We report direct measurements of the optical gain on vertical-cavity surface-emitting laser (VCSEL) material using a stripe-length method featuring segmented contacts. We utilise the similarity of the in-plane transverse electric (TE) polarised matrix element and that of the VCSEL lasing mode and a simple method to reduce round trip effects. The confinement factor is determined from cold-cavity simulations of the in-plane TE polarised slab waveguide mode and used to convert the measured in-plane modal gain into the vertical-cavity modal gain, as required for the VCSEL structure. This gives a threshold material gain of 1440 ± 140 cm −1 at 30 °C for this structure. A comparison with the threshold material gain values determined from the lasing condition, where internal optical losses due to doping induced absorption is included using parameters taken from the literature, indicates the presence of an additional source of optical loss in the experiment which increases the threshold material gain by ∼450 cm −1 . A best fit is obtained by increasing the optical loss in the n-DBR (distributed Bragg reflectors) layers to 40 cm −1 , which is consistent with previous work on additional scattering losses due to interface roughening in the n-DBR layers. To further demonstrate the utility of this method for rapid optimisation, the gain-peak wavelength is measured directly, and its temperature dependence is compared to the lasing wavelength.

We report on high resolution analysis of vertical cavity surface emitting lasers (VCSELs) to detect and assess defects in sub-surface layers. We employ a focussed ion beam scanning electron microscope (FIB-SEM) to sputter and image successive cross sections (slice and view technique) in order to produce a 3D reconstruction of the oxide aperture region. High resolution images and measurements of the multilayers and oxide apertures of VCSEL devices were obtained. The process took ∼2.5 h and produced over 270 slice SEM images for a device volume of approximately 13.2 × 16.0 × 13.8 μ m 3 , with a voxel size of 50 nm. On-wafer, single mode VCSEL devices with high and low output powers were analysed to compare their oxide apertures and distributed Bragg reflector (DBR) layer structures. It was found that the low output power VCSEL had DBR layer defects and a 41.8% reduction of effective oxide aperture area, explaining the lower power obtained. The results provide evidence that oxide aperture area and structural defects are major factors that affect the optical output power of VCSEL devices. Outcomes in this work show FIB-SEM slice and view is a valuable method for 3D reconstruction of VCSEL devices, which enables top view, cross-sectional view and angled view of the whole device region as well as designated structures such as oxide aperture or structural defects in various layers. This work demonstrates a promising technique with high resolution (50 nm) 3D imaging for analysis of complex semiconductor devices.

Compact coherent population trapping (CPT) based clocks require single-mode, low-power consumption, high-speed and polarization-stable laser sources. We report our progress in developing customized vertical cavity surface emitting laser (VCSEL) diodes designed for operating on the D1 transition of cesium, and specifically tailored for CPT-based atomic clocks. The VCSELs provide high power (>1 mW), narrow linewidths (<; 100 MHz), mode-hop free tunability over 8 nm, and are shown to be polarization stable over a wide range of operating bias conditions (average polarization suppression >15 dB). Preliminary spectroscopy has been observed.

Two-dimensional (2D) layers like graphene are subject to long-wavelength fluctuations that manifest themselves as strong height fluctuations (ripples). In order to control the ripples, their relationship with external strain needs to be established. We therefore perform molecular dynamics (MD) of suspended graphene, by the use of a newly developed force field model (MMP) that we prove to be extremely accurate for both C Diamond and Graphene. The MMP potential successfully reproduces the energy of the σ-bonds in both sp3 and sp2 configuration. Our MD simulations and experimental electron microscopy analysis reveal that ordered and static ripples form spontaneously as a direct response to external pressure. Furthermore the morphology of graphene and strain response of the crystal bonds differ depending on the particular directions where external pressure is present. Different regions of the strained graphene sheet are then investigated by tight-binding. Localised bandgap opening is reported for specific strain combinations, which also results in particular signatures in the phonon spectrum. Such controllable morphological changes can therefore provide a means to practically control and tune the electronic and transport properties of graphene for applications as optoelectronic and nanoelectromechanical devices.

