Laser synthesis of nanometric iron oxide films for thermo-sensing applications

• Large values of the S coefficient of 8.7 mV/K and ZT = 12 at 290–304 K.
• Dependence of S coefficient and ZT on substrate nature temperature and ambient oxygen pressure.
• Effective thermo-sensors operating at moderate temperature.

KrF* excimer laser pulses of 248 nm were used for the synthesis of nanometric iron oxide films with variable thickness, stoichiometry and electrical properties. Film deposition was carried out on <1 0 0> Si and SiO2 substrates. The number of laser pulses was increased from 4000 to 6000, while ambient reactive oxygen pressure varied from 0.1 to 1.0 Pa. The film thickness depends on oxygen pressure, number of laser pulses and substrate nature. All films demonstrated semiconducting temperature behaviour with variable band gap (Eg) depending on oxygen pressure, substrate nature and temperature. Eg value was less than 1.0 eV for all deposited films. XRD analysis evidenced that films deposited on Si substrate have polycrystalline structure, while films deposited on SiO2 were amorphous. The higher oxygen pressure, the lower crystallinity of the deposited film was observed, resulting in change of thermo electromotive force coefficient (S) value. For larger substrate temperature, a better crystallization was observed in the deposited films, resulting in increased S coefficient value. The largest value of the S coefficient was about 8.7 mV/K in the range 290–295 K and it decreased to 1.0–1.6 mV/K when heating temperature changed from 240 to 330 K. The figure of merit of deposited structures was ZT = 3–6 in the range 240–330 K with a maximum of 12 at 300–304 K. We have shown that thermo-sensing characteristics of the films strongly depend on their electrical and structural properties.

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