ZnO Single Crystals Research Articles

Challenges and opportunities of ZnO-related single crystalline heterostructures

Recent technological advancement in ZnO heterostructures has expanded the possibility of device functionalities to various kinds of applications. In order to extract novel device functionalities in the heterostructures, one needs to fabricate high quality films and interfaces with minimal impurities, defects, and disorder. With employing molecular-beam epitaxy and single crystal ZnO substrates, the density of residual impurities and defects can be drastically reduced and the optical and electrical properties have been dramatically improved for the ZnO films and heterostructures with MgxZn1-xO. Here, we overview such recent technological advancement from various aspects of application. Towards optoelectronic devices such as a light emitter and a photodetector in an ultraviolet region, the development of p-type ZnO and the fabrication of excellent Schottky contact, respectively, have been subjected to intensive studies for years. For the former, the fine tuning of the growth conditions to make MgxZn1-xO as intrinsic as possible has opened the possibilities of making p-type MgxZn1-xO through NH3 doping method. For the latter, conducting and transparent polymer films spin-coated on MgxZn1-xO was shown to give almost ideal Schottky junctions. The wavelength-selective detection can be realized with varying the Mg content. From the viewpoint of electronic devices, two-dimensional electrons confined at the MgxZn1-xO/ZnO interfaces are promising candidate for quantum devices because of high electron mobility and strong electron-electron correlation effect. These wonderful features and tremendous opportunities in ZnO-based heterostructures make this system unique and promising in oxide electronics and will lead to new quantum functionalities in optoelectronic devices and electronic applications with lower energy consumption and high performance.

New Membrane Architecture with High Performance: ZIF-8 Membrane Supported on Vertically Aligned ZnO Nanorods for Gas Permeation and Separation

A new ZIF-8 membrane architecture with high performance supported on vertically aligned ZnO nanorods was successfully prepared. The vertically aligned, single crystal ZnO nanorods were grown seamlessly from porous ceramic support to form an intermediate support layer for the ZIF-8 membrane. They provide multiple anchorages for the ZIF-8 membrane that are both strong and flexible. The nanorods were activated to induce a uniform nucleation of ZIF nuclei on their surface to initiate and guide the growth of a defect-free ZIF-8 membrane. Single gas permeations and binary separations carried out to investigate the transport properties of these new membrane architectures confirmed that the ZIF-8 membranes were free of defects and stable at a higher temperature (473 K).

Research progress in ZnO single-crystal: growth, scientific understanding, and device applications

Zinc oxide, a wide band-gap semiconductor, has shown extensive potential applications in high-efficiency semiconductor photoelectronic devices, semiconductor photocatalysis, and diluted magnetic semiconductors. Due to the undisputed lattice integrity, ZnO single crystals are essential for the fabrication of high-quality ZnO-based photoelectronic devices, and also believed to be ideal research subjects for understanding the underlying mechanisms of semiconductor photocatalysis and diluted magnetic semiconductors. This review, which is organized in two main parts, introduces the recent progress in growth, basic characterization, and device development of ZnO single crystals, and some related works in our group. The first part begins from the growth of ZnO single crystal, and summarizes the fundamental and applied investigations based on ZnO single crystals. These works are composed of the fabrication of homoepitaxial ZnO-based photoelectronic devices, the research on the photocatalysis mechanism, and dilute magnetic mechanism. The second part describes the fabrication of highly thermostable n-type ZnO with high mobility and high electron concentration through intentional doping. More importantly, in this part, a conceptual approach for fabricating highly thermostable p-type ZnO materials with high mobility through an integrated three-step treatment is proposed on the basis of the preliminary research.

Neutron monochromators of BeO, MgO and ZnO single crystals

The monochromatic features of BeO, MgO and ZnO single crystals are discussed in terms of orientation, mosaic spread, and thickness within the wavelength band from 0.05 up to 0.5nm. A computer program MONO, written in “FORTRAN”, has been developed to carry out the required calculations. Calculation shows that a 5mm thick MgO single crystal cut along its (200) plane having mosaic spread of 0.5° FWHM has the optimum parameters when it is used as a neutron monochromator. Moreover, at wavelengths shorter than 0.24nm the reflected monochromatic neutrons are almost free from the higher order ones. The same features are seen with BeO (002) with less reflectivity than that of the former. Also, ZnO cut along its (002) plane is preferred over the others only at wavelengths longer than 0.20nm. When the selected monochromatic wavelength is longer than 0.24nm, the neutron intensities of higher orders from a thermal reactor flux are higher than those of the first-order one. For a cold reactor flux, the first order of BeO and MgO single crystals is free from the higher orders up to 0.4nm, and ZnO at wavelengths up to 0.5nm.

