Xingzhao Liu

Properties Optimization for Perovskite Oxide Thin Films by Formation of Desired Microstructure

Perovskite oxide materials are very important for the electronics industry, because they exhibit promising properties. With an interest in the obvious applications, significant effort has been invested in the growth of highly crystalline epitaxial perovskite oxide thin films in our laboratory. And the desired structure of films was formed to achieve excellent properties. Y₁Ba₂Cu₃O 7-x (YBCO) superconducting thin films were simultaneously deposited on both sides of 3 inch wafer by inverted cylindrical sputtering. Values of microwave surface resistance R s (75 K, 145 ㎓, 0 T) smaller than 100 mΩ were reached over the whole area of YBCO thin films by pre-seeded a self-template layer. For implementation of voltage tunable high-quality varactor, A tri-layer structured SrTiO₃ (STO) thin films with different tetragonal distortion degree was prepared in order to simultaneously achieve a large relative capacitance change and a small dielectric loss. Highly a-axis textured Ba 0.65 Sr 0.35 TiO₃ (BST65/35) thin films was grown on Pt/Ti/SiO₂/Si substrate for monolithic bolometers by introducing Ba 0.65 Sr 0.35 RuO₃ (BSR65/35) thin films as buffer layer. With the buffer layer, the leakage current density of BST65/35 thin films were greatly reduced, and the pyroelectric coefficient of 7.6×10?? C ㎝?² K?¹ was achieved at 6 V/㎛ bias and room temperature.

β-Ga2O3 solar-blind deep-ultraviolet photodetector based on a four-terminal structure with or without Zener diodes

A four-terminal photodetector was fabricated on the (2¯01)-dominant β-Ga2O3 thin film which was deposited in a plasma-assisted molecular beam epitaxy system. The suitability of this film for solar-blind DUV detection was proved by its transmission spectra. Moreover, the device operating in a specific voltage-current mode can accurately detect the DUV radiation both qualitatively and quantitatively. Accordingly, a dark/photo voltage ratio of 15 was achieved, which is comparable to that of previously-reported β-Ga2O3 interdigital metal-semiconductor-metal photoconductor. More importantly, the aperture ratio of our proposed device exceeds 80%, nearly doubling that of the conventional interdigital metal-semiconductor-metal devices including photoconductor and Schottky-type photodiode, which can intensively benefit the detection efficiency. Furthermore, it was found the dark/photo voltage ratio was nearly trebled with the assistance of two Zener diodes, and further enhancement can be expected by increasing the...

Characterization of vertical Au/β-Ga2O3 single-crystal Schottky photodiodes with MBE-grown high-resistivity epitaxial layer*

High-resistivity beta-Ga2O3 thin films were grown on Si-doped n-type conductive beta-Ga2O3 single crystals by molecular beam epitaxy (MBE). Vertical-type Schottky diodes were fabricated, and the electrical properties of the Schottky diodes were studied in this letter. The ideality factor and the series resistance of the Schottky diodes were estimated to be about 1.4 and 4.6 x 10(6) Omega. The ionized donor concentration and the spreading voltage in the Schottky diodes region are about 4 x 10(18) cm(-3) and 7.6 V, respectively. The ultra-violet (UV) photo-sensitivity of the Schottky diodes was demonstrated by a low-pressure mercury lamp illumination. A photoresponsivity of 1.8 A/W and an external quantum efficiency of 8.7 x 10(2)% were observed at forward bias voltage of 3.8 V, the proper driving voltage of read-out integrated circuit for UV camera. The gain of the Schottky diode was attributed to the existence of a potential barrier in the i-n junction between the MBE-grown highly resistive beta-Ga2O3 thin films and the n-type conductive beta-Ga2O3 single-crystal substrate.

Amorphous InGaMgO Ultraviolet Photo-TFT with Ultrahigh Photosensitivity and Extremely Large Responsivity

Recently, amorphous InGaZnO ultraviolet photo thin-film transistors have exhibited great potential for application in future display technologies. Nevertheless, the transmittance of amorphous InGaZnO (~80%) is still not high enough, resulting in the relatively large sacrifice of aperture ratio for each sensor pixel. In this work, the ultraviolet photo thin-film transistor based on amorphous InGaMgO, which processes a larger bandgap and higher transmission compared to amorphous InGaZnO, was proposed and investigated. Furthermore, the effects of post-deposition annealing in oxygen on both the material and ultraviolet detection characteristics of amorphous InGaMgO were also comprehensively studied. It was found that oxygen post-deposition annealing can effectively reduce oxygen vacancies, leading to an optimized device performance, including lower dark current, higher sensitivity, and larger responsivity. We attributed it to the combined effect of the reduction in donor states and recombination centers, both of which are related to oxygen vacancies. As a result, the 240-min annealed device exhibited the lowest dark current of 1.7 × 10−10 A, the highest photosensitivity of 3.9 × 106, and the largest responsivity of 1.5 × 104 A/W. Therefore, our findings have revealed that amorphous InGaMgO photo thin-film transistors are a very promising alternative for UV detection, especially for application in touch-free interactive displays.

Novel AlN/Pt/ZnO Electrode for High Temperature SAW Sensors

In order to develop a film electrode for the surface acoustic wave (SAW) devices working in high temperature, harsh environments, novel AlN/Pt/ZnO multilayers were prepared using pulsed laser deposition (PLD) systems on langasite (LGS) substrates. The AlN film was used as a protective layer and the ZnO buffer layer was introduced to improve the crystal quality of Pt films. The results show that the resistances of Pt and AlN/Pt film electrodes violently increase above 600 °C and 800 °C, respectively, while the resistances of AlN/Pt/ZnO electrodes have more stable electrical resistance from room temperature to 1000 °C. The AlN/Pt/ZnO electrode, where the ZnO film was deposited at 600 °C, has the best temperature stability and can steadily work for 4 h at 1000 °C. The mechanism underlying the stable resistance of the AlN/Pt/ZnO electrode at a high temperature was investigated by analyzing the microstructure of the prepared samples. The proposed AlN/Pt/ZnO film electrode has great potential for applications in high temperature SAW sensors.

