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Neutron radiation detector for nuclear reactor applications plays an important role in getting information about the actual neutron flux. Such a detector must be able to operate at high neutron flux levels (>109 cm−2 s−1) and discriminate the neutron and gamma responses in the nuclear reactor’s mixed neutron–gamma environment. Silicon carbide and diamond are the most attractive semiconductor materials for neutron detection, thanks to their outstanding properties, such as high displacement threshold energy and wide bandgap energy, which allow them to operate in high radiation levels and high temperature. The aim of this article is to compare the ability to detect thermal neutrons of these two semiconductors at the same irradiation conditions. For this purpose, the neutron irradiation tests of detectors were implemented at MINERVE research reactor at CEA Cadarache. The 4H-silicon carbide (SiC) p+n diode has demonstrated better neutron–gamma discrimination at 0-V bias voltage than at −200 V which is explained by its increased sensitivity to gamma photons at −200 V caused by a wider charge collection region than at 0 V. Therefore, it is preferable to use the 4H-SiC p+n diode without an external electric field for applications in the mixed neutron–gamma environment such as nuclear reactor environment. The results show that the single-crystal chemical vapor-deposited (sCVD) diamond-based detector has better neutron to gamma discrimination, thanks to the use of 6Li as a neutron converter instead of 10B. However, the study of the radiation stability of detectors showed that the sCVD diamond-based detector suffers from the “polarization effect” when it operates at a high neutron flux (~109 cm $^{-2}\,\,\cdot \,\,\text{s}^{-1}$ ).

In Silicon, impurities introduce recombination centers and degrade the minority carrier lifetime. It is therefore important to identify the nature of these impurities through their characteristics: the capture cross sectionσand the defect levelEt. For this purpose, a study of the bulk lifetime of minority carriers can be carried out. The temperature dependence of the lifetime based on the Shockley-Read-Hall (SRH) statistic and related to recombination through defects is studied. Nickel and gold in p-type Si have been selected for the SRH lifetime modeling. The objective of the analysis is to carry out a study to evaluate gold and nickel identification prior to temperature-dependent lifetime measurements using the microwave phase-shift (μW-PS) technique. The μW-PS is derived from the PCD technique and is sensitive to lower impurity concentrations. It has been shown that both gold and nickel can be unambiguously identified from the calculated TDLS curves.

International audience; This study is interested in the local activation of Light-induced degradation (LID) defect in highly co-doped silicon wafers with boron and phosphorus. For this purpose, the experiments are focused on measuring the minority carrier lifetime before and after LID activation via a mapping technique. The LID defect density exhibits a Gaussian distribution centered on the excitation point of the laser beam; the intensity of the Gaussian distribution of the LID defect varies with the concentration of the co-dopants. The lifetime of the minority carriers decreases in all-silicon sample regions, while the excitation laser beam focuses on an area of approximately one mm2. This observation indicates that LID defects are activated even in the unexcited areas of silicon wafers, suggesting a LID diffusion phenomenon from the laser excitation point to the whole silicon wafer. We deduce that a high phosphorus doping level in silicon wafers leads to a significant reduction in the LID effect.

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We report on the investigation of a variety of metallization processes for hole selective passivating contacts, which consist in the combination of an ultra-thin silicon oxide (SiOx) interfacial film (grown by wet treatments) and a 25 nm thick boron (B) doped polysilicon layer (poly-Si). Plasma Enhanced Chemical Vapor Deposition (PECVD) of amorphous silicon (a-Si) followed by crystallization annealing is used for poly-Si elaboration. Optimization of metallization steps is required in order to (1) reach low specific contact resistances (ρc), and (2) preserve passivation quality along with structural integrity. This study focuses on the analysis of two processes: electron-beam evaporation deposition, and Physical Vapor Deposition (PVD) by sputtering. Different metals (silver (Ag) and aluminum (Al)) and annealing conditions are studied. Additionally, we investigated the effect of an additional a-Si capping layer on both (1) and (2) items. PVD of Ag appears as the most promising technique combining relatively low ρc (90 mΩ.cm2) as deposited with limited passivation degradations, and thus seems suitable for device integration as a full area back contact.

