Thin Solid Films, 222 (1992) 251 253 251 In-plane and vertical high frequency conductivity in Si/Ge short-period superlattices T. Fromherz, M. Helm and G. Bauer lnstitut fiir Halbleiterphysik, Universitiit Linz, A-4040 Linz (Austria) H. Kibbel and E. Kasper Daimler Benz Forschungsinstitut Ulm, W-7900 Ulm (Germany) Abstract The in-plane and vertical conductivities of Si/Ge short-period superlattices are determined by far-IR Fourier transform spectroscopy. It is shown that an Si6Ge 4 superlattice exhibits significant conductivity along the superlattice axis, indicating miniband transport. In contrast, the conductivity of an SisGe 8 superlattice is consistent with two-dimensional behaviour, which implies that the minibands are too narrow to permit transport along the growth axis. These conclusions can be inferred from polarization-dependent transmission measurements together with a simulation using an anisotropic dielectric function for the superlattice. Over the last few years, the epitaxial growth of multilayered structures consisting of silicon and germa- nium has made significant progress. It has become possible, for example, to grow high mobility modula- tion-doped quantum wells [1] or resonant tunnelling structures [2]. Short-period superlattices have attracted considerable interest, because one hopes to achieve a quasi-direct band gap in these systems [3-5], which could lead to optoelectronic applications of the Si/Ge system. The intraband optical properties (which are concerned with only the conduction or valence band) and the electronic properties of short-period superlat- tices have been studied much less. In the present paper, we present a far-IR spec- troscopy study of Si/Ge short-period superlattices to obtain information on their transport properties, espe- cially along the superlattice axis. IR spectroscopy of superlattices has already led to valuable physical infor- mation on miniband transport in the GaAs/A1GaAs system. For example, the miniband mass has been determined by cyclotron resonance measurements with the magnetic field in the plane of the layers [6], and the quenching of the miniband conductivity with tempera- ture has been demonstrated [7]. Even though Si/Ge superlattices of a quality comparable with that of III-V systems are not available yet, it is shown that informa- tion on vertical transport can be gained from IR mea- surements. The samples investigated were grown by molecular beam epitaxy on strain-symmetrized, partially relaxed buffer layers of Sil_xGex. The germanium content of the buffer was chosen in order to distribute the strain in the superlattice symmetrically [8]. Two superlattices were studied in detail: one SisGe8 superlattice (i.e. 8 monolayers each of silicon and germanium) with an antimony doping n = 3 × 10 ~8cm -3 containing 225 pe- riods, and one Si6Ge 4 superlattice with n =4 x 10 ~8 cm -3 (145 periods). The latter type of structure is of special interest, since a direct band gap has been pre- dicted owing to the folding of the conduction band [3, 5]. The experiments were performed in a Bruker IFS l13v Fourier transform spectrometer at low tempera- tures (T ~ 10 K) in a helium flow cryostat. The samples were prepared in a multipass waveguide geometry as indicated in the diagram shown in Fig. l(a). The radia- tion is coupled into the samples at one edge, which is wedged at an angle of 38 °, and then undergoes several total internal reflections at a reflection angle of 52 °. As will be discussed below, in this geometry p-polarized light (electric field parallel to the plane of incidence, transverse magnetic (TM) mode) couples to the elec- tron motion along the superlattice axis, whereas s- polarized light (transverse electric mode) reflects the in-plane motion of the electrons. Before the experimental spectra are presented, it ap- pears useful to discuss the electrodynamics of an an- isotropic layered medium. The layers which have to be considered are the substrate and the superlattice, which is bounded by a surface. At a semiconductor-vacuum interface the TM mode (p polarization), reflected under an angle of 52 °, undergoes a phase jump of nearly 180 ° relative to the incident wave, which means that the electric field has a node at the interface. Thus there is very low coupling of the electromagnetic wave to the 0040-6090/92/$5.00 ~, 1992 -- Elsevier Sequoia. All rights reserved