Ga2O3 Phase Research Articles

V doped Orth-Ga2O3: Half-metallic ferromagnetism, large magnetic anisotropy energy and high Curie temperature

Half-metallic ferromagnets have attracted intense interest worldwide and are considered to be an important component of spintronics devices. In this paper, an unpredicted Ga2O3 phase (named as Orth-Ga2O3) with good mechanical, dynamic, and thermodynamic stability is proposed by employing the adaptive genetic algorithm (AGA) crystal structure prediction approach. Surprisingly, V-doped Orth-Ga2O3 (V@Orth-Ga2O3) exhibits a strong ferromagnetic (FM) half-metallicity with a magnetic moment of 2 μB per formula. A detailed analysis of the density of states shows that the FM half-metallicity originates from the double exchange interaction between V-3d and O-2p orbitals. Besides, V@Orth-Ga2O3 has a big magnetic anisotropy energy (MAE) of 0.36 meV, mainly due to the (dxy, dx2-y2) orbitals of the V atom. According to Monte Carlo simulations, the Curie temperature TC of V@Orth-Ga2O3 is 225 K. The effect of carrier concentration and strain on the ferromagnetic stability of V@Orth-Ga2O3 system is also investigated. These unique magnetic properties make V@Orth-Ga2O3 an extremely promising material in spintronic nanodevices.

Heteroepitaxy of ε‐Ga2O3 thin film for artificial synaptic device

AbstractEmerging‐wide bandgap semiconductor Ga2O3 shows distinct characteristics for optoelectronic applications and a stable crystal phase of Ga2O3 is highly desired. Herein, we have first reported a metal‐semiconductor‐metal structure photonic synaptic device based on the ε‐Ga2O3 thin film. The ε‐Ga2O3 epilayer is grown on the c‐sapphire with a low temperature nucleation layer, which presents a crystal orientation relationship with the c‐sapphire (ε‐Ga2O3 <010> // c‐sapphire <1–100> and ε‐Ga2O3 <001> // c‐sapphire <0001>). The ε‐Ga2O3 photonic device was stimulated by UV pulses at different pulse widths, pulse intervals, and reading voltages. Under the UV pulse excitation, the photonic device exhibits primary synaptic functions including excitatory postsynaptic current, short term memory, pair pulse facilitation, long term memory, and STM‐to‐LTM conversion. In addition, stronger and repeated stimuli can naturally contribute to the higher learning capability, thus prolonging the memory time.

Investigation of BSCCO Superconductors with Ga Substitution by Solid State Method

Samples of (Bi2-xGax)Sr2CaCu2O2 superconductors with varying Ga substitution levels (x=0, 0.06, 0.12, 0.20) were synthesized using the solid-state reaction method. The effects of partial substitution of bismuth and gallium elements on the BSCCO structure were investigated in terms of their electrical, magnetic, structural, and mechanical properties. X-ray diffraction (XRD) was used to determine the crystalline phases, and samples A and B exhibited sharp peaks, indicating well-crystallized material with no Ga2O3 phase. SEM images were used to obtain microscopic views of all samples, and the elemental compositions of each element were determined from EDX spectra. The critical temperature was calculated from R-t measurements, and Sample A exhibited the highest Tc at 96K. Magnetic properties were investigated for all samples, including magnetic hysteresis behavior and magnetic moment. The critical current density (Jc) values were calculated using the Bean model by utilizing magnetic hysteresis.

