Thesis presented December 21, 2017

Abstract:
Within the general goal of reaching high operating temperature while maintaining strong requirements on infrared photodetector performances, the pressure on HgCdTe material quality is increasingly growing. In particular, careful attention is now being paid to stress and stress relaxation within HgCdTe photodiodes. While recent studies have focused on the lattice mismatch induced strain over areas in the order of the wafer, no experimental investigation has been able to resolve the strain at the micrometer level. The typical millimetric spatial resolution limit of standard diffraction can be overtaken using a focused synchrotron X-ray white beam. Indeed, by performing Laue microdiffraction measurements, we can map with a sub-micrometer resolution both the local deviatoric strain and lattice orientation.
This thesis focuses on the analysis of etched trenches inside HgCdTe layers with variations on passivation and annealing steps. We are able to investigate a precise mapping around the etching and appreciate the local effects of the processing steps. Diffraction peak displacement mapping evidences bending of the crystal planes around etched trenches, with strong dependence upon the processing steps.
Then, we focus on the relative position of all the peaks which are measured simultaneously. Assuming a bi-axial strain between layer and substrate, the slope of the peak relative displacement as a function of their position directly provides the strain value undergone by the material. Thus, we measure the strain through the entire sample with a precision of 3.10^-5 and map it with a sub-micronic position precision.
Finally, we show how the use of a three point calibrated bending set-up leads to the experimental determination of the plasticity threshold in HgCdTe/CdZnTe epitaxial structures together with the spatial distribution of the applied strain. The dynamics from elastic to plastic regime and the layer behavior once the plastic threshold is reached are investigated. Furthermore, the etching step creates abrupt edges inside the layer, leading to a modification of the strain field by concentrating it on the angles of the trench.

Keywords:
Infrared detectors, HgCdTe, Structural investigation

On-line thesis.