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The β-Ga2O3 (011) plane could be a potential substrate orientation for power devices because it avoids a kind of dislocation which leads to leakage current and breakdown of power devices as the dislocations propagate parallel to the (011) plane.

Ultrahigh resistivity stability over a wide temperature range of semi-insulating (SI) β-Ga2O3 is required in challenging temperature environments, such as fire prewarning, atmospheric monitoring and space exploration. A promising deep-level...

A simple sonication–heat treatment method to prepare novel urchin-like β-Ga2O3 microspheres with uniformly distributed nanowires is reported. Its unique morphology is expected to have potential in optoelectronic devices and other fields.

Owing to the advantages of ultra-wide bandgap and rich material systems, gallium oxide (Ga2O3) has emerged as a highly viable semiconductor material for new researches. This article mainly focuses on the growth processes, material characteristics, and applications of Ga2O3. Compared with single crystals and the epitaxial growth of other wide-bandgap semiconductors, large-size and high-quality β-Ga2O3 single crystals can be efficiently grown with a low cost, making them highly competitive. Thanks to the availability of high-quality single crystals, epitaxial films, and rich material systems, high-performance semiconductor devices based on Ga2O3 go through a booming development in recent years. The defects and interfaces of Ga2O3 are comprehensively analyzed owing to their significant influence on practical applications. In this study, the two most common applications of Ga2O3 materials are introduced. The high breakdown electric field, high working temperature, and excellent Baliga's figure-of-merit of Ga2O3 represent an inspiring prospect for power electronic devices. In addition, the excellent absorption in deep-ultraviolet band provides new ideas for optoelectronic detectors and ensures the dramatic progress. Finally, the summary, challenges, and prospects of the Ga2O3 materials and devices are presented and discussed.

The Ga2O3 solar-blind photodetectors (SBPDs) face the tradeoff between power consumption ( $P_{\text {C}}$ ) and photoresponse performance. Here, an ultrasensitive two-terminal photodetector based on $\boldsymbol {\beta }$ -Ga2O3 microflake with extremely-low working voltage and ${P}_{\text {C}}$ was achieved. At a working voltage of 2 V and ${P}_{\text {C}}$ of 10 fW, the Ga2O3 SBPD exhibits superexcellent photodetection performance, including a responsivity ( ${R}$ ) of ${2.3}\,\,\times \,\,{10}^{{5}}\text{A}$ /W, a detectivity ( ${D}^{ \boldsymbol {\ast }}$ ) of ${3.5}\,\,\times \,\,{10}^{{18}}$ Jones, a photo-to-dark-current ratio ( PDCR ) of ${3.2}\,\,\times \,\,{10}^{{8}}$ , and an ultra-low dark current ( ${I}_{\text {dark}}$ ) of 5 fA. Strikingly, the device keeps satisfactory performance at ultralow working voltage of 0.01 V, including a ${P}_{\text {C}}$ of 0.05 fW, a ${R}$ of ${2.4}\,\,\times \,\,{10}^{{3}}$ A/W, a ${D}^{ \boldsymbol {\ast }}$ of ${5.6}\,\,\times \,\,{10}^{{16}}$ Jones, and a PDCR of ${3.4}\,\,\times \,\,{10}^{{6}}$ . The superior solar-blind sensitivity makes it the most excellent Ga2O3 detector towards high-performance and low- $P_{\text {C}}$ SBPD applications.

This work reports a $\beta $ -Ga2O3 double-barrier Schottky barrier diode (DBSBD) with both low turn-on voltage and low reverse leakage current by using Ni and PtOx as the anode electrode. The barrier height of PtOx-based diode can be effectively modulated from 1.26 to 1.62 eV by adjusting oxygen pressure during sputtering processes. Combining the maximum work function of PtOx electrode with the optimization of the electrode ratio of Ni and PtOx, the DBSBD with an electrode diameter ratio of $\text{D}_{\text {Ni}} /\text{D}_{\text {PtOx}}= {75}/{150} \mu \text{m}$ not only exhibits a high forward current of 470.9 A/cm2 (at 3.5 V), a low on-resistance of 4.1 $\text{m}\Omega ~\cdot $ cm2 and a low turn-on voltage of 1.13 V, but also possesses a relatively low reverse leakage current of ${1.2}\times {10}^{-{7}}$ A/cm2 (at −100 V), which is more than one order of magnitude lower than that of the Ni-SBD. Silvaco TCAD simulation reveals that such optimization can be attributed to the suppression of edge leakage current due to the double-barrier contact. Therefore, the strategy of double-barrier design can balance the forward and reverse characteristics in SBD, providing a new device structure for advanced power electronics.

