Doctor of Philosophy

dissertationDiffractive optics, an important part of modern optics, involves the control of optical fields by thin microstructured elements via diffraction and interference. Although the basic theoretical understanding of diffractive optics has been known for a long time, many of its applications have not yet been explored. As a result, the field of diffractive optics is old and young at the same time. The interest in diffractive optics originates from the fact that diffractive optical elements are flat and lightweight. This makes their applications into compact optical systems more feasible compared to bulky refractive optics. Although these elements demonstrate excellent diffraction efficiency for monochromatic light, they fail to generate complex intensity profiles under broadband illumination. This is due to the fact that the degrees-of-freedom in these elements are insufficient to overcome their strong chromatic aberration. As a result, despite their so many advantages over refractive optics, their applications are somewhat limited in broadband systems. In this dissertation, a recently developed diffractive optical element, called a polychromat, is demonstrated for several broadband applications. The polychromat is comprised of linear "grooves" or square "pixels" with feature size in the micrometer scale. The grooves or pixels can have multiple height levels. Such grooved or pixelated structures with multilevel topography provide enormous degrees-of-freedom which in turn facilitates generation of complex intensity distributions with high diffraction efficiency under broadband illumination. Furthermore, the super-wavelength feature size and low aspect ratio of this micro-optic make its fabrication process simpler. Also, this diffractive element is not polarization sensitive. As a result, the polychromat holds the potential to be used in numerous technological applications. Throughout this dissertation, the broadband operation of the polychromat is demonstrated in four different areas, namely, photovoltaics, displays, lenses and holograms. Specifically, we have developed a polychromat-photovoltaic system which facilitates better photon-to-electron conversion via spectrum splitting and concentration, a modified liquid crystal display (LCD) that offers higher luminance compared to a standard LCD, a cylindrical lens that demonstrates super-achromatic focusing over the entire visible band, a planar diffractive lens that images over the visible and near-IR spectrum and broadband transmission holograms that project complex full-color images with high efficiency. In each of these applications, a unique figure of merit was defined and the height topography of the polychromat was optimized to maximize the figure of merit. The optimization was achieved with the aid of scalar diffraction theory and a modified version of direct binary search algorithm. Single step grayscale lithography was developed and optimized to fabricate these devices with the smallest possible fabrication errors. Rigorous characterization of these systems demonstrated broadband performance of the polychromat in all of the applications

Doctor of Philosophy

dissertationOptics is an old topic in physical science and engineering. Historically, bulky materials and components were dominantly used to manipulate light. A new hope arrived when Maxwell unveiled the essence of electromagnetic waves in a micro perspective. On the other side, our world recently embraced a revolutionary technology, metasurface, which modifies the properties of matter-interfaces in subwavelength scale. To complete this story, diffractive optic fills right in the gap. It enables ultrathin flat devices without invoking the concept of nanostructured metasurfaces when only scalar diffraction comes into play. This dissertation contributes to developing a new type of digital diffractive optic, called a polychromat. It consists of uniform pixels and multilevel profile in micrometer scale. Essentially, it modulates the phase of a wavefront to generate certain spatial and spectral responses. Firstly, a complete numerical model based on scalar diffraction theory was developed. In order to functionalize the optic, a nonlinear algorithm was then successfully implemented to optimize its topography. The optic can be patterned in transparent dielectric thin film by single-step grayscale lithography and it is replicable for mass production. The microstructures are 3?m wide and no more than 3?m thick, thus do not require slow and expensive nanopatterning techniques, as opposed to metasurfaces. Polychromat is also less demanding in terms of fabrication and scalability. The next theme is focused on demonstrating unprecedented performances of the diffractive optic when applied to address critical issues in modern society. Photovoltaic efficiency can be significantly enhanced using this optic to split and concentrate the solar spectrum. Focusing through a lens is no news, but we transformed our optic into a flat lens that corrects broadband chromatic aberrations. It can also serve as a phase mask for microlithography on oblique and multiplane surfaces. By introducing the powerful tool of computation, we devised two imaging prototypes, replacing the conventional Bayer filter with the diffractive optic. One system increases light sensitivity by 3 times compared to commercial color sensors. The other one renders the monochrome sensor a new function of high-resolution multispectral video-imaging

