β phase gallium oxide (β-Ga₂O₃) is an ideal semiconductor material for power devices based on its ultra-wide bandgap, high breakdown electric field, and easy preparation. However, it is still challenging to realize p-type doping of the β-Ga₂O₃ because of its relatively low energy of valence band maximum (VBM) and flat band dispersion near the VBM, which limits the development of p-n junctions and bipolar transistors. The main strategies for the p-type doping of β-Ga₂O₃ in recent research are based on size effect, defect regulation, non-equilibrium dynamic process, and solid solution. For the β-Ga₂O₃ p-n homojunction and heterojunction, improving crystal quality and reducing the interface defect states are the key issues for optimizing devices’ performances. This paper focuses on the p-type conductivity problem of β-Ga₂O₃, systematically reviews the electronic structure of β-Ga₂O₃, the experimental characterization and theoretical calculation method of doping levels, the reasons for p-type doping difficulty, and the breakthrough in research advancements for improving the p-type doping of β-Ga₂O₃. Finally, the relevant studies on the β-Ga₂O₃ p-n homojunction and heterojunction devices are briefly reviewed. It requires further exploration to realize p-type doping of bulk-phase β-Ga₂O₃ through complex-defect regulation, non-equilibrium dynamics, solid solution, and combining these schemes. The device performances of p-n homojunction and heterojunction also need further optimization.

Using a self-designed vertical Bridgman (VB) furnace, a growth furnace model was established through dynamic simulation and experimental deep coupling iterative optimization method. The optimal temperature field of the growth furnace was obtained by optimizing the temperature field, and the actual temperature field was optimized and modified based on the simulated optimal temperature field. 3-inch (1 inch=2.54 cm) diameter gallium oxide (β-Ga₂O₃) single crystal was successfully grown in this paper. 2.5 inch (100) plane β-Ga₂O₃ wafers were successfully fabricated from the as-grown crystal. The crystalline quality and optical properties of β-Ga₂O₃ crystal were characterized and tested. The test results indicate that, the β-Ga₂O₃ crystal has high crystalline quality, with cut-off edges of ultraviolet for (100) plane is 257.5 nm, corresponding to an optical bandgap of 4.78 eV. The Laue diffraction spots of the crystal are clear and symmetrical, with a minimum full width at half maximum (FWHM) of the rocking curve at 39.6″.

In this work, large-size Fe-doped β-Ga₂O₃ single crystals were grown by Czochralski method. High quality 2 inch (1 inch=2.54 cm) (010) substrates were fabricated, and the crystal quality, processing quality and electrical properties of these substrates were studied. The uniform appearance of the substrates under the polarizing strain gauge indicates the absence of twinning, cracks and other macrocopic defects, confirming their good macrocrystalline quality. The full width at half maximum (FWHM) of the X-ray rocking curve of (020) plane for these substrates is less than 29.7″, reflecting good microcrystalline quality. The surface average roughness (Ra) of the substrates is less than 0.240 nm, with a local thickness variation (LTV) less than 1.769 μm, a total thickness variation (TTV) of 5.092 μm, and a warp of 3.132 μm, suggesting superior substrate processing quality. Furthermore, the electrical resistivity of the substrate is ~7×10¹¹ Ω·cm, facilitating the development of the microwave and radio frequency devices.