Glycine adsorption and photo-reaction over ZnO(000ī) single crystal

The adsorption and reaction of the amino acid glycine (NH2CH2COOH) are studied experimentally on the polar single crystal surface of zinc oxide, ZnO(000ī), by X-ray photoelectron spectroscopy (XPS) under UV light in presence and absence of molecular O2. Deposition at 350K mainly resulted in a largely deprotonated monolayer (NH2CH2COO−(a)+OH(s); where O is surface oxygen, (a) is for adsorbed and (s) is for surface species) identified by its XPS C1s binding energy at 289.3eV (COO), 286.7eV (CH2) and XPS O1s at 531.8eV (COO). A decrease in the signals of all functional groups of the adsorbed glycine (monitored by their C1s, O1s, and N1s lines) is seen upon UV excitation in the absence and presence of O2 pressures up to 5×10−6mbar. The photoreaction cross sections extracted from the decrease in the C1s peaks were found to be =2.6×10−18 (COO(a)) and 1.4×10−18(CH2) cm2. The photoactivity of the ZnO(000ī) surface under UHV-conditions is found to be comparable to that seen in direct photolysis of amino acids in solution.

2 MeV PROTON IRRADIATION EFFECTS ON ZnO SINGLE CRYSTAL

Melt grown ZnO single crystal was irradiated with varying fluence of 2 MeV proton beam in the range of 1 × 1016 H +/cm2 to 5 × 1017 H +/cm2. The irradiated spots exhibited varying degree of discoloration due to the irradiation indicating surface and volume defects. The surface roughness of the irradiated spots was measured using atomic force microscopy indicating surface modification of the sample. The extent of the damage on the ZnO single crystal from radiation was assessed using Raman spectroscopy. The emergence of the A1( LO ) band at 579 cm-1 pointed to the significant surface alteration in the ZnO crystal due to proton bombardment.

Photosensitization of ZnO single crystal electrodes with PbS quantum dots

Mott–Schottky analysis is used to determine the band edge positions for ZnO single crystals in acetonitrile electrolytes. The band alignment between ZnO and PbS QDs suggests that electron injection can occur for PbS quantum dots (QDs) over a wide range of sizes. Sensitization of ZnO single crystal electrodes with PbS QDs is then demonstrated using ferrocene/ferrocenium as the regenerator (hole acceptor). The largest 5.5 nm QDs are sensitizing the ZnO out to 1580 nm or 0.84 eV, to our knowledge the longest wavelength sensitization measured for any oxide semiconductor. The driving force for the hole transfer from the 5.5 nm PbS QDs with a band gap of 0.84 eV using ferrocene/ferrocenium regenerator in acetonitrile is found to be close to zero, which may result in a high open circuit voltages for QDs sensitized solar cells (QDSSCs) with a spectral response extending well into the infrared.

Ohmic-Rectifying Conversion of Ni Contacts on ZnO and the Possible Determination of ZnO Thin Film Surface Polarity

The current-voltage characteristics of Ni contacts with the surfaces of ZnO thin films as well as single crystal (0001) ZnO substrate are investigated. The ZnO thin film shows a conversion from Ohmic to rectifying behavior when annealed at 800°C. Similar findings are also found on the Zn-polar surface of (0001) ZnO. The O-polar surface, however, only shows Ohmic behavior before and after annealing. The rectifying behavior observed on the Zn-polar and ZnO thin film surfaces is associated with the formation of nickel zinc oxide (Ni1-xZnxO, where x = 0.1, 0.2). The current-voltage characteristics suggest that a p-n junction is formed by Ni1-xZnxO (which is believed to be p-type) and ZnO (which is intrinsically n-type). The rectifying behavior for the ZnO thin film as a result of annealing suggests that its surface is Zn-terminated. Current-voltage measurements could possibly be used to determine the surface polarity of ZnO thin films.

Optical signatures of photoinduced Zn vacancies in ZnO single crystal

193-nm excimer laser interaction with ZnO single-crystal at 0.05–500 KW/cm2 intensities is investigated under ultra high vacuum conditions by time resolved photoluminescence (PL) spectroscopy. A dominant 3.18 eV PL emission band at 295 K is observed. This band shows unusually long 0.52 ± 0.01 μs lifetime, indicating a defect mediated emission mechanism. The demonstrated negative thermal quenching for this band confirms its free electron to acceptor type transition. The involved acceptor is attributed to zinc vacancy with ∼100 meV shallow acceptor state. This study finds that 193-nm interactions produce Zn vacancies in transient states at 0.05–50 KW/cm2 excitation intensities and in stable state at 500 KW/cm2 or above intensities. The transient zinc vacancy production at such low intensities further validates the Frenkel pair creation as mechanism for creating these defects in ZnO single-crystal.