Understanding the effects of annealing induced structural transformations on the UVC absorbance and other optical properties of RF sputter deposited Ga2O3 thin films

Gallium oxide (Ga2O3) is a transparent material with high absorption in the UVC region of the electromagnetic spectrum and hence is a very important candidate in the field of short wavelength optical device fabrication. A proper understanding of the different optical parameters is necessary for developing more efficient coatings and devices. In this work, changes in the optical behavior of Ga2O3 thin films due to post-deposition annealing (at temperatures 300 °C–900 °C) are discussed in detail. Structural, surface morphological and compositional modifications of the films are identified using the x-ray diffractometer, scanning electron microscopy and x-ray photoelectron spectrometer techniques, respectively. At 900 °C, a highly stable monoclinic β phase of Ga2O3 is obtained. The optical transmittance spectra acquired using UV–Vis spectroscopy indicate an improved UVC absorbance of the β-Ga2O3 films with an excellent visible transmittance (>80%). The structural transformation from amorphous to crystalline β-Ga2O3 phase and the associated reduction in defect density is found to modify other optical attributes, like the bandgap energy, Urbach energy, dispersion parameters, etc.

Defect accumulation in β-Ga2O3 implanted with Yb

Radiation-induced crystal lattice damage and its recovery in wide bandgap oxides, in particular beta-gallium oxide (β-Ga2O3), is a complex process. This paper presents the detailed study of defect accumulation in the β-Ga2O3 single crystal implanted with Ytterbium (Yb) ions and the impact of Rapid Thermal Annealing (RTA) on the defects formed. The (2¯01)oriented β-Ga2O3 single crystals were implanted with eleven fluences of Yb ions ranging from 1 × 1012 to 5 × 1015 at/cm2. Channeling Rutherford Backscattering Spectrometry (RBS/c) was used to study the crystal lattice damage induced by ion implantation and the level of structure recovery after annealing. The quantitative and qualitative analyses of collected spectra were performed by computer simulations. As a result, we present the first defect accumulation curve of β-Ga2O3 implanted with rare earth ion that reveals a two-step damage process. In the first stage, the damage of the β-Ga2O3 is inconspicuous, but begins to grow rapidly from the fluence of 1 × 1013 at/cm2, reaching the saturation at the random level for the Yb ion fluence of 1 × 1014 at/cm2. Further irradiation causes the damage peak to become bimodal, indicating that at least two new defect forms develop for the higher ion fluence. These two damage zones differently react to annealing, suggesting that they could origin from two phases, the amorphization phase and the new crystalline phase of Ga2O3. High-resolution x-ray diffraction (HRXRD) demonstrates the presence of strain and the γ phase of Ga2O3 after implantation, which disappear after annealing.

Zn-doping induced phase control mechanism of Ga2O3 thin films by spray pyrolysis deposition for application of solar-blind ultraviolet photodetector

Ga2O3 polymorphs have attracted considerable attention for optoelectronic and electronic device applications with high durability. Despite the many investigations, the crystal phase of Ga2O3 is difficult to control at low temperatures. In this study, the phase of Ga2O3 was controlled by Zn-doping at 500 °C by spray pyrolysis deposition. The pristine Ga2O3 showed the mixed α- and ε-phases, which disappeared with increasing Zn content from 0 to 60%. A single β-phase Ga2O3 thin film was formed when the Zn content was 60%. As the Zn content was increased to 120%, ZnO particles formed on the β-phase Ga2O3 surface. Therefore, the Zn dopants substitute for the Ga site and modify its coordination in the Ga2O3 matrix to form a β-phase Ga2O3. The β-Ga2O3 based solar-blind ultraviolet photodetector exhibited 1.645/0.581 s response times, a responsivity of 3.93 × 10−4 A W−1, and a detectivity of 2.65 × 1010 Jones under a bias voltage of 20 V.

Investigation of Optical functions, sub-bandgap transitions, and Urbach tail in the absorption spectra of Ga2O3 thin films deposited using mist-CVD

