Rivadulla

Fabrication of oxide and nitride thin-films with improved thermoelectric efficiency

Thermoelectric (TE) materials produce an electrical voltage in response to a temperature gradient. On a reverse version of this effect, thermoelectric cooling occurs when an electric current is passed through the system. Therefore, these materials can be used in thermoelectric energy conversion devices.

We are exploring the possibility of using low dimensional correlated oxides and nitrides (thin-films and multilayers) as TE materials. We try to exploit the effects of electronic and phononic confinement and interface scattering to optimize their thermoelectric performance.

We have fabricated thin-films of CrN and demonstrated an important increase in their thermoelectric efficiency with respect to the bulk material. We are now involved in the fabrication of multilayers of different oxides and nitrides by Pulsed Laser Deposition (PLD).

         Figure 1: XRD and x-ray reflectivity patterns of CrN films synthesized by sputtering (right).

Also, we have demonstrated that epitaxial strain allows an independent control of the thermoelectric power and the electrical resistivity in polaronic conductors. This has been exemplified in ultra thin-films of La₂NiO₄.

  
  Figure 2: High-resolution TEM picture of a La₂NiO₄ thin-film deposited on top of SrTiO₃ by PLD.
  

Development of chemical methods of thin-film deposition 

In the last few years, chemical solution deposition techniques emerged as a cheaper alternative to physical methods (MBE, PLD, sputtering, etc). Although the control of thickness, homogeneity and stoichiometry is still not competitive with physical techniques, the quality of the films is improving very fast.

We use polymer assisted deposition for the fabrication of thin-films of many materials with different properties, starting from environmentally friendly water solutions of biocompatible polymers.

Using this method, we were able to synthesize high quality films and multilayers of different oxides, with different properties, and over very large areas.

    
Figure 3. Left: PLD-PAD comparison. HRTEM of the crystalline quality of a thin film of LaMnO3 (15nm) deposited on SrTiO3 substrate by PAD (chemical deposition) and PLD.
Right: Epitaxial thin film of LaMnO3, 20 nm thick, deposited in a substrate of 1"-STO (chemical deposition). Optical photolithography was used to define 340 Hall bars (see inset). Results demonstrate an exceptional electrical homogeneity over the whole area.

     
Figure 4: Left: A thin film of LaMnO₃ (15 nm thick) deposited on a 1"-SrTiO₃ substrate by polymer assisted deposition. Right: High-resolution TEM picture of the LaMnO₃ / SrTiO₃ interface.

Figure 5: High-resolution microscopy of a bilayer of 20 nm thick film LaMnO₃ and 6 nm thick film of LaCoO₃ grown on top of it. The epitaxial growth is seen in the diffraction (right). The samples have been prepared by polymer assisted deposition. The independent reversal of the magnetization of both layers can be appreciated in the magnetic hysteresis loop M(H) plot.

Figure 6: High-resolution TEM picture of a 25 nm thick film of Bi_1.68Ca₂Co_1.69Oy thermoelectric material deposited on LaAlO₃ by polymer assisted deposition. The homogeneous orientation of the film can be appreciated and the different layers of CoO and BiCa of the structure are clearly resolved in the STEM-HAADF image. The thermoelectric power and electrical resistivity is shown in the right panel.

Magnetic and transport properties (electronic and thermal) in transition-metal compounds 

Metal-insulator transitions in 3d-metal compounds, mainly oxides, occur very often at unexpected electron fillings, due to electronic correlations.

When these insulators (Mott-Hubbard insulators) are doped or the correlations are reduced (by means of chemical or hydrostatic pressure), unconventional phases of matter emerge: spontaneous electronic and magnetic phase segregation, non-Fermi liquid and overall high temperature superconductivity.

We are currently studying correlated itinerant-electron systems, like NaxCoO₂, CoS₂, manganites, spinels, etc, to describe the charge and spin excitations in a regime in which magnetic, charge and lattice degrees of freedom are strongly coupled. We are also interested in the process of heat transport in magnetic systems with geometric frustration, like pyrochlores and spinels (spin liquids).