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.     

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.     

ZnO – Yb2O3 composite optical ceramics: Synthesis,structure and spectral-luminescent properties

Zinc oxide optical ceramics containing 0–2 wt% ytterbium are prepared by uniaxial hot pressing of commercial oxides at 1150 and 1180 °C. The ceramics have the main crystalline phase of hexagonal wurtzite-type ZnO. Ytterbium ions do not enter the ZnO crystals but form a cubic sesquioxide phase of Yb₂O₃ located at the ZnO grain boundaries. Yb acts as an inhibitor for the ZnO grain growth. The ceramics exhibit transmittance up to 60 % in the visible. Their transmission in the infrared is determined by the free charge carrier absorption. The Yb³⁺ ions are found in C₂ and C_3i sites in Yb₂O₃ crystals. Under X-ray excitation, the ceramics exhibit intense luminescence bands in the blue (near-band-edge emission) and green (defect emission) whose positions, intensities and decay times depend on the Yb content. Yb₂O₃ causes a redistribution of luminescence intensity in favor of the near-band-edge emission and fastens the emission decay.     

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.     

Co-doping effects of (Al,Ti, Mg) on the microstructure and electrical behavior of ZnO-based ceramics

Co-doped ZnO-based ceramics using Al, Ti, and Mg ions in different ratios were synthesized with the objective to investigate the doping effects on the crystalline features, microstructure and the electrical behavior. For Al and Ti doping, a coexistence of crystalline phases was shown with a major wurtzite ZnO structure and secondary spinel phases (ZnAl₂O₄, Zn₂TiO₄, or Zn_aTi_bAl_cO_d), while Mg doping did not alter significantly the structural features of the wurtzite ZnO phase. The electrical behavior induced by Al, Ti, and Mg co-doping in different ratios was investigated using Raman, electron paramagnetic resonance (EPR) and ²⁷Al and ⁶⁷Zn solid-state nuclear magnetic resonance (NMR). Al doping induces a high electrical conductivity compared to other doping elements. In particular, shallow donors from Zn_i-Al_Zn defect structures are inferred from the characteristic NMR signal at about 185 ppm; that is, quite far from the usual oxygen coordinated Al. The Knight shift effect emanating from a highly conducting Al-doped ZnO ceramics was considered as the origin of this observation. Oppositely, as Ti doping leads to the formation of secondary spinel phases, EPR analysis shows a high concentration of Ti³⁺ ions which limit the electrical conductivity. The correlation between the structural features at the local order, the involved defects and the electrical behavior as function of the doping process are discussed.     

Enhancement of Al-N co-dopant solubility in ZnO by high temperature thermal annealing

In this paper, we have developed a method to enhance the Al-N co-dopant solubility in bulk ZnO prepared by solid state reaction method. Reactive donor Al and acceptor N were mobilized by annealing the samples at various temperatures from 650 to 850?°C with a step of 50?°C in a programmable furnace. The solubility enhancement argument was verified by the conductivity measurements which showed that the conductivity of annealed films increases as the annealing temperature increases. The activation energy was calculated by the Arrhenius plot and was found to be (0.08?eV) very close to activation energy of shallow acceptor (nitrogen). To further strengthened our argument, we have also performed XRD, FTIR, Raman Spectroscopy and SEM measurements. XRD data suggested that only ZnO phases were present and no evidence for the presence of AlN, Al₂O₃ or Zn₃N₂ phases. We have also observed weakening and peak shifting of (002) with annealing temperature that suggested the incorporation of more acceptor defects in the crystal of ZnO. FTIR results verified the presence of Zn-O bond (437?cm^?1) along with week vibration of Al-N bond at 917?cm^?1. Raman spectroscopy data consists of 2E₂, A₁ (LO) and E_2(high) modes of ZnO but sample annealed at 800?°C has additional nitrogen related mode at 507?cm^?1. SEM images demonstrated the crystalline nature of samples having smooth surface but sample annealed at 800?°C has rough surface which indicated the enhancement of acceptor defects density.     

Zn1−xSixO: Improved optical transmission and electrical conductivity

Si addition in ZnO lattice significantly improves electrical conductivity. The extra charge of Si⁴⁺ ion (in comparison to Zn²⁺) attracts more oxygen in the lattice and reduces oxygen vacancies. Reduction of oxygen vacancies (defects) reduces strain in the lattice. Transparency of visible light (<3.0 eV) improves due to reduction of these defects in the wide bandgap (~3.3 eV: UV) of ZnO. Extra charge of Si⁴⁺ enhances carrier density in the ZnO lattice. Improved carrier density, reduced strain facilitate transport of carriers and therefore conductivity increases. Si incorporation also makes the samples moisture resistant. The material becomes more robust to operate in adverse humid conditions. An ideal transparent conductive oxide (TCO) should be conductive, transmit visible light and able to sustain humid conditions. All these properties are observed in Zn_(1−x)Si_xO material.     

