RAMAN-SCATTERING FROM FULLY STRAINED GE1-XSNX (X-LESS-THAN-OR-EQUAL-TO-0.22) ALLOYS GROWN ON GE(001)2X1 BY LOW-TEMPERATURE MOLECULAR-BEAM EPITAXY

Fully strained single-crystal Ge1-xSnx alloys (x less than or equal to 0.22) deposited on Ge(001) 2 x 1 by low-temperature molecular beam ep itaxy have been studied by Raman scattering. The results are character ized by a Ge-Ge longitudinal optical (LO) phonon line, which shifts to lower frequencies with increasing x. Samples capped with a 200-Angstr om-thick Ge layer exhibit a second Ge-Ge LO-phonon line whose position remains close to that expected from bulk Ge. For all samples, capped- and uncapped, the frequency shift Delta omega(GeSn) Of the Ge-Ge LO ph onon line from the Ge1-xSnx layer, with respect to the position for bu lk Ge, is linear with the Sn fraction x (Delta omega(GeSn) = -76.8x cm (-1)) over the entire composition range. Using the elastic constants, the Gruneisen parameter, and the shear phonon deformation parameter fo r Ge, we calculate the contribution of compressive strain to the total frequency shift to be Delta omega(strain) = 63.8 x cm(-1). Thus, the LO phonon shift in Ge1-xSnx due to substitutional-Sn-induced band stre tching in fully relaxed alloys is estimated to be Delta omega(bond) = -140.6x cm(-1). (C) 1998 American Institute of Physics. [S0021-8979(98 )09816-8].