High-performance Zinc-Tin-Oxide thin film transistors based on environment friendly solution process

Zinc-Tin-Oxide (ZTO) thin films were fabricated using a simple and eco-friendly sol-gel method and their application in thin film transistors (TFTs) was investigated. Annealing temperature has a crucial influence on the structure and electrical properties of sol-gel ZTO thin films. The ZTO thin films annealed at 300–600?°C revealed smooth and uniform surfaces with amorphous state, in addition, a high optical transparency over 90% of the ZTO films in the visible range was obtained. The electrical performance of ZTO TFTs showed obvious dependence on annealing temperature. The ZTO TFTs annealed at 500?°C showed a high carrier mobility of 5.9?cm²/V, high on/off current ratio (I_on/off) of 10⁶-10⁷, and threshold voltage (V_th) of 1.03?V. To demonstrate the application of sol-gel ZTO films in low-power display fields, we also fabricated ZTO TFTs with a solution-processed high-permittivity (high-k) ZrTiO_x dielectric layer. The ZTO/ZrTiO_x TFTs showed high mobility of 17.9?cm²/V and I_on/off of 10⁵-10⁶?at a low operation voltage of 3?V, indicating that Indium-free ZTO thin films would be potential candidates for low cost, high performance oxide TFT devices.     

室温下溅射法制备高迁移率氧化锌薄膜晶体管

为降低氧化锌薄膜晶体管(ZnO TFT)的工作电压,提高迁移率,采用磁控溅射法在氧化铟锡(ITO)导电玻璃基底上室温下依次沉积NbLaO栅介质层和ZnO半导体有源层,制备出ZnO TFT,对器件的电特性进行了表征。该ZnO TFT呈现出优异的器件性能:当栅电压为5 V、漏源电压为10 V时,器件的饱和漏电流高达2.2 m A;有效场效应饱和迁移率高达107 cm~2/(V·s),是目前所报道的室温下溅射法制备ZnO TFT的最高值,亚阈值摆幅为0.28 V/decade,开关电流比大于107。利用原子力显微镜(AFM)对NbLaO和ZnO薄膜的表面形貌进行了分析,分析了器件的低频噪声特性,对器件呈现高迁移率、低亚阈值摆幅以及迟滞现象的机理进行了讨论。     

Effect of annealing temperature on optical and electrochemical properties of chitosan–ZnO nanostructure

Chitosan–ZnO nanostructures were prepared by chemical precipitation method using different concentration of zinc chloride and sodium hydroxide solutions. Nanorod-shaped grains with hexagonal structure for samples annealed at 300 °C and porous structure with amorphous morphology for samples annealed at 600 °C were revealed in SEM analysis. X-ray diffraction patterns confirmed the hexagonal phase ZnO with crystallite size found to be in the range of ~24.15–34.83 nm. Blue shift of UV–Vis absorption shows formation of nanocrystals/nanorods of ZnO with marginal increase in band gap. Photoluminescence spectra show that blue–green emission band at 380–580 nm. The chitosan–ZnO nanostructures used on surface of a glassy carbon electrode gives the oxidation peak potential at ~0.6 V. The electrical conductivity of chitosan–ZnO composites were observed at 2.1?×?10^?5 to 2.85?×?10^?5?S/m. The nanorods with high surface area and nontoxicity nature of chitosan–ZnO nanostructures observed in samples annealed at 300 °C were suitable as a potential material for biosensing.     

Influence of Annealing Treatment on Structural,Optical, Electric,and Thermoelectric Properties of MBE-Grown ZnO

In this paper, we have reported the influence of annealing treatment on structural, optical, electrical, and thermoelectric properties of MBE-grown ZnO on Si substrate. After growth, a set of as grown ZnO was annealed in oxygen environment at 500–800°C and another set was annealed in different environments (vacuum, oxygen, zinc, and vacuum + zinc) at 600°C for one hour in a programmable furnace. X-ray diffraction (XRD) results demonstrated that all annealed samples exhibited a major diffraction peak related to (002) plane. The full width at half maximum (FWHM) of this plane decreased and crystalline size increased for oxygen annealed sample and it increased when samples were annealed in zinc, vacuum, and successively annealed in vacuum and zinc. Further, photoluminescence spectrum revealed that the intensity of band edge emission increased and defect emission decreased as annealing temperature (oxygen environment) increased while it decreased for rest of annealing ambient. It is suspected that annealing in oxygen environment causes compensation of the oxygen vacancies by the incoming oxygen flux, while annealing in zinc and vacuum generates more oxygen vacancies. Hall and Seebeck measurements are also consistent with these arguments.     

