In-situ monitoring of In0.5Ga0.5As quantum dot formation during metalorganic vapor phase epitaxy by fast-nulling ellipsometry

The successful application of ellipsometry for in-situ investigation of In0 .5Ga0.5As quantum dot (QD) growth on GaAs (001) substrates in Stranski-Kras tanow growth mode is reported. The Delta- Psi trajectory of ellipsometric s ignal during In0.5Ga0.5As growth shows a different feature compared with th e one shown in the growth of lattice matched system. The trajectory has thr ee inflection points. At the initial stage of the growth, trajectory shows rapid increase in Psi (the first inflection point) and hereafter, decrease in Delta (the second inflection point) was observed. Passing through the th ird inflection point, the trajectory shows a random feature and obvious sur face roughening was observed. Ex-situ atomic force microscope measurements (AFM) for the sample surfaces were performed for top In0.5Ga0.5As whose gro wth was stopped before and after the second inflection point. The precise c omparison between AFM and ellipsometric signal indicates that the second in flection point corresponds to the onset of QD formation. It is suggested th at the second inflection point of ellipsometric signal is deduced from the increase in the extinction coefficient, k, caused by the increase in surfac e roughness coming from the transition from two-dimensional to three-dimens ional surface morphology. (C) 1999 Elsevier Science B.V. All rights reserve d.