  The p-type doping efficiency of 4H silicon carbide (4H-SiC) is rather low due
to the large ionization energies of p-type dopants. Such an issue impedes the
exploration of the full advantage of 4H-SiC for semiconductor devices. In this
letter, we show that co-doping group-IVB elements effectively decreases the
ionization energy of the most widely used p-type dopant, i. e., aluminum (Al),
through the Coulomb repulsion between the energy levels of group-IVB elements
and that of Al in 4H-SiC. Among group-IVB elements Ti has the most prominent
effectiveness. Ti decreases the ionization energy of Al by nearly 50%, leading
to a value as low as ~ 0.13 eV. As a result, the ionization rate of Al with Ti
co-doping is up to ~ 5 times larger than that without co-doping at room
temperature when the doping concentration is up to 1018 cm-3. This work may
encourage the experimental co-doping of group-IB elements such as Ti and Al to
significantly improve the p-type doping efficiency of 4H-SiC.
