Les cellules solaires à multi-jonctions de type III-V possèdent des rendements de conversion de l'énergie très élevés (46%). Cependant, les méthodes de fabrication généralement utilisées sont complexes et coûteuses, notamment pour les cellules solaires non monolithiques associées par des techniques de collage et à structure inversée. Cette thèse vise à augmenter les rendements de conversion des cellules solaires monolithiques à l'aide de méthodes prospectives. Le travail est focalisé sur l'étude des défauts électroniquement actifs dans les matériaux constituant les cellules solaires au moyen de techniques photoélectriques et capacitives, et il peut être scindé en trois parties. La première partie traite des cellules solaires à simple jonction avec des couches absorbantes non dopées d'alliages InGaAsN de 1 eV de bande interdite de différentes épaisseurs obtenues sous forme de super-réseaux (InAS / GaAsN) par épitaxie à jets moléculaires (MBE) sur des substrats de GaAs. Pour des épaisseurs inférieures à 1200 nm, la concentration de défauts est négligeable et n'affecte pas fortement les propriétés photoélectriques, tandis que que pour une épaisseur de 1600 nm, la forte concentration de défauts détectés réduit la durée de vie des porteurs photogénérés, et conduit à une baisse significative du rendement quantique externe et des performances de la cellule. La deuxième partie du travail est consacrée à l'étude de cellules solaires à une et plusieurs jonctions avec des couches actives de (In)GaP(As)N obtenues par MBE sur des substrats respectifs de GaP et de Si. Nous avons trouvé que les cellules solaires de type p-i-n avec des couches actives de GaPAsN non dopé présentaient de meilleures performances que les cellules solaires de type p-n avec des couches actives de GaPAsN dopé n. De plus, les cellules solaires avec une couche d'absorbeur en GaPAsN non dopé présentent de meilleures propriétés photoélectriques et des concentrations de défauts plus faibles que celles avec un absorbeur obtenu à partir de super-réseaux InP / GaPN. Plusieurs niveaux de défauts ont été détectés dans la bande interdite de ces matériaux et leurs paramètres ont été décrits en détail. Nous avons montré qu'un traitement de post-croissance approprié pouvait améliorer la qualité électronique des couches et des cellules solaires. Une cellule solaire à triple jonction a été fabriquée avec des couches actives d'absorbeurs de GaPAsN et de GaPN non dopées. La valeur élevée de la tension de circuit ouvert (>2,2V) atteste du fonctionnement des 3 sous-cellules, mais la performance globale est limitée par les faibles épaisseurs de couches d'absorbeurs. Enfin, la troisième partie du travail est consacrée à l'étude de couches de GaP obtenues sur des substrats de Si à des températures inférieures à 400 ° C par une méthode originale de dépôt de couches atomiques assistée par plasma (PE-ALD). En effet, celle-ci utilise un équipement de dépôt chimique en phase vapeur assisté par plasma et elle repose sur l'interaction de la surface avec les atomes de Ga et P provenant respectivement du triméthylgallium et de la phosphine qui sont injectés alternativement. Nous avons également fait croître des couches en utilisant un processus continu (fournissant simultanément les atomes P et Ga) et observé que leurs propriétés électriques et structurelles étaient moins bonnes que celles obtenues par la méthode PE-ALD proposée. Nous avons exploré l'influence des conditions de croissance sur les hétérostructures GaP / Si. Nous avons constaté qu'une faible puissance de plasma RF conduit à de meilleures propriétés photoélectriques, structurelles et à moins de défauts, grâce à une meilleure passivation du substrat de silicium. En outre, nous avons démontré que, contrairement à des résultats de la littérature utilisant des procédés MBE, la technique PE-ALD n'affecte pas ou très peu les propriétés électroniques des substrats de silicium et aucune désactivation des dopants n'a été observée. / Multi-junction solar cells based on III-V compounds have reached very high power conversion efficiencies (46%). However, the fabrication methods that are generally used are complex and expensive for non-monolithic bonded and inverted solar cells. This thesis is devoted to the study of prospective methods to increase the efficiency of monolithic solar cells. The work is focused on the study of electronically active defects in the materials constituting the solar cells by means of photoelectric and capacitance techniques (admittance spectroscopy, DLTS,…) and it can be divided into three parts. The first part deals with single-junction solar cells wherein the absorber is made of i-layers of 1 eV bandgap InGaAsN compounds with various thicknesses grown as sub-monolayer digital alloys (SDA) of InAs/GaAsN by molecular-beam epitaxy (MBE) on GaAs wafers. The cell with 900 nm thick InGaAsN exhibits the best photovoltaic performance and no defects could be evidenced from capacitance techniques. When the thickness is increased to 1200 nm, defects were detected, but their concentration is low so it did not strongly affect the photoelectric properties. Further increase to 1600 nm of the layer thickness was shown to lead to a higher defect concentration causing a change in the band diagram of the structure and lowering the lifetime of photogenerated carriers. This could explain the drastic drop of the external quantum efficiency, and the overall poor performance of the solar cell. The second part is devoted to the study of single- and