Detection of nanometer-sized strain fields in (GaIn)(NAs) alloys by specific dark field transmission electron microscopic imaging

The N-induced large strain fields in metal organic vapor phase epitaxy grown (GaIn)(NAs) quantum wells are imaged using dark field imaging in a transmission electron microscope with two different reflections, from which one is sensitive to the chemical composition and the other one to the strain in the material. By comparing the images of the (GaIn)(NAs) to those of ternary (GaIn)As alloys, which have identical macroscopic compressive strain as the quaternary alloys, as well as to those of ternary Ga(NAs) with identical N content than the quaternary alloys, it can be shown that by using the presented technique, one indeed images the N-induced strain fields in the material. The density of the strain fields increases with increasing N content to a critical value above which the crystal undergoes a morphological transition. From the density of the strain fields one could speculate that they might be originated by N-III-N next-nearest neighbors or by a N-induced N-III-N ordering with a longer chain length.