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Comparisons of buried InAs/GaAs and InAs/InP quantum dots (QDs) utilizing transmission electron microscopy display interesting parallels and differences between the two systems. The higher 7.2% misfit in the InAs/GaAs system produces small (~20 nm diameter) QDs with a majority displaying a predominately round shape. The lower 3.2% misfit in the InAs/InP system produces larger QDs (~35 nm diameter) with the majority also displaying a predominately round shape. In both systems, the size of the QDs can be varied by changes in growth procedures and the largest QDs in any population show evidence of faceting c 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the single and multiple layers of self-assembled InAs quantum dots (QD) produced by the indium-flush technique by transmission electron microscopy. Development of a technique to reproducibly grow QD of uniform size and shape; Determination of the shapes and sizes of QD through the interpretation of the observed contrast in plan-view.

Reports on hidden symmetries in the energy levels of excitonic quantum dots, or artificial atoms. Possibilities for quantum dots that may form the basis of new generations of lasers; Importance of understanding the internal electronic structure of the dots; Observations of electrons and holes in a single quantum dot; Hidden symmetries in the energy function that describes the multi-particle system.

The optical performance of external-cavity lasers based on InAs/InGaAsP quantum dot laser diodes is investigated. The broad electroluminescence reveals a gain spectrum with full width at half maximum of at least 175 nm. By fabricating as-cleaved ridge lasers of different length and width, tuning ranges as high as 110 nm have been achieved at wavelengths encompassing 1.55 μm. The tuning ranges and efficiencies obtained are limited by internal losses and competition between the external-cavity lasing modes and the laser diode natural lasing modes. The laser diode length is found to affect both the wavelength tuning range and the threshold current density, which is consistent with a quantum-dot type density of states. [ABSTRACT FROM AUTHOR]

Five stacked layers of InAs quantum dots (QDs) embedded in quaternary InGaAsP are grown on (100) InP substrate to form a laser diode. The QD ensemble has a density of 1.5×10[sup 10] cm[sup -2] and emits light at ∼1.6 μm at 77 K. Lasing wavelength and threshold current density can be shifted by changing the cavity length of the laser diode and the latter reaches a value as low as 49 A/cm2 at 77 K for a gate size of 2000 μm×150 μm. Temperature dependence of the threshold current is observed implying the presence of thermionic emission increasing with temperature. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We study how the optical properties of InAs self-assembled quantum dots (QDs) grown on GaAs substrate are affected when using AlGaAs barriers to increase the carrier confinement. The inhomogeneous broadening of the QD ensemble is found to increase when ternary aluminum alloys are used next to or within the QDs. By growing thin GaAs spacers to separate the QDs from the AlGaAs barriers, we obtain QD ensembles which exhibit little photoluminescence quenching and well-defined excited states up to room temperature. Postgrowth rapid thermal annealing is then used to intermix these InAs/GaAs/AlGaAs QDs and diffuse the Al towards the QDs. In contrast with QDs having thick binary GaAs barriers, the inhomogeneous broadening of QDs with nearby AlGaAs barriers is not decreased with intermixing, leading to unresolved excited state peaks when the interdiffusion length becomes comparable to the GaAs spacer thickness. © 2001 American Institute of Physics. We study how the optical properties of InAs self-assembled quantum dots (QDs) grown on GaAs substrate are affected when using AlGaAs barriers to increase the carrier confinement. The inhomogeneous broadening of the QD ensemble is found to increase when ternary aluminum alloys are used next to or within the QDs. By growing thin GaAs spacers to separate the QDs from the AlGaAs barriers, we obtain QD ensembles which exhibit little photoluminescence quenching and well-defined excited states up to room temperature. Postgrowth rapid thermal annealing is then used to intermix these InAs/GaAs/AlGaAs QDs and diffuse the Al towards the QDs. In contrast with QDs having thick binary GaAs barriers, the inhomogeneous broadening of QDs with nearby AlGaAs barriers is not decreased with intermixing, leading to unresolved excited state peaks when the interdiffusion length becomes comparable to the GaAs spacer thickness. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses the effects of alloy intermixing in quantum-dot (QD) ensembles with sharp adjustable shells using the state-filling spectroscopy. Rapid thermal annealing on samples of self-assembled QD grown with different intersublevel energy spacings; Indication of the intense and sharp shell structures observed in photoluminescence.

