Infrared (IR) reflection spectroscopy has been conventionally used to examine surface vibration on metal surfaces. However, it is very difficult to apply this technique to a semiconductor surface, where a sensitivity enhancement observed on the metal surface is not expected and signal behavior is strongly dependent on the measurement angle. To overcome this problem, external reflection spectroscopy on semiconductor substrates with a "buried metal layer (BML)" structure has recently been proposed [1]. We report here the construction of an IR reflection spectroscopy system in UHV that incorporates a variable-angle mechanism, which is useful in assigning vibrational modes on surfaces. We also observe in vacuo a well-oriented adlayer on Si surfaces to demonstrate the applicability of this system to surface IR spectroscopy on semiconductor substrates with a BML.
Figure 1 shows a schematic of the variable-angle IR spectroscopy system. The moving mirrors located outside the UHV chamber are used to vary the angle of incidence within a range between 60 and 85 deg. The noise level of the 100% line after taking an average of 1000 scans is <1x10^-4 at 2000cm^-1 and <5x10^-4 at 650cm^-1.
Fig. 1: Schematic view of the variable-angle IR spectroscopy system.
Figure 2 shows typical spectra obtained from H on Si(001) BML substrates. A H-adsorbed surface was formed by adsorption of atomic hydrogen on the well-ordered clean surface of Si(001)2x1-BML substrates prepared by ex situ ion implantation and in situ Si MBE. The origins of the signals were assigned on the basis of the surface selection rules on the BML substrate [2] and are shown as M, D, and T (mono-, di-, and tri-hydrides). For the relatively thin Si overlayer (Fig. 2(a)), vibration modes were observed with only the transition moments vertical to the surface for p-polarization. They reached maximum intensity at an angle of incidence of around 80deg. No signal was observed for s-polarization. For the relatively thick Si overlayer (Fig.2(b)), modes with both lateral and vertical components showed prominent signals for p-polarization, while only lateral modes can be detected at a significant intensity for s-polarization. Moreover, Fig. 2 clearly depicts a surface reaction in which the initial phase of a mixture of M+D+T changes to a phase composed solely of M. These results show that the structure of the surface species including the orientation can be examined using the newly constructed IR spectroscopy system and the BML substrate by analyzing the spectral dependence on polarization, angle of incidence, and thickness of the Si overlayer.
Fig. 2: Typical IR spectra observed from H on Si-BML
substrates. Si thickness is 1300A(a) and 3500A(b).
[1] V. M. Bermudez, J. Vac. Sci. Technol. A10(1992)3478.
[2] Y. Kobayashi and T. Ogino, Appl. Surf.
Sci. in press.