Effects of size and shape on electronic states of quantum dots

A strain-modified, single-band, constant-potential three-dimensional model was applied to study the dependence of electronic states of InAs/GaAs quantum dots (QDs) of different shapes and sizes. The energy trend was found to decrease monotonically with increasing QD size (i.e.E ~ size ^?γ ) but exhibited minimum value at aspect ratio of approximately 0.5. The ground state energy for broad tip was found to be always lower than that of narrow tip. Thus, effort to alter the QD shape instead of the aspect ratio is proposed for longer wavelength emission with InAs/GaAs QDs. The energy dependency γ for volume was found to be approximately three times smaller than that for base length and height. A method was proposed to exploit this large difference for growth experimentalists to verify if the capped InAs QDs follow similar increase as the uncapped InAs QDs upon growth parameter variation.     

Excitonic and electronic states in ellipsoidal and semiellipsoidal quantum dots

A simple model is assumed to obtain analytical solutions of the Schrödinger equation in prolate spheroidal coordinates for the electron–hole pair confined to an ellipsoidal quantum dot (EQD) or to a semiellipsoidal quantum dot (SEQD). Numerical calculations are carried out to find the excitonic states as well as the electronic states decoupled from holes in such geometries. Their dependence on the inverse of the eccentricity of the ellipsoidal surfaces for different interfocal distances is investigated. The binding energy and the recombination radiation energy are calculated for GaAs and InAs QDs; the same dependences are also investigated. Comparison with previous calculations and experiments shows a good order-of-magnitude agreement. It is demonstrated that some of the available states in an EQD are forbidden in the SEQD and, consequently, some of the photoluminescence lines observed in the former case are suppressed in the latter geometry.     

Numerical simulation of a coupling effect on electronic states in quantum dots

Lasers operating at 1.3 μm have attracted considerable attention owing to their potential to provide efficient light sources for next-generation high-speed communication systems. InAs/GaAs quantum dots (QDs) were pointed out as a reliable low-cost way to attain this goal. However, due to the lattice mismatch, the accumulation of strain by stacking the QDs can cause dislocations that significantly degrade the performance of the lasers. In order to reduce this strain, a promising method is the use of InAs QDs embedded in InGaAs layers. The capping of the QD layer with InGaAs is able to tune the emission toward longer and controllable wave-lengths between 1.1 and 1.5 μm. In this work, using the effective-mass envelope-function theory, we investigated theoretically the optical properties of coupled InAs/GaAs strained QDs based structures emitting around 1.33 μm. The calculation was performed by the resolution of the 3D Schrödinger equation. The energy levels of confined carriers and the optical transition energy have been investigated. The oscillator strengths of this transition have been studied with and without taking into account the strain effect in the calculations. The information derived from the present study shows that the InGaAs capping layer may have profound consequences as regards the performance of an InAs/GaAs QD based laser. Based on the present results, we hope that the present work make a contribution to experimental studies of InAs/GaAs QD based structures, namely the optoelectronic applications concerning infrared and mid-infrared spectral regions as well as the solar cells.     

Influence of quantum dots shape approximation on size reconstructed from atomic force microscopy measurements

In this work we discuss the influence of the atomic force microscopy (AFM) probe tip geometry and the object — quantum dot form on the quantum dots dimension in the growth plane reconstructed from the AFM measurements. It is shown that ignoring the geometry of the probe tip and the quantum dot leads to significant differences between dimensions obtained from the AFM measurements and the real dimensions. Inaccuracies in QD size determination of the nano-objects from AFM measurements are defined.     

Effects of Coulomb interactions on spin states in vertical semiconductor quantum dots

The effects of direct Coulomb and exchange interactions on spin states are studied for quantum dots contained in circular and rectangular mesas. For a circular mesa a spin-triplet favored by these interactions is observed at zero and nonzero magnetic fields. We tune and measure the relative strengths of these interactions as a function of the number of confined electrons. We find that electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle state degeneracy, and find that the spin-configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states. For a rectangular mesa we observe no signatures of the spin-triplet at zero magnetic field. Due to the anisotropy in the lateral confinement single-particle state degeneracy present in the circular mesa is lifted, and Coulomb interactions become weak. We evaluate the degree of the anisotropy by measuring the magnetic field dependence of the energy spectrum for the ground and excited states, and find that at zero magnetic field the spin-singlet is more significantly favored by the lifting of level degeneracy than by the reduction in the Coulomb interaction. We also find that the spin-triplet is recovered by adjusting the level degeneracy with magnetic field. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Effects of annealing temperature on shape transformation and optical properties of germanium quantum dots

The influences of thermal annealing on the structural and optical features of radio frequency(rf) magnetron sputtered self-assembled Ge quantum dots(QDs) on Si(100) are investigated.Preferentially oriented structures of Ge along the(220) and(111) directions together with peak shift and reduced strain(4.9%to 2.7%) due to post-annealing at 650 ℃ are discerned from x-ray differaction(XRD) measurement.Atomic force microscopy(AFM) images for both pre-annealed and post-annealed(650 ℃) samples reveal pyramidal-shaped QDs(density ~ 0.26×10~(11) cm~(-2)) and dome-shape morphologies with relatively high density ~ 0.92×10~(11) cm~(-2),respectively.This shape transformation is attributed to the mechanism of inter-diffusion of Si in Ge interfacial intermixing and strain non-uniformity.The annealing temperature assisted QDs structural evolution is explained using the theory of nucleation and growth kinetics where free energy minimization plays a pivotal role.The observed red-shift ~ 0.05 eV in addition to the narrowing of the photoluminescence peaks results from thermal annealing,and is related to the effect of quantum confinement.Furthermore,the appearance of a blue-violet emission peak is ascribed to the recombination of the localized electrons in the Ge-QDs/SiO_2 or GeO_x and holes in the ground state of Ge dots.Raman spectra of both samples exhibit an intense Ge-Ge optical phonon mode which shifts towards higher frequency compared with those of the bulk counterpart.An experimental Raman profile is fitted to the models of phonon confinement and size distribution combined with phonon confinement to estimate the mean dot sizes.A correlation between thermal annealing and modifications of the structural and optical behavior of Ge QDs is established.Tunable growth of Ge QDs with superior properties suitable for optoelectronic applications is demonstrated.     