Radiation detectors based on wide-bandgap semi- conductors have received considerable attention in many applications such as the experiments in material testing reactors, high energy particle physics experiments, or fusion facilities for plasma diagnostics. In this paper, we compared a 4H-silicon-carbide (SiC)-based detector with a single crystal chemical vapor deposited (sCVD) diamond-based detector for 14-MeV neutron detection. For this purpose, the deuterium–tritium neutron generator of Technical University of Dresden with 14-MeV neutron output up to 1011 n/sin $4\pi $ has been used. In this paper, we interpret the results of our first measurements with both 4H-SiC and sCVD diamond detectors at low neutron flux of $9.4 \times 10^{6}\text {n}/(\mathrm {cm}^{2}\cdot \text {s})$ and at room temperature.

Neutron radiation detector for nuclear reactor applications plays an important role in getting information about the actual neutron yield and reactor environment. Such detector must be able to operate at high temperature (up to 600° C) and high neutron flux levels. It is worth nothing that a detector for industrial environment applications must have fast and stable response over considerable long period of use as well as high energy resolution. Silicon Carbide is one of the most attractive materials for neutron detection. Thanks to its outstanding properties, such as high displacement threshold energy (20-35 eV), wide band gap energy (3.27 eV) and high thermal conductivity (4.9 W/cm·K), SiC can operate in harsh environment (high temperature, high pressure and high radiation level) without additional cooling system. Our previous analyses reveal that SiC detectors, under irradiation and at elevated temperature, respond to neutrons showing consistent counting rates as function of external reverse bias voltages and radiation intensity. The counting-rate of the thermal neutron-induced peak increases with the area of the detector, and appears to be linear with respect to the reactor power. Diamond is another semi-conductor considered as one of most promising materials for radiation detection. Diamond possesses several advantages in comparison to other semiconductors such as a wider band gap (5.5 eV), higher threshold displacement energy (40-50 eV) and thermal conductivity (22 W/cm·K), which leads to low leakage current values and make it more radiation resistant that its competitors. A comparison is proposed between these two semiconductors for the ability and efficiency to detect fast neutrons. For this purpose the deuterium-tritium neutron generator of Technical University of Dresden with 14 MeV neutron output of 1010 n·s-1 is used. In the present work, we interpret the first measurements and results with both 4H-SiC and chemical vapor deposition (CVD) diamond detectors irradiated with 14 MeV neutrons at room temperature.

Neutron radiation detector for nuclear reactor applications plays an important role in getting information about the actual neutron yield and reactor environment. Such detector must be able to operate at high neutron flux levels (>109 n/(cm2s)) and discriminate the neutron and gammaresponses in the nuclear reactor’s mixed neutron-gamma environment. Moreover such a detector must have fast and stable response over considerable long period of use. Silicon Carbide and Diamond are the most attractive semiconductor materials for neutron detection in severe media (high temperature, high gamma dose rate and high neutron fluxes). Thanks to their outstanding properties, such as high displacement threshold energy and wide band gap energy, SiC and Diamond can operate in harsh environment (high temperature and high radiation level). The aim of this work is to compare the ability to detect thermal neutrons of these two semi-conductors at the same irradiation conditions. For this purpose, the neutron irradiation tests of detectors were implemented at MINERVE research reactor at CEA Cadarache. Both kinds of detectors show the linear evaluation of the count rate in the thermal neutroninduced peak with the reactor power. Our results reveal that the 4H-SiC-based neutron detector shows worse neutron to gamma discrimination in comparison to the sCVD diamond-based detector when the reverse bias voltage is applied to the 4H-SiC diode. The best neutron to gamma discrimination, though, is obtained with the 4H-SiC-based detector at 0V.

Spectral sensitivity measurements versus temperature have been carried out on irradiated SiC p+n photodiodes, fabricated using two different doping processes: Aluminium standard implantation and Boron plasma immersion ion implantation. The spectral sensitivity of Al doped photodiodes increase for incident wavelength higher than 270 nm, and are very stable below. Boron doped irradiated photodiodes show a general increase of the photoresponse for all wavelengths. In both cases, an hysteresis effect is observable when with the temperature. Results are presented and discussed.

International audience; In this work, we investigate silver (Ag) nanoparticle-related plasmonic effect on light absorption in Si substrate. Ag nanoparticles (Ag-NPs) deposited on top of Si were used to capture and couple incident light into these structures by forward scattering. We demonstrate that we can control nanoparticle size and shape while varying deposition time and annealing parameters. By the increase of the total time of the reaction process, morphology of Ag-NPs evolutes affecting the number and the width of surface plasmon resonance peaks, whereas for changed annealing parameters (temperature and time), the effect is more pronounced on the broadening and the position of peaks. Specific morphology of Ag-NPs can exhibit an interesting enhancement of optical properties which enables plasmon-related application in photovoltaic solar cells.

