Study of interfaces and thin overlayers by surface plasmon exitation

232

Study of interfaces and thin overlayers by surface plasmon exitation M. A bra ham Friedrich-Schiller-Universitat Jena, Sektion Physik, I.~ax-Wien-Platz 1, Jena 6900, GDR M. Klopfleisch, Iii. Golz, U. Trutschel, U. Schellenberger TH Ilmenau, PSF 327, Ilmenau 6325, GDR A. Tadjeddine L.E.I., C.N.R.S., F 92195 Meudon, France The optical excitation of surface plasmons (SP) at the metaldielectric interface has been proved to be a valuable tool for in-situ investigations of the interfacial properties /1/. Surface plasmons are collective excitations of the electron gas bounded to the interface between a surface plasmon active material and a dielectric. Such materials are characterized by a dominating contribution of the quasi free electrons to the optical properties. In contrast to EELS we investigate surface plasmons in the low wave vector region k N w/c where optical excitation is possible. It should be emphasized that in contrast to ellipsometry and reflectivity measurements there is a fieln enhancement at the metal/ambient interface when SP's are excited. This makes this technique especially surface sensitive. We excite SP by attenuated total reflection in Otto-configuration /1/. The measurements have been carried out by scanning the angle of incidence at fixed wavelength. SP-resonance manifests as a pronounced minimum in the reflectivity curve of parallel polarized light. Its angular position and half width are indicate of the real and imaginary parts of the SP-wave-vector (Fig. 1). When the SF-active metal surface is altered e.g. by a thin overlayer the propagation constants of the SP changes. This manifests by a shift and a damping of the ATR-minimum respectively. Using this effect we have studied interface and overlayer properties of different systems.

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234

To illustrate the method we report about the study of thin CdS layers in the vicinity of the f'und amant aj ab s or-pt i on gap. CdS layers of different thickness (5-20 lli~) have been evaporated onto Ag films (100 nm) under identical conditions. In our experiments SF's are excited at the Ag/CdS interface with Ag being the SF-active medium and CdS acting as perturbing layer. The p-polarized reflectivity curves have been measured as function of the angle of incidence for various wavelength between 1.8 and 2.6 eV. ;<;xaminating these curves we obtained the real part of the k-vector from the mi.n.i.mum position and the damping from the half width. The spectral dependence of these quantities are shown in Fig. 2 respectively Fig. 3. One observe in the spectral region around the gap of CdS a characteristic back-bending of the SF-dispersion curve (Fig.2). In the same region the damping of the resonance increases strongly due to the onset of interband transitions in the CdS-overlayer. To separate the influence of the overlayer we have drawn in Fig. 3 the measured half width of the SP-resonance minimum as function of the corresponding value for an uncoated Ag-s&~ple. Both in the dispersion curve and in the half width pattern a qualitative different behaviour is seen for the thinner films. This is better seen if the half width of SP resonance with and without film are drawn in the same figure (Fig.4): One notices a step like behaviour of the damping which becomes less pronounced if the film thickness increases. Our hypothesis is that the observed behaviour of SP-damping is due to a size quantization of the carriers in the CdS layer. In a first approximation one can assume that the contribution of the overlayer to the SF-damping is mainly proportional to the imaginary part of the overlayer dielectric function /2/. On the other hand we aSSl~e for a first rough estimation that t 2(CdS) is proportional to the joint density of states. Ve calculated the joint density of state in the framework of a simple model for the two dimensional subband structure occuring due to size quantization perpendicular to the layer /3/.

235

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Fig. 5: Calculated joint density-of-states of CdS-films of given thickness using the envelope method. Equation (6) and (7) with mc=O.2ma. my=5rna /13/ and Eg=2. 385 eV were used. The arrow indicates the bulk value of the gap assumed to be that of sample d 5•

The obtained function (Fig. 5) exhibits the main features of the observed spectral dependence of the SP_damping (Fig.2): (i) blue shift of the absorption edge as observed e.g. in the case of quan~um dots /4/; (ii) a steplike behaviour at energies of the onset of intersubband transitions. Thus we have shown that the SP-technique is sensitive enough for an optical observation of the size quantization in a very thin semiconductor layer deposited onto a metal. We only mention another interesting feature of SP-excitation are their macroscopic propagation distances in the infrared (1-2 em at 10.6/um) /1/. Hence it is possible to couple the SP by a prism and to outcouple it at a certain distance after travelling over the surface. This technique has been applied successfully to study gas adsorption at metal surfaces in UHV /6/. We have shown how this sensitive method can be applied to study the metal-electrolyte interface /5/.

Literature /1/ /2/ /3/ /4/ /5/ /6/

Surface Polaritons, V.M. Agranovitch and D.L. Mills (Eds.) North Holland Comp., Amsterdam 1982 F. Abeles, J. Physique 38 (1977) 5-67 M. Abraham, G. Paasch, A. Tadjeddine, N. Hakiki; Solid State Comm. 60 (1986) 393 A. Foijtik, H. Weller, U. Koch, A. Henglein; Ber. Bunsenges. Phys. Chem. 88 (1984) 969 U. Schellenoerger, M. Abraham, U. Trutschel; Surf. Sci. in press D.M. Riffle, I.M. Hansen, A.J. Sievers Phys. Rev. Eli (1986) 692