Oscillatory thickness dependences of the electrical conductivity, Hall coefficient, charge carrier mobility, and Seebeck coefficient were obtained at room temperature for n-type thin Bi films (d=3–300 nm) fabricated by the thermal evaporation of a bismuth crystal in a vacuum and deposition on mica substrates at 380 K. We attribute this oscillatory behavior to quantum-size effects, which are observable when the electron mean-free path and Fermi wave length exceed the film thickness d.

Quantum-size effects in n-type bismuth thin films

Antimony telluride (Sb2Te3) thin films were deposited on silicon substrates at room temperature (300 K) by radio frequency magnetron sputtering method. The effects of annealing in N2 atmosphere on their thermoelectric properties were investigated. The microstructure and composition of these films were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, respectively. The electrical transport properties of the thin films, in terms of electrical conductivity and Seebeck coefficient were determined at room temperature. The carrier concentration and mobility were calculated from the Hall coefficient measurement. Both of the Seebeck coefficient and Hall coefficient measurement showed that the prepared Sb2Te3 thin films were p-type semiconductor materials. By optimizing the annealing temperature, the power factor achieved a maximum value of 18.02 μW cm−1 K−2 when the annealing temperature was increased to 523 K for 6 h with a maximum electrical conductivity (1.17 × 103 S/cm) and moderate Seebeck coefficient (123.9 μV/K).

Effects of annealing on thermoelectric properties of Sb2Te3 thin films prepared by radio frequency magnetron sputtering

https://engineering.purdue.edu/NanoLab/PDF/2015%20Thermoelectric%20properties%20of%20electrodeposited%20tellurium%20films%20and%20the%20sodium%20lignosulfonate%20effect.pdf

Thermoelectric properties of electrodeposited tellurium films and the sodium lignosulfonate effect

We have synthesized brushlike p-Te/n-SnO2 hierarchical heterostructures by a two-step thermal vapor transport process. The morphologies of the branched Te nanostructures can be manipulated by adjusting the source temperature or the argon flow rate. The growth of the branched Te nanotubes on the SnO2 nanowire backbones can be ascribed to the vapor–solid (VS) growth mechanism, in which the inherent anisotropic nature of Te lattice and/or dislocations lying along the Te nanotubes axis should play critical roles. When exposed to CO and NO2 gases at room temperature, Te/SnO2 hierarchical heterostructures changed the resistance in the same trend and exhibited much higher responses and faster response speeds than the Te nanotube counterparts. The enhancement in gas sensing performance can be ascribed to the higher specific surface areas and formations of numerous Te/Te or TeO2/TeO2 bridging point contacts and additional p-Te/n-SnO2 heterojunctions.

Vapor-solid growth of p-Te/n-SnO2 hierarchical heterostructures and their enhanced room-temperature gas sensing properties.

Without the aid of surfactants or catalysts, one-dimensional (1D) nanostructures of Te (nanowires with a small quantity of nanotubes and nanoribbons) have been synthesized directly from template-free electrodeposition (TFED) in an aqueous solution at low temperature. As observed by electron microscopy, the as-grown Te nanomaterials are single crystalline trigonal structure and contain few defects. The preferential growth of these 1D nanostructures along the [001] direction is attributed to the intrinsic anisotropic crystal structure of Te and the special growth condition of TFED. It is anticipated that TFED could be used as a versatile approach to synthesize various 1D materials which contain intrinsic highly anisotropic crystal structures.

Template-Free Electrodeposition of One-Dimensional Nanostructures of Tellurium

Tellurium thin films of thicknesses between 25 and 200 nm have been vacuum-deposited on glass substrates at room temperature in a vacuum of 5×10−5torr. The thermoelectric power measurements on these films have been carried out, after annealing, in the temperature range from 300 to about 500 K. It is found from the study that thermoelectric power is independent of temperature and is also, apparently, independent of thickness, over the range of temperatures and thicknesses investigated. The results are discussed on the basis of size effect and thermoelectric effect theories.

Thermoelectric power of tellurium thin films and its thickness and temperature dependence

Anomalous variation of resistance in bismuth thin films and the effect of substrate temperature

Bismuth thin films of various thicknesses between 15 nm and 350 nm were vacuum deposited at room temperature on to glass substrates, immediately after which they were twice heat treated at a uniform rate. During the heat treatment, the resistance changes were monitored and, using these data, the initial lattice distortion energy spectra of as-grown bismuth thin films have been evaluated. It is found that the defects have preferential activation energy values around 1.06 eV, 1.14 eV and 1.32 ev. It is also found that ∫F0 (E) dE oscillates with thickness, which is attributed to the quantum size effect.

Lattice distortion energy spectra of as-grown bismuth thin films and their thickness dependence

Oscillatory behaviour of resistivity with thickness in bismuth thin films

Bismuth thin films of thickness 720±10 A have been vacuum deposited on clean glass substrates held at various substrate temperatures ranging between 30 ° and 185 °C at a constant deposition rate 0f 3±0.3 A s. Immediately after the formation of the films, the heat treatment was given to the films in situ. For each film, the resistance was recorded as a function of temperature during the heat treatment. It has been found that the resistance of the films decreases when the temperature increases during the first cycle of heating, and this has been attributed to the removal of defects. During the first cycle of cooling and the next cycle of heating and cooling, it is observed that the resistance first decreases when the temperature increases and then increases as the temperature increases. Using Vand’s theory, from the resistance–temperature data during heat treatment, both defect density and the initial lattice distortion energy spectra have been evaluated. It is observed from these spectra that as the substr...

N. Jayaprakash

Papers

Thermoelectric power of tellurium thin films and its thickness and temperature dependence

Anomalous variation of resistance in bismuth thin films and the effect of substrate temperature

Lattice distortion energy spectra of as-grown bismuth thin films and their thickness dependence

Oscillatory behaviour of resistivity with thickness in bismuth thin films

Effect of substrate temperature during growth on defect density in bismuth thin films