A review on synthesis and applications of gallium oxide materials

Gallium oxide (Ga2O3), as a new kind of ultra-wide band gap semiconductor material, is widely studied in many fields, such as power electronics, UV - blind photodetectors, solar cells and so on. Owing to the advantages of its excellent performance and broad application prospects in semiconductor technology, Ga2O3 materials have attracted extensive academic and technological attention. This review mainly focuses on introducing the main liquid-phase synthesis methods of Ga2O3 nanoparticles, such as direct-precipitation, chemical bath deposition, hydrothermal, solvothermal, and sol-gel method, including the characteristics in process and advantages and disadvantages of these methods. Then, the effects of reaction conditions, such as pH, capping agent, aging and calcination conditions, on the morphologies and sizes of the precursor and the final products were elucidated. Moreover, the applications of Ga2O3 particles in the fields of catalysis, gas sensors, and other devices in current research on Ga2O3 nanomaterials are discussed with the description of the basic working principle and influence factors.

Phase-Dependent Phonon Heat Transport in Nanoscale Gallium Oxide Thin Films

Different phases of Ga2O3 have been regarded as superior platforms for making new-generation high-performance electronic devices. However, understanding of thermal transport in different phases of nanoscale Ga2O3 thin-films remains challenging, owing to the lack of phonon transport models and systematic experimental investigations. Here, thermal conductivity (TC) and thermal boundary conductance (TBC) of the( 1 ¯ 010 ) $( {\bar 1010} )$ α-,( 2 ¯ 01 ) $( {\bar 201} )\;$ β-, and (001) κ-Ga2O3 thin films on sapphire are investigated. At ≈80 nm, the measured TC of α (8.8 W m-1 K-1) is ≈1.8 times and ≈3.0 times larger than that of β and κ, respectively, consistent with model based on density functional theory (DFT), whereas the model reveals a similar TC for the bulk α- and β-Ga2O3. The observed phase- and size-dependence of TC is discussed thoroughly with phonon transport properties such as phonon mean free path and group velocity. The measured TBC at Ga2O3/sapphire interface is analyzed with diffuse mismatch model using DFT-derived full phonon dispersion relation. Phonon spectral distribution of density of states, transmission coefficients, and group velocity are studied to understand the phase-dependence of TBC. This study provides insight into the fundamental phonon transport mechanism in Ga2O3 thin films and paves the way for improved thermal management of high-power Ga2O3-based devices.

A Review of β-Ga2O3 Power Diodes

As the most stable phase of gallium oxide, β-Ga2O3 can enable high-quality, large-size, low-cost, and controllably doped wafers by the melt method. It also features a bandgap of 4.7-4.9 eV, a critical electric field strength of 8 MV/cm, and a Baliga's figure of merit (BFOM) of up to 3444, which is 10 and 4 times higher than that of SiC and GaN, respectively, showing great potential for application in power devices. However, the lack of effective p-type Ga2O3 limits the development of bipolar devices. Most research has focused on unipolar devices, with breakthroughs in recent years. This review mainly summarizes the research progress fora different structures of β-Ga2O3 power diodes and gives a brief introduction to their thermal management and circuit applications.

Photoelectron Holographic Study for Atomic Site Occupancy for Si Dopants in Si-Doped κ-Ga2O3(001)

We investigated atomic site occupancy for the Si dopant in Si-doped κ-Ga2O3(001) using photoelectron spectroscopy (PES) and photoelectron holography (PEH). From PES and PEH, we found that the Si dopant had one chemical state, and three types of inequivalent Si substitutional sites (SiGa) were formed. The ratios for the inequivalent tetrahedral, pentahedral, and octahedral SiGa sites were estimated to be 55.0%, 28.1%, and 16.9%, respectively. Higher (lower) ratios for the three inequivalent SiGa sites may come from a lower (higher) formation energy. The Tetra (Octa) SiGa site has the highest (lowest) ratio of the three SiGa sites since it has the lowest (highest) formation energy. We suggest that the tetrahedral SiGa site is due to the active dopant site, whereas the pentahedral and octahedral SiGa sites can be attributed to the inactive dopant sites for Si-doped κ-Ga2O3(001).