Structural and electrical properties of fluorine-doped zinc tin oxide thin films prepared by radio-frequency magnetron sputtering

Transparent and conductive thin films of fluorine doped zinc tin oxide (FZTO) were deposited on glass substrates by radio-frequency (RF) magnetron sputtering using a 30 wt% ZnO with 70 wt% SnO₂ ceramic targets. The F-doping was carried out by introducing a mixed gas of pure Ar, CF₄, and O₂ forming gas into the sputtering chamber while sputtering ZTO target. The effect of annealing temperature on the structural, electrical and optical performances of FZTO thin films has been studied. FZTO thin film annealed at 600 °C shows the decrease in resistivity 5.47 × 10⁻³ Ω cm, carrier concentration ∼10¹⁹ cm⁻³, mobility ∼20 cm² V⁻¹ s⁻¹ and an increase in optical band gap from 3.41 to 3.60 eV with increasing the annealing temperatures which is well explained by Burstein–Moss effect. The optical transmittance of FZTO films was higher than 80% in all specimens. Work function (ϕ) of the FZTO films increase from 3.80 eV to 4.10 eV through annealing and are largely dependent on the amounts of incorporated F. FZTO is a possible potential transparent conducting oxide (TCO) alternative for application in optoelectronics.     

Use of graphene for forming Al-based p-type reflectors for near ultraviolet InGaN/AlGaN-based light-emitting diode

We demonstrated the improved performance of near UV (365 nm) InGaN/AlGaN-based LEDs using highly reflective Al-based p-type reflectors with graphene sheets as a diffusion barrier. The use of graphene sheets did not degrade the reflectance of ITO/Al contacts, viz. ∼81% at 365 nm. The ITO/graphene/Al contacts annealed at 300 °C exhibited better ohmic behavior with a specific contact resistance of 1.5 × 10⁻³ Ωcm² than the ITO/Al contact (with 9.5 × 10⁻³ Ωcm²). Near UV LEDs fabricated with the ITO/graphene/Al contact annealed at 300 °C showed a 7.2% higher light output (at 0.1 W) than LEDs with the ITO/Al reflector annealed at 300 °C. The SIMS results exhibited that, unlike the ITO/graphene/Al, the ITO/Al contacts undergo a significant indiffusion of Al atoms toward the GaN after annealing. Furthermore, both Ga and Mg atoms were also more extensively outdiffused in the ITO/Al contacts after annealing. On the basis of the SIMS and electrical results, the possible explanations for the annealing-induced degradation of the ITO/Al contacts are described and discussed.     

Electrical properties of Cd,Zn and S ion-implanted layers in GaAs

The electrical properties of cadmium, zinc, and sulfur ion-implanted layers in gallium arsenide have been measured by the van der Pauw-Hall technique. Ion implantation was performed with the substrates held at room temperature. The dependence of sheet resistivity, surface carrier concentration, and mobility on ion dose and on post-implantation anneal temperature was determined. In the case of 60 keV Cd⁺ ions implanted into n-type substrates, a measurable p-type layer resulted when samples were annealed for 10 minutes at a temperature in the range 600—900°C. After annealing at 300—900°C for 10 minutes, 100 per cent electrical activity of the Cd ions resulted for ion doses ≤ 10¹⁴/cm².

The properties of p-type layers produced by implantation of 85 keV Zn⁺ ions were similar to those of the 60 keV cadmium-implanted layers, in that no measurable p-type behavior was observed in samples annealed below a relatively high temperature. However, in samples implanted with 20 keV Zn⁺ ions a p-type layer was observed after annealing for 10 minutes at temperatures as low as 300°C.

Implantation of sulfur ions into p-type GaAs substrates at room temperature resulted in the formation of a high resistivity n-type layer, evcn before any annealing was performed. Annealing at temperatures up to 200°C or above 600°C lowered the resistivity of the layer, while annealing in the range 300—500°C eliminated the n-type layer.     

