Self-assembled Quantum Rings Research Articles

Control of complex quantum structures in droplet epitaxy

We report the controllable growth of GaAs quantum complexes in droplet molecular-beam epitaxy, and the optical properties of self-assembled AlxGaAs quantum rings embedded in a superlattice. We found that Ga droplets on a GaAs substrate can retain their geometry up to a maximum temperature of 490 °C during post-growth annealing, with an optimal temperature of 320 °C for creating uniform and symmetric droplets. Through controlling only the crystallisation temperature under As4 in the range of 450 °C to 580 °C, we can reliably control diffusion, adsorption and etching rates to produce various GaAs quantum complexes such as quantum dots, dot pairs and nanoholes. AlxGaAs quantum rings are also realised within these temperatures via the adjustment of As beam equivalent pressure. We found that crystallisation using As2 molecules in the place of As4 creates smaller diameter quantum rings at higher density. The photoluminescence of As2 grown AlxGaAs quantum rings embedded in a superlattice shows a dominant emission from the quantum rings at elevated temperatures. This observation reveals the properties of the quantum ring carrier confinement and their potential application as efficient photon emitters.

Spin Polarization of Carriers in InGaAs Self-Assembled Quantum Rings Inserted in GaAs-AlGaAs Resonant Tunneling Devices

In this work, we have investigated transport and polarization resolved photoluminescence (PL) of n-type GaAs-AlGaAs resonant tunneling diodes (RTDs) containing a layer of InGaAs self-assembled quantum rings (QRs) in the quantum well (QW). All measurements were performed under applied voltage, magnetic fields up to 15 T and using linearly polarized laser excitation. It was observed that the QRs’ PL intensity and the circular polarization degree (CPD) oscillate periodically with applied voltage under high magnetic fields at 2 K. Our results demonstrate an effective voltage control of the optical and spin properties of InGaAs QRs inserted into RTDs.

Carrier transfer in vertically stacked quantum ring-quantum dot chains

The interplay between structural properties and charge transfer in self-assembled quantum ring (QR) chains grown by molecular beam epitaxy on top of an InGaAs/GaAs quantum dot (QD) superlattice template is analyzed and characterized. The QDs and QRs are vertically stacked and laterally coupled as well as aligned within each layer due to the strain field distributions that governs the ordering. The strong interdot coupling influences the carrier transfer both along as well as between chains in the ring layer and dot template structures. A qualitative contrast between different dynamic models has been developed. By combining temperature and excitation intensity effects, the tuning of the photoluminescence gain for either the QR or the QD mode is attained. The information obtained here about relaxation parameters, energy scheme, interlayer and interdot coupling resulting in creation of 1D structures is very important for the usage of such specific QR–QD systems for applied purposes such as lasing, detection, and energy-harvesting technology of future solar panels.

Fabrication of GaSb quantum rings on GaAs(0 0 1) by droplet epitaxy

We report the achievement in the fabrication of self-assembled GaSb quantum rings (QRs) on GaAs(001) substrates by droplet epitaxy technique using molecular beam epitaxy. Surface morphology was characterized by atomic force microscopy. The QR formation can be described by the diffusion of Ga atoms out of an initial Ga droplet during crystallization with Sb flux. The understanding of the formation mechanism lights up the way to grow more complex GaSb nanostructures by droplet epitaxy. The optical properties of GaSb QRs in a GaAs matrix were investigated by photoluminescence (PL). Power- and temperature-dependent PL measurements were performed to study the carrier dynamics and to observe the characteristics of type-II band alignment.

Time evolution of self-assembled GaAs quantum rings grown by droplet epitaxy

The correlation between the formation mechanism and the optical emission of GaAs quantum rings (QRs) is studied for the first time. The GaAs QRs were fabricated by the droplet epitaxy technique on Al0.3Ga0.7As surface using molecular beam epitaxy (MBE). Time evolution of these GaAs QRs with different arsenisation time is presented. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to observe the structural and geometrical changes during the formation of the GaAs QRs. Microstructural changes of the GaAs QRs were clearly seen in photoluminescence (PL) but barely observable in traditional surface spectroscopy techniques of SEM and AFM. This provides a new pathway to observe the microstructural changes of GaAs QRs growth. PL measurement showed that the emission peaks from the QRs changes from a single sharp peak to two sharp peaks at different stages of formation of the GaAs QRs. The splitting of PL peaks is a result of bimodal distribution of GaAs QRs formed during the droplet epitaxy process.

