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- Chen, Jr-Tai, 1982-
(författare)
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MOCVD growth of GaN-based high electron mobility transistor structures
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present work was to improve the overall quality of GaN-based high electron mobility transistor (HEMT) epitaxial structures grown on semi-insulating (SI) SiC and native GaN substrates, using an approach called bottom-to-top optimization. The bottom-to-top optimization means an entire growth process optimization, from in-situ substrate pretreatment to the epitaxial growth and then the cooling process. Great effort was put to gain the understanding of the influence of growth parameters on material properties and consequently to establish an advanced and reproducible growth process. Many state-of-the-art material properties of GaN-based HEMT structures were achieved in this work, including superior structural integrity of AlN nucleation layers for ultra-low thermal boundary resistance, excellent control of residual impurities, outstanding and nearly-perfect crystalline quality of GaN epilayers grown on SiC and native GaN substrates, respectively, and record-high room temperature 2DEG mobility obtained in simple AlGaN/GaN heterostructures.The epitaxial growth of the wide bandgap III-nitride epilayers like GaN, AlN, AlGaN, and InAlN, as well as various GaN-based HEMT structures was all carried out in a hot-wall metalorganic chemical vapor deposition (MOCVD) system. A variety of structural and electrical characterizations were routinely used to provide fast feedback for adjusting growth parameters and developing improved growth processes.
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| 2. |
- Chen, Xiaoyu, 1993-
(författare)
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Theoretical Studies on CO2 Reduction Electrocatalysts
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The atmospheric CO2 concentration has increased by more than 20% since 1980s and has now reached the highest level than at any point in the past 800 000 years. Electrochemical CO2 reductions are receiving particular in- terest as the apparatus are relatively easy to maintain and cheap to operate. However, the direct reduction of CO2 into CO2 radical requires a very high over-potential, meaning a substantial waste in energy. In order to lower the over-potential required, a large number of catalysts has been synthesised and studied. Among these catalysts, three are studied in this work due to their interesting reactivities. We believe the further understanding gained in our studies will benefit the development of new and better catalysts.Ru(6-Mebpy)(tBu3-tpy) reduces CO2 at its first reduction potential and can therefore lower the over-potential required significantly. This observation is unique for Ru(tpy)(bpy) type of catalysts. Density functional theory (DFT) cal- culations revealed that the steric hindrance provided by the 6-methyl group weakens Ru-solvent interactions and hence allows solvent detachment to take place after only one reduction, which is otherwise not possible. Furthermore, we proposed a new mechanism for CO2 to CO reduction at the first reduc- tion potential and identified a cyclic intermediate by Infra-red spectroscopy in collaboration with experimentalists. Such intermediate was not reported pre- viously for Ru-based electrocatalysts.Co(TPP)/CNTs as a heterogenous catalyst exhibits superior reactivity as compared to in solution. DFT calculations with implicit solvent model ac- counts its enhanced reactivity to the increased proton concentration in water. The inverse-loading effect was studied by potential mean force (PME) sam- pling. Our results suggest that aggregation is triggered by the strong inter- molecular p - p interactions among the catalysts. Flatter nanotubes have better contact with Co(TPP) and hence reduces aggregation tendency. The same cat- alyst was also used as an example to study catalysis at interfaces in an electric field. Our full-explicit EVB -MD (Empirical Valence Bond-Molecular Dynam- ics) model illustrates that the electric double layer concentrates cations, which significantly stabilises polarised CO2 at a higher concentration and hence eases CO2 binding. Furthermore, we have also shown that either the electric field or the cations along provides only a minor, almost negligible stabilisation.In 2019, CoPc/CNTs was reported to be the first early-period transition metal complex that can catalyse CO2-to-CH3OH conversion at a decent yield. Literature search on previous work suggests that the presence of well-dispersed, monomeric CoPc is crucial to further reduce CO into CH3OH. We calculated the reaction profiles for both monomeric CoPc and dimeric CoPc, which is the simplest form of aggregates. Our DFT results demonstrate that after the formation of catalyst-CO- complex, monomers tend to go though further reac- tions to afford CH3OH while dimers tend to dissociate CO as reductions are slightly harder, which in turn, is raised from a less degree of solvation stabili- sation upon reductions.
