Effect of thickness on the photoluminescence of silicon quantum dots embedded in silicon nitride films

Abstract

In this work, the effect of film thickness on the photoluminescence (PL) spectra of Si quantum dots embedded in silicon nitride films is investigated experimentally and theoretically. The films were deposited by remote plasma enhanced chemical vapor deposition using the same SiH2Cl2/H2/Ar/NH3 mixture and deposition conditions, in order to obtain films with similar composition and approximately equal average size (∼3.1 nm) of Si quantum dots. Only the deposition times were varied to prepare five samples with different thicknesses ranging from ∼30 nm to 4500 nm. Chemical characterization by Fourier Transform Infrared Spectroscopy and X-ray Photoelectron Spectroscopy were carried out in order to check that the composition in all films was the same. The structure, average size, and size distribution of the Si quantum dots were deduced from High-Resolution Transmission Electron Microscopy. The thickness of the films was determined by ellipsometry and interferometry of UV-Vis transmission spectra. It was found experimentally that the increase of the thickness above a few hundreds of nanometers produces significant distortions of the PL spectra of the films, such as peak shifts and the appearance of shoulders and multiple peaks suggesting interference effects. Comparing the experimental results with theoretical simulations, it is shown that these distortions are mainly due to interference effects and not to intrinsic changes in the films. The approximation used to simulate the PL spectra as a function of film thickness allows improving the fitting between simulated and experimental spectra by changing some optical parameters and can be helpful to further investigate the intrinsic optical properties of the films.