Absorption Coefficient Of Silicon Research Articles

Wireless wavelength measurement system with temperature compensation using filter-free wavelength sensors

Abstract Portable wavelength detection systems have been developed for various environmental and biochemical applications. Conventional systems have some limitations as they are unable to adapt to changes in wavelength and rely on optical filters or slits to distinguish between different wavelengths. To address these limitations, a filter-free wavelength sensor system was proposed, utilizing the absorption coefficient of silicon to identify wavelengths. The proposed system comprises an analog circuit for measuring photocurrent from a filter-free wavelength sensor, integrated with a temperature sensor and microcontroller for signal processing. The proposed system can measure currents with a resolution of 1.2 pA within a 30 nA range, achieving a coefficient of determination of 0.999 for measured currents in relation to light intensity. The microcontroller features a temperature compensation algorithm, enabling wireless control and data transmission. Applying temperature compensation reduced the rate of errors in the data by 61%. By eliminating the need for optical components, a wireless measurement system was developed that can accurately identify wavelengths.

Numerical investigations of water jet-guided laser cutting of silicon

To investigate the interaction mechanism between a laser, water jet, and substrate, a model is developed to simulate the temperature field evolution and removal process during water jet-guided laser (WJGL) cutting of silicon. The model accounted for the temperature-dependent properties of the silicon absorption coefficient, as well as the physical processes of solid-liquid-gas phase change. A three-dimensional finite volume model of WJGL cutting of silicon is created, incorporating laser energy input, water jet impact-cooling, and silicon phase transition and removal. The volume of fluid (VOF) method is employed to trace the interphase interface and obtain the groove shape. The validity of the model is verified by comparing simulation results with experimental data. The simulation results show that the groove cross section is characterized by a “V” shape. The groove depth nonlinearly increases from 52 to 385 μm with an increasing number of cuts. Additionally, the residual temperature of the silicon substrate rises from 837 to 1345 K as the number of scans increases from 1 to 10. The findings offer valuable insights into WJGL cutting research, specifically shedding light on the intricate details of the laser-water jet-substrate interaction mechanism.

(Invited) Detection of Wavelength Information by Filter-Free Wavelength Sensor and Its Applications

