A new insight into ohmic contact formation on SiC: Enhanced tunneling current by ion implantation

Silicon carbide (SiC) has outstanding material characteristics, such as a high critical electric field, high thermal conductivity and high mechanical strength. Because of these properties, it is a promising semiconductor material for power semiconductor devices. It is observed that there is a large potential barrier at the metal/SiC interface which is caused by a wide bandgap of SiC and hence making it difficult to achieve good ohmic connections with low contact resistivity. Ti, W, Ni, and other commonly used electrode metals do not have enough low work function values, hence high-temperature annealing at a temperature of about 1000 °C is the usual method for lowering contact resistivity. High-temperature annealing, however, has drawbacks such as surface roughening, metal melting, and decreased device performance. Therefore, it is essential to investigate alternative strategies to increase conductivity with a limited thermal expense.

In a new study published in peer-reviewed journal Applied Physics Express by Mr. Masahiro Hara, Dr. Mitsuaki Kaneko and Professor Tsunenobu Kimoto at Kyoto University in Japan, investigated the effect of high-dose phosphorus (P) ion implantation on electrical properties of magnesium (Mg) contacts on SiC. By implanting high-density P ions, non-alloyed Schottky contacts on SiC were created, and then their current-voltage (I-V) characteristics and contact resistivity were examined. Mg was chosen as the electrode metal due to its low work function, which results in a low barrier height at a metal/SiC junction. These contacts were compared to those on nitrogen-doped (N-doped) epitaxial SiC.

P ions were implanted into SiC to produce profiles in the shape of boxes with varying the P atom density. With a surface-protective carbon cap, activation annealing was carried out for 20 minutes at 1750 °C. The research team used secondary ion mass spectrometry (SIMS) and capacitance-voltage (C-V) measurements to find and extract the depth profiles of the P atom and net donor densities. The experiment confirmed almost perfect activation of the implanted P atoms, even at a very high P atom density of 3 x 10¹⁹ cm^-3.

In their previous study, the researchers looked at the tunneling current in Schottky barrier diodes (SBDs) fabricated with heavily N-doped SiC epitaxial layers, which is described by both thermionic field emission and field emission. On the other hand, the current density at the contacts on P⁺-implanted SiC was much larger than that on epitaxial layers. Through a careful investigation of the barrier height, the authors pointed out that trap-assisted tunneling (TAT) is a possible cause to enhance the current density. Besides, very low contact resistivity (10^-6 Ωcm²) attributed to the enhanced tunneling current is demonstrated with no warm therapy.

To conclude, I-V characteristics and contact resistivity at Mg contacts formed on heavily P⁺-implanted SiC were investigated. The authors found that the contacts formed on ion-implanted SiC had a much larger current compared to those on epitaxial SiC with similar doping density. As a result, the non-alloyed Mg contacts on the ion-implanted SiC had a very low contact resistance as low as that on practical Ni-based contacts formed with a high-temperature treatment. Mr. Masahiro Hara and the colleagues showed that ion implantation is superior to epitaxial growth in terms of decreasing contact resistivity at SiC ohmic contacts. The present results could help establish a low-temperature process for SiC ohmic contacts and further improve their contact resistivity.