MEASUREMENT OF INDIUM SEGREGATION IN STRAINED INXGA1-XAS GAAS QUANTUM-WELLS BY TRANSMISSION ELECTRON-MICROSCOPY/

During the production of InxGa1-xAs quantum wells by the crystal growt h technique of molecular beam epitaxy, the indium tends to accumulate in the surface layer and to float on the crystal growth surface. This effect delays incorporation of the indium into the quantum well and re sults in a graduated value of x across the well. Detailed indium conce ntration profile information is essential if accurate theoretical mode ls are to be applied to describe these quantum wells. Therefore, to qu antify this indium segregation we combine the complementary results of two experimental techniques: transmission electron microscopy (TEM) a nd double-crystal X-ray diffraction (DCXRD). The indium concentration well profiles were determined through an analysis of the contrast vari ation in (200) dark-held TEM images. Differences of less than 0.5% ind ium concentration were discernible. The DCXRD analysis provided an acc urate value for the total amount of indium in the individual wells whi ch was used as the scaling factor for the TEM concentration profile. T o ensure that the observed contrast differences were due to variations in indium concentration and were not merely TEM sample preparation ar tefacts, high-quality cross-sectional TEM samples with parallel cleave d surfaces free of ion-milling damage were required. Accurate thicknes s values for the samples were also needed. A TEM sample preparation te chnique was developed to provide these requirements. Indium concentrat ion profiles were taken from intensity line scans across dark-held TEM images of these samples. These profiles were interpreted by a simple kinematical theory (structure factor) calculation, and then by a more rigorous model using the dynamical theory of electron diffraction. The results of these two models were then checked for consistency with th e results of the DCXRD measurement. The model profiles of indium segre gation within the quantum wells, calculated using the structure factor approach for up to 35% indium content, matched very closely the profi les produced by the use of dynamical theory. Both sets of model profil es strongly supported the DCXRD results. These results demonstrate tha t a simple structure factor calculation in conjunction with a DCXRD me asurement can allow an accurate measure of quantum well profiles and o f the abruptness of InxGa1-xAs on GaAs and GaAs on InxGa1-xAs interfac es. This information is a powerful aid in the modelling and understand ing of devices based on these structures.