Search

A drop in the efficiency of light-emitting diodes based on InGaN/GaN QWs known as the ‘green gap’ has been studied intensively over the past dozen years. Several factors were revealed to contribute to its origin, such as random fluctuations in the indium concentration or the diffusion of point defects during the growth of QWs. The aim of this paper is to demonstrate that the Kirkendall effect can be the mechanism responsible for the thermal decomposition of InGaN/GaN MQWs structures, contributing to the green gap problem. By applying density functional theory, harmonic approximation, and harmonic transition state theory, we calculated the heights of the migration energy barriers of In and Ga atoms diffusing in $\mathrm{In_\textit xGa_{1-\textit{x}}N}$ alloys ( $x = 0, 0.11, 0.22$ ), the vibrational frequencies of $\mathrm{In_\textit xGa_{1-\textit{x}}N}$ alloys in the presence of migrating point defects, the temperature dependencies of the defect migration energy barriers and diffusion coefficients of Ga and In atoms migrating in $\mathrm{In_\textit xGa_{1-\textit{x}}N}$ alloys. We demonstrated the presence of unbalanced diffusion rates of In and Ga atoms at the $\mathrm{In_\textit xGa_{1-\textit{x}}N/GaN}$ interfaces and finally explained the experimentally observed mechanism of void formation at the $\mathrm{In_\textit xGa_{1-\textit{x}}N/GaN}$ interfaces by means of the Kirkendall effect.

Bainite is a steel microstructure consisting of three phases, bainitic ferrite, austenite and carbides. It forms in two different morphologies, upper and lower bainite, where different diffusion mechanisms are dominant. The aim of this work is to simulate both transformations within a unified model. To this end, we extend an own previously published model for lower bainite with diffusion across the phase interface. As a central idea we introduce weighted Helmholtz energy functions and a weighted mobility tensor, respectively. The individual Helmholtz energy functions and mobility terms are related to the different diffusion mechanisms which are responsible for the formation of both morphologies. Two representative examples illustrate the capability of the coupled phase field/diffusion model and show the expected behaviour. [ABSTRACT FROM AUTHOR]

The formation and diffusion of point defects have a detrimental impact on the functionality of devices in which a high quality AlN/GaN heterointerface is required. The present paper demonstrated the heights of the migration energy barriers of native point defects throughout the AlN/GaN heterointerface, as well as the corresponding profiles of energy bands calculated by means of density functional theory. Both neutral and charged nitrogen, gallium, and aluminium vacancies were studied, as well as their complexes with a substitutional III-group element. Three diffusion mechanisms, that is, the vacancy mediated, direct interstitial, and indirect ones, in bulk AlN and GaN crystals, as well at the AlN/GaN heterointerface, were taken into account. We showed that metal vacancies migrated across the AlN/GaN interface, overcoming a lower potential barrier than that of the nitrogen vacancy. Additionally, we demonstrated the effect of the inversion of the electric field in the presence of charged point defects V G a 3 − and V A l 3 − at the AlN/GaN heterointerface, not reported so far. Our findings contributed to the issues of structure design, quality control, and improvement of the interfacial abruptness of the AlN/GaN heterostructures. [ABSTRACT FROM AUTHOR]

• Migration energy barriers E b of point defects are calculated in GaN and InN crystals. • An approach to compute E b across the interface of two material is shown. • Diffusion of defects across the strained and relaxed InN/GaN interfaces is studied. • All vacancies are more mobile in the InN part of the strained InN/GaN heterostructure. • The lowest migration energy barrier E b near the InN/GaN interface exhibits VIn. Diffusion of native point defects is examined in the interface region of the InN/GaN heterostructure grown in hexagonal c direction. Heights of migration energy barriers of single vacancies and their complexes with substitutional group-III element are calculated in bulk GaN and InN crystals for two diffusion mechanisms, as well as across the InN/GaN heterointerface. An impact of the in-plane strain, the influence of built-in electric field, as well as the presence of electric dipole on vacancy motion is analysed. The obtained results show that metal vacancies V Ga and V In can migrate across the InN/GaN interface overcoming a potential energy barrier lower than that of V N . Moreover, the smallest energy barrier is related to the in-plane migration of V In what can favor spatial segregation of In atoms near the InN/GaN interface. This phenomenon can significantly contribute to the thermal degradation process of InGaN/GaN QWs, often observed experimentally both during the operation of nitride devices, as well as at the stage of their growth. [ABSTRACT FROM AUTHOR]