β-Ga2O3 Single Crystals Research Articles

Floating Particles in the Melt during the Growth of β-Ga2O3 Single Crystals Using the Czochralski Method

Floating particles often appear during the Czochralski (CZ) growth of β-Ga2O3 in the Ir crucible, thereby impeding the seeding process. Identifying the floating nanoparticles and then inhibiting or removing them is critical for growing high-quality β-Ga2O3 single crystals. We grew β-Ga2O3 crystals containing floating particles using the CZ method. It is indicated that the floating particles were composed of Ir with a face-centered cubic (fcc) structure. In addition, the β-Ga2O3/Ir interface was comprehensively characterized, showing sharp and straight configuration on the whole with small fluctuations at the nanoscale. Combined with density functional theory (DFT) calculation, we found that Ir-O bonding was responsible for stabilizing the interface. Accordingly, the atomic configuration of the interface with the stablest structure, including the relaxed one, was determined. Based on the formation mechanism of the floating particles, we propose three effective strategies, including blowing sufficient oxygen into the bottom of the Ir crucible, coating a protective layer on its inwall and equipping a mechanical arm for inhibiting or removing them.

Effect of air annealing on the structural, electrical, and optical properties of V-doped β-Ga2O3 single crystals

V impurities were intentionally introduced into β-Ga2O3 crystals as n-type dopants to improve the n-type conductivity of single-crystal substrates. A high-quality 0.20 mol% V-doped β-Ga2O3 single crystal was fabricated, and the effects of air annealing on the structure and the electrical and optical performances of V-doped β-Ga2O3 single crystals were systematically studied. The V-doped β-Ga2O3 crystal exhibited a high crystal quality, smooth surface, and high carrier concentration. In comparison, the crystalline quality of β-Ga2O3 was improved, and it showed a flat surface after the annealing treatment. Compared with that before annealing, the carrier concentration decreased from 5.90 × 1018 to 9.51 × 1017 cm−3, the optical transmittance increased in the near-infrared region, and the peak intensity of the Ag(3), mid-frequency Ag(8),Ag(9), and Ag(10) phonons were changed, which were attributed to the electron traps of the gallium vacancy (VGa) and the two cation vacancies paired with one cation interstitial atom (2VGa1-Gai) complex. After annealing, the oxygen vacancy (VO) was compensated, and the Ga3+ content decreased, which were related to the decrease in the free electron concentration. The variations in the VO and VGa concentrations resulted in a decrease in the blue luminescence peak area ratio and the formation of a green luminescence peak, respectively. These results can facilitate a better comprehension of the structural and property changes in V-doped β-Ga2O3 crystals owing to air annealing.

Enhancing the luminescence yield of Cr3+ in β -Ga2O3 by proton irradiation

In situ ion-beam-induced luminescence measurements reveal a strong enhancement of the Cr3+ emission yield in electrically conductive chromium doped β-Ga2O3 single crystals upon proton irradiation. The observed effect can be explained based on the Fermi-level pinning caused by radiation defects. This pinning of the Fermi level activates deep carrier traps that can act as sensitizers of the Cr3+ emission. In agreement with this model, in semi-insulating samples, where the Fermi level lies deep in the bandgap, the Cr3+ emission is present already in as-grown samples, and no enhancement of its intensity is observed upon proton irradiation. The boost of the Cr3+ emission yield by irradiation, observed in conductive samples, is reversed by thermal annealing in argon at temperatures above 550 °C for 30 s. The results reveal a high potential of Cr-doped β-Ga2O3 for in situ and ex situ optical radiation detection and dosimetry.

Effects of temperature and oxygen partial pressure on electrical conductivity of Fe-doped β-Ga2O3 single crystals

In this work, we measure DC and AC conductivity and Hall voltage to determine the origin of electrical insulating properties of Fe-doped β-Ga2O3 single crystals, which are measured perpendicular to the 2¯01 crystallographic plane. We find that electrical conduction is predominantly controlled by free electrons in the temperature range 230–800 °C with the mutual compensation of the impurity donor (Si) and acceptor dopant (Fe), explaining the low concentration of free electrons and Fermi level pinning over a wide range of temperatures. Furthermore, the negative temperature-dependence of the carrier mobility indicates that it is limited by optical phonon scattering. Importantly, we find electrical conductivity to be largely independent of oxygen partial pressure (pO2) from air to 10−4 atm at 600 °C, but it becomes slightly dependent on pO2 at 800 °C, as intrinsic non-stoichiometric point defects begin to influence the charge balance.

