Characterization of MOVPE grown InGaAsP superlattices for modulators by electron diffraction, X-ray diffraction and Z-contrast imaging

Abstract

MOVPE grown ternary and quaternary GaxIn1−xAszP1−z heterostructures 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 especially the variation of the tetragonal distortion on a nanometer scale as well as abrupt interfaces. We investigated cross-sectional specimens from nominally strain compensated quaternary superlattices (SL) of GaxIn1−xAszP1−zGayIn1−yAszP1−z on InP substrates by convergent beam electron diffraction (CBED) and atomic number (Z)-contrast imaging using subnanometer electron probes in a field-emission scanning transmission electron microscope (STEM). While CBED allows the measurement of local strain, Z-contrast imaging combines qualitative chemical information with high spatial resolution. Additionally, X-ray diffraction (XRD) was used to characterize the layer stacks. By CBED and Z-contrast imaging we found that even small deviations from design parameters of the SL like slightly degraded interfaces and asymmetries in thicknesses and strain balance of the SL layers can give rise to degradation in device performance. The MOVPE growth process was successfully optimized to obtain abrupt interfaces in the SL, as demonstrated by XRD and Z-contrast imaging. Photocurrent spectra of this type of SL give clear evidence for the Wannier-Stark effect. So-called “oblique” transitions of the order up to 2 are observed, which demonstrates the high quality of the SL and shows the applicability as active layers in optoelectronic modulators operating at 1.55 μm.