Preparation, Properties and Applications of Free-Standing Porous Silicon Films
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ABSTRACT Using special electrochemical etching and lift-off steps, we have fabricated large-area freestanding porous silicon films in the thickness range from 0.1 gtm to 50 Jgm. Their transmission is near 100% in the near infrared which is indicative of very high porosity/low index of refraction films. These optically flat and homogeneous films exhibit no surface and bulk scattering, despite the fact that they did not undergo supercritical drying. The relationship between the absorption coefficient, the luminescence spectrum, and the chemical and structural properties is examined as a function of preparation and post-treatment conditions. Because of their superior optical properties, these films are suitable for many device applications.
INTRODUCTION Four years after Canham's discovery of bright red luminescence of porous silicon at room temperature [1], we know much more about this remarkable material including the possible luminescence mechanisms and various means of optimizing the optical properties. To date, the photoluminescense (PL) of light emitting porous silicon (LEPSi) covers the entire visible spectrum from the blue to the IR region, depending on the manufacturing conditions and post-treatments steps [2]. In the red, the luminescence efficiency can be as high 10% which is in the range of direct gap materials like GaAs and thus 4 to 5 orders of magnitude above that of bulk crystalline silicon
(c-Si).
For the potential application of porous silicon as an interface between microelectronics and optoelectronics a thorough understanding of the luminescence mechanism is essential. Freestanding LEPSi films make the direct comparison of the PL and absorption spectra feasible without any influence of the c-Si substrate [3]. This may allow us to verify or reject the models that attempt to explain the origin of the photoluminescence, and in particular to distinguish between the quantum confinement model [ 1,4] and the surface states model [5].
EXPERIMENTAL LEPSi was produced in an electrochemical etching cell containing an aqueous HF solution in which the anode is the crystalline silicon wafer and the cathode a Pt wire. The solution comtained ethanol (C2H 5 OH ), water and 100% HF (hydrofluoric acid) in a ratio of 2 to 1 to 1. We focus on thin films produced with a current density from 14 to 100 mA/cm 2 passing through a circular area of 1.76 cm 2 for a few seconds up to several minutes depending on the desired thickness. The anodization occured in the dark and the polished wafers had either low resistivity (highly Boron doped p-type, 0.01-0.005 fQcm) or higher resistivity (p-type, 3-7 flcm). For removing the porous silicon thin film from the substrate we applied an electropolishing step 333
Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
[6). To switch from toporous silicon formation to electropolishing, 2 is possible simply [3]. to increase Such a 300 mA/cm (p+ substrate, it25% HF-solution) the current density approximately might lead to structural damage high current density, applied a
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