Zn-face ZnO Research Articles

Influence of the reaction between water and ZnO on the properties of Au Schottky contacts and the photocatalytic activity of ZnO(0001) single crystals

Wet chemical treatment is a conventional surface cleaning method, and metal oxide photocatalysts are commonly used to decompose organic compounds in water. In this study, we have investigated the influence of the reaction between water and ZnO on the properties of Au Schottky contacts and the photocatalytic activity of Zn-face ZnO(0001) single crystals. The ZnO substrate was put in deionized water at temperatures between 40 °C and 90 °C (water treatment). The ZnO substrate was etched with deionized water, and the surface roughness increased with increasing water temperature. Although the water treatment had no significant influence on the surface composition of the ZnO substrate, it changed the properties of the Au contacts. Schottky contacts were formed on the as-received ZnO substrate and the ZnO substrate after the water treatment at 40 °C, whereas ohmic contacts were formed on the ZnO substrate after the water treatment at 90 °C. Photoelectron emission spectra showed that the surface Fermi level in the ZnO substrate after the water treatment at 90 °C was located just below the conduction band. However, the water treatment had no significant influence on the photocatalytic activity of the ZnO substrate. Even when the water treatment was performed at 90 °C, the surface Fermi level of the ZnO substrate in an electrolyte solution, which was estimated from photoelectrochemical measurements, was located about 0.4 eV below the conduction band.

Magnesium incorporation efficiencies in MgxZn1−xO films on ZnO substrates grown by metalorganic chemical vapor deposition**Project supported by the National Natural Science Foundation of China (Grant No. 61474121) and the Major Scientific Project of Fujian Province, China (Grant No. 2014NZ0002-2).

We investigate the magnesium (Mg) incorporation efficiencies in MgxZn1−xO films on c-plane Zn-face ZnO substrates by using metalorganic chemical vapor deposition (MOCVD) technique. In order to deposit high quality MgxZn1−xO films, atomically smooth epi-ready surfaces of the hydrothermal grown ZnO substrates are achieved by thermal annealing in O2 atmosphere and characterized by atomic force microscope (AFM). The AFM, scanning electron microscope (SEM), and x-ray diffraction (XRD) studies demonstrate that the MgxZn1−xO films each have flat surface and hexagonal wurtzite structure without phase segregation at up to Mg content of 34.4%. The effects of the growth parameters including substrate temperature, reactor pressure and VI/II ratio on Mg content in the films are investigated by XRD analysis based on Vegard’s law, and confirmed by photo-luminescence spectra and x-ray photoelectron spectroscopy as well. It is indicated that high substrate temperature, low reactor pressure, and high VI/II ratio are good for obtaining high Mg content.

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.

On quantum efficiency of photoluminescence in ZnO layers and nanostructures

Abstract In this work we studied PL in ZnO layers and nanostructures, including ZnO homoepitaxial layers on ZnO substrate and ZnO–Zn 1 − x Mg x O single quantum well (SQW) structures grown on sapphire substrates by MBE, and ZnO nanowires grown on sapphire by MOCVD. The external quantum efficiency (QE) of PL in O-face ZnO layers exceeded that in Zn-face ZnO layers by two orders of magnitude at low temperatures. In a sample with SQW the combined external QE from the 4.6-nm-wide SQW and 50-nm-thick Zn 1 − x Mg x O barriers achieved 28% at 15 K. The highest external QE was observed in one of the samples with ZnO nanowires—52% at 15 K and 2% at 300 K. Contribution of defect-related PL bands in ZnO nanowires samples was extremely low.

Structural properties of GaN grown on Zn-face ZnO at room temperature

We have grown GaN films on atomically flat Zn-face ZnO at room temperature (RT) and 700 °C, and compared their structural properties. Although the film quality of GaN grown at 700 °C was quite poor due to the serious interface reaction between GaN and ZnO, GaN with an atomically flat stepped and terraced surface grows epitaxially at RT due to the suppression of the interface reaction. The growth of GaN at RT proceeds in the layer-by-layer mode, while at 700 °C it grows three-dimensionally. Atomic force microscope observations after alkali etching of GaN surfaces revealed that GaN grown at RT exhibits an N-polarity while that grown at 700 °C has a Ga-polarity.