International audience; All silicon (Si) ingot fabrication processes share challenges to control grain structure, defect and impurity contamination during the solidification step to improve the material properties. The final grain structure and inherent structural defects issued from the solidification step are responsible for the photovoltaic (PV) properties for a large part, all the more as they are often associated with impurity distribution. Impurities play a major role as they not only can modify the development of the grain structure formation but interact as well with structural defects creating regions of deleterious minority carrier lifetime recombination. Samples containing different levels of impurities and solidified with different processes were selected and analyzed as-grown or observed by X-ray imaging during re-solidification from as-grown seeds. The growth features and relative crystallographic orientation of neighbor grains were characterized. Moreover, minority carrier lifetime measurements were performed and correlated to the growth features. The complementarity of the different techniques allows improving the understanding of phenomena at stake during the formation of grains and twins, the effect of impurities and their impact on photovoltaic properties. The results show the significant influence of light and metallic impurities such as copper on the grain structure and on the electrical properties. 2

International audience; In the framework of the I_SMART project the main aim is to develop an innovative radiation detection system based on silicon carbide technology in view to detect neutrons (thermal and fast) and photons for harsh environments. In the present work two geometries have been realized based on ion implantation of boron. In the first geometry, 10B ions have been implanted into the Al metallic contact of a p-n diode to create the neutron converter layer. In the second geometry one single process has been used to realize both the p+-layer and the neutron converter layer. The technological processes followed to fabricate these detectors, with a study of their electrical behavior and their responses under thermal neutron irradiations are addressed in this paper.

International audience; During the past years, our team developed an original lifetime measurement method for semiconductors wafers, called microwave phase-shift (µW-PS). It was successfully employed to determinate bulk lifetime and surface recombination velocities on silicon. We recently adapted the method to silicon carbide (SiC), using a continuous UV laser and acousto-optic modulator. The theory is presented, using the standard continuity equation in n-type 4H-SiC. The proposed measurement setup firstly allows to estimate a bulk lifetime ranging from 2.3 to 6.7 µs.

6th International Conference on Crystalline Silicon Photovoltaics (SiliconPV), CEA INES, Chambery, FRANCE, MAR 07-09, 2016; International audience; The paper is devoted to the identification of the metallic impurities in silicon wafers by using Temperature Dependent Lifetime Spectroscopy (TDLS). We consider the variation of all recombination mechanisms, intrinsic and extrinsic, to follow the variation of lifetime with the temperature. The extrinsic recombination mechanism is based on the standard Shockley-Read-Hall theory (SRH) [1], [2] and we simulated the variation of SRH lifetime for two impurities : gold and iron. The simulation results show that their SRH lifetime variations with the temperature are opposite and that the presence of a peak is characteristic of the impurity studied. Experimental measurements are displayed showing the identification of gold impurity by means of Phase-Shift TDLS (PS-TDLS) measurement. Thanks to these results, we demonstrate that PS-TDLS is an efficient method to identify gold and iron impurities at concentrations as low as 1.10(10)cm(-3) for a doping level of 1.10(15)cm(-3). (C) 2016 The Authors. Published by Elsevier Ltd.

The photoluminescence (PL) of highly doped and compensated Czochralski silicon has been investigated. The low-temperature PL spectra show a donor-to-acceptor zero-phonon broad band and clearly distinguished phonon replica shifting to the blue side with increasing the net doping concentration in silicon crystal. The data are consistent with DAP luminescence involving an acceptor level of about 46 meV above the valence band edge in silicon. The analysis of temperature evolution of the PL spectra indicate that the observed PL spectra are due to a donor–acceptor pair recombination with a donor level located at 46 meV. The redshift of this band with increasing excitation power was also observed. The dependence of the emission peak energy on the excitation power is characteristic of highly compensated material in which potential fluctuations influence the band structure.

