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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 have fabricated self-aligned, side-gated suspended multi-walled carbon nanotubes (MWCNTs), with nanotube-to-gate spacing of less than 10 nm. Evaporated metal forms an island on a suspended MWCNT, the island and the nanotube act as a mask shielding the substrate, and lift-off then removes the metal island, leaving a set of self-aligned side gates. Al, Cr, Au, and Ti were investigated and the best results were obtained with Cr, at a yield of over 90%.

The ability to grow carbon nanotubes/nanofibres (CNs) with a high degree of uniformity is desirable in many applications. In this paper, the structural uniformity of CNs produced by plasma enhanced chemical vapour deposition is evaluated for field emission applications. When single isolated CNs were deposited using this technology, the structures exhibited remarkable uniformity in terms of diameter and height (standard deviations were 4.1 and 6.3% respectively of the average diameter and height). The lithographic conditions to achieve a high yield of single CNs are also discussed. Using the height and diameter uniformity statistics, we show that it is indeed possible to accurately predict the average field enhancement factor and the distribution of enhancement factors of the structures, which was confirmed by electrical emission measurements on individual CNs in an array.

We describe the fabrication and digital logic operation of superconducting nanotransistors. The nanotransistor is a superconducting weak-link device that has integrated hot-phonon injector heaters to control its switching critical current. The fabrication process utilizes a self-aligned method, where the heater acts as a mask for reactive ion etching to define the device pattern. This device is much simpler to fabricate than previously reported controllable superconducting transistors and the principle of operation makes it possible to use a single nanotransistor connected to a load resistor as a NOT logic gate, allowing lower power consumption and improved levels of integration.

We report on the fabrication and field emission of carbon nanotube lateral field emitters. Due to its high aspect ratio and mechanical strength, we use vertically aligned multi-wall carbon nanotubes prepared by plasma-enhanced chemical vapour deposition as cathodes, which makes the fabrication of cantilever type lateral field emitters possible. The emission characteristics show that the field emission initiates at 11–17 V. The device has high geometrical enhancement factors (9.3 × 106 cm−1) compared to standard Spindt tips, which may be due to increased field concentration at the nanotube tip and the close proximity of the anode (

This paper gives a brief review of our recent work done in the area of nanometre-scale Coulomb blockade (CB) memory and logic devices, that enable us to realize future electron-number scalability by overcoming inherent problems to conventional semiconductor devices. We introduce multiple-tunnel junctions (MTJs), naturally formed in heavily doped semiconductor nanowires, as a key building block for our CB devices. For memory applications, the hybrid MTJ/MOS cell architecture is described, and its high-speed RAM operation is investigated. For logic applications the binary decision diagram logic is discussed as a suitable architecture for low-gain MTJ transistors.

Nanowires fabricated from a modulation-doped Si/Si0.7Ge0.3 heterostructure grown on a virtual substrate can exhibit Coulomb blockade due to single-electron charging of multiple islands. A non-blockaded behaviour in some wires is attributed to a combination of incomplete tunnel barrier formation and leakage currents. The leakage manifests itself in the form of a B2 dependence in magnetoresistance measurements. It is found that the leakage arises mainly from the substrate. In some narrow nanowires at very low temperatures, the leakage is negligible and electronic transport along the wires can be explained using the classical Coulomb blockade theory. It is believed that threading dislocations are responsible for the formation of Coulomb islands in the wires. The islands' sizes and the number of tunnel barriers separating the islands agree well with the geometrical dimensions of the fabricated wires.

Dual-gate quantum dot transistors have been fabricated in n- and p-type strained SiGe on silicon-on-insulator. In single-gate modes, the narrow and wide gates on opposing sides modulate the single-charge tunnelling events in the quantum dot. When both gates are coupled together, the resultant modulation implies that capacitive coupling via the substrate controls the tunnelling. The coupling causes constructive-destructive effects in lifting-restoring the Coulomb blockade condition in the SiGe quantum dots.

Leakage currents originating in the virtual substrates which are required in many Si heterostructure systems have been measured. Both ohmic (AuSb) and Schottky (Pt) contacts to a modulation-doped Si:SiGe heterostructure show significant leakage when the contacts cover deep pits originating from growth defects and contaminants. Shallower pits emerging later in the growth process do not contribute to extra conduction. These pits appear after growth of the graded layer which leads us to conclude that carrier transport from the contact along the dense network of dislocations formed in the graded buffer layer is responsible for the leakage found in Si:SiGe systems making use of virtual substrates.

