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<p>The energy dependent response of silicon diodes in photon beams is a known problem. A new approach to solve this problem is by correcting the response, a response model was suggested by Yin et al. (2002, 2004), and later refined by Eklund and Ahnesjö (2009). In this work a prototype software was developed to calculate correction factors for arbitrary measurement points in MLC shaped fields using fluence pencil beam kernels to calculate the spectra used by the model of Eklund and Ahnesjö (2009). This work investigate this approach for large field sizes. It was found that the relative dose measurements of the corrected unshielded diode agreed with ionization chamber measurements within 1% at the central axis. Measurements made off axis (square and irregular fields) agreed within 2%, better results were achieved within the fields when the off axis beam softening were taken into consideration. This work has also shown that this new approach is an alternitive to shielded diodes and that corrected diodes will in some cases provide more reliable results.</p>

The energy dependent response of silicon diodes in photon beams is a known problem. A new approach to solve this problem is by correcting the response, a response model was suggested by Yin et al. (2002, 2004), and later refined by Eklund and Ahnesjö (2009). In this work a prototype software was developed to calculate correction factors for arbitrary measurement points in MLC shaped fields using fluence pencil beam kernels to calculate the spectra used by the model of Eklund and Ahnesjö (2009). This work investigate this approach for large field sizes. It was found that the relative dose measurements of the corrected unshielded diode agreed with ionization chamber measurements within 1% at the central axis. Measurements made off axis (square and irregular fields) agreed within 2%, better results were achieved within the fields when the off axis beam softening were taken into consideration. This work has also shown that this new approach is an alternitive to shielded diodes and that corrected diodes will in some cases provide more reliable results.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22441号 / 工博第4702号 / 新制||工||1734(附属図書館) / 京都大学大学院工学研究科原子核工学専攻 / (主査)教授 神野 郁夫, 教授 斉藤 学, 准教授 櫻井 良憲 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

<p>Silicon diodes compared to other types of dosimeters have several attractive properties, such as an excellent spatial resolution, a high sensitivity, and clinically practical to use. These properties make silicon diodes a preferred dosimeter for relative dosimetry for several types of measurements in small field dosimetry, e.g., stereotactic treatments and intensity modulated radiotherapy (IMRT). Silicon diodes are, however, limited by an energy dependent response variation in photon beams, resulting in that the diode readout per dose to the phantom medium varies with photon spectral changes, thereby introducing a significant uncertainty in the measured data. The traditional solution for the energy dependent over-response caused by low-energy photons is to use diodes with a shielding filter of high atomic number. These shielded diodes, however, show an incorrect readout for small fields due to electrons scattered from the shielding (Griessbach <em>et al</em>. 2005). In regions with degraded lateral electron equilibrium (LEE) shielded diodes over-respond due to an increased degree of LEE, as a consequence of the high density shielding (Lee <em>et al</em>. 2002).</p><p>In this work a prototype software that corrects for the energy dependent response of a silicon diode is developed and validated for small field sizes. The developed software is based on the novel concept of Monte Carlo (MC) simulated fluence pencil beam kernels to calculate spectra (Eklund and Ahnesjö 2008), and the spectra based silicon diode response model proposed by Eklund and Ahnesjö (2009). The software was also extended to include correction of ionization chambers, for the energy dependent Spencer-Attix water/air stopping power ratio (<em>s</em>_w,air). The calculated <em>s</em>_w,air are shown to be in excellent agreement with published values to better than 0.1% for most values, the maximum deviation being 0.3%.</p><p>Measured relative depth doses, relative profiles, and output factors in water, for small square field sizes, for 6 MV and 15 MV clinical photon beams are presented in this work. The results show that the unshielded Scanditronix-Wellhöfer EFD^3G silicon diode response, corrected by the developed software, is in excellent agreement with reference ionization chamber measurements (corrected for change in <em>s</em>_w,air), the maximum deviation being 0.4%.</p><p>Measurements with two types of shielded diodes, namely Scanditronix-Wellhöfer PFD silicon diodes (FP1990 and FP2730), are also included in this work. The shielded diodes are shown to have an over-response as large as 2-3.5% for field sizes smaller than 5 cm x 5 cm. The presented results also suggest a difference in accuracy as large as 0.5-1% between the two types of shielded diodes, where the spectral composition at the measurement position dictates which type of diode is more accurate.</p><p>The fast correction of silicon diodes provided by the developed software is more accurate than shielded diodes for small field sizes, and can in radiotherapeutic clinical practice increase the dosimetric accuracy of silicon diodes.</p>

