ZnO Bulk Single Crystals Research Articles

Enhancement of band-edge photoluminescence of bulk ZnO single crystals coated with alkali halide

We have studied coating effects of bulk ZnO crystal by means of photoluminescence (PL) measurements. Bulk crystal surface of ZnO is once covered by the melt of alkali halide at high temperature and is coated with thin alkali halide crystal. At low temperature, band-edge PL intensity of the ZnO crystal coated with KCl or KI is about 300 times larger than that of a polished crystal. In the no-coating ZnO crystal treated by the same thermal conditions, the enhancement of the PL intensity is only about 20 times larger. The enhancement of band-edge PL also occurs considerably at room temperature. The experimental results including the PL spectra in the visible light region, suggesting the qualitative improvement of ZnO surface by the coating, are discussed in terms of the incorporation of halogen ions supplied from alkali halide into oxygen vacancies.

Ferromagnetism in Mn- and Co-implanted ZnO nanorods

ZnO nanorods with diameters of 15–30 nm were grown on Ag-coated Si substrates by catalyst-driven molecular beam epitaxy and then implanted with Mn+ or Co+ ions to doses of 1–5×1016 cm−2. After subsequent annealing at 700 °C for 5 min, the structural properties of the nanorods were unaffected, but they exhibited ferromagnetism that persisted to temperatures of 225–300 K. The coercive fields were ⩽100 Oe even at 10 K. The results are similar to those obtained for implantation of Mn+ or Co+ ions in bulk single-crystal ZnO and indicate promise for nanorods for nanoscale spintronic applications.

Brillouin scattering study of ZnO

Polarized Brillouin scattering measurements were carried out for a bulk ZnO single crystal. The whole set of elastic stiffness constants was determined to be c11=19.0, c12=11.0, c13=9.0, c33=19.6, c44=3.9, and c66=4.0 in units of 1011 dyn/cm2. The relationships between photoelastic constants at wavelength of 514.5 nm were also obtained: p11, p13, p44, and p66=1.8, 2.3, 0.50, and 0.38, respectively, relative to p12.

Polarized photoreflectance spectra of excitonic polaritons in a ZnO single crystal

Exciton–polariton structures in a high-quality bulk ZnO single crystal were resolved at 8 K by means of polarized photoreflectance (PR) and photoluminescence measurements. The energies of the PR resonances corresponded to those of the upper and lower exciton–polariton branches, where A, B, and C excitons couple simultaneously to an electromagnetic wave. Longitudinal-transverse splitting of ground-state exciton polaritons and resonances due to the first excited states of respective excitons were observed due to the large oscillator strength. Photoluminescence peaks due to the corresponding polarion branches were clearly resolved. The valence-band ordering was confirmed to be A-Γ9v, B-Γ7vu, and C-Γ7vl.

Visible emission from ZnO doped with rare-earth ions

We report the results of a cathodoluminescence (CL) and photoluminescence (PL) study of ZnO-bulk single crystals and epilayer thin-film samples grown on a sapphire (0001) substrate and doped by implantation with rare-earth ions (RE3+): Pr3+ , Dy3+ , Ho3+ , Er3+ , Tm3+ (bulk crystals, co-doped with Li), Sm3+, Dy3+, and Er3+ (epilayers). The PL and PL excitation (PLE) spectra of polycrystalline ZnO doped with RE3+ ions (Nd3+, Dy3+, Er3+, and Tm3+) and co-doped with Li+, Cl-, and N- ions have also been studied.

Mechanical deformation of single-crystal ZnO

The deformation behavior of bulk ZnO single crystals is studied by a combination of spherical nanoindentation and atomic force microscopy. Results show that ZnO exhibits plastic deformation for relatively low loads (≳4–13 mN with an ∼4.2 μm radius spherical indenter). Interestingly, the elastic–plastic deformation transition threshold depends on the loading rate, with faster loading resulting, on average, in larger threshold values. Multiple discontinuities (so called “pop-in” events) in force–displacement curves are observed during indentation loading. No discontinuities are observed on unloading. Slip is identified as the major mode of plastic deformation in ZnO, and pop-in events are attributed to the initiation of slip. An analysis of partial load–unload data reveals values of the hardness and Young’s modulus of 5.0±0.1 and 111.2±4.7 GPa, respectively, for a plastic penetration depth of 300 nm. Physical processes determining deformation behavior of ZnO are discussed.

Cathodoluminescence and scanning tunnelling spectroscopy of ZnO single crystals

Bulk ZnO single crystals grown by the hydrothermal (HTT) and alkali flux methods have been investigated by means of scanning tunnelling spectroscopy (STS) and time resolved cathodoluminescence (CL). Measurements were performed in the different crystalline faces. The results from these measurements show that both, surface electrical properties and luminescent characteristics depend on the face studied. Polar O-terminated surfaces show an intrinsic conduction behaviour with a surface band gap ranging from 0.4 to 0.8 eV. Zn-terminated surfaces show mainly n-type conduction. The non-polar faces present either intrinsic or p-type behaviour. CL spectra show that the relative intensity of the different components of the deep level band also depends on the atomic structure of the face under study. This complex behaviour is clearly revealed from the time resolved spectra. The differences observed are attributed to the nature of the defects present in each case and, in particular, to different impurity incorporation processes that could be mainly controlled by the atomic configuration and polarity of the planes.

