Thermoelectric properties of cold pressed samples of semiconductor (Bi₁₋xSbx)₂Te₃ solid solutions

The composition dependences of thermoelectric (TE) properties of (Bi₁₋xSbx)₂Te₃ solid solutions (0 < x < 1) produced by cold pressing and subsequent annealing were investigated at room temperature. Samples were prepared from cast polycrystals, obtained by the cooling of melt down to room temperature in evacuated quartz ampoules and subsequent annealing. It was established that cast samples exhibited p-type conductivity in the entire composition range, and an increase in the Sb₂Te₃ content led to the growth of electrical conductivity and drop of the Seebeck coefficient. The change of the conductivity type from positive to negative in the composition range x = 0 - 0.6 took place after cold pressing and composition dependencies of the properties became more complex. The maximum figure of merit value (Zmax = (3.1±0.4)·10-3 K-1) that was achieved in cold-pressed annealed samples at x = 0.8 was comparable to the values of Z for single crystals of undoped (Bi₁₋xSbx)₂Te₃ solid solutions and for polycrystalline samples produced by other methods. It follows from the data obtained that the proposed method of preparing the samples of (Bi₁₋xSbx)₂Te₃ solid solutions by cold pressing and subsequent annealing may appear to be useful in thermoelectric devices

Temperature dependences and isotherms of galoanomagnetic properties of Bi doped PbTe crystals and thin films

The temperature dependences of galvanomagnetic properties (the Hall coefficient, electrical conductivity, charge carrier mobility) of (PbTe)₁₀₀₋ₓBiₓ (x = 0-1) alloys obtained by doping PbTe with elementary Bi and thin films prepared from these alloys were studied in the temperature range 80-300 K. On the basis of the temperature dependences, the isotherms of the properties were plotted. It was established that a non-monotonic behavior of the dependences of the properties on the Bi concentration, which had been observed earlier at room temperature, is preserved at lower temperatures. This confirms our earlier suggestion about the self-organization processes taking place in the defect subsystem of the crystal at certain Bi concentrations

Plastic deformation instabilities in Bi crystals under microindentation

The dependences of microhardness Н on the load on indentor were obtained for bismuth single- and polycrystals. The general tendency to Н decreasing with Р increasing up to ~ 0.15 N is observed. After that the microhardness practically does not change. In the range of Р - 0,05-0,15 N, in the Н(Р) dependences the oscillations are detected whose presence is explained by qualitative changes in plastic deformation micromechanisms and in the crystal defect structure under increasing indentor load.Получены зависимости микротвердости Н моно- и поликристаллов висмута от величины нагрузки на индентор Р. Наблюдается общая тенденция к снижению Н с увеличением Р до ~0,15 N, после чего микротвердость практически не изменяется. В интервале Р - 0,05-0,15 N на зависимостях Н(Р) обнаружены осцилляции, наличие которых связывается с качественными изменениями в микромеханизмах пластической деформации и в дефектной структуре кристалла при увеличении нагрузки на индентор.Одержано залежностi мiкротвердостi Н моно- та полiкристалiв вiсмуту вiд величини навантаження на iндентор Р. Cпостерiгається загальна тенденцiя до зниження Н при зростаннi Р до ~0,15 N, пiсля чого мiкротвердiсть практично не змiнюється. В iнтервалi Р - 0,05-0,15 N на залежностях Н(Р) виявляються осциляцii, наявнiсть яких пов'язується з якiсними змiнами у мiкромеханiзмах пластичної деформацiї та у дефектнiй структурi кристала при зростаннi навантаження на iндентор

Concentration anomaly of heat capacity in PbTe based solid solutions

The temperature dependences of the heat capacity in the Pb₁-xMnxTe and Pb₁-xGexTe (x = 0-0.04) solid solutions based on PbTe were obtained in the temperature range of 100-670 K. Pronounced peaks were observed in the isotherms of the heat capacity in the vicinity of x ~ 0.01-0.015. The presence of the peaks is explained on the basis of the idea about the existence of concentration phase transitions of percolation type, which take place in any solid solution and are related to transformation of impurity discontinuum into impurity continuum

