Fabrication and optical characterization of macroporous silicon photonic crystals
Tipus de documentProjecte/Treball Final de Carrera
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The computer revolution experienced in recent years has been possible thanks to semiconductor materials, such as silicon, germanium and gallium arsenide. The success of the silicon-based microelectronics is due to the ability to integrate multiple elements on the same chip such as processors, memories, and interfaces. However, the increasing miniaturization and the realization of faster devices have revealed the difficulty to overcome the intrinsic limits of these materials. For example, devices have to dissipate the heat produced by the enormous power density involved, without damaging themselves , and the lengths can not be reduced indefinitely if one wants to ensure the synchronization of the signals. All these limitations have led to research into technologies that overcome the physical limits of the silicon-based microelectronics. From the outset, the most promising approach has been photonics. In photonics, the so-called photonic crystals have aroused a widespread interest. These innovative materials are fundamental for the construction of optical circuits, i.e. circuits in which the transmission of information is carried out by photons instead of electrons. These materials can be considered as the optical analogue of semiconductors: in semiconductors the presence of a periodic potential, causes the formation of electronic energy bands separated by forbidden intervals called energy gaps, where there are not electronic states. In a photonic crystal, a periodic distribution of dielectrics with different refractive index causes the formation of a so-called photonic energy gap: photons with energy values internal to the forbidden gap can not pass through the crystal and will be refected or conffined inside of it. The goal of this thesis is the fabrication of photonic crystals based on macroporous silicon technology and their optical characterization using infrared spectroscopy in order to reveal the presence of photonic band gaps. All the fabrication and characterization processes were carried out in the clean room and laboratories of the "Universitat Politècnica de Catalunya (UPC)" , thanks to the collaboration of the "Grup de Recerca en Micro i Nanotecnologies". The thesis is structured as follows: in Chapter 1 are presented the theoretical aspects related to photonic crystals, as well as the mechanism of formation of macropores in n-type silicon wafers by electrochemical etching. In chapter 2 is explained the fabrication process of the macroporous silicon photonic crystals detailing all the process steps to which the silicon wafer is subjected to obtain the desired structures. Photographs taken by a scanning electron microscope (SEM) are also reported to verify the correspondence between the obtained structures and those drawn in the design phase. In Chapter 3, using the MIT photonic band gap (MPB) software package, developed by the Massachusetts Institute of Technology (MIT) , the band diagrams of different photonic structures are computed to study how certain design parameters could affect the position and width of the photonic band gaps. Chapter 4 presents the results of the refection and thermal emission measurements, carried out by Fourier Transform Infrared spectroscopy (FT-IR) to determine the optical response and to verify the presence of photonic band gaps in the fabricated structures. Finally, in Chapter 5 are given the conclusions of this thesis.
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