Monoclinic Ga2O3 Research Articles

Ga2O3 nanowires grown on GaN–Ga2O3 core–shell nanoparticles using a new method: Structure, morphology, and composition

Ga2O3 nanowires grown on GaN–Ga2O3 core–shell nanoparticles were prepared through heat-treating GaN powder method which comprises a pre-nitridation process in the flow of N2 gas and a post-oxidation process in the air at 1200°C. XRD and EDS patterns indicated that the heat-treated GaN powders were a powder mixture of GaN and Ga2O3. SEM, TEM, HRTEM and SAED images revealed that some nanowires that grow out from the edge of the GaN–Ga2O3 core–shell nanostructures with atomically smooth interfaces were monoclinic Ga2O3. Large blue-shifts in vibration frequency of Ga–N bonds observed in the FTIR spectrum could be contributed to size confinement effect and internal strains in GaN nanoparticles.

Microstructural study of MBE-grown ZnO film on GaN/sapphire (0001) substrate

Abstract Single crystalline ZnO film is grown on GaN/sapphire (0001) substrate by molecular beam epitaxy. Ga2O3 is introduced into the ZnO/GaN heterostructure intentionally by oxygen-plasma pre-exposure on the GaN surface prior to ZnO growth. The crystalline orientation and interfacial microstructure are characterized by X-ray diffraction and transmission electron microscopy. X-ray diffraction analysis shows strong c-axis preferred orientation of the ZnO film. Cross-sectional transmission electron microscope images reveal that an additional phase is formed at the interface of ZnO/GaN. Through a comparison of diffraction patterns, we confirm that the interface layer is monoclinic Ga2O3 and the main epitaxial relationship should be $$ (0001)_{ZnO} \parallel (001)_{Ga_2 O_3 } \parallel (0001)_{GaN} $$ and $$ [2 - 1 - 10]_{ZnO} \parallel [010]_{Ga_2 O_3 } \parallel [2 - 1 - 10]_{GaN} $$.

Growth of β-Ga2O3 nanorods by ammoniating Ga2O3/V thin films on Si substrate

This paper reports that β-Ga2O3 nanorods have been synthesized by ammoniating Ga2O3 films on a V middle layer deposited on Si(111) substrates. The synthesized nanorods were confirmed as monoclinic Ga2O3 by x-ray diffraction, Fourier transform infrared spectra. Scanning electron microscopy and transmission electron microscopy reveal that the grown β-Ga2O3 nanorods have a smooth and clean surface with diameters ranging from 100 nm to 200 nm and lengths typically up to 2μm. High resolution TEM and selected-area electron diffraction shows that the nanorods are pure monoclinic Ga2O3 single crystal. The photoluminescence spectrum indicates that the Ga2O3 nanorods have a good emission property. The growth mechanism is discussed briefly.

Study of defect behaviour in Ga<SUB align=right>2O<SUB align=right>3 nanowires and nano-ribbons under reducing gas annealing conditions: applications to sensing

The growth of monoclinic Ga2O3 nanowires, nano-ribbons and nano-sheets has been investigated. Results indicate that high quality single crystal nanowires can be grown at 900?C using an Au catalyst, while single crystal nano-ribbons and nano-sheets require no metal catalyst for growth. Since bulk Ga2O3 is a promising material for high temperature gas sensing, Ga2O3 nanowires and nano-ribbons may prove to enhance the sensing capability due to the high surface area. We have investigated the nature of defects in this material using electron spin resonance, photoluminescence and scanning electron microscopy, in as grown material, as well as under annealing in a reducing gas (H2) at various high temperatures. Results indicate the presence of an oxygen deficiency in the material, resulting in a conduction electron signal that becomes enhanced with annealing. The eventual loss of this signal is attributed to hydrogen etching of the nanowires, leading to a loss of material.

Electrical Properties and Microstructures of Ultraviolet Transparent Ga<sub>2</sub>O<sub>3</sub> Thin Films

The electrical and optical properties, and microstructures of 100 nm-thick Ga2O3 films fabricated on Al2O3(0001) substrates by a sputtering deposition were investigated. The partial pressure of oxygen was controlled and the substrate temperature was kept to be 500 °C during deposition. With increasing the oxygen partial pressure, the structures of the Ga2O3 films deposited on the substrates were observed to change from amorphous to crystalline (monoclinic β-type Ga2O3). The transmittance of the Ga2O3 films was measured to be more than 80 % at the visible and ultraviolet regions although the electrical resistivity was high. In order to obtain both low electrical resistivity and high transmittance at the ultraviolet regions, the addition of active dopant elements such as Sn into the Ga2O3 films would be required.

