Strain evolution during the growth of epitaxial Ge layers between narrow oxide trenches

•Compressively strained Ge epilayers were grown on 40-nm SiO2 trench arrays.•Strain evolution in Ge films with different thicknesses was evaluated.•Asymmetric strain relaxation was observed due to the SiO2 trench structure.•Nanobeam electron diffraction confirms the compressive in-plane strain values.

We have grown the high quality and compressively strained Ge epilayers on a Si substrate with 40-nm width SiO2 trench patterns at a growth temperature of 600 °C. Based on (224) reciprocal space mapping measurements of Ge samples with a different thickness, the residual in-plane strain value along the trench direction decreased from −0.74% to −0.42% with increasing thickness of the Ge layer from 150 nm to 180 nm. In addition, the compressive strain along the trench direction (ε1¯10) was larger than that in the direction perpendicular to the trench (ε110) regardless of the thickness. For example, when Ge was overgrown on a SiO2 trench, the ε1¯10 and ε110 values were −0.42% and ~0%, respectively. We conclude that the asymmetric strain relaxation behavior of Ge is related to the SiO2 trench patterns, which prevent the dislocations from gliding. Defects such as a microtwin and/or stacking fault were generated during the coalescence of Ge films having different lattice constants in each Ge layer arising from the different relaxation values. A local strain in Ge, with a high spatial resolution of 2.5 nm, was measured along the two directions by means of a nanobeam electron diffraction method, thus confirming asymmetric strain relaxation and the results are in good agreement with reciprocal space mapping results.