Modeling, Growth and Characterization of III-V and Dilute Nitride Antimonide Materials and Solar Cells

abstract: III-V multijunction solar cells have demonstrated record efficiencies with the best device currently at 46 % under concentration. Dilute nitride materials such as GaInNAsSb have been identified as a prime choice for the development of high efficiency, monolithic and lattice-matched multijunction solar cells as they can be lattice-matched to both GaAs and Ge substrates. These types of cells have demonstrated efficiencies of 44% for terrestrial concentrators, and with their upright configuration, they are a direct drop-in product for today’s space and concentrator solar panels. The work presented in this dissertation has focused on the development of relatively novel dilute nitride antimonide (GaNAsSb) materials and solar cells using plasma-assisted molecular beam epitaxy, along with the modeling and characterization of single- and multijunction solar cells. Nitrogen-free ternary compounds such as GaInAs and GaAsSb were investigated first in order to understand their structural and optical properties prior to introducing nitrogen. The formation of extended defects and the resulting strain relaxation in these lattice-mismatched materials is investigated through extensive structural characterization. Temperature- and power-dependent photoluminescence revealed an inhomogeneous distribution of Sb in GaAsSb films, leading to carrier localization effects at low temperatures. Tuning of the growth parameters was shown to suppress these Sb-induced localized states. The introduction of nitrogen was then considered and the growth process was optimized to obtain high quality GaNAsSb films lattice-matched to GaAs. Near 1-eV single-junction GaNAsSb solar cells were produced. The best devices used a p-n heterojunction configuration and demonstrated a current density of 20.8 mA/cm2, a fill factor of 64 % and an open-circuit voltage of 0.39 V, corresponding to a bandgap-voltage offset of 0.57 V, comparable with the state-of-the-art for this type of solar cells. Post-growth annealing was found to be essential to improve Voc but was also found to degrade the material quality of the top layers. Alternatives are discussed to improve this process. Unintentional high background doping was identified as the main factor limiting the device performance. The use of Bi-surfactant mediated growth is proposed for the first time for this material system to reduce this background doping and preliminary results are presented.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Electronic Nanodevices

The start of high-volume production of field-effect transistors with a feature size below 100 nm at the end of the 20th century signaled the transition from microelectronics to nanoelectronics. Since then, downscaling in the semiconductor industry has continued until the recent development of sub-10 nm technologies. The new phenomena and issues as well as the technological challenges of the fabrication and manipulation at the nanoscale have spurred an intense theoretical and experimental research activity. New device structures, operating principles, materials, and measurement techniques have emerged, and new approaches to electronic transport and device modeling have become necessary. Examples are the introduction of vertical MOSFETs in addition to the planar ones to enable the multi-gate approach as well as the development of new tunneling, high-electron mobility, and single-electron devices. The search for new materials such as nanowires, nanotubes, and 2D materials for the transistor channel, dielectrics, and interconnects has been part of the process. New electronic devices, often consisting of nanoscale heterojunctions, have been developed for light emission, transmission, and detection in optoelectronic and photonic systems, as well for new chemical, biological, and environmental sensors. This Special Issue focuses on the design, fabrication, modeling, and demonstration of nanodevices for electronic, optoelectronic, and sensing applications

Advanced Functional Antireflection Coatings for Broadband Multijunction Solar Cells