We report on the investigation of optical properties of epitaxial Ga2O3 thin films deposited at different temperatures using mist-Chemical Vapor Deposition on c-plane sapphire. The deposited films were of highly crystalline quality, with measured full width at half maxima (FWHM) of on-axis rocking curve scans as low as 62 arcsec for α-Ga2O3 sample grown at 500 °C. Pure phase κ-Ga2O3 was found to get stabilized as temperature increased to 650 °C. The optical properties of the as-deposited films were studied using absorbance, reflectance, and transmittance spectra. Various optical functions have been calculated, such as refractive index (n), extinction coefficient (k), real part of dielectric function (ϵreal), imaginary part of dielectric function (ϵimaginary) and high-frequency dielectric constant (ϵ∞). Furthermore, absorption spectra were modeled for sub-bandgap transitions and exponential onset near band-edge using Elliott–Toyozawa and Urbach–Martienssen models, respectively. Codi’s rule was used to estimate the value of the structural disorder parameter (χ) associated with Urbach’s tail. Variation of the extracted parameters, for instance, Urbach’s Energy (EU), phonon energy (hνp), steepness parameter (σ), electron–phonon interaction strength (Ee−phonon), Codi’s structural disorder parameter (χ), bandgap (Eg) and excitonic binding energy (Eexc) were studied against deposition temperature; these parameters were found to correlate with deposition temperature and hence the crystalline quality of the film. In light of the results, it was inferred that the exponential tail is inevitable at finite temperature because of the dominant Eg phonon mode present at ∼430cm−1 in α- or α- dominated Ga2O3 phase.

(Invited) Epitaxial Growth and Characterization of Ga2O3 Polymorphs

Gallium oxide (Ga2O3) is an ultra-wide band gap semiconductor with extraordinary potential for high-power, high-frequency, and high-temperature electronics and UV optoelectronics. The existence of stable and metastable phases, or polymorphs, of Ga2O3 presents challenges and opportunities for future device development. β-Ga2O3 is the thermodynamically stable phase and occurs in melt-grown, single-crystal substrates. The metastable phases, which have been demonstrated in pure- or mixed-phase form in epitaxial films, also possess ultra-wide bandgaps. Intriguing properties, such as ferromagnetism and ferro- and/or piezoelectricity, have been reported for certain phases and could enable new types of devices based on Ga2O3. Controlled growth of different Ga2O3 polymorphs could also lead to structurally-matched alloys, e.g., with In2O3 (Eg = 3.7 eV) and Al2O3 (Eg = 8.8 eV), to modulate bandgaps. In this presentation an overview of the stable and metastable Ga2O3 phases in terms of properties and growth methods will be given. We will present results from chemical vapor deposition, highlighting how certain conditions (e.g., temperature, growth rate, substrate, and impurity content) can affect the resulting polymorph(s). Characterization of phase composition, microstructure, and morphology using x-ray diffraction, scanning electron microscopy, and scanning transmission electron microscopy will be shown for each type of film.

Size-Dependent Phase Transition in Ultrathin Ga2O3 Nanowires.

Gallium oxide (Ga2O3) has attracted extensive attention as a potential candidate for low-dimensional metal-oxide-semiconductor field-effect transistors (MOSFETs) due to its wide bandgap, controllable doping, and low cost. The structural stability of nanoscale Ga2O3 is the key parameter for designing and constructing a MOSFET, which however remains unexplored. Using in situ transmission electron microscopy, we reveal the size-dependent phase transition of sub-2 nm Ga2O3 nanowires. Based on theoretical calculations, the transformation pathways from the initial monoclinic β-phase to an intermediate cubic γ-phase and then back to the β-phase have been mapped and identified as a sequence of Ga cation migrations. Our results provide fundamental insights into the Ga2O3 phase stability within the nanoscale, which is crucial for advancing the miniaturization, light weight, and integration of wide-bandgap semiconductor devices.

Vacuum Electrodeposition of Cu(In, Ga)Se2 Thin Films and Controlling the Ga Incorporation Route

The traditional electrochemical deposition process used to prepare Cu(In, Ga)Se2 (CIGS) thin films has inherent flaws, such as the tendency to produce low-conductivity Ga2O3 phase and internal defects. In this article, CIGS thin films were prepared under vacuum (3 kPa), and the mechanism of vacuum electrodeposition CIGS was illustrated. The route of Ga incorporation into the thin films could be controlled in a vacuum environment via inhibiting pH changes at the cathode region. Through the incorporation of a low-conductivity secondary phase, Ga2O3 was inhibited at 3 kPa, as shown by Raman and X-ray photoelectron spectroscopy. The preparation process used a higher current density and a lower diffusion impedance and charge transfer impedance. The films that were produced had larger particle sizes.

