Polaronic interacceptor hopping transport in intrinsically doped nickel oxide

Abstract

In this work, we revisit the issue of the nature of electronic transport in nickel oxide (NiO) and show that the widely used model of free small polaron hopping, initially raised to characterize transport in high-purity samples, is not appropriate for modeling intrinsically doped NiO. Instead, we present extensive evidence, collected by means of temperature-and frequency-dependent measurements of the electrical conductivity σ, that the model of polaronic inter-acceptor hopping can be used to consistently explain the electronic conduction process. In this framework, holes are localized to acceptors (Ni vacancies), forming a strongly bound, polaron-like state. They can only move through the film by hopping to a neighboring, at least partially unoccupied, acceptor. This renders the spatial overlap between neighboring polaronic wave functions a highly critical parameter. The signature of this process is the occurrence of two temperature regions of the DC conductivity, separated by about half the Debye temperature θ D 2 ≈ 200 K. For T > θ D 2 , holes are transferred by phonon-assisted hopping over the potential barrier between two sites, whereas phonon-assisted tunneling through the barrier dominates below that temperature. We also show that the degree of structural and electronic disorder plays a vital role in determining the characteristics of the transport process: high disorder leads to strong energetic broadening of the acceptor states such that hopping to more distant sites may be favored over transfer to nearest neighbors (variable range hopping).The assumption of high binding energies of the charge carriers at V Ni is in accordance with the recent paradigm shift regarding the understanding of the electronic structure of NiO: holes doped into NiO couple to Ni 3d spins, thereby occupying deep polaron-like states within the band gap (Zhang-Rice bound doublets). This work is the first to explicitly take this perception into account to explain carrier transport in NiO.

Nickel vacancy acceptor in nickel oxide: Doping beyond thermodynamic equilibrium

“…(a) dc conductivity, (b): relaxation strength and (c): rate of the MWS polarization mode for temperatures above 300 K (black symbols). Representative data points from measurements below 300 K are shown in addition (gray symbols) 5. …”

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

“…In contrast, hopping conduction through acceptor levels has been proposed as an alternative model. [ 34,35 ] Karsthof et al [ 28 ] proposed the PIH model to explain the electronic conduction process in intrinsically doped NiO films. In this model, holes localized to acceptors conduct by hopping to a neighboring acceptor.…”

“…In contrast, hopping conduction through acceptor levels has been proposed as an alternative model. [34,35] Karsthof et al [28] proposed the PIH model to explain the electronic conduction Li concentration (at. %) Figure 9.…”

“…Recently, Zhang et al [ 27 ] reported that the conduction mechanism in Li‐doped NiO epitaxial films is better described by a small polaron hopping (SPH) model at temperatures above 200 K. They concluded that both the valence band maximum and an acceptor state developed with Li‐doping are composed primarily of ZR bound states. In contrast, Karsthof et al [ 28 ] reported that the conduction process in intrinsically doped NiO films could be consistently explained by a polaronic interacceptor hopping (PIH) model, based on holes trapped on nickel vacancy sites due to the formation of small polaron‐like ZR bound states and their direct transfer between such sites. However, there is still uncertainty in the literature regarding the nature of carrier transport in NiO.…”

Magnetic characteristics of epitaxial NiO films studied by Raman spectroscopy

“…3 Furthermore, low growth temperatures favor the formation of additional point defects or defect complexes as demonstrated in Ref. 36 for NiO films synthesized by pulsed laser deposition. Consequently, it is reasonable to attribute the decreasing magnitude of 1 − Q at low growth temperatures (see Table I) to an increasing density of point defects.…”