Archivio della ricerca di Triestehttps://arts.units.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Thu, 26 Nov 2020 07:01:04 GMT2020-11-26T07:01:04Z1071Gain and noise in GaAs/AlGaAs avalanche photodiodes with thin multiplication regionshttp://hdl.handle.net/11368/2943410Titolo: Gain and noise in GaAs/AlGaAs avalanche photodiodes with thin multiplication regions
Abstract: Avalanche photodiodes based on GaAs/AlGaAs with separated absorption and multi- plication regions (SAM-APDs) will be discussed in terms of capacitance, response to light (gain and noise) and time response. The structures have been fabricated by molecular beam epitaxy introducing a δ p layer doped with carbon to separate the multiplication and the absorption re- gions. The thickness of the latter layer defines the detection efficiency and the time resolution of the structure, which in turn allows tailoring the device for specific scientific applications. Within the multiplication region a periodic modulation of the bandgap is obtained by growing alternating nanometric layers of AlGaAs and GaAs with increasing Al content; this staircase structure enables the tuning of the bandgap and subsequently provides a well-defined charge multiplication. The use of such staircase hetero-junctions enhances electron multiplication and conversely reduces — at least in principle — the impact of the noise associated to hole multiplication, which should result in a decreased overall noise, when compared to p-i-n diodes composed by a single material. The first part of this paper focuses on the electrical characteristics of the grown structure and on the comparison with the simulated behaviour of such devices. In addition, gain and noise measure- ments, which have been carried out on these devices by utilizing photons from visible light to hard X-rays, will be discussed and will be compared to the results of a nonlocal history-dependent model specifically developed for staircase APDs.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11368/29434102019-01-01T00:00:00ZA new expression for the gain-noise relation of single-carrier avalanche photodiodes with arbitrary staircase multiplication regionshttp://hdl.handle.net/11368/2943548Titolo: A new expression for the gain-noise relation of single-carrier avalanche photodiodes with arbitrary staircase multiplication regions
Abstract: We propose a simple expression to relate the total excess noise factor of a single-carrier multiplication staircase avalanche photodiode (APD) to the excess noise factor and gain given by the individual conduction band discontinuities. The formula is valid when electron impact ionization dominates hole impact ionization; hence, it is especially suited for staircase APDs with In-rich multiplication regions, as opposed, for example, to GaAs/AlGaAs systems where hole ionization plays an important role. The formula has been verified by accurate means of numerical simulations based on a newly developed nonlocal history dependent impact ionization model.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11368/29435482019-01-01T00:00:00ZInter-Backbone Charge Transfer as Prerequisite for Long-Range Conductivity in Perylene Bisimide Hydrogelshttp://hdl.handle.net/11368/2928554Titolo: Inter-Backbone Charge Transfer as Prerequisite for Long-Range Conductivity in Perylene Bisimide Hydrogels
Abstract: 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.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/11368/29285542018-01-01T00:00:00ZFAvalanche photodiodes for X-ray detectors based on GaAs/AlGaAs semiconductorshttp://hdl.handle.net/11368/2967978Titolo: FAvalanche photodiodes for X-ray detectors based on GaAs/AlGaAs semiconductors
Thu, 19 Mar 2020 00:00:00 GMThttp://hdl.handle.net/11368/29679782020-03-19T00:00:00ZInvestigation of the behaviour of GaAs/AlGaAs SAM-APDs for synchrotron radiationhttp://hdl.handle.net/11368/2943546Titolo: Investigation of the behaviour of GaAs/AlGaAs SAM-APDs for synchrotron radiation
Abstract: This 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.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11368/29435462019-01-01T00:00:00ZInfluence of δ p-doping on the behaviour of GaAs/AlGaAs SAM-APDs for synchrotron radiationhttp://hdl.handle.net/11368/2917692Titolo: Influence of δ p-doping on the behaviour of GaAs/AlGaAs SAM-APDs for synchrotron radiation
Abstract: This work focuses on the development and the characterization of avalanche photodiodes with separated absorption and multiplication regions grown by molecular beam epitaxy. The i-GaAs absorption region is separated from the multiplication region by a δ p-doped layer of carbon atoms, which ensures that after applying a reverse bias, the vast majority of the potential drops in the multiplication region. Therein, thin layers of AlGaAs and GaAs alternate periodically in a so-called staircase structure to create a periodic modulation of the band gap, which under bias enables a well-defined charge multiplication and results in a low multiplication noise. The influence of the concentration of carbon atoms in the δ p-doped layer on the device characteristics was investigated and experimental data are presented together with simulation results.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11368/29176922017-01-01T00:00:00ZAn improved nonlocal history-dependent model for gain and noise in avalanche photodiodes based on energy balance equationhttp://hdl.handle.net/11368/2935842Titolo: An improved nonlocal history-dependent model for gain and noise in avalanche photodiodes based on energy balance equation
Abstract: We present a nonlocal history-dependent model for impact ionization gain and noise in avalanche photodiodes (APDs) especially suited for staircase APDs. The model uses a simple energy balance equation to define effective electric fields valid also in the presence of band discontinuities which are then used to express the ionization coefficients. The model parameters have been calibrated against literature data for gain and noise in GaAs and AlxGa1-xAs (x = 0.2, 0.6, 0.8) p-i-n diodes. Application to the experimental data for gain and noise in heterojunction and staircase separate absorption and multiplication APDs is reported to demonstrate the ability of the model in describing complex APD structures. It is found that, in spite of conduction band discontinuities being much larger than valence band ones, hole impact ionization contributes a significant degradation of the noise metrics in GaAs/AlGaAs staircase APDs. These nontrivial insights demonstrate the usefulness of the model to steer device design and optimization.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/11368/29358422018-01-01T00:00:00Z