|Title:||Heteroepitaxy and characterization of perovskite titanate thin films grown on III-V semiconductor|
Ferroelectric thin films.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Physics|
|Pages:||xvi, 128 leaves : ill. ; 30 cm.|
|Abstract:||In recent years, the integration of perovskite titanates with mature semiconductor technology has attracted extensive attention for both basic research and applications. Perovskite titanates such as (Ba, Sr)TiO₃ (BST) have relatively high dielectric tunability and moderate dielectric constant, which have found many applications for frequency and phase agile microwave devices. Up to now, the titanate-based tunable devices have been well fabricated on single-crystal oxide substrates such as MgO and LaAlO₃. III-V semiconductors such as GaAs have direct band gap and higher saturated electron mobility compared to conventional silicon (Si), making them very suitable for high-speed microelectronic and photonic devices. Hence, it is of great interest to combine ferroelectric titanates with GaAs exhibiting high-performance microwave capabilities. In the present thesis, ferroelectric BST thin films were epitaxially grown on GaAs (001) substrate using SrTiO₃ (STO) as a buffer layer by laser molecular beam epitaxy. The STO buffer layer was first deposited on GaAs substrate by setting the substrate temperature around 600 °C and the pressure of 5 x 10⁻⁵ Pa. During the BST thin film deposition, the substrate temperature was maintained at 620 °C and the oxygen partial pressure was kept at 1 Pa. After deposition, the heterostructure was annealed in 200 Pa oxygen pressure for 1 hr to reduce oxygen vacancies. X-ray diffraction measurements indicate that BST is well crystallized and epitaxially aligned with STO/GaAs. Atomic force microscope studies show that the films have atomically flat surfaces. The interface structure of STO/GaAs was studied by transmission electron microscopy (TEM). The sharp interface from TEM image provides an evidence that the STO film deposited on GaAs has good crystallinity. The chemical valences of BST thin films on STO/GaAs and energy band at the interface of STO/GaAs were investigated using photoelectron spectroscopy. A type II heterojunction with a valence band offset of 2.5 eV and conduction band offset of 0.7 eV is formed. The dielectric and ferroelectric properties of BST/STO/GaAs heterostructure are studied. The measurements were conducted by applying coplanar configurations on BST thin films. With increasing frequency from 100 kHz to 10 MHz, the dielectric constant of the film was reduced from 870 to 700 accordingly. The in-plane dielectric properties of the heteroepitaxial structure under different applying frequency were investigated from -190 to 90°C, indicating Curie temperature of the BST film to be around 52 °C. At room temperature, the dielectric constant of the heterostructure under moderate dc bias field can be tuned by 30% and K factor is found to be close to 8. By considering the high-frequency capabilities and large-size availability of commercial GaAs, this work shows that the BST/STO/GaAs heterostructure can be processed to be a new system for tunable microwave applications.|
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