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Langmuir 1997, 13, 4652-4658
Photoluminescence Quenching and the Photochemical Oxidation of Porous Silicon by Molecular Oxygen Jessica Harper and Michael J. Sailor* Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, California 92093-0358 Received June 3, 1996X Exposure of luminescent n-type porous Si to gaseous molecular oxygen results in reversible quenching of the visible photoluminescence associated with this material. Steady-state and time-resolved photoluminescence quenching follow a dynamic Stern-Volmer model. From the Stern-Volmer analysis, the quenching rate constant, kq, was found to be 26 ( 9 Torr-1 s-1. The rate constant for quenching is not strongly dependent on the chemical composition of the surface. Hydride-, deuteride-, or oxide-terminated surfaces all display similar quenching rate constants. Quenching is attributed to electron transfer from the luminescent chromophore in porous Si to an O2 molecule weakly chemisorbed to a surface defect. In parallel with the reversible quenching process but on a much longer time scale (minutes to hours depending upon light intensity), porous Si samples also slowly photooxidize. Both the intensity (measured at steady state) and lifetime (measured by nanosecond-pulsed laser excitation) of photoluminescence decrease as the surface oxide layer grows, approaching a constant value after several hours of O2 exposure. The mechanism of photochemical oxidation is proposed to involve the same photogenerated O2 species produced during quenching.
Introduction The intensity and color of the visible photoluminescence from porous Si have been shown to be extremely sensitive to a variety of adsorbates. Reversible or partially reversible quenching of the luminescence can be achieved by exposure to simple nonreacting molecules such as benzene,1,2 to Lewis bases such as trialkylamines,3-5 or to energy acceptor molecules such as anthracene.6,7 The sensitivity of this phenomenon and its compatibility with conventional Si microelectronic fabrication techniques have presented the possibility for use of this material in liquid or gas phase sensors.8 In the present work, we show that exposure of porous Si to gaseous oxygen results in reversible quenching of photoluminescence. The dependence of the quenching on O2 pressure follows a dynamic Stern-Volmer model. Quenching is ascribed to a mechanism involving transient nonradiative electron transfer from the luminescent chromophore in porous Si to a weakly chemisorbed O2 molecule. Oxidation of porous Si is currently a severe limitation for sensor and other microelectronic applications. For example, the oxidation of porous Si in electroluminescent devices leads to increased series resistances and a concomitant loss of efficiency after several hours of operation.9-18 The results presented here provide insight X
Abstract published in Advance ACS Abstracts, August 1, 1997.
(1) Lauerhaas, J. M.; Sailor, M. J. Science 1993, 261, 1567-1568. (2) Lauerhaas, J. M.; Credo, G. M.; Heinrich, J. L.; Sailor, M. J. J. Am. Chem. Soc. 1992, 114, 1911-1912. (3) Chandler-Henderson, R. R.; Sweryda-Krawiec, B.; Coffer, J. L. J. Phys. Chem. 1995, 99, 8851-8855. (4) Chun, J. K. M.; Bocarsly, A. B.; Cottrell, T. R.; Benziger, J. B.; Yee, J. C. J. Am. Chem. Soc. 1993, 115, 3024-3025. (5) Coffer, J. L.; Lilley, S. C.; Martin, R. A.; Files-Sesler, L. A. J. Appl. Phys. 1993, 74, 2094-2096. (6) Ko, M. C.; Meyer, G. J. Chem. Mater. 1995, 7, 12-14. (7) Fisher, D. L.; Harper, J.; Sailor, M. J. J. Am. Chem. Soc. 1995, 117, 7846-7847. (8) Fisher, D. L.; Gamboa, A.; Harper, J.; Lauerhaas, J. M.; Sailor, M. J. Mater. Res. Soc. Symp. Proc. 1995, 358, 507-518. (9) Bsiesy, A.; Nicolau, Y. F.; Ermolieff, A.; Muller, F.; Gaspard, F. Thin Solid Films 1995, 255, 43-48. (10) Canham, L. T.; Leong, W. Y.; Beale, M. I. J.; Cox, T. I.; Taylor, L. Appl. Phys. Lett. 1992, 61, 2563-2565. (11) Halimaoui, A.; Oules, C.; Bomchil, G.; Bsiesy, A.; Gaspard, F.; Herino, R.; Ligeon, M.; Muller, F. Appl. Phys. Lett. 1991, 59, 304-306.
