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MEDICAL PHYSICS IN <strong>THE</strong> BALTIC STATES 7 (2009) Proceedings of International Conference “Medical Physics 2009” 8 - 10 October 2009, Kaunas, Lithuania INVESTIGATION <strong>OF</strong> POROUS SILICON IRRADIATED WITH X-RAY PHOTONS Skirmantė MOCKEVIČIENĖ*, Igoris PROSYČEVAS**, Vaida KAČIULYTĖ*, Rita PIKAITĖ*, Diana ADLIENĖ* *Kaunas University of Technology, **Institute of Physical Electronics, Kaunas University of Technology Abstract: Porous silicon structures produced using vapor phase chemical etching method and exposed to 15 MeV X-ray photons have been investigated. It was found that irradiation introduces some changes of the chemical bonding structure related to the hydrogen release during exposure and is responsible for the changes of the pore growth mechanism, which could be used formatting porous structures for radiation detectors. Keywords: porous silicon, X-ray irradiation, chemical bonding structure, pores 1. Introduction Porous silicon structures are characterized through broad application spectrum, including technology and medicine due to their specific structure-related properties, simplicity and cheapness of their production [1]. One of the possible applications of porous silicon is flat panel radiation detectors. Porous silicon detectors with a large active interaction area are well known as a broad band and high sensitivity detectors. Properties of porous structures are dependent on the fabrication method and technological parameters. Modification of structures is possible via interaction of energetic particles with a target too. However to our knowledge, there is a lack of information concerning modification of porous Si due to its interaction with accelerated X-ray photons (medical energy range, 10-15 MeV). 2. Materials and methods Method of chemical vapor etching was used for the obtaining of porous structures. Si (111) samples with a surface area of 1cm 2 were placed into Teflon cell at a distance 0f 10 cm from the liquid surface of etching acids mixture. Mixture of HF and HNO3 acids of different concentration was used. The samples were fabricated varying concentrations of acids and etching time. Two groups of samples have been investigated: 1.samples, which were produces varying HF and HNO3 concentrations (1:1; 1:2; 1:6) in a mixture, but keeping the same etching time of 24 hours, and 2. samples, which were produced using the same concentration of acids in the mixture (HF:HNO3 - 4:1), but varying the time of etching (24 hr, 48 hr, 60 hr) 121 Fabricated samples were irradiated with high energy (15 MeV) X-ray photons, which were generated in medical linear accelerator Clinac 2100C (VARIAN). Dose of 2Gy was delivered to all investigated porous Si targets. Chemical bonding structure of the samples was investigated using FTIR spectrometer Nicolet 5700 equipped with 10 Spec modality (10 Degree Specular Reflectance Accessory). Optical properties of the samples were estimated using laser ellipsometer Gaertner 117 (exciting wavelength 632,8 nm) as well as optical interferometer [2]. Porosity of the samples was evaluated using Bruggeman‘s formula [3]: 2 2 2 ( 1−n PS ) ⋅ ( nSi + 2⋅nPS) 2 2 3⋅nPS ⋅( 1−nSi ) ⎡ ⎤ P = 1−⎢ ⎥⋅100% ⎢⎣ ⎥⎦ Where nSi is Si the refraction index and nPSi is the refraction index of porous silicon. Surface morphology was investigated using optical microscope MMU-3, supported wit digital camera Canon5. 3. Results and Discussion Properties of porous silicone structure depend on the concentration of the etching acids and etching time (Table1). It was found that an increase of HNO3 concentration in the mixture, when etching time is the same, leads to the formation of thinner porous layers, however the number of pores increases (Fig.1: a, b, c). It is possible to achieve pore growth if the time of etching increases (Fig.1: d, e, f).
S. Mockevičienė et al. / Medical Physics in the Baltic States 7 (2009) 121 - 122 Table1 Main characteristics of investigated porous silicone structures Samples HF: HNO3 Etching time, h Porous layer thickness, μm Refractive index Porosity, % N8 1:1 24 4.201 1.21 83.6 N10 1:2 24 2.767 1.32 75.3 N11 1:6 24 2.103 1.50 61.4 N5 4:1 24 2.013 1.43 66.8 N20 4:1 48 3.934 1.21 83.6 N4 4:1 60 4.851 1.17 86.7 a) b) c) Typical for porous silicon Si-H, Si-O-Si, SiH2 peaks were found in the initial FTIR spectra of different samples. Increase of HNO3 concentration in the acids mixture resulted in the creation of additional Si-Hx groups seen in FTIR spectrum (Fig.2). Created Si-Hx groups were responsible for the number of new formations (pores) on the sample surface. Fig.2. FTIR spectra of a sample No11 before (A) and after (B) its irradiation with X-ray photons. Spectral intensity was lower for the whole energy range after the irradiation of samples with high energy X-ray photons. In the case of the highest HNO3 concentration used for the fabrication of samples (Sample No11) it was clearly seen, that the intensity of Si-Hx bonds was lower and some bonds disappeared, thus reducing limiting property of the structure for the grow of pores into the depth, Such a behaviour most likely is related to 122 the release of hydrogen during the exposure of samples to X-ray photons. 4. Conclusions Surface morphology and porosity of porous silicon depend on the technological parameters of vapor phase etching. Increasing concentration of HNO3 in the mixture, leads to the creation of the new formations on the sample surface and limits pore growth into the depth. Radiation induced changes of properties are not significant. However, they are related to the surface reconstruction due to the hydrogen release during irradiation. Modification of porous silicon surface using high energy photon beams is possible only in the case, when HNO3concentration in the acids mixture is relatively high. 5. References 1. Z. Chuan Feng, R. Tsu. Porous silicon, World Scientific, 1994, P. 465 2. A.P. Hakhoyan, S.V. Melkonyan Features of the refractive index of porous silicon with gradient porosity. Armenian Journal of Physics, vol. 1, 2008, pp. 146-150; 3. R Herino, G Bomchil, K Barla and C Bertrand, Porosity and pore size distribution of porous silicon layers, J Electrochem Soc, Vol 134, No 8, pp 1994 - 2000 (1987). 4. H. Kaabi, N. Mliki, M. Cheynet, W. Saikaly, O. Gilbert, B. Bessaïs, B. Yangui , Structural and optical properties of vapour-etching based porous silicon, 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, P 155-157.
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