This paper presents a study of 4H-SiC UV photodetectors based on p+n thin junctions. Two kinds of p+ layers have been implemented, aiming at studying the influence of the junction elaborated by the ion implantation process (and the subsequent annealing) on the device characteristics. Aluminum and Boron dopants have been introduced by beam line and by plasma ion implantation, respectively. Dark currents are lower with Al-implanted diodes (2 pA/cm2 @ - 5 V). Accordingly to simulation results concerning the influence of the junction thickness and doping, plasma B-implanted diodes give rise to the best sensitivity values (1.5x10-1 A/W @ 330 nm).

This study focuses on the fabrication and the characterization of solar cells using highly doped boron-phosphorus compensated n-type multicrystalline silicon. Two ingots were used and compared for this study. One ingot was crystallized from a silicon feedstock purified via the metallurgical route. The second ingot was crystallized from an electronic grade silicon feedstock in order to be metallic impurity free. Both ingots were intentionally compensated with a similar boron and phosphorus amount in the initial feedstock. Chemical and electrical characterizations were carried out on each ingot and similar carrier lifetimes were reported. Large area solar cells were fabricated on wafers selected at different positions along both ingots with a reference fabrication process leading to 18.4% efficiency on FZ material. No differences in terms of cells performances were observed between both ingots. Efficiencies between 13.5% and 15.0% were reported and were found to be limited by the low resistivity values of this material. This is consistent with the carrier lifetime previously measured which underlines the weak influence of the metallic impurities initially present in the wafers. Some perspectives are given in terms of feedstock specifications to exploit the potential of this material.

This study focuses on the evolution under illumination of the electrical performances of solar cells made with n-type boron-phosphorus compensated Czochralski silicon. First, we show via carrier lifetime measurements that this material is sensitive to light-induced degradation (LID) effects due to the formation of BO(i2) complexes. Solar cells were then processed with this compensated material. The cells performances were found to be dependent on the position of the wafer along the ingot height. This dependence is due to carrier lifetime variations along the ingot height and is not directly linked with the differences in concentration between boron and phosphorus. LID experiments were then performed on the same cells. Unlike what was suggested from previous studies, we highlighted a weak decrease of the cell performances under illumination, consistent with the electrical characterizations at the wafer level. These degradations have been quantified and linked with the chemical composition of the material. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In cast multicrystalline silicon ingots impurity concentrations vary along the ingot height due to segregation phenomena during the directional solidification. It is expected that these concentrations are the highest at the top of the ingot which solidifies last. The bottom of the ingot which solidifies first, and which is longer in contact with the crucible floor is contaminated by solid state diffusion. As a consequence, lifetime ( τ n ) and diffusion length ( L n ) of minority carriers are the highest in the central part of the ingot and decrease strongly in the top as well as at the bottom. However, the impurity concentration is so high at the extremities of the ingot that additional solid state segregation phenomena occur at extended defects, which extract impurities from the adjacent grains. That is revealed at grain boundaries (GBs) by τ n and L n scan maps and also by the variation of the mobility of the majority carriers which cross GBs.

Two types of 4H-SiC semiconductor detectors (D1 and D2) are realized based on ion implantation of 10B inside the aluminum metallic contact. The first detector shows a high leakage current after 10B implantation and low signal to noise ratio. However, improvements concerning the implantation parameters led to lower leakage current and thus to higher signal to noise ratio. Moreover such detectors show their stability under different thermal neutron fluxes showing the reproducible features of the pulse height spectra and same electrical behaviour before and after irradiation.

In this work, we try to make a p-type monocristalline silicon pn junction using an easier doping method. We combined spin-coating thin film deposition method and solid doping technique. This technique can be considered as variety of the SOD method. In this study, phosphorous-based gel compounds was prepared and deposited by spin coating. Heat treatment would thus, after deposition of thin layer, diffuse phosphorus atoms into the substrate to obtain a pn diode. Study by Secondary Ions Mass Spectrometry (SIMS) showed a surface phosphorus concentration of 10 20 at/cm 3 incorporated within the silicon substrate to a depth of 300 nm. The microwave phase-shift (µW-PS) technique is used to determine the bulk lifetime ( τ b) of minority carriers. In this technique, the phase-shift between a microwave beam (10 GHz) and a sine-modulated infrared excitation is related to τb and to the surface recombination velocity (S) (Palais, Clerc, Arcari, Stemmer & Martinuzzi, 2003). The lifetime τb mean values vary from 7 µs for a p-type Silicon to 97 µs for phosphorus-diffused silicon. The surface recombination velocity S varies from around 500 to 1000 cm.s -1 .

