J. Crofton

The Physics of Ohmic Contacts to SiC

The specific contact resistance of an ohmic contact will be discussed including ways to calculate and measure this parameter. Ohmic contacts to n- and p-type hexagonal SiC will then be detailed. Low resistance n-type ohmic contacts are predominately fabricated by annealing a refractory metal, thereby forming a silicide with a lowered Schottky barrier height at the metal-SiC interface. P-type contacts on the other hand generally use Al or Al alloys which upon annealing enable Al to diffuse into the SiC thus resulting in ohmic properties. Aluminium alloys however suffer from many problems which will be discussed. Other novel contacting schemes to p-type SiC will also be reviewed.

Titanium and aluminum-titanium ohmic contacts to p-type SiC

Abstract Very low resistance ohmic contacts to p-type SiC were fabricated by depositing a 90-10 wt.% alloy of Al and Ti followed by a high temperature anneal of approximately 1000°C for 2 min. Specific contact resistances ranged from approximately 5 × 10 −6 to 3 × 10 −5 Ω cm 2 on material with a doping of 1.3 × 1019 cm−3. The initial AlTi thickness before annealing was found to be critical to controlling the AlTi sheet resistance during the anneal. In addition, chemically etching the AlTi layer after annealing revealed pitting indicative of severe reaction between the AlTi and SiC surface, as confirmed by Rutherford Backscattering. In contrast, ohmic contacts to the same SiC material were fabricated by depositing pure Ti and annealing at 800°C for 1 min. These contacts were ohmic with a specific contact resistance between 2 × 10 −5 and 4 × 10 −5 Ω cm 2 . Examination of the SiC surface after chemically etching away the annealed contact revealed a smooth surface, suggesting a much more planar Ti/SiC interface.

Finding the optimum Al–Ti alloy composition for use as an ohmic contact to p-type SiC

Abstract Alloys of aluminum and titanium have been successfully used to form low resistance ohmic contacts to p-type SiC. While the 90 wt.% Al alloy has been studied extensively, the literature does not reveal any work which indicates whether the 90/10 or any other alloy composition is the best alloy for use as an ohmic contact material to p-SiC. This work systematically looks at four different Al–Ti alloy compositions in an attempt to decide which alloy if any is superior as an ohmic contact material. The alloy compositions that were studied were chosen by examining the binary Al–Ti phase diagram and choosing specific phases prior to reaction with the SiC. It will be shown that only alloys which have some amount of a liquid phase present at the anneal temperature will form an ohmic contact to p-type SiC.

Morphological study of the Al–Ti ohmic contact to p-type SiC

Abstract The composition 70 wt.% Al was recently reported to provide low resistance Al–Ti ohmic contacts with excellent electrical uniformity on p-type SiC. Using scanning electron microscopy and atomic force microscopy, an investigation of the surface morphology and edge definition of the annealed contacts was conducted, and the morphology of the buried metal/semiconductor interface was examined by etching away the contact metallization and imaging the freshly exposed SiC surface. This information provides guidance on the suitability of the contact for devices with small feature sizes and shallow p-type epilayers. Patterned contacts exhibited good edge definition, a root-mean-square surface roughness of 11 nm, and a root-mean-square interfacial roughness of 12 nm. The deepest observed penetration of the metallization into the SiC was 65 nm, and the lateral length scale of the morphological features at the buried metal/semiconductor interface was sufficiently small compared to the active area of the contact to allow good contact-to-contact reproducibility. The interfacial reactions and ohmic contact formation mechanism are considered from the point of view of the materials characterization study presented here and the binary Al–Ti and quaternary Al–C–Si–Ti phase diagrams.

High temperature ohmic and Schottky contacts to N-type 6H-SiC nickel

We report specific contact resistances measured at elevated temperatures for Ni ohmic contacts to 6H‐SiC. The specific contact resistances were measured with the linear transmission line method at both room temperature and at 773 K and yielded values <5×10−6 Ω‐cm2 at both temperatures. The trend shows a decreasing contact resistance at higher temperatures. The annealed metal film is a nickel silicide with substantial mixing of C throughout the silicide layer. Also reported are the results of I‐V and C‐V barrier height measurements for Ni Schottky contacts to 6H‐SiC. Current‐voltage barrier heights as high as 1.2 eV have been measured, and the contacts show good electrical and physical stability following long‐term anneals at 573 K in a vacuum ambient of 10−6 torr. These ohmic and Schottky contacts have been developed by the CCDS in collaboration with the Air Force and the Westinghouse Electric Corporation, and transfer of our contact technology to the Westinghouse Science and Technology is now complete.

Ohmic Contacts to P-Type Epitexial and Imlanted 4H-SiC

Nickel ohmic contacts to p-type epitaxial and heavily implanted 4H-SiC are described. Room and elevated temperature results are presented. Elevated temperature measurements of specific contact resistance are compared to theoretical calculations. The calculations require the acceptor doping concentration and the contact’s barrier height. Epitaxial material has a known acceptor value thereby allowing the barrier height to be deduced by requiring agreement between the calculated and measured values of the contact resistance. Calculations of the contact resistance for implanted material use the barrier height from the epitaxial results along with a variable activated acceptor doping concentration which is adjusted to give agreement with measured room temperature specific contact resistances. Specific contact resistances as low as 7x10-6 ohm-cm2 fabricated on the Si face have been obtained to epitaxial 4H p-type material whereas contacts to implanted material result in much larger contact resistance values of 4x10-5 ohm-cm2. These results, when compared to theoretical calculations, indicate that activated acceptor doping concentrations in heavily implanted material are on the order of 2% of the implant concentration.

Electrical characterization and charge transport in 6H‐SIC MESFET’S

6H‐SiC MESFET’s have been fabricated and tested at temperatures up to 673 K. Fabrication procedures have been briefly outlined and electrical characteristics of these devices presented with consideration for use in high temperature electronic circuits. The extrapolated threshold voltage (VTH), transconductance (gm), and small‐signal voltage gain (Av) were plotted vs. temperature, showing that the devices retained good electrical characteristics up to 523 K. Emission currents and charge transport across the Schottky barrier gate contact were examined in relation to the maximum operating temperature of the devices. Logarithmic plots of the gate‐to‐source current vs. applied voltage demonstrated the effect of gate leakage currents and also provided information about the current conduction mechanisms in the channel.

Al/Ni And Al/Ti Ohmic Contact To P-type SiC Diffused Layer

Ohmic contacts to p-type SiC were fabricated by depositing Al/Ni and Al/Ti followed by high temperature annealing. A p-type layer was fabricated by Al or B diffusion from vapor phase into both p-type and n-type substrates. The thickness of the diffused layer was about 0.1–0.2 μm with surface carrier concentration of about 1.0×10 19 cm −3 . Metal contacts to a p-type substrate with a background doping concentration of 1.2×10 18 cm −3 , without a diffusion layer, were also formed. The values of specific contact resistance obtained by Circular Transmission Line Method (CTLM) and Transfer Length Method (TLM) for the n-type substrate, and by Cox & Strack method for p-type substrate, respectively, varied from 1.3×10 −4 Ωcm 2 to 8.8×10 −3 Ωcm 2 . The results indicate that the specific contact resistance could be significantly reduced by creating a highly doped diffused surface layer.