Daoyou Guo

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.

Effects of dopant concentration on structural and near-infrared luminescence of Nd3+-doped beta-Ga2O3 thin films

We have investigated structural and near-infrared (NIR) luminescence of Nd3+-doped β-Ga2O3 thin films (Nd:Ga2O3) with different Nd3+ doping concentrations. With an increase of Nd3+ content, the crystal lattice of the films expands, while the energy band gap shrinks. Moreover, NIR luminescence is investigated as a function of Nd3+ doping concentration. The measured results are related to the structural change and energy transfer of cross relaxation process ascribed to 4F3/2—4I9/2, 4F3/2—4I11/2, and 4F3/2—4I13/2 of the phosphor films. This work implies that the enhanced NIR luminescence and blue-shift observation are associated with the lattice distortion and the variation in the crystal field of Nd: Ga2O3.

Zero-Power-Consumption Solar-Blind Photodetector Based on β-Ga2O3/NSTO Heterojunction

A solar-blind photodetector based on β-Ga2O3/NSTO (NSTO = Nb:SrTiO3) heterojunctions were fabricated for the first time, and its photoelectric properties were investigated. The device presents a typical positive rectification in the dark, while under 254 nm UV light illumination, it shows a negative rectification, which might be caused by the generation of photoinduced electron-hole pairs in the β-Ga2O3 film layer. With zero bias, that is, zero power consumption, the photodetector shows a fast photoresponse time (decay time τd = 0.07 s) and the ratio Iphoto/Idark ≈ 20 under 254 nm light illumination with a light intensity of 45 μW/cm2. Such behaviors are attributed to the separation of photogenerated electron-hole pairs driven by the built-in electric field in the depletion region of β-Ga2O3 and the NSTO interface, and the subsequent transport toward corresponding electrodes. The photocurrent increases linearly with increasing the light intensity and applied bias, while the response time decreases with the increase of the light intensity. Under -10 V bias and 45 μW/cm2 of 254 nm light illumination, the photodetector exhibits a responsivity Rλ of 43.31 A/W and an external quantum efficiency of 2.1 × 104 %. The photo-to-electric conversion mechanism in the β-Ga2O3/NSTO heterojunction photodetector is explained in detail by energy band diagrams. The results strongly suggest that a photodetector based on β-Ga2O3 thin-film heterojunction structure can be practically used to detect weak solar-blind signals because of its high photoconductive gain.

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.

Abnormal bipolar resistive switching behavior in a Pt/GaO1.3/Pt structure

A stable and repeatable abnormal bipolar resistive switching behavior was observed in a Pt/GaO1.3/Pt sandwich structure without an electroforming process. The low resistance state (LRS) and the high resistance state (HRS) of the device can be distinguished clearly and be switched reversibly under a train of the voltage pulses. The LRS exhibits a conduction of electron tunneling, while the HRS shows a conduction of Schottky-type. The observed phenomena are considered to be related to the migration of oxygen vacancies which changes the space charge region width of the metal/semiconductor interface and results in a different electron transport mechanism.

Inhibition of unintentional extra carriers by Mn valence change for high insulating devices

For intrinsic oxide semiconductors, oxygen vacancies served as the electron donors have long been, and inevitably still are, attributed as the primary cause of conductivity, making oxide semiconductors seem hard to act as high insulating materials. Meanwhile, the presence of oxygen vacancies often leads to a persistent photoconductivity phenomenon which is not conducive to the practical use in the fast photoelectric response devices. Herein, we propose a possible way to reduce the influence of oxygen vacancies by introducing a valence change doping in the monoclinic β-Ga2O3 epitaxial thin film. The unintentional extra electrons induced by oxygen vacancies can be strongly suppressed by the change valence of the doped Mn ions from +3 to +2. The resistance for the Mn-doped Ga2O3 increases two orders of magnitude in compared with the pure Ga2O3. As a result, photodetector based on Mn-doped Ga2O3 thin films takes on a lower dark current, a higher sensitivity, and a faster photoresponse time, exhibiting a promising candidate using in high performance solar-blind photodetector. The study presents that the intentional doping of Mn may provide a convenient and reliable method of obtaining high insulating thin film in oxide semiconductor for the application of specific device.

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.

Magnetic anisotropy and deep ultraviolet photoresponse characteristics in Ga2O3:Cr vermicular nanowire thin film nanostructure

Nanostructures of magnetic doping semiconductors have attracted considerable attention due to their fantastic electronic, optical and magnetic properties as well as their unique morphology for both interconnects and functional units in fabrication of nanodevices. Herein, a Ga2O3:Cr vermicular nanowire thin film nanostructure was obtained on α-Al2O3 (0001) substrates by pulsed laser deposition. The nanostructure exhibits room temperature anisotropic ferromagnetic behavior with an easy axis perpendicular to the film plane and a Curie temperature of higher than 400 K. An obvious deep ultraviolet photoelectric response was obtained in the formed nanostructure.