Fabrication and electrical characterization of ZnO rod arrays/CuSCN heterojunctions

ZnO rod arrays/CuSCN heterojunctions are fabricated by depositing ZnO rod arrays films using two-step chemical bath deposition (CBD) and CuSCN thin films using successive ionic layer adsorption and reaction (SILAR) on ITO substrate successively. The structures and morphologies of ZnO films and CuSCN films, analyzed by X-ray diffraction (XRD) spectroscopy and metallurgical microscope, show that ZnO films are hexagonal wurtzite structure and consisted of vertical polycrystalline rods with diameter of 1 μm, CuSCN films are β-phase structure and consisted of elongated grains with length of 3 μm. Current–voltage (I–V) measurements of ZnO/CuSCN heterojunctions show good diode characteristics with rectification ratio about 48.3 at 3 V. The forward conduction is, respectively, determined by carrier recombination in the space charge region, defect-assisted tunneling and exponential distribution trap-assisted space charge limited current mechanism with the increase of forward voltage. Also, a band diagram of ZnO/CuSCN heterojunctions has been proposed to explain the transport mechanism.     

Structural and electrical properties of VO2/ZnO nanostructures

We examined the structural and electrical properties of uniformly-oriented VO₂/ZnO nanostructures. VO₂ was deposited on ZnO templates by using a direct current-sputtering deposition. Scanning electron microscope and transmission electron microscope measurements indicated that b-oriented VO₂ were uniformly crystallized on ZnO templates with different lengths. VO₂/ZnO formed nanorods on ZnO nanorods with length longer than 250 nm. X-ray absorption fine structure at the V K edge of VO₂/ZnO showed M₁ and R phases of VO₂ at 30 and 100 °C, respectively, suggesting structural-phase transition occurring between the two temperatures. Temperature-dependent resistance measurements of VO₂/ZnO nanostructures revealed metal-to-insulator transition at 65 °C and 55 °C during a heating and a cooling, respectively, regardless of ZnO length. Asymmetry behavior of resistance curves from VO₂/ZnO nanostructure during a heating and a cooling was attributed from a strong bond of VO₂ and ZnO.     

Carrier transport mechanisms in the ZnO based heterojunctions grown by MBE

This paper presents a review of models of the current transport in different kind of heterojunctions (HJs) and their characteristics. In order to effectively deduce the dominant electron transport for the HJs based on ZnO or Zn_1?xMg_xO layers grown on Si substrate by MBE a comparison is performed – which type of the HJ exhibits better electrical properties. The current–voltage characteristics for the studied HJs were measured within 280–300 K. The transport properties of the HJs are explained in terms of Anderson model with reference to aforementioned current transport models. It is found, that the mechanisms of current transport for all of the studied HJs are similar. At a low forward voltage bias the tunneling current dominates while at medium voltage bias (0.5–1 V) multitunneling capture-emission prevails with the electron trap located at 0.1–0.25 eV below the bottom of a ZnO (Zn_1?xMg_xO) conduction band. Beyond this voltage bias space charge limited current governs the current transport.     

Investigation of the luminescence properties of ZnO tetrapods and clusters grown on Si substrates

ZnO nanostructures are formed on Si substrates using Zn powder without catalyst. The substrate temperature was controlled from 450 to 600 °C, and the variation of structural and optical properties was investigated. From all samples both ZnO tetrapods and clusters were observed. Among them, no significant dispersion was observed from the ZnO tetrapods. However, ZnO clusters show considerable change in density and size. From the energy dispersive X-ray spectroscopy (EDX) results, atomic composition difference was observed. The clusters have considerable O-deficiencies, while tetrapods have almost stoichiometric composition. From all samples, strong luminescence, UV emission at 3.21 eV and green emission at 2.5 eV, were observed at room temperature. Cathodoluminescence measurements showed that the UV emission is closely related with tetrapods and the green emission is dominated from the clusters.     

Fabrication and photoluminescence of P doped ZnO nanobelts by thermal evaporation method

Uniform and flat single crystal ZnO:P nanobelts (NBs) were fabricated on Si (1 0 0) substrates by the thermal evaporation method. The growth process, free-catalyst self-assembly vapor-solid (V-S) mechanism, was described and investigated deeply in terms of thermodynamics and kinetics. Then, the photoluminescence (PL) properties of ZnO NBs were studied in a temperature range from 10 to 270 K. At 10 K the recombination of acceptor-bound exciton (A⁰X) was predominant in the PL spectrum, and was attributed to the transition of P_Zn−2V_Zn complex bound exciton. The active energy of A⁰X and acceptor binding energy were calculated to be 17.2 and 172 meV, respectively. The calculated acceptor binding energy of P doped ZnO nanostructure is in good agreement with that of P doped ZnO film.     

