Combinatorial laser MBE synthesis of 3d ion doped epitaxial ZnO thin films

Microstructure and nonohmic properties of binary ZnO-V2O5 ceramics sintered at 900 °C

The modification of the densification behavior and the grain-growth characteristics of the microwave-sintered ZnO materials, caused by the incorporation of V2O5 additives, have been systematically studied. Generally, the addition of V2O5 markedly enhances the densification rate, such that a density as high as 97.9% of the theoretical density and a grain size as large as 10 µm can be attained for a sintering temperature as low as 800°C and a soaking time as short as 10 min. Increasing the sintering temperature or soaking time does not significantly change the sintered density of the ZnO-V2O5 materials but it does monotonously increase their grain size. Varying the proportion of V2O5 in the range of 0.2-1.0 mol% does not pronouncedly modify such behavior. The leakage current density (JL) of these high-density and uniform-granular-structure samples is still large, which is amended by the incorporation of 0.3 mol% of Mn3O4 in the ZnO materials, in addition to 0.5 mol% of the V2O5 additives. Samples that are obtained using such a method possess good nonohmic characteristics (α= 23.5) and a low leakage current density (JL= 2.4 10-6 A/cm2).

https://ir.nctu.edu.tw:443/bitstream/11536/31768/1/000076895000024.pdf

Microstructure and Nonlinear Properties of Microwave‐Sintered ZnO‐V2O5 Varistors: I, Effect of V2O5 Doping

In this paper a detailed nature of the electrical potential barriers in ZnO based varistors are presented. The excellent nonlinear current–voltage (I–V) characteristic of these varistors is attributed to the potential barriers formed between the successive ZnO grains during the processing cycle. These grain boundaries in the microstructure results in a few device-related parameters such as built-in-potential, barrier height, barrier width, grain carrier density, Fermi level position in the grains, etc. Thermal activation energy is determined from the status of the leakage current of this device. Substantial studies conducted to comprehend possible conduction processes in the grain boundary regions under applied bias (electric field). It is understood that electron transport dominate the grain boundary regions based on electrical potential barrier consideration across the grain boundaries. The free electrons overcome potential barrier at elevated temperatures as well as under bias. These carriers fill trap sites (charged defect states) both in the bulk ZnO and in the grain surfaces (surface states). The formation of space charge region within the successive ZnO grains across the grain boundaries is a result of the equilibrium state of the device. Based on the findings an expression of the Schottky potential barrier width is proposed satisfying relevant expression of the current caused by the application of the electric field. It is observed that the potential barrier width is sharply reduced in the temperature range 320 K ≤ T ≤ 350 K. This reduction in the potential barrier width is likely to be associated with the relaxation process of the trap filled (trapped) carriers, usually monitored by the dielectric response (in the form of differential capacitance) of the device. Thus, the results provide an improved understanding of the nonlinear conduction behavior in ZnO varistors. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical barriers in the ZnO varistor grain boundaries

The effects of the oxide additives MnO2, Co3O4, and Sb2O3, commonly incorporated in commercial Bi2O3-doped ZnO varistors, on the current–voltage characteristics and microstructure of 0.25 mol% V2O5-doped ZnO varistors have been studied. MnO2 is the most significant additive in terms of its effects on varistor performance. Varistor performance can also be improved by increasing the V2O5 content to 0.5 mol% in a ZnO ceramic containing 1 mol% MnO2. Further increases in the V2O5 content of 1 mol% MnO2-doped material cause a deterioration in varistor behavior. The microstructure of the samples consists mainly of ZnO grains with zinc vanadates as the minority secondary phases. Additional spinel phase is formed when Sb2O3 is incorporated.

Microstructure and Current–Voltage Characteristics of Multicomponent Vanadium-Doped Zinc Oxide Varistors

The densification behavior, microstructure, and electrical properties of ZnO‐V2O5 ceramics were studied with V2O5 as the only additive ranging from 0.01 to 1.0 mol %. The addition of V2O5 to zinc oxide shows a tendency to enhance the densification rate and promote grain growth. However, a microstructure that consisted of anomalously grown grains was found for the specimens containing V2O5≥0.05 mol % when sintered at 1100 °C for 2 h. The x‐ray diffraction and SEM‐EDS microanalysis revealed that the sintered specimens had a two‐phase microstructure, i.e., a vanadium‐rich intergranular phase formed between ZnO grains. The formation of the grain boundary barrier layer was confirmed by the non‐ohmic I‐V behavior and the quick drop of apparent dielectric constant with increasing frequency of the ceramics. A nonlinearity coefficient of 2.4–2.8 was obtained at a current density of 10 mA/cm2 for a series ZnO‐V2O5 ceramics, and a Schottky barrier height of 0.44–0.47 eV (at 25 °C) was determined from the I‐V and C‐V experimental data, based on the thermionic emission theory, and the model of back‐to‐back double Schottky barriers.

Jyh‐Kuang Tsai

Papers

Non‐ohmic characteristics of ZnO‐V2O5 ceramics