Shunli Wang

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.

Construction of GaN/Ga2O3 p–n junction for an extremely high responsivity self-powered UV photodetector

A self-powered ultraviolet photodetector was constructed with GaN/Ga2O3 p–n junction by depositing n-type Ga2O3 thin film on Al2O3 single crystals substrate covered by p-type GaN thin film. The fabricated device exhibits a typical rectification behavior in dark and excellent photovoltaic characteristics under 365 nm and 254 nm light illumination. The device shows an extremely high responsivity of 54.43 mA W−1, a fast decay time of 0.08 s, a high Ilight/Idark ratio of 152 and a high detectivity of 1.23 × 1011 cm Hz1/2 W−1 under 365 nm light with a light intensity of 1.7 mW cm−2 under zero bias. Such excellent performances under zero bias are attributed to the rapid separation of photogenerated electron–hole pairs driven by built-in electric field in the interface depletion region of GaN/Ga2O3 p–n junction. The results strongly suggest that the GaN/Ga2O3 p–n junction based photodetectors are suitable for applications in secure ultraviolet communication and space detection which require high responsivity and self-sufficient functionality.

Propeller-shaped ZnO nanostructures obtained by chemical vapor deposition: photoluminescence and photocatalytic properties

Propeller-shaped and flower-shaped ZnO nanostructures on Si substrates were prepared by a one-step chemical vapor deposition technique. The propeller-shaped ZnO nanostructure consists of a set of axial nanorod (50 nm in tip, 80 nm in root and 1 µm in length), surrounded by radial-oriented nanoribbons (20-30nm in thickness and 1.5 µm in length). The morphology of flower-shaped ZnO nanostructure is similar to that of propeller-shaped ZnO, except the shape of leaves. These nanorods leaves (30nm in diameter and 1-1.5 µm in length) are aligned in a radial way and pointed toward a common center. The flower-shaped ZnO nanostructures show sharper and stronger UV emission at 378nm than the propeller-shaped ZnO, indicating a better crystal quality and fewer structural defects in flower-shaped ZnO. In comparison with flower-shaped ZnO nanostructures, the propeller-shaped ZnO nanostructures exhibited a higher photocatalytic property for the photocatalytic degradation of Rhodamine B under UV-light illumination.

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.

Cations substitution tuning phase stability in hybrid perovskite single crystals by strain relaxation

Methylammonium (MA) and formamidinium (FA) are two typical A site cations in lead halide perovskites. Instability of synthesised crystals will degrade the properties of the photoelectrical device constructed by such perovskites. MAPbI3 and FAPbI3 in cubic crystal structure have been demonstrated to be the most stable at room temperature. Herein we synthesised MA(EA)PbI3 and FA(MA)PbI3 single crystals using an inverse-temperature crystallization strategy by partially substituting the methylammonium (MA) with ethylammonium (EA) and the formamidinium (FA) with methylammonium (MA) respectively. The XRD results show that both crystal structures are cubic, which means organic incorporation can stabilize the crystal structure of lead halide perovskites. The lattice distortion decrease and strain relaxation in single crystals were considered to be the reason leading to higher stability. The single crystals of MA(EA)PbI3 and FA(MA)PbI3 with low trap state density exhibit excellent light-absorbing properties, indicating their potential applications in photoelectric devices.

Evidence for the bias-driven migration of oxygen vacancies in amorphous non-stoichiometric gallium oxide

The conductivity of gallium oxide thin films is strongly dependent on the growth temperature when they deposited by pulsed laser deposition under vacuum environment, exhibiting an insulative-to-metallic transition with the decrease of the temperature. The high conductive gallium oxide films deposited at low temperature are amorphous, non-stoichiometric, and rich in oxygen vacancy. Large changes in electrical resistance are observed in these non-stoichiometric thin films. The wide variety of hysteretic shapes in the I-V curves depend on the voltage-sweep rate, evidencing that the time-dependent redistribution of oxygen vacancy driven by bias is the controlling parameter for the resistance of gallium oxide.

Influence of oxygen vacancy on resistive switching property of Ag/Nb:SrTiO3/Ti structure

Abstract With the aim to get optimised resistive switching properties, the influence of oxygen vacancy concentration of Nb:SrTiO3 (NSTO) on the resistive switching properties of Ag/NSTO/Ti structure was studied. To obtain the NSTO with different oxygen vacancy concentrations, the substrates had been annealed in different atmospheres and temperatures before they were used to fabricate Ag/NSTO/Ti structure. The resistive switching properties were investigated by measuring I–V characteristics, endurance ability and resistance distribution at room temperature. The results show that both annealing in oxygen atmosphere and vacuum atmosphere degraded the switching properties. In our case, the device annealed in air at 700°C displayed the best resistive switching behaviour. After analysis, we suggest that both Schottky barrier height and barrier width, which can be tuned by changing the oxygen vacancy concentration, play important roles in getting high quality resistive switching property.

Characterization and Ultraviolet Photodetection Application of the Sm2O3/n-Si Heterojunction

The Sm2O3/n-Si heterojunction has been fabricated by deposition of Sm2O3 thin film on n-type (100) silicon substrate using radio frequency (RF) magnetron sputtering followed by postannealing treatment. The obtained Sm2O3 thin film exhibited C-type cubic crystal structure with bandgap ofxa0~4.1xa0eV. The Sm2O3/n-Si heterojunction showed an obvious rectification characteristic with turn-on voltage of 0.5xa0V and a decrease of resistance with increase of temperature, indicating typical semiconductor behavior. The heterojunction showed fine and stable ultraviolet photoresponse to illumination by 254-nm light. This combination of a wide-bandgap semiconductor with silicon might enable future applications in ultraviolet photodetection.

Carrier tuning the metal-insulator transition of epitaxial La0.67Sr0.33MnO3 thin film on Nb doped SrTiO3 substrate

La0.67Sr0.33MnO3 (LSMO) thin films were deposited on (001)SrTiO3(STO) and n-type doped Nb:SrTiO3(NSTO) single crystal substrates respectively. The metal to insulator transition temperature(TMI) of LSMO film on NSTO is lower than that on STO, and the TMI of LSMO can be tuned by changing the applied current in the LSMO/NSTO p-n junction. Such behaviors were considered to be related to the carrier concentration redistribution in LSMO film caused by the change of depletion layer thickness in p-n junction which depends greatly on the applied electric field. The phenomenon could be used to configure artificial devices and exploring the underlying physics.

Fabrication and characterization of a-oriented TbFeO3/Nb-doped SrTiO3 heterostructure

The a-axis oriented TbFeO3, (TFO) thin film was epitaxially grown on (001) Nb-1 wt.%-doped SrTiO3 (Nb–STO) single crystal substrate to construct a heterostructure. The heterostructure exhibited a good rectifying behavior over the temperature range of 25–300 K, and the rectification ratio was continuously enhanced with increasing temperature. In positive bias direction, transport mechanism of the heterostructure showed a crossover from the Schottky-emission-like to a space-charge-limited type at 120 K, the temperature at which the TFO shows a magnetic ordering, while in the negative bias direction it was solely dominated by the space-charge-limited mechanism. The result indicates that coupling between electrical and magnetic orders can influence the rectifying behavior of the heterostructure. The observation hints the possibility to tune rectifying properties of a heterostructure via influencing magnetic properties of the magnetic layer.