ZnO Single Crystals Research Articles

Room Temperature Ferromagnetism in D-D Neutron Irradiated ZnO Single Crystals

Room temperature ferromagnetism (RTFM) was observed in unirradiated ZnO single crystal owing to zinc vacancy (VZn) defects. D-D neutron irradiation was used to regulate the defect concentration and types avoiding interference of impurity elements. D-D neutron mainly collides elastically with ZnO single crystals, producing VZn, oxygen vacancy (VO), and other point defects, the density and kind of defect associated with neutron irradiation fluence. It can be found the saturation magnetization (Ms) first increases and then decreases with the irradiation fluence increases, meaning there is an optimum value for VZn concentration. When the neutron fluence is greater than 5× 109 n/cm2, more unstable VZn combined with VO to form zinc-oxygen divacancy (VZnO), indicating neutron irradiation changes the defect types. VZnO induces a smaller magnetic moment; thus, the Ms decreased. To further verify the origin of RTFM, the CASTEP module was employed to calculate the magnetic and structural properties of native vacancy defects in ZnO. VZn induces 2.12 μB magnetic moment, and only VZnO formed by the nearest VZn and VO has 2.09 μB magnetic moment, while 3VO cluster introduces a magnetic moment of $\sim $1 μB. Combined experiment and CASTEP calculation results, VZn is the most likely origin of RTFM.

Effects of annealing on photoluminescence and defect interplay in ZnO bombarded by heavy ions: Crucial role of the ion dose

Bombardment of ZnO with heavy ions generating dense collision cascades is of particular interest because of the formation of nontrivial damage distribution involving a defected layer located between the surface and the bulk damage regions, as seen by Rutherford backscattering spectroscopy in the channeling mode. By correlating photoluminescence and channeling data, we demonstrate that the thermal evolution of defects in wurtzite ZnO single crystals implanted with Cd ions strongly depends on the implanted dose. Specifically, the ion dose has a profound effect on the optical response in the spectral range between the near-band-edge emission and deep-level emission bands. The interplay between interstitial and vacancy type defects during annealing is discussed in relation to the evolution of the multipeak damage distribution.

Segregation and properties at curved vs straight (000) inversion boundaries in piezotronic ZnO bicrystals

AbstractTEM and SEM investigations of ZnO bicrystal interfaces were undertaken with an aim to study the correlation of local grain‐boundary structure, segregation, and electrical transport perpendicular to the interface. To this end, varistor‐like ZnO bicrystals with piezotronic characteristics were chosen with (000)║(000) tail‐to‐tail orientation with respect to the c‐axis. In order to contrast different local grain‐boundary structures with different coherency and segregation of bismuth, but identical macroscopic polarization state, two complementary processing techniques were applied. A diffusion‐bonded bicrystal with an intermediate thin film containing Zn–Bi–Co–O provided a straight interface as reference. In contrast, a ZnO bicrystal prepared by epitaxial solid‐state transformation was manufactured by bonding two ZnO single crystals with a 100 µm thick polycrystalline ZnO varistor material with a typical dopant composition including bismuth and cobalt. This structure was annealed to the point that a bicrystal was formed with the varistor concentration at the boundary, which was strongly curved due to the polycrystalline microstructure still providing a shadow image at the interface. The results highlight a distinct correlation between local interfacial morphology, degree of segregation of bismuth, and degree of nonlinearity of the electrical transport across the interface.

