ZnO Single Crystal Substrates Research Articles

Two-dimensional confinement of excitons at the interface in nonpolar MgZnO/ZnO heterostructures

We report on the polarity-dependent excitonic emission at the interface in MgZnO/ZnO heterostructures grown on both polar and nonpolar ZnO single-crystal substrates. Structural and morphological analyses confirm that the heterostructures grow homoepitaxially on nonpolar $m$-plane and $a$-plane crystal substrates. The nonpolar heterostructures are investigated by depth-resolved cathodoluminescence spectroscopy revealing an excitonic interface emission centered at 3.12 eV, identified as the $H$-band signature of indirect excitons. These results reveal the formation of robust indirect excitons confined at the MgZnO/ZnO interface due to the presence of an epitaxially strained interfacial layer near the interface in the MgZnO, which is confirmed by high-order x-ray diffraction. It is also found that the $H$-band emission originates from the self-absorption of the ZnO free exciton emission in the proximity of the interface. Temperature-dependent studies of the $H$ band yields a thermal activation energy of 47 meV for the indirect exciton in the nonpolar heterojunctions. This activation energy is the largest among all free and bound excitons in ZnO and is much higher than the values found in AlGaN/GaN and MgZnO/ZnO/sapphire hetrostructures. Our findings indicate that the optical properties of nonpolar heterostructures are strongly influenced by the piezoelectric effect.

Selective Homoepitaxial Growth of ZnO Layers on c(+)-Surface by Solvothermal Reaction in Water-Ethylene Glycol Solvent.

Solvothermal deposition of ZnO layers on the c(±)-surfaces of ZnO single crystal substrates in a water–ethylene glycol solvent was investigated. Homoepitaxial growth of nanoparticulate layers was observed on the c(+)-surface. The manner of nanoparticle deposition on the c(+)-surface was similar to that of spherical particles precipitated in the solution, in that both grew through the oriented attachment of small particles during the early growth stage. The growth of the nanoparticulate film on the c(−)-surface was much slower than that on the c(+)-surface. After aging, the top surface of the film on the c(+)-surface transformed into a layer of pyramid-like particles so that the base of the pyramids was directed toward the surface. In contrast, randomly oriented pyramidal particles covered the c(−)-surface. Ostwald ripening through dissolution–recrystallization transformed the nanoparticles into pyramid-shaped particles in the latter stage when they were in contact with the solution. The faster growth on the c(+)-surface than on the c(−)-surface and the pyramidal shape of the particles with c(+)-basal plane deposited on the c(±)-surfaces after aging confirmed that the growth of the c(+)-plane was promoted, whereas the growth of {101̅0} and c(−)-planes was inhibited in this solution.

Formation of nanoripples on ZnO flat substrates and nanorods by gas cluster ion bombardment.

In the present study Ar+ cluster ions accelerated by voltages in the range of 5–10 kV are used to irradiate single crystal ZnO substrates and nanorods to fabricate self-assembled surface nanoripple arrays. The ripple formation is observed when the incidence angle of the cluster beam is in the range of 30–70°. The influence of incidence angle, accelerating voltage, and fluence on the ripple formation is studied. Wavelength and height of the nanoripples increase with increasing accelerating voltage and fluence for both targets. The nanoripples formed on the flat substrates remind of aeolian sand ripples. The ripples formed at high ion fluences on the nanorod facets resemble well-ordered parallel steps or ribs. The more ordered ripple formation on nanorods can be associated with the confinement of the nanorod facets in comparison with the quasi-infinite surface of the flat substrates.

Design and application of gas cluster accelerator for surface smoothing and nanostructures formation

