Improvement of thermoelectric properties of Bi₂Te₃ and Sb₂Te₃ films grown on graphene substrate

Abstract

Phone: þ82 42 860 7677, Fax: þ82 42 861 4151A study of substrate effect on the thermoelectric (TE) properties of Bi 2 Te 3 (BT) and Sb 2 Te 3 (ST) thin films grown by plasma-enhanced chemical vapor deposition (PECVD) was performed. Graphene substrates which have small lattice mismatch with BT and ST were used for the preparation of highly oriented BT and ST thin films. Carrier mobility of the epitaxial BT and ST films grown on the graphene substrates increased as the deposition temperature increased, which was not observed in that of SiO 2 /Si substrates. Seebeck coefficients of the as-grown BT and ST films were observed to be maintained even though carrier concentration increased in the epitaxial BT and ST films on graphene substrate. Although Seebeck coefficient was not improved, power factor of the as-grown BT and ST films was considerably enhanced due to the increase of electrical conductivity resulting from the high carrier mobility and moderate carrier concentration in the epitaxial BT and ST films.Thermoelectric (TE) energy generators have promising applications in the operation of micro-scale electric devices and systems. Many previous researches have been focused on the enhancement of the efficiency of TE materials [1][2][3][4][5][6][7][8]. The efficiency of thermoelectric materials depends on the dimensionless thermoelectric figure of merit ZT ¼ a 2 sT/k where a, T, s, and k are the Seebeck coefficient, absolute temperature, electrical conductivity, and thermal conductivity, respectively. The recent reports explained that the enhancement of ZT was mainly due to the reduction of k by nanostructure-engineering which dramatically increases phonon boundary scattering at interfaces [1,2,[5][6][7]. However, the enhancement of ZT by increasing the thermoelectric power factor (a 2 s) has been reported [9][10][11][12].Various deposition methods have been reported for preparation of highly oriented Bi 2 Te 3 (BT) and Sb 2 Te 3 (ST) thin films, such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), and pulsed laser deposition (PLD) [13][14][15]. MOCVD has attracted the most amount of attention due to its low cost and high controllability for thin film growth, which make it most suitable for large-scale production. On the other hand, the plasma-enhanced chemical vapor deposition (PECVD) has an advantage of low deposition temperature with a relatively high deposition rate in comparison with conventional MOCVD process.It is well known that the electrical conductivity of BT and ST along c-plane is much higher than that of c-axis because of the layered structural characteristics of single crystalline BT and ST [16,17]. Furthermore, epitaxial BT and ST thin films are also expected to have higher power factors than that of polycrystalline. However, latticematched substrate for BT and ST has rarely been reported until now except expensive yet small-sized crystalline

A comprehensive review on Bi2Te3‐based thin films: Thermoelectrics and beyond

“…Among the various thin film growth parameters, the T sub and the postannealing processing could remarkably alter the formation and transformation of atomic defects and thus the electronic transport properties of n-type Bi 2 Te 3 films. Our results [101] and the studies from other laboratories [81,92,132,133] have shown that the substrate temperatures T sub in the range of 240-350°C are suitable for the stable growth of highquality Bi 2 Te 3 films. Because the thermal evaporation temperature of Te is around 220°C, which is obviously lower than the optimal T sub , Bi 2 Te 3 films are generally grown under a Te-poor environment due to a serious re-evaporation of Te from the heated substrate.…”

“…As documented by the values of PF s from different groups, see Figure 3B, it is obvious that the optimized PF s in Bi 2 Te 3 films are in the range of 2.5–3.5 mWm –1 K –2 at T sub = 240–300°C. Measurements made by Lee et al [ 81 ] and Zhang et al [ 101 ] on single crystalline Bi 2 Te 3 films with the optimal carrier concentration of 7–9 × 10 19 cm –3 attained the highest PF of ∼3.5 mWm –1 K –2 . Similar to the effect of T sub , the annealing temperature ( T ann ) and the annealing time ( t ann ) during the postannealing process can also regulate atomic defects in Bi 2 Te 3 films.…”

“…[ 78–80 ] The proposal generated much interest and provided a strong impetus for explorations of Bi 2 Te 3 ‐based thin films and SLs. Currently, various techniques are being utilized for the fabrication of Bi 2 Te 3 ‐based thin films, among them are metal‐organic chemical vapor deposition (CVD), [ 81–83 ] electrochemical deposition (ECD), [ 84,85 ] molecular beam epitaxy (MBE), [ 86–88 ] thermal coevaporation, [ 89–91 ] magnetron sputtering (MS), [ 92–95 ] and pulsed laser deposition (PLD). [ 96–98 ]…”

“…[78][79][80] The proposal generated much interest and provided a strong impetus for explorations of Bi 2 Te 3 -based thin films and SLs. Currently, various techniques are being utilized for the fabrication of Bi 2 Te 3 -based thin films, among them are metal-organic chemical vapor deposition (CVD), [81][82][83] electrochemical deposition (ECD), [84,85] molecular beam epitaxy (MBE), [86][87][88] thermal coevaporation, [89][90][91] magnetron sputtering (MS), [92][93][94][95] and pulsed laser deposition (PLD). [96][97][98] The film growth is a nonequilibrium kinetic process in which the rate-limiting steps strongly affect the growth mode, morphology, microstructure, and composition of the grown films.…”

Improved thermoelectric performance of Bi2Se3 alloyed Bi2Te3 thin films via low pressure chemical vapour deposition

“…Chemical vapour deposition (CVD) allows for the production of superior quality thin films with better conformity, coverage, and stoichiometric control compared with sputtering. Conventional deposition of binary or ternary compounds requires the use of multiple precursor systems [32], however recent work in single-source precursors (SSPs) is a promising alternative, allowing ease of handling and atom efficiency without compromising on stoichiometry or morphological control. We have previously demonstrated the viabilities of using SSPs to deposit high-quality binary (e.g.…”