Forslag til prosjektoppgaver H2018 Nanoteknologi/elektronikk

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1 Forslag til prosjektoppgaver H2018 Nanoteknologi/elektronikk Simuleringer av topologisk (Berezinskii-Kosterlitz-Thouless) faseovergang i 2D modellsystem basert på superferromagnetiske arrays av tynnfilm nanomagneter 1 student Topologiske faseoverganger vant omfattende oppmerksomhet ved tildelingen av Nobelprisen i fysikk for 2016 til John M. Kosterlitz, David J. Thouless og F. Duncan M. Haldane ( Deres teoretiske oppdagelser har åpnet nye dører for studiet av eksotiske faser/tilstander så som supraledning, supraflytende væske, magnetiske tynnfilmer og kvante-hall-effekt. Mye basert på prisvinnernes arbeider jakter vitenskapen i dag på nye, topologiske materialer/faser med håp om fremtidige anvendelser innen materialteknologi og elektronikk. Det opprinnelige arbeidet til Kosterlitz og Thouless [1] avdekket en ny type faseover-ganger i 2 dimensjoner der virvler (eng. vortices) i et 2D spinnsystem, kjent som XY-modellen, står sentralt. Ved lav temperatur opptrer disse virvlene parvis. Når temperaturen øker vil (ved en gitt kritisk temperatur, TKT) disse parene dekoples og virvlene bevege seg fritt i materialet, noe som påvirker spin-spin korrelasjonslengden, (r). Eksperimentelle studier av den topologiske faseovergangen i XY-modellen basert på 2D magnetiske system er vanskelig å finne, trolig fordi slike virvler i magnetiske tynnfilmer vil ha en ut-av-planet spinnkomponent i sentrum av virvelen, noe som bryter med den strengt planare XYmodellen. I denne oppgaven vil vi benytte mikromekaniske simuleringer (MuMax 3 ) på superferromagnetiske arrays av dipolært koplede monodoméne (tynnfilm) nanomagneter til å søke etter topologiske faseoverganger i XY-modellen. I slike arrays vil det magnetisk moment til de enkelte nanomagnetene representere XY-modellens spinn, uten noen ut-avplanet-orienterte spinn i sentrum av virvlene. Målet med oppgaven er å fastlegge betingelsene (geometri, dimensjoner, materialparametre) for eksperimentell observasjon av KT-overgangen i et slikt array av tynnfilm nanomagneter. For mer om superferromagnetiske arrays, se Slêtjes et al. [2]. For mer om Nobelprisen i fysikk for 2016 og topologiske faseoverganger, se [3]. [1] J.M. Kosterlitz and D.J. Thouless, J.Physics C: Solid State Physics 6 (7), 1181 (1973) [2] S.D. Slöetjes et al., AIP Advances 7, (2017) [3] Veileder: Sam D. Slöetjes, rom A479, e-post: sam.sloetjes@ntnu.no (adr. fram til sommeren: Berkeley, California) Faglærer: Prof. Jostein Grepstad, rom A471, e-post: grepstad@ntnu.no

