Scanning Tunneling Microscopic and Scanning Tunneling Spectroscopic Studies of Nanocrystalline CdSe Thin Film

Nanocrystalline CdSe thin film prepared by chemical solution deposition was imaged in air with a scanning tunnelling microscope(STM). Scanning tunnelling current spectroscopy(STS) was taken at a fixed tip - sample separation. Tunnelling current(i) - voltage(v) curve and differential conductance spectrum show an n-type schottky rectifying behaviour and yield a direct measure of band gap energy. An increase of bandgap energy (1.8 - 2.1eV) was measured indicating energy quantization of this particular thin film.,

Impact of low/high-κ spacer-source overlap on characteristics of tunnel dielectric based tunnel field-effect transistor

The effects of low-κ and high-κ spacer were investigated on the novel tunnel dielectric based tunnel field-effect transistor(TD-FET) mainly based upon ultra-thin dielectric direct tunneling mechanism. Drive currents consist of direct tunneling current and band-to-band tunneling(BTBT) current. Meanwhile, tunneling position of the TD-FET differs from conventional tunnel-FET in which the electron and hole tunneling occur at intermediate rather than surface in channel(or source-channel junction under gate dielectric). The 2-D nature of TD-FET current flow is also discussed that the on-current is degraded with an increase in the spacer width. BTBT current will not begin to play part in tunneling current until gate voltage is 0.2 V. We clearly identify the influence of the tunneling dielectric layer and spacer electrostatic field on the device characteristics by numerical simulations. The inserted Si_3N_4 tunnel layer between P+ region and N+ region can significantly shorten the direct and band-to-band tunneling path, so a reduced subthreshold slope(Ss) and a high on-current can be achieved. Above all the ambipolar current is effectively suppressed, thus reducing off-current. TD-FET demonstrates excellent performance for low-power applications.

Effects of Fe-Oxide and Mg Layer Insertion on Tunneling Magnetoresistance Properties of CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

To study the influence of CoFeB/MgO interface on tunneling magnetoresistance （TMR）, different structures of magnetic tunnel junctions （MTJs） are successfully prepared by the magnetron sputtering technique and characterized by atomic force microscopy, a physical property measurement system, x-ray photoelectron spectroscopy, and transmission electron microscopy. The experimental results show that TMR of the CoFeB/Mg/MgO/CoFeB structure is evidently improved in comparison with the CoFeB/MgO/CoFeB structure because the inserted Mg layer prevents Fe-oxide formation at the CoFeB/MgO interface, which occurs in CoFeB/MgO/CoFeB MTJs. The inherent properties of the CoFeB/MgO/CoFeB, CoFeB/Fe-oxide/MgO/CoFeB and CoFeB/Mg/MgO/CoFeB MTJs are simulated by using the theories of density functions and non-equilibrium Green functions. The simulated results demonstrate that TMR of CoFeB/Fe-oxide/MgO/CoFeB MTJs is severely decreased and is only half the value of the CoFeB/Mg/MgO/CoFeB MTJs. Based on the experimental results and theoretical analysis, it is believed that in CoFeB/MgO/CoFeB MTJs, the interface oxidation of the CoFeB layer is the main reason to cause a remarkable reduction of TMR, and the inserted Mg layer may play an important role in protecting Fe atoms from oxidation, and then increasing TMR.

Study on Ground Deformation during Shield Tunnel Construction

Through the systematic analysis of the ground settlement generated by the process of shield tunneling,the relationships between ground deformation and construction parameters are studied in this paper.Based on the assumption of linear small deformation,a mathematical model of the relationship between ground deformation and construction parameters is set up.The principle and method of optimization for estimating ground deformation is studied.The actual measured data are compared with the results of theoretical analysis in a case.Considering different ground formations in different construction sites with different adverse effects on surface and underground structures,the ground surface deformations caused by shield tunneling is an aimed topic in this paper.The contributions and research implications are the revealed relationships between the ground deformation and the shield tunneling parameters during construction.

