This dissertation details the study of band bending in n- and p-type GaN samples with a Kelvin probe utilizing different illumination geometries, ambients (air, oxygen, vacuum 10-6 mbar), and sample temperatures (77 – 650 K). The Kelvin probe, which is mounted inside an optical cryostat, is used to measure the surface potential. Illumination of the GaN surface with band-to-band light generates electron-hole pairs, which quickly separate in the depletion region due to a strong electric field caused by the near-surface band bending. The charge that is swept to the surface reduces the band bending and generates a surface photovoltage (SPV). Information about the band bending can be obtained by fitting the SPV measurements with a thermionic model based on the emission of charge carriers from bulk to surface and vice versa. The band bending in freestanding n-type GaN templates has been evaluated. The Ga-polar and N-polar surfaces exhibit upward band bending of about 0.74 and 0.57 eV, respectively. The surface treatment also plays a major role in the SPV behavior, where the SPV for mechanical polished surfaces restores faster than predicted by a thermionic model in dark. When measuring the photoluminescence (PL) signal, the PL from mechanically polished surfaces was about 4 orders of magnitude smaller than the PL from chemically mechanically polished surfaces. The PL and SPV behaviors were explained by the presence of a large density of defects near the surface, which quench PL and aid in the restoration of the SPV via electron hopping between defects. Temperature-dependent SPV studies have also been performed on doped n- and p-type GaN samples. In Si-doped n-type GaN, the estimated upward band bending was about 1 eV at temperatures between 295 and 500 K. However, in p-type GaN, the downward band bending appeared to increase with increasing temperature, where the magnitude of band bending increased from 0.8 eV to 2.1 eV as the temperature increased from 295 to 650 K. It appears that heating the p-type GaN samples allows for band bending values larger than 1 eV to fully restore. Pre-heating of samples was of paramount importance to measure the correct value of band bending in p-type GaN. The slope of the dependence of the SPV on excitation intensity at low temperatures was larger than expected; however, once the temperature exceeded 500 K, the slope began to reach values that are in agreement with a thermionic model.

There is an understanding of surface photovoltage (SPV) behavior for GaN, yet little is known about the SPV behavior for AlGaN. In this work, a Kelvin probe was used to measure the SPV for p-type AlGaN. Very slow SPV transients were found in AlGaN, which could not be explained with a simple thermionic model. A possible explanation of this behavior is the segregation of impurities to the surface, which causes significant reduction of the depletion region width (down to 2 nm), with carrier tunneling and hopping becoming the dominant mechanisms responsible for the SPV transients. To verify this assumption, the near-surface defective region (about 40 nm) has been removed through the ICP-RIE process. After the etching, the SPV transients became fast and increased in magnitude by about 0.6 eV. By using the thermionic model, band bending was estimated to be -1 eV.

The study of photovoltaic materials is important so as to develop new solar energy technologies: in particular, quantum-confined semiconductors could offer increased quantum efficiencies at a much lower manufacture cost. This thesis contains results from a number of pump-probe experiments designed to probe the carrier dynamics in bulk and quantum-confined photovoltaics. A THz time-domain spectrometer was designed, built and commissioned. The THz refractive indices and absorption coefficients of toluene and hexane were determined, and the spectrometer was benchmarked using a photoexcited GaAs wafer. Results are presented of time-resolved THz spectroscopy of photoexcited bulk InP as a function of laser excitation wavelength. These data were used to extract the quantum efficiency of bulk InP in order to compare with recent results for InP quantum dots. The quantum efficiency in quantum dots increases when the incident photon energy is at least twice the band gap energy, whereasthe efficiency of the bulk material is found to decrease. This is because of surface recombination, and these measurements therefore verify the potential superiority of quantum dot materials over bulk materials for use in solar energy applications. Initial measurements of quantum dots using THz spectroscopy highlighted the various experimental challenges involved and the upgrades required to study such samples in the future.The time-dependence of the photoinduced surface photovoltage (SPV) in Si was studied on nanosecond timescales by synchronizing an ultrafast laser system to a synchrotron radiation source (the SRS at Daresbury, UK), and measuring the resulting shift in the photoelectron spectrum. The equilibrium band bending was determined, and the decay of the SPV was attributed to the recombination of charge carriers across the band gap. Results are presented for the SPV in bulk ZnO and for PbS quantum dot chemically attached to ZnO. The fact that the PbS quantum dots were chemically attached to the surface without becoming oxidized was verified using X-ray photoelectron spectroscopy (XPS). The changes caused by photoexcitation occur on much longer timescales in ZnO than Si (sub-milliseconds rather than nanoseconds), and these timescales were conveniently accessed using the time-resolved XPS facility at the TEMPO beamline at Synchrotron SOLEIL (Paris, France). This is due to oxygen adsorption and desorption processes at the ZnO surface affectingthe transfer of charge carriers. The addition of PbS quantum dots to the ZnO surface was found to increase the speed of this charge transfer due to injection of carriers directly from the PbS quantum dot to the bulk ZnO conduction band.

