Thickness modulation in symmetrically strained III-V semiconductor superlattices grown by MOVPE.

Lateral thickness modulations of epitaxial layers have been observed in symmetrically strained (Galn)As/Ga(AsP) superlattices grown on misoriented (001) GaAs substrates by metallorganic vapour phase epitaxy. Systematic studies on the structural characteristics of the modulation as a function of the lattice mismatch of the individual layers and the misorientation of the substrates have been carried out by transmission electron microscopy (TEM) (cross-sectional and plan-view TEM, as well as high resolution TEM (HREM)). The modulation is periodic and anisotropic. The thickness maxima and minima of similar layers form a slab-like structure. The slabs are inclined from the growth direction and are parallel to the tilting axis of the misorientation of the substrates from (001), which is <[(1)over bar 10]> where the misorientation is towards [110], and [100] where the misorientation is towards [010]. The onset of the modulation starts earlier during deposition in the case of higher mismatch strain. The modulation amplitudes increase as the deposition proceeds, a nd a re larger when the mismatch strains are higher. The period of the modulation does not depend on the mismatch strain, and is larger when the width of the surface steps due to the misorientation is broader. The modulation is much less severe in (AlGaln)As/Ga(AsP) strained layer superlattices. Surface migration of cations is considered to be the cause of redistribution of the equally spaced surface steps into step bunching, which leads to the thickness modulations. Also discussed are the roles of substrate misorientation, lattice mismatch and the cation species in the SLS.