Fabrication and characterization of Mg-doped pencil-shaped ZnO microprisms

Two types of novel Mg-doped pencil-shaped ZnO microprisms had been successfully synthesized on Mg(NO₃)₂-coated Si (1 1 1) substrates by thermal chemical vapor deposition method. The as-prepared ZnO prisms were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), field-emission transmission electron microscope (FETEM), selected area electron diffraction (SAED), and photoluminescence (PL) spectroscopy. The straight microprisms are made up of hexagonal pyramids tips and hexagonal prisms bodies. Both of the structures are perfect single crystal and have grown along the [0 0 0 1] direction preferentially. Photoluminescence reveals a red-shift at around 387 nm which is induced by Mg doping and a green light emission peak at around 511 nm. The pencil-shaped ZnO microstructure can provide an improvement in novel ultraviolet light-emitting devices. In addition, the growth mechanism of the special ZnO microprisms is discussed briefly.     

Microstructures, optical and electrical properties of In-doped ZnO thin films prepared by sol-gel method

ZnO and indium-doped ZnO (I_xZO) thin films were prepared on silica-glass substrates by the sol-gel method. The thin films were crystallized at 600 °C and 700 °C for 1 h in 6.9 × 10⁻¹ Torr under pure O₂ atmosphere. The analyzed results were compared to investigate the structural characteristics and optical properties. The surface morphology of the I_xZO films was different from that of the ZnO films, and showed a thin overlay structure. In addition, the crystallization of I_xZO film was depleted at higher crystallized temperatures. From XRD analysis, the ZnO and I_xZO thin films possessed hexagonal structures. Notably, micro-In₂O₃ phases were observed in the I_xZO thin films using EDS. Both of In₂O₃ phases and the crystallization mechanism not only improved the peeling of structure, but also improved the electrical conductivity of I_xZO thin films. For the PL spectrum, the optical property of the I_xZO film was raised at a higher crystallization temperature. Although the In₂O₃ phases reduced the structural defects of I_xZO thin film, the optical effect of the residual In³⁺ was not enhanced completely at higher crystallized temperatures.     

MEH-PPV/ZnO纳米晶无机有机复合电致发光器件的研究

以Ⅱ一Ⅵ族无机半导体ZnO纳米颗粒为电子传输层,MEH-PPV为空穴传输层兼发光层,得到的电致发光器件比单层MEH-PPV器件的发光亮度和效率都明显提高。器件结构为ITO／MEH-PPV／ZnO／Al的电致发光光谱同单层PPV器件的光谱出现了不同,在620nm处出现了一个小的发光峰,应该是ZnO的发光。另外,双层结构器件的启亮电压由单层器件的9V降到了4V左右。由I-V曲线及发光光谱可判断出发光区域应在MEH-PPV／ZnO界面处,并且复合区域可能随着电压的变化而变化。     

The fabrication and photoresponse of ZnO/diamond film heterojunction diode

Boron-doped p-type freestanding diamond (FSD) films were prepared by hot filament chemical vapor deposition (HFCVD) method. The effect of B/C ratio on the electrical properties of FSD films was investigated by Hall effect measurement system. A ZnO/diamond heterojunction diode was fabricated successfully by depositing n-type ZnO films on the p-type FSD substrate by radio-frequency (RF) magnetron sputtering method. The wavelength dependent photoresponse properties of the heterojunction diode were investigated by studying the effect of light illumination on current-voltage (I-V) characteristics and photocurrent spectra at room temperature. The diode showed a significant discrimination between ultraviolet (UV) and the visible light under reverse bias conditions and photoresponse of the device was approximately linear related to the increasing reverse bias voltages.     

Fabrication and electrical properties of ordered two-dimensional hollow ZnO shells’ arrays

Colloidal templating and pulsed laser deposition (PLD) have been combined to fabricate arrays of ordered two-dimensional hollow ZnO cells. Based on a spin-coating method, a facile method has been developed to distribute colloidal bead in an ordered two-dimensional fashion between the fingers of inter-digital transducers. The underlying principle involved in the spin-coating method has been analyzed by obtaining Fourier transformation patterns from the arrangements of beads. The surfaces of oxide shells consist of protruding nano-crystallites, which have a würtzite structure of ZnO. Their electrical properties were measured as a function of annealing temperature. The change in the electrical properties upon grain growth has been attributed to alteration in the fraction site availability for defect formation at the grain boundaries.     

