Decomposition of amorphous Si2C by thermal annealing
Introduction
Thin films of non-stoichiometric silicon carbide (Si1 −xCx) are interesting for applications in various branches of technology. Typical examples are window layers in solar cells [1], insulating layers in thin film transistors [2], thin film light emitting diodes [3], [4], colour displays [5], UV detectors [6], Li ion battery anodes [7] and microelectro-mechanical systems (MEMS) [8]. An actual field of research is the tailored synthesis of silicon nano crystals embedded in an amorphous matrix of silicon carbide for application as tandem solar cells [9], [10]. For such a design, a fundamental understanding of the amorphous to crystalline transition is of large importance, especially for Si-rich films of composition Si1 −xCx (x < 0.5).
Besides an investigation of phase formation, phase separation and crystallization kinetics, the understanding of the chemical order of the amorphous system as well as the early stages of crystallite formation are of importance. As illustrated in the tetrahedron model of Ref. [11], for amorphous silicon–carbon alloys, three types of chemical order have to be considered: (a) complete random order where no preferential chemical bonding between Si and C atoms exists; (b) complete chemical order, which means that in Si-rich alloys a C atom in the centre of a tetrahedron is surrounded by four Si atoms and that a maximum of possible Si–C bonds is realised; and (c) complete chemical order with phase separation, which means that the Si–C bonds are clustered [11]. According to molecular dynamics simulations [12], about 40–45% homonuclear bonds might be present in stoichiometric amorphous silicon carbide. However, the same simulations also revealed a high degree of short- and medium-range order excluding a complete random order.
In a recent study [13], we investigated the thermal stability of thin films of Si2C at 800 °C under Ultra High Vacuum (UHV) conditions. In this work, the formation of Si crystals with diameters of about 500 nm on the surface of the film was observed. Besides, no significant amount of crystallized SiC was detected by grazing incidence x-ray diffractometry (GIXRD). Further investigations [14] of the crystallization kinetics of amorphous Si2C films by GIXRD at temperatures between 1200 °C and 1350 °C revealed a transient formation of crystalline Si, superimposed by stoichiometric SiC crystallization.
In this study, we focussed on the processes determining the thermal stability of amorphous Si2C at 1200 °C. Therefore, microstructure and chemical bonds in the films are investigated at different time scales of annealing by means of x-ray photoelectron spectroscopy (XPS), ex situ and in situ transmission electron microscopy (TEM), grazing incidence x-ray diffractometry (GIXRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM).
Section snippets
Experimental details
Thin amorphous films of non-stoichiometric silicon carbide (Si2C) were prepared by a r.f. co-sputtering technique using a 3″ ION'X planar magnetron source (TFC, Grafenberg, Germany) mounted on a standard DN 150 CF double cross recipient equipped with pre-sputter shutter and sample positioner allowing various distances (100–200 mm) between magnetron and substrate. Deposition was done at a rate of 5 nm/min, using argon (6.0) sputter gas at an operating pressure of 0.1 Pa, a sputtering power of 80 W
XPS fitting procedure
For a better understanding of the fitting procedure which we used for all our XPS spectra, the reader is referred to Fig. 1. It shows representative fits of the Si2p and C1s core level spectra and the corresponding relations between the different species, which have been considered. Literature studies as well as own reference measurements were used to work out a reliable fitting algorithm. The corresponding fit parameters are summarized in Table 1. Considering the tetrahedron model of amorphous
Summary
In this study, we investigated the thermal stability of amorphous Si2C thin films, deposited on silicon substrates at high temperatures. The samples were annealed at 1200 °C in a resistance tube furnace for 2 and 20 h, respectively, and afterwards analyzed by means of XPS, ex situ and in situ TEM GIXRD, AFM and SEM. Annealing at 1200 °C is accompanied by a significant change in the bonding structure and the surface topography of the film. Annealing for 2 h led to the crystallization of silicon and
Acknowledgements
This research has been supported by the German Research Foundation (DFG) under the contract Schm1569/9-2. The authors would like to thank S. Korte and G. Lilienkamp for SEM measurements (Omicron NanoSAM provided by the German Research Foundation (DFG), grant number: INST 189/158-1) and S. Dahle for experimental assistance. We would further like to thank M. Bruns for his annotations according this paper and Prof. W. Daum for providing the AFM. This work was partially carried out with the support
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