Role of defects in tailoring optical,electronic, and luminescent properties of Cu-doped ZnO films

We present a detailed characterization study on copper-doped ZnO films to correlate the films' electronic and optical properties with the existing native defects in the lattice. In addition, we describe the variation in the concentration of these defects with Cu dopant and temperature. The results of XRD confirmed the single-phase würtzite-structure of the synthesized films. The SEM images showed a homogeneous and dense grain morphology with a granular form and a signature for a hexagonal-like shape. The EDX, XPS, and UV–Vis spectra showed the proper doping of Cu ions into the lattice. The XPS analysis indicated mixed electronic states of both Cu²⁺ and Cu¹⁺ and showed a clear increase in the Cu²⁺ intensity relative to Cu¹⁺, with Cu dopant. The transmittance spectra exhibited an average value above 80% in all doped films in the visible and infrared regions. The overall results indicated a clear link between the films’ optical and electronic responses and the level of the intrinsic defects in the lattice. By increasing the Cu dopant, we find a slight reduction in the energy bandgap (E_g). This is correlated with a clear reduction in the blue emission luminescence band associated with the V_Zn and in the yellow emission band associated with the O_i. On the other hand, we observed a clear enhancement in the green emission band originating from the V_O, and in the emission band related to possible transitions from Zn_i levels to O_i levels. The slight reduction in the E_g signals a weak sp-d hybridization between the ZnO conduction band electrons and the Cu²⁺ ions, which is mediated by the intrinsic defects. With reducing the temperature, the photoluminescence temperature profiles indicated a slight increase in the E_g values and a negligible effect on the distribution of the native defects.     

Role of vanadium ions,oxygen vacancies,and interstitial zinc in room temperature ferromagnetism on ZnO-V2O5 nanoparticles

In this work, we present the role of vanadium ions (V⁺⁵ and V⁺³), oxygen vacancies (V_O), and interstitial zinc (Zn_i) to the contribution of specific magnetization for a mixture of ZnO-V₂O₅ nanoparticles (NPs). Samples were obtained by mechanical milling of dry powders and ethanol-assisted milling for 1 h with a fixed atomic ratio V/Zn?=?5% at. For comparison, pure ZnO samples were also prepared. All samples exhibit a room temperature magnetization ranging from 1.18?×?10⁻³ to 3.5?×?10⁻³ emu/gr. Pure ZnO powders (1.34?×?10⁻³ emu/gr) milled with ethanol exhibit slight increase in magnetization attributed to formation of Zn_i, while dry milled ZnO powders exhibit a decrease of magnetization due to a reduction of V_O concentration. For the ZnO-V₂O₅ system, dry milled and thermally treated samples under reducing atmosphere exhibit a large paramagnetic component associated to the formation of V₂O₃ and secondary phases containing V⁺³ ions; at the same time, an increase of V_O is observed with an abrupt fall of magnetization to σ?~?0.7?×?10⁻³ emu/gr due to segregation of V oxides and formation of secondary phases. As mechanical milling is an aggressive synthesis method, high disorder is induced at the surface of the ZnO NPs, including V_O and Zn_i depending on the chemical environment. Thermal treatment restores partially structural order at the surface of the NPs, thus reducing the amount of Zn_i at the same time that V₂O₅ NPs segregate reducing the direct contact with the surface of ZnO NPs. Additional samples were milled for longer time up to 24 h to study the effect of milling on the magnetization; 1-h milled samples have the highest magnetizations. Structural characterization was carried out using X-ray diffraction and transmission electron microscopy. Identification of V_O and Zn_i was carried out with Raman spectra, and energy-dispersive X-ray spectroscopy was used to verify that V did not diffuse into ZnO NPs as well to quantify O/Zn ratios.     

Simultaneously enhanced electrical stability and nonlinearity in ZnO varistor ceramics: Role of Si-stabilized δ-Bi2O3 phase

In the present study, simultaneously enhanced electrical stability (low degradation rate of 8.0 × 10^?3 mA? h^1/2) and high nonlinear coefficient of 56 were obtained in ZnO varistors by doping SiO₂. To clarify the mechanism of enhanced properties, comprehensive microscopic analyses were studied. Particularly, the intrinsic point defects were quantitatively characterized for the first time. Results showed that the densities of zinc interstitials (Zn_i) and oxygen vacancies (V_o) were dramatically decreased, resulting in enhanced stability. Besides, reduced Zn_i and V_o decreased the total donor density, contributing to the improved barrier height and thus leading to enhanced nonlinearity. Combined with XRD and SEM results, it is deduced that such reduced Zn_i and V_o are attributed to the Si-stabilized high oxygen conducting δ-Bi₂O₃ phase. Furthermore, this elucidated mechanism, which has been long neglected in Si-doped varistors, may provide valuable insights into further developing high-performance ZnO varistors.     

