G. Medeiros-Ribeiro

Intermixing and shape changes during the formation of InAs self-assembled quantum dots

The initial stages of GaAs overgrowth over self-assembled coherently strained InAs quantum dots (QDs) are studied. For small GaAs coverages (below 5 nm), atomic force microscopy (AFM) images show partially covered island structures with a regular size distribution which are elongated in the [011] direction. Analysis of the AFM profiles show that a large anisotropic redistribution of the island material is taking place during the initial GaAs overgrowth. Short time annealing experiments together with photoluminescence spectroscopy on annealed QDs are consistent with a Ga and In intermixing during the overgrowth. Surface QDs capped with 5 nm or more GaAs show a strong luminescence intensity indicating that surface QDs are remarkably insensitive to surface recombination effects.

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.

The dynamics of tunneling into self-assembled InAs dots

Using frequency-dependent capacitance spectroscopy, the dynamics of tunneling into arrays of self-assembled InAs quantum dots is investigated with respect to sample geometry, Coulomb interaction, and magnetic field. An equivalent resistance-capacitance circuit is derived which allows us to determine the tunneling times for each state of the dots. The different tunneling times for different many-particle states are explained by a reduced tunneling barrier and Coulomb interaction. A magnetic field applied perpendicular to the tunneling direction results in a strong suppression of the charging signal, which is attributed to enhanced localization caused by the magnetic field. Calculations for three-dimensional to zero-dimensional magnetotunneling can account for the experimental data.

Size quantization effects in InAs self-assembled quantum dots

We study size quantization effects in InAs self-assembled quantum dots (QDs) that are embedded in GaAs. Using capacitance, photoluminescence and photovoltage spectroscopy, we correlate the measured quantized level energies with the quantum dot sizes and densities obtained from transmission electron microscopy. With increasing dot size, we observe a strong redshift of the QD features in all our data. In the capacitance spectra, a band gap renormalization of the two-dimensional wetting layer system appears when the first excited QD state crosses the wetting layer ground state. The relative size dependence and absolute energetic position of the QD transitions determined with photoluminescence provide some information about the influence of lateral confinement and height of the QD.

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 ...

Single-electron charging and Coulomb interaction in InAs self-assembled quantum dot arrays

Sequential single-electron charging is observed in InAs Self-Assembled Quantum Dots using capacitance spectroscopy. In this system, the Coulomb energy is smaller than the inter-level energy spacings due to the quantum confinement and both effects can be separately identified. A theoretical model is proposed for this system and the capacitance experiments were devised in order to experimentally observe the effects of Coulomb interaction between electrons on the dots. The effects of inter- and intra-dot Coulomb interaction have been observed in the capacitanc e spectra. A good agreement between the proposed model and experiment is achieved.

Magnetic properties and imaging of Mn‐implanted GaAs semiconductors

Submicron ferromagnets have been successfully incorporated into GaAs semiconductors by Mn+ ion implantation and subsequent heat treatment. Transmission electron microscopy, x‐ray fluorescence spectrum analysis, and atomic force microscopy are used to structurally characterize the GaMn precipitates which form within the GaAs matrix. These crystallites are room‐temperature ferromagnets with controllable magnetic properties. Magnetic force microscopy images reveal that unmagnetized samples contain both magnetic dipoles and quadrupoles, but that after magnetization the single‐domain state predominates.

Luminescence quenching in InAs quantum dots

We report how photoluminescence from self-assembled InAs quantum dots depend on pumping power and vertical electric field. The InAs dots, which are embedded in a capacitor-like structure, act as efficient trapping centers for excitons. At a high enough electric field, however, the photoexcited electrons tunnel out of the dots fast enough to quench the emission. For samples with two adjacent layers of vertically aligned dots, we find that the threshold voltage for quenching depends very strongly on the optical pumping power. In total contrast to this, we find no comparable effect for samples grown with a single layer of dots. We explain this in terms of efficient storage of electrons and holes in the double-layer samples.

Storage of electrons and holes in self-assembled InAs quantum dots

We report spectroscopic measurements of charge-tunable quantum dots. The samples contain vertically aligned double dots which we can fill with electrons from a back contact. We show how we can also accumulate holes in the dots by illuminating the samples with below band gap radiation when a large negative bias is applied. We argue that this is possible through a large disparity in the electron and hole tunneling times. Interband spectroscopy reveals a strong reduction in the quantization energy for the dots in the second layer.

Coulomb-coupling in vertically aligned self-assembled InAs quantum dots

Electronic coupling effects in vertically aligned self-assembled InAs quantum dots are investigated using capacitance and far-infrared (FIR) spectroscopy. Capacitance spectra show distinct shifts of the many-particle ground state energies, which is the result of a strong electrostatic dot-dot interaction. FIR spectroscopy shows that the dynamics of the dot system is only slightly affected by Coulomb interaction. The dots in the second layer are found to have a larger diameter than those in the first.