@phdthesis{Hartmann2013, author = {Hartmann, Carsten}, title = {Aluminiumnitrid-Volumenkristallz{\"u}chtung mittels physikalischen Gasphasentransports}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-28992}, school = {BTU Cottbus - Senftenberg}, year = {2013}, abstract = {In der Promotionsarbeit wurde ein Z{\"u}chtungsverfahren zur Herstellung strukturell hochqualitativer AlN-Volumenkristalle mittels PVT-Methode entwickelt. Wesentliche Grundvoraussetzungen daf{\"u}r sind ein thermisch und chemisch stabiles Tiegelmaterial, ein AlN-Quellmaterial mit Sauerstoffverunreinigungen <300 ppm und AlN-Keime mit hoher kristalliner Perfektion. Unter den getesteten potenziellen Tiegelmaterialien (BN, TaC, TaN, NbC, NbN, TaB2, W) zeigten sich TaC und mit Abstrichen W unter AlN-Z{\"u}chtungsbedingungen ausreichend stabil und wurden f{\"u}r die Wachstumsversuche verwendet. Zur effektiven Reduzierung der Sauerstoffverunreinigungen im AlN-Quellmaterial wurde ein karbothermischer Reduktionsprozess entwickelt, welcher eine Restsauerstoffkonzentration im Quellmaterial von <300 ppm gew{\"a}hrleistet. AlN-Keime f{\"u}r die Homoepitaxie von AlN-Volumenkristallen wurden durch heteroepitaktisches Wachstum auf SiC-Substraten und {\"u}ber spontane Nukleation freistehender AlN-Kristalle hergestellt. Beim heteroepitaktischen Wachstum auf SiC zeigte sich eine starke Abh{\"a}ngigkeit von der Substratpolarit{\"a}t. Wachstum auf C-polarem SiC ist mit geringeren {\"A}tzgrubendichten von 5*10^4 - 10^6 cm^(-2) gegen{\"u}ber Si-polarem Wachstum mit {\"A}tzgrubensdichten von 5*10^6 - 10^7 cm^(-2) gekennzeichnet. F{\"u}r beide Substratpolarit{\"a}ten wurden Modelle des Anwachsstadiums entwickelt. AlN-Kristalle mit bis zu 35 mm im Durchmesser wurden gez{\"u}chtet. Die hohe Rissneigung aufgrund von Abk{\"u}hlspannungen und Si-Konzentrationen von mehreren Prozent im gewachsenen AlN-Kristall vermindern aber die Kristallqualit{\"a}t erheblich. Eine sehr hohe kristalline Perfektion kann hingegen durch spontane Nukleation freistehender AlN-Kristalle auf einer Zwischenebene in der Tiegelmitte gew{\"a}hrleistet werden. Bei Nukleationstemperaturen von 2200 °C wurden isometrische Kristalle mit 12*12*14 mm^3 gez{\"u}chtet. Die Kristalle weisen eine zonare Struktur auf, welche durch einen in [000-1]-Richtung gewachsenen Kernbereich mit Versetzungsdichten <10^2 cm^(-2) und einem senkrecht um den Kernbereich gewachsenen Randbereich mit Versetzungsdichten von 10^2 - 10^4 cm^(-2) gekennzeichnet ist. Strukturell hochqualitative (000-1)-Keime wurden aus spontan nukleierten isometrisch gewachsenen AlN-Kristallen pr{\"a}pariert und f{\"u}r die homoepitaktische Volumenkristallz{\"u}chtung verwendet. Unter Zuhilfenahme numerischer Temperaturfeldsimulationen wurde ein angepasster Keimhalter entwickelt, welcher ein leicht konvexes Temperaturfeld am Keim gew{\"a}hrleistet und Parasit{\"a}rwachstum unterdr{\"u}ckt. Somit konnten AlN-Volumenkristalle homoepitaktisch gez{\"u}chtet werden, welche eine Durchmesservergr{\"o}ßerung unter Beibehaltung der hohen strukturellen Qualit{\"a}t der Keimkristalle zeigen. Dieses Verfahren bietet die Grundlage, durch die Z{\"u}chtung mehrerer Kristallgenerationen eine Durchmesseraufweitung auf industriell relevante Gr{\"o}ßen von 1-2" zu erreichen.}, subject = {Transport; Gasphase; Kristallwachstum; Gasphasenz{\"u}chtung; Volumenkristallz{\"u}chtung; Nitrid-Halbleiter; Substrat; Vapour growth; Bulk growth; Nitride semiconductor; Substrate}, language = {de} } @phdthesis{Jia2009, author = {Jia, Guobin}, title = {Characterization of electrical and optical properties of silicon based materials}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-14030}, school = {BTU Cottbus - Senftenberg}, year = {2009}, abstract = {In this work, the electrical and luminescence properties of a series of Si based materials used for photovoltaics, microelectronics and nanoelectronics have been investigated by means of electron beam induced current (EBIC), cathodoluminescence (CL), photoluminescence (PL) and electroluminescence (EL). Photovoltaic Si produced by block casting has been investigated by EBIC on wafers sliced from different parts of the ingot. The impact of selected solar cell processing steps on the material properties has been evaluated by EBIC utilizing adjacent wafers from the ingot. The temperature dependence of dislocations' EBIC contrast was measured to assess the degree of dislocation contamination with impurities, yielding low dislocation contamination for the middle of the block and high contamination in the top and bottom regions. This is in agreement with the impurity distribution in the block. It was found that phosphorus diffusion gettering (PDG) followed by SiN firing greatly reduces the recombination activity of extended defects at room temperature, and improves the bulk property simultaneously. The improvement is attributed to both PDG of metal impurities and a passivation effect of SiN firing. In order to better understand the factors limiting the properties of thin polycrystalline Si layers prepared by the Aluminum induced layer exchange (Alile) technique, epilayers grown on (111) and (100) monocrystalline Si substrates were used as a model system to investigate the impact of processing temperature (Ts) and type of substrate. It was found that no dislocations are formed for epilayers on (100) Si, while a high density of dislocations was detected on epilayers prepared on (111) Si at 450 °C. The dislocation density decreases with increasing TS. The diffusion lengths extracted from the energy dependent EBIC collection efficiencies reveal an improvement of the epilayer quality with increasing TS during growth from 450 °C to 650 °C, and a decrease of the epilayer quality at 700 °C. This is attributed to a reduction of the dislocation density with increasing TS and a formation of precipitates during the process. Precipitate formation of at 700 °C is limited because the metal impurities are very mobile at high TS, resulting in a homogeneous distribution of the impurities. Because the impurities are effective lifetime killers of the minority carriers, so the diffusion length decreases. PL measurements on epilayers grown on Si substrates revealed no characteristic dislocation-related luminescence (DRL) lines at room temperature and 77 K, while intense characteristic DRL lines D1 - D4 have been detected in the sample prepared by the Alile technique. This indicates that dislocations in the Alile sample are relatively clean. The possible reason for the purification of the Alile samples is Al induced gettering during the polycrystalline Si layer growth. The diffusion length in the thin top layer of Si-on-insulator (SOI) samples has been successfully measured by EBIC employing suppression of the surface recombination at the buried oxide layer and at surface of the top layer by biasing. The measured diffusion length is several times larger than the layer thickness. Dislocation networks produced by Si wafer direct bonding have been investigated with regard to their electrical properties by EBIC. The networks were observed to show charge carrier collection and electrical conduction. Inhomogeneities in the charge collection were detected in n- and p-type samples under appropriate beam energy. The EBIC contrast behavior can be understood under the consideration of the positively charged oxide precipitates along with dislocations charged with majority carriers, where the appearance of the contrast in dark or bright depends strongly on the ratio of the collection and the recombination loss of the carriers.The luminescence properties of Si nanostructures (Si nanowires, Si nano rods, porous Si, and Si/SiO2 multi quantum wells (MQWs)) are another important subject of this work. Sub-bandgap infrared (IR) luminescence around 1570 nm has been found in Si nanowires, nano rods and porous Si. PL measurements with samples immersed in different liquid media, for example, in aqueous HF (50\%), concentrated H2SO4 (98\%) and H2O2 established that the sub-bandgap IR luminescence originates from the Si/SiOx interface. Its origin was explained in terms of a simple recombination model through radiative interface states. EL in the sub-bandgap IR range has been observed in simple diodes prepared on porous Si and MQWs at room temperature. The results show the possibility to fabricate an efficient light emitter around 1570 nm wavelength based on the radiative recombination at the Si/Si oxide interface. Based on the knowledge about radiative transitions via the interface states, an improved understanding of luminescence in dislocated samples was proposed.