Strain-free GaAs Quantum Dots Research Articles

Dot-Size Dependent Excitons in Droplet-Etched Cone-Shell GaAs Quantum Dots.

Strain-free GaAs quantum dots (QDs) are fabricated by filling droplet-etched nanoholes in AlGaAs. Using a template of nominally identical nanoholes, the QD size is precisely controlled by the thickness of the GaAs filling layer. Atomic force microscopy indicates that the QDs have a cone-shell shape. From single-dot photoluminescence measurements, values of the exciton emission energy (1.58...1.82 eV), the exciton–biexciton splitting (1.8...2.5 meV), the exciton radiative lifetime of bright (0.37...0.58 ns) and dark (3.2...6.7 ns) states, the quantum efficiency (0.89...0.92), and the oscillator strength (11.2...17.1) are determined as a function of the dot size. The experimental data are interpreted by comparison with an atomistic model.

Luminescence from Droplet-Etched GaAs Quantum Dots at and Close to Room Temperature.

Epitaxially grown quantum dots (QDs) are established as quantum emitters for quantum information technology, but their operation under ambient conditions remains a challenge. Therefore, we study photoluminescence (PL) emission at and close to room temperature from self-assembled strain-free GaAs quantum dots (QDs) in refilled AlGaAs nanoholes on (001)GaAs substrate. Two major obstacles for room temperature operation are observed. The first is a strong radiative background from the GaAs substrate and the second a significant loss of intensity by more than four orders of magnitude between liquid helium and room temperature. We discuss results obtained on three different sample designs and two excitation wavelengths. The PL measurements are performed at room temperature and at T = 200 K, which is obtained using an inexpensive thermoelectric cooler. An optimized sample with an AlGaAs barrier layer thicker than the penetration depth of the exciting green laser light (532 nm) demonstrates clear QD peaks already at room temperature. Samples with thin AlGaAs layers show room temperature emission from the QDs when a blue laser (405 nm) with a reduced optical penetration depth is used for excitation. A model and a fit to the experimental behavior identify dissociation of excitons in the barrier below T = 100 K and thermal escape of excitons from QDs above T = 160 K as the central processes causing PL-intensity loss.

Spectral broadening in self-assembled GaAs quantum dots with narrow size distribution

The control over the spectral broadening of an ensemble of emitters, mainly attributable to the size and shape dispersion and the homogenous broadening mechanisms, is crucial to several applications of quantum dots. We present a convenient self-assembly approach to deliver strain-free GaAs quantum dots with size dispersion below 10%, due to the control of the growth parameters during the preliminary formation of the Ga droplets. This results in an ensemble photoluminescence linewidth of 19 meV at 14 K. The narrow emission band and the absence of a wetting layer promoting dot–dot coupling allow us to deconvolve the contribution of phonon broadening in the ensemble photoluminescence and study it in a wide temperature range.

Resonance fluorescence from an atomic-quantum-memory compatible single photon source based on GaAs droplet quantum dots

Single photon sources, which are compatible with quantum memories, are important components of quantum networks. In this article, we show optical investigations on isolated GaAs/Al0.25Ga0.75As quantum dots grown via droplet epitaxy, which emit single photons on resonance with the Rb-87-D2 line (780 nm). Under continuous wave resonant excitation conditions, we observe bright, clean, and narrowband resonance fluorescence emission from such a droplet quantum dot. Furthermore, the second-order correlation measurement clearly demonstrates the single photon emission from this resonantly driven transition. Spectrally resolved resonance fluorescence of a similar quantum dot yields a linewidth as narrow as 660 MHz (2.7 μeV), which corresponds to a coherence time of 0.482 ns. The observed linewidth is the smallest reported so far for strain free GaAs quantum dots grown via the droplet method. We believe that this single photon source can be a prime candidate for applications in optical quantum networks.

Neutral, charged excitons and biexcitons in strain-free and asymmetric GaAs quantum dots fabricated by local droplet etching

We present an experimental and theoretical study of the optical properties of asymmetric strain-free GaAs quantum dots (QDs) fabricated by filling of self-organized nanoholes (NHs) created by local droplet etching. The energy levels are calculated within the effective mass approximation using as input a model anisotropic QD shape with C2v symmetry based on atomic force microscopy (AFM) profiles of NHs. The influence of the QD height and shape anisotropy on the exciton emission energy and the s-px and s-py energy splittings is studied theoretically. The experimentally observed bound nature of excitonic states is well reproduced by our theoretical approach which includes direct Coulomb energies and correlation effects. We investigate the fine-structure splitting (FSS) of the neutral exciton as a function of dot size. Theoretical calculations of the long-range electron-hole (e-h) exchange interaction predict an increase of the FSS with decreasing QD height, which describes well the experimental trend.

