A-Site Ordered, Perovskite-Like Manganites Grown by PLD or Laser-MBE: Their Growth, Structural and Physical Characteriza
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277 Mat. Res. Soc. Symp. Proc. Vol. 602 © 2000 Materials Research Society
EXPERIMENTAL Dense ceramic targets of PrMnO 3 and SrMnO 3 were prepared using standard ceramic synthesis methods. Appropriate ratios of Pr 6O 11 , SrCO 3 and MnO 2 powders were mixed and intimately ground using a semi-planetary ball mill. The powder was annealed twice for 12 h at 900'C and once for 12 h at 1100'C, with intermediate grindings. Pellets having a diameter of 2.5 cm were cold uniaxially pressed and sintered for 24 h at 1500'C. Two different deposition systems were used, a "classical" one and a laser-MBE one, and an excimer laser (Lambda Physics) with X = 248 nm (KrF) was utilized with both systems. "Classical" PLD system : Optical quality single-crystal substrates of [001] SrTiO 3 were ultrasonically cleaned in acetone and then in alcohol, and were attached to the heater using silver paste. A thermocouple was mechanically attached to the heater block close to the substrate and the temperature measured is referred to as the deposition temperature. The background pressure of the chamber was 10-5 mbar and the distance between the substrate and target was 47 mm. During deposition, the substrate was held at a constant temperature of 580'C in a dynamic vacuum of 0.3 mbar. After deposition, the oxygen pressure was increased to a static value of 500 mbar and the films were cooled to room temperature at 20°C/min. Superlattices were deposited by alternating the number of laser pulses according to the deposition rate of each material. The deposition rate was observed to be relatively constant when identical deposition parameters were used. The deposition rate for each target is determined by depositing several superlattices and determining the thickness of each layer and the overall superperiod from the satellite peaks observed in the x-ray diffraction patterns. This gives a fairly accurate deposition rate under the conditions of superlattice deposition. Laser-MBE : Optical quality single-crystal substrates of [001] SrTiO 3were also used but after the cleaning in acetone and alcohol they were etched with a NH 4F/HF buffered solution to obtain a terraced surface [11]. The substrate was then attached to the heater block with silver paste. The temperature referred to as the deposition temperature is measured by a thermocouple attached to the heater block. The base pressure of the system was 10-8 mbar and the distance between target and substrate was 6 cm. To improve the quality of the surface, prior to the deposition of the superlattices, the substrate was buffered with about 25 layers of SrTiO 3 deposited at 720'C in a dynamic vacuum of 1.5x 104 mbar of molecular oxygen. During the deposition of the superlattices, the heater was held at constant temperature in a dynamic vacuum of 7.5 x10- 4 mbar, while the gas mixture used during deposition was 94% 02 and 6% 03. A differentially pumped electron gun was used to monitor RHEED oscillations during the deposition. After deposition, the superlattices were cooled to 300'C in the same pressure and a
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