No description available.

Complex oxides are an intriguing field of solid-state research, as they can exhibit a wide variety of functional properties, such as ferroelasticity, ferroelectricity, ferro- and antiferromagnetism or an even more complicated type of magnetic ordering, the combination or interaction of those ferroic properties (multiferroicity), high spin polarization, or high-temperature superconductivity. Thus they are prospective candidates for future materials in microelectronics. It is a matter of fact that the performance of such oxide-based devices depends mainly on transport properties, which in turn depend on the distribution and density of intrinsic or extrinsic electronic interface states across the device structure. The present thesis focuses on the identification and characterization of such electronic properties by two different photoassisted spectroscopy techniques: surface photovoltage spectroscopy and photoelectron spectroscopy. This work especially deals with perovskite oxides, namely with the model perovskite strontium titanate (SrTiO3) as a substrate and three differently doped lanthanum manganite thin films (10-15 nm thickness) grown by pulsed laser deposition (PLD) on the SrTiO3 substrate(La0.7Sr0.3MnO3, La0.7Ca0.3MnO3, La0.7Ce0.3MnO3). The first part aims at the identification of electronic surface and interface states at the free SrTiO3 surface as well as at the three different lanthanum manganite/SrTiO3 interfaces. For that purpose three different experimental realizations of the surface photovoltage spectroscopy technique were implemented and employed: photoelectron spectroscopy under additional optical excitation, the capacitive detection of the photoinduced displacement current in a parallel-plate capacitor geometry under modulated optical excitation, and the classical Kelvin probe technique. The methods are evaluated comparatively with respect to their suitability to analyze the given oxidic interfaces. The main result of this first part is a map of the energetic positions and relaxation time constants of the surface states at the SrTiO3 surface as well as of the interface states at the lanthanum manganite/SrTiO3 interfaces within the SrTiO3 bandgap. The interface states were classified into film- and substrate-induced states and it could be demonstrated that an appropriate annealing procedure can dramatically decrease their densities. The second part tackles the problem of the manganese valence and the doping type of di- and tetravalent-ion-doped LaMnO3. The question whether the insulating parent compound LaMnO3 becomes an electron-doped semiconductor after doping with tetravalent cations such as Ce4+ - which would be in analogy to the well-established hole doping after partial substitution of La3+ by divalent cations such as Sr2+ or Ca2+ - has been discussed controversially in the literature so far. Due to the physics of the manganite crystal lattice the question can also be formulated in a different way: Can part of the manganese ions be driven from the Mn3+ state towards the Mn2+ state without any crystal instabilities or phase separation phenomena? In order to contribute to the clarification of this question, an extensive X-ray- and UV-photoelectron spectroscopy (XPS/UPS) investigation was performed. The three differently doped lanthanum manganite thin films were comparatively studied considering the exchange splitting of the Mn 3s core level line, which is a linear function of the Mn valence, as measured by XPS and the work function as extracted from UPS. All measurements were performed at different states of deoxygenation after heating in ultrahigh vacuum and reoxidation after heating in a pure oxygen atmosphere. Strong evidence for electron doping of the La0.7Ce0.3MnO3 film after deoxygenation was found. Furthermore, the