Single-crystal Ga2O3 Research Articles

Photochemical wet etching of (0 0 1) plane ß-phase Ga2O3, and its anisotropic etching behavior

The photochemical (PC) wet etching behavior of (001) plane β-phase Ga2O3(β-Ga2O3)single crystal was investigated using potassium hydroxide etchant solution under ultraviolet illumination in the temperature range of 80–95 °C. The etch rate was 1.65 nm/min at 80 °C, and increased up to 4.88 nm/min at 95 °C. The etch rate of the (001) plane β-Ga2O3shows strong temperature dependence with the activation energy of 0.798 eV. The PC wet etching characteristics of the (001) plane β-Ga2O3was compared with those of (2¯01) and (010) plane β-Ga2O3. The etch rates increased in order of the (010), (2¯01), and (001) plane, which is accordance with the increasing surface energy trend. The oval line-shaped grooves aligned along [010] direction were observed on the etched (001) plane β-Ga2O3 crystal surface after 30 min. PC wet etching at 80 °C.

High-Speed and Ultrasensitive Solar-Blind Ultraviolet Photodetectors Based on In Situ Grown β-Ga2O3 Single-Crystal Films.

Deep-level defects in β-Ga2O3 that worsen the response speed and dark current (Id) of photodetectors (PDs) have been a long-standing issue for its application. Herein, an in situ grown single-crystal Ga2O3 nanoparticle seed layer (NPSL) was used to shorten the response time and reduce the Id of metal-semiconductor-metal (MSM) PDs. With the NPSL, the Id was reduced by 4 magnitudes from 0.389 μA to 81.03 pA, and the decay time (τd1/τd2) decreased from 258/1690 to 62/142 μs at -5 V. In addition, the PDs with the NPSL also exhibit a high responsivity (43.5 A W-1), high specific detectivity (2.81 × 1014 Jones), and large linear dynamic range (61 dB) under 254 nm illumination. The mechanism behind the performance improvement can be attributed to the suppression of the deep-level defects (i.e., self-trapped holes) and increase of the Schottky barrier. The barrier height extracted is increased by 0.18 eV compared with the case without the NPSL. Our work contributes to understanding the relationship between defects and the performance of PDs based on heteroepitaxial β-Ga2O3 thin films and provides an important reference for the development of high-speed and ultrasensitive deep ultraviolet PDs.

Ultra-high response solar-blind UV detector based on mix-phase MgZnO thin film on fused quartz substrate

Mix-phase MgZnO thin films with different structure constitutions are deposited under different temperature with Mg0.35Zn0.65O target by PLD method. Through study on the UV response characteristics of mix-phase MgZnO with different structure constitutions, mix-phase MgZnO detector with relative higher solar-blind UV response (3 × 103A/W at 250 nm @ 25V) and high Iuv signal at solar-blind UV light (0.848 mA at 140 mW/cm2 254 nm @25V) is made with suitable c-MgZnO/h-MgZnO ratio. The internal quasi-tunnel breakdown mechanism between c-MgZnO and h-MgZnO within the mix-phase MgZnO thin film, introduced much higher deep UV response of the mix-phase MgZnO detector than previous reported MgZnO detectors, the deep UV response of the mix-phase MgZnO detector is also higher than single crystal Ga2O3 based MSM detector that deposited at 800 °C high temperature. In addition, the Iuv of the simple MSM structure device MgZnO detector at faint solar-blind UV light, is nearly the same as some heterojunction devices that made with complicated device technology and high temperature condition. Therefore, mix-phase MgZnO with suitable c-MgZnO/h-MgZnO is a solar-blind UV sensitive material in high performance deep UV detectors.

Vertical van der Waals heterojunction diodes comprising 2D semiconductors on 3D β-Ga2O3.

