High-quality ZnO Single Crystals Research Articles

Understanding the Sensing Mechanism of ZnO Nanoparticles through Identifying Intrinsic Defects

Progress in crystal growth made ZnO applicable as the next generation of inorganic light-emitting diodes and lasers, which has attracted much attention. Our previous work is a proof that Zn vacancies are occupied by three protons to form VZnH3 complexes in high-quality ZnO single crystals. In this work, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) studies on high-quality ZnO single crystals further confirm that three proton clusters or cations with three positive charges are the intrinsic defects and/or impurities related to doping in the host-lattice of high-quality ZnO single crystals. Unintentionally doped hydrogen (H) clusters might be relating to ZnO’s sensing capabilities. The sensor response of ZnO semiconductors is promoted by a decreasing size; however, the nature of the size effects has not been elucidated satisfactorily. In this work, broccoli-like ZnO nanoarrays, which can detect H2S gas in the parts per billion (ppb) region, has been synthesized under a hydrothermal method. High-resolution transmission electron microscopy (HRTEM) experimental results demonstrate that there are multiple polar faces exposed on the surfaces of ZnO nano crystals. The high catalytic active of oxygen species adsorbed on the multiple polar surfaces should respond to the high sensing performance and the high selectivity of ZnO toward reducing H2S gas. However, the enormous variations in electric current cannot be produced by releasing electrical charges from O– adsorbates into semiconductors. EPR results show that Zn+ interstitials have been produced after the reaction between H2S and O– adsorbates on ZnO. In contrast to the unintentionally doped hydrogen (H) clusters, our experimental results show that the Zn+ interstitials acting as shallow donors is what really offer the extra free-electron carriers.

Crystal growth, finite element analysis and oxygen annealing of ZnO bulk crystal grown by low-temperature-gradient chemical vapor transport

The growth of high-quality ZnO bulk crystals is an outstanding challenge, and the nature of grown-in defects in crystals is far from well understood. In this work, a specially designed low-temperature-gradient CVT growth system was developed based on finite element analysis. High-quality ZnO single crystals were successfully grown. The supersaturation front in the growth system was controlled, and internal stress in the crystal was reduced by introducing a radiation baffle and a special seed pedestal. The structural, electrical, and optical properties of ZnO crystals were studied before and after oxygen annealing. The scanning electron microscope (SEM) and optical microscope (OM) results indicate the stable growth is achieved under the Zn-rich growth condition. The X-ray rocking curve measurements reveal the high crystalline quality of the grown crystal with the full-width at half-maximum (FWHM) value of 17 arcsec for the (004) reflection. The time-of-flight secondary ion mass spectroscopy (ToF-SIMS) results prove the high purity of grown crystals and reveal the variations of trace impurity concentrations in crystals. When annealed in oxygen at 1000 °C, the optical band gap (Eg) changes from 2.50 eV to 3.15 eV. The Raman analysis indicates that oxygen vacancies (Vo) in the as-grown crystal disappear after annealing. The Hall measurements show that the room temperature (RT) mobilities before and after annealing are 197 cm2V−1c−1 and 207 cm2V−1c−1, respectively. The RT carrier concentration decreases by about two orders of magnitude to 5.45 × 1015 cm−3, while the RT resistivity increases by about one order of magnitude to 5.57 Ω cm after annealing. The positron annihilation lifetimes (PALS) results prove the presence of Zn vacancies (Vzn) after annealing. The origins of as-grown ZnO green luminescence (GL) and the temperature-dependent blue shifts of the GL peak positions are explained based on Zni and Vo defects. The fine structures of annealed ZnO GL are found to be related to Vzn native defects rather than Cu impurity.

Who make transparent ZnO colorful? – Ion implantation and thermal annealing effects

ZnO has re-attracted considerable interest as a wide band gap semiconductor and energy material in recent years. When most of the near-band-edge exciton sharp lines in the ultraviolet spectral region have been firmly identified, defect origins of broad color emissions are still open to debate. By implanting Cu+ and Zn+ ions into high quality ZnO single crystal, respectively, and investigating their photoluminescence spectra at different temperatures, we firmly show that the structured green emission band is indeed associated with copper impurity. Incorporation of excess Zn ions into ZnO results in a structureless red emission band with a nearly perfect Gaussian lineshape. The pure electronic level locations of these two color emission bands were inclusively determined by using generalized multiple Brownian oscillator model when the simultaneous multiphonon emission was taken into account. The nature of the two colorful luminescent centers, including their charge states, was discussed with the aid of the latest theoretical results on different point defects and their optical transitions in ZnO.