2 Prosjekt- og masteroppgaver om elektromagnetiske metamaterialer Metamaterialer er kunstige, periodiske strukturer der gitterkonstanten er mye mindre enn en bølgelengde. Dermed vil materialet ideelt se ut som et homogent medium, med en effektiv permeabilitet og permittivitet. Man kan oppnå permeabiliteter og permittiviteter som ikke finnes i naturlige materialer. F.eks. har det blitt demonstrert negative parametre, som igjen gir negativ brytningsindeks. Slike materialer kan brukes til å lage perfekte linser (som kan oppløse detaljer mindre enn en bølgelengde). Man kan også lage mer kompliserte, anisotrope materialer som kan implementere en vilkårlig transformasjon av det elektromagnetiske feltet, slik som den som trengs for å få til en usynlighetskappe. Fordi gitterkonstanten må være mye mindre enn en bølgelengde, er det lettest å lage metamaterialer for bruk innen radio- og mikrobølgeteknologi. Det er en rekke interessante anvendelser her, f.eks. effektive, miniatyriserte antenner. Etter hvert som man kan strukturere materialene på nanonivå, kan metamaterialene også få stor betydning innen fotonikk/optikk. Oppgave: Numerisk homogenisering av metamaterialer Når man skal designe metamaterialer og/eller forstå egenskapene deres, trengs det algoritmer for å beregne den effektive permittiviteten og permeabiliteten. Denne oppgaven går ut på å utvikle en numerisk algoritme på datamaskin for å beregne disse for et vilkårlig metamateriale som funksjon av frekvens og bølgetall. Algoritmen må virke for tilfellet der metamaterialet består av metallstrukturer og dielektriske medier. Oppgaven vil bestå i et første kort litteraturstudium, deretter analytisk utvikling og programmering av den numeriske algoritmen. Oppgave: Grensebetingelser i medier med romlig dispersjon De mest interessante metamaterialene har ofte betydelig romlig dispersjon. Romlig dispersjon betyr at polariseringstettheten og magnetiseringstettheten ikke bare er avhengig av feltene i det aktuelle punktet, men også av feltene i en liten omegn. Dette gjør det vanskelig å formulere grensebetingelser og Fresnel-ligninger for å beregne refleksjon og transmisjon. Oppgaven går ut på å bruke numeriske verktøy, særlig FDTD-metoden (finite-differencetime-domain) til å undersøke området nær en grenseflate. Målet er å komme fram til sammenhengen mellom polariseringstettheten/magnetiserings-tettheten nær grenseflaten og feltene. Dette er aktuelt for å finne modifiserte Fresnels ligninger for refleksjon og transmisjon.

3 Oppgavene passer for studenter som har hatt elektromagnetisme og som liker matematikk og/eller numerikk. Oppgavene vil kunne ende med publisering i internasjonale konferanser eller tidsskrifter. Man kan utføre oppgaven med fjern-veiledning fra Kjeller, eller man kan få kontorplass ved ITS på Kjeller. Veileder: Johannes Skaar, tlf , johannes.skaar@its.uio.no, som selv er meget interessert i disse temaene og vil følge opp studentene i stor grad. Faglærer: Jostein Grepstad, tlf , jostein.grepstad@ntnu.no, Elektroblokk A471

4 SILICON NANOPHOTONIC BIOSENSOR Introduction Photonics is currently one of the most rapidly developing branches of science that explores the properties of light for solving various problems. The list of applications is extensive and the impact it has on our lives is often compared to that of what electronics has achieved since the invention of a transistor. Information processing, ultrafast telecommunication, medical diagnosis, optical displays of various kinds, ultra highprecision sensors are only few examples where photonics has become prominent. With continuously expanding ranges of new applications and materials, this branch of research is believed to transform our societies in the next few decades. Silicon as platform for photonic devices is interesting due to its high light confinement, and because standard lithography and CMOS processes can be used to fabricate micro- and nanoscale components. Because silicon is CMOS compatible, integrated systems on a single semiconductor chip can be realized. Examples of micro- and nanophotonic components are dielectric waveguides, microresonators of various types, light modulators, couplers, periodic structures and more. Especially, a certain type of periodic structures, known as photonic crystals, due to their extraordinary ability of light confinement, possess a truly remarkable potential for applications in both fundamental research and industry. Research in silicon photonics is an ongoing activity at the Department of Electronic Systems. Both theoretical and experimental work is explored. A number of master students and PhD students have been/are designing and processing silicon-based photonic crystal and waveguide structures. Processes and recipes have been developed at the NTNU Nanolab to fabricate these silicon-based structures. Optimization is an ongoing task. The project will be integrated with the Digital Life Norway, Lab-on-a-chip biophotonic sensor platform for diagnostics project funded by the Research Council of Norway ( ). See Goals The aim of this project is to design and simulate a silicon nanophotonic biosensor. To achieve high sensitivity it is likely that the sensor will be ring resonator based or interferometric (e.g. based on a Mach-Zehnder configuration). Both waveguide and photonic crystal structures can be pursued. The component can be processed at the NTNU Nanolab as part of the master s project, or the student can continue to optimize simulations. Part of the project will be to perform a literature study on the state-of-the-art silicon photonic sensors. In particular surface functionalized biosensors will be

