Universiẗat, Diss--Kassel, 2004.

Chemnitz, Techn. Univ., Diplomarb., 2000.

Techn. Universiẗat, Diss., 2001--Berlin.

Nürnberg, Universiẗat, Diss., 2002--Erlangen.

Terahertz source technology has been an active area of research for a number of years. This has helped develop continuous wave solid-state sources that are highly desirable in a wide range of applications spanning from Earth science to medical science. However, even with advances in terahertz technology, the generation of fundamental source power at these frequencies is still challenging. Promising electronic solid-state devices fall short in overcoming source power shortage due to electronic breakdown mechanism and fabrication limits at terahertz frequencies. The fundamental physical limitation of photonic devices, such as low photon energy, force cryogenic operation which at times is impractical. Schottky diode frequency multipliers often offer a very practical solution for generating continuous wave radiation based on solid-state technology. This harmonic source technology is today a most certain candidate for many applications where compactness and room temperature operation is desired. However, despite of all the advances in Schottky diode fabrication and their use in frequency multiplication, output power falls rapidly with increasing frequency. Thermal constrains, fabrication limits, assembly errors and parasitic losses all constitute changes that affect the performance of these devices and make it difficult to reproduce experimental data. To overcome these problems and progress towards the generation of milliwatts of power at terahertz frequencies, the study of existing methods to generate and handle high power is necessary. In the first part of the thesis, the design, fabrication and development of two Schottky diode-based frequency doublers is discussed. The work focuses on the generation of high-power sources that are capable of handling higher input powers while maintaining good thermal efficiencies. A detailed study into the machining tolerances, assembly errors and temperature effects are evaluated for the frequency doublers. High frequency effect such as velocity saturation is also addressed. Depending on the design frequency and power handling, two different circuit configurations are employed for the frequency doublers. While the high-power 80/160 GHz frequency doubler used a discrete flip-chip diode configuration, the 160/320 GHz frequency doubler employed an integrated diode membrane to mitigate sensitivity issues encountered during assembly and enable correlation between simulated and measured data. The second part proposes the use of millimeter-wave Schottky diode-based radiometers for imaging of composites samples. The focus of this experiment is the introduction of an alternate EM inspection method with the use of broadband Schottky diode components. This technique combines two different fields {--} non-destructive testing and radiometry, which presents a potentially new and interesting area for research. Since no single method can qualify to be the most accurate for all inspections, and with the future integration bringing down manufacturing costs of high frequency components, this demonstration presents a new approach to consider for future material imaging and evaluation experiments.

