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- PostAnyon Colliders: A time-dependent quantum Hall particle collider to reveal fractional statistics in the Laughlin sequence(2023) Varada, Sushanth; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Splettstoesser, Janine; Acciai, Matteo; Spånslätt, ChristianElementary particles in nature (3+1 dimensions) are classified into bosons and fermions based on their exchange statistics. However, more general statistics, intermediate be- tween fermionic and bosonic, are possible in 2+1 dimensions. Quasiparticles obeying this intermediary statistics are called anyons. A particularly relevant phase of matter hosting anyons is the fractional quantum Hall effect, where anyonic statistics has recently been demonstrated. Generally, exchange statistics is expected to be accessible in interference experiments, such as in the Hong-Ou-Mandel effect. In this setup, fermions show van- ishing current correlations due to anti-bunching caused by the Pauli exclusion principle. Bosons, instead, bunch together due to Bose-Einstein statistics causing a surge in the current correlations. Can Hong-Ou-Mandel interferometry be extended to probe the frac- tional statistics of anyons? In this thesis, we investigate this question in a fractional quantum Hall setup in the Laughlin sequence (filling factor ν = 1/(2n + 1), n ∈ Z+), where two anyons collide at a quantum point contact with a tunable time delay. Previous studies investigating sim- ilar systems relate current correlations of quasiparticle collisions with braiding between injected anyons and quasi-particle-hole excitations at the tunneling quantum point con- tact, which emerge due to thermal or quantum fluctuations. However, it remains unclear whether the presently studied Hong-Ou-Mandel effect probes the universal exchange phase (θ) picked up by the quasiparticles or other parameters, such as the non-universal scaling dimension (δ). We show that θ accumulated by the incoming anyons due to interaction with quasi-particle-hole pairs at the quantum point contact cancel out in time-sensitive two-particle interferometry. Instead, the key quantity measured through current correla- tions is the non-universal δ of the quasi-particle-hole excitations.
- PostBow-tie antenna-coupled graphene FETs for direct detection at 0.6THz(2014) Zak, Audrey; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe recent demonstration of the electric field effect in graphene, a two-dimensional carbon lattice, and the measurement of its extraordinary room-temperature properties have sparked enormous interest in graphene as a material in electronics. Atomically thin and conductive, graphene has many options for integration with existing electronic devices and opportunities for the development of new devices. Graphene has a high carrier mobility and high carrier saturation velocity at room temperature, two properties which give it the potential for use in high-speed, high-frequency applications. Technologies operating at terahertz (1 1012 Hz) frequencies are becoming more numerous as an effort is made to close the so-called terahertz gap, and graphene has recently emerged as a viable terahertz material. This thesis presents terahertz (THz) direct detectors based on antenna-coupled graphene eld effect transistors (GFETs). These GFETs were fabricated using both mechanically exfoliated graphene and graphene grown by chemical vapor deposition (CVD) and integrated with split bow-tie antennae. These direct detectors are capable of room-temperature rectification of a 0.6 THz signal and achieve maximum optical responsivities of 13.0 V/W and 9.1V/W and minimum noise equivalent power (NEP) of 530 pW/Hz0:5 and 490 pW/Hz0:5 for the CVD and exfoliated detectors, respectively. These results are a significant improvement over previous work done with graphene direct detectors and are comparable to other established direct detector technologies. This is the first time room temperature direct detection has been demonstrated using CVD graphene, and because large-area graphene can be produced by CVD growth, scalable graphene detector production is now a possibility.
- PostCarbon Nanotube Networks as Thermally Conducting Layers(2022) Juteräng, David; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Liu, Johan; Fu, YifengFlexible and thermally conductive materials with microfabricated structures are important in a number of different research fields. Different approaches for integration of such materials into functioning devices have been implemented in a plethora of ways. Carbon nanotube networks have been the subject of many studies due to their remarkable physical properties, including high thermal conductivity, high electron mobility, high Young’s modulus and their flexibility, but challenges still remain. One hurdle to overcome is the lack of efficient bonds between nanotubes in meshes. In this project, the viability of a nickel/carbon nanotube network have been investigated in the context of a potential thermal spreading hybrid material. Carbon nanotubes of with different lengths were grown on silicon substrates, dispersed in acetone and mixed into solutions containing Nickel-oxide particles. The blends were deposited onto new Silicon substrates where they formed networks. The Nickel particles stuck to strands and bundles of nanotubes, forming bridges between them. Thermal treatment of the networks were performed at different time scales in order to study the effects of annealing on the networks. The characteristics of the Ni/CNT networks were finally investigated using scanning electron microscopy and Raman spectroscopy in order to study potential changes within them. An increase of the D-peak/G-peak intensity ratio corresponding to longer thermal treatment of the substrates were concluded to be a plausible indicator of increased bonding between the Ni-particles and CNTs. In addition, a simulation was made of a CNT-CNT electron tunneling junction. This was done in order to provide the theoretical backround for the challenges regarding CNT meshes. The lack of chemical bonds between tubes were calculated to increase the resistance of a square CNT thin film by approximately 150%.
