Synthesis and optical properties of amorphous Si₃N _{4- x}P_x dielectrics and complementary insights from ab initio structural simulations

Applications that require tuning of the properties of amorphous Si ₃N₄ currently make use of nonstoichiometric silicon oxynitrides or silicon nitrides. In this study compositional tuning is employed to produce a new family of amorphous dielectrics with near stoichiometric Si₃N_4-xP_x (x ∼0-1) compositions and adjustable optical response. Air stable and thermally robust materials are deposited at 650-700 °C on Si wafers with a growth rates as high as 0.2 μm/min via CVD reactions of P(SiH₃)₃ with NH _3. An alternative route involving P(SiH₃) ₃/N(SiH₃)₃ mixtures with ammonia was also explored and found to yield comparable materials. Precise control of the reactant fluxes produces alloys in which the PSi₃ molecular core of the precursor is likely incorporated intact into the covalent nitride network, leading to an assemblage of corner-shared SiN₄ and SiN₃P tetrahedra containing residual N-H bonds. The optical properties of the resultant materials have been studied by spectroscopic ellipsometry, which indicated that the incorporation of P systematically decreases the band gap while increasing the refractive index. Density-functional theory simulations were used to reconcile the molecular geometry of the precursors with local bonding in the solids and predict that the incorporated PSi₃ cores adopt a planar geometry for low P contents (up to x = 0.5) and become slightly puckered as P increases to x = 1. This implies that the more rigid Si ₃N units initially induce a planarization in their softer Si ₃P counterparts, leading to hexagonal-type ground-state structures with slightly lower symmetry than that of the β-Si₃N₄ prototype. The simulations of the band-structure for Si₃N _3.5P_0.5 and Si₃N₃P confirm the decreasing trend observed in the optical band gap as the P content is increased.