Discusses the evolution of the energy levels in quantum dot ensembles with different densities. Lateral interactions for quantum dot spacings of hundreds of nanometers; Decrease in the photoluminescence intensity.

Describes the fabrication of quantum-dot laser diodes with up to five well-defined electronic shells using self-assembled quantum dots (QD) grown by molecular-beam epitaxy. Observation of lasing in the upper QD shells for small-gain media; Progression of lasing towards the QD ground states for longer cavity lengths.

Examines the indium arsenide self-assembled quantum dots (QD) on indium phosphide substrate by molecular beam epitaxy. Range of the spectral region; Characterization of QD using photoluminescence and transmission electron microscopy; Influence of buffer layer on the island formation of residual strain.

Investigates the energy levels of nanometer size indium gallium arsenide/ GaAs quantum dots (QD) and selective excitation of the photoluminescence. Synthesis of QD structures with diameters comparable to electrons' de Broglie wavelength; Use of coherent islanding growth effect in highly strained semiconductors; Observation of positions of QD peaks.

Examines the time-resolved optical characterization of indium gallium arsenide/gallium arsenide quantum dots. Details of the photoluminescence efficiency of the quantum dots; Increase in radiative decay at low temperature due to excitonic recombination; Analysis of the emission and absorption energies of the quantum dots.

Laser annealing of InAs/GaAs quantum-dot (QD) microstructures has been investigated for selective area tuning of their electronic shell structure. Extensive blueshifts of the QD excited states were observed following 20-40 s laser irradiation. In the most extreme case, we were able to shift the position of the ground state transition by 298 meV, i.e., to the spectral region where the photoluminescence signal originates from the as-grown InAs wetting layer. A reduction from ∼50 to 8 meV of the full width at half maximum of the PL peak corresponding to this transition indicates a drastic change in the structural characteristics of the investigated QD ensemble. The attractive feature of the laser-QD-intermixing technique is that it offers the possibility of obtaining targeted blueshifts and inter-sublevel energy spacing on the lateral scale required in the fabrication of QD-based integrated optoelectronic devices and, possibly, photonic band gap crystals. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We compare the degradation of InAs/GaAs quantum well (QW) and quantum dot (QD) laser diodes following irradiation by high energy (8.56 MeV) phosphorous ions. Over a fluence range of 10[sup 8]-10[sup 11] ions/cm[sup 2], the degradation of the low temperature QD photoluminescence and electroluminescence emission is greatly suppressed relative to that of QW based devices (x100 and x1000, respectively at the highest dose studied). Irradiated QD laser diodes demonstrated lasing action over the entire range of fluences, and 2 orders of magnitude beyond the maximum dose sustainable by QW devices. The improved damage response of QD based structures results from efficient collection and localization of electrons and holes by QDs in the active region, which limit carrier transfer to nonradiative centers. This work suggests the suitability of QD device architectures for use in radiation environments, and in high power applications, wherever nonradiative processes promote the degradation or failure of traditional QW devices. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The interaction between zero-dimensional states and surface states is studied using near-surface quantum dot (QD) ensembles with well-defined electronic shells. The inhomogeneous broadening of self-assembled InAs/GaAs QDs increases from ∼30 to more than ∼46 meV as the distance of the QDs from the surface is changed from 100 to 5.0 nm. Simultaneously, a decrease of the radiative recombination intensity by ∼3 orders of magnitude, and a red-shift of ∼65 meV are observed. For QDs capped with less than ∼10 nm, remarkable charge transfers between the QD and surface states lead to optical memory effects lasting over time-scales of several minutes. [ABSTRACT FROM AUTHOR]

Artificial molecules are studied using coupled quantum-dot (QD) ensembles with well-defined electronic shells. The coupling strength between the zero-dimensional states is varied by changing the distance between two layers of stacked self-assembled InAs/GaAs QDs. For strongly coupled QDs grown with a 4 nm spacer, state-filling spectroscopy reveals a shift of the QD symmetric state to lower energies by ∼23 meV. The wetting layer states are also strongly coupled because of the shallow confinement, resulting in a redshift of its symmetric state by ∼26 meV. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have examined the effect of thermally induced interdiffusion on the luminescence emission from red and infrared emitting self-assembled III–V quantum dots. Three different combinations of dot/barrier materials have been investigated: InAlAs/AlGaAs, InGaAs/AlGaAs and InGaAs/GaAs. In all cases, thermal intermixing was found to result in significant blueshifts of the photoluminescence (PL) emission. In addition, narrowing of the linewidth of the inhomogeneously broadened PL peak was observed. Both effects were found to be strongly dependent on the material system and average dot size. InAlAs/AlGaAs quantum dots exhibited the greatest linewidth reduction after intermixing, indicating this to be a promising method of achieving narrower luminescence lines for devices such as red-emitting zero-dimensional lasers. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

