Crystal Superlattices Research Articles

X-ray diffraction study of a one-dimensional GaAs–AlAs superlattice

Low- and high-angle X-ray diffraction patterns have been obtained from one-dimensional superlattice crystals prepared artificially by alternately depositing predetermined thicknesses of GaAs and AlAs, on the (001) face of a GaAs single-crystal by molecular-beam epitaxy. The positions and intensities of several superlattice reflections obtained along the 00l, 11l and 02l reciprocal lattice rows have been recorded. The structure of the superlattice can be approximated by a model which incorporates elastic strains in the unit cell due to the lattice mismatch between GaAs and AlAs. The number of Ga and Al layers in the superlattice unit cell can be accurately determined from the low-angle scattering data while the relative intensities of the high-angle superlattice reflections are a sensitive measure of the elastic strain present in the lattice. It is shown that the elastic strain agrees with the value computed theoretically on the assumption that the strain is not relieved by dislocations at the GaAs–AlAs interfaces.

X-ray diffraction from one-dimensional superlattices in GaAs1−xPxcrystals

An X-ray diffraction analysis has been performed on superlattice crystals which were grown epitaxically by alternately depositing GaAs and GaAs0.5P0.5 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 (g 100 A) 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.