This paper introduces the wavelength and light intensity detection method using the filter-free wavelength sensor without optical filters. Optical detection methods are widely used in medical, environmental, chemical, biofield, and agricultural analyses. In the biofield, the optical detection method makes it possible to predict, diagnose, and analyze diseases by detecting the wavelength information and light intensity, such as absorption, fluorescence, and spectral shift.In order to measure a specific wavelength and intensity of light, mainly used a detection method such as a filter cube of a fluorescence microscope or a monochromator of a spectrometer system. Even if this system has high sensitivity and selectivity, it is not easy to apply to POCTs that require portability due to the high price and increased system volume due to the integration of optical filters and other parts. Therefore, various studies have reported using semiconductor processes to detect the wavelength and light intensity with a high-performance and miniaturized system.An on-chip photodetection device has been reported, such as a fluorescence detection sensor to integrate interference or absorption filters on CMOS image sensors with high selectivity and sensitivity. Since the specific wavelength of light intensity detection is possible, it can apply to biofields such as fluorescence analysis. However, detecting different wavelength changes by fluorescent reagent is difficult because the optical filter is integrated into the CMOS image sensor. A method of multi-wavelength analysis with the single-pixel without an optical filter is reported using a CMOS-buried quad p-n junction photodetector. Since the light wavelength has a different absorption depth depending on the silicon absorption coefficient, it is possible to separate wavelength by measuring the current generated at each p-n junction. Therefore, it will provide a high-space-resolution image in the biofield. However, the possibility detection wavelength band is fixed according to the buried p-n junction numbers and depth.Previously, we reported a filter-free wavelength sensor with a photogate structure. Sensor can detect light intensity and wavelength without optical filters based on the dependence of the light absorption depth in silicon. The sensor fabricates using the 1-polysilicon, 2-metal process in the LSI facility of the Toyohashi University of Technology in Japan, as shown in Fig1(a). The sensor consists of a three-layer structure where deep n-well and p-well were formed on a p-type silicon substrate, as shown in Fig1(b). Applying a voltage to the photogate and n-well forms a potential peak W as a mountain shape below the sensing area. Therefore, the photocurrent generated by the incident light was divided into the surface side (I PG), and bottom side (I n-well) based on the potential peak W and wavelength identification is possible from the current ratio (I n-well/I PG). To detect the wavelength information of multi-peak light for applications in the biofield, compare the centroid wavelength and current ratio. We experimentally confirmed that the centroid wavelength was proportional to the current ratio by the light absorption depth of silicon. Since the light absorption depth depends on the light wavelength, the current ratio is constant even when the light intensity changes. Therefore, estimation of the light intensity is possible by the sum of each current.We proposed a wavelength, spectral shift, and absorbance measure method using a filter-free wavelength sensor to apply various fields. Figure 2(a) shows a schematic for detecting the fluorescence wavelength. Without using optical filters, fluorescence can be quantified by calculating the centroid wavelength of the light even when the excitation light is emitted simultaneously. In addition, it successfully identified fluorescence emitted from Legionella bacteria [1]. Figure 2(b) shows a schematic diagram that can identify the change in the spectrum using a sensor. Integrating an LSPR biosensor into a filter-free wavelength sensor makes it possible to measure the amount of spectrum changed by molecular binding. Therefore, miniaturization of the LSPR detection system becomes possible, and we succeeded in detecting and quantifying the S-protein of SARS-CoV-2 [2]. Figure 2(c) shows a schematic diagram of the absorbance measurements method for samples that absorb a specific wavelength range. By the proposed absorbance measurement method was possible to measure the ratio of chlorophyll content and chlorophyll a/b ratio contributing to plant photosynthesis [3].The filter-free wavelength sensor fabricated by the semiconductor process has been confirmed for its application as an optical detection system. In the future, we will continue research to develop an optical sensor system that is more compact, sensitive, and selective and can be applied to various fields.[1] Biosensors, vol.12, no.11, p.1033 (2022)[2] Applied Physics Express, vol.16, no.1, p.012012 (2023)[3] Jpn. J. Appl. Phys., vol.61, p.SD1041 (2022) Figure 1

Demonstrating a Filter-Free Wavelength Sensor with Double-Well Structure and Its Application.

This study proposed a filter-free wavelength sensor with a double-well structure for detecting fluorescence without an optical filter. The impurity concentration was optimized and simulated to form a double-well-structured sensor, of which the result was consistent with the fabricated sensor. Furthermore, we proposed a novel wavelength detection method using the current ratio based on the silicon absorption coefficient. The results showed that the proposed method successfully detected single wavelengths in the 460-800 nm range. Additionally, we confirmed that quantification was possible using the current ratio of the sensor for a relatively wide band wavelength, such as fluorescence. Finally, the fluorescence that was emitted from the reagents ALEXA488, 594, and 680 was successfully identified and quantified. The proposed sensor can detect wavelengths without optical filters, which can be used in various applications in the biofield, such as POCT as a miniaturized wavelength detection sensor.

BIFACIAL (N+/P/P+) SILICON SOLAR CELL BASE THICKNESS OPTIMIZATION, WHILE ILLUMINATED BY THE REAR FACE WITH MONOCHROMATIC LIGHT OF SHORTWAVELENGTHS

The monochromatic absorption coefficient of silicon, inducing the depth of penetration of light into the base of the solar cell, is used through back surface recombination velocity expressions, to determine the optimum thickness necessary for the production of a large photocurrent. The absorption-generation-diffusion and recombination (bulk and surface) phenomena are taken into account in determining the optimum thickness of an n+ -p-p+ bifacial solar cell, for it manufacture process optimization.

