5 research outputs found
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
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
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
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