Thin Films of SiP Lamellar Alloys: A First Step toward 2D SiP

Abstract

We investigate the structural and vibrational properties of Si:P thin films obtained by coevaporation of Si and P in ultra-high vacuum at room temperature followed by rapid thermal annealing. The thermal crystallization of the films was followed by Raman spectroscopy. Annealing at temperatures larger than 950 °C leads to the formation of crystalline phases. Density functional theory calculations of the vibrational modes allow us to identify orthorhombic SiP. Electron energy loss spectroscopy combined with energy dispersive spectroscopy give evidence of a plasmon signature of the SiP phase. The distribution of the crystalline phases in the film was imaged by energy filtered transmission electron microscopy. Both Si and SiP areas having sizes of a few microns are found to coexist in the films. Highresolution scanning transmission imaging provides a clear evidence of the lamellar structure while spatially resolved electron energy loss spectroscopy allows us to obtain a chemical mapping for both Si and P atoms, which agrees quite well with the orthorhombic structure of SiP. Our results represent an important first step on the way to obtain 2D SiP, a promising new material for which a direct bandgap has been predicted.