Absorbing/emitting phonons with one dimensional MOSFETs

We consider nanowires in the field effect transistor device configuration. Modeling each nanowire as a one dimensional lattice with random site potentials, we study the heat exchanges between the nanowire electrons and the substrate phonons, when electron transport is due to phonon-assisted hops between localized states. Shifting the nanowire conduction band with a metallic gate induces different behaviors. When the Fermi potential is located near the band center, a bias voltage gives rise to small local heat exchanges which fluctuate randomly along the nanowire. When it is located near one of the band edges, the bias voltage yields heat currents which flow mainly from the substrate towards the nanowire near one boundary of the nanowire, and in the opposite direction near the other boundary. This opens interesting perspectives for heat management at submicron scales: arrays of parallel gated nanowires could be used for a field control of phonon emission/absorption.     

聚苯胺/ZnO纳米线薄膜材料作为发光层的柔性光电器件 及其发光机理

综合氧化锌纳米线(ZnO NWs)的光学活性与聚苯胺(PANI)的空穴传输特性,设计并制备了一种聚合物/ZnO纳米线电致发光材料,并对其发光特性进行了研究。通过高分子络合软模板法,将有序的单晶ZnO NWs均匀生长在覆有铟锡氧化物(ITO)涂层的柔性聚乙烯对苯二甲酸乙二醇酯(PET)衬底上并嵌入PANI薄膜,获得了电致发光薄膜材料和有机/无机异质结实验器件ITO/(ZnO NWs-PANI)。有机/无机异质结器件电致发光可调,在相对低的开启电压下呈现室温蓝紫外发光,并且ZnO NWs表面覆盖PANI增加了蓝紫外发光的强度和稳定性;而无PANI的ZnO NWs阵列具有450 nm处的缺陷发射峰,这可能是电子从扩展态锌间隙Zn_i到价带的跃迁引起的。这些结果表明,基于PANI/ZnO纳米线的复合材料在柔性光电器件方面的应用极具潜力。     

表面钝化效应对GaAs纳米线电子结构性质影响的第一性原理研究

纳米线表面存在大量的表面态,它们能够引起电子分布在纳米线表面,使得纳米线的电学性质对表面条件变得更加敏感,严重地制约器件的性能.表面钝化能够有效地移除纳米线的表面态,进而能够有效地优化器件的性能.采用基于密度泛函理论的第一性原理计算方法研究了表面钝化效应对GaAs纳米线电子结构性质的影响.考虑了不同的钝化材料,包括氢元素、氟元素、氯元素和溴元素.研究结果表明:具有小尺寸的GaAs裸纳米线的能带结构呈间接带隙特征,表面经过完全钝化后,转变为直接带隙特征;GaAs纳米线表面经过氢元素不同位置和不同比例钝化后,展示出不同的电学性质;表面钝化的物理机理是钝化原子与纳米线表面原子通过电荷补偿移除纳米线表面的电子态;与氢元素钝化相比,GaAs纳米线表面经过氟元素、氯元素和溴元素钝化后,带隙宽度较小,原因是氟元素、氯元素和溴元素在钝化过程中具有较小的电荷补偿能力,不能完全移除表面态.     

Electroluminescence from ZnO nanowire-based p-GaN/n-ZnO heterojunction light-emitting diodes

Vertically aligned ZnO nanowires were successfully grown on the sapphire substrate by nanoparticle-assisted pulsed laser deposition (NAPLD), which were employed in fabricating the ZnO nanowire-based heterojunction structures. p-GaN/n-ZnO heterojunction light-emitting diodes (LEDs) with embedded ZnO nanowires were obtained by fabricating p-GaN:Mg film/ZnO nanowire/n-ZnO film structures. The current–voltage measurements showed a typical diode characteristic with a threshold voltage of about 2.5 V. Electroluminescence (EL) emission having the wavelength of about 380 nm was observed under forward bias in the heterojunction diodes and was intensified by increasing the applied voltage up to 30 V.     

掺AlZnO纳米线阵列的光致发光特性研究

采用化学气相沉积方法，以金做催化剂，在Si (100)衬底上制备了掺AlZnO纳米线阵列.扫描电子显微镜(SEM)表征发现ZnO纳米线的直径在30nm左右.X射线衍射(XRD)图谱上只存在ZnO的(002)衍射峰，说明ZnO纳米线沿c轴择优取向.掺AlZnO纳米线阵列的室温光致发光(PL)谱中出现了3个带边激子发射峰：373nm，375nm，389nm.运用激子理论推算出掺AlZnO纳米线的禁带宽度为3.343eV ，束缚激子结合能为0.156eV；纯ZnO纳米线阵列PL谱中3个带边激子发射 关键词： 光致发光 化学气相沉积(CVD) 激子 ZnO纳米线阵列     