Band structure and thermoelectric properties of inkjet printed ZnO and ZnFe2O4 thin films

The band structure and thermoelectric properties of inkjet printed ZnO and ZnFe₂O₄ thin films have been investigated. The bulk pellets were prepared by a solid-state method and thin films were deposited using an inkjet printing method. Multiple print cycles were required to fabricate homogeneous films and the composition of the thin films can be varied by varying the relative amounts of liquid deposited. It was possible to obtain high thermoelectric properties of ZnO by controlling the ratios of dopant added and the temperature of the heat treatments. XRD analysis showed that the fabricated samples have a wurtzite structure and an additional ZnAl₂O₄ phase was formed with increasing Al content and sintering temperature. It was found that the band gap of Al doped ZnO becomes smaller with increasing Al content and thus the electrical conductivity of Al_x doped ZnO (x=0.04) thin films showed the highest electrical conductivity (114.10 S/cm). The ZnFe₂O₄ samples were compared against the ZnO samples. The formation of single phase cubic spinel structure of the sintered ZnFe₂O₄ samples was found and confirmed by X-ray diffraction technique. Secondary phase Fe₂O₃ was also detected for compositions with Zn (x≤0.4). Finally, we want to report that the electrical conductivity of Zn_xFe_3−xO₄ was lower than the conductivity of the Al-doped ZnO.     

Investigation on the comparison of the structural,electrical and optical properties between ZCO:Na and ZCO:(Na,N) films synthesized by RF magnetron sputtering

Sodium doped ZnCdO (ZCO:Na) and sodium-nitrogen co-doped ZnCdO [ZCO:(Na, N)] films have been deposited on quartz substrates by radio frequency (RF) magnetron sputtering followed by a post-annealing treatment. The Hall-effect measurement results emphasized the importance of the dopant and annealing conditions in realizing p-type conversion. The ZCO:(Na, N) film annealed at 655 °C for 30 min (denoted sample F) showed optimal p-type conduction properties, which has the carrier concentration of 7.84 × 10¹⁸ cm^?3. Compared to the best p-type conduction of the ZCO:Na film (sample C), sample F reveals an increased carrier concentration (up from 10¹⁷ to 10¹⁸ cm^?3) owing to the formation of Na_Zn and N_o dual acceptors. Furthermore, the XPS results revealed that sample F has a higher Na_Zn acceptor content than sample C. The ZCO:(Na, N) films exhibited better crystal quality compared to the ZCO:Na films based on comparison of the values of full width at half maximum and intensity. It was found that the band gap (E_g) of all ZCO:Na and ZCO:(Na, N) films were smaller than that of pure ZnO due to Cd doping, and that the E_g increased with the increase of T_ann, which is ascribed to the fact that more Cd atoms were evaporated from the films at higher T_ann. In addition, the E_g of the ZCO:(Na, N) films (samples E-G) are generally larger than that of the ZCO:Na films (samples A-D). This is attributed to the incorporation of N in ZCO:(Na, N), as the N_o acceptor impedes the formation of V_o defects, resulting in a decrease in the formation of the Cd_Zn-V_o complex, which in turn decreased the Cd concentration.     

Room-temperature violet luminescence and ultraviolet photodetection of Sb-doped ZnO/Al-doped ZnO homojunction array

A Sb-doped ZnO microrod array was fabricated on an Al-doped ZnO thin film by electrodeposition. Strong violet luminescence, originated from free electron-to-acceptor level transitions, was identified by temperature-dependent photoluminescence measurements. This acceptor-related transition was attributed to substitution of Sb dopants for Zn sites, instead of O sites, to form a complex with two Zn vacancies (V_Zn), the Sb_Zn-2V_Zn complex. This Sb_Zn-2V_Zn complex has a lower formation energy and acts as a shallow acceptor which can induce the observed strong violet luminescence. The photoresponsivity of our ZnO p-n homojunction device under a negative bias demonstrated a nearly 40-fold current gain, illustrating that our device is potentially an excellent candidate for photodetector applications in the ultraviolet wavelength region.     

