Electronic coupling and structural ordering of quantum dots using InAs quantum dot columns

In this article, recent investigations of vertically aligned quantum dot columns conducted at Stanford University are reviewed. The quantum dots are InAs in a matrix of GaAs. Both the quantum dots and quantum dot columns are formed through strain-induced islanding, without lithography. Two aspects of these columns are discussed. First, the electronic coupling of quantum dots within columns of up to ten quantum dots is demonstrated. The coupling is adjusted and improvements to a simple light-emitting diode are shown. Second, increased uniformity of a surface quantum dot layer is shown when a subsurface layer of these columns are used. The most impressive results occur when the columns contain a large number of islands. Reduced variations in average ensemble height and diameter, called size uniformity, and average nearest neighbor distances, called structural uniformity, are shown. A surface unit cell of islands is demonstrated and the lack of a surface lattice is discussed.     

The engineering and properties of InAs quantum dot molecules in a GaAs matrix

Arrays of InAs quantum dot (QD) molecules in the GaAs matrix, which consist of pairs of vertically aligned InAs QDs, have been synthesized by molecular beam epitaxy. A study of the resulting structures by transmission electron microscopy demonstrated that the vertically aligned QDs are equal in size. Photoluminescence measurements revealed that the spectra of the samples under study contain bands corresponding to electronic states in QD molecules.     

Issues of practical realization of a quantum dot register for quantum computing

We present a quantum dot structure fabricated by the lithographic positioning, which can be used as a prototype of the quantum dot register for quantum computing. Using simple model calculations we show that parameters of our quantum dot structure are very close to the ones required for two possible embodiments of a quantum computer. Results of numerical simulation of the quantum dot register, as well as discussion of materials and technological issues of fabrication of quantum logic gates are also presented.     

The quest for the quantum dot

Semiconductor devices that confine electrons within two-dimensional quantum wells are already in production. The zero-dimensional quantum dot promises super-efficient lasers with the last word in spectral purity. The author briefly discusses the definition of the electron and then goes on to discuss in more detail the quantum well obtained in the case of a GaAlAs-GaAs heterojunction. Further quantisation of the electron energy produces one dimensional quantum wires. By removing the one degree of freedom of the quantum wire leaves the quantum dot, zero degrees of freedom, the ultimate in energy quantisation     

Enhanced optical polarization anisotropy in quantum wells under anisotropic tensile strain

Anisotropic in-plane strain in quantum wells leads to an optical polarization anisotropy that can be exploited in optoelectronic devices such as modulators. A theoretical model shows that the behavior of the polarization anisotropy with increasing strain anisotropy is radically different for quantum wells under anisotropic tensile and compressive strains of equal magnitude. This strikingly different behavior arises from the different valence-subband mixing that occurs in the cases of anisotropic tensile and compressive strain. Specifically, the mixing of the first heavy- and light-hole subbands that occurs only under anisotropic tensile strain is central to the polarization anisotropy.     

Heteroepitaxial growth of InAs on Si: A new type of quantum dot

The mechanism for heteroepitaxial growth in the InAs/Si system is studied by reflection highenergy electron diffraction, scanning tunnelling microscopy, and photoluminescence. For certain growth conditions, InAs nanostructures are found to develop on the Si surface immediately during the growth process in the course of molecular beam epitaxy. The range of substrate temperatures that lead to formation of nanosized islands is determined. InAs quantum dots grown on a buffer Si layer with a silicon layer of thickness 50 nm grown on the top produced photoluminescence lines at a wavelength of 1.3 μm at 77K and 1.6 μm at 300 K. Fiz. Tekh. Poluprovodn. 33, 1066–1069 (September 1999)     

Effects of GaAs-spacer strain on vertical ordering of stacked InAs quantum dots in a GaAs matrix

Vertical ordering in stacked layers of InAs/GaAs quantum dots is currently the focus of scientific research because of its potential for optoelectronics applications. Transmission electron microscopy was applied to study InAs/GaAs stacked layers grown by molecular-beam-epitaxy with various thicknesses of GaAs spacer. Thickness dependencies of quantum dot size and their ordering were observed experimentally and, then, compared with the results of strain calculations based on the finite element method. The vertical ordering did occur when the thickness of the GaAs spacer was comparable with the dot height. The ordering was found to be associated with relatively large InAs dots on the first layer. Quantum dots tend to become larger in size and more regular in plane with increasing numbers of stacks. Our results suggest that the vertical ordering is not only affected by strain from the InAs dots on the lower layer, but by total strain configuration in the multi-stacked structure.     