reversible tunability of the Mn valence by variation of the oxygen content could be demonstrated for both tetravalent- and divalent-ion-doped lanthanum manganite films. / Oxidische Komplexverbindungen können eine Vielzahl an funktionellen Eigenschaften, wie z.B. Ferroelastizität, Ferroelektrizität, Ferro- und Antiferromagnetismus sowie kompliziertere magnetische Ordnungen, die Kombination und Interaktion solcher ferroischer Eigenschaften (Multiferroizität), hohe Spinpolarisation oder Hochtemperatursupraleitung aufweisen und gelten daher als aussichtsreiche Materialien für die zukünftige Mikroelektronik. Entscheidend für die Funktionsfähigkeit oxidischer Bauelemente sind deren elektronische Transporteigenschaften, die in äußerst sensibler Weise von der Verteilung und Dichte von ex- oder intrinsischen elektronischen Defektzuständen an Grenz- und Oberflächen innerhalb der Bauelementstruktur abhängen. Die vorliegende Arbeit beschäftigt sich mit der Spektroskopie solcher elektronischer Eigenschaften mittels photonenbasierter Methoden. Im Fokus stehen dabei perowskitische Oxide , speziell das Modellperowskit Strontiumtitanat (SrTiO3) als Substrat und darauf mittels gepulster Laserdeposition (PLD) abgeschiedene dünne Filme (10-15 nm Dicke) dotierter Lanthanmanganate (La0.7Sr0.3MnO, La0.7Ca0.3MnO3, La0.7Ce0.3MnO3). Im Rahmen einer halbleiterphysikalischen Interpretation widmet sich der erste Teilder Identifikation elektronischer Ober- und Grenzflächenzustände an der SrTiO3-Oberfläche sowie an verschiedenen Lanthanmanganat/SrTiO3-Grenzflächen mittels dreier unterschiedlicher experimenteller Methoden zur Vermessung der Oberflächenphotospannung: der Photoelektronenspektroskopie unter zusätzlicher optischer Anregung, einer kapazitiven Detektionsmethode in Plattenkondensatorgeometrie unter modulierter optischer Anregung und der optischen Kelvin-Sonde. Neben einem auf die bei oxidischen Ober- und Grenzflächen auftretenden besonderen Herausforderungen zugeschnittenen Methodenvergleich werden Grenzflächenzustände bezüglich ihrer energetischen Position in der Bandlücke des SrTiO3 und ihres Relaxationsverhaltens analysiert, als substrat- oder filminduziert klassifiziert, und die Verringerung ihrer Dichte nach geeigneter Ausheilprozedur wird nachgewiesen. Der zweite Teil der Arbeit befasst sich mit der in der Literatur bisher kontrovers diskutierten Frage, ob sich die isolierende Stammverbindung LaMnO3 durch Dotierung mit tetravalenten Kationen, wie z.B. Ce4+, in einen elektronendotierten Halbleiter verwandeln lässt - analog zur Herstellung lochdotierter Lanthanmanganate durch Dotierung mit divalenten Kationen, wie z.B. Sr2+ oder Ca2+. Die Frage ist äquivalent zur Betrachtung, ob unter Beibehaltung der Stabilität des Kristallgitters ein Teil der Manganionen vom Mn3+-Zustand in den Mn2+-Zustand übergehen kann. Um einen Beitrag zur Klärung dieses Problems zu leisten, wurden als elektronisch sensitive Methoden die Röntgen- und UV-Photoelektronenspektroskopie (XPS/UPS) gewählt. Die oben genannten Lanthanmanganatfilme wurden dazu hinsichtlich der Austauschaufspaltung der Mangan-3s-Linie im XP-Spektrum, die in linearer Weise von der Manganvalenz abhängt, und der anhand der Breite des UP-Spektrums ermittelten Austrittsarbeit jeweils nach Reinigung der Oberfläche im Ultrahochvakuum (UHV) vergleichend untersucht. Die Messungen wurden nach unterschiedlich starker Desoxidation durch Heizen im UHV und Reoxidierung durch Heizen in Sauerstoffatmosphäre durchgeführt. Es konnte nachgewiesen werden, dass eine Elektronendotierung des La0.7Ce0.3MnO3-Films bei geeigneter Einstellung des Sauerstoffgehalts tatsächlich möglich ist. Außerdem wurde gezeigt, dass sich sowohl in di- als auch in tetravalent dotierten Lanthanmanganatfilmen die Manganvalenz und damit der Dotierungstyp reversibel durchstimmen lässt.