Tähänastisesti korkein aurinkokennolla saavutettu hyötysuhde on tuotettu III‒V puolijohteisiin perustuvilla korkean hyötysuhteen moniliitoskennoilla. Nämä uusiin materiaaleihin pohjautuvat aurinkokennot mahdollistavat yli neljän kennoliitoksen rakenteet, joilla todennäköisesti ylitetään 50 %:n hyötysuhde vuosikymmenen loppuun mennessä. Tällöin koko aurinkokennon pitää olla loppuun asti optimoitu ja valmistettu, jotta vältytään ylimääräisiltä optisilta ja sähköisiltä häviöiltä. Tavoitteen saavuttaminen vaatii soveltuvia laajakaistaisia heijastuksenestopinnoiteita erilaisille moniliitosrakenteille estämään hyödyllisen auringonvalon heijastuminen kennon pinnalta. Tässä työssä on keskitytty kehittämään heijastuksenestopinnoitteita korkean hyötysuhteen moniliitoskennoille. Mitä leveämpi kaista auringonvaloa hyödynnetään, sitä vaikeammaksi tasomaisten pinnoitteiden optimointi käy. Halutun pinnoitteen täytyisi laajakaistaisuuden lisäksi olla yksinkertainen valmistaa toistettavasti erilaisille uusille aurinkokennoratkaisuille. Potentiaalisena ratkaisuna vaatimuksille on käyttää tasomaisten kerrosten lisäksi nanokuvioitua pintaa, jota hyödyntämällä voidaan suhteellisen yksinkertaisesti minimoida heijastus laajalta auringonvalon kaistalta. Työssä tutkittiin pinnoitteiden kerrosmateriaaleina matalan taitekertoimen magnesiumfluoridia ja korkean taitekertoimen tantaalipentoksidia. Näiden lisäksi tutkittiin uuden nanopinnoitusmenetelmän soveltamista monikerroksisiin heijastuksenestopinnoitteisiin. Menetelmässä alumiinioksidikerroksesta muokataan satunnainen nanokuvioitu pinta de-ionisoidussa vesihauteessa. Pinnoitetta käytettiin hilasovitetuille III‒V moniliitoskennoille, sen toimivuutta tarkasteltiin normaaleissa käyttöolosuhteissa, ja nanokuvion säilyvyyttä tutkittiin syklisen jäädyttämisen alaisena. Nanorakenteen kestävyyden parantamiseksi testattiin myös hydrofobisuuskäsittelyä päällystämällä pinnoite ohuella fluoropolymeeri-kerroksella.High efficiency III‒V semiconductor multijunction solar cells hold the record of the highest achieved conversion efficiency. Solar cells based on new materials enabling more than 4-junction architectures will most likely push the highest efficiency above 50% within the next decade. To be able to achieve this goal, every aspect of the solar cell structure needs to be designed and fabricated spot on, minimizing any possible optical and electrical losses. To this end, broadband antireflection coatings are instrumental for suppressing the amount of reflected light from the surface of the solar cell. This work contributes to the development of broadband antireflection coatings for primary use in connection with high efficiency multijunction solar cells. As the bandwidth of the utilized solar irradiation is getting increasingly wider, the antireflection coatings based on standard planar structures become harder to optimize, requiring fabrication of more complex films. On the other hand, there is a need to deploy simple and cost effective fabrication techniques to enable economical deployment of new photovoltaic technologies. This work focuses on developing multilayer antireflection coatings that utilize a nanostructured top layer to surpass the limitations of the conventional planar structures. As a first strand of work, material properties and their relation to the fabrication processes are investigated for low refractive index MgF2 films deposited by electron beam evaporation and the high refractive index Ta2O5 films deposited by ion beam sputtering. The second major part is related to the investigation of a novel technique to fabricate nanostructures with antireflective properties employing a simple de-ionized water treatment. The process is applied to form randomly distributed nanostructures on thin planar amorphous Al2O3 layer. A key result introduced in this work is the novel integration of the alumina nanostructuring with an underlying multilayer antireflection coating, specifically aimed to be used in lattice-matched III‒V semiconductor multijunction solar cells. The performance of the nanostructured coating was assessed in practical III-V multijunction solar cells, revealing its suitability for practical applaiction. Finally, the stability and durability of the nanostructrure has been improved using a hydrophobicity treatment based on fluoropolymerization, and evaluated under atmospheric icing conditions

