Compositional Characterization of in AlAs/GaAs Step-Graded Layers by Micro-Raman Scattering and Photoluminescence

Step graded composition inxal1-xas layers grown by mbe on gaas substrates have been characterised by micro-raman scattering and photoluminescence (pl). the indium composition was gradually increased in six or four intermediate layers with smaller slope of the grade. pl spectra show that the band attributed to the inalas band gap of the active layer is intense and well resolved which reflects a strong reduction of negative effects of lattice mismatch. the alloying effect in the inxal1-xas layers has been interpreted using the modified random element isodisplacement (mrei) formalism. raman spectra show that inas-like modes are not strongly dependent either on the graded layers structure or on the indium content. however, the alas-like lo phonon frequency was used to calculate the composition of the inxal1-xas active layers. in addition, the indium content of the active layers was also derived from pl measurements and the dependence of the inxal1-xas alloy gap energy on the indium composition. comparison between indium contents obtained by the two methods show a good agreement with those measured during growth of the epilayers. smaller shifts of the indium contents determined from the different techniques were obtained for sample s2 which have the lower slope of the grade. as a consequence, residual strain in the active layers is strongly reduced by using thick buffer layers with smaller grading rate. disorder activated modes have been also observed in the low frequency region of the raman spectra.