Here, we demonstrate X-ray fitting through kinematical simulations of the intensity profiles of symmetric reflections for epitaxial compositionally graded layers of AlGaN grown by molecular beam epitaxy pseudomorphically on [0001]-oriented GaN substrates. These detailed simulations depict obvious differences between changes in thickness, maximum concentration, and concentration profile of the graded layers. Through comparison of these simulations with as-grown samples, we can reliably determine these parameters, most important of which are the profiles of the concentration and strain which determine much of the electrical properties of the film. In addition to learning about these parameters for the characterization of thin film properties, these fitting techniques create opportunities to calibrate growth rates and control composition profiles of AlGaN layers with a single growth rather than multiple growths as has been done traditionally.
We report on AlxGa1-xN heterostructures resulting from the coherent growth of a positive then a negative gradient of the Al concentration on a [0001]-oriented GaN substrate. These polarization-doped p-n junction structures were characterized at the nanoscale by a combination of averaging as well as depth-resolved experimental techniques including: cross-sectional transmission electron microscopy, high-resolution X-ray diffraction, Rutherford backscattering spectrometry, and scanning probe microscopy. We observed that a small miscut in the substrate orientation along with the accumulated strain during growth led to a change in the mosaic structure of the AlxGa1-xN film, resulting in the formation of macrosteps on the surface. Moreover, we found a lateral modulation of charge carriers on the surface which were directly correlated with these steps. Finally, using nanoscale probes of the charge density in cross sections of the samples, we have directly measured, semiquantitatively, both n- and p-type polarization doping resulting from the gradient concentration of the AlxGa1-xN layers.
The instability during the growth and processing of epitaxial GeSn layers with high Sn molar fraction and high compressive strain is still to be fully studied. In this work, the relationship among strain relief, dislocations, and Sn outdiffusion in GeSn layers with a Sn content of ∼9 atom % was studied as a function of pre-existing misfit/threading dislocation (MD/TD) density and annealing time at 300 °C. For a GeSn epilayer strained to a Ge-on-Si virtual substrate (Ge-VS), an increase of strain relief by a factor of ∼2 is observed after 2 h of annealing, without a significant effect on strain relaxation for a more prolonged temperature treatment. This is explained by the limited propagation or elongation of pre-existing MDs at the interface of GeSn/Ge-VS. The onset of Sn outdiffusion and the appearance of segregation spots are observed for the GeSn epilayers with significant strain relaxation (≥50%) before annealing, for which the density of MDs ≥ 2 × 10 5 cm −1 . This is explained by generation of high-density MDs/TDs acting as preferential sites for Sn accumulation during the growth of the GeSn layer. This work explicitly provides an understanding that dislocation engineering is one of the key factors for the stability and performance of GeSn semiconductors.
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