Enhancing UV photosensitivity of a ZnO UV nanosensor using electrical stimulation at megahertz frequency

We report a novel technique to enhance the ultraviolet (UV) photosensitivity of a ZnO nanosensor with ZnO nanowires bridged on micromachined metallic electrodes. The experimental results reveal that the photoconductivity and the time response of the ZnO nanowire sensor with either Schottky or Ohmic contacts are significantly improved by electrifying the nanowire sensors using an alternating current at the frequency of megahertz. An integrated UV sensor incorporating ZnO nanowires with a constant current mode driving circuit is developed, which demonstrates promising sensitivity and time response to UV illumination with a low power consumption.     

Enhancing UV photosensitivity of ZnO UV nanosensor using electrical stimulation at megahertz frequency

We report a novel technique to enhance the ultraviolent (UV) photosensitivity of ZnO nanosensor with ZnO nanowires bridged on micromachined metallic electrodes. The experimental results reveal that the photoconductivity and the time response of the ZnO nanowire sensor with either Schottky or Ohmic contacts are significantly improved by electrifying the nanowire sensors using an alternating current at the frequency of megahertz. An integrated UV sensor incorporating ZnO nanowires with a constant current mode driving circuit is developed, which demonstrates promising sensitivity and time response to UV illumination with a low power consumption.     

背入射Au/ZnO/Al结构肖特基紫外探测器

设计制作了一种Au/ZnO/Al结构的紫外探测器,光的入射方式采用背入射式。ZnO薄膜是用磁控溅射在蓝宝石衬底上制备的。I-V测试表明:Au与ZnO形成了肖特基接触。得到探测器的光响应峰值在352nm,截止边为382nm,可见抑制比达一个量级。由于该探测器是一种垂直结构器件,对于进一步实现ZnO紫外探测器阵列及单光子探测有很好的研究价值。     

Electrical characteristics of UV photodetectors based on ZnO/diamond film structure

Ultraviolet photodetectors based on ZnO/diamond film structure were fabricated. The properties of Au/ZnO contacts and effects of grain sizes on the electrical characteristics of photodetectors were discussed. Due to the bombardment with Au atoms and the annealing process, fine ohmic contacts were formed between Au electrodes and ZnO films. Dark currents and photocurrents of the photodetectors were related to sputtering time and the grain size of ZnO films. For the photodetector with a bigger grain size, a lower dark current and a higher photocurrent were obtained under 10 V bias voltage. The time-dependent photocurrent confirmed the carrier trapping effect.     

Single Metal Ohmic and Rectifying Contacts to ZnO Nanowires: A Defect Based Approach

Ohmic and rectifying metal contacts to semiconductor nanowires are integral to electronic device structures and typically require different metals and different process techniques to form. Here we show how a noble metal ion beam of Pt commonly used to pattern conducting contacts in electron microscopes can form both ohmic and Schottky/blocking contacts on ZnO nanowires by controlling native point defects at the intimate metal‐semiconductor interface. Spatially‐resolved cathodoluminescence spectroscopy on a nanoscale both laterally and in depth gauges the nature, density, and spatial distribution of specific native point defects inside the nanowires and at their metal interfaces. Combinations of electron and ion beam deposition, annealing, and sculpting of the same nanowire provide either low contact resistivity ohmic contacts or a high Schottky/blocking barrier with a single metal source. These results highlight the importance of native point defects distributed inside nanowires and their variation near interfaces with sculpting and annealing.     

退火处理对ZnO纳米线紫外探测器性能的改善

通过介电泳方法定向排列了ZnO纳米线,制作了自组装有序的纳米线紫外探测器.为了适合在金叉指电极上排列,用水热方法设计生长了超长的ZnO纳米线,并通过700℃的热退火处理,使得可利用的表面缺陷增多.通过研究器件的光致发光光谱和光响应,发现光、暗电流比和响应恢复时间有显著提高,并分析了其中可能的机理.     