DEVELOPMENT OF METAL MATRIX COMPOSITE GRIDLINES FOR SPACE PHOTOVOLTAICS

Space vehicles today are primarily powered by multi-junction photovoltaic cells due to their high efficiency and high radiation hardness in the space environment. While multi-junction solar cells provide high efficiency, microcracks develop in the crystalline semiconductor due to a variety of reasons, including: growth defects, film stress due to lattice constant mismatch, and external mechanical stresses introduced during shipping, installation, and operation. These microcracks have the tendency to propagate through the different layers of the semiconductor reaching the metal gridlines of the cell, resulting in electrically isolated areas from the busbar region, ultimately lowering the power output of the cell and potentially reducing the lifetime of the space mission. Pre-launch inspection are often expensive and difficult to perform, in which individual cells and entire modules must be replaced. In many cases, such microcracks are difficult to examine even with a thorough inspection. While repairs are possible pre-launch of the space vehicle, and even to some extent in low-to-earth missions, they are virtually impossible for deep space missions, therefore, efforts to mitigate the effects of these microcracks have substantial impact on the cell performance and overall success of the space mission. In this effort, we have investigated the use of multi-walled carbon nanotubes as mechanical reinforcement to the metal gridlines capable of bridging gaps generated in the underlying semiconductor while providing a redundant electrical conduction pathway. The carbon nanotubes are embedded in a silver matrix to create a metal matrix composite, which are later integrated onto commercial triple-junction solar cells

Lateraalisesti seostetut III-V diodit suuren pinta-alan optoelektroniikkaa varten

Conventional light-emitting diode (LED) designs experience current crowding and efficiency droop that limit their use in high-power applications. These problems could be solved with a recently proposed diffusion-driven current transport (DDCT) concept, that utilizes electron and hole diffusion currents as an injection method. So far, DDCT devices with lateral heterojunctions (LHJ) have been realized with selective-area growth (SAG) and ion implantation of GaN. However, these methods are quite challenging, complicated and expensive. Selective diffusion doping studied in this work could provide a simpler and more cost-effective fabrication technique for realizing lateral DDCT devices based on conventional III-V compound semiconductors. This thesis investigated the performance of laterally doped GaAs/AlGaAs double heterojunction (DHJ) LEDs by simulations, and the selective-area diffusion doping methods needed to realize such devices in practice. First simulations of the LEDs studied the fundamental properties, requirements and limitations of the devices. Diffusion doping of n-type GaAs and GaAs/AlGaAs DHJ substrates from a Zn thin film dopant source, protected by a spin-on glass (SOG) film, was experimentally studied. Additionally, the viability of the developed doping method for realizing the DDCT devices was evaluated. The simulations indicated that high-efficiency LEDs with an internal quantum efficiency of over 99.4 %, with highly uniform radiative recombination within the active region of the LED, can be achieved with high quality materials. Fabricated samples were characterized with current-voltage (IV) and light emission measurements, and visually inspected with a scanning electron microscope (SEM) and an optical profilometer. Vertical LEDs were successfully fabricated on both substrates, although some indications of substrate degradation was observed in the IV-characteristics of the devices.Perinteiset hohtodiodirakenteet ovat alttiita virran ahtautumiselle sekä hyöty suhteen heikkenemiselle, mikä rajoittaa niiden käyttöä suurta tehoa vaativissa sovelluksissa. Mahdollisena ratkaisuna ongelmaan on hiljattain ehdotettu uutta virransyöttömenetelmää (DDCT engl. diffusion-driven current transport), joka käyttää elektronien ja aukkojen diffuusiovirtoja LEDin injektointimenetelmänä. Tähän mennessä DDCT:tä hyödyntävien lateraalisten GaN rakenteiden valmistamiseen on sovellettu selektiivistä epitaksiaa (SAG engl. selective-area growth) sekä ioni-istutusta. Nämä valmistustekniikat ovat kuitenkin hyvin haastavia, monimutkaisia ja kalliita. Diffuusioseostusmenetelmät voisivat tarjota yksinkertaisen ja kustannustehokkaan tekniikan DDCT laitteiden valmistamiseksi perinteisistä III-V yhdistepuolijohteista. Tässä diplomityössä tutkitaan simulaatioiden avulla lateraalisesti seostettujen GaAs/AlGaAs tuplaheteroliitos-LEDien suorituskykyä, sekä kokeellisesti selektiivisiä diffuusioseostusmenetelmiä, joilla simuloitujen laitteiden valmistaminen voisi olla mahdollista. Simulaatiot DDCT rakenteista selvittävät niiden perusominaisuuksia, vaatimuksia sekä rajoituksia. Kokeelliset tutkimukset keskittyvät n-tyyppisten GaAs ja GaAs/AlGaAs substraattien diffuusioseostamiseen sinkkiohutkalvoista, jotka suojattiin spin-on glass (SOG) kalvolla. Lisäksi arvioitiin seostusmenetelmien mahdollista käyttöä DDCT laitteiden valmistukseen. Valmistetut näytteet karakterisoitiin virta-jännite- sekä valoemissio-mittauksilla, pyyhkäisyelektronimikroskoopilla sekä optisella korkeuskarkeusmittarilla. Vertikaalisia LEDejä valmistettiin onnistuneesti molemmille substraateille, mutta joitakin merkkejä substraatin laadun heikkenemisestä havaittiin esimerkiksi IV-mittauksissa. Simulaatiot osoittivat että rakennetta käyttäen voidaan toteuttaa LEDejä, joiden sisäinen kvanttihyötysuhde on yli 99.4 %, ja joiden säteilevän rekombinaation homogeenisuus aktiivisessa alueessa on korkea

