Abstract
Interfacial reactions of Er and Tb thin films on silicon have been studied by cross-sectional and planview transmission electron microscopy, scanning electron microscopy, sheet resistance measurement, X-ray diffraction as well as differential scanning calorimetry.The growth of the amorpbous interlayer in both Er/Si and Tb/Si systems was observed to exhibit similar behaviors. The growth was found to follow a linear growth law initially in samples annealed at 190-240℃. The activation energy of the linear growth and maximum thickness of the a-interlayer were measured to be 0.5 eV, 15.5 nm and 0.35 eV, 16 nm in Er/Si and Tb/Si systems, respectively.Crystalline silicides (ErSi2-x or TbSi2-x ) were found to form at the amorphous interlayer/Si interfaces in samples after prolonged and/or high temperature annealing. Simultaneous growth of the a-interlayer and crystalline phase was observed and the growth rate of a-interlayer was faster than the growth of epitaxial ErSi2-x and TbSi2-x phases in samples annealed at 270-300 ℃ in Er/Si and Tb/Si systems, respectively. The competitive growth can be understood from energetic consideration. A high density of recessed amorphous regions were found to form between isolated epitaxial silicide regions which led to uneven silicide/Si interfaces and eventually pinholes in the silicide films at high temperatures. The formation mechanism of rough silicide/Si interface is discussed.Completely amorphized Er-Si intermixing layers were found to form in all deposited multilayer samples. After low temperature annealing, the crystalline ErSi2-x was found to form in Si-rich amorphous alloys. On the other hand, both ErSi2-x and Er2O3) were found to form in Er-rich amorphous alloys. The ErSi2-x phase, which has the lowest activation energy to nucleate, is the preferred phase in the Er-Si interfacial reactions. The stability of amorphous alloys was found to increase with metal concentration. The presence of multiple peaks of this study may also be evidence of nucleation barriers determining first phase formation in the erbium-amorphous silicon system.A new formation mechanism of pinholes in rare-earth silicide films on (111)Si was found. The epitaxial TbSi2-x layer was initially found to form continuously on (lll)Si in samples annealed at 400℃. A high density of the recessed regions were then formed randomly. The depth and dimension of the recessed regions were found to increase with annealing time. Finally, the TbSi2-x thin layer inside the recessed regions was broken to form pinholes.Epitaxial TbSi2-x films with hexagonal and orthorhombic structures on (001)Si and only hexagonal structure on (111)Si were found to grow by rapid thermal annealing. From a combination of studying planview and cross-sectional TEM samples. The 3-dimensional structures of vacancy ordering were determined. The vacancy ordering superstructure (a√3 a√3 2c) of unit cell size were found in epitaxial TbSi2-x thin films on (001)Si sample. However, the vacancy ordering superstructure (a√3 a√3 c) of unit cell size were found on epitaxial TbSi2-x thin film on (111)Si sample.The hexagonal and orthorhombic phases are polymorphic and can be converted to the other phase via a stacking fault structure. The formation of a high density of stacking faults in Tb silicides on (001)Si is attributed tc the simultaneous formation of both phases in the system.High resolution Transmission Electron Microscopy (HRTEM) was used to study the atomic structure of the Si/TbSb/(111)Si double interfaces. The unrelaxed geometrical models of Si/TbSi2/Si double- heterostructure interfaces can be systematically deduced from the dichromatic Constramed-Comcidence-Site-Lattice (CCSL) patterns. The atomic structures were determined by comparing with computer image simulation. The relationship of interface bonding and structures of reepitaxial Si/TbSi2 and TbSi2/Si interfaces was discussed.