multi-junction solar cells with active layers of (In)GaP(As)N grown by molecular beam epitaxy (MBE) on GaP and Si wafers, respectively. More precisely, the active layers were either quaternary alloys of GaPAsN or SDAs of InP/GaPN. We found that p-i-n type solar cells with active layers of i-GaPAsN showed better performance than p-n type solar cells with active layers of n-GaPAsN due to higher EQE values. Moreover, solar cells with an i-GaPAsN absorber layer show better photoelectric properties and lower defect concentrations, than those with an SDA InP/GaPN absorber layer. Different defect levels were detected by capacitance methods in these materials and their parameters were described in detail. We showed that a suitable post-growth treatment could improve the electronic quality of the GaPAsN layer and the solar cell properties. Also, a triple-junction solar cell was fabricated with active layers of i-GaPAsN and i-GaPN. All subcells were found to be operating, leading to a large open circuit voltage (>2.2 V), but the overall performance is limited by the low value of the quantum efficiency due to low thicknesses of i-layers that should be increased for better absorption. Finally, the third part is devoted to the study of GaP layers grown on Si wafers at temperatures below 400 °C using an original method called plasma-enhanced atomic-layer deposition (PE-ALD). Indeed, it uses a plasma-enhanced chemical vapor deposition equipment and it is based on the alternate interaction of the wafer surface with Ga and P atoms coming from injected trimethylgallium and phosphine, respectively. We also grew layers using a continuous process (providing simultaneously the P and Ga atoms) and observed that their electric and structural properties were poorer than that grown by the proposed PE-ALD method. The influence of growth conditions on the GaP/Si heterostructures was explored. We found that low RF-plasma power leads to better photoelectric, structural and defect-related properties, due to a better passivation of the silicon wafer. In addition, we demonstrated that, contrary to results reported in the literature using MBE processes, our growth process does not affect the electronic properties of phosphorous doped n-Si wafers, while slight changes were observed in boron-doped p-Si wafers containing Fe-related defects, however without deactivation of the doping nor strong degradation of the electronic properties.

This thesis deals with infrared studies of impurity states, ultrafast carrier dynamics as well as coherent intersubband polarizations in semiconductor quantum structures such as quantum wells and superlattices, based on the GaAs/AlGaAs material system. In the first part it is shown that the 2pz confined impurity state of a semiconductor quantum well develops into an excited impurity band in the case of a superlattice. This is studied by following theoretically the transition from a single to a multiple quantum well or superlattice by exactly diagonalizing the three-dimensional Hamiltonian for a quantum well system with random impurities. Intersubband absorption experiments, which can be nearly perfectly reproduced by the theory, corroborate this interpretation, showing that at low temperatures in the low doping density regime all optical transitions originate from impurity transitions. These results also require reinterpretation of previous experimental data. The relaxation dynamics of interminiband transitions in doped GaAs/AlGaAs superlattices in the mid-IR are studied. This involves single-color pump-probe measurements to explore the dynamics at different wavelengths, which is performed with the Rossendorf freeelectron laser (FEL), providing picosecond pulses in a range from 3-200 µm and are used for the first time within this thesis. In these experiments, a fast bleaching of the interminiband transition is observed followed by thermalization and subsequent relaxation, whose time constants are determined to be 1-2 picoseconds. This is followed by an additional component due to carrier cooling in the lower miniband. In the second part, two-color pump-probe measurements are performed, involving the FEL as the pump source and a table-top broad-band tunable THz source for probing the transmission changes. These measurements allow a separate specification of the cooling times after a strong excitation, exhibiting time constants from 230 ps to 3 ps for different excitation densities and miniband widths. In addition, the dynamics of excited electrons within the minibands is explored and their contribution quantitatively extracted from the measurements. Intersubband absorption experiments of photoexcited carriers in single quantum well structures, measured directly in the time-domain, i.e. probing coherently the polarization between the first and the second subband, are presented. From the data we can directly extract the density and temperature dependence of the intersubband dephasing time between the two lowest subbands, ranging from 50 up to 400 fs. This all optical approach gives us the ability to tune the carrier concentration over an extremely wide range which is not accessible in a doped quantum well sample. By varying the carrier density, many-body effects such as the depolarization and their influence on the spectral position as well as on the lineshape on the intersubband dephasing are studied. Also the difference of excitonic and free-carrier type excitation is discussed, and indication of an excitonic intersubband transition is found.