Comprehensive device simulation of 16.9% efficient two-terminal PbS–PbS CQD tandem solar cell

The significant losses that limit the efficiency of a single-junction solar cell are thermalization loss and transmission loss. Thus, to efficiently utilize the solar spectrum and mitigate these losses, tandem solar cells (TSCs) have significantly impacted the photovoltaic (PV) landscape. In this context, the research on perovskite/silicon tandems is currently dominating the research community. However, challenges such as stability of perovskite, pin-hole defects, conformal deposition of perovskite over textured silicon, the low absorption coefficient of silicon, and high module cost of the bottom subcell are the tailback for its mass production at the industrial level. Therefore, considering the detailed balance limit for the TSC, a low-cost solution-processed top and bottom subcells with a tunable bandgap can be well thought for the low-cost tandem design. Thus, here in this work, two different PbS–PbS colloidal quantum dot (CQD) TSCs with a conversion efficiency of 15.6% and 16.9% are proposed through comprehensive device simulations. Three different PbS-CQDs with a bandgap of 1.14eV, 1.45eV, and 1.56eV are utilized to design the TSCs under consideration. Detailed standalone and tandem analysis has been carried out in terms of absorber layer thickness variation, filtered spectrum, filtered integrated power, current matching, tandem current density voltage (J-V) curves, and tandem PV parameter to finalize the conversion efficiency. The filtered spectrums are obtained by using the transfer matrix method to account for the interfacial reflection losses and thin-film interference effects. The tandem device constructed using 1.45eV and 1.56eV based top subcell showed the open-circuit voltage V OC of as high as 1.71V and 1.83V, respectively. The comprehensive theoretical analysis of PbS–PbS CQD tandem devices proposed in this work may pave the way for developing high-efficiency TSCs for low-cost applications. • Two different PbS–PbS CQDs tandem solar cells (TSCs) with a conversion efficiency of 15.60% and 16.92% are proposed. • Detailed standalone and tandem analysis has been carried out to optimize the conversion efficiency. • Three different PbS-CQDs with a bandgap of 1.14eV, 1.45eV, and 1.56eV are utilized to design the TSCs under consideration. • TSCs constructed using 1.45eV and 1.56eV based top subcell showed the open-circuit voltage V OC of 1.71V and 1.83V, respectively. • This detailed study would open path for development of high-efficiency, low-cost perovskite-PbS CQD TSCs in future.

Research progress in the epitaxial growth of silicon-based group IV materials, and their light emitters and photodetectors

Silicon-based optoelectronic integrated circuits, which are composed of silicon-based light sources, photodetectors, light modulators, and other modules, are core technologies in the recent information era. Silicon is the cornerstone of microelectronic integration, but it encounters issues with optoelectronic integration. First, as an indirect bandgap material, silicon is difficult to be used in high-efficiency light sources, which is still currently the biggest challenge. Second, the absorption coefficient of silicon in the near-infrared communication band is very low, so it is not suitable for the near-infrared photodetectors. Silicon-compatible group IV materials can achieve direct band through band engineering. They have a high absorption coefficient in the near-infrared and have attracted extensive attention of researchers. In this paper, the research progress of epitaxial growth of silicon-based materials, and their light emitters and photodetectors, are reviewed. The growth technologies of Ge, SiGe/Ge heterojunction, quantum wells, and GeSn quantum dots are introduced. Based on the above silicon-based group IV heterostructure materials, a new method for modulating the height of the contact barrier between metal and semiconductor was proposed. Also, various photodetectors with different structures were developed. The lateral heterojunction LED and vertical resonant-cavity LED, including dual active regions compatible with traditional CMOS processes, were developed. The luminescence performance of the devices was improved effectively and the luminescence gain of strain Ge was observed.