Dans cette thèse, les diodes Schottky pour des applications en ondes millimétriques et aux fréquences térahertz sont étudiées. Une méthodologie de conception et d'optimisation est proposée pour améliorer la performance de telles diodes. La conception et les simulations sont effectuées à l'aide d'un programme basé sur un modèle analytique. Les différentes méthodes de calcul de la fréquence de coupure de la diode sont définies, étudiées et classifiées selon les applications potentielles. En utilisant un modèle de diode générique et général, une nouvelle approche pour calculer la fréquence de coupure est suggérée pour les applications de mélangeur / multiplicateur. Cette approche permet d'évaluer la tension seuil avec une précision beaucoup plus grande et proche de la réalité. En outre, la conception d’une diode Schottky en tenant compte dès le départ l’application visée (détecteur direct, mélangeur ou multiplicateur) est étudiée. Cette thèse montre que l'ingénierie de la structure épitaxiale a un impact important lorsque l’on utilise une conception de diode basée sur l’application finale comme proposée. Un procédé de microfabrication a été entièrement développé et caractérisé. Une méthode de planarisation unique est introduite pour permettre de connecter la diode par des ponts à air en minimisant les effets parasites. Afin d'éviter une coûteuse lithographie par faisceau électronique, une anode en forme de T est produite en utilisant une technique de photolithographie. Ce procédé est fiable et répétitif, est de faible coût et offre une grande souplesse en matière de conception en plus de répondre au besoin d‘une production de masse, pour laquelle la lithographie par faisceau d’électrons n’est guère possible. Le procédé final nécessite simplement deux étapes de métallisation, nombre minimal possible que nous avons atteint. En raison des exigences de recuit du contact ohmique, il est impossible d’avoir moins de deux étapes de métallisation. Le processus de planarisation proposé repose sur l'utilisation de différents taux de gravure plasma de deux résines couramment utilisées. Pour les travaux réalisés dans cette thèse, une épitaxie GaAs HBT disponible au sein du laboratoire a été utilisée. Les résultats de caractérisation de diodes réalisés dérivés des mesures DC et RF sont rapportés et comparés avec les résultats de la simulation. Les résultats de mesure montrent une réduction significative de la capacité parasite de la diode à moins de 20% de sa capacité totale. Par conséquent, le procédé de conception et de fabrication de ce travail peut fournir des diodes qui peuvent fonctionner au-delà du térahertz avec des dimensions pour l’anode plus grandes que les diodes trouvées dans la littérature et qui peuvent donc être fabriquées uniquement par des techniques de photolithographie optique. / Abstract: In this thesis, Schottky diodes for millimeter waves and terahertz application are scrutinized. A design and optimization methodology is proposed to improve the diode performance. Design and simulations are performed by using an analytical model based code. Diode cut-off frequency calculation methods are studied and classified for different applications. Considering general diode equivalent circuit model, a new approach for calculating the cut-off frequency is suggested for mixer/multiplier applications. This approach provides cut-off much closer to its practical value. Also, the diode design based on its application, direct detector and mixer/multiplier, is studied. It is shown that the epitaxial structure engineering has impact on diode application based design. For diode realization a microfabrication process is developed. Unique planarization method is introduced which provides necessary substruction for the airbridges. In order to avoid expensive e-beam lithography, a T-shaped anode is produced by employing photolithography technique. This process is repeatable, reliable, low cost, gives high flexibility in design terms, and suitable for mass production. The final process merely requires two metallization steps which is minimum possible number due to annealing requirement of ohmic contact. The proposed planarization process is based on using different plasma etching rates of two common resists. In the diode fabrication an available GaAs HBT epitaxial wafer is used. The realized diode characterization results derived from DC and RF measurements are reported and compared with the simulation results. The measurement results showed significant reduction in parasitic capacitance of the diode to under twenty percent of its total capacitance. Therefore, the design and fabrication method of this work can provide diodes to operate over one terahertz with larger anode area (that can be produced by photolithography techniques).

GaInN has the potential to revolutionize the solar cell industry, enabling higher efficiency solar cells with its wide bandgap range spanning the entire solar spectrum. However, material quality issues stemming from the large lattice mismatch between its binary endpoints and questionable range of p-type doping has thus far prevented realization of high efficiency solar cells. Nonetheless, amorphous and multi-crystalline forms of GaInN have been theorized to exhibit a defect-free bandgap, enabling GaInN alloys at any indium composition to be realized. But the range of possible p-type doping has not yet been determined and no device quality material has been demonstrated thus far. Nonetheless, a Schottky barrier design (to bypass the p-type doping issue) on single-crystal GaInN can be used to provide some insight into the future of amorphous and micro-crystalline GaInN Schottky barrier solar cells. Through demonstration of a functional single crystalline GaInN Schottky barrier solar cell and comparison of the results to the best published reports for more conventional p-i-n GaInN solar cells, this work aims to establish the feasibility of amorphous and multi-crystalline GaInN solar cells. / Ph. D.