Molecular Beam Epitaxy of β-(InxGa1-x)2O3 on β-Ga2O3 (010): Compositional Control, Layer Quality, Anisotropic Strain Relaxation, and Prospects for Two-Dimensional Electron Gas Confinement

In this work, we investigate the growth of monoclinic β-(InxGa1-x)2O3 alloys on top of (010) β-Ga2O3 substrates via plasma-assisted molecular beam epitaxy. In particular, using different in situ (reflection high-energy electron diffraction) and ex situ (atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy) characterization techniques, we discuss (i) the growth parameters that allow for In incorporation and (ii) the obtainable structural quality of the deposited layers as a function of the alloy composition. In particular, we give experimental evidence of the possibility of coherently growing (010) β-(InxGa1-x)2O3 layers on β-Ga2O3 with good structural quality for x up to ≈ 0.1. Moreover, we show that the monoclinic structure of the underlying (010) β-Ga2O3 substrate can be preserved in the β-(InxGa1-x)2O3 layers for wider concentrations of In (x ≤ 0.19). Nonetheless, the formation of a large amount of structural defects, like unexpected (10 2 ̅ ) oriented twin domains and partial segregation of In is suggested for x > 0.1. Strain relaxes anisotropically, maintaining an elastically strained unit cell along the a* direction vs plastic relaxation along the c* direction. This study provides important guidelines for the low-end side tunability of the energy bandgap of β-Ga2O3-based alloys and provides an estimate of its potential in increasing the confined carrier concentration of two-dimensional electron gases in β-(InxGa1-x)2O3/(AlyGa1-y)2O3 heterostructures.

Structural and Photoelectronic Properties of κ-Ga2O3 Thin Films Grown on Polycrystalline Diamond Substrates

Orthorhombic κ-Ga2O3 thin films were grown for the first time on polycrystalline diamond free-standing substrates by metal-organic vapor phase epitaxy at a temperature of 650 °C. Structural, morphological, electrical, and photoelectronic properties of the obtained heterostructures were evaluated by optical microscopy, X-ray diffraction, current-voltage measurements, and spectral photoconductivity, respectively. Results show that a very slow cooling, performed at low pressure (100 mbar) under a controlled He flow soon after the growth process, is mandatory to improve the quality of the κ-Ga2O3 epitaxial thin film, ensuring a good adhesion to the diamond substrate, an optimal morphology, and a lower density of electrically active defects. This paves the way for the future development of novel hybrid architectures for UV and ionizing radiation detection, exploiting the unique features of gallium oxide and diamond as wide-bandgap semiconductors.

Efficient DFT prediction of chemical and structural stability using van der Waals correction: application for A3B2Ga3O12garnets (A = Lu, Y and B = Al, Sc)

Several seamless van der Waals (vdW) correction methods available for a wide range of systems could be expected to enhance stability predictions by accounting for the vdW effect. The stability of material can be evaluated using chemical potential phase diagram (CPD) which reveals the elemental chemical potential conditions for a successful synthesis. In this work, viability of various vdW correction approaches in improving the accuracy of stability prediction for A3B2Ga3O12garnets (A = Lu, Y and B = Al, Sc) has been studied. From the results, we have found that vdW-df-cx, Grimme-D3, vdW-df-c09, and vdW-df2-c09 significantly improve ΔHprediction with MAPE of >5.0% lower than PBE, which exhibit their potential for stability prediction based on the CPD analysis. For CPD construction whose reliability is based on ΔHprediction, vdW-df-cx which can minimize the MAPE in ΔH, relative to experimental data, is selected as the best method among all studied vdW approaches. A more accurate description of total energy of O2molecule and the competing compounds with layered structure can be also acquired by incorporating vdW interaction. However, the MAPE in lattice constant reveals that there is no significant improvement of lattice constant prediction for the studied garnets and their competing compounds. The vdW method which gives the MAPE in lattice constant slightly lower than that of PBE is vdW-df2-b86r. Although we found that the vdW corrections can improve material stability prediction, there is still room for the development of a novel DFT-based vdW method capable of accurately predicting both the lattice constant and ΔHof solids, including complex materials like garnets.

Ultra-Large Compressive Plasticity of E-Ga2 O3 Thin Films at the Submicron Scale

Gallium oxide (Ga2 O3 ) usually fractures in the brittle form, and achieving large plastic deformability to avoid catastrophic failure is in high demand. Here, ε-Ga2 O3 thin films with columnar crystals and partial unoccupied Ga sites are synthesized, and it is demonstrated that the ε-Ga2 O3 at the submicron scale can be compressed to an ultra-large plastic strain of 48.5% without cracking. The compressive behavior and related mechanisms are investigated by in situ transmission electron microscope nanomechanical testing combined with atomic-resolution characterizations. The serrated plastic flow and large strain burst are two major deformation forms of ε-Ga2 O3 during compression, which are attributed to the dislocation nucleation and avalanches, formation of new grains, and amorphization. The ultra-large compressive plasticity of ε-Ga2 O3 thin films at the submicron scale can inspire new applications of Ga2 O3 in micro- or nano- electronic and optoelectronic devices, especially those that require impact resistance during processing or service.