The authors extend their previous studies of the heating by DC electric fields of submicron-diameter free-standing wires of doped GaAs at low lattice temperatures. For lower fields, equivalent to sub-nanowatt dissipated power in their samples, all thermal processes could be accounted for in terms of electron heating alone. They consider here the regime where the hot electrons deposit some of their excess energy into the host lattice, they extract a characteristic electron-phonon scattering length that is an appreciable fraction of the sample length, and the authors provide evidence for ballistic phonon transport.

The authors describe a vertical hot electron transistor structure in the GaAs/AlGaAs materials system, in which the base is a two-dimensional electron gas in GaAs. They undertake initial studies of the hot electron spectroscopy at low temperature, and corroborate the high common base current gain measured to be >100 at low temperature, the best result ever achieved for a unipolar transistor. They compare these attractive results with the less satisfactory results achieved earlier with a uniformly doped (3D) GaAs base. A number of devices have been made which allow magnetotransport and Hall measurements to be performed on the base while the transistor is operating.

We describe some of the physics peculiar to electron transport in quasi-one-dimensional systems in semiconductors, with particular reference to three systems in which it has been investigated: ultra-fine free-standing wires of doped GaAs, the split-gate high-electron-mobility transistor (with and without a back-gate to vary the carrier concentration), and vertical quantum pillars fabricated from multiple-barrier heterostructure material. They are all electron waveguide structures, characterised by < 0.1 µm feature sizes in two of the three spatial dimensions, and they all exhibit phenomena quite different from those seen in macroscopic semiconductor structures. We try to identify the subset of this physics that may be exploitable, and comment on the more general relevance of this new physics to devices.

A method is described that measures both the transmission coefficients and the Hall resistance for a narrow cross. Buttiker's formula relating these quantities has been verified. The amount of collimation is estimated from the magnetoresistance and shown to be small, with approximately=5% more electrons travelling straight on at the cross in zero magnetic field compared with a magnetic field of 0.1 T. This illustrates that the reason for the quenching of the Hall effect here is a scrambling mechanism rather than the collimation mechanism.

High 2D electron mobility narrow Hall bars, 0.8 mu m wide by 30 mu m long, were fabricated in which, after photoexcitation, the scattering mean free path was up to 4.5 mu m long at 1.3 K. The low field negative magnetoresistance arises from quantum interference (weak localization) with the flux cancellation mechanism absent, even though the mean free path is much greater than the width. The Landau levels are significantly broadened, implying that a flat-bottomed model potential is inappropriate in this particular case. It is suggested that the potential fluctuations giving rise to the broadening are responsible for the absence of the flux cancellation, and when small angle scattering contributes to the elastic mean free path. The value of this length compared with the width is no longer a sufficient criterion for the determination of the nature of the magnetoresistance.

A brief account is presented of the authors' recent work on one-dimensional ballistic transport in GaAs-AlGaAs heterojunctions in 1D. Quantization of the ballistic resistance occurs, resulting in values of 2e2/hi where i is the number of 1D subbands. Application of high-resolution electron beam lithography allows the fabrication of a number of structures which exploit various manifestations of ballistic transport and quantum interference and are described here.

For over a decade, the use of the ionised cluster beam (ICB) technique to deposit thin films of materials with applications in microelectronics has been explored. These films have been shown to be of superior quality compared to films deposited by conventional methods. With the ICB technique there is greater control over growth behaviour because there are more process parameters which influence film growth than are found in other methods. The evaporant material is heated in an almost closed crucible with a small aperture. Clusters can be produced when the gas pressure inside the crucible rises so that the evaporant expands adiabatically through the aperture into a region of low pressure. The clusters are passed through an electron beam and some of the clusters are ionised by impact. The ionised clusters are then accelerated towards the substrate, which is held at a variable potential. By altering the beam conditions, such as the ionisation current and acceleration potential, the kinetics of film growth can be modified resulting in improved film quality. This paper presents a brief review of the ICB method of thin film deposition. This is followed by review of some of the properties of films deposited by the ICB technique which have useful applications in microelectronics. Finally, some work carried out at the Cavendish Laboratory in this area is presented, including the deposition of thin (

