Yuehua An

Fabrication of β-Ga 2 O 3 thin films and solar-blind photodetectors by laser MBE technology

Laser molecular beam epitaxy technology has been employed to deposit β-gallium oxide (β-Ga2O3) on (0001) sapphire substrates. After optimizing the growth parameters, (2¯01)-oriented β-Ga2O3 thin film was obtained. Ultraviolet-visible absorption spectrum demonstrates that the prepared β-Ga2O3 thin film shows excellent solar-blind ultraviolet (UV) characteristic with a band gap of 5.02 eV. A prototype photodetector device with a metal-semiconductor-metal structure has been fabricated using high quality β-Ga2O3 film. The device exhibits obvious photoresponse under 254 nm UV light irradiation, suggesting a potential application in solar-blind photodetectors.

Oxygen vacancy tuned Ohmic-Schottky conversion for enhanced performance in β-Ga2O3 solar-blind ultraviolet photodetectors

β-Ga2O3 epitaxial thin films were deposited using laser molecular beam epitaxy technique and oxygen atmosphere in situ annealed in order to reduce the oxygen vacancy. Metal/semiconductor/metal structured photodetectors were fabricated using as-grown film and annealed film separately. Au/Ti electrodes were Ohmic contact with the as-grown films and Schottky contact with the annealed films. In compare with the Ohmic-type photodetector, the Schottky-type photodetector takes on lower dark current, higher photoresponse, and shorter switching time, which benefit from Schottky barrier controlling electron transport and the quantity of photogenerated carriers trapped by oxygen vacancy significant decreasing.

Unipolar resistive switching behavior of amorphous gallium oxide thin films for nonvolatile memory applications

Amorphous gallium oxide thin film with heavy oxygen deficiency was deposited on Pt/Ti/SiO2/Si substrate by pulsed laser deposition in order to explore the resistive switching behavior of the Pt/Ga2O3-x/Pt sandwich structure. A well unipolar resistive switching behavior was obtained in this structure, which exhibits a high resistance ratio of OFF/ON up to 104, non-overlapping switching voltages, and excellent repeatability and retention. Both I-V relation plots of ON and OFF states and temperature dependent variation resistances indicate that the observed resistive switching behavior can be explained by the formation/rupture of conductive filaments formed out of oxygen vacancies.

Room temperature ferromagnetism in (Ga1−xMnx)2O3 epitaxial thin films

Mn-doped monoclinic β-(Ga1−xMnx)2O3 thin films were epitaxially grown on α-Al2O3 (0001) substrates by alternately depositing Ga2O3 and Mn layers using the laser molecular beam epitaxy technique. The crystal lattice expands and the energy band gap shrinks with the increase of Mn content for Mn ions incorporated into the Ga site. Ferromagnetism appears even above room temperature when x ≥ 0.11 and can be remarkably enhanced with the continuous increase of Mn indicated by the increased magnetization and coercivity. This study presents a promising candidate for use in spintronic devices that are capable of working at room temperature.

Epitaxial growth and magnetic properties of ultraviolet transparent Ga2O3/(Ga1−xFex)2O3 multilayer thin films

Multilayer thin films based on the ferromagnetic and ultraviolet transparent semiconductors may be interesting because their magnetic/electronic/photonic properties can be manipulated by the high energy photons. Herein, the Ga2O3/(Ga1−xFex)2O3 multilayer epitaxial thin films were obtained by alternating depositing of wide band gap Ga2O3 layer and Fe ultrathin layer due to inter diffusion between two layers at high temperature using the laser molecular beam epitaxy technique. The multilayer films exhibits a preferred growth orientation of crystal plane, and the crystal lattice expands as Fe replaces Ga site. Fe ions with a mixed valence of Fe2+ and Fe3+ are stratified distributed in the film and exhibit obvious agglomerated areas. The multilayer films only show a sharp absorption edge at about 250 nm, indicating a high transparency for ultraviolet light. What’s more, the Ga2O3/(Ga1−xFex)2O3 multilayer epitaxial thin films also exhibits room temperature ferromagnetism deriving from the Fe doping Ga2O3.