Interfacial Properties of the SnO/κ-Ga2O3 p-n Heterojunction: A Case of Subsurface Doping Density Reduction via Thermal Treatment in κ-Ga2O3

The interfacial properties of a planar SnO/κ-Ga2O3 p-n heterojunction have been investigated by capacitance-voltage (C-V) measurements following a methodological approach that allows consideration of significant combined series resistance and parallel leakage effects. Single-frequency measurements were carried out in both series- and parallel-model measurement configurations and then compared to the dual-frequency approach, which permits us to evaluate the depletion capacitance of diode independently of leakage conductance and series resistance. It was found that in the bias region, where the dissipation factor was low enough, they give the same results and provide reliable experimental C-V data. The doping profile extracted from the C-V data shows a nonuniformity at the junction interface that was attributed to a depletion of subsurface net donors at the n-side of the diode. This attribution was corroborated by doping profiles and carrier distributions in the n and p sides of the heterojunction obtained from the simulation of the measured C-V data by the Synopsys Sentaurus-TCAD suite. Hall effect measurements and Hg-probe C-V investigation on single κ-Ga2O3 layers, either as-grown or submitted to thermal treatments, support the hypothesis of the subsurface donor reduction during the SnO deposition. This study can shed light on the subsurface doping density variation in κ-Ga2O3 due to high-temperature treatment. The investigation of the SnO/κ-Ga2O3 heterointerface provides useful hints for the fabrication of diodes based on κ-Ga2O3. The methodological approach presented here is of general interest for reliable characterization of planar diodes.

Electrical and Recombination Properties of Polar Orthorhombic κ-Ga2O3 Films Prepared by Halide Vapor Phase Epitaxy

In this study, the structural and electrical properties of orthorhombic κ-Ga₂O₃ films prepared using Halide Vapor Phase Epitaxy (HVPE) on AlN/Si and GaN/sapphire templates were studied. For κ-Ga₂O₃/AlN/Si structures, the formation of two-dimensional hole layers in the Ga₂O₃ was studied and, based on theoretical calculations, was explained by the impact of the difference in the spontaneous polarizations of κ-Ga₂O₃ and AlN. Structural studies indicated that in the thickest κ-Ga₂O₃/GaN/sapphire layer used, the formation of rotational nanodomains was suppressed. For thick (23 μm and 86 μm) κ-Ga₂O₃ films grown on GaN/sapphire, the good rectifying characteristics of Ni Schottky diodes were observed. In addition, deep trap spectra and electron beam-induced current measurements were performed for the first time in this polytype. These experiments show that the uppermost 2 µm layer of the grown films contains a high density of rather deep electron traps near E_c - 0.3 eV and E_c - 0.7 eV, whose presence results in the relatively high series resistance of the structures. The diffusion length of the excess charge carriers was measured for the first time in κ-Ga₂O₃. The film with the greatest thickness of 86 μm was irradiated with protons and the carrier removal rate was about 10 cm^-1, which is considerably lower than that for β-Ga₂O₃.

In situ X-ray diffraction study of the solvothermal formation mechanism of gallium oxide nanoparticles

Gallium oxides are of broad interest due to their wide band gaps and attractive photoelectric properties. Typically, the synthesis of gallium oxide nanoparticles is based on a combination of solvent-based methods and subsequent calcination, but detailed information about solvent based formation processes is lacking, and this limits the tailoring of materials. Here we have examined the formation mechanisms and crystal structure transformations of gallium oxides during solvothermal synthesis using in situ X-ray diffraction. γ-Ga₂O₃ readily forms over a wide range of conditions. In contrast, β-Ga₂O₃ only forms at high temperatures (T > 300 °C), and it is always preceded by γ-Ga₂O₃, indicating that γ-Ga₂O₃ is a crucial part of the formation mechanism of β-Ga₂O₃. The activation energy for formation of β-Ga₂O₃ from γ-Ga₂O₃ is determined to be 90-100 kJ mol^-1 in ethanol, water and aqueous NaOH based on kinetic modelling of phase fractions obtained from multi-temperature in situ X-ray diffraction data. At low temperatures GaOOH and Ga₅O₇OH form in aqueous solvent, but these phases are also obtained from γ-Ga₂O₃. Systematic exploration of synthesis parameters such as temperature, heating rate, solvent and reaction time reveal that they all affect the resulting product. In general, the solvent based reaction paths are different from reports on solid state calcination studies. This underlines that the solvent is an active part of the solvothermal reactions and to a high degree determines different formation mechanisms.

