Wide bandgap semiconductor material - gallium oxide (Ga₂O₃) - has emerged as a new competitor to SiC and GaN in the race toward next-generation power devices by virtue of the excellent material properties and the relative ease of mass wafer production. In this talk, following a short introduction of material properties and features of Ga₂O₃, an overview of our recent development progress in device processing and characterization of Ga₂O₃ field-effect transistors (FETs) and Schottky barrier diodes (FP-SBDs) will be reported.

State-of-the-art Ga₂O₃ metal-oxide-semiconductor FETs (MOSFETs) were fabricated with unintentionally-doped (UID) β-Ga₂O₃ (010) epitaxial layers grown on semi-insulating Fe-doped substrates by ozone molecular beam epitaxy [1]. Selective-area Si-ion implantation doping of the UID Ga₂O₃ epilayer formed the device channel and ohmic contacts, while the high resistivity of UID Ga₂O₃ was harnessed for planar device isolation without mesa etching. SiO₂-passivated depletion-mode MOSFETs with a gate-connected field plate (FP) demonstrated a high off-state breakdown voltage (V_br) of 755 V, a large drain current on/off ratio of over nine orders of magnitude, DC-RF dispersion-free output characteristics, and stable high temperature operation against thermal stress at 300°C.

We also fabricated and characterized Pt/Ga₂O₃ FP-SBDs on n^--Ga₂O₃ drift layers grown on n⁺-Ga₂O₃ (001) substrates [2], owing to the success of halide vapor phase epitaxy (HVPE) for high-speed growth of high-quality Ga₂O₃ thin films [3, 4]. 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 V_br of 1076 V. Note that this was the first demonstration of V_br of over 1 kV in any Ga₂O₃ power device.

In summary, we succeeded in fabricating depletion-mode Ga₂O₃ FP-MOSFETs and vertical Ga₂O₃ FP-SBDs on single-crystal β-Ga₂O₃ substrates. Despite the simple structures, both the FP-MOSFETs and FP-SBDs revealed excellent device characteristics and demonstrated great potential of Ga₂O₃ electron devices for power electronics applications.

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] M. H. Wong et al., IEEE Electron Device Lett 37, 212 (2016), [2] K. Konishi et al., 74th Device Research Conference IV-A.5, 2016, [3] K. Nomura et al., J. Cryst. Growth 405, 19 (2014), [4] H. Murakami et al., Appl. Phys. Express 8, 015503 (2015).

The SnO₂ NWs were grown on the Au/α-Al₂O₃(001) substrates by atmospheric-pressure CVD using Sn powder and H₂O as source materials. Both the substrate and Sn powder were heated by the horizontal furnace, whereas H₂O was vaporized in its own vaporizer. Both the vapors of Sn and H₂O were timely separated and transported onto the substrate by nitrogen carrier gaseous.

X-ray diffraction measurements and SEM observations revealed the successful growths of the SnO₂ NWs by the ASS conditions. The average diameter of the SnO₂ NWs grown under the simultaneous source supply (SSS) condition increased exponentially with increasing growth temperature (T_g), reflecting the enhancement of the contribution of the VS growth. In contrast to this, the average diameter of the NWs grown under the ASS condition was almost independent of T_g, indicating that the contribution of the VS growth is effectively suppressed using the ASS condition. PL spectra showed the increase in the intensity of the orange band emission with increasing Tg, suggesting that the increase in the O vacancies and/or Sn interstitial atoms [3]. Moreover, the NWs average diameter was found to be independent of cycle number in the cycle number range of 300-700.

This work was supported by JSPS KAKENHI Grant Number JP26390029.

[1] T. Terasako et al., Thin Solid Films 528 (2013) 237.

[2] T. Terasako et al., Thin Solid Films (to be published).

[3] D. Calestani et al., Mater. Sci. Eng. C 25 (2005) 625.

This paper discusses the transparent resistive random access memory (ReRAM) from ITO/WO₃/ZnO/ITO structure to multilayer ITO/WO_x(x<3)/WO₃/ZnO/AlN/ITO structure with oxygen concentration distribution and heat dissipating layer. The X-ray photoelectron spectroscopy (XPS) is used to confirm the existence of WO_x/WO₃ double layers. The transmission electron microscopy (TEM) images show the AlN layer has limited effect on the grain structure and the interface roughness of ZnO. Moreover, the transmittance of the multilayer ReRAM achieves 85.49% that is suitable for optoelectronic applications.

The bipolar ReRAM mechanism is based on filament model with the movement of oxygen vacancies. Because the oxygen ions may recombine with the vacancies and break the conductive path near the top electrode during the SET operation, the WO_x between electrode and WO₃ provides sufficient vacancies for efficient resistive changes. In the meanwhile, the WO₃ can limit the rupture and formation region of filaments. This gradient tungsten oxide stabilizes the low resistance states, decreases the operating voltages, and increases the endurance from < 10³ cycles to 10⁴ cycles. Since the electrical field and heat drive the movements of ions and vacancies, the inserted AlN with high thermal conductivity can dissipate the uncontrollable heat and remain the directional electrical field. This AlN layer prevents the ReRAM from the heat-activated ion movement and further masters the high resistance state, so the resistance levels of the multilayer ReRAM are tight and stable. Conclusively, in the ITO/WO_x/WO₃/ZnO/AlN/ITO ReRAM, the operating voltages of SET and RESET operations are 1.9V and -1.1V, respectively, the retention stay more than 10⁴s at 150°C, the endurance is 10⁴ cycles with resistance ratio over 20x, and both of the low and high resistance states are extremely stable during cycling.

Technological development for memory, logic IC, on-display devices and batteries is indispensable for advanced portable electronic products. Among all these devices, a reliable, fast-working, and energy-saving non-volatile memory is extremely important. There are several next generation memory under developed, RRAM, PCRAM, MRAM, MTJ, FeRAM. All these devices must use array to storage, but the sneak path current is still the problem that we can’t integrate large amount of advanced RAM into a chip. One Selector connect to one Memory is one of the solution to sneak current, and it is the most efficient method to integrate memories into array chip.

Selector can be used in any resistance-changed memory. There are large amounts of selectors developed in recent year, one of they use transition metal oxide to achieve double side diode properties. Metal insulator transition (MIT) has been widely developed because of its volatile state switch. In this article, we use Vanadium Oxide to be our device to find two factors that influences the switch characteristic. Because MIT happens in the difference of temperature, we think the thermal and electric field will influences the devices meanwhile. By current fitting and Comsol simulation, we conclude the phenomena happens in transition layer.