In situ scanning electron microscope electrical characterization of GaN nanowire nanodiodes using tungsten and tungsten/gallium nanoprobes

A lithography-free technique for measuring the electrical properties of n-type GaN nanowires has been investigated using nanoprobes mounted in a scanning electron microscope (SEM). Schottky contacts were made to the nanowires using tungsten nanoprobes, while gallium droplets placed in situ at the end of tungsten nanoprobes were found to be capable of providing Ohmic contacts to GaN nanowires. Schottky nanodiodes were fabricated based on single n-type nanowires, and measured current–voltage (I–V) results suggest that the Schottky nanodiodes deviate from ideal diodes mainly due to their nanoscopic contact area. Additionally, the effect of the SEM electron beam on the I–V characteristics was investigated and was found to impact the transport properties of the Schottky nanodiodes, possibly due to an increase in carrier density in the nanodiodes.     

AlGaN/GaN high-electron-mobility transistors with transparent gates by Al-doped ZnO

AlGaN/GaN high-electron-mobility transistors （HEMTs） with Al-doped ZnO （AZO） transparent gate electrodes are fabricated, and Ni/Au/Ni-gated HEMTs are produced in comparison. The AZO-gated HEMTs show good DC characteristics and Schottky rectifying characteristics, and the gate electrodes achieve excellent transparencies. Compared with Ni/Au/Ni-gated HEMTs, AZO-gated HEMTs show a low saturation current, high threshold voltage, high Schottky barrier height, and low gate reverse leakage current. Due to the higher gate resistivity, AZO-gated HEMTs exhibit a current-gain cutoff frequency （fT） of 10 GHz and a power gain cutoff frequency （fmax） of 5 GHz, and lower maximum oscillation frequency than Ni/Au/Ni-gated HEMTs. Moreover, the C-V characteristics are measured and the gate interface characteristics of the AZO-gated devices are investigated by a C-V dual sweep.     

The research on suspended ZnO nanowire field-effect transistor

This paper reports that a novel type of suspended ZnO nanowire field-effect transistors (FETs) were successfully fabricated using a photolithography process, and their electrical properties were characterized by I--V measurements. Single-crystalline ZnO nanowires were synthesized by a hydrothermal method, they were used as a suspended ZnO nanowire channel of back-gate field-effect transistors (FET). The fabricated suspended nanowire FETs showed a p-channel depletion mode, exhibited high on--off current ratio of ～10⁵. When V_DS=2.5 V, the peak transconductances of the suspended FETs were 0.396 μS, the oxide capacitance was found to be 1.547 fF, the pinch-off voltage V_TH was about 0.6 V, the electron mobility was on average 50.17 cm²/Vs. The resistivity of the ZnO nanowire channel was estimated to be 0.96× 10²Ω cm at V_GS = 0 V. These characteristics revealed that the suspended nanowire FET fabricated by the photolithography process had excellent performance. Better contacts between the ZnO nanowire and metal electrodes could be improved through annealing and metal deposition using a focused ion beam.     

Current--voltage characteristics of an individual helical CdS nanowire rope

This paper studies the electronic transport in an individual helically twisted CdS nanowire rope, on which platinum microleads are attached by focused-ion beam deposition. The current-voltage （I - V） characteristics are nonlinear from 300 down to 60 K. Some step-like structures in the I - V curves and oscillation peaks in the differential conductance （dI/dV - V） curves have been observed even at room temperature. It proposes that the observed behaviour can be attributed to Coulomb-blockade transport in the one-dimensional CdS nanowires with diameters of 6-10 nm.     

Comparison of bismuth emitting liquid metal ion sources

In this paper, we report on studying of ZnO nanowire mats as an electrical nanomaterial with particular interest in their interaction with various gas surroundings for gas sensing characteristics. The ZnO nanowires were synthesized on sapphire substrates using a horizontal tube furnace. The techniques of Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), X-Ray Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS) were applied to determine the as-grown ZnO nanowires’ morphological and crystal structures, chemical composition and electronic states. Four-terminal current-voltage (I–V) measurements were used to examine the electrical conductance of the ZnO nanowire mats exposed to various testing gases with reference to the vacuum condition. Gas exposure experiments were conducted in a custom-built environmental chamber, which was filled with different testing gases. We observed the current being significantly influenced with ambient CO gas. The I–V behavior of CO gas was also found to be reversible and repeatable after the chamber evacuation, which indicates that the ZnO nanowire mats can be used for gas sensing purposes. A possible interactive model of nanowires and testing gas molecules is proposed to elucidate the sensing selective and sensitive mechanism for gas sensors.     

