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0958-L08-02

Pronounced Photonic Effects of High-Pressure Water Vapor Annealing on Nanocrystalline Porous Silicon Bernard Gelloz, Takayuki Shibata, Romain Mentek, and Nobuyoshi Koshida Elec.&Elec. Eng., Tokyo Univ. A&T, 2-24-16 Nakacho, Koganei, Tokyo, 184-8588, Japan

ABSTRACT The effects of high-pressure water vapor annealing (HWA) on the refractive index of PS have been studied. HWA was conducted at 260 ºC and 1.3 MPa, for 3 h. The refractive index (real part n and imaginary part k) was estimated by fitting reflectivity spectra. HWA considerably modifies the layers refractive index. It enhances the optical transparency of PS, particularly at short wavelengths down to below 400 nm. Both n and k are significantly reduced by HWA between 400 nm and 850 nm. These results are attributed to the high level of oxidation of PS after HWA. The high transparency of the treated layers enables Si-based photonics in the full visible range and also in the near UV range. Distributed Bragg reflectors (DBRs) have been fabricated. The central wavelengths appear blue-shifted compared to untreated samples due to the reduced refractive index. Almost no changes could be found in the reflectivity properties after one year storage in air for HWA-treated DBRs. Therefore, HWA is very useful for getting stable practical photonics devices in the visible and near UV range.

INTRODUCTION Porous silicon (PS), as a system containing a large density of nanocrystalline Si dots, is a very attractive material for its various functions and tunable properties, in particular in photonics. Photoluminescence, electroluminescence, as well as various photonic filters such as distributed Bragg reflectors (DBRs) have been demonstrated [1-3]. Though DBRs based on PS can be made with high specifications from the infrared (IR) to the red part of the visible spectrum, the characteristics at shorter wavelengths is limited by light absorption in residual Si [4,5]. On the other hand, the main obstacle hindering the usage of PS in commercial products is its poor stability upon ageing. Its large opened inner surface is very sensible to ambient atmosphere and evolves as a function of time. Several methods have been implemented in order to stabilize PS. Thermal [6], chemical or electrochemical oxidation [7,8] enhances the stability of PS devices but is still not good enough for practical use. Chemical modification of the hydrogen-terminated surface of PS by organic molecules led to significantly enhanced stability due to surface protection by steric hinderance and the establishment of stable and strong Si-C covalent bonds [9,10]. Actually these layers are very stable at the scale of weeks. However, their progressive oxidation cannot be fully prevented at the scale of months and years.

Recently [11,12], we reported that high-pressure water vapor annealing (HWA) is very useful for getting stable and highly efficient photoluminescence from as-anodized and electrochemically oxidized PS samples formed on p-type substrates. Under appropriate HWA conditions, the external PL q