Growth and characterization of Ge-70(n)/Ge-74(n) isotope superlattices

We report on the molecular beam epitaxial (MBE) growth of Ge-70(n)/Ge-74(n) isotope superlattices composed of alternating layers of the stable isotope s Ge-70 and Ge-74. Samples prepared in this work have atomic layers n = 4, 8, 16, and 32. All superlattices are p-type having a net-impurity concentra tion of similar to 10(16) cm(-3). Zone-folding of optical phonons due to th e mass periodicity in the growth direction has been observed clearly for al l samples using high resolution Raman spectroscopy. The corresponding phono n mode of each Raman peak has been indexed according to theoretical calcula tions using the linear-chain model and the planar bond-charge model. The fr equency of the Raman peaks found by the experiment agree very well with tho se of the phonon modes calculated for each superlattice structure. A detail ed analysis of the Raman spectra concludes that the degree of interface mix ing between Ge-70 and Ge-74 layers for our typical growth condition is less than two monolayers. (C) 2000 Elsevier Science S.A. All rights reserved.