Titanium Tungsten (TiW) for Ohmic contacts to n-and p-type 4H-SiC
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Titanium Tungsten (TiW) for Ohmic contacts to n-and p-type 4H-SiC S.-K. Lee, C.-M. Zetterling, and M. Östling KTH, Royal Institute of Technology, Dept. of Electronics, S-164 40, Kista, SWEDEN
ABSTRACT In the present work, we investigated sputtered titanium tungsten (TiW) contacts for Ohmic contacts to both n- and p-type 4H-SiC with long-term stability under high temperature (500oC). . Epitaxial layers with a doping concentration of 1.3×1019 and 6×1018 cm-3 were used. After high temperature annealing (>950oC) sputtered TiW contacts showed Ohmic behavior with good uniform distribution of the specific contact resistance. We obtained an average specific contact resistance (ρC) of 4×10-5 Ωcm2 and 1.2 ∼1.7×10-4 Ωcm2 for p- and n-type, respectively from linear TLM measurement. We also found some variation of the specific contact resistance and the sheet resistance from our TLM measurement for p-type contacts. We will discuss this behavior with the measurement of SIMS. Long-term stability with a top-cap layer is also discussed.
INTRODUCTION Silicon carbide (SiC) is a wide band gap semiconductor with useful properties for high temperature, high temperature, and high power device application [1]. At present time, the performance of SiC devices is limited by the fabrication of high temperature stable and low resistance Ohmic contacts. For silicon carbide, many research reports have been published on electrical contacts, both Schottky and Ohmic. Lower specific contact resistance is usually obtained to n-type 4H- and 6H-SiC (∼10-4 to 10-6 Ω⋅cm-2) than to p-type 4H- and 6H-SiC (∼10-3 to 10-5 Ω⋅cm-2)[2,3]. Generally, lowering the barrier height or increasing the doping concentration can reduce specific contact resistance[4]. Nickel contacts annealed >950oC are widely used as Ohmic contacts to n-type 6H- and 4H-SiC [5,6]. The specific contact resistance (ρC) is as low as 5×10-6 Ω⋅cm-2. However, the annealing process causes undesirable features, such as broadening of the metal-SiC interface, a rough interface morphology, and substantial roughening of the contact surface [7]. We also found that Ni contacts started to react after 650oC annealing in vacuum furnace for 30 min and made the surface rougher as shown in Figure 1 (a) even though Ni contacts show excellent electrical properties (ρC ≈ 5×10-6 Ωcm2). Rough surface and interface of Ni contacts will cause ultimate device failure via contact degradation and wire bond failure under the exposure of high power and temperature operating. In addition, the contact material should be selected for low-resistance, reaction properties and compatibility to the process for SiC device application. Viewed in this light, titanium tungsten (TiW) is one of the potential candidates for Ohmic contacts to both n- and p-type 4H-SiC. It has quite low resistance, it may be wet etched, and is very inert with respect to reaction with SiC under high temperatures to both n- and p-type 4H-SiC [8]. We have also demonstrated that TiW Schottky contacts had excellent rectifying characteristics after annealing a
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