Nanocrystalline SiC formed by annealing of a-SiC:H on Si substrates: A study of dopant interdiffusion

Abstract

Nanocrystalline silicon carbide (nc-SiC) is an interesting material for electronics applications, both in its own right and as a host matrix for silicon quantum dots. When synthesized by annealing of a-SiC:H on Si substrates, interdiffusion of dopants occurs if either the a-SiC:H or the Si substrate is doped. Annealing a-SiC:H on highly boron-doped substrates at 1100 °C leads to a fairly homogeneous doping level of ≥4 × 1019 cm−3 throughout the nc-SiC film. An unexpected anomaly in secondary ion mass spectroscopy quantification is observed and a method to circumvent it is shown. The nanostructure of the nc-SiC is only weakly affected as most of the diffusion occurs after the onset of crystallization. Annealing of doped a-SiC:H on Si substrates at 1100 °C leads to strong free carrier absorption at infrared wavelengths. This is demonstrated to originate from dopants that have diffused from the a-SiC:H to the Si substrate, and a method is developed to extract from it the doping profile in the Si substrate. The detection limit of this method is estimated to be ≤6 × 1013 cm−2. Doping levels of (0.5–3.5) × 1019 cm−3 are induced at the Si substrate surface by both boron and phosphorus-doped a–SiC:H. When the Si substrate is doped opposite to the a-SiC:H p–n junctions are induced at a depth of 0.9–1.4 μm within the Si substrate for substrate resistivities of 1–10 Ω cm. Implications for different solar cell architectures are discussed. Dopant diffusion can be strongly reduced by lowering the annealing temperature to 1000 °C, albeit at the expense of reduced crystallinity.