OBSERVATION OF CONFINEMENT EFFECTS ON ACCEPTORS IN SI/SI1-XGEX SUPERLATTICES

We have used photothermal ionization spectroscopy to study boron acceptors in p-Si/Si1-xGex, MBE grown on n-Si substrates. The Si1-xGex layers were center-doped with boron in concentrations between 10(17) to 10(18) cm-3. For constant Si layer width, at Si1-xGex widths of 100 angstrom and smaller, bands of acceptor transitions appeared between 120 to 400 cm-1 depending on the width. For 100 angstrom wells, the observations are explained by the simple hydrogenic model of a confined acceptor of Bastard and the assumption that they form states associated to the heavy hole valence band. This assumption is justified by the well known fact that the valence band is splitted from the ligth hole band by the stress present in these superlattices. For the 30 angstrom superlattice, appreciable differences with respect to the Bastard model axe observed. This indicate the need to include the finite height of the confining barrier and the effect of the ligth and heavy hole band states for acceptors confined in wells of 30 angstrom in width and smaller.