Electronic Structure Of Quantum Wells Research Articles

Heat-driven acoustic phonons in lamellar nanoplatelet assemblies.

Colloidal CdSe nanoplatelets, with the electronic structure of quantum wells, self-assemble into lamellar stacks due to large co-facial van der Waals attractions. These lamellar stacks are shown to display coherent acoustic phonons that are detected from oscillatory changes in the absorption spectrum observed in infrared pump, electronic probe measurements. Rather than direct electronic excitation of the nanocrystals using a femtosecond laser, impulsive transfer of heat from the organic ligand shell, excited at C-H stretching vibrational resonances, to the inorganic core of individual nanoplatelets occurs on a time-scale of <100 ps. This heat transfer drives in-phase longitudinal acoustic phonons of the nanoplatelet lamellae, which are accompanied by subtle deformations along the nanoplatelet short axes. The frequencies of the oscillations vary from 0.7 to 2 GHz (3-8 μeV and 0.5-1 ns oscillation period) depending on the thickness of the nanoplatelets-but not their lateral areas-and the temperature of the sample. Temperature-dependence of the acoustic phonon frequency conveys a substantial stiffening of the organic ligand bonds between nanoplatelets with reduced temperature. These results demonstrate a potential for acoustic modulation of the excitonic structure of nanocrystal assemblies in self-assembled anisotropic semiconductor systems at temperatures at or above 300 K.

A room temperature continuous-wave nanolaser using colloidal quantum wells

Colloidal semiconductor nanocrystals have emerged as promising active materials for solution-processable optoelectronic and light-emitting devices. In particular, the development of nanocrystal lasers is currently experiencing rapid progress. However, these lasers require large pump powers, and realizing an efficient low-power nanocrystal laser has remained a difficult challenge. Here, we demonstrate a nanolaser using colloidal nanocrystals that exhibits a threshold input power of less than 1 μW, a very low threshold for any laser using colloidal emitters. We use CdSe/CdS core-shell nanoplatelets, which are efficient nanocrystal emitters with the electronic structure of quantum wells, coupled to a photonic-crystal nanobeam cavity that attains high coupling efficiencies. The device achieves stable continuous-wave lasing at room temperature, which is essential for many photonic and optoelectronic applications. Our results show that colloidal nanocrystals are suitable for compact and efficient optoelectronic devices based on versatile and inexpensive solution-processable materials.

Assessment of Anisotropic Semiconductor Nanorod and Nanoplatelet Heterostructures with Polarized Emission for Liquid Crystal Display Technology.

Semiconductor nanorods can emit linear-polarized light at efficiencies over 80%. Polarization of light in these systems, confirmed through single-rod spectroscopy, can be explained on the basis of the anisotropy of the transition dipole moment and dielectric confinement effects. Here we report emission polarization in macroscopic semiconductor-polymer composite films containing CdSe/CdS nanorods and colloidal CdSe nanoplatelets. Anisotropic nanocrystals dispersed in polymer films of poly butyl-co-isobutyl methacrylate (PBiBMA) can be stretched mechanically in order to obtain unidirectionally aligned arrays. A high degree of alignment, corresponding to an orientation factor of 0.87, was achieved and large areas demonstrated polarized emission, with the contrast ratio I∥/I⊥ = 5.6, making these films viable candidates for use in liquid crystal display (LCD) devices. To some surprise, we observed significant optical anisotropy and emission polarization for 2D CdSe nanoplatelets with the electronic structure of quantum wells. The aligned nanorod arrays serve as optical funnels, absorbing unpolarized light and re-emitting light from deep-green to red with quantum efficiencies over 90% and high degree of linear polarization. Our results conclusively demonstrate the benefits of anisotropic nanostructures for LCD backlighting. The polymer films with aligned CdSe/CdS dot-in-rod and rod-in-rod nanostructures show more than 2-fold enhancement of brightness compared to the emitter layers with randomly oriented nanostructures. This effect can be explained as the combination of linearly polarized luminescence and directional emission from individual nanostructures.

Red, Yellow, Green, and Blue Amplified Spontaneous Emission and Lasing Using Colloidal CdSe Nanoplatelets.

There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue-emitting NPLs, the ASE threshold is 50 μJ/cm(2), lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights the qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials.