- PostCharacterisation and optimisation of superconducting microwave resonators(2016) Boehme, Thijs; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceIn this master's thesis, I present the design, fabrication, and characterisation of superconducting microwave resonators. These resonators are quarter-wavelength coplanar waveguide resonators, operating at resonance frequencies between 4 GHz and 8 GHz. This kind of resonator is a key component in several applications, ranging from quantum computation devices to parametric amplifiers. Minimising the losses (achieving high internal quality factors) in these resonators is important to maximise performance of the applications they are used for. During the thesis work, we have studied several possible changes to improve the resonator quality. Fabrication of the resonators was done in the Nanofabrication Laboratory at Chalmers University of Technology, and consist of the following steps: cleaning a sapphire or silicon wafer, depositing a layer of superconducting niobium, photolithography, and etching the waveguide. We then measured the resonators in a cryogen-free dilution refrigerator, at a temperature of 10 mK. The measurements consisted of a power sweep, in order to quantify the quality factor at high and low excitation powers. As an example, the first resonator we fabricated had a quality factor of 4.3 x 10^4 at high power (about 10^5 photons in the resonator), and quality factor of 3.7 x 10^4 at the single-photon level. We then investigated different loss mechanisms that we expect to negatively affect the quality factor, and how to reduce this loss. Firstly, to reduce radiation losses and magnetic vortex losses, we reduced the lateral dimensions of the resonator. Secondly, as two-level system (TLS) losses are believed to mainly reside at the surfaces of the materials and interfaces between them, we etched into the substrate to selectively move a part of the TLS away from the resonator. We also studied the effect of the meandering of the resonator on the quality factor, as well as using two different etching gasses. By gathering all this data, we were then able to compare the different combinations to see the effect of the changing parameters. In conclusion, the reduced lateral dimensions and the removal of part of the substrate indeed reduce the losses, as expected from the theory. We also found interesting behaviour with regards to the meandering, namely that having a meandered structure negatively affects the quality factor. Combining all results, we can give a guideline for the fabrication of resonators in the future, which gave us a quality factor up to 4.9 x 10^5 at high excitation power, and 3.0 x 10^5 at the single photon level.
- PostCharacterisation of a travelling-wave parametric amplifier for improved qubit measurements(2019) Nilsson, Hampus Renberg; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceIn order to build a large-scale quantum computer, one of the requirements is high-fidelity multiplexed qubit readout, which in turn relies on the use of ultralow-noise amplifiers. While Josephson Parametric Amplifiers (JPAs), using a cavity with few Josephson junctions, have shown promising results for single qubit readout, they have limitations such as low saturation power and a gain-bandwidth product restriction. This makes them less suited for frequency multiplexed qubit readout. A current-pumped Josephson Travelling-Wave Parametric Amplifier (JTWPA), using 4-wave mixing in a lumped-element transmission line, features both high saturation power as well as no restriction on large bandwidth and high gain. This is possible due to multiple junctions and the lack of a cavity. However, also this current pumped JTWPA has an inherent problem, namely phase mismatch between the pump and the signal. This prevents exponential gain. In this thesis, a current pumped JTWPA using the resonant phase matching (RPM) technique to reduce the phase mismatch problem is studied and characterised. The JTWPA shows high gain, high saturation power and a large signal-to-noise ratio improvement. The JTWPA is also used for qubit readout, for which it clearly improves the readout fidelity.
