Dielectric Susceptibility and Strain in Sr 1-X Ba X Tio 3 Ferroelectric Thin Films Grown by Pulsed Laser Deposition

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73 Mat. Res. Soc. Symp. Proc. Vol. 388 01995 Materials Research Society

mLll 4

100 PM

200 m Figure 1.

SEM photomicrograph of interdigital electrodes for capacitance measurements. (Chrome/gold)

EXPERIMENTAL Thin films of SBT on (001) LaAlO 3 were grown by pulsed laser deposition for compositions x = 0, 0.35, 0.65 and 0.80. The pulsed laser deposition system used to grow these SBT films has been described previously in more detail [4,5]. A pulsed eximer laser (-30 nsec pulses, -300 mJ/pulse and ) = 248 nm focused with a 50 cm focal length lens to a fluence of -2 J/cm 2) was used to ablate the pressed powder targets of various SrTiO3 and BaTiO 3 compositions. The vaporized material was deposited onto a heated LaAIO 3 substrate approximately 3 cm away from the target. The substrate was heated to 750°C in a 350 mTorr oxygen atmosphere. The films grew at approximately 2 A/pulse to a total film thickness of 0.6 pim. One 7 pgm thick film was grown. The film composition and thickness were determined by Rutherford backscattering of 6.2 MeV He 2 ÷ ions. Capacitance measurements were made by e-beam evaporating chrome/gold interdigital electrodes on the surface of the SBT films. The electrodes were patterned by a standard lithography technique. An SEM photomicrograph of these electrodes is shown in Figure 1, where the gap spacing is 5 g.m, the finger width is 7.5 p.m and the finger length is 75 pm. The electrodes were -1500 A thick, and electrical contact was made by wire bonding two gold wires to the large electrode contact pads. One wire was attached to each pad next to the fingers. Measurements of the capacitance and relative dissipation factor as a function of temperature were made at 1 kHz using an HP4284A LCR meter. The temperature control was achieved with an APD Cryogenics closed cycle refrigerator with a Lakeshore 330 temperature controller. A reference measurement was made on an uncoated LaA10 3 substrate which showed a negligible temperature dependence. Thus changes in the capacitance as a function of temperature are due to changes in the dielectric susceptibility of the film. Non-uniform strain was determined from x-ray diffraction using a Rigaku Rotaflex diffractometer with Cu K. radiation from a rotating anode source. RESULTS AND DISCUSSION SBT films grown by pulsed laser deposition were found, by x-ray diffraction, to be single phase, well oriented and nearly epitaxial. Figure 2 shows a 0/20 scan for x = 0.35 which is typical of all the compositions investigated. The film is exclusively (00f) oriented with a (002) Cascan width of only 0.2090. The coscan widths for the (002) line of all the compositions were less than 74

106

105 4

10

--

ubstrate S N S Lattice Parameter

T

C 10

x

aA

0.00

3.9065 3.9357 3.9576

S00.35

0.65 101

3.9741

S0.80

10 20 30 40 50 60 70 80 90 100110120 20 Figure 2.

X-ray diffraction from a thin film of SBT x = 0.35 on (001) oriented LaAIO 3. The lattice parameters listed on the right are calculated from the (004) line position.

0.5'. The lattice constants of the films we