Two inch diameter, highly conducting bulk β -Ga2O3 single crystals grown by the Czochralski method

Two inch diameter, highly conducting (Si-doped) bulk β-Ga2O3 single crystals with the cylinder length up to one inch were grown by the Czochralski method. The obtained crystals revealed high structural quality characterized by narrow x-ray rocking curves (FWHM ≤ 25 arc sec) and high surface smoothness (RMS < 200 pm) of the epi-ready wafers. The free electron concentration and Hall mobility at room temperature were in the range of 1.6–9 × 1018 cm−3 and 118 – 52 cm2 V−1 s−1, respectively, which are not affected by a heat treatment at temperatures up to 1000 °C in an oxidizing atmosphere. Temperature-dependent electrical properties of the crystals revealed a degenerated semiconducting state. Both high structural quality and electrical properties make the crystals well suited as substrates for homoepitaxy and electronic device fabrication in the vertical configuration.

Effective Suppression of MIS Interface Defects Using Boron Nitride toward High-Performance Ta-Doped-β-Ga2O3 MISFETs.

β-Ga2O3 is considered an attractive candidate for next-generation high-power electronics due to its large band gap of 4.9 eV and high breakdown electrical field of 8 MV/cm. However, the relatively low carrier concentration and low electron mobility in the β-Ga2O3-based device limit its application. Herein, the high-quality β-Ga2O3 single crystal with high doping concentration of ∼3.2 × 1019 cm-3 was realized using an optical float-zone method through Ta doping. In contrast to the SiO2/β-Ga2O3 gate stack structure, we used hexagonal boron nitride as the gate insulator, which is sufficient to suppress the metal-insulator-semiconductor (MIS) interface defects of the β-Ga2O3-based MIS field-effect transistors (FETs), exhibiting outstanding performances with a low specific on-resistance of ∼6.3 mΩ·cm2, a high current on/off ratio of ∼108, and a high mobility of ∼91.0 cm2/(V s). Our findings offer a unique perspective to fabricate high-performance β-Ga2O3 FETs for next-generation high-power nanoelectronic applications.

Disordering of β-Ga2O3 upon irradiation with Si+ ions: Effect of surface orientation

Development of gallium oxide-based devices is one of the most promising areas of modern electronic technology. The method of ion implantation is widely used in electronic engineering; however, it has not yet been sufficiently studied for gallium oxide from a physical point of view. One of the interesting questions is the possible impact of surface orientation of β-Ga2O3 single crystals on the structural damage upon ion irradiation. In this work, the effect of Si+ ion irradiation on the structural properties of β-Ga2O3 crystals with (−201) and (010) surface orientations has been investigated by X-ray diffraction and small-angle reflection electronography. It is established that mechanical stresses in the irradiated layer depend on surface orientation: in the case of (−201) orientation, the stress in lateral to the surface direction is compressive, while it is tensile for the (010) orientation. At one and the same irradiation dose, the degree of damage for (010) orientation is shown to be stronger than that for (−201). Possible explanation is suggested for the surface orientation effect.

Crucial Role of Oxygen Vacancies in Scintillation and Optical Properties of Undoped and Al-Doped β-Ga2O3 Single Crystals

In this paper, the effects of oxygen vacancy and gallium vacancy on the optical and scintillation properties of undoped β-Ga2O3 crystal and 2.5 mol % Al doped gallium oxide were investigated. For the undoped β-Ga2O3, the transmittance is improved after annealing in oxygen or nitrogen atmosphere. After the introduction of Al element, the absorption cutoff appears slightly blue shift, and the band gap increases. For the undoped as-grown β-Ga2O3 single crystals, the decay time consists of a fast component (τ1) of the order of nanoseconds, and two slow components (τ2, τ3) of tens to hundreds of nanoseconds. The contribution of the fast decay time component in the decay times is 2.78%. While for Al-doped β-Ga2O3, the faster (τ1) time is 2.33 ns for the as-grown one, and the contribution is 68.02%. However, the pulse height spectrum shows that the introduction of 2.5 mol % Al will reduce the light yield of the β-Ga2O3 crystal.

Defect energy levels in monoclinic β-Ga2O3

The defect levels in semiconductor are crucial to the performance of related devices. However, the positions of the defect levels in monoclinic β-Ga2O3, such as that for oxygen vacancy (VO), Ga–O divacancy, and self-trapped hole (STH), are still controversial. In this paper, we systematically investigated the photoluminescence and photoluminescence excitation spectra of undoped, Si-doped, and Mg-doped β-Ga2O3 single crystals grown by the edge-defined film-fed growth method. It is found that the oxygen vacancies are deep donors and their levels are related to the coordinations of the vacancies. For each crystal, the recombination between VO and Ga–O divacancy levels is the origin of the blue emission, and the red emission originates from the transition from VO to STH levels. The location of STH level is also related to the geometrical position of oxygen. For the Si-doped crystal, no red emission is observed due to the annihilation of STHs. While a Mg impurity level is produced at ∼1 eV above the valence band maximum, which causes an extra red emission in the spectra of Mg-doped crystal. Our results strongly support the recent theoretical results obtained via first-principle calculations [Q. D. Ho et al., Phys. Rev. B 97 (2018) 115163] and could be benefit for designing β-Ga2O3 related devices.

