ZnO Bulk Single Crystals Research Articles

Ultra-wide spectral range (0.4–8 μm) transparent conductive ZnO bulk single crystals: a leading runner for mid-infrared optoelectronics

Transparent conductive oxides have been widely applied to solar cells, liquid crystal displays, and other optoelectronic devices because of their excellent transparent conductive properties in the visible-near-infrared (Vis-NIR) region. At present, the transparent conductive oxide films (TCOFs) such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and aluminum-doped zinc oxide (AZO) are widely noticed and used. However, a high carrier concentration (≥1020 cm−3) of these TCOFs leads to a result that their transparency will be affected by the plasmons effect, and thus their transparent range will generally end around 2–3 μm. As such, they are more applicable in the Vis-NIR region but unable to meet the requirements of mid-infrared (MIR) optoelectronic devices. Faced with this situation, herein, we reported the native ZnO bulk single crystals with low background carrier concentration (1015–1016 cm−3) and high carrier mobility (μ > 300 cm2 V−1 s−1). These single crystals boast excellent transparent conductive properties (IR cutoff edge: 6–8 μm, Rsheet: 3.7–28.5 Ω/sq) with an advantage that they can be massively produced by a hydrothermal method at a low cost. Based on these merits, the native ZnO bulk single crystals bring a new possibility of breaking the bottleneck of MIR transparent conductivity.

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.

Surface ferromagnetism in ZnO single crystal

We report on the room temperature surface ferromagnetism in undoped ZnO bulk single crystal. Typical ferromagnetic saturation behavior is clearly demonstrated by magnetization loops, and the saturated magnetization can be manipulated by annealing under argon atmosphere. A strongly correlation, between the saturated magnetization and the concentration of singly occupied oxygen vacancies on the surface of ZnO single crystal, is established by means of the Raman spectra, photoluminescence, electron paramagnetic resonance and atomic force microscope. It is suggested that the observed room temperature ferromagnetism in ZnO single crystal should be attributed to the effect of surface oxygen vacancy defects. This finding may help to get further insight into the ferromagnetic origin in other undoped oxide semiconductor materials.

Gamma-ray irradiation effect on ZnO bulk single crystal: Origin of low resistivity

We report the origin of low resistivity in ZnO bulk single crystals with thicknesses of 500 μm caused by gamma-ray irradiation. The crystals are irradiated at room temperature with gamma-rays of 1.17–1.33 MeV from a cobalt-60 source. The gamma-ray dose is 170 kGy. The resistivity at room temperature decreases from 4.1 × 104 Ω cm to 3.1 × 102 Ω cm by the irradiation. After the irradiation, slight increase in green luminescence at around 530 nm is observed, suggesting that zinc vacancy (VZn), oxygen vacancy (VO) and oxygen interstitial (Oi) are induced by Compton electrons emitted by the gamma-ray irradiation because this PL peak is a superposition consisting of the emissions relating to the VZn (∼490 nm), VO (∼530 nm), and Oi (∼580 nm). The PL intensity of VO is eight times larger than that of VZn. The existence of VO+ (g value is 1.996) is also observed by the electron paramagnetic resonance. This signal disappears by illuminating red LED with a wavelength of 654 nm, indicating VO+ to VO2+ transition. The existence of VZn would suggest the formation of zinc interstitial (Zni). In analogy with the low resistivity after Al-implanted ZnO, the origin of the low resistivity in gamma-ray irradiated ZnO is attributed to the Zni located at ∼30 meV below the conduction band.

Zn Vacancy Formation Energy and Diffusion Coefficient of CVT ZnO Crystals in the Sub-Surface Micron Region

By using positron annihilation spectroscopy methods, we have experimentally demonstrated the creation of isolated zinc vacancy concentrations >1020 cm−3 in chemical vapor transport (CVT)-grown ZnO bulk single crystals. X-ray diffraction ω-rocking curve (XRC) shows the good quality of ZnO single crystal with (110) orientation. The depth analysis of Auger electron spectroscopy indicates the atomic concentrations of Zn and O are almost stoichiometric and constant throughout the measurement. Boltzmann statistics are applied to calculate the zinc vacancy formation energies (Ef) of ~1.3–1.52 eV in the sub-surface micron region. We have also applied Fick’s 2nd law to calculate the zinc diffusion coefficient to be ~1.07 × 10−14 cm2/s at 1100 °C. The zinc vacancies began annealing out at 300 °C and, by heating in the air, were completely annealed out at 700 °C.

