InAs/GaAs Quantum Dots Covered by Graded GaAsSb Strain Reducing Layer

Abstract

Two systems of QDs are mainly used to extend operating wavelength of the GaAs based optoelectronic devices towards telecommunication wavelengths 1.3 μm or 1.55 μm, In(Ga)As/GaAs and GaSb/GaAs QDs (Cui et al, Physica E 45:173–176, 2012). The most successful QD struc-tures combine InAs QDs with InGaAs or GaAsSb strain reducing layer (SRL) to red shift the QD emission wavelength (Zhang et al, Sci Rep 2:477, 2012; Liu et al, Appl Phys Lett 86(14):143108, 2005; Hospodková, Capping of InAs/GaAs quantum dots for gaas based lasers. In: Al-Ahmadi A (ed) Quantum dots: a variety of new applications. InTech, pp 27–46, 2012; Hospodková et al, J Crys Growth 370:303–306, 2013). The tasks of the SRL are to re-duce the strain in QDs, to conserve their height and shape and to prevent their dis-solution. SRL also changes the hole localization: depending on the amount of Sb in GaAsSb, the hole may be localized in the QD (type-I heterostructure, less than about 14 % of Sb) or outside the QD in the SRL (type-II, more than 14 % of Sb); electrons are in both cases inside the QD. In this work, the decrease of the ground state emission energy was studied while keeping type-I band alignment between InAs QDs and GaAsSb SRL. The InAs/GaAs QD structures were prepared by low-pressure Metalorganic Vapour Phase Epitaxy (LP MOVPE) in Stranski-Krastanow growth mode. All samples were covered by the GaAsSb SRL with different Sb content and gradation and were studied by photoluminescence (PL). The theoreti-cal simulations of the QD structures were done and compared with the PL results. As both theoretical and experimental results confirm, the type-I to type-II transi-tion with increasing Sb content in SRL depends on the size of QDs; for smaller QDs, less Sb is needed to obtain the type-II band alignment (13 % Sb in the SRL for 2 nm high QDs vs. 15 % Sb for 8 nm high QDs). The redshift of PL wavelength is remarkable with increasing Sb amount and is more pronounced for type-II het-erostructures. Furthermore, the influence of Sb gradation in SRL on the QD prop-erties was studied. Decreasing Sb concentration creates a triangular barrier in the valence band of the structure, which may split the ground state energy of holes. For low total Sb content, the hole ground state is localized in the QD and for high content, the ground state of holes is in the SRL, which means that the type-I to type-II transition may be tuned by the Sb content. If the concentration of Sb is increasing in the SRL, the hole ground state energy is the same in the QD and in SRL, which shifts the probability density of finding the hole towards the top of the QD (to the SRL). The QD structures with graded Sb content (with 5 nm thick SRL and total amount of Sb in SRL 10 %) were prepared and the PL was measured. The shortest PL wavelength was measured for samples with constant Sb content in the SRL, which means that both types of SRL gradation are efficient to extend the PL wavelength. The longest PL wavelength (1,391 nm) of type-I alignment was achieved for samples with decreasing Sb content, which is in accordance with the theoretical model for low Sb content (Fig. 52.1).