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- Post1D Edge Contacts to 2D Material Heterostructures(2021) Karpiak, Bogdan; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Zhao Ternehäll, Huan; Prasad Dash, SarojGraphene has been in the focus of research in many fields of applications due to its unique properties. In particular, the 2D nature, low charge carrier concentration and high mobility of carriers are promising properties for the use in magnetic Hall sensors. At the same time, low spin-orbit coupling and negligible hyperfine interactions make it interesting for spin-polarized electron transport. However, single graphene layer, if unprotected, is prone to defects introduced during fabrication processes and also defects due to interfaces with other insulators or contact materials. These factors would inevitably lead to decrease of graphene device performance. By encapsulating graphene in hexagonal boron nitride (h-BN), another insulating atomically flat twodimensional (2D) material with superior interface properties with graphene, one can fabricate heterostructures for robust and high-performance devices. Utilizing one-dimensional (1D) edge contacts to graphene sheet in such devices based on 2D materials would also allow to minimize contacts-induced degradation of channel properties. The graphene/h-BN heterostructures for studied devices were prepared both by exfoliation from bulk crystals and by transfer of CVD-grown materials over large area. After patterning the 2D heterostructures, 1D edge contacts were fabricated by means of electron or laser beam lithography and electron beam evaporation of metals. In these devices, proof-of-concept for batch fabrication of Hall elements on large area all-CVD h-BN/graphene/h-BN heterostructures is demonstrated. Such 1D edge contacts of ferromagnetic materials to graphene/h-BN heterostructures are also explored for spin injection into graphene in devices with novel design. The findings described in this thesis allow to advance the graphene Hall elements fabrication technology towards large-scale, industry-compatible manufacturing and lay basis for understanding and further optimization of the phenomena that drive and influence the operation of graphene spin-based devices with novel design involving 1D edge contacts.
- PostA 200 GHz Subharmonic Resistive Mixer and an IF Amplifier Based on GFETs(2016) Zhang, Yaxin; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceIn recent years, graphene, a two-dimensional monolayer of carbon atoms, has rapidly attracted great attention in high-speed electronics. The promising property such as the high intrinsic carrier mobility as well as high carrier saturation velocity make graphene a potential candidate for high-speed transistors operating in the millimeter wave and the terahertz frequency ranges. In this thesis, a 200 GHz subharmonic resistive mixer and a microwave amplifier based on graphene FET (GFET) are presented. The mixer is designed to down convert 200 GHz to 1 GHz with LO frequency of 100.5 GHz, and the amplifier is operating at 1 GHz. A large-signal GFET model is set up in a standard circuit simulator for the mixer and amplifier device optimisation as well as circuit-device integrative simulation. The device of the amplifier has a gate length 1 μm and a width 2 x 120 μm, and the mixer GFET is designed as Lg · Wg = 0.5 · 80 μm2. An array of bow-tie structured graphene nanoconstructions is applied in the mixer GFET channel to obtain simultaneously a right impedance level as well as a higher current on-off ratio. Chemical vapor deposition (CVD) method is utilised for graphene preparation, and the mixer and amplifier circuit are realised in coplanar waveguide (CPW) technology on a 100 μm thick high resistive silicon substrate. A planar inductor is applied in the amplifier design for the purpose of input matching as well as circuit’s integration. Metal air-bridges are added in final layout for reducing circuit discontinuities and parasitic mode propagation at the circuit T junction. Full-wave EM simulations are used for the passive circuits design. The first version of integrated receiver circuit including the designed mixer and amplifier is also fabricated. The conversion loss (CL) of the mixer over the RF frequencies from 190 to 210 GHz is measured to be 34 dB ± 3 dB, with the minimum CL of 31.5 dB at 190 GHz and 10 dBm LO pump power. The amplifier power gain is measured to be 6 dB at 1 GHz.
- PostA categorical order theory of pulse scheduling in gate-based quantum computing(2023) Andersson, Axel; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Bylander, Jonas; Dobsicek, MiroslavWe define a preorder on a vector space of complex valued integrable functions on the non-negative real numbers. This preorder is then used to develop a scheduling theory for microwave pulse schedules with an application for quantum computer experiments on superconducting circuits. The scheduling theory is further developed in a categorical framework using a subcategory of Ord, the category of preordered sets and order-preserving mappings between them. The developed theory is then applied to create a Python library which translates IBM OpenPulse schedules to Quantify schedules. Further, this library was then used to conduct single qubit characterisation experiments. Performed experiments include: resonator spectroscopy, two-tone spectroscopy, Rabi oscillation, relaxation time (T1) and qubit state discrimination experiments.
