M. A. Migliorato

Structural and optical studies of vertically aligned InAs/GaAs self-assembled quantum dots

We report a structural and optical spectroscopic investigation of multiple layer InAs/GaAs self-assembled quantum dots, studied as a function of the GaAs thickness between the quantum dot layers. With decreasing GaAs thickness the positions of dots in different layers exhibit a transition from no correlation to full correlation. Optically the dots in uncorrelated and fully correlated structures are found to exhibit very distinct and different properties. With increasing laser power the photoluminescence of the correlated structure exhibits a high energy, asymmetrical broadening, an effect absent in the uncorrelated structure. In photoluminescence excitation multiple-LO-phonon carrier relaxation features are observed in the spectra of the uncorrelated structure but not in the spectra of the correlated structure. These differences are explained in terms of nonresonant carrier tunneling between the dots in the correlated dot structures.

Influence of composition on the piezoelectric effect and on the conduction band energy levels of InxGa1-xAs/GaAs quantum dots

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.

Enhancement of efficiency of InGaN-based light emitting diodes through strain and piezoelectric field management

We report calculations of the strain dependence of the piezoelectric field within InGaN multi-quantum wells light emitting diodes. Such fields are well known to be a strong limiting factor of the device performance. By taking into account the nonlinear piezoelectric coefficients, which in particular cases predict opposite trends compared to the commonly used linear coefficients, a significant improvement of the spontaneous emission rate can be achieved as a result of a reduction of the internal field. We propose that such reduction of the field can be obtained by including a metamorphic InGaN layer below the multiple quantum well active region.

Tunability of the piezoelectric fields in strained III-V semiconductors

In this work we show that tetragonal strain can be used to create a sign reversal of the piezoelectric field in InAs/GaAs semiconductor heterostructures. The strain dependence of the internal displacement of the cation-anion pairs and of the bond polarity are taken into account, beyond the linear model, within an ab initio scheme. The reported tunability of the piezoelectric field is a concept that can be exploited in optoelectronic devices.

Non-linear piezoelectricity in zinc blende GaAs and InAs semiconductors

We investigate the strain dependence of piezoelectric effect, both linear and non linear, in zincblende GaAs and InAs semiconductors. We expanded the polarization in terms of the ionic and dipole charges, internal displacement and the exploited the ab-initio Density Functional Theory (DFT) to evaluate the dependence of all quantities on the strain tensor. By this detailed study of the non linear piezoelectric effect, we report that even third order effects are significant.

Atomistic simulation of InxGa1-xAs/GaAs quantum dots with nonuniform composition

An atomistic model of an InxGa1-xAs/GaAs quantum dot with nonuniform composition is investigated. An empirical inter-atomic potential, the Tersoff potential, is used to obtain dynamic relaxation through energy minimisation. Bond deformations are analysed in order to predict the components of the strain tensor with resolution on the atomic scale, revealing the nature of the lattice distortion in both the dot pyramid and the capping layer. The piezoelectric charges are then computed directly from the off diagonal components of the strain, revealing a wider distribution of the dipoles compared to those previously reported by other groups

Elastic and vibrational properties of group IV semiconductors in empirical potential modelling

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-Grüneisen 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.

Empirical interatomic potential for the mechanical, vibrational and thermodynamic properties of semiconductors

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.

Using Quantum Confinement to Uniquely Identify Devices

Modern technology unintentionally provides resources that enable the trust of everyday interactions to be undermined. Some authentication schemes address this issue using devices that give a unique output in response to a challenge. These signatures are generated by hard-to-predict physical responses derived from structural characteristics, which lend themselves to two different architectures, known as unique objects (UNOs) and physically unclonable functions (PUFs). The classical design of UNOs and PUFs limits their size and, in some cases, their security. Here we show that quantum confinement lends itself to the provision of unique identities at the nanoscale, by using fluctuations in tunnelling measurements through quantum wells in resonant tunnelling diodes (RTDs). This provides an uncomplicated measurement of identity without conventional resource limitations whilst providing robust security. The confined energy levels are highly sensitive to the specific nanostructure within each RTD, resulting in a distinct tunnelling spectrum for every device, as they contain a unique and unpredictable structure that is presently impossible to clone. This new class of authentication device operates with minimal resources in simple electronic structures above room temperature.

A review of non linear piezoelectricity in semiconductors

The piezoelectric effect in polar semiconductor has seen increased interest in recent years because of the prospect of exploiting semiconducting behavior and piezoelectric response, i.e. generating electric fields in response to pressure, in novel optoelectronic devices with applications as pressure sensors and energy harvesting. In this paper we review the basic concepts and recent findings related to the novel concept of non-linear piezoelectricity, which can be exploited in composite nanostructured materials to increase the piezoelectric response compared to bulk materials. Applications to light emitting diodes and nanowires will also be discussed. We will show how the non-linear theory of piezoelectricity can in some cases lead to opposite predictions compared to the classic linear theory.