Precession electron diffraction (PED) was used to measure the long-range order parameter in lattice-mismatched AlInP epitaxial films under investigation for solid-state-lighting applications. Both double- and single-variant films grown at 620, 650 and 680 °C were analysed in TEM cross-section. PED patterns were acquired in selected-area-diffraction mode through external microscope control using serial acquisition, which allows inline image processing. The integrated peak intensities from experimental patterns were fit using dynamical simulations of diffraction from the ordered domain structures. Included in the structure-factor calculations were mean atomic displacements of the anions (P) due to ordering, which were found by valence-force-field calculations to have a nearly linear dependence on order parameter. A maximum order parameter of S = 0.36 was measured for a double-variant specimen grown at 650 °C.
Compound semiconductors play a central role in current light-emitting diodes (LED) technology, but improvements in the red- and amber-emitting components are needed. The semiconductor alloy AlInP offers advantages over incumbent materials by making use of an arrangement in the crystal structure, called ‘atomic ordering’, that occurs spontaneously under certain deposition conditions. Quantitative measurement of the extent to which the ordering phenomenon occurs is needed to fully exploit the properties of the ordered material. Transmission electron diffraction offers a means to directly probe the ordered structures, but the quantification of electron-diffraction data has been a long-standing challenge, due to multiple scattering processes, referred to as ‘dynamical’ diffraction. The method of precession electron diffraction (PED) addresses this problem and has found numerous applications in crystallography. We have applied PED to ordered AlInP films, using computer-controlled acquisition to perform alignments and construct data sets during collection. A model of the microscopic, ordered domain structure was developed to compare the diffraction data to simulations. Samples grown at different temperatures, and ordered along either one or two directions, were evaluated. The strongest ordering was observed in a sample grown at 650 °C with ordering along two directions.
Keywords: atomic ordering; light-emitting diodes; precession electron diffraction; semiconductors; solid-state lighting; transmission electron microscopy.
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