Zn-polar Face Research Articles

Enhancing Photoluminescence Spectra for Doped ZnO Using Neutron Irradiation.

This study investigates the effect of neutron irradiation on zinc oxide (ZnO) crystal bulk substrates that have been ion-implanted with gadolinium (Gd) and iron (Fe) in different concentrations in the Zn-polar (0001) face of the crystal to produce Gd:ZnO and Fe:ZnO crystal bulk samples. The neutron beams of the 241Am-Be flux were moderated to be 7 × 104 n/s cm2 by a 4.5 cm-thick wax. The implanted face was left to face the neutron beams on top of the wax for 90 h. The results show that the nuclear reaction between thermal neutrons and the implanted material produced a large amount of heat, which significantly enhanced the photoluminescence spectra of the crystal bulk surface after the surface damage caused by ion implantation. This study provides insights into the benefits of having a doped material in the ZnO crystal after irradiation and highlights the potential of neutron irradiation as a method for enhancing the properties of implanted crystal bulk substrates.

Neutron Irradiation to Transmute Zinc into Gallium.

Modified 241Am-Be neutron beams showed an ability to change the optical properties of zinc oxide (ZnO) photoluminescence (PL) spectra by transmuting zinc (Zn) into gallium (Ga) after irradiation. This study investigates the time required by slow neutron irradiation to register the transmutation of the Zn into Ga. Two series of samples from different suppliers hydrothermally (HT) grown by TEW Tokyo Denpa Co. Ltd., Tokyo, Japan, and MTI corporation, China, are irradiated for 6, 12, 18, and 24 days on the Zn-polar face of each sample to specify the relationship between the irradiation intensity and transmutation.

Tailorable properties of Nd-doped ZnO epitaxial thin films for optoelectronic and plasmonic devices

Transparent conductive oxides with both low resistivity and high transparency in the visible and near-infrared wavelengths are required for solar cells and plasmonic applications. In this work we demonstrate that highly conducting and transparent Nd:ZnO thin films have been grown on c-cut sapphire single crystal substrates by pulsed electron beam deposition under argon (10−2 mbar). At 2% at. Nd doping, these films crystallize in the wurtzite structure at T > 300 °C, with well-defined epitaxial relationships with the c-cut sapphire as a function of the substrate temperature. The improvement of the film structural quality with increasing growth temperature led to the lowest resistivity value of 5.12 × 10−4 Ωcm and highest transparency that were maintained more than 36 months under ambient conditions. X-ray photoelectron valence band measurements revealed that the dominant Zn-polar face of Nd:ZnO thin films explains the improved electrical properties. The dielectric functions of films were determined and show that these films are suitable as transparent contact electrodes for solar cell devices or tunable epsilon-near-zero materials for plasmonics.

Focused ion beam assisted prototyping of graphene/ZnO devices on Zn-polar and O-polar faces of ZnO bulk crystals

We demonstrate an experimental approach for prototyping heterojunctions formed between graphene and bulk semiconductor substrates. This approach employs focused ion beam milling to fabricate microscale area heterojunctions and in-situ electrical measurements in the chamber of the scanning electron microscope to measure their electrical characteristics. The aim is to limit the impact of defects in graphene on the electrical characteristics of the junctions. The approach is demonstrated on graphene/ZnO structures with different polar faces. On these structures, theoretical predictions pointing to differences in charge transport are experimentally validated.

Microwave synthesis of ZnO microcrystals with novel asymmetric morphology

This paper reports a discovery of novel morphological features of ZnO asymmetric micron-sized single crystals synthesized by microwave irradiation of W/O emulsions containing a Zn source in the inner aqueous phase. The influence of the zinc ion concentration on the size and shape is studied in detail. The morphological and structural properties show that the obtained ZnO powders are almost mono-dispersed ZnO single crystals with a novel morphology of truncated hexagonal pyramids ca. 1 μm in diameter. A scanning precession electron diffraction analysis reveals that the asymmetrically shaped ZnO single crystals thus obtained have a crystallographic c-axis perpendicular to the bottom hexagonal flat surfaces, and annular bright-field imaging indicates that the bottom surfaces are Zn-terminated ZnO(0001)-Zn polar faces. These new findings are led from a combination of well-controllable liquid-phase emulsion synthesis of the oxide powders and highly advanced atomic-resolution TEM imaging technique, suggesting for the first time a highly probable relationship between the polarity of the wurtzite-type crystal structure and the novel asymmetric morphology of the ZnO microcrystals obtained in this study.

