Exchange interaction in p-type GaAs/Al_{x}Ga_{1-x}As heterostructures studied by magnetotransport

Low-temperature magnetotransport experiments have been performed on a p-type GaAs/AlxGa1-xAs quantum well. From activation measurements on Shubnikov–de Haas conduction minima it was found that exchange interactions can be of great importance for both odd and even filling factors and strongly influence the observed periodicity. Furthermore, it was found that the temperature dependence of Shubnikov–de Haas oscillations in the low-magnetic-field regime could not be explained within a single-particle model based on a solution of the full Luttinger Hamiltonian in a magnetic field. Numerical simulations of Shubnikov–de Haas spectra, based on a model that treats hole exchange interactions in a simplified manner, show unambiguously that exchange driven enhancement of hole "spin" splittings are extremely important at magnetic fields as low as 1.5 T. Also, the inclusion of a valence-band warping in the calculations is shown to be essential. Qualitatively, most experimental observations could be described within the presented model. Our results imply that, in any hole system, the effective masses obtained from temperature-dependent SdH measurements are to be treated with extreme care as they can deviate from their single-particle value by as much as a factor of 2

Passen en meten op de schaal van atomen en in het onderwijs

Inleiding We zeggen vaak ‘het is een kwestie van passen en meten’ en daarmee bedoelen we dat we zoeken naar een oplossing. Dat is de kern van wetenschappelijk onderzoek, maar ‘passen en meten’ op de schaal van atomen is ook in de letterlijke zin van toepassing op mijn vakgebied omdat we gebruik maken van het passen en mispassen van kristalroosters en omdat we op de atomaire schaal meten om de invloed van roosterspanningen te bestuderen. Ook in omgekeerde richting is ‘passen en meten’ van toepassing bij de studie van moderne halfgeleiderstructuren. Als we meten aan moderne halfgeleiderstructuren dan blijkt dat heel veel eigenschappen alleen in exact afgepaste vorm voorkomen. Deze eigenschap is uitermate interessant in toepassingen van halfgeleiderstructuren. Het is duidelijk dat onderwijs geven en organiseren ook een kwestie is van ‘passen en meten’

Long wavelength (> 1.55 mu m) room temperature emission and anomalous structural properties of InAs/GaAs quantum dots obtained by conversion of In nanocrystals

We demonstrate that molecular beam epitaxy-grown InAs quantum dots (QDs) on (100) GaAs obtained by conversion of In nanocrystals enable long wavelength emission in the InAs/GaAs material system. At room temperature they exhibit a broad photoluminescence band that extends well beyond 1.55 mu m. We correlate this finding with cross-sectional scanning tunneling microscopy measurements. They reveal that the QDs are composed of pure InAs which is in agreement with their long-wavelength emission. Additionally, the measurements reveal that the QDs have an anomalously undulated top surface which is very different to that observed for Stranski-Krastanow grown QDs

Analysis of the shallow and deep center occupancies in silicon-doped aluminum gallium arsenide using a multilevel donor model

The concentration of occupied deep centers in Si-doped AlxGa1-xAs for x=0.2 has been calculated from a three-level donor model, in which the shallow levels are treated as excited states of the deep (DX) ground state. The deep level is assumed to be tied to the L valley, and the shallow levels to the G and X valleys. The behavior of the free-electron density and the thermal activation energy as function of composition is in good agreement with experimental results reported in the literature. In this model of dependent donor levels the deep-level occupancy can be directly calculated without needing deep-level transient spectroscopy measurements. A two-level donor model is used to calculate the pressure dependence of the deep level from a hydrostatic pressure experiment on a GaAs/Al0.3Ga0.7As heterostructure reported in the literature. We assume a shallow level tied to the G valley and an arbitrary deep level which is not coupled to any of the conduction bands. The calculation of the position of the deep level relative to the G valley as a function of pressure confirms the coupling of the deep level to the L valley. In this dependent donor model no large compensation is needed to fit the experimental data

