Intense Rt Visible Photoluminescence from Anodized Amorphous and Nanocrystalline Silicon Films
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INTENSE RT VISIBLE PHOTOLUMINESCENCE FROM ANODIZED AMORPHOUS AND NANOCRYSTALLINE SILICON FILMS
E. BUSTARRET', J.C. BRUYERE', F. MULLER*, M. LIGEON*"
"LEPES-CNRS, BP166, F-38042 Grenoble Cedex 9, France "LSP-UJF, BP 807, F-38402 St Martin d'H~res Cedex, France.
ABSTRACT
Heavily Boron-doped micrometer-thick amorphous (a-Si:B:H) or nanocrystalline (nc-Si) silicon layers have been deposited on a variety of conductive substrates by the 50 KHz PECVD of SiH 4 / B2 H6 / H2 mixtures at 320 0C. These films have been partially electro-oxidized in a HF solution and then anodically oxidized in water in a manner similar to that yielding luminescent porous monocrystalline silicon layers (PcSL). Although the anodized films are x-ray amorphous, they yield intense luminescence properties at room temperature very similar to those of anodically oxidized PcSL with a similar vibrational spectrum. In both anodically oxidized materials, aging is shown to improve the external quantum efficiency. In amorphous anodized layers, optical microscopy under UV excitation showed strongly luminescent strain-related heterogeneities (20 lam in diameter) connected by non luminescent channels. The incidence of our results on the current debate about the origin of visible room-temperature luminescence in porous silicon and Si:O:H systems is discussed. INTRODUCTION
A measure of the strong potential for optoelectronic applications of the recently discovered 1,2 room-temperature (RT) visible light emission from anodized silicon is the robustness of the effect : two years of intensive research have shown that it is observed both in the HF solution, where the surface is highly hydrogenated and even traces of silicon oxide are absent, and in partially oxidized layers resulting from a spontaneous reaction with ambient air, or from more controlled electrochemical 3 , chemical 4 or thermal 5 processes. In fact, it has also been shown that stain etches 6 and spark erosion 7 of Si wafers led to similar observations. Moreover, we have shown recently 8 .9 that silicon does not even need to be crystalline for the treated layer to yield an intense luminescence inthe visible. These results should be compared both with earlier reports on the visible RT photoluminescence (PL) from as-deposited a-SiHx 10 or a-Si:O:H 11,12 layers, and with the striking properties of siloxene derivates 13. Even though it is not obvious that all these experimental observations in silicon-based systems have one same origin, they have led to the attribution of the striking PL properties of nanoporous Si to (i) quantum size confinement in nanocrystalline Si regions 1 - 4, (ii) surface silicon
hydrides 14 -16, (iii) an amorphous Si phase 17 , 18 , and last to (iv) siloxene derivates 12. In this context, after a short review of the main properties of our Porous amorphous Si Layers (PaSL), we shall also compare the long term stability of the two types of layers before discussing the incidence of our results on the aboveoutlined debate. Finally, we shall also describe and characterize stress-r
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