- PostDeposition and Characterization of Dielectric Distributed Bragg Reflectors(2016) Lapp, Michael; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe Vertical Cavity Surface Emitting Laser (VCSEL) is a laser well suited for data centre applications due to its low cost, high speeds, good fiber coupling, and low threshold current The current standard wavelength for short-haul is 850 nm, however switching to 980 nm can improve transmission distance in optical fiber by lowering absorption and chromatic dispersion. To improve the performance of VCSELs, mirrors with less optical loss can be used. VCSELs require mirrors with > 99 % reflectance, and Distributed Bragg Reflectors (DBRs) are used to achieve this. Today, epitaxially-grown semiconductor DBRs are used, however these have low refractive index contrast, scattering from dopants, and free-carrier absorption. These DBRs can be replaced by dielectric ones which can offer higher refractive index contrast and less absorption. These properties can increase optical confinement in the gain region and decrease losses in the mirrors leading to VCSELs with more output power and lower threshold currents. DBRs using dielectric materials to reflect 980 nm light above 99 % are therefore of key interest. In this work DBRs with SiO2/TiO2 layers as well as SiO2/a-Si were deposited by sputtering. Mirrors were fabricated with 2, 4, 6, and 8 pairs for each set of materials. A measurement setup was built to measure reflectances above 99 % with ±0.03 % accuracy to evaluate the mirrors. DBRs of SiO2/TiO2 were found to achieve above 99 % reflectance for 6 and 8 pairs with measured reflectances of 99.42 ± 0.03 % and 99.80 ± 0.03 % respectively with negligible extinction coefficient. SiO2/a-Si DBRs achieved over 99 % reflectance for 4, 6, and 8 pairs with measured reflectances of 99.36 ± 0.03 %, 99.02 ± 0.03 %, and 99.17 ± 0.03 % respectively with extinction coefficient for a-Si between 0.0010 and 0.0015 at 980 nm All DBRs agreed well with simulated results. SiO2/TiO2 mirrors were found to have more reliable deposition and achieved higher reflectances. SiO2/a-Si DBRs on the other hand were found to have less reliable deposition and their reflectance saturated above 6 pairs due to the higher extinction coefficient in a-Si, indicating little to no light is transmitted out of these DBRs, limiting their usefulness for VCSELs. A 6-pair SiO2/TiO2 DBR appears very promising to use in VCSELs due to the superior reliability of the materials and increased output power from a slightly lower reflectance than the 8-pair mirror.
- PostElectrical and Optoelectronic Properties of Two-Dimensional Lateral Heterostructure Semiconductors(2024) Krishna Kumar, Baskar; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Prasad Dash, Saroj; Prasad Dash, Saroj; Md Hoque, AnamulTwo-Dimensional (2D) semiconductors are promising materials for atomically thin electronics and optoelectronics. Specifically, p-n junctions and their gate voltagecontrolled effects in the lateral heterostructures of such 2D semiconductors offer several advantages because of their atomically thin in-plane superlattices. 2D p-n junctions have great potential for application in low-power, high-performance electro-optical devices, such as tunnel transistors, light-emitting diodes, photodetectors and photovoltaic cells. Although vertical heterojunctions are promising for electro-optical devices, the use of mechanical exfoliation process to obtain the vertical heterostructure is unsuitable for wafer-scale fabrication. The in-situ growth of high-quality lateral heterostructures with multiple junctions has just started to be explored. In this master’s thesis, we fabricated field-effect transistors (FETs) based on MoS2- WS2 lateral heterostructures and performed their electrical and optoelectronic characterization. The lateral heterostructures grown using the water-assisted one-pot chemical vapour deposition (CVD) are used to fabricate the back-gated FETs on Si-SiO2 substrates with Ti/Au contacts. We characterized the individual MoS2 and WS2 channels and their heterojunctions. The junctions show diodic behaviour, which could be understood by the formation of n-n+ junction. The transistor parameters are extracted for MoS2, WS2 and MoS2-WS2 heterojunction. Furthermore, we observed a persistent photoconductivity (PPC) effect with a time constant of 10 hrs at the heterojunction. The PPC effect is being explored for applications such as optoelectronic synapses, optical memory, artificial vision etc.