L’objet de cette thèse est d’étudier le dopage ex-situ de nanofils semiconducteurs IV pour des applications en électronique, spintronique ou encore thermoélectricité. Deux techniques de dopage ont été explorées : l’implantation par faisceaux d’ions et le Spin-On-Doping (SOD).L’implantation d’ions Mn a été testée dans les nanofils de Ge avec l’objectif de synthétiser un matériau semi-conducteur ferromagnétique dilué. Une concentration en Mn de quelques pourcents peut être atteinte sans amorphisation du fil ni formation de précipités, ce qui est très encourageant. Lors d’expériences d’implantation réalisées in situ dans un microscope électronique en transmission, une forte exaltation de la pulvérisation sous irradiation électronique a été constatée.La technique SOD consiste à faire diffuser thermiquement les impuretés de type p ou n contenues une résine de type HSQ (Hydrogen silsesquioxane) qui enrobe les nanofils. Le recuit de la HSQ (dopée ou non) engendre une modification structurale des nanofils (bien que cette technique soit considérée comme non destructive). Lors du recuit, une transformation partielle de la phase diamant 3C vers la phase hexagonale 2H, a en effet été observée dans les nanofils de Si et de Ge, au-delà de 500 et 400°C respectivement. Les paramètres essentiels de la transformation de phase sont la contrainte de cisaillement résultant de la densification de la résine et le budget thermique. Les nanofils de Ge deviennent amorphes au-delà de 650°C, ce qui interdit en pratique leur dopage par SOD.Les caractérisations électriques ont été réalisées sur des nanofils de Si réalisés par gravure ionique réactive sur substrats orientés (111) et contactés en matrice ou individuellement. Pour le contactage de nanofils uniques en configuration NW-FET (nanowire field effect transistors), un procédé technologique basé sur la lithographie électronique a dû être développé. Les difficultés à surmonter étaient relatives à la faible longueur des nanofils. Diverses techniques de caractérisation ont été mises en œuvre (I-V en configuration verticale ou horizontale de type TLM (Transient Linear Measurement), SSRM (scanning spreading resistance microscopy), EBIC (electron beam induced current). Les mesures collectives concernent des ensembles de nanofils de type p enrobés dans une résine qu’elle soit dopante ou non. Pour observer un courant notable dans la structure, un recuit est nécessaire. Au-delà d’une température de recuit de 600°C, une polarisation négative du substrat induit un comportement conforme au mécanisme SCLC (space charge limited current) attendu pour des nanofils faiblement dopés enrobés dans une matrice isolante. En positif, on observe une caractéristique I(V) ohmique et une densité de courant jusqu’à 500 fois plus élevée dans les nanofils que dans le substrat. Ce comportement pourrait être dû à l’influence des états d’interface provenant de la technique de gravure. Cette hypothèse est confortée par le fait qu’après recuit à 900°C, le courant en direct s’explique en considérant dans les fils un dopage proche de celui du substrat, et surtout par l’observation en SSRM d’une couche conductrice interfaciale entre fils et HSQ. Elle permet aussi d’interpréter les mesures sous pointes faites sur les fils de type n. Le mode de transport SCLC a également été observé pour des nanofils individuels contactés sous pointe ou par lithographie. Ces mesures n’ont pas mis directement en évidence l’influence de la transformation de phase.Le dopage de type n ou p par SOD s’avère efficace après recuit à 900°C. Dans ce cas, les comportements observés, contacts ohmiques et jonctions p-n, peuvent être interprétés plus simplement en considérant des niveaux de dopage supérieurs à 3×10¹⁶ cm⁻³ en type p et 2×10¹⁶ cm⁻³ en type n. Ces valeurs déduites des résistivités mesurées sont sans doute très sous-estimées puisque la mobilité dans les fils est sans doute inférieure à celle du volume. / This