Manipulating the Dielectric Properties of Crystalline Perovskite Films through Isovalent A-site Cation Substitution

Abstract

Atomic layer deposition (ALD) offers a viable route for the growth of thin and conformal films over 3-D topographies and is becoming attractive as a method to grow films thin enough, and with sufficient dielectric constants (k), for the fabrication of next-generation dynamic random memories (DRAMs). Through isovalent A-site substitution of Sr for Ba in the ABO3 perovskite the dielectric constant can be tuned to be orders of magnitude greater than either SrTiO3 or BaTiO3 near ambient temperatures. We used ALD to grow thin (≤ 15 nm) Ba x Sr1-x TiO3 (BST) films that are epitaxially integrated to SrTiO3 (001) (STO) and Zintl-templated Ge (001). Films of three compositions, which are x ~ 0.7, 0.5 and 0.3, and thicknesses of 7.8 to 14.9 nm were grown at 1.05 Torr and 225 °C using barium bis(triisopropylcyclopentadienyl), strontium bis(triisopropylcyclopentadienyl), titanium tetraisopropoxide and H2O. Film compositions were controlled by changing cycle ratios (Ba:Sr, Ba:Ti and Sr:Ti) and confirmed by in situ X-ray photoelectron spectroscopy (XPS). Films were amorphous as deposited and required post-deposition vacuum annealing at 650-710 °C to crystallize. Epitaxy was confirmed with X-ray diffraction and transmission electron microscopy. Only BST (00l) out-of-plane diffraction signals were detected. Capacitance-voltage (C-V) measurements revealed that BST thin films grown by ALD on STO (001) have dielectric constant values ranging from 210 for Ba0.71Sr0.26TiO3 to 368 for Ba0.48Sr0.43TiO3. The dielectric constant k increased with thickness with x in the range of 0.27 ≤ x ≤ 0.31. Interfacial effects inherent to the ~ 10 nm Ba x Sr1-x TiO3 films on Ge (001) affect the capacitance measurements leading to k of 87 and 140 for 10.9 and 14.6 nm films, respectively. The epitaxial films have high k in the bulk. Using capacitance measurements for Ba x Sr1-x TiO3 films (x~0.5) 13 to 18.4 nm thick, a bulk k of 3200 at RT and a low interfacial capacitance density (C/A) of 100 fF/mm2 were extracted from thickness dependent relationships.