No description available.

Complex oxides are an intriguing field of solid-state research, as they can exhibit a wide variety of functional properties, such as ferroelasticity, ferroelectricity, ferro- and antiferromagnetism or an even more complicated type of magnetic ordering, the combination or interaction of those ferroic properties (multiferroicity), high spin polarization, or high-temperature superconductivity. Thus they are prospective candidates for future materials in microelectronics. It is a matter of fact that the performance of such oxide-based devices depends mainly on transport properties, which in turn depend on the distribution and density of intrinsic or extrinsic electronic interface states across the device structure. The present thesis focuses on the identification and characterization of such electronic properties by two different photoassisted spectroscopy techniques: surface photovoltage spectroscopy and photoelectron spectroscopy. This work especially deals with perovskite oxides, namely with the model perovskite strontium titanate (SrTiO3) as a substrate and three differently doped lanthanum manganite thin films (10-15 nm thickness) grown by pulsed laser deposition (PLD) on the SrTiO3 substrate(La0.7Sr0.3MnO3, La0.7Ca0.3MnO3, La0.7Ce0.3MnO3). The first part aims at the identification of electronic surface and interface states at the free SrTiO3 surface as well as at the three different lanthanum manganite/SrTiO3 interfaces. For that purpose three different experimental realizations of the surface photovoltage spectroscopy technique were implemented and employed: photoelectron spectroscopy under additional optical excitation, the capacitive detection of the photoinduced displacement current in a parallel-plate capacitor geometry under modulated optical excitation, and the classical Kelvin probe technique. The methods are evaluated comparatively with respect to their suitability to analyze the given oxidic interfaces. The main result of this first part is a map of the energetic positions and relaxation time constants of the surface states at the SrTiO3 surface as well as of the interface states at the lanthanum manganite/SrTiO3 interfaces within the SrTiO3 bandgap. The interface states were classified into film- and substrate-induced states and it could be demonstrated that an appropriate annealing procedure can dramatically decrease their densities. The second part tackles the problem of the manganese valence and the doping type of di- and tetravalent-ion-doped LaMnO3. The question whether the insulating parent compound LaMnO3 becomes an electron-doped semiconductor after doping with tetravalent cations such as Ce4+ - which would be in analogy to the well-established hole doping after partial substitution of La3+ by divalent cations such as Sr2+ or Ca2+ - has been discussed controversially in the literature so far. Due to the physics of the manganite crystal lattice the question can also be formulated in a different way: Can part of the manganese ions be driven from the Mn3+ state towards the Mn2+ state without any crystal instabilities or phase separation phenomena? In order to contribute to the clarification of this question, an extensive X-ray- and UV-photoelectron spectroscopy (XPS/UPS) investigation was performed. The three differently doped lanthanum manganite thin films were comparatively studied considering the exchange splitting of the Mn 3s core level line, which is a linear function of the Mn valence, as measured by XPS and the work function as extracted from UPS. All measurements were performed at different states of deoxygenation after heating in ultrahigh vacuum and reoxidation after heating in a pure oxygen atmosphere. Strong evidence for electron doping of the La0.7Ce0.3MnO3 film after deoxygenation was found. Furthermore, the