Doping-induced transition from polycrystals to single crystals in ZnO thin films

The growth of high quality ZnO thin films is a hot research topic in the field of optoelectronic materials. In this study, high quality ZnO thin films with a double-layer structure were obtained by sol-gel method. Al-doping induced transition from polycrystals to single crystals in the ZnO thin films was found. The results show that Al-doping improves the c-axis orientation of ZnO thin films. When the Al-doping concentration is higher than 4at%, the frustum of hexagonal pyramid (FHP) shaped ZnO nanostructures are formed on the polycrystalline ZnO thin films surface. As the Al-doping concentration further rises, the areal density of the FHP-shaped nanostructures also gradually increases and the size becomes more uniform. Both SAED patterns and high-resolution TEM images indicate that the FHP-shaped ZnO nanostructures are single crystals. It is of great scientific significance to transform polycrystalline ZnO into single crystal ZnO by Al-doping combined with a simple heat-treatment process. Compared with pure ZnO thin films, the FHP-shaped nanostructures on the surface show better UV emission performance, suggesting these materials are ideal ones for the preparation of ZnO-based UV emitters. The authors propose that the growth mechanism of FHP-shaped ZnO nanostructure is a multi-step solid-solid reaction dominated by surface diffusion of zinc atoms.

The role of surface depletion layer effects on the enhancement of the UV emission in ZnO induced by a nanostructured Al surface coating

The UV enhancement of Al-coated ZnO single crystals with a wide range of carrier densities is systematically studied using depth-resolved cathodoluminescence and photoluminescence as well as valence band X-ray photoemission spectroscopy (VB-XPS). An up to 17-fold enhanced PL UV emission for Al-coated ZnO with the highest carrier density was measured, which falls to a 12-fold increase for the lowest carrier density. Depth-resolved cathodoluminescence measurements confirm that the enhancement is strongest near the Al-ZnO interface consistent with an increased UV emission due to an exciton-localized surface plasmon coupling mechanism. Correlative cathodoluminescence, photoluminescence and VB-XPS studies reveal that a number of additional effects related to the presence of the Al surface coating also contribute to the UV enhancement factor. These include increased UV enhancement due to the formation of a surface depletion layer induced by the Al coating, which also passivates competitive non-radiative surface recombination channels found in uncoated ZnO. Significantly, it was established that the magnitude of the emission enhancement factor can be raised in a controlled way by reducing the thickness of the depletion layer by increasing the carrier density. The contribution of these effects collectively provides an explanation for the large span of enhancement factors reported in the literature.

Er implantation into various cuts of ZnO – experimental study and DFT modelling

This paper presents a DFT study of the positions of erbium ions in the ZnO structure as well as experimental study of Er ion implantation into different crystallographic cuts of ZnO. The theoretical simulations have compared cohesive energies and defect-formation energies for different Er positions in the wurtzite ZnO structure. In the experimental part, single-crystal ZnO substrates were implanted with Er+ ions at an energy of 190 keV using fluences of 1 × 1016 and 5 × 1016 ion.cm−2. The ion implantation was followed by the application of the thermal annealing in oxygen atmosphere at 1000 °C for 5 h. The RBS/channelling method with fine scans was used to study the positions of erbium ions and structure recovery during the annealing process. The Er-doped ZnO samples exhibited clear luminescence in the NIR spectral region at around 1.5 μm. This work provides new results on the comparison of erbium ion implantation into different polar and non-polar crystallographic cuts of ZnO, resulting in different luminescence properties of the samples. Experimental and theoretical results are compared as well.

Spin-lattice relaxation processes of transition metal ions in a heavily cobalt doped ZnO: Phonon heating effect

Inversion recovery with electron spin-echo detection has been used to study the electron spin-lattice relaxation rates 1/T1 for transition metal impurities in heavily cobalt-doped hydrothermally grown ZnO single crystals. The relaxation dynamics of Co2+ ions dominates the phonon bottleneck effect in the Orbach-Aminov process, which involves the modulation of the zero-field-splitting tensor. The relaxation mechanism may be treated in terms of phonon heating with fast rate of energy pump from Co2+ spins into the lattice phonon modes. The measurements reveal the cross-relaxation process in which the single Co2+ ions cross-relax to exchange-coupled clusters of cobalt ions. The higher temperature relaxation of Co2+ indicates an additional Orbach-Aminov process via a state of ∼226 cm−1 above the ground state Kramers doublet. It is shown that in this system Co2+ ions play a role of the rapidly relaxing centers, strongly mediating the spin-lattice relaxation of the other transition metal impurities, such as Mn2+ and Fe3+.