Vapor-Phase Synthesis of ZIF-8 MOF Thick Film by Conversion of ZnO Nanorod Array.

ZIF-8 metal organic framework "micrometer-thick" films were constructed from ZnO precursor by a vapor-phase synthesis. The ZnO-to-ZIF-8 crystal transformation proceeded in the presence of 2-methylimidazole (Hmim) vapor. Continuous coatings of intergrown ZIF-8 crystals require control of a nucleation density. The dependence of ZnO crystal plane on the ZnO-to-ZIF-8 crystal transformation was investigated using four bulk ZnO single crystals: a-plane (11-20), c-plane (0001), m-plane (10-10), and r-plane (10-11). It was revealed that the m-plane (10-10) of ZnO is more effectively transformed into ZIF-8. In this work, highly oriented ZnO nanorod array film was used to provide the transport pathway of Hmim molecules and volume expansion space of ZnO-to-ZIF-8 crystal transformation for nucleation and crystal intergrowth. The high conversion of ZnO nanorod array into ZIF-8 in a short time could be achieved because (1) such mass transfer is easy due to the uniform internanorod distance being maintained during reaction and (2) the surface of the nanorod array is dominated by the highly reactive m-plane (10-10).

Optical and Electrical Properties of Sn-Doped Zinc Oxide Single Crystals

Sn dopant in ZnO may significantly improve the n-type conductivity of ZnO through a characteristic double effect. However, studies on bulk Sn-doped ZnO are rare, and the effect of Sn doping on the optoelectronic properties of bulk ZnO is not well understood. In this work, the effect of Sn doping on the optical and electrical properties of ZnO bulk single crystals was investigated through optical absorption spectroscopy, Hall-effect measurements, and thermoluminescence (TL) spectroscopy. Undoped and Sn-doped ZnO single crystals were grown by chemical vapor transport method and characterized by x-ray diffraction analysis. The Sn doping level in the crystals was evaluated by inductively coupled plasma mass spectroscopy measurements. Hall-effect measurements revealed an increase in conductivity and carrier concentration with increasing Sn doping, while TL measurements identified a few donor species in the crystals with donor ionization energy ranging from 35 meV to 118 meV. Increasing Sn doping was also associated with a color change of single crystals from colorless to dark blue.

Strong yellow emission of high-conductivity bulk ZnO single crystals irradiated with high-power gyrotron beam

We report the strong yellow emission of bulk ZnO single crystals irradiated with the high-power gyrotron beam. Hydrothermally grown bulk crystals with high conductivity are irradiated at room temperature with up to 60-W output of a sub-terahertz gyrotron wave source. During gyrotron irradiation, the high-conductivity crystals exhibit intense emissions with a peak of around 2 eV (600 nm) and a longer-wavelength tail. The sample temperatures were also elevated from room temperature to above 1000 K by irradiation. However, when heated up to 1250 K using a heater without irradiation, the ZnO crystals do not exhibit similar visible emissions. We then use the generalized Planck's radiation in non-equilibrium states as an explanation of our experimental observations. The emission peak intensity can be enhanced by the gyrotron-induced non-equilibrium states, and the emission peak position can be related to the Urbach energy. With high intensities in the visible wavelengths, the emissions of the irradiated crystals can be readily observed with our bare eyes or with inexpensive digital cameras. As the spatial distribution of the yellow emission reflects the gyrotron beam pattern, the bulk ZnO single crystals can then be utilized for the quick diagnosis of gyrotron beam patterns and positions.