III-nitrides on oxygen- and zinc-face ZnO substrates

The characteristics of III-nitrides grown on zinc- and oxygen-face ZnO by plasma-assisted molecular beam epitaxy were investigated. The reflection high-energy electron diffraction pattern indicates formation of a cubic phase at the interface between III-nitride and both Zn- and O-face ZnO. The polarity indicates that Zn-face ZnO leads to a single polarity, while O-face ZnO forms mixed polarity of III-nitrides. Furthermore, by using a vicinal ZnO substrate, the terrace-step growth of GaN was realized with a reduction by two orders of magnitude in the dislocation-related etch pit density to ∼108cm−2, while a dislocation density of ∼1010cm−2 was obtained on the on-axis ZnO substrates.

GaN epitaxy on thermally treated c-plane bulk ZnO substrates with O and Zn faces

ZnO is considered as a promising substrate for GaN epitaxy because of stacking match and close lattice match to GaN. Traditionally, however, it suffered from poor surface preparation which hampered epitaxial growth in general and GaN in particular. In this work, ZnO substrates with atomically flat and terrace-like features were attained by annealing at high temperature in air. GaN epitaxial layers on such thermally treated basal plane ZnO with Zn and O polarity have been grown by molecular beam epitaxy, and two-dimensional growth mode was achieved as indicated by reflection high-energy electron diffraction. We observed well-resolved ZnO and GaN peaks in the high-resolution x-ray diffraction scans, with no Ga2ZnO4 phase detectable. Low-temperature photoluminescence results indicate that high-quality GaN can be achieved on both O- and Zn-face ZnO.

High-quality ZnO epilayers grown on Zn-face ZnO substrates by plasma-assisted molecular beam epitaxy

High-quality ZnO epilayers have been grown on Zn-face ZnO substrates by plasma-assisted molecular beam epitaxy. With increasing O/Zn flux ratio from the stoichiometric to the O-rich, the growth mode changed from three- to two-dimensional growth and the line widths of (0 0 0 2) and (1 0 1 ̄ 0) ω-rocking curves showed dramatic narrowing from 471 to 42 arcsec and from 1635 to 46 arcsec, respectively. These values are narrower compared to those of ZnO on sapphire and also those of device-grade MOCVD-grown GaN. Moreover, A-, B-excitons (FE A, FE B), and the n=2 state of FE A at 3.378, 3.393, and 3.424 eV, respectively, were clearly observed in the low-temperature (4.2 K) photoluminescence spectrum of ZnO grown under O-rich flux conditions. Our results show that growth under high O-rich flux conditions is required to produce high-quality Zn-polar ZnO films.

Epitaxy of highly optical efficient GaN on O and Zn face ZnO

ABSTRACTZnO is a highly efficient photon emitter, has optical and piezoelectric properties that are attractive for a variety of applications. Due to its stacking order and close lattice to GaN, it is also considered as a substrate material for GaN epitaxy. In the past the poor preparation of ZnO surface has been a major handicap to GaN epitaxy. However, proper treatment we developed recently can make both faces of ZnO smooth with atomic level terraces. Epitaxy of GaN on O-face and Zn-face ZnO by reactive molecular beam epitaxy was performed. We used low-temperature RF growth of GaN buffer layer on ZnO surface to protect it from both ammonia and Ga. No Ga2ZnO4, an oxide with the spinel structure formed due to reaction of ZnO with Ga, was found, in contrast to earlier reports. The low-temperature photoluminescence (PL) indicates that both faces of ZnO can provide a high quality GaN with high radiative efficiency. In previous research it has been reported that O-face ZnO is slightly better for GaN epitaxy. Our new finding demonstrates that high-quality GaN epilayers can be grown on Zn face of ZnO.

Growth of Ga-face and N-face GaN films using ZnO Substrates

We have used plasma molecular beam epitaxy on (0 0 0 1) and (0 0 0 ) ZnO substrates to induce epitaxial growth of GaN of a known polarity. The polarity of the ZnO substrates can be easily and unambiguously determined by measuring the sign of the piezoelectric coefficient. If we assume that N-face GaN grows on O face ZnO and that Ga-face GaN grows on Zn face ZnO, then we can study the growth of both Ga and N faces. The most striking difference is the doping behavior of the two faces. Growth on the Ga-face is characterized by a higher carrier concentration and a lower threshold for Ga droplet formation.