Four-electron quantum dots in magnetic fields

Making use of hyperspherical coordinates, the energy spectrum of a four-electron configuration in a harmonic quantum dot (QD) as a function of the dot size and the strength of a magnetic field are investigated. Discontinuous ground-state transitions induced by an external magnetic field and the quantum dot size effect are shown. An important aspect of the size effect is the crossover of energy levels. The present results are useful to understand the optical and magnetic properties of QD materials.     

Inelastic light scattering from electronic excitations in quantum dots

A review is given of progress in our understanding of the electronic excitations in semiconductor quantum dots as studied by inelastic light scattering spectroscopy. Such experiments have revealed the characteristics of single particle, charge density, and spin density excitations of many-electron dots in zero and applied magnetic fields. Theoretical calculations, which reveal an electronic shell structure, are in general accord with the experimental results. Although the behaviour in a magnetic field is extremely complex, it is now feasible to study collective excitations from a range of strongly correlated ground states using this technique.     

Charge accumulation of quantum dots at room temperature

The accumulation of charge in InGaAs quantum dots has been measured at room temperature by the photoelectrochemical capacitance-voltage (CV) technique for the first time. A carrier per quantum dot ratio greater than four has been observed. The use of atomic force microscopy and low temperature and room temperature photoluminescence (PL) confirm the existence of quantum dots. Also, a possible excited state is indicated by room temperature PL in a sample with small quantum dots.     

Inelastic light scattering from electronic excitations in deep-etched quantum dots and wires

Resonant Raman spectroscopy of modulation-doped GaAs/AlGaAs multiple quantum well dots and wires is reported. Deep etching with a SiCl₄ reactive ion etching process achieved an excellent aspect ratio (>10:1) and low surface damage for dots and wires of sizes in the range 60–250 nm. A rich spectrum of single particle excitations was observed at Raman shifts in the range 1–35 meV for both dots and wires. Sharp resonances were found for the Raman intensities. The electronic scattering in wires exhibits distinct polarization properties in agreement with theoretical predictions and the spin density excitation energies are in reasonable agreement with Hartree approximation calculations. The dispersion of the intrasubband plasmon collective mode in 60 nm wires has been determined. The excitations in dots show a systematic shift to higher energy with decreasing dot diameter consistent with increased confinement. Magneto-Raman scattering from dot samples was also investigated at magnetic fields up to 12 T and the excitation spectra show level splitting, level crossing and mode softening with increasing magnetic field.     

Charge transport in lead sulfide quantum dots/phthalocyanines hybrid nanocomposites

A hybrid composite of non-aggregated lead sulfide (PbS) nanoparticles of average size 5.8 ± 1 nm embedded within a film of an octaalkyl substituted metal-free phthalocyanine (Compound 2) was prepared on interdigitated gold electrodes by mild acidic treatment of newly synthesised octasubstituted lead phthalocyanine analogue (Compound 1) in solid state phase. This nanocomposite film shows an enhancement of in-plane electrical conductivity over that of a film of octaalkyl substituted metal-free phthalocyanine alone by nearly 65%. This observation is consistent with the formation of charge complex compound as indicated by Raman and XPS data. The presence of PbS in the composite was examined on the basis of XRD peak positions which are comparable with those of bulk PbS. A band gap of 2.22 eV was calculated from optical absorption data using Tauc's law, implying quantum confinement. The mono dispersal behaviour of PbS nanoparticles was established from TEM and XRD studies. The hopping conduction mechanism is found to be primarily responsible for charge transport in the hybrid nanocomposite film with the hopping distance larger than PbS diameter.     

Conductivity of quantum wires in uniform magnetic fields

The features of the de conductivity of quantum wires in longitudinal and transverse magnetic fields are studied for degenerate and nondegenerate electron gas. The conductivity is calculated on the basis of the Kubo formalism with regard to the elastic scattering of charge carriers at long-wavelength lattice vibrations. The final theoretical results for the conductivity are compared to the experimental data. The suggested model of quantum wires allows, among other things, an interpretation of the nonmonotonic dependence of the transverse magnetoresistance on the magnetic field.     

Electron transport in unipolar heterostructure transistors with quantum dots in strong electric fields

A model for the explaining specific features of the electron transport in strong electric fields in the quantum-dot unipolar heterostructure transistor (AlGaAs/GaAs/InAs/GaAs/InAs) is presented. It is shown that the two-step shape of the output current-voltage characteristic I _D (V _D ) and the anomalous dependence of the drain current I _D on the gate voltage V _G are caused by the ionization of quantum dots in the strong electric field at the drain gate edge. The ionization of quantum dots sets in at the drain voltage V _D that exceeds the V_D1 value, at which the I _D (V _D ) dependence is saturated (the first step of the I-V characteristic). With the subsequent increase in V _D , i.e., for V _D >V_D1, the I _D (V _D ) dependence has a second abrupt rise due to the ionization of quantum dots, and then, for V _D =V_D2>V_D1, the current I _D is saturated for the second time (the second step in the current-voltage characteristic). It is suggested to use this phenomenon for the determining the population of quantum dots with electrons. The model presented also describes the twice-repeated variation in the sign of transconductance g _m =dI _D /dV _G as a function of V _G .     

