STRAIN AND RELAXATION EFFECTS IN INASP INP MULTIPLE-QUANTUM-WELL OPTICAL MODULATOR DEVICES GROWN BY METAL-ORGANIC VAPOR-PHASE EPITAXY/

Strained-layer multiple quantum well (MQW) InAsP/InP optical modulator s have been fabricated from layers grown by metal-organic vapor phase epitaxy. The devices are a series of p-i(MQW)-n photodiodes in which t he active core regions consist nominally of 25 periods of 10 nm InAsP quantum wells of 4.4%, 10.0%, 15.6%, and 26.4% As composition separate d by 10 nm InP barriers. Structural parameters for the samples were ob tained using high-resolution x-ray diffraction rocking curves and tran smission electron microscopy. The series contains samples with both co herently strained and partially relaxed multi-layers where the relaxat ion is characterized by misfit dislocations. The band offsets for the heterostructures were determined by fitting the energy positions of th e optical absorption peaks with those computed using the Martin-Bastar d model for strained-layer superlattices [as in M. Beaudoin et al., Ph ys. Rev. B 53, 1990 (1996)]. The conduction band discontinuities thus obtained are linear in the As composition (7.5+/-0.08 meV per As % in the InAsP layer) at low and room temperature for As concentrations up to 39%, and up to 17% average relaxation. Comparisons between the cohe rently strained and partially relaxed samples demonstrated a broadenin g of optical transition linewidths due to relaxation which appears to be of minor consequence for optical modulator devices as the essential optical and electrical properties remain intact. The electric field-d ependent red-shift of the n=1 electron-heavy hole transition was measu red by a photocurrent method and found to be enhanced in structures wi th lower barrier heights. (C) 1997 American Institute of Physics.