Fabrication and characterization of n-ZnO on p-SiC heterojunction diodes on 4H-SiC substrates

We report on the growth and characterization of n-ZnO/p-4H-SiC heterojunction diodes. Our n-ZnO layers were grown with radical-source molecular beam epitaxy (RS-MBE) on p-4H-SiC epilayers, which was previously prepared in a horizontal hot-wall reactor by chemical vapour deposition (CVD) on the n-type 4H-SiC wafers. Details on the n-ZnO growth on 8-off 4H-SiC wafers, the quality of the layers and the nature of realized p–n structures are discussed. Mesa diode structures were fabricated. Al was sputtered through a circle mask with diameter 1 mm and annealed to form Ohmic contacts to p-SiC. Ohmic contacts to the n-ZnO were formed by 30 nm/300 nm Ti/Au sputtered by electron beam evaporation. Electrical properties of the structures obtained have been studied with Hall measurements, and current–voltage measurements (I–V). I–V measurements of the device showed good rectifying behavior, from which a turn-on voltage of about 2 V was obtained.     

Preparation and characterization of nanocrystalline ZnO particles from a hydrothermal process

Rod-like ZnO nanoparticles were prepared by the hydrolysis of zinc acetate under heating in diethylene glycol (DEG). Structural characterization of the synthesized powders was investigated by XRD, FT-IR, electron paramagnetic resonance (EPR) and transmission electron microscopy (TEM). The size of the particles increased as the amount of H₂O added increased in the nano size range. The average crystallite size calculated from the XRD patterns varied from 6 to 64 nm corresponding to the amount of H₂O added. The ZnO nanopartilces possess the wurtzite type crystallographic structure. It was found that these ZnO nanoparticles had singly ionized oxygen vacancy defect () and superoxide ions from the EPR investigations. A strong near UV emission of the ZnO nanoparticles at about 380 nm was observed and its intensity decreased as the amount of H₂O increased. This emission of ZnO nanoparticles is found to be particles size dependent due to the confinement effect. A green emission at about 540 nm due to the recombination of electrons trapped at singly ionized oxygen vacancies defect () appeared when the amount of H₂O increased. The intensity of the green emission increases as the concentration of increases.     

Fabrication of high quality silicon–polyaniline heterojunctions

A high quality silicon–polyaniline heterojunction is produced by spin-coating of soluble polyaniline on silicon substrates. The devices have excellent reproducibility of their electrical characteristics and high rectification ratio. The rectification ratio is 60,000 at ±1.0 V at room temperature, and typical reverse current at −1.0 V is 3 nA. A G/I×G plot is used to analyze the current–voltage characteristics, yielding typical series resistance of 4 kΩ and ideality factor in a range from 1.0 to 2.0. The devices present great potential for use as radiation and/or gas sensors.     

Growth and characterization of thin ZnO films deposited on glass substrates by electrodeposition technique

Electrodeposition technique was used in order to produce nanometric zinc oxide films on glass insulating substrates. The effect of electrolyte concentration and applied current density on the formation and growth of electrodeposited Zn thin films in aqueous solutions of ZnSO₄ were studied. After a thermal oxidation, a characterization of the structural morphology of the films deposited was carried out by optical microscopy (OM), atomic force microscopy (AFM), scanning electron microscopy (SEM) and by grazing incidence X-rays diffraction (GIXD). These characterization techniques show that the grains size of the films after oxidation at temperature 450 °C is between 5 and 15 nm, as well as the structure is polycrystalline nature with several orientations. UV/vis spectrophotometry confirms that it is possible to obtain transparent good ZnO films with an average transmittance of approximately 80% within the visible wavelength region, as well as the optical gap of obtained ZnO films is 3.17 eV.     

Preparation and characterization of Mg-doped ZnO thin films by sol-gel method

Undoped and Mg-doped ZnO thin films were deposited on Si(1 0 0) and quartz substrates by the sol-gel method. The thin films were annealed at 873 K for 60 min. Microstructure, surface topography and optical properties of the thin films have been measured by X-ray diffraction (XRD), atomic force microscope (AFM), UV-vis spectrophotometer, and fluorophotometer (FL), respectively. The XRD results show that the polycrystalline with hexagonal wurtzite structure are observed for the ZnO thin film with Mg:Zn = 0.0, 0.02, and 0.04, while a secondary phase of MgO is evolved for the thin film with Mg:Zn = 0.08. The ZnO:Mg-2% thin film exhibits high c-axis preferred orientation. AFM studies reveal that rms roughness of the thin films changes from 7.89 nm to 16.9 nm with increasing Mg concentrations. PL spectra show that the UV-violet emission band around 386-402 nm and the blue emission peak about 460 nm are observed. The optical band gap calculated from absorption spectra and the resistivity of the ZnO thin films increase with increasing Mg concentration. In addition, the effects of Mg concentrations on microstructure, surface topography, PL spectra and electrical properties are discussed.     

