3 research outputs found

    Investigation of the behaviour of GaAs/AlGaAs SAM-APDs for synchrotron radiation

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    11noThis work reports on the fabrication and characterization of a novel high-speed, low-noise X-ray Avalanche Photodiode based on III-V compound semiconductors operating over an extended photon energy range. These materials were suggested as their higher atomic numbers allow for the absorption of higher photon energies; hence, shorter response times can be achieved by growing APDs with thinner active regions. In addition, the use of staircase hetero-junctions enhances electron multiplication and results in lower noise if compared with conventional p-i-n diodes. In this work, molecular beam epitaxy was used to produce GaAs/AlGaAs APDs with separated absorption and multiplication regions. The multiplication region, separated from the absorption region by a δ p-doped layer of carbon, contains a staircase structure composed of nanometric layers of AlGaAs and GaAs, which alternate periodically. The periodic modulation of the band gap enables a well-defined charge multiplication and results in low multiplication noise. Several devices were characterized in terms of dark current, photocurrents generated utilizing visible and hard X-ray sources as well as noise generated under laser light.openopenNichetti, Camilla; Steinhartova, Tereza; Antonelli, Matias; Cautero, Giuseppe; Menk, Ralf Hendrik; Pilotto, Alessandro; Driussi, Francesco; Palestri, Pierpaolo; Selmi, Luca; Arfelli, Fulvia; Biasiol, GiorgioNichetti, Camilla; Steinhartova, Tereza; Antonelli, Matias; Cautero, Giuseppe; Menk, Ralf Hendrik; Pilotto, Alessandro; Driussi, Francesco; Palestri, Pierpaolo; Selmi, Luca; Arfelli, Fulvia; Biasiol, Giorgi

    Synchrotron Radiation Study of Gain, Noise, and Collection Efficiency of GaAs SAM-APDs with Staircase Structure

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    In hard X-ray applications that require high detection efficiency and short response times, such as synchrotron radiation-based Mössbauer absorption spectroscopy and time-resolved fluorescence or photon beam position monitoring, III–V-compound semiconductors, and dedicated alloys offer some advantages over the Si-based technologies traditionally used in solid-state photodetectors. Amongst them, gallium arsenide (GaAs) is one of the most valuable materials thanks to its unique characteristics. At the same time, implementing charge-multiplication mechanisms within the sensor may become of critical importance in cases where the photogenerated signal needs an intrinsic amplification before being acquired by the front-end electronics, such as in the case of a very weak photon flux or when single-photon detection is required. Some GaAs-based avalanche photodiodes (APDs) were grown by a molecular beam epitaxy to fulfill these needs; by means of band gap engineering, we realised devices with separate absorption and multiplication region(s) (SAM), the latter featuring a so-called staircase structure to reduce the multiplication noise. This work reports on the experimental characterisations of gain, noise, and charge collection efficiencies of three series of GaAs APDs featuring different thicknesses of the absorption regions. These devices have been developed to investigate the role of such thicknesses and the presence of traps or defects at the metal–semiconductor interfaces responsible for charge loss, in order to lay the groundwork for the future development of very thick GaAs devices (thicker than 100 [Formula: see text] m) for hard X-rays. Several measurements were carried out on such devices with both lasers and synchrotron light sources, inducing photon absorption with X-ray microbeams at variable and controlled depths. In this way, we verified both the role of the thickness of the absorption region in the collection efficiency and the possibility of using the APDs without reaching the punch-through voltage, thus preventing the noise induced by charge multiplication in the absorption region. These devices, with thicknesses suitable for soft X-ray detection, have also shown good characteristics in terms of internal amplification and reduction of multiplication noise, in line with numerical simulations

    Inter-Backbone Charge Transfer as Prerequisite for Long-Range Conductivity in Perylene Bisimide Hydrogels

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    Hydrogelation, the self-assembly of molecules into soft, water-loaded networks, is one way to bridge the structural gap between single molecules and functional materials. The potential of hydrogels, such as those based on perylene bisimides, lies in their chemical, physical, optical, and electronic properties, which are governed by the supramolecular structure of the gel. However, the structural motifs and their precise role for long-range conductivity are yet to be explored. Here, we present a comprehensive structural picture of a perylene bisimide hydrogel, suggesting that its long-range conductivity is limited by charge transfer between electronic backbones. We reveal nanocrystalline ribbon-like structures as the electronic and structural backbone units between which charge transfer is mediated by polar solvent bridges. We exemplify this effect with sensing, where exposure to polar vapor enhances conductivity by 5 orders of magnitude, emphasizing the crucial role of the interplay between structural motif and surrounding medium for the rational design of devices based on nanocrystalline hydrogels
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