Electrodeposited Ni/Ge and germanide schottky barriers for nanoelectronics applications

In recent years metal/semiconductor Schottky barriers have found numerous applications in nanoelectronics. The work presented in this thesis focuses on the improvement of a few of the relevant devices using electrodeposition of metal on Ge for Schottky barrier fabrication. This low energy metallisation technique offers numerous advantages over the physical vapour deposition techniques. Electrical characteristics of the grown diodes show a high quality rectifying behaviour with extremely low leakage currents even on highly doped Ge. A non-Arrhenius behaviour of the temperature dependence is observed for the grown Ni/Ge diodes on lowly doped Ge that is explained by a spatial variation of the barrier heights. The inhomogeneity of the barrier hights is explained in line with an intrinsic surface states model for Ge. The understanding of the intrinsic surface states will help to create ohmic contacts for doped n-MOSFETs. NiGe were formed single phase by annealing. Results reveal that by using these high-quality germanide Schottky barriers as the source/drain, the subthreshold leakage currents of a Schottky barrier MOSFET could be minimised, in particular, due to the very low drain/body junction leakage current exhibited by the electrodeposited diodes. The Ni/Ge diodes on highly doped Ge show negative differential conductance at low temperature. This effect is attributed to the intervalley electron transfer in Ge conduction band to a low mobility valley. The results show experimentally that Schottky junctions could be used for hot electron injection in transferred-electron devices. A vertical Co/Ni/Si structure has been fabricated for spin injection and detection in Si. It is shown that the system functions electrically well although no magnetoresistance indicative of spin injection was observed

