Band 1   ISBN 3-89685-346-5
ISSN 1615-9284

Direkte, breitbandige II-VI- und III-V-Halbleiterstrukturen werden vorrangig als Basismaterialien für optoelektronische Bauelemente im blauen und blaugrünen Spektralbereich verwendet. Die zentrale Eigenschaft dieser Materialsysteme bildet dabei das Relaxationsverhalten nach optischer bzw. elektronischer Anregung. Hierbei treten nichtstrahlende Prozesse in Konkurrenz zu den strahlenden. Da diese nicht nur zu einer Reduzierung der Lichtausbeute sondern auch zur Erwärmung und damit zu einer Verringerung der Lebensdauer des Bauelements führen, ist die quantitative Bestimmung der nichtstrahlenden Relaxationsprozesse von besonderer Bedeutung. Unter Verwendung des Meßprinzips der Calorimetrischen Absorptionsspektroskopie bei mK-Temperaturen wurden in dieser Arbeit erstmals die nichtstrahlenden Relaxationsprozesse bandkantennaher Zustände systematisch untersucht. Dabei zeigte sich, daß auch bei Kristallen hoher optischer Qualität aufgrund von Reabsorptionen stets ein Großteil (> 70 %) der Relaxationsprozesse nichtstrahlend abläuft. Speziell im Hinblick auf die Untersuchung von Gruppe III-Nitriden konnten neben einem verschwindenden 'Bandgap-Bowing' in unverspanntem AlGaN erstmalig direkte Zusammenhänge zwischen der Lichtausbeute und verschiedenen Materialeigenschaften der Halbleiterstrukturen aufgezeigt werden.

Band 2   ISBN 3-89685-347-3
ISSN 1615-9284

Zentrales Thema dieser Arbeit ist die Fragestellung, welchen Einfluß die Dotierung auf das strukturelle und transiente Verhalten bandkantennaher Energiezustände in ZnSe-basierten Halbleiterstrukturen hat. Hierzu wurden zusätzlich zu Lumineszenz- und Absorptionsexperimenten systematische Analysen über das transiente Verhalten kohärenter und inkohärenter Energierelaxationsprozesse an n- und p- dotierten ZnSe-Hetero- und Quanten-Trog Strukturen durchgeführt. Experimentell wurden zur Zeitaufgelösten Lumineszenz komplementäre, auf Zwei-Strahl-Techniken basierende, Spektroskopie-Techniken realisiert. In stark dotierten Proben konnte das Emissionsverhalten der Donator-Akzeptor-Paarbande im Gegensatz zu dem bisherigen Modell einer dynamischen Potentialfluktuation durch eine statische, exponentielle Verteilung der Störstellen-Zustandsdichte erklärt werden. In moderat dotierten Proben konnte gezeigt werden, daß die Rekombinationskinetik der exzitonischen Komplexe durch das Umladungsverhalten der Donatoren und Akzeptoren wesentlich beeinflußt wird. In schwach n-dotierten Quantentopf-Strukturen wurden erstmals Trionen-Singulett und Triplettzustände in ZnSe nachgewiesen. Sowohl die Feinstruktur als auch das dynamische Verhalten der Trionenzustände wurden charakterisiert.

Band 3   ISBN 3-89685-350-3
ISSN 1615-9284

In this work, dynamic properties of InGaAs/GaAs quantum-dot lasers with emission wavelengths near 1.0 or 1.3 µm are investigated. Using time-resolved spectroscopy, a very small chirp of 0.007 Å/mA is demonstrated experimentally for quantum-dot lasers under direct current modulation. A linewidth enhancement factor a of 2.7 is measured in spite of the theoretical prediction of chirp-free behavior (a = 0) of quantum-dot lasers. This non-vanishing value is attributed to the asymmetric differential gain spectrum due to the contribution of excited states of quantum dots. However, the measured value is already among the best values reported for quantum-well lasers with similar sample structures. If the influence of excited states is reduced, even smaller a is expected.
The transient behavior of quantum-dot lasers is also studied here. Due to the small carrier lifetimes, quantum-dot lasers show very fast turn-on under zero bias condition. Relaxation oscillations are measured using either a streak camera or a fast MSM-photodetector. The highest relaxation oscillation frequency for the present quantum-dot lasers at room temperature is found to be 5.3 GHz, corresponding to a 3 dB modulation bandwidth of 8.2 GHz. These results on the dynamic properties indicate the important potential of quantum-dot lasers for applications in optical communication systems.

