Fabricação de multicamadas de Quantum Dots de PbTe por laser ablation

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

This thesis was dedicated to the fabrication of thin films, quantum dots and multilayer ofIV-VI semi conducting materiaIs by pulsed-Iaser deposition (PLD). In the first part we report on the construction of a "home made" facility for pulsed laser deposition. The basic experimental set-up fabricated for PLD consists of a vacuum chamber pumped to a base pressure of ~ 1 x 10-7 mbar by a turbo molecular pump. Laser ablation of the target is performed using the second harmonic of a Q-Switched Quantel Nd:Y AG laser (532 nm, 5ns, 20Hz, 1 65mJ/pulse). The pulsed laser beam is focused on an incidence angle of 45° on a target, using a 30 cm lens. The target holder is rotated around the target normal in arder to reduce target degradation by multiple irradiation of a single spot. Afterwards we report on the fabrication and characterization of PbTe and HgTe thin films by pulsed laser deposition in an argon atmosphere. The influence of the growth parameter like target-substrate distance, laser fluence, backing gas pressure and substrate temperature on the structural and morphological properties of the films was studied. Highly oriented, polycrystalline and almost splashing free films were fabricated provided the correct parameter were chosen. We also investigated the expansion dynamics of the plasma induced by the laser ablation of PbTe by analyzing the light emitted from the plume. Space and time-resolved optical spectroscopy measurements indicate the presence of both, neutral (PbI, TeI) and ionized (PbII, TeII) species. The velocities of the species remain unchanged for oxygen pressures up to 0.1 mbar, which suggest that the expansion of the plume occurs without further collisions with the foreign gas in this pressure range. For pressures of 0.5 an above, a region of increasing emission intensity between target and substrate is found when a gas environment is present. This region is related to an increasing of the species density and a compression of the plume. A model to explain this feature was proposed. Finally, we report on the fabrication of SiO2/PbTe multilayers. The gemi conducting nanoparticles were grown by laser ablation of a PbTe target. The glass matrix was fabricated by a plasma chemical vapor deposition (PECVD) method using tetramethoxysilane (TMOS) as precursor. This alternating growth was achieved with a computer-controlled interface using a Lab View code. Cross-section TEM image clearly showed QD s layer well separated from each other with glass matrix layers. The influence of the ablation time and the laser fluence on the density and size distribution of the quantum dots was studied. Despite most of the nanoparticles observed by HRTEM were oriented in the (200) or (220) directions, electron diffiaction patterns of the multilayer reveal that there is no preferential orientation of the QD s. Absorption, photo luminescence and relaxation time of the multilayer were also too measured

ASSUNTO(S)

filmes finos pecvd plasma produzido por laser pontos quanticos otica não-linear

ACESSO AO ARTIGO

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