Quantitative C lattice site distributions in epitaxial Ge1-yCy/Ge(001) layers

Epitaxial metastable Ge1-yCy alloy layers with y less than or equal to0.035 were grown on Ge(001) from hyperthermal Ge and C atomic beams at depositio n temperatures T-s of 250 and 300 circleC. The use of hyperthermal beams al lows us to controllably vary the concentration of C incorporated as Ge-C sp lit interstitials. Ge1-yCy layers grown with incident Ge-atom energy distri butions corresponding to less than or equal to0.14 lattice displacement per incident atom (dpa) are in a state of in-plane tension and contain signifi cant concentrations of C atoms incorporated in substitutional sites. Increa sing the dpa to 0.24 yields layers in compression with C incorporated prima rily as Ge-C split interstitials. Ab initio density functional calculations of the formation energies and strain coefficients associated with C atomic arrangements in Ge show that configurations containing multiple C atoms, r eferred to collectively as C nanoclusters, are energetically more favorable than substitutional C and Ge-C split interstitials and yield a nearly zero average strain. In contrast, substitutional C and Ge-C split interstitials produce large tensile and compressive strains, respectively. Using the cal culated strain coefficients, measured layer strains obtained from high-reso lution reciprocal lattice maps, and substitutional C concentrations determi ned by Raman spectroscopy, we obtain the fraction of C atoms incorporated i n substitutional, Ge-C split interstitial, and nanocluster sites as a funct ion of the total C concentration y and T-s. We find that at low y and T-s v alues, all C atoms are incorporated in single-C configurations: substitutio nal C and Ge-C split interstitials. Their relative concentrations are contr olled by the dpa through the production of near-surface Ge self-interstitia ls which are trapped by substitutional C atoms to form Ge-C split interstit ials. Increasing y and T-s, irrespective of the dpa, leads to an increase i n the fraction of C nanoclusters, while the fractions of substitutional C a nd Ge-C split interstitials decrease, due to the higher C-C encounter proba bility at the growth surface. (C) 2001 American Institute of Physics.