Table of contents

These conference proceedings contain the written papers of the contributions presented at the 2^nd International Conference on: Theory, Modelling and Computational methods for Semiconductors. The conference was held at the St Williams College, York, UK on 13^th–15^th Jan 2010. The previous conference in this series took place in 2008 at the University of Manchester, UK.

The scope of this conference embraces modelling, theory and the use of sophisticated computational tools in Semiconductor science and technology, where there is a substantial potential for time saving in R&D. The development of high speed computer architectures is finally allowing the routine use of accurate methods for calculating the structural, thermodynamic, vibrational and electronic properties of semiconductors and their heterostructures. This workshop ran for three days, with the objective of bringing together UK and international leading experts in the field of theory of group IV, III-V and II-VI semiconductors together with postdocs and students in the early stages of their careers. The first day focused on providing an introduction and overview of this vast field, aimed particularly at students at this influential point in their careers.

We would like to thank all participants for their contribution to the conference programme and these proceedings. We would also like to acknowledge the financial support from the Institute of Physics (Computational Physics group and Semiconductor Physics group), the UK Car-Parrinello Consortium, Accelrys (distributors of Materials Studio) and Quantumwise (distributors of Atomistix).

The EditorsAcknowledgements Conference Organising Committee: Dr Matt Probert (University of York) and Dr Max Migliorato (University of Manchester) Programme Committee: Dr Marco Califano (University of Leeds), Dr Jacob Gavartin (Accelrys Ltd, Cambridge), Dr Stanko Tomic (STFC Daresbury Laboratory), Dr Gabi Slavcheva (Imperial College London) Proceedings edited and compiled by Dr Max Migliorato and Dr Matt Probert

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

We present a short review of methods of evaluating of strain from atomistic models in the context of linear elasticity.

The replacement of As by N in GaAs introduces several pertubations, including both a change in potential at the N site and also long range strain relaxation in the crystal. It can be very useful to describe this perturbation in terms of the change in the Hamiltonian, ΔH, due to the introduction of N. Using plane-wave approaches, ΔH treats explicitly all atomic displacements in the structure. By contrast, a tight-binding approach allows a much simpler analysis. We illustrate the benefits of the tight-binding method by considering Ga(P,As:Bi) and GaNAs. We show that the tight-binding method provides a clear and quantitative explanation for many of the unusual electronic properties of these and related alloys, including the nonmonotonic variation of electron effective mass, m_e, and gyromagnetic ratio, g_e, in GaNAs.

A method of calculating scattering cross-sections based on first principles electronic structure calculations, previously used for alloy scattering in SiGe is presented and applied to substitutional carbon in silicon. It is found that the intravalley scattering is the predominant scattering mechanism with a contribution to the scattering rate around a factor of 4 greater than intravalley alloy scattering in SiGe.

The methodology for the calculation of charged defects using the CRYSTAL program is discussed. Two example calculations are used to illustrate the methodology: He⁺ ions in a vacuum and two intrinsic charged defects, Cu vacancies and Ga substitution for Cu, in the chalcopyrite CuGaS₂.

DFTB⁺ is a recent general purpose implementation of density-functional based tight binding. One of the early motivators to develop this code was to investigate lanthanide impurities in nitride semiconductors, leading to a series of successful studies into structure and electrical properties of these systems. Here we describe our general framework to treat the physical effects needed for these problematic impurities within a tight-binding formalism, additionally discussing forces and stresses in DFTB. We also present an approach to evaluate the general case of Slater-Koster transforms and all of their derivatives in Cartesian coordinates. These developments are illustrated by simulating isolated Gd impurities in GaN.

A theoretical framework and dynamical model for description of the natural optical activity and Faraday rotation in an individual chiral single-walled carbon nanotube in the highly nonlinear coherent regime is proposed. The model is based on a discrete-level representation of the optically active states near the band edge. Chirality is modelled by a system Hamiltonian corresponding to energy-level configurations, specific for each handedness, that are mirror reflections of each other. An axial magnetic field is introduced through the Aharonov-Bohm and Zeeman energy-level shifts.The time evolution of the quantum system is studied using the coherent vector Maxwell-pseudospin equations. Giant natural and magneto-optical gyrotropy, exceeding the one of the artificial photonic metamaterials, is numerically demonstrated for a single (5,4) carbon nanotube and an estimate of the magnitude of the natural circular dichroism and specific optical rotatory power is obtained. The model provides a framework for investigation of chirality and magnetic field dependence of the ultrafast nonlinear optical response of a single carbon nanotube.

The 3 and 4-electron states of a gated semiconducting carbon nanotube quantum dot are calculated by exact diagonalisation of a modified effective mass Hamiltonian. A typical nanotube quantum dot is examined and the few-electron states are Wigner molecule-like. The exact diagonalisation method and the rate of convergence of the calculation are discussed.

A model for calculating the current and spin polarization in a double-barrier InGaAs resonant tunnelling structure is described with the aim to account for phase-breaking scattering. It is based on the nonequilibrium Green's function method with both elastic and inelastic (LO-phonon) scattering described within the self-consistent first Born approximation. It has been found that the maximum current spin polarization of around 0.4 in the ballistic limit decreases to around 0.1 for scattering transport with scattering-induced broadening of quasi-bound states of around 4meV.

