Modeling, production and optimization of a-Si:H multicolordiode for an improved color separation

Gegenstand der verfassten Dissertationsschrift ist die Entwicklung neuartiger, lichtsensitiver und spektralselektiver Photodioden aus amorphem, hydrogenisiertem Silizium sowie amorphen Siliziumlegierungen. Üblicherweise erreicht man Farbselektivität in Bildaufnehmern aus kristallinem Silizium durch Aufbringen optischer Filterschichten auf einen helligkeitsempfindlichen Sensor. Nutzt man die wellenlängenabhängige optische Absorption des amorphen Materials aus und erzeugt zudem einen mehrschichtigen Halbleiterstapel, dessen Ladungsträgerdrifteigenschaften sich für unterschiedliche Schichttiefen unterscheiden, wird eine Feldsteuerung der spektralen Empfindlichkeit ermöglicht. Anhand der vorgestellten Bauelementarchitekturen kann eine beliebige Anzahl ortsaufgelöster Farbsignale in nur einem einzelnen Sensorelement generiert werden. Es wird der Entwicklungsprozess von Helligkeitssensoren über diskret bandabstandsoptimierte Mehrfarbsensoren bis hin zu höchstkomplexen, kontinuierlich bandveränderten, Hyperspektralsensoren beschrieben. Die genaue Kenntnis elektrooptischer Eigenschaften der implementierten Einzelschichten in derartigen Detektoren - diese Dünnschichten sind Absorptions-, Dotierungs- und Kontaktschichten - ist im Entwicklungsprozess unabdingbar. Im Rahmen dieser Arbeit findet eine ausführliche elektrische und optische Charakterisierung einer Vielzahl diskreter Dünnschichten mittels Strom-/Spannungs-, spektraler Empfindlichkeits- und konstanter Photostrommessungen statt. Zudem werden fundamentale Ergebnisse aus optischen Simulationen, insbesondere Absorptions-, Reflexions- und Transmissionsspektren sowie Schichtdickenanalysen, präsentiert und diskutiert. Weiterhin wird das transiente und das kapazitive Verhalten der hergestellten Farbsensoren beleuchtet. Um die Funktion der Sensoren anhand von Transport-, Generations- und Rekombinationsmechanismen von Ladungsträgern im amorphen Silizium physikalisch nachvollziehen zu können, werden zwei Modelle, ein stark vereinfachtes, analytisches Modell und ein komplexerer, numerischer Modellansatz zur Berechnung diverser halbleiterphysikalischer Parameter des Sensorelementes pin-Diode, gegenübergestellt. Besonderes Augenmerk wird in diesem Zusammenhang auf den örtlichen Verlauf des elektrischen Feldes und dem Produkt aus Ladungsträgerbeweglichkeit und -lebensdauer, dem mutau-Produkt, gelegt. Diese Parameter definieren die Driftlänge, die fundamentale Variable zur Steuerung der spektralen Empfindlichkeit. Die vorgestellten Modelle sind zur Beschreibung der spektralen Empfindlichkeit ungeeignet, jedoch tragen sie in großem Maße zum Verständnis der Funktionsweise der entwickelten Bauelemente bei und unterstützen die Interpretation der entstandenen Messergebnisse. Ein selbst entwickeltes Modell zur Beschreibung des Kathodenmaterialeinflusses auf Interferenzerscheinungen in der fallenden Flanke der Spektralantwort stellt ein eigenständiges Kapitel der Arbeit dar. Die Herstellung eines Hyperspektralsensors, dessen Bauelementstruktur sowohl die Detektion des nahezu kompletten sichtbaren Spektralbereiches als auch die Abtastung im nahen UV-Bereich ermöglicht, ist eines der wissenschaftlichen Kernresultate der Dissertation. Ein neuartiges Prozessierungsverfahren, welches eine textit{in situ}-Strukturierung von Dünnschichten im Hochvakuum-Herstellungsprozess ermöglicht, ohne diese mit Flüssigkeiten zu ätzen, wird ebenso präsentiert. Unmittelbar vor Fertigstellung der Arbeit konnte erfolgreich die Integration eines vergrabenen, diskreten a-Si:H-Dünnschichtfilters, welcher gesondert die Grünempfindlichkeit eines Mehrfarbdetektors unterdrückt, demonstriert werden. Nicht zuletzt belegen die vorgestellten Dünnschichtbauelemente, dass diese innovative Sensortechnologie zur Erkennung koloristischer Merkmale erhebliches Potential der Weiterentwicklung aufweist. Ein potentielles Einsatzgebiet der neu entwickelten Sensoren liegt im Bereich der Sicherheitstechnologie, zum Beispiel in der Erkennung von Gefahrstoffen, wie Sprengstoffen oder illegalen Drogen. Ebenso sind Anwendungen in der Medizin- und Umwelttechnik vorstellbar, etwa bei der Klassifizierung von Abfällen oder bei Untersuchungen von Gewebeproben. Mögliche Anwendungsfelder