O2 and CO sensing of Ga2O3 multiple nanowire gas sensors

Abstract Gallium oxide nanowires were synthesized by a chemical thermal evaporation method using gallium metal as a source material. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy characterizations indicate that the obtained nanowires are well-crystallized single phase monoclinic Ga2O3. Multiple nanowire gas sensors were fabricated by dispensing the Ga2O3 nanowires on an interdigitated Pt-electrode. The Ga2O3 nanowire gas sensors show reversible response to O2 and CO gases in a working temperature range of 100–500 °C. A peak response is found at 300 °C for O2 gas and the peak response appears at 200 °C for CO gas. For both kinds of gases, the sensor response increases empirically with an increase of gas concentration. The results demonstrate the possibility of using the Ga2O3-based gas sensor at low working temperature field.

A Study of Ga<sub>2</sub>O<sub>3</sub> Nanomaterials Synthesized by the Thermal Evaporation of GaN Powders

We have synthesized gallium oxide (Ga2O3) nanomaterials at two different growth temperatures on iridium (Ir)-coated substrates by thermal evaporation of GaN powders. The products consist mainly of nanobelts, with some additional nanosheets. The nanobelts were of a single-crystalline monoclinic Ga2O3 structure. The broad emission photoluminescence band of 900°C-products had a different peak position from that of the 970°C-products.

Growth and optical characterization of Ga2O3 nanobelts and nanosheets

Monoclinic Ga2O3 nanobelts and nanosheets are obtained by a simple chemical route involved with H2O at 950 °C in Ar atmosphere. Electron microscopy observations reveal that the as-synthesized Ga2O3 nanobelts and nanosheets are structurally uniform, single crystalline, and most of them are free of defects and dislocations. The nanobelts are growing along with [001] facets, and the nanosheets are stacked up by (011) facets. The Raman scattering spectrum of Ga2O3 nanostructures shows a 30 cm−1 redshift at high wave numbers in comparison with that of bulk Ga2O3 powder. The photoluminescence spectrum reveals that there exists a stable blue emission band centered at 460 nm, which is mainly attributed to the oxygen vacancies in the Ga2O3 nanostructures.

Interface Engineering in ZnO Epitaxy

We report interface engineering in ZnO epitaxy to grow high-quality layers by plasma-assisted molecular beam epitaxy. Through interface engineering, we have succeeded in two-dimensional layer-by-layer growth of ZnO both on sapphire and GaN, and control of lattice polarity of ZnO films on a Ga-polar GaN template. MgO buffer has been used to convert the growth mode from a three-dimensional to a two-dimensional mode in ZnO epitaxy on sapphire. O-plasma pre-exposure on Ga-polar GaN templates has been employed to form a monoclinic Ga2O3 interface layer at the ZnO/GaN heterointerface, while Zn pre-exposure prevents oxidation of the GaN surface resulting in a ZnO/GaN heterointerface without an interface layer. The lattice polarity of ZnO films on Ga-polar GaN templates with and without a Ga2O3 interface layer with inversion symmetry have been revealed as Zn- and O-polar, respectively, by coaxial impact collision ion scattering spectroscopy. Structural properties of ZnO films grown with MgO buffer on sapphire or with Zn pre-exposure on GaN templates are better than those grown without MgO buffer or with an O-plasma pre-exposure, respectively. A buffer mechanism of MgO is discussed based on reflection high-energy electron diffraction and high-resolution X-ray diffraction analyses, while the mechanism for controlling the polarity in ZnO epitaxy is discussed using an interface layer with inversion symmetry. It is suggested that the present method offers a general approach to control the crystal polarity.

