STEM EELS plasmon imaging ( SEPI ) for mixed phase silicon / silicon‐oxides systems

Abstract

Plasmon imaging using energy filtered transmission electron microscopy (EFTEM) has been a well‐established technique for investigating mixed phase silicon systems for more than a decade [1]. To image the silicon distribution typically an energy window of 4 eV centered at 17 eV is used. With this approach the signal contains significant contributions of the silicon monoxide and dioxide plasmons which deteriorates contrast and prohibits quantitative imaging. As alternative approach we developed SEPI. A method, based on the combination of scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS). SEPI allows to separate the contributions of Si, SiO and SiO 2 (see Fig. 1(a)) and, thereby, to map them individually. This was achieved by a spectrum‐by‐spectrum evaluation routine of EELS data cubes including the extraction of the single scattering distribution (SSD) and the subsequent fitting with reference spectra of silicon monoxide and dioxide and the analytical Drude model for silicon. In this work we present the successful application of SEPI for investigations of three silicon / silicon oxides systems. All measurements were performed on FEI Tecnai Osiris microscope equipped with a FS‐1 electron energy spectrometer. In Fig. 1(b) the silicon nanostructure of a nanocrystalline hydrogenated SiO x (nc‐SiO x :H) layer deposited on a silicon wafer using PECVD is shown. During the last years nc‐SiO x :H was under investigation for several applications in silicon thin film solar cells [2], as it offers high electronic conductivity compared to other silicon alloys. The electrical properties are commonly explained by the presence of silicon nanofilaments [3], which can be seen in Fig. 1(b). In Fig. 2, a line scan across the interface of a 14.5 nm thick silicon oxide layer on a silicon wafer is shown. The oxide layer was thermally grown at 900 °C partially using HCl atmosphere. The distribution of the three components Si, SiO, and SiO 2 demonstrate that between the silicon substrate and the SiO 2 layer an intermediate SiO layer exists. Fig. 3 (a) shows the oxygen distribution measured using energy dispersive X‐ray spectroscopy (EDX) of an oxygen precipitate in silicon. Oxygen precipitates in silicon are generated by precipitation of the supersaturated interstitial oxygen during thermal processing. Their stoichiometry was under discussion for decades and the compositions found were ranging from SiO 2 to SiO. Recently, it was shown that the center of the precipitate consists of SiO 2 [4]. The SiO plasmon ratio distribution of the oxygen precipitate, shown in Fig. 3 (b), demonstrates that the oxygen precipitate is also surrounded by an SiO layer similar to the oxide layer.