Phosphorus oxides in heavily doped polysilicon films

Abstract

In tunneling oxide passivation contact (n-TOPCon) photovoltaic devices, poly-Si (n⁺) films with high-concentration phosphorus doping are the key materials for electron selective passivation. Its optical and electronic properties strongly depend on the chemical configuration and physical phase, and also on high temperature annealing and structural relaxation in the recrystallization process. The poly-Si (n⁺) films grown on SiO_<i>x</i>/n-Si substrates by low pressure chemical vapor deposition technology are investigated, while the microstructure of the film is studied by using X-ray photoelectron spectroscopy with depth etching, high-resolution transmission electron microscopy and X-ray diffraction analysis. It is found that the binding energy values of the two fitted peaks (O2 and O3) of O 1s state of the thin film are situated at 532.1 and 533.7 eV, corresponding to the bonding of O—Si and O—P, respectively. The binding energy values of the two fitted peaks (P2 and P3) of P 2p state are located at 132.4 and 135.1 eV, corresponding to O—P* bonding with the same origin. Electronic microscopy and light diffraction analyses show that the polycrystalline silicon film has the characteristic of (111) preferential orientation, and the space of crystal plane is 0.313 nm, for which the average grain size is in a range of about 43.6–55.0 nm. However, the mechanical deformation and grain boundaries are generated in the annealing process at 920 ℃ along (111) crystal cluster, resulting in the localized monocrystalline state within large grains. The comprehensive analyses of thermodynamic function parameters of formation enthalpy, reaction entropy, heat capacity, formation energy and Gibbs free energy and energy minimum principle analysis indicate that there exist conditions for forming Si—O and P—O bonds in the polysilicon film, and thus the bonding state of silicon and phosphorus oxides are formed.