Influence of oxygen vacancies on the photoresponse of β-Ga2O3/SiC n–n type heterojunctions

β-Ga2O3/4H-SiC n–n type heterojunctions have been fabricated by depositing high quality β-Ga2O3 films on c-axis orientation n-type 4H-SiC substrates using laser molecular beam expitaxy. The influences of oxygen vacancies on the junction performances are investigated. It is found that the existence of oxygen vacancies degrades the rectifying and photoresponse properties of the heterojunctions. A large rectification ratio of 1900, a high 254 nm ultraviolet photosensitivity of 6308% and a zero response of 365 nm ultraviolet have been achieved by reducing oxygen vacancies. It is supposed that the oxygen vacancies in Ga2O3 affect the depletion layer of the n–n junction greatly.

Growth and characterization of α-phase Ga2−x Sn x O3 thin films for solar-blind ultraviolet applications

Ga2−x Sn x O3 thin films on m-plane (300) sapphire substrates were deposited by laser molecular beam epitaxy technology. Corundum-structured α-phase Ga2−x Sn x O3 thin films with different Sn content were obtained at 850 °C in vacuum pressure of 5 × 10−5 Pa. The band-gap energy of the α-phase Ga2−x Sn x O3 thin films, determined from the absorption spectrum, decreases linearly with increasing Sn content. A metal–semiconductor–metal structured solar-blind photodetector based α-phase Ga2−x Sn x O3 thin films was fabricated, and excellent solar-blind ultraviolet characteristics were demonstrated. The ratio of I 254/I dark was up to 1.40 × 102 and the responsivity increased to 9.55 × 10−2 A W−1. The results suggest that α-phase Ga2−x Sn x O3 thin films are promising candidates for use in solar-blind photodetectors.

Dual-band photodetector with a hybrid Au-nanoparticles/β-Ga2O3 structure

Hybrid Au-nanoparticles (NPs)/β-Ga2O3 thin films were fabricated by laser molecular beam epitaxy of high quality β-Ga2O3 thin films on c-plane sapphire substrates followed by spin coating and post annealing of Au NPs. Photodetectors (PDs) made of hybrid Au-NPs/β-Ga2O3 thin films and bare β-Ga2O3 thin films are comparatively studied. It is found that Au-NPs can obviously improve the performances of PDs. Lower dark current, higher photoresponse and faster switching time under 254 nm light illumination are obtained by introducing Au-NPs. The localized surface plasmonic resonance from Au-NPs could be attributed to the enhanced performance. Meanwhile, a photoelectric response to 532 nm light illumination is observed in an Au NPs/β-Ga2O3 based PD. It is suggested that our hybrid Au-NPs/β-Ga2O3 PD could serve as a dual-band detector in one device.

Reversible transition between bipolar and unipolar resistive switching in Cu2O/Ga2O3 binary oxide stacked layer

Both unipolar resistive switching (URS) and bipolar resistive switching (BRS) behaviors are observed in Cu2O/Ga2O3 stacked layer. The conversion between BRS and URS is controllable and reversible. The switching operations in BRS mode requires smaller voltage than that in the URS mode. The oxygen vacancies closed to the Cu2O/Ga2O3 interface contributes to the BRS, and the bias-controlling filament formation/rupture in depletion layer is considered to contribute to the URS. The URS happens only in the negative voltage part due to the nature of directionality of the p-n junction. The process reported here can be developed to design memory device.

Interface induced transition from bipolar resistive switching to unipolar resistive switching in Au/Ti/GaOx/NiOx/ITO structures

We report the transition from bipolar resistive switching (BRS) to unipolar resistive switching (URS) in the Au/Ti/GaOx/NiOx/ITO device at room temperature. After the proper soft breakdown of the p–n junctions (GaOx/NiOx), the switching operation could be easily transferred from BRS to URS mode. The BRS and URS behaviors are possibly related to the interfacial variation of the Ti/GaOx Schottky junction barrier and GaOx/NiOx p–n junction barrier, respectively. The high/low resistance state can be distinguished clearly and be switched reversibly under a train of voltage pulses in both BRS and URS modes. The endurance characteristics show good reliability of the stored resistance state. These results suggest a potential device for the next generation of nonvolatile memory applications.