Search

Der gennemgås grundlæggende teori for plastisk og elastisk opførsel i Selective Area Growth (SAG) nanowires (NWs), som er groet i en heterostruktur med uoverensstemmelse mellem gitterkonstanterne. Vi fokuserer på III/V NWs af InAs på en buffer af enten InGaAs eller GaAs, og gennemgår litteraturen for analytiske forsøg på at beskrive deres vækst og udvikling, særligt med henblik på plastiske urenheder såsom misfit dislocations (MDs). Disse forsøg er fænomenologiske og kvalitativt korrekte, men de viser sig at være svære at bruge til kvantitative forudsigelser omkring kritiske højder og egenskaber af SAG NWs. Fra dette udgangspunkt udvikles et omfattende simulationsapparat som bruger finite element method (FEM) til at simulere relaksering i SAG heterostrukturer. Vi præsenterer en nyskabende teknik for også at simulere MDs, som kan generaliseres til alle morfologier og sammensætninger, og som skaber et strain field vi kan bekræfte teoretisk. Vi forudsiger en række kritiske højder og konkluderer, at den optimale indførelse af MDs er en kollektiv indførelse af adskillige på samme tid., We review the phenomenon of misfit dislocations (MDs) in Selective Area Growth (SAG) nanowires (NWs) which have been grown as heterostructures with an epitaxial mismatch. We research current analytical attempts at describing their formation alongside growth stages, focusing on III/V NWs of InAs on InGaAs and GaAs buffers. These phenomenological and qualitative descriptions of dislocation formation are found unhelpful in quantitative predictions, specifically for critical dimensions and controlling the quality and properties of SAG NWs. From here we develop an extensive finite element method (FEM) framework to simulate strain energy within SAG heterostructures. We present a novel technique for simulating MDs, transferable to any morphology and composition, and yielding verifiable strain fields. We predict several critical heights and find the optimal formation of MDs to be a collective one.

Der gennemgås grundlæggende teori for plastisk og elastisk opførsel i Selective Area Growth (SAG) nanowires (NWs), som er groet i en heterostruktur med uoverensstemmelse mellem gitterkonstanterne. Vi fokuserer på III/V NWs af InAs på en buffer af enten InGaAs eller GaAs, og gennemgår litteraturen for analytiske forsøg på at beskrive deres vækst og udvikling, særligt med henblik på plastiske urenheder såsom misfit dislocations (MDs). Disse forsøg er fænomenologiske og kvalitativt korrekte, men de viser sig at være svære at bruge til kvantitative forudsigelser omkring kritiske højder og egenskaber af SAG NWs. Fra dette udgangspunkt udvikles et omfattende simulationsapparat som bruger finite element method (FEM) til at simulere relaksering i SAG heterostrukturer. Vi præsenterer en nyskabende teknik for også at simulere MDs, som kan generaliseres til alle morfologier og sammensætninger, og som skaber et strain field vi kan bekræfte teoretisk. Vi forudsiger en række kritiske højder og konkluderer, at den optimale indførelse af MDs er en kollektiv indførelse af adskillige på samme tid., We review the phenomenon of misfit dislocations (MDs) in Selective Area Growth (SAG) nanowires (NWs) which have been grown as heterostructures with an epitaxial mismatch. We research current analytical attempts at describing their formation alongside growth stages, focusing on III/V NWs of InAs on InGaAs and GaAs buffers. These phenomenological and qualitative descriptions of dislocation formation are found unhelpful in quantitative predictions, specifically for critical dimensions and controlling the quality and properties of SAG NWs. From here we develop an extensive finite element method (FEM) framework to simulate strain energy within SAG heterostructures. We present a novel technique for simulating MDs, transferable to any morphology and composition, and yielding verifiable strain fields. We predict several critical heights and find the optimal formation of MDs to be a collective one.

We investigate effects of quasiparticle poisoning in a Majorana island with strong tunnel coupling to normal-metal leads. In addition to the main Coulomb blockade diamonds, "shadow" diamonds appear, shifted by 1e in gate voltage, consistent with transport through an excited (poisoned) state of the island. Comparison to a simple model yields an estimate of parity lifetime for the strongly coupled island (~ 1 {\mu}s) and sets a bound for a weakly coupled island (> 10 {\mu}s). Fluctuations in the gate-voltage spacing of Coulomb peaks at high field, reflecting Majorana hybridization, are enhanced by the reduced lever arm at strong coupling. In energy units, fluctuations are consistent with previous measurements.

GaAs was central to the development of quantum devices but is rarely used for nanowire-based quantum devices with InAs, InSb and SiGe instead taking the leading role. p-type GaAs nanowires offer a path to studying strongly confined 0D and 1D hole systems with strong spin–orbit effects, motivating our development of nanowire transistors featuring Be-doped p-type GaAs nanowires, AuBe alloy contacts and patterned local gate electrodes towards making nanowire-based quantum hole devices. We report on nanowire transistors with traditional substrate back-gates and EBL-defined metal/oxide top-gates produced using GaAs nanowires with three different Be-doping densities and various AuBe contact processing recipes. We show that contact annealing only brings small improvements for the moderately doped devices under conditions of lower anneal temperature and short anneal time. We only obtain good transistor performance for moderate doping, with conduction freezing out at low temperature for lowly doped nanowires and inability to reach a clear off-state under gating for the highly doped nanowires. Our best devices give on-state conductivity 95 nS, off-state conductivity 2 pS, on-off ratio $\sim {10}^{4}$, and sub-threshold slope 50 mV/dec at $T=4$ K. Lastly, we made a device featuring a moderately doped nanowire with annealed contacts and multiple top-gates. Top-gate sweeps show a plateau in the sub-threshold region that is reproducible in separate cool-downs and indicative of possible conductance quantisation highlighting the potential for future quantum device studies in this material system

Majorana fermions in condensed matter systems have attracted a lot of attention since first strong signatures came from Mourik et. al. of Majorana bound states in proximitized nanowires. The main interest in these quasiparticles that they exhibit non-Abelian exchange statistics and can be encoded as a topologically protected qubits. Main focus of the thesis, was to develop a nanowire based system, where charge readout can be done on Majorana fusion rule experiment devices. With a successful device material and geometry combination, such devices were fabricated and investigated at low temperatures. The devices that were measured are first of their kind, were charge sensing of each and individual nanowire superconducting islands is possible. The devices have the ability to be tuned from single to double quantum dots and vice versa, while taking charge sensing data. Even/odd effect at finite magnetic fields was also observed, indicating that epitaxially grown Al, fully proximitized the semiconductor. Majorana search in these devices was also conducted and presented in this thesis, with a zero bias conductance peak observed at 275 mT.

Majorana fermions in condensed matter systems have attracted a lot of attention since first strong signatures came from Mourik et. al. of Majorana bound states in proximitized nanowires. The main interest in these quasiparticles that they exhibit non-Abelian exchange statistics and can be encoded as a topologically protected qubits. Main focus of the thesis, was to develop a nanowire based system, where charge readout can be done on Majorana fusion rule experiment devices. With a successful device material and geometry combination, such devices were fabricated and investigated at low temperatures. The devices that were measured are first of their kind, were charge sensing of each and individual nanowire superconducting islands is possible. The devices have the ability to be tuned from single to double quantum dots and vice versa, while taking charge sensing data. Even/odd effect at finite magnetic fields was also observed, indicating that epitaxially grown Al, fully proximitized the semiconductor. Majorana search in these devices was also conducted and presented in this thesis, with a zero bias conductance peak observed at 275 mT.