Molecular beam epitaxy growth and optical characterization of GaN/AlGaN nanowire heterostructures emitting in the ultraviolet

Abstract : Using specific growth conditions, AlGaN nanowire (NW) sections can be grown in epitaxy on top of GaN NW templates. Such NW growth, performed by plasma-assisted molecular beam epitaxy in the present case, allows the subsequent characterization of very small volume of material free of extended defects commonly observed in planar structures. This absence of defects makes these NWs very promising for optoelectronic devices operating in the ultraviolet. However, achieving such devices requires a better understanding of the NW fundamental properties.The issue of alloy inhomogeneity at nanoscale has notably remained obscure so far. In order to make it clearer, the latter has been first investigated in the present work, especially through optical characterization. For our experiments, non-intentionally doped (NID) AlGaN NWs have been grown in various conditions in order to potentially tune the compositional fluctuations within the AlGaN alloy and therefore possibly probe for carrier localization centers of different size and Al composition. It has been firstly observed through structural characterization that the length of Al-rich sections preferentially nucleating on top of GaN NWs can be tuned by varying the growth kinetical parameters, emphasizing a growth mechanism governed by kinetics. Optical studies have then evidenced that compositional fluctuations induce carrier localization and exhibit a quantum dot-like behavior. The latter has been observed whatever the growth conditions explored in this work. Our results are consistent with the spontaneous formation during growth of tiny Ga-richer regions shown to share similar micro-optical features over a given emission wavelength range for all investigated growth conditions. Such regions exhibiting the single-photon emission character are present at very small scale, as signs of their existence have been also evidenced in thin NID AlGaN quantum disks.In addition, doping in Al(Ga)N NW, especially p-type, is far from being fully comprehended. In particular, the issue of dopant incorporation as well as optical and electrical activation in such NWs remains unclear. The latter has been examined in Al(Ga)N NW pn junctions doped with Mg and Si atoms. First, signatures specific to dopant incorporation in NWs have been highlighted through structural characterization, before evidencing AlGaN pn junctions electrically. Moreover, optical analysis have exhibited optically active both dopant types. Nonetheless, Mg dopants are but partially active electrically due to passivation by hydrogen emphasized by the observation of Mg-H complexes. To cope with the latter issue, post-growth annealing experiments have been attempted. Concomitantly, AlN NW pn junctions have been also preliminarily investigated and present interesting morphological features. Indeed, deep hollows have been observed in NWs and associated with Mg doping carried out at low growth temperature. The NW morphology can be tuned by varying growth kinetical parameters and by using the surfactant effect of Mg atoms. When increasing growth temperature, these hollows disappear, while the NW top shape has been observed to switch from hexagonal to star-like, emphasizing growth conditions very far from thermodynamical equilibrium. Electrical activation of dopants has not been evidenced so far in AlN NW pn junctions.
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Submitted on : Tuesday, December 12, 2017 - 1:21:09 AM
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Matthias Belloeil. Molecular beam epitaxy growth and optical characterization of GaN/AlGaN nanowire heterostructures emitting in the ultraviolet. Materials Science [cond-mat.mtrl-sci]. Université Grenoble Alpes, 2017. English. ⟨NNT : 2017GREAY021⟩. ⟨tel-01661587⟩

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