Directional Carrier Transport in Micrometer-Thick Gallium Oxide Films for High-Performance Deep-Ultraviolet Photodetection

Incorporating emerging ultrawide bandgap semiconductors with a metal-semiconductor-metal (MSM) architecture is highly desired for deep-ultraviolet (DUV) photodetection. However, synthesis-induced defects in semiconductors complicate the rational design of MSM DUV photodetectors due to their dual role as carrier donors and trap centers, leading to a commonly observed trade-off between responsivity and response time. Here, we demonstrate a simultaneous improvement of these two parameters in ε-Ga₂O₃ MSM photodetectors by establishing a low-defect diffusion barrier for directional carrier transport. Specifically, using a micrometer thickness far exceeding its effective light absorption depth, the ε-Ga₂O₃ MSM photodetector achieves over 18-fold enhancement of responsivity and simultaneous reduction of the response time, which exhibits a state-of-the-art photo-to-dark current ratio near 10⁸, a superior responsivity of >1300 A/W, an ultrahigh detectivity of >10¹⁶ Jones, and a decay time of 123 ms. Combined depth-profile spectroscopic and microscopic analysis reveals the existence of a broad defective region near the lattice-mismatched interface followed by a more defect-free dark region, while the latter one serves as a diffusion barrier to assist frontward carrier transport for substantially enhancing the photodetector performance. This work reveals the critical role of the semiconductor defect profile in tuning carrier transport for fabricating high-performance MSM DUV photodetectors.

Structure and Thermal Stability of ε/κ-Ga2O3 Films Deposited by Liquid-Injection MOCVD

We report on crystal structure and thermal stability of epitaxial ε/κ-Ga2O3 thin films grown by liquid-injection metal−organic chemical vapor deposition (LI-MOCVD). Si-doped Ga2O3 films with a thickness of 120 nm and root mean square surface roughness of ~1 nm were grown using gallium-tetramethylheptanedionate (Ga(thd)3) and tetraethyl orthosilicate (TEOS) as Ga and Si precursor, respectively, on c-plane sapphire substrates at 600 °C. In particular, the possibility to discriminate between ε and κ-phase Ga2O3 using X-ray diffraction (XRD) φ-scan analysis or electron diffraction analysis using conventional TEM was investigated. It is shown that the hexagonal ε-phase can be unambiguously identified by XRD or TEM only in the case that the orthorhombic κ-phase is completely suppressed. Additionally, thermal stability of prepared ε/κ-Ga2O3 films was studied by in situ and ex situ XRD analysis and atomic force microscopy. The films were found to preserve their crystal structure at temperatures as high as 1100 °C for 5 min or annealing at 900 °C for 10 min in vacuum ambient (<1 mBar). Prolonged annealing at these temperatures led to partial transformation to β-phase Ga2O3 and possible amorphization of the films.

ε-Ga2 O3 : An Emerging Wide Bandgap Piezoelectric Semiconductor for Application in Radio Frequency Resonators

The explosion of mobile data from the internet of things (IoT) is leading to the emergence of 5G technology with dramatic frequency band expansion and efficient band allocations. Along with this, the demand for high-performance filters for 5G radio frequency (RF) front-ends keeps growing. The most popular 5G filters are constructed by piezoelectric resonators based on AlN semiconductor. However, AlN possesses a piezoelectric constant d₃₃ lower than 5 pm V^-1 and it becomes necessary to develop novel semiconductors with larger piezoelectric constant. In this work, it is shown that strong piezoelectricity exists in ε-Ga₂ O₃ . High-quality phase-pure ε-Ga₂ O₃ thin films with a relatively low residual stress are prepared. A switching spectroscopy piezoelectric force microscope (SS-PFM) measurement is carried out and the piezoelectric constant d₃₃ of ε-Ga₂ O₃ is determined to be ≈10.8-11.2 pm V^-1 , which is twice as large as that of AlN. For the first time, surface acoustic wave (SAW) resonators are demonstrated on the ε-Ga₂ O₃ thin films and different vibration modes resonating in the GHz range are observed. The results suggest that ε-Ga₂ O₃ is a great material candidate for application in piezoelectric devices, thanks to its wide bandgap, strong piezoelectric property, small acoustic impedance, and low residual stress.

Gallium Oxide for Gas Sensor Applications: A Comprehensive Review

Ga₂O₃ has emerged as a promising ultrawide bandgap semiconductor for numerous device applications owing to its excellent material properties. In this paper, we present a comprehensive review on major advances achieved over the past thirty years in the field of Ga₂O₃-based gas sensors. We begin with a brief introduction of the polymorphs and basic electric properties of Ga₂O₃. Next, we provide an overview of the typical preparation methods for the fabrication of Ga₂O₃-sensing material developed so far. Then, we will concentrate our discussion on the state-of-the-art Ga₂O₃-based gas sensor devices and put an emphasis on seven sophisticated strategies to improve their gas-sensing performance in terms of material engineering and device optimization. Finally, we give some concluding remarks and put forward some suggestions, including (i) construction of hybrid structures with two-dimensional materials and organic polymers, (ii) combination with density functional theoretical calculations and machine learning, and (iii) development of optical sensors using the characteristic optical spectra for the future development of novel Ga₂O₃-based gas sensors.