- PostElectronic Transport Measurements of Two-Dimensional Semiconductor Homostructures(2024) Blomqvist, Jonathan; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Prasad Dash, Saroj; Prasad Dash, SarojThe ever-increasing demand for emerging technologies requires advancements in semiconductor devices beyond what is hitherto envisaged. Two-dimensional (2D) semiconductors have recently gained considerable attention for field-effect transistor technologies. However, there are various challenges regarding the growth of 2D semiconductors and the optimisation of their channel properties. Specifically, the growth of 2D semiconductors by chemical vapour deposition (CVD) can result in bilayer patches and twisted layered structures, leading to the formation of homojunctions. These homojunctions may exhibit non-uniform charge transport due to various growth-related defects. Therefore, it is imperative to investigate charge transport across such homojunctions in 2D semiconductor field-effect transistor devices. In this thesis, charge transport across monolayer-bilayer homojunctions in twisted 2D WS2-homostructures is demonstrated. Devices were fabricated from WS2 flakes containing monolayer-bilayer junctions with twist angles of 0◦ and 60◦. Transport measurements conducted at room temperature revealed rectifying behaviour across monolayer-bilayer WS2-homojunctions. This rectifying behaviour is attributed to the different band gaps and work functions of monolayer WS2 and bilayer WS2. Additionally, it is likely that intrinsic defects around the nucleation site, originating from the CVD growth, increased the doping concentration in the bilayer region. Together, these factors affect the charge transport across the homojunctions and result in the observed diodic behaviour. These findings contribute to a deeper understanding of the physics of 2D semiconductor devices, thereby highlighting the pivotal role of these devices in shaping future technological developments.
- PostEncoding of Qubit States in Resonators With Cat Codes(2019) Winther, Johan; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceQuantum computing has gained a lot of interest in recent years and commercial products are just now entering the market. However one of the main challenges in realising a quantum computer is noise and one key technology to remedy this is quantum error correction (QEC). One way of performing QEC exploits the storage of the quantum information of a qubit in a resonator as cat codes. To do this one needs to apply encoding pulses to the coupled qubit-resonator system in order to perform a state transfer from the qubit to the resonator. These pulses need to be numerically obtained by simulation. This thesis studies the potential of using a gradient-based optimization method, the so called Krotov's method, to numerically optimize encoding pulses for encoding arbitrary qubit states in cat codes. The Python package Krotov, a package for quantum optimal control using the method, is first used to perform state evolution from |0> to |1> and |0> to |2> of an anharmonic resonator in order to familiarise with the package and optimal control in general. It is shown that, assuming a maximum drive amplitude and no dissipation, the method can realise a |0> to |1> evolution with fidelity F > 0.99999 and a total pulse length of only 10.75 ns. For the |0> to |2> evolution a total pulse length of 30 ns is needed to reach the same fidelity. Finally, the Krotov is used to optimize pulses for transferring qubit states into the resonator as cat codes. The transfer of six states were simultaneously optimized in order to approximate a unitary which transfers an arbitrary qubit state into the resonator as a cat code. Using mostly experimentally realistic parameters, it is shown that the method can optimize pulses which realise the encoding of arbitrary qubit states to cat codes with a fidelity of at least F > 0.998900. Although plenty of challenges still remain to prove this can be done in experiments, the results points to the Krotov package as a viable tool for encoding pulse optimization.
- PostFabrication and Characterisation of Thin-Film Superconducting Nanowire Superinductors for Novel Quantum Devices(2014) Niepce, David; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceIn this master's thesis, we present the design, fabrication and characterisation of superconducting thin-film nanowire superinductances for use in quantum information processing and quantum metrology. In order to characterise the superconducting nanowire, it is placed inside a step-impedance lambda/2 microwave resonator. In a step-impedance resonator, the strong wave impedance mismatch between the feed lines (50 ohm) and the nanowire (ca. 5 kohm) results in the formation of standing waves within the resonator. By measuring the transmission and reflection parameters of the resonator, we can probe the nanowire properties. We have shown that the high kinetic inductance of thin NbN films can be used to fabricate superinductors that exhibit an inductance two to three orders of magnitude larger than an ordinary geometric inductance of the same size. An inductance of 787 nH is demonstrated for a 200nm x 1mm nanowire. This corresponds to a reactive microwave impedance of 25 kohm at 5 GHz, which is higher than the resistance quantum h/(4e^2) = 6.5 kohm.
