Epitaxial CrN Thin Films with High Thermoelectric Figure of Merit

Abstract

The design of efficient thermoelectric (TE) devices for energy harvesting and advanced cooling applications is one of the current challenges in materials science. [1] So far, the most common materials used in commercial TE devices are rock-salt IV-VI (PbTe, PbSe) and distorted rock-salt V2-VI3 (Bi2Te3, Bi2Se3) semiconductors. [2] One of the key factors behind the high TE performance of these materials is their abnormally low lattice thermal conductivity (κl), 2 which is one of the fundamental parameters that define the dimensionless TE figure of merit zT = S 2 σT/(κl+κe), in which S is the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κe the electronic thermal conductivity.In a recent paper, Lee et al. [3] suggested that the main reason for the low lattice thermal conductivity in rock-salt IV-VI compounds is the resonant bonding (RB) effect: the p-orbitals with 3 electrons per atom cannot form the six saturated bonds of the rock-salt lattice, and therefore an RB structure is established. [ 4 ] Using first-principles calculations, they demonstrated that the large electronic polarizability of the resonant bonds introduces long-range interactions and a softening of the transverse optical phonon mode. This ultimately causes acoustic phonon scattering and is responsible for the low lattice thermal conductivity in IV-VI and V2-VI3 compounds. An interesting question is whether or not the concept of RB can be extrapolated to transition-metal (TM) compounds with a rock-salt structure. The versatility of the oxidation states and ionic sizes shown by TM ions would offer enormous possibilities for tuning their TE properties, which would allow for new approaches regarding the design of TE materials with improved capabilities.In this communication we demonstrate that rock-salt CrN shows intrinsic lattice instabilities that suppress its thermal conductivity. Using ab-initio calculations, we determined that the origin of these instabilities is similar to that observed in IV-VI compounds with RB states. [3,5] Through the fabrication of high quality epitaxial (001) CrN thin films we report a 250% increase in the zT at room temperature compared to bulk CrN. [6] These results along with its high thermal stability, resistance to corrosion, and exceptional mechanical properties, make CrN a promising n-type material for high-temperature TE applications.The presence of extrinsic factors, such as N-vacancies or epitaxial constrains, are likely behind the large variety of structural and transport properties previously reported for CrN films. [7] In the case of polycrystalline bulk CrN, the intergrain contribution to the electrical and 3 thermal conductivities can be significant enough to mask its intrinsic transport properties and, ultimately, its thermoelectric performance. Therefore, in order to access the intrinsic thermoelectric properties of CrN, it is necessary to develop the fabrication of epitaxial, stoichiometric, and fully relaxed CrN films. The results discussed in this pap...

Microstructure and thermoelectric properties of CrN and CrN/Cr2N thin films

“…In addition to the more traditional applications and properties of CrN, studies on the thermoelectric properties have shown that CrN has a moderate electrical resistivity, low thermal conductivity, and high Seebeck coefficient values. Quintela et al [7] reported on 1:1 stoichiometric CrN thin films, showing a Seebeck coefficient of approximately −120 µ VK −1 and a dimensionless figure of merit ( zT ) of 0.12 at room temperature (compared to 0.1 for PbTe:Tl). Their work emphasizes the fact that annealed samples have better electrical and thermal conductivity, resulting in films with enhanced thermoelectric properties.…”

Reduction of the thermal conductivity of the thermoelectric material ScN by Nb alloying

“…[1][2][3][4][5][6][7][8] Scandium nitride (ScN) is one of them and is an n-type semiconductor with a reported indirect band gap of around 0.9 eV 9,10 . It possesses both a high carrier concentration (10 and has a low electrical resistivity of around 300 µΩcm.…”

Design of Transition-Metal Nitride Thin Films for Thermoelectrics

“…Thus Gall et al concluded that both N vacancies and the density of crystalline defects that can generate during synthesis are the main reasons for the variation of electrical transport in CrN. 160 These suggestions are confirmed by the recent report from Rivadulla et al 144 They studies thermoelectric effect of non-stoichiometric and stoichiometric CrN(001) thin films on MgO(001) growth by DC magnetron sputtering and annealed in ammonia (NH3) gas for 2 h at 800°C to obtain stoichiometric CrN films.Ṫhe results shows that there is a factor of 100 different in the electrical resistivity. The main reason of such a improvement is due to improvement of crystalline quality yielding an improvement of mobility.…”

“…[136][137][138][139][140][141][142] CrN is easy to synthesis as a thin film by magnetron sputtering (DC, pulsed DC, middle frequency RF, or High Power Impulse). [141][142][143][144][145][146][147] CrN thin films can be grown with a very low internal stresses, thus it can be synthesized in various thickness from thin to thick coatings (100 nm to 100 µm) on a variety of steels and engineering ceramics with very good adhesion. 137,141,142,148 Moreover there are number of reports on possibility on synthesis CrN in a bulk form as well.…”

“…The main reason of such a improvement is due to improvement of crystalline quality yielding an improvement of mobility. 144 In science, the theoretical descriptions of nature can be formed by analyzing the result of experiment, or by performing calculations based on existing theories and axioms. It is well acknowledged that theoretical modeling is a powerful tool for gaining information about the nature of materials in materials science.…”