Sensitive photodetectors that operate at a wavelength of 2 μm are required for applications in sensing and imaging but state-of-the-art devices are severely limited by high dark current density ...(Jdark). The narrow-bandgap materials required for mid-infrared (2–5 µm) detection are plagued by carrier recombination and band-to-band tunnelling; as a result, detectors must be operated at cryogenic temperatures. HgCdTe is currently the most commonly used materials system for these applications and has achieved Jdark = 3 × 10−4 A cm–2 at a gain of 10 while operating at 125 K. Here, we report the details and results for avalanche photodiodes for 2-μm detection based on a separate absorption, charge, and multiplication design in the AlxIn1–xAsySb1–y materials system. We achieve comparable Jdark between 200–220 K and demonstrate very low excess noise (k ≈ 0.01) and gain >100 at room temperature. Such avalanche photodiodes could prove useful for receivers for eye-safe light imaging, detection and ranging.Highly sensitive avalanche photodiodes that operate at near-infrared wavelengths of up to 2 μm could prove useful for eye-safe light imaging, detection and ranging, and other applications.
Al x In1–x As y Sb1–y digital alloys lattice-matched to GaSb were grown within the miscibility gap by molecular beam epitaxy, with aluminum fractions ranging from 0% to 80%. Photoluminescence spectra ...from Al x In1–x As y Sb1–y films and from Al x In1–x As y Sb1–y /GaSb type-II superlattices were used to determine the direct bandgap and the band offsets as a function of the aluminum fraction. Varying the aluminum content tuned the direct bandgap from 0.25 eV (0% aluminum) to 1.24 eV (75% aluminum), corresponding to photon wavelengths from 5000 to 1000 nm, with the transition from direct-gap to indirect-gap occurring at ∼1.18 eV (∼72% aluminum), or 1050 nm. This direct-gap tuning range of 0.93 eV is the largest reported for a III–V alloy lattice-matched to a commercially available substrate. The broadly tunable bandgap and type-I band alignments of this lattice-matched quaternary make it attractive for advanced mid-infrared and near-infrared detectors and sources.
In 1982, Capasso and co-workers proposed the solid-state analogue of the photomultiplier tube, termed the staircase avalanche photodiode. Through a combination of compositional grading and small ...applied bias, the conduction band profile is arranged into a series of steps that function similar to the dynodes of a photomultiplier tube, with twofold gain arising at each step via impact ionization. A single-step staircase was previously reported but did not demonstrate gain scaling through cascading multiple steps or report noise properties. Here we demonstrate gain scaling of up to three steps; measurements show the expected 2N scaling with the number of staircase steps, N. Furthermore, measured noise increased more slowly with gain than for photomultiplier tubes, probably due to the lower stochasticity of impact ionization across well-designed heterojunctions as compared with the secondary electron emission from metals. Excellent agreement was found between the experiments and Monte Carlo simulations for both gain and noise.A three-step staircase avalanche diode was demonstrated and pre-cited gain scaling was confirmed. The technology may be considered as a solid-state analogue to the photomultiplier tube.
Characterizing the immune response to pneumococcal proteins is critical in understanding this bacterium’s epidemiology and vaccinology. Probing a custom-designed proteome microarray with sera from 35 ...healthy US adults revealed a continuous distribution of IgG affinities for 2,190 potential antigens from the species-wide pangenome. Reproducibly elevated IgG binding was elicited by 208 “antibody binding targets” (ABTs), which included 109 variants of the diverse pneumococcal surface proteins A and C (PspA and PspC) and zinc metalloprotease A and B (ZmpA and ZmpB) proteins. Functional analysis found ABTs were enriched in motifs for secretion and cell surface association, with extensive representation of cell wall synthesis machinery, adhesins, transporter solute-binding proteins, and degradative enzymes. ABTs were associated with stronger evidence for evolving under positive selection, although this varied between functional categories, as did rates of diversification through recombination. Particularly rapid variation was observed at some immunogenic accessory loci, including a phage protein and a phase-variable glycosyltransferase ubiquitous among the diverse set of genomic islands encoding the serine-rich PsrP glycoprotein. Nevertheless, many antigens were conserved in the core genome, and strains’ antigenic profiles were generally stable. No strong evidence was found for any epistasis between antigens driving population dynamics, or redundancy between functionally similar accessory ABTs, or age stratification of antigen profiles. These results highlight the paradox of why substantial variation is observed in only a subset of epitopes. This result may indicate only some interactions between immunoglobulins and ABTs clear pneumococcal colonization or that acquired immunity to pneumococci is an accumulation of individually weak responses to ABTs evolving under different levels of functional constraint.
Digital alloy AlInAsSb avalanche photodiodes exhibit low excess noise comparable to those fabricated from Si. The electron and hole ionization coefficients are critical parameters for simulation and ...analysis of high-sensitivity receivers. We report ionization coefficients using a mixed injection technique that employs measurement of the gain for different incident wavelengths and a simulation algorithm.
We report the first direct dry transfer of a single-crystalline thin film grown by molecular beam epitaxy. A double cantilever beam fracture technique was used to transfer epitaxial bismuth thin ...films grown on silicon (111) to silicon strips coated with epoxy. The transferred bismuth films retained electrical, optical, and structural properties comparable to the as-grown epitaxial films. Additionally, we isolated the bismuth thin films on freestanding flexible cured-epoxy post-transfer. The adhesion energy at the bismuth/silicon interface was measured to be ∼1 J/m2, comparable to that of exfoliated and wet transferred graphene. This low adhesion energy and ease of transfer is unexpected for an epitaxially grown film and may enable the study of bismuth’s unique electronic and spintronic properties on arbitrary substrates. Moreover, this method suggests a route to integrate other group-V epitaxial films (i.e., phosphorus) with arbitrary substrates, as well as potentially to isolate bismuthene, the atomic thin-film limit of bismuth.
Single-photon avalanche diodes (SPADs) that are sensitive to photons in the Short-wave infrared and extended short-wave infrared (SWIR and eSWIR) spectra are important components for communication, ...ranging, and low-light level imaging. The high gain, low excess noise factor, and widely tunable bandgap of Al
In
As
Sb
avalanche photodiodes (APDs) make them a suitable candidate for these applications. In this work, we report single-photon-counting results for a separate absorption, charge, and multiplication (SACM) Geiger-mode SPAD within a gated-quenching circuit. The single-photon avalanche probabilities surpass 80% at 80 K, corresponding with single-photon detection efficiencies of 33% and 12% at 1.55 µm and 2 µm, respectively.
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