Band 4   ISBN 3-89685-356-2
ISSN 1615-9284

Novel epitaxial low-resistivity contacts to silicon (Si) as well as the self-organized formation of nanostructures are of particular importance for future Si device technology. In this work growth, chemical composition, and Schottky-barrier formation of lanthanide (Ln) silicides on n-type Si(111)7x7 and Si(001)2x1 are studied by means of photoemission (PE), also with sub-µm resolution, STM, and LEED. On Si(111)7x7 hexagonal silicide layers are found. An extremely low band bending is observed in the monolayer regime with a Fermi-level position of only 0.08±0.05 eV below the conduction-band minimum, representing the lowest value ever observed on metal/n-Si interfaces. With increasing coverage a final Schottky-barrier height of 0.32±0.05 eV is obtained. Epitaxial Si-overgrowth of the monolayer was found to conserve the Fermi-level position to a large extent allowing to exploit the flat-band conditions for device applications. In contrast to the two-dimensional silicide growth mode on Si(111)7x7, the growth of free-standing wires, wire assemblies, and µm-sized silicide clusters was observed on Si(001)2x1, indicating a strong influence of kinetic effects and of the anisotropy of the (001)-substrate surface. In PE a number of different silicide components and a band bending similar to the one on Si(111) were observed. Due to these physical properties Ln silicides on Si could play an important role in future device technology.

Band 5   ISBN 3-89685-353-0
ISSN 1615-9284

Three main contributions to the surface optical anisotropy of Cu, Ag, and Au(110) could be separated: Transitions between localized surface states at points of high symmetry in the surface Brillouin zone were found on the clean and oxygen covered Cu(110) and Ag(110) surfaces and possibly on clean Au(110). Secondly, transitions involving the bulk d and sp bands at the surface contribute to the surface optical properties of all metals investigated. Finally, on rough Ag(110) surfaces, the excitation of surface plasmons clearly contribute to the surface optical properties. On the clean Cu and Ag(110) surfaces the surface state transitions lead to strong resonances in the optical spectra. This surface peak overlaps with a bulk transition in the case of copper. On Ag(110), on the other hand, the corresponding transition is energetically separated from the bulk transitions.

Band 6   ISBN 3-89685-364-3
ISSN 1615-9284

Group-III Nitrides are the new basic material for a whole new generation of optoelectronic devices, operating from the ultraviolett to the red spectral range. Their properties make them extremly interesting for applications such as light-emitting diodes or laser diodes, for high-power electronics and UV-detectors. Despite these technical advances the basic physics of this material system remained barely understood. Fundamental issues like the origin of the emission processes and the mechanisms of optical amplification are still subject of discussion.
Therefore, the main focus of this work is the systematic investigation of processes concerning the emission and lasing in group-III nitrides. These investigations comprehend the basic material GaN as well as its alloys, AlGaN and InGaN. Excitonic processes like the biexciton decay, inelastic exciton-exciton- and free-carrier-scattering represent the fundamental mechanisms of optical gain in hexagonal and cubic GaN epitaxial layers. The recombination and gain processes of the cubic phase of GaN and its alloys were investigated for the first time.
Furthermore, this work contains a systematic study of the optical amplification of AlGaN epitaxial layers. From experiments on AlGaN/GaN quantum well structures it was shown, that the magnitude and direction of polarization fields determine the emission wavelength. This quantum-confined Stark effect is the origin of increased threshold values for lasing. From pump&probe experimens an excellent agreement with investigations on optical amplification by means of variation-of-stripelength-method were obtained.
In this work it was evidenced that localization of carriers in low-dimensional structures is the driving force for optical amplification in InGaN-based devices. The optical properties under high excitation as well as the lasing processes are dominated by transitions from localized carriers, which was also confirmed by intensity-dependent pump&probe measurements. From all these results an explanation was given for the high-efficiency of InGaN-based devices. The mechanism of carrier localization prevails the detrimental influence of strong piezo-electric fields.

Band 7   ISBN 3-89685-366-X
ISSN 1615-9284

Theoretical predictions of the electronic properties of semiconductor quantum dots and wires provide a link between investigations of the structural and optical properties of such systems. A complex numerical model enables the simulation of the impact which shape, chemical composition, strain, and piezoelectricity have on the electronic and optical properties of nanostructures, eventually leading to a comprehensive understanding of a manifold of prevailing experimental data. The eight-band k·p model provides a fast and transparent connection between the electronic structure of quantum dots or wires and the bulk properties of the constituent materials. Absorption spectra and fine-structure properties of few-particle states are subsequently calculated within a coherent framework. The predictive power of the method is demonstrated by comparison with previous second-principles calculations and various experiments, yielding outstanding agreement between calculated and measured quantities. As specific nanostructures, capped InAs quantum dots on GaAs (001) grown in the Stranski-Krastanow mode, and V-groove quantum wires of InGaAs fabricated by growth on patterned substrates of AlGaAs or InP are modeled.

