OPTICAL-PROPERTIES OF STRAINED AND RELAXED SI1-XGEX ALLOYS

Complex dielectric functions of strained and relaxed Si1-xGex(0.07 les s than or equal to x less than or equal to 0.23) alloys grown epitaxia lly on Si(001) substrates have been measured ellipsometrically at room temperature in the 2-6 eV photon energy region. For the relaxed Si1-x Gex alloys, the structure at about 3.4 eV due to the E(0)('),E(1), and E(1)+Delta(1) interband transitions is found to shift gradually to lo wer energies as the Ge composition increases while the 4.2 eV structur e due to the E(2) interband transitions remains at about the same ener gy. For the strained alloys, the rate of the low-energy shift of the 3 .4-eV structure for increasing Ge composition is found to be smaller t han that for the relaxed alloys. A line-shape analysis on the numerica l second derivative spectra of the dielectric functions has been perfo rmed, and the critical-point energies of the E(0)(,)E(1), E(1)+Delta(1 ), and E(2) transition edges for the strained and the relaxed samples was obtained. Results of the line-shape analysis show that the rates o f decrease of the E(1) and E(1)+Delta(1) energies for increasing Ge co mposition are bigger than that of the E(0)(') energy for both the rela xed and the strained alloys. Also, the E(0)('), E(1), and E(1)+Delta(1 ), transition edges for the strained alloys are located at higher ener gies than those for the relaxed alloys. Such strain-induced shifts in the transition energies are explained in terms of the changes in the e lectronic band structure of Si1-xGex alloys predicted by the deformati on potential theory.