Magnetic resonance studies of ZnO

We have used EPR and ODMR to study state-of-the-art bulk ZnO single crystals. Most of the samples are n-type; however, under certain conditions (e-irradiated or annealed), we observe a center due to residual nitrogen ( g ∥=1.9953, g ⊥=1.9633 and A iso=1.225 mT, A aniso=0.864 mT). The N center is a nitrogen p-orbital along the c-axis, very similar to P or As in ZnSe, which have a Jahn-Teller distorted, deep state, seen in EPR. The intensity of the N center is strongly anisotropic, as would be expected for shallow heavy-hole acceptors with m J=± 3 2 in which the magnetic dipole transition is forbidden for B ∥ c and becomes allowed away from that orientation due to mixing with m J=± 1 2 states. The EPR spectra for the n-type samples are dominated by delocalized donors ( g ∥=1.9570 and g ⊥=1.9551). In one sample, the donor appears to be localized. These crystals also exhibit a broad PL band at 2.5 eV. ODMR on the 2.5 eV “green” PL band reveals an S=1 triplet center (not seen in EPR) with spin-Hamiltonian parameters of D=0.763 GHz, g ∥=1.9710, and g ⊥=2.0224. ODMR of shallow donors is also observed as a luminescence-decreasing feature on the 2.5 eV green emission band.

Electrochemical schottky characteristics of ZnO for capacitance-voltage measurements

Water-based Schottky contacts are examined for flatband potential on bulk single-crystal ZnO material. The Zn and O faces are studied as well as the a-axis. Also, depletion capacitance-voltage (CV) measurements are made on a bulk ZnO sample that has been exposed to extensive electron irradiation. The CV measurements show that the ZnO (O-face) surface is only slightly affected by electron irradiation up to 2 MeV.

Cathodoluminescence microscopy of hydrothermal and flux grown ZnOsingle crystals

Bulk ZnO single crystals grown by the hydrothermal and flux methodshave been characterized by steady-state and time resolved cathodoluminescencemeasurements performed on the different crystalline faces. A shift of thepeak near band edge towards lower energies is observed in spectrarecorded with increasing delay times. This behaviour is often observed in theetch pit regions in alkali flux grown crystals, which suggests the presence ofa band related to dislocations or to the point defects surrounding thedislocations. In the low-energy region, cathodoluminescence spectra show thatthe relative intensity of the different components of the deep level band alsodepends on the atomic structure of the face under study. This complexbehaviour is clearly revealed from the time resolved spectra. The differencesobserved are attributed to the nature of the defects present in each face and,in particular, to different impurity incorporation processes that could bemainly controlled by the atomic configuration and polarity of the planes.

Scanning tunnelling spectroscopycharacterization of ZnO single crystals

Bulk ZnO single crystals grown by the hydrothermal andflux methods have been characterized by scanning tunnellingspectroscopy performed in the different crystalline faces.Normalized differential conductance has been found to depend onthe face considered. Polar O-terminated surfaces show anintrinsic conduction behaviour and surface bandgap in the range0.4-0.8 eV, which depends on the position probed. The Znpolar surfaces show mainly n-type conduction. The non-polar mregions present either intrinsic or p-type behaviour. Thedifferences observed are attributed to the nature of impuritiesand defects appearing in the polar and non-polar surfaces duringcrystal growth.

Structural, optical, and surface acoustic wave properties of epitaxial ZnO films grown on (011̄2) sapphire by metalorganic chemical vapor deposition

High-quality ZnO films are receiving increased interest for use in low-loss high-frequency surface acoustic wave (SAW) devices, acousto-optic and optical modulators, as buffer layers for III-nitride growth, and as the active material in ultraviolet solid state lasers. In this work, high quality epitaxial ZnO films were grown on R-plane sapphire substrates by metalorganic chemical vapor deposition. The structural, piezoelectric, and optical properties of the ZnO films on R sapphire have been investigated. The epitaxial relationship between ZnO and R-Al2O3 was found to be (112̄0) ZnO∥(011̄2) Al2O3, and [0001] ZnO∥[01̄11] Al2O3. The interface between as-grown ZnO and R sapphire was atomically sharp and semicoherent, as evaluated by transmission electron microscopy. On annealing the films at temperatures above 850 °C, a solid state reaction occurred between ZnO and Al2O3, resulting in the formation of ZnAl2O4 (spinel) at the interface. A 15–20 nm spinel layer formed when the ZnO film was annealed at 850 °C for 30 min, whereas a 150 nm layer formed when the film was annealed at 1000 °C for 150 min. To prevent this reaction from occurring, the maximum process temperature should be below 750 °C. The surface acoustic wave properties of the piezoelectric ZnO were evaluated by fabricating SAW devices on (112̄0) ZnO/(011̄2) Al2O3. An effective electromechanical coupling coefficient, keff2, of 6% was achieved for a 1.5 μm thick ZnO film, which is close to the value for bulk single-crystal ZnO. The photoluminescence spectra were obtained both at room temperature and at 11 K. The full width at half maximum of the 3.363 eV band edge emission photoluminescence peak measured at 11 K was 6 meV, which is close to that for single-crystal ZnO. We also evaluated the anisotropic absorption characteristics of the (112̄0) ZnO film, which can be used for a high contrast ultraviolet light modulator.