The conduction mechanism in heavily Al-doped or heavily Al-and N-codoped p-type 4H-SiC epilayers was investigated. In both the singly-doped and codoped samples with an Al concentration (CAl) between 4x1019 and 2x1020 cm-3, band and nearest-neighbor hopping (NNH) conductions appeared in high and low temperature ranges, respectively. The codoping of N donors makes the NNH conduction dominant at temperatures higher than in the singly-doped samples. In both the singly-doped and codoped samples with CAl between 1x1019 and 4x1019 cm-3, an unexpected conduction appeared between the regions of the band and NNH conductions.

To reduce the resistivity of heavily Al-doped 4H-SiC epilayers, the conduction mechanisms were investigated for Al concentrations (CAl) between 2.4 × 1019 and 4.7 × 1020 cm−3 and measurement temperatures (T) between 20 and 600 K. The results elucidated the relationship between the conduction mechanisms and the values of T and CAl. For the epilayers with CAl ∼ 3 × 1019 cm−3, an unexpected additional conduction mechanism was observed between the band and nearest-neighbor-hopping conduction regions, for which two plausible conduction models are proposed. To reduce the resistivity of epilayers with CAl > 2 × 1020 cm−3, it is essential to fabricate samples with low lattice distortion.

We investigate the transition of the conduction mechanism from band and nearest-neighbor hopping (NNH) conduction to variable-range hopping (VRH) conduction in heavily Al-doped 4H-SiC epilayers with increasing Al concentration (C Al). In a sample with C Al of 1.8 × 1020 cm−3, the dominant conduction mechanisms at high and low temperatures were band and VRH conduction, respectively, whereas in samples with lower C Al values they were band and NNH conduction, respectively, and in samples with higher C Al values VRH conduction was dominant over the entire range of measurement temperatures examined (20–600 K).

The temperature dependencies of the resistivity and Hall coefficient for heavily Al-doped 4H-SiC epilayers with Al concentration (CAl) higher than 2×1019 cm-3 were investigated. The signs of measured Hall coefficients (RH) change from positive to negative at low temperatures. For the epilayers with CAl < 3×1019 cm-3 the sign inversion occurred in the hopping conduction region, which was reported to be explicable using the model for amorphous semiconductors. For the epilayers with CAl > 3×1019 cm-3, on the other hand, the sign inversion occurred in the band conduction region, which is a striking feature, because the movement of free holes in the valence band should make RH positive. The sign-inversion temperature increased with increasing CAl, while the dominant-conduction-mechanism-change temperature was almost independent of CAl.

We have observed negative Hall coefficients [R H(T)] in a nearest-neighbor hopping (NNH) conduction region in epilayers of heavily Al-doped or Al–N co-doped p-type 4H-SiC grown on n-type 4H-SiC substrates by CVD or in wafers of heavily Al–N co-doped p-type 4H-SiC fabricated by solution growth. We propose a simple physical model to explain the sign of R H(T) in NNH conduction. According to this model, R H(T) becomes positive when the Fermi level (E F) is higher than the Al acceptor level (E Al), that is, the Fermi–Dirac distribution function f(E Al) is greater than 0.5, whereas R H(T) becomes negative when E F is lower than E Al, which occurs at low temperatures. Because the dominant conduction mechanisms in heavily Al-doped or Al–N co-doped p-type 4H-SiC with Al concentrations on the order of 1019 cm−3 are band and NNH conduction at high and low temperatures, respectively, the proposed model can explain why R H(T) becomes negative at low temperatures.