- PostA Class-J Power Amplifier with Varactor Based Dynamic Load Modulation(2014) Hallberg, William; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceIn order to reach increasingly higher data rates and energy requirements in mobile networks, energy efficiency and broadband operation in power amplifiers have been driving parameter in the research of wireless transmitters. This thesis presents the theory for a broadband design of a dynamic load modulated class-J power amplifier. Calculations show that a drain efficiency higher than 70% down to an output power back off of 7.7 dB can be maintained for a fractional bandwidth of 36% by tuning the transistor load reactance during the appropriate operating conditions. The modulation of the transistor load reactance can, for example, be achieved with varactors. The concept is demonstrated in a gallium nitride high electron mobility transistor power amplifier with silicon carbide varactors. The power amplifier achieved a power adde iency over 50% for 1.70 to 1.80 GHz down to 5 dB output power back off, with the maximum output power of 40.4dBm for continuous wave measurements. For modulated signals, the power amplifier showed excellent linearity and high efficiency. For a 3.84MHz 6:6 dB peak to average power ratio W-CDMA signal at 1.75 GHz, the power amplifier achieved an adjacent channel leakage ratio of -48 dBc, an average power added efficiency of 44.9% and an average power of 33.1 dBm. The correlation between theory, simulated results and measured results is discussed to show the potential of the broadband, dynamic load modulated class-J power amplifier concept.
- PostA Fast Wafer-Level Reliability Study of Multi-Time Programmable (MTP) Memory Devices(2011) Kulshreshta, Kopal; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe task of this project was to devise a method for fast wafer level reliability testing of non-volatile memories for process development and qualification, in both siliconon-insulator (SOI) as well as CMOS bulk substrate technologies, with an aim to engineer out the conventional wafer baking step using an oven, thus, reducing the time and cost for reliability testing. This master thesis has been written during my internship in the reliability group of the front-end innovation department at NXP Semiconductors, Nijmegen, The Netherlands. The thesis report is divided into the following chapters. - Chapter 2 provides a basic overview of the types of non volatile memories relevant to this thesis - Chapter 3 is dedicated to multi-time programmable (MTP) memories; the device under study in this thesis project - Chapter 4 concerns the existing design for fast Wafer Level Reliability testing of MTP (fWLR-MTP) memory devices. - Chapter 5 discusses the thermal simulations performed on the existing 3-finger fWLR-MTP test structure using the COMSOL Multiphysics environment. - Chapter 6 focuses on the improvement of the existing fWLR MTP design to meet the project goals i.e. a temperature of 250 oC in the MTP device region. Also, the designs of the layout for the improved designs, which were sent for fabrication, are presented in this chapter. - Chapter 7 presents a summary of the project and also discusses the future impact of it.
- PostA High Efficiency and Wideband Doherty Power Amplifier for 5G(2017) Hünerli, Halil Volkan; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceIn today’s wireless communications, mobile networks need high data rates and low power consumption. For this purpose, novel wideband and energy efficient power amplifiers should be designed. This thesis is concerned with this problem. Doherty Power Amplifiers (DPAs) are popular architectures for obtaining high average efficiency for a large range of output power levels. In this work, a DPA is designed using WIN Semiconductor’s 50μm GaAs pHEMT process and a monolithic microwave integrated circuit (MMIC) layout ready for tape-out fabrication in Ka-band is created. In this thesis, a power amplifier consisting of two stages; a DPA and a pre-amlifier for improved gain, is designed and simulated. Main and auxiliary cells of the DPA are fed through an unequal Wilkinson power splitter. The simulations show that peak power added efficiency (PAE) of 40% and gain > 15 dB is achieved for the 26.5-31.5 GHz band. The PAE levels of 26% at 6 dB back-off and 18% at 9 dB back off is achieved at the center frequency of 29 GHz. Output power is larger than 26 dBm for the defined band. These properties make this design a promising candidate for future 5G applications.
