Correlation Between Bulk Morphology and Luminescence in Porous Silicon Investigated by Evaporation Induced Pore Collapse
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Correlation Between Bulk Morphology and Luminescence in Porous Silicon Investigated By Evaporation Induced Pore Collapse Donald J. Sirbuly, Michael D. Mason, and Steven K. Buratto Department of Chemistry and Biochemistry University of California, Santa Barbara, CA 93106 ABSTRACT We use a combination of scanning electron microscopy, laser scanning confocal microscopy and luminescence spectroscopy to correlate the emission properties of anodized porous silicon (PS) with film morphology in samples that have undergone evaporation induced collapse of the underlying porous structure. Several PS samples were investigated as a function of the current density (J) and total etch time, while the total charge (Q) injected per unit area (and the total Si removed) was kept constant during etching. From this data two classes of PS samples emerge. Porous silicon samples produced at high current density have a 3-dimensional pore network with a narrow distribution of blue-green emitting chromophores. In contrast, low current density samples form a 2-dimensional pore network normal to the Si substrate with larger chromophores and exhibit broad red luminescence. INTRODUCTION Visible light emission from Si via anodic etching in aqueous HF, called porous silicon (PS), has stimulated tremendous interest over the past several years due to its potential application in opto-electronic devices such as light emitting diodes (LEDs) and lasers, and the ability to be integrated with current Si processing technology.[1, 2] To date much of the research on PS has been devoted to developing a detailed understanding of the emission mechanism, which has been attributed primarily to an excited state confinement effect.[3] Although a quantum confinement model for the emission mechanism of as-prepared porous silicon has gained widespread support, the high degree of structural inhomogeneity and parametric tunability of etched samples have made a direct correlation between chromophore size and optical bandgap difficult. In most cases the “diameter” of the silicon framework within the layer is estimated as a linear function of sample porosity. Support for this conclusion is drawn from a host of fluorescence data, specifically a shift towards higher emission energies with an increase in exposure to the electrolyte, an increase in current density, or an increase in HF concentration. One common characteristic of as-prepared porous silicon is its tendency to deform and crack during drying. However, the particular morphologies that are produced during solvent evaporation provide insight into the geometry of the pore structure, and PS network, of the in situ sample. In this study PS samples were prepared by carefully controlling the total charge (Q) supplied during etching. In this way, samples with the same pore volume were made, under different etch parameters, and their resultant morphologies after drying were compared. Scanning electron microscopy (SEM) was used to directly image the sample structure, while laser scanning confocal microscopy (LSCM) and fluores
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