ANALYSIS OF LASER POWER CONVERTERS IN LASER BASED POWER SUPPLIES

Napajanje elektronskih naprav v ekstremnih in industrijskih okoljih pogosto zahteva uporabo visoko zanesljivih električnih napajalnikov, imunih na raznovrstne okolijske in elektromagnete motenje. Zahtevane specifikacije takšnih napajalnikov je mogoče doseči z uporabo sistemov, ki za izvor energije uporabljajo svetlobo laserskih virov. Energija v obliki monokromatske svetlobe je na oddaljeno mesto vodena skozi električno neprevodni medij, s čimer je dosežena inherentna neobčutljivost takšnih napajalnih sistemov na vse vrste elektromagnetih motenj. Lasersko svetlobo vodimo bodisi brezkontaktno po zraku ali priporočljivejše po električno neprevodnem optičnem vlaknu. V slednjem govorimo o sistemih za prenos »moči po optičnem vlaknu« (ang. Power–over–Fiber systems, PoF). Monokromatsko svetlobo je za napajanje elektronskih naprav potrebno pretvoriti v enosmerno električno energijo, kar storimo s fotonapetostnimi pretvorniki optimiziranimi za pretvorbo monokromatske svetlobe laserskih virov – »pretvorniki laserske moči« (ang. Laser Power Converter, LPC). PoF sistem je zaključen s priključitvijo podpornega elektronskega vezja na izhod pretvornika laserske moči, ki poskrbi za prilagoditev napetostnega nivoja za zanesljivo napajanje elektronskih naprav. PoF sistemi napajanja elektronskih naprav so našli svoje mesto v ekstremnih in industrijskih okoljih zaradi lastnosti kot so: • imunost na elektromagnetne motnje (enosmerna in izmenična električna in magnetna polja, razelektritve ozračja, radiofrekvenčne motnje, …), • velika prebojna trdnost med izvorom energije in napajano napravo, • majhna teža vodnikov energije (optična vlakna), • pri poškodbi vodnikov energije ne prihaja do iskrenja, … Zaradi omenjenih lastnosti so bili PoF sistemi razviti in uporabljeni za napajanje: • senzorjev za merjenje parametrov visokonapetostnih daljnovodov, • elektronskih merilnikov pod vodno gladino, • elektronskih podsklopov naprav za magnetno resonanco, • brezpilotnih letal, • elektronskih implantatov v človeškem telesu, • kontrolnih podsistemov v satelitih, • nadzornih video kamer, • merilnikov obratovalnih parametrov vetrnih turbin, … Kljub uspešni implementaciji PoF sistemov v nekaterih nišnih aplikacijah, je prenos energije z lasersko svetlobo še vedno razmeroma neznana tehnološka rešitev. Razlogov za to je veliko, verjetno pa je eden glavnih nizek izkoristek takšnega prenosa energije, ki se v praksi na sistemski ravni giblje nekje med 10 % in 30 %. Največ vložene energije se izgubi pri pretvorbi elektrike v svetlobo, pri čemer sodobne laserske diode dosegajo izkoristke med 40 % in 70 % ter nadalje pri pretvorbi laserske svetlobe nazaj v elektriko, pri čemer najboljši pretvorniki laserske moči dosegajo učinkovitost pretvorbe med 40 % in 60 %. V večini praktičnih aplikacij izgube pri prvotni pretvorbi energije iz elektrike v svetlobo s sistemskega vidika niso problematične, saj je laser postavljen na mestu, kjer je zagotovljena oskrba s potrebno električno energijo. Večje omejitve predstavljajo približno polovične izgube energije pri pretvorbi laserske svetlobe v električno energijo, preostanek energije pa je še dodatno zmanjšan za 10 % do 20 % zaradi izgub na podporni elektroniki. Tako v praksi izgube na sprejemni strani omejujejo največjo električno moč, ki jo lahko napajani napravi zanesljivo zagotovi en pretvornik laserske moči, na približno 1 W. Takšna omejitev največje dovedene moči ne predstavlja večjih problemov za napajanje nizkoenergijskih senzorjev, vendar omejuje doseg splošne uporabnosti PoF sistemov. V želji po razširitvi uporabnosti PoF sistemov se pričajoča doktorska naloga osredotoča na odkrivanje glavnih izgubnih mehanizmov v pretvornikih laserske moči in podporne elektronike. Rezultati sistematične analize in kvantitativnega ovrednotenja izgub so pripeljali do konceptualnih predlogov za izboljšanje sedanjih pretvornikov laserske moči.Electronic devices in extreme and industrial environments often require specialized power supplies immune to a variety of environmental and electromagnetic interferences. Such requirements can be met with power supplies that use lasers as an energy source. The laser light can be transmitted to a powered electronic device either wirelessly through the air or preferably through electrically nonconductive optical fiber. In the latter case, such power supplies are commonly known as Power–over–Fiber (PoF) systems. Energy in the form of monochromatic light must be transformed into electrical energy to power electronic devices. This energy transformation is achieved with photovoltaic (PV) devices optimized for conversion of monochromatic laser light called Laser Power Converters (LPC). Theoretically possible light-to-electricity conversion efficiency of LPCs is impaired by a variety of optical and electrical losses and light energy that is not converted into electrical energy results in energy loss, which in return reduces PoF systems efficiency. For high system efficiencies, LPCs must be made out of an appropriately selected high-quality III-V semiconductors and currently, the best manufactured LPCs exceed 60% conversion efficiency at strictly controlled laboratory conditions. Even thou such a figure is unheard of for the solar cells, an optimized PV converter illuminated with monochromatic light can theoretically convert more than 75% of impinged light to electricity, under the same conditions as the stated manufactured LPC. In this thesis, the reason for such a discrepancy between theoretical and practical conversion efficiency is studied in details and further, novel supporting electronics for LPCs in PoF systems are devised and analyzed in order to increase the system efficiency

AlGaN/GaN Dual Channel HFETs and Realization of GaN Devices on different substrates