Application of a Genetic Algorithm in Four-Terminal Perovskite/Crystalline-Silicon Tandem Devices

Perovskite/crystalline-silicon (c-Si) tandem solar cells offer a viable roadmap for reaching power conversion efficiencies beyond 30%. In this configuration, however, the silicon cell now mainly receives an infrared rich illumination spectrum where the absorption coefficient of silicon is poor. To boost the light absorption in this wavelength interval, transmitted through the top cell, a solution is to introduce a dedicated nanoscale texture on the front side of the silicon bottom cell and tweaking the optical elements in its design. These optical elements are manifold, with tunable geometrical dimensions, layer thicknesses, and refractive indices. For optically optimizing nanostructured silicon solar cells, electromagnetic wave solving methods, such as the rigorous coupled-wave analysis (RCWA), are normally used but they become computationally infeasible when several design variables are involved in an optimization problem. In this work, a natural selection algorithm, known as the genetic algorithm, is coupled with RCWA for extracting the optimal values of various design parameters in four-terminal perovskite/c-Si tandem devices in parallel. Both two-side-contacted and interdigitated back-contacted silicon heterojunction cell structures, featuring an inverse nanopyramid grating texture on the front, are optimized for five interdependent variables using a genetic algorithm for the bottom cell application and benchmarked against their random pyramid textured analogs. Our study shows that an optimized inverse nanopyramid grating texture can outperform the standard random pyramid texture when considering incidence angle variations. More importantly, the study illustrates how a genetic algorithm can support modeling complex solar cell structures with numerous degrees of freedom.

Optofluidic Formaldehyde Sensing: Towards On-Chip Integration.

Formaldehyde (HCHO), a chemical compound used in the fabrication process of a broad range of household products, is present indoors as an airborne pollutant due to its high volatility caused by its low boiling point ( °C). Miniaturization of analytical systems towards palm-held devices has the potential to provide more efficient and more sensitive tools for real-time monitoring of this hazardous air pollutant. This work presents the initial steps and results of the prototyping process towards on-chip integration of HCHO sensing, based on the Hantzsch reaction coupled to the fluorescence optical sensing methodology. This challenge was divided into two individually addressed problems: (1) efficient airborne HCHO trapping into a microfluidic context and (2) 3,5–diacetyl-1,4-dihydrolutidine (DDL) molecular sensing in low interrogation volumes. Part (2) was addressed in this paper by proposing, fabricating, and testing a fluorescence detection system based on an ultra-low light Complementary metal-oxide-semiconductor (CMOS) image sensor. Two three-layer fluidic cell configurations (quartz–SU-8–quartz and silicon–SU-8–quartz) were tested, with both possessing a 3.5 µL interrogation volume. Finally, the CMOS-based fluorescence system proved the capability to detect an initial 10 µg/L formaldehyde concentration fully derivatized into DDL for both the quartz and silicon fluidic cells, but with a higher signal-to-noise ratio (SNR) for the silicon fluidic cell () when compared to the quartz fluidic cell (). The signal intensity enhancement in the silicon fluidic cell was mainly due to the silicon absorption coefficient at the excitation wavelength,, which is approximately five times higher than the absorption coefficient at the fluorescence emission wavelength, .

In-depth investigation and applications of novel silicon photonics microstructures supporting optical vorticity and waveguiding for ultra-narrowband near-infrared perfect absorption

We propose a novel concept of designing silicon photonics metamaterials for perfect near-infrared light absorption. The study’s emphasis is an in-depth investigation of various physical mechanisms behind the ∼ 100 % ultra-narrowband record peak absorptance of the designed structures, comprising an ultrathin silicon absorber. The electromagnetic power transport, described by the Poynting vector, is innovatively explored, which shows combined vortex and crossed-junction two-dimensional waveguide-like flows as outcomes of optical field singularities. These flows, though peculiar for each of the designed structures, turn out to be key factors of the perfect resonant optical absorption. The electromagnetic fields show tight two-dimensional confinement: a sharp vertical confinement of the resonant-cavity type combined with a lateral metasurface supported confinement. The silicon-absorbing layer and its oxide environment are confined between two subwavelength metasurfaces such that the entire design is well compatible with silicon-on-insulator microelectronics. The design concept and its outcomes meet the extensive challenges of ultrathin absorbers for minimum noise and an ultra-narrowband absorptance spectrum, while maintaining an overall very thin structure for planar integration. With these materials and such objectives, the proposed designs seem essential, as standard approaches fail, mainly due to a very low silicon absorption coefficient over the near-infrared range. Tolerance tests for fabrication errors show fair tolerability while maintaining a high absorptance peak, along with a controllable deviation off the central-design wavelength. Various applications are suggested and analyzed, which include but are not limited to: efficient photodetectors for focal plane array and on-chip integrated silicon photonics, high-precision spectroscopic chemical and angular-position sensing, and wavelength-division multiplexing.

