Photodetectors are semiconductor devices that can convert optical signals into electrical signals. There is a wide range of photodetector applications such as fiber optics communication, infrared heat camera sensors, as well as in equipment used for medical and military purposes. Nanowires are thin needle-shaped structures made of semiconductor materials, e.g. gallium arsenide (GaAs), indium phosphide (InP) or silicon (Si). Their small size, well-controlled crystal structure and composition as well as the possibility to fabricate them monolithically on silicon make them ideally suited for sensitive photodetectors with low noise. In this project, Fourier Transform Infrared (FTIR) Spectroscopy is used to investigate the optical characteristics of InP nanowire-based PIN photodetectors. The corresponding electrical characteristics are also measured using very sensitive instrumentation. A total of 4 samples consisting of processed nanowires with 80 nm diameter but different density and length have been examined. The experiments were conducted from 78K (-196oC) to room temperature 300K (27oC). The spectrally resolved photocurrent and current-voltage (IV) curves (in darkness & under illumination) for different temperatures have been studied and analyzed. The samples show excellent IV performance with very low leakage currents. The photocurrent scales with the number of nanowires, from which we conclude that most photocurrent is generated in the substrate. Spectrally resolved photocurrent data, recorded at different temperatures, display strong absorption in the near-infrared region with interesting peaks that reveal the underlying optical processes in the substrate and nanowires, respectively. The nature of the absorption peaks is discussed in detail. This study is an important step towards integration of optically efficient III-V nanoscale devices on cheap silicon substrates for applications e.g. on-chip optical communication and solar cells for energy harvesting.

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La diffusion Raman est réalisée dans la géométrie inhabituelle de diffusion « en avant » (fonctionnant schématiquement en ‘mode de transmission’) pour explorer la nature et les propriétés des modes phonon-polaritons (polaires) de divers cristaux mixtes A1-xBxC à base de ZnSe. Un aperçu général est recherché en sélectionnant des systèmes qui se rapportent au même composé parent par souci de cohérence - à savoir ZnSe - mais avec différentes structures cristallines, i.e. de type zincblende (cubique : Zn1-xBexSe, ZnSe1-xSx, ZnxCd1-xSe) et de type wurtzite (hexagonal : Zn1-xMgxSe). Les systèmes retenus englobent toute la panoplie des comportements dans le régime de phonons natifs (non polaires) des phonons-polaritons, y compris les déviations sur-diversifiées [1 × (AB), 2 × (AC)] et sous-diversifiées de 1 × (AB, AC) par rapport au type -nominal [1 × (AB), 1 × (AC)], également dénommées multi-mode, 1-mode-mixte et 2-mode, respectivement, dans la classification admise des spectres Raman classiques de cristaux mixtes obtenus dans la géométrie conventionnelle de rétrodiffusion (fonctionnant schématiquement dans en 'mode réflexion'). La modélisation du contour des spectres Raman phonon-polariton obtenus est réalisée dans le cadre de la théorie de la réponse diélectrique linéaire en se basant sur des mesures d’ellipsométrie de l'indice de réfraction, avec des calculs ab initio en appui réalisés sur des motifs d'impureté prototypes dans les limites diluées (x~0,1), pour sécuriser le jeu réduit de paramètres d'entrée qui régissent le comportement phonon de base (non polaire) des cristaux mixtes étudiés. La discussion des spectres Raman obtenus en rétrodiffusion /avant est effectuée dans le cadre du modèle de percolation développé au sein de l’équipe pour une compréhension renouvelée des spectres optiques de vibration des cristaux mixtes. Ce modèle formalise une vision des choses selon laquelle les liaisons chimiques d'une espèce donnée vibrent à des fréquences différentes dans un cristal mixte selon que leur environnement est de même type ou de type différent à l'échelle très locale (des premiers, voire des seconds voisins). [...] / Inelastic Raman scattering is implemented in the unusual (near-)forward scattering geometry (schematically operating in the ‘transmission mode’) to explore the nature and properties of the (polar) phonon-polariton modes of various ZnSe-based A1-xBxC mixed crystals. An overall insight is searched by selecting systems that relate to the same parent compound for the sake of consistency – namely ZnSe – but with different crystal structures, i.e. of the zincblende (cubic: Zn1-xBexSe, ZnSe1-xSx, ZnxCd1-xSe) and wurtzite (hexagonal: Zn1-xMgxSe) types. Most of all, altogether the retained systems span the full variety of behavior in the native (non polar) phonon regime of the phonon-polaritons, including the over-diversified [1×(A−B),2×(A−C)] and sub-diversified 1×(A−B,A−C) deviations with respect to the nominal [1×(A−B),1×(A−C)] type, also referred to as the multi-mode, 1-mixed-mode and 2-mode types, respectively, in the admitted classification of the conventional Raman spectra of mixed crystals taken in the backscattering geometry (schematically operating in the ‘reflection mode’). Fair contour modeling of the obtained phonon-polariton Raman spectra is achieved within the linear dielectric response theory based on ellipsometry measurements of the refractive index and with ab initio calculations in support done on prototypal impurity motifs in both dilute limits (x~0,1), as needed to secure the reduced set of input parameters that govern the native (non polar) phonon mode behavior of the used mixed crystals. The backward/near-forward Raman spectra are discussed within the scope of the so-called percolation model developed within our group for a renewed understanding of the optical vibration spectra of the mixed crystals. This model formalizes a view that the chemical bonds of a given species vibrate at different frequencies in a mixed crystal depending on their like or foreign environment at the very local (first- or second-neighbor) scale. This introduces a generic 1-bond→2-mode phonon behavior for a mixed crystal, presumably a universal one. The main results enunciate as follows. [...]