Single electron transport in a free-standing quantum dot

We have fabricated a single quantum dot in the two-dimensional electron gas of a free-standing GaAs/AlGaAs heterostructure. Standard processing techniques are used to define the dot on a narrow mesa and selective etching undercuts the device to form a 120 nm thick free-standing beam connected to the substrate by source and drain regions. Work by Weig et al. [Phys. Rev. Lett. 92(4) (2004) 046804] on similar devices has provided evidence for phonon blockade which suppresses conduction at low temperatures and bias. Further studies are presented here that can only partially be explained by this theory. A similar gap in conductance has been seen, but evolution of the structure in a magnetic field suggests an alternative explanation of multiple dots in series, may better explain the results.     

Characteristic temperature of a tunneling-injection quantum dot laser: Effect of out-tunneling from quantum dots

A laser structure is studied, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs). An extended theoretical model is developed allowing for out-tunneling leakage of carriers from QDs into the opposite-to-injection-side QWs (electrons into the p-side QW and holes into the n-side QW). Due to out-tunneling leakage, parasitic recombination of electron-hole pairs occurs outside QDs – in the QWs and optical confinement layer. The threshold current density j_th and the characteristic temperature T₀ are shown to be mainly controlled by the recombination in the QWs. Even in the presence of out-tunneling from QDs and recombination outside QDs, a tunneling-injection laser shows potential for significant improvement of temperature stability of j_th – the characteristic temperature T₀ remains very high (above 300 K at room temperature) and not significantly affected by the QD size fluctuations.     

High-performance 980 nm quantum dot lasers for high-powerapplications

High-performance quantum dot lasers emitting at 980 nm with output powers of up to 4 W CW from a single facet (AR/HR coating, 100 μm stripe width) have been fabricated. Wall-plug efficiencies >50%, were achieved at room temperature. Owing to an improved carrier confinement output powers as high as 1 W CW can be obtained from the fundamental dot transition even at temperatures as high as 110°C     

Coherence and phase sensitive measurements in a quantum dot

Via a novel interference experiment, which measures magnitude and phase of the transmission coefficient through a quantum dot in the Coulomb regime, we prove directly, for the first time, that transport through the dot has a coherent component. We find the same phase of the transmission coefficient at successive Coulomb peaks, each representing a different number of electrons in the dot, however, as we scan through a single Coulomb peak we find an abrupt phase change of π. The abrupt phase change can be explained in the framework of a one-dimensional model, which we develop.     

Electric-field sensor based on a double quantum dot in a microcavity

A scheme for an optical quantum external-electric-field sensor based on a double quantum dot placed in a high-Q semiconductor microcavity is proposed. A model of the dynamic processes occurring in this system is developed, its spectral characteristics are investigated, and the noise stability of the sensor is examined. It is demonstrated that, owing to design features, the device has a number of advantages, including high sensitivity, the presence of different excitation and measurement channels, and the possibility of accurate determination of the spatial field distribution.     

Polaron effect-dependent third-order optical susceptibility in a ZnS/CdSe quantum dot quantum well

The effect of the electron-phonon interaction on the third-harmonic is investigated theoretically for electrons confined in a core-shell quantum dot. The interactions of electrons with different phonon modes in the core-shell system, including the confined longitudinal optical (LO) and the interface optical (IO) phonon modes, are investigated. We carried a detailed calculation of third-harmonic generation (THG) process on a ZnS/CdSe core-shell quantum dot as a function of pump photon energy with different incident photon energy and under different sizes. The results reveal that the polaron effects are quite important especially around the peak value of the third-order susceptibility. By increasing the size of the quantum dots, the peaks of χ_THG⁽³⁾ will shift to lower energy, and the intensities of the peaks will increase.     