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.     

Electrical and optical properties of Al-doped ZnO and ZnAl2O4 films prepared by atomic layer deposition

ZnO/Al₂O₃ multilayers were prepared by alternating atomic layer deposition (ALD) at 150°C using diethylzinc, trimethylaluminum, and water. The growth process, crystallinity, and electrical and optical properties of the multilayers were studied with a variety of the cycle ratios of ZnO and Al₂O₃ sublayers. Transparent conductive Al-doped ZnO films were prepared with the minimum resistivity of 2.4 × 10⁻³ Ω·cm at a low Al doping concentration of 2.26%. Photoluminescence spectroscopy in conjunction with X-ray diffraction analysis revealed that the thickness of ZnO sublayers plays an important role on the priority for selective crystallization of ZnAl₂O₄ and ZnO phases during high-temperature annealing ZnO/Al₂O₃ multilayers. It was found that pure ZnAl₂O₄ film was synthesized by annealing the specific composite film containing alternative monocycle of ZnO and Al₂O₃ sublayers, which could only be deposited precisely by utilizing ALD technology.     

Photophysical investigation of the formation of defect levels in P doped ZnO thin films

Zinc oxide (ZnO) offers a major disadvantage of asymmetry doping in terms of reliability, stability, and reproducibility of p-type doping, which is the main hindrance in realization of optoelectronic devices. The problem is even more complicated due to formation of various native defects in unintentionally doped n-type ZnO. The realization of p-type conductivity in doped ZnO requires an in-depth understanding of the formation of an effective shallow acceptor, as well as donor-acceptor compensation. Photophysical properties such as photoconductivity along with photoluminescence (PL) studies have unprecedentedly and effectively been utilized in this work to monitor the evolution of various in-gap defects. Phosphorus (P) doped ZnO thin films have been grown by RF magnetron sputtering under various Ar to O₂ gas ratios to investigate the effect of O₂ on the donor-acceptor compensation by comprehensive photoconductivity measurements supported by the PL studies. Initial elemental analyses indicate presence of abundant zinc vacancies (V_Zn) in O-rich ambience. The results predict that P sits in the zinc (Zn) site rather than the oxygen (O) site causing the formation of P_Zn–2V_Zn acceptor-like defects, which compensates the donor defects in P doped ZnO films. Photocurrent spectra uniquely reveal presence of more oxygen vacancies (V_O) defects states in lower O₂ flow, which gets compensated with an increase in the O₂ flow. Successive photocurrent transients indicate probable presence of more V_O in the films grown with lower O₂ flow and more V_Zn in higher O₂ flow. Overall the photosensitivity measurements clearly present that O-rich ambience expedites the formation of acceptor defects which are compensated, thereby lowering the dark current and enhancing the ultraviolet photosensitivity.     

Effect of thermal annealing on electrical degradation characteristics of Sb–Bi–Mn–Co-added ZnO varistors

The effects of thermally annealing Bi–Mn–Co–Sb₂O₃-added ZnO varistors on their electrical degradation were investigated. For the samples added with 0.01 mol% Sb₂O₃ and without Sb₂O₃, no marked difference in the nonlinearity index α of the voltage–current (V–I) characteristics was observed upon electrical degradation for the annealed and nonannealed samples. Upon increasing the amount of Sb₂O₃ added, the values of α increased after electrical degradation for the annealed samples. Moreover, the value of α after electrical degradation was proportional to the width of gauss function (width) of the X-ray diffraction peak for Zn_2.33Sb_0.67O₄-type spinel particles under various annealing conditions. The added Sb₂O₃ did not dissolve in the ZnO grains but became segregated at grain boundaries. Therefore, it is speculated that the increase in the width of the spinel particles is due to the increase in the numbers of fine spinel particles at grain boundaries and triple points. Furthermore, it is suggested that the improvement of the electrical degradation is due to the decrease in the mobility of oxide ions or Zn²⁺ ions owing to their being blocked by uniformly dispersed fine spinel particles at grain boundaries.     

Controllable microstructure tailoring for regulating conductivity in Al-doped ZnO ceramics

Structural and electrical behavior of Al₂O₃ doped ZnO-based ceramics were investigated as function of the aluminum doping ratios under reducing sintering atmosphere (N₂+CO). With Al₂O₃ doping from 0.1 mol% to 0.55 mol%, the electrical conductivity increases firstly to a maximum (1.52 × 10⁵ S·m⁻¹) at 0.25 mol%, and then decreases gradually. The increased conductivity is explained by the formation of shallow donors as Al_Zn-Zn_i complexes with doping to 0.25 mol%. As Al₂O₃ doping further increasing to 0.55 mol%, ZnAl₂O₄ spinel phase and more ZnO-ZnO grain boundaries are formed, hindering charge carriers transport, to decrease charge carrier mobility, thus to decrease the conductivity of ZnO ceramics. Therefore, the Al_Zn-Zn_i complexes, grain boundaries and ZnAl₂O₄ spinel can be adjusted by doping different Al₂O₃ amount, thus the carriers’ concentration and their mobility are optimized to increase the conductivity. Our work, as a fundamental research, is of great significance to control conductivity by regulating Al₂O₃ doping.     