This letter reports a high-performance solar-blind phototransistor based on N2-annealed $\beta $ -Ga2O3 microflake for weak light detection. The phototransistor exhibits an ultra-low dark current of 27 fA, a high external quantum efficiency of $8.36\times 10^{7}$ %, a photo-to-dark-current ratio of $1.08\times 10 ^{7}$ , a low power consumption of 2.92 pW, and a narrow-band response with a cut off wavelength of 263 nm. In addition, the spectral selectivity can be well modulated by the gate bias and reaches a maximum ${R} _{240}/{R} _{400}$ ratio of $2.4\times 10 ^{4}$ . Especially, an ultra-high responsivity ( ${R}$ ) of $1.71\times 10^{5}$ A/W and a record-high detectivity ( ${D} ^{\ast }$ ) of $1.19\times 10^{18}$ Jones have been achieved under 254nm illumination of $4.1~\mu \text{W}$ /cm2. The superior weak-light-detection performance of the device makes it one of the best Ga2O3 detectors towards solar-blind photodetection application.

We designed an original and effective method to study the laser damage mechanism of a β-Ga2O3 single crystal grown by the edge-defined film-fed growth (EFG) method. Structure destruction under irradiation with a high light field intensity of 1064 nm using an Nd:YAG laser was systematically studied in the β-Ga2O3 crystal. Four typical crystal defects, namely, voids, amorphous and nanocrystalline layers and high density stacking faults, were found using a focused ion beam–dual beam scanning electron microscope (FIB-SEM) and a transmission electron microscope (TEM). For the first time, a laser damage mechanism in the β-Ga2O3 subsurface layer was built up and illuminated by microstructure analysis. The stacking fault relaxation of the β-Ga2O3 single crystal was observed in situ for the first time under electron beam irradiation. The cathodoluminescence (CL) spectrum further revealed the luminescence properties of the stacking faults.

Beta-type gallium oxide (β-Ga2O3) has attracted enormous attention for application as a transparent conductive oxide (TCO) semiconductor due to its wide bandgap at around 4.9 eV. However, unintentionally doped β-Ga2O3 crystals by the edge-defined film-fed growth (EFG) method have blue coloration in some cases, making their application as TCO a challenge. Herein, we focus on the investigation of the blue color center in β-Ga2O3 crystals grown by EFG. Glow discharge mass spectrometry (GDMS) was used to measure the impurity concentrations. X-ray diffraction revealed contraction of the crystal lattice in the blue area. The wet etching method was used to demonstrate the density of etch pits. The blue emission with high intensity from cathodoluminescence (CL) proved the existence of a sufficient number of vacancies in the blue area. X-ray photoelectron spectroscopy (XPS) analysis showed that GaI3d (Ga+) abounded in the blue area. Transmission spectroscopy combined with capacitance–voltage (C–V) measurement provided evidence that the blue area with lower transmittance from UV to NIR was more conductive. GaI3d (Ga+) induced by vacancies was speculated to be the blue color center. The blue color center can be removed by high-temperature annealing in an oxygen atmosphere.

The sesquioxide Lu2O3 single crystal has enormous potential applications as host material for solid-state lasers operating at high average powers, and its development is limited by the lack of large sized single crystals with high quality. Compared with the traditional Czochralski (Cz) method, the edge-defined film-fed growth (EFG) method, employing a die or a shaper, doesn't make the grown crystal come into contact with the melt in crucible directly during the growth process. Therefore, the growth interface is stable and the serious bottleneck problems of a “w”-shaped isotherm in the Czochralski method could be avoided effectively. Herein, the EFG method may be a promising technique to obtain ultra-high temperature sesquioxide single crystals with a large size, and more importantly, high quality. Accordingly, the sesquioxide Lu2O3 single crystal has been grown controllably with an oriented seed by the EFG method for the first time and the EFG method is promoted to an extremely high temperature of 2450 °C. The dimensions of the grown crystal are Φ 25 × 30 mm3, and the difficulties encountered during the crystal growth have been discussed in detail. Furthermore, the quality of the crystal has been evaluated. The optical properties of the grown crystal have been investigated.