Piezotronic effect at Schottky barrier of a metal-ZnO single crystal interface

ZnO is considered as one of the most promising semiconductor materials for future applications based on the piezotronic effect. Intense studies on ZnO nanowires had been carried out to understand the modulation of the Schottky barrier height at the metal ZnO interface via piezoelectricity. However, an experimental investigation on bulk ZnO single crystals and a fundamental comparison of the modification of the barrier height determined experimentally and theoretically are still missing. Therefore, an adjustment of the electrostatic potential barrier height at metal-ZnO single crystal interfaces due to stress induced piezoelectric charges was conducted, using both O- and Zn-terminated surfaces. In-situ stress dependent impedance and current-voltage measurements were utilized to extract the electrical properties of the potential barrier and to determine the reduction of the barrier height. The decrease of the interface resistance and increase of the capacitance reveal the presence of stress induced piezoelectric charges. The experimentally evaluated reduction of the barrier height reveals a moderate change of about 9 meV at 70 MPa and supports prior work on metal-ZnO nanowires. This change was found to be in good agreement with theoretical calculations based on the imperfect screening model if a thickness of the interface layer is assumed to be ∼2 Å.

Critical increase in Na-doping facilitates acceptor band movements that yields ~180\u2009meV shallow hole conduction in ZnO bulk crystals

Stable p-type conduction in ZnO has been a long time obstacle in utilizing its full potential such as in opto-electronic devices. We designed a unique experimental set-up in the laboratory for high Na-doping by thermal diffusion in the bulk ZnO single crystals. SIMS measurement shows that Na concentration increases by 3 orders of magnitude, to ~3 × 1020 cm−3 as doping temperature increases to 1200 °C. Electronic infrared absorption was measured for Na-acceptors. Absorption bands were observed near (0.20–0.24) eV. Absorption bands blue shifted by 0.04 eV when doped at 1200 °C giving rise to shallow acceptor level. NaZn band movements as a function of doping temperature are also seen in Photoluminescence emission (PL), Photoluminescence excitation (PLE) and UV-Vis transmission measurements. Variable temperature Hall measurements show stable p-type conduction with hole binding energy ~0.18 eV in ZnO samples that were Na-doped at 1200 °C.

Hydrogen-related excitons and their excited-state transitions in ZnO

The role of hydrogen in the photoluminescence (PL) of ZnO was investigated using four different types of bulk ZnO single crystal, with varying concentrations of unintentional hydrogen donor and Group I acceptor impurities. Photoluminescence spectra were measured at 3 K, with emission energies determined to \ifmmode\pm\else\textpm\fi{}50 \ensuremath{\mu}eV, before and after separate annealing in ${\mathrm{O}}_{2}$, ${\mathrm{N}}_{2}$, and ${\mathrm{H}}_{2}$ atmospheres. Using this approach, several new hydrogen-related neutral-donor-bound excitons, and their corresponding B exciton, ionized donor, and two electron satellite (TES) excited state transitions were identified and their properties further investigated using hydrogen and deuterium ion implantation. The commonly observed ${I}_{4}$ (3.36272 eV) emission due to excitons bound to multicoordinated hydrogen inside an oxygen vacancy (${\mathrm{H}}_{\mathrm{O}}$), that is present in most ZnO material, was noticeably absent in hydrothermally grown (HT) ZnO and instead was replaced by a doublet of two closely lying recombination lines ${I}_{4b,c}$ (3.36219, 3.36237 eV) due to a hydrogen-related donor with a binding energy (${E}_{D}$) of 47.7 meV. A new and usually dominant recombination line ${I}_{6\text{\ensuremath{-}}H}$ (3.36085 eV) due to a different hydrogen-related defect complex with an ${E}_{D}$ of 49.5 meV was also identified in HT ZnO. Here, ${I}_{4b,c}$ and ${I}_{6\text{\ensuremath{-}}H}$ were stable up to approximately 400 and 600 \ifmmode^\circ\else\textdegree\fi{}C, respectively, indicating that they are likely to contribute to the unintentional $n$-type conductivity of ZnO. Another doublet ${I}_{5}$ (3.36137, 3.36148 eV) was identified in hydrogenated HT ZnO single crystals with low Li concentrations, and this was associated with a defect complex with an ${E}_{D}$ of 49.1 meV. A broad near band edge (NBE) emission centered at 3.366 eV was associated with excitons bound to subsurface hydrogen. We further demonstrate that hydrogen incorporates on different lattice sites for different annealing conditions and show that the new features ${I}_{4b,c}$, ${I}_{6\text{\ensuremath{-}}H}$, and ${I}_{5}$ most likely originate from the lithium-hydrogen defect complexes $\mathrm{L}{\mathrm{i}}_{\mathrm{Zn}}\text{\ensuremath{-}}{\mathrm{H}}_{\mathrm{O}}$, $\mathrm{A}{\mathrm{l}}_{\mathrm{Zn}}\text{\ensuremath{-}}{\mathrm{H}}_{\mathrm{O}}\text{\ensuremath{-}}\mathrm{L}{\mathrm{i}}_{\mathrm{Zn}}$, and ${\mathrm{V}}_{\mathrm{Zn}}\text{\ensuremath{-}}{\mathrm{H}}_{3,4}$, respectively.