Effects of crystal shape on the energy levels of zero-dimensional PbS quantum dots

Nanometer-size PbS quantum dots have been made by electrodeposition on a Au(111) substrate. The deposited nanocrystals have a flattened cubic shape. We probed the single-electron energy-level spectrum of individual quantum dots by scanning tunneling spectroscopy and found that it deviates strongly from that of spherical PbS quantum dots. The measured energy-level spectrum is successfully explained by considering strong confinement in a flattened cubic box.     

Half-integer filling-factor states in quantum dots

The emergence of half-integer filling-factor states, such as upsilon=5/2 and 7/2, is found in quantum dots by using numerical many-electron methods. These states have interesting similarities and differences with their counterstates found in the two-dimensional electron gas. The upsilon=1/2 states in quantum dots are shown to have high overlaps with the composite fermion states. The lower overlap of the Pfaffian state indicates that electrons might not be paired in quantum dot geometry. The predicted upsilon=5/2 state has a high spin polarization, which may have an impact on the spin transport through quantum dot devices.     

Effects of nanocrystal shape on the physical properties of colloidal ZnO quantum dots

The effects of both nanocrystal shape and applied magnetic field on the electron energy spectra of colloidal ZnO quantum dots have been investigated in the frame of finite element method, using nonuniform triangular elements. Four shapes of quantum dots (spherical, ellipsoidal, rod-shaped, and lens-shaped) were studied. It was found that the physical properties of the semiconductor quantum dots could be manipulated by changing their size and/or their shape. The energies of an electron increase as one reduces the quantum dot shape symmetry from spherical towards the lens-shaped. The magnetic field effect strongly interplays with the nanocrystal size and the nanocrystal shape effects. Such interplay has been attributed to the competition of the quantum confinement effect introduced by the barrier potential and the quantum confinement effect introduced by the applied magnetic field.     

Optical switching of quantum states inside self-assembled quantum dots

Photoluminescence and excitation of photoluminescence spectroscopy have been performed for two kinds of single InAs self-assembled quantum dots grown on GaAs. The presence of unintentional impurities (donors and acceptors) offers the possibility to switch from negative to positively charged excitons by selectively exciting impurity related optical transitions.     

External electric field effects on the electronic and hydrogenic impurity states in ellipsoidal and semi-ellipsoidal quantum dots

In the effective mass approximation, energy eigenvalues of an electron confined in ellipsoidal and semi-ellipsoidal quantum dots, with and without hydrogenic impurity, under the influence of an external electric field have been investigated, using the matrix diagonalization method. The lower-laying states of the electron as functions of the electric field strength, the dot size and its geometry are calculated. Our results show that the electronic states are strongly affected by the applied electric field, the size and the geometry of the dot.     

Electronic states and shapes of silicon quantum dots

A curviform surface breaks the symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in the bandgap. The calculation results show that the bonding energy and electronic states of silicon quantum dots are different on various curved surfaces, for example, a Si-O-Si bridge bond on curved surface provides localized levels in bandgap and its bonding energy is shallower than that on the facet. The red-shifting ofthe photoluminescence spectrum on smaller silicon quantum dots can be explained by the curved surface effect. Experiments demonstrate that silicon quantum dots are activated for emission due to the localized levels provided by the curved surface effect.     

尺寸分布对量子点激发态发光性质的影响

采用有效质量近似方法研究了量子点的激发态光致发光峰的展宽问题. 对尺寸不均匀分布下量子点各能级发光峰与平均尺寸量子点发光峰的能量偏差进行了计算，定性地描述了尺寸分布对量子点基态和激发态发光峰展宽的影响. 研究表明，量子点的高度、直径以及体积等不均匀分布使量子点具有不同的垂直、平面方向的量子束缚. 这两种量子限制的相互作用决定了量子点激发态发光峰的宽度相对于基态发光峰的大小. 在各种不同性质的尺寸分布下，量子点激发态发光峰的展宽有可能大于、等于或小于基态发光峰的展宽. 关键词： 量子点 尺寸分布 激发态     

Electric-field-tunable electronic properties of graphene quantum dots

The tight-binding method is employed to investigate the electronic properties of a square graphene quantum dot subject to an in-plane electric field (F). The electronic properties are strongly modified by tuning the field strength or altering the field direction. F will change state energies, alter energy gaps, and induce energy gap modulations. State energies show oscillatory behavior with the change of the field strength. The oscillating amplitude and period are further modulated by the change of the field direction. The field-orientation-dependent electronic properties originate in the geometrical anisotropy of the square graphene quantum dot. Moreover, the density of states (DOS), exhibiting many discrete peaks, directly reveals the characteristic of the electric-field-tunable electronic properties. The number and frequencies of DOS peaks are significantly dependent on the field strength and direction.