High quality columnar β-Ga2O3 crystals were successfully grown via an edge-defined film-fed growth (EFG) method equipped with columnar Ir die. The effects of the pulling rate and die height on the structural quality and crystal shape of β-Ga2O3 are summarized and discussed in detail. The main problems of spiral and polycrystal formation and incomplete columnar quality in the β-Ga2O3 crystal growth were solved successfully. Suitable values for the growth parameters such as pulling rate and Ir height were determined to be in the range between 2–10 mm h−1 and 30–40 mm (0.6–0.8 mm longer than the crucible height), respectively. Simultaneously, the die diameter should be 25 mm (0.5 mm wider than the crucible diameter) to obtain columnar β-Ga2O3 crystals in good quality. Furthermore, the crystalline quality of the as-grown crystal was confirmed via high-resolution X-ray diffraction (HRXRD) with a full-width at half-maximum (FWHM) of 69.3 arcsec. The defects of β-Ga2O3 were studied via a chemical etching method. Two types of etch pits were observed: (1) a triangle-shaped etch pit and (2) a quadrangle-shaped etch pit. The average defect density was estimated at a lower order of magnitude of −104 cm−2.

A novel disordered structure crystal has drawn intense interest as a promising ultrafast laser crystal host for its broad absorption and emission bands. Herein, a novel disordered crystal of Nd3+-doped CaYAl3O7 (CYAM) was successfully grown using the Czochralski method for the first time. The effective segregation coefficient of Nd3+ ions in CaYAl3O7 was calculated to be 0.76. To better understand the intrinsic optical properties, the transmission spectra and refractive index of pure CaYAl3O7 were measured. A systematic investigation of the crystal structure characteristics, fluorescence lifetime, and polarized absorption and emission cross-sections of the Nd:CYAM crystal at RT was reported. The maximal absorption cross-sections are 3.51 and 5.34 × 10−20 cm2 for π-polarization and σ-polarization, with a FWHM of 18 nm. The emission cross-sections of π- and σ-polarization are 5.97 and 4.55 × 10−20 cm2 with FWHM values of 14 and 14.4 nm, respectively. The results show that the Nd:CYAM crystal has broad, strong absorption and emission cross-sections. In particular, effective continuous-wave laser operation was realized with this crystal in the a-cut sample for the first time. A maximum output power of 714 mW was achieved, corresponding to an optical-to-optical conversion efficiency of 7.14% and a slope efficiency of 8.6%. Collectively, these results show that the Nd:CaYAl3O7 crystal is a promising disordered laser host.

High performance $\beta-\text{Ga}_{2}\mathrm{O}_{3}$ field-effect transistor (FET) solar-blind photodetectors (SBPDs) with record detectivity (D*) of $3.87\times 10^{17}$ Jones, responsivity (R) of $2.50\times 10^{4}\mathrm{A}/\mathrm{W}$ , photo-to-dark-current ratio (PDCR) of $7.87\times 10^{6}$ , and external quantum efficiency (EQE) of $1.22\times 10^{7}\%$ are reported based on mechanically exfoliated $\beta-\text{Ga}_{2}\mathrm{O}_{3}$ micro-flakes. The excellent performance of the $\beta-\text{Ga}_{2}\mathrm{O}_{3}$ detector under $\mu \mathrm{W}/\text{cm}^{2}$ level illumination makes it one of the best Ga 2 O 3 SBPDs towards weak light detection, sharp imaging, and safe communication application.

