FR4 Oskar Kleins Auditorium
Friday 14 October
10:30 - 13:30
This work covers the implementation of highly specialized epitaxial techniques enabled by the near-equilibrium hydride vapor-phase epitaxy growth process in III-V/Si integration for Si-based tandem solar cells and photoelectrochemical reactions, quasi phase matching GaP structures on GaAs substrates, and regrowth of InP:Fe on quantum cascade lasing structures.
III-V/Si integration is an important topic in several fields of research with a significant one being solar energy harvesting. Combining the economic benefits of Si with the advantageous and flexible optical and electronic properties of III-V’s represents significant improvements in both photovoltaic and photoelectrochemical applications. GaAsP is a promising candidate for Si-based tandem solar cells, but it has seen much less research compared to other III-V compounds. Establishing a cost-efficient method for integrating GaAsP on Si could pave the way for significant improvements in photovoltaics. However, there are many technological challenges with this integration, some of which are addressed in this work using advanced epitaxial techniques. A fabrication process for full 2” wafer GaAsP/Si templates is developed. This is based on the epitaxial lateral overgrowth technique to reduce misfit dislocations, which utilizes the inherent selectivity and high growth rate of hydride vapor phase epitaxy. Extensive work is also done to establish control of the crystalline quality and composition of planar and laterally grown GaAsP on both GaAs and Si. Planar GaAsP/Si solar cell structures are grown based on the optimization work, and a process for fabricating devices out of these is developed. Work on III-V/Si integration is also done towards photoelectrochemical applications; p-GaP is grown on Si by direct heteroepitaxy, which requires a specialized technique called vapor mixing epitaxy. This utilizes a specialized precursor injection scheme in order to allow the growth to take place at a much lower temperature than during conventional growth. Both GaAs and GaP are grown directly on Si using the low-temperature process, investigating the impact of substrate orientation and temperature, before a more extensive investigation is carried out for Zn-doped p-type GaP on Si. The p-GaP/Si growth is also used along with p-GaP/GaAs and p-GaP/GaP reference samples to perform hydrogen evolution and CO2 reduction reactions. Selective area growth of GaP and GaAs directly on SiO2/Si templates using the low-temperature process is also demonstrated.
Despite the many applications in communication and security, there are relatively few direct sources of coherent radiation in the mid-infrared and terahertz spectral ranges. One method of accessing these frequencies is the down-conversion of more well-established sources using non-linear optical processes. Quasi phase matched semiconductor structures are a promising pathway for this, and orientation-patterned GaP/GaAs has been identified as one of the top candidates. However, the fabrication of these structures puts high demands on the epitaxial processes used, which are investigated in this work. Additionally, wafer-bonded GaAs templates have seen less previous research than templates fabricated using molecular beam epitaxy. A homoepitaxial selective area growth study of GaP is performed as a pre-study to investigate how the growth-rate anisotropy can be controlled using the growth conditions. Subsequently, two methods for maintaining vertical domain boundaries during heteroepitaxy of orientation patterned GaP on wafer bonded GaAs templates are established. One method is to use two sets of growth conditions where the first set forms tilted facets on the top of both domains, followed by a second set of conditions with higher growth rate. The second method is to suppress the formation of misfit dislocations by increasing the GaCl flow, which reduces the lateral growth rate enhancement caused by such defects. The impact of GaCl flow on misfit dislocation formation is studied in more detail, confirming the effectiveness of this approach. Initial results of an on-going investigation are also presented, where growth on different types of GaAs templates are studied using temporally resolved growth, showing the evolution of the growth profile.
Another source for coherent mid-infrared and terahertz radiation that has seen extensive research and development is the quantum cascade laser, which utilizes inter-subband transitions in carefully engineered semiconductor multi-quantum well structures. This approach offers both high output power and wide frequency tunability, but inherently generates more heat compared to typical inter-band lasing transitions. This is most often addressed by employing buried heterostructures that maximizes the thermal conduction away from the lasing region. The regrowth of InP:Fe on InGaAs/AlGaAs structures has been demonstrated to be very effective, despite the unconventional geometric requirements put on the epitaxial process. In this work, InP:Fe regrowth on a novel hexagonally close-packed photonic crystal structure is studied. This structure is designed to enable power scaling of terahertz radiation emission while maintaining a single optical mode. The impact on thermal management is investigated by thermal dissipation simulations using a finite element method. It is found that, as also seen on well-established ridge structures, the thermal dissipation is greatly enhanced by the regrowth of InP:Fe compared to other structural materials with poorer thermal properties. Regrowth of InP:Fe on a photonic crystal quantum cascade laser sample is presented, utilizing the high growth rate anisotropy inherent to hydride vapor phase epitaxy to achieve full planarization around the 12 µm tall structure in 13 min of growth. Additionally, regrowth is also performed on a more conventional ridge-style laser structure, utilizing a tapered design to increase confinement and increase output power. While tapered designs have been investigated previously, this utilizes a novel design that emits from the tapered end of the ridge in order to mitigate heating effects. The L-I-V characteristics and beam stability of these structures were analyzed during room-temperature quasi-continuous lasing, achieving 1.4 watt peak output power.
This work covers a number of advanced epitaxial methods and their usage for applications in different fields based on leveraging the strengths or mitigating the drawbacks of hydride vapor phase epitaxy. The economic benefits of the technique in combination with the unique solutions provided by its key features demonstrates potential for several applications based on III-V/Si integration and mid-infrared radiation generation.