隧道结的发光效应

一、基本原理根据量子力学原理,如果被真空或绝缘物分开的二电极间间隙足够窄,窄到仅有几个原子直径的距离,这时即使电子没有足够的能量“越过”由这个间隙形成的势垒,由于电子的波动性它们也能经“隧道”穿过势垒．这种现象称作电子隧道效应．观察电子隧道效应的实验模型是隧道结：一层约２０Ａ厚的绝缘物夹在两层金属薄膜之间．当在这两层金属膜之间加上小的偏压Ｖ时,便有电子隧道穿过绝缘层形成隧道电流,此时电子获得?...     

外电场下STM钨针尖电子结构的理论研究

采用离散变分局域密度泛函方法，研究了外电场对钨(111)面针尖电子结构的影响.详细计算和分析了在不同偏压和距离条件下，钨针尖的隧道激活轨道和电荷分布.研究结果表明：隧道激活轨道中针尖原子的成分对外偏压的极性、大小以及针尖与样品之间的距离都较敏感.与过去理论计算结果不同，钨针尖原子的5d_z²轨道对隧道激活轨道有一定贡献，但并不是最主要的.在加正偏压时对隧道激活轨道贡献最大的为5d_xz和5d_yz轨道，而在加负偏压时则为包括 关键词：     

自旋极化电子从铁磁金属注入半导体时自旋极化的计算

对自旋极化电子从铁磁金属通过绝缘层薄膜注入半导体时的自旋极化率与绝缘层厚度以及所加偏压的关系等作了计算.所得结果与最新实验结果相符，并发现偏压适中、绝缘层较厚时 有较大的电流自旋极化率，偏压很小时电流自旋极化率几乎为零. 关键词： 自旋极化电子注入 Slonczewski模型 隧道磁电阻 非零偏压     

处理具有任意形状势垒的磁性隧道结中电子输运的一个简单方法

在Slonczewski自由电子模型的基础上,提出了一个可用于处理具有任意形状势垒的磁性隧道结中磁电子输运的简单方法,并以三种常见构形的势垒,即梯形势垒,计入了镜像势的梯形势垒和抛物线势垒为例,讨论了势垒形状对隧穿磁电阻及其随偏压变化的影响. 关键词： 磁性隧道结 隧穿磁电阻 任意形状势垒 非零偏压     

Au表面等离子体激元激发的扫描探针电子能谱

结合扫描隧道显微镜（STM）与电子能谱仪是实现表面微区元素分析的途径之一．我们将环形电子能量分析器和三维扫描探针系统相结合,建立了一台扫描探针电子能谱仪(SPEES)．通过测量针尖近场发射束流激发的Au表面能量损失谱,我们用研究了Au原子的等离子体激元激发现象．进一步通过改变针尖－样品距离,我们研究了Au等离子体激元峰与弹性散射峰的强度比随针尖－样品距离变化的关系．研究结果发现该强度比与针尖－样品距离的关系并不是单调变化,而是在一个特定位置存在极大．     

电场下用扫描隧道显微镜对针尖原子扩散的观察

用扫描隧道显微镜（STM）研究了在金针尖和金样品间施加大偏压时所发生的各种不同的现象．在缓变大偏压的作用下，观察到针尖原子会发生场致扩散，导致针尖形状发生变化，并且还观察到了场发射和共振隧穿现象．提出了针尖原子的场致扩散是偏压电场使针尖表面极化引起的这一机理，并且指出了这种场致扩散在用大脉冲偏压作表面加工中起着重要的作用 关键词：     

MgO基磁性隧道结温度-偏压相图的理论研究

MgO基磁性隧道结是自旋电子器件研究的热点问题,其温度特性和偏压特性在实际应用中极其重要.因此,亟需在理论上计算得到MgO基磁性隧道结的温度-偏压相图.本文构建了适用于单晶势垒层磁性隧道结的理论.该理论将单晶势垒层视作周期性光栅,利用光学衍射理论处理势垒层对隧穿电子的衍射,因此可以很好地计入隧穿电子波的相干性.根据此理论,同时计入温度和偏压的影响计算了MgO基磁性隧道结的温度-偏压相图.理论结果表明,通过调节MgO基磁性隧道结的铁磁电极半交换劈裂能D、化学势μ以及势垒层周期势v(Kh)可以优化其温度特性和偏压特性.该结果为MgO基磁性隧道结的应用提供了坚实的理论基础.     