Self-assembling growth of (Ba,Sr)TiO3 epitaxial thin films

Ferroelectric thin films of SrTiO3, BaTiO3, and (Ba,Sr)TiO3 were fabricated epitaxially on SrTiO3(100) and Si(111) single crystal substrates in an ultrahigh vacuum (10−5 Pa) by laser molecular beam epitaxy. The lowest crystallization temperatures of homoepitaxial thin films of SrTiO3 and heteroepitaxial thin films of BaTiO3 and (Ba,Sr)TiO3 on SrTiO3(100) substrates obtained were as low as 280, 330, and 340 °C, respectively. With the growth rate lower than 0.17 A/s, it was discovered that the thickness of each layer of BTO film was an integral of the BTO unit cell height. When the growth rate was decreased from 0.17 to 0.05 A/s, the thickness of each layer varied from nine unit cells to one unit cell. It is very important to construct a nanometer structure of inorganic ABO3 perovskite oxides by this kind of self-assembly.

Dielectric thin films for GaN-based high-electron-mobility transistors

Abstract The effects of dielectric thin films on the performance of GaN-based high-electron-mobility transistors (HEMTs) were reviewed in this work. Firstly, the nonpolar dielectric thin films which act as both the surface passivation layers and the gate insulators of the high-frequency GaN-based high-electron-mobility transistors were presented. Furthermore, the influences of dielectric thin films on the electrical properties of two-dimensional electron gas (2DEG) in the AlGaN/GaN hetero-structures were analyzed. It was found that the additional in-plane biaxial tensile stress was another important factor besides the change in surface potential profile for the device performance improvement of the AlGaN/GaN HEMTs with dielectric thin films as both passivation layers and gate dielectrics. Then, two kinds of polar gate dielectric thin films, the ferroelectric LiNbO3 and the fluorinated Al2O3, were compared for the enhancement-mode GaN-based HEMTs, and an innovative process was proposed. At last, high-permittivity dielectric thin films were adopted as passivation layers to modulate the electric field and accordingly increase the breakdown voltage of GaN-based HEMTs. Moreover, the polyimide embedded with Cr particles effectively increased the breakdown voltage of GaN-based HEMTs. Finally, the effects of high-permittivity dielectric thin films on the potential distribution in the drift region were simulated, which showed an expanded electric field peak at the drain-side edge of gate electrode.

Residual Stresses In Oblique Incidence Deposited Alumina Thin Film

Residual stresses of alumina thin film deposited on silicon substrate by using electron beam evaporation with oblique angle deposition (OAD) method are studied. The growth parameters that affect the residual stresses of alumina thin film, such as the substrate temperature, the deposition rate, the film thickness, the inclined angle, and the testing temperature are discussed. The results show that the tensile stress value decreases with the increasing substrate temperature, and the compressive stress value increases with the increasing substrate temperature at various inclined angles. Along with the deposition rate increasing, the residual stress value decreases at various inclined angles. With the increasing film thickness, the residual stress value decreases at various inclined angles. With the increasing testing temperature, the residual stress value increases at various inclined angles. While the alumina thin film residual stress value is small at high inclined angle. By choosing the appropriate film preparation parameters, the alumina thin film residual stress is effectively controlled.

YBa2Cu3O7−δ thin films with low surface resistance prepared by self-template sputtering method

Abstract Highly c -axis-oriented epitaxial YBa 2 Cu 3 O 7− δ (YBCO) thin films prepared on LaAlO 3 substrates by a self-template technique have been investigated for microwave applications. By varying the conditions of the two-step self-template method, the properties of YBCO thin films could be improved significantly. With this method, it was easy to reproducibly obtain YBCO thin films with T c 0 ≥90 K, Δ T c ≤1 K, R s (77 K, 10 GHz)∼500 μΩ. The best sample grown under optimal conditions had a low R s of 330 μΩ at 77 K and 10 GHz. In addition, the correlation between the microwave surface resistance, R s , and the crystal structure is discussed by studying the electron channeling pattern (ECP) of thin films. In conclusion, the microwave losses can strongly be affected by structural imperfections.

Optimization of Tc of epitaxial GdBaCuO growth superconducting thin film by inverted cylindrical sputtering

Thin films of GdBaCuO(GBCO) compound have been deposited in situ onto LaAlO3 single crystal substrates by inverted cylindrical sputtering.The variation of superconductive properties of the thin films with the substrate temperature and sputtering pressure has been systematic investigated.By the optimization of deposition parameter,high quality c-axis epitaxial GBCO thin films of Tc0>92K were reproducibly grown at the substrate temperature of 720—820℃ with a low deposition total pressure (40Pa).The Tc0 of the best sample is as high as 93.2K.Only by changing the target composition into GdBa2Cu4Oy (Gd124),was it observed that the samples always contain some a-axis oriented films,impling that excess copper would favour a-axis growth in thin films,and the Tc of the samples is lower than that with Gd123 target.The superconductivity of the thin films under high substrate temperature was clearly improved by the procedure of special post oxygenization at 400℃ in ozone atmosphere.These results are very useful to further prepare large area thin films of GBCO.