Nickel vacancy acceptor in nickel oxide: Doping beyond thermodynamic equilibrium

This work reports on temperature-induced out-diffusion and concentration decay of the prominent intrinsic point defect ${\mathrm{V}}_{\mathrm{Ni}}$ (nickel vacancy) in the wide-gap $p$-type semiconductor nickel oxide (NiO). ${\mathrm{V}}_{\mathrm{Ni}}$ can easily be introduced into NiO thin films by offering high oxygen reactivity during film growth, rendering nonstoichiometric semiconducting material. However, exposure to lower oxygen reactivity after growth, e.g., in a standard atmosphere, usually leads to a gradual decrease of film conductivity, because the vacancy concentration returns to its thermodynamic equilibrium value. In this study we observe this process in situ by performing temperature-dependent measurements of the electrical conductivity on a room temperature-grown NiO film. At a temperature of 420 K under exclusion of oxygen, the doping level decreases by a factor of 8 while the associated room temperature DC conductivity drops by six orders of magnitude. At the same time, out-diffusion of the mobile ${\mathrm{V}}_{\mathrm{Ni}}$ species can be indirectly observed through the occurrence of electrode polarization characteristics.

Figure 1

Current density-voltage relations of selected Pt/NiO/Pt (symmetric behavior) and FTO/NiO/Pt (rectifying behavior) structures in semi-logarithmic and linear (inset) scale.

Figure 2

(a) Real part ${\sigma }^{\prime }$ of the complex conductivity function ${\sigma }^{*}={\sigma }^{\prime }+i{\sigma }^{″}$ of a Pt/NiO/Pt structure, measured by BDS in a temperature cycle between room temperature and 420 K. (b) Dielectric loss representation of the same data set for selected temperatures, including fits to the MWS polarization according to (3) (shaded areas).

Figure 3

(a) DC conductivity, (b) relaxation strength, and (c) rate of the MWS polarization mode for temperatures above 300 K (black symbols), obtained from fits of ${\varepsilon }^{″}\left(\omega \right)$ according to (3). Representative data points from measurements below 300 K are shown in addition (gray symbols) [8]. Red lines: Arrhenius-type fits used to extract the shown activation energies. Growth and epoxy hardening temperatures indicated by vertical dashed lines.

Figure 4

Doping profiles of a selected NiO/FTO contact before and after annealing at $420{}^{\circ }\mathrm{C}$ in ${\mathrm{N}}_2$. The measurement frequency was 5 kHz. Doping levels measured under $V=0\mathrm{V}$ indicated by horizontal lines.

Figure 5

Annealing-induced effects of ${\mathrm{V}}_{\mathrm{Ni}}$ out-diffusion. (a) Schematic cross-sectional view of the film before annealing with high density of ${\mathrm{V}}_{\mathrm{Ni}}$, (b) polarization of mobile charges within grains under AC electric field leads to dielectric loss peak (Maxwell-Wagner-Sillars polarization, MWS), and (c) interacceptor barriers for hopping transport $W_{\mathrm{H},\mathrm{pre}}$ are low. (d) During annealing, the ${\mathrm{V}}_{\mathrm{Ni}}$ concentration decreases due to out-diffusion to the film/electrode interface, where (e) oxygen is released from the film, leaving behind uncompensated negative charges that lead to electrode polarization (EP). (f) Lower ${\mathrm{V}}_{\mathrm{Ni}}$ concentration results in enhanced hopping barriers $W_{\mathrm{H},\mathrm{post}}$ between adjacent acceptors.