Imaging interfaces in epitaxial heterostructures.

Abstract

We present high resolution electron density maps of two types of systems investigated. The first type consists of two InAs films grown on GaSb (001) substrates (one grown using As₄, the other one grown using As₂) and two GaSb films grown on InAs (001). The semiconductor samples were 9 monolayers thick and were grown using MBE. The three dimensional electron densities were obtained measuring the intensities along the Bragg rods and analyzing them with the newly developed Coherent Bragg Rod Analysis (COBRA) phase retrieval method. The study represents the first attempt to use a phase retrieval method in the characterization of a buried semiconductor heterostructure. The semiconductor was treated as a quaternary of form Ga<italic>_m</italic>In_1-<italic>_m</italic>As<italic>_n</italic>Sb_1-<italic>_n</italic>. The <italic>m</italic>(<italic>z</italic>) and <italic>n</italic>(<italic> z</italic>) compositional fractions and the vertical lattice spacing profiles were extracted. The results tend to indicate that the transition regions have a quaternary composition and are relatively narrow (1.5 unit cells). In the case of the GaSb film, significant presence of As is observed. The lattice constant in the film is not constant and has a minimum &sim;1.5 unit cells below the surface. The second type of system investigated consists of a thin film of PbTiO₃ on SrTiO₃ (001), with and without a Pt electrode on top. The 50 A thick PbTiO₃ film was grown by sputtering. The ED maps were investigated for peak position shifts. The O1 lattice seems to be shifted toward the film substrate interface in the case of the bare film. In the case of the Pt electrode region, its behavior is more complex. The O2 sublattice exhibits a shift toward the surface of the film, with a maximum of &sim;0.25 A at 12 unit cells away from the substrate. The effect relaxes toward the surface. In the case of the bare film, the Ti peak width is slightly increased in the vicinity of both film interfaces. The presence of the Pt electrode seems to inhibit this behavior, fact that can be correlated with the inhibition of the presence of surface charge.