An extended X-ray absorption fine structure study of amorphous and crystalline germanium

Abstract

Extended X-ray absorption fine structure (EXAFS) results are presented for amorphous and crystalline Ge at temperatures ranging from 83 to 1085 K. Specific tests for the detection of asymmetry in the distribution of atoms are examined. In amorphous Ge the distribution of nearest neighbours is found to be Gaussian. In crystalline Ge at the highest temperatures studied a small asymmetry in the distribution of nearest atoms, attributed to anharmonicity in the effective two-body potential, must be included explicitly in the EXAFS analysis to determine changes in nearest neighbour distances to an accuracy of ± 0.01 Å. The temperature dependence of the disorder parameter σ12 in crystalline Ge is found to obey an Einstein model, contrary to earlier work. The structure of amorphous Ge is found to be consistent with a continuous random network of distorted tetrahedra in which the dynamic contribution to σ12(T) is given by the same force-constant law as in crystalline Ge. The static structural disorder provides an additional contribution to a σ12(T) which decreases as the crystalline temperature is approached. Many-body contributions to the reduction of the amplitude of the EXAFS interference function in amorphous and crystalline Ge are also determined empirically.