(71a) Manipulating the Optical and Dielectric Properties of Crystalline Thin Perovskite Films

This talk will address strategies and mechanisms for growth of ultra-thin ternary and quaternary single crystal oxides – oxides whose properties derive from composition, orientation and interfacial effects. This talk will also address strategies to manipulate the properties and mitigate the defects inherent in these thin films.

Thin BaTiO₃ (BTO) films have been shown to be a promising candidate for on-chip photonic devices due to BTO’s 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 crystalline BTO thin films with Si, specifically, atomic layer deposition (ALD) and chemical solution deposition (CSD). The epitaxial ALD lattice is defined by the Si (100) substrate and films as thick as 66 nm are under compressive strain with the BTO c-axis oriented in the out-of-plane direction. 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 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 provide 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 display an EO coefficient of 89 pm/V for ~85 nm films.

Through isovalent A-site substitution of Sr for Ba in the bulk ABO₃ perovskite, the dielectric constant can be tuned to be orders of magnitude greater than either SrTiO₃ or BaTiO₃ near ambient temperatures. We use 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). 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). Crystalline order is established with X-ray diffraction and transmission electron microscopy. Despite the presence of cation defects (i.e., A:B ≈ 1.0), oxygen anion vacancies, and dislocation defects, capacitance-voltage (C-V) measurements reveal 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 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 on Ge(001), a bulk k of 3,200 at RT was extracted from thickness dependent relationships. Interfacial effects (i.e., possible GeOx at the interface with Ge (001), and dielectric screening) inherent to ~ 10 nm Ba_xSr_1-xTiO₃ films on Ge (001) affect the capacitance measurements leading to k of 87 for 10.9 nm films. Mitigation strategies are discussed to increase k for ultra-thin films.