Temperature dependent distinct coupling and dispersions of heavy- and light-hole excitonic polaritons in ZnO

Distinct coupling behavior of heavy- and light-hole excitonic polaritons in ZnO was unveiled by investigating the optical reflectance spectra of a high quality ZnO single crystal as a function of temperature both experimentally and theoretically. A resonance like coupling region was found at a temperature of around 50 K at which several relevant physical quantities such as the transverse exciton transition energy, polarizability, and damping parameters of the two kinds of excitonic polaritons were revealed to overturn. Calculated dispersions correctly reflect the nature of coupled photon and exciton and reproduce the spectral structures of the interacting polaritons.

Formation Dynamics of Excitons and Temporal Behaviors of Fano Resonance Due to the Exciton–Impurity–Phonon Configuration Interaction in ZnO

Formation dynamics of free and neutral donor bound excitons (FX and D(0)X) in a high quality ZnO single crystal are studied by means of time-resolved photoluminescence (TRPL) at various temperatures. At low-temperatures, FX and D(0)X formation times are determined to be ~5 and ~10 ps, respectively, by fitting the rise process with the Boltzmann sigmoidal function. Temporal information of FX- and D(0)X-longitudinal optical (LO) phonon coupling is also acquired by measuring TRPL spectra of the first-order LO phonon-assisted FX and D(0)X transitions. In particular, interesting time evolution of luminescence intensity in the Fano resonance region due to the configuration interaction of exciton-impurity-phonon is explored.

Ion-implantation induced nano distortion layer and its influence on nonlinear optical properties of ZnO single crystals

Second harmonic generation (SHG) and X-ray diffraction rocking curves of high-quality ZnO single crystals implanted by different ions (He, Cu, and Zn) were investigated. Interestingly, it was found that both He- and Zn-implanted samples show a convinced increment in SHG efficiency while the Cu-implanted one does not. X-ray diffraction rocking curves of the samples show satellite structures, and the simulations firmly reveal the formation of quasi-interfaces inside He- and Zn-implanted crystals. These quasi-interfaces lead to SHG improvement in the two samples. Polarization dependence of SHG of the samples on the excitation light also evidences this conclusion.

Nanoindentation and photoluminescence characterization of ZnO thin films and single crystals

In this work an optical and micromechanical properties of ZnO films deposited on glass by simple method based on the mechanoactivated oxidation have been investigated and compared with those of bulk ZnO single crystal and commercial ZnO films. Results showed that investigated ZnO films have stable high adhesion with glass and can form grained structure with hardness 8–10 GPa or whiskers nanostructure with high hardness 18–20 GPa. Young’s modulus is in range from 80 to 120 GPa. Optical properties of obtained ZnO films with whiskers structure are typical for transparent high quality ZnO single crystal. These coatings also exhibit a photo-electric response which reflects on the change in resistivity.

Nanocrystals of ZnO formed by the hot isostatic pressure method

We present here a novel and simple physical method for obtaining ZnO nanocrystals, using the pressure cycle method and the membrane diamond anvil cell (MDAC) up to 13 GPa at 500 K. The recovered nanocrystals in the wurtzitic ZnO phase (w-ZnO) were characterized by the high-resolution Raman scattering and X-ray diffraction. A single crystal of w-ZnO was used in this work. The high-pressure cell was a MDAC specifically designed for high-temperature studies. The Raman spectra of the high-quality ZnO single crystal and nanocrystals were compared, and we observed both a softening and an asymmetric broadening of the Raman peaks, which are in good agreement with the effect of relaxation of the q-vector selection rule due to quantum size confinement effects. The experimental results confirm the existence of ZnO nanocrystals in the w-ZnO phase with an average diameter of 17 nm.