Wide bandgap semiconductors such as gallium oxide (Ga2O3) have attracted much attention for their use in next-generation high-power electronics. Although single-crystal Ga2O3 substrates can be routinely grown from melt along various orientations, the influence of such orientations has been seldom reported. Further, making rectifying p-n diodes from Ga2O3 has been difficult due to lack of p-type doping. In this study, we fabricated and optimized 2D/3D vertical diodes on β-Ga2O3 by varying the following three factors: substrate planar orientation, choice of 2D material and metal contacts. The quality of our devices was validated using high-temperature dependent measurements, atomic-force microscopy (AFM) techniques and technology computer-aided design (TCAD) simulations. Our findings suggest that 2D/3D β-Ga2O3 vertical heterojunctions are optimized by substrate planar orientation (-201), combined with 2D WS2 exfoliated layers and Ti contacts, and show record rectification ratios (>106) concurrently with ON-Current density (>103 A cm-2) for application in power rectifiers.

New near-infrared optical modulator of Co2+:β-Ga2O3 single crystal

As a binary oxide, β-Ga2O3 is a promising host material with high thermal conductivity compared with the traditional laser materials. In this work, Co2+-doped β-Ga2O3 single crystal was successfully grown by the edge-defined film-fed growth (EFG) method and firstly designed as a near-infrared (NIR) optical modulator by doping with cobalt. The doping concentration of Co2+ ions was determined to be 0.029 at.% by X-ray fluorescence. The thermal conductivity was determined to be 13.0 W·m-1·K-1 along a* direction. There were two typical broadband absorption peaks around 1173 nm and 1588 nm related to the transitions of transitions of 4A2(4F)→4T1(4F). By means of Z-scan technique, the third-order nonlinear refractive index of Co2+:β-Ga2O3 was measured to be 9.14×10−13 (esu), which was much larger than that of Cr4+:YAG crystal. It was further successfully employed as a saturable absorber for passively Q-switched lasers at 1342 nm. In the Q-switched regime, the maximum average output power of 35 mW was obtained with the shortest pulse width of 280 ns and repetition rate of 181 kHz. The results indicated that Co2+:β-Ga2O3 crystals have important potential to be used as optical modulators for passive Q-switching laser generation at the NIR band.

Controlled growth of 2D ultrathin Ga2O3 crystals on liquid metal.

2D metal oxides (2DMOs) have drawn intensive interest in the past few years owing to their rich surface chemistry and unique electronic structures. Striving for large-scale and high-quality novel 2DMOs is of great significance for developing future nano-enabled technologies. In this work, we demonstrate for the first time controllable growth of highly crystalline 2D ultrathin Ga2O3 single crystals on liquid Ga by the chemical vapor deposition approach. With the introduction of oxygen into the growth process, large-area hexagonal α-Ga2O3 crystals with a uniform size distribution have been produced. At high temperature, fast diffusion of oxygen atoms onto the liquid surface facilitates reaction with Ga and thus leads to in situ formation of 2D ultrathin crystals. By precisely controlling the amount of oxygen, the vertical growth of the Ga2O3 single crystal has been realized. Furthermore, phase engineering can be achieved and thus 2D β-Ga2O3 crystals were also prepared by precisely tuning the growth temperature. The controlled growth of 2D Ga2O3 crystals offers an applicable avenue for fabrication of other 2D metal oxides and can further open up possibilities for future electronics.

2-Dimensional Ga2O3 Based Field Effect Transistor for Hydrogen Sensing Application

β-phase gallium oxide (β-Ga2O3) has attracted a lot of interest for the applications of high-power electronics, gas sensors, and solar-blind photodetectors. β-Ga2O3 is an alternative semiconducting material for high-power and high-temperature devices due to a large bandgap energy of 4.9 eV and high critical breakdown field (> 8 MV/cm). The Baliga’s figure of merit is 3400, which is 4-10 times higher than those of GaN and SiC. Ga2O3 based devices is expected to be more efficient with reduced size and high level of integration because the theoretical on-state resistance is almost one order magnitude lower than those of GaN and SiC devices at the same breakdown voltage. When compared to GaN and SiC, single crystal Ga2O3 wafers are available at low cost with floating zone and the edge-defined film-fed growth methods. The monoclinic β-Ga2O3 is the most stable form among five different crystal phases of Ga2O3. The large lattice constant of 12.23 Å of β-Ga2O3 along [100] direction enables to achieve the facile cleavage of β-Ga2O3 crystal into 2-dimensional flake though β-Ga2O3 is not a Van der Waals material. The thin channel of Ga2O3 flake is beneficial to the FET (field effect transistor) type gas sensor. In this study, the fabrication of 2-dimensional β-Ga2O3 flake based field effect transistor and its hydrogen sensing characteristics for hydrogen sensor application will be presented.