International audience; Plasmonics is a new light trapping method used in photovoltaic (PV) solar cells. A significant enhancement of the scattered and absorbed incident light due to the use of silver nanoparticles (Ag-NPs) was observed, which yield to the exaltation of the electromagnetic field in the vicinity of these NPs. In this context, we investigate optically and morphologically the effect of the NPs size dependence on the localized surface plasmon resonance. Extinction, absorption and scattering cross sections are calculated using Mie theory.

Multicrystalline silicon wafers (mc-Si) are characterized by high densities of crystallographic defects and impurities like oxygen and heavy metals. Recombination centres are generated at grain boundaries and dislocations by segregated impurities and other centres are created in the grains by dissolved impurities. The electrical properties of mc-Si are inhomogeneous and their characterisation in terms of lifetime ( τ ) and diffusion length ( L ) of minority carriers requires application of mapping techniques. Maps are obtained by means of reflected microwave intensity, spectroscopic photoluminescence or light beam-induced current techniques. P-type mc-Si wafers were intensively investigated in the last years, but n-type was not, although it is of a particular interest. Indeed, the capture cross sections of metallic impurities are frequently smaller than those found in p-type and it is expected that L p are higher than L n . It is shown that the electrical properties of p-type wafers are well described by τ and L scan maps, which are well correlated with the features of defects. However, in as cut n-type wafers L values are so high that they are overestimated due to the lateral diffusion of photocarriers. Photoluminescence (PL) maps are well correlated with lifetime scan maps and they indicate that dislocation-related defects have low recombination activity, which is confirmed by negligible intensity of the room-temperature “defect” PL band with the maximum at 1,560 nm.

Radiation detectors based on radiation-hardened semiconductor such as silicon carbide (SiC), have received considerable attention in many applications such as in outer space, high energy physics experiments, gas and oil prospection, and nuclear reactors. For the first time it was demonstrated the reliability of thermal neutron detectors realized by standard ion implantation of boron layer as a neutron converter layer. Moreover, these detectors respond to thermal neutrons and gamma rays showing different counting rates at different voltages and under different types of shielding.

International audience; Experimental investigation of photoconductivity in Si-rich silicon oxide (SRSO)/SiO2 multilayer (ML) structures prepared by magnetron reactive sputtering is reported. Photocurrent (PC) measurements show that the PC threshold increases with decreasing the thickness of SRSO layer. Photo-conduction processes in our samples are shown to be dominated by carrier transport through quantum-confined silicon nanocrystals embedded in the SiO2 host. In addition, the observed bias-dependence of photocurrent intensity is consistent with a model in which carrier transport occurs by both tunneling and hopping through defect states in the silicon oxide matrix. A photocurrent density Jph of 1–2 mA cm−2 is extracted from our results. Although this photocurrent density along the ML absorber film is relatively low, the results presented in this work are believed to be a valuable contribution toward the implementation of all-Si tandem solar cells.

International audience; A comparison is made between two kinds of Nitrogen implantations for the formation of thin n+p junctions in p-type Silicon Carbide (SiC) epitaxial layers. The standard beam ion implantations and PULSIONTM processes were performed at two distinct energies (700 eV and 7 keV) and the subsequent annealing was held at 1600°C in a resistive furnace specifically adapted to SiC material. Positron Annihilation Spectroscopy (PAS) and unpolarized infrared reflectivity (IR) measurements were carried out before and after the annealing, respecively. Despite the presence of deep vacancy clusters near the as-implanted sample surfaces, no extended defects were detected after the annealing. Plasma implanted samples prove to contain a lower point defect concentration than beam implanted samples. The concentration of these defects (resulting from plasma process) is higher in the plane parallel to the optical axis, which denotes an energy spreading alongside the dopant distribution.

This paper deals with the simulation of the reverse current density of 4H-SiC ultraviolet- (UV) photodetector devices based on p-i-n diodes. Simulations using the finite-element method presented in this paper lead to an understanding of the photodetector current density levels in dark field, as well as under UV exposure. The theoretical study coming from these simulations allows us to propose some UV-photodetector performance enhancements. A way to improve the UV-photodetector performance is to increase the current density at reverse bias. This study demonstrates the improvement in electrical characteristics achievable by either optical or electrical means. Optical simulations prove that an increase in photon harvesting is possible by using a specific patterned surface, a surface grating which behaves as a photonic crystal. In addition to the absorption enhancement achieved caused by this kind of surface, we point out wavelength selectivity. Nevertheless, the electrical simulation confirms that the electric field placement is of primary importance. To ensure that the photon absorption is carried out inside the space-charge region, the simulation study leads us to propose a UV-photodetector comprising a “flat-top pyramid” surface.