Defects and microstructure of highly conducting Al-doped ZnO ceramics obtained via spark plasma sintering

Al-doped ZnO ceramics were sintered by conventional sintering method and spark plasma sintering (SPS) respectively. Electrical properties and microstructure have been investigated by various measurements. The samples sintered via SPS exhibit a huge electrical conductivity, up to 3.0 × 10⁵ S/m at room temperature, which was much higher than that of the sample sintered via the conventional sintering. Structural and morphorlogical characterizations pointed out that the further incorporation of Al ions and the absence of a secondary phase, contribute to the increase of the carrier concentration. Raman spectroscopy revealed the occurrence of structural distortions and a disorder induced by Al doping. Photoluminescence spectra were interpreted by different electronic active defects such as the defect complexes (Al_Zn-Zn_i) which play a key for the high electrical conductivity. Thus, SPS and Al doping modified the microstructure and the concentration of the electronic active defects to ensure high electrical conductivities in doped ZnO-based ceramics.     

Photoluminescence mechanism of annealed ZnO/Zn/Al2O3 sandwich structures deposited on glass substrates

ZnO/Zn/Al₂O₃ sandwich structures are grown on glass substrates by magnetron sputtering. The effect of Al₂O₃ layers on optical properties of ZnO/Zn/Al₂O₃ sandwich structures is investigated. Results indicated that as the deposition time of Al₂O₃ increases, violet peak centered at 402 nm gradually shifted to 412 nm and the intensity firstly decreases and then increases. We discuss the intensity change and shift of violet peak relating to V_Zn defects and the band alignment of ZnO/Zn/Al₂O₃ sandwich structures, respectively. We proposed that ZnO/Zn/Al₂O₃ sandwich structures can be approximately regarded as a quasiquantum-well-like structure. So the electron tunneling from Zn to Al₂O₃ layer is suppressed and the photogenerated carriers can be confined in the Zn Fermi level. In order to further understand the effect of posttreatment on optical properties of samples, samples are annealed in vacuum at 350 °C for 1 h. PL emissions are weakened with the increase of Al₂O₃ deposition time. Interestingly, at a same deposition condition, PL emissions are still improved after posttreatment. Combined Al₂O₃ layer modulation with annealing treatment, steady PL properties can be effectively improved.     

Effects of thermal annealing on the optical properties of Ar ion irradiated ZnS films

Ar-ion-implantation to a dose of 1×10¹⁷ ions/cm² was performed on cubic ZnS thin films with (111) preferred orientation deposited on fused silica glass substrates by vacuum evaporation. After ion implantation, ZnS films were annealed in flowing argon at different temperatures from 400 to 800 °C. The effects of ion implantation and post-thermal annealing on the structural and optical properties of ZnS films were investigated by X-ray diffraction (XRD), photoluminescence (PL) and optical transmittance measurements. XRD reveals that the diffraction peaks recover at ∼500 °C. The optical transmittances show that the bandgap of ZnS films blueshifts when annealed below 500 °C, and redshifts when annealed above 500 °C. PL results show that the intrinsic defect related emissions decrease with increasing annealing temperature from 400 to 500 °C, and increase with increasing annealing temperature from 500 to 800 °C. The observed PL emissions at 414 and 439 nm are attributed to the transitions of Zn_i→V_Zn and V_S→VBM, respectively.     

Thermal Evaporation Synthesis and Properties of ZnO Nano/Microstructures Using Carbon Group Elements as the Reducing Agents

ZnO nano/microstructures have been formed by thermal evaporation method using ZnO powders mixed with carbon group elements (C, Si, Ge, Sn, or Pb) as the reducing agent. For cases of mixed precursors of ZnO/C, ZnO/Si, and ZnO/Ge, the pure ZnO nano/microstructures are realized, while for ZnO/Sn (ZnO/Pb) systems, the phase of Pb₂O₃ (Zn₂SnO₄) generally are represented in the ZnO products. The appearance of Pb₂O₃ (Zn₂SnO₄) is attributed to the lower melting point and higher vapor pressure of Sn (Pb) in the heating and evaporation processes. The morphologies and sizes of the products are controlled by adjusting the growth regions and/or introducing gaseous argon. Room temperature (RT) photoluminescence spectra indicate that the intensity (peak position) of the ultraviolet emission is increased (redshift) due to the existence of Zn₂SnO₄ phase in the ZnO products. The Pb₂O₃ (Zn₂SnO₄) phase in ZnO nano/microstructures plays a important role in enhancing the saturation magnetizations of RT ferromagnetism with respect to the case of pure ZnO products fabricated by the precursor of mixed ZnO and graphite.     

