PbTe Quantum Research Articles

Influence of external factors on the self-organization of lead and tin telluride nanostructures on the BaF2(111) surface under conditions close to the thermodynamic equilibrium

The morphology of PbTe and SnTe nanostructures grown on BaF2(111) substrates from the vapor phase in a vacuum under conditions close to the thermodynamic equilibrium has been investigated using atomic force microscopy. The equilibrium shape of PbTe and SnTe quantum dots and the statistical parameters of arrays of these quantum dots have been studied as a function of the thermodynamic conditions of growth, the crystal lattice mismatch between the materials of the quantum dots and substrate, and elastic properties of these materials. It has been established that, when the BaF2(111) substrate is deformed under external mechanical loading, the self-organization of dislocations on the BaF2(111) surface can result in the formation of a nanoscale ordered strain relief, which can be used for the fabrication of nanostructures. The morphology of this relief depends on the external load and on the temperature at which the substrate is deformed. It has been shown that the deformation effect on the surface of the substrate and light irradiation of the growth zone of nanostructures affect the nucleation of islands and kinetic processes occurring on the surface of the substrate during their growth. Using the influence of external factors on the BaF2(111) surface under certain thermodynamic conditions, it is possible to grow SnTe and PbTe nanostructures with different morphologies: continuous epitaxial layers with a thickness of less than 10 nm, homogeneous arrays of quantum dots with a high lateral density (more than 1011 cm2), quasi-periodic lateral nanostructures (nanowires), “single” and “coupled” quantum dots, and “molecules” of quantum dots.

Linear Optical Properties in PbTe Quantum Dots

Normal 0 21 false false false PT-BR X-NONE X-NONE MicrosoftInternetExplorer4 Neste trabalho estuda-se teoricamente as propriedades ópticas lineares, como o coeficiente de absorção, a função dielétrica e o índice de refração, em um ponto quânticos esférico e semi-esférico de PbTe. Com o objetivo de compreender como a mudança de geometria e de tamanho afetam as propriedades lineares, foi realizado um estudo sistemático dos autovalores de energia, das funções de onda e das simetrias dominantes dentro do modelo k.p 4×4 de Kane-Dimmock . /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Tabela normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;}

Absence of nonlocal resistance in microstructures of PbTe quantum wells

AbstractWe report on experiments allowing to set an upper limit on the magnitude of the spin Hall effect and the conductance by edge channels in quantum wells of PbTe embedded between PbEuTe barriers. We reexamine previous data obtained for epitaxial microstructures of n‐type PbSe and PbTe, in which pronounced nonlocal effects and reproducible magnetoresistance oscillations were found. Here we show that these effects are brought about by a quasi‐periodic network of threading dislocations adjacent to the BaF2 substrate, which give rise to a p‐type interfacial layer and an associated parasitic parallel conductance. We then present results of transport measurements on microstructures of modulation doped PbTe/(Pb,Eu)Te:Bi heterostructures for which the influence of parasitic parallel conductance is minimized, and for which quantum Hall transport had been observed, on similar samples, previously. These structures are of H‐shaped geometry and they are patterned of 12 nm thick strained PbTe quantum wells embedded between Pb0.92Eu0.08Te barriers. The structures have different lateral sizes corresponding to both diffusive and ballistic electron transport in non‐equivalent L valleys. For these structures no nonlocal resistance is detected confirming that PbTe is a trivial insulator. The magnitude of spin Hall angle γ is estimated to be smaller than 0.02 for PbTe/PbEuTe microstructures in the diffusive regime.

Twisted ZB–CdTe/RS–PbTe (111) heterojunction as a metastable interface structure

The heterostructures of (zinc-blende)–CdTe/(rock-salt)–PbTe are typically found to have their common cubic axes aligned to each other, as in the case of PbTe quantum dots embedded in a CdTe matrix. In this work, we perform both theoretical and experimental studies on the CdTe/PbTe heterostructure in a different geometry: a planar CdTe/PbTe (111) heterostructure. We simulate the epitaxial growth of CdTe (PbTe) on the (111) PbTe (CdTe) substrate, using a density-functional theory. A twisted CdTe/PbTe (111) interface structure has been predicted in the layer-by-layer epitaxial growth on the (111) substrate, in contrast to the non-twisted CdTe/PbTe (111) interface reported in the literature. This predicted structure has been confirmed experimentally in the heterostructure grown by molecular beam epitaxy, using a high-resolution transmission electron microscope. The twisted interface has a lower binding energy than the non-twisted one, indicating that the twisted structure is a metastable phase formed in the non-equilibrium growth process. Additionally, the interface reconstructions of the CdTe/PbTe (111) heterostructure observed by reflection high-energy electron diffraction are explained using the twisted interface model.

