Height Of Quantum Dots Research Articles

Electric field effects on excitons in cylindrical quantum dots with asymmetric axial potential. Influence on the nonlinear optical properties

The exciton binding energy in an asymmetrical GaAs–Ga1−x AlxAs cylindrical quantum dot is studied with the use of the effective mass approximation and a variational calculation procedure. The influence on these properties of the application of an electric field along the growth direction of the cylinder is particularly considered. It is shown that for zero applied electric field the binding energy is a decreasing function of the quantum dot radius and height. Given a fixed geometric configuration, this quantity becomes a decreasing function of the electric field strength. In addition, the exciton-related nonlinear optical absorption and optical rectification resonant peaks are reported as functions of quantum dots dimensions and the aluminum molar fraction in the potential barrier regions.

Correlated electron—hole transitions in wurtzite GaN quantum dots: the effects of strain and hydrostatic pressure

Within the effective-mass and finite-height potential barrier approximation, a theoretical study of the effects of strain and hydrostatic pressure on the exciton emission wavelength and electron—hole recombination rate in wurtzite cylindrical GaN/AlxGa1−xN quantum dots (QDs) is performed using a variational approach. Numerical results show that the emission wavelength with strain effect is higher than that without strain effect when the QD height is large (> 3.8 nm), but the status is opposite when the QD height is small (< 3.8 nm). The height of GaN QDs must be less than 5.5 nm for an efficient electron—hole recombination process due to the strain effect. The emission wavelength decreases linearly and the electron—hole recombination rate increases almost linearly with applied hydrostatic pressure. The hydrostatic pressure has a remarkable influence on the emission wavelength for large QDs, and has a significant influence on the electron—hole recombination rate for small QDs. Furthermore, the present numerical outcomes are in qualitative agreement with previous experimental findings under zero pressure.

Temperature dependence of the optical properties of high-density GaAs quantum dots

We investigate the effect of the quantum dot (QD) density on the thermal escape and the retrapping processes of carriers for unstrained GaAs/AlGaAs QDs through temperature-dependent photoluminescence measurements. We fabricated high-density GaAs QDs (8.4 × 1010/cm2, dot-dot distance ∼34 nm) on an Al0.3Ga0.7As/GaAs (111)A surface by using droplet epitaxy. The average lateral size and height of the GaAs QDs are 24 and 6 nm, respectively. Temperature-dependent photoluminescence (PL) studies show that high-density GaAs QDs undergo a sigmoidal-shape energy shift. The sigmoidal dependence of the PL peak energy can be explained by thermal escaping of carriers followed by re-trapping by QDs. Our analysis indicates that the re-trapping probability of thermally-escaped carriers increases with decreasing dot-to-dot distance (corresponding to an increase in the QD density).

Kinetic Monte Carlo simulations and cross-sectional scanning tunneling microscopy as tools to investigate the heteroepitaxial capping of self-assembled quantum dots

The growth of self-assembled quantum dots has been intensively studied in the last decade. Despite substantial efforts, a number of details of the growth process remain unknown. The reason is the inability of current characterization techniques to image the growth process in real time. In this work, this limitation is alleviated by the use of kinetic Monte Carlo simulations in conjunction with cross-sectional scanning tunneling microscopy. The two techniques are used to study the method of strain engineering as a procedure to control the height of quantum dots. We show that fully three-dimensional kinetic Monte Carlo simulations can be matched with the experimentally obtained morphology of buried quantum dots and that combination of the two techniques provides details of the growth process that hitherto could not be obtained.

Gfactors and diamagnetic coefficients of electrons, holes, and excitons in InAs/InP quantum dots

The electron, hole, and exciton $g$ factors and diamagnetic coefficients have been calculated using envelope-function theory for cylindrical InAs/InP quantum dots in the presence of a magnetic field parallel to the dot symmetry axis. A clear connection is established between the electron $g$ factor and the amplitude of those valence-state envelope functions that possess nonzero orbital momentum associated with the envelope function. The dependence of the exciton diamagnetic coefficients on the quantum dot height is found to correlate with the energy dependence of the effective mass. Calculated exciton $g$ factor and diamagnetic coefficients, constructed from the values associated with the electron and hole constituents of the exciton, match experimental data well, however including the Coulomb interaction between the electron and hole states improves the agreement. Remote-band contributions to the valence-band electronic structure, included perturbatively, reduce the agreement between theory and experiment.