5 studied. To make the sensor specific to a target biomolecule, the surface is coated with antibodies via chemically activated functional groups. These surface coated antibodies act as specific capture agents and protein binding to these surface receptors induces a sensor response. The antigen-antibody binding causes a surface refractive index change of the functionalized sensor and the concentration of the binding antigens can be measured. In parallel with the literature study the student will learn the basics of the simulation software. The main goal of the project will be to design the photonic sensor by the aid of numerical simulations using COMSOL Multiphysics, which is based on Finite Element Method (FEM). Other simulation tools may also be utilized such as, MIT Photonic Bandgaps (MPB) to calculate eigenmodes and MIT Electromagnetic Equation Propagation (MEEP), which is finite-difference time-domain (FDTD) based. If time allows during the Fall semester, cleanroom courses will be taken at the Nanolab to qualify the student to fabricate the device in the NTNU Nanolab. It is also possible to take these courses at the beginning of the master s project. Figure 1: Examples of structures simulated using FEM, COMSOL Multiphysics. Supervisors Astrid Aksnes, Prof., room B413, Astrid.Aksnes@ntnu.no, tel Mukesh Yadav, PhD student, room B417, Mukesh.Yadav@ntnu.no

6 Student/master projects in nano-optoelectronics 2018/19 The nano-optoelectronics group at the Department of Electronic Systems (IES) focuses on growth, characterization and processing of III-V semiconductors for use in photonics, solar cell technology and sensor applications. A strong focus is being made on III-V semiconductor nanowires (NWs) grown by molecular beam epitaxy (MBE lab). Fabrication of NW devices is being explored by electron beam lithography as well as by nanoimprint and focused ion beam techniques using NTNU NanoLab. Optoelectronic characterization of individual nanosystems using techniques such as low temperature photocurrent and electroluminescence (Nanophotonics lab) is done in order to understand, improve and utilize quantum size effects for future nanophotonic device applications. In collaboration with Prof. Ton van Helvoort at IFY we also perform detailed structural characterization of the nanowires using the TEM Gemini Center. Normally you will work in close collaboration with a postdoc or a PhD student of the group to achieve your goals in the project. The projects normally have an applied character and can be both theoretical and practical, and some include clean room work in NTNU NanoLab. The projects are most suited for students from the nano-electronics and nanomaterials branch of the Nanotechnology program and the nano-electronics and photonics branch of the Electronics Systems Design and Innovation program, but other students with a strong background in these fields are also welcome. Faculty supervisors for the projects are: Prof. Helge Weman, helge.weman@ntnu.no, Room B415 in Electro. Prof. Bjørn-Ove Fimland, bjorn.fimland@ntnu.no, Room A381 in Electro. Suggested projects: 1. Single III-V semiconductor nanowire Hall bar and field effect devices. (involves work in NanoLab) III-V semiconductor NWs are promising candidate for high efficiency solar cells and light emitting diodes (LEDs). Our group has a long history in developing III-V semiconductor nanowire devices ranging from GaAs-based NW array solar cells to AlGaN-based NW array UV-LEDs. In order to realize such NW array devices, it is very important to understand the opto-electrical properties of each single NW. In addition, we need an accurate control of doping in the NW to take the NW technology to a commercially competitive level. However, the NWs are inherently challenging to characterize because of their small dimensions and complex geometry. Through this project, the student will have a great opportunity to participate in the fabrication and characterization of single NW Hall bar and field effect devices. She/he will start to learn various fabrication processes, especially e-beam lithography (EBL) in NanoLab, and finally carry out opto-electrical measurements of the devices at Nanophotonics lab in IES. Supervisor: Ph.D student: Anjan Mukherjee, anjan.mukherjee@ntnu.no