Hydrothermal Growth of ZnO Single Crystals with High Carrier Mobility

Thirty millimeter ZnO single crystals have been grown by the hydrothermal method with new mineralizers and a low-cost liner. A sharp X-ray rocking curve with the full width at half-maximum of 36 arcsec has been obtained for the (002) reflection, indicating a good crystallinity of the crystal. Low temperature photoluminescence spectrum revealed the crystal has a narrow and strong free exciton emission band (3.359 eV) at 10 K, with the absence of a green-yellow emission band that generally is induced by the impurities or lattice vacancies. Compared to ZnO crystals hydrothermally grown from conventional mineralizers (LiOH and KOH), this crystal has a unique feature that the room-temperature carrier mobility is close to the intrinsic value, with the carrier concentration maintained as high as 4.09 × 1016 cm−3. Two main donors which may relate to the H impurity were indicated. Analysis revealed that the diffusion of H in the ZnO lattice might be responsible for the decrease in conductivity of the ZnO crystal after annealing. The reason why the new hydrothermal method yielded low-resistance ZnO crystals could be interpreted as a much higher concentration of H impurity and much lower concentration of Li impurity incorporated into the ZnO lattice, compared with samples yielded by the conventional hydrothermal method. We anticipate the reported high-quality ZnO single crystals can not only be suitable objects for studying the intrinsic properties of ZnO, but also be potential substrates for fabricating ZnO light emitting diodes devices.

Growth of high-quality ZnO single crystals by seeded CVT using the newly designed ampoule

The high-quality ZnO single crystals were grown by seeded chemical vapor transport (CVT) in a newly designed ampoule using carbon as a transport agent. The well-faceted crystal of about 5×5×5 mm 3 can be grown reproducibly. Secondary ion mass spectroscopy (SIMS) analysis, X-ray rocking curve (XRC) and photoluminescence (PL) measurements demonstrate that the grown single crystal is of high purity and high crystallinity.

NANOMECHANICAL PROPERTIES AND POSSIBLE APPLICATIONS OF MECHANOACTIVATED ZNO COATINGS

NANOMECHANICAL PROPERTIES AND POSSIBLE APPLICATIONS OF MECHANOACTIVATED ZNO COATINGSA simple and cost-effective mechanoactivated oxidation method (MAOM) has been developed for producing ZnO coatings on glass. To achieve the transparency of initially opaque mechanoactivated ZnO coatings they were annealed at the temperatures modifying the surface morphology of nanostructured coatings. Investigation of nanomechanical properties has revealed that MAOM coatings in as-obtained state have higher hardness (up to 3 GPa) as compared with bulk metallic zinc (160 MPa). In the annealed state the coatings show stabily high adhesion with glass, with the ability to form a grained structure (hardness 8-10 GPa) or a whiskers nanostructure (hardness 18-20 GPa). The Young modulus of coatings ranged from 80 to 120 GPa. The nanomechanical and optical properties were compared with those of bulk ZnO single crystal and commercial ZnO films. The optical properties of ZnO films with a whiskers structure are typical of a transparent high-quality ZnO single crystal. The coatings obtained have high electrical resistance, which decreases in response to the UV exposure. The photocurrent increases by two orders of magnitude, which is easily detectable and makes it possible to use such coatings as UV emission detectors.

Comparative characterization of differently grown ZnO single crystals by positron annihilation and Hall effect

Abstract High-quality ZnO single crystals of dimensions 10×10×0.3 mm 3 , grown either using a pressurized melt or a hydrothermal growth approach, have been investigated in their as-received state and are compared regarding their properties revealed by positron annihilation and Hall effect measurements. By positron annihilation performed at room temperature it is found that the pressurized melt grown crystals contain a certain amount of Zn+O divacancies, but no Zn vacancies are detected, whereas the hydrothermally grown crystals contain a dominating defect yet unknown in its structure but possibly connected to the Zn vacancy. Furthermore, the influence of an additional refined chemical–mechanical polishing of the crystal surface by a special procedure on the depth distribution of vacancy-type defects is demonstrated. Hall measurements, performed in the temperature range 20–325 K, showed that the crystal growth method has a strong influence on the carrier mobility, and the estimated acceptor densities also differ significantly in both types of crystal.

Photoluminescence studies of high-quality ZnO single crystals by hydrothermal method

Photoluminescence and reflectivity spectral measurement have been carried out between liquid helium (4.2 K) and room temperature to evaluate the high-quality ZnO wafers grown by hydrothermal method. The + c, − c and m surface of the ZnO wafer, illuminated by 325 nm ultraviolet light at 4.2 K, emitted the visible luminescence of pale green, pale yellow and pale green, respectively. The broad and weak emissions appeared at 2.43 eV in the + c and m surface, and at 2.29 eV in the − c surface. These peaks were caused by the cation atoms incorporated into the ZnO crystal during the crystal growth. The excitation density dependence of the PL spectra was also investigated, and it was found that no donor–acceptor pair emissions included in the PL spectra of the + c, − c and m surface. The thermal quenching process of the neutral donor-bound and free exciton emissions were observed in the temperature range of 13–300 K, and then the thermal activation energy of A-free exciton was estimated to be 63 and 66 meV in the + c and m surface, respectively.