Metal-organic chemical-vapor deposition of (Ba,Sr)TiO3: Nucleation and growth on Pt-(111)

Abstract

Thin films of the ͑Ba x Sr 1−x ͒TiO 3 ͑BST͒ solid solution series were grown by metal-organic chemical-vapor deposition ͑MOCVD͒ on platinized silicon wafers with emphasis to ͑Ba 0.7 Sr 0.3 ͒TiO 3 and SrTiO 3 . The nucleation behavior and the size of the stable nuclei were investigated by different scanning probe microscope techniques including local conductivity scanning. The characteristic differences were observed for different deposition temperatures, i.e., a homogeneous nucleation of small BST grains on the larger Pt grains at 565°C and a dominating grain-boundary nucleation at 655°C. X-ray photoelectron spectroscopy indicates a stoichiometric composition from the beginning. The microstructural evolution was investigated by high-resolution transmission electron microscopy and revealed randomly oriented grains ͑typical in-plane size 10-20 nm͒ with a high density of twins at 565°C and ͑100͒-oriented defect-free grains of slightly increased size at 655°C. This remarkably stable ͑100͒ texture seems specific for MOCVD as it is not observed for other deposition methods. The grain structure and surface morphology and their dependence on film stoichiometry ͑group-II/Ti ratio͒ are investigated. The relation between the structural and the electrical properties, capacitance, and leakage current, is finally discussed.

Electrical properties of (Ba,Sr)TiO3 thin films revisited: The case of chemical vapor deposited films on Pt electrodes

“…60 For the thickest films of ϳ100 nm there are even larger variations, nevertheless, there is some indication that the group-II rich films show the highest leakage. This leakage behavior might be directly related to the higher surface roughness of these films 20 ͑the larger hillocks of the Ti-rich films wont hurt, but the more deep valleys at the grain boundaries of the Gr-II rich films do͒. Hence, these results for the thicker films reproduce earlier conclusions, 2 which reported better leakage behavior for Tirich films based on data from ϳ60 nm thick films.…”

“…Again Gr-II rich BST is competitive only for thin films. This correlates well with the development of the grain structure and surface roughness 20 and indicates an inhomogeneous current for these films. In summarizing, the close interrelation between the reduction of effective DK with thickness and the decrease of the leakage current yields quite different consequences: a good news for the future application at low voltages, e.g., DRAMs, as there is no increase of leakage with reduction of film thickness; a bad news for the high-application of thicker films as the higher DK is related to higher leakage and as there is also an increased tendency for degradation under current stress.…”

“…Details on deposition and structural characterization have been reported previously. 20 For electrical measurements, Pt electrodes ͑ϳ100 nm thick͒ were sputter deposited on top of the BST films and patterned using shadow masks or lift-off lithography. Pad sizes of 1.0, 0.20, 0.05, 0.018, and 0.008 mm 2 were produced.…”

“…In spite of the scatter of the data there is an indication for a reproducible asymmetry for the thickest films ͑ϳ100 nm͒, and this behavior could be reproduced with some examples for complete loops on one pad. The surface topology and rms roughness of the thinner films is mainly determined by the roughness of the Pt electrode and a significant increase in roughness is observed only for films thicker than ϳ60 nm, hence, the difference in the leakage behavior of the thickest films might be related to the increased roughness of the films, 20 i.e., the thinner films are quite well conformal covering the Pt structure and only for the thicker films the grain structure of the BST films themselves becomes dominant. Remarkably, the higher currents are observed for electron injection from the bottom electrode and the roughness of the electrode structure becomes important as soon as there is no longer a conformal coverage.…”

“…19 Hence, a consistent investigation of the dependence of the electrical properties on film stoichiometry, microstructure, and thickness under identical conditions seemed necessary for revealing some generally valid trends as well as their limitations. We have reported on the structural film properties previously, 20 and concentrate on the electrical properties here. As the best and most reproducible data were obtained for films deposited at temperatures ജ600°C, we selected such films, although lower temperatures seem necessary for technological applications in order to achieve good conformal deposition on small structures 21,22 and to avoid degradation of the contact plugs.…”