Investigation of the Composition of the Si/SiO2 Interface in Oxide Precipitates and Oxide Layers on Silicon by STEM/EELS

Abstract

In a previous work, we found that both the interface between an oxide precipitate and the surrounding silicon matrix and the interface between a silicon substrate and a thermal oxide layer are of the same nature [1]. In both cases, between SiO2 existing in the center of the precipitate and in the oxide layer and Si of the matrix and the substrate a suboxide region of 2-3 nm exists. These results were obtained by electron energy loss spectrometry (EELS) carried out by scanning transmission electron microscopy (STEM). In the low loss region, it is possible to distinguish between Si, SiO, and SiO2 which all exhibit different maxima of the plasmon loss energy. By deconvolution, the local composition of the phase can be determined with the help of reference spectra of the three components. The stoichiometry of the oxide precipitates (SiO x ) was debated for a long time and application of different methods on different samples containing oxygen precipitates resulted in different values for x ranging from 1 to 2. Recently, it was demonstrated by applying several direct and indirect methods that the oxide precipitates consist of SiO2 [2]. Inspired by the results in Ref. 1 and 2, a layer model was proposed explaining the different values for x for oxide precipitates of different geometry [3]. Especially, for plate-like precipitates with a large surface the lower x values could be explained. Now the question came up if the width of the interface between Si and SiO2 is constant for all growth temperatures because from interface physics it could be expected that its width increases with temperature. Another point is, if the thickness of oxide layer or oxide precipitates plays a role. For this reason, we again used thermal oxide layers on (100) silicon of different thickness grown at different temperature to investigate the composition profile across the interface. We also compared the results with profiles from plate-like oxide precipitates grown at different temperatures. A scanning transmission electron microscope (STEM) FEI Tecnai Osiris equipped with tools for energy dispersive X-ray spectroscopy (EDX) analysis and EELS was used to analyze the interface and composition of a plate-like precipitates located at a very thin place of the lamella prepared from the sample by grinding, polishing, and ion milling. The profiles of the x values across the interfaces of the oxide layers and the precipitates were determined from the deconvolution of the Si, SiO, and SiO2 EELS spectra. The results of these investigations demonstrate that the stoichiometry of SiO2 (x=2) cannot be reached if the oxide layer thickness is lower than 10 nm for thermal oxides grown at 900 °C. The lower the thickness of the layer is the lower is the maximum x value. This is due to an interface layer of equal maximum slope for all oxide layers. The slope was obtained from fitting by sigmoid functions. Such interface layers were also found for the oxide precipitates but the maximum slope is lower. It was found to increase with increasing diffusion length of interstitial oxygen. The maximum of the x profiles also depends on the thickness of the plate-like oxide precipitates. The results will be discussed with respect to the phase separation process leading to precipitation of interstitial oxygen in silicon.