Im Rahmen der vorliegenden Arbeit wurden Schottky-Dioden mittels Rolle-zu-Rolle-Verfahren hergestellt und charakterisiert. Die Dioden bestanden dabei aus einer Kathode (Aluminium oder Kupfer), die durch Sputtern aufgebracht wurde, einer Halbleiterschicht aus Polytriphenylaminen (PTPA3), die mittels Tiefdruck aufgebracht wurde und einer im Flexodruck hergestellten Anode (PEDOT:PSS, Pani oder Carbon Black). Aus elektrischer Sicht wiesen dabei Dioden mit Kupfer und Carbon Black die besten Eigenschaften auf. Mit Hilfe dieser Elektrodenmaterialien und bei Halbleiterschichtdicken von ca. 200 nm konnten Grenzfrequenzen der Dioden von über 1 MHz realisiert werden. Weiterhin wiesen diese Dioden eine gute Langzeitstabilität sowie eine gute Stabilität gegenüber UV-Licht, Feuchtigkeit und Temperatur auf. / Aim of this work was to demonstrate that Schottky-Diodes can be fabricated by means of Roll-to-Roll-Methods and to characterize these diodes. The diodes consists of a sputtered cathode (Aluminum or Copper), a gravure printed semiconducting layer of Polytriphenylamine (PTPA3) and a flexo printed anode (PEDOT:PSS, Pani, Carbon Black). Best electrical characteristics were obtained with diodes consisting Copper and Carbon Black as electrodes. With a thickness of the semiconducting layer of ~200 nm diodes with a cut-off frequency above 1 MHz could be demonstrated. These diodes showed also a good stability when exposed to UV-light, moisture and temperature.

In this work Scanning Kelvin Probe Microscopy (SKPM) was used to characterize surface states and device surface charging in nitride materials. Samples grown by Molecular Beam Epitaxy (MBE), Metal Organic Chemical Vapor Deposition (MOCVD) and Hydride Vapor Phase Epitaxy (HVPE) typically show a high surface band bending of about 1 eV. In an n-type sample with 3X1017 cm-3 carrier concentration, 1 eV upward band bending corresponds to 1.7X1012 cm-2 trapped charge density in the surface states. Under continuous ultraviolet (UV) illumination up to 0.6 eV surface photo voltage effect could be observed in some samples, which further indicates that surface band bending is very likely larger than 0.6 eV, i.e. close to 1 eV. Reactive Ion Etching (RIE)damage was observed to increase surface band bending by about 0.4 eV where as surface treatments in organic solvents and inorganic acids did not affect surface band bending significantly. These results indicate presence of high density of surface states in devices fabricated in nitride materials. Surface potential measurements immediately after turning off a reverse bias to the Schottky contact of a GaN Schottky diode as well as an AlGaN/GaN Hetero-junction Field Effect Transistor (HFET) show an increase of band bending near the Schottky contact edge. For an applied reverse bias of 4 V, about 0.5 eV increase of band bending was observed. This increase of band bending was caused by tunneling of electrons from the Schottky contact and their subsequent capture by surface states near the contact edge. In case of the HFET, the increase of band bending for a bias that caused no current flow through the device was similar to a bias that did. This showed that hot electron injection from the channel did not play a significant role in increasing surface band bending. The accumulated charge near the gate edge of a HFET can deplete the channel, which would cause the drain current to decrease. The total times of accumulation and dissipation of excess surface charge near the gate edge of the HFET were comparable to the time scales of drain current transients of current collapse and recovery. From this observation we attributed current collapse phenomena to charge accumulation near the edge of the reverse biased gate contact of a HFET.

The aim of this master thesis was to develop a process for fabricating Schottky diodes, using techniques that are suitable for cheap large volume mass production e.g. printing, with tetrapod structured ZnO as the semiconductor. Part of the work involved selecting suitable metals for ohmic and Schottky contact and identification of a binder that can be used for dispersion of the Zinc Oxide (ZnO). ZnO is a II-VI compound semiconductor with a wide band gap (3,4 eV). The Schottky diode is used as a rectifier. A rectifier serves the purpose to turn Alternating Current (AC) to Direct Current (DC). The Schottky diode should only conduct current in the forward direction, in the reverse direction the current should be blocked. In this thesis printed diodes were used to construct different types of rectifiers for example half wave rectifiers and full wave rectifiers. Aside from electrical properties, adhesion properties have also been investigated. Adhesion was showed to depend on not only the choice of binder, but also UV-dose and annealing temperature. Aluminum and silver together with ZnO proved to be the best materials combination with a rectification ratio up to 105−106. Different sizes of Schottky diodes were printed and the smaller diodes with an area of 0,5x0,5mm^2 performed best as a half wave rectifiers while the larger size,1x1mm^2, performed best as a full wave rectifier.