- PostFabrication of double-sided Fresnel zone plates and absolute measurements of their diffraction efficiency in an X-ray-tube-based setup(2019) Yurgens, Viktoria; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceDiffractive lenses for X-rays, so-called Fresnel zone plates (FZPs), are used as focusing elements in X-ray imaging applications in synchrotrons all over the world. One of the main characteristics of FZPs is their diffraction efficiency. Measurements of the absolute diffraction efficiency as a function of the X-ray energy can reliably be done using synchrotron radiation, but require beamtime at synchrotrons which is not available on a routine basis. Here, the design, construction and characterization of an in-lab X-ray-tube-based setup for measuring the absolute diffraction efficiency of single- and double-sided FZPs is presented. The broad spectrum from the Xray tube is monochromatized using a set of copper- and nickel filters, giving an approximate X-ray energy of 8.4 keV; the spectrum is simulated and confirmed through spectrometer measurements. Single- and double-sided line-doubled Fresnel zone plates with different characteristics have been fabricated and measured using the setup, yielding absolute diffraction efficiencies of up to 6.4%. Diffraction efficiency maps, showing the spatial distribution of the diffraction efficiency, have been obtained for each zone plate. The measurements have given important information regarding issues with the fabrication process related to the alignment between the two halves of double-sided zone plates, and these issues have been investigated further. Finally, measurements have been performed where the misalignment in double-sided zone plates has been determined by employing a functionality of the setup where the measured zone plates can be tilted around two different axes.
- PostFabrication of High Temperature Thermoelectric Energy Harvesters for Wireless Sensors(2013) Köhler, Elof; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceImplementing energy harvesters and wireless sensors in turbofan jet engines could simplify development procedure and decrease costs. Some of the more difficult areas to measure with wired sensors due to cable wiring, can with wireless sensors and energy harvesters be easily accessed from the cooling channels where the temperature is between 500-900C. This thesis covers the design, analytic modeling, power calculations, synthesis and fabrication of a suitable thermoelectric energy harvester. Known high temperature solutions are based on materials with peak efficiency in lower or higher temperature regions. The modules in this paper are optimized for the temperature range between 600-800C. Chosen materials are n-type Ba8Ga16Ge30 and p-type La-doped Yb14MnSb11, having the highest measured zT value in this region. The use of these materials is uncommon and commercial modules based on these two materials do not exist yet. The total module area was set to 1cm2 and the design goal was to be able to maintain a temperature gradient of at least 200C with as high specific power as possible. The material choices and other design variables like the number of couples, thickness of electrodes, area and height of thermoelectric legs were selected using a model created in MATLAB. The La-doped Yb14MnSb11 was synthesized and identified from x-ray scattering. A measurement of properties was not possible due to insufficient size of the crystals. Ba8Ga16Ge30 was synthesized and resulted in an approximated zT value of 0.83 at 700C. Calculations based on 17 couples and 1mm height of the thermoelectric legs gives a power output of 1100mW/g or 600mW/cm2 with a temperature gradient of 200C. The calculated power from one module yields more than the required power for a single wireless sensor, connecting several modules in series would be preferable, reducing the losses from DC-DC conversion and makes it possible to power entire sensor networks.
- PostGeometric scaling of dielectric loss in superconducting coplanar waveguides(2023) Emil, Rehnman; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Bylander, Jonas; Bylander, JonasFor superconducting qubits, coplanar waveguide (CPW) resonators play an imprtant role in qubit readout and control. They can also be used as a probe for loss in qubits. Loss in CPWs is mainly caused by two-level systems (TLS), radiation and quasi-particles. These competing loss sources are all affected by the resonator size. By measuring the power dependence of the internal quality factor (Qi) of resonators with varying geometry we find that for narrow resonators, TLS loss is limiting low-power Q, and radiation is the main source of high-power loss. Wide resonators show a weaker Qi power dependence, which suggests a smaller contribution from TLS loss. Changing the thin film deposition method greatly improved Qi for these resonators indicating that quasi-particle loss was limiting Qi. Another parameter that affects TLS loss is the gap between the CPW centre conductor and ground plane. Using a large gap, we measured single-photon level Qi of 1.5 million. The larger gap reduces the TLS loss by diluting the electric filed in the TLS hosting regions. We also investigate the loss in so-called trenched resonators. By removing the silicon substrate in the resonator gap, one of the TLS hosting regions can be moved away from the areas with the highest electric field strengths. A trenching recipe using deep reactive ion etching was developed, and trenched resonators were fabricated and measured. The low-power quality factor in the trenched resonators was significantly higher compared to the standrad resonator, and we achieved a single-photon level Qi of 1.9 million.