Ambipolarity Regulation of Deep‐UV Photocurrent by Controlling Crystalline Phases in Ga2O3 Nanostructure for Switchable Logic Applications

AbstractPhotoelectrochemical photocurrent switching (PEPS) effect can regulate the polarity of photocurrent, which has significant potential applications in areas such as logic gates, photosynapse, and artificial intelligence. In this work, it is reported for the first time that a pure Ga2O3 photoelectrochemical system exhibits ambipolar photocurrent behavior induced by deep ultraviolet, which is closely linked to the crystalline phase of Ga2O3 (α or β) and the surface states of oxygen vacancies. Spongy porous nanorod arrays (NRAs) of Ga2O3 designed here not only increase the contact area of Ga2O3 with the electrolyte but also can lower largely the reflection of light and improve light‐trapping capacity. For α phase Ga2O3, the photocurrent is in a forward direction under positive bias and shows a backward direction under negative bias in NaOH solution, exhibiting a distinct ambipolar photocurrent phenomenon, which can be attributed to more oxygen vacancy surface states and lower potential barrier at the semiconductor/electrolyte interface. Furtherly, the effect of the surface states on the ambipolar photocurrent behavior of α‐Ga2O3 NRAs is demonstrated by various treatment times of oxygen plasma, whose switching point moves from 0 V to −0.19 V with treatment for 30 min and continues to move in the negative direction with the increase of treatment time. Moreover, based on the ambipolar photocurrent behavior of α‐Ga2O3 NRAs, adjustable Boolean logic gates with voltage are prepared as the input source, offering a new path for the photoelectric device multifunctional integration needed in the Post‐Moore era, with a high accuracy manipulated by solar‐blind deep ultraviolet light.

High conductivity β-Ga2O3 formed by hot Si ion implantation

This work demonstrates the advantage of carrying out silicon ion (Si+) implantation at high temperatures for forming controlled heavily doped regions in gallium oxide. Room temperature (RT, 25 °C) and high temperature (HT, 600 °C) Si implants were carried out into MBE grown (010) β-Ga2O3 films to form ∼350 nm deep Si-doped layers with average concentrations up to ∼1.2 × 1020 cm−3. For such high concentrations, the RT sample was too resistive for measurement, but the HT samples had 82.1% Si dopant activation efficiency with a high sheet electron concentration of 3.3 × 1015 cm−2 and an excellent mobility of 92.8 cm2/V·s at room temperature. X-ray diffraction measurements indicate that HT implantation prevents the formation of other Ga2O3 phases and results in reduced structural defects and lattice damage. These results are highly encouraging for achieving ultra-low resistance heavily doped Ga2O3 layers using ion implantation.

Thickness dependent optical properties of amorphous/polycrystalline Ga2O3 thin films grown by plasma-enhanced atomic layer deposition

Amorphous/polycrystalline Ga2O3 is attracting considerable attention because its low cost and convenience of use make it a promising material for applications of solar blind ultraviolet detectors and power electronics, except for the widely investigated α/β phase Ga2O3. Spectroscopic ellipsometry (SE) is the preferred technique for determining optical constants that are vital for device design. The bandgap energy of Ga2O3 is close to the upper energy limit of commercial ellipsometers, making it challenging to determine the optical constants of Ga2O3 films. Considering the results obtained using the point-by-point method, we developed a strategy using 2 Tauc-Lorentz (2TL) oscillators that reasonably describes the optical behavior of the amorphous/polycrystalline Ga2O3 films deposited by plasma-enhanced atomic layer deposition on Si, sapphire, and Si/SiO2 substrates. The refractive index of the Ga2O3 films increases after annealing due to the reduced thickness and polycrystalline structure of the films. The polycrystalline samples have larger bandgaps than amorphous samples. The optical constants of Ga2O3 films with thicknesses ranging from ∼ 3 to ∼ 28 nm were extracted through SE fitting by the developed 2TL method. For both the as-deposited and annealed films, the bandgap increases as the film thickness decreases. The obtained results are useful for the design of photodetectors and power devices.

