Monoclinic Gallium Oxide Research Articles

Dominating migration barrier for intrinsic defects in gallium oxide: Dose-rate effect measurements

Ion bombardment provides an opportunity to study basic properties of intrinsic defects in materials since the radiation-induced disorder accumulation depends on the balance between defect generation and migration rates. In particular, variation of such parameters as irradiation temperature and ion flux, known in the literature as dose-rate effect, interconnects the macroscopically measured lattice disorder with the migration barrier of the dominating defects. In this work, we measured the dose-rate effect in monoclinic gallium oxide (β-Ga2O3) and extracted its activation energy of 0.8 ± 0.1 eV in the range of 25–250 °C. Taking into account that the measurements were performed in the Ga-sublattice and considering 0.8 ± 0.1 eV in the context of theoretical data, we interpreted it as the migration barrier for Ga vacancies in β-Ga2O3, limiting the process. Additionally, we observed and took into account an interesting form of the lattice relaxation due to radiation-induced disorder buildup, interpreted in terms of the compressive strain accumulation, potentially trigging phase transitions in Ga2O3 lattice.

Zero-biased solar-blind photodetectors based on AlN/β-Ga2O3 heterojunctions

Monoclinic gallium oxide (β-Ga2O3) has drawn much attention in solar-blind detection because of its unique characteristics such as good thermal and chemical stability, intrinsic visible/solar blind, high breakdown electric field, etc. However, the relatively slow response hinders the actual applications of β-Ga2O3 photodetectors. In this work, AlN/β-Ga2O3 heterojunction photodetectors with a fast response speed of 320 ns have been developed. Moreover, the AlN/β-Ga2O3 heterojunction photodetector can work at 0 V with a responsivity of 7.0 mA W−1 and a detectivity of 1.25 × 1013 cm Hz1/2W−1. The results reported in this work indicate that AlN/β-Ga2O3 heterojunctions are feasible for solar-blind photodetectors with fast response and high sensitivity.

Reduction of MOS interfacial states between β -Ga2O3 and Al2O3 insulator by self-reaction etching with Ga flux

In this Letter, self-reaction etching (SRE) with Ga flux demonstrates the capability of eliminating surface contaminations and damage, as well as improving the electrical characteristics of the interface between monoclinic gallium oxide (β-Ga2O3) and the insulator. Compared to post-tetramethyl ammonium hydroxide wet chemical treatment, SRE is a low damage repair method that effectively removes surface contaminants introduced during previous inductively coupled plasma etching of β-Ga2O3 without surface damage. As a consequence, the surface band bending on the β-Ga2O3 surface decreased as demonstrated by the core-level peak shifts of x-ray photoemission spectroscopy, which indicated fewer negative charges remained on the surface. Furthermore, the interface state density (Dit) between β-Ga2O3 and an Al2O3 insulator was determined by using high-temperature conductance and photoassisted C–V measurements. The Dit dropped significantly for samples treated by SRE as compared with other treatments. These results suggested that SRE is an attractive etching candidate for future Ga2O3 power device fabrication.

Self-catalyst β-Ga2O3 semiconductor lateral nanowire networks synthesis on the insulating substrate for deep ultraviolet photodetectors.

Monoclinic gallium oxide (β-Ga2O3) is a super-wide bandgap semiconductor with excellent chemical and thermal stability, which is an ideal candidate for detecting deep ultraviolet (DUV) radiation (100–280 nm). The growth of β-Ga2O3 is challenging and most methods require Au as the catalyst and a long reacting time (more than 1 hour). In this work, the self-catalyst β-Ga2O3 lateral nanowire networks were synthesized on an insulating substrate rapidly by a simple low-cost Chemical Vapor Deposition (CVD) method. A thin film of β-Ga2O3 nanowire networks was synthesized within a reacting time of 15 minutes, which possesses a huge possibility for the rapid growth of β-Ga2O3 metal oxide nanowires networks and application in the future solar-blind photodetector. MSM (metal–semiconductor–metal) photodetectors based on the β-Ga2O3 nanowire networks revealed fast response (on–off ratios is about 103), which is attributed to the unique cross-junction barrier-dominated conductance of the nanowire networks. In addition, the self-catalyst β-Ga2O3 nanowires grown on insulating SiO2 are achieved and could be expected to find important applications in a bottom-up way of fabricating the next generation semiconductor nanoelectronics.