}, subject = {Siliciumverbindungen; Werkstoffkunde; Silicium; Elektrische Eigenschaft; Optische Eigenschaft; Versetzungsnetzwerk; Elektrische Leitf{\"a}higkeit; Diffusionsl{\"a}nge in SOI-Schichten; Solarzellen; Infrarote Lumineszenz im Sub-bandgap-Bereich; Dislocation network; Electrical conductivity; Diffusion length in SOI layer; Solar cells; Sub-bandgap infrared luminescence from Si/SiOx interface}, language = {en} } @phdthesis{Oriwol2014, author = {Oriwol, Daniel}, title = {Die Versetzungsstruktur von multikristallinem Silicium aus der industriellen VGF-Blockkristallisation}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-31553}, school = {BTU Cottbus - Senftenberg}, year = {2014}, abstract = {Die vorliegende Arbeit befasst sich mit der Struktur und der Entwicklung von Versetzungen in multikristallinen Siliciumbl{\"o}cken aus der gerichteten Blockkristallisation. Versetzungen k{\"o}nnen f{\"u}r die Rekombination von Ladungstr{\"a}gern verantwortlich sein und damit den Wirkungsgrad von Solarzellen und -modulen mindern. Die experimentelle Arbeit gliedert sich bez{\"u}glich ihrer Fragenstellung und der Methodenwahl in drei Teile und wird in den Kapiteln 2 bis 4 behandelt. In Kapitel 2 wird die Defektstruktur vieler Siliciumbl{\"o}cke mittels automatisierten Verfahren untersucht. Die Analysemethoden umfassen dabei die Auswertung der Wafertextur und der Infrarot-Durchlichtbilder sowie {\"A}tzgrubendichtez{\"a}hlung (EPD) und Photolumineszenz (PL). Der Betrachtungsabstand ist makroskopisch und die Ergebnisse geben das Verhalten von vielen Versetzungen wieder. Die Versetzungsstruktur in multikristallinem Silicium ist gepr{\"a}gt durch scharf abgegrenzte Bereiche mit sehr hoher Versetzungsdichte. Diese sog. Versetzungscluster bestehen aus einem Netzwerk aus Versetzungs-Pile-ups (Kleinwinkelkorngrenzen) und k{\"o}nnen in sogenannte leichte und dichte Cluster unterschieden werden. Die Versetzungscluster haben im Siliciumblock einen Ausgangspunkt, von dem aus sie sich ausbreiten. Dieser befindet sich haupts{\"a}chlich an Korngrenzen und generiert sich spontan w{\"a}hrend der Erstarrung in der N{\"a}he der Phasengrenze. Das Auftreten von leichten und dichten Clustern ist abh{\"a}ngig von der Kornorientierung parallel zur Wachstumsrichtung. K{\"o}rner mit Orientierungen nahe <111>, <211> und <311> neigen dazu leichte Cluster auszubilden, w{\"a}hrend K{\"o}rner mit Orientierungen um <110>, <331> und <531> eher dichte Cluster ausbilden. Kornorientierungen um <100> und <511> sind unauff{\"a}llig bzgl. der Ausbildung von Versetzungsclustern. Der Zusammenhang liegt in der Orientierung der Gleitebenen begr{\"u}ndet. Kapitel 3 behandelt die strukturelle Untersuchung der Versetzungen mit der Transmissionselektronenmikroskopie (TEM) sowie der Synchrotron- R{\"o}ntgentopographie (WB-XRT). Die Versetzungen formen streng geordnete Pile-ups, welche letztendlich Kleinwinkelkorngrenzen entlang der Wachstumsrichtung ausbilden. Der Versetzungsabstand betr{\"a}gt etwa 30 bis 800 nm, was mit einer Verkippung in der Kristallorientierung von 0,3 bis 0,07 ° korrespondiert. Die Rotation der Kristallorientierung verl{\"a}uft haupts{\"a}chlich um eine Achse parallel zur Wachstumsrichtung. Anhand dieser Beobachtungen wurde ein Modell zur Absch{\"a}tzung der Versetzungsdichte aufgestellt. F{\"u}r die leichten Cluster betr{\"a}gt diese ca. 2*10^5 cm^2 und f{\"u}r die dichten Cluster etwa 3*10^7 cm^2. Die Auswirkungen der Versetzungsstruktur auf die elektrischen Eigenschaften werden in Kapitel 4 behandelt. Mit Electron Beam Induced Current (EBIC) und Dunkel-Lock-In-Thermographie (DLIT) wurde herausgestellt, dass vor allem solche Versetzungen elektrisch aktiv sind, welche sich zu Kleinwinkelkorngrenzen angeordnet haben. Ein niedriger Versetzungsabstand innerhalb der Subkorngrenzen scheint nur eine Bedingung f{\"u}r eine elektrische Aktivierung zu sein. Ein Zusammenhang zwischen dem Betrag der Verkippung einer Subkorngrenze und dem EBIC-Kontrast konnte nicht hergestellt werden. In der abschließenden Diskussion (Kap. 5) wird ein Modell zur Entstehung und Evolution von Versetzungsclustern vorgeschlagen. Die Versetzungscluster generieren sich haupts{\"a}chlich an Korngrenzen. Dabei werden die Kleinwinkelkorngrenzen durch die Restrukturierung von Versetzungen sekund{\"a}r gebildet. Die Ergebnisse legen nahe, dass dies w{\"a}hrend der Erstarrung unmittelbar nach der Phasengrenze geschieht. Die treibenden Kr{\"a}fte sind demnach thermische Spannungen an der Erstarrungsfront. Die Versetzungen und damit die Subkorn-Strukturen erreichen die Phasengrenze und bleiben bei der weiteren Kristallisation erhalten, sodass die kontinuierlich auftretenden thermischen Spannungen mit neuen Versetzungen und damit mit erneuter Bildung von Versetzungen und Subkorngrenzen abgebaut werden m{\"u}ssen. Durch diesen Vererbungseffekt erh{\"o}ht sich die Versetzungsdichte stetig mit steigender Blockh{\"o}he. Im Abschluss werden Maßnahmen zur Reduktion der Versetzungsdichte im Kristallisationsprozess diskutiert. Beim Ankeimen am Tiegelboden oder w{\"a}hrend der Erstarrung sollten geeignete Kornorientierungen bevorzugt werden. Eine weitere Maßnahme ist die Reduktion der radialen thermischen Spannungen w{\"a}hrend der Kristallisation. Unbekannt bleibt die genaue, atomare Ursache der erh{\"o}hten Versetzungsgeneration an Korngrenzen und die damit verbundene spontane Bildung von Versetzungsclustern. Weiterhin bleibt offen, ob und in welchem Umfang Lomer-Cottrell-Versetzungen innerhalb der Subkorngrenzen gebildet werden und welchen Einfluss diese auf die elektrische Aktivit{\"a}t haben.}, subject = {Versetzungen; Multikristallines Silicium; Gerichtete Erstarrung; Synchrotron R{\"o}ntgentopographie; Transmissionselektronenmikroskopie; Dislocation; Multicrystalline silicon; Directional soldification; Synchrotron X-Ray topography; Transmission electron microscopy; Silicium; Polykristall; Versetzung }, language = {de} } @phdthesis{Costache2006, author = {Costache, Florenta}, title = {Dynamics of ultra-short laser pulse interaction with solids at the origin of nanoscale surface modification}, isbn = {978-3-8322-6465-9}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-3674}, school = {BTU Cottbus - Senftenberg}, year = {2006}, abstract = {This thesis addresses fundamental physical processes which take place at the surface region of a target during and after the interaction with ultra-short laser pulses. The general goal is to bring together different phenomena and discuss the non-equilibrium nature of the interaction of femtosecond laser pulses (tp < 100 fs) with various materials, in particular dielectrics and semiconductors. Different experiments, using various techniques, are designed to explore the basic mechanisms of laser ionization, defect creation, electron-lattice energetic transfer, charged particles desorption, optical breakdown, phase transformations and surface morphological changes. Such processes are shown to depend strongly on the laser intensity. Thus, they are analyzed for intensities over four orders of magnitude (10^11-10^14 W/cm2), around the surface optical breakdown (damage) threshold intensity. First, experimental studies using time-of-flight mass spectrometry indicate that non-resonant intense ultra-short laser pulses can efficiently ionize a dielectric (semiconducting) material leading to emission of electrons as well as charged particles, i.e. atomic ions and large clusters, and neutral particles. Under these irradiation conditions, the ionization processes can be at best described by