Hongtao Cao

Microstructural, optical, and electrical properties of SnO thin films prepared on quartz via a two-step method.

A simple, cost-effective, two-step method was proposed for preparing single-phase SnO polycrystalline thin films on quartz. X-ray diffraction (XRD) analysis demonstrated that the annealed films were consisted of polycrystalline alpha-SnO phase without preferred orientation, and chemical composition analysis of the single phase in nature was analyzed using X-ray photoelectron spectroscopy (XPS). Transmittance spectra in UV-vis-IR range indicated that the average transmittance of both the as-deposited and the annealed SnO thin films was up to 70%. The optical band gap decreased from 3.20 to 2.77 eV after the annealing process, which was attributed to the crystalline size related quantum size effect. Photoluminescence (PL) spectrum of the annealed film showed only a weak peak at 585 nm, and no intrinsic optical transition emission was observed. Moreover, the p-type conductivity of SnO film was confirmed through Hall effect measurement, with Hall mobility of 1.4 cm(2) V(-1) s(-1) and hole concentration of 2.8 x 10(16) cm(-3).

Microstructure, optical, and electrical properties of p-type SnO thin films

SnO thin films were fabricated by electron beam evaporation on (100) Si and c- and r-plane Al2O3 substrates. The films grown at 25 °C are nanocrystalline, while the films grown at 600 °C are epitaxial on r-plane Al2O3 and (001) textured on Si and c-plane Al2O3. The SnO films have an optical band gap of 2.82–2.97 eV and p-type conductivity, according to Hall measurements, with resistivities of 0.5–110 Ω cm, hole concentrations of 1017–1019 cm−3, and Hall mobilities of 0.1–2.6 cm2/Vs. The p-type conductivity, which appears to correlate with VSn, can be enhanced via Y- and Sb-doping. Defect complexes of SbSn−2VSn are suggested to be the acceptors in Sb-(or Y-) doped SnO films.

Characterization of SnO2 nanowires as an anode material for Li-ion batteries

SnO2 nanowires synthesized by thermal evaporation method are investigated as a possible anode electrode for Li-ion batteries. In the first discharge process, the capacity of Li ions is 2133mAhg−1, which is much more than the theoretical total capacity of the bulk SnO2, 1494mAhg−1. During the successive 15cycles, the reversible capacity stays in the range of 1250–700mAhg−1 with a capacity fading of 3.89%percycle at a constant current density of 0.5mAcm−2. These results demonstrate that SnO2 nanowires are a promising anode material for Li-ion battery applications.

Room temperature ferromagnetism in transition metal (V, Cr, Ti) doped In2O3

Indium oxide is chosen as the host material for doping Ti, V, and Cr transition metal ions. Theoretical calculations based on density functional theory within a local spin density approximation show that V–V separation of 5.6A is more stable with a strong ferromagnetic coupling. Our calculations clearly predict that substitution of vanadium for indium should yield ferromagnetism in In2O3. Experimentally, (In0.95TM0.05)O3 (TM=Ti,V,Cr) were prepared using sol-gel as well as solid state reaction methods. Superconducting quantum interference device magnetization measurements as a function of field and temperature clearly showed that the V and Cr doped samples are ferromagnetic with Curie temperature well above room temperature. Thin films deposited by pulsed laser ablation using these materials on sapphire substrates exhibit a preferred 222 orientation normal to the plane of the film. The magnetic moment for (In0.95V0.05)O3 film deposited in 0.1mbar oxygen pressure was estimated to be 1.7μB∕V and is comparabl...

Facile and environmentally friendly solution-processed aluminum oxide dielectric for low-temperature, high-performance oxide thin-film transistors.

We developed a facile and environmentally friendly solution-processed method for aluminum oxide (AlOx) dielectrics. The formation and properties of AlOx thin films under various annealing temperatures were intensively investigated by thermogravimetric analysis-differential scanning calorimetry (TGA-DSC), X-ray diffraction (XRD), spectroscopic ellipsometry, atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), impedance spectroscopy, and leakage current measurements. The sol-gel-derived AlOx thin film undergoes the decomposition of organic residuals and nitrate groups, as well as conversion of aluminum hydroxides to form aluminum oxide, as the annealing temperature increases. Finally, the AlOx film is used as gate dielectric for a variety of low-temperature solution-processed oxide TFTs. Above all, the In2O3 and InZnO TFTs exhibited high average mobilities of 57.2 cm(2) V(-1) s(-1) and 10.1 cm(2) V(-1) s(-1), as well as an on/off current ratio of ∼10(5) and low operating voltages of 4 V at a maximum processing temperature of 300 °C. Therefore, the solution-processable AlOx could be a promising candidate dielectric for low-cost, low-temperature, and high-performance oxide electronics.