thesis aims at studying the ex-situ doping of semiconducting nanowires (NWs) for applications in electronics, spintronics or thermoelectricity. Two widely used techniques have been envisaged: ion beam implantation and Spin-On-Doping (SOD).The ion beam implantation of Mn ions has been tested in Ge NWs in an attempt to form a 1D diluted magnetic semiconductor structure. A Mn concentration of few percents can be achieved without amorphization of the nanowire nor clustering, what is very promizing. During implantation done in situ in a transmission electron microscope, a strong enhancement of the sputtering under electron irradiation has been observed.The doping by SOD results from the thermal diffusion of p-type or n-type impurities contained in a HSQ (Hydrogen silsesquioxane) resist in which the NWs are embedded. The curing of the HSQ resist (doped or not) leads to a structural modification of nanowires (while SOD is generally assumed to be non-destructive). As a result of the annealing, a partial transformation of the 3C diamond phase towards the 2H hexagonal phase is observed in Si and Ge nanowires, above 500 et 400°C respectively. The main parameters of that phase transformation are the shear stress due to the HSQ densification and the thermal budget. Ge NWs are found to turn to amorphous above 650°C, what renders SOD practically unusable for Ge NWs. Two methods are currently used for the fabrication of nanowires, the VLS (vapor-liquid-solid) growth and reactive ion etching of (111) Si wafers. For practical reasons, etched NWs were used for the study of their electrical properties.The electrical characterizations were done on arrays of Si NWs embedded in a HSQ matrix or on single NWs. For contacting single NWs in the NW-FET(nanowire field effect transistors) configuration , a process based on electron beam lithography has been developed. The issues to be solved were related to the low length of NWs. Various measurement techniques were used: I-V in two tips or TLM (Transient Linear Measurement) arrangement, SSRM (scanning spreading resistance microscopy), EBIC (electron beam induced current). Collective measurements were done on arrays of p-type NWs embedded in a HSQ resist, doped or not. It was firstly observed that an annealing is needed to observe a noticeable current in the structure. Above an annealing temperature of 600°C, for a negative bias applied to the substrate, the observed behavior can be described by the SCLC (space charge limited current) mechanism expected for poorly doped NWs in an isolating matrix, while a positive bias applied to the substrate results in an ohmic characteristic and in a current density up to 500 times higher in the NWs than in the substrate. This unexpectedly high intensity in direct bias may be attributed to electrically active surface states resulting from the etching process. This hypothesis is conforted by the fact that an annealing at 900°C (without extra doping) the measured intensity can be explained by assuming the same doping level in NWS than in the substrate. In addition, an interfacial conductive between resist and nanowires can be observed by SSRM. These interfacial states can be also involved for understanding the measurements done on n-type NWS. The SCLC mechanism of transport has been also observed for single NWs contacted by tips or by lithographied contacts. These measurements were not able to evidence the effect of the phase transformation on the electrical properties.P-type and n-type doping by SOD becomes effective after annealing at 900°C. After doping, ohmic or rectifying behaviours on p-type substrate are observed as expected. That renders more easy the interpretation of results, by assuming doping levels in the NWs of 3×10¹⁶ cm⁻³ and 2×10¹⁶ cm⁻³ for p-type and n-type respectively. These values as deduced from resistivities are probably very underestimated as the mobilities in NW are probably much lower than in the bulk.

