Strain effects on optical properties of self-assembled InAs/GaAs quantum dots grown by epitaxy are investigated. Since a capping layer is added after the self-assembly process of the quantum dots, it might be reasonable to assume that the capping layer neither experiences nor affects the induced deformation of quantum dots during the self-assembly process. A new two-step model is proposed to analyse the three-dimensional induced strain fields of quantum dots. The model is based on the theory of linear elasticity and takes into account the sequence of the fabrication process of quantum dots. In the first step, the heterostructure system of quantum dots without the capping layer is considered. The mismatch of lattice constants between the wetting layer and the substrate is the driving source for the induced elastic strain. The strain field obtained in the first step is then treated as an initial strain for the whole heterostructure system, with the capping layer, in the second step. The strain from the two-step analysis is then incorporated into a steady-state effective-mass Schrödinger equation. The energy levels as well as the wavefunctions of both the electron and the hole are calculated. The numerical results show that the strain field from this new two-step model is significantly different from models where the sequence of the fabrication process is completely omitted. The calculated optical wavelength from this new model agrees well with previous experimental photoluminescence data from other studies. It seems reasonable to conclude that the proposed two-step strain analysis is crucial for future optical analysis and applications.

This paper investigates the differences between the fully coupled and the semi-coupled piezoelectric models for determining strain fields, piezoelectric potentials and optical properties of wurtzite InGaN quantum dots (QDs) in three different shapes. Through the calculations, we show that the relative difference of the x-component strain inside the dot remains almost unchanged regardless of the shapes and the sizes of the dot. On the other hand, a large relative difference for the z-component strain is obtained with the use of the semi-coupled model. We also find that the semi-coupled model clearly overestimates the piezoelectric potential, and the transition energy difference increased with increases in the dot size and indium composition. Consequently, the semi-coupled model causes a great amount of distortion in predicting the optical properties of InGaN QDs. It is thus evident that the fully coupled model for calculating the electromechanical fields and optical properties of InGaN QDs may be more appropriate according to our numerical examples.

This paper investigates the influence of thickness of template layer on strain fields and transition energies in lens-shaped self-assembled SiGe/ Si quantum dots. This study analyzes strain fields in and around quantum dots on the basis of the theory of linear elasticity. Strain fields are then incorporated into the steady-state effective-mass Schrödinger equation. Energy levels and wavefunctions of both electrons and holes are calculated. The calculated results of strain-induced phonon frequency are consistent with previous results obtained by Raman spectroscopy. Moreover, the calculated transition energy agrees well with previous experimental photoluminescence data. Numerical results also suggest that transition energy decreases as the template layer thickness increases.

In this study, we investigate the influences of different composition distributions on the electronic structures of truncated cone-shaped InGaN quantum dots (QDs). A varying parameter, a, is defined as w 1 =w 2 , in which w 1 and w 2 are the top and base diameter of QD, respectively. This factor is set to study the role of similar truncated cones with the same base diameter and height on electronic structures of QD. Three indium composition distributions are considered: ellipsoid, uniform, and linear. The single-band effective-mass equation and six-band kÁp theory are used to calculate the transition energies of electrons and holes, respectively. The numerical results reveal that the parameter a plays an important role in changing the piezoelectric potential. In addition, an InGaN quantum dot with a linear distribution of indium has the maximum transition energy, whereas one with an ellipsoidal distribution has the minimum value. It is noteworthy that the binding energy greatly decreases as a increases for different indium distributions.

This study theoretically investigates the effect of the sign of the shear piezoelectric constant on the optical properties of wurtzite InGaN quantum dots (QDs) that are grown on polar, semipolar and nonpolar GaN substrates. The strain fields, electric potentials and singleparticle state energies are analyzed using the theory of piezoelectricity and the strained k • p Hamiltonian. Calculations reveal that the sign of e 15 greatly affects the electric potentials and optical properties, especially of larger QDs. The change in electron energy is particularly sensitive to the height of QDs for either sign of e 15 . A positive e 15 causes a greater decrease (increase) in electron (hole) energies than a negative e 15 . Furthermore, the exciton binding energy of polar QDs is sensitive to dot height, unlike semipolar and nonpolar QDs, which have weak binding energies. The transition energies of InGaN QDs in semipolar or nonpolar substrates are greatly increased. However, the overlap of the electron-hole wavefunctions is clearly greater when the indium content is lower. Based on the results herein, QDs grown on (1 1 0 1) planes should be preferred to those grown on polar and nonpolar planes for optical applications.