A custom-built gas cluster ion source with energy up to 50 keV is constructed, and Ar, CO<sub>2</sub>, N<sub>2</sub>, and O<sub>2</sub> are used as the working gases. The clusters are formed by a metal supersonic conical nozzle with critical diameter in a range of 65–135 μm and a cone angle of 14°. The nozzle is powered in the pulsed mode, which improves the pumping conditions, and also makes it possible to increase the gas pressure in the stagnation zone to 15 atm and thereby obtain larger clusters. Based on the principle of ultrasonic expansion, gas cluster ions with an average size of 3000 atoms are obtained. The cluster beam current of 50 μA is obtained. The Ar cluster beam, which is less reactive, is used for treating surface, namely, surface smoothing and formation of self-assembled nanostructures. The Ar cluster bombardment perpendicular to the surface of the substrate is used to demonstrate the smoothing of the surface of Si wafers, Ti coating, and Au film. For the initial Si wafer, its root-mean-square (RMS) roughness of 1.92 nm decreases down to 0.5 nm after cluster beam treatment. The cleaning effect of the cluster beam is also observed very well. The one-dimensional (1D) isotropic power spectral density of the Si surface topography before and after smoothing are also discussed. The off-normal irradiation Ar cluster beam is also used to form self-assembled surface nanoripple arrays on the surface of flat ZnO single crystal substrates. The ripple formation is observed when the incident angle of the cluster beam is in a range of 30°–60°. The process of nanoripple fabrication is significantly governed by the cluster beam incident angle, energy and dose. The nano-ripples formed on the flat substrates remain eolian sand ripples and their formation starts at the incident angle of 30°. The most developed nanoripples are observed at the incident angle within a range of 45°–60°. The surface morphology and characteristic distribution of the nano-structures on the flat ZnO substrate are also analyzed by the two-dimensional (2D) power spectral density function. Next, Ar cluster beam is used for irradiating the ZnO nanorod arrays grown on the Si substrate. Due to various angles between the nanorod’s axis and the substrate normal, the conditions of the ripple formation on the nanorod facets are also realized. The dependence of wavelength on the accelerating voltage of the cluster ions and the dose are studied. Similar dependence of wavelength on accelerating voltage and dose are found for nanorods. Comparing with the flat ZnO surface, nanoripples on the ZnO nanorod faces at high irradiation doses demonstrate an ordering effect, and morphology of the ripples resembles that of more parallel steps rather than eolian ripples.

Er implantation into various cuts of ZnO – experimental study and DFT modelling

This paper presents a DFT study of the positions of erbium ions in the ZnO structure as well as experimental study of Er ion implantation into different crystallographic cuts of ZnO. The theoretical simulations have compared cohesive energies and defect-formation energies for different Er positions in the wurtzite ZnO structure. In the experimental part, single-crystal ZnO substrates were implanted with Er+ ions at an energy of 190 keV using fluences of 1 × 1016 and 5 × 1016 ion.cm−2. The ion implantation was followed by the application of the thermal annealing in oxygen atmosphere at 1000 °C for 5 h. The RBS/channelling method with fine scans was used to study the positions of erbium ions and structure recovery during the annealing process. The Er-doped ZnO samples exhibited clear luminescence in the NIR spectral region at around 1.5 μm. This work provides new results on the comparison of erbium ion implantation into different polar and non-polar crystallographic cuts of ZnO, resulting in different luminescence properties of the samples. Experimental and theoretical results are compared as well.

Creation of Acceptor Centers in ZnO Single Crystals by Annealing in Sb Vapor

We have investigated the influence of Sb on the carrier concentration of ZnO single crystals. ZnO single crystal substrates were doped in a sealed ampule by annealing in the Sb vapor at temperatures from 700 to 900 °C to create acceptor centers in ZnO. The dopant concentration profiles were determined by secondary ion mass spectrometry. A significant distinction in dopant concentrations as well as depth of penetration was observed for different temperatures of annealing. Electrical properties of the samples were studied by the Hall Effect and resistivity measurements. Valence states of Sb in the samples were investigated by the X-ray photoelectron spectroscopy method to prove the incorporation of the latter in the matrix of ZnO before and after oxygen annealing. The evolution of the carrier concentration of ZnO along with the valence state of Sb is explained.

Homogeneous ZnO nanowire arrays p-n junction for blue light-emitting diode applications.

ZnO is a promising short-wavelength light-emitting materials for its wide bandgap (3.37 eV) and large exciton binding energy (∼60 meV), however, practical p-type doped ZnO is the main challenge in this field. Here, Blue light-emitting diodes (LEDs) based on the homogeneous junctions of Sb doped ZnO nanowire arrays grown on Ga doped ZnO single crystal substrate are fabricated. Element analysis, FET and Hall-effect measurements demonstrate that the Sb atom has been successfully doped into ZnO nanowires to from p-type conductivity. On the benefit of high quality of nano-size homojunction, the fabricated LED shows low turn-on voltage turn-on voltage as low as 3.4 V and strong blue emission peak located at 425 nm at room temperature, which originate from interfacial recombination of ZnO nanowire p-n homojunctions. The present blue LED based on ZnO material may have potential applications in short-wavelength optoelectronic devices.

A sensitive ultraviolet light photodiode based on graphene-on-zinc oxide Schottky junction

Abstract In this study, we present a simple ultraviolet (UV) light photodiode by transferring a layer of graphene film on single-crystal ZnO substrate. The as-fabricated heterojunction exhibited typical rectifying behavior, with a Schottky barrier height of 0.623 eV. Further optoelectronic characterization revealed that the graphene-ZnO Schottky junction photodiode displayed obvious sensitivity to 365-nm light illumination with good reproducibility. The responsivity and photoconductive gain were estimated to be 3×104 A/W and 105, respectively, which were much higher than other ZnO nanostructure-based devices. In addition, it was found that the on/off ratio of the present device can be considerably improved from 2.09 to 12.1, when the device was passivated by a layer of AlOx film. These results suggest that the present simply structured graphene-ZnO UV photodiode may find potential application in future optoelectronic devices.

