Passivation capping of InAs surface quantum dots by TMA/Al2O3: PL enhancement and blueshift suppression

Abstract

Passivation capping that enhances the photoluminescence (PL) of molecular beam epitaxy (MBE)-grown InAs surface quantum dots (SQDs) is realized by ex situ low-temperature atomic layer deposition (ALD)-grown Al2O3. As the Al2O3 cap thickness increased from 2 to 30 nm, the PL intensity was enhanced by 2.7-fold and the blue shift was suppressed. This is in strong contrast to wet chemistry passivation and in situ GaAs capping by MBE, both of which resulted in significant PL blueshift, due to etching in the former, and In/Ga intermixing and strain in the latter. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) reveal that the Al2O3 cap layer mimics the shape of the underlying SQDs. The cross-sectional transmission electron microscopy (TEM) further reveals that the SQD size and shape remained unchanged after Al2O3 capping, which is in strong contrast to MBE-capping. The passivation mechanisms and native oxide reduction by trimethylaluminum (TMA), including self-clean-up reaction through ligand exchange, are discussed based on the results by x-ray photoelectron spectroscopy (XPS). A detailed comparison between Al2O3 and ZnO cap layers reveals the significance of the cap properties on the SQD size/shape and PL blueshift. While the size/shape of SQDs is preserved by Al2O3 capping, the apex is slightly removed by ZnO capping, resulting in a stronger PL blueshift compared to Al2O3.