One-dimensional self-consistent solution of modulation doped nanocrystalline/crystalline Si heterojunctions

A kind of modulation doped structure of n-type nanocrystalline hydrogenated silicon (nc-Si:H) film with intrinsic nc-Si:H layer with p-type bulk Si 100 substrate was proposed. The numerical self-consistent solutions of one-dimensional Schrödinger and Poisson equations along the direction normal to the heterojunction were performed to calculate the distribution of electron density and profiles of conduction-band as a function of ionized donor concentration in doped film, thickness of intrinsic layer, and other device parameters in the junctions. The calculated results are shown to be in agreement with experimental data. The relation of mobility vs sheet density of two-dimensional electron gases under different scattering mechanisms was analyzed. The obtained consequences may be used to evaluate optimum design for the modulation doped nc-Si:H-based devices.     

Structural characterization of supported nanocrystalline ZnO thin films prepared by dip-coating

Nanocrystalline ZnO thin films prepared by the sol-gel dip-coating technique were characterized by grazing incidence X-ray diffraction (GIXD), atomic force microscopy (AFM), X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS). The structures of several thin films subjected to (i) isochronous annealing at 350, 450 and 550 °C, and (ii) isothermal annealing at 450 °C during different time periods, were characterized. The studied thin films are composed of ZnO nanocrystals as revealed by analysing several GIXD patterns, from which their average sizes were determined. Thin film thickness and roughness were determined from quantitative analyses of AFM images and XR patterns. The analysis of XR patterns also yielded the average density of the studied films. Our GISAXS study indicates that the studied ZnO thin films contain nanopores with an ellipsoidal shape, and flattened along the direction normal to the substrate surface. The thin film annealed at the highest temperature, T = 550 °C, exhibits higher density and lower thickness and nanoporosity volume fraction, than those annealed at 350 and 450 °C. These results indicate that thermal annealing at the highest temperature (550 °C) induces a noticeable compaction effect on the structure of the studied thin films.     

ZnO纳米棒／MEH-PPV异质结的复合电致隧穿发光

利用低温水热法生长的ZnO纳米棒（ZnO-NRs）,和p型有机半导体材料聚[2-甲氧基-5-（2-乙基己氧基）-1,4-苯撑乙烯撑]（MEH-PPV）复合制备了结构为“ITO/ZnO晶种/ZnO-NRs/MEH-PPV/Al”的发光器件。测试结果发现,该器件具有非常好的二极管整流特性。对ZnO-NRs/M EH-PPV异质结施加超过17 V的反向偏压时,可同时获得两种半导体材料的发光,且ZnO近紫外光（380 nm ）发射强度远大于 M EH-PPV的红橙光强度,发光功率随着反向偏压的增加迅速增强,然而施加正向偏压时未探测到发光。该器件的发光机理不同于其他文献报道的正偏压发光,而属于反偏压发光器件,其发光机理归因于有机无机复合异质结的界面特殊性和ZnO-NRs的纳米尺寸效应,反偏压下器件实现的是载流子隧穿发光,而正偏压时载流子以表面态的无辐射复合及漏电流方式消耗。     

The crystallization and physical properties of Al-doped ZnO nanoparticles

Un-doped Al (0-9 at.%) nanoparticles and doped ZnO powders were prepared by the sol-gel method. The nanoparticles were heated at 700-800 °C for 1 h in air and then analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectra and photoluminescence (PL). The results of un-doped (ZnO) and Al-doped ZnO (AZO) nanoparticles were also compared to investigate the structural characteristics and physical properties. XRD patterns of AZO powders were similar to those of ZnO powders, indicating that micro-Al ions were substituted for Zn atoms and there were no variations in the structure of the ZnO nanoparticles. From the XRD and SEM data, the grain size of the AZO nanoparticles increased from 34.41 to 40.14 nm when the annealing temperature was increased. The Raman intensity of the AZO nanoparticles (Al = 5 at.%) increased when the annealing temperature was increased. Increasing the degree of crystalline not only reduced the residual stress, but also improved the physical properties of the nanoparticles.