Epitaxial growth and photoluminescence of Si/pure-Ge/Si quantum structures on Si(311) substrates

Si/pure-Ge/Si quantum structures were grown on Si(311) substrates by gas-source molecular beam epitaxy at various growth temperatures (T-g) and their optical properties were investigated by photoluminescence (PL) spectroscopy. At T-g higher than 740 degrees C, uniform steps which align towards the [<(2)over bar 33>] direction were found to have formed after growing a 2000 Angstrom Si buffer layer. A series of Si/pure-Ge/Si layers on the surface steps with various Ge coverage (Q) exhibited intense PL and showed systematic energy shift due to the quantum confinement effect even with Q less than one monolayer (ML), which indicates the existence of the lateral confinement effect. With increasing incident laser power, the PL energy of samples with Q larger than 1 ML shifted to higher energies as can usually be seen for indirect quantum wells (QWs). On the other hand, below 1 ML, PL peaks did not show any blue shift with increasing laser power. This can be explained by a bunched density of states at the band edge in quantum wires (QWRs) which were realized by preferential growth of Ge atoms at the step edges. At T-g of 700 degrees C, Ge QWRs were not realized even with Q less than 1 ML as evidenced by a spectral blue-shift with increasing excitation power. This is presumably due to the insufficient surface migration length and the resultant incorporation of Ge atoms at the terraces, leading to the formation of SiGe alloy QWs with 1 ML thickness. For Q larger than 3.0 ML, the growth mode changed to being three-dimensional and isosceles-triangle-shaped Ge islands were formed. However, PL mainly came from the wetting layer since the sizes of the islands were too large and the crystal quality was probably poor.