Non-steady-state photo-EMF in β-Ga2O3 crystals at λ = 457 nm

The non-steady-state photoelectromotive force is excited in a monoclinic gallium oxide crystal at wavelength λ = 457 nm. The crystal grown in an oxygen atmosphere is insulating and highly transparent for a visible light, nevertheless, the formation of dynamic space-charge gratings and observation of the photo-EMF signal is achieved without application of any electric field to the sample. The dependencies of the signal amplitude on the frequency of phase modulation, light intensity, spatial frequency and light polarization are measured. The material demonstrates the anisotropy along the [100] and [010] directions, namely, there is a small difference in the transport parameters and a pronounced polarization dependence of the signal. The crystal's photoconductivity, responsivity and diffusion length of electrons are estimated for the chosen light wavelength and compared with the ones for other wide-bandgap crystals.

Investigation of β‐Ga2O3 Film Growth Mechanism on c‐Plane Sapphire Substrate by Ozone Molecular Beam Epitaxy

Growth mechanism and temperature of monoclinic gallium oxide (β‐Ga2O3) films grown by ozone molecular beam epitaxy (MBE) are investigated herein. Phase‐pure () β‐Ga2O3 films can be grown on c‐plane sapphire substrates when the growth temperature exceeds 500 °C, and the grain size increases with the increase in the growth temperature. Sixfold rotational domains are obtained based on the epitaxial relationships (Ga2O3 < 010>//Al2O3<> and Ga2O3 < 102>//Al2O3 < 110>). For the film grown at 700 °C, the threefold facets are observed for the first time, which are resulted from the easy‐cleaved (100) plane and rotational domains. In addition, the cathodoluminescence spectra of the β‐Ga2O3 films show that the proportion of ultraviolet emission increases with the increase in the growth temperature, which is attributed to the modulation of defect. Finally, the growth rate can be modulated by the ratio of Ga flux and ozone pressure. A reference is provided for heteroepitaxy of β‐Ga2O3 films and better understanding of the ozone MBE growth mechanism of oxides.

Localized UV emitters on the surface of \u03b2-Ga2O3

Monoclinic gallium oxide (β-Ga2O3) is attracting intense focus as a material for power electronics, thanks to its ultra-wide bandgap (4.5–4.8 eV) and ability to be easily doped n-type. Because the holes self-trap, the band-edge luminescence is weak; hence, β-Ga2O3 has not been regarded as a promising material for light emission. In this work, optical and structural imaging methods revealed the presence of localized surface defects that emit in the near-UV (3.27 eV, 380 nm) when excited by sub-bandgap light. The PL emission of these centers is extremely bright—50 times brighter than that of single-crystal ZnO, a direct-gap semiconductor that has been touted as an active material for UV devices.

Tuning the responsivity of monoclinic solar-blind photodetectors grown by metal organic chemical vapor deposition

We report on the fabrication and characterization of solar-blind photodetectors based on metal organic chemical vapor deposition grown polycrystalline monoclinic indium gallium oxide alloys on sapphire using N2O for oxidation. The effects of growth conditions on indium incorporation efficiency and oxygen vacancies of the alloy, photo-to-dark current ratio (PDR), gain and responsivity of the fabricated photodetectors were investigated. The optical bandgap of the films was found to decrease due to the indium incorporation (x = 20.3%, 17.7%, 10.6% for samples A, B, and C, respectively) into the lattice of gallium oxide. By increasing the indium content incorporated into the lattice of , we demonstrated solar-blind photodetectors whose peak responsivity increased from 0.79 A/W () to 319.1 A/W, 66.1 A/W and 27.7 A/W for samples A, B and C, respectively at 5 V applied bias with the cut off wavelength below 280 nm. Increasing in content resulted in a higher concentration of oxygen vacancies in as-grown films. Increased oxygen vacancies as a result of the change in growth conditions lead to higher photoconductive gain, higher responsivities, and lower PDR, demonstrating a trade-off between responsivity and the PDR. To the best of our knowledge, the peak responsivity value reported in this work is the highest for based solar-blind photodetectors. Fast rise and fall times in the order of 100 ms have been measured for the photodetectors.

