Optical properties of low-strained InxGa1−xAs∕GaAs quantum dot structures at the two-dimensional–three-dimensional growth transition

Abstract

In x Ga 1 − x As ∕ Ga As quantum dots (QDs) were grown by solid source molecular beam epitaxy for indium contents of around 30%, which assures the QD growth in the very low strain limit. The structures were fabricated for a constant nominal InxGa1−xAs layer thickness but varying content (strain) from below to far above the critical thickness conditions, which has allowed to detect the onset of three-dimensional island formation and their evolution with the increasing material amount (for higher In contents the critical thickness for island formation is smaller and hence a larger fraction of the InxGa1−xAs layer is spent on dot formation). In order to investigate the properties of such an uncommon QD system, photoreflectance and photoluminescence, combined with scanning electron microscopy, have been used. Optical transitions connected with the ternary layer have been observed and followed from the lowest content quantum well case through the transformation into three-dimensional islands on the wetting layer (WL) and a coexistence of the QD-related and WL-related transitions. Due to the observation of both heavy hole and light hole related transitions in photoreflectance spectra, the thickness of the wetting layer versus changed indium content could be determined, comparing the experimental data with the results of the effective mass envelope function calculations.

Impact of wetting-layer density of states on the carrier relaxation process in low indium content self-assembled (In,Ga)As/GaAs quantum dots

“…Due to the strain release and surface atom migration anisotropy, strongly elongated QDs are formed, with typical sizes of 25 nm in width, several nanometers in height, and up to 100 nm in length for uncapped QDs. 34,35 They are preferentially aligned along the [110] direction and exhibit a rather low planar density of ∼5×10 9 cm −2 . For the PL and TRPL experiments, the sample was placed in a helium-flow cryostat with a precise (better than 0.1 K) control of the temperature in the range of 4.5-300 K. The structure was excited nonresonantly by a pulse train from a Ti:sapphire mode-locked laser with a repetition frequency of 76 MHz and 140 fs pulse width.…”

Toward weak confinement regime in epitaxial nanostructures: Interdependence of spatial character of quantum confinement and wave function extension in large and elongated quantum dots

“…Previous optical studies on such nanostructures have revealed that not only is the spatial confinement weaker than in standard QDs due to the increased nanostructure size [17], but that also the confining potential is very shallow (in the sense of the potential depth with respect to the barrier height). This is manifested by a low-temperature emission energy difference between the QD and WL emission bands as low as 25 meV [19]. It has important implications, especially for carrier dynamics [18,21], and allows for thermally activated energy/carrier transfer between the WL and the QD ensemble as well as the WL-mediated carrier redistribution [18].…”

“…The investigated structures were realized by submonolayer deposition of 30 alternating In 0.12 Ga 0.88 As/InAs sequences of 0.12 nm and 0.03 nm thicknesses, respectively. The deposited material is estimated to be 4.5 nm thick based on the growth conditions, of which 2.7 nm constitutes the wetting layer (WL) [19]. Further details of the growth procedure are given elsewhere [20].…”

Probing the carrier transfer processes in a self-assembled system with In 0.3 Ga 0.7 As/GaAs quantum dots by photoluminescence excitation spectroscopy

“…A QD layer with nominal thickness of 4.5 nm was deposited on a 300-nmthick GaAs buffer layer, where 2.7 nm of the nominal thickness is wasted to form the wetting layer, and the remaining material constituted self-assembled quantum dots [26]. The indium content of 30% results in low lattice mismatch (2%) between the deposited InGaAs layer and the GaAs matrix, and the QD self-assembly takes place in very low strain conditions.…”