- PostA Novel Method for the In-Situ Mechanical Characterization of Single Living Yeast Cells in an ESEM(2012) Ram, Abilash; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceWater transport is a very important activity in living cells. Normally, water transport in living cells occurs by the process of osmosis. However, there are cases where faster water transport is necessary. Water channel proteins are expressed in cells where faster water transport is required. Aquaporin (AQP) an example of water channel proteins. They are produced by astrocytic cells in the human brain. When a human brain experiences oedema as a result of physical trauma, the increased inter-cranial pressure can in some circumstances result in a coma. Since oedema consists of fluids including water, there is a suspicion that the water channel proteins could be involved. A study of the protein and its effect on the rate of water transport in single cells can lead to the clinical and/or pharmaceutical development of suitable protein inhibitors. This thesis establishes a method for the study of single living yeast cells using an Atomic Force Microscopy sensor as part of a nanometer resolution and precision characterization system inside an Environmental Scanning Electron Microscope (ESEM). First, different preparation methods, for the yeast cell sample, were tested until a specific kind known as the liquid cell culture was identified as being suitable for the purpose of this thesis. Next, experiments for characterization of the AFM sensor were performed followed by the characterization of a single cell. Results show that the AFM sensor works best with the electron beam of the microscope turned off. Single yeast cells were also successfully characterized. There are more cases and effects to investigate, evaluate and overcome before the system is completely suitable for the study of single living yeast cells. However, the system shows promise and can become an easy and systematic procedure for the study of AQP and its effects on the rate of water transport in single living yeast cells.
- PostActive electronically controlled IFF-antenna for L-band(2018) Nilsson, Albin; Schultze, David; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe projects purpose is to design, build and measure a transmitter and receiver module (TRM) and antenna prototype for an IFF/SSR system using Active Electronically Scanned Array (AESA) technique, which is unique in these systems. The project concludes the design, build and measurement of a TRM split in its basic blocks in the form of test circuits. An antenna array with 10+4 active elements are designed, build and measured in an anechoic chamber. The project successfully resulted in a theoretical power to each element of 953W at 1dB compression. The 2nd and 3rd harmonic generated from the system could be kept within the IFF standard limit with the manufactured filter in this project. The antenna constructed using dipoles has an active reflection factor of around 10 dB for the worst element at 0 steering angle, and at 60 steering angle it was 5dB for the worst element and edge frequency. The complete TRM was only in the design stage in this master thesis, the size of the design resulted in the dimensions 250x117mm. In conclusion, the power requirement for 700W per module was met and the power in 1 dB compression were 953W with losses after the power amplifier accounted for. The linear power from the amplifier yielded a power of 800W, also meeting the power requirements. The antenna did not meet the requirement of 10 dB return loss, however, the antenna design in itself resulted in a small and light antenna array which was desirable attributes in this project. The requirement for the size of the complete TRM was 250 mm x 120 mm and this requirement was met in the design stage for this project. There were two different phase shifters in this project and the conclusion was to go for the phase shifter designed with transmission lines instead of the IC circuit. This decision was made because the designed phase shifter had a lower and more even insertion loss. It also had a more predicable phase shift for 180°.
- PostAn Antenna Integrated Low-Noise Receiver for mm-Wave Wideband, High-Datarate Communication(2015) Campion, James; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe continued growth in mobile network data traffic, forecast to increase at a compound rate of 54% annually, has created the need for wireless networks which can handle data rates orders of magnitude greater than is possible with current systems. This growth in data traffic necessitates the use of advanced techniques such as carrier-aggregation and MIMO in order to increase the capacity of wireless networks. In spite of these techniques, current wireless networks are insufficient to handle the predicted future levels of data traffic as their capacity is inherently limited by their narrow bandwidth. This dilemma has led to a surge in interest in the creation of wireless links at carrier frequencies far higher than currently used, where wide swathes of continuous bandwidth are readily available. One such band of frequencies, known as the H-band, lying between 200-325 GHz, has recently been allocated for use in wireless communication links by the Federal Communications Commission in the United States of America. The development of compound semiconductor materials, such as Indium Phosphide (InP), and advances in fabrication and processing techniques over the course of the past decade has enabled the creation of solid-state circuits at such frequencies. To date, many front-end low-noise amplifiers (LNAs) targeting H-band frequencies have reported noise figures of the order of 10 dB, with only moderate values of gain, thereby limiting the potential capacity of receiver systems. In light of this, this thesis presents the design of a mixer-first receiver for use at H-band frequencies which contains no front-end RF amplifier. Instead, the proposed receiver utilises a single-balanced topology consisting of an input RF quadrature hybrid, a single-balanced transconductance mixer, a pair of IF amplifiers and an active IF balun to perform down-conversion and amplification. An antenna is also integrated on chip to provide the input RF signal. The proposed receiver has IF and RF bandwidths of 42 and 138 GHz respectively and requires only 0 dBm of LO drive power to operate. Careful co-design of the transconductance mixer and IF amplifier ensures wideband IF operation of the receiver. The total conversion gain of the receiver is 23 dB and the simulated noise figure is between 13-15 dB over the entire IF bandwidth. The theoretical capacity of the receiver is somewhat greater than previously reported H-band receiver designs as a result of its wide IF bandwidth. The receiver is implemented using the TSC 250 InP double-heterojunction bipolar transistor (DHBT) process from Teledyne Scientific Corporation and consumes an area of 0.9x1.19 mm2. The design of each component of the receiver is presented, as well as a discussion of the trade-offs made in the design of the complete receiver, followed by a characterisation of the complete receiver and discussion of its performance.