In dieser Dissertation wird die spektrale und örtliche Verteilung der Lumineszenz von Heterostrukturen in selbstorganisierten Nanodrähten (ND) mit Hilfe von Kathodolumineszenz-Spektroskopie (KL) im Rasterelektronenmikroskop untersucht. Diese Methode wird ergänzt durch Messungen der kontinuierlichen und zeitaufgelösten Mikro-Photolumineszenz. Drei verschiedene Strukturen werden behandelt: (i) GaAs-ND bestehend aus Segmenten der Wurtzit (WZ) bzw. Zinkblende (ZB) Kristallstrukturen, (ii) auf GaN-ND überwachsene GaN-Mikrokristalle und (iii) (In,Ga)N Einschlüsse in GaN-ND. Die gemischte Kristallstruktur der GaAs-ND führt zu komplexen Emissionsspektren. Dabei wird entweder ausschließlich Lumineszenz bei Energien unterhalb der ZB Bandlücke, oder aber zusätzlich bei höheren Energien, gemessen. Diese Differenz wird durch unterschiedliche Dicken der ZB und WZ Segmente erklärt. Messungen bei Raumtemperatur zeigen, dass die Bandlücke von WZ-GaAs mindestens 55 meV größer als die von ZB-GaAs ist. Die Lumineszenz-Spektren der GaN-Mikrokristalle enthalten verschiedene Emissionslinien, die auf Stapelfehler (SF) zurückzuführen sind. SF sind ZB Quantentöpfe verschiedener Dicke in einem WZ-Kristall und es wird gezeigt, dass ihre Emissionsenergie durch die spontane Polarisation bestimmt wird. Aus einer detaillierten statistischen Analyse der Emissionsenergien der verschiedenen SF-Typen werden Emissionsenergien von 3.42, 3.35 und 3.29 eV für die intrinsischen (I1 und I2) sowie für extrinsische SF ermittelt. Aus den entsprechenden Energiedifferenzen wird -0.022C/m² als experimenteller Wert für die spontane Polarisation von GaN bestimmt. Die Bedeutung sowohl der piezoelektrischen Polarisation als auch die der Lokalisierung von Ladungsträgern wird für (In,Ga)N-Einschlüsse in GaN-ND gezeigt. Hierbei spielt nicht nur die Lokalisierung von Exzitonen, sondern auch die individueller Elektronen und Löcher an unterschiedlichen Potentialminima eine Rolle. / In this thesis, the spectral and spatial luminescence distribution of heterostructures in self-induced nanowires (NWs) is investigated by cathodoluminescence spectroscopy in a scanning electron microscope. This method is complemented by data from both continuous and time-resolved micro-photoluminescence measurements. Three different structures are considered: (i) GaAs NWs containing segments of the wurtzite (WZ) and zincblende (ZB) polytypes, (ii) GaN microcrystals overgrown on GaN NWs, and (iii) (In,Ga)N insertions embedded in GaN NWs. The polytypism of GaAs NWs results in complex emission spectra. The observation of luminescence either exclusively at energies below the ZB band gap or also at higher energies is explained by differences in the distribution of ZB and WZ segment thicknesses. Measurements at room temperature suggest that the band gap of WZ GaAs is at least 55 meV larger than that of the ZB phase. The luminescence spectra of the GaN microcrystals contain distinct emission lines associated with stacking faults (SFs). SFs essentially constitute ZB quantum wells of varying thickness in a WZ matrix and it is shown that their emission energy is dominated by the spontaneous polarization. Through a detailed statistical analysis of the emission energies of the different SF types, emission energies of 3.42, 3.35 and 3.29 eV are determined for the intrinsic (I1 and I2) as well as the extrinsic SFs, respectively. From the corresponding energy differences, an experimental value of -0.022C/m² is derived for the spontaneous polarization of GaN. The importance of both carrier localization and the quantum confined Stark effect induced by the piezoelectric polarization is shown for the luminescence of (In,Ga)N insertions in GaN NWs. Not only localized excitons, but also electrons and holes individually localized at different potential minima contribute to the observed emission.