BCl3-Based Dry Etching of Exfoliated (100) ß-Ga2O3 Flakes

We report on BCl3-based dry etching characteristics of (100) β-Ga2O3 flakes using inductively coupled plasma etching technique, which were prepared by mechanical exfoliation from Ga2O3 single crystal. The etch rate of (100) Ga2O3 flake increased with increasing bias rf powers up to 52 nm min−1 using 25 sccm BCl3/15 sccm N2 gas chemistry. We also examined the etch rate dependence of Ga2O3 on crystal orientation. The (100) Ga2O3 flake has the lowest etch rate when compared with (010) and Ga2O3 surfaces. The lowest etch rate of (100) Ga2O3 surface results from less Ga-O bond breaking efficiency and low dangling bond density on the surface. It is notable that Ga2O3 surface is etched at higher rate than (010) and (100) Ga2O3 surfaces. The surface morphologies of etched (100) Ga2O3 surface exhibited little change, and rms roughness values remained less than 1.5 nm over a broad range of etch conditions.

Scintillation Properties of $\beta$ -Ga2O3 Single Crystal Excited by $\alpha$ -Ray

High-quantity and large-size β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single crystals were grown by the optimal edge-defined film-fed method. Under the excitation of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">241</sup> Am α-ray, the scintillation light yields of the as-grown and after-annealed β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single crystal are estimated to be 4797 and 6788 ph/5.5 MeV-α, respectively. The energy resolutions of the as-grown and after-annealed β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single crystal are 21.8% and 19.1%, respectively. Annealing in the air can eliminate some color centers and decrease the concentration of free electrons, which can lead to the near-infrared absorption. The light yield and the energy resolution are some slightly worse than that of the commercial Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (BGO) scintillator. However, the fast component with the time constant of 7 ns for the as-grown and after-annealed β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single crystal reaches 25% of the total yield, suggesting that the β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single crystal grown by the present method is a promising fast scintillator, which could be used in the ultrafast timing resolution detection systems.

Growth behavior of β-Ga2O3 nanowires synthesized by chemical vapor deposition

Gallium oxide (Ga2O3) nanowires deposited on Si substrates were synthesized by chemical vapor deposition (CVD) of Ga powders with assisted catalyzed by nickel chloride (NiCl2). The microstructure of these nanowires was investigated using high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). Four major types of morphology were observed for these Ga2O3 nanomaterials, namely, the particle-fused nanowires, single-crystal nanowires, nanorods and core–shell nanowires. Single crystal and core–shell nanowires were mainly studied. EELS indicated that the amorphous shell was Ga2O3. Because metal catalysts were introduced in the growth process, a vapor-solid (VS) growth mechanism was proposed to explain the single crystal Ga2O3 nanowires.

Effect of temperature on photoresponse properties of solar-blind Schottky barrier diode photodetector based on single crystal Ga2O3**Project supported by National Key Research and Development Plan of China (Grant Nos. 2016YFB0400600 and 2016YFB0400601), the National Natural Science Foundation of China (Grant Nos. 61574026, 11675198, 61774072, and 11405017), the Natural Science Foundation of Liaoning Province, China (Grant Nos. 201602453 and 201602176), China

A solar-blind photodetector is fabricated on single crystal Ga2O3 based on vertical structure Schottky barrier diode. A Cu Schottky contact electrode is prepared in a honeycomb porous structure to increase the ultraviolet (UV) transmittance. The quantum efficiency is about 400% at 42 V. The Ga2O3 photodetector shows a sharp cutoff wavelength at 259 nm with high solar-blind/visible (=3213) and solar-blind/UV (=834) rejection ratio. Time-resolved photoresponse of the photodetector is investigated at 253-nm illumination from room temperature (RT) to 85.8 °C. The photodetector maintains a high reversibility and response speed, even at high temperatures.