multiphoton ionization and ionization at defects sites. The structural defects provide the means for an increased positive ion desorption rate. A multiple pulse incubation effect in the ion yield can be well related with the reduction of the multi-pulse damage threshold with increasing intensity. Following the initial electron excitation and emission, positive ions are released from the surface in a substantial amount with high ion velocities indicative of a localized microscopic electrostatic expulsion. With increasing intensity, the amount of ions gets larger and larger and their velocity distribution exhibits a bimodal structure. Also, in these conditions, negative ions are detected. The ion desorption can arise from a combination of a localized electrostatic repulsion (macroscopic Coulomb explosion) and a thermal 'explosive' mechanism. The later becomes more important with increasing intensity. The very fast energy input and particle emission result in a transient perturbation and deformation of the target lattice. Using pump-probe experiments the temporal evolution of lattice dynamics can be analyzed upon single-pulse excitation for many different target materials. This deformation is indicated to be a material characteristic. It is associated with the generation of transient defects in dielectrics or fast phase transitions in semiconductors and metals. Therefore, it could well give estimates of lifetime of transient defect states or electron-phonon relaxation times.At last the surface morphology after ablation is analyzed, with emphasis on the laser-induced surface periodic patterns (ripples). The patterns observed appear to be very different from the 'classical' ripples formed after long pulse ablation. They can have periods much smaller than the incident wavelength and are rather insensitive to the variation of the laser wavelength and angle of incidence. We show that control factors are laser beam polarization and the irradiation dose. Additionally, the patterns exhibit features pointing toward a chaotic origin. Their possible formation mechanism is likely linked with the non-equilibrium nature of the interaction.}, subject = {Ultrakurzer Lichtimpuls; Nanostruktur; Oberfl{\"a}chenstruktur; Ultra-kurze Laser Pulse; Flugzeit-Massenspektrometrie; Teilchen-Emission; Ultraschnelle Gitterdynamik; Ripples; Femtosecond laser pulses; Time-of-Flight mass spectrometry; Particle emission kinetics; Ultrafast lattice dynamics; Ripples}, language = {en} } @phdthesis{Arguirov2007, author = {Arguirov, Tzanimir Vladimirov}, title = {Electro-optical properties of dislocations in silicon and their possible application for light emitters}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-5837}, school = {BTU Cottbus - Senftenberg}, year = {2007}, abstract = {This thesis addresses the electro-optical properties of silicon, containing dislocations. The interest in those properties is driven mainly by two practical reasons. One is the optical characterisation of multicrystalline silicon for solar cells, and the other is the design of light emitting diodes based on silicon by enhancement of silicon radiative properties via introduction of dislocations. The work demonstrates that dislocation specific radiation may provide a means for optical diagnostics of solar cell grade silicon. It provides insight into the mechanisms governing the dislocation recombination activity, their radiation, and how are they influenced by other defects present in silicon. We demonstrate that photoluminescence mapping is useful for monitoring the recombination activity in solar cell grade silicon and can be applied for identification of contaminants, based on their photoluminescence signatures. It is shown that the recombination at dislocations is strongly influenced by the presence of metals at the dislocation sites. The dislocation radiation activity correlates with their electrical activity. Thus, photoluminescence mapping at room temperature may provide a means for revealing and characterising of dislocation-rich regions in multicrystalline silicon. It is shown that the dislocation and band-to-band luminescence are essentially anti-correlated. The band-to-band intensity being related to the diffusion length of minority carriers can be used for measurements of diffusion length, as long as the surface recombination rate is controlled. Moreover, photoluminescence mapping can be used for the detection of optically active defects in solar grade materials. Thus, betaFeSi2 precipitates, with a luminescence at 0.8 eV, were detected within the grains of block cast materials. They exhibit a characteristic feature of quantum dots, namely blinking. The second aspect of the thesis concerns the topic of silicon based light emitters for on-chip optical interconnects. The goal is an enhancement of sub-band-gap or band-to-band radiation by controlled formation of dislocation-rich areas in microelectronics-grade silicon as well as understanding of the processes governing such enhancement. For light emitters based on band-to-band emission it is shown, that internal quantum efficiency of nearly 2 \% can be achieved, but the emission is essentially generated in the bulk of the wafer. On the other hand, light emitters utilizing the emission from dislocation-rich areas of a well localized wafer depth were explored. Three different methods for reproducible formation of a dislocation-rich region beneath the wafer surface were investigated and evaluated in view of their room temperature sub-band-gap radiation: (1) silicon implantation and annealing, (2) epitaxially grown SiGe buffer, and (3) direct wafer bonding. The most promising dislocation-based emitter appears the utilization of a dislocation network produced by wafer bonding. It is shown, that monochromatic D1 radiation (wavelength 1.5 µm) can be generated in a well localised depth of the wafer. The radiation is not absorbed in silicon and such localized emitter can, potentially, be coupled with silicon waveguides and Ge-based detectors for optical interconnects.}, subject = {Silicium; Versetzung ; Lumineszenz; Lumineszenz; Versetzungen; Silizium; Luminescence; Dislocations; Silicon}, language = {en} } @phdthesis{Trushin2011, author = {Trushin, Maxim}, title = {Electronic properties of interfaces produced by silicon wafer hydrophilic bonding}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-22841}, school = {BTU Cottbus - Senftenberg}, year = {2011}, abstract = {The thesis presents the results of the investigations of electronic properties and defect states of dislocation networks (DNs) in silicon produced by wafers direct bonding technique. Practical interest for the investigations in this area issued - first of all - from the potential application of such dislocation networks in microelectronics as all-Si light emitter for on-chip interconnection. Besides, dislocation networks may serve as a perfect model object to get new information about the fundamental properties of dislocations and grain boundaries in Si, what is of particular importance for multicrystalline silicon solar cells performance. Despite of a long story of studying of dislocations in silicon, a new insight into the understanding of their very attractive properties was succeeded due to the usage of a new, recently developed silicon wafer direct bonding technique, allowing to create regular dislocation networks with predefined dislocation types and densities. Samples for the investigations were prepared by hydrophilic bonding of p-type Si (100) wafers with same small misorientation tilt angle (~0,5°), but with four different twist misorientation angles Atw (being of <1°, 3°, 6° and 30°, respectively), thus giving rise to the different DN microstructure on every particular sample. The main experimental approach of this work was the measurements of current and capacitance of Schottky diodes prepared on the samples which contained the dislocation network at a depth that allowed one to realize all capabilities of different methods of space charge region spectroscopy (such as CV/IV, DLTS, ITS, etc.). The