Les semi-conducteurs à base de composés III-V de type GaN présentent de nombreux avantages – liés essentiellement à leur grande bande interdite - par rapport aux semi-conducteurs traditionnels Si, ou III-V des filières GaAs. De plus, il est possible de former, comme pour les semi-conducteurs traditionnels III-V des hétérojonctions de type HEMT (High Electron Mobility Transistor) AlGaN/GaN permettant d'obtenir à la fois une forte densité de porteurs confinés à l'hétérojonction et des mobilités électroniques élevées. Ces composants sont à l'heure actuelle les candidats les plus prometteurs pour des applications hyperfréquences de puissance. Cependant, l'échauffement observé au cours du fonctionnement et les différentes étapes de réalisation des composants ont un impact anormal sur les performances intrinsèques du composant. La microspectrométrie Raman est une technique nondestructive et sans contact avec une résolution spatiale submicronique, adaptée à l'étude des HEMTs AlGaN/GaN en fonctionnement. L'utilisation de différentes longueurs d'onde excitatrices visible et UV permet de sonder les hétérostructures à différentes profondeurs de pénétration. Les informations obtenues avec cette technique d'analyse sont la composition de l'hétérostructure, les contraintes entre les différentes couches, la résistance thermique aux interfaces, la qualité cristalline des différentes couches, le dopage et le comportement thermique des différentes couches. Le développement d'un système de microspectrométrie Raman UV résolue en temps a permis d'analyser le comportement thermique transitoire des HEMTs AlGaN/GaN en fonctionnement et plus particulièrement dans la zone active du composant.

At present, focus of the SiC crystal growth development is on improving the crystalline quality without polytype inclusions, micropipes and the occurrence of extended defects. The purity of the grown material, as well as intentional doping must be well controlled and the processes understood. High-quality substrates will significantly improve device performance and yield. One of the aims of the thesis is further understanding of polytype inclusion formation as well as impurity control in SiC bulk crystals grown using PVT method also termed seeded sublimation method. Carbonization of the source was identified as a major reason behind the polytype inclusion occurrence during the growth. The aim of this work was further understanding of sublimation growth process of 4H-SiC bulk crystals in vacuum, in absence of an inert gas. For comparison growth in argon atmosphere (at 5 mbar) was performed. The effect of the ambient on the impurity incorporation was studied for different growth temperatures. For better control of the process in vacuum, tantalum as a carbon getter was utilized. The focus of the SiC part of the thesis was put on further understanding of the PVT epitaxy with an emphasis on the high growth rate and purity of grown layers. High resistivity 4H-SiC samples grown by sublimation with high growth rate were studied. The measurements show resistivity values up to high 104 cm. By correlation between the growth conditions and SIMS results, a model was applied in which it is proposed that an isolated carbon vacancy donor-like level is a possible candidate responsible for compensation of the shallow acceptors in p-type 4H-SiC. A relation between cathodoluminescence (CL) and DLTS data is taken into account to support the model. To meet the requirements for high voltage blocking devices such as high voltage Schottky diodes and MOSFETs, 4H-SiC epitaxial layers have to exhibit low doping concentration in order to block reverse voltages up to few keV and at the same time have a low on-state resistance (Ron). High Ron leads to enhanced power consumption in the operation mode of the devices. In growth of thick layers for high voltage blocking devices, the conditions to achieve good on-state characteristics become more challenging due to the low doping and pronounced thicknesses needed, preferably in short growth periods. In case of high-speed epitaxy such as the sublimation, the need to apply higher growth temperature to yield the high growth rate, results in an increased concentration of background impurities in the layers as well as an influence on the intrinsic defects. On-state resistance Ron estimated from current density-voltage characteristics of Schottky diodes on thick sublimation layers exhibits variations from tens of mΩ.cm2 to tens of Ω.cm2 for different doping levels. In order to understand the occurrence of high on-state resistance, Schottky barrier heights were first estimated for both forward and reverse bias with the application of thermionic emission theory and were in agreement with literature reported values. Decrease in mobility with increasing temperature was observed and its dependencies of T–1.3 and T–2.0 for moderately doped and low doped samples, respectively, were estimated. From deep level measurements by Minority Carrier Transient Spectroscopy (MCTS), an influence of shallow boron related levels and D-center on the on-state resistance was observed, being more pronounced in low doped samples. Similar tendency was observed in depth profiling of Ron. This suggests a major role of boron in a compensation mechanism. In the second part of the thesis growth and characterization of GaN is presented. Excellent electron transport properties with high electron saturate drift velocity make GaN an excellent candidate for electronic devices. Especially, AlGaN/GaN based high electron mobility transistors (HEMT) have received an increased attention in last years due to their attractive properties. The presence of strong spontaneous and piezoelectric polarization due to the lattice mismatch between AlGaN and GaN is responsible for high free electrons concentrations present in the vicinity of the interface. Due to the spatial separation of electrons and ionized donors or surface states, 2DEG electron gas formed near the interface of the heterostructure exhibits high sheet carrier density and high mobility of electrons. Al0.23Ga0.77N/GaN based HEMT structures with an AlN exclusion layer on 100 mm semiinsulating 4H-SiC substrates have been grown by hot-wall MOCVD. The electrical properties of the two-dimensional electron gas (2DEG) such as electron mobility, sheet carrier density and sheet resistance were obtained from Hall measurements, capacitance-voltage and contact-less eddy-current techniques. The effect of different scattering mechanisms on the mobility have been taken into account and compared to the experimental data. Hall measurements were performed in the range of 80 to 600 K. Hall electron mobility is equal to 17140 cm2(Vs)-1 at 80 K, 2310 cm2(Vs)-1 at room temperature, and as high as 800 cm2(Vs)-1 at 450 K, while the sheet carrier density is 1.04x1013 cm-2 at room temperature and does not vary very much with temperature. Estimation of different electron scattering mechanisms reveals that at temperatures higher than room temperature, experimental mobility data is mainly limited by optical phonon scattering. At relevant high power device temperature (450 K) there is still an increase of mobility due to the AlN exclusion layer. We have studied the behaviour of Ga-face GaN epilayers after in-situ thermal treatment in different gas mixtures in a hot-wall MOCVD reactor. Influence of N2, N2+NH3 and N2+NH3+H2 ambient on the morphology was investigated in this work. The most stable thermal treatment conditions were obtained in the case of N2+NH3 gas ambients. We have also studied the effect of the increased molar ratio of hydrogen in order to establish proper etching conditions for hot-wall MOCVD growth.