The authors have fabricated a 2D array of 1D narrow channels in a square lattice configuration on a high mobility GaAs/GaAlAs heterostructure. When a gate voltage is applied the underlying two-dimensional electron gas is depleted creating 4000 dots, of size 0.2 mu m, leaving a grid-like conducting path of quasi one-dimensional channels that are linked every 0.5 mu m. For low values of magnetic field the Hall voltage measured on either side of the device is quenched across its entire width of 25 mu m. The longitudinal magneto-resistance reveals Aharonov-Bohm oscillations with a flux period of h/e for magnetic fields up to 0.4 T. The conductance versus gate voltage shows structure that is consistent with Bragg reflections of the electron waves when the one-dimensional subband wavelength at the Fermi energy, in the direction of current flow, is equal to one superlattice period. Dips in the conductance when this criteria is met show a ln T temperature dependence, similar in origin to a 2D quantum interference. A magnetic field quenches these dips when the cyclotron orbit is comparable with the superlattice period.

The authors present studies in the DC voltage bias regime of a split-gate device utilising the high-mobility electron gas of a GaAs/AlGaAs heterojunction. They find oscillations in the I-V characteristics of a one-dimensional channel which they relate to single-electron tunnelling events in the channel. The effects are explained using a mesoscopic model possibly involving the presence of an impurity in the channel and the role of the device capacitance. The particular aspect of this system of importance is that unlike in conventional capacitors, the capacitance and resistance of depleted regions varies with the trapped charge.

The authors report on experiments, and their analysis, on electron heating effects in 3.2 mu m long free-standing fine wires (triangular cross-section with approximately=0.5 mu m side) of single-crystal GaAs doped at 1017 cm-3. The data can be used to extract a thermal conductivity that has a linear temperature dependence, consistent with a contribution from electrons and/or one-dimensional phonons they show that the electronic contribution dominates.

Low-temperature electrical and magnetoresistance measurements have been performed on free-standing and supported wires of n-type GaAs doped to 1017 cm-3 over the temperature range 0.47-4.2 K. These wires were triangular in cross-section, with widths of 600-900 nm and lengths of 3.2-10 mu m. The authors report that the observed increase in the resistance for temperatures below 4.2 K can be interpreted as being due to a combination of weak localisation of 3D electron-electron interaction effects. They also show how the results described here can be used as the basis for a determination of the phonon conductivity and dimensionality in free-standing structures.

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 authors present results on the quantum conduction properties of a one-dimensional channel, 0.7 mu m long and 0.3 mu m wide, containing two potential barriers. The barriers are 0.5 mu m apart and 0.18 mu m wide and are defined by a gate on a high-mobility GaAs-AlGaAs heterojunction. The device behaves like a one-dimensional ballistic point contact with two narrow barriers in the middle. Reducing the width by electrostatic squeezing reveals quasi-periodic peaks in the resistance due to the resonance. At higher temperatures the effect of the barriers is removed so that the peaks disappear leaving the quantised plateaus associated with a one-dimensional ballistic point contact. A magnetic field removes the structure when the level broadening becomes comparable to the level separation.

The authors present experimental results, and a supporting theory, showing that a one-dimensional system in which transport is ballistic possesses a quantised resistance, h/2ie2, where i is the number of occupied 1D sub-bands and the spin degeneracy is two. A short narrow channel is defined in the 2DEG of a GaAs-AlGaAs heterojunction and as the width of the system is changed, the sub-bands pass through the Fermi energy and the resistance jumps between quantised values. The value of the quantised resistance is derived and the accuracy of the quantisation is discussed. The effect can be strong at temperatures approximately 0.1 K, with up to 17 sub-bands being observed. The action of a transverse magnetic field is to depopulate the sub-bands and form hybrid levels; a parallel field lifts the spin degeneracy and brings about a further quantisation of resistance at values h/2(i+1/2)e2.