Dielectrophoretically aligned GaN nanowire rectifiers

We demonstrate GaN nanowire (NW) current rectifiers which were formed by assembling n-GaN nanowires on a patterned p-Si substrate by means of alternating current (ac) dielectrophoresis. The dielectrophoresis was accomplished at a frequency of 10 kHz with three different ac bias voltages (5, 10, and 15 V_p–p), indicating that the number of aligned GaN nanowires increased with increasing ac bias voltage. The n-GaN NW/p-Si diodes showed well-defined current rectifying behavior with a forward voltage drop of 1.2–1.5 V at a current density of 200 A/cm². We observed that the GaN NW diode functioned well as a half-wave rectifier. PACS 71.20.Nr; 73.40.Cg; 73.40.Ei; 73.40.Kp     

紫外光辐照对TiO2纳米线电输运性能的影响及磁阻效应研究

采用溶胶凝胶法以及静电纺丝法, 利用热处理工艺, 成功制备出了多晶锐钛矿型TiO₂纳米线, 通过两线法在室温下测试单根TiO₂纳米线的V-I曲线来研究其电输运性能及磁阻效应. 结果表明: 在无光照环境下其V-I曲线为不过零点的直线, 零场电阻较大, 在磁场作用下电阻下降, 表现出负磁阻效应; 紫外光辐照环境下TiO₂纳米线载流子浓度增加使得电阻变小, 然而在磁场作用下电阻增大, 表现为正磁阻效应. 紫外光辐照导致的载流子浓度变化, 使得负磁阻转变为正磁阻, 我们将磁阻变化归结为d电子局域导致的负磁阻与能带劈裂导致的正磁阻两种机理相互竞争的结果.     

The influence of wire shape on surface plasmon mode distribution

The influence of the nanowire shape on the excitation of surface plasmon polaritons at metallic nanowire arrays is studied numerically. For a system of silver nanowires housed on a polymer substrate, nanowires with rectangular and elliptical cross sections are compared. It was found that in the case of rectangular nanowires the excitation efficiency is higher for surface plasmons at the polymer–metal interface than for surface plasmons at the air–metal interface. Conversely, in the case of elliptical nanowires the air–metal plasmon modes are stronger. Further, it is noted that the nanowire shape directly influences the position of the surface plasmon resonance.     

片上制备横向结构ZnO纳米线阵列紫外探测器件

将纳米技术与传统的微电子工艺相结合, 片上制备了横向结构氧化锌(ZnO)纳米线阵列紫外探测器件, 纳米线由水热法直接自组织横向生长于叉指电极之间, 再除去斜向的多余纳米线, 其余工艺步骤与传统工艺相同. 分别尝试了铬(Cr)和金(Au)两种金属电极的器件结构: 由于Cr电极对其上纵向生长的纳米线有抑制作用, 导致横向生长纳米线长度可到达对侧电极, 光电响应方式为受表面氧离子吸附控制的光电导效应, 光电流大但增益低, 响应速度慢, 经二次电极加固, 纳米线根部与电极金属直接形成肖特基接触, 光电响应方式变为光伏效应, 增益和速度得到了极大改善; 由于Au电极对其上纵向生长的纳米线有催化作用, 导致溶质资源的竞争, 相同时间内横向生长的纳米线不能到达对侧, 而是交叉桥接, 但却形成了紫外光诱导的纳米线间势垒结高度调控机理, 得到的器件特性为最优, 在波长为365 nm的20 mW/cm²紫外光照下, 1 V电压时暗电流为10^-9 A, 光增益可达8×10⁵, 响应时间和恢复时间分别为1.1 s和1.3 s.     

A study of optoelectronics in photonic nanowires made from photonic crystals

We have studied optoelectronic properties of photonic nanowires doped with an ensemble of four-level nanoparticles. Nanowires are made from two photonic crystals A and B where crystal A is embedded in B. Photons are confined with the photonic nanowire due to the band structure engineering of crystals A and B. A probe field is applied to monitor the absorption spectrum, and a control field is applied to shift the position of absorption peak. It is considered that nanoparticles are interacting with bound photon states of the nanowire. It is found that the number of bound states in the wire depends on the size and the energy depth of the wire. It is also found that when the resonance energy lies near the bound state, the system goes from absorbing state to the transparent state. This is due to the strong coupling between nanoparticles and bound photons in the wire. The control field switches the system from the transparent state to the absorbing state by changing the location of the resonance energy. The present findings can be used to make new types of optoelectronic switches.     