Synthesis and Characterization of Monoclinic Gallium Oxide Nanomaterials for High-Concentration Ethanol Vapor Detection

This study was conducted to synthesize Gallium Oxide nanomaterials from its bulk form using a low-cost and repeatable method and investigate on its potential as sensing element for Ethanol (EtOH). Horizontal Vapor Phase Growth (HVPG) Technique was utilized to synthesize monoclinic Gallium Oxide (β-Ga2O3) nanowires. Images obtained from the Scanning Electron Microscope showed high density nanowires specifically in the area of highest temperature gradient. SEM also showed that the grown structures using HVPG Technique were at the nanoscale with diameters ranging from 51.60 to 908.38 nm. Higher surface-to-volume ratio was also noted in the area of highest temperature gradient which was subjected to an applied magnetics field. The Gallium to Oxygen ratio was verified via EDX to be approximately 2:3 which agrees with the atomic ratio of Ga2O3. The monoclinic structure of the grown nanomaterials was investigated using Raman Spectroscopy. Raman peaks of the samples were at 199 and 486 cm−1 which was accounted to the presence of two Raman-active modes of the b-polymorph of Ga2O3. The Raman spectrum of the grown nanowires confirmed that the material is monoclinic in structure and belongs to C2/m space group. The I-V Curve of the grown nanowires were also determined using two-point probe which illustrates a non-linear curve similar to that of a semiconductor material. Furthermore, additional fundamental properties such as resistivity and specific conductance of the materials were also determined via van der Pauw Technique. Results showed that the material has high specific conductance and low resistivity. The synthesized nanomaterial was responsive to Ethanol vapor. Exposure to the said compound increased its resistance. Graphs showed that there is a significant increase in the resistance after exposure to ethanol vapor.

Polarization detection in deep-ultraviolet light with monoclinic gallium oxide nanobelts

Detection of polarization in deep-ultraviolet (DUV) wavelength is of great importance, especially in secure UV communication. In this paper, we report DUV polarization detectors based on ultra-wide bandgap β-Ga2O3 nanobelts, which belong to a monoclinic system with a strong anisotropic lattice structure. Single-crystalline β-Ga2O3 nanobelts are synthesized at high-temperature via chemical vapor deposition (CVD). Crystallographic investigation is performed to determine the crystal orientation of the nanobelts, by the combination of selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), crystal modeling and diffraction simulation. The photoresponse to unpolarized DUV light shows a high responsivity of 335 A W−1 and high sensitivity even to a low illumination power of pW. Strong anisotropy in responsivity and response speed, depending on incident light polarization, is observed. The underlying mechanism is attributed to the combination of internal dichroism and 1D morphology, as indicated by the DFT calculation and FDTD simulation. This work shows a way of DUV polarization detection using CVD grown Ga2O3 nanobelts, which could broaden the investigation of the Ga2O3 material and DUV photodetection.

Density functional tight binding study of β-Ga2O3: Electronic structure, surface energy, and native point defects.

A new parameter set to model monoclinic gallium oxide, β-Ga2O3, with the density functional tight binding (DFTB) method is developed. Using this new parameter set, DFTB calculations of bulk electronic band structure, surface energy of low-index surfaces, and formation energy of native point vacancy defects are performed and compared with the state-of-the-art density functional theory (DFT) calculations using the advanced hybrid exchange correlation functional. DFTB calculates the bandgap energy of 4.87 eV around the Fermi energy with the conduction band approximately following the DFT study by Peelaers and Van de Walle [Phys. Status Solidi B 252, 828 (2015)]. The surface energies calculated feature the correct order of stability among low index surfaces with surface energies in semiquantitative agreement with Bermudez' report [Chem. Phys. 323, 193 (2006)]. Oxygen and gallium vacancy defect formation energies and respective transition levels calculated using DFTB with a new parameter set are in semiquantitative agreement with the previous DFT reports by Varley et al. and Zacherle et al. [Appl. Phys. Lett. 97, 142106 (2010); Phys. Rev. B 87, 235206 (2013)]. This new semiempirical parameter set for β-Ga2O3, validated in bulk, surface, and point properties, would be useful for large spatiotemporal quantum chemical calculations regarding β-Ga2O3.

