Exciton radiative lifetime in sub-monlayer and stranskii-Krastanow grown InGaAs/GaAs quantum dots

Submonolayer (SML) growth of quantum dots (QDs) presents an alternative approach to Stranskii-Krastanow (SK) grown QDs [1-3]. SML growth has been applied to fabricate QD-based high-power edge-emitting lasers [1] as well as single-mode vertical-cavity surface-emitting lasers with high modulation bandwidth [3]. The superior performance of SML QD lasers has usually been attributed to the high density and uniformity of SML QDs [2]. However, another important parameter governing the maximum modal gain from optical transitions in QDs is the oscillator strength, inversely proportional to the exciton radiative lifetime rad [4]. Here, we compare the exciton radiative lifetime of SML-grown and SK-grown InGaAs/GaAs QDs at 10 K, via time-resolved photoluminescence (PL). The SML-QDs, were formed by 10 cycles of alternate deposition of 0.5 monolayer (ML) InAs and 2.5 ML GaAs, at 500 °C. The SK-QDs were formed by depositing 4.1 ML In0.6Ga0.4As on GaAs. The PL spectra indicate that the SML-QDs show better uniformity than the SK-QDs. Transient PL for SML and SK QDs at 10 K, where the PL decay time d is dominated by the exciton radiative lifetime, show d = 90 ps for SML-QDs which is much shorter than for SK-QDs (d = 600 ps). By a detailed analysis of the temperature dependence we find that the radiative lifetime rad of SML-QDs is short (90 ps) and almost independent of temperature below 50K and increase to about 800 ps at room temperature [5]. The short exciton radiative lifetime of SML-QDs could be one of the key reasons for the observed high performance of the SML QD lasers.