High-Performance Ga2O3 Diode Based on Tin Oxide Schottky Contact

A high-performance Schottky diode based on a 600-μm-thick Cr-doped β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single crystal has been fabricated using SnOx as the Schottky contact. The SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> film was deposited in argon/oxygen mixture gas to ensure an oxygen-rich stoichiometry in Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> near the Schottky interface, thus reducing oxygen deficiency-related interface state density. The SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> film included three components: Sn, SnO, and SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , as revealed by X-ray photoelectron spectroscopy characterization. The high-quality Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single crystal grown by an edge-defined film-fed method has a carrier concentration of 1.0×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and an electron mobility of ~90 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> . The frequency-dependent capacitance and conductance analysis revealed that the maximum active interface state density is 2.46×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> at a frequency of 500 kHz.

Erratum: “Three-dimensional anisotropic thermal conductivity tensor of single crystalline β-Ga2O3” [Appl. Phys. Lett. 113, 232105 (2018)

\b{eta}-Ga2O3 has attracted considerable interest in recent years for high power electronics, where thermal properties of \b{eta}-Ga2O3 play a critical role. The thermal conductivity of \b{eta}-Ga2O3 is expected to be three-dimensionally (3D) anisotropic due to the monoclinic lattice structure. In this work, the 3D anisotropic thermal conductivity tensor of a (010)-oriented \b{eta}-Ga2O3 single crystal was measured by using a novel time-domain thermoreflectance (TDTR) method with a highly elliptical pump beam. Our measured results suggest that at room temperature, the highest in-plane thermal conductivity is along a direction between [001] and [102], with a value of 13.3+/-1.8 W/mK, and the lowest in-plane thermal conductivity is close to the [100] direction, with a value of 9.5+/-1.8 W/mK. The through-plane thermal conductivity, which is along the [010] direction, has the highest value of 22+/-2.5 W/mK among all the directions. Temperature-dependent thermal conductivity of \b{eta}-Ga2O3 was also measured and compared with a modified Callaway model calculation to understand the temperature dependence and the role of impurity scattering.

Inductively Coupled Plasma Etching and Electrically Active Damage of Bulk, Single-Crystal Ga2O3

Monoclinic β phase Ga2O3 is attracting great attention for high power electronics, military applications, and solar blind photodetectors for deep-UV detection, all because of its large direct bandgap (~4.9 eV) and theoretical critical electric field (EC) strength of 8 MV/cm. [1,2,3] There has been a tremendous achievement in both bulk and epitaxial growth of β-Ga2O3. The large bulk β-Ga2O3 is commercially available, and n-type epistructures are also coming onto the market in limited quantities.[4] Record high Vertical diode fabricated on β -Ga2O3 with a reverse breakdown voltage of 1600 V was demonstrated[5], and Ga2O3 based variety of electronic devices have also been reported including metal-semiconductor field-effect transistors, depletion-mode metal-oxide-semiconductor field-effect transistors, and finfets fabricated on either bulk or thin films.