Preparation of Compositionally GradientThin Films by Ion-Beam Evaporation, страница 2

FIGURE 1. Schematic illustration of experimental setup for preparation of compositionally gradient Si-Ge thin films

In this investigation, the preparation of compositionally gradient Si-Ge thin films was carried out with Si and Ge plates as the target. Figure 1 shows the schematic illustration of experimental setup for preparation of compositionally gradient Si-Ge thin films. After irradiating the ion beam on the Si and Ge plates, the compositionally gradient Si-Ge thin films were prepared on stainless steel (SUS304) substrates, which were located at the position of 120 mm from the target. Experimental conditions for the preparation of the thin films are listed in Table 1.

The composition of the compositionally gradient Si-Ge thin films was determined by an energy dispersive X-ray spectroscope (EDS). The EDS was equipped with a scanning electron microscope (SEM), which was operated at the acceleration voltage of 10 kV. By this voltage, the incident depth of electrons corresponded to the thickness (approximately 300 nm) of the compositionally gradient Si-Ge thin films. Thus, the intensity of X-ray from the substrate was almost same as that of the background of EDS spectra. Moreover, the composition of the thin films was determined by Φ (ρ, z) method using the Cliff-Lorimer factor [5], which was obtained from the spectrum of a standard Si-50 at. % Ge bulk.

RESULTS AND DISCUSSION

Figure 2 shows substrate position (r) dependence of the composition of the thin film that is prepared by an ion-beam irradiation on Si and Ge plates. The content of Si and Ge atoms in the thin film was measured by EDS, and the ratio of Si atoms for Ge ones could change almost continuously in r ranging from –75 to 0 mm. Thus, the compositionally gradient Si-Ge thin film was confirmed to be synthesized for the irradiation of ion beam at once. Additionally, in Figure 2, the Si content in the thin film changed from 0 to 70 at. % on a substrate, and the compositionally gradient Si-Ge thin film was observed to have low value as a=0.8 at. %/mm. Therefore, in the XRD measurement of the thin film with a=0.8 at. %/mm, the difference in composition is approximately 5 at. % at the irradiation area of X-ray. This value was found to become an eighth of that of the previous work [3].

TABLE 2. r’ and a of the compositionally gradient Si-Ge thin films that are prepared in x ranging from 0 to 10 mm.

x (mm)

r’ (mm)

a (at. %/mm)

0

–58

0.8

5

–17

1.5

10

–4

3.0