key tasks for the investigations were specified as the exploration of the DN-related gap states, their variations with gradually increasing twist angle Atw, investigation of the electrical field impact on the carrier emission from the dislocation-related states, as well as the establishing of the correlation between the electrical (DLTS), optical (photoluminescence PL) and structural (TEM) properties of DNs. The most important conclusions drawn from the experimental investigations and theoretical calculations can be formulated as follows: - DLTS measurements have revealed a great difference in the electronic structure of small-angle (SA) and large-angle (LA) bonded interfaces: dominating shallow level and a set of 6-7 deep levels were found in SA-samples with Atw of 1° and 3°, whereas the prevalent deep levels - in LA-samples with Atw of 6° and 30°. The critical twist misorientation angle separating SA- and LA- interfaces was estimated as Atw*≈ 3,5±0,5°, what agrees quiet well with the results of previous PL and TEM investigations. - For the dominating shallow traps in SA-samples (denoted as ST1/ST3 traps) a new phenomenon - that is 'giant Poole-Frenkel effect' of enhanced carrier emission due to dislocations elastic strain field was observed for the first time. Performed theoretical calculations have shown that in the investigated samples such an effect should be ascribed to the row of 60° dislocations rather than to the mesh of screw ones. In this respect, shallow traps ST1/ST3 were identified either with shallow 1D bands (directly or as being coupled with them) or with shallow stacking fault states on splitted 60° dislocation. - From the comparison and correlations of measured DLTS spectra with the results of PL and TEM investigations it was established, that shallow ST1/ST3 traps participate in D1 radiative recombination and that the structural elements, responsible for D1 luminescence of small-angle DNs, are the triple knots (intersections with screw dislocations) along the 60° dislocations. However, the optimal density of 60° dislocations as well as of triple knots, in other words - the optimal tilt and twist misorientation angles for maximal D1 intensity - needs further clarification.}, subject = {Elektronische Eigenschaft; Siliciumbauelement; Bonden; Wafer; Silicium; Versetzungsnetzwerk; Poole-Frenkel Effekt; DLTS; D1 Lumineszenz Band; Dislocation networks; DLTS; Poole-Frenkel effect; D1 Luminescence band}, language = {en} } @phdthesis{Mai2010, author = {Mai, Andreas}, title = {Integration von LDMOS-Transistoren f{\"u}r Hochfrequenzanwendungen in eine 0.13µm-SiGe:C-BiCMOS-Technologie}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-20740}, school = {BTU Cottbus - Senftenberg}, year = {2010}, abstract = {Die vorliegende Arbeit befaßt sich mit der Entwicklung von MOS-Transistoren mit lateral erweitertem Drainanschluss (LDMOS) und deren Integration in eine 0.13µm SiGe:C-BiCMOS-Technologie. In dieser Technologie stehen neben SiGe-Heterobipolartransistoren (HBT) auch komplement{\"a}re MOS Feldeffekttransistoren (MOSFET) f{\"u}r Betriebsspannungen von 1.2V und 3.3V sowie passive Bauelemente, wie z.B. integrierte Kondensatoren, Widerst{\"a}nde und Spulen, zur Verf{\"u}gung. Die 0.13µm-BiCMOS-Technologie verbindet so die Vorteile eines skalierten CMOS-Prozesses, z.B. f{\"u}r Digitalschaltungen mit hohen Rechenleistungen, mit den sehr guten Hochfrequenzeigenschaften der SiGe-HBTs. Damit erm{\"o}glicht sie neuartige Anwendungen im Millimeterwellenbereich, wie z.B. in optischen Netzwerken mit {\"U}bertragungsraten {\"u}ber 100 Gb/s oder in drahtlosen Kommunikationssystemen. Die zus{\"a}tzliche Integration von Hochvolt-MOSFETs, mit der sich diese Arbeit befasst, erm{\"o}glicht eine erweiterte Funktionalit{\"a}t der mittels der BiCMOS-Technologie herstellbaren Schaltkreise. In der Regel verlangt die Herstellung von Hochvolttransistoren in etablierten BiCMOS- oder CMOS-Umgebungen meist zus{\"a}tzlichen Prozessaufwand zu den Standardabl{\"a}ufen. Ein wesentliches Ziel dieser Arbeit war die Entwicklung von Integrationskonzepten die diesen technologischen Mehraufwand minimieren. Im ersten Teil dieser Arbeit wird ein Integrationskonzept entwickelt, welches die Herstellung komplement{\"a}rer LDMOS-Transistoren erlaubt, d.h. LDMOS-Transistoren mit n-Kanal (NLDMOS) als auch p-Kanal (PLDMOS), und einen zus{\"a}tzlichen Maskenschritt pro Transistortyp im Vergleich zum Basisprozess erfordert. Dabei wird speziell die Driftregion des NLDMOS-Transistors und insbesondere der Einfluss eines innerhalb der n-dotierten Driftregion realisierten p-dotierten Gebietes untersucht. Im zweiten Teil dieser Arbeit wird ein neuartiges Prinzip zur Realisierung von Hochvolttransistoren ohne zus{\"a}tzlichen Prozessaufwand vorgestellt. Dabei wird die schwach dotierte Driftregion durch eine spezielle Kombination von Implantationen des Basis-CMOS-Prozesses hergestellt. Das vorgestellte Konzept wird so optimiert, dass es auch die Realisierung von komplement{\"a}ren Hochvolttransistoren erlaubt. Trotz des minimalen bzw. keines technologischen Mehraufwandes bei der Realisierung der komplement{\"a}ren Transistoren erzielen die Bauelemente Spitzenwerte in den Grenzfrequenzen bei den entsprechenden maximalen Betriebs- und Durchbruchspannungen f{\"u}r Si-basierte LDMOS-Transistoren.}, subject = {CMOS-Schaltung; Digitale integrierte Schaltung; SiGe-BICMOS; LDMOS-Transistoren; High-voltage; Si-LDMOS; Radio frequency; SiGe-BiCMOS}, language = {de} } @phdthesis{Krause2015, author = {Krause, Christoph}, title = {Investigation of particular crystal defects in solar silicon materials using electron beam techniques}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-34846}, school = {BTU Cottbus - Senftenberg}, year = {2015}, abstract = {The aim of this work is to describe and explain the properties of defects in multicrystalline (mc) and thin-film solar silicon (Si). For this reason, investigations with scanning electron microscope methods were performed, namely cathodoluminescence (CL), electron beam induced current (EBIC), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Additionally, photoluminescence (PL) and reverse-biased electro luminescence (ReBEL) measurements were also conducted. Through correlation of PL, ReBEL and EBIC, it was possible to localize breakdown sites at mc-Si solar cells. Problems that occurred during the thin-film EBIC investigations could be demonstrated and explained. For the first time cross sectional EBIC investigations could be performed on thin-film silicon tandem cells. At mc-Si, it was possible to observe the oxygen related P-line next to the common D1-line luminescence at 10 K clearly distinguishable from each other at once. Furthermore, a hitherto not comprehensively discussed intense luminescence line at 0.93 eV could be described in detail. Through correlation of PL, CL, EBIC, EBSD, and TEM measurements, the origin of the now named Di luminescence at 0.93 eV is postulated to be in connection with Frank partial dislocations, with two energetic levels inside the band gap, one at 112±9 meV below the conduction band and the other at 93±10 meV above the valence band. Finally, it was attempted to explain the behavior of twin boundaries at temperatures below 30 K, where these show an enhanced collection efficiency in comparison to the surrounding grains. An alteration of the local "freeze out" temperature, possibly by a local band gap narrowing, is suggested as a reason. Another conceivable explanation is a breakdown of the diode potential at the grains.