Infiltration of ZnO in Mesoporous Silicon by Isothermal Zn Annealing and Oxidation

Abstract

In this work a two-step procedure is reported for the formation of ZnO/porous silicon (PS) composites in which ZnO is embedded in the pores of sponge like mesoporous silicon. The procedure consists of an isothermal annealing of the PS layer in Zn vapors using a close space configuration and a subsequent oxidation of the Zn infiltrated in the pores. The oxidation agent and the annealing duration are optimized for a complete oxidation of the infiltrated Zn. Structure, morphology and composition of the samples were characterized by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Rutherford backscattering spectrometry (RBS) and photoluminescence (PL). The ZnO/PS composite was observed to exhibit a broad luminescent band covering almost all the visible range. Porous silicon (PS) has received a lot of interest after the discovery by L. T. Canham 1 of its efficient luminescence at room temperature, which makes PS a promising material in the field of optoelectronics. Furthermore, the nanometric size of the pores allows its use as template for fabrication of nanostructures.2-5 On the other hand, the internal surface of PS can be as high as 1000 m 2 /cm 3 and its refractive index can be tuned depending upon its porosity and the embedded material. All of these properties make the PS promising in sensor technology. [6][7][8] Infiltration of ZnO into PS is primarily motivated by potential applications of ZnO in the field of optoelectronics, due to its wide bandgap of 3.37 eV at 300 K. The efficient luminescence and functionality of ZnO nanostructures make them useful in several applications such as UV luminescence devices, 9 piezoelectric devices, 10ZnO -based transparent thin-film transistors, 11 chemical sensors, 12,13 photovoltaic devices 14 and solid state lighting technology. 15 Usually, white emission is obtained by combining LEDs emitting in the blue, green and red region of the electromagnetic spectrum. However, a broad band in the entire visible range, due to the coupling between UV and yellow-green emission of ZnO with the red one of PS, has been observed in ZnO/PS structures. 16,17 ZnO has been grown on top of PS using sol-gel, 3 electrochemical deposition, 18 pulsed laser deposition, 17 spray pyrolysis 19 and sputtering. 20,21 However, up to the best knowledge of the authors, ZnO infiltration inside the pores has been demonstrated only by using a combination of sol-gel deposition and annealing. 22 In fact, special provisions have to be taken for the infiltration of mesoporous materials because pores entrance obstruction can prevent the completion of process. In the case of sponge-like mesoporous silicon, infiltration becomes particularly difficult due to the irregular and intricate arrangement of the pores. In this paper, we propose a two-step procedure consisting of using isothermal close space sublimation (ICSS) 23 for the Zn infiltration and then vapor transport of water for its subsequent oxidation. The advant...