High-efficiency photovoltaics through mechanically stacked integration of solar cells based on the InP lattice constant

Solar Energy is a clean and abundant energy source that can help reduce reliance on fossil fuels around which questions still persist about their contribution to climate and long-term availability. Monolithic triple-junction solar cells are currently the state of the art photovoltaic devices with champion cell efficiencies exceeding 40%, but their ultimate efficiency is restricted by the current-matching constraint of series-connected cells. The objective of this thesis was to investigate the use of solar cells with lattice constants equal to InP in order to reduce the constraint of current matching in multi-junction solar cells. This was addressed by two approaches: Firstly, the formation of mechanically stacked solar cells (MSSC) was investigated through the addition of separate connections to individual cells that make up a multi-junction device. An electrical and optical modelling approach identified separately connected InGaAs bottom cells stacked under dual-junction GaAs based top cells as a route to high efficiency. An InGaAs solar cell was fabricated on an InP substrate with a measured 1-Sun conversion efficiency of 9.3%. A comparative study of adhesives found benzocyclobutene to be the most suitable for bonding component cells in a mechanically stacked configuration owing to its higher thermal conductivity and refractive index when compared to other candidate adhesives. A flip-chip process was developed to bond single-junction GaAs and InGaAs cells with a measured 4-terminal MSSC efficiency of 25.2% under 1-Sun conditions. Additionally, a novel InAlAs solar cell was identified, which can be used to provide an alternative to the well established GaAs solar cell. As wide bandgap InAlAs solar cells have not been extensively investigated for use in photovoltaics, single-junction cells were fabricated and their properties relevant to PV operation analysed. Minority carrier diffusion lengths in the micrometre range were extracted, confirming InAlAs as a suitable material for use in III-V solar cells, and a 1-Sun conversion efficiency of 6.6% measured for cells with 800 nm thick absorber layers. Given the cost and small diameter of commercially available InP wafers, InGaAs and InAlAs solar cells were fabricated on alternative substrates, namely GaAs. As a first demonstration the lattice constant of a GaAs substrate was graded to InP using an InxGa1-xAs metamorphic buffer layer onto which cells were grown. This was the first demonstration of an InAlAs solar cell on an alternative substrate and an initial step towards fabricating these cells on Si. The results presented offer a route to developing multi-junction solar cell devices based on the InP lattice parameter, thus extending the range of available bandgaps for high efficiency cells