Toby D. Young

Morphology and strain of self-assembled semipolar GaN quantum dots in (112¯2) AlN

GaN quantum dots (QDs) grown in semipolar (112¯2) AlN by plasma-assisted molecular-beam epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy techniques. The embedded (112¯2)-grown QDs exhibited pyramidal or truncated-pyramidal morphology consistent with the symmetry of the nucleating plane, and were delimited by nonpolar and semipolar nanofacets. It was also found that, in addition to the (112¯2) surface, QDs nucleated at depressions comprising {101¯1} facets. This was justified by ab initio density functional theory calculations showing that such GaN/AlN facets are of lower energy compared to (112¯2). Based on quantitative high-resolution TEM strain measurements, the three-dimensional QD strain state was analyzed using finite-element simulations. The internal electrostatic field was then estimated, showing small potential drop along the growth direction, and limited localization at most QD interfaces.

3D modelling of misfit networks in the interface region of heterostructures

We present a methodology for the stress–strain analysis of a film/substrate interface by combining crystallographic and continuum modelling. Starting from measurements of lattice parameters available from experimental observations, the heterostructure is recast initially in the form of a crystallographic model and finally as a continuum elastic model. The derived method is capable of handling dense arrays of misfit dislocations as well as large areas of the interface between two crystal structures. As an application we consider the misfit dislocation network in the GaN/Al2O3 interface region through determination of strain relaxation and associated residual stresses. Our calculated results are referred back to and found to be in good agreement with the experimental observations of misfit dislocation arrays obtained from high resolution transmission electron microscopy.

Influence of hydrostatic pressure on the built-in electric field in ZnO/ZnMgO quantum wells

We used high hydrostatic pressure to perform photoluminescence measurements on polar ZnO/ZnMgO quantum well structures. Our structure oriented along the c-direction (polar direction) was grown by plasma-assisted molecular beam epitaxy on a-plane sapphire. Due to the intrinsic electric field, which exists in polar wurtzite structure at ambient pressure, we observed a red shift of the emission related to the quantum-confined Stark effect. In the high hydrostatic pressure experiment, we observed a strong decrease of the quantum well pressure coefficients with increased thickness of the quantum wells. Generally, a narrower quantum well gave a higher pressure coefficient, closer to the band-gap pressure coefficient of bulk material 20 meV/GPa for ZnO, while for wider quantum wells it is much lower. We observed a pressure coefficient of 19.4 meV/GPa for a 1.5 nm quantum well, while for an 8 nm quantum well the pressure coefficient was equal to 8.9 meV/GPa only. This is explained by taking into account the press...

A qualitative semi?classical treatment of an isolated semi?polar quantum dot

To qualitatively determine the behaviour of micro-macro properties of a quantum dot grown in a non-polar direction, we propose a simple semi-classical model based on well established ideas. We take into account the following empirical phenomena: (i) The displacement and induced strain at heterojunctions; (ii) The electrostatic potential arising from piezoelectric and spontaneous polarisation; and (iii) The localisation of excitons (particle-hole pairs) arising from quantum confinement. After some algebraic manipulation used to cast the formalism into an arbitrarily rotated frame, a numerical model is developed for the case of a semi-polar wurtzite GaN quantum dot buried in a wurtzite AlN matrix. This scheme is found to provide a satisfying qualitative description of an isolated semi-polar quantum dot in a way that is accessible to further physical interpretation and quantification.

Nonlinear Finite Element and Atomistic Modelling of Dislocations in Heterostructures

A continuum and atomistic approach to the modelling of dislocations observed by the High Resolution Transmission Electron Microscopy (HRTEM) is discussed. We present a methodology for the analysis of dislocations, stacking faults and interfacial regions in crystal heterostructures by using experimental measurements, and atomistic/continuum models. The derived method is capable of handling dense arrays of dislocations as well as large areas of the interface between two crystal structures. As an example, we consider stacking faults in bulk GaN and a misfit dislocation network in a GaN/sapphire interfacial region through relaxation of strain and associated residual stresses; where the quantification of the latter is beyond the reach of experiment alone.

A monolithic white-light LED based on GaN doped with Be

In this communication, the use of gallium nitride doped with beryllium as an efficient converter for white light emitting diode is proposed. Until now beryllium in this material was mostly studied as a potential p-type dopant. Unfortunately, the realization of p-type conductivity in such a way seems impossible. However, due to a very intensive yellow emission, bulk crystals doped with beryllium can be used as light converters. In this communication, it is demonstrated that realisation of such diode is possible and realisation of a colour rendering index is close to that necessary for white light emission.

Exemplifying Quantum Systems in a Finite Element Basis

This paper presents a description of the abstractions required for the expression and solution of the linear single‐particle Schrodinger equation in a finite element basis. This paper consists of two disparate themes: First, to layout and establish the foundations of finite element analysis as an approximate numerical solution to extendable quantum mechanical systems; and second, to promote a high‐performance open‐source computational model for the approximate numerical solution to quantum mechanical systems. The structural foundation of the one‐and two‐dimensional time‐independent Schrodinger equation describing an infinite potential well is explored and a brief overview of the hierarchal design of the computational library written in C++ is given.

A Non‐Singular Computational Method for Modelling Defects in Nanocrystalline Materials

In the synthesis of crystalline materials the atomic structure of crystalline systems are not found to be ideal. They exhibit, for example, to a variety of forms of defects such as point‐ or line‐defects, impurities, grain boundaries, pop‐ins, and cracks. In this paper a truncation‐interpolation model of the structural properties of defects is considered for the general case for line‐defects, namely, the mixed‐dislocation. In this work an analysis of the dislocation density is undertaken, in particular, in the limit of the core where the dislocation core becomes important. One feature of this analysis is to explore a modification of the traditional classical field theory of dislocations. The modification presented here was invoked as a scheme to investigate the dislocation density of the mixed dislocations in nanostructures which remains ill‐defined in the traditional classical field theory of dislocations. Starting from a quasi‐analytical classical field theory our modelling method is referred back to another scheme that determines the dislocation density starting from experimental observations from high‐resolution transmission electron microscopy.

A method for atomistic/continuum analysis of defects in large HRTEM images

The capability to extract continuous lattice distortion fields from High Resolution transmission Electron Microscopy (HRTEM) images is invaluable for treating a large number of material problems, and methods such as Geometrical Phase Analysis (GPA) or peak-finding have been employed for this purpose. However, it is difficult to use such methods to obtain experimental strain maps from HRTEM images of large areas containing many dislocations due to localized changes of the imaging conditions imposed by the strain fields or by variations of the specimen thickness “Figure 1.a”.