Direct fabrication of graphene/zinc oxide composite film and its characterizations

Graphene-based composites represent a new class of materials with potential for many applications. Graphene can be attached to a metal, a semiconductor, or any polymer for enhancing properties. In this work, a new mixed dispersion approach for graphene-based composite has taken on. Graphene flakes (<4 layers) and a well-known semiconductor zinc oxide (ZnO) (<50 nm particle size) have dispersed in N-methyl-pyrrolidone. We deposited graphene/ZnO composite thin film by a simple, low-cost, environmentally friendly and non-vacuum electrohydrodynamic atomization process on silicone substrate. Experiments have been carried out by changing flow rate and applied potential while keeping stand-off distance and substrate velocity constant, to discover the optimum conditions for obtaining a high-quality thin film. It has been explored that high-quality thin composite film is obtained at optimum flow rate of 300 μl/h at 6.3 kV applied potential after curing for 2 h at 300 °C. Graphene/ZnO thin composite film has been characterized using Field emission scanning electron microscopy, Ultra-violet Visible near Infra Red spectroscopy, X-ray diffraction, Raman Spectroscopy and 3D-Nanomap. For electrical behavior analysis, a simple diode Indium tin oxide/(poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/polydioctylfluorene-benzothiadiazole(F8BT)/(Graphene/ZnO) has fabricated. It is observed that at voltage of 0.3 V, the current in organic structure is at low value of 1.20 × 10^?3 Amp/cm² and after that as further voltage was applied, the device current increased by the order of 110 and reaches up to 1.32 × 10^?1 Amp/cm² at voltage 2 V.     

A low-temperature and short-annealing process for metal oxide thin film transistors using deep ultraviolet light for roll-to-roll processing

Solution-processed metal oxide semiconductors have superior electron mobility and stability than solution-processed organic semiconductors. However, their fabrication requires a very-high-temperature and long-time annealing process. In this study, we utilized deep ultraviolet (DUV) light to decrease both the temperature and time of the annealing process. High external energy is required to break the organic bonds in a metal oxide film, which is generally supplied by a high-temperature annealing process carried out for a long duration.Alternatively, the required high energy can be supplied more efficiently by irradiating the metal oxide film with DUV light for a shorter duration. In this work, we used DUV light whose peaks at 172 nm instead of the generally used mercury lamp, peaking at 254 and 185 nm. Owing to this difference, thin film transistors (TFTs) could be fabricated on silicon wafers at a lower temperature and shorter duration as compared to the conditions used in previous studies. Various conditions, such as the heating temperature, duration of DUV irradiation, and N₂ flow rate, were optimized to control the heating temperature so as to achieve a mobility of 4.44 cm²/V·s and on–off ratio of 2 × 10⁷, which are much higher than those of a transistor annealed at 300 °C for 30 min (mobility, 1.31 cm²/V·s and on–off ratio, 7 × 10⁵).     

Synthesis of nanocrystalline ZnO nanobelts via pyrolytic decomposition of zinc acetate nanobelts and their gas sensing behavior

The pyrolytic decomposition of layered basic zinc acetate (LBZA) nanobelts (NBs) into nanocrystalline ZnO NBs is investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). We also report on the gas sensing response of the resulting ZnO nanomaterial to CO. The LBZA NBs are grown at 65 °C in an aqueous solution of zinc acetate dihydrate. AFM and SEM results show as-grown products possess the characteristic layered structure of the LBZA crystals. XRD and XPS results show that annealing as-grown products at 210 °C in air causes a transformation from zinc acetate to nanocrystalline ZnO NBs via thermal decomposition. The ZnO crystalline domain size increases with temperature from 9.2 nm at 200 °C to 94 nm at 1000 °C, as measured from XRD. SEM shows evidence of sintering at 600 °C. The thickness of the NBs, determined via AFM, ranges from 10 to 50 nm and remains approximately constant with annealing temperature. XPS confirmed the chemical transformation from zinc acetate to ZnO and showed a significant remaining zinc hydroxide component for the ZnO NBs consistent with published results. PL measurements at room temperature show a blue shift in peak emission as the nanobelts change from LBZA to ZnO at 200 °C. Above this transition temperature, the ZnO nanobelts possess strong band edge emission at 390 nm and little broad band emission in the visible region. The AFM and SEM images reveal that the crystallites within the nanobelts orientate in rows along the long axis during annealing. This structure provides a high surface area to volume ratio of aligned nanoparticles which is beneficial for gas sensing applications. Gas sensors fabricated from 400 °C annealed nanobelts showed a response of 1.62 when exposed to 200 ppm of CO in dry air at 400 °C, as defined by the ratio of resistance before and during exposure. This indicates that ZnO nanostructures obtained by thermal decomposition of LBZA NBs could provide a cost effective route to high sensitivity gas sensors.     