Magnetostatic interactions in dense nanowire arrays

We demonstrate the importance of magnetostatic interactions in dense arrays of ferromagnetic nanowires. Beginning from a simple micromagnetic model, we have calculated the interaction field for saturated magnetization in the plane of the array (perpendicular to the axes of the wires) and normal to the plane, using a hybrid (numerical and analytical) strategy. The slope of interaction field versus wire length changes dramatically at the transition between a dipolar regime (at very small lengths) and a monopolar regime (for longer nanowires). We present the interaction fields and the applied fields at saturation for large nanowire arrays. These results are compared with experiment for electrodeposited arrays, and very good agreement is obtained. This shows that the high field behavior of such arrays is dominated by magnetostatic effects and that a nanowire array behaves like a double-sided distribution of magnetic monopoles.     

Magnetization reversal dynamics in Co nanowires with competing magnetic anisotropies

We present the magnetization reversal dynamics of Co nanowires with competing magnetic anisotropies. The aspect ratio (R) of the nanowires is varied between 2.5 and 60, and we observe a cross-over of the directions of the magnetic easy and hard axes at R=6.8. Micromagnetic simulations qualitatively reproduce the observed cross-over and give detailed insight into the reversal mechanisms associated with the cross-over. The reversal mechanism for a field applied along the long axis of the nanowire exhibits a quasi-coherent rotation mode and a corkscrew-like mode, respectively, above and below the cross-over, with the formation of a Bloch domain near the cross-over region. For a field applied along the short axis, the reversal occurs by nucleation and propagation of reversed domains from the two ends of the nanowires for very high values of the aspect ratio down to the cross-over region, but it transforms into quasi-coherent rotation mode for smaller aspect ratios (below the cross-over region).     

Germanium nanowires: from synthesis, surface chemistry, and assembly to devices

A low temperature synthesis of single crystalline Ge nanowires via chemical vapor deposition is enabled by balancing the feedstock and its diffusion in growth seeds. Understanding and optimizing the synthetic chemistry leads to deterministic nanowire growth at well-defined locations and bulk quantity production of homogeneous nanowires, both of which greatly facilitate the assembly toward parallel nanowire arrays. Surface chemistry studies reveal that p- and n-type Ge nanowires undergo different oxidation routes and the surface oxide induced states cause opposite band bending for nanowires with different doping. Furthermore, long chain alkanethiols form a dense and uniform protection layer on Ge nanowire surfaces and therefore afford excellent oxidation resistance. Finally, high performance field effect transistors are constructed on Ge nanowires with both thermally grown SiO₂ and atomic layer deposited HfO₂ as gate dielectrics. PACS 73.63.-b; 73.63.B6; 73.22.-f; 73.20.At; 73.90.+f     

The stability and electronic properties of wurtzite and zinc-blende ZnS nanowires

The stability and electronic properties of the pristine wurtzite (WZ) and zinc-blende (ZB) structural ZnS nanowires are investigated and compared by using first-principles approaches. It shows that the WZ-ZnS nanowire is more stable energetically than the ZB-ZnS nanowire. The two kinds of ZnS nanowires have different electronic properties due to both the quantum confinement effect and the surface effect. The band gaps of pristine WZ nanowires become larger than that of the corresponding bulk ZnS, while those of ZB nanowires are smaller. The electronic properties of the hydrogen-passivated WZ-ZnS and ZB-ZnS nanowires are further calculated. The underlying physical reason for their energetic and electronic structures is elucidated.     

电子束诱导沉积纳米线的电阻与场发射特性

"运用电子束诱导沉积技术在钨针尖表面沉积钨纳米线．在透射电子显微镜中,原位测量单根纳米线的电阻与场发射特性,并观察其显微结构变化．样品台为特制的电性能测试样品台,包括步进电机和压电陶瓷驱动的装置．导电铜片作为与纳米线相对的另一极．自行设计制作锁相放大器电路测量纳米线的电阻．结果表明,纳米线的电阻为0.1*10-3 -m量级．纳米线头部的几何缺陷将影响其场发射特性．纳安级电流将改变纳米线头部的几何结构与微观结构．场发射开启电压比结构变化前低11 V左右．"     

Mn掺杂ZnO纳米线磁性质的第一性原理研究

本文采用第一性原理方法系统研究了Mn原子单掺杂和双掺杂ZnO纳米线的稳定性和磁性质.所有掺杂纳米线的束缚能都为负值,表明掺杂增强了纳米线的稳定性.表面掺杂纳米线显示了直接带隙半导体特性,而中间掺杂纳米线显示了间接带隙半导体特性.纳米线的总磁矩主要来源于Mn原子3d轨道的贡献.由于杂化,相邻的O原子和Zn原子也产生了少量自旋.在超原胞内,Mn原子和O原子磁矩平行排列,表明它们之间是铁磁耦合.     

