Growth and characterization of ⁷⁰Ge_n/⁷⁴Ge_n isotope superlattices

We report on the molecular beam epitaxial (MBE) growth of ⁷⁰Ge_n/⁷⁴Ge_n isotope superlattices composed of alternating layers of the stable isotopes ⁷⁰Ge and ⁷⁴Ge. Samples prepared in this work have atomic layers n = 4, 8, 16, and 32. All superlattices are p-type having a netimpurity concentration of approximately 10¹⁶ cm^-3. Zone-folding of optical phonons due to the mass periodicity in the growth direction has been observed clearly for all samples using high resolution Raman spectroscopy. The corresponding phonon mode of each Raman peak has been indexed according to theoretical calculations using the linear-chain model and the planar bond-charge model. The frequency of the Raman peaks found by the experiment agree very well with those of the phonon modes calculated for each superlattice structure. A detailed analysis of the Raman spectra concludes that the degree of interface mixing between ⁷⁰Ge and ⁷⁴Ge layers for our typical growth condition is less than two monolayers.