Serge A. Charlebois

Magnesium diboride nanobridges fabricated by electron-beam lithography

MgB2 nanobridges were fabricated by e-beam lithography and Ar-ion beam milling. Nanobridges of widths ranging from 60 nm to 1 μm and 3 μm in length were realized by Ar-ion beam milling using amorphous carbon as etching mask. The processing did not harm the superconducting properties appreciably. High values of the critical current density, more than 10 MA cm2, were measured for bridges with widths down to 60 nm. Current-voltage (I-V) characteristics showed a behavior typical of a bridge going normal, after the critical current is exceeded, and remaining normal as the current is decreased to a lower switch back value due to Joule heating. We could also observe switching behavior in some bridges indicating formation of normal hotspots in the bridges before they returned to their superconducting state. Alternative explanations may include natural grain boundaries in the film or the movement of Abrikosov vortices. The current-voltage (I-V) characteristics showing critical current densities up to 5× 107 A/cm2 indicates excellent film properties in the nanobridges.

Dynamical effects of an unconventional current-phase relation in YBCO dc-SQUIDs

The predominant d-wave pairing symmetry in high-temperature superconductors allows for a variety of current-phase relations in Josephson junctions, which is to a certain degree fabrication controlled. In this Letter, we report on direct experimental observations of the effects of a nonsinusoidal current-phase dependence in YBCO dc SQUIDs, which agree with the theoretical description of the system.

Development of a single photon avalanche diode (SPAD) array in high voltage CMOS 0.8 µm dedicated to a 3D integrated circuit (3DIC)

We present the realization of Single Photon Avalanche Diode (SPA D) arrays for Positron Emission Tomography (PET). These SPAD arrays are designed in Teledyne BALSA High Voltage (HV) CMOS technology targeted for a 3 dimensional (3D) heterogeneous integration with deep-submicron CMOS readout electronics to realize a 3D Single Photon Counting Module (3DSPCM). We are developing a post-process Through Silicon Vias (TSV) technology at Universite de Sherbrooke to implement the 3D heterogeneous bonding. The 3D integration of SPAD and electronics reduces the SPAD interconnect parasitic capacitance, greatly increases the photosensitive area and improves overall performances. Also, SPAD are known for their excellent timing performance which enables Time of Flight capabilities in PET, significantly improving image contrast and spatial resolution. For this purpose, we designed, fabricated and characterized SPAD arrays with active quenching circuit using the HV CMOS 0.8 μm technology. The chosen structure is a p+ anode in an n-well using a p-well isolation layer to achieve 54 % of fill factor and takes full advantage of the 3D integration scheme. SPAD evaluation results show a Photodetection Efficiency (PDE) up to 49 % at 480 nm, Dark Count Rate (DCR) of 50 kcps, afterpulsing probability <;2 %, crosstalk probability <;1 %, and timing jitter of 100 ps at room temperature.

High-Resistivity Silicon Dielectric Ribbon Waveguide for Single-Mode Low-Loss Propagation at F/G-Bands

In this paper, dielectric ribbon waveguides (DRWs) are designed and characterized to work in the F (90-140 GHz) and G (140-220 GHz) frequency bands. High resistivity silicon (HR-silicon) due to its high transparency in these bands, and its low-cost fabrication process, were selected for the fabrication of these waveguides. The guided modes of DRW are studied using Marcatili and multi-line methods. A design procedure for optimal single-mode propagation was introduced. The wave attenuation inside the waveguide is discussed. Our experiments show that HR-silicon DRW has less than 0.087 dB/cm (0.01 cm-1) of loss over the guiding bandwidth. The designed DRWs were realized using microfabrication techniques. The measurements are in very good agreement with theoretical predictions.

Parallel plate waveguide with anisotropic graphene plates: Effect of electric and magnetic biases

The performances of a parallel plate waveguide (PPWG) supported by perfect electric conductor (PEC)-graphene and graphene-graphene plates are evaluated. The graphene plate behavior is modeled as an anisotropic medium with both diagonal and Hall conductivities derived from Kubo formula. The PPWG modes supported by PEC-graphene and graphene-graphene plates are studied. Maxwells equations are solved for these two waveguides, while the graphene layers are biased with an electric field only and with both electric and magnetic fields. It is shown that when both electric and magnetic biases are applied to the graphene, a hybrid mode (simultaneous transverse electric (TE) and transverse magnetic (TM) modes) will propagate inside the waveguide. The intensity of each TE and TM modes can be adjusted with the applied external bias fields. Study of different waveguides demonstrates that by decreasing the plate separation (d), the wave confinement improves. However, it increases the waveguide attenuation. A dielectric...