Band 8   ISBN 3-89685-375-9
ISSN 1615-9284

The group iii nitrides gan, inn and aln and their related ternary and quaternary alloys have emerged as important materials for the development of optoelectronic devices such as light emitting devices, laser diodes and detectors. the technological progress culminated in the fabrication of the first continuous-wave laser diode at room temperature for the ultraviolet spectral range in 1996. some major physical problems appearing for the group III nitrides had to be solved and gave rise to the investigations reported in this thesis. Here, the main focus is on the optical, vibrational and crystalline properties of various nitride structures. Different optical methods, such as (resonant) Raman scattering, (time-resolved) photoluminescence spectroscopy and cathodoluminescence microscopy, were applied.

The hole concentration of Mg-doped GaN is limited to some 1018 cm-3 due to self compensation. The energetic position of the compensating donor levels could be determined as 240, 350 and 850 meV, respectively, below the conduction band. Furthermore, it is known that hydrogen compensates the Mg-acceptors, which makes a post-growth treatment necessary. Compensation by the formation of MgGa-N-H-complexes can be monitored by observing a local vibrational mode at 3125 cm-1. However, a variety of modes around 2200 cm-1 and in the range of the host phonons appears only for p-conducting GaN.

The lack of a proper substrate material for the heteroepitaxy of GaN lead to the development of the epitaxial lateral overgrowth method in order to produce material with low dislocation density. Microscopic investigations show that strong (unintentional) doping and strain gradients are present in such overgrown structures. Free electron concentrations up to 1.4·1019 cm-3 were found above the masks independently of the mask material. Compressive stress of 0.8 GPa was determined near the substrate interface. Finite-element simulations were performed for the understanding of the stress relaxation.

The formation of nitride alloys is also afflicted with difficulties. Composition fluctuations and phase separation occur in InGaN. In this work it is shown how optical methods can be used to determine the degree of these fluctuations. In addition, the mechanism of stress relaxation by cracking in AlGaN/GaN-heterostructures is described.

Band 9   ISBN 3-89685-385-6
ISSN 1615-9284

Die epitaktische Herstellung von (In,Ga,Al)N-Halbleiterschichten auf Silizium-Substraten mittels metallorganischer Gasphasenepitaxie und die Charakterisierung solcher Schichten ist Gegenstand des vorliegenden Werkes. Diese Nitrid-Verbindungen sind die Grundlage für elektronische und optoelektronische Bauelemente wie Transistoren, Photodetektoren, LEDs und Laser mit vielfältigen Anwendungen im sichtbaren bis ultravioletten Spektralbereich. Die Abscheidung der ersten GaN-Schicht auf dem Silizium-Substrat ist hierfür von zentraler Bedeutung, da deren Qualität maßgeblich die Eigenschaften aller nachfolgenden Schichten bestimmt. Die in dieser Arbeit erstmals angewendete Nukleationstechnik fährt, beruhend auf der Abscheidung einer AlAs-Schicht, die nachfolgend in AlN umgewandelt wird, zu einer sehr guten, für die Realisierung von Bauelementen wie z.B. LEDs notwendigen und hinreichenden Schichtqualität. Die Abscheidung ternärer Nitrid-Verbindungen wie InGaN und AlGaN wird exemplarisch durchgeführt und letztlich eine InGaN/AlGaN/GaN-LED mit einer Emissionswellenlänge bei 430 nm realisiert.
Zur Reduktion der hohen Versetzungsdichte in den GaN-Pufferschichten wird eine neue Methode präsentiert, die auf dem lateralen Überwachsen strukturierter Si-Substrate beruht. Anders als bei bisherigen Verfahren, wird die Versetzungsreduktion in einem unterbrechungsfreien Wachstumsprozeß und ohne Oberflächenmaskierung erreicht.

The focus of this work is on the Metalorganic Chemical Vapor Phase Deposition (MOCVD) of (In,Ga,Al)N-semiconductor layers on Silicon substrates. These layers are the basis of modern electronic and optoelectronic devices like transistors, photodetectors, LEDs and Laserdiodes with many applications in the visible and ultraviolet spectral range. The deposition of the initial GaN layer on the Silicon substrate is of crucial importance, since the quality of all subsequently grown layers depends on the perfection of this buffer layer. The newly developed growth process as described herein is based on an AlAs nucleation layer and its subsequent conversion to AlN and leads to comparable well buffer layer properties as in the case of nitride epitaxy on sapphire substrates. Doping with Si and Mg as well as the growth of ternary alloys like InGaN and AlGaN on such GaN buffer layers were also investigated and, finally, concluded to a simple LED-device structure with an emission wavelength around 435 nm.
Furthermore, a new approach to reduce the very high dislocation density in the GaN buffer layers is successfully realized by using structured Si substrates. Unlike the previously used techniques, the reduction of the dislocation density is achieved by an interruption-free and mask-free growth process.