We have developed an interatomic potential that with a single set of parameters is able to accurately describe at the same time the elastic, vibrational and thermodynamics properties of semiconductors. The simultaneous inclusion of radial and angular forces of the interacting atom pairs (short range) together with the influence of the broken crystal symmetry when the atomic arrangement is out of equilibrium (long range) results in correct predictions of all of the phonon dispersion spectrum and mode-Gruneisen parameters of silicon and germanium. The long range interactions are taken into account up to the second nearest neighbours, to correctly influence the elastic and vibrational properties, and therefore represent only a marginal computational cost compared to the full treatment of other proposed potentials.Results of molecular dynamics simulations are compared with those of ab initio calculations, showing that when our proposed potential is used to perform the initial stages of the structural relaxation, a significant reduction of the computational time needed during the geometry optimization of density functional theory simulations is observed.

We present an atomistic interatomic potential that with a single set of parameters is able to accurately describe at the same time the elastic, vibrational and thermodynamics properties of semiconductors. We also show that the correct inclusion in the potential of short and long range interactions provides a model for the force field that accurately performs Static Dynamics and Molecular Dynamics.

Empirical models are widely used to simulate large atomic structures where instead ab initio methods are not practical because of computational limitations. However models such as Tersoff potential [8], [9], Valence Force Field [13], [14] or Stillinger- Weber potential [15] have some restrictions in correctly predicting simultaneously both elastic and vibrational properties of the crystals [18]. Thus, extension of the functional form of the potentials by including further atomic interactions [20] [21] compared to the simple 2- and 3-body terms, is required. An empirical interatomic potential is proposed which represents a substantial improvement of the Tersoff potential for semiconductors modelling. The new model includes multi-bond interactions and the volume dependency by considering the tetrahedron distortions of the covalent crystal.

We propose a tight-binding model for the polarization that considers direct and dipole contributions and employs microscopic quantities that can be calculated by first-principles methods, e.g. by employing Density Functional Theory (DFT). Applying our model to In x Ga 1-x As alloys allows us to settle discrepancies between the values of e 14 as obtained from experiments and from linear interpolations between the values of InAs and GaAs. Our calculated piezoelectric coefficient is in very good agreement with photo current measurements of InAs/GaAs(111) quantum well samples.

We address the issue of accurate parametrization for the Abell-Tersoff empirical potential applied to tetrahedrally bonded semiconductor materials. Empirical potential methods for structural relaxation are widely used for group IV semiconductors while, with few notable exceptions, work on III-V materials has not been extensive. In the case of the Abell-Tersoff potential parametrizations exist only for III-As and III-N, and are designed to correctly predict only a limited number of cohesive and elastic properties. In this work we show how by fitting to a larger set of cohesive and elastic properties calculated from density functional theory, we are able to obtain parameters for III-As, III-N, III-P, and III-Sb zinc blende semiconductors, which can also correctly predict important nonlinear effects in the strain.

Atomistic simulations that use the Tersoff empirical potential accurately reproduce the effects of the presence of compositional disorder in strained semiconductor alloys. This method is applied to InGaAs quantum dot islands, for which gradients in the In composition distribution have been observed and accurately measured, and we demonstrate that the internal piezoelectric fields contribute strongly to the nature of the electron wavefunctions. The theoretical predictions are supported by experimental evidence: intersubband absorption measurements confirm that the p-states degeneracy for the electron first excited state is lifted and a minimum splitting of at least 5meV is to be generally expected.

We address fundamental issues relating to the symmetry of the shape and the nonuniform composition of InGaAs quantum dot islands. Using atomistic simulations in the framework of the Tersoff empirical potential, we study the effect of compositional gradients in the In distribution on the piezoelectric effect in quantum dots. We demonstrate that the internal piezoelectric fields contribute strongly to the experimentally observed optical anisotropies. This is confirmed by accurate high-resolution transmission electron microscopy analysis over hundreds of islands grown in different conditions that reveals the absence of structural anisotropy under our growth conditions.