CoSb3 Thin Films Research Articles

High-performance In filled CoSb3 nano thin films fabricated by multi-step co-sputtering method

In filled CoSb3 thin films with various In content were prepared on flexible substrates by multi-step co-sputtering method, which provided a promising way to prepare low-cost and high performance thin films. The influence of In content on the microstructure and thermoelectric properties of the CoSb3 based thin films are systematically investigated. The thin film transforms from P-type to N-type with the increasing of In content. Both of the electrical conductivity and the Seebeck coefficient of the thin films increase after In filled. The thermal conductivity of the In filled thin film significantly decreases and the ZT value of the In-filled thin films are much higher than that of the unfilled thin film. It can be observed that the In filled thin films are single CoSb3 phase and the grain size increases after In filled. The average grain size of all the thin films are less than 50nm, indicates the thin films are nano-particles structure.

Thermoelectric properties of cobalt–antimonide thin films prepared by radio frequency co-sputtering

Co–Sb thin films with an Sb content in the range 65–76at.%, were deposited on a thermally oxidized Si (100) substrate preheated at 200°C using radio-frequency co-sputtering. Evaluation using scanning electron microscopy images and X-ray diffraction reveals that the films were polycrystalline, with a grain size in the range 100–250nm. Energy-dispersive spectroscopy analysis indicates single-phase CoSb2 and CoSb3 films, as well as multiphase thin films with either CoSb2 or CoSb3 as the dominant phase. The electrical and thermoelectric properties were measured and found to be strongly dependent on the observed phases and the defect concentrations. The CoSb2 thin films were found to exhibit a significant n-type thermoelectric effect, which, coupled with the very low electrical resistivity, resulted in a larger power factor than that of the CoSb3 thin films. We find power factors of 0.73mWm−1K−2 and 0.67mWm−1K−2 for the CoSb2 and CoSb3 thin films, respectively.

Thermoelectric Properties of Cobalt Antimony Thin Films Deposited on Flexible Substrates by Radio Frequency Magnetron Sputtering

Cobalt antimony thin films were deposited on flexible substrates at room temperature by radio frequency magnetron sputtering with different sputtering power. The atomic ratio of Co to Sb in the cobalt antimony thin film was closest to 1:3 when the sputtering power was 55 W. The thermoelectric properties of the thin films deposited at room temperature were inconspicuous due to their amorphous microstructure which was characterized by x-ray diffraction. To enhance the thermoelectric properties of the thin films, cobalt antimony thin film deposited by sputtering power of 55 W was annealed at various temperatures ranging from 443 K to 593 K. It was found that all the thin films had n-type conductivity and the CoSb3 thin films annealed at 493–593 K were polycrystalline with (310) preferential orientation. The Seebeck coefficient of CoSb3 thin films annealed at 543 K increased with the raising of the measuring temperature (323–473 K), and the maximum Seebeck coefficient was −88 μV/K, which is the highest value for CoSb3 thin films deposited on flexible substrate.

Structural properties of thermoelectric CoSb3 skutterudite thin films prepared by molecular beam deposition

The research field of thermoelectricity was renewed by the progress made in nanostructuring approaches and by the investigation of new material groups as skutterudites, whose most promising representative is CoSb3. In this work Co–Sb thin films with a thickness of 30nm were deposited by molecular beam deposition at different substrate temperatures as well as on non-heated substrates followed by a post-annealing step. An extended investigation of the phase formation in dependence of deposition method and parameters, film composition, and post-treatment is given. The presented results provide different routes to achieve high quality single phase films. It was also demonstrated that the grain size of the CoSb3 thin films is very sensitive to the used deposition method and especially on the substrate temperature during deposition. A controllable grain size by changing the deposition parameters could be a key feature for the optimization of the thermoelectric properties, since especially the thermal conductivity should strongly depend on the grain size due to enhanced grain boundary scattering of phonons.

