Structural and optical investigation of InAsxP1−x/InP strained superlattices

We report a complete characterization of InAsxP1−x/InP (0.05<x<0.59) superlattices epitaxially grown by low pressure metalorganic chemical vapor deposition and by chemical beam epitaxy. Samples were obtained by both conventional growth procedures and by periodically exposing the just-grown InP surface to an AsH3 flux. Using the latter procedure, very thin InAsxP1−x/InP layers (10–20 Å) are obtained by P↔As substitutions effects. Arsenic composition of the so obtained layers depends both on AsH3 flux intensity and exposure times. Samples have been characterized by means of high resolution x-ray diffraction, high resolution transmission electron microscopy, 4 K photoluminescence, and extended x ray absorption fine structure spectroscopy. The combined use of high resolution x-ray diffraction and of 4 K photoluminescence, with related simulations, allows us to predict both InAsP composition and width, which are qualitatively confirmed by electron microscopy. Our study indicates that the effect of the formation of thin InAsP layers is due to the As incorporation onto the InP surface exposed to the As flux during the AsH3 exposure, rather than the residual As pressure in the growth chamber during InP growth. Arsenic K-edge extended x-ray absorption fine structure analysis shows that the first shell environment of As at these interfaces is similar to that found in bulk InAsxP1−x alloys of similar composition. In particular we measure an almost constant As–In bond length (within 0.02 Å), independent of As concentration; this confirms that epitaxy with InP is accompanied by local structural distortions, such as bond angle variations, which accommodate the nearly constant As–In bond length. In our investigation we characterize not only very high quality heterostructures but also samples showing serious interface problems such as nonplanarity and/or a consistent chemical spread along the growth axis. In the study presented here we thus propose a general method, based on several independent techniques, for the characterization of the interface quality of semiconductor superlattices.