Size controlled synthesis of monodisperse PbTe quantum dots: using oleylamine as the capping ligand

We report a simple method to synthesize monodisperse hydrophobic PbTe quantum dots followed by a stability study of the as-synthesized quantum dots in air. We provide evidence that air-stable PbTe quantum dots can be synthesized using this method. PbCl2 complexed with oleylamine was used as the Pb precursor. Tellurium powder dissolved in tri-n-octylphosphine was used as the Te precursor. Oleylamine was used as the only capping ligand. The size and shape of the PbTe quantum dots were controlled by changing variables such as injection temperature, growth temperature, and growth time. Both Pb to oleylamine and Pb to Te feed mole ratios have been examined to obtain the optimal synthetic conditions. The PbTe quantum dots can be changed from hydrophobic to hydrophilic through ligand exchange using 4-mercaptopyridine to replace oleylamine as the capping ligand. The colloidal PbTe quantum dots were characterized by transmission electron microscopy, high resolution transmission electron microscopy, selected area X-ray diffraction, energy-dispersive X-ray spectroscopy, FT-IR spectrometer, 31PNMR spectrometer, and powder X-ray diffraction. The sizes of the PbTe quantum dots synthesized ranged from 2.6 nm to 14.0 nm with a relative standard deviation of ∼5.6 to 9.1%. The shape of the PbTe quantum dots was either spherical ( ∼9 to 10 nm).

Midinfrared electroluminescence from PbTe/CdTe quantum dot light-emitting diodes

Midinfrared electroluminescence of epitaxial PbTe quantum dots in CdTe with emission in the 2–3 μm wavelength range is demonstrated up to room temperature. The light-emitting diode structures were grown by molecular beam epitaxy with the active PbTe quantum dots embedded in the intrinsic zone of a CdTe/CdZnTe p-i-n junction on GaAs (100) substrates. The current and temperature dependences of the electroluminescence emission are presented. The comparison with photoluminescence measurements shows that midinfrared light-emission from the diodes originates from the quantum dots.

Ab initio characterization of the electronic properties of PbTe quantum dots embedded in a CdTe matrix

The electronic properties of PbTe nanocrystals (NCs) embedded in a CdTe matrix are investigated using an ab initio pseudopotential method and a repeated supercell approximation. The asymmetric distribution of polar dot-matrix interfaces leads to the presence of an electrostatic field within the NC region. The consequences of this field on the electronic properties of the dot-matrix system are discussed in detail. The resulting spatial separation of electrons and holes inside the NC region gives rise to a quantum-confined Stark effect. It influences the photoluminesence of the NCs.

Contact superconductivity in In–PbTe junctions

We report on electron transport studies on superconductor/semiconductor hybrid structures of indium and n-type PbTe quantum wells embedded between Pb0.92Eu0.08Te barriers. In/PbTe contacts form by spontaneous alloying, which occurs already at room temperature. The alloyed phase penetrates deeply into the semiconductor and forms metallic contacts even in the presence of depletion layers at the semiconductor’s surface. Although the detailed structure of this phase is unknown, we observe that it exhibits a superconducting transition at temperatures below 7 K. This causes such substantial reduction in the contact resistances that they even become comparable to those predicted for ideal superconductor-normal conductor contacts. Our findings suggest that the new superconducting phase is result of Josephson coupling arising between tiny superconducting precipitates randomly distributed in the interface region. In analogy to the granular superconducting systems, the coupling would lead to the occurrence of the global superconductivity only when the Coulomb charging energies of the precipitates are sufficiently small. This condition is exceptionally well fulfilled in PbTe because of a huge static dielectric constant of this material, ε>1000.

Particle size control of PbTe quantum dots incorporated in the germano-silicate glass optical fiber by heat treatment

The effect of heat treatment of the germano-silicate glass optical fiber incorporated with PbTe quantum dots on the absorption characteristics was investigated. Incorporation of the PbTe QDs in the fiber core was confirmed by the absorption peaks that appeared at 687 nm and 1055 nm and their shift upon heat treatment. The absorption peak was found to shift linearly toward a longer wavelength after heat treatment at 700 °C, 1000 °C, and 1100 °C for 1 h. The red shift of the excitonic absorption peak was attributed to the increase in the average size of the PbTe QDs in the fiber core.