erschließen sich zudem in Bereichen der Photovoltaik, spektrophotometrischer und hyperspektraler Messsysteme sowie in chemischen Analyseverfahren.The object of this PhD thesis is the development of novel light sensitive and spectral selective photodiodes based on amorphous silicon and its alloys. Typically, color selectivity of state of the art crystalline silicon sensors is achieved by stacking additional optical filter arrays on sensors surfaces being just sensitive to light intensity. The wavelength dependent optical absorption in amorphous silicon combined with an adequate $mutau$-engineering of the absorbing multilayer stack, results in a field-adjustable drift-profile. This enables the opportunity to manipulate the spectral response. The sensor architectures developed are capable to generate almost an infinite number of space-resolved color information signals in just one single sensing device. The development process presented ranges from simple light intensity detectors to multicolor sensors with an optimized discrete bandgap structure. At the end of the process, characteristics of high complex and continuous bandgap adjusted hyperspectral sensors are discussed. Detector development requires an exact and well-founded knowledge of electro-/optical characteristics of the thin film layers; these are absorbing-, doping- and contact thin films. In this work, a variety of layers is examined by current-/voltage-, spectral sensitivity- and constant photocurrent measurements. Beneath optical simulations to analyze absorption-, reflection- and transmission spectra, scanning electron microscopy studies were performed to monitor film thicknesses, etching processes and the structural quality of the deposited thin films. An additional focus is put on the transient and capacitive behavior of the devices developed. To understand physical fundamentals like transport-, generation- and recombination processes in an amorphous hydrogenated silicon pin-photosensor, a simple analytical model and a more complex numerical model of a-Si:H devices are compared to calculate essential parameters like current density. Furthermore, those models support the interpretation of the measurement results. Especially, the course of the electrical field and the product of charge carrier mobility and life-time, the mutau-product, give an insight in understanding the functionality of the devices presented. This special transport parameter defines the drift-length, the fundamental value enabling a voltage adjustable spectral response. Those models do not allow a fully characterization of the spectral response curves, but they support the fundamental understanding of the devices functionality and help in interpreting the measurement results. In this work, a model describing the correlation between the cathode material and the interference fringes, occurring in the falling edge of the spectral response, is developed and highlighted in a separate chapter. One of the main research results being presented in this work is the realization of an innovative hyperspectral sensor being not just sensitive in the visible spectral range, but being able to scan optical signals in the near ultraviolet, too. Another highlight presented, is the development of a novel process, enabling an in-situ structuring of thin films in a high vacuum process, without etching the material with a fluid solvent. At the end of this thesis, an innovative sensor structure comprising an internal, discrete a-Si:H filter structure which successfully suppresses the green sensitivity of a multicolor sensor is characterized. All thin film detectors presented, reveal that this kind of innovative sensor technology is predestined to be used in color recognition systems. Therefore, they offer a lot of potential for improvement and redevelopment. Possible sensor applications may exist in fields of civil security technologies, for example in the detection of hazardous materials, like explosives or illegal drugs. Moreover, applications in fields of environmental engineering are imaginable to classify waste or tissue samples. These sensors also may be useful for improving photovoltaic cells, medical diagnostic tools, fluorescence and spectrophotometric measurement systems, chemical analysis tools or colorimetric and hyperspectral imagery systems

