Visible and Deep-Ultraviolet Raman Spectroscopy as a Tool for Investigation of Structural Changes and Redistribution of Carbon in Ni-Based Ohmic Contacts on Silicon Carbide
Three samples of 4H polytype of silicon carbide (4H-SiC) covered with the following sequence of layers: carbon/nickel/silicon/nickel/silicon were investigated with micro-Raman spectroscopy. Different thermal treatments of each sample result in differences of carbon layer structure and migration of carbon atoms thorough silicide layer. Two ranges of Raman shift were investigated. The first one is placed between 1000 and 2000 . The main carbon bands D and G are observed in this range. Analysis of the positions of these bands and their intensity ratio enables one to determine the graphitization degree of carbon layer. Additional information about the changes of the carbon layer structure was derived from analysis of 2D band placed around 2700 . Application of deep ultraviolet excitation delivered information about the structure of carbon layer formed on the free surface of silicides and the distribution of the carbon inside the silicide layer. 1. Introduction The following combination of silicon carbide (SiC) physical properties: wide band gap, high critical electric field, simple method of dielectric layer fabrication (surface oxidation), and large thermal conductivity makes from SiC very promising material for fabrication high power, high-temperature, and high-frequency electronic devices [1]. The crucial issue in technology of manufacturing SiC-based devices is the capability to form reproducible and reliable ohmic contacts with a low specific contact resistance [2–4]. Formation of ohmic contacts to SiC is typically manufactured by the deposition of metals followed by high-temperature annealing (HTA: ~1000°C) [3–5]. As was shown previously [4–11] metal/SiC structures are not thermally stable. The following reactions of metals with SiC are possible:(i)SiC + refractory metals (Ti, Ta, W, etc.)→HTA→carbides (TiC, TaC, WC, etc.) + silicides (TiSix, , , etc.) + ternary phases ( , , , etc.);(ii)SiC + other metals (Ni, Pd, Pt, etc.)→HTA→silicides ( , , ) + C. Nickel is the most widely used metal for fabrication of ohmic contacts to n-type SiC due to the formation of contacts with very low specific contact resistance (~10？6 Ω？cm2) [2, 3]. In spite of numerous publications and progress in study of interaction between Ni and SiC, there are still many open questions concerning mechanism of contact formation and its reliability. There are different versions of the ohmic contact formation mechanism:(i)inhomogeneity of metal-SiC Schottky barrier [12];(ii)creation of carbon vacancies [1, 13] or defect states [10] near the interface region;(iii)“snowplow effect” of
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