- PostAn In-Band OSNR Monitoring Method for Polarization Multiplexed QPSK Signals Using Stokes Parameters(2013) Lundberg, Lars; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe optical signal-to-noise ratio (OSNR) is an important parameter for measuring signal quality in optical communications systems. Due to the recent development of polarization multiplexed systems, Nyquist filtered systems and reconfigurable systems, the traditional optical spectrum analysis method for estimating the OSNR cannot be used. Several other methods have been proposed, with various shortcomings, so there is a need for other methods. In this work, an in-band OSNR monitoring method for polarization multiplexed signals based on a Stokes polarimeter has been investigated through theoretical studies, simulations and measurements. For the measurements, a 90 degree hybrid based polarimeter was constructed and used for measurements on noise loaded 28 GBd DP-QPSK signals. The impact of chromatic dispersion (CD) and polarization-mode dispersion (PMD) on the method was also investigated. The method was successfully used to estimate the OSNR within 1dB for OSNR values up to 25 dB. Through simulations it was shown that the tolerable amount of CD increased if the bandwidth used for the ADC was decreased. In the measurements of this work, a bandwidth of 10 MHz was used, which should tolerate over 1000 km of SMF with D = 17 ps/nm/km according to the simulations. However, the method was shown to be sensitive to PMD, tolerating a differential group delay of less than a tenth of the symbol time. Also developed in this work was a method for compensating for a non-ideal 90 degree hybrid. A provisional patent application has been filed for the method.
- PostAnalysis and improvement of phase noise performance of a PLL-based RF synthesizer(2015) Persson, Björn; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe goal of this thesis work was to analyze, model and improve the phase noise performance of a wideband synthesizer prototype. The analyzed synthesizer is based on a phase-locked loop (PLL) with an active loop filter, where the output frequency range is 2340MHz − 4420MHz in steps of 260MHz. The noise analysis carried out in the thesis places emphasis on the loop filter and proposes a model for a model for simulation of phase noise at the output of the PLL. The model is verified through measurements and a new filter design is proposed. The new filter is designed for reduced thermal noise which reduces the PLL output phase noise. At an output frequency 3120MHz, the simulated phase noise of the synthesizer with the new filter design is −124.4 dBc/Hz at offset frequency 1MHz, which is an improvement of more than 7.5 dB compared to the synthesizer with the original filter design.
- PostAnalysis, Construction and Evaluation of a Radial Power Divider/Combiner(2017) Kläppevik, Ida; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceRadial N-way combiners and dividers are efficient and compact structures for power combining and dividing, which combines or divides the power of N inputs in one single step. The use of this method results in low losses and therefore presents an interesting alternative to often used combiner and divider networks, such as binary tree structures consisting of 2- or 3-way splitters. In this thesis project, the construction of radial N-way cavity combiners and dividers is analysed and an 8-port TM020 cylindrical cavity combiner/divider with SMA-connector input and output ports is designed and fabricated. The characteristics of the fabricated prototype is evaluated and compared to the simulated design. The average return loss at the peripheral ports is higher than 18.4 dB and the return loss at the centre port is higher than 17.6 dB for a 10 % bandwidth around the centre design frequency of 3.1 GHz. The measured insertion loss is lower than 0.16 dB for the same band. The divider and combiner are used in an active test setup with 8 InGaP HBT amplifiers, by which a study of degradation in the event of amplifier failure is performed. Methods to reach a more graceful degradation are discussed. The thesis report also presents a brief overview of different types of N-way combiners and a comparison between the characteristics of some published designs. In conclusion, radial N-way combiners are a very attractive alternative for combining in high power systems due to their low-loss performance and the gradual loss of power in the event of amplifier failure. If they are used together with solid state power amplifiers, they can be used to replace other high power RF amplifier solutions such as the travelling wave tube amplifier.
