(Invited) Tunneling Phenomena and Ohmic Contact Formation at Non-Alloyed Metal/Heavily-Doped SiC Interfaces
Authors: Hara Masahiro , Kitawaki Takeaki , Kuwahara Kotaro , Tanaka Hajime , Kaneko Mitsuaki , Kimoto Tsunenobu 
Published: December 2024

To clarify the formation mechanism of alloyed ohmic contacts on SiC by high-temperature sintering (~1000°C) and to improve the fabrication process of SiC electronic devices, a physical understanding of fundamental properties at non-sintered (non-alloyed) metal/heavily-doped SiC interfaces is indispensable. This paper reviews tunneling phenomena through a thin Schottky barrier at non-alloyed contacts on heavily-doped SiC, and design guidelines toward low-resistance ohmic contacts are proposed based on transport modeling.

The first target was a Schottky contact formed on a heavily nitrogen-doped n-type SiC epitaxial layer. Vertical Schottky barrier diode (SBD) structures were fabricated by systematically varying the donor density in the epitaxial layers (up to 1019 cm−3), and the current-voltage (I-V) characteristics were analyzed based on a numerical calculation of direct tunneling (DT) current, in which energy integration of carrier distribution and tunneling probability is performed. Note that the DT transport describes both the thermionic field emission (TFE) and field emission (FE). The numerical calculation reproduces well the experimental I-V curves for vast ranges of the donor density, barrier height, and applied voltage. The authors also formed Schottky contacts on heavily aluminum-doped p-type SiC epitaxial layers and revealed unique DT transport associated with the complicated valence band structure of SiC. The valence band in SiC has a split-off band with a light effective mass (m
* = 0.2m
0), which is located at the energy of several tens of meV above the topmost heavy- and light-hole bands (m
* = 1.6m
0). Reflecting the exponential contribution of the effective mass to the tunneling probability, carrier transport through non-alloyed contacts on heavily-doped p-type epitaxial layers is dominated by hole tunneling through the split-off band.

The next step was to clarify how different the interface properties are between the cases of epitaxial and ion-implanted layers. It was found that vertical SBDs fabricated with heavily phosphorus- and aluminum-implanted n- and p-type SiC, respectively, both exhibit several orders of magnitude larger current than those with epitaxial layers having almost identical doping densities. Systematic analysis of the Schottky barrier height through capacitance-voltage (C-V) measurement revealed that there is no significant difference in the barrier height at the contacts on ion-implanted and epitaxial layers, indicating a contribution of a carrier transport process other than DT. The enhanced current is likely explained by trap-assisted tunneling (TAT) through implantation-induced deep levels.

Toward modeling of contact resistivity based on the deep understanding of the tunneling phenomena (DT and TAT) gained above, experimental characterization and numerical analysis of the contact resistivity at non-alloyed ohmic contacts formed on heavily ion-implanted n- and p-type SiC were performed. At non-sintered Mg and Ti contacts on n-type SiC (donor density: 2×1020 cm−3), corresponding to the barrier height of 0.7 and 1.0 eV, respectively, an extremely low contact resistivity of 1-2×10−7 Ωcm2 was achieved without performing any thermal treatment after the electrode formation. As for p-type SiC with the acceptor density of about 1×1020 cm−3, a Ni contact (barrier height: 1.7 eV) exhibited a relatively low contact resistivity of 9.8×10−3 Ωcm2 without sintering. Through comparison between the contact resistivities experimentally and numerically obtained, how the dominant tunneling process changes depending on the doping density is carefully discussed, and a physics-based model considering the contributions of DT and TAT to predict the contact resistivity at non-alloyed ohmic contacts on implanted SiC is proposed as follows: the contact resistivity is significantly reduced thanks to a contribution of the TAT when the doping density is below about 1020 cm−3, and the contact resistivity at further high doping density sharply decreases and is well predictable based on the DT theory. The proposed model provided several quantitative guidelines regarding the doping density and barrier height for designing low-resistance ohmic contacts on SiC.

The model to predict the contact resistivity standing on the physics of tunneling is beneficial not only for a deeper understanding of the ohmic contact formation on SiC but also for exploring a novel low-temperature fabrication process toward low-resistance ohmic contacts. Besides, the high-field tunneling phenomena revealed in this study are expected to be commonly observed at contacts formed on other wide-bandgap semiconductors. Thus, the present data and model can also give a critical insight into the ohmic contact formation on these materials.