0D Band Gap Engineering by MBE Quantum Rings: Fabrication and Optical Properties

Abstract

In this chapter we show how it is possible to modify the shape and size of InAs on GaAs self assembled quantum dots grown by Molecular Beam Epitaxy (MBE) by introducing a pause during the capping process, also known as partial overgrowth technique (García et al. in Appl. Phys. Lett. 71:2014, 1997; Appl. Phys. Lett. 72:3172, 1998; Granados and García in Appl. Phys. Lett. 82:2401, 2003). Under certain growth-pause capping conditions it is possible to obtain self-assembled quantum rings. The changes in shape and size lead to a modification of the quantum confinement potential and enables the control over fundamental physical properties, such as the optical emission energy from ground or excited states, the magnitude of its fine structure splitting or the sign of its permanent electric dipole moment.

The partial capping technique has played a key role in the engineering of 0D nanostructures with tailor made properties (Michler et al. in Science 290(5500):2282, 2000; Kiravittaya et al. in Rep. Prog. Phys. 72(4):046502, 2009). For example, it has allowed to fabricate a single-photon source that is based on a single self assembled quantum nanostructure embedded in a high-quality factor microcavity structure (Michler et al. in Science 290(5500):2282, 2000). Another example is the possibility to engineer what has been called “the smallest rings of electricity” (see Chap. 2), which unveil novel magnetic properties associated to non-trivial topologies at the nanoscale (Fomin (ed.) in J. Nanoelectron. Optoelectron., vol. 6. American Scientific, 2011) (see Chaps. 2, 4, 14, 17, and 18).

Typically, the formation of uniform and high quality self-assembled quantum dots, requires fixed growing parameters that does not allow to control independently the size, shape and overall density of the ensemble. The partial capping technique allows to have two separate sets of growing parameters, or “control knobs”: one for optimum QD nucleation and another employed for tuning size and shape during partial capping.

It is well known that the accumulated stress during growth of heteroepitaxial materials systems can lead to the self-assembly of quantum dots. But these driving forces are the very same ones responsible for a disassembling process that takes places during the capping process. Embedding an ensemble of elastically relaxed islands into a matrix material with a smaller lattice parameter, puts into play forces that compete dynamically with the capping process. Some examples of these processes are: atomic segregation, material interchange, surface reconstructions changes, stress-induced melting and de-wetting. The islands on the surface (either pyramid-, dome- or lens-shaped), will not preserve intact their structure after being capped. That is why it is so crucial to understand and control in detail the growth and embedding mechanisms of stressed materials. A way to achieve atomic level control of these dissociation mechanisms is to introduce a growth pause during capping to let the system relax.

This chapter focuses on the understanding and control of the capping process of quantum dots, and how under certain capping conditions, it is possible to obtain quantum rings and other nanostructures with quantum properties engineered at will.

We show in situ, real time, accumulated stress and Reflection High Energy Electron Diffraction (RHEED) measurements during InAs on GaAs(001) growth that shed light on the complicated processes that take place during growth and capping of lattice mismatched, and therefore strained, nanostructures. The experiments show that a large amount of indium melts due to the stress accumulation. This highly mobile material plays a key role on the transformations of self assembled nanostructures. For example, this liquid indium strongly segregates during the capping with GaAs, resulting in asymmetrical final soft barrier potentials.

We present a model to explain the formation of quantum rings under certain partial capping conditions which takes into account the competition processes between de-wetting [see Chap. 2], stress-induced melting of InAs and In/Ga exchange/alloying. We present Atomic Force Microscope (AFM) results of nanostructures with different shapes obtained under various partial capping conditions. The role of As₂ in the final formation of rings is also discussed.

Changes in the size and shape of the self-assembled nanostructures induced during the partial capping process allows additional 0D band gap engineering that will change accordingly their optical properties. This is based on measurements of continuous wave photoluminescence (PL), time-resolved PL (TRPL) and photoluminescence excitation (PLE) obtained both, in QRs ensembles, and in single quantum rings. Nanostructures capped under different conditions and with different emission energies are compared. As the diameter to height ratio increases, the radiative lifetime and the splitting between ground and excited states decreases leading to sizeable effects in the recombination dynamics as a function of the temperature. We show that a smaller height and a strong In/Ga exchange induced by the partial capping process are responsible of the reduction of the electrical polarizability and the inversion of the permanent dipole moment in these QRs. Finally, the voltage dependent micro-PL and micro-PLE spectra of charge tunable QRs are presented and analyzed in the framework of a central parabolic confinement model including coulomb interactions in the strong confinement regime.