Thin-film epitaxial structures of BaSi2, BaO, and BaTiO3, have been grown on the (001) face of silicon using ultrahigh vacuum, molecular beam epitaxy (MBE) methods. Source shuttering for the metal species coordinated with a pulsed, or cyclic, oxygen arrival at the growing oxide surfaces significantly improves film quality. The epitaxial growth of BaO is accomplished without silica formation at the BaO/Si interface by stabilizing BaSi2 as a submonolayer template structure. In situ ellipsometric measurements of the indices of refraction for BaO and for BaTiO3 in a BaTiO3/BaO/Si multilayer gave n=1.96 for BaO and n=2.2 for the BaTiO3, within 10% of their bulk values. These values suggest that this structure can be developed as an optical waveguide. BaO is impermeable to silicon for films as thin as 10 nm at temperatures as high as 800 °C, and good epitaxy can be obtained from room temperature to 800 °C. The epitaxy is such that BaTiO3(001)∥BaO(001)∥Si(001) and BaTiO 3〈110〉∥BaO〈100〉∥Si〈100〉.

Effect of interface defect formation on carrier diffusion and luminescence in In0.2Ga0.8As/Al x Ga1−x As quantum wells

A commercially available spin-on glass ͑methylsilsesquioxane, MSQ͒ was modified by the introduction of porosity. The porosity reduced the effective dielectric constant of the MSQ by the incorporation of air. The pores were created by adding a sacrificial polymer ͑substituted norbornene polymer͒ to the silsesquioxane matrix. The sacrificial material was thermally decomposed to form nanosize voids within the films. The physical and electrical properties of the porous films were studied as a function of the reactivity of the sacrificial polymer with the glass, and the loading and molecular weight of the sacrificial polymer. Transmission electron microscopy was used to evaluate the porous microstructure. Cross-sectional images show pores of nearly spherical geometry with 5-20 nm diam. The dielectric constant and the index of refraction of the porous MSQ were lower after the decomposition of the sacrificial material. The dielectric constant decreased from 2.7 for a nonporous MSQ film to ϳ2.2 for a film with 30 wt % loading of the sacrificial polymer. In a similar way, the index of refraction was reduced from 1.42 to 1.29 for the porous MSQ film. The mechanical properties were evaluated using nanoindentation techniques. This paper focuses on the significant improvements observed upon introduction of porosity to the films. The fracture toughness, or the resistance to crack propagation, increased dramatically with porosity, as compared with the nonporous MSQ films. As a result, thicker MSQ films can be fabricated without spontaneous cracking. The elastic modulus and the hardness of the porous films were measured and showed a reduction in both properties with increasing porosity in the film.

Thin film perovskite-type oxide SrTiO3 has been grown epitaxially on Si(001) substrate by molecular beam epitaxy. Reflection high energy electron diffraction and x-ray diffraction analysis indicate high quality SrTiO3 heteroepitaxy on Si substrate with SrTiO3(001)//Si(001) and SrTiO3[010]//Si[110]. The SrTiO3 surface is atomically as smooth as the starting substrate surface, with a root mean square roughness of 1.2 Å observed by atomic force microscopy. The thickness of the amorphous interfacial layer between SrTiO3 and Si has been engineered to minimize the device short channel effect. An effective oxide thickness <10 Å has been obtained for a 110 Å thick dielectric film. The interface state density between SrTiO3 and Si is 6.4×1010 cm−2 eV−1, and the inversion layer carrier mobilities are 221 and 62 cm2 V−1 s−1 for n- and p-channel metal–oxide–semiconductor devices with 1.2 μm effective channel length, respectively. The gate leakage in these devices is two orders of magnitude smaller than a comparable SiO2 gate dielectric metal–oxide–semiconductor field effect transistors.