X-ray diffraction from one-dimensional superlattices in GaAs_1−xP_x crystals

An X-ray diffraction analysis has been performed on superlattice crystals which were grown epitaxically by alternately depositing GaAs and GaAs_0.5P_0.5 layers on a gallium arsenide substrate. The periodic change of compositions causes a one-dimensional periodic lattice distortion along the direction of growth. The periodic lattice distortion gives rise to satellite peaks close to the Bragg reflections of the sphalerite-type lattice of gallium arsenide. Two models are employed to calculate the X-ray diffraction intensities: in one the lattice distortion is described by two longitudinal phonons with frequency ω = 0, in the other a periodic step function is used. The X-ray diffraction data obtained on two samples show that the two alternating layers of different composition are coherent and, therefore, constitute one single-crystal with its unit cell large (≳ 100 Å) along the growth direction. From the fit of the two models to the data one can determine the period and amplitude of the lattice distortion, the relative thickness of the two layers and the size of the domains which diffract coherently. These parameters are discussed in relation to the physical growth conditions.