D. Leonard

Direct formation of quantum‐sized dots from uniform coherent islands of InGaAs on GaAs surfaces

The 2D–3D growth mode transition during the initial stages of growth of highly strained InGaAs on GaAs is used to obtain quantum‐sized dot structures. Transmission electron micrographs reveal that when the growth of In0.5Ga0.5As is interrupted exactly at the onset of this 2D–3D transition, dislocation‐free islands (dots) of the InGaAs result. Size distributions indicate that these dots are ∼300 A in diameter and remarkably uniform to within 10% of this average size. The areal dot densities can be varied between 109 and 1011 cm−2. The uniformity of the dot sizes is explained by a mechanism based on reduction in adatom attachment probabilities due to strain. We unambiguously demonstrate photoluminescence at ∼1.2 eV from these islands by comparing samples with and without dots. The luminescent intensities of the dots are greater than or equal to those of the underlying reference quantum wells.

Time‐resolved optical characterization of InGaAs/GaAs quantum dots

We report on the optical characterization of the strained InGaAs/GaAs quantum dots (QDs). The temperature dependence of the photoluminescence (PL) indicates that the onset energy of the thermal quenching in ∼20‐nm‐diam QDs is enhanced by a factor of ∼2 as compared to a quantum well (QW), due to the additional confinement. At low temperature, an increased carrier lifetime is observed for the QDs as compared to a reference QW (880 vs 330 ps). The carrier lifetime in the QDs was found to be independent of the temperature for T<30 K. In addition to this different dynamics of the localized excitons, we find that in the steady state PL and PL excitation, there is virtually no overlap between the emission and the absorption energies.

Surface migration induced self‐aligned InAs islands grown by molecular beam epitaxy

We utilize the sensitivity of the two‐ to three‐dimensional growth transition of InAs self‐assembled islands on InAs coverage to demonstrate the growth of self‐aligned InAs islands on etched GaAs ridges by molecular beam epitaxy. The different migration behavior of In adatoms on different crystal planes of etched ridges is used to spatially modulate the supply of In adatoms. The ridges are oriented either along the [011] and [011] direction on (100) substrates with grating spacing of 0.28, 1, and 5 μm. Atomic force microscopy reveals that the InAs islands are self‐aligned along the ridges and they have a typical size of 400 A in diameter and 120 A in height. In samples with [011] oriented ridges, the islands are located on the sidewalls. On the other hand, for [011] oriented ridges the islands are on the (100) planes on and at the foot of the mesa. On samples with a grating pitch of 0.28 μm, all the islands are located either on the sidewalls or at the bottom of the ‘‘V groove’’ for both grating orienta...

Electron and hole energy levels in InAs self‐assembled quantum dots

Capacitance spectroscopy is used to determine the allowed energy levels for electrons and holes in InAs self‐assembled quantum dots embedded in GaAs. Using this technique, the relative energy of the electron and hole states is measured with respect to their respective energy band minima in the GaAs. This allows the construction of an energy level diagram for these quantum dots which correlates well with previously observed photoluminescence data. By tuning the device geometry, a fine structure in the electron ground state is revealed and attributed to Coulomb charging effects.

Spatially resolved visible luminescence of self-assembled semiconductor quantum dots.

Ensembles of defect-free InAIAs islands of ultrasmall dimensions embedded in AIGaAs have been grown by molecular beam epitaxy. Cathodoluminescence was used to directly image the spatial distribution of the quantum dots by mapping their luminescence and to spectrally resolve very sharp peaks from small groups of dots, thus providing experimental verification for the discrete density of states in a zero-dimensional quantum structure. Visible luminescence is produced by different nominal compositions of InxAI(1–x)As-AIyGa(1–y)As.

Visible luminescence from semiconductor quantum dots in large ensembles

Visible luminescence has been obtained from ensembles of defect‐free, InxAl(1−x)As islands of ultrasmall dimensions embedded in AlyGa(1−y)As cladding layers. These structures were grown by molecular beam epitaxy and studied with low‐temperature photoluminescence (PL) and transmission electron microscopy. Visible luminescence is produced using various compositions of InxAl(1−x)As/AlyGa(1−y)As. Quantum dot size distributions, planar densities, dot heights, and wetting layer thicknesses are presented and correlated with the PL spectra.

Selective excitation of the photoluminescence and the energy levels of ultrasmall InGaAs/GaAs quantum dots

The energy levels of nanometer size InGaAs quantum dots epitaxially grown on GaAs by the coherent islanding effect are probed using selectively excited photoluminescence (PL), and PL excitation. A lateral‐confinement‐induced interlevel spacing of ∼30 meV between the first two states can be deduced from the spectra.

Temperature effects on the radiative recombination in self-assembled quantum dots

Abstract Several ensembles of self-assembled quantum dots (QDs) based on the AlInAsAl/GaAs and InGaAs/GaAs material systems have been investigated using photoluminescence (PL), PL excitation (PLE) and time-resolved PL (TRPL). The influence of the temperature is measured by monitoring sharp spectral features (as narrow as ∼ 90 μeV) obtained when probing the PL of small QD ensembles (few hundreds QDs). Thermionic emission of the photocarriers out of the QD potential is found to be the dominant mechanism leading to the thermal quenching of the PL and temperature-independent linewidths are observed up to the onset of the PL quenching.

Structural and optical characterization of InAs/InGaAs self‐assembled quantum dots grown on (311)B GaAs

InAs/InGaAs self‐assembled quantum dots (SADs) are fabricated on (311)B GaAs by molecular‐beam epitaxy using the Stranski–Krastanow growth mode. The critical thickness for the SAD formation on (311)B substrates is almost the same as that on (100) surface, characterized by in situ reflection high‐energy electron‐diffraction pattern changes. Atomic force microscopy observation on In0.5Ga0.5As SADs showed uniform nanometer scale quantum dots with a typical dot diameter of around 25 nm ±2 nm and a typical height of 13.7 nm ±2.2 nm. In photoluminescence measurements, an intense peak from the SADs is observed with a sharp luminescence from the wetting layer that exists two‐dimensionally underneath the SAD structure. A compound SAD structure is proposed where the composition of the wetting layer and the SAD layer are different. This is intended to suppress the luminescence from the wetting layer. The narrowest full widths at half‐maximum of the luminescence from the compound SADs on (311)B are about 35 meV at 2 ...

Spiral growth of GaSb on (001) GaAs using molecular beam epitaxy

Atomic force microscopy is employed to obtain images of the surface of GaSb epilayers grown on (001) GaAs using molecular beam epitaxy. The images reveal a surface that consists of micron size mounds that are approximately 4 nm high. A stepped surface is clearly observed on the mounds with a single step edge that originates from a screw dislocation at the center of the mound and moves out to the edge in a spiral fashion. The surface structure of the spiral mounds is observed to depend on the growth temperature of the GaSb epilayer, presumably as a result of a shorter diffusion length of the group III adatoms for lower substrate temperatures.