The improvement of thermoelectric property of bulk ZnO via ZnS addition: Influence of intrinsic defects

As in any semiconducting solids, intrinsic defects can affect the properties of ZnO, such as the electrical and thermal conductivities. Defect engineering is usually focused on optimizing the materials’ synthesis or annealing parameters, i.e., temperature, atmosphere, etc. Here we report an approach to change the intrinsic defects of ZnO by adding a small amount of ZnS. During the sintering process, ZnS was decomposed. Apart from the formation of SO₂, the decomposed S and Zn can also be simultaneously doped onto O and Zn sites to change the intrinsic defects in ZnO. For instance, some of the S was converted to SO₂ and led to the formation of V_o (oxygen vacancy); meanwhile, Zn may move to the V_Zn (Zn vacancy) site and decrease the concentration of Zn vacancy. Due to the changes in these native defects, the carrier concentration increased and the thermal conductivity decreased when the content of ZnS was increased to x = 0.01. This sample had an optimal zT value, which was twice that of undoped ZnO. However, with further increase in ZnS, the carrier concentration was reduced. These results suggest a method to tune the intrinsic defects of ZnO via doping technology and bring potential opportunities to improve the thermoelectric performance of this oxide.     

Selectively enhanced oxygen vacancies in undoped polycrystalline ZnO as a consequence of Multi-Step Sintering

With a motivation to investigate the simultaneous effect of sintering temperature and sintering time in tandem on defect distributions in polycrystalline undoped ZnO a new technique called Multi-step sintering (MSS) has been deliberately designed. Systematic investigations on structural parameters suggest the rigorous defect propagation along a-axis of the unit cell. We report the unwavering nature of Zinc interstitials (Zn_i) and the complexes of Zn_i with oxygen vacancies (V_o) with sintering temperature. The enhanced and broad green emission in Photoluminescence spectroscopy reveals the existence of surplus V_o in the sample. The band gap narrowing at 800 °C and enhanced Raman modes in Raman scattering experiment confirms the existence of excess oxygen vacancies and make MSS successful technique to tailor the intrinsic defects without any extrinsic doping.     

Fabrication of tridoped p-ZnO thin film and homojunction by RF magnetron sputtering

Tridoping (Al–As–N) into ZnO has been proposed to realize low resistive and stable p-ZnO thin film for the fabrication of ZnO homojunction by RF magnetron sputtering. The tridoped films have been grown by sputtering the AlN mixed ZnO ceramic targets (0, 0.5, 1 and 2 mol%) on GaAs substrate at 450 °C. Here, Al and N from the target, and As from the GaAs substrate (back diffusion) takes part into tridoping. The grown films have been characterized by Hall measurement, X-ray diffraction, photoluminescence, time-of-flight secondary ion mass spectroscopy and X-ray photoelectron spectroscopy. It has been found that all the films showed p-conductivity except for 2 mol% AlN doped film. The obtained resistivity (8.6×10⁻² Ω cm) and hole concentration (4.7×10²⁰ cm⁻³) for the best tridoped film (1 mol% AlN) is much better than that of monodoped and codoped ZnO films. It has been predicted that [(As_Zn−2V_Zn)+N_O] acceptor complex is responsible for the p-conduction. The homojunction fabricated using the best tridoped ZnO film showed typical rectifying characteristics of a diode. The junction parameters have been determined for the fabricated homojunction by Norde's and Cheung's method.     

Elucidating the unintentional p-type nature of spinel Co3O4: A defect study using ab-initio calculation

Co₃O₄ is one of the most widely used materials in energy and environmental field due to its unintentional p-type nature, which depends on the preparation conditions. In this study, we investigated the origin of the unintentional p-type conductivity of Co₃O₄ by calculating all possible intrinsic point defects. We found that the octahedral cobalt vacancy and tetrahedral cobalt vacancy are the sources of unintentional p-type doping. Using charge balance theory, we analyzed the effect of preparation condition on intrinsic defect-induced doping. In most of preparation condition, the formation of these cobalt vacancies plays a dominant role and the spontaneous formation of p-type doping is unavoidable. However, if there is ample oxygen and the temperature is low during the preparation, the unintentional p-type doping can be avoided. This theoretical work on defects provides a crucial clue to optimize Co₃O₄ for various electrochemical applications.     