Semiconductor quantum dots

Since the invention of semiconductor lasers, huge improvements in device performance have been achieved, and a large variety of specialized designs for different applications were conceived. Two major steps have played a key role in the improvement of device properties. The first step was the application of semiconductor heterostructures that allowed the separate optimization of optical and carrier confinement. The second step was the introduction of quantum films, also called quantum wells, in the carrier recombination zone (started in the 1980s). This permitted a strong reduction of threshold current density due to an increased density of states at the laser energy. This effect of increased density of states is related to the partial discretization of the allowed energy states of carriers, i.e., electrons and holes, and is based on quantum mechanical principles. One major advantage of quantum-dot structures results from the full three-dimensional carrier confinement on a nanometer scale. Therefore, a semiconductor quantum dots, InAs dots embedded in GaAs, behave like non- or weakly interacting single atoms. In addition, the realization of device-quality quantum dot structures became possible by the introduction of self-organized growth. Both, molecular beam epitaxy (MBE) and metal organic vapor phase epitaxy (MOVPE) techniques, which are capable of the controlled deposition of a fraction of an atomic monolayer, can be used.     

Long-time photoluminescence kinetics of InAs/AlAs quantum dots in a magnetic field

The influence of a magnetic field on the low-temperature photoluminescence (PL) kinetics of InAs/AlAs quantum dots is studied. It is found that the PL decay becomes faster upon application of the magnetic field. The results obtained are explained in the context of a model that considers the fine structure of exciton levels and their Zeeman splitting in the magnetic field.     

Self-assembled quantum dots

The greatest success in semiconductor lasers has been brought by the ability to artificially structure new materials on an atomic scale by using advanced crystal growth methods such as MBE and MOVPE. Laser performance successes gained using quantum wells in optoelectronic devices can be extended by adopting quantum wire and quantum dot structures. There have been several reports of successful lasing action in semiconductor dot structures within the past year. This article reviews the recent progress in the development of quantum dot lasers.     

Dynamic equilibrium of magnetic ions in Cd(Mn)Te quantum dots

Magneto optical micro-spectroscopy allowed us to access the magnetic field dependency of the photoluminescence of Cd(Mn)Te/ZnTe single quantum dots. We report a strongly non-linear behavior of the exciton and biexciton energies as a function of magnetic field which clearly differs from the expected and usually observed giant Zeeman effect. We demonstrate that the existence of a positive feedback loop between photocarriers and Mn spins results in heating the Mn system and can explain the observed phenomenon. This shows that controlling the coupling of the dot with its environment is a corner stone to achieve spin manipulation in quantum dots.     

Size dependence of the magnetic properties of electrochemically self-assembled Fe quantum dots

A quasi-periodic array of Fe quantum dots of cylindrical shape has been synthesized by electrodeposition of Fe in porous alumina. By controlling the fabrication parameters, we have controlled the length, diameter, and spacing of the dots. The magnetic properties are shown to depend on these parameters. It has been found that at room temperature, there exists a critical diameter of the dots for which the coercivity is a maximum. The largest value of coercivity obtained at room temperature is 2640 Oe which rises to 2900 Oe on annealing. At a low temperature of 5K, an increase in coercivity is observed for most of the samples. The largest value is 3800 Oe which rises to a value of 4100 Oe in the corresponding annealed counterpart. At 5K, no maximum is seen in the coercivity as a function of diameter. Instead, the coercivity is found to decrease with increasing diameter. This dependence of the coercivity on diameter is discussed in terms of localized reversal effects.     

Built-in electric fields in InAs quantum dots grown on (N11) GaAs substrates

We report on the optical properties and carrier kinetics of a set of InAs self-assembled quantum dots on (N11)A/B GaAs substrates by means of cw and time-resolved PL. The cw-PL spectra show a blue shift of the PL band on different (N11) QD structures when increasing the carrier photoinjection. This is attributed to a photoinduced screening of the quantum confined Stark shift of the QD optical transition due to a large built-in electric field. The presence of an internal electric field also induces intrinsic optical non-linearity in time-resolved measurements. The analysis of the recombination kinetics shows that the carrier screening occurs inside the QDs, thus demonstrating the intrinsic nature of the built-in field. The dependence of the internal field on the substrate orientation and termination agrees with the presence of piezoelectric field and permanent dipole moment inside the QDs.     

Effects of self-consistent electrostatic potential in quantum wells with several quantum confinement levels in high magnetic fields

The effect of self-consistent electrostatic potential on the spectrum of two-dimensional electron states in high magnetic fields is studied under the conditions where more than one quantum confinement subband is filled. The cases of magnetic field directed perpendicular to the quantum well plane and tilted magnetic field are considered. In the case of perpendicular magnetic field it is shown that two or more Landau levels that belong to different quantum confinement subbands can be degenerate in some ranges of concentrations (magnetic fields). The inclination of the magnetic field with respect to the growth direction produces opening of the energy gap between these levels; however, the gap as a function of concentration (magnetic field) remains almost constant in the same range of parameters.     

Effects of external fields on the excitonic emission from single InAs/GaAs quantum dots

A low-temperature micro-photoluminescence (μ-PL) investigation of InAs/GaAs quantum dots (QDs) exposed to a lateral external electric field is reported. It is demonstrated that the QDs PL signal could be increased several times by altering the external and/or the internal electric field. The internal field in the vicinity of the dots could be altered by means of an additional infra-red laser. We propose a model, which is based on an essentially faster lateral transport of the charge carriers achieved in an external electric field. Consequently, also the capture probability into the dots and subsequently the dot luminescence is also enhanced. The results obtained suggest that the lateral electric fields play a major role for the dot luminescence intensity measured in our experiment.     

Applications of colloidal quantum dots

This paper addresses a number of major trends underlying the continuing effort to realize practical optoelectronic, electronic, and information-processing devices based on ensembles of quantum dots assembled in a variety of matrix materials. The great diversity of such structures makes it possible to fabricate numerous ensemble-based devices for applications underlying photoluminescent devices, light-emitting diodes, displays, photodetectors, photovoltaic devices, and solar cells. In addition, the application of colloidal quantum dots to allied technologies such as nanobiotechnology is considered for the case of monitoring conformational changes in biomolecules using luminescent quantum dots.