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Descripción
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| Nanocrystals of Group IV semiconductors embedded in a dielectric SiO2 matrix are of interest because of their applications in optoelectronic devices. In this work, Si oxide films with a controlled excess of Si were deposited by LPCVD and thermally annealed to form Si nanocrystals embedded in SiO2. The deposition was carried out on Si and SiO2 wafers using Si2H6 and O2. To model the growth rate and the excess of Si as a function of the process temperature, pressure and Si2H6/O2 flow ratio, a 2k factorial design experiment was implemented. The pressure was varied from 185 to 300 mTorr, the deposition temperature from 250 to 450 ºC and the gas flow ratio from 0.2 to 5. The as-deposited samples were annealed in a furnace at temperatures up to 1100 ºC for times up to 1 hour to segregate the Si in excess. The composition of the as-deposited and annealed oxides was qualitatively analyzed by Fourier transform infrared spectroscopy (FTIR). Spectroscopic ellipsometry (SE) measurements were carried out, and the thickness, the optical constants and the Si content of the films were derived from simulations of the results. A model of the sample structure formed by one or several layers, with optical properties represented either by Cauchy functions or by Maxwell-Garnett mixtures of the optical properties of Si and SiO2, on top of the substrate was used. The presence of nanocrystals in the annealed samples was studied by Raman spectroscopy and cathodoluminescence (CL). The Raman spectra were acquired using UV excitation (325 nm) to enhance the sensitivity of the technique to small amounts of crystalline material. The influence of the first order spectrum of a Si substrate is eliminated by using the samples deposited on SiO2 wafers for these purposes. The CL emission of the samples was measured at temperatures from 80 K to RT. As the gas flow ratio increases, a peak attributed to Si-H bond bending appears at 887 cm-1 in the FTIR spectra and its intensity increases with the gas flow ratio, indicating that the oxide films are incorporating an excess of Si atoms. The SE measurements indicate that the growth rate ranges from 0.15 to 1.5 nm/min and the volume of Si in excess ranges from 0 to 50 %. The growth rate depends on the interaction between the temperature and the gas flow ratio, while it only depends slightly on the main variables. The excess of Si depends on the temperature and on the gas flow ratio, as well as on their interaction, but only slightly on the pressure. The design of the appropriate deposition processes to obtain films with the desired thickness and excess of Si can be carried out using these results. The FTIR spectra of the annealed layers corresponds to stoichiometric SiO2 for annealing temperatures above 900 ºC. SE shows that the volume fraction of the excess of Si is reduced after annealing. The main features of the Raman and CL spectra and their dependence on the structure and processing conditions of the samples will be discussed. | |
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Internacional
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Si |
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JCR del ISI
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No |
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Título de la revista
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Amorphous and Polycrystalline Silicon Science and Technology |
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ISSN
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13:978-1-55899-949-7 |
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Factor de impacto JCR
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0 |
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Información de impacto
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Volumen
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DOI
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Número de revista
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989 |
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06 |
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