Preparation and thermoelectric properties of RF co-sputtered CoSb3 skutteruddite thin films

Various skutterudites have been under investigation because of their potential application in thermoelectric materials and devices. These studies indicate that bulk materials with a skutterudite-type structure show reasonable thermoelectric behaviors at elevated temperatures, which make them suitable for high-temperature thermoelectric applications. The results of thermoelectric-property measurements on skutterudite-phase CoSb3 thin films are presented in this study in order to extend the bulk material concept to micro-device applications by using thin film deposition technology. Thin films are deposited on oxidized silicon substrates by using the RF co-sputtering method. The film’s composition is found to depend strongly on the deposition conditions. The temperature at which the deposited films transition from an amorphous state to a crystalline state has been reported to be about 153 °C. Therefore, some experiments are performed with the substrate temperature kept at 200 °C to obtain polycrystalline films. The crystal structure of the film is evaluated by using x-ray diffraction (XRD) measurements. Energy dispersive spectroscopy (EDS) is used to determine the film’s composition whereas the surface morphology and the thickness are investigated and measured by using scanning electron microscopy (SEM). Finally, the thermoelectric properties, namely, the electrical resistivity and the Seebeck coefficient, are used to calculate the power factor of the deposited thin films and, their variations with temperature are measured. We report a maximum power factor of 0.41 mW/mK2 for the film deposited at a 200 °C substrate temperature.

A voltammetric study of the underpotential deposition of cobalt and antimony on gold

This study reports on preliminary experiments which are required to define the experimental conditions for depositing CoSb3 thin films by the electrochemical atomic layer epitaxy (EC-ALE) method. The underpotential deposition (UPD) of cobalt and antimony on polycrystalline gold substrates has been investigated by means of cyclic voltammetry, anodic potentiodynamic scanning and coulometry. The UPD of Sb on Co-covered Au and of Co on Sb-covered Au have also been studied and compared to those obtained on bare Au substrate.The UPD potential values for both Co and Sb are reported in this paper, and owing to the flexibility of the EC-ALE equipment, new insights on the Co UPD on gold substrate are presented. This study shows that the subsequent alternate deposition of Co and Sb monolayers is feasible and thus that the EC-ALE method can be used to elaborate CoSb3 thin films.

Influence of the substrate thermal expansion coefficient on the morphology and elastic stress of CoSb3 thin films

AbstractDuring the post‐annealing and cooling process of CoSb3 thin films deposited on thermally oxidized Si(100) substrates, cracks occur at the surface of the films, which can be caused by the difference in thermal expansion coefficient of the substrate and the film. To investigate the crack formation, 40‐nm‐thick CoSb3 films were deposited at room temperature under ultra‐high vacuum (UHV) conditions onto various substrates, exhibiting different thermal expansion coefficients (2 × 10−6 to 12 × 10−6 K−1). All samples were post‐annealed in UHV at 500 °C for 1 h. The composition of the films was verified by Rutherford backscattering spectrometry. The phase formation and elastic stress of the films were analyzed by X‐ray diffraction, confirming the formation of the desired skutterudite phase, while the individual grains were studied by electron backscatter diffraction. In addition, the surface morphology and the roughness of the films were investigated by atomic force microscopy. For substrates with a thermal expansion coefficient between 9 × 10−6 and 11 × 10−6 K−1, crack formation can be prevented and a minimum in roughness was found, resulting also in a minimal value of the electrical resistivity.

