Caracterização e modificação de heteroestruturas de nitretos do grupo III

Os nitretos binários semicondutores do grupo III, e respetivos compostos, são vastamente estudados devido à sua possível aplicabilidade em dispositivos optoeletrónicos, tais como díodos emissores de luz (LEDs) e LASERs, assim como dispositivos para a eletrónica de elevadas temperatura, potência e frequência. Enquanto se concretizou a comercialização na última década de LEDs e LASERs recorrendo ao ternário In1-yGayN, estudos das propriedades fundamentais estruturais e óticas, assim como de técnicas de processamento no desenvolvimento de novas aplicações de outros ternários do grupo III-N encontram-se na sua fase inicial. Esta tese apresenta a investigação experimental de filmes finos epitaxiais de Al1-xInxN crescidos sobre camadas tampão de GaN e de Al1-yGayN e o estudo do recozimento e implantação de super-redes (SL) compostas por pontos quânticos de GaN (QD) envolvidos por camadas de AlN. Apesar do hiato energético do Al1-xInxN poder variar entre os 0,7 eV e os 6,2 eV e, por isso, numa gama, consideravelmente superior à dos ternários Al1-yGayN e InyGa1-yN, o primeiro é o menos estudado devido a dificuldades no crescimento de filmes com elevada qualidade cristalina. É efetuada, nesta tese, uma caracterização estrutural e composicional de filmes finos de Al1-xInxN crescidos sobre camadas tampão de GaN e de Al1-yGayN usando técnicas de raios-X, feixe de iões e de microscopia. Mostra-se que o Al1-xInxN pode ser crescido com elevada qualidade cristalina quando a epitaxia do crescimento se aproxima da condição de rede combinada do Al1-xInxN e da camada tampão (GaN ou Al1-yGayN), isto é, com conteúdo de InN de ~18%, quando crescido sobre uma camada de GaN. Quando o conteúdo de InN é inferior/superior à condição de rede combinada, fenómenos de relaxação de tensão e deterioração do cristal tais como o aumento da rugosidade de superfície prejudicam a qualidade cristalina do filme de Al1-xInxN. Observou-se que a qualidade dos filmes de Al1-xInxN depende fortemente da qualidade cristalina da camada tampão e, em particular, da sua morfologia e densidade de deslocações. Verificou-se que, dentro da exatidão experimental, os parâmetros de rede do ternário seguem a lei empírica de Vegard, ou seja, variam linearmente com o conteúdo de InN. Contudo, em algumas amostras, a composição determinada via espetrometria de retrodispersão de Rutherford e difração e raios-X mostra valores discrepantes. Esta discrepância pode ser atribuída a defeitos ou impurezas capazes de alterar os parâmetros de rede do ternário. No que diz respeito às SL dos QD e camadas de AlN, estudos de recozimento mostraram elevada estabilidade térmica dos QD de GaN quando estes se encontram inseridos numa matriz de AlN. Por implantação iónica, incorporou-se európio nestas estruturas e, promoveu-se a ativação ótica dos iões de Eu3+ através de tratamentos térmicos. Foram investigados os efeitos da intermistura e da relaxação da tensão ocorridos durante o recozimento e implantação nas propriedades estruturais e óticas. Verificou-se que para fluências elevadas os defeitos gerados por implantação são de difícil remoção. Contudo, a implantação com baixa fluência de Eu, seguida de tratamento térmico, promove uma elevada eficiência e estabilidade térmica da emissão vermelha do ião lantanídeo incorporado nos QD de GaN. Estes resultados são, particularmente relevantes, pois, na região espetral indicada, a eficiência quântica dos LEDs convencionais de InGaN é baixa.

Asymmetric rolling of 5182 aluminium alloy and interstitial free steel sheets

This Ph.D. research focuses on asymmetric rolling (ASR), as an alternative method for improving mechanical responses of aluminium-magnesium alloy and interstitial free (IF) steel regarding industrial requirements. Aluminium alloys are attractive materials in various industries due to their appropriate properties such as low density and corrosion resistance; however, their low formability has limited their applications. As formability of aluminium alloys can be improved through texture development, part of this dissertation is dedicated to producing the desired crystallographic texture with the ASR process. Two types of ASR (i.e. reverse and continuous asymmetric rolling) were investigated. The impact of shear deformation imposed by ASR processes on developing the desirable texture and consequently on mechanical behaviours was observed. The developed shear texture increased the normal and also planar anisotropy. Texture evolution during plastic deformation as well as induced mechanical behaviour were simulated using the “self-consistent” and Taylor models. Interstitial free (IF) steel was the second material selected in this dissertation. Since IF steel is one of the most often used materials in automotive industries it was chosen to investigate the effect of shear deformation through ASR on its properties. Two types of reverse and continuous asymmetric rolling were carried out to deform IF steel sheets. The results of optical microscopy and atomic force microscopy observations showed no significant difference between the grains’ morphology of asymmetric and conventionally rolled samples, whereas the obtained results of transmission electron microscopy indicated that fine and equiaxed dislocation cells were formed through the asymmetric rolling process. This structure is due to imposed shear deformation during the ASR process. Furthermore, the mechanical behaviour of deformed and annealed sheets was evaluated through uniaxial tensile tests. Results showed that at low thickness reductions (18%) the asymmetric rolled sample presented higher stress than that of the conventionally rolled sheet; while for higher thickness reductions (60%) the trend was reversed. The texture analyses indicated that intense rolling texture components which developed through 60% thickness reduction of conventional rolling cause a relatively higher stress; on the contrary the fine structure resulting from ASR appears to be the source of higher stress observed after pre-deformation of 18%.

Severe plastic deformation of Al–Zn alloys

In this work, the R&D work mainly focused on the mechanical and microstructural analysis of severe plastic deformation (SPD) of Al–Zn alloys and the development of microstructure–based models to explain the observed behaviors is presented. Evolution of the microstructure and mechanical properties of Al–30wt% Zn alloy after the SPD by the high–pressure torsion (HPT) has been investigated in detail regarding the increasing amount of deformation. SPD leads to the gradual grain refinement and decomposition of the Al–based supersaturated solid solution. The initial microstructure of the Al–30wt% Zn alloy contains Al and Zn phases with grains sizes respectively of 15 and 1 micron. The SPD in compression leads to a gradual decrease of the Al and Zn phase grain sizes down to 4 microns and 252 nm, respectively, until a plastic strain of 0.25 is reached. At the same time, the average size of the Zn particles in the bulk of the Al grains increases from 20 to 60 nm and that of the Zn precipitates near or at the grain boundaries increases as well. This microstructure transformation is accompanied at the macroscopic scale by a marked softening of the alloy. The SPD produced by HPT is conducted up to a shear strain of 314. The final Al and Zn grains refine down to the nanoscale with sizes of 370 nm and 170 nm, respectively. As a result of HPT, the Zn–rich (Al) supersaturated solid solution decomposes completely and reaches the equilibrium state corresponding to room temperature and its leads to the material softening. A new microstructure–based model is proposed to describe the softening process occurring during the compression of the supersaturated Al–30wt% Zn alloy. The model successfully describes the above–mentioned phenomena based on a new evolution law expressing the dislocation mean free path as a function of the plastic strain. The softening of the material behavior during HPT process is captured very well by the proposed model that takes into consideration the effects of solid solution hardening and its decomposition, Orowan looping and dislocation density evolution. In particular, it is demonstrated that the softening process that occurs during HPT can be attributed mainly to the decomposition of the supersaturated solid solution and, in a lesser extent, to the evolution of the dislocation mean free path with plastic strain.