La thèse porte sur l'étude théorique des propriétés électroniques à la surface de l’oxyde de métal de transition SrTiO3. Ce matériau est la pierre angulaire de l'électronique des oxydes, un nouveau domaine de recherche qui a pour but d'enquêter sur les oxydes de métaux de transition en tant que candidats post-silicium pour une émergence future de nouveaux composants électroniques. Le SrTiO3 est en soi un système étonnant : dans sa plus pure composition chimique, c’est un bon isolant avec une large bande interdite. Cependant, en le dopant avec de petites quantités d'autres éléments, il se transforme en un métal à haute mobilité d'électrons. Le SrTiO3 a également saisi l'attention en raison de sa capacité à accueillir des gaz d'électrons bidimensionnels (2DEGs) quand il est interfacé avec certains oxydes polaires. Ces 2DEGs présentent des propriétés fascinantes, la plus visible étant la coexistence du magnétisme et de la supraconductivité.La surface du SrTiO3 peut également accueillir des 2DEGs, sans avoir besoin de s'interfacer avec d'autres matériaux ; dans ce cas, les électrons participant aux transports sont générés par des lacunes d'oxygène créées à la surface. Cette observation est remarquable, car le SrTiO3 offre une structure simple où les propriétés des 2DEGs peuvent être étudiées.Cette thèse s’articule autour des deux axes. Tout d'abord, elle étudie la bicouche STO orientée 111, formée de seulement deux cellules unitaires. Deuxièmement, elle analyse les puits quantiques générés par les postes vacants de l'oxygène à la surface 111 du STO. Les deux sujets sont abordés en utilisant des modèles de liaison forte, dans lesquels le Hamiltonien incorpore différents termes liés aux énergies sur place, aux interactions de saut et au couplage spin-orbite. A partir de ces calculs, j’ai réalisé une analyse exhaustive des propriétés, du caractère et de la parité des orbitaux des bandes de valence et de conduction, ainsi que des états de bord dans la bicouche 111. / The thesis is focused on the theoretical study of the electronic properties at the surface of the transition metal oxide STO. This material is the cornerstone of oxide electronics, an emerging research area that has the goal of investigating transition metal oxides as post-silicon candidates for a future emerging new electronics. STO is in itself an astounding system; in its purest chemical composition is a good ban-insulator with a wide bandgap. Nevertheless, upon doing it with tiny amounts of other elements it transforms itself in a metal with high electron mobility. Even more remarkably, at the lowest temperatures, typically below 300mK, it goes superconductor. And adding to these properties, strain induces also ferroelectricity in this material. Over the last years, STO has also grabbed attention because of its ability of hosting two-dimensional electron gas (2DEGs) when it is interfaced with some polar oxides. Such 2DEGs exhibit fascinating properties, the most conspicuous is the coexistence of magnetism and superconductivity.The surface of STO can host 2DEGs too, without need of interfacing it to other materials; in this case the electrons participating in transport are generated by oxygen vacancies created at the surface. This is remarkable observation, as it affords a simpler structure where the 2DEGs properties can be studied. In spite of the accumulated knowledge, still a better fundamental comprehension is required of the electronic structure of the quantum wells at the surfaces oriented along the 111 direction, for which the perovskite structure is reminiscent of the celebrated honeycomb-like structure of graphene. Contrary to the latter, in which electrons are in s- and p- states, 111 quantum wells in STO would host electrons in d-bands. Higher electronic correlations are then expected, that may bring new fascinating physics.The outline of this Thesis has two main branches: first, it studies the 111-oriented STO bilayer, formed by just two unit cells; secondly it analyzes the quantum wells generated by Oxygen vacancies at the 111-surface of STO. Both subjects are approached using tight-binding models in which the Hamiltonian incorporates different terms related to on-site energies, hopping interactions or spin-orbit coupling. From these calculations, I have carried out an exhaustive analysis of the orbital character and parity properties of valence and conduction bands, as well as edge states in the 111 bilayer. Tight-binding calculations have also shed light on the orbital character, space location and extension and energy of electronic states generated by oxygen vacancies at the 001 surface of STO.

No description available.

The family of semiconducting 2H-phase group-VI transition metal dichalcogenides (TMDs) have been suggested to be promising candidates for hosting optically accessible spin qubits due to their desirable optical and electrical properties, however, experimental progress towards this goal has been impeded by the difficulties associated with the fabrication of clean structures with quality contacts. In this thesis, we present the complex process for obtaining functional contacts to two particular TMDs, molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), from which we use as the foundation for the fabrication of three important gate defined quantum structures: quantum dots, a charge detector, and a long 1D channel. These structures all play an important role in furthering the understanding of these materials and are the building blocks for achieving functional spin qubits. More precisely, we investigate the contact resistances associated with various cleaning procedures and contact architectures and report a recipe that results in an ultra-low contact resistance even at cryogenic temperatures. We then demonstrate electrical control of hole quantum dots, the host of the spin qubit, in gated heterostructure devices based on monolayer WSe2 and study its properties. With a similar structure, we demonstrate that a gate-defined nano-constriction is sensitive to the charge occupation of a nearby quantum dot and is therefore suitable to be used as a charge sensor, a valuable component of elaborate quantum circuits. Finally, we demonstrate the realization of a gate-defined quantum confined 1D channel in a high mobility monolayer WSe2 sample and observe an anomalous conductance quantization in units of e2/h. These results pave the way for the development of quantum devices based on electrostatically confined quantum dots defined in semiconducting TMDs and push forward our understanding of their electronic properties.