The thesis consists of eight chapters of which the first chapter presents a brief overview of inorganic nanostructures. Synthesis and magnetic properties of MnO and NiO nanocrystals are described in Chapter 2, with emphasis on the low-temperature ferromagnetic interactions in these antiferromagnetic oxides. Chapter 3 deals with the synthesis and characterizations of nanocrystals of ReO3, RuO2 and IrO2 which are oxides with metallic properties. Pressure-induced phase transitions of ReO3 nanocrystals and the use of the nanocrystals for carrying out surface-enhanced Raman spectroscopy of the molecules form Chapter 4. Use of ionic liquids to synthesize different nanostructures of semiconducting metal sulfides and selenides is described in Chapter 5. Synthesis of Mn-doped GaN nanocrystals and their magnetic properties are described in Chapter 6. A detailed investigation has been carried out on the growth kinetics of nanostructures of a few inorganic materials by using small-angle X-ray scattering and other techniques (Chapter 7). The study includes the growth kinetics of nanocrystals of Au, CdS and CdSe as well as of nanorods of ZnO. Results of a synchrotron X-ray study of the formation of nanocrystalline gold films at the organic-aqueous interface are also included in this chapter. Chapter 8 discuses the use of the organic-aqueous interface to generate Janus nanocrystalline films of inorganic materials where one side of the film is hydrophobic and other side is hydrophilic. This chapter also includes the formation of nanostructured peptide fibrils at the organic-aqueous interface and their use as templates to prepare inorganic nanotubes.

Nous avons étudié par spectroscopie optique les propriétés électroniques de boîtes quantiques (BQs) de GaN/AlN crûes en épitaxie par jets moléculaires selon la direction non-polaire [11-20] (axe a). Dans cette orientation on attend une forte réduction des effets du champ électrique interne existant dans les hétérostructures d'axe [0001] (axe c), ce que nous avons vérifié expérimentalement par des expériences de photoluminescence (PL) résolue et intégrée en temps d'un ensemble de boîtes.<br />La réalisation d'un montage de microPL optimisé pour l'UV nous a permis d'isoler les premiers spectres de BQs uniques de GaN plan a. Des effets de charges locales responsables de l'élargissement des raies de PL (diffusion spectrale) ont pu être mises en évidence et partiellement contrôlés par l'application d'un champ électrique vertical.<br />La dépendance en température des raies de BQs uniques nous a permis d'étudier le couplage de l'exciton confiné avec les phonons acoustiques. La modélisation de ce mécanisme ainsi que les valeurs des temps de déclin indiquent que la localisation latérale de l'exciton est plus forte que celle imposée par la boîte.

