New insight into the microstructure and doping of unintentionally n-type microcrystalline silicon carbide

Microcrystalline silicon carbide (μc-SiC:H) deposited by hot wire chemical vapor deposition (HWCVD) and plasma-enhanced chemical vapor deposition (PECVD) provide advantageous opto-electronic properties, making it attractive as a window layer material in silicon thin-film and silicon heterojunction solar cells. However, it is still not clear which electrical transport mechanisms yield dark conductivities up to 10⁻³ S/cm without the active use of any doping gas and how the transport mechanisms are related to the morphology of μc-SiC:H. To investigate these open questions systematically, we investigated HWCVD and PECVD grown layers that provide a very extensive range of dark conductivity values from 10⁻¹² S/cm to 10⁻³ S/cm. We found out by secondary ion mass spectrometry measurements that no direct correlation exists between oxygen or nitrogen concentrations and high dark conductivity σ_d, high charge carrier density n, and low activation energy Ea. Higher σ_d seems to rise from lower hydrogen concentrations or/and larger coherent domain sizes L_SiC. On the one hand, the decrease of σ_d with increasing hydrogen concentration might be due to the inactivation of donors by hydrogen passivation that gives rise to decreased n. On the other hand, qualitatively consistent with the Seto model, the lower σ_d and lower n might be caused by smaller L_SiC, since the fraction of depleted grain boundaries with higher Ea increases accordingly.