Ground reaction curves for circular excavations in non-homogeneous,axisymmetric strain-softening rock masses

Fast methods to solve the unloading problem of a cylindrical cavity or tunnel excavated in elasto-perfectly plastic, elasto-brittle or strain-softening materials under a hydrostatic stress feld can be derived based on the self-similarity of the solution. As a consequence, they only apply when the rock mass is homogeneous and so exclude many cases of practical interest. We describe a robust and fast numerical technique that solves the tunnel unloading problem and estimates the ground reaction curve for a cylindrical cavity excavated in a rock mass with properties depending on the radial coordinate, where the solution is no longer self-similar. The solution is based on a continuation-like approach(associated with the unloading and with the incremental formulation of the elasto-plastic behavior), fnite element spatial discretization and a combination of explicit sub-stepping schemes and implicit techniques to integrate the constitutive law, so as to tackle the diffculties associated with both strong strain-softening and elasto-brittle behaviors. The developed algorithm is used for two practical ground reaction curve computation applications. The frst application refers to a tunnel surrounded by an aureole of material damaged by blasting and the second to a tunnel surrounded by a ring-like zone of reinforced(rock-bolted) material.

Centrifuge modelling of tunnelling below existing twin tunnels with different types of support

Tunnel excavation below existing tunnels produces ultimate and serviceability problems to the existing tunnels.The behaviours of induced stresses on the existing tunnels haven’t yet been fully recognized.In this study,a centrifuge model test was adopted to investigate the effects of new tunnelling on two existing overlying tunnels.One existing tunnel model simulated a prototype composite lining tunnel and the other simulated a prototype segmental lining tunnel.The volume loss produced by new tunnel excavation was modelled by an in-flight actuator system.The surface settlements,the existing tunnels settlements,the soil pressures on existing tunnels,the bending movements of existing tunnels,and the joint behaviours of existing tunnels were monitored.The volume of surface settlement trough was much smaller than the soil volume moving into the tunnel,due to the heave of the tunnel bottom and the dilation of sand during shearing.The maximum settlement of the segmental lining model was larger than that of the composite lining model as the equivalent bending stiffness of the composite lining model was larger than that of the former.Due to new tunnel excavation,the soil pressures on different positions of the existing tunnel behaved differently,and the bending movements of the existing tunnels decreased.Moreover,the joint deformation of existing tunnel caused by new tunnel excavation could be classified into three types:(1)translation,(2)rotation,and(3)combination of both.

Vertical-dual-source tunnel FETs with steeper subthreshold swing

In order to improve the drive current and subthreshold swing（SS）, a novel vertical-dual-source tunneling field-effect transistor（VDSTFET） device is proposed in this paper. The influence of source height, channel length and channel thickness on the device are investigated through two-dimensional numerical simulations. Si-VDSTFET have greater tunneling area and thinner channel, showing an on-current as high as 1.24 A at gate voltage of 0.8 V and drain voltage of 0.5 V, off-current of less than 0.1 f A, an improved average subthreshold swing of 14 m V/dec,and a minimum point slope of 4 m V/dec.

Conductance hysteresis and inelastic excitations at hydrogen decorated cerium atoms and clusters in a tunnel junction

Voltage-controlled conductance and switching induced by single molecules or atoms are ideally studied in scanning tunneling microscope （STM） tunnel junctions. While the objects under consideration are mostly used in their original form, little is known of the possibilities of in situ adjustments of their properties. Here, we evidence properties of a tunnel junction made of a Ce atom/cluster built by atomic manipulation on Au（111） at a temperature of 4.6 K in the presence of H2. The conductance through the object is characterized by a switching voltage corresponding to an opening or closing of an inelastic electron tunneling conductance channel at 50 mV for a Ce atom and 140 mV for a Ce cluster and by charging. We demonstrate that the electronic properties of an STM junction can be engineered in a simple way by in situ guiding of the H2 pinning at an atomic cluster.