Interactions of intrinsic defects formed in ZnO and their contribution to electric conductivity

The formation process of lattice defects in single crystals of ZnO and their contribution to electric conduction were investigated by means of electric conductivity measurements and positron annihilation lifetime spectroscopy (PALS). For the ZnO samples once heated at high temperatures between 1073 K and 1273 K, we observed defect-induced n-type conductivity by sample warming or light irradiation to the sample, suggesting that interstitial zinc atoms (Zni) and oxygen vacancies (VO) as shallow and deep donors, respectively, were formed by the high-temperature heating. After heat treatment at 1373 K, however, the conductivity arises only from thermal excitation by sample warming, demonstrating that only Zni contributes to the conductivity but VO does not. Based on the above observation and the results of PALS measurements for the samples heated at high temperature, we propose a mechanism of the n-type conduction in ZnO that carrier electrons are provided by interstitial zinc atoms as a result of electron-hole cancellation through interaction between zinc vacancies (VZn) and VO.

Energetics, migration and trapping of Zn interstitials in ZnO

Zn interstitial (Zni) is one of the fundamental intrinsic defects in ZnO and prominently affects the physical properties of the material. Here, the energetics and migration properties of Zni have been studied in ion implanted ZnO using a new approach based on the Li marker diffusion. Specifically, ZnO single crystals were implanted with 3.2 keV/amu B and BF2 ions and the release of Zni from the implanted regions during annealing was correlated with the advance of the characteristic Li depleted region into the bulk of the samples. Using this methodology, we calculate the activation energy of 1.45 eV to govern the process for B implants. Assuming that the migration energy of Zni to be of ~0.6 eV, as discussed previously in literature, a barrier for releasing Zni from the implanted region may be estimated as ~0.8–0.9 eV. Meanwhile, in the BF2 implanted samples, the migration of Zni is less efficient, as interpreted from the Li redistribution features in these samples; in particular, it is argued that Zni may be trapped by defect complexes related to the presence of F.

Understanding the thermal evolution of defects in carbon-implanted ZnO single crystal

Defects and impurities play a major role in controlling the electrical and optical properties of semiconductor materials. Herein, hydrothermally grown ZnO single crystals have been implanted with carbon (C) dopants at room temperature and then annealed in argon atmosphere at various temperatures between 400 and 800 °C. The thermal evolution of C-related defects and their effects on the structural, optical and electrical properties of ZnO single crystals were systematically characterized and discussed. The results show ion implantation induces serious lattice disorder, and post-implantation annealing could promote the lattice renormalization, accompanied by an increase in crystal quality and average visible transmittance. Furthermore, it is found that the diffusion of octahedral carbon interstitial (Ci) along parallel to c-axis facilitates the growth of carbon sp2 clusters due to its low migration barrier during annealing, which energetically contribute to the decrease of the resistivity. Meanwhile, abundant Ci will be able to enter into VZn to form CZn or combine with lattice O to form (CO)O donor defects upon annealing, dominating the increase of electron carrier concentration and enhancing the anomalous Raman mode at 510-525 cm−1. These findings strengthen the fundamental understanding of the donor behavior of C impurities in ZnO.

Ethanol Gas Sensors Based on ZnO Nanotube Structures Fabricated By Atomic Layer Deposition

