SPECTROSCOPY OF BAND-TO-BAND OPTICAL-TRANSITIONS IN SI-GE ALLOYS AND SUPERLATTICES

We report the results of an extensive study of band-to-band optical tr ansitions in Si-Ge (n:m) superlattices and alloys where n approximate to m. Our samples were grown by molecular-beam epitaxy on (001) silico n using symmetrically strained layers and characterized by high-resolu tion x-ray diffraction and transmission electron microscopy. This grow th procedure permits the synthesis of continuous Si-Ge superlattices w ith a thickness of several thousand Angstrom. Optical absorption was s tudied by photocurrent spectroscopy at 300, 77, and 4.2 K. These resul ts were analyzed to determine the dependence of the photocurrent on th e photon energy. The energy dependence of absorption was also measured by optical transmission spectroscopy. Analysis of these experiments g ives approximate agreement with photoconductivity experiments on the v alue of the energy gap, but also shows that thr energy dependence of t he absorption coefficient varies linearly with the photon energy, whil e photoconductivity experiments show that the photocurrent increases w ith the fourth power of the energy. The absorption coefficient, and it s dependence on the photon energy, are calculated directly from the jo int density of states which is extracted from the electronic band stru cture. Our calculations show that the dependence of optical absorption on photon energy is linear for perfect superlattices: alpha(((h) over bar omega)=A(0))((h) over bar omega-E-g)(x), where x=1, with the expo nent increasing above 1 in the presence of disorder such as from atomi c steps, interface roughness, and similar defects. [S0163-1829(98)0671 5-0].