Lattice dynamics and substrate-dependent transport properties of (In, Yb)-doped CoSb3 skutterudite thin films

Lattice dynamics, low-temperature electrical transport, and high-temperature thermoelectric properties of (In, Yb)-doped CoSb3 thin films on different substrates are reported. Pulsed laser deposition under optimized conditions yielded single-phase polycrystalline skutterudite films. Raman spectroscopy studies suggested that In and Yb dopants occupy the cage sites in the skutterudite lattice. Low-temperature electrical transport studies revealed the n-type semiconducting nature of the films with extrinsic and intrinsic conduction mechanisms, in sharp contrast to the degenerate nature reported for identical bulk samples. Calculations yielded a direct bandgap close to 50 meV with no evidence of an indirect gap. The carrier concentration of the films was identical to that reported for the bulk and increased with temperature beyond 250 K. The higher resistivity exhibited is attributed to the enhanced grain boundary scattering in films with a high concentration of grains. The maximum power factor of ∼0.68 W m−1 K−1 obtained at 660 K for the film on glass is found to be nearly four times smaller compared to that reported for the bulk. The observed difference in the power factors of the films on different substrates is explained on the basis of the diffusion of oxygen from the substrates and the formation of highly conducting CoSb2 phase upon the oxidation of CoSb3.

Pulsed laser deposition and thermoelectric properties of In- and Yb-doped CoSb3 skutterudite thin films

Abstract

Low Thermal Conductivity and Related Thermoelectric Properties of Zn4Sb3 and CoSb3 Thin Films

ABSTRACTThermal conductivity and other thermoelectric properties were investigated on β-Zn4Sb3 and CoSb3 thin films. They are prepared by co-deposition of the specific sets of Zn, Sb and Co targets by RF magnetron sputtering. Thermal conductivity measurement was performed based on an AC calorimetry method using LaserPIT. The Zn4Sb3 thin films investigated are Zn-rich with respect to the stoichiometric composition. Their thermal conductivity decrement is proportional to the reciprocal film thickness. A nano-scale grain size in a ∼350 nm-thick film specimen gives arise to an almost 50% reduction in its thermal conductivity (∼0.5W/mK) at room temperature. Low electrical resistivity and high Seebeck coefficient can be achieved simultaneously in the film specimen with properly controlled thickness and microstructure. A ZT of 1.2 at ∼460K has been obtained for the ∼350nm thick Zn4Sb3 film specimen. The minimum thermal conductivity of about 1.1 W/mK was obtained in a ∼325 nm-thick CoSb3 film specimen at room temperature.

Transport and structural properties of binary skutterudite CoSb3 thin films grown by dc magnetron sputtering technique

The results of the studies of structural, electrical, and thermoelectric properties of binary skutterudite CoSb3 thin films are presented. The n-type films were prepared without intentional doping on oxidized Si(100) and ceramic Al2O3 substrates using magnetron dc sputtering technique. The electron probe microanalysis has shown that the composition of the films is strongly affected by the preparation conditions. The x-ray measurements indicate the films to be amorphous if they are deposited on substrates kept at room temperature. On the basis of the differential thermal and x-ray analyses data, the temperature of the transition from the amorphous to crystalline state was established to be (153±1)°C. The films are polycrystalline with a preferential orientation of the growth along the (310) direction of the CoSb3 lattice if grown on substrates kept at 200 °C. The electrical resistivity ρ(T) and the Seebeck coefficient S(T) were measured from room temperature up to about 700 K. The value of ρ(T) decreases with increasing temperature. The maximum absolute value of the Seebeck coefficient is found to be −250 μV/K at around 550 K for both substrates. This is the highest value of the Seebeck coefficient for CoSb3 films deposited by dc magnetron sputtering. The predominant mechanisms of the scattering in CoSb3 are discussed on the basis of the temperature dependence of the Hall mobility. The carrier concentration n(T) is found to be around 1.2×1019 cm−3 at 300 K and is almost constant over the temperature range of (250–400) K. The room temperature measurements of the thermal conductivity carried out by the 3ω method have shown a value of about 3 W/(m K), which is significantly lower than that reported for bulk material.