Adsorption and Electronic Structure of CdSe Quantum Dots on Single Crystal ZnO: A Basic Study of Quantum Dot-Sensitization System

We describe the morphology and consider the optical absorption and the ground state energy level of CdSe quantum dots (QDs) on single crystal ZnO substrates with various crystal orientations. The morphologies and crystal growth of CdSe QDs were analyzed by atomic force microscopy (AFM), photoacoustic (PA) spectroscopy for the optical absorption measurements, and photoelectron yield (PY) spectroscopy to evaluate the ground state energy level. The adsorption and crystal growth of CdSe QDs depend on both the adsorption time and the surface orientation of the ZnO substrate. The change in average diameter of the QDs on ZnO with adsorption time is not related to the crystal surface orientation, and the growth is a two-step process, different from that on single crystal rutile-TiO2. This is characteristic of the difference between CdSe QDs grown on ZnO and those grown on rutile-TiO2. The crystal quality of the QDs on ZnO is poorer than those grown on rutile-TiO2, indicating that the reaction for crystal growth o...

High-temperature reactivity and wetting characteristics of Al/ZnO system related to the zinc oxide single crystal orientation

Wettability and reactivity of the Al/ZnO couple at a temperature of 1273 K in vacuum have been studied with application of modified sessile drop method. The results obtained for the ZnO single crystal substrates of various orientations evidenced non-wetting by liquid aluminium, showing high contact angles of 100°–107°. Detailed transmission electron microscopy characterization unambiguously proved that the creation of two interfacial layers revealing highly epitaxial growth took place at high-temperature interaction between liquid Al drop and ZnO substrate. The layers of columnar grains consisted of reactively formed Al2O3 and ZnAl2O4, the second one growing at the expense of the alumina. The alumina was identified as either cubic γ-, tetragonal δ- or monoclinic λ-Al2O3, depending on subtle changes in chemistry of the surface top layer of the ZnO substrates.

Evidence for photogenerated intermediate hole polarons in ZnO

Despite their pronounced importance for oxide-based photochemistry, optoelectronics and photovoltaics, only fairly little is known about the polaron lifetimes and binding energies. Polarons represent a crucial intermediate step populated immediately after dissociation of the excitons formed in the primary photoabsorption process. Here we present a novel approach to studying photoexcited polarons in an important photoactive oxide, ZnO, using infrared (IR) reflection-absorption spectroscopy (IRRAS) with a time resolution of 100 ms. For well-defined (10-10) oriented ZnO single-crystal substrates, we observe intense IR absorption bands at around 200 meV exhibiting a pronounced temperature dependence. On the basis of first-principles-based electronic structure calculations, we assign these features to hole polarons of intermediate coupling strength.

Tuning Energy Level Alignment At Organic/Semiconductor Interfaces Using a Built-In Dipole in Chromophore–Bridge–Anchor Compounds

A chromophore–bridge–anchor molecular architecture is used to manipulate the molecular level energy position, with respect to the band edges of the substrate, of a chromophore bound to a surface via an anchor group. An energy shift of the chromophore’s frontier orbitals is induced by the addition of an oriented molecular dipole into the bridge part of the compound. This principle has been tested using three Zinc Tetraphenylporphyrin derivatives of comparable structure: two of which possess a dipole, but pointing in opposite directions and, for comparison, a compound without a dipole. UV–vis absorption and emission spectroscopies have been used to probe the electronic structure of the compounds in solution, while UV photoemission spectroscopy has been used to measure the relative position of the molecular levels of the chromophore with respect to the band edges of a ZnO(11–20) single crystal substrate. It is shown that the introduction of a molecular dipole does not alter the chromophore’s HOMO–LUMO gap, and that the molecular level alignment of the compounds bound to the ZnO surface follows the behavior predicted by a simple parallel-plate capacitor model.

Optical characterization by variable angle spectroscopic ellipsometry of nitrogen-doped MgxZn1 − xO thin films prepared by the plasma-assisted reactive evaporation method

Non-doped MgxZn1−xO films (MgxZn1−xO films) and nitrogen-doped MgxZn1−xO films (MgxZn1−xO:N films) were grown epitaxially on Zn faces of ZnO single crystal substrates by the plasma-assisted reactive evaporation (PARE) method using ZnMg alloys. Optical parameters, refractive index n, extinction coefficient k, and absorption coefficient α of these films and ZnO substrate were easily estimated by variable angle spectroscopic ellipsometry. The k values of these films abruptly increased with increase in photon energy near the absorption edge region. Dispersions of n of these films were sharper with larger peak values than that of a ZnO substrate. MgxZn1−xO films and MgxZn1−xO:N films grown on a ZnO substrate by the PARE method are of high quality with less defects than those of a ZnO substrate. Maximum value of α of these films calculated from k was about 2×105cm−1. The values of intercepts on photon energy hν axis of (αhν)2‐hν plots for these films and the ZnO substrate agree with peak energy of PL spectra, the origin of which is free excitons. These results indicate that absorption in those materials is dominated by exciton absorption.