UV and IR emission intensity in ZnO films, nanorods, and bulk single crystals doped with Er and additionally introduced impurities

For ZnO films, nanorods, and bulk single crystals doped with Er+ ions, it is shown that the effect of codopants introduced into the cation and ion sublattices and the observation of a high-intensity emission band at the wavelength λmax = 1535 nm are defined by the local environment of the Er+ ion. Doping of the films and single crystals with Er+ ions by diffusion brings about an infrared (IR) emission band with a low intensity because of an inadequate concentration of impurity ions. The emission intensity of this band can be raised by introducing additional Ag, Au, or N+ impurities into the ZnO films. The UV-emission intensity of the Er-doped films and single crystals at λmax = 368–372 nm is identical to that of the undoped films. ZnO nanorods doped with Er only or together with Al or Ga codopants exhibit only one IR band (at λmax = 1535 nm), whose intensity decreases upon the introduction of codopants. Doping of the nanorods with the N+ gaseous impurity during growth (930 < T < 960°C) and then with the Er+ impurity by diffusion does not yield a substantial increase in the IR-emission intensity compared to the that of the corresponding band for nanorods not doped with the N+ impurity. In the Er-doped nanorods, whose photoluminescence spectra exhibit a high-intensity band at λmax = 1535 nm, the UV emission band at λmax = 372 nm is practically lacking.

Green photoluminescence in ZnO crystals: a combined study using positron annihilation, photoluminescence, and hall measurements

The 510–525 nm green emission in melt grown Cermet bulk ZnO single crystals is investigated. It was observed that the green emission intensity is strongly influenced by the presence of zinc vacancies. Green emission intensity was found to decrease by more than two orders of magnitude by reducing zinc vacancy concentration upon annealing in cadmium metal environment. Positron annihilation spectroscopy, photoluminescence and hall measurements support this argument.

Hydrogen interstitial in H-ion implanted ZnO bulk single crystals: Evaluation by elastic recoil detection analysis and electron paramagnetic resonance

The origins of low resistivity in H ion-implanted ZnO bulk single crystals are evaluated by elastic recoil detection analysis (ERDA), electron paramagnetic resonance (EPR), and Van der Pauw methods. The H-ion implantation (peak concentration: 5.0×1015cm−2) into ZnO is performed using a 500keV implanter. The maximum of the concentration of the implanted H estimated by a TRIM simulation is at 3600nm in depth. The resistivity decreases from ∼103Ωcm for un implanted ZnO to 6.5Ωcm for as-implanted, 2.3×10−1Ωcm for 200°C annealed, and 3.2×10−1Ωcm for 400°C annealed samples. The ERDA measurements can evaluate the concentration of hydrogens which move to the vicinity of the surface (surface to 300nm or 100nm) because of the diffusion by the annealing at 200°C and 400°C. The hydrogen concentration near the surface estimated using the 2.0MeV helium beam is ∼3.8×1013cm−2 for annealed samples. From EPR measurements, the oxygen vacancy of +charge state (Vo+) is observed in as-implanted samples. The Vo+ related signal (g=1.96) observed under no illumination disappears after successive illumination with a red LED and appears again with a blue light illumination. The activation energy of as-implanted, 200°C annealed, and 400°C annealed samples estimated from the temperature dependence of carrier concentration lies between 29meV and 23meV, suggesting the existence of H interstitial as a shallow donor level.