In this paper we present further details of a predictive Keldysh nonequilibrium many body Green's functions theory for quantum transport including high order electron-electron, electron-phonon, electron-impurity and interface roughness scattering processes. Our approach is fully frequency and momentum dependent. Local current conservation is observed even if only few states are considered. The resulting algorithm has led to good agreement with experimental observations.

The gradual transition of the band-gap at the Si-SiO2 interface affects quantisation and leakage characteristics of MOS inversion layer. We establish a link between first principles DFT simulations of the interface, and continuum simulations in the effective mass approximation, in order to obtain a realistic description of the band-gap transition for device modelling. The simplistic approach of obtaining real-space-dependent band-gap profile from the ab initio calculated electronic structure results in uncertainty of the simulated device characteristics. This uncertainty is small however, when compared to the magnitude of the simulated impact of the transition layer. A linear transition of the band-gap over 6 – 7 Å in the oxide approximates well the effects simulated with the realistic band-gap profile from DFT.

The ensemble Monte Carlo device simulations are employed to obtain I-V characteristics for the 25 nm gate length template Si MOSFETs designed by the SiNANO consortium. The simulated ID-VG characteristics are compared against previous results from various Monte Carlo device codes [Fiegna C et al., in Proc. SISPAD 2007, pp. 57-60 (Springer Vienna, 2007)]. After this verification, we have scaled the transistor laterally only from a gate length of 25 nm to gate lengths of 20, 15, 10 and 5 nm. We have then monitored the average electron velocity and sheet density along the channel at a supply voltage of 1.0 V in order to gain an insight into the degradation of the injection velocity experimentally observed in various Si transistor architectures when the gate length is scaled into deep sub-50 nm dimensions. We have found a substantial decrease of the overall velocity profile along the channel including a decrease of the peak velocity when the channel is smaller than 15 nm while the drive current is able to sustain its increase thanks to the increasing velocity at the drain side.

The electronic structure and the intraband optical properties of self-assembled quantum rods were analysed using a theoretical model based on strain-dependent eight-band k · p method. It was found that the intraband optical absorption spectrum for the radiation polarised in the growth direction has one strong peak caused by the transition from the ground state to the state confined in the rod with one node in the growth direction. On the other hand the absorption of in-plane polarised radiation is weaker and is determined by the transitions to mixed quantum well - quantum rod states. The most prominent transitions fall into the technologically relevant terahertz region of the electromagnetic spectrum and can be tailored by engineering the quantum rod height.

Much effort has been committed to development of quantum-dot-based infrared photodetectors owing to their potential for normal-incidence absorption and low dark current. Quantum-dot-in-well structures offer additional advantages, such as better wavelength tunability and improved carrier collection. This system presents a challenge for modeling of electronic structure, as it requires solution for a complex system (quantum dot plus quantum well) with both discrete levels and the continuum energy spectrum. The Green's function method, mostly used for such problems, has very high computational cost. Here we use the Finite Element Method to model intraband absorption spectra of quantum-dot-in-well structures within the effective mass approximation.

We consider the formation energies and stabilities of dopants in semiconductor alloys. We show that they are not bounded by the formation energies in the related pure materials. On the contrary, by tuning the alloy composition, dopant solubility can be increased significantly above that in the pure materials. Furthermore, it is not always necessary to carry out full defect calculations in alloy supercells, since good estimates of the formation energies at the most stable substitution sites can be obtained by calculating the formation energies in the various component pure materials, but strained to the lattice parameter of the alloy.

In this work we have implemented a numerical simulator and analytical model to study the dependence of the tunnelling current on the polarization ratio of the carrier spin for a degenerate and ferromagnetic heterojunction. We have applied these models to study the behaviour of a magnetically doped GaAs/ZnO PN junction and the current transport in a PN heterojunction where the polarization of the spin of the charge carriers is also a control variable.

In this paper, using the Non-Equilibrium Green Function (NEGF) formalism, we have calculated the electron transport through a silicon nanowire with a rough interface. The nanowire was orientated in the [100] direction. Two different wire diameters, D₁ = 2.18 nm and D₂ = 3.21 nm, have been used to analyse the effect of the cross section in the transmission coefficients. We used a full-band description of the electron state by using a tight binding formalism to describe the electron Hamiltonian. The calculation makes full use of the recursive algorithm to calculate the transmission. We compare several nanowires that differ in the microscopic realization of interface roughness. The energy shift between the onset of the transmission of the rough and the pristine nanowire are 200 meV and 50 meV for D1 and D2 respectively. Furthermore, the transmissions of some nanowires show resonances due to a resonant cavity created by the interface surface. This is particularly relevant as effective-mass-approximation NEGF simulations of similar nanowire transistors show the same resonances. These resonances play a role in increasing the off-current of nanowire transistors.

The use of alternative channel materials to maintain device performance with scaling for CMOS technology is an active area of research, with Germanium offering an extremely attractive possibility for pMOSFETs in CMOS. In this paper we use full band Monte Carlo transport simulations to investigate the impact of substrate orientation and biaxial strain on hole mobility in bulk Germanium helping to establish a preferential substrate channel orientation that can maximize carrier mobility for these devices.