Tunable Near-Infrared Emission from CdSe/CdTe/CdSe Core/Shell/Shell Quantum Dots.

The strong piezoelectric field in the InxGa1−xN/GaN quantum well (QW) LEDs, separates the electrons and holes spatially, which decreases the luminescence. Various shapes and compositions of such QWs are studied to improve the performance. We have studied the transition energy (TE), overlap of electron and hole wave functions, band structures and field distributions of the parabolic QWs (PQW) through the self consistent solutions of Schrodinger and Poisson equations. The shape of the PQW is varied along with the compositions and dopings. The square of the overlap of electron and hole wave functions i.e. the transition probability (TP) is strikingly increased, compared to the rectangular QW and it is even higher than the symmetrically staggered QW. At a particular current density, for the same TE, the TP of the PQW increases more than two times that of the rectangular QWs. An important feature, desirable for the QW LEDs emerge. The change of the TE with increase in the current density is minimized. A brief theory, computational procedures and the results will be presented in details with suitable discussions.

Cathodoluminescence (CL) measurements of the ground-state excitonic transition of vertically stacked $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ quantum dots (QDs) exhibited an in-plane linear polarization anisotropy in close proximity to microcracks. Microcracks form as a result of a mismatch of the thermal expansion coefficient between the $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ layers and the Si(111) substrate. In close proximity to the cracks, the layers are found to be under uniaxial tensile stress, whereas the film is under biaxial tensile stress for distances greater than $\ensuremath{\sim}3\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ from the cracks. The microcracks serve as an excellent stressor through which the strain tensor of the $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ QDs can be reproducibly modified for studies of strain-induced changes in the optical and electronic properties by using a spatially resolved probe, such as with CL. Changes in the optical properties of the QDs are attributed to stress-dependent variations of the band edges and the electric field along [0001], which is caused by charge polarization. Such changes in the field will subsequently affect the oscillator strength between electrons and holes. Three-dimensional $6\ifmmode\times\else\texttimes\fi{}6$ $\mathbit{k}\ensuremath{\cdot}\mathbit{p}$ calculations of the QD electron and hole wave functions and eigenstates were performed to examine the influence of biaxial and uniaxial tensile stresses on the polarization-dependent momentum matrix element in varying proximity to the microcracks. The model reveals that a change from biaxial to uniaxial stress alters the admixture of ${p}_{x}$ and ${p}_{y}$ characters of the band edges and the ground-state hole wave function, changes the shape and direction of elongation of the hole isosurfaces, and accounts well for the subsequent anisotropy in the polarization dependent optical transitions.

Using a many-body approach based on single-particle pseudopotential wave functions, we calculate the dependence of the optical transitions in CdSe nanocrystals on the presence of ‘‘spectator’’ electrons or holes. We find that~i! as a result of the different localization of the electron and hole wave functions, the absorption lines shift by as much as 22 meV/unit charge when electrons or holes are loaded into the quantum dot. ~ii! The lowest emission line is significantly red shifted with respect to the lowest allowed absorption line. ~iii! Trapping of a ‘‘spectator’’ hole in a surface state is predicted to lead to dramatic changes in the absorption spectrum, including the appearance of new transitions. Semiconductor quantum dots can be charged by deliberate injection of carriers ~via electrical contacts, 1 or via a scanning-tunneling-microscopy tip, 2 ! by photoionization processes removing one or more carriers from the quantum dot, 3 or by capture of external charges. 4 The effects of charging on the optical properties of self-assembled InAs/GaAs quantum dots have been recently measured both in absorption 5 and emission. 6 It was found that when electrons are progressively loaded into the quantum dots, the absorption and emission energies are redshifted relative to the neutral dots. Furthermore, low-energy lines disappear from the absorption spectrum, 5 while new high-energy lines appear in the photoluminescence spectrum. 6 In colloidal quantum dots, charging of surface states is believed to be at the origin of a variety of unusual phenomena, including the occurrence of a permanent dipole moment even in zincblende dots, 7 intermittency ~blinking! of photoluminescence, 3 spectral diffusion and Stark shift, 8 upconversion of photoluminescence, 9 and possibly even the occurrence of long spin lifetimes. 10 However, there are still no reports on the absorption or emission spectra of charged colloidal dots. The effects of charging on the interband optical transitions can be examined using a screened Hartree-Fock model, where the initial and final states are expressed as Slater determinants. The energy DEh,e(Nes ) required to optically excite an electron from the valence-band state h to the conduction-band state e in the presence of N es ‘‘spectator electrons’’ ( e s ) is:

Nowadays the search for an efficient silicon based light emitting source attracts a lot of attention, because the niche of the light emitter device for chip to chip communications is not yet occupied [1]. Low quantum efficiency of radiative recombination in the bulk Si caused by an indirect nature of its band gap is the challenge to be bypassed. Up to the date the hybrid III–V on Si technology dominates, but other approaches are intensively investigated. Among of them there are promising SiGe/Si heterostructures, which can be grown by epitaxy on Si (100) substrates, e.g., in the form of quantum wells or quantum dots (QD). Thereby a Si p-n junction can be used to supply carriers to the light-emitting Ge/Si nanostructure. In spite of the fact that the bulk Ge itself has an indirect band gap, the energy band diagram of strained SiGe on Si is modified due to the elastic strain and the quantum confinement effect. The hetero junction SiGe/Si is characterized by so-called ‘type-II’ band alignment, where carriers are located on the opposite sides of the junction: electrons - in Si, but holes - in Ge. This fact might lead to a small overlap of the electron and hole wave functions and a low oscillator strength causing a low light emission. A possible solution of the latter problem might be found, if the carriers are strongly confined at the sharp Si/Ge hetero-interface. This can be realized via small Ge QDs of a very high density. In particular, high density of Ge QDs leads to the higher luminescence probability of Δ xy - HH transitions [2]. Here Δ xy are electronic states localized in the compressively strained Si between the QDs, HH is the heavy hole state in the QD.

For visible range emitting particles, which are relevant for display and additional applications, Cd-chalcogenide nanocrystals have reached the highest degree of control and performance. Considering potential toxicity and regulatory limitations, there is a challenge to successfully develop Cd-free emitting nanocrystals and, in particular, heterostructures with desirable properties. Herein, we report a colloidal synthesis of fluorescent heavy-metal-free Zn-chalcogenide semiconductor nanodumbbells (NDBs), in which ZnSe tips were selectively grown on the apexes of ZnTe rods, as evidenced by a variety of methods. The fluorescence of the NDBs can be tuned between ∼500 and 585 nm by changing the ZnSe tip size. The emission quantum yield can be greatly increased through chloride surface treatment and reaches more than 30%. Simulations within an effective-mass-based model show that the hole wave function is spread over the ZnTe nanorods, while the electron wave function is localized on the ZnSe tips. Quantitative agreement for the red-shifted emission wavelength is obtained between the simulations and the experiments. Additionally, the changes in radiative lifetimes correlate well with the calculated decrease in electron-hole overlap upon growth of larger ZnSe tips. The heavy-metal-free ZnTe/ZnSe NDBs may be relevant for optoelectronic applications such as displays or light-emitting diodes.

We analyze time-resolved spontaneous emission from excitons confined in self-assembled InAs quantum dots placed at various distances to a semiconductor-air interface. The modification of the local density of optical states due to the proximity of the interface enables unambiguous determination of the radiative and nonradiative decay rates of the excitons. From measurements at various emission energies, we obtain the frequency dependence of the radiative decay rate, which is only revealed due to the separation of the radiative and nonradiative parts. It contains detailed information about the dependence of the exciton wave function on quantum dot size. We derive the quantum optics theory of a solid-state emitter in an inhomogeneous environment and compare this theory to our experimental results. Using this model, we extract the frequency dependence of the overlap between the electron and hole wave functions. We furthermore discuss three models of quantum dot strain and compare the measured wave-function overlap to these models. The observed frequency dependence of the wave-function overlap can be understood qualitatively in terms of the different compressibility of electrons and holes originating from their different effective masses and binding energies.

Atomistic calculations of the electronic states in In/sub 0.2/Ga/sub 0.8/N/GaN and GaN/AlN quantum dots (QDs) have been carried out using a polarization-potential-dependent sp/sup 3/ tight-binding method. A valence-force-field method is used for the strain distribution, and a finite-difference method for the polarization-induced potential and field. We find that a strong built-in electric field is induced in the QDs due to the polarization. The field causes the quantum-confined Stark effect in the QDs; i.e., red-shifted transition energies and a spatial separation of electron and hole wave functions.