Among various Metal Oxide Semiconductor (MOS) gas sensors, ZnO nanostructure gas sensors have been widely used due to its large exciton binding energy, wide band gape of 3.37 eV, large exciton binding energy of 60 meV at room temperature, good electrical conductivity, low cost, and high mechanical stability. Furthermore, a variety of morphology has been developed to improve the properties and applications. In this work, ZnO nanotube nanostructures were fabricated as gas sensors to sense ethanol vapor due to its high electrochemical stability, nontoxicity, and, especially, high surface-to-volume ratio. Several coaxial ZnO nanotubes were synthesized on porous substrate to further enhance the sensing response to ethanol vapor with increased reaction surfaces in ZnO nanostructures. In this study, ZnO nanotubes were synthesized by Atomic Layer Deposition (ALD) on Anodic Aluminum Oxide (AAO) templates with sacrificial Al2O3 layers. The second Al2O3 sacrificial layer followed by ZnO layer was synthesized on AAO samples by ALD with TMA (Al2(CH3)6) and DI water as precursors. In addition, the third ZnO thin film layer were deposited on the surface of the sacrificial layer of Al2O3 with precisely controlled thickness by ALD. The entire procedure outlined above for fabricating ZnO-Al2O3-ZnO structures is considered as one super cycle. More super cycles were applied to synthesize additional coaxial ZnO nanotubes. To eliminate the sacrificial layer, a Precision Ion Polishing System (PIPS) was used to remove the top cover as shown in Figure 1. Therefore, the Al2O3 sacrificial layers were exposed for Sodium Hydroxide (NaOH) wet etching. After the sacrificial layers were removed by NaOH, ZnO nanotube gas sensors were formed as shown in Figure 1 (b). To investigate the sensing performance of ZnO nanotube gas sensors to ethanol vapors, a gas sensor testing system was developed with a sealed reaction chamber and a control system with stable temperature control and accurate gas concentration control. The sensing performance of the MOS gas sensors consisting of only core single crystal ZnO nanotubes, was compared to sensors consisting of double nested coaxial ZnO nanotubes, and further compared to three nested tube-in-tube coaxial ZnO nanotube structures and analyzed under various ethanol vapor concentrations at different temperatures. The ZnO nanostructures sensing results on ethanol vapor demonstrated the sensing performance was considerably enhanced due to the increased surface to volume ratio in the ZnO tube structures. Figure 1

Au incorporation into various ZnO crystallographic cuts realised by ion implantation – ZnO damage characterization

Non-polar surfaces, such as a-plane (11–20) and m-plane (10-10), for ZnO have become more attractive as numerous efforts have recently been made to grow non-polar ZnO facets for applications in nanoscale photonic devices. Noble-metal incorporation into transparent semiconductors such as ZnO has been investigated because of the non-linear optical response of such structures. This paper presents a study of defect evolution in various ZnO crystallographic cuts caused by Au implantation. The investigation has focused on ZnO structure characterisation, Au distribution and the interior morphology of the a-, m- and c-planes ZnO single crystals implanted with 400 keV Au+ ions at the ion fluences of 5 × 1014 and 1 × 1015 cm−2 and subsequently annealed at 600 °C in O2. The structure modification was studied using Rutherford backscattering spectrometry (RBS) in the channelling mode (RBS/C) and Raman spectroscopy. After the ion-implantation process, low surface damage was observed in all ZnO orientations unlike deep structural damage. Deep structural damage grew with increased Au-ion fluence and Au did not exhibit strong out-diffusion from the depth to the surface during the post-implantation annealing. Small but noticeable differences were observed between different ZnO orientations. RBS measurements during ion implantation revealed more progressive deep-damage formation in the c- and m-planes than in the a-plane ZnO. Simultaneously, the smallest Zn sub-lattice disorder deduced from RBS/C measurements was observed in the a-plane ZnO. During post-implantation annealing, a slight structure recovery (about 4%) was observed in all orientations. Raman spectroscopy confirmed the increasing structure disorder with the enhanced ion fluence for all as-implanted ZnO orientations and a partial reconstruction of the ZnO structure during annealing, when the intensity of E2 phonons was increased and that of longitudinal optical (LO) phonons was suppressed because of the disorder recovery. E2 (high) and E1(LO) Raman phonon modes connected with oxygen sub-lattice ordering/disordering have been investigated in detail – they show a significant modification mainly in the m-plane. The cause of the different behaviour of ZnO planes as well as the differences in the incorporation and movement of Au and Er atoms in the ZnO structure are discussed in the work.

