CHARACTERIZATION OF MOVPE GROWN INGAASP SUPERLATTICES FOR MODULATORS BY ELECTRON-DIFFRACTION, X-RAY-DIFFRACTION AND Z-CONTRAST IMAGING

MOVPE grown ternary and quaternary GaxIn1 - xAszP1 - z heterostructure s on InP substrates are increasingly used for the fabrication of, e.g. optoelectronic devices like modulators. Two of the key parameters for the performance of such devices are the crystalline quality and espec ially the variation of the tetragonal distortion on a nanometer scale as well as abrupt interfaces. We investigated cross-sectional specimen s from nominally strain compensated quaternary superlattices (SL) of G axIn1 - xAszP1 - z/GayIn1 - yAszP1 - z on InP substrates by convergent beam electron diffraction (CBED) and atomic number (Z)-contrast imagi ng using subnanometer electron probes in a field-emission scanning tra nsmission electron microscope (STEM). While CBED allows the measuremen t of local strain, Z-contrast imaging combines qualitative chemical in formation with high spatial resolution. Additionally, X-ray diffractio n (XRD) was used to characterize the layer stacks. By CBED and Z-contr ast imaging we found that even small deviations from design parameters of the SL like slightly degraded interfaces and asymmetries in thickn esses and strain balance of the SL layers can give rise to degradation in device performance. The MOVPE growth process was successfully opti mized to obtain abrupt interfaces in the SL, as demonstrated by XRD an d Z-contrast imaging. Photocurrent spectra of this type of SL give cle ar evidence for the Wannier-Stark effect. So-called ''oblique'' transi tions of the order up to 2 are observed, which demonstrates the high q uality of the SL and shows the applicability as active layers in optoe lectronic modulators operating at 1.55 mu m.