7 2. InGaN/GaN nanowires on graphene for flexible LEDs (involves work in NanoLab) InGaN/GaN light emitting diodes (LEDs) is primarily dominant for high-quality solidstate lightening in both industry and academia. However, high-quality thin film devices suffer from defects, such as threading dislocations, point defects etc., that severely deteriorate the device efficiency. By introduction of InGaN/GaN nanowires instead of thin film, these LEDs can be defect-free and monolithically integrated with silicon substrate or even 2D materials, e.g. graphene. In this project, the student will use the newly installed molecular beam epitaxy (MBE) system equipped with a nitrogen plasma source to grow InGaN quantum wells inside the GaN nanowires on graphene glass substrates. These structures can then be lifted off and integrated on arbitrary substrates, e.g. plastic films, for flexible LEDs. The student will aid in growing these InGaN/GaNbased heterostructured nanowires, and be involved in the fabrication of the LED devices. In addition, the student will use high-resolution scanning electron microscopy (SEM) with energy dispersive x-ray (EDX) elemental analysis in NTNU Nanolab to characterize the morphology and composition of these nanowires. Supervisor: Postdoc: Dingding Ren, dingding.ren@ntnu.no PhD student: Anjan Mukherjee, anjan.mukherjee@ntnu.no 3. Growth study of AlGaN nanowires on graphene for UV LED applications (involves work in NanoLab) GaN and its alloys (InGaN and AlGaN) have become one of the most important materials in the semiconductor industry, particularly for light emitting applications. Compared to InGaN, which has been well-exploited for visible LEDs, the development of AlGaN based UV LEDs is much more challenging. Such LEDs are in high demand for UV light based sterilization and disinfection. The utilization of nanowire structures significantly enhance the crystal quality of the active AlGaN material, with no dependence on the crystalline nature of underlying substrate, not achievable using conventional thin film structures. Graphene will be used as an epitaxial substrate for the AlGaN nanowire growth and transparent electrode. Such combined AlGaN nanowire/graphene structure could be a key factor in advancing the progress of future UV LEDs. The student will start the project with a nucleation study of GaN and further AlGaN nanowires grown by molecular beam epitaxy on graphene. Different characterization techniques (SEM, XRD, Raman, PL and electrical characterization) will be employed in order to optimize the structural and electrical properties of the grown AlGaN nanowires. Fully processed UV LED structures can be fabricated and studied as the part of a master thesis project in the spring Supervisors: Postdoc: Dingding Ren, dingding.ren@ntnu.no PhD student Anjan Mukherjee, anjan.mukherjee@ntnu.no

8 4. GaAsSb-based superlattice nanowire laser arrays Semiconductor nanowires (NW) have been successfully applied for optoelectronic devices such as lasers and light emitting diodes (LEDs). GaAs NWs with multiple axial GaAsSb/GaAs heterostructured superlattice inserts have been fabricated in the Nanowire group at NTNU and found to be lasing in the near infrared spectral region. However, all experiments conducted so far have been on single NWs, detached from the growth substrate. The next step towards achieving a NW-based laser device is to study as-grown NW laser arrays with in-plane optical excitation and head-on collection of the luminescence signal. The student will work in the Nanophotonics lab at NTNU on developing an optical setup that will enable conducting this experiment. The project involves working with Class 4 lasers and cryogenic liquids (i.e. liquid He), and has a strong focus on nanophotonics and solid state physics. Supervisors: Postdoc: Lyubomir Ahtapodov, lyubomir.ahtapodov@ntnu.no Postdoc: Dingding Ren, dingding.ren@ntnu.no 5. Optical characterization of III-nitride nanostructures. GaN/InGaN and GaN/AlGaN nanowires (NW) and nanopyramids grown on graphene hold great potential for fabricating light emitting devices such as LEDs and lasers with emission wavelengths from the visible all the way down to the UVC spectral region. In this project, the student will conduct micro-photoluminescence spectroscopy (µ-pl) and optical pumping of single nanostructures, as well as as-grown nanostructure arrays. Of particular interest are 1) understanding and optimization of the light emission process in GaN/AlGaN heterostructures, and 2) understanding the waveguiding properties of single GaN/AlGaN nanopyramids. The student will mainly work in the Nanophotonics lab at NTNU. The project involves using Class 4 lasers in order to achieve UV excitation of the samples in the range nm and, in addition, data analysis and numerical simulations using Matlab (or similar) in order to create a theoretical model that builds on the acquired experimental results. The project has a strong focus on optics and spectroscopy of advanced nanomaterials. Supervisors: Postdoc: Lyubomir Ahtapodov, lyubomir.ahtapodov@ntnu.no Postdoc: Dingding Ren, dingding.ren@ntnu.no