}, subject = {Defects; Silicon; EBIC; CL; Di luminescence; Defekte; Silizium; EBIC; CL; Di Lumineszenz; Solarzelle; Silicium; Zuverl{\"a}ssigkeit}, language = {en} } @phdthesis{Ohlerich2009, author = {Ohlerich, Martin}, title = {Investigations of the physics potential and detector development for the ILC}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-14409}, school = {BTU Cottbus - Senftenberg}, year = {2009}, abstract = {The International Linear Collider offers a lot of different interesting challenges concerning the physics of elementary particles as well as the development of accelerator and detector technologies. In this thesis, we investigate two rather separate topics - the precision measurement of the Higgs boson mass and of its coupling to the neutral gauge boson Z and the research and development of sensors for BeamCal, which is a sub-detector system of the ILC detector. After the Higgs boson has been found, it is important to determine its properties with high precision. We employ the Higgs-strahlung process for this purpose. A virtual Z boson is created in the electron-positron collisions, which emits a Higgs-boson while becoming on-shell. Using the so-called recoil technique, we determine the Higgs boson mass by reconstructing the Z boson momentum and using the center-of-mass energy of the colliding leptons. This technique allows to measure the Higgs boson mass without considering the Higgs boson decay, i.e. it can be applied even to a Higgs boson invisibly decaying. Monte-Carlo studies including a full detector simulation and a full event reconstruction were performed to simulate the impact of a realistic detector model on the precision of the Higgs boson mass and production cross-section measurement. Also, an analytical estimate of the influence of a given detector performance on the Higgs boson mass measurement uncertainty is given. We included a complete sample of background events predicted by the Standard Model, which may have a detector response similar to the signal events. A probabilistic method is used for the signal-background separation. Several other probabilistic methods were used to investigate and improve the measurement of the Higgs-strahlung cross-section and the Higgs boson mass from the recoil mass spectrum obtained after the signal-background separation. For a Higgs boson mass of 120 GeV, a center-of-mass energy of 250 GeV and an integrated luminosity of 50/fb, a relative uncertainty of 10\% is obtained for the cross-section measurement, and a precision of 118 MeV for the Higgs boson mass. The original motivation to use the recoil technique for a Higgs boson mass measurement independent on its decay modes could not be completely confirmed. For a Higgs boson mass of 180 GeV and 350 GeV, a statistics corresponding to 50/fb is not sufficient to achieve the necessary significance of the recoil mass peak above the background. The BeamCal is a calorimeter in the very forward region, about 3 m away from the nominal interaction point and surrounding the beam pipe. Due to its location, a lot of beamstrahlung pair particles will hit this calorimeter, representing a challenge for the operational reliability of the sensors under such harsh radiation conditions. We investigated single-crystal and polycrystalline CVD diamond, gallium arsenide and radiation-hard silicon as sensor candidates for their radiation hardness and found that diamond and gallium arsenide are promising. We used a 10 MeV electron beam of few nA to irradiate the samples under investigation up to doses of 5 MGy for diamond, up to about 1.5 MGy for gallium arsenide and up to about 90 kGy for silicon. We measured in regular periods the CCD to characterize the impact of the absorbed dose on the size of the signal, which is generated by electrons of a Sr-90 source crossing the sensor. Additional measurements such as the dark current and the CCD as functions of the voltage completed the characterization of the sensor candidates. For the single-crystal CVD diamond, also the thermally stimulated current was measured to determine amongst others the defect density created by irradiation. In the diamond samples, evidence for strong polarization effects inside the material was found and investigated in more detail. A phenomenological model based on semi-conductor physics was developed to describe the sensor properties as a function of the applied electric field, the dose and the dose rate. Its predictions were compared with the results of the measurements. Several parameters such as time scales and cross-sections were determined using this model, which led to ongoing investigations.}, subject = {Detektor; Speicherring; Higgs-Teilchen; Higgs-Bosonmasse; Detektor; Detektorentwicklung; ILC; Higgs boson mass; Detector development; ILC}, language = {en} } @phdthesis{Ratzke2012, author = {Ratzke, Markus}, title = {Kraftmikroskopie als Werkzeug zur Charakterisierung elektrischer Oberfl{\"a}cheneigenschaften}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-24440}, school = {BTU Cottbus - Senftenberg}, year = {2012}, abstract = {Die stetig fortschreitende Miniaturisierung in der Halbleiterindustrie macht es notwendig, Oberfl{\"a}chenparameter mit Aufl{\"o}sung im Nanometerbereich zu messen und auch abzubilden. Von gr{\"o}ßtem Interesse sind hierbei das Oberfl{\"a}chenpotential und die Kapazit{\"a}t der oberfl{\"a}chennahen Bereiche, da diese Aussagen {\"u}ber die elektronische Struktur erlauben. Hierbei muss großes Augenmerk auf die M{\"o}glichkeit der zerst{\"o}rungsfreien und pr{\"a}parationsarmen Messung gelegt werden, da jede Behandlung der zu untersuchenden Materialien deren Oberfl{\"a}cheneigenschaften {\"a}ndert. Im Rahmen dieser Arbeit wurden auf der Atomkraftmikroskopie basierende Methoden sowohl experimentell als auch mit Hilfe von Simulationen auf ihre Anwendbarkeit f{\"u}r die Untersuchung von Halbleiteroberfl{\"a}chen evaluiert. Es stellt sich heraus, dass die kontaktfreien Methoden „Scanning Kelvin Probe Microscopy" und „Scanning Capacitance Microscopy" sehr gut geeignet sind, um die elektronische Struktur der Probenoberfl{\"a}che qualitativ zu beurteilen. Allerdings muss f{\"u}r quantitative Aussagen ein recht großer rechentechnischer Aufwand betrieben werden.}, subject = {Kraftmikroskopie; Halbleiteroberfl{\"a}che; Atomkraftmikroskopie; Silizium; Scanning Kelvin Probe Microscopy; Scanning Capacitance Microscopy; Atomic force microscopy; Silicon; Scanning Kelvin Probe Microscopy; Scanning Capacitance Microscopy}, language = {de} } @phdthesis{Mankovics2015, author = {Mankovics, Daniel}, title = {Luminescence investigation of bulk solar silicon and silicon thin films on glass substrate}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-35196}, school = {BTU Cottbus - Senftenberg}, year = {2015}, abstract = {The aim of this work is to study the optical properties of crystal defects in multicrystalline solar silicon and poly-/microcrystalline silicon thin films on glass substrate. First a setup for photoluminescence imaging on multicrystalline silicon solar wafers was developed. This system is suitable for detecting band-to-band luminescence as well as defect-related luminescence at room temperature on large-scale wafers at different stages of their processing. Spectroscopic photoluminescence investigations of multicrystalline silicon solar wafers indicated a new intense luminescence line at ≈ 0.91 eV at room temperature. The origin of this line is probably found in a specific grain boundary. Furthermore, luminescence in the region of 0.8 eV was investigated in detail, and it was found that probably oxygen is responsible for a peak at 0.77 eV at 80 K. Electroluminescence investigations at room temperature at both materials exhibit extended defect structures such as grain boundaries. Furthermore, it can be concluded that electroluminescence imaging in reverse bias mode indicate on serious breakdown points in solar cells, which can lead to destruction of solar cells and modules. By comparing defect-related and reverse bias electroluminescence images, a difference in the spatial distribution of defects emitting D1 radiation and defects emitting light under reverse bias beyond -12 V is detectable. In addition, there seems to be a correlation in the distribution of non-doping impurities and photoluminescence. Concerning this, vertical slabs of two silicon blocks were examined by means of Fourier-transform infrared spectroscopy and photoluminescence. A correlation of the distributions of interstitial oxygen and the band-to-band luminescence profiles could be found. Additionally, a correlation between D3/D4 luminescence profile and nitrogen distribution in the blocks was observed. Finally, the growth process, particularly the transition from amorphous to microcrystalline silicon by PECVD, was studied by combined photoluminescence and Raman investigations. Formation of silicon nano-grains was detected by means of photoluminescence and Raman spectroscopy.