Excitons in cylindrical GaAs–Ga1−xAlxAs quantum dots under applied electric field

The exciton binding energy and photoluminescence energy transition in a GaAs-Ga1−xAlxAs cylindrical quantum dot are studied with the use of the effective mass approximation and a variational calculation procedure. The influence of these properties on the application of an electric field along the growth direction of the cylinder is particularly considered. It is shown that for zero applied field the binding energy and the photoluminescence energy transition are decreasing functions of the quantum dot radius and height. Given a fixed geometric configuration, both quantities then become decreasing functions of the electric field strength as well.

Coexistence of type-I and type-II band alignments in antimony-incorporated InAsSb quantum dot nanostructures

Antimony-incorporated InAsSb quantum dots (QDs) are grown by molecular beam epitaxy on GaAs(001) substrates. The QD density increases ∼7 times while the QD height decreases ∼50% due to the increase of QD nucleation sites after Sb incorporation into the GaAs buffer layer and into the InAs QDs. These Sb-incorporated InAsSb QDs show red-shift in the photoluminescence (PL) spectrum and large energy separation between confined energy levels. More interestingly, besides the typical type-I QD transition, an additional peak from the recombination at wetting layer interface develops as the excitation laser intensity increases. This peak clearly exhibits type-II characteristics from the measurement of a large blue-shift of the PL peak and a long PL decay time. Finally, the mechanism of the coexistence of type-I and type-II band alignments is discussed.

Growth evolution of Ge quantum dot modulated by the atom bombardment during ion beam sputtering deposition

The Ge quantum dots on Si substrate are prepared by ion beam sputtering deposition (IBSD). The growth evolution is observed to experience two stages with Ge coverage (θ) increasing. When θ increases from 6 monolayers (ML) to 10.5 ML, the average base width and height of quantum dots both increase, and the dome shape dots with small aspect ratio values are obtained. As the dots grow up, Ge atoms are also accumulated in the wetting layer, which contributes to the observed quantum dot density increasing mildly during this stage. When θ is in a range from 11.5 ML to 17 ML, vertical growth dominates the dot evolution. Another dome shape quantum dots are prepared with large aspect ratio values. Ge coverage gain results in the dot density increasing rapidly. A wetting layer decomposition process is demonstrated to give significant effect on that. The growth transition occurs as θ increases from 10.5 ML to 11.5 ML, and the dot density is enhanced 6.4 times in this course. So it is concluded that the evolution of Ge quantum dot prepared by IBSD is very different from that deposited on the thermal equilibrium condition. The observed characters of the dot shape and size distribution result from the kinetic behaviors of the surface atoms which are restricted by the thermodynamic limitation. Ge coverage is the one of the most important factors which can change the free energy. On the other hand, the energic sputtered atom bombardment enhances surface diffusion and defers nucleation of three-dimensional islands until the superstrain wetting layer is formed, which can also change the system free energy and the surface atom kinetic behaviors. So the growth evolution of Ge quantum dots prepared by IBSD is related so much with the effect of atom bombardment on the quantum dot growth.

Exciton states and optical properties in zinc-blende GaN/AlGaN quantum dot

Within the framework of effective-mass approximation, exciton states confined in zinc-blende GaN/AlGaN quantum dot (QD) are investigated by a variational approach, including the three-dimensional confinement of electron and hole in the QD and the finite band offset. Numerical results show that both the exciton binding energy and the interband emission energy decrease when QD height (or radius) increases. Our theoretical results are in agreement with the experimental measurements.

Density Increase of Upper Quantum Dots in Dual InGaN Quantum-Dot Layers

Single and dual layers of InGaN quantum dots (QDs) are grown by metal organic chemical vapor deposition. In the former, the density, average height and diameter of QDs are 1.3 × 109 cm−2, 0.93nm and 65.1 nm, respectively. The latter is grown under the same conditions and possesses a 20 nm low-temperature grown GaN barrier between two layers. The density, average height and diameter of QDs in the upper layer are 2.6 × 1010 cm−2, 4.6 nm and 81.3 nm, respectively. Two reasons are proposed to explain the QD density increase in the upper layer. First, the strain accumulation in the upper layer is higher, leading to a stronger three-dimensional growth. Second, the GaN barrier beneath the upper layer is so rough it induces growth QDs.