Impedance analysis of secondary phases in a Co-implanted ZnO single crystal.

Co ions with 100 keV energy with a fluence of 1 × 10(15) cm(-2) are implanted into ZnO(0001) single crystals at 300 °C under vacuum. The resulting Co-implanted ZnO single crystals and the subsequent 750 °C and 900 °C annealed samples are analysed with respect to their structural, optical, electronic, magnetic and ac electrical properties. Photoluminescence and X-ray photoelectron spectroscopy results indicate the signatures of the Co(2+) state and its substitution at the tetrahedrally coordinated Zn-sites. X-ray diffraction and X-ray photoelectron spectroscopy identify the presence of the ZnCo2O4 and Co3O4 phases in the 900 °C annealed sample. By comparing the resistance response of the identified phases towards different magnetic environments, the impedance spectroscopy results successfully identify two magnetic phases (ZnCo2O4 and Co3O4) and a paramagnetic (CoZn) phase for the 750 °C and 900 °C annealed samples, implying the extrinsic nature of room temperature ferromagnetism. The observed ferromagnetism in each sample is not of single origin, instead the mutual effects of the secondary phases embedded in the paramagnetic host matrix are in competition with each other.

Morphology and crystallinity control of wet chemically grown ZnO nanorods

The controlled growth of ZnO nanorods on technologically relevant substrates is essential for their practical integration into next generation optoelectronic and piezoelectric devices. In this report, highly ordered and uniform vertical ZnO nanorod arrays were synthesized using a facile, low temperature selective area wet chemical growth process. The nanorods were grown through nucleation windows that were patterned in a PMMA mask using electron beam lithography. At first, the technique was demonstrated on `ideal' ZnO single crystal substrates, where the geometrical parameters of the highly uniform and crystallographically coherent nanorods were dictated by the nucleation pattern, the polarity of the substrate, and the growth conditions. The obtained geometry was then compared to 4 further arrays corresponding to different ZnO seed layers deposited on Si and sapphire substrates. Scanning electron microscopy showed that the crystal orientation and the alignment of the nanorods were determined by the underlying seed layer. The piezoresponse force microscopy revealed that the d_{33} piezoelectric tensor component of the wet chemically grown nanorods was comparable (6-12 pm/V) to that of the highest value measured on ZnO single crystal (12.4 pm/V). The presented nanorod arrays have several potential applications, from nanorod based light emitting devices to CMOS compatible piezoelectric nanoforce sensors.

A conductive ZnO–ZnGaON nanowire-array-on-a-film photoanode for stable and efficient sunlight water splitting

We report highly stable and efficient sunlight water splitting on a ZnO–ZnGaON nanowire-array-on-a-film photoanode without the assistance of any co-catalyst. The single crystalline ZnO–ZnGaON nanowire-array-on-a-film photoanode was synthesized via a high-temperature vapor-phase diffusion reaction of gallium (Ga) and nitrogen (N) on a single crystal domain ZnO nanowire-array-on-a-film structure. The synthesized ZnO–ZnGaON photoanode offers visible light absorption through N incorporation, improved electrical conductivity via Ga incorporation, and structural advantages with the high-aspect-ratio nanowire array. Compared to the chemically unstable ZnO nanowire photoanode, the ZnO–ZnGaON nanowire photoanode significantly improves the anti-photocorrosive ability for water splitting. A highly stable photocurrent density of ∼1.5 mA cm−2 is obtained with the ZnO–ZnGaON nanowire photoanode at an applied bias of 0.8 VRHE under continuous sunlight illumination over five hours without noticeable degradation.

A DLTS study of a ZnO microwire, a thin film and bulk material

ABSTRACTWe have investigated the electrical properties of a ZnO microwire grown by carbo-thermal evaporation, a ZnO thin film grown by pulsed-laser deposition on an a-plane sapphire, and a hydrothermally grown Zn-face ZnO single crystal (Tokyo Denpa Co. Ltd.). The samples were investigated by means of current-voltage measurements, capacitance-voltage measurements, and deep-level transient spectroscopy.The defects T2 [1,2] and E3 [1,3,4] were identified in all three sample types. Additionally, in the single crystal and thin film samples E64 [5] and E4 [1] were detected. These findings support the common opinion that T2 is an intrinsic defect since it is found in all the samples investigated and thus its occurrence is not related to any growth technique.