In this study, the spin-splitting energy of the lowest conduction bands in bulk zincblende and wurtzite structures of III-V semiconductors had been investigated by the linear combination of atomic orbital (LCAO) method, the atomic bond-orbital model (ABOM), and the two-band k¡Dp (2KP) model. Spin-splitting calculation for zincblende structures: We develop a 16-band atomic bond-orbital model (ABOM) to compute the spin splitting induced by bulk inversion asymmetry in zincblende materials. This model is derived from the linear combination of atomic orbital (LCAO) scheme such that the characteristics of the real atomic orbitals can be preserved to calculate the spin splitting. The Hamiltonian of 16-band center-zone ABOM (CZABOM) is based on a similarity transformation performed on the nearest-neighbor LCAO Hamiltonian with a second-order Taylor expansion over k at the £F point. The spin-splitting energies in bulk zincblende semiconductors, GaAs and InSb, are calculated, and the results agree with the LCAO and first-principles calculations. However, we find that the spin-orbit coupling between bonding and antibonding p-like states, evaluated by the 16CZABOM, dominates the spin splitting of the lowest conduction bands in the zincblende materials. Spin-splitting calculation for wurtzite structures: The spin-splitting energies in biaxially strained bulk wurtzite material AlN are calculated using the linear combination of atomic orbital (LCAO) method, and the equi-spin-splitting distributions in k-space near the minimum-spin-splitting (MSS) surfaces are illustrated. These data are compared with those derived analytically by two-band k¡Dp (2KP) model. It is found that the results from these two methods are in good agreement for small k. However, the ellipsoidal MSS surface under biaxial compressive strain does not exist in the 2KP model, because the data points are far from the £F point. Instead, three basic shapes of the MSS surface occur in the wurtzite Brillouin zone: a hyperboloid of two sheets, a hexagonal cone, and a hyperboloid of one sheet, evaluated from the LCAO method across the range of biaxial strains from compressive to tensile. The shapes of the equi-spin-splitting (ESS) surfaces near these MSS surfaces have also three types: a hyperboloid of one sheet, an approximate, asymmetric hyperboloid surface, and an opposing hyperboloid of one sheet.

