Quantum Dots

The following sections are included:

• Contents

In 1932, H. P. Rocksby ¹ discovered that the red or yellow color of some silicate glasses could be linked to microscopic inclusions of CdSe and CdS. It was not until 1985 when these changes in color were linked to the energy states determined by quantum confinement in these CdS or CdSe “quantum dots” ². Since that time, the number of experimental studies of quantum dots has literally exploded, stimulated, in particular, by the development of mature Molecular Beam Epitaxy technology, which rapidly became the technology of choice for growing quantum dots in different materials systems. Fig. 1 clearly illustrates the increased interest in quantum dots…

We describe a procedure for calculating the electronic structure of semiconductor quantum dots containing over one million atoms. The single particle electron levels are calculated by solving a Hamiltonian constructed from screened atomic pseudopotentials. Effects beyond the single particle level such as electron and hole exchange and correlation interactions are described using a configuration interaction (CI) approach. Application of these methods to the calculation of the optical absorption spectrum, Coulomb repulsions and multi-exciton binding energies of InGaAs self-assembled quantum dots are presented.

We review the concepts and principal experimental results pertaining to the self-assembly and self-ordering of quantum dots in semiconductor systems. We focus on the kinetics and thermodynamics of the formation and evolution of coherently strained 3D islands, and the effects of strain on nucleation, growth, and island shape. We also discuss ongoing research on methods to control the density, size, and size distributions of strained islands, both within a single strained layer and in quantum dot (QD) multilayers.

This article reviews the advances in the growth of III-nitride quantum dots achieved in the last few years and their unique properties. The growth techniques and the structural and optical properties associated with quantum confinement, strain, and polarization in GaN/Al_xGa_1-xN and In_xGa_1-xN/GaN quantum dots are discussed in detail.

Gain and threshold current of a quantum dot (QD) laser are analyzed theoretically taking into account the inhomogeneous line broadening caused by fluctuations in QD parameters. Two regimes of QD filling by carriers, nonequilibrium and equilibrium, are identified, depending on temperature, barrier heights and QD size. Critical tolerable parameters of the QD structure, at which lasing becomes impossible to attain, are shown to exist. The minimum threshold current density and optimum parameters are calculated. Violation of charge neutrality in QDs is revealed, which affects significantly the threshold current and its temperature dependence. The gain-current dependence is calculated. The voltage dependences of the electron and hole level occupancies in QDs, gain and current are obtained. The observed temperature dependence of threshold current (constant at low temperatures and rapid increase above a certain temperature) is predicted and explained. Violation of charge neutrality is shown to give rise to the slight temperature dependence of the current component associated with the recombination in QDs. The characteristic temperature T₀ is calculated considering carrier recombination in the optical confinement layer and violation of charge neutrality in QDs. The inclusion of violation of charge neutrality is shown to be critical for the correct calculation of T₀ at low T. T₀ is shown to fall off profoundly with increasing T. Theoretical estimations confirm the possibility of a significant reduction of the threshold currents and enhancement of T₀ of QD lasers as compared with the conventional quantum well (QW) lasers. Longitudinal spatial hole burning is analyzed. Unlike QW lasers, thermally excited escapes of carriers from QDs, rather than diffusion, are shown to control the smoothing-out of the spatially nonuniform population inversion and multimode generation in QD lasers. A decrease in the QD size dispersion is shown not only to decrease the threshold current but to increase considerably the relative multimode generation threshold as well. Concurrent with the increase of threshold current, an increase of the multimode generation threshold is shown to occur with a rise in temperature. Ways to optimize the QD laser, aimed at maximizing the multimode generation threshold, are outlined.

Quantum Dots (QD) provide unique opportunities to extend all the basic properties of heterostructure lasers and move further their applications. Practical fabrication of QD lasers became possible when techniques for self-organized growth allowed fabrication of dense and uniform arrays of narrow-gap nanodomains, coherently inserted in a semiconductor crystal matrix. Using of InAs QD lasers enabled significant improvement of device performance and extension of the spectral range on GaAs substrates to mainstream telecom wavelengths. Continuous wave 1.3 µm room-temperature output power of ∼300 mW single mode for edge-emitters and of 1.2 mW multimode for vertical-cavity surface-emitting lasers are realized. Long operation lifetimes are manifested. The breaktrough became possible both due to development of self-organized growth and defect-reduction techniques in QD Technology.