GaN-based HFETs demonstrate ubiquitous high power and high frequency performance and attract tremendous research efforts. Even though significant advances have been achieved, there still exist some critical issues needed to be investigated and solved. In particular, high defect densities due to inhomogeneous growth and operation under high power conditions bring many unique problems which are not so critical in the conventional Si and GaAs materials systems. In order to reduce the defect density and heat dissipation of GaN-based HFETs, research work on the realization of GaN-based HFETs on bulk GaN substrate has been carried out and the key problems have been identified and solved. Hot phonon scattering is the bottleneck which limits the enhancement of electron velocity in the GaN 2DEG channel. It is found that the plasmon-phonon coupling is the mechanism for converting of hot phonons into high group velocity acoustic phonons. In order to push more electrons into the GaN 2DEG channel in the plasmon-phonon coupling regime and to further reduce the hot phonon lifetime, a novel AlGaN/GaN dual channel HFET structure has been proposed. The growth, fabrication and characterization of such a AlGaN/GaN dual channel HFET structure has been carried out. Conventionally GaN-based light emitting diodes and laser diodes are grown and fabricated using the c-plane III-nitride expitaxy layers. In c-plane III-nitride epi-layers, the polarization-induced electric field introduces spatial separation of electron and hole wave functions in quantum wells (QW)s used LEDs and laser diodes LDs and degrades quantum efficiency. As well, blueshift in the emission wavelength becomes inevitable with increasing injection current unless very thin QWs are employed. The use of nonpolar orientations, namely, m-plane or a-plane GaN, would solve this problem. So far, m-plane GaN has been obtained on LiAlO2 (100), m-plane SiC substrates, and m-plane bulk GaN, which all have limited availability and/or high cost. Silicon substrates are very attractive for the growth of GaN due to their high quality, good thermal conductivity, low cost, availability in large size, and ease with which they can be selectively removed before packaging for better light extraction and heat transfer when needed To realize the low cost and improve the internal quantum efficiency of GaN based light emitting diodes, the process for m-plane GaN growth on Si (112) substrates has been studied and optimized. The continuous m-plane GaN film is successfully grown on Si (112) substrates

Extraction of Dzyaloshinksii-Moriya interaction from propagating spin waves validated

The interfacial Dzyaloshinksii-Moriya interaction (iDMI) is of great interest in thin-film magnetism because of its ability to stabilize chiral spin textures. It can be quantified by investigating the frequency non-reciprocity of oppositely propagating spin waves. However, as the iDMI is an interface interaction the relative effect reduces when the films become thicker making quantification more difficult. Here, we utilize all-electrical Propagating Spin Wave Spectroscopy (PSWS) to disentangle multiple contributions to spin wave frequency non-reciprocity to determine the iDMI. This is done by investigating non-reciprocities across a wide range of magnetic layer thicknesses (from 4 to 26 nm) in Pt/Co/Ir, Pt/Co/Pt, and Ir/Co/Pt stacks. We find the expected sign change in the iDMI when inverting the stack order, and a negligible iDMI for the symmetric Pt/Co/Pt. We additionally extract a difference in surface anisotropies and find a large contribution due to the formation of different crystalline phases of the Co, which is corroborated using nuclear magnetic resonance and high-resolution transmission-electron-microscopy measurements. These insights will open up new avenues to investigate, quantify and disentangle the fundamental mechanisms governing the iDMI, and pave a way towards engineered large spin-wave non-reciprocities for magnonic applications.Comment: 12 pages, 2 figure

Modeling and simulation of disordered light management structures in optoelectronic devices

