Search

Comunicação em poster no simposio M - More than Moore: Novel materials approaches for functionalized Silicon based Microelectronics, Si and Ge nanocrystals (NCs) embedded in a dielectric matrix, such as SiO2, have attracted great interest for many relevant technological applications. There are many studies about Ge NCs embedded in single layer of SiO2 [1]. However, there are only a few published results about Ge NCs embedded in SiO2 multilayer [2]. Using a multilayered superlattice approach one can achieve a higher density and more uniform size and spatial distributions of NCs, which is very important for the collective behavior effects. In this work we investigate the structural properties of Ge quantum dot (QD) lattices formed in amorphous silica matrix by magnetron sputtering deposition of (Ge+SiO2)/SiO2 multilayer. We demonstrate the dependencies of QD shape, size, separation and spatial arrangement type on Ge-rich (Ge+SiO2) layer thickness. We show that the formed QDs are elongated in direction perpendicular to the multilayer surface (vertical direction). The vertical size of the QDs and their separation in the vertical direction can be tuned by changing the Ge-rich layer thickness. The mean values of the QDs lateral size of their lateral separation are not affected by the thickness of the Ge-rich layer. However, the thickness of Ge-rich layer significantly affects the regularity in QD ordering. In addition, we investigate the dependence of the multilayer atomic composition and QD inner crystalline structure on the deposition parameters, which are important for future device applications. [1] S. Takeoka, K. Toshikio, M. Fujii, S. Hayashi, and K. Yamamoto, Phys. Rev. B 61, 15988 (2000) [2] F. Gao, M. A. Green, G. Conibeer, E. C. Cho, Y. D. Yang, I. Pere-Wurfl, and C. Flynn, Nanotechnology 19, 455611 (2008)

Comunicação oral, Semiconductor nanoparticles (NPs) are of great interest for multiple applications in the fields of photonics, optoelectronics, photovoltaics and biomedicine [1]. The ability to manage properly the size distribution and density in the fabrication process is of crucial relevance. In the literature, it can be found a large number of strategies for NPs production, including physical vapour deposition (PVD) methods. The main advantage of this technique, with respect to others, is the possibility to produce NPs in a free-contaminant inert atmosphere. In particular, pulsed laser deposition (PLD) technique is a versatile technique for producing NPs at room temperature (RT) when an inert gas atmosphere is introduced in the growth chamber [2]. Shadow mask or eclipse method has been demonstrated as an excellent approach for NPs size filtering, avoiding direct deposition of micron-sized particles usually produced in the ablation process [3]. In the nanometer scale, high-resolution microscopy techniques are essential to evaluate NPs sizes and density among other features, in order to optimize the NPs production process. In this work, we present scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies of NP films produced by PLD in an inert Argon gas atmosphere and under different deposition conditions, namely gas pressure, energy density and laser pulse frequency. We have combined shadow mask and off-axis configurations for NPs deposition. It is shown that, both NPs sizes and their distribution can be controlled precisely using adequate deposition conditions. WSxM software was used for AFM image processing and analysis [4]. [1] Andrey Rogach, Springer Wien New York (2010). [2] L. Patrone, D. Nelson, V. I. Safarov, M. Sentis, W. Marine, S. Giorgio, J. Appl. Phys. 87 (2000) 3829. [3] J. Martín-Sánchez, L. Marques, E.M.F. Vieira, Q.T. Doan, A. Marchand, A. El Hdiy, A.G. Rolo, S.R.C. Pinto, M.M.D. Ramos, A. Chahboun, M.J.M. Gomes, submitted to J. Nanopart. Res. [4] I. Horcas, R. Fernandez, J.M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero, A.M. Baro, Review of Scientific Instruments, 78 (2007) 013705.