reversible tunability of the Mn valence by variation of the oxygen content could be demonstrated for both tetravalent- and divalent-ion-doped lanthanum manganite films. / Oxidische Komplexverbindungen können eine Vielzahl an funktionellen Eigenschaften, wie z.B. Ferroelastizität, Ferroelektrizität, Ferro- und Antiferromagnetismus sowie kompliziertere magnetische Ordnungen, die Kombination und Interaktion solcher ferroischer Eigenschaften (Multiferroizität), hohe Spinpolarisation oder Hochtemperatursupraleitung aufweisen und gelten daher als aussichtsreiche Materialien für die zukünftige Mikroelektronik. Entscheidend für die Funktionsfähigkeit oxidischer Bauelemente sind deren elektronische Transporteigenschaften, die in äußerst sensibler Weise von der Verteilung und Dichte von ex- oder intrinsischen elektronischen Defektzuständen an Grenz- und Oberflächen innerhalb der Bauelementstruktur abhängen. Die vorliegende Arbeit beschäftigt sich mit der Spektroskopie solcher elektronischer Eigenschaften mittels photonenbasierter Methoden. Im Fokus stehen dabei perowskitische Oxide , speziell das Modellperowskit Strontiumtitanat (SrTiO3) als Substrat und darauf mittels gepulster Laserdeposition (PLD) abgeschiedene dünne Filme (10-15 nm Dicke) dotierter Lanthanmanganate (La0.7Sr0.3MnO, La0.7Ca0.3MnO3, La0.7Ce0.3MnO3). Im Rahmen einer halbleiterphysikalischen Interpretation widmet sich der erste Teilder Identifikation elektronischer Ober- und Grenzflächenzustände an der SrTiO3-Oberfläche sowie an verschiedenen Lanthanmanganat/SrTiO3-Grenzflächen mittels dreier unterschiedlicher experimenteller Methoden zur Vermessung der Oberflächenphotospannung: der Photoelektronenspektroskopie unter zusätzlicher optischer Anregung, einer kapazitiven Detektionsmethode in Plattenkondensatorgeometrie unter modulierter optischer Anregung und der optischen Kelvin-Sonde. Neben einem auf die bei oxidischen Ober- und Grenzflächen auftretenden besonderen Herausforderungen zugeschnittenen Methodenvergleich werden Grenzflächenzustände bezüglich ihrer energetischen Position in der Bandlücke des SrTiO3 und ihres Relaxationsverhaltens analysiert, als substrat- oder filminduziert klassifiziert, und die Verringerung ihrer Dichte nach geeigneter Ausheilprozedur wird nachgewiesen. Der zweite Teil der Arbeit befasst sich mit der in der Literatur bisher kontrovers diskutierten Frage, ob sich die isolierende Stammverbindung LaMnO3 durch Dotierung mit tetravalenten Kationen, wie z.B. Ce4+, in einen elektronendotierten Halbleiter verwandeln lässt - analog zur Herstellung lochdotierter Lanthanmanganate durch Dotierung mit divalenten Kationen, wie z.B. Sr2+ oder Ca2+. Die Frage ist äquivalent zur Betrachtung, ob unter Beibehaltung der Stabilität des Kristallgitters ein Teil der Manganionen vom Mn3+-Zustand in den Mn2+-Zustand übergehen kann. Um einen Beitrag zur Klärung dieses Problems zu leisten, wurden als elektronisch sensitive Methoden die Röntgen- und UV-Photoelektronenspektroskopie (XPS/UPS) gewählt. Die oben genannten Lanthanmanganatfilme wurden dazu hinsichtlich der Austauschaufspaltung der Mangan-3s-Linie im XP-Spektrum, die in linearer Weise von der Manganvalenz abhängt, und der anhand der Breite des UP-Spektrums ermittelten Austrittsarbeit jeweils nach Reinigung der Oberfläche im Ultrahochvakuum (UHV) vergleichend untersucht. Die Messungen wurden nach unterschiedlich starker Desoxidation durch Heizen im UHV und Reoxidierung durch Heizen in Sauerstoffatmosphäre durchgeführt. Es konnte nachgewiesen werden, dass eine Elektronendotierung des La0.7Ce0.3MnO3-Films bei geeigneter Einstellung des Sauerstoffgehalts tatsächlich möglich ist. Außerdem wurde gezeigt, dass sich sowohl in di- als auch in tetravalent dotierten Lanthanmanganatfilmen die Manganvalenz und damit der Dotierungstyp reversibel durchstimmen lässt.