Structural, Chemical, Optical, and Electrical Evolution of SnOx Films Deposited by Reactive rf Magnetron Sputtering

In this paper, SnO(x) films were produced by reactive radio frequency magnetron sputtering under various oxygen partial pressure (P(O)) in conjunction with a thermal annealing at 200 °C afterwards. The obtained SnO(x) films were systematically studied by means of various techniques, including X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, spectroscopic ellipsometry, and Hall-effect measurement. The structural, chemical, and electrical evolution of the SnO(x) films was found to experience three stages: polycrystalline SnO phase dominated section with p-type conduction at P(O) ≤ 9.9%; amorphous SnO(2) phase dominated area at P(O) ≥ 12.3%, exhibiting n-type characteristics; and conductivity dilemma area in between the above mentioned sections, featuring the coexistence of SnO and SnO(2) phases with compatible and opposite contribution to the conductivity. The polycrystalline to amorphous film structure transition was ascribed to the enhanced crystallization temperature due to the perturbed structural disorder by incorporating Sn(4+) into the SnO matrix. The inversion from p-type to n-type conduction with P(O) variation is believed to result from the competition between the donor and acceptor generation process, i.e., the n-type behavior would be present if the donor effect is overwhelming, and vice versa. In addition, with increasing P(O), the refractive index decreased from 3.0 to 1.8 and the band gaps increased from 1.5 to 3.5 eV, respectively.

Ambipolar inverters using SnO thin-film transistors with balanced electron and hole mobilities

Ambipolar thin film transistors have attracted increasing research interests due to their promising applications in complementary logic circuits and the dissipative charge transporting devices. Here, we report the fabrication of an ambipolar transistor using tin mono-oxide (SnO) as a channel, which possesses balanced electron and hole field-effect mobilities. A complementary metal oxide semiconductor-like inverter using the SnO dual operation transistors is demonstrated with a maximum gain up to 30 and long-term air stability. Such logic device configuration would simplify the circuit design and fabrication process, offering more opportunities for designing and constructing oxide-based logic circuits.

The structural, optical and electrical properties of Y-doped SnO thin films and their p-type TFT application

Unintentionally doped and Y-doped SnO thin films were prepared and characterized by x-ray diffraction, spectroscopic ellipsometry, and Hall-effect measurements. SnO-based thin-film transistors were also fabricated and investigated. Preferred (0 0 l) grain orientation present in the undoped films is alleviated by Y doping, inducing the deterioration of crystallinity as well as the decrease in Hall-effect and saturation field-effect mobilities. However, both the films and the transistor devices always possess p-type characteristics in this study. As the Y content increases, the optical band gap, the real part of the dielectric constant of the films and the on/off current ratio of the devices increase. Moreover, the threshold voltage was observed to shift towards the positive direction as more yttrium content is introduced. These results give evidence that the yttrium element is incorporated into the SnO lattice successfully and higher hole concentration can be generated.

Mechanism for resistive switching in chalcogenide-based electrochemical metallization memory cells

It has been reported that in chalcogenide-based electrochemical metallization (ECM) memory cells (e.g., As2S3:Ag, GeS:Cu, and Ag2S), the metal filament grows from the cathode (e.g., Pt and W) towards the anode (e.g., Cu and Ag), whereas filament growth along the opposite direction has been observed in oxide-based ECM cells (e.g., ZnO, ZrO2, and SiO2). The growth direction difference has been ascribed to a high ion diffusion coefficient in chalcogenides in comparison with oxides. In this paper, upon analysis of OFF state I–V characteristics of ZnS-based ECM cells, we find that the metal filament grows from the anode towards the cathode and the filament rupture and rejuvenation occur at the cathodic interface, similar to the case of oxide-based ECM cells. It is inferred that in ECM cells based on the chalcogenides such as As2S3:Ag, GeS:Cu, and Ag2S, the filament growth from the cathode towards the anode is due to the existence of an abundance of ready-made mobile metal ions in the chalcogenides rather than ...

Aqueous Solution-Deposited Gallium Oxide Dielectric for Low-Temperature, Low-Operating-Voltage Indium Oxide Thin-Film Transistors: A Facile Route to Green Oxide Electronics

We reported a novel aqueous route to fabricate Ga2O3 dielectric at low temperature. The formation and properties of Ga2O3 were investigated by a wide range of characterization techniques, revealing that Ga2O3 films could effectively block leakage current even after annealing in air at 200 °C. Furthermore, all aqueous solution-processed In2O3/Ga2O3 TFTs fabricated at 200 and 250 °C showed mobilities of 1.0 and 4.1 cm2 V(-1) s(-1), on/off current ratio of ∼10(5), low operating voltages of 4 V, and negligible hysteresis. Our study represents a significant step toward the development of low-cost, low-temperature, and large-area green oxide electronics.