ (dashed lines).

Figure 3

Measured transmission intensity for the $p$-type undoped and doped O-rich $\mathrm{Ni}{\mathrm{O}}_{1+\delta }$ thin films with thickness of 100 nm. Stoichiometric NiO transmission intensity spectrum is also shown for comparison.

Figure 4

Temperature dependence of (a) resistivity, (b) hole concentration, and (c) mobility of undoped and doped $\mathrm{Ni}{\mathrm{O}}_{1+\delta }$ thin films.

Figure 5

Temperature-dependent conductivity plotted in (a) the small polaron hopping conduction model (b) Arrhenius conduction model.

Figure 6

XPS valence-band spectra of NiO and doped NiO. The dashed line spectra are fits to the XPS spectra with a step function convoluted with a Gaussian function used to determine the $\left(E_F-E_V\right)$ value of the films.