Performance comparison of Pt/Au and Ni/Au Schottky contacts on AlxGa1-x N/GaN heterostructures at high temperatures

In contrast with Au/Ni/Al 0.25 Ga 0.75 N/GaN Schottky contacts,this paper systematically investigates the effect of thermal annealing of Au/Pt/Al 0.25 Ga 0.75 N/GaN structures on electrical properties of the two-dimensional electron gas in Al 0.25 Ga 0.75 N/GaN heterostructures by means of temperature-dependent Hall and temperature-dependent current-voltage measurements.The two-dimensional electron gas density of the samples with Pt cap layer increases after annealing in N 2 ambience at 600℃ while the annealing treatment has little effect on the two-dimensional electron gas mobility in comparison with the samples with Ni cap layer.The experimental results indicate that the Au/Pt/Al 0.25 Ga 0.75 N/GaN Schottky contacts reduce the reverse leakage current density at high annealing temperatures of 400-600℃.As a conclusion,the better thermal stability of the Au/Pt/Al 0.25 Ga 0.75 N/GaN Schottky contacts than the Au/Ni/Al 0.25 Ga 0.75 N/GaN Schottky contacts at high temperatures can be attributed to the inertness of the interface between Pt and AlxGa1-xN.     

Patterned horizontal growth of ZnO nanowires on SiO2 surface

We report patterned horizontal growth of ZnO nanowires on SiO₂ surface for the study of electrical and luminescent characteristics of individual nanowires and for device applications. Patterns of gold catalytic seed islands with barrier layers which suppress vertical growth were employed to facilitate horizontal growth on SiO₂ surface. After the growth, ZnO nanowire devices are fabricated by patterning electrodes aligned over the seed islands and their device characteristics are investigated. We could also investigate history of synthesis conditions by obtaining local luminescence characteristics along individual nanowires.     

ZnO纳米线场效应管的制备及I-V特性研究

采用静电探针和原子力探针技术,将化学气相沉积工艺制备的,长度为30—200 μm,直径80—750 nm的单根半导体ZnO纳米线搭接在Au,Zn,Al不同功函的金属隔离沟道两端,构建出了最基本的ZnO纳米线绝缘栅场效应管. 研究了沟道类型、纳米线直径、退火温度和外加栅压对ZnO纳米线场效应管I-V特性的影响. 利用半导体与金属材料的肖特基接触、欧姆接触的产生机理及电子输运理论,对结果进行了分析和讨论. 关键词： ZnO纳米线 场效应管 I-V特性')" href="#">I-V特性     

Micropatterned ZnO rod arrays prepared by Au‐catalyzed electroless deposition

Micropatterned ZnO was synthesized by an electroless deposition process using Au stripes as catalytic surfaces. The Au‐patterned electrodes were prepared on SiO₂/Si wafers using photolithography. The site‐selective deposition of patterned ZnO hexagonal rod arrays is confirmed by scanning electron microscopy. The ZnO micropatterned surface revealed a conversion of wettability from hydrophilic to superhydrophobic depending on the deposition reaction param‐ eters. The electrical measurements carried out at room temperature before and after exposure to ammonia vapors of the patterned ZnO arrays show a resistance variation with exposure time. Highly reproducible, easy scalable and low‐cost, photolithography and electroless deposition techniques could provide a facile approach to fabricate functionalized micropatterns, for a wide range of applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Pt/Au/n-InGaN肖特基接触的电流输运机理

基于热电子发射和热电子场发射模式,利用I-V方法研究了Pt/Au/n-InGaN肖特基接触的势垒特性和电流输运机理,结果表明,在不同背景载流子浓度下,Pt/Au/n-InGaN肖特基势垒特性差异明显.研究发现,较低生长温度制备的InGaN中存在的高密度施主态氮空位（V_N）缺陷导致背景载流子浓度增高,同时通过热电子发射模式拟合得到高背景载流子浓度的InGaN肖特基势垒高度和理想因子与热电子场发射模式下的结果差别很大,表明V_N缺陷诱发了隧穿机理并降低了肖特基势垒高度,相应的隧穿电流显著增大了肖特基势垒总的输运电流,证实热电子发射和缺陷辅助的隧穿机理共同构成了肖特基势垒的电流输运机理.低背景载流子浓度的InGaN肖特基势垒在热电子发射和热电子场发射模式下拟合的结果接近一致,表明热电子发射是其主导的电流输运机理.     

