Search

In this paper, we are discussing about the effect of parameters on roughness, such as initial resist thickness, molecular weight etc.

Multiple tunnel junctions have been realized in silicon using highly doped nanowires in silicon-on-insulator material by making use of pattern-dependent local oxidation. The electrical characteristics show clear Coulomb blockade oscillations of a single dot. On depleting the nanowires with side gates, peak pairing is observed in the oscillations. This characteristic of double dot structures can be explained by the creation of additional tunnel barriers in the nanowire due to the potential distribution caused by the dopants.

We report on the theoretical investigation of how geometrically uniform highly doped silicon nanowires can break up into a series of islands that exhibit Coulomb blockade. By using a newly developed numerical simulation in which random ionized dopants are introduced explicitly and the electron distribution is calculated self-consistently under the Thomas-Fermi approximation, we demonstrate natural formation of electron islands in the nanowires owing to the random dopant potential. We study the quasi-one-dimensional nature of the electron islands formed in the nanowires. The offset charge effects on the current threshold of the nanowire transistors are then investigated by feeding the derived structural parameters such as inter-island capacitance and tunnel resistance into a Monte Carlo single electron transport simulator. We show that the overall threshold voltage distribution can roughly be described as a two-`macro'-island system despite a complex series of multiple electron islands.

We have fabricated nanometer-scale point-contact devices in 50-nm-thick poly-Si films with a grain size of from 150 nm to 20 nm. Both linear and nonlinear I ds-V ds characteristics were observed in these devices, corresponding to a channel without a grain boundary (GB) and that with a single or a few GBs, respectively. The temperature dependence of resistivity indicated that the effective potential barrier height q V B of the GBs for the devices which show the nonlinear I ds-V ds characteristics ranges from 30 meV to 80 meV. We discussed percolation conduction of electrons through a few GBs due to nonuniform GB properties in heavily doped poly-Si films.

Electron pumps based on multiple tunnel junctions have been realized in silicon using highly doped nanowires in silicon-on-insulator material. The electron pump characteristics show a clear transfer of the pattern of Coulomb blockade oscillations to one of a pumping action. Furthermore, the pump current tends to scale linearly with the frequency. The results can be explained by a pumping mechanism in asymmetric multiple tunnel junctions. These devices allow sufficient control of electron packets which are potentially useful as basic building blocks of silicon-based binary-decision-diagram logic circuits.

A novel technique for single atom lithography has been developed for the fabrication of nanostructures. It is based on a focused ion-beam with a retarding field stage, which enables the ion impact energy to be reduced to any level down to 0 eV. An ion detector has been designed and incorporated in the system which has a quantum detector efficiency close to 100% for high energy ion impacts and is capable of detecting the arrival of single Au+ ions with energies as low as 30 eV. The system has been used to deposit nanostructures in the size range 1–5 nm in diameter. The relation of size to surface morphology and to beam parameters has been studied. A new system operating in UHV conditions has been commissioned to enhance the study, particularly with respect to nucleation of particles and surface mobility effects. Initial experiments on a Coulomb blockade device with islands fabricated with the system have been conducted.

We have studied the transport properties over a range of temperatures of two quantum dots connected in series formed in silicon inversion layers. The inter-dot interaction as well as the electron number can be changed by means of the field effect in our device. For weak inter-dot interactions, the device shows a periodic current oscillation and a biquadratic temperature-dependent current due to co-tunneling. On increasing the inter-dot interaction, we have observed splitting of peaks and estimated the typical inter-dot capacitance to be 1.3×10-18 F.

In order to investigate anisotropic transport in various types of lattices, two-dimensional electron gas in the hexagonal and rectangular antidot lattices are measured. Magnetoresistance is isotropic in hexagonal lattice, but anisotropic in the rectangular lattice with circular antidots. The influence of antidot shapes upon anisotropic transport is also investigated in the rectangular lattice. It is found that the anisotropic transport is sensitive to the arrangement and the shape of antidot potential and anomalous peaks are observed in rectangular lattice with ellipsoidal antidots. The origin of the anomalous peaks is discussed. In both lattices quenching of the Hall effect is observed and the positions of commensurate peaks in Hall resistance shift to higher field than that in magnetoresistance. These effects cannot be explained by the pinned orbit model.