Band 10   ISBN 3-89685-388-0
ISSN 1615-9284

Cross-sectional scanning tunneling microscopy (XSTM) of buried semiconductor nanostructures is the most important method to directly provide detailed information of their atomic composition. This work concentrates on two main subjects: Firstly, it is shown how to use strain relaxation as contrast mechanism in XSTM to evaluate the atomic structure not only at the cleavage surface but also underneath. By comparison with numerical simulations based on continuum mechanical strain theory the shape, size and stoichiometry of InAs/GaAs quantum dots can be determined much better than without. Furthermore it is shown, that this contrast mechanism plays a major role only in the case of cleaved quantum dots (and wires as well), whereas it is much less distinctive for quantum wells. Secondly, different InAs/GaAs quantum dot samples are observed. It is shown that the dot formation process and the spatial and chemical properties of the dots are sensitively dependent on the growth parameters. Especially the growth rate and the arrangement of growth interruptions are important for the quantum dot formation. In one case distinctive quantum dots are found, for which the shape, size and stoichiometry could be determined in detail. In another case only strongly segregated InGaAs layers with fluctuation of the InAs concentration are present. Additionally, a small part of this work is attended to the concentration profile in InGaAs/GaAs quantum dots, at which a so-called reversed cone arrangement can be confirmed by strain relaxation simulations.

Band 11   ISBN 3-89685-395-3
ISSN 1615-9284

ln this work several scanning nearfieId optical microscopes (SNOM) were deveIoped and realized to study the photoluminescence behavior of individual quantum dots in the temperature range between 4 K and 300 K. The different experimental designs are presented in detail. With these instruments, several properties of the InGaAs quantum dots were observed for the first time, which lead to a strongIy improved understanding of both the physical nature of optical transitions in quantum dots and the interaction between the fiber tip and the sample. It is shown that the photoIuminescence signal of an individual quantum dot is described by a temperature-dependent Lorentzian spectral Iíne shape with line widths of 1O-20 meV at 300 K decreasing to less than 1 meV at 4 K. This behavior is assigned to a temperature-induced lifetime effect. AIso multiexciton effects have been studied and characterized. Binding energies of about 16 meV for biexcitons and 10 meV for trions have been found, which are unexpectedly high for III-V semiconductor quantum dots. Furthermore, ring-like emission profiles have been found in some photoIuminescence scanning images of individual quantum dots. This novel effect is related to a Stark effect induced by an electrically charged tip. Theoretical simulations yieId a very good agreement with the experimental results.

Band 12   ISBN 3-89685-398-8
ISSN 1615-9284

Modern epitaxy methods enabled the fabrication of nanostructures formed in a self-organized way, referred to as quantum dots. These structures are considerably interesting for the use in semiconductor devices such as lasers and detectors. This work focusses on the growth of self-organized GaSb quantum dots in GaAs matrix employing Metalorganic Chemical Vapor Phase Deposition. Important issues such as the formation process of these quantum dots during the growth interruption after the deposition of the layer are investigated. The results are of general relevance also for the understanding of quantum dot formation in other material systems.

The electronic type II structure of GaSb/GaAs quantum dots implies the localization of holes within the quantum dots with large localization energies and the localization of the electrons in the surrounding of the quantum dots. The specific electronic properties are investigated by and optical and electrical means, e. g. excitation-, temperature- and time dependent photoluminescence measurements and time dependent capacitance measurements.

Furthermore, the combination of quantum dots of different materials such as lnAs and GaSD in GaAs opens up a new pathway for the modification of structural and electronic properties of the combined layers. Such important issues as the density of the upper quantum dot layer and the emission wavelength of the quantum dot stack as compared to a single quantum dot layer are assessed in this work.