Back Surface Recombination Velocity Dependent of Absorption Coefficient as Applied to Determine Base Optimum Thickness of an n+/p/p+ Silicon Solar Cell

The monochromatic absorption coefficient of silicon, inducing the light penetration depth into the base of the solar cell, is used to determine the optimum thickness necessary for the production of a large photocurrent. The absorption-generation-diffusion and recombination (bulk and surface) phenomena are taken into account in the excess minority carrier continuity equation. The solution of this equation gives the photocurrent according to absorption and electronic parameters. Then from the obtained short circuit photocurrent expression, excess minority carrier back surface recombination velocity is determined, function of the monochromatic absorption coefficient at a given wavelength. This latter plotted versus base thickness yields the optimum thickness of an n+-p-p+ solar cell, for each wavelength, which is in the range close to the energy band gap of the silicon material. This study provides a tool for improvement solar cell manufacture processes, through the mathematical relationship obtained from the thickness limit according to the absorption coefficient that allows base width optimization.

Rear contact silicon solar cells with a-SiCX:H based front surface passivation for near-ultraviolet radiation stability

Surface recombination (due to dangling bonds) and lower absorption (due to the low absorption coefficient of silicon (Si)) are the major hindrances in silicon-based photovoltaic (PV) devices. To overcome this, numerous complex texturing schemes are projected to enhance the light trapping. However nanostructured cells are not efficient due to the large surface to volume ratio which enhances surface recombination. Thus, the nanostructured cells require additional passivation scheme to mitigate the recombination losses. Here, we have designed a nontextured, 15% efficient, amorphous silicon carbide hydrogenated (a-SiCX:H) passivated, 10-μm thick rear contact Si solar cell device. Considerable reduction in photo reflectance is obtained in the near ultraviolet (UV)/visible spectral region together with near UV stability at higher surface recombination velocity (SRV). External quantum efficiency (EQE) > 90% is achieved by the a-SiCX:H based device (within the wavelength spectrum of 480–620 nm). Improvement in spectrum response give rise to 28.1 mA cm−2 short circuit current density (JSC). Further, the performance of a-SiCX:H passivated device is compared with a conventional dielectric anti-reflective coating (ARC) and high-low junction-based surface passivation techniques. Results indicate that the presence of a-SiCX:H reduces the hole concentration near the front surface which eventually decreases the surface recombination. Highly efficient and reliable solar cells can be achieved by the design schemes reported in this paper, which balance both the photonic and electronic effects together.

Absorption coefficients of silicon: A theoretical treatment

A theoretical model with explicit formulas for calculating the optical absorption and gain coefficients of silicon is presented. It incorporates direct and indirect interband transitions and considers the effects of occupied/unoccupied carrier states. The indirect interband transition is calculated from the second-order time-independent perturbation theory of quantum mechanics by incorporating all eight possible routes of absorption or emission of photons and phonons. Absorption coefficients of silicon are calculated from these formulas. The agreements and discrepancies among the calculated results, the Rajkanan-Singh-Shewchun (RSS) formula, and Green's data are investigated and discussed. For example, the RSS formula tends to overestimate the contributions of indirect transitions for cases with high photon energy. The results show that the state occupied/unoccupied effect is almost negligible for silicon absorption coefficients up to the onset of the optical gain condition where the energy separation of Quasi-Femi levels between electrons and holes is larger than the band-gap energy. The usefulness of using the physics-based formulas, rather than semi-empirical fitting ones, for absorption coefficients in theoretical studies of photovoltaic devices is also discussed.