Crystallization behavior of CoSb3thin films

Crystallization behavior of CoSb₃thin films

Pulsed laser deposition of skutterudite thin films

Skutterudite (CoSb3 and IrSb3) thin films of high phase purity and crystallinity have been prepared by pulsed laser deposition. It is found that while the crystallinity of the films increases with increasing temperature, the phase purity decreases due to antimony volatility. The skutterudite phase can be stabilized to higher temperature by addition of excess antimony to the pulsed laser deposition target. Target-to-substrate distance must be optimized to obtain stoichiometric skutterudite films with low stress. A decrease in the flux of energetic particles and changes in particle stoichiometry with increasing distance caused by plume tilting results in lower film stress, but the diantimonide phase is nucleated if the distance is too large. It is possible to further reduce compressive stress by growing films in an inert background gas to decrease the flux of energetic particles. Overall, the highest quality films are grown from targets with excess antimony at a temperature of 270 °C with a target-to-substrate distance of 7 cm and a background argon gas pressure of 10 mTorr. The resistivity, mobility, and carrier concentration of these films are indicative of a degenerate p-type semiconductor with hole concentrations in the range of previously measured bulk values.

MnドープCoSb<SUB>3</SUB>薄膜の作製と熱電特性

We have prepared Mn-doped CoSb3 thin films by the annealing of ion-beam sputtered Co/Mn/Sb multi-layers at 673 K for 1h in vacuum, and studied the effects of the amount of doped Mn on the thermoelectric properties of the films. Mn atoms are assumed to be substituted for Co sites since the Mn-doped samples indicated no apparent shifts in the lattice parameter from the non-doped sample obtained by X-ray diffraction. Thermoelectric properties of the thin films were evaluated by measuring DC conductivity and Seebeck coefficients at elevated temperatures. The non-doped CoSb3 samples showed a p-type semiconducting character with the conductivity of about 7×103/Ωm irrespective of their chemical composition to be Sb-rich or Sb-deficient. The Seebeck coefficient of the sample was about 160 μV/K at 540 K. In contrast, the samples doped with 1-2 at%Mn exhibited a rather low conductivity of about 1×103/Ωm and their conduction behavior was changed to n-type with the Seebeck coefficient of about −70 μV/K at 500 K.

ＲＦ‐スパッタ法による熱電材料ＣｏＳｂ３薄膜の作製

A CoSb3 compound with a skutterudite structure (MX3) is one of promising thermoelectric materials. In this study, the preparation of CoSb3 thin films on glass substrates by the RF-magnetoron sputtering method has been researched. By using a Co-target and a Sb-target, simultaneous and alternate sputtering methods were carried out. Single phase CoSb3 thin films have been successfully obtained under the following conditions; (1) annealing at 200°C in a vacuum after simultaneous sputtering, (2) annealing at 400°C in a vacuum after making multilayered Co/Sb/Co/Sb films by alternate sputtering.

Physical Properties Of Single-Phase Skutterudite Thin-Films (CoSb3 and IrSb3)

AbstractBulk skutterudite phases based on the CoAs3 structure have yielded compositions with a high thermoelectric figure-of-merit (“ZT”) through the use of doping and substitutional alloying. It is postulated that further enhancements in ZT may be attained in artificially structured skutterudites by engineering the microstructure to enhance carrier mobility while suppressing the phonon component of the thermal conductivity. In this work the growth and properties of singlephase CoSb3 and IrSb3 skutterudite thin films are reported. The films are synthesized by pulsed laser deposition (PLD) where the crystallinity can be controlled by the deposition temperature. Powder X-ray diffraction (PXRD), Transmission electron microscopy (TEM) and Rutherford- Back Scattering (RBS) were used to probe phase, structure, morphology and stoichiometry of the films as functions of growth parameters and substrate type. A substrate temperature of 250°C was found to be optimal for the deposition of the skutterudites from stoichiometric targets. Above this temperature the film is depleted of antimony due to its high vapor pressure eventually reaching a composition where the skutterudite structure is no longer stable. However, when films are grown from antimony-rich targets the substrate temperature can be increased to at least 350°C while maintaining the skutterudite phase. In addition, adhesion properties of the films are explored in terms of the growth mode and substrate interaction. Finally, preliminary room temperature electrical and thermal measurements are reported.