ß-Ga2O3 flake based Schottky diode hydrogen sensor

2D (2-dimensional) (100) plane β phase Ga2O3 (β-Ga2O3) flake based Pt Schottky diode is demonstrated for hydrogen sensing application. The (100) plane β-Ga2O3 flake was formed from the side wall of a mother (2̅01) plane β-Ga2O3 bulk substrate by the cost-effective mechanical exfoliation method. The hydrogen response of the (100) plane Ga2O3 flake based diode with the catalytic Pt Schottky electrode for hydrogen decomposition reaction was measured in the wide concentration range of 50–40,000 ppm hydrogen. The β-Ga2O3 flake based hydrogen sensor showed excellent responsivity of 2.32 × 108% for the 500 ppm hydrogen exposure at 25°C, and exhibited a reliable hydrogen sensing behavior up to 400°C. The decent cross-selectivity of the (100) plane β-Ga2O3 flake based Pt Schottky diode sensor over the other gases including N2, CO, CO2, CH4, NO2, NH3, and O2 was observed. In addition, the effect of the humidity on hydrogen sensing was investigated.

A comprehensive study of defects in gallium oxide by density functional theory

Ultrawide bandgap gallium oxide (Ga2O3) is a promising material for power semiconductor devices and deep ultraviolet (UV) solar-blind photodetectors. Understanding the properties of point defects in Ga2O3 is necessary to realize better-performing devices. A comprehensive study based on density functional theory (DFT), using the generalized gradient approximation (GGA): Perdew-Burke-Ernzerhof (PBE) exchange–correlation functional, of point defects in corundum (α), monoclinic (β), and orthorhombic (ε) phases of Ga2O3 is presented. The point defects include vacancies, interstitials, antisites, and extrinsic impurities in various phases of Ga2O3. Defect formation energies, charge transition energy levels, and defect concentrations variation with temperature are listed and presented under both gallium-rich (Ga-rich) and oxygen-rich (O-rich) growth conditions. The formation energy diagrams predict that the Ga2O3 phases favor the formation of Ga and O vacancies and the incorporation of extrinsic impurities. The calculations also show that the charge transition levels are deep inside the bandgap regardless of the Ga2O3 phase and growth environment. The impacts of temperature on the intrinsic point defects are analyzed by probing the vibrational modes of β-Ga2O3 thin films grown in a metal–organic chemical vapor deposition (MOCVD) system at 700 °C temperature and annealed at 1100 °C in an O-rich environment.

Low MOCVD growth temperature controlled phase transition of Ga2O3 films for ultraviolet sensing

The influences of low growth temperatures (420–490 °C) on phase transition of Ga2O3 films grown on Al2O3 substrates by means of metal-organic chemical vapor deposition (MOCVD) are investigated here. The changes of crystal structure, phase composition, surface and cross-sectional morphologies are carried out by X-ray diffraction, atomic force microscope and field-emission scanning electron microscope to reveal the detail evolution from ε-phase to β-phase in Ga2O3 films. The fluctuant Gibbs free energy change relying on growth temperature induces different growth mechanism of Ga2O3 films. Furthermore, the ultraviolet photoresponse properties of different phase compositions for Ga2O3 films are compared by constructing metal-semiconductor-metal photodetectors. It is found that mixture phase Ga2O3 films express better photodetection performance than pure phase Ga2O3 films due to the formation of built-in field within phase junction. This proposed regulation principle of Ga2O3 phase transition provides a new route to high-performance photoelectric material.

