Ohmic contacts to SiC

Low resistance, reliable, temperature-stable ohmic contacts are a prerequisite for the commercialization of SiC device technology. Still, these contacts are not yet satisfactory for a variety of reasons the annealing temperature is too high, the contacts penetrate too deep, they deteriorate when devices operate at elevated temperature, and the contact resistance is quite high. The problem of stable contacts to SiC may be resolved by using refractory metals. Refractory metals can be used to form both carbides and silicides. Silicides appear to provide a stable resistance if carbides are not present [1]. Contact systems based on such metals as nickel, chromium, titanium, cobalt and tungsten have been demonstrated for n-type SiC. Most contact systems for p-type SiC are Al-based and this imposes a limitation on the operating temperature of SiC devices with p-type contacts. 

Recently, Kelner and co-workers [2] obtained a specific ohmic contact resistance of 3.5 x 10 6Qcm2 with a carrier concentration of 4x 10l8cm 3 to 6H-SiC. The contacts were resistively evaporated nickel that was rapidly thermally annealed at 1000°C, for 30 s, in a forming gas atmosphere. Crofton et al [3] recently reported a contact resistance as low as 10 5Qcm2 to p-type SiC. Another problem to be solved is the fabrication of low resistivity Au or Al overlays for device interconnects. These overlays have to be separated from contacts by a diffusion barrier layer (for example, W, Cr, Ti, or conducting nitrides). 

All in all, SiC technology is progressing. A summary of contacts to the various polytypes is listed in TABLES 1 to 3. As mentioned above, the problem of reliable high temperature low resistance ohmic contacts is the most important problem to be solved. A second issue is the development of semi-insulating SiC for microwave and/or integrated circuits. SiC is now the prime candidate for commercial applications in high temperature devices and circuits and for devices and circuits operating in a harsh environment. 

Type of contact Specific contact resistance rc (Q cm2) Annealing conditions Comments Ref 

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Cole M. W., Joshi P. C., Hubbard C. W., Wood M.C., Ervin M. H., Greil B. and Ren F., Improvement Ni based composite ohmic contact to n-SiC for high temperature and high power device applications, J. Appl. Phys. 88 (2000) pp.2652-2657. 

Nakshima et al. have fabricated p-n junction devices by employing A1 implantation to yield a p-doped layer in n-type 6H-SiC [66]. A Pt layer on top of the p-type ohmic contact (PtSi) provided both protection and a catalytic metal contact to create a chemical gas sensor device. A response (30 and 60 mV, respectively) was obtained to both 50 ppm and 100 ppm of ammonia in nitrogen at 500°C. 

Figure 6.12 Cross-sectional view of the 4H-SiC power BJT fabrication, (a) Starting epilayer structure. (b) Dry etching of emitter and base epilayers. (c) p implantation for guard rings and contacts to p-base. (d) Formation of ohmic contacts, (e) Over layer metal deposition, (f) Double metal process.

Nickel Schottky contacts with a diameter of 300 am and a thickness of 1500 A were deposited on the porous SiC by magnetron deposition followed by photolithography patterning. A blanket ohmic contact was formed by Ni deposition on the backside and rapid thermal annealing at 1000 °C was done prior to anodization. The schematic cross-section of the formed structure is shown in Figure 2.21. Note, nickel contacts were deposited on a porous substrate with the skin layer which is characterized by low porosity and pore diameters of <20 nm. Thus, the effect of contact... 

State-of-the-art Schottky diodes operate up to 400°C [28] see FIGURE 5. The stability of the Schottky diodes limits the maximum temperature of operation of SiC MESFETs since SiC itself can withstand much higher temperatures. State-of-the-art ohmic contacts can operate at significantly higher temperatures than Schottky contacts. 

High ohmic contact resistances in SiC devices present a serious limitation for high frequency performance. Furthermore, the problem of ohmic and Schottky contact thermal stability has not been solved. Contacts (and, sometimes, packages) usually limit the maximum SiC device operating temperatures. The existence of micropipes in 6H- and 4H-SiC material leads to a low yield for power devices. Improvements in material quality, the development of bulk 3C-SiC for 3C-SiC homoepitaxy, and the development of better contacts are of primary importance for the advancement of the SiC device technology. 

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