[6,7,8,9] It is quite clear that more organized study of etching is needed for intradevice isolation or for exposing layers in gate recess processing for subsequent Schottky gate formation. While numerous wet etchants have been reported for Ga2O3, including HNO3/HCl, H2SO4, H3PO4 and HF-based solutions,[10,11] not too much is known about its dry etching characteristics and the associated mechanisms and effects on the optical properties of the material. In this study, we will report the effect of dry etching conditions on etching rates and damage induced by the ion bombardment on vertical β-Ga2O3 rectifiers. A Plasma Therm 760 Inductively Coupled Plasma (ICP) system was employed for dry etching by varying the ICP power ranging from 100 to 800W, chuck power (15-400 W) with either Cl2/Ar or BCl3/Ar discharges. The highest etch rate achieved was ~130 nm/min using 800 W ICP source power and 200 W chuck power (13.56 MHz). The etched Ga2O3 surfaces become oxygen-deficient under strong ion-bombardment conditions, and the electrically active damage introduced during etching was quantified using Schottky barrier height and diode ideality factor measurements. For low etch rate conditions (~12 nm/min) at low powers (150 W ICP source power at 2 MHz and 15 W rf chuck power at 13.56 MHz). There was only a small decrease in reverse breakdown voltage (~6%) while the Schottky barrier height decreased from 1.2 eV to 1.01 eV and the ideality factor increased from 1.00 to 1.06. Under higher etch rate (~70 nm/min) and power (400 W ICP and 200 W rf power) conditions, the damage was more significant, with the reverse breakdown voltage decreasing by ~35%, the barrier height was reduced to 0.86 eV and the ideality factor increased to 1.2. By annealing the etched samples at 450°C, the Schottky barrier height, diode ideality factor and reverse breakdown voltage were restored as compared to the reference sample. I. Stepanov, V.I. Nikolaev, V. E. Bougrov and A.E. Romanov, Rev. Adv. Mater. Sci., 44, 63 (2016).Higashiwaki, K. Sasaki, H. Murakami, Y. Kumagai, A. Koukitu, A. Kuramata, T. Masui and S. Yamakosh, Semicond. Sci. Technol., 31, 034001 (2016).Higashiwaki, K. Sasaki, A. Kuramata, T. Masui and S. Yamakoshi, Appl. Phys. Lett., 100, 013504 (2012).Higashiwaki, K. Sasaki, H. Murakami, Y. Kumagai, A. Koukitu, A. Kuramata, T. Masui and S. Yamakosh, Semicond. Sci. Technol., 31, 034001 (2016).Yang, S. Ahn, F. Ren, S. J. Pearton, S. Jang, and A. Kuramata, IEEE Electron Device Lett,. 38(7), 906 (2017).H. Wong, K. Sasaki, A. Kuramata, S. Yamakoshi and M. Higashiwaki, IEEE Electr. Dev. Lett., 37, 212 (2016).Michele Baldini, Martin Albrecht, Andreas Fiedler, Klaus Irmscher, Robert Schewski, and Günter Wagner, ECS J. Solid State Sci. Technol. 6, Q3040 (2017).Higashiwaki, K. Sasaki, H. Murakami, Y. Kumagai, A. Koukitu, A. Kuramata, T. Masui and S. Yamakosh, Semicond. Sci. Technol., 31, 034001 (2016).Shihyun Ahn, Fan Ren, Janghyuk Kim, Sooyeoun Oh, Jihyun Kim, Michael A. Mastro and S. J. Pearton, Appl. Phys. Lett.109, 062102 (2016).Takayoshi Oshima, Takeya Okuno, Naoki Arai, Yasushi Kobayashi and Shizuo Fujita, Jpn. J. Appl. Phys. 48, 040208 (2009)Shigeo Ohira and Naoki Arai, Phys. Stat. Sol. (c) 5, No. 9, 3116-3118 (2008)