A high-performance Schottky diode based on a 600- ${\mu }\text{m}$ -thick Cr-doped ${\beta }$ -Ga2O3 single crystal has been fabricated using SnOx as the Schottky contact. The SnOx film was deposited in argon/oxygen mixture gas to ensure an oxygen-rich stoichiometry in Ga2O3 near the Schottky interface, thus reducing oxygen deficiency-related interface state density. The SnOx film included three components: Sn, SnO, and SnO2, as revealed by X-ray photoelectron spectroscopy characterization. The high-quality Ga2O3 single crystal grown by an edge-defined film-fed method has a carrier concentration of $1.0 \times 10^{18}$ cm $^{-3}$ and an electron mobility of ~90 cm2/Vs. The current density–voltage characteristics of the Schottky diode demonstrated high performance with a large barrier height of 1.19 eV, a close-to-unity ideality factor of 1.02, and a high rectification ratio beyond $10^{10}$ . The frequency-dependent capacitance and conductance analysis revealed that the maximum active interface state density is $2.46 \times 10^{12}$ eV $^{-1}$ cm $^{-2}$ at a frequency of 500 kHz.

The edge-defined film-fed growth (EFG) is a promising method to grow large β-Ga2O3 crystal. The thermal field in the EFG system should be designed to provide proper conditions for growing large-size and high-quality crystals. 2D calculation of the meniscus shape and 3D global simulations are carried out to investigate and design the thermal fields in an EFG system for the growth of β-Ga2O3 ribbon crystals. Effects of the induction coil position on the thermal field before seeding are firstly studied. Results indicate that it is difficult to improve all features of the thermal field just by changing the coil position. An improved design of adding a crucible cover is then proposed and investigated. It turns out that the crucible cover is a feasible design to improve the thermal field for the EFG growth of β-Ga2O3 ribbon crystals comprehensively. The studies are expected to provide essential knowledge for further design and optimization of the EFG system.

Herein, bulk single crystals of Er:Y3Sc2Ga3O12 (Er:YSGG) with a maximum diameter of 35 mm or a maximum length of 100 mm were successfully grown by the Czochralski method using a 60 mm diameter crucible. The high-resolution X-ray diffraction and optical interferometry results indicated that the as-grown crystals were of high quality. The crystalline perfection of the as-grown crystal was evaluated to be 20.58′′ via the FWHM of the rocking curve, and the optical homogeneity RMS was in the order of 10−6. A continuous wave laser of the Er:YSGG crystal operating at 2798.7 nm has been demonstrated. Moreover, a maximum output power of 745 mW has been realized. The achieved maximum slope efficiency is 31.5%, which is, to date, the highest slope efficiency in the Er:YSGG crystal lasers operating at ∼2.8 μm.

Herein, the solid–liquid interface and bulk crystal growth habit of β-Ga2O3 were studied during the Czochralski and the modified edge-defined film-fed growth (EFG) methods, respectively. The technical characteristics and process optimizations of the two methods were systematically compared and explored during the crystal growth of β-Ga2O3. In the Czochralski method, the phenomenon of spiral growth was explained based on a concave solid–liquid interface and heat transfer at high temperatures. To overcome the problem of spiral growth, a square Ir die was designed and used for the growth of β-Ga2O3 in the EFG method, and high quality β-Ga2O3 single crystals were obtained with a flat interface. The full-width at half-maximum (FWHM) of the X-ray rocking curve on the (100)-faced β-Ga2O3 grown by the EFG method was only 35.6 arcsec, indicating high crystalline quality. The carrier source of the unintentionally doped β-Ga2O3 was determined via impurity elemental analysis. A relationship between the carrier concentrations and mid-IR transmission was established. Furthermore, the PL and Raman spectra of the as-grown and annealed crystals were obtained to reveal the energy structure and stress in the crystal.

