Une sonde électrostatique de Langmuir cylindrique a été utilisée pour caractériser une post-décharge d’un plasma d’ondes de surface de N2-O2 par la mesure de la densité des ions et électrons ainsi que la température des électrons dérivée de la fonction de distribution en énergie des électrons (EEDF). Une densité maximale des électrons au centre de la early afterglow de l’ordre de 1013 m-3 a été déterminée, alors que celle-ci a chuté à 1011 m-3 au début de la late afterglow. Tout au long du profil de la post-décharge, une densité des ions supérieure à celle des électrons indique la présence d’un milieu non macroscopiquement neutre. La post-décharge est caractérisée par une EEDF quasi maxwellienne avec une température des électrons de 0.5±0.1 eV, alors qu’elle grimpe à 1.1 ±0.2 eV dans la early afterglow due à la contribution des collisions vibrationnelles-électroniques (V-E) particulièrement importantes. L’ajout d’O2 dans la décharge principale entraîne un rehaussement des espèces chargées et de la température des électrons suivi d’une chute avec l’augmentation de la concentration d’O2. Le changement de la composition électrique de la post-décharge par la création de NO+ au détriment des ions N2+ est à l’origine du phénomène. Le recours à cette post-décharge de N2 pour la modification des propriétés d’émission optique de nanofils purs de GaN et avec des inclusions d’InGaN a été étudié par photoluminescence (PL). Bien que l’émission provenant des nanofils de GaN et de la matrice de GaN recouvrant les inclusions diminue suite à la création de sites de recombinaison non radiatifs, celle provenant des inclusions d’InGaN augmente fortement. Des mesures de PL par excitation indiquent que cet effet n’est pas attribuable à un changement de l’absorption de la surface de GaN. Ceci suggère un recuit dynamique induit par la désexcitation des métastables de N2 suite à leur collision à la surface des nanofils et la possibilité de passiver les défauts de surface tels que des lacunes d’azote par l’action d’atomes de N2 réactifs provenant de la post-décharge. L’incorporation d’O2 induit les mêmes effets en plus d’un décalage vers le rouge de la bande d’émission des inclusions, suggérant l’action des espèces d’O2 au sein même des nanostructures. / A cylindrical electrostatic Langmuir probe was used to characterize the flowing afterglow of a N2-O2 surface wave plasma. The spatial distribution of the number density of positive and electrons as well as the EEDF were measured. A maximum of the number density of electrons in the mid 1013 m-3 was obtained in the center of the early afterglow, while it decreased at 1011 m-3 early in the late afterglow, thus indicating non-macroscopically neutral media all along the flowing afterglow. It is characterized by an EEDF close to a Maxwellian with an electron temperature of 0.5±0.1 eV, while it increased at 1.1±0.2 eV in the early afterglow due to the contribution of important vibration-electron collisions. After addition of small amounts of O2 in the main N2 microwave discharge, the charged particles densities and electron temperature first strongly increased then decreased with increasing O2 concentration. A change in the charged population in the afterglow by the creation of NO+ to the detriment of the N2+ ions is responsible of this phenomenon. This N2 flowing afterglow was later used for plasma-induced modification of pure GaN nanowires and InGaN/GaN dot-in-a-wire heterostructures and characterized by PL. While the band edge emission from GaN nanowires and the GaN matrix of the InGaN/GaN nanowires strongly decreased due to the creation of non-radiative recombination centers in the near-surface region, the emission from the InGaN inclusions strongly increased. PL excitation measurements show that this increase cannot be explained by a plasma-induced shift of the GaN absorption edge. Instead a dynamical annealing process induced by the desexcitation of N2 metastables following their collision with the nanowire surface and the passivation of surface defects such as nitrogen vacancies by the highly reactive nitrogen atoms in the afterglow are responsible of the increase of the PL intensity. The addition of O2 gives the same results as the pure N2 treatment, but a redshift of the emission band related to the InGaN inclusions is also observed, suggesting an important contribution of the oxygen species.

Group III-A nitrides (GaN, AlN, InN and alloys) are materials of considerable contemporary interest and currently enable a wide variety of optoelectronic and high-power, high-frequency electronic applications. All of these applications utilize device structures that employ a single or multiple hetero-junctions, with material compositions varying across the interface. For example, the workhorse of GaN based electronic devices is the high electron mobility transistor (HEMT) which is usually composed of an AlGaN/GaN hetero-junction, where a two-dimensional electron gas (2DEG) is formed due to differences in polarization between the two layers. In addition to such hetero-junctions in the same material family, formation of hetero-interfaces in nitrides begins right from the epitaxy of the very first layer due to the lack of native substrates for their growth. The consequences of such "dissimilar" hetero-junctions typically manifest as large defect densities at this interface which in turn gives rise to defective films. Additionally, if the substrate is also a semiconductor, the electrical properties at such dissimilar semiconductor-nitride hetero-junctions are particularly important in terms of their influence on the performance of nitride devices. Nevertheless, the large defect densities at such dissimilar 3D-3D semiconductor interfaces, which translate into more trap states, also prevents them from being used as active device layers to say nothing of reliability considerations arising because of these defects. Recently, the advent of 2D materials such as graphene and MoS2 has opened up avenues for Van der Waal’s epitaxy of these layered films with practically any other material. Such defect-free integration enables dissimilar semiconductor hetero-junctions to be used as active device layers with carrier transport across the 2D-3D hetero-interface. This thesis deals with hetero-epitaxial growth platforms for reducing defect densities, and the material and electrical properties of dissimilar hetero-junctions with the group III-A nitride material system.