Variable range hopping in TiO2 insulating layers for oxide electronic devices

Abstract

TiO2 thin films are of importance in oxide electronics, e.g., Pt/TiO2/Pt for memristors and Co-TiO2/TiO2/Co-TiO2 for spin tunneling devices. When such structures are deposited at a variety of oxygen pressures, how does TiO2 behave as an insulator? We report the discovery of an anomalous resistivity minimum in a TiO2 film at low pressure (not strongly dependent on deposition temperature). Hall measurements rule out band transport and in most of the pressure range the transport is variable range hopping (VRH) though below 20 K it was difficult to differentiate between Mott and Efros-Shklovskii's (ES) mechanism. Magnetoresistance (MR) of the sample with lowest resistivity was positive at low temperature (for VRH) but negative above 10 K indicating quantum interference effects

Photocatalytic Graphene-TiO2 Thin Films Fabricated by Low-Temperature Ultrasonic Vibration-Assisted Spin and Spray Coating in a Sol-Gel Process

“…The dark current-voltage (I-V) and sheet resistance curves are affected by the presence of TiO 2 in graphene-TiO 2 thin films [2,38], because TiO 2 behaves as an insulator in the dark. The I-V curves shown in Figure 7a confirm that TiO 2 thin film creates no current in the dark, whereas graphene thin film is highly conductive.…”

Low-temperature conductivity of silicon doped with antimony

“…The temperature dependences of the specific resistance for these mechanisms are described by the well known expression (1) where ρ 0 is the pre exponential factor, while T* and p are the characteristic temperature and the exponent, which are defined by the mechanism of hopping con In the three dimensional case, expression (1) charac terizes the VRH mode [3,5] with characteristic tem peratures T* = T M = β 0 /k B g 0 a 3 for the Mott mecha nism and the exponent p = 1/4 and, correspondingly, T* = T 0 = β 1 q 2 /(k B aκ) at p = 1/2 for the ShklovskiiEfros mechanism.…”

“…This makes it possible to determine the value of T* (from the slope angle) and also the exponent p (by exhaustively searching through known model values). In the second approach [16,17], researchers search for the temperature region corresponding to linearization of the reduced activation energy w(T) = -∂logρ/∂logT, which formally makes it possible to determine the exponent p in relation (1). In both approaches, the correctness of the determination of T* and p is appre ciably affected by the following factors: the accuracy of temperature stabilization at the experimental points ρ(T), the number of mechanisms in the crossover's temperature range under study, and the temperature dependence of pre exponential factors ρ 0 in expres sions of type (1).…”

“…At present, there are no signs of the lessening of interest in the study of mechanisms of conduction in semiconducting materials at low temperatures, including such materials as traditional heavily doped semicon ductors in the vicinity of the metal-insulator transi tion [1,2]. This is related both to fundamental prob lems of electron transport in the vicinity of these tran sitions and to some applied aspects.…”

Electrical property measurements of Cr-N codoped TiO2 epitaxial thin films grown by pulsed laser deposition

“…16 With the combination of increasing disorder and the presence of localized states created by doping, the variable range hopping (VRH) model becomes a viable description. 17 The VRH model was recently found by Zhao et al 18 to provide a good description for the conductivity of PLD grown TiO 2 films at variable oxygen pressure. In contrast, the conductivity in 5 of our anatase films occurs at higher temperatures than that reported above, and none of the remaining 3 films with low temperature conductivity exhibit VRH type behavior.…”