}, subject = {Silicon; Defects; Photoluminescence; Luminescence imaging; Silicon thin films; Silizium; Defekte; Photolumineszenz; Lumineszenz-Imaging; Silizium-D{\"u}nnfilme; Silicium; D{\"u}nnschichttechnik; Solarzelle; Fehleranalyse}, language = {en} } @phdthesis{Schmidtbauer2013, author = {Schmidtbauer, Jan}, title = {MBE growth and characterization of germanium nanowires}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-28660}, school = {BTU Cottbus - Senftenberg}, year = {2013}, abstract = {Semiconductor nanowires, also called nanorods or nanowhiskers, are of particular interest for various applications in nanotechnology. Especially, germanium as a CMOS compatible material with its good electronic properties has gained renewed interest in recent years due to the availability of modern gate dielectrics. The present work deals with the vapor-liquid-solid growth of germanium nanowires and their characterization. The Growth has been carried out by means of molecular beam epitaxy using differently oriented germanium and silicon substrates whereas gold has been used to create metal catalyst droplets with radii of typically 100 nm and below. All stages from the substrate preparation to the final growth have been investigated in the frame of this work to find significant control parameters that influence the growth result. The droplet formation by means of gold evaporation onto the heated substrates has been investigated extensively on different substrates and for different surface preparations to identify parameters that are crucial for the resulting size distribution. Thereby sticking effects of the droplet circumference turned out to influence the radius distribution significantly. Germanium nanowires have been observed to grow preferentially along the <011> crystallographic directions on all utilized substrate orientations leading to defined possible inclinations of the wires with respect to the substrate normal. In contrast to the faceting known from silicon wires, the sidewalls mainly exhibit four flat {111} facets whereas the tip is roof shaped consisting of another two {111} facets. Different models which describe the inclined growth are presented and discussed. Furthermore, the material transport during the growth has been investigated. The nanowire length was found to be up to eight times larger than the nominal layer thickness according to the total amount of deposited germanium which is explained by surface diffusion towards the nanowires. The diffusion dominated growth regime was confirmed by length-radius-plot showing a decrease of the nanowire length at increasing radii. A temperature dependent diffusion model has been utilized to describe the observed nanowire length as a function of the substrate temperature. Beside conventional nanowires, so-called in-plane nanowires which grow along the substrate surface have been studied. Like their vertically growing counterparts, they also tend to grow along <011> in-plane directions which is particularly distinct on Ge(011) substrates. However, the fraction of nanowires which are aligned along <011> is influenced by substrate imperfections which was intentionally affected by means of wet-chemical substrate preparation. In addition to the nanowire growth, techniques for selective catalyst removal as well as for nanowire embedding in an insulating, transparent matrix have been established which can be important prerequisites for further nanowire processing in terms of electric or optoelectronic applications.}, subject = {Nanodraht; Germanium; Molekularstrahlepitaxie; Nanodr{\"a}hte; Molekularstrahlepitaxie; Germanium; Oberfl{\"a}chendiffusion; Nanostrukturen; Nanowire; Molecular beam epitaxy; Germanium; Surface diffusion; Nucleation; Nanostructures}, language = {en} } @phdthesis{Klossek2013, author = {Klossek, Andr{\´e}}, title = {Optical characterization of thin-film Si solar cells and knowledge transfer from bulk mc-Si}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-29350}, school = {BTU Cottbus - Senftenberg}, year = {2013}, abstract = {The aim of this work is to establish tools for optical characterization of defects in thin-film silicon solar cells. This is related to a challenging process of setup adjustments and careful interpretation of the measured raw data because of several artifacts and effects, which are typical for thin films. They are caused by the low layer/sample thickness and the related high impact of interfaces. Therefore, different thin-film samples were investigated to establish a process to correct/minimize these thin-film effects. The possibility of a knowledge transfer from mc-Si wafers with bulk thickness to thin Si films was checked. This would simplify a successful interpretation of the corrected data. Defects in mc-Si were investigated for many decades without the parasitic impact of thin films. Other Si phases, which are limited to thin-film samples, were investigated to learn details about their specific physical properties. These Si phases are amorphous and microcrystalline silicon. Additional to that electroluminescence investigations were performed on mc-Si solar cells. These investigation deals with topics, which are not even understood on bulk materials up to now. This could offer a basic for further knowledge transfers to thin-film Si.}, subject = {D{\"u}nnschichtsolarzelle; Photolumineszenz; D{\"u}nnschicht Solarzellen; Optische Charakterisierung; Photolumineszenz; Thin-film solar cells; Optical characterization; Photoluminescence}, language = {en} } @phdthesis{Fahr2008, author = {Fahr, Martin}, title = {Parasit{\"a}re W{\"a}rmefl{\"u}sse und Verunreinigungen in Reinstmetall-Fixpunktzellen der Internationalen Temperaturskala}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-4639}, school = {BTU Cottbus - Senftenberg}, year = {2008}, abstract = {Die internationale Temperaturskala von 1990 wird im Bereich von -38,8344°C bis 961,78°C neben dem Wasser- und dem Quecksilber-Tripelpunkt durch Schmelz- und Erstarrungstemperaturen von Reinstmetallen definiert. Die Unsicherheiten bei der Darstellung und Weitergabe der Basiseinheit Kelvin des Internationalen Einheitensystems in diesem Bereich (und somit auch der Einheit Grad Celsius und aller abgeleiteten Einheiten) werden dominiert von den Unsicherheiten der Realisierung dieser Fixpunkte und diese wiederum von einem Beitrag, der vom Einfluss in den Metallen gel{\"o}ster Verunreinigungen herr{\"u}hrt und bis jetzt nur pauschal abgesch{\"a}tzt wurde. Zwar wird versucht, noch reineres Fixpunktmetall zu verwenden, aber wesentliche Verbesserungen bez{\"u}glich der Reproduzierbarkeit der Fixpunkttemperaturen k{\"o}nnen nur durch eine Korrektion dieser Beitr{\"a}ge erreicht werden. Voraussetzungen f{\"u}r diese Korrektion sind nicht nur eine chemische Spurenanalyse der Fixpunktmetalle mit Unsicherheiten und Detektionsgrenzen am aktuellen technischen Limit, sondern auch die Kenntnis der Auswirkungen jeder einzelnen Verunreinigung auf die Fixpunkttemperatur. Diese Abh{\"a}ngigkeit der Phasen{\"u}bergangstemperatur von der Konzentration der jeweiligen Verunreinigung wird auf Grund fehlender Kenntnisse im Bereich kleinster Konzentrationen von 10^-9 bis 10^-5 (mol/mol) experimentell durch gezielte Dotierung der Fixpunktmetalle bestimmt. Um die dargestellten Probleme grundlegend zu l{\"o}sen, wurden im Rahmen dieser Arbeit zwei verschiedene Fixpunktzelldesigns entwickelt. Beide erlauben eine Messung und Kontrolle des Drucks der Gasatmosph{\"a}re und sind außerdem wiederverschließbar, wodurch sie eine chemische Analyse des Fixpunktmaterials erm{\"o}glichen, das sich in der Zusammensetzung wesentlich von dem unterscheiden kann, mit dem die Zelle bef{\"u}llt worden ist. W{\"a}hrend der eine