Tuning ZnO nanorods photoluminescence through atmospheric plasma treatments

Room temperature atmospheric plasma treatments are widely used to activate and control chemical functionalities at surfaces. Here, we investigated the effect of atmospheric pressure plasma jet (APPJ) treatments in reducing atmosphere (Ar/1‰ H2 mixture) on the photoluminescence (PL) properties of single crystal ZnO nanorods (NRs) grown through hydrothermal synthesis on fluorine-doped tin oxide glass substrates. The results were compared with a standard annealing process in air at 300 °C. Steady-state photoluminescence showed strong suppression of the defect emission in ZnO NRs for both plasma and thermal treatments. On the other side, the APPJ process induced an increase in PL quantum efficiency (QE), while the annealing does not show any improvement. The QE in the plasma treated samples was mainly determined by the near band-edge emission, which increased 5–6 fold compared to the as-prepared samples. This behavior suggests that the quenching of the defect emission is related to the substitution of hydrogen probably in zinc vacancies (VZn), while the enhancement of UV emission is due to doping originated by interstitial hydrogen (Hi), which diffuses out during annealing. Our results demonstrate that atmospheric pressure plasma can induce a similar hydrogen doping as ordinarily used vacuum processes and highlight that the APPJ treatments are not limited to the surfaces but can lead to subsurface modifications. APPJ processes at room temperature and under ambient air conditions are stable, convenient, and efficient methods, compared to thermal treatments to improve the optical and surface properties of ZnO NRs, and remarkably increase the efficiency of UV emission.

Creation of Acceptor Centers in ZnO Single Crystals by Annealing in Sb Vapor

We have investigated the influence of Sb on the carrier concentration of ZnO single crystals. ZnO single crystal substrates were doped in a sealed ampule by annealing in the Sb vapor at temperatures from 700 to 900 °C to create acceptor centers in ZnO. The dopant concentration profiles were determined by secondary ion mass spectrometry. A significant distinction in dopant concentrations as well as depth of penetration was observed for different temperatures of annealing. Electrical properties of the samples were studied by the Hall Effect and resistivity measurements. Valence states of Sb in the samples were investigated by the X-ray photoelectron spectroscopy method to prove the incorporation of the latter in the matrix of ZnO before and after oxygen annealing. The evolution of the carrier concentration of ZnO along with the valence state of Sb is explained.

Application of a ZnO UV sensor for a scintillation-type radiation detector

Radiation detection characteristics of a device combining a YAP:Ce (cerium-doped yttrium aluminum perovskite) scintillator and a ZnO UV sensor were studied using X-ray radiation. Both ZnO and YAP have high resistance to radiation. A photoconductive-type UV sensor with comb-shaped Al electrodes with a gap of 50 μm was fabricated on the Zn-face of a nitrogen-doped ZnO single crystal. A photocurrent spectrum of the ZnO UV sensor showed a peak at wavelength of about 370 nm, and the sensor did not show a response in wavelengths of the visible light region. Scintillation characteristics of YAP:Ce were investigated under the condition of X-ray radiation. Scintillation luminescence of YAP:Ce showed a spectrum with a peak at a wavelength of about 360 nm, which almost corresponds to the peak wavelength of a photocurrent spectrum of the ZnO UV sensor. A device having a structure in which the YAP:Ce scintillator is placed above the ZnO UV sensor was fabricated. When the YAP:Ce scintillator was irradiated by X-rays, the photocurrent of the ZnO UV sensor was proportional to the X-ray intensity. The sensitivity of the device was improved by deposition of a reflective Al film on the YAP:Ce scintillator. The device using the combination of a YAP:Ce scintillator and a ZnO UV sensor can be used as a radiation detector with high radiation resistance.

Effect of Ar Post-irradiations on Magnetic Properties of Cu-Implanted ZnO Single Crystals

Ar+ post-irradiation effects on magnetic behavior of Cu+-implanted ZnO single crystals were studied. It was found that Cu+-implanted and Ar+-irradiated samples, even the primary sample, showed a magnetic moment (MM), but the saturation magnetization (MS) of Ar+ post-irradiated on Cu+ as-implanted one drastically decreased. Based on the first-principle method, it was drawn to the incomplete filling electronic configuration of the substituted Cu ion whose 3d orbital hybridized with O-2p contributing to the magnetism. The MM induced by the substituted Cu+ ion was much less than that induced by the substituted Cu2+ and Cu3+ ion, so the ratio of Cu2+,3+/Cu+ decreased after Ar+ post-irradiation, leading to the decrease of MS. The excessive concentration of O vacancy induced by post-irradiation also weakened the MM by their compensating electrons, and decreased the effective magnetic exchange coupling interaction. Cu-related clusters dissolution was also a critical reason for the reduction of MS.

