Showing papers by "Andrej Yu. Kuznetsov published in 2015"

Maskless inverted pyramid texturization of silicon

Abstract: We discovered a technical solution of such outstanding importance that it can trigger new approaches in silicon wet etching processing and, in particular, photovoltaic cell manufacturing. The so called inverted pyramid arrays, outperforming conventional pyramid textures and black silicon because of their superior light-trapping and structure characteristics, can currently only be achieved using more complex techniques involving lithography, laser processing, etc. Importantly, our data demonstrate a feasibility of inverted pyramidal texturization of silicon by maskless Cu-nanoparticles assisted etching in Cu(NO3)2 / HF / H2O2 / H2O solutions and as such may have significant impacts on communities of fellow researchers and industrialists.

Probing defects in nitrogen-doped Cu2O.

Abstract: Nitrogen doping is a promising method of engineering the electronic structure of a metal oxide to modify its optical and electrical properties; however, the doping effect strongly depends on the types of defects introduced. Herein, we report a comparative study of nitrogen-doping-induced defects in Cu2O. Even in the lightly doped samples, a considerable number of nitrogen interstitials (Ni) formed, accompanied by nitrogen substitutions (NO) and oxygen vacancies (VO). In the course of high-temperature annealing, these Ni atoms interacted with VO, resulting in an increase in NO and decreases in Ni and VO. The properties of the annealed sample were significantly modified as a result. Our results suggest that Ni is a significant defect type in nitrogen-doped Cu2O.

Back-illuminated Si photocathode: a combined experimental and theoretical study for photocatalytic hydrogen evolution

Abstract: Si is an excellent absorber material for use in 2-photon photoelectrochemical hydrogen production. So far nearly all studies of silicon photoelectrodes have employed frontal illumination despite the fact that in most water-splitting 2-photon device concepts the silicon is the “bottom” cell in the tandem stack and therefore illuminated from the back with respect to the electrolyte. In the present work, we investigate back-illuminated Si photoelectrodes experimentally, as well as by modelling, the dependence of induced photocurrent on various parameters, such as carrier diffusion length (Le) and surface recombination velocity (vs) to quantify their relative importance. A bifacial light absorbing structure (p+pn+ Si) is tested under back-illumination conditions which mimic the actual working environment in a tandem water splitting device. The thickness of the absorbing Si layer is varied from 30 to 350 μm to assess the impact of the diffusion length/thickness ratio (Le/L) on photocatalytic performance. It is shown how the induced photocurrent (JL) of a back-illuminated sample increases as wafer thickness decreases. Compared to the 350 μm thick sample, a thinned 50 μm thick sample shows a 2.7-fold increase in JL, and consequently also a higher open circuit voltage. An analytical model is developed to quantify how the relative Le/L-ratio affects the maximum JL under back-illumination, and the result agrees well with experimental results. JL increases with the Le/L-ratio only up to a certain point, beyond which the surface recombination velocity becomes the dominant loss mechanism. This implies that further efforts should to be focused on reduction of surface recombination. The present study is the first experimental demonstration of a Si wafer based photocathode under back-illumination. Moreover, the comparative experimental and theoretical treatment also highlights which photoabsorber properties merit the most attention in the further development towards full tandem water splitting devices.

Optical activity and defect/dopant evolution in ZnO implanted with Er

Abstract: The effects of annealing on the optical properties and defect/dopant evolution in wurtzite (0001) ZnO single crystals implanted with Er ions are studied using a combination of Rutherford backscattering/channeling spectrometry and photoluminescence measurements. The results suggest a lattice recovery behavior dependent on ion dose and involving formation/evolution of an anomalous multipeak defect distribution, thermal stability of optically active Er complexes, and Er outdiffusion. An intermediate defect band occurring between the surface and ion-induced defects in the bulk is stable up to 900 °C and has a photoluminescence signature around 420 nm well corresponding to Zn interstitials. The optical activity of the Er atoms reaches a maximum after annealing at 700 °C but is not directly associated to the ideal Zn site configuration, since the Er substitutional fraction is maximal already in the as-implanted state. In its turn, annealing at temperatures above 700 °C leads to dissociation of the optically active Er complexes with subsequent outdiffusion of Er accompanied by the efficient lattice recovery.

Fluorine doping: a feasible solution to enhancing the conductivity of high-resistance wide bandgap Mg0.51Zn0.49O active components.

Abstract: N-type doping of high-resistance wide bandgap semiconductors, wurtzite high-Mg-content MgxZn1–xO for instance, has always been a fundamental application-motivated research issue. Herein, we report a solution to enhancing the conductivity of high-resistance Mg0.51Zn0.49O active components, which has been reliably achieved by fluorine doping via radio-frequency plasma assisted molecular beam epitaxial growth. Fluorine dopants were demonstrated to be effective donors in Mg0.51Zn0.49O single crystal film having a solar-blind 4.43 eV bandgap, with an average concentration of 1.0 × 1019 F/cm3.The dramatically increased carrier concentration (2.85 × 1017 cm−3 vs ~1014 cm−3) and decreased resistivity (129 Ω · cm vs ~106 Ω cm) indicate that the electrical properties of semi-insulating Mg0.51Zn0.49O film can be delicately regulated by F doping. Interestingly, two donor levels (17 meV and 74 meV) associated with F were revealed by temperature-dependent Hall measurements. A Schottky type metal-semiconductor-metal ultraviolet photodetector manifests a remarkably enhanced photocurrent, two orders of magnitude higher than that of the undoped counterpart. The responsivity is greatly enhanced from 0.34 mA/W to 52 mA/W under 10 V bias. The detectivity increases from 1.89 × 109 cm Hz1/2/W to 3.58 × 1010 cm Hz1/2/W under 10 V bias at room temperature.These results exhibit F doping serves as a promising pathway for improving the performance of high-Mg-content MgxZn1-xO-based devices.

Germanium-based quantum emitters towards a time-reordering entanglement scheme with degenerate exciton and biexciton states

Abstract: We address the radiative emission of individual germanium extrinsic centers in ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ epilayers grown on germanium substrates. Microphotoluminescence experiments demonstrate the capability of high temperature emission (70 K) and complex exciton configurations (neutral exciton $X$ and biexciton $XX$, positive ${X}^{+}$ and negative ${X}^{\ensuremath{-}}$ charged excitons) of these quantum emitters. Finally, we investigate the renormalization of each energy level showing a large and systematic change of the binding energy of $XX$ and ${X}^{+}$ from positive to negative values (from $\ensuremath{\sim}+5$ meV up to $\ensuremath{\sim}\ensuremath{-}7$ meV covering $\ensuremath{\sim}70$ meV of the emission energy) with increasing quantum confinement. These light emitters, grown on a silicon substrate, may exhibit energy-degenerate $X$ and $XX$ energy levels. Furthermore, they emit at the highest detection efficiency window of Si-based single-photon detectors. These features render them a promising device platform for the generation of entangled photons in the time-reordering scheme.

4H-SiC Neutron Sensors Based on Ion Implanted 10 B Neutron Converter Layer

Abstract: In the framework of the I_SMART project the main aim is to develop an innovative complete 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.

Study of the Stability of 4H-SiC Detectors by Thermal Neutron Irradiation

Abstract: 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.