Phase separation and atomic ordering in epitaxial semiconductor alloys studied by transmission electron microscopy.

摘要

Transmission electron microscopy (TEM) is used to examine the microstructure of epitaxial II-VI and III-V ternary semiconductor alloys. Emphasis is placed on the crystallographic aspects of alloy ordering and phase separation and the influence of growth parameters. Quantitative methods of image and diffraction pattern analysis are examined.; TEM studies of novel II-VI pseudobinary alloys grown by molecular beam epitaxy are presented. The microstructure of the ZnSe{dollar}sb{lcub}1-x{rcub}{dollar}Te{dollar}sb{lcub}x{rcub}{dollar} ({dollar}xapprox0.5{dollar}) alloy is examined and related to the optical properties. A vertical composition modulation between Se- and Te-rich alloy compositions with a wavelength of 14-30 A is observed for growth on vicinal (001) GaAs substrates miscut toward (111). The composition profile appears to be nearly sinusoidal, as revealed by single, sharp diffraction satellites. We present evidence that the modulation occurs at the growth surface and remains stable in the bulk during annealing for 1-2 hr. at 450{dollar}spcirc{dollar}C. X-ray diffraction experiments confirm the long-range uniformity of the modulations and provide quantitative structural information. We contrast the modulation to related effects that have been reported in the literature. We find that the strain associated with the modulation appears to be less than expected from simple elastic considerations. The structural and optical properties of the modulated alloy are shown to differ considerably from those of ZnSe/ZnTe strained-layer superlattices. We also examine the influence of synthetic ordering by beam shuttering on blue-green emission from ZnCd{dollar}sb{lcub}0.2{rcub}{dollar}Se{dollar}sb{lcub}0.8{rcub}{dollar}/ZnSe quantum wells. Novel structural features of II-VI-based diluted magnetic semiconductors are also demonstrated.; TEM examinations of atomic ordering in Ga{dollar}sb{lcub}1-x{rcub}{dollar}In{dollar}#sb{lcub}x{rcub}{dollar}As and Ga{dollar}sb{lcub}1-x{rcub}{dollar}In{dollar}sb{lcub}x{rcub}{dollar}P ({dollar}xapprox0.5{dollar}) alloys grown by metal-organic chemical vapor deposition are presented. Deviations from perfect CuPt ordering are revealed by irregular domain profiles and diffraction streaks. Dynamical diffraction simulations are performed to compute order parameters based on electron-diffraction intensities. We investigate models in which the ordering is imperfect and uniformly distributed, or nearly perfect and isolated in large domains within random material.; Computer algorithms used for the analysis of electron images and diffraction patterns are discussed in an appendix. Additional appendices contain computed strain energy and structure factors for composition-modulated ZnSe{dollar}sb{lcub}0.5{rcub}{dollar}Te{dollar}sb{lcub}0.5{rcub}{dollar}.