It can be defined as follows : where r α (r β ) is the fraction of α-sites find more (β-sites) occupied by the right atom A (B), x A (x B) is the atom fraction of A (B) and y β (y α ) denote the fraction of β − sites (α − sites). For a completely random crystal, r α = x A and S =
0, while for a perfectly ordered structure, S = 1. Numerous studies have been conducted to determine the degree of ordering through different techniques, such as nuclear magnetic resonance , PL  and X-ray diffraction . In X-ray and electron diffraction methods, LRO parameters have been determined from the ratio of superlattice and fundamental PLX-4720 datasheet reflection intensities weighted by their structure factors by applying kinematical diffraction theory . In general, the electron GDC-0973 manufacturer diffraction method to determine structure factors of alloys does not always allow determination of the LRO parameters
because superlattice reflections of ordering alloys are not amenable to critical voltage techniques . Conventional TEM has also been used in this way; however, the weak intensity of extra reflections makes it impossible to carry out a study of image intensity similar to that described by Baxter et al. . To circumvent this, an estimation of the order parameter from the HRTEM images taken at different zones inside the GaAsBi layer was carried out. It is well known that HRTEM images are a two-dimensional
intensity pattern produced from a complex interference of the electron beams exiting from the analysed sample. These images carry quantitative information of the sample, Methocarbamol namely atomic structure, lattice parameters/strain and chemical information . Furthermore, FFT reconstruction of HRTEM images provides information about the periodicity of the atomic structure which can be correlated to the electron diffraction patterns registered at the back focal plane of the objective lens . In the following, we interpret the bright spots in the FFT images as diffraction spots (reflections) from crystallographic planes of the crystalline phases in the structures. CuPtB ordering in zinc-blende GaAsBi occurs in the alternating 111 planes of group V atoms resulting in a diffraction spot at ½ (111). The intensity of the extra reflections depends on the level of said ordering; hence, the higher the grade of ordering the more intense in the extra reflection in the FFT. Thus, an estimation of S is given by : where I s and I 111 are the intensity of the ½(111) and (111) spots, respectively; F s, is the structure factor for a fully ordered alloy and is given by F s = 2(f As − f Bi) and F 111 = 4(f III − if V) is the structure factor for the 111 reflections. The absolute diffracted intensity is subject to errors due to several experimental parameters.