Eigenenergies and eigenfunctions in coupled electron waveguides with Schottky gates are numerically calculated by solving the Schrödinger equation and the Poisson equation self-consistently in order to estimate the strength of coupling between two electron waveguides. It is concluded that very low middle-gate voltage is required for strong coupling. In order to realize strong coupling, a modified structure which has a middle gate with a small gap is proposed and fabricated using electron beam lithography and the lift-off technique.

The multiple-tunnel junction (MTJ) produced by forming a side-gated constriction in δ-doped GaAs is demonstrated as a basic component of single electronics. A single-electron memory cell, in which one bit of information is represented by the excess or lack of a precise number of electrons, was fabricated and the operation was confirmed at liquid helium temperature. A new switching element, the variable-barrier MTJ, and a new circuit, the single-charge injection logic circuit, are proposed.

We report the fabrication and performance of a new gallium arsenide (GaAs) metal semiconductor field-effect transistor (MESFET) device. The new device combines the concept of step-doping, where the doping of the channel is changed in a step, from high towards the source to low towards the drain, of the MESFET channels with that using a buried p-layer under the channel. Focused ion beams (FIBs) of silicon (Si) and beryllium (Be) were used, respectively, to implant the n-layer for the stepped channel and the p-layer at the channel substrate interface. A range of Be co-implantation doses and energies were studied for 150 keV channel implantation. The Be co-implanted layer improves the transconductance and pinch-off voltage of the devices by increasing the abruptness of the channel layer and reducing the substrate current. The available power from a step-doped device is high compared to uniform-channel devices with either low or high dopant concentration.

A rapid electron beam annealing technique has been used to induce the reaction of 1150±50 Å deposited tantalum films on single crystal silicon. The annealing was carried out at temperatures ranging from 750°C to 1000°C, in 25°C increments, for a dwell time of 2 s in each case. The tantalum silicide films were characterized by sheet resistance measurements and Auger electron spectroscopy depth profiling. The resistivity of the completely reacted thin tantalum silicide films was 45-50 µΩ-cm. The very accurate control of the time-temperature profile that was possible in the electron beam system enabled a detailed study of the growth rate of tantalum silicide. An activation energy of 3.0-0.2 eV was deduced for the growth of tantalum disilicide from the solid-phase reaction between the metal and the silicon substrate.

Free-standing wire samples have been fabricated that enable low dimensional conducting wires to be examined by transmission electron microscopy (TEM). These structures have been fabricated using a combination of gas-assisted ion beam etching and electron beam lithography, and have cross-sectional dimensions between 70 nm and 1 µm, and lengths of up to 50 µm. The free-standing wires have been examined by TEM before and after ion implantation and annealing. The results indicate that no extended defects are created in the wires, even for high doses (1016cm-3) or high beam energies (>50 keV). The reason for this is not fully understood. In addition, the fabrication process has been extended so that low temperature electrical measurements can be performed on the TEM samples. In this way it is hoped eventually to study the role of specific crystalline defects in limiting the electronic and phonon transport.

The electrical and structural characteristics of tantalum-titanium bilayers on silicon reacted by electron beam heating have been investigated over a wide range of temperature and time conditions. The reacted layers exhibit low sheet resistance and stable electrical characteristics up to at least 1100degC. Titanium starts reacting from 750degC onwards for 100 milliseconds reaction times whereas tantalum starts reacting only above 900degC for such short reaction times. RBS results confirm that silicon is the major diffusing species and there is no evidence for the formation of ternary silicides. Reactions have also been explored on millisecond time scales by non-isothermal heating.