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| 3. |
- Puttisong, Yuttapoom, 1984-
(författare)
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Room-temperature defect-engineered spin functionalities in Ga(In)NAs alloys
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor spintronics is one of the most interesting research fields that exploits both charge and spin properties for future photonics and electronic devices. Among many challenges of using spin in semiconductors, efficient generation of electron spin polarization at room temperature (RT) remains difficult. Recently, a new approach using defect-mediated spin filtering effect, employing -interstitial defects in Ga(In)NAs alloys, has been shown to turn the material into an efficient spin-polarized source capable of generating >40% conduction electron spin polarization at RT without an application of external fields. In order to fully explore the defectengineered spin functionalities, a better understanding and control of the spin filtering effects is required. This thesis work thus aims to advance our understanding, in terms of both physical and material insights, of the recently discovered spin filtering defects in Ga(In)NAs alloys. We have focused on the important issues of optimization and applications of the spin filtering effects.To improve spin filtering efficiency, important material and defect parameters must be addressed. Therefore, in Papers I–III formation of the defects in Ga(In)NAs alloys has been examined under different growth and post-growth treatment conditions, as well as in different structures. We found that the defects were the dominant and important nonradiative recombination centers in Ga(In)NAs epilayers and GaNAs/GaAs multiple quantum wells, independent of growth conditions and post-growth annealing. However, by varying growth and post-growth conditions, up to four configurations of the defects, exhibiting different hyperfine interaction (HFI) strengths between defect electron and nuclear (e-n) spins, have been found. This difference was attributed to different interstitial sites and/or complexes of . Further studiesfocused on the effect of post-growth hydrogen (H) irradiation on the spin filtering effect. Beside the roles of H passivation of N resulting in bandgap reopening of the alloys, H treatment was shown to lead to complete quenching of the spin filtering effect, accompanied by strong suppression in the concentrations of the defects. We concluded that the observed effect was due to the passivation of the defects by H, most probably due to the formation of H- complexes.Optimizing spin filtering efficiency also requires detailed knowledge of spin interactions at the defect centers. This issue was addressed in Papers IV and V. From both experimental and theoretical studies, we were able to conclude that the HFI between e-n spins at the defects led to e-n spin mixing, which degraded spin filtering efficiency at zero field. Moreover, we have identified the microscopic origin of electron spin relaxation (T1) at the defect centers, that is, hyperfine-induced e-n spin cross-relaxation. Our finding thus provided a guideline to improve spin filtering efficiency by selectively incorporating the defects with weak HFI by optimizing growth and post-growth treatment conditions, or by searching for new spin filtering defect centers containing zero nuclear spin.The implementation of the defect-engineered spin filtering effect has been addressed in Papers VI–VIII. First, we experimentally demonstrated for the first time at RT an efficient electron spin amplifier employing the defects in Ga(In)NAs alloys, capable of amplifying a weak spin signal up to 27 times with a high cut-off frequency of 1 GHz. We further showed that the defectmediated spin amplification effect could turn the GaNAs alloy into an efficient RT optical spin detector. This enabled us to reliably conduct in-depth spin injection studies across a semiconductor heterointerface at RT. We found a strong reduction of electron spin polarization after optical spin injection from a GaAs layer into an adjacent GaNAs layer. This observation was attributed to severe spin loss across the heterointerface due to structural inversion asymmetry and probably also interfacial point defects.Finally, we went beyond the generation of strongly polarized electron spins. In Paper IX we focused on an interesting aspect of using strongly polarized electron spins to induce strong nuclear spin polarization at RT, relevant to solid-state quantum computation using a defect nuclear spin of long spin memory as a quantum bit (qubit). By combining the spin filtering effect and the HFI, we obtained a sizeable nuclear spin polarization of ~15% at RT that could be sensed by conduction electrons. This demonstrated the feasibility of controlling defect nuclear spins via conduction electrons even at RT, the first case ever being demonstrated in a semiconductor.
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