β-Ga₂O₃, a semiconductor with a wide bandgap, has garnered significant attention from researchers owing to its remarkable optoelectronic properties. The exploration of how elemental doping affects the spectral properties of β-Ga₂O₃ constitutes a pivotal research area within materials science, offering substantial research value and promising application prospects. In this study, β-Ga₂O₃∶6%Bi single crystal was successfully synthesized in the CO₂ atmosphere through the utilization of the optical floating zone (OFZ) method. The primary focus of this investigation was to delve into the spectral properties of the Bi-doped β-Ga₂O₃ single crystal. To gain a thorough understanding of the samples, a battery of sophisticated characterization techniques was employed. These included X-ray diffraction (XRD) for analyzing the crystal structure, Raman spectroscopy for probing vibrational modes, scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) for examining surface morphology and elemental composition, X-ray photoelectron spectroscopy (XPS) for determining chemical states, and both transmission and fluorescence spectroscopy for assessing optical properties. The experimental outcomes unveiled that it is difficult for Bi ions to be doped into the β-Ga₂O₃ crystal lattice due to the large difference in ionic radii. The doped Bi ions predominantly occupied the sites of Ga ions within the GaO₆ octahedra. Compared with unintentionally doped β-Ga₂O₃, Bi-doped β-Ga₂O₃ single crystals exhibit a decrease in transmittance in the infrared region and an increase in carrier concentration; the emission spectral intensity is reduced, and the fluorescence decay time is shortened. These groundbreaking discoveries not only enhance our comprehension of the spectral properties of Bi-doped β-Ga₂O₃ single crystals but also offer technical insights for the potential application of this material in diverse fields, including scintillator materials and radiation detection systems.

In order to investigate the conductive characteristics of the intrinsic defects, the first-principles calculation method was used to calculate ε-Ga₂O₃ in this paper. Firstly, the lattice constant, band gap, density of states, and band structure of ε-Ga₂O₃ were calculated. Then, the density of states and band structure of ε-Ga₂O₃ with intrinsic defects were calculated, and their electrical properties were analyzed. The results show that ε-Ga₂O₃ is a direct bandgap semiconductor with a bandgap of 4.26 eV, the light absorption peak is around 80 nm and the light absorption coefficient approaches zero at 450 nm. In intrinsic defects, Ga vacancy defects at different sites result in p-type conductivity of ε-Ga₂O₃, while O vacancy defects at different sites do not change the conductivity of ε-Ga₂O₃; after O replaced Ga, ε-Ga₂O₃ exhibits p-type conductivity; after Ga replaced O, ε-Ga₂O₃ exhibits n-type conductivity; The interstitial O atom don’t change the conductivity of ε-Ga₂O₃; The ε-Ga₂O₃ with interstitial Ga atom exhibits n-type conductivity.

The achievement of single crystalline gallium oxide (Ga₂O₃) homoepitaxial layers with atomic-level smoothness is fundamental for the fabrication of high-performance Ga₂O₃-based power electronics or ultraviolet photodetectors. In this study, metal organic vapor phase epitaxy (MOVPE) technique was employed to comprehensively control the thermodynamic conditions and kinetic factors of epitaxial growth, resulting in the production of unintentionally doped, device-grade Ga₂O₃ single crystal films with a thickness of 1.0 μm on Ga₂O₃ substrates. Characterizations of the Ga₂O₃ samples were performed to investigate phase composition, surface morphology, crystal quality, and electrical properties. The Ga₂O₃ homoepilayer exhibits a single β phase with a preferential orientation matching the (100) plane of the substrate. Atomic force microscopy (AFM) analysis reveals a typical step-flow morphology, with a surface roughness of 0.166 nm and a step height of 0.6 nm (a/2), indicating atomic-level smoothness. High-resolution X-ray diffraction (HRXRD) rocking curve analysis was conducted to further evaluate the crystallinity of the Ga₂O₃ epilayers. The full width at half maximum (FWHM) of the epilayers is lower than that of the single crystal substrate, indicating superior quality of the Ga₂O₃ epilayers grown on the lattice-matched substrate. Hall effect measurements indicate an electron mobility of 92.1 cm²/(V·s) and a carrier concentration of 2.65×10¹⁶ cm^-3. Our results demonstrate that high-growth-rate 2D step-flow growth on commonly used non-intentionally miscut substrates can be achieved as long as the critical thermodynamic conditions, such as temperature, pressure, and the Ⅵ/Ⅲ ratio—are finely tuned to ensure that the lateral diffusion rate of the core kinetic parameters is sufficiently greater than the vertical deposition rate. The exceptional crystal quality and electrical properties highlight the significant potential of these (100)-oriented homoepitaxial films in the development of high-performance Ga₂O₃-based power electronics.