Excitonic complexes in strain-free and highly symmetric GaAs quantum dots fabricated by filling of self-assembled nanoholes

We perform a theoretical study of the optical transitions for different excitonic complexes in highly symmetric strain-free GaAs quantum dots (QDs) fabricated by epitaxially filling nanoholes (NHs) in an AlGaAs surface. NHs are formed by local droplet etching. As a first step, we propose a QD shape modeling consistent with atomic force microscopy (AFM) profiles and an experimental growth procedure. We investigate the QD height dependence of s- and p- shell exciton recombination energies in the framework of the effective mass approximation with an exact numerical diagonalization method. A comparison between theoretical results and available spectroscopic data is carried out. Systematic evolution of the binding energies of neutral (X), charged excitons (X–, X+) and the biexciton (XX), with QD height, is interpreted in terms of a balance between the Coulomb interactions and charge carrier correlation effects. Our calculations demonstrate the important role of the correlation energies in elucidating the bound character of all few-particle states especially the biexciton.

Excitonic states in GaAs quantum dots fabricated by local droplet etching

The influence of dot size and shape on the excitonic structure of GaAs/AlGaAs quantum dots (QDs) is studied experimentally and theoretically. Almost strain-free GaAs QDs are fabricated by epitaxially filling of nanoholes in an AlGaAs surface. The nanoholes are formed in a self-assembling fashion by local droplet etching. As an important point, the size of the QDs can be adjusted by the hole filling level. As a consequence, the exciton recombination energy can be controllably varied over a range of 130 meV by the dot size. We present micro- and macro-photoluminescence measurements of the $s$-shell recombinations. With the hole shape determined using atomic force microscopy and the amount of GaAs filled into the holes, we have quite precise input parameters for a simulation of the excitonic states in the QDs. The measured exciton and biexciton recombination energies are well reproduced by calculations based on eight band $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ theory and configuration interaction scheme with a deviation of less than 7 meV.

Nuclear magnetization in gallium arsenide quantum dots at zero magnetic field

Optical and electrical control of the nuclear spin system allows enhancing the sensitivity of NMR applications and spin-based information storage and processing. Dynamic nuclear polarization in semiconductors is commonly achieved in the presence of a stabilizing external magnetic field. Here we report efficient optical pumping of nuclear spins at zero magnetic field in strain-free GaAs quantum dots. The strong interaction of a single, optically injected electron spin with the nuclear spins acts as a stabilizing, effective magnetic field (Knight field) on the nuclei. We optically tune the Knight field amplitude and direction. In combination with a small transverse magnetic field, we are able to control the longitudinal and transverse components of the nuclear spin polarization in the absence of lattice strain—that is, in dots with strongly reduced static nuclear quadrupole effects, as reproduced by our model calculations.

Decay dynamics and exciton localization in large GaAs quantum dots grown by droplet epitaxy

We investigate the optical emission and decay dynamics of excitons confined in large strain-free GaAs quantum dots grown by droplet epitaxy. From time-resolved measurements combined with a theoretical model we show that droplet-epitaxy quantum dots have a quantum efficiency of about 75% and an oscillator strength between 8 and 10. The quantum dots are found to be fully described by a model for strongly-confined excitons, in contrast to the theoretical prediction that excitons in large quantum dots exhibit the so-called giant oscillator strength. We attribute these findings to localized ground-state excitons in potential minima created by material intermixing during growth. We provide further evidence for the strong-confinement regime of excitons by extracting the size of electron and hole wavefunctions from the phonon-broadened photoluminescence spectra. Furthermore, we explore the temperature dependence of the decay dynamics and, for some quantum dots, observe a pronounced reduction in the effective transition strength with temperature. We quantify and explain these effects as being an intrinsic property of large quantum dots owing to thermal excitation of the ground-state exciton. Our results provide a detailed understanding of the optical properties of large quantum dots in general, and of quantum dots grown by droplet epitaxy in particular.

Geometry of epitaxial GaAs/(Al,Ga)As quantum dots as seen by excitonic spectroscopy

It is shown that exciton and multiexciton emission lines (``spectral barcode'') of a quantum dot conceal nontrivial structural information on the shape and size of the dot, information which can be uncovered by comparison with atomistic many-body theory. Application to the newly established strain-free GaAs quantum dots grown via ``droplet epitaxy'' onto AlGaAs matrix reveal the shape and size as ``seen'' by spectroscopy. The results show that the previously determined dot height ($\ensuremath{\sim}14$ nm) as ``seen'' by cross-sectional scanning tunneling microscopy (XSTM) could not possibly be consistent with the excitonic signature (1.7--1.9 eV), as the latter must reflect dot height of 1--4 nm. Multiexciton ``barcode'' and fine structure spitting suggest GaAs/AlGaAs dots are in Gaussian-shape in agreement with XSTM measurement. Both spectroscopy and XSTM measurements were done on GaAs dots capped by the Al${}_{0.3}$Ga${}_{0.7}$As barrier layer. The fact that XSTM sees tall dots and spectroscopy sees short dots is thus not because the dot change its height in one experiment relative to the other but because different dots must have been used. This was uncovered by theoretical simulation of the experiment showing that the two experiments could not possibly correspond to the same dot.

