Structural investigations of superconducting multilayers consisting of semiconducting materials

There are rather exotic semiconducting superlattices (SL) consisting of monochalcogenides of Pb, Sn and rare-earth metals which exhibit superconductivity at temperature as high as 6 K. Here we report the results of precision x-ray diffractometry and TEM investigations as well as Auger spectroscopy data obtained on some of the epitaxially grown superconducting semiconducting SLs. It is established that essential features of the SL structure determining the appearance of superconductivity are the perfect single-crystallinity of the samples and the presence of continuous dense grids of misfit dislocations on the interfaces between two semiconductors. The segregation of free Pb which was observed in some cases does not correlate, according to experimental data, with the appearance of superconductivity

Misfit dislocation superlattices in IV-VI multilayered compounds as zero-dimensional quantum boxes

Quantum dots can be generated by a new method based on the formation of misfit dislocations in a perfectly periodic array network during the epitaxial growth of IV-VI compound superlattices. Infrared photoluminescence spectra of IV-VI superlattices are measured at temperatures T=5-90 K. A large blue shift and temperature independent line positions and width were observed in PbSe-PbS/(001)KCl superlattices with a three-dimensional periodic arrays of misfit dislocations. It is supposed that these features are due to quantum dot luminescence

Size effects in thin n-PbTe films

The effect of the film thickness d on the Seebeck coefficient S, the Hall coefficient RH, electrical conductivity σ, charge carrier mobility μH and thermoelectric power factor S²σ of thin films (d = 7-235 nm) prepared by thermal evaporation of n-type PbTe polycrystals doped with InTe in vacuum onto (001)KCl substrates was investigated. It was established that at d ≈ 20 nm, an inversion of the conductivity type (p → n) occurs, which is attributed to a change in the thermodynamic equilibrium conditions in films as compared with bulk crystals and\or to partial re-evaporation of In atoms. In the thickness range d 20 nm, the thickness dependences of the properties exhibit an oscillatory behavior with the period Δ d ≈ 12 nm. The observed oscillatory character of the thickness dependences of the kinetic coefficients is attributed to the manifestation of quantum size effects. The theoretical S(d) dependence calculated in the approximation of size quantization taking into account d-dependences of the Fermi energy and a number of subbands is in good agreement with the experimental one with regard to the oscillation period

Interdiffusion in EuS -based epitaxial superlattice nanostructures

The diffusion intermixing of layers during annealing of EuS -based epitaxial superlattice nanostructures was studied by X-ray diffraction technique. The interdiffusion coefficients for EuS —PbS , EuS —PbSe , EuS —SrS superlattices were determined basing on change of the intensity of near-Bragg reflection satellites in X-ray diffraction pattern. There are no layer intermixing in EuS —PbTe superlattice was observed

Investigation of the growth mechanism, structure, and thermoelectric properties of thin PbTe films grown on mica

The growth mechanisms, structure and thermoelectric properties of thin PbTe films prepared by thermal evaporation in vacuum and subsequent deposition on mica substrates at temperatures Ts = 375, 525 and 635 К were studied. The films were prepared from charge with different electron concentrations (n = 10¹⁷ and n = 10²⁰ cm⁻³). The film thickness was varied in the range d =4-500 nm. Electron microscopy study showed that PbTe grows on mica epitaxially in an island like fashion predominantly in the (111) orientation. It is established that in PbTe films there exists a critical thickness at which the transition from electron to hole conductivity with decreasing d is observed. Covering films with a protective layer, lowering the substrate temperature and increasing electron concentration in the charge result in narrowing of the thickness range corresponding to hole conductivity. It is shown that electron concentrations n in the charge and in thick PbTe films grown at the substrate temperature Ts = 525 К differ, the character and magnitude of this difference depending on n in the charge.Исследованы механизм роста, структура и термоэлектрические свойства тонких пленок РЬТе, полученных термическим испарением в вакууме на подложки из слюды. Варьировались толщина пленок (d = 4-500 нм), температура подложки (Т3 = 375, 525 и 635 К) и концентрация носителей заряда в исходной шихте (п = 10¹⁷ и п = 10²⁰ см⁻³). Методом электронной микроскопии установлено, что РbТе растет на слюде эпитаксиально по островковому механизму преимущественно в ориентации (111). Установлено, что в пленках РbТе существует критическая толщина, при которой наблюдается переход от электронной к дырочной проводимости при уменьшении d. Нанесение на пленки защитного покрытия, снижение температуры подложки и увеличение концентрации электронов в шихте приводят к сужению интервала толщин, соответствующих дырочной проводимости. Показано, что значения концентрации электронов п в шихте и толстых пленках РbТе, полученных при температуре подложки Ts = 525К, различаются, причем характер и величина этого изменения зависят от n в шихте.Досліджено механізм росту, структура і термоелектричні властивості тонких плівок РbТе, одержаних термічним випаровуванням у вакуумі на підкладки із слюди. Варіювались товщина плівок (d = 4-500 нм), температура підкладки (Т3 = 375, 525 и 635 К) та концентрація носіїв заряду у вихідній шихті (п = 10¹⁷ и п = 10²⁰ см⁻³). Методом електронної мікроскопії встановлено, що РbТе росте на слюді епітаксіально за острівковим механізмом переважно в орієнтації (111). Встановлено, що у плівках РЬТе існує критична товщина, при якій спостерігається перехід від електронної до діркової провідності при зменшенні d. Нанесення на плівки захисного покриття, зниження температури підкладки та збільшення концентрації електронів у шихті приводять до зменшення інтервалу товщин, який відповідає дірковій провідності. Показано, що значення концентрації електронів n у шихті і товстих плінках РЬТе, одержаних при температурі підкладки Ts = 525 К, відрізняються, а характер і величина цієї відмінності залежать від n у шихті

