High Real-Space Resolution Measurement of the Local Structure ofGa1−xInxAsUsi

Abstract

High real-space resolution atomic pair distribution functions (PDF)s from the alloy series Ga1−xInxAs have been obtained using high-energy x-ray diffraction. The first peak in the PDF is resolved as a doublet due to the presence of two nearest neighbor bond lengths, Ga-As and In-As, as previously observed using XAFS. The widths of nearest, and higher, neighbor pairs are analyzed by separating the strain broadening from the thermal motion. The strain broadening is five times larger for distant atomic neighbors as compared to nearest neighbors. The results are in agreement with model calculations.The average atomic arrangement of crystalline semiconductor alloys is usually obtained from the position and intensities of the Bragg peaks in a diffraction experiment [1], and the actual nearest neighbor and sometimes next nearest neighbor distances for various pairs of atoms by XAFS measurements [2]. In this Letter we show how high energy x-ray diffraction and the resulting highresolution atomic pair distribution functions (PDF)s can be used for studying the internal strain in Ga 1−x In x As alloys. We show that the first peak in the PDFs can be resolved as a doublet and, hence, the mean position and also the widths of the Ga-As and In-As bond length distributions determined. The detailed structure in the PDF can be followed out to very large distances and the widths of the various peaks obtained. We use the concentration dependence of the peak widths to separate the strain broadening from the thermal broadening. At large distances the strain broadening is shown to be about five times larger than for nearest neighbor pairs. Using a simple valence force field model, we get good agreement with the experimental results.

Chemical ordering rather than random alloying in SbAs

“…Local-structure PDF studies of the supposed solid solutions (In,Ga)As, Zn(Se,Te), and (Li,Na)AsSe 2 have shown that materials with single-phase Bragg diffraction patterns can exhibit nanoscale clustering of the end members that is evidenced by split nearest-neighbor bond distances in the PDF. [30][31][32] For extensive nanoclustering of Sb and As we would see short bonds corresponding to As-As (2.52Å), plus long Sb-Sb bonds (2.91Å). The low-r region of the (Color online) Least-squares refinements to the SbAs PDF collected at room temperature using (a,b) x-rays and (c) neutrons.…”

“…The PDFs do not show any short As-As distances which would signal the onset of nanoscale phase separation that is invisible to Bragg diffraction. [30][31][32][33] Hightemperature Bragg diffraction finds the atomic mixing to become disordered above 550 K. Unlike GeTe, the compound SbAs does not become cubic upon heating.…”

High real-space resolution structure of materials by high-energy x-ray diffraction

“…Extra positional disorder, manifested by enlarged temperature factors, has been found on both metal and As sites as well. Both the static displacement and the positional disorder have been found to peak at a composition x = 0.5 [6,7].…”

“…More experimental details can be found in refs. [6,7]. Experimental reduced structure factors and the corresponding atomic PDFs G(r) are shown in Figs.…”

Total scattering atomic pair distribution function: new methodology for nanostructure determination

“…However, peaks at higher-r region exhibit significant broadening ( Figure 5(b)) indicating that there is some structural relaxations taking place as bond-bending distortions, as has been observed before in semiconductor alloys. [80,81] This homogeneous strain increases rapidly for the ultra-small (CdSe_QD1) nanoparticles revealing that they have a significant compressive strain. Comparable behaviour is also seen with increases in the PDF peak broadening with decreasing nanoparticle size indicating that the inhomogeneous strain, or width of the bond-length distribution in the nanoparticles, also increases.…”