State-of-the-Art β-Ga2O3 Field-Effect Transistors for Power Electronics

Due to the emergence of electric vehicles, power electronics have become the new focal point of research. Compared to commercialized semiconductors, such as Si, GaN, and SiC, power devices based on β-Ga₂O₃ are capable of handling high voltages in smaller dimensions and with higher efficiencies, because of the ultrawide bandgap (4.9 eV) and large breakdown electric field (8 MV cm^-1). Furthermore, the β-Ga₂O₃ bulk crystals can be synthesized by the relatively low-cost melt growth methods, making the single-crystal substrates and epitaxial layers readily accessible for fabricating high-performance power devices. In this article, we first provide a comprehensive review on the material properties, crystal growth, and deposition methods of β-Ga₂O₃, and then focus on the state-of-the-art depletion mode, enhancement mode, and nanomembrane field-effect transistors (FETs) based on β-Ga₂O₃ for high-power switching and high-frequency amplification applications. In the meantime, device-level approaches to cope with the two main issues of β-Ga₂O₃, namely, the lack of p-type doping and the relatively low thermal conductivity, will be discussed and compared.

A Review on Gallium Oxide Materials from Solution Processes

Gallium oxide (Ga₂O₃) materials can be fabricated via various methods or processes. It is often mentioned that it possesses different polymorphs (α-, β-, γ-, δ- and ε-Ga₂O₃) and excellent physical and chemical properties. The basic properties, crystalline structure, band gap, density of states, and other properties of Ga₂O₃ will be discussed in this article. This article extensively discusses synthesis of pure Ga₂O₃, co-doped Ga₂O₃ and Ga₂O₃-metal oxide composite and Ga₂O₃/metal oxide heterostructure nanomaterials via solution-based methods mainly sol-gel, hydrothermal, chemical bath methods, solvothermal, forced hydrolysis, reflux condensation, and electrochemical deposition methods. The influence of the type of precursor solution and the synthesis conditions on the morphology, size, and properties of final products is thoroughly described. Furthermore, the applications of Ga₂O₃ will be introduced and discussed from these solution processes, such as deep ultraviolet photodetector, gas sensors, pH sensors, photocatalytic and photodegradation, and other applications. In addition, research progress and future outlook are identified.

Electronic, Thermal, and Thermoelectric Transport Properties of ε-Ga2O3 from First Principles

The electronic, thermal, and thermoelectric transport properties of ε-Ga₂O₃ have been obtained from first-principles calculation. The band structure and electron effective mass tensor of ε-Ga₂O₃ were investigated by density functional theory. The Born effective charge and dielectric tensor were calculated by density perturbation functional theory. The thermal properties, including the heat capacity, thermal expansion coefficient, bulk modulus, and mode Grüneisen parameters, were obtained using the finite displacement method together with the quasi-harmonic approximation. The results for the relationship between the Seebeck coefficient and the temperature and carrier concentration of ε-Ga₂O₃ are presented according to the ab initio band energies and maximally localized Wannier function. When the carrier concentration of ε-Ga₂O₃ increases, the electrical conductivity increases but the Seebeck coefficient decreases. However, the figure of merit of thermoelectric application can still increase with the carrier concentration.

Low-energy Ga2O3 polymorphs with low electron effective masses

We predict three Ga₂O₃ polymorphs with P2₁/c or Pnma symmetry. The formation energies of P2₁/c Ga₂O₃, Pnma-I Ga₂O₃, and Pnma-II Ga₂O₃ are 57 meV per atom, 51 meV per atom, and 23 meV per atom higher than that of β-Ga₂O₃, respectively. All the polymorphs are shown to be dynamically and mechanically stable. P2₁/c Ga₂O₃ is a quasi-direct wide band gap semiconductor (3.83 eV), while Pnma-I Ga₂O₃ and Pnma-II Ga₂O₃ are direct wide band gap semiconductors (3.60 eV and 3.70 eV, respectively). Simulated X-ray diffraction patterns are provided for experimental confirmation of the predicted structures. The polymorphs turn out to provide low electron effective masses, which is of great benefit to high-power electronic devices.

α-Gallium Oxide Films on Microcavity-Embedded Sapphire Substrates Grown by Mist Chemical Vapor Deposition for High-Breakdown Voltage Schottky Diodes