Heterojunction Hybrid Devices from Vapor Phase Grown MoS2_{2}

We investigate a vertically-stacked hybrid photodiode consisting of a thin n-type molybdenum disulfide (MoS$_{2}$) layer transferred onto p-type silicon. The fabrication is scalable as the MoS$_{2}$ is grown by a controlled and tunable vapor phase sulfurization process. The obtained large-scale p-n heterojunction diodes exhibit notable photoconductivity which can be tuned by modifying the thickness of the MoS$_{2}$ layer. The diodes have a broad spectral response due to direct and indirect band transitions of the nanoscale MoS$_{2}$. Further, we observe a blue-shift of the spectral response into the visible range. The results are a significant step towards scalable fabrication of vertical devices from two-dimensional materials and constitute a new paradigm for materials engineering.Comment: 23 pages with 4 figures. This article has been published in Scientific Reports. (26 June 2014, doi:10.1038/srep05458

Graphene and two-dimensional materials for optoelectronic applications

This article reviews optoelectronic devices based on graphene and related two-dimensional (2D) materials. The review includes basic considerations of process technology, including demonstrations of 2D heterostructure growth, and comments on the scalability and manufacturability of the growth methods. We then assess the potential of graphene-based transparent conducting electrodes. A major part of the review describes photodetectors based on lateral graphene p-n junctions and Schottky diodes. Finally, the progress in vertical devices made from 2D/3D heterojunctions, as well as all-2D heterostructures is discussed

Contact Resistance Study of Various Metal Electrodes with CVD Graphene

In this study, the contact resistance of various metals to chemical vapour deposited (CVD) monolayer graphene is investigated. Transfer length method (TLM) structures with varying widths and separation between contacts have been fabricated and electrically characterized in ambient air and vacuum condition. Electrical contacts are made with five metals: gold, nickel, nickel/gold, palladium and platinum/gold. The lowest value of 92 {\Omega}{\mu}m is observed for the contact resistance between graphene and gold, extracted from back-gated devices at an applied back-gate bias of -40 V. Measurements carried out under vacuum show larger contact resistance values when compared with measurements carried out in ambient conditions. Post processing annealing at 450{\deg}C for 1 hour in argon-95% / hydrogen-5% atmosphere results in lowering the contact resistance value which is attributed to the enhancement of the adhesion between metal and graphene. The results presented in this work provide an overview for potential contact engineering for high performance graphene-based electronic devices

Optimizing the Optical and Electrical Properties of Graphene Ink Thin Films by Laser-annealing

We demonstrate a facile fabrication technique for graphene-based transparent conductive films. Highly flat and uniform graphene films are obtained through the incorporation of an efficient laser annealing technique with one-time drop casting of high-concentration graphene ink. The resulting thin films are uniform and exhibit a transparency of more than 85% at 550 nm and a sheet resistance of about 30 k{\Omega}/sq. These values constitute an increase of 45% in transparency, a reduction of surface roughness by a factor of four and a decrease of 70% in sheet resistance compared to unannealed films.Comment: 18 pages, 4 figure

Impedance characterization of hydrothermally synthesized nickel zinc ferrite nanoparticles for electronic application

This study comprehensively investigates the structural, morphological, dielectric, and conductivity properties of Ni0.5Zn0.5Fe2O4 nanoparticles synthesized through a hydrothermal method, focusing on their suitability for technological applications. The nanoparticles exhibited a cubic structure with an average grain size of approximately 19 nm. The dielectric properties were analyzed with respect to frequency and temperature, showcasing behaviors consistent with Maxwell-Wagner and Koop's theories. The dielectric plane plots, corresponding to the impedance circuit in the Smith Chart, were found to align with the Davidson-Cole relaxation model. Moreover, the conductivity properties adhered to the Jonscher Power law, resembling conductive properties akin to semiconductors in accordance with the band theory. Notably, the s parameter, indicative of the DC conduction mechanism, displayed temperature-dependent variations, suggesting compatibility with the small polar and correlated hopping barrier conduction models. The thermal activation energies of the Ni0.5Zn0.5Fe2O4 nanoparticles at 102, 103, 104, 105, and 106 rad/s frequencies have been recorded at 0.115, 0.141, 0.157, 0.133 and 0.121 eV, respectively. The experimental results strongly suggest that Ni0.5Zn0.5Fe2O4 nanoparticles hold promise as an inspiring material for electronic circuit applications.Scientific Research Projects Unit of Yildiz Technical University [FBA -2021-4395]The authors thank Scientific Research Projects Unit of Yildiz Technical University, Tuerkiye (Grant/project no: FBA -2021-4395) for financial support of this study