InAs/GaP semiconductor quantum dots (QDs) were spontaneously formed using direct island growth (Volmer–Weber) rather than Stranski–Krastanow (S-K) growth. Structural investigations of InAs/GaP QDs suggest kinetically limited growth and show a broad size distribution. Photoluminescence and cathodoluminescence spectroscopy reveal large inhomogeneous broadening with the emission peak centering at 1.7 eV. Device applications exploiting broad optical emission in QDs are discussed. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Surface photovoltage (SPV) measurements were used to study InAs/GaAs multilayered structures. Two types of sample grown by molecular beam epitaxy were investigated: a quantum dot sample containing a fifteen-layer stack of InAs quantum dots (QD), each separated by GaAs layers, and a multi-quantum well sample. SPV spectra revealed two well-defined transitions and a broad peak. These features were associated with phenomena occurring in the InAs wetting layer-based quantum wells, the InAs-based QDs, and carrier transport within the GaAs buffer layer. A fitting of the Aspnes line shape function to the first derivative of the SPV curves was used to determine the energy and broadening parameter of each transition. Temperature-dependent SPV measurements also revealed that the activation mechanism for the low energy peak was due to carrier emission over the quantum well hetero-barrier. In summary, SPV was shown to be a powerful technique for studying carrier generation and recombination, as well as transport in low dimensional structures. [Copyright &y& Elsevier]

We present results of calculations and experiments on photo-current spectroscopy of modulation doped InAs self-assembled quantum dots. We derive an expression for the transition probability describing the dependence of the photo-current on the parameters of the dot, the number of carriers, and the polarization and energy of incident photons. The results of preliminary calculations are compared with experiments, which clearly show the dependence on photon energy and polarization. [Copyright &y& Elsevier]

Abstract: We measure the dephasing time of the exciton ground state transition in InGaAs quantum dots (QD) and quantum dot molecules (QDM) using a sensitive four-wave mixing technique. In the QDs we find experimental evidence that the dephasing time is given only by the radiative lifetime at low temperatures. We demonstrate the tunability of the radiatively limited dephasing time from 400ps up to 2ns in a series of annealed QDs with increasing energy separation of 69–330meV from the wetting layer continuum. Furthermore, the distribution of the fine-structure splitting and of the biexciton binding energy is measured. decreases from 96 to with increasing annealing temperature, indicating an improving circular symmetry of the in-plane confinement potential. The biexciton binding energy shows only a weak dependence on the confinement energy, which we attribute to a compensation between decreasing confinement and decreasing separation of electron and hole. In the QDM we measured the exciton dephasing as function of interdot barrier thickness in the temperature range from 5 to 60K. At 5K dephasing times of several hundred picoseconds are found. Moreover, a systematic dependence of the dephasing dynamics on the barrier thickness is observed, showing how the quantum mechanical coupling in the molecules affects the exciton lifetime and acoustic-phonon interaction. [Copyright &y& Elsevier]

Abstract: The optical performance of InAs/InGaAsP quantum dot (QD) lasers grown on (100) InP was studied for three different material structures. The most efficient QD laser structure, having a threshold current of ∼107mA and an external differential quantum efficiency of 9.4% at room temperature, was used to form the active region of a grating-coupled external cavity tunable laser. A tuning range of 110nm was demonstrated, which was mainly limited by the mirror and internal losses of the uncoated laser diode. Rapid state-filling of the QDs was also demonstrated by observing the evolution of the spectra with increasing injected current. [Copyright &y& Elsevier]

Abstract: During the recent years semiconductor nanostructures have attracted considerable interest with respect to potential applications in quantum information processing. In particular, quantum dot molecules have been suggested to provide the building block of a quantum computer: forming quantum gates due to coherent coupling of two dots. The characteristic dependence of the splitting of ‘bonding’ and ‘anti-bonding’ states suggests coherent coupling of two InAs/GaAs quantum dots. Anti-crossings in the fine structure of excitons due to mixing of optically bright and dark states have been observed in Faraday configuration. In Voigt configuration the diamagnetic shift of the quantum dot molecule is enhanced compared to a single quantum dot. These findings altogether demonstrate the coherent coupling of exciton states in quantum dot molecules. [Copyright &y& Elsevier]