The authors have constructed a gated device on a high mobility GaAs-GaAlAs heterostructure which acts as a ballistic channel within a resonant cavity defined by reflectors at each end. Both the channel width and the separation of the reflectors are smaller than the electron mean free path, such that when the channel is defined, oscillations in the resistance are observed as the distance between the reflectors is reduced. When the width of the channel is wide in comparison with the width of the voltage, or current probes, non-local voltage fluctuations result in the measured resistance going to zero. At intermediate channel widths the resistance oscillations are periodic in the separation of the reflectors with a period equal to half the Fermi wavelength. In this regime gradually changing the width enables one to calculate the change in phase of the electron wavefunction as it passes through a one-dimensional subband, this being found to be in good agreement with experiment. When the channel is very narrow, the resistance shows peaks of up to 200% which arise because of the overall quantisation imposed by the cavity.

The authors have defined, in parallel configuration, two split gate devices 0.3 mu m long by 0.3 mu m wide with a centre-to-centre separation of 0.5 mu m on a high mobility heterojunction. At low temperatures, when the phase coherence length is longer than the channel length, the conduction through each one is ballistic and shows steps in conductance of 2e2/h. If the phase coherence length is also greater than the channel separation, transport through both channels is no longer independent, but instead the channels act co-operatively, leading to coherent steps in the conductance of 4e2/h; this can be seen even if the channels do not have the same widths. The device shows Aharonov-Bohm oscillators in resistance of amplitude 10%, arising from a current circulating via edge states through both channels, indicating that the electron wavefunctions is indeed coherent through both.

The authors provide evidence for both voltage- and current-controlled negative differential resistance instabilities in the one-dimensional ballistic resistor driven into the hot-electron regime. Quantum reflections are used to explain the former effect, while extra electron subbands provide the current 'filaments' in the latter case.

The authors present experimental results showing that the quantised nature of the ballistic resistance in narrow channels is preserved when the electrons pass ballistically through two narrow constrictions. As the width of each narrow channel is varied independently, the resistance of the pair is equal to the resistance of the narrowest; this is explained by the conservation of quantum (sub-band) number. The absolute quantisation is not as accurate as observed in a single constriction and is modified by an anomalous resistance whose origin the authors discuss.

Thin titanium layers on silicon were ion-beam mixed with arsenic and antimony ions. The layers were reacted by electron beam heating under different time and temperature conditions and the properties of the reacted layers were explored by sheet resistance measurements, SEM and Rutherford backscattering spectrometry (RBS). Reaction times as short as a few milliseconds have resulted in the simultaneous formation of stable TiSi2 layers and shallow n+p junctions with acceptable electrical characteristics. RBS analysis confirms that arsenic diffuses readily through the silicide layer while antimony shows a tendency to pile up at the interface.

The authors have observed Aharonov-Bohm oscillations using a split gate GaAs-AlGaAs heterojunction FET of the type used to demonstrate the quantised, one-dimensional, ballistic resistance. The periodicity of the Aharonov-Bohm oscillations allowed extraction of the channel area and, hence, by using values of width derived from magnetic depopulation data, the channel length could be determined. It is found that the effective channel length increases as the area, and width, decreases in agreement with a simple model based on the depletion approximation.

Low temperature electrical measurements on particular semiconductor structures can exhibit effects due to the spatial confinement of carriers and reduced dimensionality. Examples are provided by studies of tunnelling in confining structures, transport in narrow channels, ballistic transport through constrictions and quantum interference in rings. These observations rely on the growth of layers by molecular beam epitaxy (MBE) to provide both two-dimensional electron gases (2DEGs) and the very high material quality essential for the observation of some quantum effects. To define the specific geometries necessary for the above experiments electrostatic "squeezing" can be used to confine the planar carrier gas to regions of controllable extent. Recent results obtained from MBE GaAs/AlGaAs 2DEG structures can now be compared with measurements made on hole gases and the use of in situ ion beams in fabrication promises a new class of structures for investigation. However, it is rarely sufficient merely for MBE to provide a carrier gas with the highest maximum carrier mobility. The particular requirements of an experiment affect not only the design of the structure itself but also the MBE growth strategy and the optimum deposition conditions. Growth rate, substrate temperature and source material flux ratios are MBE variables and the suitability of layers for specific postgrowth physics experiments can be shown to depend upon these variables in a systematic manner. Moreover, further requirements for the grown material can include the need to provide specific sheet carrier concentrations, high mobilities in unilluminated samples and the complete absence of conducting paths in parallel with the 2DEG. There are often compromises inherent in attempting to simultaneously optimise several MBE variables and careful management of the MBE machine itself is essential.

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.