Nucleation and eventual coalescence of Ge islands, grown out of 5 to 7nm diameter openings in chemical SiO2 template and epitaxially registered to the underlying Si substrate, were shown to generate a low density of threading dislocations (<< 106/cm2). This result compared favourably with a threading dislocation density exceeding 108/cm2 in Ge films grown directly on Si. However, the coalesced Ge film contained a relatively high density of stacking faults (5 x 107/cm2), and subsequent growth of GaAs led to an adverse root-mean-square roughness of 36nm and a reduced photoluminescence intensity at 20% compared to GaAs grown on Ge or GaAs substrates. Herein, it was found that annealing the Ge islands at 1073K for 30min before their coalescence into a contiguous film completely removed the stacking faults. However, the anneal step undesirably desorbs any SiO2 not covered by existing Ge islands. Further Ge growth results in a threading dislocation density of 5 x 107/cm2, but without any stacking faults. Threading dislocations were believed to result from the later Ge growth on the newly exposed Si where the SiO2 had desorbed from areas uncovered by Ge islands. The morphology and photoluminescence intensity of GaAs grown on the annealed Ge was comparable to films grown on GaAs or Ge substrates. Despite this improvement, the GaAs films grown on the annealed Ge/Si exhibited a threading dislocation density of 2 x 107/cm2 and a minority carrier lifetime of 67ps compared to 4 to 5ns for GaAs on Ge or GaAs substrates. A second oxidation step after the high temperature anneal of the Ge islands was proposed to reconstitute the SiO2 template and subsequently improve the quality of Ge film.

Removal of Stacking Faults in Ge Grown on Si through Nanoscale Openings in Chemical SiO2. D.Leonhardt, J.Sheng, J.G.Cederberg, M.S.Carroll, Q.Li, M.J.Romero, D.Kuciauskas, D.J.Friedman, S.M.Han: Thin Solid Films, 2011, 519[22], 7664-71