Absence of ferromagnetism in transition metal (Co, Ni and Cu) doped ZnO films

Structural, optical and magnetic properties of oriented Zn0.95TM0.05O (TM = Co, Ni, Cu) films prepared on amorphous quartz substrate using RF magnetron sputtering have been studied. X-ray diffraction patterns show that all films are single phase and are preferably oriented in (002) direction. An analysis of transmission spectra shows that the optical band gap is reduced for all doped ZnO films with respect to that of bulk ZnO single crystal. With increase in thickness of Cu doped ZnO films, the band gap reduces further (3.00 eV for 200 nm film and 2.86 eV for 400 nm film). A strong absorption is observed at 665 nm for Co doped ZnO film due to transitions of tetrahedrally coordinated Co2+ from 4A2 (4F) → 2E (2G). A pre-edge peak observed in TM K-edge XANES spectra of each film confirms that TM ions get substituted in Zn site of ZnO hexagonal wurtzite structure. No room temperature ferromagnetism is observed in any of doped films.

Determination of Diffusion Coefficient of Copper in ZnO (001) Single Crystals at 1000 °C

Copper from a solid source was diffused into undoped n-type bulk ZnO (001) single crystals at 1000 °C under a nitrogen atmosphere at different diffusion times. The Cu diffusion profiles were obtained by Secondary ion mass spectroscopy (SIMS), and the fitting reveals a diffusion case from a constant concentration source. A value for the diffusion coefficient of 2.42(±0.2) × 10−12 cm2∙s−1 was obtained. Electrical measurements present an increment of carrier concentration with diffusion time, but remains n-type which indicates an increase in the donor levels produced by structural defects in ZnO. Photoluminescence (PL) spectra showed an increment of green emission intensity associated with Cu incorporation.

Electrodeposition of ZnO nanorods onto GaN towards enhanced H2S sensing

Zinc oxide nanorods (ZnO NRs) were grown on gallium nitride (GaN) substrates by electrodeposition. HRTEM and XRD analyses were performed to determine crystalline structure. Single crystal ZnO NRs with an average size of 240 nm were grown along the c-axis direction. The resulting H2S sensors based on ZnO NRs/GaN displayed excellent selectivity and response, particularly in the concentration range of 1–50 ppm of H2S. In the concentration of 50 ppm H2S, a sensing response time of 82 s and a recovery time as low as 48 s can be achieved at 240 °C. In addition to ZnO NRs forming porous network channels for gas diffusion in and out, the H2S-sensing performance of ZnO NRs/GaN heterojunctions could be attributed to the enhanced electron transport and the favorable charge transfer resulting from the excellent transport capability of GaN. The in-situ fabrication of sensing materials makes our sensors extremely attractive in the field of H2S detection.

Electronic processes in doped ZnO nanopowders

ZnO nanocrystals, undoped and doped with Iridium or Indium were prepared by solar irradiation in Heliotron reactor in PROMES CNRS facilities, France. The comparative analysis of the excitonic spectra of ZnO single crystal and ZnO nanocrystals (NCs) doped with In and Ir was performed. It is shown that the excitonic processes in Ir doped nanocrystals coincide well with electronic processes in undoped NC and single crystal; however, the electronic processes in In-doped nanocrystals are significantly different from those in single crystal. The radioluminescence spectra of ZnO:In was analysed and additional luminescence band at ∼3.18 eV was detected due to In-doping. The luminescence decay time depends on In concentration in nanocrystals and is significantly less in ZnO:In compared with undoped nanocrystals. The fast scintillation of ZnO:In makes this material promising for application