Near Infrared InAs/GaAsSb Quantum Dot Light Emitting Diodes

A series of light-emitting diodes (LEDs) with active layers based on InAs quantum dots (QDs) covered by GaAsSb capping layers is presented. Varying the Sb content in the capping layer from <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\sim}{2}$</tex></formula> to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}28\%$</tex></formula> , room temperature electroluminescence (EL) from 1.15 to 1.5 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> is obtained. The external efficiency of the devices, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\eta_{\rm ext}$</tex></formula> , increases as the Sb is increased up to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{15\%}$</tex></formula> and then decreases for higher Sb contents, consistently with the reported increase of QD height with the Sb content up to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{15\%}$</tex></formula> and the band alignment transition from type I to type II above <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{15\%}$</tex></formula> Sb. An analysis of the EL and photocurrent spectra shows that the emission from type I LEDs originates from the recombination between electrons and holes confined in the QDs. On the other hand, the EL from the type II devices is the combination of two different processes. First, recombination between electrons confined in the QDs and holes at the capping layer. Second, a type I-like recombination of electrons from the QDs and holes residing in extended levels of the quantum well composed by the capping layer and the QDs. The mechanisms responsible for the thermal quenching of the EL are also studied. Escape of holes from the QD to the capping layer is identified as the dominant mechanism for the type I devices, whereas in type II structures it is the escape of electrons from QD excited levels to the barrier which dominates.

Nucleation Sequence of InAs Quantum Dots on Cross-Hatch Patterns

InAs quantum dots (QDs) are grown via molecular beam epitaxy on cross-hatch pattern (CHP) templates that result from lattice-mismatched epitaxy of In(x)Ga(1-x)As on (100)-GaAs substrates. Growth of InAs on low-(x = 0.10) and medium-(x = 0.13) mismatch CHPs with InAs thickness grading from sub- to beyond critical thickness show different stages of QD nucleation that is dictated mainly by surface steps. Tangential surface stress fields arising from the buried network of (110) misfit dislocations (MDs) at the InGaAs/GaAs interface are simulated in two dimensions and found to have a direct correlation to QD height at various locations, implying sequential QD nucleation at the surface intersection of the glide plane of dislocation T-section, cross-hatch intersection, threading dislocation, [1-10] MD line, and [110] MD line, followed by nucleation on the flat areas. Deviations from this nominal sequence is possible due to material anisotropy and are accounted for in the stress calculation by dislocation-specific scaling factors.

Strong suppression of internal electric field in GaN/AlGaN multi-layer quantum dots in nanowires

Photoluminescence (PL) studies of GaN/AlxGa1−xN quantum dots (QDs) in nanowires demonstrate an efficient carrier confinement, resulting in thermally stable decay times up to 300 K. The evolution of the PL transition energy as a function of both the QD height and the Al mole fraction in the barriers, as well as the evolution of the decay time as a function of the QD height, point out that a built-in electric field is significantly smaller than the value expected from the spontaneous polarization discontinuity. This is explained by the uniaxial compressive strain resulting from the spontaneously formed Al-rich shell that envelops the QD stack.

Exciton states and interband optical transitions in wurtzite InGaN/GaN quantum dot nanowire heterostructures

Within the framework of the effective-mass approximation, the exciton states and interband optical transitions in In x Ga 1− x N/GaN strained quantum dot (QD) nanowire heterostructures are investigated using a variational method, in which the important built-in electric field (BEF) effects, dielectric-constant mismatch and three-dimensional confinement of the electron and hole in In x Ga 1− x N QDs are considered. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the exciton oscillator strength.

Voltage-controlled electron tunneling from a single self-assembled quantum dot embedded in a two-dimensional-electron-gas-based photovoltaic cell

We perform high-resolution photocurrent (PC) spectroscopy to investigate resonantly the neutral exciton ground-state (X0) in a single InAs/GaAs self-assembled quantum dot (QD) embedded in the intrinsic region of an n-i-Schottky photodiode based on a two-dimensional electron gas (2DEG), which was formed from a Si δ-doped GaAs layer. Using such a device, a single-QD PC spectrum of X0 is measured by sweeping the bias-dependent X0 transition energy through that of a fixed narrow-bandwidth laser via the quantum-confined Stark effect (QCSE). By repeating such a measurement for a series of laser energies, a precise relationship between the X0 transition energy and bias voltage is then obtained. Taking into account power broadening of the X0 absorption peak, this allows for high-resolution measurements of the X0 homogeneous linewidth and, hence, the electron tunneling rate. The electron tunneling rate is measured as a function of the vertical electric field and described accurately by a theoretical model, yielding information about the electron confinement energy and QD height. We demonstrate that our devices can operate as 2DEG-based QD photovoltaic cells and conclude by proposing two optical spintronic devices that are now feasible.