Surface morphology study of recrystallization dynamics of amorphous ZnO layers prepared on different substrates

Amorphous oxides-based devices are exposed, during fabrication, to different processing conditions affecting their properties. Zinc oxide is a prospective candidate for transparent amorphous oxides, but its structure is changing under the influence of temperature. We investigated surface recrystallization of amorphous zinc oxide layers deposited onto fused silica, sapphire and Si substrates by pulsed laser deposition. The prepared three series of layers had highly nonequilibrium phase structures. Using atomic force microscopy and scanning electron microscopy, the effect was studied of subsequent annealing at 200, 400, 600, 800 °C for 60 min upon the surface structural properties of the layers. The following parameters were analyzed: average roughness, RMS roughness and size of formed grains on selected places with 1 × 1 μm² area. Surface structural analysis revealed that annealing led to recrystallization of the prepared layers and roughening of the structural features on the surface. With increasing annealing temperature, the calculated parameters were increasing. The average surface roughness of zinc oxide layers annealed at 800 °C is three times higher than that of the layers annealed at lower temperatures for all substrates used. The process dynamics of thermally caused recrystallization of the layers was different for each of the substrates used.     

Local structure around Zn and Ga in solution‐processed In–Ga–Zn–O and implications for electronic properties

We study by X‐ray absorption spectroscopy the local structure around Zn and Ga in solution‐processed In–Ga–Zn–O thin films as a function of thermal annealing. Zn and Ga environments are amorphous up to 450 °C. At 200 °C and 450 °C, the Ga atoms are in a β‐Ga₂O₃ like structure, mostly tetrahedral gallium oxide phase. Above 300 °C, the Zn atoms are in a tetrahedral ZnO phase for atoms inside the nanoclusters. The observed formation of the inorganic structure above 300 °C may be correlated to the rise of the mobility for IGZO TFTs. The Zn atoms localized at the nanocluster boundary are undercoordinated with O. Such ZnO cluster boundary could be responsible for electronic defect levels. Such defect levels were put in evidence in the upper half of the band gap. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effect of annealing temperature on the characteristics of ZnO thin films

Effect of annealing temperature on characteristics of sol–gel driven ZnO thin film spin-coated on Si substrate was studied. The UV–visible transmittance of the sol decreased with the increase of the aging time and drastically reduced after 20 days aging time. Granular shape of ZnO crystallites was observed on the surface of the films annealed at 550, 650, and 750 °C, and the crystallite size increased with the increase of the annealing temperature. Consequently nodular shape of crystallites was formed upon increasing the annealing temperature to 850 °C and above. The current–voltage characteristics of the Schottky diodes fabricated with ZnO thin films with various annealing temperatures were measured and analyzed. It is found that, ZnO films showed the Schottky characteristics up to 750 °C annealing temperature. The Schottky diode characteristics were diminished upon increasing the annealing temperature above 850 °C. XPS analysis suggested that the absence of oxygen atoms in its oxidized state in stoichiometric surrounding, might be responsible for the diminished forward current of the Schottky diode when annealed above 850 °C.     

Preferred orientation of ZnO nanoparticles formed by post-thermal annealing zinc implanted silica

High quality zinc oxide nanoparticles with (002) preferred orientation were prepared by post-thermal annealing zinc implanted silica at 700 °C using two methods. One method was annealing zinc implanted silica at 700 °C for 2 h in oxygen ambient; the other method was sequentially annealing zinc implanted silica at 700 °C in nitrogen and oxygen ambient for 1 h, respectively. X-ray diffraction (XRD), absorption and microphotoluminescence (micro-PL) results indicated that the latter method could create high quality ZnO nanoparticles with (002) preferred orientation and narrow size-distribution. X-ray photoelectron spectra (XPS) showed the formation of ZnO nanoparticles on a silica surface, where the ZnO nanoparticle content increased with increasing oxidation time in an oxygen environment. The processes of the transformation from Zn to ZnO are discussed.     

High‐performance In–Zn–O thin‐film transistors with a soluble processed ZrO2 gate insulator

Spin‐coated zirconium oxide films were used as a gate dielectric for low‐voltage, high performance indium zinc oxide (IZO) thin‐film transistors (TFTs). The ZrO₂ films annealed at 400 °C showed a low gate leakage current density of 2 × 10^–8 A/cm² at an electric field of 2 MV/cm. This was attributed to the low impurity content and high crystalline quality. Therefore, the IZO TFTs with a soluble ZrO₂ gate insulator exhibited a high field effect mobility of 23.4 cm²/V s, excellent subthreshold gate swing of 70 mV/decade and a reasonable I_on/off ratio of ～10⁶. These TFTs operated at low voltages (～3.0 V) and showed high drain current drive capability, enabling oxide TFTs with a soluble processed high‐k dielectric for use in backplane electronics for low‐power mobile display applications. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

XRD study on the effect of the deposition condition on pulsed laser deposition of ZnO films

