Properties of Porous Silicon with Photoluminescence Enhanced by a Remote-Plasma Treatment
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PROPERTIES OF POROUS SILICON WITH PHOTOLUMINESCENCE ENHANCED BY A REMOTE-PLASMA TREATMENT
Y. XIAO*,**, M.J. HEBEN*, J.I. PANKOVE*,**, AND Y.S. TSUO* *National Renewable Energy Laboratory, Golden, CO 80401 "**University of Colorado, Dept. of Electrical and Computer Engineering, Boulder, CO 80309
ABSTRACT We have developed remote-plasma and argon-ion laser-illumination treatments that enhance and stabilize the photoluminescent emission from porous Si (PS) that was electrochemically etched and thermally annealed. Infrared absorption spectroscopy revealed that the bonded oxygen content in the film was increased and the density of surface silicon dihydride bonds was reduced by these processing steps. The photolumninescence (PL) intensity of the treated films was initially about 15 times higher than that of the as-etched films, and increased further with the time of laser irradiation in air until a steady-state intensity was reached. The emission from the treated samples increases with decreasing temperature to a maximum intensity at about 100 K. Below 100 K, the photoluminescence intensity of the treated PS samples remains constant with decreasing temperature down to 7.7 K, whereas the as-etched samples, as well as siloxene, show a rapid decrease in photoluminescence intensity with decreasing temperature in this temperature range. Electron spin resonance data show that dangling bond densities in annealed PS films are reduced by the remote-plasma treatment. INTRODUCTION Thermal, optical, electrochemical, and plasma post-formation processing of porous Si has been employed by several groups to alter the surface chemical composition of porous silicon. Such treatments effect the nature and degree to which the PS surfaces are electronically passivated, and therefore effect the non-radiative recombination rates of photo-generated carriers [1]. An understanding of the relationship between surface passivation and the quantum efficiency of luminescence is of vital importance for the understanding of the fundamental mechanisms of light emission from silicon nanostructures. Furthermore, the development of robust passivating surface terminations will be necessary for the use of porous silicon in optoelectronic applications. Studies have found that the PL from PS degrades with illumination time due to the incorporation of oxygen onto hydrogen-rich as-etched-PS surfaces [2], while others have shown that oxygen incorporation by conventional high temperature oxidation [3], electrochemical oxidation [1], or rapid thermal annealing [4] can result in a PL enhancement. Evidently, the manner in which oxygen is incorporated onto PS surfaces dictates whether a beneficial or detrimental function is performed. Remote-plasma processing is a convenient means by which monoatomic species may be made to interact with porous silicon surfaces at a relatively low temperature. Placement of the samples outside of the plasma insures minimal damage to fragile Si nanostructures by energetic atoms and ions. This technique has often been employed to passivat
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