Rapid synthesis of nc-Si/a-SiNx:H QD thin films by plasma processing for their cost effective applications in photonic and photovoltaic devices

Abstract

A rapid and single step synthesis of nc-Si/a-SiN_x:H QD thin films, where nc-Si QDs of size ∼9–2.6 nm and number density ∼10¹¹–10¹² cm⁻² are embedded in a-SiN_x:H matrix, has been made possible from a (SiH₄ + NH₃) gas mixture, with the advent of high density low pressure planar inductively coupled plasma processing. The nc-Si/a-SiN_x:H QD films of high crystallinity (∼80%) with preferred orientation I_〈220〉/I_〈111〉 ∼1, along with the distinct presence of α-Si₃N₄ and β-Si₃N₄ components have been obtained at a competitive deposition rate of ∼20.6 nm min⁻¹ from an extremely low flow rate (∼2.0 sccm) of the feed gas SiH₄. The significance of the result lies in attaining such a material from pure SiH₄ plasma without H₂-dilution, and that even at a low substrate temperature (∼250 °C) compatible for device fabrication. Tunable and intense visible photoluminescence (PL), with a variety of individual colors in the range 1.55–3.10 eV, has been demonstrated as a consequence of band-to-band recombination due to quantum confinement effects (QCE). The high magnitude of the confinement parameter (∼13.5 eV nm²) has been correlated to the core–shell-like structure of the QDs within a dielectric matrix; the amorphous shell-like component surrounding the rigid network of the nc-Si QD core has been revealed by the HR-TEM micrograph. With all their various properties e.g., high crystallinity, favored orientation, wide optical gap due to the quantum confinement in the Si-ncs and a significant amount of nitrogen bonding in the matrix, as well as their photoluminescence, which is tunable over a wide range, the nc-Si/a-SiN_x:H quantum dot (QD) thin films obtained at high growth rate have enormous promise for their cost effective applications in silicon based photonic and photovoltaic devices.