(Keynote) Gallium Oxide Electronics: Beyond SiC and GaN

Ultrawide bandgap (UWBG) semiconductors, which are defined in terms of a bandgap energy larger than those of SiC and GaN, are getting attention as emerging materials. Gallium oxide (Ga2O3) is one of the typical UWBG materials and has attractive material properties represented by an extremely large bandgap of about 4.5 eV [1]. Furthermore, availability of high-quality, large-area native wafers manufactured from bulk single crystals synthesized by melt growth methods is another important material feature of Ga2O3 [2]. In this talk, I will introduce our state-of-the-art Ga2O3 metal-oxide-semiconductor field-effect transistor (MOSFET) and Schottky barrier diode (SBD) technologies developed for applications to power and harsh environment electronics. The first single-crystal Ga2O3 transistors by means of metal-semiconductor FETs were demonstrated in 2011 [3]. As a result of continuous efforts, we developed lateral depletion-mode (D-mode) Ga2O3 MOSFETs with a gate-connected field plate (FP) [4]. The devices showed a large off-state breakdown voltage of 755 V, a large drain current on/off ratio of over nine orders of magnitude, stable device operation at temperatures up to 300°C, and negligible DC–RF dispersion. A high gamma-ray tolerance was also demonstrated for the bulk Ga2O3 channel by virtue of weak radiation effects on their output current and threshold voltage. Lateral enhancement-mode Ga2O3 MOSFETs enabling normally-off operation were fabricated by Si-ion implantation doping in an undoped Ga2O3 layer to form source/drain contacts and access regions [5]. The undoped Ga2O3 channel layer that was fully depleted at thermal equilibrium gave rise to positive threshold voltage. A decent on-state drain current of 1.4 mA/mm and a sufficiently large drain current on/off ratio of six orders of magnitude were also achieved. Recently, our FET development took a step forward to vertical D-mode Ga2O3 MOSFETs. In the device structure, the source was electrically isolated from the drain by a current blocking layer formed by Mg-ion implantation except at an aperture opening for drain current conduction. Despite a large source–drain leakage current due to an imperfect function of the current blocking layer, the drain current modulation by applied gate bias was successfully demonstrated. We have also been developing vertical Ga2O3 SBDs. Ga2O3 FP-SBDs were fabricated on n --Ga2O3 drift layers grown by halide vapor phase epitaxy on n +-Ga2O3 (001) substrates [6]. The illustrative device with a net donor concentration of 1.8×1016 cm-3 exhibited a specific on-resistance of 5.1 mΩ·cm2 and an ideality factor of 1.05 at room temperature. Successful FP engineering resulted in a high breakdown voltage of 1076 V. Note that at the breakdown condition, the peak electric field in the Ga2O3 drift layer was estimated to be over 5 MV/cm, which was much larger than theoretical limits for SiC and GaN. Steady and rapid progress has been made on developing building blocks such as bulk wafers, epitaxial films, and device process techniques for realization of Ga2O3 electronics. We fabricated and characterized relatively simple Ga2O3 MOSFET and SBD structures. Both types of developed devices revealed excellent characteristics and thus demonstrated great potential of Ga2O3. Research and development in the next decade is certainly crucial for the road to industrialization of Ga2O3 electronic devices. This work was partially supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics” (funding agency: NEDO). [1] T. Onuma, S. Saito, K. Sasaki, T. Masui, T. Yamaguchi, T. Honda, and M. Higashiwaki, Jpn. J. Appl. Phys. 54, 112601 (2015). [2] A. Kuramata, K. Koshi, S. Watanabe, Y. Yamaoka, T. Masui, and S. Yamakoshi, Jpn. J. Appl. Phys. 55, 1202A2 (2016). [3] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, and S. Yamakoshi, Appl. Phys. Lett. 100, 013504 (2012). [4] M. H. Wong, K. Sasaki, A. Kuramata, S. Yamakoshi, and M. Higashiwaki, IEEE Electron Device Lett. 37, 212 (2016). [5] M. H. Wong, Y. Nakata, A. Kuramata, S. Yamakoshi, and M. Higashiwaki, Appl. Phys. Express 10, 041101 (2017). [6] K. Konishi, K. Goto, H. Murakami, Y. Kumagai, A. Kuramata, S. Yamakoshi, and M. Higashiwaki, Appl. Phys. Lett. 110, 103506 (2017).

Room‐Temperature Fabricated Amorphous Ga2O3 High‐Response‐Speed Solar‐Blind Photodetector on Rigid and Flexible Substrates

A solution to the fabrication of amorphous Ga2O3 solar‐blind photodetectors on rigid and flexible substrates at room temperature is reported. A robust improvement in the response speed is achieved by delicately controlling the oxygen flux in the reactive radio frequency magnetron sputtering process. Temporal response measurements show that the detector on quartz has a fast decay time of 19.1 µs and a responsivity of 0.19 A W−1 as well, which are even better than those single crystal Ga2O3 counterparts prepared at high temperatures. X‐ray photoelectron spectroscopy and current–voltage tests suggest that the reduced oxygen vacancy concentration and the increased Schottky barrier height jointly contribute to the faster response speed. Amorphous Ga2O3 solar‐blind photodetector is further constructed on polyethylene naphthalate substrate. The flexible devices demonstrate similar photoresponse behavior as the rigid ones, and no significant degradation of the device performance is observed in bending states and fatigue tests. The results reveal the importance of finely tuned oxygen processing gas in promoting the device performance and the applicability of room‐temperature synthesized amorphous Ga2O3 in fabrication of flexible solar‐blind photodetectors.

The process and mechanism of the GaN nanoparticles formed by nitridation of β-Ga2O3 crystal

The process of conversion from [Formula: see text]-Ga2O3 single crystal to gallium nitride (GaN) in an atmosphere of NH3 by chemical vapor deposition is investigated. The surface morphology and microstructure of the GaN nanoparticles are observed by scanning electron microscope, which indicates that the growth of GaN is via the Volmer–Weber mechanism. The [Formula: see text]-Ga2O3 is firstly evaporated at high temperature to form the porous layer, followed by the surface-defect induced GaN nucleation formation. The crystalline structure and epitaxial relationship of the GaN nanoparticles are investigated by X-ray diffraction (XRD) via [Formula: see text]–[Formula: see text], showing GaN (0002) and (0004) diffraction peaks in the XRD spectra. It is concluded that the polycrystalline GaN film with hexagonal structure has a strong c-axis preferential orientation.

