Electron energy loss spectroscopy of silicon nanostructures fabricated in a scanning transmission electron microscope

Saifullah MSM, Boothroyd CB, Botton GA and Humphreys CJ

Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom

The reduction of amorphous oxides and fluorides using the high intensity electron beam inside a VG HB501 scanning transmission electron microscope has been used for the direct formation of lithographic structures as small as 2-5 nm (1,2). Recently it has been shown that for a high enough dose, even SiO₂ can be reduced to silicon, allowing the production of silicon dots and lines as small as 2 nm across (3). Such small dimensions are comparable with the particle size in porous silicon, suggesting the interesting possibility that silicon dots formed by electron irradiation may share some of the luminescence properties of porous silicon. Here we investigate the low and core loss electron energy loss spectra of silicon particles formed by the electron beam irradiation of 20 nm thick electron beam evaporated SiO₂.

Figure 1: Electron energy loss spectra showing
the change in fine structure of the silicon L edges
as a function of electron dose.

The formation of silicon from SiO₂ under a 100 keV electron probe can be followed by observing the Si L edge as a function of electron dose (Figure 1). For low doses the characteristic edge structure of SiO₂ with an edge onset at 106 eV and strong peaks at 108 and 114 eV are seen. With increasing electron irradiation, the Si-L_3,2 edge characteristic of silicon at 99 eV appears and the strong SiO₂ peaks diminish, while the Si-L₁ edge at 152 eV replaces that of SiO₂ at 157 eV. At the same time, a sharp silicon plasmon appears at 16.8 eV to replace the much broader SiO₂ plasmon at 23 eV. Energy filtered images taken using these two plasmon peaks (Figure 2) confirm that small particles of silicon have been formed in the centre of the irradiated regions and earlier work has established that such silicon dots are amorphous (3). The composition of the central silicon particle in figure 2a can be obtained from the silicon L and oxygen K edge spectra in figure 3 taken from an undamaged area of SiO₂ and from the centre of the particle. The composition of the particle is found to be SiO_0.3. Although oxygen is still present, the shape of the silicon L edge and the presence of a silicon plasmon both show that silicon is present with a coating of the oxide, rather than the whole particle having a uniform composition of SiO_0.3.

Figure 2: Energy filtered images of silicon particles formed on SiO₂ using (a) the Si plasmon at 17 eV and (b) the SiO₂ plasmon at 23 eV energy loss.

Figure 3: Energy loss spectra showing (a) Si L and (b) the oxygen K edges from the unirradiated SiO₂ and a heavily irradiated silicon particle.

The low loss spectrum of the silicon particle resembles closest the spectrum of amorphous silicon, except that there is a small edge at 10 eV from the remaining SiO₂ coating the particle (Figure 4). Albu-Yaron et al. (4) found that their luminescent nanoporous crystalline silicon exhibited a distinct peak at 4.8 eV which is not present in bulk crystalline silicon. It is interesting that we find no evidence for such a 4.8 eV peak and this correlates with our amorphous silicon dots being non-luminescent. Work is in progress to evaluate the changes in the low loss spectra during crystallisation of these structures.

Figure 4: Low loss spectra from the unirradiated SiO₂,
heavily irradiated silicon particle and nanoporous silicon.
The porous silicon spectrum is from Albu-Yaron et al. (4).

MSMS acknowledges Cambridge Nehru Trust for the financial support.

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