Modelling of heating and photoexcitation of single-crystal silicon under multipulse irradiation by a nanosecond laser at 1.06 μm

We report a theoretical study of heating and photoexcitation of single-crystal silicon by nanosecond laser radiation at a wavelength of 1.06 μm. The proposed physicomathematical model of heating takes into account the complex nonlinear dynamics of the interband absorption coefficient of silicon and the contribution of the radial heat removal to the cooling of silicon between pulses under multipulse irradiation, which allows one to obtain a satisfactory agreement between theoretical predictions of silicon melting thresholds at different nanosecond pulse durations and experimental data (both under single-pulse and multipulse irradiation). It is found that under irradiation by nanosecond pulses at a wavelength of 1.06 μm, the dynamic Burshtein – Moss effect can play an important role in processes of photoexcitation and heating. It is shown that with the regimes typical for laser multipulse microprocessing of silicon (the laser spot diameter is less than 100 μm, and the repetition rate of pulses is about 100 kHz), the radial heat removal cannot be neglected in the analysis of heat accumulation processes.

Optical constants of silicon in the range of 30–10000 cm–1

A spectrophotometric method of determining the optical constants of materials is proposed that is based on the correction of absorption in spectra. With this method, specimens of silicon of different grades have been examined. The refractive index and the absorption coefficient of silicon in the spectral range of 30–10000 cm–1 have been determined.

Effect of layer thickness on device response of silicon heavily supersaturated with sulfur

We report on a simple experiment in which the thickness of a hyperdoped silicon layer, supersaturated with sulfur by ion implantation followed by pulsed laser melting and rapid solidification, is systematically varied at constant average sulfur concentration, by varying the implantation energy, dose, and laser fluence. Contacts are deposited and the external quantum efficiency (EQE) is measured for visible wavelengths. We posit that the sulfur layer primarily absorbs light but contributes negligible photocurrent, and we seek to support this by analyzing the EQE data for the different layer thicknesses in two interlocking ways. In the first, we use the measured concentration depth profiles to obtain the approximate layer thicknesses, and, for each wavelength, fit the EQE vs. layer thickness curve to obtain the absorption coefficient of hyperdoped silicon for that wavelength. Comparison to literature values for the hyperdoped silicon absorption coefficients [S.H. Pan et al. Applied Physics Letters 98, 121913 (2011)] shows good agreement. Next, we essentially run this process in reverse; we fit with Beer’s law the curves of EQE vs. hyperdoped silicon absorption coefficient for those wavelengths that are primarily absorbed in the hyperdoped silicon layer, and find that the layer thicknesses obtained from the fit are in good agreement with the original values obtained from the depth profiles. We conclude that the data support our interpretation of the hyperdoped silicon layer as providing negligible photocurrent at high S concentrations. This work validates the absorption data of Pan et al. [Applied Physics Letters 98, 121913 (2011)], and is consistent with reports of short mobility-lifetime products in hyperdoped layers. It suggests that for optoelectronic devices containing hyperdoped layers, the most important contribution to the above band gap photoresponse may be due to photons absorbed below the hyperdoped layer.

Stochastic Approach to Phonon-Assisted Optical Absorption.

We develop a first-principles theory of phonon-assisted optical absorption in semiconductors and insulators which incorporates the temperature dependence of the electronic structure. We show that the Hall-Bardeen-Blatt theory of indirect optical absorption and the Allen-Heine theory of temperature-dependent band structures can be derived from the present formalism by retaining only one-phonon processes. We demonstrate this method by calculating the optical absorption coefficient of silicon using an importance sampling MonteCarlo scheme, and we obtain temperature-dependent line shapes and band gaps in good agreement with experiment. The present approach opens the way to predictive calculations of the optical properties of solids at finite temperature.

