Guided self-assembly of lateral InAs/GaAs quantum-dot molecules for single molecule spectroscopy

We report on the growth and characterization of lateral InAs/GaAs (001) quantum-dot molecules (QDMs) suitable for single QDM optical spectroscopy. The QDMs, forming by depositing InAs on GaAs surfaces with self-assembled nanoholes, are aligned along the [ 1 " 10] direction. The relative number of isolated single quantum dots (QDs) is substantially reduced by performing the growth on GaAs surfaces containing stepped mounds. Surface morphology and X-ray measurements suggest that the strain produced by InGaAs-filled nanoholes superimposed to the strain relaxation at the step edges are responsible for the improved QDM properties. QDMs are Ga-richer compared to single QDs, consistent with strain-enhanced intermixing. The high optical quality of single QDMs is probed by micro-photoluminescence spectroscopy in samples with QDM densities lower than 10 8 cm –2. Keywords Lateral quantum-dot molecules AE Quantum dots AE Quantum dot composition AE Self-assembled growth Semiconductor quantum dots (QDs) have attracted extensive attention in recent years mostly motivated by the perspective of using them as a medium for storage and manipulation of quantum bits (qubits) [1]. To realize operations between qubits, coupled QD systems are essential [2]. The simplest case consists of a quantum-dot molecule (QDM) composed of two interacting QDs. A well-developed technique to fabricate QDMs with high structural and optical quality is based on the epitaxial growth of vertically stacked self-assembled QDs [3–6]. Two coupling mechanisms can be distinguished, i.e., electron and/or hole tunneling [3, 4, 6] or electromagnetic coupling, typically dipole– dipole interaction of excitons [5, 7]. Since the QDs composing a QDM are usually different, the tuning into resonance is often achieved by using an external tuning parameter such as an electric field [4, 6]. Ideally, also the barrier between the QDs composing a QDM should be tuned independently. This may be achieved by positioning a gate electrode between the two QDs. While such a task is technologically challenging for the vertical geometry, it may be easier for a lateral geometry as suggested previously [8]. Since self-assembled QDs are typically characterized by a flat geometry, the nature of the lateral coupling may differ appreciably from that of vertical coupling, rendering the study of laterally coupled QDs of fundamental interest. The growth of laterally coupled QDs is mostly based on the random occurrence of spatially close QDs [9]. Recently, we have invented a fabrication technique allowing the controlled growth of lateral QDMs [8, 10– 12]. The method is based on the growth …