Atomic scale structure and morphology of (In,Ga)As-capped InAs quantum dots

The atomic scale morphology of In As ∕ Ga As quantum dots (QDs) capped with In 0.1 Ga 0.9 As layers of different thickness are studied using in situ scanning tunneling microscopy (STM) and compared to the effects of capping with a pure GaAs layer. QDs capped with a 5 nm In 0.1 Ga 0.9 As layer exhibit a longer photoluminescence emission wavelength than those capped with GaAs. STM studies show that the QDs capped with In 0.1 Ga 0.9 As retain their height during the initial stages of capping (up to ∼ 2 nm ), whereas the GaAs-capped QDs collapse as material migrates from their tops onto the cap surface. After deposition of a 25 nm GaAs cap the surface is still far from flat, whereas the In 0.1 Ga 0.9 As capping layer is planar after just 5 nm deposition. High-resolution STM images, supported by reflection high-energy electron diffraction measurements, reveal a ( 4 × 3 ) ∕ c ( 4 × 6 ) reconstruction for the In 0.1 Ga 0.9 As cap, whereas the GaAs cap layer rapidly exhibits a c ( 4 × 4 ) reconstruction after the first few monolayers of deposition. The planar morphology is a consequence of enhanced In adatom diffusion on the InGaAs alloy surface.