InAs Quantum Dot Formation Studied at the Atomic Scale by Cross-sectional Scanning Tunnelling Microscopy

Abstract

Introduction Quantum dot formationSelf-assembled quantum dots (QDs) have attracted much attention in the last years [1,2]. These nanostructures are very interesting from a scientific point of view because they form nearly ideal zero-dimensional systems in which quantum confinement effects become very important. These unique properties also make them very interesting from a technological point of view. For example, InAs QDs are employed in QD lasers [3,4], single electron transistors [5], midinfrared detectors [6,7], single-photon sources [8,9], etc. InAs QDs are commonly created by the Stranski-Krastanov growth mode when InAs is deposited on a substrate with a bigger lattice constant, like GaAs or InP [10]. Above a certain critical thickness of InAs, three-dimensional islands are spontaneously formed on top of a wetting layer (WL) to reduce the strain energy. Once created, the QDs are subsequently capped, a step which is required for any device application.For any application based on InAs QDs, an accurate control of their electronic properties is required. The electron and hole states in the dot are very sensitive to the QD size, shape and composition (as well as to the surrounding material), and therefore an accurate control of the QD structural properties is necessary for applications. This explains the strong effort dedicated in the last years to the structural characterization of QDs [11], which is essential in order to have a deep understanding of the QD formation process. Although a lot of effort has been dedicated to study surface QDs, there are relatively few studies focused on the effect of the capping process [12][13][14][15][16][17][18][19][20]. Some of these studies have already shown significant differences in size, shape and composition between uncapped and capped QDs. For example, an important collapse of the QD height has been reported for InAs/GaAs QDs capped with GaAs [14,15,[17][18][19], revealing the big influence of the capping process on the structural properties of the QDs. Indeed, critical issues affecting the dot take place during capping like dot decomposition, intermixing, segregation, As/P exchange and composition modulation in the capping layer. This means that the real buried QDs must be studied in order to understand the complete QD formation process. Cross-sectional scanning tunnelling microscopy is an especially useful technique for this purpose, because it allows the structure of the capped dots at the atomic scale to be assessed. Cross-sectional scanning tunnelling microscopy (X-STM)The scanning tunnelling microscope is part of the family of scanning probe microscopes, which are able to provide direct real space information of a physical property at the atomic scale. This

Quadrupolar spectra of nuclear spins in strained InxGa1−xAs quantum dots

“…38, and specifically for the InAs QDs, see Ref. 39. Guided by the samples used in recent nuclear spin experiments, [13][14][15][16][17][18][19][20][21][22][23][24][25] we center our discussion around a QD of a truncated cone shape 59 with a base (top) diameter 25 nm (10 nm), placed over a 0.5 nm wetting layer, all fully embedded in a GaAs host lattice.…”

“…Note that due to the truncated cone shape, an anisotropy always remains regardless of the height of the QD. In self-assembled QDs, while the height is controlled to a very high precision, such as by capping and double capping techniques, 39 this is not the case for the lateral dimension, which is essentially determined by the local growth TABLE II. Effect of alloy composition on the Gaussian envelope-square-weighted statistics for the same atomistic quantities considered in Table I.…”

“…Therefrom, this simply suggests that the NMR spectra, especially in the Faraday geometry, can be utilized to extract the indium mole fraction, which is one of the key unknown material parameters for a specific QD under consideration. 38,39 D. NCSHFI-mediated nuclear spin depolarization Finally, we discuss within the Faraday configuration the NCSHFI-mediated nuclear spin depolarization, or for that matter the polarization process as well, as it is also governed by the same matrix element [cf. Eq.…”

“…Considering different magnetic field orientations and the effect of random alloy mixing, i.e., In x Ga 1−x As QDs, critical insight is gained on the vulnerability of each nuclear spin transition in the NMR spectrum under inhomogeneous broadening. An important complication is that the mole fraction of the constituents can vary appreciably from dot to dot within the same sample, 38,39 rendering the in situ compositional identification of a targeted QD so far not practical. We demonstrate that this can be readily extracted from the main span of the NMR spectra.…”

MaterialVis: Material visualization tool using direct volume and surface rendering techniques

“…Quantum dots have possible uses in various areas such as quantum computing, solar cells, medical imaging, LEDs and transistors. Biasiol and Heun [37] and Ulloa et al [38] present in-depth information about the structure and physical properties of quantum dots.…”