Parametric study on the controllable growth of ZnO nanostructures with tunable dimensions using catalyst-free metal organic chemical vapor deposition

In this work we demonstrated the feasibility of controlling the morphology and crystallinity features of ZnO nanostructures by adjusting the growth temperature and reaction pressure in the appropriate windows. The microstructure of the films was made to change from a three-dimensional nanorods form to a two-dimensional nanowall networks form, and then to a two-dimensional dense film form. The controllable morphology evolution of the ZnO nanostructures was studied by analyzing the growth mechanisms and growth kinetics within different temperature and pressure windows. We also investigated the differences on the crystallinity and optical characteristics of three typical ZnO nanostructures. By optimizing the growth conditions, single-crystal ZnO films with a low full width at half-maximum (221 arcsecond) of the (0002) ω-rocking curve was obtained. It is believed that the promising results will provide valuable information for the fabrication of dimension controllable ZnO nanostructures that can be employed in reliable ZnO-based nanodevices.

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.

Intrinsic magnetic order and inhomogeneous transport in Gd-implanted zinc oxide

We report the results from magnetic, resistivity, and Hall effect measurements on a ferromagnetically ordered 5% Gd low energy implanted ZnO single crystal. Temperature-dependent magnetization measurements show that the Gd ions do not contribute to the magnetic order; hence, the magnetic order is intrinsic. The electronic transport in the Gd-implanted region is inhomogeneous, and there is a nonlinear Hall resistance. The nonlinear Hall resistance is likely to be a consequence of the inhomogeneous transport and not due to an anomalous Hall effect.

Self-assembly epitaxial growth of nanorods on nanowalls in hierarchical ZnO hexagonal nanocastle

Three-dimensional (3D) hierarchical por- ous nanostructure materials with controlled morphol- ogy have attracted much attention for their unique properties and wide applications. In the present work, a novel 3D hierarchical ZnO nanostructure was prepared through chemical vapor deposition, which is self-assembled by an array of vertical 1D nanorods on top of 2D nanowalls network in 3D hexagonal nanocastle. As revealed by transmission electron microscopy, the hierarchical nanostructures are com- posed of single crystal ZnO along the (0001) direction, and the preferred surfaces of nanowalls are f10 10g. Based on those experimental results, a growth mech- anism which involves a Zn-self-catalytic vapor- liquid-solid and vapor-solid mode is proposed to explain the formation of nanorods on nanowalls in 3D nanocastle. Raman and photoluminescence spectrum demonstrated that these 3D hierarchical ZnO nanostructures are nearly free of strain and exhibit an intense ultraviolet emission at 377.6 nm with negligible defect-related green bands.

On quantitative analysis of interband recombination dynamics: Theory and application to bulk ZnO

The issue of the quantitative analysis of time-resolved photoluminescence experiments is addressed by developing and describing two approaches for determination of unimolecular lifetime, bimolecular recombination coefficient, and equilibrium free-carrier concentration, based on a quite general second-order expression of the electron-hole recombination rate. Application to the case of band-edge emission of ZnO single crystals is reported, evidencing the signature of sub-nanosecond second-order recombination dynamics for optical transitions close to the interband excitation edge. The resulting findings are in good agreement with the model prediction and further confirm the presence, formerly evidenced in literature by non-optical methods, of near-surface conductive layers in ZnO crystals with sheet charge densities of about 3–5×1013 cm−2.

Application of ZnO single crystals for light-induced water splitting under UV irradiation

This paper presents experimental results of implementation of ZnO single crystals as photoanodes in photoelectrochemical (PEC) cells for hydrogen generation through the process of water splitting. Both, as-grown and O2-annealed ZnO single crystals were investigated for this purpose. A 351 nm argon laser line was used as the light source. The XRD investigations showed that used ZnO crystals are of good crystalline quality. It was found that the as-grown ZnO single crystals possess higher conversion efficiencies comparing to the O2-annealed one. The photocurrent density was found to increase significantly with the increase of external bias applied and excitation light intensity. Time dependent photocurrent density characteristics showed that the decay of photocurrent density was not observed within the measurement time. The differences in behaviour of the as-grown and the annealed in O2 ZnO single crystals are discussed in terms of crystals intrinsic defects.

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.