Laurent Ottaviani

Charge trapping dynamics in PbS colloidal quantum dot photovoltaic devices.

The efficiency of solution-processed colloidal quantum dot (QD) based solar cells is limited by poor charge transport in the active layer of the device, which originates from multiple trapping sites provided by QD surface defects. We apply a recently developed ultrafast electro-optical technique, pump-push photocurrent spectroscopy, to elucidate the charge trapping dynamics in PbS colloidal-QD photovoltaic devices at working conditions. We show that IR photoinduced absorption of QD in the 0.2-0.5 eV region is partly associated with immobile charges, which can be optically detrapped in our experiment. Using this absorption as a probe, we observe that the early trapping dynamics strongly depend on the nature of the ligands used for QD passivation, while it depends only slightly on the nature of the electron-accepting layer. We find that weakly bound states, with a photon-activation energy of 0.2 eV, are populated instantaneously upon photoexcitation. This indicates that the photogenerated states show an intrinsically bound-state character, arguably similar to charge-transfer states formation in organic photovoltaic materials. Sequential population of deeper traps (activation energy 0.3-0.5 eV) is observed on the ~0.1-10 ns time scales, indicating that most of carrier trapping occurs only after substantial charge relaxation/transport. The reported study disentangles fundamentally different contributions to charge trapping dynamics in the nanocrystal-based optoelectronic devices and can serve as a useful tool for QD solar cell development.

Structural characterization of 6H- and 4H-SiC polytypes by means of cathodoluminescence and x-ray topography

The cathodoluminescence (CL) technique is used to analyse the radiative recombination properties of four distinct silicon carbide (SiC) samples: a 6H-SiC n+-type Lely wafer, two off-axis 4H-SiC epitaxial layers of n type and p type, and a ()-oriented 4H-SiC n+-type substrate. The CL spectra, recorded at various temperatures and at various excitation conditions, show strong differences between the polytypes, indicating a better homogeneous distribution of radiative centres inside the 6H polytype than in the 4H one, and also between the different orientations. For the ()-oriented 4H sample, luminescence features decrease when the excitation intensity increases, probably due to a more significant indirect transition band. The CL spectra also vary for the same sample, due to the impurity and the microscopic defect density variations. Comparisons between two local spectra taken in two distinct areas of the ()-oriented 4H sample, and with images obtained by x-ray topography in the same areas, allow us to establish that some structural defects are involved in luminescence centres. A deep centre involved in green luminescence (at 1.80?eV) is found to be associated with basal plane dislocations with the Burgers vector .

A Comparative Study of High-Temperature Aluminum Post-Implantation Annealing in 6H- and 4H-SiC, Non-Uniform Temperature Effects

4H-and 6H-SiC small samples were implanted by keV Al + ions at room temperature and annealed in an induction heating furnace, at the center of the susceptor, for different temperatures and times in the range 1600-1800°C and 5-60 min, respectively. The implanted layers were amorphous but the SiC crystalline structures were recovered after annealing, as measured by Rutherford Back-Scattering analyses in Channeling geometry. Al + electrical activation determined by sheet resistance and Hall effect measurements increases with the annealing temperature or time, on both polytypes. When whole SiC wafers were annealed in the same induction heating furnace, sheet resistance mapping systematically presented a radial gradient from the center to the periphery of the wafer. The measured linear dependence between sheet resistance and temperature allowed us to rebuild the radial temperature gradient at the crucible-susceptor furnace during the annealing process. Introduction Silicon carbide (SiC) is envisaged as a promising semiconductor material for a wide variety of high-temperature, high-power and high-frequency electronic applications. Ion implantation, an indispensable technique to locally dope silicon carbide still presents many problems in particular for p-type zone creation. High ionization energy of dopants imposes to raise the implanted dose above the amorphization threshold for room temperature implantations. Structure recrystallization and electrical activation of dopants, i.e. their incorporation in active SiC atomic sites, require high temperature annealing, about 1700°C in special configuration, with an overpressure of silicon and carbide. In this work p-type 6H and 4H-SiC layers created by Aluminum (Al) ion implantations followed by high temperature annealings are studied in order to realize efficient p +-n junctions for bipolar power diodes. Dopant electrical activation dependence on the post-implantation annealing conditions is discussed considering the non-uniform temperature at the SiC sample surface during this process.