14th International Conference on Defects-Recognition, Imaging and Physics in Semiconductors, Miyazaki, JAPAN, SEP 25-29, 2011; International audience; Light-induced defect generation seriously reduces the minority-carrier lifetime of crystalline silicon (c-Si) wafers which causes a decrease in solar cell efficiency. In this paper we investigate the impact of boron-oxygen complexes and iron impurities on the light induced minority-carrier lifetime degradation in c-Si, comparing electronic grade and upgraded metallurgical grade materials. For the latter, the characteristic of the decay process is shown to be composed of a fast initial decay and a subsequent slow asymptotic decay. We conclude that the dissociation of iron-boron pairs must be taken into account to explain the light-induced lifetime reduction.

International audience; This paper deals with the study of the photoresponse properties of 4H-SiC UV-photodetector devices based on a thin junction following their testing in darkness and under UV light over the 200 to 400 nm range. An increase of the carrier harvesting for low implanted layer thickness was shown by simulation. Thus, an implantation at low energy (27 keV) was carried out on our samples. Because of the thin junction architecture, the photocurrent at 280 nm was found to be four orders of magnitude larger than the dark current. A spectral responsivity of our photodetectors shows a peak at 280 nm with a value of 0.03 A/W.

International audience; This paper focuses on the formation of thin n+p junctions in p-type Silicon Carbide (SiC) epitaxial layers using two kinds of Nitrogen implantations. The standard beam ion implantations and PULSIONTM processes were performed at two distinct energies (700 eV and 7 keV), and the subsequent annealing was held at 1600°C in a resistive furnace specifically adapted to SiC material. No measurable electrical activity was obtained for both implantations performed at 700 eV, due to some outdiffusion of N dopants during the annealing despite a low surface roughness (rms ~ 1.4 nm) and no residual damage detected by RBS/C. A higher sheet resistance was measured in plasma-implanted samples at 7 keV (in comparison with beam-line implanted samples), which is partly related to N outdiffusion. The profiles of N atoms beam-implanted at 7 keV are not affected by the annealing. The corresponding electrical activation is fully completed.

In this paper, we deal with the study of Ultra Violet (UV) photodetector device based on SiC material undergoing a p-i-n structure process. Current density-voltage (J-V) measurements in reverse and forward bias, are performed on the UV photodetector device. Due to a very thin p+-type doping layer, a high reactivation annealing and the metallic contact deposit, experimental measurements point out Junction Barrier Schottky (JBS) device behaviour in spite of the p-i-n structure device process. To understand this involuntary phenomenon, these experimental characteristics are accompanied with an experimental study by the SIMS analysis.

International audience; This paper focuses on UV-photodetector simulation. The calculus method description and the physical equations which occur in this model are presented as well as the UV-photodetector structure (p+n--n+ diode). Based on the Finite Element Method the electrical part solves the continuity and Poisson equation, and the optical part solves by Maxwell’s equation, FDTD [1]. Simulation works point out the influence of the p+-type layer on the electrical characteristics such as the current densities versus reverse bias. Indeed, simulation results show the current density increase with the decrease doping concentration or the p+-type layer thickness.

Inadvertent contaminations of silicon wafers during processing steps are simulated by in-diffusion of gold and iron. Impurity concentration mappings are deduced from lifetime scan maps, with a lateral resolution of 50 μm. Such scan maps are obtained by the contactless phase shift technique, when the sample surfaces are passivated by an acqueous solution of polyvidone. The local value of gold or iron concentration are found to be in agreement with concentrations computed from deep-level transient spectroscopy results determined by means of arrays of small metal–semiconductor diodes.