Using a pulsed laser deposition (PLD) process on a ZnO target in an oxygen atmosphere, thin films of this material have been deposited on Si(111) substrates. An Nd: YAG pulsed laser with a wavelength of 1064 nm was used as the laser source. The influences of the deposition temperature, laser energy, annealing temperature and focus lens position on the crystallinity of ZnO films were analyzed by X-ray diffraction. The results show that the ZnO thin films obtained at the deposition temperature of 400°C and the laser energy of 250 mJ have the best crystalline quality in our experimental conditions. The ZnO thin films fabricated at substrate temperature 400°C were annealed at the temperatures from 400°C to 800°C in an atmosphere of N₂. The results show that crystalline quality has been improved by annealing, the optimum temperature being 600°C. The position of the focusing lens has a strong influence on pulsed laser deposition of the ZnO thin films and the optimum position is 59.5 cm from the target surface for optics with a focal length of 70 cm.      

Influence of annealing temperature on optical properties of zinc oxide thin films analyzed by spectroscopic ellipsometry method

Zinc oxide (ZnO) thin films were sol–gel spin coated on glass substrates, annealed at various temperatures 300 °C, 400 °C and 500 °C and characterized by spectroscopic ellipsometry method. The optical properties of the films such as transmittance, refractive index, extinction coefficient, dielectric constant and optical band gap energy were determined from ellipsometric data recorded over the spectral range of 300–800 nm. The effect of annealing temperature in air on optical properties of the sol–gel derived ZnO thin films was studied. The transmission values of the annealed films were about 65% within the visible range. The optical band gap of the ZnO thin films were measured between 3.25 eV and 3.45 eV. Also the dispersion parameters such as single oscillator energy and dispersive energy were determined from the transmittance graph using the Wemple and DiDomenico model.     

Influence of annealing temperature on structural and optical properties of ZnO: In thin films prepared by ultrasonic spray technique

Transparent conducting indium doped zinc oxide was deposited on glass substrate by ultrasonic spray method. The In doped ZnO samples with indium concentration of 3 wt.% were deposited at 300, 350 and 400 °C with 2 min of deposition time. The effects of substrate temperature and annealing temperature on the structural, electrical and optical properties were examined. The DRX analyses indicated that In doped ZnO films have polycrystalline nature and hexagonal wurtzite structure with (0 0 2) preferential orientation and the maximum average crystallite size of ZnO: In before and annealed at 500 °C were 45.78 and 55.47 nm at a substrate temperature of 350 °C. The crystallinity of the thin films increased by increasing the substrate temperature up 350 °C, the crystallinity improved after annealing temperature at 500 °C. The film annealed at 500 °C and deposited at 350 °C show lower absorption within the visible wavelength region. The band gap energy increased from E_g = 3.25 to 3.36 eV for without annealing and annealed films at 500 °C, respectively, indicating that the increase in the transition tail width. This is due to the increase in the electrical conductivity of the films after annealing temperature.     

Solution processed Al-doped ZnO and its performance in dye sensitized solar cells

Al-doped ZnO rods of nanometer to sub-micrometer size range have been successfully synthesized by a simple yet cost-effective solution processed sonochemical technique. Systematic XRD analysis established the solid solubility limit for Al in the ZnO lattice to be ca. 3 mol% at an elevated annealing temperature of 800 °C. The secondary ZnAl₂O₄ phase appears with increasing dopant concentrations and at lower annealing temperatures. Significant variations in the optoelectronic properties are induced by modifications in the surface defects of ZnO rods as a result of Al doping. As a consequence, an improved fill factor (FF) of 74.78 and 75.76% with a conversion efficiency (η) of 1.59 and 1.79% have been achieved for the fabricated DSSC devices made of the 800 °C annealed ZnO rods doped by 1 and 3 mol% Al, respectively.     

N离子注入富氧ZnO薄膜的p型导电及拉曼特性研究

采用射频磁控溅射法在富氧环境下制备ZnO薄膜, 继而结合N离子注入及热退火实现薄膜的N掺杂及p 型转变, 借助霍尔测试和拉曼光谱研究了N离子注入富氧ZnO薄膜的p型导电及拉曼特性. 结果表明, 在 600 ℃温度下退火120 min可获得性能较优的p-ZnO: N薄膜, 其空穴浓度约为2.527×10¹⁷ cm^-3. N离子注入ZnO引入了三个附加拉曼振动模, 分别位于274.2, 506.7和640.4 cm^-1. 结合电学及拉曼光谱的分析发现, 退火过程中施主缺陷与N受主之间的相互作用对p-ZnO的形成产生重要影响.