Role of V/III ratio on atomic ordering and surface morphology of InGaP layers grown by chemical beam epitaxy

In this work, the ordering of In0.5Ga0.5P layers grown by chemical beam epitaxy on GaAs(0 0 1) substrates was studied as a function of V/III ratio. Samples were characterized by photoluminescence measurements at 77 K, transmission electron microscopy (TEM) (diffraction patterns and dark field images) and atomic force microscopy (AFM). Photoluminescence peak energy decreases from 1.917 to 1.902 eV for increasing V/III ratios indicating an enhancement of ordering degree. Electron diffraction pattern presents diffuse scattering for all samples with maximum intensity at [1/2, 1/2, 1/2]B positions; spots at [1/2, 1/2, 1/2]B positions are also observed for samples grown with higher V/III ratio. TEM-dark field images show that the ordered regions become larger, elongated and tilted as V/III ratio increases. The angle between ordered regions and the [1̄ 1 0] direction increases from 25° to 60° with increasing V/III ratio. Surface morphologies, observed by AFM consist of large-scale structures superimposed by small-scale (ellipsoidal) structures elongated along the [1̄ 1 0] direction. A size correlation of ordered regions and small-scale structures was observed. As the V/III ratio is increased the ellipsoidal structures become larger and denser, and a larger number of bilayer steps is clearly observed. These results are consistent with models where a close correlation between step structures (monolayer versus bilayer) and ordering is suggested.