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X-ray diffraction from thin film structures : characterization and modeling

Abstract

InAs/GaSb superlattices grown via molecular beam epitaxy, and containing InSb-like interfacial layers, were analyzed with a combination of x-ray diffraction (XRD) and structural refinement techniques. The superlattice refinement from x-rays (SUPREX) method determines with high accuracy the average thicknesses and d-spacings of the individual InAs and GaSb layers in addition to standard structural parameters usually obtained by XRD, such as the modulation length (periodicity), average out- of-plane interplanar spacings, and total thickness. The combined SUPREX/XRD experiments show that the absence of certain odd order satellite features in the x-ray data is due to asymmetric and inhomogeneous lattice strain. \ textit}Ex situ}-annealed InAs/GaSb superlattices were studied using atomic force microscopy (AFM) and XRD methods. Results show that annealing at temperatures between 200 ̕C and 300 ̕C for 1 hour in HVAC improves the structural quality of these superlattices. Strain relaxation occurs during the annealing process indicating that there are chemical intermixing and anion segregation in the superlattices. The effect of the inner-molecular electron density on the x-ray diffraction profile of a layer-stacked thin film is studied. Important phase information contained in the x-ray diffraction profile of highly anisotropic molecular-based thin films is charaterized. The experimental and calculated results show that the intensity distribution of the diffraction peaks belonging to the same lattice orientation provides important structural information. For example, tilt angle and core electron density of a molecule can be determined from the intensity distribution. The out-of-plane tilt angle relaxation is studied numerically. The results show that the relaxation can only occur at the first phthalocyanine monolayer above the substrate. The lateral grain size effect and the polar angle anisotropy are studied using a three-dimensional model. The FWHM of the center peak in associated rocking curves gives lateral coherence length or lateral grain size, and the ratio of the intensities from the diffraction peaks in normal diffraction curves shows the uniaxial angular anisotropy of the phthalocyanine thin films

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