Nanofabrication of a quantum dot array: Atomic force microscopy of electropolished aluminum

One step required for the fabrication of a quantum dot array on an aluminum substrate is the preparation of a flat aluminum surface. To enable the optimization of the electropolishing procedure, atomic force microscopy was used to examine the morphology of electropolished polycrystalline aluminum surfaces that were prepared under different electropolishing conditions. The electropolishing voltage, time, and temperature were varied. Two distinctly different surface morphologies were observed for different electropolishing conditions and transitional structures were observed for intermediate conditions. It was found that the type of surface morphology and the surface roughness could be controlled primarily with the electropolishing voltage while temperature and time had relatively little effect over the range examined in this study.     

Numerical simulation of optical feedback on a quantum dot lasers

We use multi-population rate equations model to study feedback oscillations in the quantum dot laser. This model takes into account all peculiar characteristics in the quantum dots such as inhomogeneous broadening of the gain spectrum, the presence of the excited states on the quantum dot and the non-confined states due to the presence of wetting layer and the barrier. The contribution of quantum dot groups, which cannot follow by other models, is simulated. The results obtained from this model show the feedback oscillations, the periodic oscillations which evolves to chaos at higher injection current of higher feedback levels. The frequency fluctuation is attributed mainly to wetting layer with a considerable contribution from excited states. The simulation shows that is must be not using simple rate equation models to express quantum dots working at excited state transition.     

Excitonic dynamics of a quantum dot coupled to a laser-driven semiconductor microcavity

Within the density matrix formalism, we report on the quantum control of the excitonic coherences in quantum dots coupled to a single mode field resonant semiconductor cavity. We use an external classical laser field to drive the dynamical response of the excitonic states. Dissipation mechanisms associated with the cavity field and the excitonic states are explicitly included in the model. Our numerical simulations of the excitonic dynamics are in good agreement with recent experimental reports. Furthermore, we compute and show how to tailor such a dynamics in the presence of the laser field by means of controlling the detuning between the laser and the cavity field frequencies. The results are analyzed with a view to implementing quantum control of local qubit operations.     

Nano-optical probing of exciton wave-functions confined in a GaAs quantum dot

We have enhanced the performance of near-field scanning optical microscopy (NSOM) in terms of the spatial resolution and the sensitivity in signal detection. A careful preparation of an aperture-NSOM probe provides us with a spatial resolution as high as 30 nm in fluorescence imaging spectroscopy. We have applied this technique to map out the center-of-mass wave functions of an exciton confined in a GaAs quantum dot (a monolayer-high island formed in a quantum well). The spatial profile of the exciton emission, which reflects the shape of the island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. A theoretical calculation of the spatial distribution of the polarization field quantitatively reproduced the experimental result. Furthermore, mapping of an excited state wave-function with a node structure is also demonstrated. The novel technique can be extensively applied to wave-function engineering in the design and fabrication of quantum devices.     

Quantum-confined Stark shift in electroreflectance of a cylindrical GaN quantum dot

The electroreflectance (ER) spectra in the presence of the modulated electric field have been employed to study the fine structure of a cylindrical GaN quantum dot (QD), including the light hole and heavy hole interband transitions, and the ER spectra exhibit Franz-Keldysh oscillation characteristics with abscissa of energy (E−E_g). The quantum-confined Stark shift (QCSS) happened when the electric field intensity increased and the light and heavy holes dependent characteristics have been shown. The three-dimensional Schrödinger equation of QD has been calculated within the framework of effective-mass approximation, and the ER indices have been obtained from modulation absorption coefficients using the Seraphin coefficients and the Kramer-Kroning relation.