Simultaneous Formation of Ni/Al Ohmic Contacts to Both n- and p-Type 4H-SiC

Abstract

The fabrication procedure for silicon carbide power metal oxide semiconductor field-effect transistors can be improved through simultaneous formation (i.e., using the same contact materials and a one-step annealing process) of ohmic contacts on both the n-source and p-well regions. We have succeeded in the simultaneous formation of Ni/Al ohmic contacts to n-and p-type SiC after annealing at 1000°C for 5 min in an ultrahigh vacuum. Ohmic contacts to n-type SiC were found when the Al-layer thickness was less than about 6 nm, while ohmic contacts to p-type SiC were observed for an Al-layer thickness greater than about 5 nm. Only the contacts with an Al-layer thickness in the range of 5 nm to 6 nm exhibited ohmic behavior to both n-and p-type SiC, with a specific contact resistance of 1.8 9 10 -4 X cm 2 and 1.2 9 10 -2 X cm 2 for n-and p-type SiC, respectively. An about 100-nm-thick contact layer was uniformly formed on the SiC substrate, and polycrystalline d-Ni 2 Si(Al) grains were formed at the contact/SiC interface. In the samples that exhibited ohmic behavior to both n-and p-type SiC, the distribution of the Al/Ni ratios in the d-Ni 2 Si(Al) grains was larger than that observed for any of the samples that showed ohmic behavior to either n-or p-type SiC. Furthermore, the grain size of the d-Ni 2 Si(Al) grains in the samples showing ohmic behavior to both n-and p-type SiC was smaller than the grains in any of the samples that showed ohmic behavior to either n-or p-type SiC. Thus, the large distribution in the Al/Ni ratios and a fine microstructure were found to be characteristic of the ohmic contacts to both n-and p-type SiC. Grains with a low Al concentration correspond to ohmic contacts to n-type SiC, while grains with a high Al concentration correspond to ohmic contacts to p-type SiC.

Ohmic contacts on silicon carbide: The first monolayer and its electronic effect

“…64,65 This can be understood by considering that the potential induced by applied bias drops largely at an contact interface between metals and semiconductors, thus hindering the current flow. The annealed system, however, exhibits a typical ohmic nature, as its I-V curve is nearly linear and the current increases sharply with the rise of applied bias.…”

Low Temperature Ni-Al Ohmic Contacts to p-Type 4H-SiC Using Semi-Salicide Processing

“…Unlike the conventional Ni-Al contact that requires anneals up to 1000 °C [2][3][4][5], the semi-salicide processed contacts only require that the second-step anneal is higher than 500 °C to become ohmic (upper limit not determined). The samples annealed at 400 °C and 450 °C showed rectifying behavior.…”

“…The contacts typically require annealing temperatures up to 1000 °C to form low-resistance contacts (~10 -4 Ω cm 2 ), with the exceptions of Ge-Ti-Al (600 °C, ultra-high vacuum anneal for 1800 s) [9], titanium (700 °C, surface recrystallization as 3C-SiC after high-dose ion-implantation, contact resistivity higher than 10 -3 Ω cm 2 ) [14], and Ni-Ti-Al (800 °C using rapid thermal processing (RTP), narrow temperature window) [6][7][8]. Of the cited articles [2][3][4][5][6][7][8][9][10][11][12][13][14], all except [12][13][14] explicitly state that evaporation and lift-off (LO) is used to pattern the contact stack, which is unsuitable for volume production.…”

Long-term stability of nickel-based ohmic contacts with n-type and p-type 4H-SiC in a high-temperature environment

“…Forming ohmic contacts to both n-type and p-type regions by the same process has been studied in the CMOS fabrication. [27][28][29][30][31][32] Linear-transmission-line-model samples, which have Ni-based ohmic contacts to implanted n-type and p-type SiC, showed ρ c in the range of 10 −6 Ω cm 2 and 10 −3 Ω cm 2 , respectively at room temperature. 31) Furthermore, the samples demonstrated high thermal stability in nitrogen atmosphere.…”