Investigation of gate-all-around silicon nanowire transistors for ultimately scaled CMOS technology from top-down approach

The gate-all-around (GAA) silicon nanowire transistor (SNWT) is considered one of the best candidates for ultimately scaled CMOS devices at the end of the technology roadmap. This paper reviews our recent work on the key issues regarding SNWTs from the top-down approach including process integration, carrier transport, and fluctuation and variability in these unique one-dimensional stronglyconfined nanowire devices. A new process integration scheme for SNWTs is discussed, which features a fully-Si-bulk substrate, an epi-free process, a self-aligned structure and a large source/drain fan-out. The physical characteristics of the fabricated devices with 10-nm-diameter nanowires are further investigated. The carrier transport performance in SNWTs is experimentally estimated, with a modified extraction methodology which takes into account the impact of temperature dependence of parasitic source resistance. SNWTs with sub-40-nm gate lengths exhibit high ballistic efficiency at room temperature, indicating great potential for SNWTs as an alternative device structure for near-ballistic transport. For heat transfer in SNWTs, the self-heating effect is also characterized. However, due to the one-dimensional (1-D) nature of nanowires and increased phonon-boundary scattering in the GAA structure, the self-heating effect in SNWTs based on the bulk substrate is comparable or even a little bit worse than SOI devices, which may limit the ultimate performance of SNWT-based circuits and thus, special design consideration is expected. On the other hand, random variation has become a practical problem at nano-scale. The characteristic variability of SNWTs is experimentally studied in detail. The results of extracted variation sources indicate that, with suppressed random dopant fluctuations in the intrinsic channel, variations in radius and metal-gate work function of SNWTs dominate both the threshold voltage and on-current fluctuations. Comparing with conventional planar MOS devices, SNWT based SRAM cells exhibit better stability due to the superior electrostatic control in SNWTs.     

Vacancies in GaN bulk and nanowires: effect of self-interaction corrections

We investigate gallium and nitrogen vacancies in gallium nitride (GaN) bulk and nanowires using self-interaction corrected pseudopotentials (SIC). In particular, we examine the band structures to compare and contrast differences between the SIC results and standard density functional theory (DFT) results using a generalized gradient approximation (GGA) (Perdew et al 1996 Phys. Rev. Lett. 77 3865) functional. For pure nanowires, we observed similar trends in the bandgap behaviour, with the gap decreasing for increasing nanowire diameters (with larger bandgaps using SIC pseudopotentials). For gallium vacancies in bulk GaN and GaN nanowires, SIC results are similar to DFT-GGA results, albeit with larger bandgaps. Nitrogen vacancies in bulk GaN show similar defect-induced states near the conduction band, whilst a lower lying defect state is observed below the valence band for the DFT-GGA calculations and above the valence band for the SIC results. For nitrogen vacancies in GaN nanowires, similar defect states are observed near the conduction band, however, while the SIC calculations also show a defect state/s above the valence band, we were unable to locate this state for the DFT-GGA calculations (possibly because it is hybridized with edge states and buried below the valence band).     

High-speed scanning photocurrent imaging techniques on nanoscale devices

We report the characterization of individual carbon nanotube and Si nanowire field-effect transistors through high-speed scanning photocurrent microscopy with a scanning speed of 1 frame/s and a photocurrent sensitivity of less than 1 pA. This enables us to record photocurrent images that are free from hysteresis effects that modify the field configurations applied by the gate bias voltage. We can clearly resolve the photocurrent signals with polarity inversion near the metallic contacts under gate bias conditions which cause severe hysteresis effects in the nanowire devices. We also studied the dynamics of the hysteresis effects for different gate bias configurations. This high-speed photocurrent imaging technique is particularly useful for obtaining two-dimensional, localized optoelectronic characteristics and their correlation with overall device performance without encountering undesired dynamic responses.     

Fano resonance in asymmetric gold nano-dimers with square and rectangular sections

The Fano resonances of the asymmetric dimers of gold nanowires with square and rectangular sections are investigated by using the finite-difference time-domain (FDTD) method. It is found that the Fano resonance peak can be switched on and off by laying or removing the rectangular section nanowire aside the square section one. There is only one dipole resonance mode of a single gold nanowire with a square section on the dielectric substrate, and it redshifts obviously accompanied with the FWHM being broadened along with the side width of the nanowire increasing. A Fano resonance appears when another gold nanowire with the rectangular section is laid aside the one with a square section. The peak value and FWHM of the Fano resonance mode increase obviously with the distance between the two nanowires getting larger. Meanwhile, it can be modulated by the height of the rectangular section nanowire. In addition, they can be regulated by the width and rotating angle of the rectangular section nanowire, but the peak value stays the same. The mechanisms for these behaviors are associated with the interaction of the superradiant and subradiant modes, and the corresponding electric field distributions are plotted to verify this. It is expected that the results will be useful for the design of wavelength biosensing and other new optical devices.