The formation of self-assembled InAs/GaAs quantum dots emitting at 1.3 μm followed by photoreflectance spectroscopy

摘要

Photoreflectance (PR) modulation spectroscopy, supported by photoluminescence (PL) and atomic force microscopy, was applied to the study of the optical properties of InAs/GaAs structures at the transition from the typical two-dimensional epitaxial growth to three-dimensional Stranski-Krastanov mode of InAs self-assembled quantum dot (QD) formation. Room temperature photoreflectance was measured on several molecular-beam epitaxy (MBE) grown structures, with growth conditions optimized for the 1.3 μm emission (an important window for telecommunication applications), differing in the nominal thickness of InAs layer from 1 to 2.5 ML (monolayer). The evolution of optical transitions connected with energy levels confined in a very thin InAs/GaAs quantum well was observed. For a small InAs nominal layer thickness (up to the critical thickness for the formation of three-dimensional islands), the heavy (light)-hole level to electron level transitions shift towards lower energy, indicating the increase in the quantum well width. When the nominal InAs layer thickness exceeds the critical value, the transition energies remain constant. It implies the formation of the so-called wetting layer, whose thickness is independent of the amount of deposited InAs material (fully driven by the strain). Its energy level structure was calculated (exploiting the effective mass approximation, with strain effects) in order to determine the actual wetting layer thickness, which was found to be approximately 1.6 ML. The features connected with the transitions between levels confined in QDs appear in PR and PL spectra for the amount of InAs material exceeding this number. The energies of the QD transitions shift to the red when the InAs layer nominal thickness is increased from 1.7 to 2 ML (indicating the increase in the average dot sizes) but remain constant for thicker layers, which is the evidence that the additional InAs material increases the density of QDs, but not their sizes. It points out at the existence of size limitation effect in the MBE growth of self-assembled QDs.