Nonperovskites KTa1XNbxO3 Bi4Ti3O12 LiNbO3

Many of these materials contain volatile components such as lead (Ref 72), potassium (Ref 73), or lithium (Ref 74). These elements present unique problems for obtaining crystalline thin films. The crystallization temperatures (500 to 700 °C) can lead to changes in the film composition because of evaporation of the more volatile components. In spite of large differences between component vapor pressures, high-quality ferroelectric films have been successfully deposited by PLD.

In PLD, stoichiometric evaporation and transfer of the target material does not guarantee a stoichiometric thin film. That is to say, the surface temperatures required to ensure adequate surface mobility may give rise to nonstoichiometric films because of evaporation. One simple solution to this problem has been to overcompensate in the target for volatile species that may be lost from the film (i.e., add excess PbO to the target to minimize the loss of lead from the film) (Ref 75). The technique may work in some cases but is undesirable, because the compensating factors may change depending on the conditions.

It has been observed that in several systems the PLD deposition conditions can be adjusted to compensate for the loss of volatile components at elevated substrate temperatures. It appears that in these systems, higher ambient pressures (> 300 mtorr) minimize the loss of these components. This is probably the result of several factors. Higher gas pressures lead to a reduction in the kinetic energy of the arriving vapor (increasing the sticking coefficient). Higher pressures favor increased oxidation, either in the gas phase or as a result of gas-surface collisions. In the case of lead, it is believed that PbO has a lower equilibrium vapor pressure than lead and is therefore retained at elevated oxygen deposition pressures.

As with HTS materials, single-layer ferroelectric films by themselves are not easily incorporated into a device. The ferroelectric properties of interest arise as a result of electric field effects and as such have presented a unique challenge for their implementation. Electrodes are required that are noninteracting under film growth conditions and that do not degrade the ferroelectric film behavior under repeated cycling. Although noble metals such as platinum or gold are commonly used, the interface between the metal and the ferroelectric film causes the ferroelectric to degrade in time. Oxide conductors such as YBCO (Ref 76), (La,Sr)CoO3 (Ref 77), and (Sr,Ca)RuO3 (Ref 78) are ideally suited for ferroelectric integration. These oxide conductors are lattice-matched perovskites. Multilayer structures have been successfully deposited by PLD that have demonstrated superior characteristics over noble metal structures.

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