Halide vapor phase epitaxy (HVPE) is mainly utilized to obtain β-Ga₂O₃ homoepitaxial wafers because of its advantages such as high growth rate and efficient impurity doping controllability. In this paper, the homoepitaxialβ-Ga₂O₃ thick films were grown by a vertical HVPE system. The effects of different growth pressures on the growth rate and epitaxial quality were investigated. It is found thatwhen growing epitaxial β-Ga₂O₃ films of the same thickness, although reducing the growth pressure slows down the growth rate, it can easily obtain high crystalline quality β-Ga₂O₃ thick films with unbroken microstep arrays. The source of unintentional nitrogen impurities in the epitaxial films was analyzed, and the possibility of nitrogen decomposition was ruled out. By adjusting the Ⅵ/Ⅲ ratio, specifically by increasing the oxygen partial pressure, the concentration of nitrogen impurities in β-Ga₂O₃ epitaxial films can be effectively reduced from 8×10¹⁶ cm^-3 to 1×10¹⁶ cm^-3. Finally, 2-inch high-quality HVPE β-Ga₂O₃ epitaxial wafer has been successfully achieved with optimized epitaxial growth parameters. The film thickness and carrier concentration are 15.8 μm and 1.5×10¹⁶ cm^-3, the inhomogeneity of which are 3.6% and 7.6%, respectively.

Ultra-wide bandgap semiconductor gallium oxide (Ga₂O₃) has important applications in power electronics and information sensing, its efficient and economical preparation is important to realize its industrial promotion. In this paper, a Sn-assisted mist chemical vapor deposition (mist CVD) technique is reported, based on which high-quality pure-phase α-Ga₂O₃ thin films were successfully epitaxially grown on c-plane sapphire substrates by this non-vacuum, low-cost method. The mist CVD growth temperature regulation experiment shows that the temperature for epitaxial growth of pure-phase α-Ga₂O₃ thin films is between 500 and 600 ℃. The physical phase, morphology, optical features, elemental content and valence of the pure-phase α-Ga₂O₃ thin films were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-visible spectrophotometer, and X-ray photoelectron spectrometer (XPS) techniques. The results indicate that the α-Ga₂O₃ thin films grown at the temperature of 600 ℃ possess a higher degree of crystallinity, a denser and flatter surface morphology. Further, the deep-UV (DUV) photoresponse properties of α-Ga₂O₃ thin films were investigated by constructing photodetectors with metal-semiconductor-metal (MSM) structures. For α-Ga₂O₃ thin films prepared at 500 and 600 ℃, the photodetectors show photo-to-dark current ratios (PDCR) of 5.85×10⁵ and 7.48×10³, external quantum efficiencies (EQE) of 21.8% and 520%, and responsivities of 0.044 and 1.09 A/W, respectively. Under 6 V bias voltage and 254 nm illumination, the response time is 0.97/0.36 s for α-Ga₂O₃ thin film grown at 500 ℃, while it increases to 2.89/4.92 s for the sample grown at 600 ℃, which may be ascribed to the formation of donor impurities within the α-Ga₂O₃ thin films by Sn-assisted growth, and affecting the carrier transport efficiency.

In this work, the electronic transport properties of unintentionally doped (UID) and tin-doped β-Ga₂O₃ homoepitaxial thin films grown by molecular beam epitaxy (MBE) are reported, with electron densities ranging from 3.2×10¹⁶ to 2.9×10¹⁹ cm^-3. The UID thin film with an electron density of 3.2×10¹⁶ cm^-3 exhibits an excellent room-temperature mobility of 125 cm²·V^-1·s^-1 and a peak mobility of 875 cm²·V^-1·s^-1 at 80 K, reaching the advanced standard of MBE-grown Ga₂O₃ thin films. Temperature-dependent Hall measurements were utilized to characterize the electronic transport properties of the homoepitaxial thin films, yielding a tin dopant activation energy of 76.2 meV. By fitting the scattering model, the electronic scattering properties of this series of homoepitaxial thin films were analyzed, revealing that ionized impurity (II) scattering from intrinsic defects and polar optical phonon (POP) scattering from Coulombic forces between cations and anions in the crystal limit the mobility growth at low and high temperatures, respectively.