Silicon diodes compared to other types of dosimeters have several attractive properties, such as an excellent spatial resolution, a high sensitivity, and clinically practical to use. These properties make silicon diodes a preferred dosimeter for relative dosimetry for several types of measurements in small field dosimetry, e.g., stereotactic treatments and intensity modulated radiotherapy (IMRT). Silicon diodes are, however, limited by an energy dependent response variation in photon beams, resulting in that the diode readout per dose to the phantom medium varies with photon spectral changes, thereby introducing a significant uncertainty in the measured data. The traditional solution for the energy dependent over-response caused by low-energy photons is to use diodes with a shielding filter of high atomic number. These shielded diodes, however, show an incorrect readout for small fields due to electrons scattered from the shielding (Griessbach et al. 2005). In regions with degraded lateral electron equilibrium (LEE) shielded diodes over-respond due to an increased degree of LEE, as a consequence of the high density shielding (Lee et al. 2002). In this work a prototype software that corrects for the energy dependent response of a silicon diode is developed and validated for small field sizes. The developed software is based on the novel concept of Monte Carlo (MC) simulated fluence pencil beam kernels to calculate spectra (Eklund and Ahnesjö 2008), and the spectra based silicon diode response model proposed by Eklund and Ahnesjö (2009). The software was also extended to include correction of ionization chambers, for the energy dependent Spencer-Attix water/air stopping power ratio (sw,air). The calculated sw,air are shown to be in excellent agreement with published values to better than 0.1% for most values, the maximum deviation being 0.3%. Measured relative depth doses, relative profiles, and output factors in water, for small square field sizes, for 6 MV and 15 MV clinical photon beams are presented in this work. The results show that the unshielded Scanditronix-Wellhöfer EFD3G silicon diode response, corrected by the developed software, is in excellent agreement with reference ionization chamber measurements (corrected for change in sw,air), the maximum deviation being 0.4%. Measurements with two types of shielded diodes, namely Scanditronix-Wellhöfer PFD silicon diodes (FP1990 and FP2730), are also included in this work. The shielded diodes are shown to have an over-response as large as 2-3.5% for field sizes smaller than 5 cm x 5 cm. The presented results also suggest a difference in accuracy as large as 0.5-1% between the two types of shielded diodes, where the spectral composition at the measurement position dictates which type of diode is more accurate. The fast correction of silicon diodes provided by the developed software is more accurate than shielded diodes for small field sizes, and can in radiotherapeutic clinical practice increase the dosimetric accuracy of silicon diodes.

Realising Si-based electrically driven light emitters in a process technology compatible with mainstream microelectronics CMOS technology is key requirement for the implementation of low-cost Si-based optoelectronics and thus one of the big challenges of semiconductor technology. This work has focused on the development of microcavity enhanced silicon LEDs (MCLEDs), including their design, fabrication, and experimental as well as theoretical analysis. As a light emitting layer the abrupt pn-junction of a Si-diode was used, which was fabricated by ion implantation of boron into n-type silicon. Such forward biased pn-junctions exhibit room-temperature EL at a wavelength of 1138 nm with a reasonably high power efficiency of 0.1% [1]. Two MCLEDs emitting light at the resonant wavelength about 1150 nm were demonstrated: a) 1 MCLED with the resonator formed by 90 nm thin metallic CoSi2 mirror at the bottom and semitranparent distributed Bragg reflector (DBR) on the top; b) 5:5 MCLED with the resonator formed by high reflecting DBR at the bottom and semitransparent top DBR. Using the appoach of the 5:5 MCLED with two DBRs the extraction efficiency is enhanced by about 65% compared to the silicon bulk pn-junction diode.

