Purification of silicon powder by the formation of thin porous layer followed byphoto-thermal annealing
© Khalifa et al.; licensee Springer. 2012
Received: 30 April 2012
Accepted: 23 July 2012
Published: 8 August 2012
Porous silicon has been prepared using a vapor-etching based technique on a commercial silicon powder. Strong visible emission was observed in all samples. Obtained silicon powder with a thin porous layer at the surface was subjected to a photo-thermal annealing at different temperatures under oxygen atmosphere followed by a chemical treatment. Inductively coupled plasma atomic emission spectrometry results indicate that silicon purity is improved from 99.1% to 99.994% after annealing at 900°C.
Porous silicon (PSi) is a nano-structured material that can be obtained by electrochemical , stain etching [2, 3], or vapor phase etching of silicon wafers [4, 5]. The main advantages of stain etching and vapor etching methods, if compared with electrochemical one, are their simplicity and capability to produce large area porous silicon layers. Porous silicon elaborated by different methods is extensively used in photovoltaic applications as an antireflection coating or as a gettering layer due to its large specific surface and chemical reactivity. Gettering of impurities by the formation of a thin porous silicon layer followed by a thermal annealing in a nitrogen, oxygen, or SiCl4 atmosphere has been used [6, 7]. It was found that porous layer play a crucial role in the gettering process. Porous silicon was also used in combination with phosphorous or aluminum gettering of unwanted impurity in silicon [8–10].
In this paper, we present the possibility of gettering impurities from commercial silicon powder (SPw) by photo-thermal annealing in oxygen atmosphere using a thin porous silicon layer on the surface of silicon grains. The gettering effect was studied using the inductively coupled plasma atomic emission spectrometry (ICP-AES).
Porous silicon thin layer was formed by exposing silicon powder to the vapor of an acid mixture composed of HF/HNO3 with 1:3 composition volume. The etching time was varied from 2 to 20 min. The obtained material was rinsed in deionized water, dried, and then analyzed using Fourier transform infrared spectroscopy (FTIR) and photoluminescence (PL) measurements.
The silicon powder with thin porous silicon layer is then subjected to a photo-thermal annealing stage under oxygen atmosphere in the aim to remove the unwanted impurities. The annealing temperature was varied from 700°C to 900°C for a fixed duration of 1 h. After this purification step, silicon powder was chemically cleaned in NaOH (1 M) solution in order to remove the porous layer and rinsed in deionized water. The purification process was evaluated by ICP-AES method.
Results and discussions
Impurity concentrations (ppm) before (SPw) and after thermal annealing
The improvement of the silicon powder purity is attributed to the migration of unwanted impurities from the volume of silicon grains composing the powder to the PS layer, at the surface of the grain, where they can be easily removed by chemical etching [6, 7]. This gettering technique is an easy and efficient way to improve the quality of silicon intended for solar grade silicon production from metallurgical grade silicon powder.
This work presents an easy, inexpensive, and efficient method for the removal of impurities from silicon powder. Obtained results show that the purity of silicon powder can be improved from 99.1% for the untreated powder to 99.995% after annealing at 900°C under oxygen atmosphere. This method is very interesting for the production of solar grade silicon from metallurgical grade silicon powder.
MK is a Ph.D. student in the Laboratory for Photovoltaic, CRTEn. MH is an assistant professor in the ISECS and a researcher in the Laboratory for Photovoltaic, CRTEn. EH is a professor in the Laboratory for Photovoltaic, CRTEn.
This work was supported by the Ministry of Higher Education and Scientific Research, Tunisia.
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