(757b) Manipulating the Optical and Dielectric Properties of Crystalline Perovskite Films through Strain and Isovalent a-Site Cation Substitution

Recent investigations into thin film BaTiO₃ (BTO) show it is a promising candidate for on-chip photonic devices due to its large linear electro-optic (EO) coefficient (r > 100-1000 pm/V) relative to more conventional photonic materials such as LiNbO₃ (~30 pm/V). We report chemical routes to the monolithic integration of electro-optically active BTO thin films with Si, specifically, atomic layer deposition (ALD) of crystalline BTO films at 225 °C and chemical solution deposition (CSD) of crystalline BTO films on SrTiO₃ (STO) (001) templates on Si (001). X-ray diffraction confirms compressive strain in BTO films after the low temperature ALD growth for films as thick as 66 nm, with the BTO c-axis oriented in the out-of-plane direction. Post-deposition annealing above 650 °C leads to in-plane c-axis orientation. Piezoresponse force microscopy was used to verify the ferroelectric switching behavior of ALD-grown films. Electrical and electro-optic measurements confirm BTO film ferroelectric behavior in out-of-plane and in-plane directions, respectively, at the µm scale. The CSD studies demonstrate the ability for this relatively simple and scalable solution deposition approach to integrate epitaxial, electro-optically active oxides with silicon and provides an understanding into the relationship between the resulting morphology and EO behavior. The CSD films are only under tensile strain with the c-axis oriented in plane. ALD-grown films have an effective linear EO coefficient of 26 pm/V for 40 nm films. CSD-grown BTO films that have been annealed to 750 °C displayed an EO coefficient of 89 pm/V for ~85 nm films.

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_xSr_1-xTiO₃ (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 Ba_0.71­Sr_0.26TiO₃ to 368 for Ba_0.48Sr_0.43TiO₃. 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_xSr_1-xTiO₃ 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_xSr_1-xTiO₃ 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.