Synthesis and characterization of Cu1-xZnxO composite thin films for sensor application

Cu_1-xZn_xO composite thin films were prepared using industrially applicable spray pyrolysis technique for volatile organic compound (VOCs) sensor application. Sensing properties for different concentration of VOCs such as acetone, ethanol and methanol were studied at different sensor operating temperature. XRD studies on prepared thin films confirmed formation of CuOZnO composite thin films with presence of different peaks for monoclinic structured CuO and hexagonal structure ZnO, it was also observed that formation of composite material improves sensing property towards VOCs. Granular morphology observed from SEM images were also contributed to enhance sensitivity of Cu_1-xZn_xO thin films. Hot probe experiment reveals that all the thin films were p-type in conductivity nature. Maximum electrical conductivity was achieved for Cu_0.75Zn_0.25O composite thin films, which also showed highest sensing property for VOCs. Cu_0.75Zn_0.25O thin films were selective towards ethanol and were capable of detecting 1 ppm of ethanol at operating temperature of 290 °C.     

Electronic active defects and local order in doped ZnO ceramics inferred from EPR and 27Al NMR investigations

Doped ZnO based ceramics were fabricated by using a solid state reaction of ZnO co-doped by TiO_2, Al₂O₃ and MgO and sintered in different atmospheres (Air, N₂, N₂ + CO). The crystalline structures consist in wurtzite ZnO and a minor spinel phase Zn₂TiO₄. The electrical conductivity is modulated by the sintering conditions with the highest value (˜10⁵ S m⁻¹) obtained in the reducing atmosphere (N₂ + CO). The role of defects and vacancies on the electrical behavior was exhaustively investigated by Raman, electron paramagnetic resonance (EPR) and solid state NMR methods. The paramagnetic centres inferred from EPR studies show a Pauli-like spin susceptibility. Their origin was assigned to shallow donors from interstitial defects (Zn_i) favored by substitutional Al ions (Al_Zn). The NMR spectral features with a characteristic 185 ppm line which correlates with the electrical conductivity are presumed to be caused by the Knight shift effect from the conduction electrons and the involved paramagnetic centres.     

Stability scheme of ZnO-thin film resistive switching memory: influence of defects by controllable oxygen pressure ratio

We report a stability scheme of resistive switching devices based on ZnO films deposited by radio frequency (RF) sputtering process at different oxygen pressure ratios. I-V measurements and statistical results indicate that the operating stability of ZnO resistive random access memory (ReRAM) devices is highly dependent on oxygen conditions. Data indicates that the ZnO film ReRAM device fabricated at 10% O₂ pressure ratio exhibits the best performance. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) of ZnO at different O₂ pressure ratios were investigated to reflect influence of structure to the stable switching behaviors. In addition, PL and XPS results were measured to investigate the different charge states triggered in ZnO by oxygen vacancies, which affect the stability of the switching behavior.     

Cost-effective large-scale synthesis of oxygen-defective ZnO photocatalyst with superior activities under UV and visible light

A cost-effective solution method was developed to produce ZnO photocatalyst in large quantity, through the conversion of ε-Zn(OH)₂ to ZnO in NaOH solutions. Experimental results indicated that the concentrated NaOH solution (4 mol L⁻¹) promoted the rapid formation of ZnO owing to the enhanced dissolution-precipitation reactions. The large-scale synthesis was also achieved with high-yield and solvent-recyclability. Structural analysis based on X-ray photoelectron spectroscopy, electron spin resonance and photoluminescence revealed that the as-prepared ZnO photocatalyst was rich in oxygen vacancies (V_O). The V_O-rich ZnO photocatalyst exhibited improved visible-light absorption, higher photocurrent responses and superior activities toward the degradation of rhodamine B under both UV (λ~254 nm) and visible-light illumination (λ>420 nm) compared to commercial ZnO and P25 TiO₂ powders, as well as good cycle stability. Based on the results of photoluminescence and active species detection, the V_O-enhanced photocatalytic activity was attributed to the generation of V_O-isolated level in the band structure. Under UV light, the V_O-level could promote charge separation by trapping the photoinduced electrons, while under visible-light, the V_O-level improved visible-light absorption and facilitated the charge generation. The presently developed synthesis may potentially benefit the large-scale production and low-cost application of ZnO photocatalyst for solar energy utilization.     