Band 13   ISBN 3-89685-201-9
ISSN 1615-9284

The demand for higher processing speeds has driven the semiconductor device fabrication field to seek out new ideas and realize new techniques. Implementing spinpolarized current transport in semiconductor devices is one such concept in spintransistor design that can provide respectable power gain and the possibility to switch devices via an externally applied magnetic field. This so called spininjection can be realized by combination of magnetic and semiconducting materials but is limited by scattering at interfacial defects (e.g. chemical reactions). In this work, the growth of epitaxial cobalt contacts on the polar (OO1) surfaces of GaAs and InGaP is presented. tn order to find optimal conditions for the growth of closed and ordered Co films the characterization of the crystalline and chemical properties of the interface is investigated. Depending on different growth parameters, like substrate temperature and stoichiometry, the chemicaI reactions, as well as the structural properties like roughness and island size are monitored. Also important in this system is the introduction of a diffusion barrier, ErAs, between the Co and semiconductor, which limits the interfacial reactions between the materials. For the analysis a multi-technique approach using Scanning Trunneling Microscopy, Reflectance Anisotropy Spectroscopy, Electron Diffraction, and Photoemission Spectroscopy is chosen. The wide variety of techniques allows for a detailed picture of the interface formation to be created. The characterization of the initial adsorption processes on an atomic scale, subsequent interface formation, and finally thick Co film growth will lead to a better understanding of metal contacts and how to limit interface reactions.

Band 14   ISBN 3-89685-202-7
ISSN 1615-9284

The (001) planes of III-V compound semiconductor surfaces have been investigated for many years now. Recently, the surfaces comprising phosphorus and the group-III elements In and Ga are attracting considerable interest due to their potential as optoelectronic devices in telecommunication systems or in consumer electronics such as DVD players. However, before commencement of this thesis, little was known about the technologically important (001) planes of these surfaces. To this end, under group-III rich surface conditions, InP, GaP and In_xGa_1-xP (001) surfaces have been investigated in some detail, each giving rise to a (2x4) reconstruction. Based on STM ond SXPS results it has been shown that for all three materials, a new type of model, the ,,mixed-dimer model'', provides a clear description of the atomic structure of the (2x4) reconstrucrion. for P-rich InP(001) it is conclusively shown that two structurally different (2x1)/(2x2) surfaces are observed on samples grown by MOVPE, which both refer to hydrogen terminated surface structures. The role of hydrogen in the formation of these surfaces is proven by MBE growth with and without hydrogen assistance, demonstrating its importance in the growth process itself.

Band 15   ISBN 3-89820-629-7
ISSN 1613-6411

In dieser Arbeit werden die optischen und elektronischen Eigenschaften von In(Ga)As/GaAs Quantenpunkten mit Hilfe von spektroskopischen Methoden, im Besonderen der Photolumineszenz Anregungsspektroskopie (PLE), untersucht. Es werden die Unterschiede zwischen den Einteilchen- und Mehrteilchenzuständen in neutralen und geladenen Quantenpunkten für Grund- und angeregte Zustände aufgezeigt. Die Renormalisierungseffekte werden quantifiziert und die für die Renormalisierung verantwortlichen Parameter identifiziert. Darüber hinaus erlauben die Untersuchungen die Zuordnung der an den Übergängen beteiligten Elektronen- und Lochniveaus. Des Weiteren wird der Einfluss von gezielt eingebrachten Defekten durch Protonenbeschuss auf die elektronischen Eigenschaften von Quantenpunkten untersucht. Dabei konnte die Bestrahlungsunempfindlichkeit des Quantenpunktgrundzustandes nachgewiesen sowie die für die Ladungsträgerdynamik verantwortlichen Relaxationsprozesse identifiziert werden. Die Verspannung der untersuchten Halbleiter-Nanostrukturen, die auch für deren selbstorganisiertes Wachstum verantwortlich ist, hat einen entscheidenden Einfluss auf die elektronischen Eigenschaften der Quantenpunkte. Der Vergleich zwischen optischer Untersuchung und theoretischer Modellierung von InAs Quantenpunkten, die mit einem In_xGa_1-xAs-Quantenfilm (x≤0,25) überwachsen wurden, zeigt die zwei wesentlichen Faktoren auf, die für die erwünschte Rotverschiebung der Quantenpunktlumineszenz verantwortlich sind: Die Dekomposition des In_xGa_1-xAs-Quantenfilms und die veränderte Verzerrungsverteilung sowie deren Auswirkung auf die elektronischen Eigenschaften der Quantenpunktstruktur. Die Stärke der einzelnen Effekte wird quantifiziert. Schließlich wird aufgezeigt, dass aus optischen Ergebnissen (für Proben unter bestimmten Wachstumsbedingungen) Rückschlüsse auf wesentliche strukturelle Eigenschaften der untersuchten Quantenpunkte gezogen werden können: Aus der bis zu achtfach strukturierten Photolumineszenz kann auf eine multimodale Verteilung von flachen InAs/GaAs Quantenpunkten mit scharfen, ebenen Grenzflächen, die sich in Monolagenstufen in der Quantenpunktgröße unterscheiden, geschlossen werden.