In this Letter, we report on establishing high performance hysteresis-free and μs-switching depletion/enhancement-mode (D/E-mode) β-Ga2O3 heterojunction (HJ) field effect transistors (FETs) with the state-of-art power figure-of-merit (P-FOM). By optimizing the p-NiOx/n-Ga2O3 interface and n-Ga2O3 recess technology, a positive threshold voltage (VT) as well as a low subthreshold slope can be substantially achieved. The trade-off between the on-resistance (Ron,sp) and breakdown voltage (BV) is improved by incorporation of T-shaped NiOx, resulting in the Ron,sp of 6.24/13.75 mΩ cm2 and the breakdown voltage (BV) of 2145/1977 V for D/E-mode devices and yielding the P-FOM = BV2/Ron,sp to be 0.74/0.28 GW/cm2. To the best of all the authors' knowledge, those P-FOMs are the highest ones among all published lateral Ga2O3 FETs. Benefited from the high-quality interface, a negligible hysteresis of 4 mV and μs-switching can be essentially achieved, showing the great promise of Ga2O3 HJ-FETs for future high-power, high-efficiency, and high-speed power electronics.

The cylindrical Sn: β-Ga2O3 crystal with high crystalline quality was successfully designed and grown by the innovative edge-defined film-fed growth (EFG) method equipped with a cylindrical Iridium die. The challenges for the growth of Sn: β-Ga2O3 crystals were overcome by optimizing the design of an afterheater. The growth morphology of cylindrical β-Ga2O3 crystal was studied using a theoretical model and the results from experimental crystal growth. The order of importance of growth conditions affecting β-Ga2O3 crystal growth morphology was examined, based on the morphological features of cylindrical β-Ga2O3 crystals obtained by the EFG and Czochralski methods. The iridium inclusions with three shapes were observed in bulk β-Ga2O3 crystal, and the formation mechanism was carefully discussed. The optical bandgap and valence band maximum (VBM) of Sn: β-Ga2O3 crystal were calculated to be 4.74 eV and 3.49 eV by absorption spectra and X-ray photoelectron spectroscopy (XPS), respectively. The corresponding surface barrier height (Φsurf) was 1.25 eV. The carrier concentration of 5.95 × 1018 cm-3 was characterized by capacitance-voltage (C-V) measurement. By the Hall measurement, the carrier mobility and resistivity were estimated to be around 51 cm2 V-1 s-1 and 3.55×10–2 Ω cm, respectively.

We report a self-Q-switched mode-locked Nd:LGGG waveguide laser with tunable dual-wavelength emission fabricated by femtosecond laser direct writing (FsLDW). The waveguide laser delivers pulses with a minimum temporal duration of approximately 30 ps at a fundamental repetition rate of up to 8.03 GHz, without using any modulation element. The maximum output power is determined to be 226 mW with a slope efficiency of 25.38%. This work indicates the promising applications of waveguide lasers based on mixed crystals for integrated optics.

We have grown β-(In0.04Ga0.96)2O3 thin film by pulsed laser deposition(PLD) on Sn doped β-Ga2O3(-201) substrates and fabricated the Au/Ni/β-(InGa)2O3 Schottky diodes on it. The electrical characteristics of the diodes were investigated within the temperature range from 300K to 400K . Based on the theory of thermal electron emission (TE), the extracted barrier height ϕb and ideality factor n are strongly temperature dependent. ϕb increasing from 0.83 eV to 0.93 eV while n decreasing from 2.82 to 1.83, respectively. By taking the Gaussian distribution of the Schottky barrier height into account , the average barrier height was calculated to be 1.18 eV with a standard deviation of 0.135 V and the effective Richardson constant A* was also determined to be 42.34 Acm−2K−2.

A Schottky barrier rectifier was fabricated with a (100)-oriented $\beta $ -Ga2O3 substrate grown by the edge-defined film-fed method. The Sn-doped $\beta $ -Ga2O3 substrate had an effective donor concentration of approximately $2\times 10^{17}$ cm−3. High performance parameters were obtained, such as a high forward current (421 A/cm2 at 2 V), low ON-resistance (2.9 $\text{m}\Omega \cdot $ cm2), and short reverse recovery time (20 ns). Furthermore, the dynamic behavior of the device is characterized through test on the half-wave rectification of ac voltages at different frequency. The diode worked well at 100 kHz. At the end of the letter, we discuss how Ga2O3 Schottky rectifier can operate at high frequency.

