There is considerable interest in the development of inexpensive lithography techniques for applications in the area of nanoscale electronics. The semiconductor industry is pursuing the development of photolithography techniques such as extreme UV and x-ray. However these techniques are extremely expensive and not suitable for smaller scale applications. In this paper we describe research on the feasibility of exploiting x-ray propagation within carbon nanotubes (CNTs) for the fabrication and characterisation of nanoscale devices. A description is given of a test structure designed to explore experimentally the possibility of x-ray propagation in carbon nanotubes. As x-ray propagation requires a grazing angle of incidence the nanotubes need to be straight and reproducible. In order to alleviate this problem the possibility of using Bragg reflection is investigated. This approach to the problem is stimulated by the inherent Bragg structure of multiwall carbon nanotubes. It is further encouraged by the recent development of coatings using materials such as WS2. Results from simulations presented in this paper show that although Bragg reflection in as-grown multiwall carbon nanotubes is weak the potential for exploitation of this phenomenon in suitably coated nanotubes exists.

Interest in carbon-based electronics has been stimulated in recent years, initially through the discovery of carbon nanotubes, but recently with the formation of graphene layers. In this paper metal-oxide-semiconductor (MOS) systems based on these carbon structures are used to model and compare charge transport within them. Schrödinger’s equation is solved self-consistently with Poisson’s equation, using the scattering matrix method. A tight-binding model is used to determine the energy band structure in graphene. The current-voltage characteristics of MOS devices based on graphene and those based on carbon nanotubes demonstrate significant differences associated with their respective transmission probabilities.

In the past few years it has been shown theoretically that carbon nanotubes coated with various materials have the potential to act as waveguides at x-ray frequencies. At these frequencies the angle of incidence relative to the axis of the nanotube is a few milliradians, creating significant challenges for the experimental confirmation of mode formation. Recent developments in the growth of multiwalled carbon nanotubes with WS2 walls suggest that they have the potential to act as Bragg fibers at x-ray frequencies. In this paper we use the scattering matrix method to study mode formation in multiwalled carbon nanotubes coated with gold. It is found that they are capable of acting as Bragg fibers but the wall thickness and the number of bilayers must be increased in order to obtain mode confinement.

The maintenance of the growth of the multibillion-dollar semiconductor industry requires the development of techniques for the fabrication and characterisation of nanoscale devices. Consequently, there is great interest in photolithography techniques such as extreme UV and x-ray. Both of these techniques are extremely expensive and technologically very demanding. In this paper we describe research on the feasibility of exploiting x-ray propagation within carbon nanotubes (CNT's) for the fabrication and characterisation of nanoscale devices. This work discusses the parameters determining the design space available. To demonstrate experimentally the feasibility of x-ray propagation, arrays of carbon nanotubes have been grown on silicon membranes. The latter are required to provide structural support for the CNT's while minimising energy loss. To form a waveguide metal is deposited between the nanotubes to block x-ray transmission in this region at the same time as cladding the CNT's. The major challenge has been to fill the spaces between the CNT's with material of sufficient thickness to block x-ray transmission while maintaining the structural integrity of the CNT's. Various techniques have been employed to fill the gaps between the nanotubes including electroplating, sputtering and evaporation. This work highlights challenges encountered in optimising the process.

The maintenance of the growth of the multibillion-dollar semiconductor industry requires the development of techniques for the fabrication and characterisation of nanoscale devices. Consequently, there is great interest in photolithography techniques such as extreme UV and x-ray. Both of these techniques are extremely expensive and technologically very demanding. In this paper we describe research on the feasibility of exploiting x-ray propagation within carbon nanotubes (CNT's) for the fabrication and characterisation of nanoscale devices. This work discusses the parameters determining the design space available. To demonstrate experimentally the feasibility of x-ray propagation, arrays of carbon nanotubes have been grown on silicon membranes. The latter are required to provide structural support for the CNT's while minimising energy loss. To form a waveguide metal is deposited between the nanotubes to block x-ray transmission in this region at the same time as cladding the CNT's. The major challenge has been to fill the spaces between the CNT's with material of sufficient thickness to block x-ray transmission while maintaining the structural integrity of the CNT's. Various techniques have been employed to fill the gaps between the nanotubes including electroplating, sputtering and evaporation. This work highlights challenges encountered in optimising the process.

