Transmission electron microscopy analysis of novel iii-v nanostructures with sb and bi for optoelectronic applications

In the constant quest to improve the efficiency and functionality of optoelectronic telecommunication devices and high-efficiency solar cells, the development of new nanostructured materials has emerged as a research area of paramount importance for technological innovation towards a more sustainable and energy-efficient future. III-V dilute antimony and dilute bismuth semiconductor materials have been proposed for the fabrication of such devices due to their excellent properties such as tuneable bandgap, high electron mobility and low exciton binding energy. In particular, GaAsSb superlattices and the incorporation of Bi into InAs quantum dots are proposed as promising nanostructures for the development of advanced photonic and optoelectronic devices. With respect to antimonides, the third chapter explores the feasibility of improving the interface quality of GaAsSb/GaAsN type II superlattices, which are promising for multi-junction solar cells, by implementing different growth strategies. In such superlattices, the challenge lies in the segregation of Sb during growth, which alters the expected compositional profiles. Therefore, growth interruption strategies with soaking and desorption steps are applied to improve these profiles and the quality of the interlayers. Sb soaking and As4 desorption were shown to affect Sb segregation and surface roughness, with a greater effect on the thinner layers. Using advanced electron microscopy techniques and the three-layer kinetic fluid model, the exponential decay of the segregation energy was observed to influence the curvature of the Sb profiles. Continuing this study, the fourth chapter investigates the effect of growth strategies involving soaking and desorption steps separately and together on GaAsSb ultrathin films. Here, by inserting AlAs markers into the structure, state-of-the-art transmission electron microscopy techniques were used to accurately monitor Sb incorporation/segregation. Sb soaking proved to be a crucial process, as it significantly promoted the incorporation of Sb into the films and significantly increased the composition gradient compared to samples without soaking. On the other hand, desorption also had a significant effect on the Sb distribution. Although this process only increased the composition gradient at the upper interface, removing a fixed amount of Sb and erasing thinner layers in the process, its contribution to improving the interface quality should not be underestimated. Desorption reduced surface roughness and allowed for a more uniform distribution of Sb in the film. However, the true potential was revealed by combining the two methods. This integrated approach resulted in GaAsSb films with steeper interfaces and no delay in the onset of incorporation, indicating a significant improvement in interface quality and Sb distribution compared to samples using only one of the methods. Finally, segregation simulations show that the segregation energy changes during film growth, being initially higher but stabilising around the same value for all cases. Sb-soaked samples reach segregation steady-state at 5 ML from the origin, much earlier than unsoaked samples (10-15 ML). The desorption effect causes a large perturbation of the segregation steady state, which is much larger in the sample with only the desorption step than when combined with soaking. Regarding bismuthides, chapters 5 and 6 present two ways of introducing Bi into the InAs/GaAs QDs system. Firstly, by addition during the growth of the QDs and secondly as a GaAsBi capping layer after growth. According to the results obtained in chapter 5, at high temperatures (510 °C) Bi acts mainly as a surfactant, controlling the diffusion of In during the growth process. This led to the formation of coherent and defect-free QDs with an inverted cone-shaped In distribution. At low temperatures, however, Bi played a more active role, promoting a change in the growth mode from two-dimensional to three-dimensional. In this regime, the QDs grew exponentially in size with Bi supply. In addition, it was observed that larger QDs tended to relax plastically, forming misfit dislocations. The most interesting finding was the detection of Bi incorporation in the larger QDs in the form of clusters. Secondly, the study in chapter 6 investigated the effect of GaAsBi as strain reduction layer on the thermal stability of InAs QDs. This was done using two different processes, the first using different Bi fluxes in the growth of the GaAsBi layer, and the second using growth interruption strategies in the capping process. From the first study, we conclude that low temperature GaAs capping reduces QD decomposition and results in larger pyramidal dots, but also increases dislocation density. By adding Bi to the capping layer, a significant reduction in dislocation density is observed, but unexpected structural changes also occur. By increasing the Bi flux, an earlier incorporation of Bi occurs, but not an increase in Bi content (the maximum Bi content in all layers is 2.4 %), but an increase in layer thickness. Furthermore, an additional InGaAs layer is formed on top of the GaAsBi layer due to the segregation of In by decomposition of the QDs. Finally, regions of high segregation were detected in the most Bi rich samples, where we observed regions of horizontal Bi-free nanowires within the epilayers. In the second study, we applied a growth interruption time after QD growth of 120 s, which is divided into dwell time, cooling time, and rise time to the final capping temperature of 370 °C. Three different time-temperature routes (TTRs) were investigated by varying the weight of each of these times. Two of the TTRs revealed defective regions of bismuth-free nanotracks in the GaAsBi layer, associated with the formation of bismuth-rich droplets moving laterally towards the surface. In addition, in the TTRs with the longest quench time, new icosahedral nanoparticles appeared embedded at the first interface. After a detailed characterisation, it was found that these nanoparticles are composed of three different phases, including rhombohedral Bi, pure Ga and a new In4Bi phase, which has not been described experimentally so far. This work presents a new technique for the incorporation of plasmonic nanoparticle arrays made of non-noble metals into the interfaces of buried semiconductor layers, which offers a greater degree of flexibility in the design of devices.

Author dc:creator

• Flores Gallegos, Sara

Advisors dc:contributor.advisor

• González Robledo, David
• Fernández de los Reyes, Daniel

Statement dc:rights

• Attribution-NonCommercial-NoDerivatives 4.0 Internacional

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc-nd/4.0/

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10498/36198

OAI identifier oai:identifier

oai:rodin.uca.es:10498/36198