Quantum wells in polar-nonpolar oxide heterojunction systems

We address the electronic structure of quantum wells in polar-nonpolar oxide heterojunction systems focusing on the case of nonpolar ${\text{BaVO}}_{3}$ wells surrounded by polar ${\text{LaTiO}}_{3}$ barriers. Our discussion is based on a density-functional description using the local spin-density approximation with local-correlation corrections $(\text{LSDA}+U)$. We conclude that a variety of quite different two-dimensional electron systems can occur at interfaces between insulating materials depending on band lineups and on the geometrical arrangement of polarity discontinuities.

Measurements of composition and electronic structure in an operating light-emitting diode using analytical electron microscopy

Scanning transmission electron microscopy (STEM) is a useful technique for the study of the morphology, composition, and electronic structure of quantum wells. However, most previous studies have been on epitaxially grown structures, before they had been used in devices. In this work we show that, with careful specimen preparation, advanced STEM techniques can be used to study a packaged commercially available light-emitting diode. The composition and morphology of both the quantum wells and the superlattices in this device have been determined and the electronic structure was measured with electron energy-loss spectroscopy.

Interband transitions of quantum wells and device structures containing Ga(N, As) and (Ga, In)(N, As)

The unusual N-induced band formation and band structure of Ga(N, As) and (Ga, In)(N, As) alloys are also reflected in the electronic structure of quantum wells (QWS) and device structures containing these non-amalgamation-type alloys. This review is divided into three parts. The first part deals with band structure aspects of bulk Ga(N, As) and motivates the possibility of a k · p-like parameterization of the band structure in terms of the level repulsion model between the conduction band edge of the host and a localized N-level. The second part presents experimental studies of interband transitions in Ga(N, As)/GaAs and (Ga, In)(N, As)/GaAs QW structures addressing band offsets, electron effective mass changes and an intrinsic mechanism contributing to the blueshift of the (Ga, In)(N, As) band gap on annealing. The observed interband transitions can be well described using a ten-band k · p model based on the level repulsion scheme. The third part deals with (Ga, In)(N, As)-based laser devices. The electronic structure of the active region of vertical-cavity surface-emitting laser and edge-emitter laser structures is studied by modulation spectroscopy. The gain of such structures is measured by optical methods and analysed in terms of a model combining the ten-band k · p description of the band structure and generalized Bloch equations.

Electronic structure of quantum wells embedded inshort-period superlattices with graded interfaces

In the present study we investigate the effect ofcomponent interdiffusion across the interfaces on theelectronic structure of a quantum well embedded in ashort-period superlattice. We calculate the energies of thebound states for an 18 monolayer (5 nm) thick GaAs quantum wellembedded in an (AlAs)4/(GaAs)8 superlattice incomparison with two AlxGa1-xAs/GaAs(x = 1 and 0.33) single quantum wells with the samethickness. The calculations are made within the framework of theenvelope-function approximation. The behaviour of the lowestelectron and hole bound states is studied for values of thediffusion length from 0 to 4 monolayers. The contribution of theX-point conduction band minima to the electronic structure isdiscussed. The calculated transition energies are shown to be inagreement with photoluminescence spectra of such structures. Thepresented approach can be used to assess the effect of thecomponent intermixing on the electronic structure of variouscomplex multilayered systems.

Segregation, interface morphology, and the optical properties ofGaAs/AlAsquantum wells: A theoretical study

We investigate optical signatures of interface segregation phenomena in epitaxially grown narrow quantum wells. Short-period step formation and segregation during growth are simulated via a generalized kinetic-limited segregation model that allows for combined vertical and lateral atomic exchanges with respect to the surface. Segregation always leads to more abrupt inverted (GaAs-on-AlAs) interfaces as compared to normal (AlAs-on-GaAs) interfaces, where a wider alloy region is present. The electronic structure of quantum wells is determined in the tight-binding approach applied to supercells $(\ensuremath{\sim}{10}^{4} \mathrm{atoms})$ with periodic boundary conditions. The atomic configurations are numerically generated according to the two-dimensional composition profiles obtained from the generalized kinetic-limited segregation model. Disorder is incorporated through an ensemble of such generated configurations, over which averaged electronic quantities are obtained. The correlation between the interface degree of roughness, alloying, and its optical properties is demonstrated. We show that segregation may improve the light-emission properties of quantum wells by reducing the roughness of stepped interfaces.