Zelltyp f{\"u}r Kalibrierzwecke konstruiert worden ist und den herk{\"o}mmlichen Zellen {\"a}hnelt, wurde der zweite Typ auf die Verwendung von deutlich weniger Fixpunktmetall hin optimiert, so dass er sich unter anderem gut f{\"u}r die erw{\"a}hnten Dotierungsexperimente eignet. Zun{\"a}chst allerdings bringen die Ver{\"a}nderungen im Design gegen{\"u}ber den {\"u}blichen Fixpunktzellen unvermeidlich eine Vergr{\"o}ßerung unerw{\"u}nschter parasit{\"a}rer W{\"a}rmefl{\"u}sse mit sich, welche als sogenannte thermische Effekte die gemessene Fixpunkttemperatur verf{\"a}lschen. Diese fallen bei der angestrebten Verwendung von h{\"o}chstreinem Fixpunktmetall relativ zu den anderen Einflussgr{\"o}ßen umso mehr ins Gewicht, deswegen wurde ihre Untersuchung zu einem weiteren Schwerpunkt der Arbeit. Dabei konnte ein Verfahren zur Quantifizierung und Korrektion und ein besseres Verst{\"a}ndnis dieser Effekte entwickelt werden, welche f{\"u}r Metallfixpunktzellen jeder Bauart n{\"u}tzlich sind. Im Ergebnis liegen die Unsicherheiten bei den Messungen mit den verkleinerten Zellen unver{\"a}ndert im Bereich weniger hundert Mikrokelvin. Die Zellen eignen sich daher sehr gut, die Beeinflussung der Phasen{\"u}bergangstemperatur der Metallfixpunkte durch die Dotierung mit anderen Elementen im Konzentrationsbereich von 10^-7 bis 10^-5 zu bestimmen. Bevor dies abschließend auch experimentell mit Untersuchungen an ausgew{\"a}hlten bin{\"a}ren Systemen gezeigt wird, werden thermodynamische Berechnungen dargestellt, die erstmals eine Vielzahl von m{\"o}glichen Verunreinigungen in den Fixpunktzellen ausschließen, so dass sich nicht nur die Anzahl zu untersuchender m{\"o}glicher Verunreinigungen in den Fixpunkten erheblich reduziert, sondern auch die Anzahl zu ber{\"u}cksichtigender Beitr{\"a}ge bei der angestrebten Korrektion der Fixpunkttemperaturen. Alles in allem steht nun eine Methodik zur Verf{\"u}gung, die Unsicherheit bei der Darstellung der internationalen Temperaturskala im industriell besonders wichtigen Bereich von -40°C bis 1000°C mindestens um den Faktor 3 zu reduzieren.}, subject = {Metall; Verunreinigung; W{\"a}rmestrom; Internationale Temperaturskala; Temperaturfixpunkte; ITS-90; Verunreinigungen in ultrahochreinen Metallen; Thermische Effekte; Strahlungstransport; Temperature fixed-points; ITS-90; Impurities in ultra-pure metals; Thermal effects; Radiation transport}, language = {de} } @phdthesis{Henyk2003, author = {Henyk, Matthias}, title = {Partikelemission bei der Ablation dielektrischer Materialien mit ultrakurzen Laserpulsen}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-000000371}, school = {BTU Cottbus - Senftenberg}, year = {2003}, abstract = {Das Ziel der vorliegenden Arbeit ist es, einen Beitrag zum Verst{\"a}ndnis mikroskopischer Prozesse zu leisten welche an der Laser-Ablation dielektrischer Kristalle mit ultrakurzen Laserpulsen beteiligt sind. Die von den Targets emittierten elektrisch geladenen Partikel werden mit einem Flugzeitmassenspektrometer nachgewiesen, und die resultierende Morphologie der Targetoberfl{\"a}che wird ex-situ mit Methoden der optischen Mikroskopie bzw. der Raster-Elektronenmikroskopie charakterisiert. Die Absorption der Laserstrahlung f{\"u}hrt zu Multiphotonen-Ionisationsprozessen, so dass es zu einer Coulomb-Explosion der Oberfl{\"a}che kommt. Bei hohen Laserintensit{\"a}ten gibt es außerdem thermische Beitr{\"a}ge zu Ablation (Phasen-Explosion). F{\"u}r die resonante Unterst{\"u}tzung der Photoelektronenemission sind materialabh{\"a}ngige Oberfl{\"a}chenzust{\"a}nde (z. Bsp. F-Zentren) wichtig. An BaF2 (111) Oberfl{\"a}chen wird ein „Layer-by-Layer" Abtrag beobachtet. Die Partikelemission hinterl{\"a}sst eine instabile Oberfl{\"a}che, deshalb kann es im Ablationskrater zur Ausbreitung von Oberfl{\"a}chenwellen und der Bildung von Ripples durch selbst-organisierte Prozesse kommen.}, subject = {Ionenkristall; Laserablation; Laserimpuls; Femtosekundenbereich; Flugzeitmassenspektrometrie; Laser; Ablation; Dielektrisches Kristall; Laserpuls}, language = {de} } @phdthesis{Zwierz2014, author = {Zwierz, Radoslaw}, title = {Plasma enhanced growth of GaN single crystalline layers from vapour phase}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-30710}, school = {BTU Cottbus - Senftenberg}, year = {2014}, abstract = {Gallium nitride (GaN) is a III-V semiconductor, characterized by direct, wide band gap of 3.4 eV at RT. As a material of particular interest for opto- and power electronics applications, it has been thoroughly studied in recent years. Utilization of GaN homoepitaxy in manufacturing of laser diodes (LDs), light-emitting diodes (LEDs), power devices, etc. would be beneficial in terms of reducing defect density, thus improving their lifetime and performance. Yet cost-effective process for providing native GaN substrates has not been established so far. The focus of this work is put on development of a new method to grow single crystalline GaN layers from Ga vapour. Our approach exploits microwave (MW) plasma as a source of excited nitrogen species, in contrast to classical physical vapour transport (PVT)-based technique, in which ammonia (NH3) serves as a source of reactive nitrogen. Novelty of MW plasma enhanced growth of GaN from vapour lies in MW nitrogen plasma formation in the vicinity of the seed, at moderate pressure (200 - 800 mbar range), and concurrent physical vapour transport of Ga to the growth zone. Simulations of the growth setup (HEpiGaN software) and of the MW plasma source (CST Microwave software) have followed the extensive investigations of material properties. The growth setup and the MW plasma source, with the resonance cavity being its crucial part, have been constructed and implemented into the existing growth reactor. The stability of MW plasma in function of temperature and pressure has been studied along with its influence on the seed temperature, and thus on the growth conditions. Furthermore, optical emission spectroscopy (OES) has been utilized for in-situ characterization of the growth atmosphere. Studies on the interaction of Ga vapour with the nitrogen discharge were interpreted on the basis of the level structure of lower excited states of Ga. Deposition experiments have been conducted, using sapphire seeds, GaN, AlN and AlGaN templates, while GaN single crystalline layers have been grown on sapphire and GaN templates. Characterization of GaN layers have been done by various methods, i.e. structure of layers by scanning electron microscopy (SEM), their composition by energy dispersive X-ray spectroscopy (EDX) and secondary ion mass spectrometry (SIMS), and crystal quality by high resolution X-ray diffraction (HRXRD). Results of the characterization together with outcome of OES measurements revealed importance of carbon for the sub-atmospheric MW plasma enhanced growth of GaN from vapour. In addition, this fact was confirmed by experiments in the setup with reduced carbon content. Possible routes for GaN synthesis have been discussed, with the most probable being CN-assisted GaN formation. While CN was detected in the plasma spectra, there was no evidence for the existence of GaN molecules in vapour phase.