International audience; We report on topographical, structural and electrical measurements of aluminum-implanted and annealed 4H-SiC epitaxial samples. The influence of heating-up and cooling-down temperature rates on the SiC surface roughness, the crystal volume reordering and the dopant electrical activation was particularly studied. A higher heating-rate was found to preserve the rms roughness for annealing temperatures lower than 1700°C, and to improve the sheet resistance whatever the annealing temperature due to a better dopant activation (except for 1600°C process, which induced a dark zone in the sample volume). A complete activation was calculated for an annealing at 1700°C during 30 minutes, with a ramp-up at 20°C/s. Rising the cooling-down rate appeared to increase the sheet resistance, probably due to a higher concentration of point defects in the implanted layer.

The size of silicon quantum dots (Si QDs) embedded in silicon nitride (SiN(x)) has been controlled by varying the total pressure in the plasma-enhanced chemical vapor deposition (PECVD) reactor. This is evidenced by transmission electron microscopy and results in a shift in the light emission peak of the quantum dots. We show that the luminescence in our structures is attributed to the quantum confinement effect. These findings give a strong indication that the quality (density and size distribution) of Si QDs can be improved by optimizing the deposition parameters which opens a route to the fabrication of an all-Si tandem solar cell.

This work is a contribution towards the understanding of the properties of hydrogenated silicon nitride (SiN x :H) that lead to efficient surface and bulk passivation of the silicon substrate. Considering the deposition system used (low-frequency plasma-enhanced chemical vapour deposition (PECVD)), we report very low values of surface recombination velocity S eff . As-deposited Si-rich SiN x :H leads to the best results (n-type Si: S eff =4 cm/s – p-type Si: S eff =14 cm/s). After annealing, the surface passivation quality is drastically deteriorated for Si-rich SiN x :H whereas it is lightly improved for low refractive index SiN x :H ( n ∼2–2.1). The chemical analysis of the layers highlighted a high hydrogen concentration, regardless the SiN x :H stoichiometry. However, the involved H-bond types as well as the hydrogen desorption kinetics are strongly dependent on the SiN x :H composition. Furthermore, “N-rich” SiN x :H appears to be denser and thermally more stable than Si-rich SiN x :H. When subjected to a high-temperature treatment, such a layer is believed to induce the release of hydrogen in its atomic form, which consequently leads to an efficient passivation of surface and bulk defects of the Si substrate. The results are discussed and compared with the literature data reported for the different configurations of PECVD reactors.

7th European Conference on Silicon Carbide and Related Materials, Barcelona, SPAIN, SEP 07-11, 2008; International audience; We report oil measurements of the minority carrier lifetime for different epitaxial 4H-SiC layers by using the microwave photoconductivity decay (mu-PCD) method. This is a non-contacting, non-destructive method very useful for the monitoring of recombination processes in semiconductor material, Distinct samples have been analyzed, giving different lifetime values. Transmittance and absorption spectra have also been carried out. The n-type layers, giving rise to a specific absorption peak near 470 nm, are not sensitive to optical excitation for the used wavelengths, as opposite to p-type layers whose lifetime values depend on thickness and doping.

International audience; Si-rich-SiO2(SRSO)/SiO2 multilayers (MLs) have been grown by reactive magnetron sputtering. The presence of silicon nanoclusters (Si-ncls) within the SRSO sublayer and annealing temperature influence optical absorption as well as photoluminescence. The optimized annealing temperature has been found to be 1100 °C, which allows the recovery of defects and thus enhances photoluminescence. Four MLs with Si-ncl size ranging from 1.5 to 8 nm have been annealed using the optimized conditions and then studied by transmission measurements. Optical absorption has been modeled so that a size effect in the linear absorption coefficient α (in cm−1) has been evidenced and correlated with TEM observations. It is demonstrated that amorphous Si-ncl absorption is fourfold higher than that of crystalline Si-ncls.

The influence of intentional iron bulk contamination on the performances of boron doped p-type multicrystalline silicon solar cells was investigated. Solar cells were made from iron contaminated wafers, with an initial dissolved iron concentration 100 times higher than that of standard wafers. Nevertheless, the conversion efficiency of these cells was not impacted by this intentional contamination. We showed that this tolerance toward iron was due to the efficiency of the gettering and hydrogenation effects, complementary in this material. While phosphorus diffusion (extracting more than 99% of the iron from the bulk) is slightly limited in regions of high dislocation density, hydrogen diffuses through the whole thickness of the wafer and passivates defects and remaining impurities, with its diffusion being faster along extended defects