No description available.

No description available.

Cette thèse propose, décrit et utilise un ensemble de techniques basées sur la microscopie à force atomique sous ultravide pour la cartographie simultanée, à l'échelle nanométrique, de la topographie de surface et des dynamiques temporelles des photo-porteurs. Ainsi, en contrôlant la dépendance du photo-potentiel de surface moyen mesuré par microscopie à force de sonde Kelvin en fonction de la fréquence de répétition d'une source lumineuse externe d'excitation, le dispositif expérimental permet d’accéder aux dynamiques temporelles du photo-potentiel de surface qui, à leur tour, permettent d'étudier les dynamiques des photo-porteurs sur une large gamme d'échantillons. Afin de permettre le processus de nano-imagerie bidimensionnelle, ces mesures sont acquises de manière répétée en enregistrant des courbes spectroscopiques en chaque point d’une grille prédéfinie. En utilisant une procédure d'ajustement mathématique automatique, les dynamiques temporelles des photo-porteurs sont extraites à partir des données expérimentales.Cet ensemble de nouvelles méthodes est utilisé pour l’étude de plusieurs types d'échantillons issus de différentes technologies photovoltaïques telles que des couches minces en silicium poly-cristallin à petits grains, des cellules de troisième génération à nano cristaux de silicium, des cellules photovoltaïques organiques et des cellules à base de matériaux de structure pérovskite. Dans chaque cas, on décrit les processus de recombinaison des photo-porteurs ainsi que leur lien avec la morphologie et la structuration du matériau. Enfin, les aspects techniques de ces nouvelles méthodes d’analyse sont présentés, ainsi que leurs limites, notamment celles concernant l'interprétation des résultats. / This thesis is directed towards the proposition and demonstration of a set of novel advanced atomic force microscopy based techniques under ultra-high vacuum conditions, enabling to map simultaneously the surface topography and the photo-carrier dynamics at the nanometre scale. In fact, by monitoring the dependence of the average surface photo-voltage measured with Kelvin probe force microscopy, as a function of the repetition frequency of a modulated excitation source, we will access the built-up and decay dynamics of the surface photo-voltage response which in turn will allows us to study the photo-carrier dynamics over a wide range of samples. In order to enable the 2-dimensional nano-imaging process, Kelvin probe force microscopy under modulated illumination measurements are acquired repeatedly over each point of a predefined grid area over the sample acquiring a set of spectroscopy curves. Then, using an automatic mathematical fit procedure, spectroscopy curves are translated into pixels of the photo-carrier dynamic time-constant images.Moreover, these set of novel techniques will be used to investigate the surface photo-voltage dynamics in several kinds of photovoltaic samples from different technological branches such as small grain polycrystalline silicon thin films, silicon nanocrystal-based third generation cells, bulk heterojunction donor-acceptor organic photovoltaics and organic-inorganic hybrid perovskite single crystal cells, discussing in each case the photo-carrier recombination process and its relation with the material’s structuration/morphology. Finally, technical aspects of these novel techniques will be discussed as well as their limitations and remaining open question regarding results interpretation.