Complex Metal Oxide Thin Film Growth by Metalorganic Chemical Vapor Deposition

Abstract

The phenomenon of ferroelectricity recently attracted great attention with the successful advances in the development of thin-film fabrication. This development enables the integration of ferroelectric materials directly into device fabrication processes such as MEMS and FeRAMs. The stringent need for high-density, high-speed, and low-power memory devices has prompted an immense interest in studying the size effects in ferroic systems. The thickness dependence of ferroelectricity and critical thickness, which is the thickness limit when the ferroelectricity disappears, has become an issue of tremendous interest for both scientific and technological point of view. In parallel, current nonvolatile memory manufacturing processes inevitably require a scalable process such as metal-organic chemical vapor deposition (MOCVD) to deposit the ferroelectric layers.

The process controls as well as the ferroelectric and piezoelectric properties of two ferroelectric material systems, lead zirconate titanate (PZT) and bismuth ferrite (BFO), prepared by MOCVD is presented in this dissertation. A systematic study on deposition process control such as stoichiometric composition, structure change and growth temperature was carried out.

The scaling of ferroelectric properties with film thickness in PZT films has been investigated. PZT films show bulk-like properties for thickness above ~20 nm. It was observed that a progressive decrease in the ferroelectric polarization as well as the piezo-response as the thickness is decreased; films as thin as 3.6 nm are piezoelectric. In this work, the interpretation of the origins of this decrease as well as results of the MOCVD processing studies is studied.

Epitaxial BFO thin film, which is an interesting candidate for Pb-free ferro / piezoelectrics, was grown by MOCVD as the first challenge. The film composition and phase equilibrium are sensitive to the Bi:Fe supply ratio of precursors. In the optimized condition, an epitaxial single perovskite phase thin films were obtained. Electrical measurements using both quasi-static hysteresis and pulsed polarization measurements confirm the existence of ferroelectricity with a switching polarization of 110-120 mC/cm2, DP (= P* - P^). Out-of plane piezoelectric (d33) measurements using an atomic force microscope yield a value of 50 - 55 pm/V.