C-Axis Oriented Ferroelectric thin Films of PbTiO 3 on Silicon Substrate by Pulsed Laser Ablation
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20 Fig. 1. X-ray diffraction patterns of PbTiO 3 thin film on Si at different substrate temperature, 200 mtorr oxygen pressure and 3.5 J/cm 2 energy density. Pb2 Ti20 6 impurity phase is indicated by '.'.
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25 ns and a pulse repetition rate of 10 Hz were used. The silicon substrates were mounted on a heater block parallel to the target . The substrate temperature was measured by thermocouple embedded into the heater block. The target was scanned by a stepper motor controlled target scanner to avoid local heating. The target to substrate distance was maintained at 5.5 cm. The substrate temperature was controlled by a Eurotherm P808 controller. The ablation was carried out in oxygen ambient. At the termination of the ablation, the chamber was flooded with oxygen and the films were allowed to cool at the normal rate. In order to obtain PbTiO 3 films with best possible ferroelectric properties, the deposition conditions were optimized in terms of substrate temperature, oxygen pressure and energy density of the laser beam. The nominal film thickness was maintained to ; 1000 A. The phase purity and orientation of the films was checked by X-ray Diffractometer (XRD) using Cu(Ka) radiation. The microstructure of the films was studied by Atomic Force Microscopy (AFM). The relative values of Pb/Ti ratio were obtained from Energy Dispersive Xray (EDX) analysis. The capacitance of the films was measured using a LCR meter with a standard parallel plate geometry with silver electrodes. The measurements were done at 500 kHz frequency. RESULTS Figure 1 shows the XRD spectra of the films grown at temperatures in the range of 600800'C at 200 mtorr oxygen pressure and laser pulse energy of 3.5 J/cm 2 It is clear from the figure that keeping in situ growth temperature above 700 0 C helps to avoid the formation of metastable, nonferroelectric Pb2Ti 2 0 6 pyrochlore phase at the interface. It also indicates that the films are preferentially oriented with c-axis perpendicular to the substrate. It is clear from the figure 1 that as the substrate temperature increases the crystalline nature improves. The calculated lattice constant c, did not change with substrate temperature. The c-parameter was found to be 4.138A which is very close to that of reported bulk value of 4.15.A. The small decrease in the value of c-parameter is possibly occurring due to grain size [12]. It is important to note that the key to the realization of pure perovskite PbTiO 3 phase is the optimized substrate temperature. It is reasonable to assume that a thin layer of Si0 2 will be formed on Si during the initial stage of ablation in oxygen atmosphere. But this layer is likely to be absorbed by the growing PbTiO 3 film. This conjecture is based on our own work on SiO 2 doped PbTiO 3 bulk samples which have resulted in a perovskite phase at calcination temperature above 700'C. Neutron diffraction studies on bulk sample shows that Si in fact occupies the interstitial posi
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