α-Gallium oxide, with its large band gap energy, is a promising material for utilization in power devices. Sapphire, which has the same crystal structure as α-Ga₂O₃, has been used as a substrate for α-Ga₂O₃ epitaxial growth. However, lattice and thermal expansion coefficient mismatches generate a high density of threading dislocations (TDs) and cracks in films. Here, we demonstrated the growth of α-Ga₂O₃ films with reduced TD density and residual stress on microcavity-embedded sapphire substrates (MESS). We fabricated the two types of substrates with microcavities: diameters of 1.5 and 2.2 μm, respectively. We confirmed that round conical-shaped cavities with smaller diameters are beneficial for the lateral overgrowth of α-Ga₂O₃ crystals with lower TD densities by mist chemical vapor deposition. We could obtain crack-free high-crystallinity α-Ga₂O₃ films on MESS, while the direct growth on a bare sapphire substrate resulted in an α-Ga₂O₃ film with a number of cracks. TD densities of α-Ga₂O₃ films on MESS with 1.5 and 2.2 μm cavities were measured to be 1.77 and 6.47 × 10⁸ cm^-2, respectively. Furthermore, cavities in MESS were certified to mitigate the residual stress via the redshifted Raman peaks of α-Ga₂O₃ films. Finally, we fabricated Schottky diodes based on α-Ga₂O₃ films grown on MESS with 1.5 and 2.2 μm cavities, which exhibited high breakdown voltages of 679 and 532 V, respectively. This research paves the way to fabricating Schottky diodes with high breakdown voltages based on high-quality α-Ga₂O₃ films.

Magnetron Sputter-Deposited β-Ga2O3 Films on c-Sapphire Substrate: Effect of Rapid Thermal Annealing Temperature on Crystalline Quality

Gallium oxide (Ga2O3) is a semiconductor with a wide bandgap of ~5.0 eV and large breakdown voltages (>8 MV·cm−1). Among the crystal phases of Ga2O3, the monoclinic β-Ga2O3 is well known to be suitable for many device applications because of its chemical and thermal stability. The crystalline quality of polycrystalline β-Ga2O3 films on c-plane sapphire substrates was studied by rapid thermal annealing (RTA) following magnetron sputtering deposition at room temperature. Polycrystalline β-Ga2O3 films are relatively simple to prepare; however, their crystalline quality needs enhancement. The β-phase was achieved at 900 °C with a crystallite size and d-spacing of 26.02 and 0.2350 nm, respectively, when a mixture of ε- and β-phases was observed at temperatures up to 800 °C. The strain was released in the annealed Ga2O3 films at 900 °C; however, the clear and uniform orientation was not perfect because of the increased oxygen vacancy in the film at that temperature. The improved polycrystalline β-Ga2O3 films with dominant (−402)-oriented crystals were obtained at 900 °C for 45 min under a N2 gas atmosphere.

Disorder-Induced Ordering in Gallium Oxide Polymorphs

Polymorphs are common in nature and can be stabilized by applying external pressure in materials. The pressure and strain can also be induced by the gradually accumulated radiation disorder. However, in semiconductors, the radiation disorder accumulation typically results in the amorphization instead of engaging polymorphism. By studying these phenomena in gallium oxide we found that the amorphization may be prominently suppressed by the monoclinic to orthorhombic phase transition. Utilizing this discovery, a highly oriented single-phase orthorhombic film on the top of the monoclinic gallium oxide substrate was fabricated. Exploring this system, a novel mode of the lateral polymorphic regrowth, not previously observed in solids, was detected. In combination, these data envisage a new direction of research on polymorphs in Ga_{2}O_{3} and, potentially, for similar polymorphic families in other materials.

Observing the microstructure of (001) κ-Ga2O3 thin film grown on a (-201) β-Ga2O3 substrate using automated crystal orientation mapping transmission electron microscopy

We grew (001) κ-Ga2O3 thin films on (-201) β-Ga2O3 substrates using mist chemical vapor deposition. X-ray diffraction analysis revealed that the thin films grown at 450–800 °C showed (004) κ-Ga2O3...

Wet Etching in β-Ga2O3 Bulk Single Crystal

Beta-phase gallium oxide (β-Ga2O3) bulk single crystal has received increasing attentions due to their fantastic performances and widespread use in power devices and solar-blind photodetectors. Wet etching has proved to...

Sn-Induced Phase Stabilization and Enhanced Thermal Stability of κ-Ga2O3 Grown by Mist Chemical Vapor Deposition

Tin (Sn)-doped orthorhombic gallium oxide (κ-Ga₂O₃) films were grown on (0001) sapphire by mist chemical vapor deposition. It is known that κ-Ga₂O₃ is more stable than α-Ga₂O₃ (corundum) but less stable than β-Ga₂O₃ (monoclinic). This thermodynamic stability means an optimal growth temperature (T _g) of the κ-phase (600-650 °C) is also in between the two. At first, it was observed that Sn doping induced the κ-phase during the growth of the β-phase (T _g = 700 °C). Interestingly, Sn could also promote the κ-phase even under the growth condition that strongly favors the α-phase (T _g = 450 °C). The postgrowth annealing tests at 800-1000 °C showed that the thermal stability of the κ-phase depends on the Sn concentration. The higher the Sn concentration, the more stable the phase. The one with the highest Sn content showed no phase transition from κ to β after annealing at 800, 900, and 1000 °C for 30 min each. This enhancement of thermal stability promises more reliable high-power and high-frequency devices for which κ-Ga₂O₃ is suitable. Although there was no correlation between Sn-induced phase stabilization and the crystal quality, cathodoluminescence revealed that increasing Sn concentration led to the strong suppression of the radiative recombination at 340 nm from the vacancy-related donor-acceptor pairs. This observation suggests that the phase stabilization by Sn could be related to a specific Ga site Sn replaces in the orthorhombic structure.