International audience; The bulk lifetime tau(n) and diffusion length L-n of minority carriers vary through the height of a cast multicrystalline silicon ( mc-Si) block. This variation is due to the segregation of metallic impurities during the directional solidification and the native impurity concentrations increase from the bottom to the top of the ingot, which is solidified last, while the ingot bottom, which is solidified first, is contaminated by the contact with the crucible floor. It is of interest to verify if a correlation exists between the bulk lifetime tau of as cut wafers and the conversion eficiency. of solar cells. In a very large ingot (> 310 kg), it was found that tau(0), in raw wafers, tau(dif) in phosphorus diffused ones and Ln in diffused wafers are smaller in the top and in the bottom of the ingot. The same evolution is observed in solar cells, however the diffusion length values L-cel in the central part of the ingot are markedly higher than those found in diffused wafers, due to the in-diffusion of hydrogen from the SiN-H antireflection coating layer. The variations of. and those of tau(0), along the ingot height, are well correlated, suggesting that the evaluation of tau(0) can predict the properties of the devices. In addition, segregation phenomena around the grain boundaries are observed at the bottom of the ingots, due to a marked contamination by the crucible floor, and at its top where impurities are accumulated. These phenomena are linked to the long duration of the solidification process and the large amount of imperfect silicon used to cast the ingot.

The influence of a gold bulk contamination on the performances of boron doped p-type crystalline silicon solar cells is investigated for different base doping levels and different kinds of materials, such as float zone Si, Czochralski Si, and multicrystalline Si. Solar cells are made from intentionally contaminated silicon wafers. By monitoring the evolution of the electrically active substitutional gold concentration by means of bulk lifetime and minority carrier diffusion length measurements, this paper highlights the eventual gettering or hydrogenation effects occurring throughout the whole process but also of the danger of such an impurity in materials containing large densities of extended defects generating recombination centers by means of the impurity-defect interaction.

We present experimental and theoretical results on InAsSbP∕InAs photodetectors for infrared detection. An unexpected hysteresis has been shown during the current-voltage I(V) investigations (300K). Drastic changes in the hysteresis cycle are observed depending on the temperature of the heterostructure. Additionally, the time step of the measurements has an influence on the hysteresis form. The appearance of such hysteresis can be explained by a tunneling effect and by considering the temperature of the structure. Experimental I(V) characteristics are in good agreement with the tunnel current coupled to a thermal approach.

International audience; N-type silicon presents several advantages compared to p-type material, among them, the most important is the small capture cross sections of metallic impurities, which are neatly smaller. As a consequence lifetime and also diffusion length of minority carriers should be neatly higher in n-type than in p-type, for a given impurity concentration. This is of a paramount interest for multicrystalline silicon wafers, in which the impurity-extended crystallographic defects interaction governs the recombination strength of minority carriers. It is experimentally verified that in 1.2 cm raw wafers lifetimes about 200 s and diffusion lengths around 220 m are measured. These values increase strongly after gettering treatments like phosphorus diffusion or Al-Si alloying. Scan maps reveal that extended defects are poorly active, although in regions where the density of dislocations is higher than 106 cm-2. Abrupt junctions are obtained by Al-Si alloying after annealing between 850 and 900 °C, which could be used for rear junction cells. Such cells can be processed by means of similar processing steps used to make conventional p-type base cells.

International audience; The kinetics of "metal-acceptor" pairing in boron-doped multicrystalline samples is investigated by means of contactless lifetime measurement. The case of FeB and CrB pairs is discussed and the influence of extended defect is evidenced for FeB. It is found that the reassociation of the "metal-acceptor" pairs is fast immediately after the dissociation and then slows down for several days. The results allow the identification of mixed contamination by Cr and Fe.

In this paper, the impact of iron contamination on the conversion efficiency of single-crystalline p-type silicon solar cells is investigated by means of the combination of numerical simulations and experimental data, taking into account the more recent results about the properties of iron in single-crystalline silicon. Numerical simulations highlight the fill factor losses due to the injection-level dependence of the bulk lifetime, which attenuates the decrease of the open circuit voltage and thus that of the solar cell conversion efficiency with iron concentration. Gettering and hydrogenation effects are quantified by means of experimental results obtained from voluntarily contaminated solar cells and integrated in the simulations. The results show that iron appears to be a metallic impurity rather well tolerated in p-type single-crystalline silicon solar cells, because its injection-level dependent bulk lifetime, like its abilities to be gettered and to be passivated by hydrogenation, limits its influe...