Thermodynamically metastable α-, ε- (or κ-), and γ-Ga2O3: From material growth to device applications

Gallium oxide (Ga2O3) has attracted tremendous attention in power electronics and ultraviolet photodetectors because of the large bandgap of 4.9–5.3 eV available to all polymorphs, as well as its high electric breakdown voltage. Recently, there has been increasing research interest in thermodynamically metastable phases such as α-, ε- (or κ-), and γ-Ga2O3, because they are predicted to exhibit superior properties compared with β-Ga2O3, the most stable phase of Ga2O3. For example, α-Ga2O3 (bandgap, Eg = 5.3 eV; expected breakdown field, Ec = ∼10 MV/cm) is expected to be a better potential candidate in power electronics than β-Ga2O3 (Eg = 4.5–4.8 eV; Ec = 8 MV/cm) because of its larger bandgap and higher breakdown field. Because these thermodynamically metastable phases cannot be grown using melt-growth techniques, they are grown heteroepitaxially on foreign substrates. We extensively illustrate the growth of these metastable phases and their alloys by employing various growth techniques and then discuss their doping and electronic properties. Finally, we emphasize their applications in devices, including power devices and solar-blind ultraviolet photodetectors.

Tight-binding band structure of β- and α-phase Ga2O3 and Al2O3

Rapid design and development of the emergent ultrawide-bandgap semiconductors Ga2O3 and Al2O3 require a compact model of their electronic structures, accurate over the broad energy range accessed in future high-field, high-frequency, and high-temperature electronics and visible and ultraviolet photonics. A minimal tight-binding model is developed to reproduce the first-principles electronic structures of the β- and α-phases of Ga2O3 and Al2O3 throughout their reciprocal spaces. Application of this model to α-Ga2O3/α-Al2O3 superlattices reveals that intersubband transitions can be engineered to the 1.55μm telecommunications wavelength, opening new directions in oxide photonics. Furthermore, by accurately reproducing the bandgap, orbital character, effective mass, and high-energy features of the conduction band, this compact model will assist in the investigation and design of the electrical and optical properties of bulk materials, devices, and quantum confined heterostructures.

Growth of α-(AlGa)2O3 alloy thin films on c-sapphire substrates by mist chemical vapor deposition using acetylacetonated Al and Ga solutions

α-Ga2O3 is a semi-stable phase of Ga2O3 and is known as an ultra-wide-bandgap semiconductor material. α-(AlGa)2O3 alloys are important for their applications in electronic and optoelectronic devices. We investigated the growth mechanism and process of α-(AlGa)2O3 alloys by mist chemical vapor deposition using acetylacetonated Al and Ga aqueous solutions. The contribution of acetylacetonated Al ions to the epitaxial growth was investigated. The effects of an anchoring mechanism on the mosaic spread were experimentally evaluated. Investigating X-ray diffraction profiles, strain relaxation processes in film formations are discussed.

RETRACTED: Effect of energetic ions irradiation on deposited gallium oxide thin film on n-Si (1 0 0) substrate

Gallium oxide (Ga2O3) thin film was deposited on n-Si (100) substrate using electron beam evaporation technique. Deposited thin film was annealed at 900 °C with oxygen flow at rate 200 ml/minute for 30 min in a tabular furnace. Thereafter, Ag9+ ion irradiated with with ion fluence (Φ) of 1E11, 5E11, 1E12 and 5E12 ions/cm2. The crystallographic structure of pristine and irradiated sample was studied. Presence of (004) (−401), (202), (−112), and (−601) planes implies presence of monoclinic (β) polycrystalline phase of Ga2O3. The average crystallite size decreases from ∼ 13.6 nm for pristine (Φ = 0 to ∼ 8.1 nm for the thin film irradiated at Φ = 5E12 ions/cm2. With increasing Φ a reduction in peak intensity and rise in peak width (FWHM) from 0.60 for pristine (Φ = 0) to 0.93 (in degree) for Φ = 5E12 ions/cm2 of the XRD peaks has been observed. The obtained response can be elucidated on the basis of introduction of defects in thin film as outcome of deposition of ion-energy which happens mainly in the form of electronic energy loss (Se).