ZnO and GaN nanostructures for optoelectronic applications: synthesis and characterization

• Main Author: Rodrigues, Joana Catarina Ferreira
• Publication Date: 2015
• Format: Article
• Language: eng
• Source: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Download full: http://hdl.handle.net/10773/25193
• topic: ZnO; GaN; AlxGa1-xN; CNTs; CdxZn1-xO; Rare-earth ions; Laser assisted flow deposition (LAFD); Photoluminescence; Photovoltaic; Photocatalysis

Wide bandgap semiconductors, such as GaN and ZnO, are materials with a wide range of applications in several important technological areas including lighting, transparent electronics, sensors, catalysis or photovoltaics. This thesis focuses on the study of GaN and ZnO, including related compounds. In the first case, the emphasis is given to the incorporation of rare-earth (RE) ions (4fn) into the nitride hosts envisaging to contribute for the development of “all-nitride” solid state lighting devices. GaN and related III-nitrides ternary alloys appear as excellent hosts for the incorporation of these ions. The use of RE ions is motivated by the electromagnetic widespread spectral range (from the ultraviolet to the near infrared) covered by the intraionic radiative relaxation of the trivalent charged ions. Ion implantation appears as an alternative approach to doping since it allows the introduction of impurities in a controlled way and without solubility limits. GaN samples with different dimensionalities were analysed and their influence in the luminescence properties of the RE3+ was investigated. Photoluminescence (PL) measurements revealed that after thermal annealing a successful optical activation of the RE3+ was achieved for the samples implanted with the different RE3+. A detailed spectroscopic analysis of RE3+ luminescent tarnsitions is presented by using temperature dependent steady-state PL, room temperature PL excitation and time resolved PL. This thesis also aims to the growth and characterization of ZnO micro and nanostructures, through a new growth technique designated by laser assisted flow deposition (LAFD). LAFD is a very high yield method based on a vapour-solid mechanism that enables the growth of ZnO crystals in a very short timescale. LAFD was used in the growth of wurtzite micro/nanocrystalline ZnO with different morphologies (nanoparticles, tetrapods and microrods) as revealed by the extensive morphological characterization. Moreover, structural analysis evidenced the high crystalline quality of the produced crystals. The optical properties of the as-grown ZnO crystals were fully investigated by luminescence techniques, which revealed a high optical quality of the LAFD produced ZnO. In addition to the unintentionally doped micro/nanocrystals, ZnO/Ag and ZnO/carbon nanotubes (CNT) composite structures were also synthesized by LAFD. Silver-related spherical particles were found to be inhomogeneously distributed at the microrods surface, accumulating at the rods tips and promoting the ZnO nanorods re-nucleation. For the case of the ZnO/CNT composites two main approaches were adopted: i) a direct deposition of ZnO particles on the surface of vertically aligned multi-walled carbon nanotubes (VACNTs) forests without employing any additional catalyst and ii) ZnO/CNT buckypaper nanocomposites. It was found that the use of the LAFD technique carried out in framework of the first approach preserves the CNTs structure, their alignment, and avoids the collapse of the VACNTs array, which is a major advantage of this method. Additionally, taking into account that a crucial step in designing modern optoelectronic devices is to accomplish bandgap engineering, the optical properties of CdxZn1-xO alloy were also evaluated. A tuning of the ZnO bandgap towards the visible spectral region was accomplished by alloying this semiconductor with CdO. Finally, the potential application of the LAFD produced ZnO structures in the photocatalysis and photovoltaic fields was tested.