Self-consistent electronic structure of quantum wells by invariant embedding method

A new approach based on the invariant embedding method for the self-consistent calculation of electronic structure of quantum wells is presented and is applied to both neutral quantum well and parabolic quantum well. Numerical results obtained for these structures agree very well with those of previous theoretical and experiment studies. The present approach is expected to lead to a more efficient and stable scheme for the calculation of electronic band structure of quantum structures. Realistic boundary conditions are naturally taken into account in the present calculation which provides a convenient way for studying boundary effects.

Resonant tunnelling used as an optical probe to the electronic structure of quantum wells

We have studied theoretically the influence of crossed electric and magnetic fields on the resonant tunnelling of photogenerated electrons and holes in an asymmetric double quantum well system. The electronic structure of this system has been calculated using the k·p multi-band model within a six-component envelope-function formalism. For a given value of applied electric field we determine the magnetic field necessary to bring into resonance the bottom (top) of the electron (hole) ground state of the narrow quantum well with a state in the wide well. We show that the obtained electric versus resonant magnetic field curves can be used to map out the dispersion relation of the wide quantum well.

Pseudopotential-based multibandk⋅pmethod for∼250 000-atom nanostructure systems

The electronic structure of quantum wells, wires, and dots is conventionally described by the envelope-function eight-band k\ensuremath{\cdot}p method (the ``standard k\ensuremath{\cdot}p model'') whereby coupling with bands other than the highest valence and lowest conduction bands is neglected. There is now accumulated evidence that coupling with other bands and a correct description of far-from-\ensuremath{\Gamma} bulk states is crucial for quantitative modeling of nanostructure. While multiband generalization of the k\ensuremath{\cdot}p exists for bulk solids, such approaches for nanostructures are rare. Starting with a pseudopotential plane-wave representation, we develop an efficient method for electronic-structure calculations of nanostructures in which (i) multiband coupling is included throughout the Brillouin zone and (ii) the underlying bulk band structure is described correctly even for far-from-\ensuremath{\Gamma} states. A previously neglected interband overlap matrix now appears in the k\ensuremath{\cdot}p formalism, permitting correct intervalley couplings. The method can be applied either using self-consistent potentials taken from ab initio calculations on prototype small systems or from the empirical pseudopotential method. Application to both short- and long-period (GaAs${)}_{\mathit{p}}$/(AlAs${)}_{\mathit{p}}$ superlattices (SL) recovers (i) the bending down (``deconfinement'') of the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}(\ensuremath{\Gamma}) energy level of (001) SL at small periods p; (ii) the type-II--type-I crossover at p\ensuremath{\approxeq}8 SL, and (iii) the even-odd oscillation of the energies of the R\ifmmode\bar\else\textasciimacron\fi{}/X\ifmmode\bar\else\textasciimacron\fi{}(L) state of (001) SL and \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}(L) state of (111) SL. Introducing a few justified approximations, this method can be used to calculate the eigenstates of physical interest for large nanostructures. Application to spherical GaAs quantum dots embedded in an AlAs barrier (with \ensuremath{\sim}250 000 atoms) shows a type-II--type-I crossover for a dot diameter of 70 \AA{}, with an almost zero \ensuremath{\Gamma}-X repulsion at the crossing point. Such a calculation takes less than 30 min on an IBM/6000 workstation model 590. \textcopyright{} 1996 The American Physical Society.

Study of electric field effects on the electronic structure of quantum wells by resonant Raman scattering

Resonant Raman scattering measurements in an external electric field have been performed in GaAs/Al x Ga 1− x As quantum wells. Changes in both the energy and intensities have been observed as a function of the applied electric field and the width of the wells. The variation of intensities depends significantly on the “allowed” ( δn = 0) or “forbidden” ( δn≠0) character of the transitions. Photoluminescence excitation spectra are presented for comparison. All the results are explained by a model calculation based on a tight-binding scheme including mixing of valence states. The agreement between theory and experimental results is good for the dependence of the transition energies and probabilities on both the applied field and the well widths.

Electronic structure of quantum wells with in-plane magnetic fields

The energy spectrum and the wave functions of quantum wells in strong magnetic fields parallel to the potential walls are calculated analytically by means of a new, graph supported method. This “Arrow Train Method” allows to solve the recurrence relations which originate in the evaluation of eigenvalue determinants of infinite order. The energy eigenvalues for infinite barrier height are computed as a power series in the magnetic fieldB and the center of orbit coordinatez0. The power series is evaluated up to the 18th order inB2 for the first four levels and for cyclotron radii comparable to or considerable less than the well width. The corresponding wave functions and the field dependent center of mass shifts are obtained.