Semiconductor nanowires have attracted great interest as building blocks for future electronic and optoelectronic devices. The variability of the growth process opens the opportunity to control and combine the various properties tailoring for specific application. It was shown that the electrical and optical characteristics of the nanowires are strongly connected with their structure. Despite intensive research in this field, the growth process is still not fully understood. In particular, extensive real structure investigations are required. Most of the reports dedicated on the structural researches are based on the results of scanning electron microscopy (SEM) or transmission electron microscopy (TEM). SEM provides an image of the surface with nanostructures and is mainly used to describe the morphology of the sample, but it does not bring information about the internal structure, phase composition and defect structure. At the same time, the internal structure can be examined by TEM down to atomic scale. TEM image of good quality are very expensive due to the efforts in sample preparation and in localisation of a single object. All these aspects make the statistical structural analysis difficult.
In the present work, X-ray diffraction analysis has been applied for structural investigation of InAs nanowires grown by different techniques. Using various X-ray diffraction geometries, the nanowire systems were investigated in terms of the lattice parameters, phase composition, strains and displacement fields and stacking defects.
In particular, realizing grazing incidence diffraction and controlling the penetration depth of X-ray beam, we characterized sample series grown by Au-assisted metal organic phase epitaxy on GaAs B substrate with different growth time. According to the results of SEM and X-ray investigations, a model of the growth process has been proposed.
A more detailed analysis was performed on InAs nanowires grown by molecular beam epitaxy (MBE) on Si substrate. MBE provides the opportunity to combine a group III-V material with nearly any semiconductor substrate independent from lattice mismatch. Vertically aligned nanowire ensembles were studied performing X-ray diffraction experiments in different scattering geometries. Considering the nanowires are composed by structural units of zinc-blende and wurtzite the latter one was found to be affected by a high density of stacking faults already at nanowires with short growth time. The stacking faults density was estimated by Monte-Carlo simulations based on model of ensemble average. A strong signal of unique zinc-blende reflection was observed as well. Coherent X-ray diffraction experiments with the use of a nano-focus setup have shown ‘bar-code’ patterning due to stacking fault arrangement within the nanowire. The found highly defective structure cannot be attributed to wurtzite or zinc-blende phases alone. Also parasitic islands were found on the samples surfaces and characterized as pure zinc-blende objects.