α-Ga₂O₃ heteroepitaxial thin films on sapphire substrates with different crystal planes were prepared by mist-chemical vapor deposition (Mist-CVD) technique. The phase, optical properties and surface morphology of the samples were investigated by XRD, SEM and UV-Vis. The temperature windows for growing pure phase α-Ga₂O₃ thin films on C, M, A and R-plane sapphire substrates within 600 ℃ are 420~480, 480~550, 590~600 and 540~600 ℃, and optical band gaps of the pure phase α-Ga₂O₃ thin film are 5.12, 5.23, 5.25 and 5.21 eV, respectively. Compared to C-plane sapphire substrates, epitaxial α-Ga₂O₃ thin films on M, A and R-plane sapphire substrates require higher growth temperatures, and the bandgap width of the films obtained on M, A and R-plane sapphire substrates are larger. The SEM results of the sample surface show significant differences in the surface morphology of α-Ga₂O₃ thin films with different crystal planes, and there is a "continuous layer+large island" structure on the α-Ga₂O₃ thin film grown on C-plane sapphire substrate. The study of epitaxial α-Ga₂O₃ thin films on sapphire substrates with different crystal planes in this article provide valuable reference for the application of α-Ga₂O₃ materials.

Gallium oxide (β-Ga₂O₃) exhibits an ultra-wide bandgap (E_g=4.5~4.9 eV) and a high critical breakdown electric field (E_br=8 MV/cm). The Baliga's figure of merit for β-Ga₂O₃-based devices is theoretically approximate four times and ten times as large as that of SiC- and GaN-based devices, respectively. Nevertheless, the breakdown voltage of β-Ga₂O₃ power devices is considerably below the theoretical limit; and there are few studies on high-power devices and their thermal stability. In addition, the low thermal conductivity of β-Ga₂O₃ materials and the presence of multiple defects result in many reliability issues, including the shift of electrical characteristics and the accelerated degradation of device performance. First, this work presents our researches on novel structures of β-Ga₂O₃ power devices, including the analysis of experimental results measured from the fabricated devices and the investigation of their electro-thermal characteristics. Second, this work studies the electro-thermal reliability of β-Ga₂O₃ mental-oxide-semiconductor field-effect transistor (MOSFET) and heterojunction field-effect transistor (HJFET). The ionized traps model and interface dipole ionization model are proposed to explain the degradation mechanism of β-Ga₂O₃ MOSFET and HJFET. Additionally, a novel reliability reinforcement technology is proposed to enhance the electro-thermal reliability of β-Ga₂O₃ HJFET. These studies indicate the considerable potential of β-Ga₂O₃ power devices in high-voltage, low-loss and high-reliability applications. Further, these studies provide novel insights into the design and optimisation of β-Ga₂O₃ power devices, and effectively advance the practical development of β-Ga₂O₃ power devices.

The detection of weak ultraviolet light has garnered significant attention for critical applications such as missile tracking, flame warning, secure communication, and environmental monitoring. Avalanche photodetectors (APDs) are a major research direction for UV detection due to their lightweight, low power consumption, high quantum efficiency, and monolithic integration. In recent years, wide bandgap and ultra-wide bandgap semiconductor materials have been regarded as ideal materials for APD design due to their large bandgap, high electron saturation drift velocity, high breakdown field strength, high thermal conductivity, and good chemical stability. Among the reported materials, Ga₂O₃ stands out as a new material of interest due to its larger bandgap, higher breakdown field strength, higher Baliga’s figure-of-merit, and shorter absorption cutoff edge. Ga₂O₃-based APDs, with advantages such as an ultra-wide bandgap, high breakdown electric field, controllable gain, and excellent thermal stability, exhibit high responsivity and internal gain, making them a hot topic in this field. This paper reviews the research progress of Ga₂O₃-based APDs, introducing the device structure, performance, development history, and research improvements of Ga₂O₃ APDs.

