Fabrication of Nano-Crystalline Porous Silicon on Si Substrates by a Plasma Enhanced Hydrogenation Technique
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Fabrication of Nano-Crystalline Porous Silicon on Si Substrates by a Plasma Enhanced Hydrogenation Technique Y. Abdi1,2, P. Hashemi1, F. Karbassian1, F.D. Nayeri1, A. Behnam1, S. Mohajerzadeh1, J. Koohsorhki1,2, M.D. Robertson3 and E. Arzi2 1 Thin Film Laboratory, University of Tehran, Tehran, Iran, +98-21 801 1235, e-mail: [email protected] 2 Department of Physics, University of Tehran, Tehran, Iran. 3 Department of Physics, Acadia University, Wolfville, NS, Canada. ABSTRACT A novel plasma hydrogenation method for the fabrication of nano-crystalline structures of silicon as well as the photoluminescence and structural properties of these porous structures is presented. We have observed that the hydrogenation process followed by an annealing treatment results in the formation of nano-crystalline silicon structures where increased temperatures during hydrogenation reduces the grain size. Furthermore, by increasing the time of the hydrogenation process, the density of the silicon grains is increased. Photoluminescence (PL) spectroscopy demonstrated the presence of a direct gap in the visible light range where materials with a smaller grain size emitted light at lower wavelengths, and a higher density of grains resulted in higher amplitudes in the PL spectrum. TEM and SEM characterization of these samples and the structure-emission relationship are also presented. INTRODUCTION There are numerous applications of nano-crystalline porous silicon in the fabrication of electronic and photonic devices making it an important technology [1] in the semiconductor industry. In addition, nano-crystalline porous silicon can be used to fabricate high efficiency solar cells [2] and photo-diodes [3,4]. In conventional preparation methods, these structures are formed using electro-chemical etching techniques [5-7]; however, the destructive properties of chemical etching is one of the major disadvantages of this technique. Also, the formation of such structures has been recently realized by means of ion implantation [8]. In this paper, we report for the first time an inexpensive method for the fabrication of nanocrystalline Si using RF plasma hydrogenation, which causes less damage to the substrate and is cost effective compared to ion implantation. EXPERIMENTAL DETAILS The process begins with the thermal growth of a 0.1 µm-thick SiO2 layer on an n-type silicon wafer at a temperature of 1200 °C. Next, 1000 Å of silicon is deposited by ebeam evaporation at a base pressure of 2×10-6 torr and a temperature of 250°C. The as-grown materials are amorphous and are then placed in a DC-PECVD reactor for hydrogenation treatment. A range of hydrogenation steps was studied using a base pressure of 3 millitorr, power densities ranging from 4.5 to 6.5 W/cm2, and temperatures ranging from 300°C to 400°C. The duration of the hydrogenation step was either 15 or 30 minutes and was followed by in-situ annealing at a temperature 70°C higher than that used for hydrogenation. Figure 1
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presents a schematic of the hydrogenation set
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