Optical Spectroscopy of Functionalized Semiconductor Heterostructures - Investigation of III-N-V/Silicon and III-Sb-V/GaAs Heterostructures for Laser Applications
Semiconductor lasers are widely used in all areas of everyday life. They can be found in personal computers, TVs, CD and DVD players, printers and laser pointers, just to name a few. However, a very important field they are used in is optical communication. This thesis tackles two of the major chall...
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|Zusammenfassung:||Semiconductor lasers are widely used in all areas of everyday life. They can be found in personal computers, TVs, CD and DVD players, printers and laser pointers, just to name a few. However, a very important field they are used in is optical communication. This thesis tackles two of the major challenges in this field. First, on short distances, i.e. on-chip or chip-to-chip, data is usually transmitted using electrical wires. However, the interconnects between different parts of the processor are actually the limiting factor for device performance. Additionally, the power dissipated due to the interconnects on a chip is significant. Therefore, it is reasonable to consider other approaches to transfer data on-chip and between chips. One possibility is changing the means of data transfer from electrical to optical providing faster interconnects and a higher energy efficiency. To do so, efficient and stable lasers on silicon substrates are needed. Even though optically driven silicon lasing has been demonstrated in the past, silicon is not the first choice because of its indirect band gap. Using other semiconductor materials such as Ga(N,As,P), which is investigated here, is a reasonable choice for optoelectronic integration for the following reasons. For As contents exceeding 70%, the quaternary Ga(N,As,P) is a direct band gap semiconductor that can be grown lattice matched to silicon using standard MOVPE techniques. Furthermore, laser operation at up 150 K - and at RT on GaP substrates - has already been demonstrated. However, from this it becomes clear that further device improvements are necessary to reach RT lasing. This thesis investigates the interplay of optical properties, alloy disorder and structural changes in Ga(N,As,P)/Si heterostructures to get a better understanding of this material. The influence of several growth and annealing parameters and processes on the optical properties is investigated as well as the influence of different sample structures and heterostructure layouts. To reveal the optical properties photoluminescence spectroscopy experiments are performed. In conjunction with structural investigation by Transmission Electron Microscopy the role of structural changes due to growth and annealing procedures and their influence on the optical properties is revealed and discussed. These results not only yield a better understanding of the complex interplay of growth parameters/structural design and optical response, but also can be used as feedback for subsequent growth of further samples leading to better device performance. Furthermore, the band offset, which is a critical parameter of a heterostructure for laser operation, is determined experimentally for the first time. The second part of this thesis deals with light sources for long-range optical communication, which is usually done using optical fibers. These optical fibers are operated best at 1.3 µm or 1.55 µm where the losses are minimal and the dispersion is closest to zero. Semiconductor lasers operating in this wavelength regime are usually made of (Ga,In)(As,P) or (Al,Ga,In)As on InP substrates. Their efficiency is somewhat poor as much of the pump power is converted to heat due to non-radiative processes. One of the most dominant processes is the Auger recombination, where the recombining electron and hole transfer their energy to another charge carrier as kinetic energy rather than creating a photon, which is emitted. To overcome these issues it is helpful to use type-II devices where the recombining electrons and holes are confined in different materials and spatially separated. In such a device the optical properties can be optimized while the Auger losses can be lowered. Additionally, such systems offer more degrees of freedom for device design as more and different materials can be used. In this thesis are investigated (Ga,In)As/Ga(N,As) heterostructures, which are used as type-II light sources in the range of 1.3 µm to 1.55 µm. Furthermore, (Ga,In)As/Ga(As,Sb) heterostructures serving the same purpose are investigated. Particular attention is paid to the influence of the interface these type-II devices inevitably have. Especially, reports on the influence of interfaces on the optical properties of these materials are lacking in the literature. The type-II PL is used as a non-destructive probe for the optical properties of such systems and their changes upon changing the interfaces. It is aimed to reveal the influence of the internal interfaces on the cw-photoluminescence as well as on the recombination dynamics and charge carrier (re-)distribution in the heterostructures. In particular, the latter ones are scarcely investigated in the literature. The optical properties will be correlated with different interface properties such as thickness and morphology. The influence of the interface on disorder is also investigated. Finally, the measurements are also used to determine the Ga(N,As)/GaAs and Ga(As,Sb)/GaAs band offsets, which are disputed in the literature.|