Growth and Characterization of Strain-engineered Si/SiGe Heterostructures Prepared by Molecular Beam Epitaxy
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
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  direction. It was concluded that this was associated to the formation and propagation of conventional a/2<110> dislocations oriented along . This was different from the relaxation observed during growth, which was mainly along in-plane .
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.
Kisel(Si)-baserad mikroelektronik har utvecklats under en femtioårsperiod till att bli basen för vår nuvarande informationsteknologi. Förutom att integrera fler och mindre komponenter på varje kisel-chip så utvecklas metoder att modifiera och förbättra materialegenskaperna för att förbättra prestanda ytterligare. Ett sätt att göra detta är att kombinera kisel med germanium (Ge) bl.a. för att skapa kvantstrukturer av nanometer-storlek. Eftersom Ge-atomerna är större än Si-atomerna kan man skapa en töjning i materialet vilket kan förbättra egenskaperna, ex.vis hur snabbt laddningarna (elektronerna) rör sig i materialet. Genom att variera Gekoncentrationen i tunna skikt kan man skapa skikt som är antingen komprimerade eller expanderade och därmed ger möjlighet att göra strukturer för tillverkning av nya typer av komponenter för mikroelektronik eller optoelektronik. I detta avhandlingsarbete har Si/SiGe nanostrukturer tillverkats med molekylstråle-epitaxi-teknik (molecular beam epitaxy, MBE). Med denna teknik byggs materialet upp på ett substrat, atomlager för atomlager, med mycket god kontroll på sammansättningen av varje skikt. Samtidigt kan töjningen av materialet designas så att inga defekter skapas alternativt många defekter genereras på ett kontrollerat sätt. I denna avhandling beskrivs detaljerade studier av hur töjda i/SiGe-strukturer kan tillverkas och ge nya potentiella tillämpningar ex.vis som källa för infraröd strålning. Studierna av de olika töjda skikten har framför allt gjorts med avancerade röntgendiffraktionsmätningar och transmissionselektronmikroskopi.
Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2008. , 69 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1176
Si/SiGe, Strain engineering, Molecular beam epitaxy, THz, Quantum cascade, Strain relaxation
IdentifiersURN: urn:nbn:se:liu:diva-11746ISBN: 978-91-7393-911-9OAI: oai:DiVA.org:liu-11746DiVA: diva2:18174
2008-06-02, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Wang, Kang L., Professor
Ni, Wei-XinHansson, Göran
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