Water-assisted pulsed Er:YAG laser interaction with silicon

Silicon is virtually transparent to the Er:YAG laser with a wavelength of 2.94 μm. In this study, we report that moderately doped silicon (1–10 Ω cm) can be processed by a pulsed Er:YAG laser with a pulse duration of 350 μs and a peak laser intensity of 1.7 × 105 W/cm2 by applying a thin water layer on top of silicon as a light absorbing medium. In this way, water is heated first by strongly absorbing the laser energy and then heats up the silicon wafer indirectly. As the silicon temperature rises, the free carrier concentration and therefore the absorption coefficient of silicon will increase significantly, which may enable the silicon to get directly processed by the Er:YAG laser when the water is vaporized completely. We also believe that the change in surface morphology after melting could contribute to the increase in the laser beam absorptance. It was observed that 525 nm-thick p-type wafer specimens were fully penetrated after 15 laser pulses were irradiated. Bright yellow flames were observed duri...

Light absorption by a layered structure of silicon particles as applied to the solar cells: Theoretical study

Absorption of a layered system (multilayer) consisting of monolayers of monodisperse crystalline silicon (c-Si) spherical particles is studied using the transfer matrix method. The wavelength range from 0.28µm to 1.1µm and the particle size range from 1µm to 1000µm are considered. The results for the particulate multilayer are compared with the ones for a homogeneous plane-parallel plate of the equivalent amount of silicon (equivalent plate). It is shown that at strong absorption of silicon the absorption coefficient of the plate is larger than the one of a single monolayer of particles. In the spectral range of small and middle absorption index of silicon absorption coefficient of the monolayer can exceed the one of the plate. Absorption coefficient of the system consisting of three and more monolayers exceeds the one of the equivalent plate. The integrated over the solar spectral irradiance (“Global tilt” ASTM G173-03) absorption coefficient is calculated. It is shown that the integral absorption of the particulate structure consisting of three and more monolayers of silicon particles is more than the one of the equivalent plate. Integral absorption coefficient of the four-monolayer system composed of particles with diameter of 1µm is approximately 1.7 times larger than the one of the plate. It increases with the particles size and the number of monolayers increasing. The maximum value is close to 0.9. The scheme of the possible design of solar cell based on the particulate structure of active layer is proposed. The described method can be applied for light absorption optimization in solar cells based on silicon and other materials.

SU - E-T-80: Energy Dependence of the Response of a Silicon Diode Designed for Small-Field Dosimetry

Purpose: To investigate the energy dependence/spectral sensitivity of silicon diodes designed for small‐field dosimetry and obtain correction factors for arbitrary photon spectra using Monte Carlo simulations. Methods: The dose to the active volume of a stereotactic diode field detector (IBA Dosimetry) was measured for several low‐energy photon spectra (mean energy 40 to 200 keV) as well as for Co60‐radiation (mean energy 1.25 MeV) and an Elekta Synergy beam (mean energy 2.9 MeV) under reference conditions. The corresponding dose to water was obtained by a PTW 31010 ionization chamber. Monte Carlo calculations corresponding to the experiments were carried out by the EGS user code DOSRZnrc. Dose to the volume of the diode as well as dose to water were calculated using the photon spectra of the measurements. Results: At the lowest photon energies used, the diode over‐responds by a factor of more than 5 due to the increasing mass energy absorption coefficient of silicon compared to that of water. This general behavior is reproduced in both our measurements and simulations. Energy correction factors for determination of dose‐to‐water obtained by experiments and simulations differ by less than 5% at mean energies below and <1% at energies above 1 MeV. Conclusion: While at high photon energies the change of response is marginal, it is extreme at low energies. Therefore it is desirable to benchmark response calculations also in the low energy domain. If calculations are accurate in this region, it is likely that they are also accurate for a realistic high energy spectrum.Our first results, with a simple geometry, indicate that validation of the simulations by the experimental results is achievable. Further refinement of the Monte Carlo modeling, as well as extension to other diode detectors is in progress. In a second step, corrections required for small field sizes will be studied.