Realising Si-based electrically driven light emitters in a process technology compatible with mainstream microelectronics CMOS technology is key requirement for the implementation of low-cost Si-based optoelectronics and thus one of the big challenges of semiconductor technology. This work has focused on the development of microcavity enhanced silicon LEDs (MCLEDs), including their design, fabrication, and experimental as well as theoretical analysis. As a light emitting layer the abrupt pn-junction of a Si-diode was used, which was fabricated by ion implantation of boron into n-type silicon. Such forward biased pn-junctions exhibit room-temperature EL at a wavelength of 1138 nm with a reasonably high power efficiency of 0.1% [1]. Two MCLEDs emitting light at the resonant wavelength about 1150 nm were demonstrated: a) 1 MCLED with the resonator formed by 90 nm thin metallic CoSi2 mirror at the bottom and semitranparent distributed Bragg reflector (DBR) on the top; b) 5:5 MCLED with the resonator formed by high reflecting DBR at the bottom and semitransparent top DBR. Using the appoach of the 5:5 MCLED with two DBRs the extraction efficiency is enhanced by about 65% compared to the silicon bulk pn-junction diode.:List of Abbreviations and Symbols 1 Introduction and motivation 2 Theory 2.1 Electronic band structure of semiconductors 2.2 Light emitting diodes (LED) 2.2.1 History of LED 2.2.2 Mechanisms of light emission 2.2.3 Electrical properties of LED 2.2.4 LED e ciency 2.3 Si based light emitters 2.4 Microcavity enhanced light emitting pn-diode 2.4.1 Bragg reflectors 2.4.2 Fabry-Perot resonators 2.4.3 Optical mode density and emission enhancement in coplanar Fabry-Perot resonator 2.4.4 Design and optical properties of a Si microcavity LED 3 Preparation and characterisation methods 3.1 Preparation techniques 3.1.1 Thermal oxidation of silicon 3.1.2 Photolithography 3.1.3 Wet chemical cleaning and etching 3.1.4 Ion implantation 3.1.5 Plasma Enhanced Chemical Vapour Deposition (PECVD) of silicon nitride 3.1.6 Magnetron sputter deposition 3.2 Characterization techniques 3.2.1 Variable Angle Spectroscopic Ellipsometry (VASE) 3.2.2 Fourier Transform Infrared Spectroscopy (FTIR) 3.2.3 Microscopy 3.2.4 Electroluminescence and photoluminescence measurements 4 Experiments, results and discussion 4.1 Used substrates 4.1.1 Silicon substrates 4.1.2 Silicon-On-Insulator (SOI) substrates 4.2 Fabrication and characterization of distributed Bragg reflectors 4.2.1 Deposition and characterization of SiO2 4.2.2 Deposition of Si 4.2.3 Distributed Bragg Reflectors (DBR) 4.2.4 Conclusions 4.3 Design of Si pn-junction LED 4.4 Resonant microcavity LED with CoSi2 bottom mirror 4.4.1 Device preparation 4.4.2 Electrical Si diode characteristics 4.4.3 EL spectra 4.4.4 Conclusions 4.5 Si based microcavity LED with two DBRs 4.5.1 Test device 4.5.2 Device fabrication 4.5.3 LED on SOI versus MCLED 4.5.4 Conclusions 5 Summary and outlook 5.1 Summary 5.2 Outlook A Appendix A.1 The parametrization of optical constants A.1.1 Kramers-Kronig relations A.1.2 Forouhi-Bloomer dispersion formula A.1.3 Tauc-Lorentz dispersion formula A.1.4 Sellmeier dispersion formula A.2 Wafer holder List of publications Acknowledgements Declaration / Versicherung

Realising Si-based electrically driven light emitters in a process technology compatible with mainstream microelectronics CMOS technology is key requirement for the implementation of low-cost Si-based optoelectronics and thus one of the big challenges of semiconductor technology. This work has focused on the development of microcavity enhanced silicon LEDs (MCLEDs), including their design, fabrication, and experimental as well as theoretical analysis. As a light emitting layer the abrupt pn-junction of a Si diode was used, which was fabricated by ion implantation of boron into n-type silicon. Such forward biased pn-junctions exhibit room-temperature EL at a wavelength of 1138 nm with a reasonably high power efficiency of 0.1%. Two MCLEDs emitting light at the resonant wavelength about 1150 nm were demonstrated: a) 1-lambda MCLED with the resonator formed by 90 nm thin metallic CoSi2 mirror at the bottom and semitransparent distributed Bragg reflector (DBR) on the top; b) 5.5-lambda MCLED with the resonator formed by high reflecting DBR at the bottom and semitransparent top DBR. Using the appoach of the 5.5-lambda MCLED with two DBRs the extraction efficiency is enhanced by about 65% compared to the silicon bulk pn-junction diode.

Realising Si-based electrically driven light emitters in a process technology compatible with mainstream microelectronics CMOS technology is key requirement for the implementation of low-cost Si-based optoelectronics and thus one of the big challenges of semiconductor technology. This work has focused on the development of microcavity enhanced silicon LEDs (MCLEDs), including their design, fabrication, and experimental as well as theoretical analysis. As a light emitting layer the abrupt pn-junction of a Si diode was used, which was fabricated by ion implantation of boron into n-type silicon. Such forward biased pn-junctions exhibit room-temperature EL at a wavelength of 1138 nm with a reasonably high power efficiency of 0.1%. Two MCLEDs emitting light at the resonant wavelength about 1150 nm were demonstrated: a) 1-lambda MCLED with the resonator formed by 90 nm thin metallic CoSi2 mirror at the bottom and semitransparent distributed Bragg reflector (DBR) on the top; b) 5.5-lambda MCLED with the resonator formed by high reflecting DBR at the bottom and semitransparent top DBR. Using the appoach of the 5.5-lambda MCLED with two DBRs the extraction efficiency is enhanced by about 65% compared to the silicon bulk pn-junction diode.