The low thermal conductivity of ultrawide bandgap (UWBG) gallium oxide (β-Ga₂O₃) is the most significant bottleneck restricting the development of its power devices, posing a huge challenge for efficient heat dissipation under high-power density conditions. Therefore, developing new thermal management and packaging technologies is extremely urgent. It is crucial to alleviate the performance and reliability issues caused by self-heating through thermal management at the material, device, and packaging levels. This paper provides a timely review of the state of the art in thermal management of UWBG β-Ga₂O₃ power devices, discussing related challenges, potential solutions, and research opportunities. The paper firstly introduces the characteristics of UWBG β-Ga₂O₃ and its significance in electronic devices, and elaborates on the crucial firstly importance of thermal management in β-Ga₂O₃ devices. Then, various thermal management techniques, including substrate-related methods and junction-side thermal management techniques, are thoroughly examined, and their impact on the electrical properties of β-Ga₂O₃ devices is analyzed. Finally, the future development trends of thermal management for UWBG β-Ga₂O₃ devices are prospected. Multi-dimensional thermal management strategies are proposed, focusing on “material-device-packaging” electrothermal collaborative design, near junction heterogeneous integration, and novel external packaging, aiming to arouse relevant research and accelerate the development and industrialization process of UWBG β-Ga₂O₃ power devices.

In this paper, the Ga₂O₃ field-effect transistors were fabricated on the Ga₂O₃ epitaxial material grown by MOCVD, and their performances were studied. In order to reduce the on-resistances of the transistors, the epitaxial layer was optimally designed and the doping concentration was increased to above 1×10¹⁸ cm^-3. The electron concentration and field-effect mobility of the epitaxial layer extracted from the long channel transistor were 2×10¹⁸ cm^-3 and 55 cm²/(V·s), respectively, which result in a corresponding channel sheet resistance of 10.3 kΩ/sq. The specific on-resistances of the Ga₂O₃ MOSFETs with gate to drain spacings of 2 and 16 μm were 2.3 and 40.0 mΩ·cm², and the corresponding breakdown voltages were 458 and 2 324 V, respectively. In order to further improve the breakdown voltages of the transistors, the p-type NiO gates were employed. The on-resistances of the fabricated Ga₂O₃ JFETs were significantly increased, but the breakdown voltages were improved to 755 V and above 3 000 V, respectively. The power figure of merits (P-FOMs) of the transistors with different gate to drain spacings were calculated, and it was found that they increased first and then decreased with the increase of the gate to drain spacings. The Ga₂O₃ MOSFET with a gate to drain spacing of 8 μm achieved the highest P-FOM, which was 192 MW/cm², indicating the MOCVD epitaxial technology demonstrates an important application prospect for Ga₂O₃ power transistors.

In this work, β-Ga₂O₃ based metal-semiconductor-metal (MSM) solar-blind ultraviolet photodetector was successfully fabricated. At room temperature and a bias voltage of 5 V, the responsivity of the device with high-quality epitaxy is 469.6 mA/W (corresponding to an external quantum efficiency (EQE) of 229.2%), and the photocurrent-dark current ratio is 5.26×10³. In order to study the potential application of the β-Ga₂O₃ based MSM type solar-blind ultraviolet detector in high temperature environment, the current-voltage (I-V) and photoresponse (I-T) characteristic of the device at high temperature were tested, and the carrier transport mechanism of the device at high temperature was analyzed. The results indicate that the dark current of the device is mainly dominated by thermionic-field emission (TFE) at low voltage and Poole-Frenkel emission (PFE) at high voltage between 300 and 375 K. The I-V curve fitted by the PFE model shows that PFE is caused by defects near 0.200 eV below the conduction band. According to the fitting results of the photoresponse characteristic, the activation energy fitted by the decay time is 0.280 eV, and activation energy fitted by the rise time is 0.036 eV. According to the analysis results, the transport process of photocurrent is as follows: photogenerated electrons are first captured by defect energy levels near 0.200 eV to 0.280 eV below the conduction band and emitted into the conduction band through PFE to generate photocurrent; the recombination process of photogenerated charge carriers is that photoelectrons tend to be captured by defect energy levels near 0.036 eV below the conduction band, and then recombine with photogenerated holes in the valence band.

