Darin T. Thomas

Preferred Orientation, Phase Formation and the Electrical Properties of Pulsed Laser Deposited SrBi2Ta2O9 Thin Films.

Systematic studies on the preferred orientation, phase formation and the electrical properties of pulsed laser deposited SrBi2Ta2O9 (SBT) have been performed. At a substrate temperature of 600°C, the film on SiO2 was not crystallized. Above 650°C, very strong diffraction from a non-ferroelectric, (111)-oriented fluorite phase was obtained. Its crystallinity increased with increasing Bi content in the target. The fluorite phase was also observed in the film on Pt deposited below 600°C, which together with the pyrochlore phase resulted in the formation of an interfacial reaction product, Bi2Pt. Above 650°C, the films tend to grow as the perovskite SBT phase with a c-axis orientation which is not the polarization axis. In contrast, the formation of the fluorite and pyrochlore phases can be suppressed on Ir electrodes.

Growth and Fabrication of 2 inch Free-standing GaN Substrates via the Boule Growth Method

High quality, single crystal GaN substrates have been demonstrated using a boule growth process. Here we report on the crystalline boules that were formed during the growth process and their material characterization. Using hydride vapor phase epitaxy process, GaN crystals were grown at growth rates greater than 200 μm/hr. Boules greater than 3 mm thick were grown and processed into free-standing substrates. Rocking curve measurements using high-resolution X-ray diffraction were performed on the substrates with FWHM values of 92 and 146 arcsec for the (002) and (102) reflections, respectively. Atomic force microscope images, etch pit studies, and transmission electron micrographs of the GaN material show high quality material quality with a dislocation density in the range of 5×10 6 to 1×10 7 cm -2 .

Pulsed laser ablation synthesis and characterization of layered SrBi2Ta2O9 films and integration into capacitors for non-volatile memories

Abstract Pulsed laser ablation deposition (PLAD) was used to synthesize SrBi2Ta2O9 (SBT) layered ferroelectric thin films to integrate them into capacitors with top and bottom Pt electrodes produce by an ion beam sputter-deposition technique. SBT layers produced by laser ablation of stoichiometric and Bi-rich SBT targets exhibited marked different orientations. In addition, Pt/SBT/Pt capacitors fabricated with the distinctly oriented SBT layers exhibited substantial difference in the shape and parameters of polarization hysteresis loops, although they exhibited practically no polarization fatigue. The results presented in this paper indicate that the composition, microstructure and properties of SBT layers and SBT-based capacitors depend on the target composition and more studies are needed to understand these dependencies.

Composition and Electrode Effects on the Electrical Properties of SrBi 2 Ta 2 O 9

SrBi 2 Ta 2 O 9 has attracted great interest for non-volatile memory applications due to its minimal polarization fatigue. This paper describes systematic studies, using pulsed laser deposition, on the effect of deposition conditions on the Bi-Pt reaction and on the potential for low temperature processing. Changing the deposition temperature (T s ) and oxygen gas pressure during deposition can control the Bi content in the films. At a T s of 600°C, the films have excess Bi and do not fully crystallize to SBT, resulting in poor remnant polarization (P r ). These films consist mostly of the pyrochlore phase, plus a small amount of disordered, c-oriented layered perovskite SBT. By annealing over 750°C, the films show improved P r , but further Pt - Bi interactions occur. At a T s of 700°C, the as-deposited films are fully crystallized and show saturated hysteresis loops. However, Bi deficiency through alloying results in reduced remnant polarization (2P r = 7.0μC/cm 2 ). Films on Ir/Pt show reduced electrode reactions and improved properties.

Sr/Bi ratio effects for Sr/sub x/Bi/sub y/Ta/sub 2/O/sub 9/ grown by pulsed laser ablation

In this study we report the effect of variations in the Sr/Bi ratio on the properties of thin film strontium bismuth tantalate (SBT), one of the primary candidates for ferroelectric nonvolatile memories. The composition was varied according to the formulation Sr/sub 1-z/Bi/sub 2.1+2/3z/Ta/sub 2/O/sub 9/. Films were prepared by pulsed laser ablation from ceramic targets. The largest switched polarizations were measured for compositions with z=0. Results are compared to other studies where off-stoichiometric compositions yielded the largest polarizations. In attempting to reduce the processing temperatures, moderate ferroelectric properties were achieved at 650/spl deg/C.