COMPARISON BETWEEN THE ELECTRONIC DIELECTRIC FUNCTIONS OF A GAAS ALASSUPERLATTICE AND ITS BULK COMPONENTS BY SPECTROSCOPIC ELLIPSOMETRY USING CORE LEVELS/

The dielectric functions of a GaAs/AlAs superlattice (SL) as well as i t its bulk constituents GaAs and AlAs were determined at T = 90 K by m eans of spectroscopic ellipsometry with rotating analyzer using synchr otron radiation in the photon-energy range between 3 less than or equa l to HBAR omega less than or equal to 25 eV. This broad spectral range gives access to excitations from the Ga 3d core levels lying approxim ately 20 eV below the conduction-band minimum. These levels are atomic like with virtually no dispersion throughout the Brillouin zone (BZ). They can thus be used as an energy reference to investigate confinemen t effects and level splittings on interband transitions in heterostruc tures compared to their bulk counterparts. The samples were grown by m olecular-beam epitaxy and maintained under ultrahigh vacuum conditions from growth to measurement, avoiding the effects of cap or oxide laye rs, especially for AlAs. The investigation concerns transitions at L- and X-related points in the BZ which is tetragonal in the case of the SL and face-centered-cubic for the bulk samples. For the latter it is found that the L-related E(1) structure consists of two spin-orbit-spl it groups of three transitions. For a (GaAs)(m)(AlAs)(n)SL with m = 9, n = 7 these two groups are blueshifted by about 200 meV compared with GaAs due to confinement effects. Similar shifts are not observed for transitions from the Ga 3d levels to the conduction-band valleys. Ther efore, the confinement acts on the highest valence bands. The major pa rt of the SL E(1)' structure is AlAs-like, but the minor one moves to higher energies by 165 and 80 meV compared to AlAs and GaAs, respectiv ely. Since the two parts of the E(1)' structure exhibit a different co nfinement behavior, they cannot have a common origin in k space. The a nalysis of X-related E(2) structures exhibits a blueshift for the SL t ransitions by about 55 meV compared with AlAs which is consistent with a confinement effect on the lowest conduction band. In correspondence to previous theoretical work a transition above the E(2) critical poi nt is found which was attributed to a SL specific transition due to th e BZ folding. The comparison with the results for the bulk samples sug gests a relation to the E(2)(P-2) critical point. Beyond that, an anal ysis of the interband transition line shapes reveals for ah E(1),E(1)' structures, and the AlAs-E(2) one, that a modified Lorentzian profile indicating final-state interaction is more appropriate than two- or t hree-dimensional critical-point line shapes. This points to deviation from the one-electron band-structure picture even for transitions far above the fundamental absorption edge.