We report on micro-photoluminescence studies on InGaN/ AlGaN quantum dots grown by metal-organic vapor phase epitaxy. The excitonic emission lines can be observed up to 100 K. Investigations of the quantum-confined Stark effect in dependence on an external electric field show a partial compensation of internal fields and an enhancement of the overlap between electron and hole wave function in the quantum dots. As a consequence, an increase of the PL intensity and a blue shift of the single-quantum-dot emission line is observed. Furthermore, we show how the internal electric field strongly affects the photoluminescence intensity of the emission line up to 40 K. In addition, by investigating the excitonic and biexcitonic states, we demonstrate that the partial compensation of the internal field increases the biexciton binding energy and influences the intensity of its emission.

Semiconductor devices often require band-gap engineering, which in turn requires the use of alloys and the tuning of their composition to achieve the required electrical or optical behavior. While their compositional fluctuations are generally treated as a perturbation in most conventional III-V semiconductors, the effects of alloy disorder are much more significant in the III-nitrides. Here, the effects of alloy disorder on carrier localization are compared for different III-V semiconductors, particularly for holes for which localization effects are more significant due to their heavier effective mass. This study is conducted using three-dimensional computation for III-V alloys with natural (random) compositional fluctuations. Given the complexity of the problem, we carry out the computations relying on a simplified Hamiltonian in the envelope wave-function approximation with a single heavy-hole valence band. We investigate the effects of compositional fluctuations on carrier localization using two methods. The first method, based on the localization landscape theory, is used to solve the localization landscape equations and obtain the effective potentials acting on carriers. This potential acts as a confining potential that predicts the regions of spatial localization of carriers, and thus allows a comparison of the effects of alloy fluctuations between the conventional III-V and the III-nitride semiconductors. We find that the effective potential of the III-nitride semiconductors exhibits much larger fluctuations compared to the other III-V semiconductors. This might point to a higher degree of carrier localization in the nitrides, particularly for holes. This is verified through the second method, solving Schr\"odinger's equation and obtaining the electron and hole wave functions. We find that for ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ the electron wave functions are delocalized even for the ground state, whereas the low-energy hole states are localized. This is in contrast with the behavior of holes in the common alloy ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$, which are found to be always delocalized. Thus, our study shows the importance of accounting for alloy disorder in the nitrides.

Atomistic tight-binding computations of the electronic properties of ZnSe/ZnS core/shell nanocrystals under applied electric field

Nanoindentation effect on the optical properties of self-assembled quantum dots

Electronic structure of self-assembled quantum dots in high magnetic fields

Confined electrons and holes in Si nanocrystals: Theoretical modeling of the energy spectrum and radiative transitions

AlGaN‐based deep‐ultraviolet (DUV) light‐emitting diodes (LEDs) hold great promise for a wide range of applications, including water sterilization, biochemical sensing, and UV curing. However, their efficiency is fundamentally limited by the quantum‐confined Stark effect (QCSE) originating from strong spontaneous and piezoelectric polarization fields in c‐plane quantum wells (QWs). These internal fields induce pronounced band bending and spatial separation of electron and hole wavefunctions. In this work, a multiple quantum well (MQW) design with a linearly graded‐aluminum composition is introduced along the [0001] direction of the QWs. The grading suppresses polarization discontinuities at quantum barrier (QB)/quantum well (QW) interfaces, reducing interface‐bound charges, and simultaneously generates distributed bulk polarization charges that create a reverse internal electric field. This counter‐field mitigates QCSE, resulting in flattened band profiles and improved wavefunction overlap. Technology computer‐aided design (TCAD) simulations using the SILVACO Advanced Technology Large‐Area Simulator (ATLAS) platform show an increase in electron–hole overlap from 92.45% to 98.26%, a 44.16% enhancement in light output power, a 63.6% reduction in internal electric field, and a 0.7 V decrease in forward voltage. The novelty of this work lies in applying in‐well linear compositional grading as a polarization‐field engineering technique for AlGaN QWs, enabling simultaneous enhancement of optical and electrical performance.In this work, a practical, scalable, and simulation‐verified pathway toward high‐efficiency AlGaN‐based DUV LEDs is reported.

Tunable near-infrared emission and fluorescent lifetime of PbSe quantum dot-doped borosilicate glass