Temperature dependences of SnTe linear expansion coefficient

The influence of kinetic factors on the temperature dependences of the linear expansion coefficient (a) for SnTe crystals with different degrees of deviation from stoichiometry was studied. The a(T) dependences were obtained by using stationary and dynamic regimes. In the case of the stationary regime, an increase in with increasing temperature (4.2-300 K) was registered, and anomalies in the a(T) dependences were observed and attributed to phase transitions. After quick plunging into liquid nitrogen and subsequent heating the samples up to 300 K without keeping them for a long time at fixed temperatures (a dynamic regime), the a(T) dependences exhibited an oscillatory behavior, most pronounced in the sample with 50.4 at.% Te. It is suggested that the observed behavior of the a(T) dependences is connected with an oscillatory process of approaching the equilibrium in the intrinsic defect subsystem and with overlapping of relaxation processes and temperature phase transitions

Temperature and concentration dependences of specific heat of Bi₁-xSbx solid solutions

For Bi₁-xSbx solid solutions, the concentration (x = 0 - 0.12) and temperature (170-525 K) dependences of specific heat Cp were obtained. At all temperatures studied, three peaks of Cp were observed near x = 0.015, x = 0.037, and x = 0.07. The observed effects were attributed to critical phenomena accompanying the second-order phase transitions: percolation transition from dilute to concentrated solid solutions, the transition to a gapless state, and the semimetal-semiconductor transition, respectively. It was shown that the values of critical indexes (α = 0.11±0.01) are the same not only for different peaks but also for different temperatures and correspond to the values theoretically calculated within the framework of scale-invariant theory for three-dimensional (3D) models

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

Вплив відхилення від стехіометрії на термоелектричні властивості полікристалів і тонких плівок Bi2Te3 в температурному інтервалі 77-300 K

Напівпровідникова сполука Bi2Te3 та тверді розчини на її основі на цей час відносяться до числа найкращих низькотемпературних термоелектричних матеріалів. Одним із методів керування типом провідності та властивостями Bi2Te3 є зміна стехіометрії. Раніше ми одержали залежності механічних та термоелектричних властивостей полікристалів Bi2Te3 від ступеня відхилу від стехіометрії при кімнатній температурі. Мета цієї роботи – дослідження характеру цих залежностей при інших температурах. Було одержано полікристали телуриду вісмуту із складами в інтервалі 59,6-67,5 ат. % Те, і для всіх кристалів було проведено вимірювання коефіцієнта Зеєбека, коефіцієнта Холла, електропровідності та рухливості носіїв заряду у температурному інтервалі 77-300 K. На основі температурних залежностей було побудовано ізотерми кінетичних коефіцієнтів. Було встановлено, що подібно ізотермам при кімнатній температурі ізотерми при більш низьких температурах мають немонотонний характер: між 60,5 і 61,0 має місце інверсія типу провідності, а поблизу 60.0 і 63 ат. % Те спостерігаються екстремуми. Дана інтерпретація експериментальних даних з урахуванням змін у зонній та дефектній структурі Bi2Te3 при зміні стехіометрії. Одержані результати дають можливість керувати термоелектричними властивостями полікристалів Bi2Te3 у температурному інтервалі 77- 300 K, змінюючи ступінь відхилення від стехіометрії.Bi2Te3 semiconductor compound and Bi2Te3-based solid solutions are presently among the best lowtemperature thermoelectric materials. One of the methods of controlling the conductivity type and properties of Bi2Te3 is changing the stoichiometry of this compound. Earlier, we have obtained the room-temperature dependences of mechanical and thermoelectric properties of Bi2Te3 polycrystals on the degree of deviation from stoichiometry. The goal of this work is to investigate the behavior of such dependences at other temperatures. Bismuth telluride polycrystals with compositions in the range of 59.6-67.5 at. % Te were obtained, and for all the crystals the Seebeck coefficient, the Hall coefficient, electrical conductivity and charge carrier mobility were measured in the temperature range 77-300 K. On the basis of the temperature dependences, the isotherms of kinetic coefficients were plotted. It was found that similar to the room-temperature isotherms, the isotherms at lower temperatures were non-monotonic: they exhibited inversion of the conductivity sign between 60.5 and 61.0 at. % Te and extrema near 60.0 and 63.0 at. % Te. The experimental data are interpreted taking into account changes in the band and defect structures of Bi2Te3 under varying stoichiometry. The obtained results make it possible to control thermoelectric properties of Bi2Te3 polycrystals in the temperature range 77-300 K by changing the degree of deviation from stoichiometry

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