This thesis deals with the research work carried out on the properties and applications such as GaN nanoparticles, Graphene etc. Chapter 1 of the thesis gives introduction to nanomaterials and various aspects of the thesis. Chapter 2 of the thesis describes the synthesis of GaN nanocrystals and their use as white light sources and as room temperature gas sensors. It also discusses negative differential resistance above room temperature exhibited by GaN. Electroluminescence from GaN-polymer heterojunction forms the last section of this chapter. Chapter 3 demonstrates the role of defect concentration on the photodetecting properties of ZnO nanorods with different defects prepared at different temperatures. Chapter 4 presents remarkable infrared and ultraviolet photodetector properties of reduced graphene oxide and graphene nanoribbons. Chapter 5 presents the infrared detecting properties of graphene-like few-layer MoS2. The summary of the thesis is given at the end of the thesis.

La fabrication des composants semi-conducteurs à base de nitrure de gallium (GaN) connaît actuellement une grande expansion. Ce matériau, par ces propriétés physico-chimiques intéressantes, est un très bon candidat pour la fabrication des composants de puissance à haute fréquence de fonctionnement. Dans la pratique, avant d'intégrer ces composants dans un système électronique, l'analyse de leur fiabilité est une étape nécessaire pour valider la technologie de fabrication utilisée. L'objectif de ce travail est la détermination des mécanismes physiques responsables de la dégradation des performances des Transistors à Haute Mobilité Electronique (HEMT) à base d'hétérostructures AlGaN/GaN et AlInN/GaN. Dans un premier temps, la caractérisation en régime statique des composants, par des mesures de courant et de capacité à différentes températures, nous a permis de repérer certaines anomalies dans les caractéristiques des composants. Cette non-idéalité liée aux effets thermiques semble provenir des mécanismes de piégeage des porteurs par les défauts dans le matériau. Dans le but d'analyser ces mécanismes, des mesures de spectroscopie de défauts profonds (DLTS) ont été effectuées sur la capacité de type Schottky du contact de la grille. L'étape suivante a consisté à mesurer les pièges profonds dans les HEMTs par DLTS en courant de drain, de façon à déterminer quels défauts influencent directement le courant dans ces dispositifs. Cette étude a été effectuée sur différents composants avec différentes géométries pour analyser au mieux le comportement de ces pièges. L'étude du contact de grille est aussi une étape importante pour déterminer les origines de défaillance des composants. Pour cela, nous avons réalisé une étude approfondie sur les différents mécanismes de transport à travers la barrière métal/semi-conducteur. Cette étude nous a permis de conclure sur la stabilité du contact de grille après les tests de vieillissement accélérés.