Dielectric function of colloidal lead chalcogenide quantum dots obtained by a Kramers-Krönig analysis of the absorbance spectrum

We combined the Maxwell-Garnett effective medium theory with the Kramers-Kr\"onig relations to obtain the complex dielectric function $ϵ$ of colloidal PbS, PbSe, and PbTe quantum dots (Qdots). The method allows extracting both real $({ϵ}_{R})$ and imaginary $({ϵ}_{I})$ parts of the dielectric function from the experimental absorption spectrum. This enables the quantification of the size-dependent oscillator strength of the optical transitions at different critical points in the Brillouin zone, strongly improving our understanding of quantum confinement effects in these materials. In addition, the static-limit sum rule yields the electronic dielectric constant from the ${ϵ}_{I}$ spectrum. Interestingly, values for lead chalcogenide Qdots remain close to the bulk dielectric constant. To verify these trends, we determined the dielectric constant of thin lead chalcogenide layers by ab initio calculations, and the results agree with the experimental data.

Millimeter and Sub-millimeter wave Response of Two-Dimensional Hot Electrons in delta doped PbTe Quantum Well

Millimeter and sub-millimeter wave response of two-dimensional hot electrons in double delta doped GaAs quantum well is studied here incorporating deformation potential acoustic, polar optic, ionized background and remote impurity scatterings in the framework of heated drifted Fermi-Dirac distribution function. The inclusion of delta doping is found to enhance the two-dimensional electron density which in turn improves the ac mobility in the GaAs quantum wells thereby providing scope of getting higher speed devices in future.

Formation of self assembled PbTe quantum dots in CdTe on Si(111)

We describe the growth and formation of self assembled PbTe quantum dots in a CdTe host on a silicon (111) substrate. Annealing yields different photoluminescence spectra depending on initial PbTe layer thickness, thickness of the CdTe cap layer and annealing temperature. Generally two distinct emission peaks at ∼0.3 eV and ∼0.45 eV are visible. Model calculations explaining their temperature dependence are performed. The dot size corresponds well with the estimated sizes from electron microscopy images. The quantum dots may be used as absorber within a mid-infrared detector.

PbTe and SnTe quantum dot precipitates in a CdTe matrix fabricated by ion implantation

We present rock-salt IV-VI semiconductor quantum dots fabricated by implantation of Pb+, Te+, or Sn+ ions into epitaxial zinc-blende CdTe layers. PbTe and SnTe nanoprecipitates of high structural quality are formed after implantation by thermal annealing due to the immiscibility of dot and matrix materials. For samples implanted only with Pb+, intense continuous-wave photoluminescence peaked at 1.6 μm at 300 K is found. In contrast, for PbTe quantum dots fabricated by coimplantation of Pb+ and Te+, the 300 K emission peak is observed at 2.9 μm, indicating luminescence from much larger dots.

Coherent {001} interfaces between rocksalt and zinc-blende crystal structures

We study coherent, two-dimensional (2D) rocksalt PbTe epilayers in a zinc-blende CdTe matrix. The embedded material transforms during an annealing step into highly symmetric quantum dots with coherent interfaces. The atomic structure of the polar {001} interfaces between the two tellurides is investigated by high-resolution transmission electron microscopy in combination with multislice simulations. Both the original 2D PbTe epilayers and the resulting PbTe quantum dots are terminated by distinctively different cation-terminated [(001)A] and anion-terminated [(001)B] interfaces at opposite faces, whereas for the structurally similar ErAs/InGaAs heterosystem as yet only the (001)A interface was reported in the literature. Ab initio simulations were performed to assess these seeming differences, which were found to be mainly caused by the different polar bonding character of the involved II-VI and III-V compounds.

Temperature-dependent midinfrared photoluminescence of epitaxial PbTe/CdTe quantum dots and calculation of the corresponding transition energy

We present the temperature dependence of continuous-wave midinfrared photoluminescence of PbTe quantum dots in a CdTe matrix. The quantum dots are formed by epitaxial precipitation of a two-dimensional PbTe layer upon thermal annealing. A strong shift of the emission to longer wavelengths with decreasing temperature is found. This shift is not only caused by the strong temperature dependence of the band gap of PbTe but also by the strain in the dot as well as by the change in quantization energies, both being temperature dependent due to the thermal-expansion mismatch between PbTe and CdTe, and via the effective masses of PbTe. At low temperatures we observed an increase in the emission intensity with rising temperature, depending on dot size. This is attributed to the presence of a dark ground state, as was also observed for lead salt nanocrystals. The influence of the excitation power on the emission spectra at various temperatures indicates carrier redistribution between the dots. Furthermore, an analytical calculation of the ground-state transition energy in the dots is performed using a spherical dot shape, and including the temperature-dependent strain in the dots and the matrix as well as the temperature dependence of the effective masses of PbTe. From these model calculations, a good agreement to the experimental data is obtained over the whole temperature range from 20 to 300 K.

