Abstract
Abstract Interfacial reactions of Yb thin films on silicon have been studied by Auger electron spectroscopy (AES), glancing incidence X-ray diffractometry (GIXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive analysis of X-ray (EDAX), high-resolution transmission electron microscopy (HRTEM) in conjunction with auto-correlation function (ACF) analysis as well as computer simulation. The growth of the amorphous interlayer in both Yb/(111)Si and Yb/(001)Si systems was observed to exhibit similar behaviors. The growth was found to follow a linear growth law initially in samples anneled at 150-215 oC. The activation energies of the linear growth and maximum thicknesses of a-interlayers were measured to be 0.49 eV, 8 nm and 0.47 eV, 7 nm in Yb/(111)Si and Yb/(001)Si systems, respectively. Crystalline phases were found to embed in the amorphous interlayer by ACF analysis. Simultaneous growth of the a-interlayer and crystalline phase was observed and the growth rate of a-interlayer was faster than that of crystalline phases. The competitive growth can be understood from energetic consideration. The dominant diffusing species in the formation of amorphous interlayer between Yb thin films and crystalline Si substrates have been determined by a Mo cluster marker experiment. Metal thin films with multilayered structures were deposited on both (111) and (001)Si substrates in an ultrahigh vacuum (UHV) electron beam evaporation system. The positions of the Mo cluster markers relative to the Si substrates, before and after heat treatment, were determined by HRTEM and EDAX as well as ACF analysis. The displacement of the Mo cluster markers in the amorphous interlayer during the Yb-Si interdiffusion indicates that Si atoms constitute the dominant diffusing species during the growth of the amorphous interlayer. The interfacial reactions of Yb-Si multilayers were studied by HRTEM and ACF analysis. Amorphous mixtures were found to form at Yb/Si interfaces in as-deposited samples. After annealing the samples at 300 oC for 30 min, Yb-Si intermixing layers were found to completely homogenized. Small crystallites were found to embed in the amorphous mixture by ACF analysis. After annealing at higher than 400 oC, the crystalline YbSi2-x was found to form in Si-rich amorphous alloys. On the other hand, both YbSi2-x and Yb2O3 were found to form in Yb-rich amorphous alloys. The YbSi2-x phase, which has the lowest activation energy to nucleate, is the preferred phase in the Yb-Si interfacial reactions. Epitaxial ytterbium silicide thin films were grown on (111)Si by UHV deposition and subsequent thermal annealing. In samples annealed at 300 ℃ for 30 min, the appearance of additional diffraction spots is attributed to the formation of an ordered vacancy superstructure in the epitaxial YbSi2-x thin films. In samples annealed at 400-1000 ℃ by RTA, the split of extra diffraction spots is correlated to the formation of two sets of out-of-step structures. The distribution of these two sets of out-of-step structures varied with annealing temperature which correlates to a change in vacancy concentration so that the compressive stress was relaxed. Computer simulation was carried out to determine the lattice structure of vacancy ordering structures. From studying the planview and cross-sectional TEM samples, the 3-dimentional structures of vacancy ordering were determined. Planar defects in YbSi2-x films were analyzed to be stacking faults on {10 0} planes with 1/6< 2 3> displacement vectors. The stacking faults were annihilated by high temperature annealing. Pinholes were also formed in the epitaxial YbSi2-x thin films and could be avoided by appropriate processing step. Epi-Si/epi-YbSi2-x /(111)Si double heteroepitaxial structures were fabricated in UHV chambers. The dominant defects in overgrown epi-Si layers were determined to be microtwins. Polycrystalline ytterbium silicide thin films were grown on (001)Si by UHV deposition and subsequent thermal annealing. The silicide grains were identified to be YbSi2-x by selected area electron diffraction (SAED). The dominant diffusing species in the formation of ytterbium silicide have been determined by SiNx-island marker experiments in conjunction with TEM observation. Polycrystalline YbSi2-x thin films were formed in a-Si/Mo/Yb/SiNx/(001)Si samples after annealing at 500 oC for 30 min. The direction of SiNx marker displacement indicates that Si atoms constitute the dominant diffusing species during the formation of YbSi2-x thin films on (001)Si. Epitaxial ytterbium silicide thin films were grown on (001)Si by in-situ high temperature UHV deposition. The epitaxial silicide layer consists of double domains which have two different azimuthal orientations making an angle of 90o to each other, since there is an anisotropy in lattice mismatch at the interface. In samples deposited at 500 oC, the appearance of additional diffraction spots is attributed to the formation of an ordered vacancy superstructure in the epitaxial YbSi2-x thin films. After annealing the samples at 700-1000 oC, the period of vacancy ordering along c-axis changes. Out-of-step structures were observed in cross-sectional electron diffraction patterns. From studying the planview and cross-sectional TEM samples, the 3-dimentional structures of vacancy ordering were determined. A high density of pinholes was found to form by SEM and XTEM observation.