- 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.
- PostAssessment of Different Epitaxial Transfer Techniques for High Frequency III-V Devices(2010) Tavakoli Dastjerdi, M. Hadi; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe main aim of this work is to transfer the epitaxially grown, indium phosphide based material structure of heterostructure barrier varactor onto silicon substrate. However, these techniques can be also employed for other high frequency and optoelectronics devices such as laser diodes -hybird lasers-, transistors, photodetectors and photonics interconnects. Different wafer bonding techniques were explored and compared. Test diodes were fabricated on the bonded samples and electrical measurements (I-V and CV) were compared to the results with the reference samples on the original indium phosphide substrate. These measurements verified the quality of epitaxial transfer. Also in order to adapt the remaining processing steps to the initial wafer bonding, a study of ohmic contacts was performed to measure the specific contact resistances for alloyed and non-alloyed ohmic contacts.
- PostAtomskaliga beräkningar för adsorption av kloroform på grafenoxid(2014) Gillberg, Christoffer; Hansson, Fredrik; Idh, Sebastian; Lindberg, Benjamin; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceI denna studie har möjligheten att använda grafenoxid för att separera kloroform från bruksvatten undersökts. Kloroform bildas som en toxisk biprodukt av dagens reningsprocesser då man neutraliser patogena mikroorganismer i dricksvatten. Bindningsenergin mellan kloroform och grafenoxid beräknades med en beräkningsmetod som baseras på DFT där vdW-DF används för att approximera utbytes- och korrelationseffekter. Resultaten visar att kloroform binder till grafenoxid med energier som varierar mellan 220-300 meV. Resultaten indikerar att grafenoxid kan användas för att separera kloroform från vatten. Dock påverkas adsorption av fler effekter än bara förmågan för kloroform att binda till grafenoxid. Därför behövs en djupare undersökning, som tar hänsyn till dessa effekter, för att ge en fullständig slutsats.
- PostBallistic Electronic Transport through an Oscillating Barrier in Graphene(2011) Korniyenko, Yevgeniy; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceGraphene is one of the prospective materials for usage in new electronic devices. Its various properties and potential appli- cations are still under large-scale studies. One of the active re- search areas is oriented towards development of a graphene-based transistor which technical characteristics would top the current Si-based designs. This thesis concentrates on a scattering prob- lem in one of such devices driven with static and time-periodic external voltages. Electrical conductance through a four-terminal graphene de- vice is examined from a quantum-mechanical point of view. A high static barrier scattering problem is solved and demonstrated to be in agreement with previous results. Differences between graphene-based and Schro ̈dinger-type systems are highlighted. Time-dependent transport is studied. Formation of energy sidebands and their relation to driving parameters is examined in details. Corrections to the static conductance are obtained for AC perturbation to static potential. Formation of bound states is investigated in the framework of first-order perturbation theory, adiabatic limit is presented. Noise calculation is presented for the static-driving case. Re- sults for zero-frequency noise and Fano factor are obtained and compared to graphene-based systems with different layouts.
- PostBandgap för två lager grafen vid asymmetrisk dopning och pålagt elektriskt fält(2018) Bossér, John; Engström, Arimande; Gulliksson, Martin; Engström, Joakim Hveisel-Ditlevsen; Rosendal, Victor; Tidekrans, Isabel Vrethed; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceSyftet med projektet var att studera algoritmerna bakom täthetsfunktionalteori (DFT) och att använda DFT för att bestämma bandgap och laddningstäthet för tvålagrigt grafen, vilket gjordes med programvaran Quantum ESPRESSO. Många tillämpningar kräver ett bandgap vilket ej finns naturligt i materialet. Syftet med denna studie var därför att introducera det utan att sänka laddningsmobiliteten. Grafenlagren modifierades antingen genom att applicera ett elektriskt fält vinkelrätt över de två lagren med en styrka som varierades mellan 0,51V/nm och 10,28V/nm, eller genom dopning av det ena lagret. Dopningen skedde genom att använda superceller där en kolatom i det övre lagret byttes ut mot en bor- eller kväveatom, vilket är atomer med massa liknande kol men där en elektron har tagits bort eller lagts till. Dopkoncentrationer mellan 0,50% och 5,56% användes i det övre lagret. Vid dopning fann vi att laddningstätheten centrerades runt dopatomen. Laddningstäthetsskillnaden mellan det dopade och det odopade grafenet var i det undre lagret 10% av skillnaden i det övre lagret nära dopatomen. Värden för bandgapet låg runt 0,3 eV. Det var högst vid låg dopkoncentration och minskade därefter när koncentrationen ökades. För det odopade lagret ökade bandgapet mer än för hela systemet. Vidare upptäcktes en förändring av bandstruktur för celler som bestod av 3N ×3N primitiva celler, där N är ett heltal, vilket överensstämmer med tidigare artiklar. När ett elektriskt fält applicerades vinkelrätt över odopat grafen ökade bandgapet med fältstyrkan tills bandgapet nådde ett maximum på 0,3 eV. Trenderna för bandgapen som hittats bygger vidare på kunskap inom området och visar att beteendet för det undre lagrets bandgap skiljer sig från resten av systemet. Att isolera det undre lagrets bandstruktur kan därför övervägas i framtida studier.