Strain and band-mixing effects on the excitonic Aharonov-Bohm effect in In(Ga)As/GaAs ringlike quantum dots

Neutral excitons in strained axially symmetric In(Ga)As/GaAs quantum dots with ringlike shape are investigated. Similar to experimental self-assembled quantum rings, the analyzed quantum dots have volcano-like shapes. The continuum mechanical model is employed to determine the strain distribution, and the single-band envelope function approach is adopted to compute the electron states. The hole states are determined by the axially symmetric multiband Luttinger-Kohn Hamiltonian, and the exciton states are obtained from an exact diagonalization. We found that the presence of the inner layer covering the ring opening enhances the excitonic Aharonov-Bohm (AB) oscillations. The reason is that the hole becomes mainly localized in the inner part of the quantum dot due to strain, whereas the electron resides mainly inside the ring-shaped rim. Interestingly, larger AB oscillations are found in the analyzed quantum dot than in a fully opened quantum ring of the same width. Comparison with the unstrained ring-like quantum dot shows that the amplitude of the excitonic Aharonov-Bohm oscillations are almost doubled in the presence of strain. The computed oscillations of the exciton energy levels are comparable in magnitude to the oscillations measured in recent experiments.

Nanoscale electrical property studies of individual GeSi quantum rings by conductive scanning probe microscopy

The nanoscale electrical properties of individual self-assembled GeSi quantum rings (QRs) were studied by scanning probe microscopy-based techniques. The surface potential distributions of individual GeSi QRs are obtained by scanning Kelvin microscopy (SKM). Ring-shaped work function distributions are observed, presenting that the QRs' rim has a larger work function than the QRs' central hole. By combining the SKM results with those obtained by conductive atomic force microscopy and scanning capacitance microscopy, the correlations between the surface potential, conductance, and carrier density distributions are revealed, and a possible interpretation for the QRs' conductance distributions is suggested.

Large size self-assembled quantum rings: quantum size effect and modulation on the surface diffusion.

We demonstrate experimentally the submicron size self-assembled (SA) GaAs quantum rings (QRs) by quantum size effect (QSE). An ultrathin In0.1 Ga0.9As layer with different thickness is deposited on the GaAs to modulate the surface nucleus diffusion barrier, and then the SA QRs are grown. It is found that the density of QRs is affected significantly by the thickness of inserted In0.1 Ga0.9As, and the diffusion barrier modulation reflects mainly on the first five monolayer . The physical mechanism behind is discussed. The further analysis shows that about 160 meV decrease in diffusion barrier can be achieved, which allows the SA QRs with density of as low as one QR per 6 μm2. Finally, the QRs with diameters of 438 nm and outer diameters of 736 nm are fabricated using QSE.

Electron energy and geometry parameters of InGaAs/GaAs quantum rings: an interpretation of C-V data

ABSTRACTWe investigate the electronic properties of InAs/GaAs quantum rings (QRs) in a magnetic field using an original effective potential model based on a single band kp-approximation with an energy dependent effective mass. We used two sets of geometrical parameters for the selfassembled QRs. The first is the experimentally proposed geometry; the second follows from the oscillator model due to the relation between the model parameters and the real sizes of the quantum objects. The energy of an electron in a magnetic field, calculated for each of the geometries, is compared with C-V experimental data. We show that the results of the calculation obtained for the second geometry fit the experimental data rather well. Interpretation of the recent C-V data given by W. Lei et al. (Appl. Phys. Lett. 96 (2010) 033111) on the basis of the oscillator model is discussed.

