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Register a new userAvalanche photodiodes based on MoS2/Si heterojunctions
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Avalanche photodiodes (APDs) are the semiconducting analogue of photomultiplier tubes offering very high internal current gain and fast response. APDs are interesting for a wide range of applications in communications1, laser ranging2, biological imaging3, and medical imaging4 where they offer speed and sensitivity superior to those of classical p-n junction-based photodetectors. The APD principle of operation is based on photocurrent multiplication through impact ionization in reverse-biased p-n junctions. APDs can either operate in proportional mode, where the bias voltage is below breakdown, or in Geiger mode, where the bias voltage is above breakdown. In proportional mode, the multiplication gain is finite, thus allowing for photon energy discrimination, while in Geiger mode of operation the multiplication gain is virtually infinite and a self-sustaining avalanche may be triggered, thus allowing detection of single photons5. Here, we demonstrate APDs based on vertically stacked monolayer MoS2 and p-Si, forming an abrupt p-n heterojunction. With this device, we demonstrate carrier multiplication exceeding 1000. Even though such multiplication factors in APDs are commonly accompanied by high noise, our devices show extremely low noise levels comparable with those in regular photodiodes. These heterostructures allow the realization of simple and inexpensive high-performance and low-noise photon counters based on transition metal dichalcogenides.
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We propose and demonstrate a scheme to realize a high-efficiency truly quantum random number generator (RNG) at room temperature (RT). Using an effective extractor with simple time bin encoding method, the avalanche pulses of avalanche photodiode (APD) are converted into high-quality random numbers (RNs) that are robust to slow varying noise such as fluctuations of pulse intensity and temperature. A light source is compatible but not necessary in this scheme. Therefor the robustness of the system is effective enhanced. The random bits generation rate of this proof-of-principle system is 0.69 Mbps with double APDs and 0.34 Mbps with single APD. The results indicate that a high-speed RNG chip based on the scheme is potentially available with an integrable APD array.
A series of III-V ternary and quarternary digital alloy avalanche photodiodes (APDs) have recently been seen to exhibit very low excess noise. Using band inversion of an environment-dependent atomistic tight binding description of short period superlattices, we argue that a combination of increased effective mass, minigaps and band split-off are primarily responsible for the observed superior performance. These properties significantly limit the ionization rate of one carrier type, either holes or electrons, making the avalanche multiplication process unipolar in nature. The unipolar behavior in turn reduces the stochasticity of the multiplication gain. The effects of band folding on carrier transport are studied using the Non-Equilibrium Greens Function Method that accounts for quantum tunneling, and Boltzmann Transport Equation model for scattering. It is shown here that carrier transport by intraband tunneling and optical phonon scattering are reduced in materials with low excess noise. Based on our calculations, we propose five simple inequalities that can be used to approximately evaluate the suitability of digital alloys for designing low noise photodetectors. We evaluate the performance of multiple digital alloys using these criteria and demonstrate their validity.
We demonstrate a graphene-MoS2 architecture integrating multiple field-effect transistors and we independently probe and correlate the conducting properties of van der Waals coupled graphene-MoS2 contacts with the ones of the MoS2 channels. Devices are fabricated starting from high-quality single-crystal monolayers grown by chemical vapor deposition and characterized by scanning Raman and photoluminescence spectroscopies. Transconductance curves of MoS2 are compared with the current-voltage characteristics of graphene contact stripes, revealing a significant suppression of transport on the n-side of the transconductance curve. Based on ab-initio modeling, the effect is understood in terms of trapping by sulfur vacancies, which counter-intuitively depends on the field-effect, even though the graphene contact layer is positioned between the backgate and the MoS2 channel.
The upgrades of ATLAS and CMS for the High Luminosity LHC (HL-LHC) highlighted physics objects timing as a tool to resolve primary interactions within a bunch crossing. Since the expected pile-up is around 200, with an r.m.s. time spread of 180 ps, a time resolution of about 30 ps is needed. The timing detectors will experience a 1-MeV neutron equivalent fluence of about $Phi_{eq}=10^{14}$ and $10^{15}$ cm$^{-2}$ for the barrel and end-cap regions, respectively. In this contribution, deep diffused Avalanche Photo Diodes (APDs) produced by Radiation Monitoring Devices are examined as candidate timing detectors for HL-LHC applications. To improve the detectors timing performance, the APDs are used to directly detect the traversing particles, without a radiator medium where light is produced. Devices with an active area of $8times8$ mm$^2$ were characterized in beam tests. The timing performance and signal properties were measured as a function of position on the detector using a beam telescope and a microchannel plate photomultiplier (MCP-PMT). Devices with an active area of $2times2$ mm$^2$ were used to determine the effects of radiation damage and characterized using a ps pulsed laser. These detectors were irradiated with neutrons up to $Phi_{eq}=10^{15}$ cm$^{-2}$.
We report operation and characterization of a lab-assembled single-photon detector based on commercial silicon avalanche photodiodes (PerkinElmer C30902SH, C30921SH). Dark count rate as low as 5 Hz was achieved by cooling the photodiodes down to -80 C. While afterpulsing increased as the photodiode temperature was decreased, total afterpulse probability did not become significant due to detectors relatively long deadtime in a passively-quenched scheme. We measured photon detection efficiency higher than 50% at 806 nm.
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