β-Ga2O3, a semiconductor material, has attracted considerable attention given its potential applications in high-power devices, such as high-performance field-effect transistors. For decades, β-Ga2O3 has been processed through chemical mechanical polishing (CMP). Nevertheless, the understanding of the effect of OH− on β-Ga2O3 processed through CMP with an alkaline slurry remains limited. In this study, β-Ga2O3 substrates were successively subjected to mechanical polishing (MP), CMP and etching. Then, to investigate the changes that occurred on the surfaces of the samples, samples were characterised through atomic force microscopy (AFM), three-dimensional laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). LSCM and SEM results showed that β-Ga2O3 is highly vulnerable to brittle fracture during MP. AFM revealed that an ultrasmooth and nondamaged surface with a low Ra of approximately 0.18 nm could be obtained through CMP. XPS results indicated that a metamorphic layer, which mainly contains soluble gallium salt (Ga(OH)4−), formed on the β-Ga2O3 surface through a chemical reaction. A dendritic pattern appeared on the surface of β-Ga2O3 after chemical etching. This phenomenon indicated that the chemical reaction on the β-Ga2O3 surface occurred in a nonuniform and selective manner. The results of this study will aid the optimization of slurry preparation and CMP.

This work studied the structural, thermal, optical and electrical properties of β-Ga2O3 (101) plane with high-density surface dangling bonds for the first time. The high quality (101)-oriented substrate with a full width at half maximum (FWHM) of 90″ in X-ray rocking curve was grown by the edge-defined film-fed growth (EFG) method. The atomic arrangement of (101) surface was terminated either by Ga or O. The dangling bond densities of Ga- and O-terminated surfaces were calculated to be 2.230 × 1015 cm−2 and 2.376 × 1015 cm−2, respectively. The thermal property of the (101) plane was characterized. The Raman active modes of the (101) plane were allowed for selective observation with the variation of polarizing angle. The surface and Schottky barrier height of the (101) plane were estimated to be as low as 1.14 eV and 1.07 eV by optical and electrical measurements, respectively. The activation energy for wet chemical etching was determined to be 0.451 eV for (101) plane and the high etching rate was obtained. By comprehensive comparisons with other common planes, the potential applications of β-Ga2O3 (101) plane should be comparable to the widely used 2 01 plane.

Device-size Nd3+:(LuxGd1−x)3Ga5O12 (Nd:LGGG) single crystal plates have been grown by edge-defined film-fed growth (EFG) method for the first time. The problems encountered during the crystal growth have been discussed and solved, resulting in a single crystal plate with a length of 180 mm. In particular, the evaporation loss of Ga2O3 composition during the crystal growing has been depressed efficiently by using an Ir lid. The crystal perfection was confirmed by X-ray rocking curve with a FWHM of the 32 arcsec, meaning a high crystalline quality. It was very interesting to find that the distribution of Nd3+ in the crystal grown by EFG method was more homogeneous than that in Cz method, benefitting from the larger segregation coefficient of Nd3+ in EFG method. The thermal conductivity was measured to be 8.1 W m−1 K−1 at room temperature. All the properties showed that the Nd:LGGG crystal plates grown by EFG method were promising for high power laser application.

In order to further improve the Nd3+:(LuxGd1-x)3Ga5O12 (Nd:LGGG) crystal performance in high power laser field, a long Nd:LGGG crystal with dimensions of Φ 23 × 112 mm3 has been grown successfully by the Czochralski (Cz) method for laser rod fabrication. Compared with the normal size LGGG crystals (like 30–50 mm in length), we overcame several difficulties in the growth of longer ones, including crystal cracking by a large longitudinal temperature gradient, spiral growth by a small radial temperature gradient, and growth instability and even constitutional super cooling by Ga2O3 volatilizing continuously. The doping concentrations of Nd3+ and Lu3+ in the as-grown crystal and the crystal optical quality have been measured. The performance of diode-side-pumped Nd:LGGG rod laser has been preliminarily tested for the first time, simply by replacing the Nd:YAG crystal rod inside a commercial laser module. Under an incident pump power of 160 W, the maximum continuous wave output power of 38 W has been obtained, corresponding to an optical-optical conversion efficiency of 23.8% and a slope efficiency of 40.8%, respectively.