Structure of thermally evaporated bismuth selenide thin films

The Bi₂Se₃ thin films with thicknesses d = 7-420 nm were grown by thermal evaporation in vacuum of stoichiometric n-Bi₂Se₃ crystals onto heated glass substrates under optimal technological conditions determined by the authors. The growth mechanism, microstructure, and crystal structure of the prepared thin films were studied using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. It was established that the prepared thin films were polycrystalline, with composition close to the stoichiometric one, did not contain any phases apart from Bi₂Se₃, were of a high structural quality, and the preferential growth direction [001] corresponded to the direction of a trigonal axis C₃ in a hexagonal lattice. The films, like the initial crystal, exhibited n-type conductivity. It was shown that with increasing film thickness, the grain size and the film roughness remain practically the same at thicknesses d << 100 nm, and after that increase, reaching their saturation values at d ~ 300 nm. It follows from the results obtained in this work that using the method of thermal evaporation in vacuum from a single source, one can prepare thin n-Bi₂Se₃ films of a sufficiently high structural quality with a composition close to the stoichiometric one and the preferential growth orientation

Interfacial superconductivity in semiconducting monochalcogenide superlattices

Superconducting and structural properties of superconducting semiconducting multilayers are investigated. These layered systems are obtained by epitaxial growth of the isomorphic monochalcogenides of Pb, Sn, and rare-earth elements on a KCl substrate. Some of these compounds are narrow-gap semiconductors (PbTe, PbS, PbSe, SnTe). Layered structures containing one or two narrow-gap semiconductors have a metallic type of conductivity and a transition to a superconducting state at temperatures in the range of 2.5–6 K. Structures containing only wide-gap semiconductors (YbS, EuS, EuSe) do not demonstrate such properties. All superconducting layered systems are type-II superconductors. The critical magnetic fields and the resistive behavior in the mixed state reveal features characteristic of other layered superconductors. However, data obtained in magnetic fields testify that the period of the superstructure corresponds to half of that obtained from x-ray-diffractometry investigations. This is evidence that the superconducting layers in these samples are confined to the interfaces between two semiconductors. Electron microscopy studies reveal that in the case of epitaxial growth the interfaces contain regular grids of misfit dislocations covering all the interface area. These samples appear to undergo a superconducting transition if they have a metallic type of conductivity in the normal state. Samples with island-type dislocation grids only reveal partial superconducting transitions. The correlations between the appearance of superconductivity and the presence of dislocations, which have been found experimentally, lead to the conclusion that the normal metallic conductivity as well as the superconductivity are induced by the elastic deformation fields created by the misfit dislocation grids. A theoretical model is proposed for the description of the narrow-gap semiconductor metallization, which is due to a band inversion effect and the appearance of electron- or hole-type inversion layers near the interfaces. For different combinations of the semiconductors, such inversion layers in the superlattices can have different shapes and topology. In particular, they can form multiply connected periodic nets having a repetition period coinciding with that of the dislocation grids. Numerical estimates show that such a scenario for the appearance of superconductivity is quite likely. It is shown that the new type of metallic and superconducting nanoscale two-dimensional structures with unusual properties may be obtained from monochalcogenide semiconductors