Exchange-correlation energy of a hole gas including valence band coupling

We have calculated an accurate exchange-correlation energy of a hole gas, including the complexities related to the valence band coupling as occurring in semiconductors like GaAs, but excluding the band warping. A parametrization for the dependence on the density and the ratio between light- and heavy-hole masses is given. We apply our results to a hole gas in an AlxGa1-xAs/GaAs/AlxGa1-xAs quantum well and calculate the two-dimensional band structure and the band-gap renormalization. The inclusion of the valence band coupling in the calculation of the exchange-correlation potentials for holes and electrons leads to a much better agreement between theoretical and experimental data than when it is omitted

Influence of the tip work function on scanning tunneling microscopy and spectroscopy on zinc doped GaAs

The authors investigated the influence of the tip work function on the signatures of zinc in gallium arsenide with scanning tunneling microscopy and spectroscopy. By deliberately inducing tip modifications, the authors can change the tip work function between 3.9 and 5.5 eV, which corresponds to the expected range for tungsten of 3.5–6 eV. The related change in flatband voltage has a drastic effect on both the dI/dV spectra and on the voltage where the typical triangular contrast appears in the topography images. The authors propose a model to explain the differences in the dI/dV spectra for the different tip work functions. By linking the topography images to the spectroscopy data, the authors confirm the generally believed idea that the triangles appear when tunneling into the conduction band is mainly suppressed

Structural, electronic, and magnetic properties of single MnAs nanoclusters in GaAs

MnAs nanoclusters in GaAs were investigated with cross-sectional scanning tunneling microscopy. The topographic images reveal that the small clusters have the same zinc-blende crystal structure as the host material, while the larger clusters grow in a hexagonal crystal phase. The initial Mn concentration during molecular beam epitaxy growth has a strong influence on the size of the clusters that form during the annealing step. The local band structure of a single MnAs cluster is probed with scanning tunneling spectroscopy, revealing a Coulomb blockade effect that correlates with the size of the cluster. With a spin-sensitive tip, for the smaller clusters, superparamagnetic switching between two distinct states is observed at T¿=¿77¿K. The larger clusters do not change their magnetic state at this temperature, i.e., they are superferromagnetic, confirming that they are responsible for the ferromagnetic behavior of this material at room-temperature

The role of dot height in determining exciton lifetimes in shallow InAs/GaAs quantum dots

The spectral dependence of the photoluminescence recombination lifetime has been measured for individual self-assembled InGaAs/GaAs quantum dots, over the entire emission envelope. The measurements show a rising trend with increasing emission wavelength, increasing from 680 ps at 900 nm to about 1020 ps at 990 nm. Measurements of the out-of-plane diamagnetic coefficients for the dots show almost no correlation with wavelength. As a result, the rising trend in the lifetimes with wavelength is interpreted in terms of the emission energy being predominantly determined by the dot height, with higher dots exhibiting longer lifetimes. © 2010 American Institute of Physics. U7 - Export Date: 2 August 2010 U7 - Source: Scopus U7 - Art. No.: 03310

Strong electrically tunable exciton g-factors in an individual quantum dots due to hole orbital angular momentum quenching

Strong electrically tunable exciton g-factors are observed in individual (Ga)InAs self-assembled quantum dots and the microscopic origin of the effect is explained. Realistic eight band k.p simulations quantitatively account for our observations, simultaneously reproducing the exciton transition energy, DC Stark shift, diamagnetic shift and g-factor tunability for model dots with the measured size and a comparatively low In-composition of x(In)~35% near the dot apex. We show that the observed g-factor tunability is dominated by the hole, the electron contributing only weakly. The electric field induced perturbation of the hole wavefunction is shown to impact upon the g-factor via orbital angular momentum quenching, the change of the In:Ga composition inside the envelope function playing only a minor role. Our results provide design rules for growing self-assembled quantum dots for electrical spin manipulation via electrical g-factor modulation