β-Ga2O3 thin film grown on sapphire substrate by plasma-assisted molecular beam epitaxy

Monoclinic gallium oxide (Ga2O3) has been grown on (0001) sapphire (Al2O3) substrate by plasma-assisted molecular beam epitaxy (PA-MBE). The epitaxial relationship has been confirmed to be [010]( ) β-Ga2O3||[ ](0001)Al2O3 via in-situ reflection high energy electron diffraction (RHEED) monitoring and ex-situ X-ray diffraction (XRD) measurement. Crystalline quality is improved and surface becomes flatter with increasing growth temperature, with a best full width at half maximum (FWHM) of XRD ω-rocking curve of ( ) plane and root mean square (RMS) roughness of 0.68° and 2.04 nm for the sample grown at 730 °C, respectively. Room temperature cathodoluminescence measurement shows an emission at ~417 nm, which is most likely originated from recombination of donor–acceptor pair (DAP).

High-Quality Schottky Barrier Diodes on β-Gallium Oxide Thin Films on Glass Substrate

Structural, electrical and optical properties of monoclinic gallium oxide thin films, deposited by pulsed laser deposition on glass substrates, are reported. The influence of growth temperature on thin film properties was investigated. Further, Ga2O3 buffer layers, grown and/or annealed at different temperatures, can be used to reduce the surface roughness of subsequently grown thin films. On thin films obtained from optimized growth parameters, Schottky barrier diodes were fabricated by reactive magnetron sputtering of Pt. Interestingly, Schottky diodes realized on thin films on glass substrate (without optimized buffer layer) have similarly good properties as diodes realized on thin films grown epitaxially on c-plane sapphire. The results stimulate realization of e. g. solar-blind photo detectors on low cost and possibly large area glass substrates.

Role of defect states in high bandgap gallium oxide identified using two-color spectroscopy

A recombination center model suggests role of crystal defects for photoexcited carriers in monoclinic symmetry gallium oxide.

Structural and Luminescence Properties of Ga2O3:Zn Micro‐ and Nanostructures

High quality Zn‐doped monoclinic gallium oxide micro‐ and nanostructures are obtained by a thermal treatment based on vapor–solid (VS) growth mechanism. Nanowires and ribbons are formed, the latter being the more abundant. The microstructural features are assessed by micro‐Raman and transmission electron microscopy revealing their crystal structure properties, such as the [‐110] growth direction for ribbons and being single crystals. In particular, a strong‐scattered light polarization dependence is reflected in the detected Raman modes. Luminescence of both Zn doped and undoped Ga2O3 samples is thoroughly studied by several techniques, exciting light with electrons or photons. Cathodoluminescence (CL) at a wide temperature range reveals that a band centered at around 2.7 eV, assigned to Zn doping in monoclinic Ga2O3, is thermally activated at temperatures above 200 K and dominates at room temperature. Besides, the characteristic bands at 3.4 and 3.0 eV of undoped Ga2O3 are obtained as well but with less relative intensity. Photoluminescence (PL) analysis at room temperature shows a similar set of bands, but slightly redshifted with respect to CL. Their relative intensities are strongly dependent on the excitation conditions and their time decays are studied by time‐resolved PL (TRPL).

Optimization of Growth Temperature of β-Ga₂O₃ Thin Films for Solar-Blind Photodetectors.

Monoclinic gallium oxide thin films were deposited on c-plane sapphire substrates at various substrate temperatures ranging from 450 °C to 700 °C by radio frequency magnetron sputtering technology. X-ray diffraction results showed that the deposited β-Ga2O3 films were oriented at (<mml:math display="block"> <mml:mover accent="true"> <mml:mn>2</mml:mn> <mml:mo>¯</mml:mo> </mml:mover></mml:math>01) direction. As the substrate temperature increased, the intensity of β-Ga2O3 peaks increased and bandgap decreased accordingly. Metal/semiconductor/metal structured solar-blind photodetectors based on β-Ga2O3 thin films growing at various substrate temperatures had been fabricated. The growth temperatures of thin films had no obvious influence on dark current and response to 365 nm light illuminations. The photoelectric properties such as responsivity and response speed of the thin films to 254 nm light illuminations were growth temperature dependent. At an applied bias of 50 V, the photodetectors prepared with 450 °C grown film had the highest responsivity of 2.18 A/W, and the photodetectors prepared with 700 °C grown film had the shortest rising time of 0.95 s under 254 nm light illuminations.