Correlation between crystal structure and magnetism in PLD grown epitaxial films of ε-Fe2O3 on GaN

In the present paper we discuss correlations between crystal structure and magnetic properties of epitaxial ε-Fe₂O₃ films grown on GaN. The large magnetocrystalline anisotropy and room temperature multiferroic properties of this exotic iron oxide polymorph, make it a perspective material for the development of low power consumption magnetic media storage devices. Extending our recent progress in PLD growth of ε-Fe₂O₃ on the surface of technologically important nitride semiconductors, we apply reciprocal space tomography by electron and x-ray diffraction to investigate the break of crystallographic symmetry occurring at the oxide-nitride interface resulting in the appearance of anisotropic crystallographic disorder in the sub-100 nm ε-Fe₂O₃ films. The orthorhombic-on-hexagonal nucleation scenario is shown responsible for the development of a peculiar columnar structure observed in ε-Fe₂O₃ by means of HRTEM and AFM. The complementary information on the direct and reciprocal space structure of the columnar ε-Fe₂O₃ films is obtained by various techniques and correlated to their magnetic properties. The peculiar temperature dependence of magnetization studied by the small-field magnetization derivative method and by neutron diffraction reveals the existence of a magnetic softening below 150 K, similar to the one observed earlier solely in nanoparticles. The magnetization reversal in ε-Fe₂O₃ films probed by X-ray magnetic circular dichroism is found different from the behavior of the bulk averaged magnetization measured by conventional magnetometry. The presented results fill the gap between the numerous studies performed on randomly oriented ε-Fe₂O₃ nanoparticles and much less frequent investigations of epitaxial epsilon ferrite films with lattice orientation fixed by the substrate.

Single-Domain and Atomically Flat Surface of κ-Ga2O3 Thin Films on FZ-Grown ε-GaFeO3 Substrates via Step-Flow Growth Mode

Herein, single-domain κ-Ga₂O₃ thin films were grown on FZ-grown ε-GaFeO₃ substrates via a step-flow growth mode. The ε-GaFeO₃ possessing the same crystal structure and similar lattice parameters as those of the orthorhombic κ-Ga₂O₃ facilitated the growth of κ-Ga₂O₃ thin films, as observed by the X-ray diffraction (XRD) analysis. Furthermore, the surface morphologies of the κ-Ga₂O₃ thin films exhibited a step-terrace and atomically flat structure. XRD φ-scan and transmission electron microscopy with selected area electron diffraction revealed that there is no occurrence of in-plane rotational domains in the κ-Ga₂O₃ thin films on ε-GaFeO₃ substrates and that the κ-Ga₂O₃ thin film comprised a single domain. TEM analysis revealed that there were no clear dislocations in the observation area. Moreover, high-resolution TEM observation showed that the atomic arrangements of the film and the substrate were continuous without the presence of an intermediate layer along the growth direction and were well-aligned in the in-plane direction.

Low Thermal Budget Heteroepitaxial Gallium Oxide Thin Films Enabled by Atomic Layer Deposition

This work explores the applicability of atomic layer deposition (ALD) in producing highly oriented crystalline gallium oxide films on foreign substrates at low thermal budgets. The effects of substrate, deposition temperature, and annealing process on formation of crystalline gallium oxide are discussed. The Ga₂O₃ films exhibited a strong preferred orientation on the c-plane sapphire substrate. The onset of formation of crystalline gallium oxide is determined, at which only two sets of planes, i.e., α-Ga₂O₃ (006) and β-Ga₂O₃ (4̅02), are present parallel to the surface. More specifically, this work reports, for the first time, that epitaxial gallium oxide films on sapphire start to form at deposition temperatures ≥ 190 °C by using an optimized plasma-enhanced ALD process such that α-Ga₂O₃ (006)∥α-Al₂O₃ (006) and β-Ga₂O₃ (2̅01)∥α-Al₂O₃ (006). Both α-Ga₂O₃ (006) and β-Ga₂O₃ (2̅01) planes are polar planes (i.e., consisting of only one type of atom, either Ga or O) and, therefore, favorable to form by ALD at such low deposition temperatures. Ellipsometry and van der Pauw measurements confirmed that the crystalline films have optical and electrical properties close to bulk gallium oxide. The film grown at 277 °C was determined to have superior properties among as-deposited films. Using TEM to locate α-Ga₂O₃ and β-Ga₂O₃ domains in the as-deposited crystalline films, we proposed a short annealing scheme to limit the development of α-Ga₂O₃ domains in the film and produce pure β-Ga₂O₃ films via an energy-efficient process. A pure β-Ga₂O₃ phase on sapphire with β-Ga₂O₃ (2̅01)∥α-Al₂O₃ (006) was successfully achieved by using the proposed process at the low annealing temperature of 550 °C preceded by the low deposition temperature of 190 °C. The results of this work enable epitaxial growth of gallium oxide thin films, with superior material properties offered by ALD, not only with potential applications as a high-performance material in reducing global energy consumption but also with an energy-efficient fabrication process.