Electrically tunable hole tunnelling from a single self-assembled quantum dot embedded in an n-i-Schottky photovoltaic cell

We perform excitation-intensity-dependent measurements of the neutral exciton (X0) photocurrent (PC) peak amplitude from a single InAs/GaAs self-assembled quantum dot (QD) embedded in the intrinsic region of an n-i-Schottky photodiode. Since resonant laser-excitation of the X0 transition cannot occur until the comparatively slow hole tunnels out of the QD, we observe a saturation of the PC peak amplitude towards high excitation-intensities, allowing us to determine the hole tunnelling time by fitting with an appropriate theoretical model. By repeating this measurement for a range of bias voltages, we obtain the hole tunnelling time as a function of vertical electric field, showing that it can be tuned by nearly two orders of magnitude. Finally, we find that the hole tunnelling rate can be described accurately by a theoretical model based on a Wentzel-Kramers-Brillouin approximation to yield precise values for the QD height and hole confinement potential.

Radiative life time of an exciton confined in a strained GaN/Ga1-xAlxN cylindrical dot: built-in electric field effects

The binding energy of an exciton in a wurtzite GaN/GaAlN strained cylindrical quantum dot is investigated theoretically. The strong built-in electric field due to the spontaneous and piezoelectric polarizations of a GaN/GaAlN quantum dot is included. Numerical calculations are performed using a variational procedure within the single band effective mass approximation. Valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The exciton oscillator strength and the exciton lifetime for radiative recombination each as a function of dot radius have been computed. The result elucidates that the strong built-in electric field influences the oscillator strength and the recombination life time of the exciton. It is observed that the ground state exciton binding energy and the interband emission energy increase when the cylindrical quantum dot height or radius is decreased, and that the exciton binding energy, the oscillator strength and the radiative lifetime each as a function of structural parameters (height and radius) sensitively depend on the strong built-in electric field. The obtained results are useful for the design of some opto-photoelectronic devices.

Two-phonon processes of intraband relaxation in the terahertz regime in quantumdots

We theoretically investigate the intraband relaxation of quantum dots in the terahertzregime due to two acoustic phonon scattering by applying a lattice relaxationapproach based on the deformation potential coupling between electrons andacoustic phonons. In particular, we find that the relaxation time depends stronglyon the ratio of two acoustic phonons. The influences of the energy separationbetween the ground and first excited state, the quantum dot height, and the latticetemperature on the relaxation time are also discussed. Our theoretical results not onlygive a reasonable explanation for the current experimental measurement but alsoprovide some insight into two-phonon intraband relaxation in quantum dots.

Exciton states and optical transitions in InGaN/GaN quantum dot nanowire heterostructures: Strong built-in electric field and dielectric mismatch effects

Considering the strong built-in electric field (BEF), dielectric-constant mismatch and 3D confinement of the electron and hole, the exciton states and interband optical transitions in [0 0 0 1]-oriented Ga-rich wurtzite In x Ga 1− x N/GaN strained quantum dot (QD) nanowire heterostructures are investigated theoretically using a variational approach under the effective mass approximation. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron–hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the radiative decay time. Our calculations show that the radiative decay time of the redshifted transitions is large and increases almost exponentially when the QD height increases, which is in good agreement with the previous experimental and theoretical results.

Energy Gap Dependence on Mn Content in a Diluted Magnetic Quantum Dot

Positively charged donor exciton binding energy is computed as a function of quantum-dot size within the single band effective mass approximation for different Mn contents in Cd1−xinMnxin Te/Cd1−xoutMnxout Te. The exciton bound polaron is computed for 0 ≤ x ≤ 0.08, on the Mn mole fraction. We determine the energy gap using the mean field approximation and incorporate the exchange interaction between the carrier and the magnetic impurity. The interband emission energy is studied with the height and radius of the cylindrical quantum dot. Valence-band anisotropy is included in our theoretical model using different hole masses in different spatial directions. Spin polaronic shifts as functions of quantum-dot radius and Mn concentration are estimated using the mean field theory. It is found that (i) the energy gap depends on the Mn mole fraction, (ii) it increases linearly with an increase in Mn content, and (iii) the effect is more pronounced for a narrow dot, showing the quantum size effects. Our results are in good agreement with other recently published reports.