Self consistent Monte Carlo simulations which include impact ionization are used to study the high-speed potential of InSb field-effect transistors. It is found that the impact ionization has a strong influence on the performance of InSb for high speed. The ionization leads to a high electron drift velocity and substrate bias can be used to extract the holes which are generated in the channel. Residual hole density within the channel, however, limits the maximum speed. The substrate bias and buffer doping are critical for extracting holes from the channel without inducing excess ionization. Simulations yield a peak cutoff frequency of 820 GHz with a 0.03125-/spl mu/m gate, a source to drain voltage of 0.58, and a sheet doping density of 1.7/spl times/10/sup 12/ cm/sup -2/.

Self-consistent Monte Carlo simulations are used to study the low noise and high gain potential of InSb avalanche photodiodes. It is found that for an electron-initiated avalanche, excess noise factors well below the minimum McIntyre value persist up to gain values of around 60 for a 3.2 /spl mu/m avalanche region. For these very low noise values, it is found that multiplication has a very unusual voltage dependence which may be exploited for highly efficient novel low noise planar arrays operating at low voltage.

Temperature profiles in the source/drain (S/D) opening of a single finger AlGaN-GaN heterostructure field-effect transistor were studied at increasing S/D voltages by micro-Raman spectroscopy with <1 /spl mu/m spatial resolution. These profiles imply high field regions near the gate edge of length /spl sim/0.4 /spl mu/m for S/D voltages between 45 and 75 V. Electric field strengths of /spl sim/1.2 and /spl sim/1.9 MV/cm are estimated for 45 and 75 V S/D voltage. The experimental results are in excellent agreement with 2-D Monte Carlo simulations.

Photomultiplication initiated by electrons and holes has been measured in submicron Si p+–i–n+ and n+–i–p+ diodes with nominal intrinsic region thicknesses between 0.8 and 0.1 μm. A local analysis of the thinner devices gives values of the electron and hole ionization coefficients (α and β, respectively) smaller than those in the literature, especially at low values of multiplication because of dead space effects. The dead space in Si appears to be less significant than in GaAs structures of similar dimensions.

Impact avalanche transit time (IMPATT) diodes are an important source of radio-frequency power at millimeter and submillimeter wavelengths. However, exploitation of these devices has been restricted, as they are commonly believed to suffer from high noise levels. In this article, we demonstrate that a heterostructure IMPATT diode has the potential for almost noise-free operation. Our analysis is based on a Monte Carlo simulation of oscillating IMPATT devices.

Drift mobility as a function of gate voltage has been measured in HFETs fabricated by MOVPE to a common layer structure, but with varying pinch-off voltage attributed to varying deep level defect densities. The mobility reached a peak of 2000 cm(2)/Vs for the best samples at room temperature. At high number density the layers showed a common mobility against gate voltage curve, which is attributed to the mobility being related to surface electric field rather than number density.

Self-consistent Monte Carlo simulations are reported for AlGaN/GaN HFETs. Hot-carrier scattering rates are determined by fitting experimental ionization coefficients and the doping character of the GaN is obtained from substrate bias measurements. Preliminary simulations for a simple model of the AlGaN surface are described and results are found to be consistent with experimental data. The high-frequency response of short-gate-length transistors is found to be sensitive to the charge state of the free AlGaN surface and it is proposed that current-slump phenomena may also be related to deep levels at this surface. Breakdown calculations show interesting two-dimensional effects close to the drain contact.

The effect of the ionization threshold softness on the temperature variation of impact ionization coefficients is examined theoretically. It is found that increasing the softness reduces the temperature dependence of ionization, because temperature induced heating or cooling of the carrier distribution results in a smaller change in the ionization scattering rate sampled. This may explain the wide variation in the temperature dependence of breakdown voltage reported in the literature and the difficulty in modelling accurately the temperature dependence of the ionization process.