Reverse manipulation of intrinsic point defects in ZnO-based varistor ceramics through Zr-stabilized high ionic conducting βIII-Bi2O3 intergranular phase

Intrinsic point defect structure plays a crucial role in functional ceramics with a grain-grain boundary microstructure. In the present study, a novel method of reversely manipulating intrinsic point defects (oxygen vacancy, V_o and zinc interstitial, Zn_i) in ZnO-based varistor ceramics is proposed, which makes use of Zr-stabilized high ionic conducting β_III-Bi₂O₃ intergranular phase. It is found that Zr-doping not only modifies the grain growth by the formation of secondary Zr-rich phase, but also influences the intrinsic point defect structure via the stabilized β_III-Bi₂O₃ phase, resulting in reduced V_o density but increased Zn_i density. The reverse manipulation is unambiguously demonstrated by broadband dielectric spectroscopy and further confirmed by the accelerated ageing experiment. The proposed intrinsic point defect dynamics unveil an important but usually neglected function of the dopant that has little solid solubility in ZnO grains, which opens up a promising way to tailor the material property.     

Photoluminescent and electrical properties of novel Nd3+ doped ZnV2O6 and Zn2V2O7

Nd³⁺ doped ZnV₂O₆ and Zn₂V₂O₇ samples were synthetized by using melt-quenching method. X-ray diffraction patterns indicate that both samples are polycrystalline. The crystallinity was also verified by Raman scattering, from which the different vibrational modes of ZnV₂O₆ and Zn₂V₂O₇ were detected. Electron dispersive spectroscopy (EDS) analysis shows that the Nd³⁺ incorporation into the ZnV₂O₆ and Zn₂V₂O₇ hosts is around 0.9±0.1 and 0.2±0.1 at%, respectively. The micrographs obtained by Scanning Electron Microscopy, reveal that the Nd³⁺ doped ZnV₂O₆ sample is predominantly composed by micro-rods, whereas the Nd³⁺ doped Zn₂V₂O₇ one is only composed by irregular blocks. The band gap energies (E_g) were calculated from the diffuse reflectance spectra by the Kubelka–Munk equation; E_g values resulted to be 2.24 and 2.86 eV for the Nd³⁺ doped ZnV₂O₆ and Zn₂V₂O₇ samples, respectively. By means of two points dark conductivity measurements, conductivity values in the 10⁻⁴–10⁻⁶ and 10⁻⁶–10⁻⁸(Ω cm)⁻¹range for the Nd³⁺ doped ZnV₂O₆ and Zn₂V₂O₇ samples were measured, respectively. The conductivity as a function of the temperature indicated a semiconductor behavior. The photoluminescence spectra upon Ar⁺ laser excitation at 488 nm, exhibited the Nd³⁺ characteristics emissions. For instance, the Nd³⁺ doped ZnV₂O₆ sample displayed the Nd^{3+ 4}F_5/2→⁴I_9/2 and ⁴F_3/2→⁴I_9/2 emissions; while the Nd³⁺ doped Zn₂V₂O₇ one showed the Nd³⁺ characteristic emissions associated with the ⁴G_7/2, ⁴F_9/2, ⁴F_5/2 and ⁴F_3/2→⁴I_9/2 transitions. The lifetimes were 80 and 130 µs for the Nd³⁺ doped ZnV₂O₆ and Zn₂V₂O₇ samples, respectively. All these results suggest a successful synthesis of Nd³⁺ doped zinc vanadate compounds by the melt-quenching technique.     