Electroreduction of Aryldiazonium Ion at the Polar and Non-Polar Faces of ZnO: Characterisation of the Grafted Films and Their Influence on Near-Surface Band Bending.

ZnO is a strong candidate for transparent electronic devices due to its wide band gap and earth-abundance, yet its practical use is limited by its surface metallicity arising from a surface electron accumulation layer (SEAL). The SEAL forms by hydroxylation of the surface under normal atmospheric conditions, and is present at all crystal faces of ZnO, although with differing hydroxyl structures. Multilayer aryl films grafted from aryldiazonium salts have previously been shown to decrease the downward bending at O-polar ZnO thin films, with Zn-O-C bonds anchoring the aryl films to the substrate. Herein we show that the Zn-polar (0001), O-polar (000 ), and non-polar m-plane (10 0) faces of ZnO single crystals, can also be successfully electrografted with nitrophenyl (NP) films. In all cases, X-ray photoelectron spectroscopy (XPS) measurements reveal that the downward surface band bending decreases after modification. XPS provides strong evidence for Zn-O-C bonding at each face. Electrochemical reduction of NP films on O-polar ZnO single crystals converts the film to a mainly aminophenyl layer, although with negligible further change in band bending. This contrasts with the large upward shifts in band bending caused by X-ray induced reduction.

Thermal stability of oxidized noble metal Schottky contacts to ZnO

Thermally stable oxidized palladium (PdOx, x=1.4) and oxidized iridium (IrOx, x=1.9) Schottky contacts (SCs) that showed strong rectification at 180°C were fabricated on the Zn-polar (0001) face of hydrothermally-grown single crystal ZnO wafers, using reactive rf sputtering and reactive eclipse pulsed laser deposition, respectively. These PdOx and IrOx SCs were highly rectifying with effective barrier heights of 1.25eV and 1.02eV, respectively, more than 12 orders of magnitude of room-temperature current rectification (at biases of ±2.0V), and were almost ‘ideal’ with ideality factors of 1.03 and 1.02, respectively, that were very close to the image-force-controlled limit for laterally homogeneous contacts. The strong rectifying performance of these SCs was maintained at 180°C, with current rectification in excess of 7 and 6 orders of magnitude for the PdOx and IrOx SCs, respectively, indicating their suitability for high temperature applications. Current-voltage characteristics over the temperature range 20–180°C were used to extract Richardson constant values of 39±15 Acm−2K−2 and 9±1 Acm−2K−2 for the PdOx and IrOx SCs, respectively, close to the theoretical value of 32 Acm−2K−2.

Tuning the Band Bending and Controlling the Surface Reactivity at Polar and Nonpolar Surfaces of ZnO through Phosphonic Acid Binding.

ZnO is a prime candidate for future use in transparent electronics; however, development of practical materials requires attention to factors including control of its unusual surface band bending and surface reactivity. In this work, we have modified the O-polar (0001̅), Zn-polar (0001), and m-plane (101̅0) surfaces of ZnO with phosphonic acid (PA) derivatives and measured the effect on the surface band bending and surface sensitivity to atmospheric oxygen. Core level and valence band synchrotron X-ray photoemission spectroscopy was used to measure the surface band bending introduced by PA modifiers with substituents of opposite polarity dipole moment: octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylphosphonic acid (F13OPA). Both PAs act as surface electron donors, increasing the downward band bending and the strength of the two-dimensional surface electron accumulation layer on all of the ZnO surfaces investigated. On the O-polar (0001̅) and m-plane (101̅0) surfaces, the ODPA modifier produced the largest increase in downward band bending relative to the hydroxyl-terminated unmodified surface of 0.55 and 0.35 eV, respectively. On the Zn-polar (0001) face, the F13OPA modifier gave the largest increase (by 0.50 eV) producing a total downward band bending of 1.00 eV, representing ∼30% of the ZnO band gap. Ultraviolet (UV) photoinduced surface wettability and photoconductivity measurements demonstrated that the PA modifiers are effective at decreasing the sensitivity of the surface toward atmospheric oxygen. Modification with PA derivatives produced a large increase in the persistence of UV-induced photoconductivity and a large reduction in UV-induced changes in surface wettability.

