Determination of carrier lifetime and diffusion length in Al-doped 4H-SiC epilayers by time-resolved optical techniques

A series of p-type 4H-SiC epilayers with aluminium concentration ranging from 2 x 10(16) to 8 x 10(19) cm(-3) were investigated by time-resolved optical techniques in order to determine the effect of aluminium doping on high-injection carrier lifetime at room temperature and the diffusion coefficient at different injections (from approximate to 3 x 10(18) to approximate to 5 x 10(19) cm(-3)) and temperatures (from 78 to 730 K). We find that the defect limited carrier lifetime tau(SRH) decreases from 20 ns in the low-doped samples down to approximate to 0.6 ns in the heavily doped epilayers. Accordingly, the ambipolar diffusion coefficient decreases from D-a = 3.5 cm(2) s(-1) down to approximate to 0.6 cm(2) s(-1), corresponding to the hole mobility of mu h = 70 cm(2) Vs(-1) and 12 cm(2) Vs(-1), respectively. In the highly doped epilayers, the injection-induced decrease of the diffusion coefficient, due to the transition from the minority carrier diffusion to the ambipolar diffusion, provided the electron diffusion coefficient of De approximate to 3 cm(2) s(-1). The Al-doping resulted in the gradual decrease of the ambipolar diffusion length, from L-D = 2.7 mu m down to L-D = 0.25 mu m in the epilayers with the lowest and highest aluminium concentrations.