β-Ga2O3 is an ultra wide-bandgap semiconductor with promising applications in high voltage and temperature electronic devices and deep-ultraviolet optoelectronic devices, and has attracted lots of attention recently. However, there are still many unknowns about the basic properties needed to be explored urgently, which are very important and necessary for the devices design and application. In this work, bulk β-Ga2O3 single crystals with a width of 1 inch were grown by using an optimized edge-defined film-fed growth (EFG) method under Ar plus 50% CO2 atmosphere. High crystalline quality of the as grown crystal has been demonstrated by the high-resolution X-ray diffraction with 43.2 arcsec of the full width at half maximum (FWHM). The fundamental physical properties of different orientated samples, including thermal properties as a function of temperature up to 500 °C, electrical properties, refractive index and hardness were investigated systematically. Moreover, the measurement showed that aluminum turned out to be a good Ohimc electrode for n-type β-Ga2O3 single crystal.

High-quality bulk β-Ga2O3 single crystals have been grown by optimized edge-defined film-fed growth (EFG) method. The problems of cracking and polycrystals have been effectively solved by using a high-quality 1 inch-wide seed. The crystalline quality of the as-grown crystal has been confirmed by X-ray rocking curve with a full-width at half-maximum (FWHM) of 42.1 arcsec. Moreover, an economical and efficient one-step mechanical exfoliation method has been proposed to get epi-ready β-Ga2O3 wafers directly. The root mean square (RMS) roughness of the wafers is noted to be around 0.1 nm, which means they are comparable or even better than wafers processed by chemical mechanical polishing (CMP). The mechanism of the exfoliation method is analyzed from a crystallographic view. A photodetector is fabricated on the exfoliated wafer directly and the device is proved to show good performance in solar-blind band.

Considering their easy growth and doping in bulk crystal growth, and good crystalline quality, ?-Ga2O3 single crystals, a very important wide-bandgap semiconductor, are now also considered to be a promising optical crystal candidate. In this work, a Cr4+:?-Ga2O3 single crystal has been grown successfully by the edge-defined film-fed growth method. The thermal conductivity of Cr4+:?-Ga2O3 has been measured along the a* direction at room temperature obtaining 16.2 W m-1 K-1, and it was much larger than that of the usually used Cr4+-doped crystals, such as Y3Al5O12 (YAG) or YVO4. The Raman spectrum indicated that the cutoff phonon energy of the ?-Ga2O3 crystal was 767.8 cm-1. A passively Q-switched nanosecond pulsed Nd:YAG laser based on a Cr4+:?-Ga2O3 saturable absorber was experimentally demonstrated for the first time to our knowledge and its mechanism is explained by first-principles calculations. By inserting the Cr4+:?-Ga2O3 crystal into the Nd:YAG laser cavity, a Q-switched laser operation was obtained with a maximum average output power of 50 mW. The corresponding pulse repetition rate, pulse width, and pulse energy were determined to be of 421.5 kHz, 235.2 ns and 0.12 ?J, respectively.

High performance Ga2O3 Schottky barrier diodes (SBDs) are achieved by adjusting chemical composition of SnOx Schottky electrode. The SnOx is produced by radio frequency sputtering under various oxygen partial pressures (PO). A synergic study on Raman, X-ray photoelectron, and optical transmission spectroscopy of the SnOx films illustrates that: the films with PO = 0 ~ 3.1% consist mainly SnO and Sn with high conductivity; the films with PO = 4.6 ~ 5.4% are composed of both p-type SnO and n-type SnO2 with high resistance; the films with PO = 10.0 ~ 13.1% are mainly dominated by n-type SnO2 with low resistivity. In addition, as PO increased from 0 to 3.1%, the SBDs performances are significantly improved due to that the SnOdominated films reduce effectively the oxygen-deficiency at the Ga2O3 interfaces and the related interface state density. With PO = 5.4%, the high resistive SnOx results in degraded diode performance because that the SnO2 component with oxygen vacancy defects may aggravate the oxygen-deficiency at the Schottky interface. With the optimized PO of 3.1%, the SBD shows high performance with a large barrier height of 1.12 eV, a near unity ideality factor of 1.22, and a high rectification ratio of > 1010.