Tuning the properties of InAs/GaAs quantum dots through a modified capping layerapplication to optoelectronic devices

Resumen

Quantum-dot (QD) technology has become a very transversal technology finding application in an increasing number of scientific and industrial fields. At the forefront of this development is the well-studied InAs/GaAs QD system. However, the limita-tions imposed by the fixed InAs-GaAs band offsets and by the difficulties to control the QD morphology due to the capping process still difficult the precise control of QD band structure that would allow the required design in different applications. The use of a certain capping layer (CL) material different than GaAs has been particularly employed to tune the ground state energy of InAs/GaAs QDs through strain and band structure engineering, so achieving the long-wavelength telecommunication windows as one of the most pursued targets. This work is mainly focused on the achievement of a higher tunability of the properties of InAs/GaAs QDs by the application of thin GaAs(Sb)(N) CLs and the optimization of the capping process in order to improve their suitability to any optoelectronic device, and more in particular to laser diodes and solar cells. GaAs(Sb)(N)/InAs/GaAs QDs are a highly versatile system in which the use of Sb and N allows for the tunability of the QD ground state while providing a huge degree of freedom regarding the QD-CL band-alignment, according to the re-quirements of the field of application. As starting point, a very straightforward approach is shown to controllably tune the structural and optical properties of InAs/GaAs QDs without the need for strain-engineering strategies. The QD dissolution process, induced by the surface In-Ga in-termixing taking place during overgrowth, is shown to be kinetically controlled by the mere adjust of the capping rate. This parameter allows not only for the control of the final structural properties of the QD and, therefore, of its optical properties, but also for the control of the wetting layer (WL) thickness, known to directly affect the per-formance of QD-based devices. This approach therefore represents an additional degree of freedom in the tunability of the QD properties, which can be combined with alternative approaches, such as the application of different CL materials. The use of GaAsSb CLs is found to improve the characteristics of both QD lasers and solar cells. On one side, GaAsSb CLs are shown reduce the threshold current density of QD lasers while extending the lasing wavelength with the Sb content. This structure improves the external differential quantum efficiency in a type-I QD-CL band alignment configuration and provides the devices with a higher thermal stability in a type-II configuration. On the other side, GaAsSb CLs are shown to tune the ab-sorption edge of QD solar cells, enhancing infrared photoresponse, while providing themselves strong additional photocurrent. Such a device acts therefore as a hybrid QD-quantum well solar cell. This, along with an improved carrier collection arising from the type-II WL-CL band alignment, yields improved conversion efficiencies, even for high Sb contents giving rise to the formation of extended defects. This sug-gests the possibility to achieve improved performance in combination with strain-balancing techniques, such as the addition of N to the CL. Despite the promising versatility of GaAsSbN CLs, an optimization of the growth process is required prior to their use in any QD-based device. The search for the op-timum CL growth conditions reveals the great impact of the capping rate, improving significantly the growth of the quaternary GaAsSbN CLs on InAs/GaAs QDs, leading to the achievement of room-temperature emission. On the other hand, the idea of an ultrashort-period GaAsSb/GaAsN superlattice CL, leads to the achievement of further extended emission at room temperature and efficient LED emission, comparable to that of LEDs containing GaAsSb CLs, known to be very efficient. The addition of N is shown to allow for the compensation of the accumulated strain in stacked QD structures for solar cells, not only in ternary GaAsN CLs, but also in quaternary GaAsSbN CLs, relieving the accumulated strain introduced by the presence of Sb. In addition, the use of N in the CL is found to extend the photore-sponse of a QD solar cell, further extending it in combination with Sb through an independent control of the conduction and valence band confinement within the QD. However, although N-containing CLs provide an enhanced absorption, the presence of N induces electron trapping effects, worsening carrier collection efficiency. Alter-native approaches are therefore proposed in order to improve the conversion effi-ciency. In particular, the CL potential depth, thickness, and structure, as well as the resulting band alignment, are found to play an important role in carrier extraction and transport in QD solar cells.