Effect of Spontaneous Polarization Charges on the Electron Mobility in ZnO Surface Quantum Wells

We present a theoretical study of the effect due to spontaneous polarization of ZnO on the low-temperature mobility of the two-dimensional electron gas (2DEG) in a ZnO surface quantum well (SFQW). We proved that for the O-polar face this causes an attraction of electrons by the positive charges bound on the surface, while for the Zn-polar face a repulsion of them far away therefrom by the negative bound charges of the same magnitude. Accordingly, surface roughness scattering is drastically enhanced in the former case, but reduced in the latter one. Therefore, the low-% temperature 2DEG mobility in ZnO SFQWs with O-polar face is found to be dominated by surface roughness. Our theory was illustrated for the sample prepared by bombardment of the O-polar face by 100-eV hydrogen ions. The surface roughness scattering enables an explanation of the 2DEG mobility, especially, the reason of low values for the mobility in the dependence from the carrier density which has not been understood when starting from impurity scattering.

Influence of polarity and hydroxyl termination on the band bending at ZnO surfaces

Surface sensitive synchrotron x-ray photoelectron spectroscopy (XPS) and real-time in situ XPS were used to study the thermal stability of the hydroxyl termination and downward band bending on the polar surfaces of ZnO single crystals. On the O-polar face, the position of the Fermi level could be reversibly cycled between the conduction band and the band gap over an energetic distance of approximately 0.8 eV (\ensuremath{\sim}1/4 of the band gap) by controlling the surface H coverage using simple ultrahigh vacuum (UHV) heat treatments up to 750 \ifmmode^\circ\else\textdegree\fi{}C, dosing with HO/H and atmospheric exposure. A metallic to semiconductorlike transition in the electronic nature of the O-polar face was observed at an H coverage of approximately 0.9 monolayers. For H coverage less than this, semiconducting (depleted) O-polar surfaces were created that were reasonably stable in UHV conditions. In contrast, the downward band bending on the Zn-polar face was significantly more resilient, and depleted surfaces could not be prepared by heat treatment alone.

Polarity effects in the optical properties of hydrothermal ZnO

Significant polarity-related differences in the near-band-edge photoluminescence from the Zn-polar and O-polar faces of hydrothermally grown ZnO single crystals, particularly in the ionized donor bound and free exciton recombination regions, were initially enhanced and then extinguished on annealing in oxygen at 400 °C and 600 °C, respectively. Polarity effects were also observed in the defect band emission with a structured green band associated with deep copper acceptor impurities appearing at lower annealing temperatures on the O-polar face. The loss of hydrogen is implicated in both these behaviors and in a sudden semiconductor-to-insulator transition between 200–300 °C.

Current-Voltage and Capacitance-Voltage Characteristics of Pd Schottky Diodes Fabricated on ZnO Grown along Zn- and O-Faces

Temperature dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements were utilized to understand the transport mechanism of Pd Schottky diodes fabricated on Zn- and O-faces of ZnO. From I-V measurements, in accordance with the thermionic emission mechanism theory, it was found that the series resistance Rsand the ideality factor n were strongly temperature dependent that decreased with increasing temperature for both the faces (Zn and O-face) of ZnO revealing that the thermionic emission is not the dominant process. The barrier height øB(I-V)increased with increasing temperature for both faces. The measured values of ideality factor, barrier height and series resistance for Zn- and O-faces at room temperature were 4.4, 0.60 eV, 217 Ω and 2.8, 0.49 eV, 251 Ω respectively. The capacitance-voltage (C–V) measurements were used to determine the doping concentration Nd, the built-in-potential Vbi, and the barrier height øB(C-V). The doping concentration was found to be decreased with increasing depth. The barrier height øB(C-V)calculated for O-polar and Zn-polar faces decreases with increasing temperature. The values of barrier height øB(C-V)determined from C-V measurements were found higher than the values of barrier height øB(I-V). Keeping in view the calculated values of ideality factor, barrier height, and series resistance shows that O-polar face is qualitatively better than Zn-polar face.

Naturally asymmetrical double-Schottky barrier model: Based on observation of bicrystal

ZnO bicrystals with specific coincidence site lattice twist boundaries are manufactured for investigation. Asymmetrical I-V behaviors occur with maximum nonlinear coefficient of forward direction tenfold of reverse one. Energy dispersion spectra measurement reveals the nonuniform distributions of elements such as bismuth and oxygen in the grain boundary region. This reflects a higher Schottky barrier formed at Zn-polar face than that at O-polar face, which is confirmed by capacitance-voltage method. Such natural asymmetry may result from the effect of spontaneous polarization of polar surfaces of ZnO. Finally, simulation results prove the validity of the asymmetrical model proposed.