Microstructure effects on the local order and electronic defects in (Al,Ti, Mg) co-doped ZnO conductive ceramics

Fine particles with different sizes of (Al, Ti, Mg) oxide co-doped ZnO ceramics were realized by a suitable grinding and a mechanical ball milling, respectively. The investigations were devoted to understand the origin of the electrical conductivity evolution with the microstructure by analyzing the local order and defect involvement in the crystalline sites of ZnO ceramics. Experimental investigations of particles were conducted to probe the local order and electronic defects with an emphasis on the electrical behavior of ceramics. Particularly, Al doping is intimately correlated with the high conductivity induced from the stabilization of particular Al_Zn-Zn_i complexes. The conduction electrons are probed through the induced Knight Shift on NMR spectra and also from the particular relaxation mechanisms of paramagnetic centers revealed by EPR studies. The correlation between the electronic active defects, the microstructure in small sized particles of ZnO based ceramics and the electrical behavior are pointed out and discussed.     

The effect of zinc substitution on the magnetism of magnesium ferrite nanostructures crystallized from borate glasses

Glasses in the system 51.7 B₂O₃/9.3 K₂O/1 P₂O₅/10.4 Fe₂O₃/(27.6–x) MgO/ x ZnO (with x=0, 5, 10, 13.8 and 20 mol%) were prepared by the conventional melt quenching method. The as prepared glass samples were thermally treated at 560 °C for 3 or 6 h. The effect of substituting MgO by ZnO in the glass network on the crystallized phase was studied. The resulting magnetic glass ceramics were characterized using X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and transmission electron microscopy (TEM) including energy dispersive X-ray analysis (EDX). The substitution of Mg by Zn resulted in a larger lattice parameter of the precipitated crystals, while the crystallite size does not change significantly. TEM micrographs, recorded from extracted particles, showed the formation of small aggregates with about 30 nm in diameter. These agglomerates contain crystals with sizes in the range from 7 to 9 nm. EDX measurements proved the incorporation of Zn²⁺ ions into the crystal phase. Room temperature magnetic measurements of the samples with up to 10 mol% ZnO showed hysteresis loops which are characteristic for super paramagnetic (SPM) behavior. A magnetic contribution was not detected for samples with higher ZnO concentrations. The maximum magnetization varied with the composition of the glass ceramics to a great extent.     

Grain‐Boundary and Thermally Stimulated Current Characteristics of Y2O3‐Doped ZnO Varistor

The doping of rare‐earth oxides can greatly improve the electrical characteristics of ZnO varistors. Thermally stimulated current (TSC) characteristic test, capacitance voltage (C–V) characteristic test, scanning electron microscope (SEM) test, and voltage current (V–I) test were carried out to study the influence of Y₂O₃ content on the electrical properties of ZnO varistors in this study. The results show that the grain size decreases while the voltage gradient increases as the Y₂O₃ content is increased. The reaction of Y₂O₃ with other additives leads to the decrease in grain‐boundary defects, which accounts for the decrement of barrier height, donor density, and surface state density. The trap level and trapped charge of ZnO varistors decrease as the Y₂O₃ content is increased from 0.3 to 0.9 mol%, which means the shallow traps inside ZnO varistors reduce, and the Y₂O₃ additive can greatly improve the TSC characteristic of ZnO varistors.     

Effect of ZnO on the microstructure and electrical properties of WO3–Bi4Ti3O12 ceramics

The aim of the present work is to explore the possibility of incorporate a small amount of ZnO to improve the microstructure control of W-doped BIT-based materials. Two different processing routes have been used according to previous results reported for other materials: reaction and sintering in one single step and a previous calcination step. The sintering behaviour of the samples, the obtained crystalline phases and the microstructure analysis indicate that the reaction between ZnO and Bi₂O₃ plays a critical role during sintering. Both Bi₂Ti₂O₇ and Zn₂TiO₄ secondary phases are stabilized when adding ZnO. Actually, when WO₃ and ZnO are incorporated simultaneously to BIT materials, they interact stabilizing the Bi₂Ti₂O₇ phase and avoiding the incorporation of W⁶⁺ into the BIT lattice. As a consequence, the electrical conductivity of the samples with ZnO is two orders of magnitude higher than that of the samples doped only with WO₃, suggesting that WO₃ does not form a solid solution with BIT. The curve dielectric constant vs temperature also reveals the role played by the Bi₂Ti₂O₇ phase.