Tese de doutoramento em Ciências (área de especialização em Física), Neste trabalho, apresenta-se um estudo da produção de nanocristais (NCs) e o estudo das propriedades estruturais de semicondutores imersos em matrizes dieléctricas . Os NCs foram produzidos pela técnica de pulverização catódica por rádio-frequência, posteriormente, foram sujeitos a tratamentos térmicos em diferentes atmosferas e temperaturas. Estudámos as condições que levam à formação de NCs dispersos de forma homogénea na matriz, de tamanho uniforme, elevada densidade e boa cristalinidade, e a ordem na posição dos NCs nas diferentes matrizes. As amostras com NCs embebidos em matrizes dieléctricas foram caracterizados pelas técnicas de difracção de raios-X (XRD), difusão de raios-X de incidência rasante (GISAXS), espectroscopia Raman, microscopia de transmissão electrónica de alta resolução (HRTEM) e espectrometria de Retrodifusão de Rutherford (RBS). O tamanho dos NCs, distribuição de tamanhos e arranjo espacial foram investigados a partir de medidas GISAXS e TEM, enquanto o acesso à composição química dos NCs e à sua estrutura cristalina foi obtido por medidas Raman, HRTEM e difracção de raios - X. Os resultados demonstram que o tamanho médio e a composição química dos NCs são fortemente sensíveis aos parâmetros de deposição e tratamentos térmicos utilizados. Verifica-se que, em amostras com NCs de Ge de tamanhos grandes em matriz de Al2O3 podem ficar sujeitos a tensões de compressão. Atribuímos este efeito à existência da matriz de Al2O3, que em certas condições pode cristalizar e induzir tensões nos cristais durante o processo de aquecimento/arrefecimento. Em alguns casos, durante o tratamento térmico o Ge oxida e forma-se GeOx, que sendo volátil desaparece do filme. Assim, a rede ordenada, previamente formada, dá origem a uma rede de espaços vazios de tamanho nanométrico. Relativamente ao sistema Si1−xGex, observa-se a formação de NCs com diferentes fracções de Ge (0 < x ≤ 1), de tamanho pequeno, com uma estreita distribuição de tamanhos e bem distribuídos na matriz de SiO2. Neste estudo, é apresentado a variação da frequência Raman dos modos vibracionais Ge-Ge e Si-Ge, em função da composição da liga. Foram produzidas estruturas de multicamadas de SiO2/Si1−xGex+SiO2/SiO2, as quais foram submetidas a tratamentos térmicos a temperaturas entre 700 oC a 1000 oC. Verificamos que, para temperaturas maiores que 700 oC, a estrutura de multicamadas é destruída pela difusão dos átomos de Ge. Para temperaturas de 700 oC, os NCs formados apresentam estrutura cristalina de boa qualidade. É efectuada uma comparação da ordem e do arranjo espacial dos NCs formados nas diferentes matrizes a partir dos resultados de medidas GISAXS. Observa-se a formação espontânea de nanopartículas de Ge ordenadas regularmente. Até agora, estas estruturas foram observadas e explicadas satisfatoriamente apenas em sistemas crescidos em matrizes cristalinas. Os resultados aqui apresentados mostram a possibilidade de formação em matrizes amorfas, com QDs de Ge auto-organizados numa rede de tridimensional do tipo BCt. Os fenómenos observados são explicados por modelo simplificado de Monte-Carlo. Os resultados mostram, também, a influência da correlação espacial nas propriedades físicas das nanopartículas formadas, tais como as suas dimensões e propriedades de comportamento colectivo resultante de um arranjo regular espacial dos QDs., The work described in this dissertation is devoted to study the structural properties of semiconductor nanocrystals (NCs) embedded in dielectric matrices.NCs were produced by RF-magnetron sputtering technique, and subsequently heat treatments in different atmospheres and temperatures. The aim was to find the optimal experimental conditions that promote the formation of NCs with uniform size, high density and good crystallinity, and we study the arragement of the NCs in different matrix. NCs embedded in dielectric matrices were characterized by X-ray diffraction (XRD), grazing incident small angle X-ray scattering (GISAXS), Raman spectroscopy, high resolution transmission electron microscopy (HRTEM) and Rutherford backscattering spectrometry. The NCs size, size distribution and the spatial arrangement were determined by GISAXS and TEM measurements, while Raman, HRTEM and diffraction measurements provided the access to the NCs composition and crystalline structure. The results of analysis show that the average size of NCs and the composition of Si1−xGex heterostructure are very sensitive to the parameters of deposition and annealing procedures. We verified that Ge NCs in Al2O3 matrix are under compressive stress for big sizes. In fact, under determinate conditions alumina matrix can crystallize and promote the stress in crystal during the heating/cooling. In some cases, during the thermal treatment the Ge oxidizes and forms GeO, which is volatile and disappears from the film. The previously formed lattice remained as a lattice of nanometric voids. For Si1−xGex system, we observed the formation of NCs with different Ge composition (0 < x ≤ 1), small sizes, narrow size distribution and well distributed in SiO2 matrix. The study of Raman frequencies of the vibrational modes Ge-Ge and Si-Ge as a function of alloy composition is presented. SiO2/Si1−xGex+SiO2/SiO2 multilayer structures were produced. The multilayers were annealed at temperature ranging between 700 oC to 1000 oC. It is shown that for temperatures higher than 700 oC the multilayers structures are destroyed to the diffusion, but the NCs formed still present good cristallinity. Finally, a comparison of the order and the spatial arrangement of NCs formed in different matrices was performed by GISAXS analysis. Especially interesting is the observation of spontaneous formation of regularly ordered Ge nanoparticles. Until now, such structures were observed and explained satisfactorily only in crystalline systems. Our results show the possibility of Ge NCs formation in amorphous matrices where the structure and ordering mechanisms are completely differents. The observed phenomena are explained and simulated by a simple Monte-Carlo model. The results also show the influence of spatial correlation on physical properties of formed NCs such as their sizes and very interesting collective behavior properties induced by a regular spatial arrangement.