PROCEEDINGS OF THE 7 INTERNATIONAL ... - Fizika

More documents

Recommendations

Info

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.
Page 1 and 2:
2009 PROCEEDINGS OF THE 7 th INTERN
Page 3 and 4:
Executive editor: D.Adlienė CONFER
Page 5 and 6:
11.15-11.30 Daiva Leščiute, Valda
Page 7 and 8:
MEDICAL PHYSICS IN THE BALTIC STATE
Page 9 and 10:
S. Mattsson / Medical Physics in th
Page 11 and 12:
S. Mattsson / Medical Physics in th
Page 13 and 14:
L. Bumbure et al. / Medical Physics
Page 15 and 16:
Average brightness Brightness range
Page 17 and 18:
Page 19 and 20:
D. Lesciute et al. / Medical Physic
Page 21 and 22:
Page 23 and 24:
D. Kaskelyte et al. / Medical Physi
Page 25 and 26:
Page 27 and 28:
V. Poderys et al. / Medical Physics
Page 29 and 30:
E D A B C V. Poderys et al. / Medic
Page 31 and 32:
Page 33 and 34:
A. Kulbickas / Medical Physics in t
Page 35 and 36:
Page 37 and 38:
M. Franckevicius / Medical Physics
Page 39 and 40:
M. Franckevicius / Medical Physics
Page 41 and 42:
J. Puiso et al. / Medical Physics i
Page 43 and 44:
J. Puiso et al. / Medical Physics i
Page 45 and 46:
T. Landberg et al. / Medical Physic
Page 47 and 48:
Page 49 and 50:
Page 51 and 52:
Page 53 and 54:
Page 55 and 56:
S. Plaude et al. / Medical Physics
Page 57 and 58:
Page 59 and 60:
Table 1. Optimization of number of
Page 61 and 62:
Page 63 and 64:
C.-M. Nilsson / Medical Physics in
Page 65 and 66:
Page 67 and 68:
S. Bernhardsson et al. / Medical Ph
Page 69 and 70:
S. Bernhardsson et al. / Medical Ph
Page 71 and 72: 2.3. Biological test. Y. Dekhtyar e
Page 73 and 74: MEDICAL PHYSICS IN THE BALTIC STATE
Page 75 and 76: Scatter dose ( Gy /mA s) 1.55E-12 1
Page 79 and 80: N. Kazuchits et al. / Medical Physi
Page 81 and 82: V. Minialga et al. / Medical Physic
Page 83 and 84: Temperature, o C 36 35 34 33 32 31
Page 85 and 86: O. Klepalov et al. / Medical Physic
Page 89 and 90: G. Adlys / Medical Physics in the B
Page 91 and 92: G. Adlys / Medical Physics in the B
Page 93 and 94: D. Mikšys / Medical Physics in the
Page 95 and 96: L. Krynke et al.. / Medical Physics
Page 99 and 100: M. Pučetaitė et al. / Medical Phy
Page 105 and 106: D. Bernatavičius et al. / Medical
Page 109 and 110: N. Vaiciunaite / Medical Physics in
Page 111 and 112: D. Adlienė et al. / Medical Physic
Page 113 and 114: MP Competences Generic Competences
Page 117 and 118: S. Tabakov et al. / Medical Physics
Page 119 and 120: J. Laurikaitienė et al. / Medical
Page 121: R, % 100,0 99,5 99,0 98,5 98,0 97,5
show all

PROCEEDINGS OF THE 7 INTERNATIONAL ... - Fizika

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?