用扫描隧道显微镜针尖操纵金岛

用扫描隧道显微镜，在小隧道阻抗的条件下（小偏压和大隧道电流），通过移动针尖，实现了在室温下对真空蒸镀在高定向石墨上的、由几万个原子组成的纳米尺度金岛的操纵．在大隧道阻抗的情形下，用同一个针尖可对操纵的结果进行观察，而不会对金岛产生扰动．这种可控的操纵是通过当钨针尖与金岛间距离很近时形成的金属间黏附力大于金岛与石墨间的摩擦力而实现的 关键词：     

单层有机器件的电子传输特性的数值模拟

在漂移扩散模型的基础上建立了单层有机器件的模型,包括了电荷注入、传输、空间电荷效应和陷阱的影响.电荷注入考虑了热电子发射电流和隧道电流.模拟得到的结果和文献中报道的实验测试数据一致.模拟研究了各个因素对器件J-V曲线的影响,电流和器件长度成反比,电流随着空穴注入势垒的减小而增加.电子注入势垒从1.7 eV减少到0.5 eV时,电流随着电子注入势垒的减小而减小,这主要是因为有机材料中电子迁移率太小,电子注入电流的增加可以忽略,而电子注入势垒的减小使内建势增加,在同样的电压下,场强 关键词： 有机器件 传输特性 数值模拟     

载流子穿越具有双Mot势垒的半导体晶界的输运行为

讨论了穿越具有双Mot势垒的n型半导体晶界的载流子输运行为，重点分析了受主缺陷扩散层对偏压下晶界势垒、直流电流、非线性特性和电容等的作用．晶界势垒在偏压下的变化决定了载流子穿越晶界的输运行为分为预击穿、击穿和回复三个区域．受主缺陷扩散层的存在改变了势垒及其偏压关系，使电流的变化和非线性特性大幅度加强，很大程度上决定了预击穿区的漏电流；同时也使势垒加宽而减小高频电容，但使直流偏压下因晶界电荷的共振响应而产生的电容峰值增大 关键词：     

Observation of electronic states on Si(111)-(7 x 7) through short-range attractive force with noncontact atomic force spectroscopy

We experimentally reveal that the short-range attractive force between a Si tip and a Si(111)-(7 x 7) surface is enhanced at specified bias voltages; we conduct force spectroscopy based on noncontact atomic force microscopy with changing bias voltage at a fixed separation. The spectra exhibit prominent peaks and a broad peak, which are attributed to quantum mechanical resonance as the energy levels of sample surface states are tuned to those of the tip states by shifting the Fermi level through changing bias voltage, and to the resonating states over a lowered tunneling barrier, respectively.     

Three-dimensional scanning force/tunneling spectroscopy at room temperature

We simultaneously measured the force and tunneling current in three-dimensional (3D) space on the Si(111)-(7 × 7) surface using scanning force/tunneling microscopy at room temperature. The observables, the frequency shift and the time-averaged tunneling current were converted to the physical quantities of interest, i.e. the interaction force and the instantaneous tunneling current. Using the same tip, the local density of states (LDOS) was mapped on the same surface area at constant height by measuring the time-averaged tunneling current as a function of the bias voltage at every lateral position. LDOS images at negative sample voltages indicate that the tip apex is covered with Si atoms, which is consistent with the Si-Si covalent bonding mechanism for AFM imaging. A measurement technique for 3D force/current mapping and LDOS imaging on the equivalent surface area using the same tip was thus demonstrated.     

Theory for photon emission from a scanning tunneling microscope

We have calculated the rate of light emission from a scanning tunneling microscope with an Ir tip probing a silver film. In the calculation we model the tip by a sphere. We find a considerable enhancement of the light emission compared with for example inverse photoemission experiments. This enhancement is explained as the result of an amplification of the electromagnetic field in the area below the microscope tip due to a localised interface plasmon. We estimate that one out of 10⁴ tunneling electrons will emit a photon in the visible range. Due to an electromagnetic decoupling of the sphere from the sample the enhanced emission is lost for photon energies above a certain value. We also find that the experimentally observed maximum in the light emission as a function of bias voltage is related to the behavior of tip-sample separation versus bias voltage.     

Absence of zero-bias anomaly in spin-polarized vacuum tunneling in Co(0001)

In a joint experimental and theoretical study, we investigate the bias-voltage dependence of the tunnel magnetoresistance (TMR) through a vacuum barrier. The TMR observed by spin-polarized scanning tunneling microscopy between an amorphous magnetic tip and a Co(0001) sample is almost independent of the bias voltage at large tip-sample separations. Whereas qualitative understanding is achieved by means of the electronic surface structure of Co, the experimental findings are compared quantitatively with bias-voltage dependent first-principles calculations for ballistic tunneling. At small tip-sample separations, a pronounced minimum in the experimental TMR was found at +200 mV bias.     