The present work has been focused on the growth of Group III-nitride epitaxial layers and nanostructures on Si (111) substrates by plasma-assisted molecular beam epitaxy. Silicon is regarded as a promising substrate for III-nitrides, since it is available in large quantity, at low cost and compatible to microelectronics device processing. However, three-dimensional island growth is unavoidable for the direct growth of GaN on Si (111) because of the extreme lattice and thermal expansion coefficient mismatch. To overcome these difficulties, by introducing β-Si3N4 buffer layer, the yellow luminescence free GaN can be grow on Si (111) substrate. The overall research work carried out in the present study comprises of five main parts. In the first part, high quality, crack free and smooth surface of GaN and InN epilayers were grown on Si(111) substrate using the substrate nitridation process. Crystalline quality and surface roughness of the GaN and InN layers are extremely sensitive to nitridation conditions such as nitridation temperature and time. Raman and PL studies indicate that the GaN film obtained by the nitridation sequences has less tensile stress and optically good. The optical band gaps of InN are obtained between ~0.73 to 0.78 eV and the blueshift of absorption edge can be induced by background electron concentration. The higher electron concentration brings in the larger blueshift, due to a possible Burstein–Moss effect. InN epilayers were also grown on GaN/Si(111) substrate by varying the growth parameters such as indium flux, substrate temperature and RF power. In the second part, InGaN/Si, GaN/Si3N4/n-Si and InN/Si3N4/n-Si heterostructures were fabricated and temperature dependent electrical transport behaviors were studied. Current density-voltage plots (J-V-T) of InGaN/Si heterostructure revealed that the ideality factor and Schottky barrier height are temperature dependent and the incorrect values of the Richardson’s constant produced, suggests an inhomogeneous barrier at the heterostructure interface. The higher value of the ideality factor compared to the ideal value and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission rather than thermionic emission. The valence band offset of GaN/β-Si3N4/Si and InGaN/Si heterojunctions were determined by X-ray photoemission spectroscopy. InN QDs on Si(111) substrate by droplet epitaxy and S-K growth method were grown in the third part. Single-crystalline structure of InN QDs (droplet epitaxy) was verified by TEM and the chemical bonding configurations of InN QDs were examined by XPS. The interdigitated electrode pattern was created and (I-V) characteristics of InN QDs were studied in a metal–semiconductor–metal configuration in the temperature range of 80–300 K. The I-V characteristics of lateral grown InN QDs were explained by using the trap model. A systematic manipulation of the morphology, optical emission and structural properties of InN/Si (111) QDs (S-K method) is demonstrated by changing the growth kinetics parameters such as flux rate and growth time. The growth kinetics of the QDs has been studied through the scaling method and observed that the distribution of dot sizes, for samples grown under varying conditions, has followed the scaling function. In the fourth part, InN nanorods (NRs) were grown on Si(111) and current transport properties of NRs/Si heterojunctions were studied. The rapid rise and decay of infrared on/off characteristics of InN NRs/Si heterojunction indicate that the device is highly sensitive to the IR light. Self-aligned GaN nanodots were grown on semi-insulating Si(111) substrate. The interdigitated electrode pattern was created on nanodots using photolithography and dark as well as UV photocurrent were studied. Surface band gaps of InN QDs were estimated from scanning tunneling spectroscopy (STS) I-V curves in the last part. It is found that band gap is strongly dependent on the size of InN QDs. The observed size-dependent STS band gap energy blueshifts as the QD’s diameter or height was reduced.

Cette thèse est consacrée à l'étude de l'outil d'épitaxie HVPE (Hydride Vapour Phase Epitaxy) pour la synthèse avec et sans catalyseur de nanofils semiconducteurs GaN et GaAs. Une étude systématique de l'influence des conditions expérimentales sur la croissance des fils de GaN est effectuée, afin de démontrer la faisabilité de cette croissance sur la surface des substrats saphir plan-c et silicium sans aucun traitement de la surface préalablement à la croissance. Nous avons démontré la croissance par VLS-HVPE, de nanofils de GaN de diamètres constants de 40 à 200 nm, de longueurs supérieures à 60 μm et présentant des qualités optique et cristallographique remarquables. Pour les nanofils de GaAs, la stabilité, inédite, de la phase cubique zinc-blende pour des diamètres de 10 nm a été démontrée par le procédé de croissance VLS-HVPE sur des longueurs de quelques dizaines de micromètres. Les mécanismes de croissance sont discutés à partir des diagrammes de phase et de la physique de la croissance HVPE qui met en oeuvre des précurseurs gazeux chlorés. Pour les semiconducteurs III-V, cette étude permet d'envisager des applications liées aux nanofils longs qui jusque là n'étaient exploitées que pour le silicium. Ces travaux montrent que dans le contexte des Nanosciences, la HVPE, outil épitaxial à fortes vitesses de croissance, mérite une audience élargie, et peut s'inscrire comme un outil complémentaire efficace aux procédés MOVPE et MBE pour le façonnage contrôlé de la matière à l'échelle nanométrique.