Thermal precipitation of self‐organized PbTe quantum dots in CdTe host matrix

AbstractThis paper describes a selective growth of PbTe/CdTe quantum wells (QWs) and quantum dots (QDs) in CdTe host matrix by molecular beam epitaxy. These two tellurides possess almost identical lattice constants, but differ fundamentally in their lattice structure. Owing to a strong phase separation by the lattice‐type mismatch, insertion of a PbTe thin layer in CdTe matrix at higher temperatures than 280 °C was resulted in a self‐organized growth of coherent and three‐dimensionally isotropic QDs. The same growth procedure at lower temperatures, on the other hand, yielded a conventional QW structure with sharp heterointerface. This QW structure, however, was found to precipitate well‐ordered QD‐array by a postgrowth annealing, indicating that the QD formation is induced by the minimization of interface energy between the inmiscible two tellurides. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical gain in PbTe/CdTe quantum dots

A theoretical model is developed to study the optical transition and optical gain of PbTe/CdTe quantum dots. The model is based on the k·p envelope function approach, and anisotropic band structure characteristics of PbTe are taken into consideration. The relationships of optical gain of PbTe/CdTe quantum dots vs dot size and injection carrier density are given. The theoretical results suggest that PbTe/CdTe quantum dots with dot size of 15—20nm are promising materials for mid-infrared lasers, which may produce optical gain higher than 5000cm-1 when the injection carrier density ranges (0.3—3)×1018cm-3. The optical gain decreases with dot size increasing. However, higher injection carrier density is required for PbTe/CdTe quantum dots with smaller sizes (<15nm). Therefore the optimal PbTe quantum dot sizes are 15—20nm.

All-optical switching device for infrared based on PbTe quantum dots

Multilayers of PbTe quantum dots embedded in SiO 2 were fabricated by alternate use of Pulsed Laser Deposition (PLD) and Plasma Enhanced Chemical Vapor Deposition (PECVD) techniques. The morphological properties of the nanostructured material were studied by means of High Resolution Transmission Electron Microscopy (HRTEM), Grazing-Incidence Small-Angle X-ray scattering (GISAXS) and X-ray Reflectometry (XRR) techniques. A preliminary analysis of the GISAXS spectra provided information about the multilayer periodicity and its relationship to the size of the deposited PbTe nanoparticles. Finally multilayers were fabricated inside a Fabry–Perot cavity. The device was characterized by means of Scanning Electron Microscopy (SEM). Transmittance measurements show the device functionality in the infrared region.

Quantum dots with coherent interfaces between rocksalt-PbTe and zincblende-CdTe

The formation of PbTe quantum dots (QDs) in a crystalline CdTe host matrix is demonstrated by the annealing of a coherent, heteroepitaxial PbTe layer clad between CdTe layers. The resulting QDs have a centrosymmetric shape and they exhibit intense room-temperature mid-infrared photoluminescence due to an electron-hole pair recombination in the narrow-gap PbTe. The intense luminescence approves the high quality of the QD interfaces, between the sixfold coordinated rocksalt structure of PbTe and the fourfold coordinated zincblende structure of CdTe. To gain further insight into the structural interface properties, we compare quantitatively multislice simulations of HRTEM images with first-principles total-energy calculations in the repeated-slab approximation. The most drastic effect occurs at the electrostatically neutral (110) interface, where we find a lateral spatial offset between the two crystal halves due to rebonding across the interface. For the two polar (001) interfaces, significantly different lattice plane spacings are observed, depending on whether the polar CdTe (001) face is cation or anion-terminated. The agreement between the first-principles calculations and the HRTEM data is excellent.

Quantum ballistic phenomena in nanostructures of paraelectric PbTe

This article reviews recent developments in the physics of lead telluride nanostructures. PbTe is a IV–VI narrow gap paraelectric semiconductor, characterized by the huge static dielectric constant ε>1000 at helium temperatures. Methods of gating and nanostructure fabrication of modulation doped PbTe∕Pb1−xEuxTe quantum wells are described. Specific properties of PbTe, make it possible to put into evidences a number of interesting effects in the quantum ballistic regime, not encountered in standard semiconductor nanostructures. It is found that conductance of narrow constrictions shows precise zero-field quantization in 2e2∕h units, despite a significant amount of charged defects in the vicinity of the conducting channel. This unusual result is a consequence of a strong suppression of Coulomb potential fluctuations in PbTe, an effect confirmed by numerical simulations. Furthermore, orbital degeneracy of electron waveguide modes can be controlled by the width of PbTe∕Pb1−xEuxTe quantum wells, so that unusual sequences of plateau conductance are observed. Finally, because of a rather large Zeeman splitting in PbTe, significant spin splitting of the conductance plateaus is observed already in the magnetic fields of about 0.2 T. It is shown that PbTe quantum wires act as efficient local spin-filters, in which the spin polarized current is carried by a large number of wire modes.