Variable-range-hopping conduction processes in oxygen deficient polycrystalline ZnO films

Abstract

We have fabricated oxygen deficient polycrystalline ZnO films by the rf sputtering deposition method. To systematically investigate the charge transport mechanisms in these samples, the electrical resistivities have been measured over a wide range of temperature from 300 K down to liquid-helium temperatures. We found that below about 100 K, the variable-range-hopping (VRH) conduction processes govern the charge transport properties.In particular, the Mott VRH conduction process dominates at higher temperatures, while crossing over to the Efros-Shklovskii (ES) VRH conduction process at lower temperatures.The crossover occurred at temperatures as high as a few tens degrees Kelvin. Moreover, the temperature behavior of resistivity over the entire VRH conduction regime from the Mott-type to the ES-type process can be well described by a universal scaling law.

Impact of metal electrode on charge transport behavior of metal-Gd2O3 systems

“…In this regard, the temperature dependent electrical conductivity measurements can reveal the underlying charge transport mechanism. Previously, temperature dependent behaviors of electrical properties of oxide semiconductors such as CeO 2 , ZnO and SrTiO 3 have been reported in the literature [7,8]. However, no report has been found in the literature about the hopping conduction mechanism of Gd 2 O 3 with different metal electrodes according to the best of our knowledge.…”

“…It has wide band gap (5.4 eV), high dielectric constant (k ¼ 18) and high refractive index (n ¼ 2) [2,3], therefore can be used for sensors, optoelectronics, data storage devices and luminescence applications [4e6]. Apart from optical, mechanical and chemical properties, Gd 2 O 3 has rich chemistry of intrinsic defects such as oxygen vacancies and Gd interstitials [7]. Therefore it is of crucial importance to understand charge transport processes in both crystalline and amorphous Gd 2 O 3 .…”

Localized tail state distribution and hopping transport in ultrathin zinc-tin-oxide thin film transistor

“…From the above analyses, we can find out that at the ZTO/dielectric interface, trapped carriers occupy the localized tail states below the Fermi level at T ¼ 310 K. This implies the good opportunity of carriers transport through unoccupied localized tail states above the Fermi level. 12 This result echoes the linear dependency of log I D vs. T À1/3 curves, indicating that the variable range hopping should be the dominant electrical transport mechanism in ZTO.…”

“…All log I D vs. T À1/3 curves exhibit a linear dependency, indicating that the variable range hopping should be the dominant electrical transport mechanism in ZTO for both above-threshold and sub-threshold regions. 12 The transport mechanisms of channel electrons are discussed based on the distribution of conducting carriers within the energy band gap of ZTO. From the distribution of conducting carriers, we can further determine the density of tail states.…”

Study of dielectric relaxation processes in printable zinc oxide films on transparent substrates

“…The study of charge transport mechanisms in a ZnO thin film containing defects, both for doped and undoped cases, is very important to determine the suitability of its electrical, optoelectronic properties for specific applications [8]. Variable range hopping (VRH) was found to be responsible for charge transport in polycrystalline ZnO sputtered films on a glass substrate under different oxygen pressures showed over a wide range of temperature from liquid helium temperature to 300 K. However, the exponential dependence of steady state conductivity r DC on temperature T followed a T À1=2 law at relatively high temperatures but a T À1=4 dependency became prevalent at low temperatures due to the presence of a Coulomb gap in oxygen deficient films [9]. Similarly, temperature dependence of r DC of chemical vapour deposited ZnO films was investigated in the presence of oxygen containing water vapour for the temperature range from 87 to 297 K, using the four probe method.…”