Growth mode changeover during gas source molecular beam epitaxy of Ge
on Si(100) is explored by photoluminescence (PL) spectroscopy with a s
ubatomic layer resolution. By observing confinement effect and develop
ment of island-related emissions with increasing Ge coverage in Si/pur
e-Ge/Si quantum wells, the onset of the island formation is determined
to be 3.7 monolayers (ML). Furthermore, the equilibrium critical thic
kness for island formation is found to be lower, i.e. 3.0 ML, by adopt
ing growth interruption after the Ge growth. Intense quantum-confined
PL is clearly observed at room temperature from the islanded Ge of qua
ntum dot character, demonstrating the potential of the Ge islands as e
fficient light emitters.