Shear-induced mechanical failure of β−Ga2O3 from quantum mechanics simulations

Monoclinic gallium oxide ($\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$) has important applications in power devices and deep UV optoelectronic devices because of such novel properties as a wide band gap, high breakdown electric field, and a wide range of $n$-type doping conductivity. However, the intrinsic failure mechanisms of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ remain unknown, which limits the fabrication and packaging of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$-based electronic devices. Here we used density-functional theory at the Perdew-Burke-Ernzerhof level to examine the shear-induced failure mechanisms of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ along various plausible slip systems. We found that the $(001)/\ensuremath{\langle}010\ensuremath{\rangle}$ slip system has the lowest ideal shear strength of 3.8 GPa among five plausible slip systems, suggesting that $(001)/\ensuremath{\langle}010\ensuremath{\rangle}$ is the most plausible activated slip system. This slip leads to an intrinsic failure mechanism arising from breaking the longest Ga\ensuremath{-}O bond between octahedral Ga and fourfold-coordinated O. Then we identified the same failure mechanism of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ under biaxial shear deformation that mimics indentation stress conditions. Finally, the general stacking fault energy (SFE) surface is calculated for the (001) surface from which we concluded that there is no intrinsic stacking fault structure for $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$. The deformation modes and SFE calculations are essential to understand the intrinsic mechanical processes of this semiconductor material, which provides insightful guidance for designing high-performance semiconductor devices.

Ga2O3 nanowires preparation at atmospheric pressure

Abstract An attempt has been undertaken to produce gallium oxide nanowires by thermal synthesis from metallic gallium source at atmospheric pressure. Silicon substrates of (1 0 0) and (1 1 1) orientation with and without silicon oxide layers (0.5 μm) were used as support. Evaporated thin gold films were deposited on the top of those silicon carriers as a catalytic agent. After thermal treatment by Rapid Thermal Processing RTP (at various temperatures and times), which was applied to make small Au islands with the diameters of about several tens of nanometers, the substrate surfaces were observed by SEM. The Ga2O3 syntheses were made at various conditions: time, temperature and gas mixture were changed. As a result, monoclinic gallium oxide β-Ga2O3 nanostructures with dominant [1 1 1] and [0 0 2] growth directions were grown. The obtained nanostructures of several tens micrometers length were studied by SEM, PL and X-ray methods.

Synthesis of monoclinic structure gallium oxide film on sapphire substrate by magnetron sputtering

Gallium oxide (Ga2O3) films were deposited on singlecrystalline sapphire (0001) substrate by radio frequency (RF) magnetron sputtering technique in the temperature range of 300—500 °C. The microstructure of the β-Ga2O3 films were investigated in detail using X-ray diffractometer (XRD) and scanning electron microscope (SEM). The results show that the film prepared at 500 °C exhibits the best crystallinity with a monoclinic structure (β-Ga2O3). Structure analysis reveals a clear out-of-plane orientation of β-Ga2O3 (201) ǁ Al2O3 (0001). The average transmittance of these films in the visible wavelength range exceeds 90%, and the optical band gap of the films varies from 4.68 eV to 4.94 eV which were measured by an ultraviolet-visible-near infrared (UV-vis-NIR) spectrophotometer. Therefore, it is hopeful that the β-Ga2O3 film can be used in the UV optoelectronic devices.

Effect of post-deposition annealing on low temperature metalorganic chemical vapor deposited gallium oxide related materials

Low temperature metalorganic chemical vapor deposition using trimethylgallium and water was investigated. The surface morphology of the film was almost flat at a deposition temperature below 182°C. This flat film was a mixture of nanocrystalline and amorphous phase. The film deposited at a temperature of 272°C resulted in a nanowire structure. X-ray diffraction measurements revealed that the nanowire film was a mixture of gallium hydroxide, gallium oxyhydroxide, and gallium tohdite or gallium oxide. We also found that post-deposition annealing above 600°C significantly changed the crystal structure of the both flat and nanowire films. Monoclinic gallium oxide phase was dominant after the post-deposition annealing above 600°C.