- PostBeräkning av bindningsenergier i atomära system med Bi2Te3 och SiO2 – Simuleringar med täthetsfunktionalteori(2012) Arvidsson, Adam; Liljeblad, Alexander; Espegren, Fredrik; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and Nanoscience
- PostBeräkningar av bindningsenergier mellan grafenoxid och vattenföroreningar med Density Functional Theory: En inledande studie för utvärdering av grafenoxid som vattenrenare(2015) Barker, David; Fors, Angelica; Lindgren, Emelie; Olesund, Axel; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceAccess to clean water is a necessity for humans, but despite today's advanced treatment of drinking water there are still many harmful molecules left when the water is to be used. Current water purifying methods are insufficent and thus, there is need for more research in the area. This study therefore examines if graphene oxide could be used to remove perfluorinated substances and trihalomethanes from drinking water. The study has been carried out using quantum mechanical calculations to examine the binding energies between the harmful molecules and graphene oxide. The calculations are based on Density Functional Theory which numerically calculates the energy of an atomic scale system. The study's results are entirely based on calculations done by software designed for that purpose and could be the basis for further research in the area. The results indicate that the absolute value of the binding energies between graphene oxide and the investigated molecules are in the range of 400-1200 meV. These binding energies are similar to the binding energies from other studies where adsorption onto graphene oxide has been investigated, indicating that graphene oxide has the potential to separate the molecules of interest from the water. Significant contribution to the binding energies comes from hydrogen bonding which occurs between the graphene oxide's functional groups and the hydrogen and fluorine atoms of the molecules. To verify that graphene oxide can actually be used as a water purifier it is necessary to perform calculations that include a water environment and to carry out experimental measurements.
- PostBias Circuit for RF Power Amplifiers(2012) Yohannes Worku, Feven; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceBias circuits often affect the instantaneous bandwidth (the bandwidth of the signal that is taken through the PA at a given time) over which a pre-distorter can linearize the transmitter’s output signal. The resulting nonlinearity is difficult to remove completely even by most advanced pre- distortion techniques. In order to keep up with the increasing data rates in telecom industry, there is a desire to increase the mentioned bandwidth. This thesis describes clearly the effects of bias circuit which causes the baseband currents to generate significant variations in the baseband voltages at the transistor terminals. This paper proposes internal decoupling (large capacitance inside the drain package of the transistor) to lessen baseband resonance problem by moving the resonance to much lower frequency with a weaken amplitude. It also proposes the use of snubber circuit to attenuate the weak resonance. Two models of packaged RF power transistor were available. These were identical except that one had internal drain baseband decoupling and the other not. Both were internally pre-matched Si LDMOS transistors designed mainly for class AB operation in the 1.8 GHz band. The transistor with internal decoupling seems to alleviate the baseband impedance variation problem thus making it easier to pre-distort. Simulation result gave a baseband impedance of 0.3Ω at 11 MHz and 11Ω at 194 MHz for the transistor with internal decoupling and for the standard transistor respectively. The measurement results gave 2Ω(manufacturers measured with un-optimized board and it was 16Ω) at 242 MHz and 4Ω at 164 MHz again for the transistor with internal decoupling and for the standard transistor respectively. For RF performance, both transistors were designed for best efficiency and gave 58% for the transistor with internal decoupling and 55% for the standard transistor. Both transistors gave output power more than 100W.