First tests of silicon-carbide semiconductors as candidate neutron detector for the ITER Test Blanket Modules

The tritium breeding blanket is an essential part of a future DT fusion power reactor. Test Blanket Modules (TBM) will be installed in the experimental reactor ITER with the aim to investigate the nuclear performance of different designs. Currently there is no qualified neutronics instrumentation for the TBMs which is able to withstand the harsh environment conditions such as high temperature and, depending on the operation scenario, intense radiation.

Radiation silicon carbide detectors based on ion implantation of boron

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.

Influence of Heating and Cooling Rates of Post-Implantation Annealing Process on Al-Implanted 4H-SiC Epitaxial Samples

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.

Effect of introducing gettering sites and subsequent Au diffusion on the thermal conductivity and the free carrier concentration in n-type 4H-SiC

We report on the application of introducing gettering sites by helium implantation prior to an annealing and subsequent gold diffusion as an approach to control the thermal conductivity and the charge carrier concentration in n-type 4H-SiC. Rutherford backscattering spectroscopy showed high diffusion of gold impurities to the introduced gettering sites which implied a success in introducing gettering sites. Data obtained from the measurements of the specific heat, thermal diffusivity, and room temperature density were used to deduce the thermal conductivity of the samples investigated. The thermal conductivity modeling showed that introducing gettering sites can increase the thermal conductivity in n-type 4H-SiC due to the reduction of phonon-impurity scattering. Raman measurements showed the presence of the desired defects introduted by ion implantation at 20°C. The analysis of the Fourier transform infrared reflectivity showed that the gettering sites can act as majority carrier traps and reduce the rec...

Detection of 14 MeV neutrons in high temperature environment up to 500 °C using 4H-SiC based diode detector

In reactor technology and industrial applications, detection of fast and thermal neutrons plays a crucial role in getting relevant information about the reactor environment and neutron yield. The inevitable elevated temperatures make neutron yield measurements problematic. Out of the currently available semiconductors 4H-SiC seems to be the most suitable neutron detector material under extreme conditions due to its high heat and radiation resistance, large band-gap and lower production cost than the competing diamond detectors. Some of future using and interesting applications of such SiC detector devices-for non-charged particles (photons and/or neutrons) are expected in the frame of non-destructive assays, nuclear reactor monitoring, safeguards, oil and gas prospections [1,2,3]. In the framework of the European I-Smart project, optimal 4H-SiC based diode geometries were developed for high temperature neutron detection. Irradiation tests were conducted with 14 MeV fast neutrons supplied by a deuterium-tritium neutron generator with an average neutron yield of 4.04 × 1010-5.25 × 1010 n/s at Neutron Laboratory of the Technical University of Dresden in Germany. In the present work, we interpret the first measurement of SiC detector irradited with fast neutrons from room temperature up to 500 degrees Celsius. These experiments are serving also the first simulation of the harsh environmental condition measurements in the tritium breeding blanket of the ITER fusion reactor, which is one of the most prominent planned location of high temperature neutron flux characterization studies in the near future.

Nuclear radiation detector based on ion implanted p-n junction in 4H-SiC

In this paper, we propose a new device detector based on ion implanted p-n junction in 4H-SiC for nuclear instrumentation. We showed the interest to use 10Boron as a Neutron Converter Layer in order to detect thermal neutrons. We present the main results obtained during irradiation tests performed in the Belgian Reactor 1. We show the capability of our detector by means of first results of the detector response at different reverse voltage biases and at different reactor power.

Irradiation of 4H-SiC UV detectors with heavy ions

Ultraviolet (UV) photodetectors based on Schottky barriers to 4H-SiC are formed on lightly doped n-type epitaxial layers grown by the chemical vapor deposition method on commercial substrates. The diode structures are irradiated at 25°C by 167-MeV Xe ions with a mass of 131 amu at a fluence of 6 × 109 cm−2. Comparative studies of the optical and electrical properties of as-grown and irradiated structures with Schottky barriers are carried out in the temperature range 23–180°C. The specific features of changes in the photosensitivity and electrical characteristics of the detector structures are accounted for by the capture of photogenerated carriers into traps formed due to fluctuations of the conduction-band bottom and valence-band top, with subsequent thermal dissociation.