Investigation of the transition phases from amorphous silicon‐based multilayers to silicon nanostructures by in situ X‐ray diffraction

Abstract

We report the investigation of the formation phases of silicon nanostructures obtained from thermal annealing of hydrogenated amorphous silicon (a-Si:H)-based multilayers using in situ X-ray diffractometry, XRD. The multilayers composed of alternating layers of a-Si:H and silicon oxide were deposited on [100]-oriented crystalline silicon substrates using a plasmaenhanced chemical vapor deposition. Our results indicate that crystallization only starts after hydrogen effusion at a 500 8C annealing temperature, independent of the a-Si:H sublayer thickness. Hydrogen effusion results in the reordering of the silicon structure from the tetragonal to the diamond structure, while the films maintain their amorphous medium-range order.The onset of crystallization and phase separation occur at different annealing temperatures depending on the a-Si:H sublayer thickness. The fraction of the crystallized nanostructure are found to also depend on the a-Si:H sublayer thickness. Phase separation and formation of silicon nanostructures is observed by the shift of the SiO stretching vibration to higher wave numbers in the Fourier transform infrared (FTIR) absorption spectra. The different crystallization phases as observed with in situ XRD will be corroborated by measurements from Raman and FTIR spectroscopies. The crystallite size obtained from XRD compares well with that obtained from transmission electron microscopy.

Impact of sublayer thickness and annealing on silicon nanostructures formation in a-Si:H/a-SiNx:H superlattices for photovoltaics

“…In order to distinguish the mechanisms of luminescence in silicon superlattices, firstly, it is important to prepare well-controlled Si nanostructures and to investigate their structural and optical properties, which are desirable for understanding of the mechanisms in superlattices. In most previous studies, optical and structural properties from Si/SiO 2 multilayers have been investigated [4,10,11,13,17]. Nevertheless, the high band offsets of the SiO 2 matrix with nanocrystalline silicon nanocrystals can suppress the tunneling probability of carriers into the NCs in devices designed for electroluminescent and photovoltaic applications.…”

“…This very active field of research has been since the 1990s with the observation of strong light emission in the visible range from porous silicon at room temperature [6,7]. In nano-scaled superlattice structures, Si sublayers and Si dielectric sublayers (Si 3 N 4 [8,9] or SiO 2 [10][11][12]) are alternately deposited in a stack and subsequently treated with a post-deposition high temperature (~ 1100 °C) annealing. Silicon sublayer thickness should not exceed the critical value predicted by the quantum confinement model.…”

“…In addition, the thickness of the Si sublayer can be adjusted very accurately and a controllable size and distribution of silicon nanocrystals can be obtained by changing of deposition conditions and parameters [8,10,11,13,14]. Recently, different approaches have been used to fabricate Si-NCs, such as molecular beam epitaxy (MBE) [7], magnetron sputtering [3], cosputtering [15], thermal oxidation [3], plasma-enhanced chemical vapor deposition [10,11]. Plasmaenhanced chemical vapour deposition (PECVD) is a powerful method to prepare superlattices required for the fabrication of optoelectronic device and solar cells [16].…”