ZnO-activated formation of phase pure perovskite Pb(In1/2Nb1/2)O3-Pb(Zn1/3Nb2/3)O3-PbTiO3 powder

This paper reports on the phase formation of perovskite Pb(In_1/2Nb_1/2)O₃-Pb(Zn_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PZN-PT) powder when doped with 0.04 to 0.83 mol% ZnO. Air calcination of undoped powder mixtures for 4 hours at 800°C resulted in a mixture of Pb₂Zn_0.29Nb_1.71O_6.565 pyrochlore, PIN-PZN-PT perovskite, and In₂O₃. ZnO dopant concentrations as low as 0.04 mol% increased the rate of perovskite formation and resulted in near phase pure perovskite powder of 0.5 μm particle size when heated at 800°C in air. In all cases PbTiO₃ and Pb(In_1/2Nb_1/2)O₃ formed prior to PIN-PZN-PT formation. ZnO doping promotes perovskite phase formation by increasing the reactivity of the intermediate pyrochlore phase by substituting Zn²⁺ on Nb⁵⁺ sites and forming oxygen vacancies when heated in air. Heating in high resulted in an incomplete reaction and a mixture of perovskite and pyrochlore whereas low resulted in phase separation into a mixture of rhombohedral perovskite, tetragonal perovskite, and pyrochlore. The sensitivity clearly shows that oxygen vacancies due to ZnO-doping are critical for synthesis of phase pure PIN-PZN-PT powder.     

Multi-electrochromism behavior and electrochromic mechanism of electrodeposited molybdenum doped vanadium pentoxide films

Molybdenum doped vanadium pentoxide (Mo doped V₂O₅) films are prepared by cathodic electrodeposition on indium tin oxide substrate from Mo doped V₂O₅ sol. As an anodic and cathodic coloration electrochromic material, the electrodeposited Mo doped V₂O₅ film presents a better cycling stability, reversibility and multi-electrochromic behavior (orange-yellow-green-blue) with an optical modulation of 60-90% in the spectral region 550-900 nm, which can be expected as a result of enhanced electron intervalence transfer between Mo⁶⁺ and V⁵⁺, V⁴⁺ states, in addition to V⁵⁺ and V⁴⁺ transition. The electrochromic mechanism of Mo doped V₂O₅ films is investigated with atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The surface roughness of the film increase at the different coloration states due to the increasing crystallinity degree. The change of the interlayer spacing for the host V₂O₅ and the change of the C and Li element states verify the insertion of organic solvent into the interlayer of the host V₂O₅ and some of the Li⁺ ions into the sites in the V-O layers. The electrochromic kinetics process indicates that the electrochromism of Mo doped V₂O₅ films can be considered as a reversible reduction/oxidation process accompanying the insertion/extraction of Li⁺ ions and electrons.     

Li-doped Cu2O/ZnO heterojunction for flexible and semi-transparent piezoelectric nanogenerators

We have investigated the characteristics of p-type Li-doped Cu₂O (LCO) films grown by radio frequency magnetron sputtering to use as p-n heterojunction for flexible and semi-transparent piezoelectric nanogenerators (PENGs). Electrical, optical, morphological properties of the LCO films were examined as a function of Ar/O₂ flow ratio as well as work function. The LCO films grown at Ar/O₂ ratio of 20/4 sccm film showed a p-type behavior with resistivity of 2.12 Ω-cm, mobility of 0.364 cm²/V-s, and carrier concentration of 8.07×10¹⁹ cm^-3. To overcome the piezoelectric potential screening effect of conventional ZnO-based PENGs, the p-type LCO layer was employed. Due to the enhanced piezoelectric potential coupled with the reduced total capacitance, the PENG with a p-LCO/n-ZnO heterojunction demonstrates the much higher output power up to ~52 μW than PENG only with ZnO layer (7 μW). The improved output power of PENGs indicates that sputtering of the p-type LCO layer on the n-type ZnO is the effective method to overcome the limit of the ZnO-based PENGs.     

Microstructural, crystal structure and electrical characteristics of shock-consolidated Ga2O3 doped ZnO bulk

Ga₂O₃ (5 wt.%) doped zinc oxide (ZnO, 95 wt.%) bulk was fabricated by underwater shock compaction technique. The microstructural, crystal structure and electrical properties of shock-consolidated samples were investigated and compared to a commercially available sintered Ga₂O₃ (5 wt.%) doped ZnO (95 wt.%). The relative density of shock-consolidated sample was about 97% of the theoretical density, and no grain growth and lattice defects were confirmed. The grain boundary resistance was remarkably higher than that of commercial sintered Ga₂O₃ doped ZnO and nonlinear current-voltage (I-V) characteristics of shock-consolidated ZnO and Ga₂O₃ doped ZnO were very lower than that of commercial ZnO varistor.