Während über hexagonales (alpha) GaN zum ersten Mal 1932 berichtet wurde, gelang erst 1989 die Synthese einer mit Molekularstrahlepitaxie (MBE) auf 3C-SiC epitaktisch gewachsenen, metastabilen kubischen (eta) GaN Schicht. Die vorliegende Arbeit befaßt sich mit der Herstellung der Verbindungen eta-(In,Ga,Al)N mittels RF-Plasma unterstützter MBE auf GaAs(001) und den mikrostrukturellen sowie optischen Eigenschaften dieses neuartigen Materialsystems. Im Vergleich zur hexagonalen bietet die kubische Kristallstruktur auf Grund ihrer höheren Symmetrie potentielle Vorteile für die Anwendung in optischen und elektronischen Bauelementen. Viele wichtige Materialgrößen der kubischen Nitride sind jedoch noch gänzlich unbekannt, da sich die Synthese einkristalliner Schichten als sehr schwierig erweist. Das Ziel dieser Arbeit ist es daher erstens, die technologischen Grenzen der Herstellung von bauelementrelevanten kubischen (In,Ga,Al)N Heterostrukturen auszuweiten und zweitens, einen Beitrag zur Aufklärung der bis dato wenig bekannten optischen und elektronischen Eigenschaften des GaN und der Mischkristalle In GaN zu leisten. Zunächst wird ein optimierter MBE Prozess unter Einsatz einer Plasmaquelle hohen Stickstofflusses vorgestellt, welcher nicht nur die reproduzierbare Epitaxie glatter, einphasiger GaN Nukleationsschichten auf GaAs ermöglicht. Vielmehr können damit auch dicke GaN. Schichten mit glatter Oberflächenmorphologie hergestellt werden, welche die Grundlage komplizierterer eta-(In,Ga,Al)N Strukturen bilden. An einer solchen GaN Schicht mit einer mittleren Rauhigkeit von nur 1.5 nm werden dann temperaturabhängige Reflexions- und Transmissionsmessungen durchgeführt. Zur Auswertung der Daten wird ein numerisches Verfahren entwickelt, welches die Berechnung des kompletten Satzes von optischen Konstanten im Spektralgebiet 2.0 = 0.4 wären grün-gelbe Laserdioden. Zusammenfassung in PostScript / While the earliest report on wurtzite (alpha) GaN dates back to 1932, it was not until 1989 that the first epitaxial layer of metastable zincblende (eta) GaN has been synthesized by molecular beam epitaxy (MBE) on a 3C-SiC substrate. The present work focuses on radio frequency (RF) plasma-assisted MBE growth, microstructure, and optical properties of the eta-(In,Ga,Al)N material system on GaAs(001). Due to their higher crystal symmetry, these cubic nitrides are expected to be intrinsically superior for (opto-) electronic applications than the widely employed wurtzite counterparts. Owing to the difficulties of obtaining single-phase crystals, many important material constants are essentially unknown for the cubic nitrides. The aim of this work is therefore, first, to push the technological limits of synthesizing device-relevant zincblende (In,Ga,Al)N heterostructures and, second, to determine the basic optical and electronic properties of GaN as well as to investigate the hardly explored alloy InGaN. An optimized MBE growth process is presented which allows not only the reproducible nucleation of smooth, monocrystalline GaN layers on GaAs using a high-nitrogen-flow RF plasma source. In particular, thick single-phase GaN layers with smooth surface morphology are obtained being a prerequisite for the synthesis of ternary eta-(Ga,In,Al)N structures. Temperature dependent reflectance and transmittance measurements are carried out on such a GaN film having a RMS surface roughness as little as 1.5 nm. A numerical method is developed which allows to extract from these data the complete set of optical constants for photon energies covering the transparent as well as the strongly absorbing spectral range (2.0 -- 3.8 eV). Inhomogeneities in the refractive index leading to finite coherence effects are quantitatively analyzed by means of Monte Carlo simulations. The fundamental band gap EG(T) of GaN is determined for 5 < T < 300 K and the room temperature density of states is investigated. Systematic studies of the band edge photoluminescence (PL) in terms of transition energies, lineshapes, linewidths, and intensities are carried out for both alpha- and GaN as a function of temperature. Average phonon energies and coupling constants, activation energies for thermal broadening and quenching are determined. Excitation density dependent PL measurements are carried out for both phases in order to study the impact of nonradiative recombination processes at 300 K. A recombination model is applied to estimate the internal quantum efficiency, the (non)radiative lifetimes, as well as the ratio of the electron to hole capture coefficients for both polytypes. It is seen that the dominant nonradiative centers in the n-type material investigated act as hole traps which, however, can be saturated at already modest carrier injection rates. In summary, despite large defect densities in GaN due to highly mismatched heteroepitaxy on GaAs, band edge luminescence is observed up to 500 K with intensities comparable to those of state-of-the-art alpha-GaN. For the first time, thick InGaN films are fabricated on which blue and green luminescence can be observed up to 400 K for x=0.17 and x=0.4, respectively. Apart from bulk-like InGaN films, the first coherently strained InGaN/GaN (multi) quantum wells with In contents as high as 50 % and abrupt interfaces are grown. This achievement shows that a ternary alloy can be synthesized in a metastable crystal structure far beyond the miscibility limit of its binary constituents despite the handicap of highly lattice mismatched heteroepitaxy. The well widths of these structures range between 4 and 7 nm and are thus beyond the theoretically expected critical thickness for the strain values observed. It is to be expected that even higher In contents can be reached for film thicknesses below 5 nm. The potential application of such InGaN/GaN multi quantum wells with x >= 0.4 would thus be diode lasers operating in the green-yellow range. abstract in PostScript