ZnO epilayers on GaN templates: Polarity control and valence-band offset

We report on the growth of polarity-controlled ZnO epilayers by plasma-assisted molecular-beam epitaxy and the measurement of valence-band offset at the ZnO/GaN heterointerface. The polarity of ZnO epilayers is determined by coaxial-impact-collision ion-scattering spectroscopy. The band offset is determined by ultraviolet and x-ray photoelectron spectroscopy. The high-resolution transmission electron microscopy study reveals the formation of an interface layer between the ZnO and GaN epilayers in O-plasma preexposed samples, while no interface layer is formed in Zn preexposed samples. Zn preexposure prior to ZnO growth results in Zn-polar ZnO epilayers (Zn face), while O-plasma preexposure leads to the growth of O-polar ZnO epilayers (O face). The interface layer is identified to be single-crystalline, monoclinic Ga2O3. The estimated valence band offset at the ZnO/GaN(0001) heterojunction with Zn preexposure is 0.8 eV with a type-II band alignment.

Control of polarity of ZnO films grown by plasma-assisted molecular-beam epitaxy: Zn- and O-polar ZnO films on Ga-polar GaN templates

We report on the growth of polarity-controlled ZnO films by plasma-assisted molecular-beam epitaxy. Different polar (Zn- and O-polar) ZnO films on unipolar (Ga-polar) GaN epilayers are selectively grown. Polarity of ZnO films is evaluated by coaxial impact collision ion scattering spectroscopy. Zn preexposure prior to ZnO growth results in Zn-polar ZnO films (Zn face), while O-plasma preexposure leads to the growth of O-polar ZnO films (O face). High-resolution transmission electron microscopy reveals the formation of an interface layer between ZnO and GaN epilayers in O-plasma preexposed samples, while no interface layer is observed in Zn preexposed samples. The interface layer is identified as single crystalline, monoclinic Ga2O3. We propose models for interface configurations at ZnO/GaN heterointerfaces, which can successfully explain the different polarities of the ZnO films.

ZnO/GaN heterointerfaces and ZnO films grown by plasma-assisted molecular beam epitaxy on (0001) GaN/Al2O3

We report the structures of ZnO/GaN heterointerfaces and properties of ZnO films grown by plasma-assisted molecular beam epitaxy (P-MBE) on (0001) GaN/Al2O3 substrates. At first, different preparations (zinc or oxygen-plasma pre-exposures) of the GaN surface are examined and the resultant heterointerface structures and properties of ZnO films are studied. The oxygen-plasma pre-exposure of the GaN surface results in the formation of an interface layer, while the formation of interface layers is prevented by zinc pre-exposure. The interface layer is identified as single-crystalline monoclinic Ga2O3 through extensive transmission electron microscopy (TEM) study. The orientation relationship between monoclinic Ga2O3 of the interface layer and GaN and ZnO epilayers is investigated as (0001) GaN∥(001) Ga2O3∥(0001) ZnO and [2-1-10] GaN∥[010] Ga2O3∥[2-1-10] ZnO. Based on structural investigations and stability of bondings, we propose interface-bonding sequences at the ZnO/GaN heterointerfaces in zinc or oxygen-plasma pre-exposed samples: “N–Ga–O–Zn” bonding sequence for zinc pre-exposed samples and “N–Ga–(O-terminated Ga2O3 layer)–Zn–O” for oxygen-plasma pre-exposed ones. Lattice misfit of Ga2O3 against underlying GaN (against upper ZnO) is estimated as −4.7% (−6.5%) and 10.7% (8.6%) along the [010] and [100] directions of the Ga2O3, respectively. The structural and optical properties of ZnO films evaluated by TEM, high-resolution x-ray diffraction and photoluminescence spectroscopy consistently reveal better crystal quality of zinc pre-exposed ZnO films than oxygen-plasma pre-exposed ones. Second, the effects of high temperature annealing on ZnO films grown at low temperature on zinc pre-exposed GaN surface are studied. Surface morphology is dramatically changed by high temperature annealing; three-dimensional island morphology on as-grown ZnO layers changes into atomically flat morphology with atomic level steps and terraces through high temperature annealing. Optical properties are also improved by high temperature annealing: (i) The intensity of band edge emissions from the ZnO films increases with their full width at half maximum values being decreased. (ii) Free exciton emission from annealed films is considerably enhanced in intensity compared to as grown films. These results indicate that high temperature annealing greatly improve the crystal quality of ZnO films grown at low temperature.