硅酸盐学报

钛酸钡(BaTiO₃, BTO)薄膜因其优异的电光效应在集成光子器件中具有重要的应用潜力。然而,在硅衬底上实现高质量BTO薄膜的外延生长并有效调控其面内极化以实现电光系数最大化仍面临关键挑战。采用氧化物分子束外延技术,结合原位反射高能电子衍射(RHEED)实时监控,Si基片上的外延钛酸锶(SrTiO₃,STO)缓冲层在12.5个单胞层(ML)后完成晶格弛豫,显著降低了BTO薄膜与Si衬底的晶格失配度,为Si基片上外延BTO提供了良好的过渡层。在STO上外延更大晶格常数的BTO薄膜时,晶格在界面至30 nm之间完成弛豫过程。降温时,由于硅衬底的热膨胀系数较低,BTO薄膜受衬底约束,保持了弛豫后的大面内晶格常数,从而获得了面内铁电极化。基于此方法制备的2in的300 nm BTO晶圆膜展现出优异的晶体质量,摇摆曲线半峰宽为0.45°,表面粗糙度小于0.2 nm,c/a=0.994 <1,并具有显著的面内多畴结构。上述原位监控技术为分析BTO薄膜外延过程中的晶格弛豫及畴结构调控提供了关键信息,为硅基BTO电光器件集成奠定了材料基础。

Introduction Silicon photonics has emerged as a critical technology for high-performance communication and computing, with electro–optic modulators serving as essential components. Barium titanate(BaTiO₃, BTO) exhibits exceptional promise due to its outstanding electro–optic coefficient(r₄₂ ≈ 923 pm/V) and compatibility with silicon-based fabrication processes. However, achieving high-performance BTO devices requires overcoming key materials integration challenges. The primary obstacles include the formation of amorphous SiO₂ interfacial layer during high-temperature, oxygen-rich BTO growth, which prevents effective epitaxy, and the substantial lattice mismatch between BTO(a = b = 3.992 ?, c = 4.036 ?) and Si(5.431 ?), resulting in approximately 4.0% mismatch even with 45° rotational epitaxy. This mismatch induces high-density dislocations and strain relaxation, compromising film quality. Strontium titanate(SrTiO₃, STO) buffer layers(3.905 ?) can reduce lattice mismatch to 1.7%, providing stress relief and serving as an epitaxial template. The electro-optic effect in BTO strongly depends on ferroelectric domain structure and crystallographic orientation, with in-plane polarization achieving maximum coefficients. While domain structures can be controlled through strain engineering and growth conditions, current analyses rely on post-growth characterization, lacking real-time monitoring of domain evolution processes and critical transition mechanisms. Methods Oxide molecular beam epitaxy(MBE) combined with in situ reflection high-energy electron diffraction(RHEED) for real-time monitoring was employed to achieve controlled growth of high-quality BTO films at 2 in wafer scale. The systematic investigation focused on lattice relaxation behavior during the growth of STO buffer layers and BTO layers with varying thicknesses. Silicon substrates underwent hydrofluoric acid(HF, 10% mass fraction) treatment to remove surface SiO₂ amorphous layers, followed by rapid transfer to ultra-high vacuum chambers(background pressure ≤ 5×10^–8 Pa). STO epitaxial growth employed a two-stage process: first, 2.5 monolayers(ML) of Sr and Ti metals were deposited at a substrate temperature of 200 ℃ under ultra-high vacuum(< 5×10^–7 Pa) with precisely controlled oxygen partial pressure(< 5×10^–6 Pa). The second stage involved in situ annealing at 500 ℃ for 8 minutes under ultra-high vacuum conditions(< 1×10^–6 Pa). After three such cycles, additional STO layers were epitaxially grown using co-deposition at a thermocouple temperature of 600 ℃ under an oxygen partial pressure of 6×10^–5 Pa. BTO layers were continuously co-deposited at 700 ℃ thermocouple temperature under 6×10^–5 Pa oxygen partial pressure, ultimately obtaining 310 nm BTO/5 nm STO/Si heterostructures. Comprehensive characterization employed high-resolution four-circle X-ray diffraction(XRD) for 2θ–ω scanning and rocking curve analysis, spectroscopic ellipsometry for determining optical constants and thickness, atomic force microscopy(AFM) for surface morphology assessment, and piezoresponse force microscopy(PFM) for ferroelectric domain characterization. Results and discussion Systematic RHEED diffraction pattern analysis during film growth enable quantitative analysis of lattice relaxation processes in both STO and BTO epitaxial films. STO buffer layers completed relaxation after 12.5 ML, reducing the lattice mismatch between BTO and Si from 4.0% to 2.2%. BTO films underwent progressive relaxation from the interface to a depth of 30 nm, with in-plane lattice constants evolving from 3.905 ?(STO-templated) to 4.013 ? at 30 nm, then stabilizing. During cooling from 700 ℃ to room temperature, thermal constraints from Si substrate significantly modified BTO behavior. The effective thermal expansion coefficient was ～1.5×10^–6 K^–1, much lower than that of bulk BTO(～14.6×10^–6 K^–1) but comparable to Si(2.6×10^–6 K-1), indicating strong substrate constraint effects. The resulting 300 nm BTO films exhibited excellent quality: a rocking curve full width at half maximum(FWHM) of 0.45°, surface roughness < 0.2 nm, and critical lattice parameters a = 4.010 ?, c = 3.990 ?(c/a = 0.994 < 1). This represents a 0.45% in-plane expansion and 1.2% out-of-plane contraction compared to bulk BTO, indicating a tetragonal phase reorientation with the long-axis in-plane. This anisotropic distortion creates residual tensile stress in-plane and compression stress out-of-plane, driving polarization reorientation from out-of-plane to in-plane throughout the entire film. PFM characterization confirmed this structure: lateral PFM(LPFM) signals significantly exceeded vertical PFM(VPFM) signals, demonstrating dominant in-plane polarization components. Optical characterization at 1550 nm revealed ideal performance with a refractive index n = 2.28 and an extinction coefficient k ～10^–6, indicating extremely low optical loss suitable for high-performance electro-optic modulators. Conclusions High-quality BTO epitaxial growth on 2-inch silicon wafers was achieved via molecular beam epitaxy. In situ RHEED monitoring enabled real-time observation of lattice relaxation processes during STO buffer layer and BTO film growth, revealing that STO lattice relaxation occurs within the first 12.5 ML from the interface, while BTO lattice relaxation occurs within the first 30 nm from the interface. Based on this relaxation behavior, the STO intermediate layer provides BTO with a low-mismatch template, while BTO lattice relaxation is critical for ultimately achieving in-plane polarized films. In situ RHEED data analysis demonstrated that constraints from low thermal expansion coefficient Si substrate enable BTO films to maintain larger in-plane lattice constants during cooling to room temperature, thereby stabilizing domain structures with predominantly in-plane polarization. This in situ monitoring technique provides crucial information for precise analysis of BTO film lattice relaxation processes and domain engineering, offering significant implications for developing silicon-based BTO electro–optic modulators, high-speed communication devices, and optical quantum computing chips.