Strain relaxation mechanism of semiconductor thin films

For the atoms at clean surface of crystal, the atomic bonding environment is very different from that of deep inside bulk crystal. This fact forces those atoms to rearrange themselves in such a way that reconstruction occurs with the total energy being minimized. Reconstruction also takes place when foreign atoms are deposited on a clean surface. This strain experienced by individual atoms will propagate into a bulk crystal down to a few layers, sometimes even to several layers. Using x-ray diffraction, the positions as well as thermal vibrational amplitudes of these strained atoms can be determined. In the first part of this thesis, we present the surface reconstruction and a strain analysis of Sb/Si(ll1) using synchrotron radiation x-ray diffraction. Another case when we can observe macroscopic strain is heteroepitaxial film growth. Even though there is lattice mismatch between substrate and film, pseudomorphic film growth is usually possible at the initial stage of growth, so the film is grown under strain. However, it becomes energetically favorable to introduce misfit dislocations above a certain thickness of film growth, which is called a critical thickness. In this case, strain is considered in the continuum elastic sense, defined by the lattice constant of the material. From measurement of the positions of multiple Bragg reflections, accurate values of lattice constant can be obtained using x-ray diffraction as a function of film thickness and a critical thickness can be determined. The second part of the thesis is a study of III-nitride thin films grown on sapphire (0001) substrate with molecular beam epitaxy. Irradiation experiments were also performed to investigate whether strain relaxation takes place in SiGe films and GaN films to look for indications of metastability.U of I Onlydissertation/thesi