Self-assembled InAs/GaAs quantum dot molecules have been studied spectroscopically in order to obtain a comprehensive picture of the exciton states confined in them. Photoluminescence studies show a characteristic splitting of the quantum dot shells, which clearly hints at a tunnel coupling of the dots. This coupling was confirmed by exciton fine structure studies, where distinct anticrossings are observed in its magnetic field dispersion. These avoided crossings would not occur if the two dots were decoupled. Four-wave mixing studies show a strong shortening of the exciton dephasing in the molecules as compared to quantum dots which appears to be problematic for application of excitons as the genuine bits in quantum information processing. Finally, we demonstrate that the molecule coupling can be controlled by applying an electric field along the molecule axis. [Copyright &y& Elsevier]

We explore growth effects leading to size and compositional limitations in the production of self-assembled quantum dots (QD) emitting at long wavelengths. Molecular beam epitaxy grown QDs are studied as a function of arsenic pressure at a specific InAs coverage, and as a function of InAs coverage for three arsenic pressures. As a function of increasing the arsenic pressure used in QD growth, the photoluminescence (PL) of capped QDs is first redshifted at low arsenic pressures, and then blueshifted at high arsenic pressures. Microscopy of uncapped QDs shows that as the arsenic pressure increases, the QD density increases while the average QD width and height decrease monotonically; these trends are consistent with the shift in PL for the high arsenic pressure samples, but are inconsistent with the shift in PL for the low-pressure samples. This points to a modification of the QDs during capping. We discuss prior reports pertaining to arsenic pressure and capping effects, and in this context describe our observations of the effects of adjusting the arsenic pressure on the formation of QDs and the mechanism by which QDs may be modified during capping. [Copyright &y& Elsevier]

We summarize here results of calculations and experiments on electron and valence hole states in a single pair of vertically stacked and electronically coupled InAs self-assembled quantum dots. In perfectly aligned quantum dots one can relate an electron–hole complex to a pair of entangled qubits. The information carried by individual qubit is related to the quantum dot index (isospin) of individual carrier. The quality of fabricated quantum dot molecules is identified from the exciton fine structure in a magnetic field. [Copyright &y& Elsevier]

We have performed magneto-photoluminescence spectroscopy of self-assembled In(Ga)As/GaAs single quantum dots. Both strength and orientation of the magnetic field have been varied. Symmetry of the dot structures as well as its breaking cause characteristic features in the optical spectra, which are determined by the electron–hole exchange and the Zeeman interaction of the carriers. The symmetry breaking can be either dot inherent due to geometry asymmetries or it can be obtained by applying a magnetic field with an orientation different from the dot symmetry axis. [Copyright &y& Elsevier]

The exciton states in self-assembled InAs/GaAs quantum dot molecules have been studied by optical spectroscopy and the results have been compared to detailed theoretical calculations. We demonstrate that excitons in symmetric molecule structures form entangled quantum states with the degree of entanglement controlled by tunneling and Coulomb interaction. [Copyright &y& Elsevier]

Five stacked layers of InAs quantum dots (QDs) embedded in quaternary InGaAsP were grown on (100) InP substrate to form a laser diode. Two methods were used to achieve tuning: changing the laser cavity length or varying the temperature. Stimulated emission between ∼1.51 and ∼1.64 μm was observed depending on the cavity length and the QD barrier height. The threshold current density was decreased for longer cavities to a value as low as 49 A/cm2 at 77 K. The relation between temperature and lasing peak wavelength was measured to be ∼0.21 nm/K leading to room temperature lasing at ∼1.61 μm. [Copyright &y& Elsevier]

We discuss key issues related to quantum dot infrared photodetectors. These are the normal incidence response, the dark current, and the responsivity and detectivity. It is argued that the present devices have not fully demonstrated the potential advantages. The dominant infrared response in devices so far is polarized in the growth direction. The observed dark currents are several orders of magnitude higher than those for quantum well photodetectors; while ideally they should be lower. The areas that need improvements are pointed out. [Copyright &y& Elsevier]