Thin Films Of Sip Lamellar Alloys: A First Step Toward 2d Sip

We investigate the structural and vibrational properties of Si:P thin films obtained by co-evaporation of Si and P in ultrahigh 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 degrees 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 gives 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. High-resolution scanning transmission imaging provides a clear evidence of the lamellar structure, while spatially resolved electron energy loss spectroscopy allows us to achieve 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 to obtain two-dimensional (2D) SiP, a promising new material for which a direct band gap has been predicted.