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into the mechanism of photooxidation of porous Si that are consistent with data from single-crystal Si and also point out the potential for interference of O2 with environmental chemical sensors based on luminescence quenching of porous Si. Experimental Section Sample Preparation and Treatment. Luminescent porous Si samples were prepared by galvanostatic photoetch of polished crystalline n-Si (phosphorus doped; 0.5-5 Ω‚cm resistivity; (100) orientation) or p-Si (boron doped; 0.5-5 Ω‚cm resistivity; (100) orientation), supplied by International Wafer Service. The etching solution was prepared by adding an equal volume of pure ethanol (Quantum Chemicals) to an aqueous solution of HF (48% by weight; Fisher Scientific). The etching cell was constructed of Teflon and was open to air. Si wafers were cut into squares with a diamond scribe and mounted in the bottom of the Teflon cell with a Viton O-ring seal, exposing 0.3 cm2 of the Si surface. Electrical contact was made to the back side of the Si wafer with a strip of heavy Al foil. A loop of Pt wire was used as a counter electrode. For the n-type samples, the exposed Si face was illuminated with approximately 120 mW/cm2 of white light from a 300 W tungsten lamp for the duration of the etch. p-type samples were etched in the dark. Etching was carried out as a two-electrode experiment at an anodic current density of 5 mA/cm2 for 33 min. After etching, the samples were rinsed in ethanol, dried under a stream of N2(g), and placed in a 100 °C oven in air for 15 min. The samples were then mounted in a glass dosing chamber connected to a glass-and-Teflon Schlenk line19 via an O-ring joint. The Schlenk line was connected to a direct-drive vacuum pump. High-purity O2 (Parsons, 99.5%) and N2 (Air Liquide, 99.995%) were passed through a drying column (12) Halimaoui, A. Appl. Phys. Lett. 1993, 63, 1264-1266. (13) Koshida, N.; Koyama, H. Appl. Phys. Lett. 1992, 60, 347-349. (14) Kozlowski, F.; Sauter, M.; Steiner, P.; Richter, A.; Sandmaier, H.; Lang, W. Thin Solid Films 1992, 222, 196-199. (15) Loni, A.; Simons, A. J.; Cox, T. I.; Calcott, P. D. J.; Canham, L. T. Electron. Lett. 1995, 31, 1288-1289. (16) Tsybeskov, L.; Duttagupta, S. P.; Fauchet, P. M. Solid State Commun. 1995, 95, 429-433. (17) Richter, A.; Steiner, P.; Kozlowski, F.; Lang, W. IEEE Electron. Device Lett. 1991, 12, 691-692. (18) Steiner, P.; Kozlowski, F.; Lang, W. Appl. Phys. Lett. 1993, 62, 2700-2702. (19) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-Sensitive Compounds, 2nd ed.; John Wiley and Sons, Inc.: New York, 1986.
© 1997 American Chemical Society
Quenching and Oxidation of Porous Silicon containing activated 4Å molecular sieves before being supplied to the Schlenk manifold. An additional column containing activated Ridox catalyst (Fisher Scientific) was used to remove residual O2 from the N2 stream. Pressure inside the manifold was monitored using an MKS Baratron pressure gauge, containing two separate pressure transducers (Model 626A) calibrated for the pressure ranges from 10-5 to 1 Torr and from 1 to 1000 Torr. The chamber was pumped to
