T Twan van Lippen

Self-organized lattice of ordered quantum dot molecules

Ordered groups of InAs quantum dots (QDs), lateral QD molecules, are created by self-organized anisotropic strain engineering of a (In,Ga)As/GaAs superlattice (SL) template on GaAs (311)B in molecular-beam epitaxy. During stacking, the SL template self-organizes into a two-dimensionally ordered strain modulated network on a mesoscopic length scale. InAs QDs preferentially grow on top of the nodes of the network due to local strain recognition. The QDs form a lattice of separated groups of closely spaced ordered QDs whose number can be controlled by the GaAs separation layer thickness on top of the SL template. The QD groups exhibit excellent optical properties up to room temperature.

Temperature-Dependent Photoluminescence of Self-Assembled (In,Ga)As Quantum Dots on GaAs (100): Carrier Redistribution through Low-Energy Continuous States

Temperature-dependent photoluminescence (PL) studies of an ensemble of self-assembled (In,Ga)As quantum dots (QDs) on GaAs (100) provide insight into the nature of the continuous states between the wetting layer (WL) and QDs. In addition to the well-known anomalous temperature dependence of the PL peak position and width around 90 K due to carrier (electron–hole pair) redistribution through the WL, we observe a similar behavior at much lower temperatures around 30 K. This behavior is attributed to carrier redistribution through the low-energy continuous states between the WL and QDs, directly proving their quasi-two-dimensional character. The smaller changes in the PL spectra than the WL-induced ones, however, indicate that the carrier redistribution and, thus, the spatial extent of the continuous states are restricted to a limited area around the QDs. This is also supported by the constant integrated PL intensity in this temperature range due to the absence of nonradiative recombination within these areas.

Electrical isolation of AlxGa1–xAs by ion irradiation

The evolution of sheet resistance Rs of n-type and p-type conductive AlxGa1−xAs layers (x=0.3, 0.6, and 1.0) during proton irradiation was investigated. The threshold dose Dth to convert a conductive layer to a highly resistive one is slightly different for n- and p-type samples with similar initial free carrier concentration and does not depend on the Al content. The thermal stability of the isolation, i.e., the temperature range for which the Rs is maintained at ≈109 Ω/sq, was found to be dependent on the ratio of the carrier trap concentration to the original carrier concentration. The thermal stability of isolated p-type samples is limited to temperatures lower than 450 °C. The temperature of ≈600 °C is the upper limit for the n-type samples thermal stability.

Observation of excited (bi-)exciton and multi-exciton features in locally homogeneous quantum dots

We investigate micro-photoluminescence spectra of arrays of locally homogeneous quantum dots (QDs). Our spectra indicate that the QDs exhibit excellent homogeneity in size on a length scale extending up to several microns. Using a Hartree–Fock based calculation including Coulomb and exchange energies, the observed spectra can be assigned to excitonic and multi-excitonic transitions, including excited exciton and excited bi-exciton transitions. We explain the strong excited-state recombinations due to the existence of a phonon bottleneck in our strain-free QD-arrays.

Self-organized ordered quantum dot molecules and single quantum dots

Ordered groups of InAs quantum dots (QDs), lateral QD molecules, are created by self-organized anisotropic strain engineering of a (In,Ga)As/GaAs superlattice (SL) template on GaAs (311)B by molecular beam epitaxy (MBE). During stacking the SL template self-organizes into a highly ordered two-dimensional (In,Ga)As and, thus, strain field modulation on a mesoscopic length scale, constituting a Turing pattern in solid state. InAs QDs preferentially grow on top of the SL template nodes due to local strain recognition, forming a lattice of separated groups of closely spaced ordered QDs. The SL template and InAs QD growth conditions like number of SL periods, growth temperatures, amount and composition of deposited (In,Ga)As, and insertion of Al-containing layers are studied in detail for optimized QD ordering within and among the InAs QD molecules on the SL template nodes, which is evaluated by atomic force microscopy (AFM). The average number of InAs QDs within the molecules is controlled by the thickness of the upper GaAs separation layer on the SL template and the (In,Ga)As growth temperature in the SL. The strain correlated growth in SL template formation and QD ordering is directly confirmed by high-resolution X-ray diffraction (XRD). Ordered arrays of single InAs QDs on the SL template nodes are realized for elevated SL template and InAs QD growth temperatures together with the insertion of a second InAs QD layer. The InAs QD molecules exhibit strong photoluminescence (PL) emission up to room temperature. Temperature dependent PL measurements exhibit an unusual behavior of the full-width at half-maximum, indicating carrier redistribution solely within the QD molecules.

Self‐Organized Lattice of Ordered Quantum Dot Molecules

Ordered groups of InAs quantum dots (QDs), lateral QD molecules, are created by self‐organized anisotropic strain engineering of a (In,Ga)As/GaAs superlattice (SL) template on GaAs (311)B by molecular‐beam epitaxy. During stacking, the SL template self‐organizes into a two‐dimensionally ordered strain modulated network on a mesoscopic length scale. The QDs form a lattice of separated groups of closely spaced ordered QDs whose number can be controlled by the GaAs separation layer thickness on top of the SL template. The QD groups exhibit excellent optical properties up to room temperature. Temperature dependent photoluminescence measurements exhibit an unusual behaviour of the full‐width at half‐maximum revealing carrier redistribution solely within the QD groups.