Polarity-dependent photoemission of in situ cleaved zinc oxide single crystals

Abstract

Semimetal graphite/ZnO Schottky diodes and their use for hydrogen sensing

A method is described for the fabrication of highly rectifying Schottky contacts on n-type ZnO (O and Zn polar face) single crystals, both bare and partially covered with Pt nanoparticles, coated with mechanically deposited colloidal graphite. A layer of Pt nanoparticles deposited by in situ pulsed electrophoretic deposition from isooctane colloid solutions is inserted between the graphite and the ZnO surface serves to dissociate hydrogen molecules in hydrogen sensing elements based on the highly rectifying Schottky barriers. The sensing elements are sensitive to gas mixtures with a low hydrogen concentration down to 10ppm and show an extremely fast response above 1000ppm.

Zn- and O-polar surface effects on Raman mode activation in homoepitaxial ZnO thin films

ZnO thin films were grown by metalorganic chemical vapor deposition on (0001) Zn- and O-polar surfaces of ZnO substrate. The surface morphology of these films was investigated by micro-Raman scattering and scanning electron microscopy. In the case of Zinc-polar face and using back scattering configuration, two new Raman bands were observed at 398 and 584.5 cm−1. They have been interpreted as quasi phonons TO (qTO) and LO (qLO) respectively. The origin of these modes was attributed to the particular morphology of ZnO films grown on Zn-polar face, which consists in c-axis nano-platelets, randomly tilted from the substrate. qTO and qLO frequencies have been calculated using the model of Loudon. SEM observation is in agreement with our Raman interpretation.

Silver Schottky contacts to Zn-polar and O-polar bulk ZnO grown by pressurized melt-growth method

The current transport mechanisms of Ag Schottky contacts to Zn-polar and O-polar bulk ZnO single crystals were investigated over the temperature range of 100–300 K. Using the thermionic emission (TE) model, Schottky contacts to Zn-polar face were found to have higher barrier heights (lower ideality factors) than those to O-polar face. Compared to the theoretical value of n-type ZnO, the higher Richardson constant was obtained for both polar faces in the modified Richardson plot, indicating that the TE model, which considers barrier inhomogeneity, cannot adequately explain the current transport. Temperature-dependent tunneling characteristics showed that the tunneling current was dominant for the Zn-polar face over the entire temperature range (100–300 K). For the O-polar face, the tunneling current was dominant mainly at low temperatures (100–200 K) and the TE component contributed strongly to the current transport above 200 K.

Polarity effects in the x-ray photoemission of ZnO and other wurtzite semiconductors

Significant polarity-related effects were observed in the near-surface atomic composition and valence band electronic structure of ZnO single crystals, investigated by x-ray photoemission spectroscopy using both Al Kα (1486.6 eV) and synchrotron radiation (150 to 1486 eV). In particular, photoemission from the lowest binding energy valence band states was found to be significantly more intense on the Zn-polar face compared to the O-polar face. This is a consistent effect that can be used as a simple, nondestructive indicator of crystallographic polarity in ZnO and other wurtzite semiconductors.

A comparative study of photoluminescence of Zn-polar and O-polar faces in single crystal ZnO using moment analysis

We report on differences in photoluminescence (PL) spectra between Zn-polar and O-polar faces in single crystal ZnO. The PL intensity ratios of one phonon to two phonon replicas of free-excitons in the two polar faces were found to be the same. This result clearly indicates that exciton-phonon coupling strengths in both faces are the same. From moment analysis, however, the relative PL intensity of the zero-phonon free-excitons in the O-polar face was larger than that in the Zn-polar face. We propose that the opposite band bending at the two polar faces causes the difference in the PL properties.

Controlled Etching Behavior of O-Polar and Zn-Polar ZnO Single Crystals

We report on optimized wet chemical processes for both the O-polar and Zn-polar faces of wurtzite bulk ZnO single crystals. Different solutions were tested to achieve controllable etching. For the O-polar ZnO surface, a controlled etch rate of was observed using an acid mixture of as an etchant. Fine-patterning of the O-polar surface with moderate etch rates and high reproducibility can be obtained using an aqueous 5% solution. In comparison, the Zn-polar ZnO surface etches significantly slower in HCl solution and exhibits strong dependence. Nevertheless, control enables reproducible etching even of the Zn-polar surface.