Observation of Optical Cavity Modes in Photoluminescent Porous Silicon Films

Abstract

Uniform layers of porous Si have been produced that show fine structure in their photoluminescence (PL) spectra characteristic of longitudinal optical cavity modes. This mode spacing can be modified in a predictable way by immersing the porous Si layer in heptane, which changes the average refractive index of the porous Si layer. The 5 ~m thick cavity is generated by a photoelectrochemical etch of single-crystal (100) p-St wafers (3 mA/cm 2, 9 C/cm ~, irradiated with 57 ~W/cm 2 of 500 nm light for the duration of the etch). Electron micrographs show that the layer is homogeneous on a submicron scale and that the St/porous Si and porous St/air transitions are abrupt (<0.5 ~m). On thinner porous Si films, optical interference leads to a significant distortion of the emission spectrum. The relevance of interference-induced spectral changes to measurements of the intrinsic emission spectrum of porous Si is discussed.The observation of visible photoluminescence (PL) 1 and electroluminescence ~-~ from porous Si has generated interest in this material for its potential optoelectronic applications. In this work, we show that films of luminescent porous Si can be fabricated that display well-resolved Fabry-Perot fringes in their luminescence spectra. The production of a luminescent porous Si layer that is optically uniform enough to exhibit these interference fringes may be important for the design of etalons and other optical components utilizing porous St. 6 This work demonstrates that the apparent lineshape and peak position of the PL spectrum of porous Si can be modified significantly by thin-film interference phenomena. Such interferencebased shifts can be used to modulate the inherent emission spectrum of porous St. On the other hand, these interference effects could be mistaken for changes in the intrinsic emission spectrum of porous St. Similar conclusions have recently been drawn from PL spectra of inhomogeneeusly (laterally) anodized porous Si layers by Fujiwara et al.7 Experimental ProceduresSingle-crystal polished (100) wafers of boron-doped (ptype) Si of 3 fl-cm resistivity were cut into rectangles with areas of approximately 0.5 cm 2. These were ohmically contacted on the back by scratching with Ga/In eutectic and affixing a Cu wire with Ag print. The ohmic nature of the contact method was checked by running a current-voltage curve of a sample containing two such Ga/In contacts. The I-V curve was linear over a +50 mA current range, with a net resistance less than 4 ~. The ohmic contacts were coated with Hysol white epoxy, and the resulting devices were used in a two-electrode electrochemical cell configuration. A Pt flag with a press-contacted Pt wire attached was used as the counterelectrode. The etching bath was a 50:50 (by volume) solution of aqueous 49% HF (Fisher Scientific, Electronic Grade) and 95% ethanol (Quantum Chemical Company). The hexane and heptane used in the study were obtained from Fisher Scientific (reagent grade) and stored over dried molecular sieves. The porous Si samples were prepared ...

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