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- Zhao, Ming, 1975-
(författare)
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Growth and Characterization of Strain-engineered Si/SiGe Heterostructures Prepared by Molecular Beam Epitaxy
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The strain introduced by lattice mismatch is a built-in characteristic in Si/SiGe heterostructures, which has significant influences on various material properties. Proper design and precise control of strain within Si/SiGe heterostructures, i.e. the so-called “strain engineering”, have become a very important way not only for substantial performance enhancement of conventional microelectronic devices, but also to allow novel device concepts to be integrated with Si chips for new functions, e.g. Si-based optoelectronics. This thesis thus describes studies on two subjects of such strain-engineered Si/SiGe heterostructures grown by molecular beam epitaxy (MBE). The first one focuses on the growth and characterizations of delicately strain-symmetrized Si/SiGe multi-quantum-well/superlattice structures on fully relaxed SiGe virtual substrates for light emission in the THz frequency range. The second one investigates the strain relaxation mechanism of thin SiGe layers during MBE growth and post-growth processes in non-conventional conditions.Two types of THz emitters, based on different quantum cascade (QC) intersubband transition schemes, were studied. The QC emitters using the diagonal transition between two adjacent wells were grown with Si/Si0.7Ge0.3 superlattices up to 100 periods. It was shown that nearly perfect strain symmetry in the superlattice with a high material quality was obtained. The layer parameters were precisely controlled with deviations of ≤ 2 Å in layer thickness and ≤ 1.5 at. % in Ge composition from the designed values. The fabricated emitter devices exhibited a dominating emission peak at ~13 meV (~3 THz), which was consistent with the design. An attempt to produce the first QC THz emitter based on the bound-to-continuum transition was made. The structures with a complicated design of 20 periods of active units were extremely challenging for the growth. Each unit contained 16 Si/Si0.724Ge0.276 superlattice layers, in which the thinnest one was only 8 Å. The growth parameters were carefully studied, and several samples with different boron δ-doping concentrations were grown at optimized conditions. Extensive material characterizations revealed a high crystalline quality of the grown structures with an excellent growth control, while the heavy δ-doping may introduce layer undulations as a result of the non-uniformity in the strain field. Moreover, carrier lifetime dynamics, which is crucial for the THz QC structure design, was also investigated. Strain-symmetrized Si/SiGe multi-quantum-well structures, designed for probing the carrier lifetime of intersubband transitions inside a well between heavy hole 1 (HH1) and light hole 1 (LH1) states with transition energies below the optical phonon energy, were grown on SiGe virtual substrates. The lifetime of the LH1 excited state was determined directly with pump-probe spectroscopy. The measurements indicated an increase of lifetime by a factor of ~2 due to the increasingly unconfined LH1 state, which agreed very well with the theory. It also showed a very long lifetime of several hundred picoseconds for the holes excited out of the well to transit back to the well through a diagonal process.Strained SiGe grown on Si (110) substrates has promising potentials for high-speed microelectronics devices due to the enhanced carrier mobility. Strain relaxation of SiGe/Si(110) subjected to different annealing treatments was studied by X-ray reciprocal space mapping. The in-plane lattice mismatch was found to be asymmetric with the major strain relaxation observed in the lateral [001] direction. It was concluded that this was associated to the formation and propagation of conventional a/2<110> dislocations oriented along [110]. This was different from the relaxation observed during growth, which was mainly along in-plane [110].A novel MBE growth process to fabricate thin strain-relaxed Si0.6Ge0.4 virtual substrates involving low-temperature (LT) buffer layers was investigated. At a certain LT-buffer growth temperature, a dramatic increase in the strain relaxation accompanied with a decrease of surface roughness was observed in the top SiGe, together with a cross-hatch/cross-hatch-free transition in the surface morphology. It was explained by the association with a certain onset stage of the ordered/disordered transition during the growth of the LT-SiGe buffer.
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| 2. |
- Zhao, Wei
(författare)
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Aqueous graphene dispersions for paper packaging
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene is widely touted as the thinnest and the most versatile material available. As an atomically thin layer of carbon atoms arranged in a hexagonal configuration, graphene has a combination of technologically important properties, such as thermal and electrical conductivity, mechanical strength, and impermeability to gases. From an industrial perspective on applications, water as a dispersing media for graphene offers safer handling and environmental benefits compared with conventional organic solvents. However, the high surface tension of water and the attractive forces between graphene surfaces drive the sheets to aggregation. Although surfactants have been an important stepping stone in the advancement of aqueous graphene dispersions, these surface-active molecules are often needed in excess and have adverse effects on coatings during film formation. These challenges limit the industrial relevance of graphene as an effective barrier in composites. In general, gas barriers against both oxygen and water vapour, made from a single coating formulation, is seemingly a holy grail for the packaging industry. In this thesis work, the aim was to gain a fundamental understanding of aqueous graphene dispersions for gas barriers used in paper packaging. Biobased materials were systematically investigated as dispersing agents for graphene based on dispersing conditions and functional barrier performance. Flavin mononucleotide (FMN), a food additive, dispersed graphene using a relatively low amount of FMN and showed intriguing spectroscopic signatures of π-π interactions with graphene. Starch nanoparticles (SNPs) realised concentrated and stable aqueous graphene dispersions for composite films. The SNP-stabilized graphene sheets in starch films lowered the gas permeability of both oxygen and water vapour simultaneously by over 70% under all the conditions tested. In general, a combined gas barrier performance is unusual for both bioplastics and common petrochemical-based plastics used in the packaging industry. Motivated by the graphene network leading to the extraordinary barrier performance, the aqueous SNP-graphene dispersion was modified for inkjet printing. The printed patterns were flexible and electrically conductive in the order of 104 S m-1 that is on par with the highest reported values in the literature. These surfactant-free aqueous SNP-graphene dispersions have the potential and versatility for paper-based gas barriers with integrated electronics. Multifunctional composite films made from these dispersions, when optimized, could become competitive with commercial plastics, and meet the current and future demands of the packaging industry.
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