Surface photoabsorption study of the effect of V/III ratio on ordering in GaInP

Cu-Pt ordering is widely observed in Ga0.5In0.5P layers grown by organometallic vapor phase epitaxy. The degree of order is a strong function of the input partial pressure of the phosphorus precursor, i.e., the V/III ratio, during growth. By observing the surface structure using in situ surface photoabsorption ~SPA! measurements, the concentration of @1 ¯ 10#-oriented P dimers, characteristic of the ~234! reconstructed surface, has been measured as a function of the growth conditions. For growth at 670 °C, the P-dimer concentration is found to increase systematically as the input tertiarybutylphosphine pressure is increased from 10 to 200 Pa. This corresponds directly to a monotonic increase in the degree of order, measured using transmission electron microscopy and low-temperature photoluminescence. These data strongly suggest that the ~234! surface reconstruction is necessary for formation of the Cu-Pt structure. The step structure at the surface was also observed for these layers using atomic force microscopy. For high V/III ratios the structure of the layers grown on exactly ~001! oriented GaAs substrates consists of islands surrounded mainly by bilayer ~5.7 Å! steps. As the V/III ratio is reduced, the step height transforms to 2.8 Å ~one monolayer!.