In this paper, high quality metastable α-phase gallium oxide (α-Ga₂O₃) films were epitaxial grown on m surface sapphire substrate by pulsed laser deposition (PLD) technique. The effects of different growth temperature and oxygen partial pressure on the morphology and crystallinity of the films were studied by X-ray diffraction and scanning electron microscopy. Based on the heteroepitaxial α-Ga₂O₃ films grown under optimized conditions, a metal-semiconductor-metal (MSM) solar blind UV photodetector was fabricated. Due to the good crystal quality and few defects of the film,the device demonstrates high performances under 254 nm illumination. The device has 10^-6 A photo current and 10^-10 A dark current at 5 V bias, achieving a photodark current ratio of 10⁴. The maximum responsivity is 36.7 A/W, the maximum external quantum efficiency is 1.79×10⁴%, and the maximum detectivity is 2.45×10¹⁴ Jones.

Due to the absence of p-type gallium oxide (Ga₂O₃), p-type nickel oxide (p-NiO_x) was commonly employed in Ga₂O₃ Schottky barrier diode (SBD), which typically utilized either junction termination extension (JTE) or hetero-junction barrier Schottky (HJBS) structure. However, the influence of the NiO_x on device performance has been insufficiently explored. In this work, the effects of NiO_x in JTE and HJBS were investigated by Sentaurus TCAD. In JTE structure, breakdown voltage (BV) presented positive correlation with NiO_x doping concentration and negative correlation with the NiO_x tilt angle. In HJBS structure, BV increased with the width and depth of the NiO_x field ring (FR), while decreased with the spacing between the FR and the anode edge. The optimal configuration was identified, consisting of 10° tilt angle and doping concentration of 3×10¹⁹ cm^-3 for the NiO_x JTE, as well as NiO_x rings of 5 μm width, 1.5 μm depth, and 1 μm spacing for NiO_x HJBS. This configuration achieved the BV of 4.52 kV, specific on-resistance (R_on,sp) of 5.68 mΩ·cm² and performance figure of merit (PFOM) value of 3.57 GW/cm², representing enhancements of 113% in BV and 132% in PFOM compared to experimental reports. This study contributed a design approach for vertical Ga₂O₃ SBD utilizing NiO_x JTE and HJBS structures, enhancing BV and PFOM in SBD.

As a new generation of wide bandgap semiconductor, gallium oxide has a larger bandgap width(4.4~4.8 eV) and higher breakdown field strength (8 MV/cm), making it an ideal material for fabricating high voltage, high frequency, and high power electronic devices. However, due to the edge concentration effect of the terminal electric field of gallium oxide Schottky diodes, the device fails due to premature breakdown, thus limiting the application of gallium oxide. A new type of composite terminal is designed in this paper, which is formed by combining a high-resistance region that can alleviate the edge concentration effect of the electric field and an electron barrier layer that suppresses reverse leakage. The simulation results show that the peak electric field near the edge of the device electrode that introduces the high-resistance region terminal structure drops from 3.650 MV/cm to 0.246 MV/cm, which can effectively alleviate the edge concentration effect of the electrode electric field. When the Mg ion implantation concentration in the high-resistance region is 10¹⁹ cm^-3, the breakdown voltage increases from 725 V to 2 115 V, the Baliga figure of merit increases from 0.060 GW/cm² to 0.247 GW/cm², and the critical breakdown field strength increases by 50.7% (from 3.650 MV/cm to 5.500 MV/cm); at the same time, the introduction of the electronic barrier layer AlN greatly reduces the reverse leakage current of the device, and the reverse breakdown voltage increases to 2 690 V, which is beneficial to the new composite terminal. The new composite structure can not only effectively suppress the reverse leakage current of the device but also effectively increase the reverse breakdown voltage of the device. This research lays a theoretical foundation for the development of high voltage resistant, low reverse leakage current gallium oxide Schottky diodes.

In this paper, large-size Eu³⁺∶CaF₂ laser crystals with Eu³⁺ doping concentrations of 250×10^-6, 1 000×10^-6 and 2 500×10^-6, respectively, were prepared by an automatic crystal control method based on Bridgman crystal growth method. The blank size of the ingots is ø210 mm×80 mm, which demonstrates it the largest Eu³⁺∶CaF₂ laser crystal reported in this field till now.