Inductively coupled plasma etch damage in (-201) Ga2O3 Schottky diodes

Bulk, single-crystal Ga2O3 was etched in BCl3/Ar inductively coupled plasmas as a function of ion impact energy. For pure Ar, the etch rate (R) was found to increase with ion energy (E) as predicted from a model of ion enhanced sputtering by a collision-cascade process, R ∝(E0.5 – ETH0.5), where the threshold energy for Ga2O3, ETH, was experimentally determined to be ∼75 eV. When BCl3 was added, the complexity of the ion energy distribution precluded, obtaining an equivalent threshold. Electrically active damage introduced during etching was quantified using Schottky barrier height and diode ideality factor measurements obtained by evaporating Ni/Au rectifying contacts through stencil masks onto the etched surfaces. For low etch rate conditions (∼120 Å min−1) at low powers (150 W of the 2 MHz ICP source power and 15 W rf of 13.56 MHz chuck power), there was only a small decrease in reverse breakdown voltage (∼6%), while the barrier height decreased from 1.2 eV to 1.01 eV and the ideality factor increased from 1.00 to 1.06. Under higher etch rate (∼700 Å min−1) and power (400 W ICP and 200 W rf) conditions, the damage was more significant, with the reverse breakdown voltage decreasing by ∼35%, the barrier height was reduced to 0.86 eV, and the ideality factor increased to 1.2. This shows that there is a trade-off between the etch rate and near-surface damage.

Synthesis and characterization of Ga2O3 nanosheets on 3C-SiC-on-Si by low pressure chemical vapor deposition

This study presents the synthesis of single crystalline β-Ga2O3 nanosheets on SiC by low pressure chemical vapor deposition. High purity gallium (Ga) metal and oxygen as source materials and argon as carrier gas were utilized for the synthesis of the nanosheets on a 3C-SiC-on-Si substrate. These single-crystal Ga2O3 nanosheets are free-standing 2D extrusions from their 1D rods, typically 1.5–7 μm in lateral size and 20–140 nm in thickness, featuring aspect ratios ranging from ∼10 to 350. Structural studies based on transmission electron microscopy and Raman spectroscopy revealed the monoclinic phase of Ga2O3 with a single crystalline nature. High resolution transmission electron microscopy with a selected area electron diffraction pattern recorded on a single β-Ga2O3 nanosheet further confirmed their single crystalline nature, with a growth direction perpendicular to (111) crystallographic plane. The growth process governing the formation of these nanosheets is a vapor-solid growth mechanism since no metal catalyst was used. These β-Ga2O3 nanosheets exhibit new possibilities and potential for future functional nanodevices that would benefit from their extremely large surface area to volume ratios.

Deuterium incorporation and diffusivity in plasma-exposed bulk Ga2O3

Deuterium incorporation depths of ∼0.13–0.65 μm were obtained in bulk, single-crystal Ga2O3 during exposure to 2H plasmas for 0.5 h at 100–270 °C. The data were fit using the Florida Object Oriented Process Simulator simulation code. The estimated diffusivities were in the range of 2.3 × 10−14–6 × 10−13 cm2/V·s over this temperature range. The activation energy for diffusion was 0.30 ± 0.08 eV, suggesting that the diffusing deuterium migrates as an interstitial. The solubility of the deuterium exhibited an activation energy of 0.35 eV. Subsequent annealing at 500 °C removed ∼85% of the deuterium out of the Ga2O3, with an activation energy of 1.35 eV. This indicates that the outdiffusion is mediated through defects or molecule formation. The thermal stability of deuterium retention is lower (∼150 °C shift to lower temperatures) than when the deuterium is incorporated in the Ga2O3 by direct ion implantation, due to trapping at residual damage in the latter case. The incorporation depths of deuterium in the Ga2O3 are significantly less than in bulk ZnO under the same conditions, where we observed incorporation to ∼25 μm.