No description available.

No description available.

<p>InP nanowires with short InAs segments were grown on InP (111)B substrates by Au assisted vapour-liquid-solid growth in a gas source molecular beam epitaxy system. Nanowire crystal structure and morphology were investigated by transmission electron microscopy as a function of temperature, growth rate, and V/III flux ratio. At 370C predominantly kinked nanowires with random morphology and low areal density were observed with a rough parasitic 2D film. At 440C, nanowire density was also reduced but the 2D film growth was smoother and nanowires grew straight without kinking. An optimum temperature of 400C maximized areal density with uniform nanowire morphology. At the optimum temperature of 400C, an increase in V/III flux ratio changed the nanowire morphology from rod-shaped to pencil like indicating increased radial growth. Growth rate did not affect the crystal structure of InP nanowires. For InAs nanowires, changing the growth rate from 1 to 0.5 μm/hr reduced the presence of stacking faults to as low as one per nanowire. Short InAs segments in InP nanowires were found to grow through two mechanisms for nanowires of length L and diameter D. The first mechanism described the supply of In to the growth front via purging of In from the Au droplet where L was proportional to D. The second mechanism involved direct deposition of adatoms on the nanowire sidewall and subsequent diffusion to the growth front where L was proportional to 1/D. For intermediate growth durations, a transition between these two mechanisms was observed. For InP and InAs nanowires, the growth mode was varied from axial to radial through the inclusion of Al to form a core shell structure. Al_xIn_1-xAs(P) shells were grown on InAs cores with Al alloy fractions between 0.53 and 0.2. These nanowires were examined by transmission electron microscopy and it was found, for all values of x in InAs-Al_xIn_1-xP structures, that relaxation had occurred through the introduction of dislocations. For InAs-Al_xIn_1-xAs structures, all values except x=0.2 had relaxed through dislocation formation. A critical thickness model was developed to determine the core-shell coherency limits which confirmed the experimental observation of strain relaxation. The effects of passivation on the electronic transport and the optical properties were examined as a function of structural core-shell passivation and chemical passivation. The mechanisms for the observed improvement in mobility for core-shell versus bare InAs nanowires was due to the reduction in ionized impurity scattering from surface states. Similarly an increase in photoluminescence intensity after ammonium sulfide passivation was explained by the reduction of donor type surface states.</p> / Doctor of Philosophy (PhD)

Ternary InGaN and AlGaN alloys have been sought after for the application of various optoelectronic devices spanning a large spectral range between the deep ultraviolet and infrared, including light-emitting diodes, and laser diodes. Their non-ideal alloy mixing, and differences in bond energy and in adatom diffusion are established as the cause for various types of nanoscale compositional inhomogeneity commonly observed in nitride thin films. Growth in a nanowire geometry can overcome the phase separation, surface segregation, and chemical ordering by providing enhanced strain relaxation of the large lattice mismatch at the free surfaces. In this dissertation, the spectral and spatial luminescence distributions of ternary III-N alloy nanowire heterostructures are investigated and correlated to structural and chemical properties with scanning transmission electron microscopy. Quantitative elemental mapping of InGaN/GaN dot-in-a-wire structures using electron energy-loss spectroscopy revealed compositional non-uniformity between successive quantum dots. Local strain mapping of the heterostructure showed a dependence of the incorporation of indium on the magnitude of the out-of-plane compressive strain within the underlying GaN barrier layer. Cathodoluminescence spectroscopy on individual nanowires presented diverse emission properties, nevertheless, the In-content variability could be directly correlated to the broad range of peak emission energies. Atomic-level chemical ordering within the InGaN was then reported, and attributed to the faceted growth surface in nanowires that promotes preferential site incorporation by In-atoms that allows for better strain relaxation. Distinct atomic-scale alloy inhomogeneities were also investigated in AlGaN nanowires, which evidenced spatial localization of carriers taking place at the resulting energy band fluctuations. A high spectral density of narrow emission lines arose from such compositional modulations, whose luminescence behaviours exhibit a dependence on the nature of the compositional fluctuations from which they originate. / Thesis / Doctor of Philosophy (PhD)