Electronic and ionic conductivity in β-Ga2O3 single crystals

In this work, we quantify electronic and ionic contributions to conductivity in the bulk and depletion widths of back-to-back sputtered Pt Schottky contacts on single crystals of β-Ga2O3. We also demonstrate asymmetric changes to these contacts induced by DC bias at temperatures as low as 200 °C, which has obvious bearing on the performance and reliability of devices. Crystals, which were unintentionally doped, doped with Zr donors, and doped with Mg acceptors, were characterized from room temperature to 900 °C. Electrochemical impedance spectroscopy, current–voltage (IV), capacitance–voltage, and the Wagner DC polarization method were employed to characterize changes in conductivity, doping, and built-in potentials with temperature and bias. This work demonstrates that ionic conductivity can be on-par with electronic conductivity in multiple circumstances in bulk crystal samples and leads to changes in Schottky contacts with an applied bias. While it has not been demonstrated that these phenomena are endemic to all melt-grown β-Ga2O3 crystals and epitaxially grown layers were not investigated, these results highlight the importance of understanding and controlling ionic conductivity and defect reactions involving impurities and native defects to enable reliable and long-lived β-Ga2O3 power devices.

Solubility of iridium into β-Ga2O3 single crystals grown by floating zone method

To reveal the solubility of Ir into β-Ga2O3 phase, various Ir-doped β-Ga2O3 single crystals were grown by the floating zone method using an optical image furnace. The single crystals grown with the nominal composition of (Ga1-xIrx)2O3+δ (x = 3 × 10−4) were inclusion-free whereas the single crystals grown with x = 5 × 10−4 and 1 × 10−3 contained inclusions of Ir metal with the typical size of 10–20 μm. The solubility of Ir into the β-Ga2O3 phase in the crystals grown with above x = 5 × 10−4 was estimated to be 5.0 ± 2.9 × 1016 cm−3 (x = 2.6 ± 1.5 × 10−6).

Growth of bulk β-Ga2O3 single crystals by the Czochralski method

The present Tutorial provides a description of the growth of bulk β-Ga2O3 single crystals by the Czochralski method with a focus on the critical growth aspects. In particular, it details the thermodynamics of Ga2O3 as the key factor for crystal growth along with comprehensive solutions that are essential for crystal size scale-up. Also, free carrier absorption is discussed, as it has a great impact on growth stability. The crystal growth direction and intentional doping, as important factors for crystal growth, are described as well. All of the aspects of the crystal growth are accompanied by resulting crystals, their structural quality, and basic physical properties.

Three-photon absorption and Kerr nonlinearity in pristine and doped β-Ga2O3 single crystals

We report the spectral dependence of the three-photon absorption coefficient γ and third-order nonlinear refractive index n2 in pristine, Sn-doped and Fe-doped β-Ga2O3 crystals using 190 fs pulses in the range of 540–750 nm, i.e., photon energy between half and one-third of the bandgap Eg. The nonlinear coefficient n2 is found to be independent of Sn or Fe doping, and the maximum value of n2 is about 4 × 10−19 m2/W when the photon energy is near Eg/2. However, we observe that Fe doping can significantly enhance the γ value in the range of 540–600 nm (near Eg/2) while doping has negligible impact on γ for wavelength larger than 620 nm. Femtosecond pump-probe measurements show that Fe doping induced a fast (∼50 ps) carrier decay process after photo excitation, which could be attributed to the Fe induced deep trap states. We obtain three-photon figures of merit that allow operational intensities up to 100 GW/cm2. Our results indicate that β-Ga2O3 has great potential for all-optical switching applications in red and near IR regimes.

