Analysis of p-type SiO_x layers as a boron diffusion source for n-type c-Si substrates

We evaluate the use of p-type silicon oxide (p-SiO_x) dielectric layers as a boron diffusion source for n-type crystalline silicon (c-Si) substrates. The p-SiO_x layers grown on n-type c-Si substrates by plasma enhanced chemical vapor deposition using a gas mixture of He/hexamethyldisiloxane/CO₂/B₂H₆ are thermally stable and do not peel off during annealing up to 1050 °C, making them effective diffusion sources. The layers were examined before and after annealing with characterization techniques including spectroscopic ellipsometry and secondary ion mass spectrometry. We observe that there is a reduction in the thickness of the p-SiO_x layer after annealing by about 50%, and that boron diffuses into the n-type c-Si substrate, forming a p⁺ layer, limited by the formation of a carbon-rich layer above the c-Si surface. The concentration of holes in the diffused region was measured by the electrochemical capacitance–voltage technique, and it was found that essentially all the boron that diffused into the n-type c-Si was active, unaffected by the presence of carbon and oxygen atoms. The concentration of carriers can be controlled by the initial thickness of the p-SiO_x layers and the depth of the p⁺/n junction can be controlled by the time of annealing. A surface carrier concentration of 3 × 10¹⁹ at cm⁻³ and a sheet resistance of the order of 120 Ω sq⁻¹ was obtained upon annealing at 1050 °C for 30 min.