- PostImpact of Equilibration on the Heat Conductance and Noise of non-Abelian fractional Quantum Hall Edges(2022) Hein, Michael; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Splettstoesser, Janine; Spånslätt Rugarn, Christian; Splettstoesser, JaninePerforming resistance measurements in a cold 2D electron gas allows to observe the quantum Hall effect. It comes along with a quantized transverse and simultaneously vanishing longitudinal resistance as well as transport along the edge in chiral channels. Some of the discovered fractional quantum Hall states are predicted to host non-Abelian quasi particles that obey exotic exchange statistics with potential use for quantum computation. An essential step towards the manipulation of these particles is to uncover the edge structure of the underlying state and thus verifying the usability of their non-Abelian properties. Recently, a novel method to distinguish between potential candidates for the fractional quantum Hall edge at filling 5/2 has been established using a combination of heat transport and noise arguments. In this thesis, the role of equilibration between counter-propagating edge modes on the heat conductance and the generation of noise at the 5/2 edge is investigated theoretically. This includes an analysis of potential structures describing the 5/2 edge within a common transport scheme and a comparison to experimental results. It is furthermore shown that the heat conductance of the most promising candidate is expected to be quantized to different values of the quantum of heat κ0 = π2kB2 /(3h) depending on the degree of thermal equilibration between the involved modes. Performing experiments with controlled thermal equilibration are therefore predicted to uncover even more details of the underlying structure.
- PostInvestigation of Transmon Qubit Designs - A Study of Plasma Frequency Predictability(2010) Krantz, Philip; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThis master thesis was dedicated to design, fabricate, and characterize subsystems of superconducting transmon qubits. Especially, focus was on investigating the predictability of the plasma frequency of the transmon, from which its operational properties are govern. In turn, the plasma frequency is governed by the normal state resistance of the two Josephson junctions along with its total capacitance. In previous experiments, this frequency predictability relied entirely upon successful simulations of the system. However, even small simulation errors prove to generate severe consequences for the system and a better, more systematic control is desired. In order to identify and minimize this uncertainty, subsystems of the transmon were isolated and investigated. Test structures were fabricated and used to measure the normal state resistance of tunnel junctions with different areas and oxidation parameters using a four-point probe setup. A regime within which the reproducibility is increased was statistically identified. The shunt capacitance, i.e. the largest contribution to the total capacitance, of transmons was extracted by coupling identical capacitors to resonators. The coupling capacitance of a resonator is related to its external quality factor which was measured from analyzing the reflected microwave signal at cryogenic temperatures. The smallest standard deviation of normal state resistances for 110 junction pairs was measured for junctions with an area 2.37 um^2 to be 1.48%, corresponding to an uncertainty contribution of 2 MHz for a designed plasma frequency of 6 GHz. Six resonators were measured throughout the shunt capacitance experiment and a linear t was used to compare the results with a written model based on conformal mapping.
- PostInvestigation of two dimensional material production with spark erosion(2017) Lindgren, Emelie; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThis project examines the possibility to produce material of two dimensions with the electrochemical method of spark erosion. Three transition metal dichalcogenides were examined, namely MoS2, MoSe2 and WS2. Their layered crystal structure and semiconductor behavior make them interesting for nanoelectronic devices. The method of spark erosion could give a cost efficient and large scale production method.Spark erosion has previously shown to be a very effective method to exfoliate nanoparticles. This project investigates the possibility to reduce the dimensions of the produced material and create 2D nanoflakes. The project includes a thorough description of the experiments conducted. Many different fields and methods were examined in order to find reliable results and conclusions. It includes electrophysics and electrochemisty, as well as nanosize characterization. The setup used to support spark erosion have been adjusted and altered to support the production of 2D materials, but it has not been optimized. It was possible to produce microparticles of WS2 but no single layer material was observed. A challenge was to separate the particles from the dielectric liquid of the setup, namely oleic acid. Centrifugation and chemical cleaning was used. To further examine the 2D material production with spark erosion the current and voltage of the setup needs to be tuned more precisely. This will effect the energy which is used to exfoliate the material.