Cross-sectional transmission electron microscopy of GaAs quantum dots fabricated by filling of droplet-etched nanoholes

We investigate strain-free GaAs quantum dots (QDs) fabricated by filling of nanoholes in semiconductor surfaces. The nanoholes are created in a self-organized fashion by local droplet etching with Al droplets as etchants. High resolution transmission electron microscopy (TEM) demonstrates that the quantum dots are free of extended defects. Elemental mapping using local electron energy loss spectroscopy (EELS) shows that the walls surrounding the nanohole openings consist of AlAs. This result confirms that the walls are optically inactive.

Optical Properties of GaAs Quantum Dots Fabricated by Filling of Self-Assembled Nanoholes.

Experimental results of the local droplet etching technique for the self-assembled formation of nanoholes and quantum rings on semiconductor surfaces are discussed. Dependent on the sample design and the process parameters, filling of nanoholes in AlGaAs generates strain-free GaAs quantum dots with either broadband optical emission or sharp photoluminescence (PL) lines. Broadband emission is found for samples with completely filled flat holes, which have a very broad depth distribution. On the other hand, partly filling of deep holes yield highly uniform quantum dots with very sharp PL lines.

Highly uniform and strain-free GaAs quantum dots fabricated by filling of self-assembled nanoholes

We demonstrate the self-assembled creation of a novel type of strain-free semiconductor quantum dot (QD) by local droplet etching (LDE) with Al to form nanoholes in AlGaAs or AlAs surfaces and subsequent filling with GaAs. Since the holes are filled with a precisely defined filling level, we achieve ultrauniform LDE QD ensembles with extremely narrow photoluminescence (PL) linewidth of less than 10 meV. The PL peaks agree with a slightly anisotropic parabolic potential. Small QDs reveal indications for transitions between electron and hole states with different quantization numbers. For large QDs, a very small fine-structure splitting is observed.

Optical orientation of electron and nuclear spins in strain free GaAs quantum dots grown by droplet epitaxy

AbstractWe report optical orientation experiments in individual, strain free GaAs quantum dots in AlGaAs grown by modified droplet epitaxy. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence at 4 K. Optical injection of spin polarized electrons into the dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build‐up time of the nuclear polarization in a single GaAs dot in the order of a second. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optically monitored nuclear spin dynamics in individual GaAs quantum dots grown by droplet epitaxy

We report on optical orientation experiments in individual strain-free GaAs quantum dots in AlGaAs grown by droplet epitaxy. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence at 4 K. Optical injection of spin-polarized electrons into a dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and we present a direct measurement of the buildup time of the nuclear polarization in a single GaAs dot in the order of 1 s.

Phonon sideband recombination kinetics in single quantum dots

We report an experimental study on the recombination kinetics of single strain-free GaAs quantum dots (QDs) grown by modified droplet epitaxy. The different bands composing the single quantum dot emission line at high temperature show identical dynamics, proving the common origin of all contributions. Our results thus agree with the interpretation of the broad pedestal band appearing when increasing the temperature as originated from the phonon replica. Finally, the relative weight of the phonon replicas depends both on temperature and on the QD size, in agreement with the theoretical predictions.

Regimes of GaAs quantum dot self-assembly by droplet epitaxy

Two regimes are observed for the density of strain-free GaAs quantum dots (QDs) grown by Ga droplet epitaxy. QDs grown from liquid Ga droplets deposited at temperatures up to $200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ exhibit densities that qualitatively agree with classical nucleation theory and are quantitatively reproduced by a rate equations based growth model under consideration of dimer break off. In contrast, at higher growth temperatures, the onset of coarsening by Ostwald ripening [Z. Phys. Chem., Stoechiom. Verwandtschaftsl. 34, 495 (1900)] causes drastically reduced QD densities. Extension of the growth models and consideration of Ostwald ripening allow the quantitative prediction of QD densities in this regime, as well.

Faceting during GaAs quantum dot self-assembly by droplet epitaxy

Strain-free GaAs quantum dots (QDs) are grown in a self-assembled fashion by applying Ga droplet epitaxy. The QDs are studied using electron diffraction and atomic force microscopy. Two distinct regimes are observed for the QD shape. QDs whose volume exceeds approximately 3×105 Ga atoms are shaped like truncated pyramids with side facets having an angle of about 55°. Smaller QDs are pyramidlike with 25° facets.