Band Offsets at κ-([Al,In]xGa1-x)2O3/MgO Interfaces

Conduction and valence band offsets are among the most crucial material parameters for semiconductor heterostructure device design, such as for high-electron mobility transistors or quantum well infrared photodetectors (QWIP). Because of its expected high spontaneous electrical polarization and the possibility of polarization doping at heterointerfaces similar to the AlGaN/InGaN/GaN system, the metastable orthorhombic κ-phase of Ga₂O₃ and its indium and aluminum alloy systems are a promising alternative for such device applications. However, respective band offsets to any dielectric are unknown, as well as the evolution of the bands within the alloy systems. We report on the valence and conduction band offsets of orthorhombic κ-(Al_xGa_1-x)₂O₃ and κ-(In_xGa_1-x)₂O₃ thin films to MgO as reference dielectric by X-ray photoelectron spectroscopy. The thin films with compositions x_In ≤ 0.27 and x_Al ≤ 0.55 were grown by pulsed laser deposition utilizing tin-doped and radially segmented targets. The determined band alignments reveal the formation of a type I heterojunction to MgO for all compositions with conduction band offsets of at least 1.4 eV, providing excellent electron confinement. Only low valence band offsets with a maximum of ∼300 meV were observed. Nevertheless, this renders MgO as a promising gate dielectric for metal-oxide-semiconductor transistors in the orthorhombic modification. We further found that the conduction band offsets in the alloy systems are mainly determined by the evolution of the band gaps, which can be tuned by the composition in a wide range between 4.1 and 6.2 eV, because the energy position of the valence band maximum remains almost constant over the complete composition range investigated. Therefore, tunable conduction band offsets of up to 1.1 eV within the alloy systems allow for subniveau transition energies in (Al_xGa_1-x)₂O₃/(In_xGa_1-x)₂O₃/(Al_xGa_1-x)₂O₃ quantum wells from the infrared to the visible regime, which are promising for application in QWIPs.

High-κ dielectric ε-Ga2O3 stabilized in a transparent heteroepitaxial structure grown by mist CVD at atmospheric pressure

The dielectric constant of metastable ε-Ga₂O₃ was evaluated for the first time by using a transparent heteroepitaxial structure of ε-Ga₂O₃/indium tin oxide/yttria-stabilized zirconia.

Strain-Induced Metastable Phase Stabilization in Ga2O3 Thin Films

It is well known that metastable and transient structures in bulk can be stabilized in thin films via epitaxial strain (heteroepitaxy) and appropriate growth conditions that are often far from equilibrium. However, the mechanism of heteroepitaxy, particularly how the nominally unstable or metastable phase gets stabilized, remains largely unclear. This is especially intriguing for thin-film Ga₂O₃, where multiple crystal phases may exist under varied growth conditions with spatial and dimensional constraints. Herein, the development and distribution of epitaxial strain at the Ga₂O₃/Al₂O₃ film-substrate interfaces is revealed down to the atomic resolution along different orientations, with an aberration-corrected scanning transmission electron microscope. Just a few layers of metastable α-Ga₂O₃ structure were found to accommodate the misfit strain in direct contact with the substrate. Following an epitaxial α-Ga₂O₃ structure of about couple unit cells, several layers (4-5) of transient phase appear as the intermediate structure to release the misfit strain. Subsequent to this transient crystal phase, the nominally unstable κ-Ga₂O₃ phase is stabilized as the major thin-film phase form. We show that the epitaxial strain is gracefully accommodated by rearrangement of the oxygen polyhedra. When the structure is under large compressive strain, Ga³⁺ ions occupy only the oxygen octahedral sites to form a dense structure. With gradual release of the compressive strain, more and more Ga³⁺ ions occupy the oxygen tetrahedral sites, leading to volumetric expansion and the phase transformation. The structure of the transition phase is identified by high-resolution electron microscopy observation, complemented by the density functional theory calculations. This study provides insights from the atomic scale and their implications for the design of functional thin-film materials using epitaxial engineering.

Heteroepitaxial growth of single-phase ε-Ga2O3 thin films on c-plane sapphire by mist chemical vapor deposition using a NiO buffer layer

In this study, single-phase ε-gallium oxide (Ga₂O₃) thin films were heteroepitaxially grown on c-plane sapphire substrates.