瞬态光电流对有机薄膜光伏器件中肖特基接触的研究

制备了结构为氧化铟锡(ITO)/有机半导体/金属的有机薄膜光伏器件,电流--电压曲线显示其具有整流特性但有机半导体和电极间肖特基接触的内建电场方向很难判定.为了研究有机半导体和电极的肖特基接触特性,分别制备了结构为ITO/有机绝缘层/有机半导体/金属和ITO/有机半导体/有机绝缘层/金属的器件,通过调制激光照射下器件的瞬态光电流方向可容易判断有机半导体和电极间肖特基接触的内建电场方向,外加偏压下瞬态光电流的强度变化进一步证实了判断的正确性.     

Nanostructure size evolution during Au-catalysed growth by carbo-thermal evaporation of well-aligned ZnO nanowires on (100)Si

We report the structural and morphological properties of well-aligned ZnO nanowires grown at 750 °C on Au-deposited and annealed (100)Si substrates using carbo-thermal evaporation. As-grown nanowires are made of wurtzite ZnO, have cylindrical shape and carry droplet-like nanoparticles (NPs) at their tips, as expected for vapour–liquid–solid (VLS) growth. Grazing incidence X-ray diffraction measurements demonstrate that the NPs are made of pure fcc Au. No secondary Au/Zn alloy phases were detected. Bragg diffraction patterns confirmed that the nanowires were grown with their crystal c-axes parallel to the [100] direction of Si (i.e. normal to the substrate surface), while Au NPs are mostly (111)-oriented. The diameter distribution of ZnO nanowires mimics that of the Au NPs at their tips. A quantitative study of the nanostructure size distribution after sequential annealing and growth steps evidences the occurrence of three nanoscale processes: (i) Ostwald ripening and/or coalescence of Au NPs before nanowire nucleation, (ii) Au-catalysed VLS nucleation and axial growth of ZnO nanowires and (iii) radial growth of nanowires by a vapour–solid process. These processes originate the NP and nanowire size evolution during the experiments. The present findings are interpreted in terms of Zn vapour pressure changes during carbo-thermal evaporation. PACS 61.46.+w; 68.65.-k; 81.16.Dn     

电极肖特基接触对有机光敏晶体管性能的影响

有机光敏晶体管是一种在有机场效应管结构中引入光控“栅极”的新型光探测器件,其光灵敏度、光响应度性能参数与源/漏电极和有源层的接触情况关系密切。本文通过真空蒸发法分别制备了采用金电极和铝电极的单层并五苯及酞菁铜有机光敏晶体管。研究了它们在黑暗和光照条件下的输出及转移特性。结果表明,高迁移率的并五苯有源层更适合搭配接触特性较好的金电极,该器件具有和铝电极器件相同高水平的光灵敏度~3×104,但其光响应度是铝电极器件的13倍;而低迁移率的酞菁铜薄膜较适合搭配能够和有源层形成肖特基接触的铝电极,有利于抑制暗电流、增强激子解离效率、提高光电流,进一步使器件在获得和金电极器件同数量级光响应度的同时,其光灵敏度是金电极器件的102倍。本文对光照下电极/有源层肖特基接触的能带变化做了理论分析,总结归纳了有机光敏晶体管电极材料和有源层材料的初步筛选规律。     

Characteristics of ZnO MSM UV photodetector with Ni contact electrodes on poly propylene carbonate (PPC) plastic substrate

In this study, we report the fabrication of ZnO metal-semiconductor-metal UV photodetector (MSM UV PD) by deposition ZnO thin film on poly propylene carbonate (PPC) plastic substrate using direct current (DC) sputtering technique, and Nickel (Ni) contact as electrodes. The structural, optical and electrical properties of the ZnO thin film were investigated by using atomic force microscopy (AFM), X-Ray diffraction (XRD) measurement, and photoluminescence (PL). The electrical characteristics of the detector were investigated using the current–voltage (I–V) measurements, the dark- and photo-currents were found to be 1.04 and 93.80 μA, respectively. Using forward dark conditions at 5 volt; the barrier height Φ_B was calculated to be 0.675 eV. Under incident wavelength of 385 nm, it was found that the maximum responsivity (R) of the Ni/ZnO/Ni MSM PD was found to be 1.59 A/W.