Ohmic contacts to beta silicon carbide : electrical and metallurgical characterizations

Citation

Chen, Jen-Sue (1995) Ohmic contacts to beta silicon carbide : electrical and metallurgical characterizations. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/yk40-pj23. https://resolver.caltech.edu/CaltechETD:etd-09172007-133201

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Thermally induced reactions between a sputter-deposited Re, Pt or Ta film and a single crystalline (001) [beta]-SiC substrate in vacuum at temperatures from 300 to 1200°C for 30 min or 1 h are investigated by MeV [...] backscattering spectrometry, x-ray diffraction, secondary ion mass spectrometry, transmission and scanning electron microscopies. The contact resistivities of the as-deposited and the annealed Pt, Re, and Ta contacts to [beta]-SiC are characterized using circular transmission line. No reaction between Re and SiC is observed for all annealing conditions. The average grain size of the as-deposited Re film is 220 nm and increases to 280 nm after annealing at 1100°C for 30 min. A strong {0001}[...] fiber texture is also observed after annealing. The chemical stability of Re thin films on SiC is consistent with an earlier study in the ternary Re-Si-C system which shows that Re and its silicides have tie lines with SiC at 1600°C. This finding also coincides with our calculations of the free energy of reactions from critically assessed thermodynamic data for rhenium silicides and SiC. Backscattering spectrometry shows that Pt reacts with SiC at 500°C. The product phase identified by x-ray diffraction is Pt3Si. At 600-900°C, the main reaction product is Pt2Si but the depth distribution of the Pt atoms changes with annealing temperature. Samples annealed at 500-900°C have a double-layer structure with a silicide surface layer and a carbon-silicide mixed layer below. At 900°C, the phases in the mixed layer become stratified, with a continuous carbon layer in direct contact with the substrate. When the sample is annealed at 1000°C, the surface morphology deteriorates with the formation of some dendrite-like hillocks; both Pt2Si and PtSi are detected by x-ray diffraction. The SiC-Pt interaction is resolved at an atomic scale with high-resolution electron microscopy. It is found that the grains of the sputtered Pt film first align themselves preferentially along an orientation of {111}[...]//{001}[...] without reaction between Pt and SiC. A thin amorphous interlayer then forms at 400°C. At 450°C, a new crystalline phase nucleates discretely at the Pt-interlayer interface and projects into or across the amorphous interlayer towards the SiC, while the undisturbed amorphous interlayer between the newly-formed crystallites maintains its thickness. These nuclei grow extensively down into the substrate region at 500°C and the rest of the Pt film is converted to Pt3Si. No significant reaction between Ta and SiC is observed at 800°C or below. At 900°C, the main product phases are Ta2C and carbon-stabilized Ta5Si3. A minor amount of unreacted Ta is also present. After annealing at 1000°C, all the tantalum has reacted; the reaction zone possesses a multilayered structure of [beta]-SiC/TaC/carbon-stabilized Ta5Si3/[alpha]-Ta5Si3/Ta2/C. At 1100°C, the reacted layer has an interface with the SiC substrate that is still quite flat, but has a rough surface due to the formation of macroscopic voids within the reacted layer. The equilibrium products predicted by the phase diagram are TaC and [TaSi2. This final state is reached upon annealing at 1200°C for 1 h. At that point, the reacted layer has a laterally very uneven structure and morphology. Contact resistivities of the as-deposited and annealed Pt, Re, and Ta films on n-type, single-crystalline (001) [beta]-SiC are determined using a circular contact pattern and the method of the circular transmission line. The [beta]-SiC substrates used in the experiment are n-type doped either non-intentionally to a carrier concentration of [...], or by nitrogen implantation and annealing to a concentration of [...]. A resistance-network model is developed to derive an analytic expression for the angular dependence of the voltage distribution along the rings when they have a non-negligible resistance. By applying this model, the resistance between two concentric contacts can be extracted and will yield the correct contact resistivity when one uses the formulae of the circular transmission line model that ignores the finite resistance of the rings. On the non-intentionally doped [beta]-SiC substrates, Pt contacts are non-ohmic regardless of the heat treatment. The as-deposited Ta and Re contacts are ohmic with contact resistivities of [...] and [...], respectively. Upon annealing at 500°C for 30 min, the resistivity of Ta increases slightly while that of Re decreases slightly. Both Ta and Re contacts become non-ohmic by annealing at 900°C for 30 min. The as-deposited Ta, Pt, and Re contacts are all ohmic on the nitrogen-implanted [beta]-SiC substrate. The contact resistivity of the as-deposited Ta contact is the lowest and in the order of high [...], stays about the same at 500°C and degrades to [...] at 1000°C. The as-deposited Re contact has the highest contact resistivity of [...] but it improves to [...] upon annealing at 900°C. The contact resistivity of the as-deposited Pt contacts is [...] and increases to [...] at 500°C. After annealing at 900°C for 30 min, the Pt contact on the nitrogen-implanted [beta]-SiC is no longer ohmic. The results are compared with the reactions that take place in those systems.

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