Implementation Study of Single Photon Avalanche Diodes (SPAD) in

Single Photon Avalanche Diodes (SPAD) are known for their excellent timing performance which enables Time of Flight capabilities in positron emission tomography (PET). However, current array architectures juxtapose the SPAD with its ancillary electronics at the expense of a poor fill factor of the SPAD array. The 3D vertical integration of SPADs and readout electronics represents a solution to the aforementioned problem. Compared to systems with external electronics readout, 3D vertical integration reduces the SPAD interconnect parasitic capacitance while greatly increasing the photosensitive area and improving overall performances. This paper presents the implementation of two SPAD structures designed for PET. The SPAD structures are designed using Teledyne DALSA high voltage (HV) CMOS technology targeted for a 3-dimensional single photon counting module (3DSPCM). SPAD with two types of guard ring (diffusion-based and virtual guard ring) are designed, fabricated and characterized. All structures are based on a p + anode in an n-well cathode and are implemented along with active quenching circuits for proper characterization. The results show that the contact distribution and the anode-cathode spacing impact the dark count rate (DCR). The design of SPADs with a diffusion guard ring have a DCR down to 3 s- 1μm-2 at room temperature, afterpulsing probability of , timing resolution of 27 ps FWHM and PDE of 49% at 480 nm.

0.8~\mu\hbox{m}

In this work we compare the characteristics of asymmetrically excited small-aperture antenna-type pulsed terahertz emitters fabricated using an ion implantation process. Our photoconductive materials consist of high resistivity GaAs substrates. Multienergy implantations of arsenic (1.2 and 2MeV) and oxygen (180, 450, and 700keV) have been used to obtain an almost uniform density of vacancies over the optical absorption depth in bulk GaAs substrates. Terahertz pulses are generated by exciting our devices with ultrashort laser pulses. Ion implantation followed by a thermal annealing process introduces nonradiative centers in our substrates which reduce the carrier lifetime and modify the shape of our terahertz pulses. Results obtained as functions of the laser excitation power and bias voltage are discussed and a comparison of the performance of these devices with conventional small-aperture antennas is given.

HV CMOS Technology

The 3D vertical integration of SPAD and readout electronics is a promising avenue to high performance photodetectors. This approach will minimize most limitations of current SiPM and lead to better performances in terms of effective PDE, timing and added functionalities. In this paper, we present a new integrated digital SiPM electronic architecture. This specific architecture aims to replace conventional analog SiPM with the added benefits of a significantly lower and constant output capacitance as well as immunity to SPAD to SPAD gain variation on the single photon resolution while providing an analog-like output signal waveform representing the sum of all triggered SPADs. This electronic architecture also offers options to turn off noisy SPADs, to reduce afterpulsing through a programmable hold-off time and to adjust the dynamic range of the output depending on the photon flux. While the 3D integration process development is underway, we realized a 2D version as a proof of principle of the electronic architecture. The 2D detector presented in this article is composed of an array of 242 pixels for a size of 1.1 × 1.1 mm2. The output capacitance of the device is 5.6 pF/mm2, the timing jitter associated to the quenching circuit is about 40 ps FWHM and the maximum timing skew within the array is about 70 ps, which could still be optimized.

Terahertz emission properties of arsenic and oxygen ion-implanted GaAs based photoconductive pulsed sources

In this paper, a tunable parallel plate waveguide with a metal plate and a sheet of graphene is presented. The anisotropic conductivity of the graphene sheet submitted to a magnetic field is modeled by a conductivity tensor in which the diagonal and Hall conductivities are derived from the Kubo formula. The Maxwell equations are solved assuming the graphene sheet been biased both with an electric and a magnetic fields. It is shown that in this case a hybrid mode propagates inside the waveguide. The amount of power which propagates in each TE and TM modes can be adjusted by the magnitude of applied fields. Considering the attenuation, the proposed waveguide demonstrate good characteristics in comparison to metal waveguides in the 0.4 to 1 THz frequency band.

A 2D Proof of Principle Towards a 3D Digital SiPM in HV CMOS With Low Output Capacitance

Most proposed realizations of a high temperature superconductor (HTS) qubit require the use of very small Josephson junctions. The properties of bicrystal junctions are especially interesting since they make it possible to implement several types of flux qubits in a relatively simple way. We have developed a technique that allows us to produce high quality sub-micrometer junctions in a reproducible way using bicrystal technology. We have successfully fabricated and characterized a large number of YBCO junctions and SQUIDs with bridge width as small as 0.2 micrometer on 0/spl deg/-3/spl deg/, 0/spl deg/-40/spl deg/ and 0/spl deg/-45/spl deg/ bicrystal STO substrates. The properties of these junctions have been extensively examined at temperatures down to 20 mK. The effects of external magnetic fields on these structures have been investigated. Figures of merit for the proposed qubits were also extracted from these measurements.