Note publique d'information : Cette étude a portée sur l'élaboration en épitaxie par jets moléculaires des matériaux
nitrures d'éléments III en phase cubique sur des pseudo-substrats de SiC cubique dans
le but de mettre en évidence les divers atouts de ces matériaux pour les applications
aux dispositifs optoélectroniques. Nous avons pu obtenir des résultats sur le contrôle
de la pureté et la qualité structurale des films épais de nitrures binaires qui se
situent à l'état de l'art. Les limitations trouvées sur l'amélioration des propriétés
des ces matériaux ont été dues à la morphologie des pseudo-substrats. Nous avons également
fabriqué des films épais d'alliage AlGaN en phase cubique dans la gamme des compositions
de 0<XAl<0,70%. D'autre part, le contrôle des modes de relaxation des couches de GaN
sur AlN a permis la réalisation de boîtes quantiques et de puits quantiques de nitrure
en phase cubique. L'étude optique a mis en évidence l'absence de polarisation spontanée
et piezo-électrique dans ces nanostructures de nitrure en phase cubique. Cependant
la forte densité de défauts structuraux limite leur performance. Finalement, nous
avons travaillé sur le dopage intentionnel des films de GaN. Nous avons démontré la
réalisation de diodes à jonction p-n émettant dans le bleu-vert mais l'interaction
des défauts structuraux et donneurs résiduels avec le dopant de type p que nous avons
utilisé a donné des problèmes de reproductibilité. Des travaux sur l'amélioration
de la morphologie et de l'homogénéité des substrats SiC s'impose pour l'amélioration
des couches de nitrure en phase cubique et les applications aux dispositifs optoélectroniques
ou à micro-ondes.
Note publique d'information : This work concern the growth of III-group nitrides (GaN, AlN, AlxGa1-xN and InxGa1-xN)
in cubic phase by molecular beam epitaxy on cubic pseudo-substrates SiC(001). The
III-nitrides are very promising semiconductors for their applications in optoelectronics,
high temperature and high speed devices. For example, optical properties of GaN crystallized
in the zinc blende structure (c-GaN), which exhibits higher crystallographic symmetry,
are expected to present some advantages compared to the hexagonal phase due to the
absence of internal electric field. In addition, the possibility of an electronic
integration with GaAs or Si devices, makes cubic GaN (AlN) very attractive. However,
due to the difficulty to grow this metastable phase, a relatively limited amount to
work has been devoted to the growth of these materials and to the fabrication of light
emitting diodes. The aim of this work is to put in evidence these different theoretical
advantages. Among the remaining problems which hinder further progress, the influence
of extended defects on the structural and electrical properties and also the apparent
lack of understanding of the mechanisms underlying impurity incorporation are a major
importance. The high concentration of defects which often dominates the optical and
electrical characteristics, a consequence of the lack of adapted substrates, makes
the reproductibility of devices difficult. Indeed, the major limitation in the growth
of c-GaN originates from polytypism, i.e. a tendency to form hexagonal subdomains
within the cubic crydtal. Another issue to fabricate GaN-based devices is the control
of doping with intentional impurities. N-type doping of c-GaN with Si is currently
performed. By contrast, p-type doping of c-GaN layers presents much more difficulties:
the chemical concentration roughly follows the Mg flux, but the hole concentration
deviates strongly from it, showing the difficulty to incorporate Mg in the Ga site.
In addition, the high O content in the layers causes that Mg still always electrically
inactive. Stacking faults-induced out-diffusion of background impurities from the
substrates is the other sources of n-type background impurities from the substrates
is the other sources of n-type background conductivity in thin GaN and AlN layers.
We have explored another way of application of this materials: quantum dots and quantum
wells. It was demonstrated that the optical properties of these structures have not
influences by the strong piezoelectric field like in nitrides crystallized in hexagonal
phase. On the other hand, we have obtained AlGaN alloys having Al content in the range
of 0 to 70%. Ours results are placed as the betters when compared with other groups
in the world.
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