Strong Enhancement of Porous Silicon Photoluminescence by Dry Photo-Chemical Surface Treatment
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Strong Enhancement of Porous Silicon Photoluminescence by Dry PhotoChemical Surface Treatment
S. Stolyarova, A. El-Bahar, Y. Nemirovsky Dept. of Electrical Engineering, Technion-Israel Institute of Technology, Haifa, 32000, Israel Tel: (972)-4-8294688 Fax: (972)-4-932341 E-mail: [email protected]
Abstract Porous silicon has been subjected to NF3/UV photochemical surface treatment in the range of 300-4000C. This treatment is found to enhance strongly the photoluminescence of porous silicon by couple orders of magnitude. The effect of photoluminescence enhancement has been observed only when the NF3 treated porous silicon was exposed to air environment. The photoluminescence intensity continued to grow with the aging time in air, even at the room temperature. Using atomic force microscope, scanning electron microscope and Auger measurements, it is found that the photoluminescence enhancement correlated with the formation of crystalline oxide layer at the porous silicon surface. This oxide layer SiOx (x≅2) is much more stoichiometric as compared to the native oxide SiOx (x≅1) covering the reference porous silicon surface (not treated by NF3). The rapid growth of the SiO2 layer is supposed to be due to the NF3/UV photo-thermal etching of the as-formed native oxide as well as to the cleaning and passivation of the porous silicon surface with fluorine. Introduction Recently, we reported a strong effect of porous silicon (PS) photoluminescence (PL) enhancement after NF3 photo-thermal treatment [1]. The NF3 is known as an etchant gas capable at certain conditions to remove native oxide from silicon surface and/or to etch the silicon [2]. In the presence of UV photons, provided in this study by an excimer lamp, the activity of the reactions at the surface is significantly enhanced [1,3]. In the current paper we present results of further investigations of the observed effect in order to understand better the nature of the PL enhancement caused by NF3/UV impact. The aging effects as well as the role of porous silicon parameters such as porosity and thickness of PS layer, in the increase of PL intensity have been studied. Experimental The p-type PS samples were prepared by anodisation of p-type silicon wafers in HF/ethanol (1:1) solution, rinsing in ethanol and drying by pentane. The 3µm and 15µm thick layers having 65% - 95% porosity were prepared with current density of 30 and 100 mA/cm2 respectively. The treatments of PS were carried out in a MOCVD system with a horizontal quartz reactor. The samples were placed onto graphite susceptor, heated by infrared lamps. The PS treatment was performed by introducing of a mixture NF3/N2 (1:1) at 50 – 400°C, 60Torr total pressure and 500sccm total flow F5.23.1
rate, with and without UV excitation. An excimer lamp situated above the samples provided the UV irradiation at 222nm. Photoluminescence, atomic force microscopy (AFM) and Auger spectrometry measurements were used to characterize the porous silicon. The PL was measured at room temperature by Dylor spectrometer using
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