Gamma-ray irradiation effects on the optical properties of bulk ZnO single crystals

Hydrothermal-grown bulk ZnO single crystals are investigated before and after gamma-ray irradiation. The irradiation does not alter the optical transparency in the visible region. The gamma rays only induce modified near-band-edge UV emission with blue-shifted peaks and shortened response times. From the initial values before irradiation, the peaks shift by 5 to 6 nm, and the response times shorten by 140 to 440 ps. We attribute these observations to the radiation-induced defects on the bulk crystals. Our results nevertheless lead to the realization of short-wavelength ZnO scintillators that can be utilized in high-energy-radiation environments.

Potassium acceptor doping of ZnO crystals

ZnO bulk single crystals were doped with potassium by diffusion at 950°C. Positron annihilation spectroscopy confirms the filling of zinc vacancies and a different trapping center for positrons. Secondary ion mass spectroscopy measurements show the diffusion of potassium up to 10 μm with concentration ∼1 × 1016 cm−3. IR measurements show a local vibrational mode (LVM) at 3226 cm−1, at a temperature of 9 K, in a potassium doped sample that was subsequently hydrogenated. The LVM is attributed to an O–H bond-stretching mode adjacent to a potassium acceptor. When deuterium substitutes for hydrogen, a peak is observed at 2378 cm−1. The O-H peak is much broader than the O-D peak, perhaps due to an unusually low vibrational lifetime. The isotopic frequency ratio is similar to values found in other hydrogen complexes. Potassium doping increases the resistivity up to 3 orders of magnitude at room temperature. The doped sample has a donor level at 0.30 eV.

Sodium doping in ZnO crystals

ZnO bulk single crystals were doped with sodium by thermal diffusion. Positron annihilations spectroscopy confirms the filling of zinc vacancies, to &amp;gt;6 μm deep in the bulk. Secondary-ion mass spectrometry measurement shows the diffusion of sodium up to 8 μm with concentration (1–3.5) × 1017 cm−3. Broad photoluminescence excitation peak at 3.1 eV, with onset appearance at 3.15 eV in Na:ZnO, is attributed to an electronic transition from a NaZn level at ∼(220–270) meV to the conduction band. Resistivity in Na doped ZnO crystals increases up to (4–5) orders of magnitude at room temperature.

Structural peculiarities and point defects of bulk-ZnO single crystals

ZnO single crystals are related to promising direct wide band gap semiconductor materials belonging to the AIIBVI type of compounds with wurtzite structure. “Unintentional” n-type conductivity in ZnO may be caused by zinc and oxygen vacancies, and interstitial zinc atoms. To date, the comprehensive structural investigation and analysis of point defects in ZnO is absent in literature. Green, light green and almost colorless ZnO single crystals grown by the hydrothermal method in concentrated alkali solutions 4М(КОН)+1М(LiOH)+0.1M(NH4OH) on monohedral seeds [0001] at crystallization temperatures in the range of 330–350°C and pressures in the range of 30–50MPa have been firstly investigated by neutron diffraction. It was revealed the presence of additional reflections (∼12–∼16%) for all the crystals caused by kinetic growth effects that give grounds to assign them to the space group P3 rather than to P63mc. Analysis of the refined compositions together with the color of ZnO crystals does not rule out the relationship between the color and vacancies in the zinc and oxygen positions whose concentration decreases with the discoloration of the samples. The analysis of the photoinduced variation of the total and on-axis transmittance versus the CW laser intensity showed that the colored samples have profound deep defects related to oxygen vacancies.

Green emission in carbon doped ZnO films

The emission behavior of C-doped ZnO films, which were prepared by implantation of carbon into ZnO films, is investigated. Orange/red emission is observed for the films with the thickness of 60–100 nm. However, the film with thickness of 200 nm shows strong green emission. Further investigations by annealing bulk ZnO single crystals under different environments, i.e. Ar, Zn or C vapor, indicated that the complex defects based on Zn interstitials are responsible for the strong green emission. The existence of complex defects was confirmed by electron spin resonance (ESR) and low temperature photoluminescence (PL) measurement.