Um die Lichtausbreitung innerhalb optoelektronischer Bauelemente gezielt zu manipulieren greift Lichtmanagement zunehmend auf ungeordnete Strukturen und Materialien zurück. Die quantitative Beschreibung dieser ungeordneten Teilchensysteme wird jedoch maßgeblich durch das Fehlen von Symmetrien erschwert. Hierdurch verlangt insbesondere die Diskrepanz der einzelnen Größenordnungen innerhalb eines Systems Modellierungswerkzeuge mit einem breiten Anwendungsbereich. Um die Streuprobleme in den typischen Dünnschichtsystemen optoelektronischer Bauelemente abzubilden, wird in dieser Arbeit eine Simulationsmethode genutzt, welche die gestreuten elektromagnetischen Felder in Kugelwellen abbildet und mit einem Formalismus für ebene Wellen kombiniert. Im Vergleich zu den etablierten differentiellen Methoden und Integralansätzen profitiert der gewählte Reihenansatz maßgeblich von einer stark reduzierten Anzahl an Unbekannten, erweist sich allerdings im Falle komplexer Streugeometrien bisher als nicht ausreichend flexibel. Bei Streuanordnungen aus nichtkugelförmigen Partikeln erfordert die T-Matrix-Methode beispielsweise einen Mindestabstand zwischen benachbarten Partikeln, um die Mehrfachstreuung richtig auflösen zu können und erweist sich daher ungeeignet für das Modellieren von dichten Partikelanhäufungen. In der Praxis kann die Methode zur optischen Modellierung somit nicht immer ihrem Ziel der Optimierung und Unterstützung der Bauelementeherstellung gerecht werden. In dieser Arbeit wird ein alternatives Verfahren zur Berücksichtigung direkter Wechselwirkungen zwischen nichtkugelförmigen Teilchen vorgestellt. Der Formalismus basiert auf einer zwischenzeitlichen Umwandlung des Translationsoperators für Kugelwellen in ein System ebener Wellen. Hierdurch können die sich ausbreitenden Felder vom evaneszenten Feld getrennt und die direkten Wechselwirkungen zwischen nichtsphärischen, konvexen Partikeln für beliebige Abstände ermittelt werden. Um den Rechenaufwand weiter zu reduzieren, werden periodische Randbedingungen für die T-matrix-Methode auf Basis von Ewald-Summen in das bestehende Modell integriert. Neben der Modellierung streng periodische Systeme kann der Reihenansatz somit ebenfalls auf die Untersuchung großer, periodischer Einheitszellen erweitert werden. Es wird untersucht, inwieweit sich eine weitreichende Periodizität auf die lokale Unordnung innerhalb der Einheitszellen auswirkt und unter welchen Bedingungen solch eine Periodizität geeignet ist um ungeordnete Partikelsysteme zu beschreiben. Die numerischen Herausforderungen der vorgestellten Techniken zur optischen Modellierung ungeordneter Partikelsysteme werden erörtert und anschließend anhand zweier praxisrelevanter Fallbeispiele illustriert. Zunächst wird ein Vergleich zwischen planarisierten Extraktionsschichten mit niedrigem und hohen Brechungsindex zur Auskopplung von Licht aus einer organischen Leuchtdiode für unterschiedliche Dichten der Streutextur gezogen. Anschließend werden poröse Polymere in eine Perowskit-Solarzelle integriert um eine diffuse und breitbandige Reflexion zu ermöglichen, wie sie für die Gebäudeintegration von Photovoltaikanlagen wünschenswert sein kann

Impact of adjacent dielectrics on the high-frequency performance of graphene field-effect transistors

Transistors operating at high frequencies are the basic building blocks of millimeter wave communication and sensor systems. The high velocity and mobility of carriers in graphene can open ways for development of ultra-fast group IV transistors with similar or even better performance than that achieved with III-V based semiconductors. However, the progress of high-speed graphene transistors has been hampered by limitations associated with fabrication, influence of adjacent materials and self-heating effects.This thesis work presents results of the comprehensive analysis of the influence of material imperfections, self-heating and limitations of the charge carrier velocity, imposed by adjacent dielectrics, on the transit frequency, fT, and the maximum frequency of oscillation, fmax, of graphene field-effect transistors (GFETs). The analysis allowed for better understanding and developing a strategy for addressing the limitations.In particular, it was shown that the GFET high-frequency performance can be enhanced by utilizing the gate and substrate dielectric materials with higher optical phonon (OP) energy, allowing for higher saturation velocity and, hence, higher fT and fmax. This approach was experimentally verified by demonstration of enhancement in the fT and fmax in GFETs with graphene channel encapsulated by the Al2O3 layers. As a further step, GFETs on diamond, material with highest OP energy and thermal conductivity, were introduced, developed and fabricated, showing the extrinsic fmax up to 50 GHz, at the gate length of 0.5 \ub5m, which is highest reported so far among the best published graphene and semiconductor counterparts.The main achievements of this thesis work are as follows: (i) comprehensive study of correlations between graphene-dielectric material quality, small-signal equivalent circuit parameters and high-frequency performance of the GFETs; (ii) experimental verification of the concept of improving the GFET high- frequency performance via selection of adjacent dielectric materials with high OP energy; (iii) introducing the diamond as a most promising dielectric material for high-frequency GFETs; (iv) development of technology and demonstration of fully integrated X and Ku band GFET IC amplifiers with state-of-the art performance.In conclusion, the routes of future development depicted in this thesis work may allow for enhancing the high-frequency performance of GFETs up to the level or even higher than that of the modern III-V semiconductor counterparts