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.

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.

Near-surface structure of polar ZnO surfaces prepared by pulsed laser deposition

Polarity-controlled homoepitaxial ZnO films were prepared in order to investigate the near-surface and the electronic structures of ZnO films with Zn-polar (0001) and O-polar 0001¯ surfaces. Neutral atom scattering spectroscopy revealed that the near surface atomic structure and the polarity of the single crystal substrate with a step and terrace structure was maintained even at the surface of the epitaxial films through substrate-mediated growth without a defect structure. X-ray photoelectron spectroscopy study also revealed that the surfaces of ZnO films and single crystal substrates showed similar band bending regardless of the surface polarity. The state of surface band bending is discussed and compared with other reports regarding ZnO and GaN.

Aluminum doping induced columnar growth of homoepitaxial ZnO films by metalorganic chemical vapor deposition

The effect of aluminum doping on the growth of ZnO films on c-plane ZnO (0001) single crystal substrates during metalorganic chemical vapor deposition was investigated. It was found that aluminum doping induces a growth mode of three-dimensional columnar growth. X-ray photoemission spectroscopy demonstrates that partial aluminum is segregated to the growth front. A combined experimental contact angle measurements and theoretical first-principle calculations suggest that the surface energy of the films is promoted by aluminum doping. Besides, aluminum doping also tends to decrease the adatoms diffusion mobility. We conclude that aluminum acts as an antisurfactant element during the homoepitaxial growth, and it increases the difficulty in obtaining high quality n-type ZnO films.

Fabrication and characterization of a room-temperature ZnO polariton laser

A ZnO planar optical microcavity displaying room-temperature polariton lasing over a wide range of cavity-exciton detunings has been fabricated. The cavity combines optimum crystalline quality, given by a ZnO single-crystal substrate, and optimum photonic quality, obtained by the use of two dielectric SiO2/HfO2 Bragg mirrors. A maximum cavity quality factor of about 4000 has been measured. Typically, the polariton lasing transition is accompanied by an increase of the output intensity by more than two orders of magnitude, a reduction of the emission linewidth and a relatively small blueshift of the lower polariton branch (less than 5% of the Rabi splitting).

FIB milling of single-crystal and sputtered ZnO: SEM and AFM characterization

We report on the morphological study of focused ion beam (FIB) milling of both sputter-deposited and single crystal (0001) ZnO substrates. The surface roughness and the FIB induced sputtering yield were measured as a function of the ion dose and angle. Smoothing effect of rough films deposited by sputtering has been found at low ion doses. FIB milling at non-normal incidence produced roughness increase and the formation of ordered terraces on (0001)-oriented sputtered ZnO. Conversely, surface roughness and FIB yield on single crystal ZnO substrates is basically unaffected by ion dose and milling angle.

Formation of a ZnO2 layer on the surface of single crystal ZnO substrates with oxygen atoms by hydrogen peroxide treatment

Formation of a ZnO2 layer by H2O2 treatment for single crystal ZnO (0001) substrates was studied. X-ray diffraction (XRD) peaks of ZnO2 with a pyrite structure were observed in XRD 2θ-ω scan patterns of the O-face of single crystal ZnO (0001) substrates with H2O2 treatment, but these peaks were not observed in patterns of the Zn-face of ZnO (0001) substrates with H2O2 treatment. XRD ω scan patterns of the ZnO (0002) plane of the O-face of single crystal ZnO (0001) substrates were broadened at the tail of the pattern by H2O2 treatment, but such broadening was not observed in that plane of the Zn-face. Grain structure of ZnO2 layers was clearly observed in atomic force microscopy (AFM) images for the O-face of ZnO (0001) substrates with H2O2 treatment. Spectra of X-ray photoelectron spectroscopy (XPS) of the O-face of ZnO (0001) substrates with H2O2 treatment showed a definite peak shift of the O 1s peak. It is thought that a pyrite structure of ZnO2 is easily formed around an O atom of the O-face of ZnO (0001) substrates. Results of XRD measurements, the AFM image, and XPS measurement of the H2O2-treated single crystal ZnO (101¯0) substrate that has oxygen atoms on the surface appeared to be the same as those of the O-face of ZnO (0001) substrates.