Optical selection rules in light emission from the scanning tunneling microscope

It is reported that optical selection rules still apply in light emission from the scanning tunneling microscope (STM). Linear polarization of isochromat light emitted from the tunneling gap between a STM tip made of tungsten (W) and a silicon (Si) sample with a (001) clean surface strongly depends on the bias voltage between tip and sample. The results show that pi* and sigma* surface states, for example, of the Si(001) sample contribute to emission of p- and s-polarized light, respectively, in accordance with optical selection rules.     

Reversible local-modification of surface structure on clean Ge(0 0 1) by scanning tunneling microscopy below 80 K

The structure of the clean Ge(0 0 1) surface is locally and reversibly changed between c(4×2) and p(2×2) by controlling the bias voltage of a scanning tunneling microscopy (STM) below 80 K. It shows hysteresis for the direction of the sample bias voltage change. The c(4×2) structure is observed with the sample bias voltage V_b?−0.7 V. This structure is maintained at V_b?0.7 V with increasing the bias voltage from −0.7 V. When V_b is higher than 0.8 V, the structure changes to p(2×2). This structure is then maintained at V_b?−0.6 V with decreasing the bias voltage from +0.8 V. The area of the structure change can be confined in the single dimer row just under the STM tip using a bias voltage pulse. In particular, the minimum transformed length is four dimers along the dimer row in the transformation from p(2×2) to c(4×2). The observed local change of the reconstruction with hysteresis is attributed to the energy transfer process from the tunneling electron to the Ge lattice in the local electric field due to the STM bias voltage. A phenomenological model is proposed for the structure changes. It is based on a cascade inversion of the dimer buckling orientation along the dimer row.     

Phase Manipulation between c(4 x 2) and p(2 x 2) on the Si(100) surface at 4.2 K

Phase manipulation between c(4x2) and p(2x2) on the Si(100) surface has been demonstrated at 4.2 K for the first time using a low-temperature scanning tunneling microscope. We have discovered that it is possible to change the c(4x2) surface into the p(2x2) surface, artificially, through a flip-flop motion of the buckling dimers by using a sample bias voltage control. Also, scanning at a negative bias voltage or applying a pulse voltage can restore the c(4x2) surface. The STM images as a function of bias voltage and tunneling current reveal the interesting dynamics of the buckling dimers on the long debated surface. Our results will show that energetic tunneling electrons are most likely responsible for the observed phase transition from c(4x2) to p(2x2).     

Effect of substrate doping concentration on quantum well states of Pb island grown on Si(1 1 1)

The interaction between the metallic film/island and the semiconductor substrate is important to the electronic properties of metallic nanostructure grown on semiconductor substrate. Here, we report a series of comparison experiments to investigate the effect of doping concentration of Si substrates on the quantum well state (QWS). Using scanning tunneling microscopy, we observed that the apparent QWS energy positions show a strong dependence on the substrate used and on the sample temperature. Further experimental results by varying the height of scanning tunneling microscope tip over the Pb island uncovered that the observed apparent QWS energy position changes mainly come from the partial bias voltage drop on the combined resistance of the Pb wetting layer and the substrate, which is comparable with the vacuum tunneling resistance at low temperatures.     

Energy-exchange processes by tunneling electrons

Resistive heating, emission heating or cooling (e.g., the Nottingham effect), and thermal fluctuation radiation are examples of energy exchange processes which are fundamental in electron field emission and in tunneling junctions of scanning tunneling microscopy. These exchange processes are analyzed for both electronic tunneling processes. We first discuss the energy delivered by a monoatomic tip in the field emission process. Strong phonon excitation is expected for field emission currents exceeding 1 nA. Secondly we present a theoretical calculation of the thermal deposition associated with the Nottingham effect in a tunneling junction. The calculation is based on the free electron model for the electrode materials and the tunneling process across a planar vacuum gap. Our results show that the thermal power is deposited not only at the electron receiving electrode but also at the emitting electrode. This originates from a finite probability for electrons below the Fermi level to tunnel through the tunneling barrier replaced by electrons starting from the Fermi level. The comparison between the calculations and the recent STM measurements is given. Finally we discuss the other energy exchange processes in the tunneling junction, and conclude that the thermal coupling between the tip and the sample of STM is extremely small under UHV conditions. This is important for high temperature STM.