P. G. Li

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.

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.

Bias tuning charge-releasing leading to negative differential resistance in amorphous gallium oxide/Nb:SrTiO3 heterostructure

Negative differential resistance (NDR) and bipolar resistive switching (RS) phenomena were observed in Au/Ga2O3−x/Nb:SrTiO3/Au heterostructures fabricated by growing amorphous gallium oxide thin films on 0.7%Nb-doped SrTiO3 substrates using pulsed laser deposition technique. The RS behavior is reproducible and stable without the forming process. The NDR phenomenon happened during the course of RS from low resistance state to high resistance state and was dependent much on the applied forward bias. The bias dependent charge releasing from oxygen vacancies was considered to contribute to the NDR behavior. The results show that there is a very close relationship between NDR and RS.

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.

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.

Carrier tuned rectifying-like behavior in superconducting La1.8Sr0.2CuO4/La1.9Sr0.1CuO4 bilayers

A bilayer structure of superconducting overdoped La1.8Sr0.2CuO4 and underdoped La1.9Sr0.1CuO4 films was fabricated using pulsed laser deposition and wet chemical etching techniques. The difference in carrier concentrations in the two layers causes rectifying-like behavior when the films are cooled below a critical temperature Tc. This behavior can be interpreted within the framework of quenched superconducting films, with carrier migration occurring between the two layers at very low positive current. These results suggest that electric field-controlled carrier migration between two layers in a bilayer structure can yield new interfacial properties, which would be of interest for device applications.

Decrease of oxygen vacancy by Zn-doped for improving solar-blind photoelectric performance in β-Ga2O3 thin films

Highly (201) oriented Zn-doped β-Ga2O3 thin films with different dopant concentrations were grown on (0001) sapphire substrates by radio frequency magnetron sputtering. With the increase of Zn dopant concentration, the crystal lattice expands, the energy band gap shrinks, and the oxygen vacancy concentration decreases. Both the metal semiconductor metal (MSM) structure photodetectors based on the pure and Zn-doped β-Ga2O3 thin films exhibit solar blind UV photoelectric property. Compared to the pure β-Ga2O3 photodetector, the Zn-doped one exhibits a lower dark current, a higher photo/dark current ratio, a faster photoresponse speed, which can be attributed to the decreases of oxygen vacancy concentration.

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.