Visible Light Emission from Porous Silicon Examined by Photoluminescence and Raman Spectroscopy
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VISIBLE LIGHT EMISSION FROM POROUS SILICON EXAMINED BY PHOTOLUMINESCENCE AND RAMAN SPECTROSCOPY TERRY R. GUILINGER, MICHAEL J. KELLY, DAVID R. TALLANT, DAVID A. REDMAN, AND DAVID M. FOLLSTAEDT Sandia National Laboratories, Albuquerque, New Mexico 87185 ABSTRACT We describe the acquisition of Raman and photoluminescence (PL) spectra on porous silicon (PS) samples that emit visible light. Spectra were acquired in both ex situ experiments (after exposure to air) and in situ experiments (with the PS covered either with the hydrofluoric acid electrolyte used in the formation process or water). Our results generally show a correlation of blue-shifted PL with increased oxidation. In one set of ex situ experiments, however, we observed an inconsistency in the shift of the wavelength of maximum luminescence intensity for PS samples that exhibit oxygenated character in the Raman spectra. A higher anodization current density produced a red shift in the PL spectra in one experiment, while chemical dissolution of the PS by hydrofluoric acid produced the well-known blue shift in the other case. In two in situ experiments, we observed very weak and red-shifted PL for a PS sample immersed in HF (compared to the same sample measured later in air) while in another we immersed air-exposed PS in water and observed a 15-fold increase in PL intensity along with a blue shift in the luminescence maximum. INTRODUCTION Although the property of visible luminescence from porous silicon (PS) was discovered more than 30 years ago [1], a renewed interest due to the work of Canham [2] has resulted in numerous publications examining the composition and morphology of PS and their relation to the mechanism of PS photoluminescence (PL). As yet, no clear model for PS photoluminescence has been accepted. Proposed mechanisms include: (1) quantum size effects in the PS nanostructures affecting the electronic band structure of silicon [2,3]; (2) luminescence from Si-O-H polymeric compounds such as siloxene, [4]; (3) PL due to the presence of SiHX species, particularly Si 2 [5]; and (4) light emission from hydrogen-terminated amorphous silicon structures [6]. Techniques such as Raman and Fourier-transform infrared (FTIR) spectroscopies have been used to examine the chemical species present in PS and associate these chemical species with PS photoluminescence. FTIR measurements of air-exposed PS films indicate that H-bonding to the Si yields SiHx surface species that may be important in PS photoluminescence since the peak PL intensity increases with increasing SiH. concentration [5]. Conversely, another FTIR study reports that the growth of a native oxide, and subsequent consumption of the SiHx from the PS, alters only slightly the PL, thus precluding SiHx as the species responsible for visible PL [7]. Another FTIR study demonstrated that the disappearance of the PL with annealing temperature did not scale directly with H2 desorption and concluded that PL from PS is not directly related to the presence of SiHR species on the PS surface [8]. Raman spectroscop
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