Electrical properties of individual self-assembled GeSi quantum rings

The nanoscale electrical properties of self-assembled GeSi quantum rings (QRs) were investigated by conductive scanning probe microscopy at room temperature. The current distribution of individual GeSi QRs measured by conductive atomic force microscopy (CAFM) shows a low conductivity at the central hole as compared to the rim; however, the QRs’ composition distribution obtained by selective chemical etching combined with AFM observation reveals that within the QRs’ central holes, the Ge content is high, which should lead to a high conductivity instead of a low one as observed. Together with the results obtained by scanning capacitance microscopy (SCM) and electrostatic force microscopy (EFM), it is supposed that the GeSi QRs’ electrical properties are mainly determined by the ring-shaped topography, rather than by the complete oxidation of the QRs’ central hole or their composition distributions.

On Optical Properties of CdTe/ZnTe Quantum Rings

The optical properties of CdTe/ZnTe self-assembled quantum rings as functions of the height and inner diameter are investigated with a finite element method based on the linear elasticity theory of solids and the eight-band k·p Hamiltonian. We find that the quantum ring height significantly alters the bi-axial strain, while it has less effect on the hydrostatic strain. It was shown that the interband transition energy increases with increasing inner diameter (or decreasing quantum ring volume), while it decreases as the height increases. We find that the matrix element for the x-polarization is much larger than that of the z-polarization, in contrast to the result of the truncated quantum dot heterostructure. It is found that the gain peak is redshifted as the height of quantum ring increases, while it is blueshifted with increasing intensity as the inner diameter of quantum ring increases.

Photoemission Microscopy Studies of Quantum Dots and Rings

This review discusses the application of photoemission-based microscopy techniques to the compositional characterization of semiconductor quantum dots and rings. The experimental technique is discussed in detail. Using this technique, self-assembled III―V and Ge/Si quantum dots have been studied. These results will be discussed, both for randomly nucleated and site-controlled quantum dots. Furthermore, results obtained on InAs/GaAs self-assembled quantum rings will be reviewed.

Influencing factors on the size uniformity of self-assembled SiGe quantum rings grown bymolecular beam epitaxy

The size uniformity of self-assembled SiGe quantum rings, which are formed by cappingSiGe quantum dots with a thin Si layer, is found to be greatly influenced by the growthtemperature and the areal density of SiGe quantum dots. Higher growth temperaturebenefits the size uniformity of quantum dots, but results in low Ge concentration as well asasymmetric Ge distribution in the dots, which induces the subsequently formedquantum rings to be asymmetric in shape or even broken somewhere in the ridge ofrings. Low growth temperature degrades the size uniformity of quantum dots, andthus that of quantum rings. A high areal density results in the expansion andcoalescence of neighboring quantum dots to form a chain, rather than quantum rings.Uniform quantum rings with a size dispersion of 4.6% and an areal density of7.8×108 cm − 2 are obtained at the optimized growth temperature of640°C.

Oscillatory persistent currents in quantum rings: Semiconductors versus superconductors

Persistent currents are a hallmark of superconductivity in metals. To observe those dissipationless currents in a non-superconducting ring, the circumference of the ring must be short enough so that the phase coherence of the electronic wave functions is preserved around the loop. Recent progress in the fabrication of self-assembled semiconductor quantum rings (SAQRs), which can be filled with only a few (1–2) electrons, has offered the unique possibility to study the magnetic-field-induced oscillations in the persistent current carried by a single electron. In this paper, we discuss similarities and distinctions between the behavior of persistent currents in semiconductor and superconductor samples and give an overview of the recent results for oscillatory persistent currents in SAQRs. Although the real SAQR shape differs strongly from an idealized circular-symmetric open ring structure, the Aharonov–Bohm oscillations of the magnetization survive, as observed in low temperature magnetization measurements on In x Ga 1− x As/GaAs SAQRs.

Impacts of coulomb interactions on the magnetic responses of excitonic complexes in single semiconductor nanostructures.