Anisotropies of angle-resolved polarized Raman response identifying in low miller index β-Ga2O3 single crystal

Beta-phase gallium oxide (β-Ga2O3) with a monoclinic lattice structure is a research hotspot in the field of ultra-wide bandgap semiconductors in the world and displays intriguing anisotropic properties. Here, we synthesize β-Ga2O3 single-crystal (SC) using the edge-defined film-fed growth method and systematically investigate the anisotropic structural and vibrational properties. The anisotropic nature of β-Ga2O3 SC is revealed by angle-resolved polarized Raman spectroscopy under linearly polarized excitations, in which different vibration modes exhibit pronounced periodic variations in intensity. In the detection of different excitation wavelengths, not only the Raman intensity show significant change, but also the strongest main peak of the Raman modes also shows significant difference. In addition, the angular dependence of the peak intensities of the Ag(4), Ag(6), Ag(7), Ag(10) and Bg modes will also be affected by different low Miller index. All the experimental results above are beneficial to the understanding of inelastic light-scattering process of β-Ga2O3 SC.

Impact of H+, O+ and Electron Radiation on Optoelectronic Properties of Β-Ga2o3 Single Crystals

Impact of H+, O+ and Electron Radiation on Optoelectronic Properties of Β-Ga2o3 Single Crystals

Controllability of β-Ga2O3 single crystal conductivity by V doping

Strategy for realizing the tunability of the effective carrier concentration of β-Ga2O3 single crystals using V as a dopant.

Impact of H+, O+ and Electron Radiation on Optoelectronic Properties of Β-Ga2o3 Single Crystals

Impact of H+, O+ and Electron Radiation on Optoelectronic Properties of Β-Ga2o3 Single Crystals

Oxygen diffusion in β-Ga2O3 single crystals at high temperatures

Monoclinic gallium oxide (β-Ga2O3) is an ultra-wide bandgap semiconductor with importance in various technological areas. We investigated oxygen tracer self-diffusion in (100) oriented β-Ga2O3 single crystals at high temperatures between 1200 °C and 1600 °C. Isotope enriched 18O2 gas was used as a tracer source. The isotope exchanged samples were analyzed by secondary ion mass spectrometry in depth profile mode. The diffusivities can be described by the Arrhenius law with an activation enthalpy of (3.2 ± 0.4) eV. Possible diffusion mechanisms are discussed using defect equilibria and density functional theory calculations as found in the literature. As a result, oxygen interstitials are more likely than vacancies as defects governing diffusion.

Atomic-scale characterization of structural and electronic properties of Hf doped β-Ga2O3

In this Letter, we investigate the atomic and electronic structure of a Hf-doped beta-gallium oxide (β-Ga2O3) single crystal using high resolution scanning transmission electron microscopy imaging and electron energy loss spectroscopy. Ultraviolet-visible (UV-Vis)-near-infrared absorption measurements and density functional theory calculations are performed to further connect the nanoscale observation to the macroscale properties arising from the atomic structure. The Hf-doped sample was grown from the melt with a nominal Hf concentration of 0.5 at. %. We show that the Hf dopants prefer to occupy octahedral over tetrahedral sites by 0.68 eV and have some resistance to form precipitates due to a repulsive interaction of 0.17 eV between Hf atoms on neighboring sites. Also, the presence of Hf atoms on either tetrahedral or octahedral sites do not significantly affect the crystal structure of β-Ga2O3. Finally, the bandgap values of the Hf doped β-Ga2O3 obtained by electron energy loss spectroscopy and UV-Vis-spectroscopy were Eg = 4.83 ± 0.1 and 4.75 ± 0.02 eV, respectively, similar to the values reported for unintentionally doped β-Ga2O3 crystals. All these results make Hf an excellent dopant candidate for β-Ga2O3.

Band Gap Engineering in β-Ga2O3 for a High-Performance X-ray Detector

Gallium oxide (Ga2O3) attracts great attention in the field of X-ray detection because of its ultrawide band gap, high breakdown electric field, and high X-ray absorption coefficient. However, unintentionally doped Ga2O3 tends to have low resistivity because of the shallow donors provided by unintentionally doped impurity elements or intrinsic defects. Iron and magnesium ion doping can increase the resistivity of β-Ga2O3, but the carrier drift length and carrier collection efficiency are greatly reduced because of the introduction of deep-level impurities. Here, Al-doped β-Ga2O3 (β-Ga2O3:Al) single crystals with high resistivity are obtained through band gap engineering. The mechanism by which Al3+ doping can increase the resistivity of the β-Ga2O3 crystal is discussed. A β-Ga2O3:15%Al-based X-ray detector with high resistivity and quality is prepared. The detector demonstrates a high sensitivity of 851.6 μC Gyair–1 cm–2, which is 42 times higher than that of the commercial amorphous Se X-ray detector. Furthermore, the detector exhibits a fast response speed and both rise time and decay time of less than 0.05 s. The high performance of the detector is attributed to the high resistivity and high quality of the crystal. This work presents a method to obtain high-performance X-ray detectors based on β-Ga2O3 single crystals through band gap engineering.