Electrical Conductivity Studies in Sol–Gel‐Derived Li‐Doped NiO Epitaxial Thin Films

Abstract

The electrical properties of transparent Li‐doped nickel oxide (NiO) epitaxial thin films grown on MgO (100) substrates using a sol–gel spin‐coating technique are investigated as to the Li concentration as determined by secondary ion mass spectroscopy. The epitaxial growth of the cubic Li‐doped NiO films with a cube‐on‐cube relationship is confirmed by X‐ray diffraction measurements. The electrical conductivity increases superlinearly with increasing Li concentration from 0.028 to 7.5 at%. The temperature dependence of the conductivity and the linear dependence of the activation energy on the mean distance between Li ions suggest that the conductivity can be explained within a hopping conduction framework. However, the significant Hall voltages observed in Hall effect measurements suggest the existence of nonhopping carriers in addition to hopping carriers. The Li concentration dependence of the conductivity cannot be well explained by the widely used model of small polaron hopping in which small polarons conduct through thermally activated hopping between nearest‐neighbor Ni sites. Instead, the observed dependence is well explained quantitatively by a model combining polaronic interacceptor hopping and nonhopping conductions. Polaronic interacceptor hopping conduction dominates for Li concentrations below 1 at%, whereas nonhopping conduction dominates for higher Li concentrations.

Identification of LiNi and VNi acceptor levels in doped nickel oxide

“…However, for a low-mobility material like NiO, this assumption should be critically reviewed. It was recently shown 20,25 that the electronic transport in both Li-doped and Ni-deficient NiO can be modeled using a theoretical framework in which the charge carriers form polarons on the acceptor sites, and move through the crystal by hopping between closely spaced acceptors (polaronic interacceptor hopping, PIH). This process was shown to determine the DC conductivity of an undepleted, doped NiO film to a large extent.…”

“…Using the data of Ref. 25 as reference, an intersite separation larger than 4 nm, or a Li Ni concentration of below 0.03 at.%, can be inferred. This agrees well with the intended Li concentration and the net doping measured on our films.…”

Ultraviolet Photodetector Based on a Beta-Gallium Oxide/Nickel Oxide/Beta-Gallium Oxide Heterojunction Structure

“…The resistivity of the 10% Li-doped NiO film was about 0.5 Ω•cm. The concentration and mobility of holes in the doped NiO were ~3 × 10 20 cm −3 and 0.05 cm 2 /Vs, respectively [6,21]. The absorption properties of the Li-doped NiO film are shown in our previous work [21].…”

Magnesium Diffusion from MgO Substrates in Sol–Gel‐Derived NiO Epitaxial Films: Effects of Heat Treatment Temperature and Li Doping

“…In general, p-type conductivity can be improved by introducing Ni vacancies through a high oxygen supply during a NiO-film growth process or by doping with monovalent cations such as Li þ , Cu þ , and Ag þ . [3][4][5][6][7] The controversy regarding the carrier transport mechanism in NiO has continued for many years, [1,[8][9][10][11] but according to recent studies, [12,13] the carrier transport in both Li-doped and Ni-deficient NiO, except for heavily Li-doped NiO, can be explained by the polaronic interacceptor hopping model instead of the commonly used smallpolaron hopping model.…”