With real-time in situ x-ray diffraction (RIX) we developed a novel method to monitor strain and strain relaxation during epitaxial growth. In our setup an extremely asymmetric 113 reflection is measured using divergent X-rays. A CCD camera serves as two-dimensional detector. Thus we are able to obtain the vertical lattice constant - which is otherwise typically measured by a θ-2θ scan - without rotating either the sample or the detector. Since the measurement time is just a few seconds, we can take time-resolved spectra which result in a relative thickness resolution in the order of 1nm.
By using a Johannson-Monochromator, we obtain convergent monochromatic X-rays focussed onto the sample. This setup provides for instance direct access to the layer thickness and hence the growth rate through the analysis of finite thickness fringes. By translating a slit through the convergent primary X-ray beam, we can separate small angular segments from the convergent beam which corresponds to a conventional ω scan. Using a stationary setup with only one moving slit, we are able to record reciprocal space maps with RIX within minutes.
We apply RIX to monitor the epitaxial growth of ZnSe on (001)GaAs and the relaxation process during growth, post-growth annealing and a cooling phase. The critical thickness of the epitaxial layer and its thermal evolution was determined and will be discussed. Inhomogeneous strain relaxation during growth and considerable relaxation during cooling down are some examples for processes that occur during heteroepitaxy. They become accessible by the novel RIX analysis tool.