Ce travail porte sur la spectroscopie magnéto-optique de semi-conducteurs magnétiques dilués (DMS) : ZnO et GaN dopés manganese, fer et cobalt. Les deux semi-conducteurs hôtes, ZnO et GaN ont une grande bande interdite, une structure wurtzite, une faible interaction spin - orbite et une forte interaction d'échange excitonique entre trous et électrons. En présence de champs magnétiques, les ions magnétiques induisent un effet Zeeman géant dont l'interprétation est complexe : les excitons s'anti-croisent et leurs énergies de transition et leurs forces d'oscillateur sont fortement influencées par l'effet Zeeman géant.<br />On a mesuré expérimentalement le splitting Zeeman géant des excitons A et B avec des couches epitaxiées sur saphir (0001) et une propagation de la lumiere parallele a l'axe c du cristal et au champ magnétique (configuration Faraday). Le splitting Zeeman géant diminue avec la température et augmente non linéairement avec le champ magnétique en accord avec l'aimantation calculée des spins isolés. Une analyse quantitative nous a permis d'analyser les propriétés magnétiques et de mesurer les intégrales d'échange pour l'ensemble des matériaux étudiés. Pour des ions avec une configuration d5 (Mn2+ et Fe3+), l'aimantation suit une fonction de Brillouin, mais pour les configurations d7 et d4 (Co2+ ou Mn3+) l'interaction spin-orbite et le champ cristallin trigonal induisent une aimantation anisotrope, en accord avec l'analyse des transitions internes des ions mesurées en spectroscopie infrarouge. Pour (Ga,Mn)N, et (Ga,Fe)N, nous avons trouvé un signe positive pour l'intégrale d'échange entre trous et spins localisés (beta). En supposant une symétrie de la bande de valence dans ZnO correspondant a une interaction spin-orbite positive (Gamma_9, Gamma_7, Gamma_7), nous trouvons un signe négative de beta pour (Zn,Co)O, et beta est de pres de zéro pour (Zn,Mn)O. Toutefois, dans l'hypothese avec spin-orbite négative, nous trouvons un signe positif de beta. Les signes et les valeurs des intégrales d'échange déterminées a partir de nos mesures magnéto-optiques ne peuvent pas etre expliqués par des tendances matérielles et des modeles basés sur l'approximation de cristal virtuel. Ceci suggere que l'échange p-d en DMS a large bande interdite, soit dans le régime de couplage fort, et la nature de splitting Zeeman géant observé est différente qu'en semi-conducteurs magnétiques dilués classiques.

In this work, the electronic band structure of III (In, Al, Ga) - V (N, As, Sb) compounds and their ternary alloys have been investigated by density functional theory (DFT) within generalized gradient approximation (GGA) and empirical tight binding (ETB) calculations, respectively. The present DFT-GGA calculations have shown direct band gap structures in zinc-blende phase for InN, InAs, InSb, GaN, and GaAs. However, indirect band gap structures have been obtained for cubic AlN, AlSb and AlAs com- pounds / here, the conduction band minima of both AlN and AlAs are located at X symmetry point, while that of AlSb is at a position lying along Gamma- X direction. An important part of this work consists of ETB calculations which have been parameterized for sp3d2 basis and nearest neighbor interactions to study the band gap bowing of III(In / Al)- V(N / As / Sb) ternary alloys. This ETB model provides a satisfactory electronic properties of alloys within reasonable calculation time compared to the calculations of DFT. Since the present ETB energy parameters reproduce successfully the band structures of the compounds at &iexcl / and X symme- try points, they are considered reliable for the band gap bowing calculations of the ternary alloys. In the present work, the band gap engineering of InNxAs1&iexcl / x, InNxSb1&iexcl / x, InAsxSb1&iexcl / x, Al1&iexcl / xInxN, Al1&iexcl / xInxSb and Al1&iexcl / xInxAs alloys has been studied for total range of constituents (0 &lt / x &lt / 1). The downward band gap bowing seems the largest in InNxAs1&iexcl / x alloys among the alloys considered in this work. A metallic character of InNxAs1&iexcl / x, InNxSb1&iexcl / x and InAsxSb1&iexcl / x has been ob- tained in the present calculations for certain concentration range of constituents (N / As) as predicted in the literature. Even for a small amount of contents (x), a decrease of the electronic e&reg / ective mass around &iexcl / symmetry point appears for InNxAs1-x, InNxSb1-x and InAsxSb1-x alloys manifesting itself by an increase of the band curvature. The calculated cross over from indirect to direct band gap of ternary Al alloys has been found to be consistent with the measurements. As a last summary, the determinations of the band gaps of alloys as a function of contents, the concentration range of con- stituents leading to metallic character of the alloys, the change of the electronic effective mass around the Brillioun zone center (Gamma) as a function of alloy contents, the cross over from indirect to direct band gap of the alloys which are direct on one end, indirect on the other end, are main achievements in this work, indispensable for the development of mate- rials leading to new modern circuit components.