- PostLarge-angle deflection lens using supercell metasurfaces(2024) Vahlin, Kerstin; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Haglund, Åsa; Strandberg, ErikA metasurface is an ultra-thin flat optical component, containing nanostructures that scatter light. Due to the planar structure it can be monolithically integrated with a vertical cavity surface-emitting laser (VCSEL), which removes alignment issues. With metasurfaces, full control of the wavefront is possible and hence a light beam can be shaped in almost anyway. For instance focusing metalenses, lenses that deflect light or vortex generators can be made. In many applications, for example in bio-photonics, it is desirable to have a lens with high numerical aperture, which entails deflection of light with large angels. The conventional way of designing a metasurface is to use phase mapping, however that approach has a fundamental limitation when it comes to creating steep angles. Therefor another type of metasurface based on diffracting elements have been implemented in this project. More specifically, these metasurfaces are made in GaAs to enable integration with GaAs VCSELs and the diffracting elements (supercells) are structured by nano-holes to get the desired light shape and circumvent the issue of aspect ratio dependent etching (ARDE). The metasurfaces were first simulated in COMSOL, then fabricated using electronbeam lithography and dry etching, and finally characterized. Metalenses utilizing supercells that deflect light up to 85◦ have been achieved. The lenses deflect with the desired angle and show a focusing effect. However the focus of the lenses is not as tight as desired and calls for further investigation.
- PostLow Frequency Dispersion in InP HEMTs(2013) Yang, Qiaoran; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe InAlAs/InGaAs/InP HEMT is the state-of-the-art technology for design of ultralow noise amplifiers for radio astronomy and deep space communication. One of the reasons that make this technology very attractive is the outstanding cut-off frequency fT and maximum frequency of oscillation fmax. However, a low frequency dispersion problem has been found for InP HEMTs working at very low temperature. This is a concern for receivers working in cryogenic condition to achieve better noise performance. No study has been published on this phenomenon before. In this thesis, a full study of low frequency dispersion in InP HEMTs is presented. The study is based on different measurement techniques including DC, RF and pulse characterization as well as analyzing spectrum measurements. Moreover, this thesis is focused on the influence of the temperature and device structure on the low frequency dispersion problem. Three mechanisms, traps, odd mode oscillation and impact ionization, have been found closely associated with the observation of low frequency dispersion in InP HEMTs.
- PostNoise Parameter Characterisation of Graphene Field Effect Transistors in the 2-8 GHz Frequency Range(2013) Tanzid, Mehbuba; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceGraphene is promising for being used as a channel material in high frequency and low noise field effect transistors (FETs). This is facilitated by its superior near-room temperature mobility (10^5 cm^2/V-s) for both type of carriers and predicted high value of saturation velocity (4*10^7 cm/s). This thesis presents noise parameter characterisation of graphene field effect transistors (GFETs) using source-pull measurement technique in the 2 to 8 GHz frequency range. Commencing from cleanroom fabrication, all stages of the work including measurement and modelling for device characterisation are dealt with in this study. In the first part of the thesis, a procedure for GFET fabrication utilising CVD graphene is developed. The device properties such as gate leakage, contact resistance, and annealing condition are optimised. The obtained contact resistance is 135 ohm-µm which is state-of-the-art. GFETs fabricated using CVD graphene on silicon dioxide (300 nm)/silicon substrate with 1 µm long and 230 µm wide graphene channels are characterised to obtain the noise performance at device level. The cut-off frequency and the maximum frequency of oscillation of the GFETs are on the order of 10.5 GHz and 13 GHz, respectively. The measured minimum noise figure was 2.4 to 4.9 dB for the extrinsic device with a corresponding associated gain of 10.6 to 2 dB in this frequency range. The intrinsic device has minimum noise figure of 0.8 to 4.3 dB after de-embedding the parasitic noise contribution using noise correlation matrices. Subsequent application of Pospieszalski two-temperature noise model provided a drain noise temperature of 1950 K and a gate noise temperature of 700 K.
- PostParametric frequency conversion in two coupled superconducting resonators(2015) Bengtsson, Andreas; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceIn this work, we have designed, fabricated, and characterized two coupled superconducting microwave resonators. Using this system, we investigate a parametric frequency conversion process in the microwave regime. We modulate the resonance frequency at the difference frequency between two modes, i.e. by three-wave mixing. An incoming signal, resonant with one of the modes, is then converted to the other mode. We demonstrate this behavior in several different schemes, e.g. using one or two modulation tones. We report a conversion efficiency of 50%, meaning that 50% of the incoming wave amplitude gets converted to the other frequency. Finally, we discuss the possibilities to use such a system in a quantum computing architecture.