}, subject = {GaN; Vapour growth; Microwave plasma; Z{\"u}chtung aus der Gasphase; Plasma; Galliumnitrid; Gasphase; Plasma}, language = {en} } @phdthesis{Varlamova2013, author = {Varlamova, Olga}, title = {Self-organized surface patterns originating from femtosecond laser-induced instability}, isbn = {978-3-95404-604-1}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-29530}, school = {BTU Cottbus - Senftenberg}, year = {2013}, abstract = {The phenomenon of laser-induced periodic surface structures (LIPSS), or ripples, generated by near-infrared radiation with the central wavelength around 800 nanometer (λlaser ≈ 800 nm) and pulse duration about of 100 femtosecond (τpulse ≈ 100 fs) on solid targets is considered in this dissertation. The main aim of the work is a better understanding of the fundamental processes of laser-matter interaction resulting in pattern formation by femtosecond (fs) laser ablation. The problem is of great interest both in fundamental and applied science. The knowledge of the underlying physical mechanisms will provide the opportunity to control surface nanostructuring, which has a big application potential in many modern technologies. Femtosecond LIPSS observed at the bottom of ablation crater reveal a large variety of features including nanostructures with periods below 100 nm. Moreover, the ripple size depends mainly on the irradiation dose/absorbed laser energy and is rather insensitive to the variation of laser wavelength or incidence angle. The orientation of the structures is dictated by laser polarization. All these experimental observations and an astounding similarity of the structures to other patterns originating from instabilities led to the idea to attribute the femtosecond laser nanostructuring to a self-organized pattern formation from laser-induced surface instability. In this dissertation, surface pattern formation upon femtosecond laser ablation is considered in the framework of an adopted surface erosion model, based on the description of spontaneous pattern formation on surfaces bombarded with high-energy ions. We exploit the similarity to ion-beam sputtering and extend a corresponding model for laser ablation by including laser polarization. It has been found that an asymmetry in the deposition and dissipation of the incident laser energy, related to the laser polarization, results in a corresponding dependence of coefficients in a nonlinear equation of the Kuramoto-Sivashinsky type. The surface morphologies calculated in the framework of this model for different configurations of the incident laser electric field show an excellent qualitative agreement with structures observed in ultra-short pulse ablation experiments. In this work, properties of the periodic surface structures induced upon femtosecond laser ablation are studied in detail, focusing on a systematic investigation of the main control parameters regulating the pattern formation process. The results support the non-linear self-organization mechanism of pattern formation from laser-induced surface instability.}, subject = {Oberfl{\"a}chenstruktur; Mikromechanik; Laserablation; Ultrakurzzeitlaser; Femtosekundenlaserablation; Laser-induzierte Oberfl{\"a}chenstrukturierung; Ripples; Selbstorganisierte Strukturbildung; Femtosecond laser ablation; Laser-induced surface patterning; Ripples; Selforganized pattern formation}, language = {en} } @phdthesis{Lublow2009, author = {Lublow, Michael}, title = {Surface analytical characterization of horizontal and vertical nanotopographies at the silicon/silicon oxide/electrolyte phase boundaries}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-14390}, school = {BTU Cottbus - Senftenberg}, year = {2009}, abstract = {Nanotopography development induced by photoelectrochemical in situ conditioning of silicon is followed using a combination of surface sensitive analysis techniques. In an etching study, vertical nanostructure analysis reveals a buried stressed layer within silicon, identified by Brewster-angle analysis (BAA). In conjunction with in system synchrotron radiation photoelectron spectroscopy (SRPES), a superior quality hydrogen terminated Si(111) surface could be prepared by obliteration of the intermediate stressed layer. Using a novel photoelectrochemical structure formation method, a variety of vertical nanotopographies has been generated and analyzed by in situ Brewster-angle reflectometry (BAR) and scanning probe microscopy (SPM). Shaping of the nanostructures became possible by real-time monitoring using BAR. Appearances range from aligned single nanoislands with improved aspect ratio to connected Si nano-networks. A model was developed to describe the nanostructure formation based on stress-induced selective oxidation. Increased local photo-oxidation is found to result in the formation of extended horizontal micro- and nanostructures with fractal properties. Within a defined light intensity range, the structures reveal the azimuthal symmetry of the investigated crystal planes (111), (100), (110) and (113). The observed features could be reproduced using a model that is based on the interplay of stress in silicon, oxidation by light generated excess holes and locally increased etching in fluoride containing solution.}, subject = {Siliciumdioxid; Oberfl{\"a}chenanalyse; Silicium; Oberfl{\"a}chenanalyse; Selbstorganisierte elektrochemische Systeme; Optik; Silicon; Surface analysis; Self-organized electrochemical systems; Optics}, language = {en} } @phdthesis{Wiatrek2013, author = {Wiatrek, Andrzej}, title = {Untersuchung der Eigenschaften der ges{\"a}ttigten stimulierten Brillouin-Streuung und ihrer Anwendungsm{\"o}glichkeiten}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus-29577}, school = {BTU Cottbus - Senftenberg}, year = {2013}, abstract = {Es ist wohl gerade die Einfachheit seiner Erzeugung, welche den nichtlinearen optischen Effekt der stimulierten Brillouin-Streuung so interessant f{\"u}r viele Anwendungen in der optischen Signalverarbeitung, der Fasersensorik, der hochaufl{\"o}senden Spektroskopie und auch der Signalverz{\"o}gerung und -speicherung macht. Die geringe Verst{\"a}rkungsbandbreite kann zur selektiven Filterung oder Abtastung optischer Signale eingesetzt werden, was die Grundlage der optischen Spektroskopie aber auch der Lichtspeicherung ist. Dar{\"u}ber hinaus werden Signale innerhalb dieser Bandbreite nicht nur verst{\"a}rkt, sondern auch abh{\"a}ngig vom Gewinn bzw. der eingestellten Pumpleistung verz{\"o}gert. Diese Eigenschaft ist die Grundlage der sogenannten Slow-Light-Systeme. Damit kann mit einem Brillouin-Verst{\"a}rker ein durchstimmbarer optischer Kurzzeitpuffer realisiert werden. In der vorliegenden Arbeit werden mit Hilfe gezielt ges{\"a}ttigter Brillouin-Verst{\"a}rker grundlegende Beschr{\"a}nkungen des Brillouin-basierten Slow-Light und der Brillouin-basierten optischen Filterung aufgehoben, was mit den herk{\"o}mmlichen linearen Ans{\"a}tzen nicht m{\"o}glich ist. Der Vorteil der S{\"a}ttigung des Brillouin-Verst{\"a}rkers ist, dass sie nur zu einer vorhersagbaren Begrenzung des Amplitudenspektrums aber nicht zur Entstehung neuer Signalfrequenzen f{\"u}hrt. Die spektrale Begrenzung des verst{\"a}rkten Signals und ein nichtlinearer Verlauf der Gruppenlaufzeit im spektralen Randbereich des Brillouin-Verst{\"a}rkers f{\"u}hren zu einer zeitlichen Verbreiterung der verz{\"o}gerten Lichtpulse. Diese Verbreiterung ist insofern problematisch, dass sie die Unterscheidbarkeit der verschiedenen Signalpegel f{\"u}r logische Einsen und Nullen signifikant verschlechtert. Mit Hilfe des vorgestellten ges{\"a}ttigten Systems erfolgt eine gezielte R{\"u}ckformung der verz{\"o}gerten Pulse, was erstmalig zu einer verbreiterungsfreien und damit auch verlustarmen Pulsspeicherung f{\"u}hrt. Dar{\"u}ber hinaus kann auf Basis der Nachbildung der ges{\"a}ttigten Filtercharakteristik mit einem linearen Brillouin-Verst{\"a}rker ebenfalls eine verbreiterungsfreie Pulsverz{\"o}gerung nachgewiesen werden. Ein optischer Speicher kann aber auch durch Unterabtastung eines Signalspektrums mit einem Brillouin-basierten Kammfilter realisiert werden. Die maximale Speicherzeit ist dabei durch die nat{\"u}rliche Brillouin Gewinnbandbreite begrenzt. Dieser Wert ist abh{\"a}ngig von der Pump- und Signalleistung, der Umgebungstemperatur, von mechanischem Stress und vom Fasermaterial. Unter Ausnutzung der S{\"a}ttigungseigenschaften des Brillouin-Gewinns l{\"a}sst sich die Gewinnbandbreite durch {\"U}berlagerung einer s{\"a}ttigenden Spektralblende nahezu beliebig verkleinern. In dieser Arbeit wird auf Basis dieser Methode eine Bandbreitenreduktion des Brillouin-Filters um etwa eine Gr{\"o}ßenordnung nachgewiesen, wobei das Potential zur Verringerung um mehrere Gr{\"o}ßenordnungen besteht.}, subject = {Nichtlineare Optik; Brillouin-Streuung; Nichtlineare Optik; Stimulierte Brillouin-Streuung; Slow-Light; Nonlinear optics; Stimulated Brillouin scattering; Slow light}, language = {de} }