The effect of strain field seeding on the epitaxial growth of Ge islands on Si(001)

The effect of strain, due to a buried, nominally 6 ML Ge quantum dot layer, upon the growth of subsequent Ge layers grown by gas source molecular beam epitaxy has been investigated. A series of samples were grown at 700°C with a nominally 6 ML Ge layer followed by a 30 nm Si spacer and then a second, thinner Ge layer. In each sample, the thickness of the second Ge layer was varied (2, 3, and 4 ML). Atomic force microscopy shows that in the second Ge layer islands form at thicknesses below the established critical thickness for this material system. This is confirmed by transmission electron microscopy images which also show the quantum dots in the second layers are stacked above those in the first layer, the island growth in the thin Ge layer being seeded by the strain field from the buried Ge islands. Photoluminescence results show a luminescence feature attributed to the strain-controlled quantum dots in the thin Ge layer. This band has properties similar to the frequently observed Ge dot luminescence but is observed at higher energies, depending upon the nominal thickness of the second Ge layer.