We have measured avalanche multiplication and noise in Si p- i-n diodes with avalanche widths, w, of 0.12 micrometers , 0.18 micrometers and 0.32 micrometers , both for pure electron and mixed carrier injection. Multiplication and excess noise measurements were also performed with hole injection on a n+-i-p+ diode with w equals 0.84 micrometers . Pure electron initiated avalanche noise results were found to be almost indistinguishable in all three layers. The excess noise factor increases dramatically with increasing w when the injection is mixed.

The avalanche multiplication and excess noise properties of a range of submicron Si diodes have been investigated. In these thin diodes the excess noise is found to fall below that predicted by conventional local noise theory. Modeling of the multiplication and excess noise using a recurrence method, which includes the dead space for carrier ionization, gives good agreement with experiment. This suggests that the dead space can reduce the excess noise in submicron Si diodes.

Long wavelength Si0.8Ge0.2 /Si quantum well infrared photodetectors (QWIPs) grown by low pressure CVD have been fabricated both as discrete devices and integrated onto a CMOS readout circuit to produce a monolithic Si-based sensor circuit for detection of thermal radiation. The peak photoresponse of the detectors near 8 mu m is dominated by transitions to unbound final states associated with the spin-orbit split-off valence band. These optical transitions are allowed by symmetry reduction in the quantum wells, which is also evident in the electrical properties. The electrical noise is nearly ideal for temperatures up to 70K, with no excess low frequency flicker noise. The capture probability for photoexcited holes into the quantum wells is similar to 0.55 at low temperature. The external 500K black body responsivities for both the discrete and the monolithically integrated QWIPs are similar to 1.8mA W-1 at 1V bias, corresponding to a single pass of the radiation through the detectors. There is no degradation of either the CMOS transistors or the QWIPs caused by the integration process to create the monolithic sensor circuit.

We have used Monte Carlo simulation methods employing both realistic band structure and a simpler analytical approximation to investigate impact ionization in bulk AlxGa1-xAs and also submicron p+in+ diodes for x40%. The calculated impact ionization rates in bulk AlxGa1-xAs compared well with previous experiments and the electron- and hole-initiated current multiplication characteristics of the p+in+ diodes were found to agree very well with our experimental results for both the analytical and the numerical models.

The electron and hole photomultiplication characteristics M-e and M-h have been measured in a series of Ga0.52In0.48P devices with high field regions ranging from 2.0 mu m down to the depletion width of a heavily doped p-n junction, The hole ionization coefficient beta is found to be slightly higher than the electron ionization coefficient alpha at low fields but at high fields they approach one another. alpha and beta are found to be significantly lower than in GaAs across the entire range of electric fields studied, and the breakdown voltage of Ga0.52In0.48P is approximately 1.9 times higher than for similar GaAs structures, Contrary to the behavior observed in GaAs, the multiplication characteristics in all except the thinnest structures appear to be relatively unaffected by the dead space, the minimum distance required to gain sufficient energy to initiate impact ionization. In these very thin structures, a local description of multiplication cannot account for the ionization behavior accurately, and therefore, a Monte Carlo (MC) model has been used to reproduce the measured multiplication characteristics and extract the ionization coefficients.

A full scale microscopic calculation of Auger rates is performed on SixGe1−x/Si superlattice structures with widely different parameters. We show that Auger rates are reasonably independent of superlattice parameters with Auger rates of some two orders of magnitude larger than that of bulk silicon. Bandstructure is found to be the dominant factor in the calculation and a simple geometrical model is devised to investigate different types of structure.

Hole mobilities in relaxed and strained undoped SiGe layers have been calculated with a one-dimensional self-consistent bipolar Monte Carlo simulation code. We have adopted a novel bandstructure model that incorporates strain effects in the alloy valence band. Both alloying and strain enhance the hole mobility compared with bulk Si and we find that alloy scattering is the dominant scattering mechanism. An alloy potential of 1.4 eV was obtained by matching our Monte Carlo data on drift mobilities to experimental Hall mobility measurements. Uncertainties in this value arise from scatter in the experimental data and a lack of detailed knowledge of the Hall factor.