X-ray rocking curve analysis of tetragonally distorted ternary semiconductors on mismatched (001) substrates

For ternary heteroepitaxial layers, the independent determination of the co mposition and state of strain requires x-ray rocking curve measurements for at least two different hkl reflections because the relaxed lattice constan t is a function of the composition. The usual approach involves the use of one symmetric reflection and one asymmetric reflection. Two rocking curves are measured at opposing azimuths for each hkl reflection. Thus, it is poss ible to account for tilting of the hkl planes in the epitaxial layer with r espect to the hkl planes in the substrate, by averaging the peak separation s obtained at the opposing azimuths. This procedure presents a practical pr oblem in the case of asymmetric reflections, for which the tilting can only be canceled if the rocking curve for one azimuth is obtained using theta-p hi incidence. A preferable approach, which provides sharper, more intense r ocking curves and greater experimental accuracy, is to measure both asymmet ric rocking curves at theta+phi incidence. This approach requires that the data be corrected for the tilting of the asymmetric planes introduced by te tragonal distortion. Here we have presented a new analytic procedure that i ncorporates the tilting of asymmetric diffracting planes due to tetragonal distortion. The new procedure allows the measurement of all rocking curves at theta+phi incidence. We have applied this new method to the case of ZnSy Se1-y grown heteroepitaxially on GaAs (001), using 004 and 044 x-ray rockin g curves. We have shown that neglect of the tilting in asymmetric planes re sults in gross errors in the calculated values of composition (as much as 3 5 times) and in-plane strain (as much as 2.6 times) for this material. (C) 2000 American Vacuum Society. [S0734-211X(00)06403-9].