We report on the diamagnetic responses of different exciton complexes in single InAs/GaAs self-assembled quantum dots (QDs) and quantum rings (QRs). For QDs, the imbalanced magnetic responses of inter-particle Coulomb interactions play a crucial role in the diamagnetic shifts of excitons (X), biexcitons (XX), and positive trions (X−). For negative trions (X−) in QDs, anomalous magnetic responses are observed, which cannot be described by the conventional quadratic energy shift with the magnetic field. The anomalous behavior is attributed to the apparent change in the electron wave function extent after photon emission due to the strong Coulomb attraction by the hole in its initial state. In QRs, the diamagnetic responses of X and XX also show different behaviors. Unlike QDs, the diamagnetic shift of XX in QRs is considerably larger than that of X. The inherent structural asymmetry combined with the inter-particle Coulomb interactions makes the wave function distribution of XX very different from that of X in QRs. Our results suggest that the phase coherence of XX in QRs may survive from the wave function localization due to the structural asymmetry or imperfections.

Electronic structure of self-assembled InGaAs/GaAs quantum rings studied by capacitance-voltage spectroscopy

Self-assembled InGaAs quantum rings, embedded in a GaAs matrix, were investigated using magneto-capacitance-voltage spectroscopy. The magnetic-field dispersion of the charging energies exhibits characteristic features for both the first and second electron, which can be attributed to a ground state transition from l=0 into l=−1, and a ground state transition from l=−1 into l=−2, respectively. Furthermore, using a combination of capacitance-voltage spectroscopy and one-dimensional numerical simulations, the conduction band structure of these InGaAs quantum rings was determined.

Excitonic behavior in self-assembled InAs/GaAs quantum rings in high magnetic fields

We investigate the exciton energy level structure of a large ensemble of InAs/GaAs quantum rings by photoluminescence spectroscopy in magnetic fields up to 30 T for different excitation densities. The confinement of an electron and a hole in these type I quantum rings along with the Coulomb interaction suppress the excitonic Aharonov-Bohm effect. We show that the exciton energy levels are nonequidistant and split up in only two levels in magnetic field, reflecting the ringlike geometry. A model, based on realistic parameters of the self-assembled quantum rings, allows us to interpret the essential features of the observed PL spectra in terms of the calculated optical transition probabilities.

The RHEED tracking of the droplet epitaxial grown quantum dot and ring structures

Self-assembled strain-free GaAs quantum dot and quantum ring structures grown by droplet epitaxy were investigated by the RHEED technique. The temporal evolution of these nanometer-scale objects were tracked in situ manner during the growth process. The initial stage of the surfaces was the same for both kinds of quantum objects. We have compared step by step the RHEED pattern in [ 1 1 ¯ 0 ] direction in the case of both quantum objects. After the Ga deposition the pattern becomes diffuse. Almost at the same time with the offering of arsenic the RHEED pattern changed from diffuse to spotty and later changed slowly to spots with chevrons in case of the quantum dot. In case of the quantum ring the situation is quite different. The pattern, which consists of streaks and spots, evaluate slowly and continuously. The different patterns and their different temporal evaluations can be explained with the shape and the kinetic of development of different quantum objects.

Rapid thermal annealing effects on self-assembled quantum dot and quantum ring structures

Low temperature photoluminescence spectroscopy is used to analyze the effects of the postgrowth thermal annealing on the electronic structure ad carrier dynamics of GaAs∕AlGaAs quantum dot and quantum ring structures grown by droplet epitaxy. All the samples show a large increase in the photoluminescence efficiencies after the thermal treatment due to sizeable reduction in the material defectivity. Modifications of the photoluminescence band, which depend on thermal annealing temperature, are found and quantitatively interpreted by means of a simple model based on the Al–Ga interdiffusion.

Electron and exciton energy spectra in self‐assembled InGaAs/GaAs ring‐like nanostructures

AbstractTheoretical analysis of the electron energy spectrum and the magnetization of an electron in a strained Inx Ga1–x As/GaAs self‐assembled quantum ring (SAQR) is performed with realistic parameters, determined from the cross‐sectional scanning‐tunneling microscopy characterization of that nanostructure. The effects of the Coulomb interaction on the energy spectrum of an exciton in a SAQR as well as on the optical‐transition probabilities are studied within our model of a SAQR. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)