Diode | EPFL Graph Search

vignette|Différents types de diodes. La diode (du grec di deux, double ; odos voie, chemin) est un composant électronique. C'est un dipôle non linéaire et polarisé (ou non symétrique). Le sens de branchement d'une diode détermine le fonctionnement du circuit électronique dans lequel elle est placée. Sans précision ce mot désigne un dipôle qui ne laisse passer le courant électrique que dans un sens. Ce dipôle est appelé diode de redressement lorsqu'il est utilisé pour réaliser les redresseurs qui permettent de transformer le courant alternatif en courant continu. Généralités Pour aborder l'électricité, on fait souvent appel à une analogie hydraulique. Si on compare l'électricité à un liquide circulant dans une canalisation, la diode est un clapet anti-retour. Dans un sens, à partir d'une certaine pression du fluide, le clapet laisse passer le fluide (analogie avec la tension de seuil). dans l'autre sens, le fluide ne fera pas ouvrir le clapet, sauf si la pression est trop

Electronic transport properties of vertical and lateral graphene-based heterostructures

Roberto Urcuyo Solorzano

Graphene has some drawbacks, like the absence of an electronic band gap which leads to only small gate switching ratios, and its fast carrier recombination that limits its use in optoelectronics. In order to overcome these issues, the aim of this thesis was to tune the properties of graphene by two different modification approaches. The first approach involved chemical functionalization to create lateral graphene heterostructures, whereas the second approach was directed toward vertical heterostructures that combine graphene with another 2D material. For the former case, two novel chemical functionalization methods were investigated. The first method relied upon hyperthermal molecular ion collisions with 4,4¿-azobis(pyridine). This enabled the covalent functionalization of graphene with a functionalization degree of up to 3%, as demonstrated by Raman in combination with XPS. AFM studies revealed that the functionalized sheets retain their topographic integrity. Thus obtained stripe-like patterns of covalently graphene within extended graphene sheets enabled enhanced on/off ratios upon gate switching. In the second functionalization, OsO4 was used to selectively introduce hydroxyl groups to graphene. Best results were obtained using UV light activation, which yielded graphene of high covalent functionalization degree, as concluded from its optical transparency and gate-induced on/off switching ratios of up to 500. Temperature dependent electrical measurements revealed 2D hopping as the dominant transport mechanism. The first type of vertical heterostructure, gr-TiOx-Ti diodes were fabricated and their electrical properties in the dark and under visible light illumination studied. In contrast to conventional MIM diodes, the performance of the graphene-based diodes was found to increase with decreasing thickness of the oxide insulator. Bias-induced modulation of the work function of graphene was identified as the key to the operation mechanism of the graphene diodes, enabling them to reach a very high asymmetry and nonlinearity (9000 and 8). The diodes compete well with state-of-the-art MIM diodes. Furthermore, the graphene-TiOx-Ti diodes could be operated as photovoltaic cell, with a maximum open-circuit voltage of 0.3 V and a short-circuit photocurrent of 14 nA under global illuminationt. This finding constitutes the first proof-of-principle of hot carrier extraction from graphene, based on the photovoltaic effect. The second type of vertical heterostructures was based on black phosphorus (BP). As a first step, thin BP sheets were combined with n-GaAs into novel pn devices. They displayed pronounced rectification behavior that, in the low bias regime, approaches that of ideal diodes. Moreover, when operated as photodiodes they reached external quantum efficiencies (EQE) above 30% under higher reverse bias. On this basis, the bP was then combined with gr into a p+/p heterojunction. Thus obtained devices achieved a maximum internal quantum efficiency (IQE) and responsivity of 13% and 10.7 mA/W, respectively. This IQE is the highest thus far reported for diodes comprising BP and another 2D material. Moreover, with the aid of a top gate it was possible to modulate both, the photo-conversion efficiency and photocurrent generation mechanism of the black phosphorous-graphene diodes. Together with the possibility to tune the photocurrent response by the thickness of the black phosphorous, these diodes emerge as promising photodetectors.

EPFL2016

Experiments and modelling for glow discharge plasmas applied to niobium sputter deposition in superconducting radiofrequency cavities

Thibaut François Hugo Richard

Thin film coatings are ubiquitous in modern daily lives, from the semi-conductor industry to aesthetic applications, and rely on the modification of a given material to enhance its surface properties. At CERN (European Organization for Nuclear Research), thin film coatings are used for several applications related to the accelerator technology field, such as electron cloud mitigation, continuous pumping or radio-frequency (RF) superconductivity. This PhD thesis aims at studying the use of a Particle-in-Cell Monte Carlo/ Direct Simulation Monte Carlo (PICMC/DSMC) numerical code applied to the modelling of deposition processes. The present study focuses on niobium thin film deposition on copper RF cavities of peculiar geometries and large sizes. Numerical simulations can help to understand and improve process parameters in existing systems, and guide the design of future coating systems such that thin ï¬lm characteristics can be predicted and expensive times of Research and Development can be reduced. In the first part of this thesis, critical parameters are identified to provide an accurate modelling of glow discharge plasmas commonly used in coating applications with the PICMC module of the numerical code. A dedicated experimental system is designed to enable both DC diode and DC magnetron operation with a coaxial cylindrical plasma source. Suitable ion-induced secondary electron emission yield and energy distribution are numerically assessed by matching simulated discharge voltages and currents with experimental ones. Simulated local plasma parameters, such as electron density and energy, are compared with experimental measurements obtained with a Langmuir probe. In the second part, transport simulations of sputtered neutral niobium atoms performed with the DSMC method are validated by comparing simulated deposition rates on a substrate with experimental ones with a compact hollow cathode magnetron sputtering source. At last, the validated ab initio methodology for coating process modelling, from plasma simulation and extraction of sputtering profiles to sputtered niobium atoms transport, is applied to typical coating systems used at CERN. The scalability of low discharge power simulation results to realistic powers is first demonstrated for a planar magnetron source in terms of cathode erosion and deposited thin film thickness profiles. Then, an elliptical RF cavity is used as a case study to apply the full methodology in a real substrate of complex shape. PICMC and DSMC simulation results are further extended towards thin film growth modelling, and simulated thin film morphology is compared with experimental Focused Ion Beam/Scanning Electron Microscopy imaging of the niobium layer.

EPFL2019

Ultra-Low-Temperature Silicon and Germanium-on-Silicon Avalanche Photodiodes

Mahdi Aminian

In this thesis we propose the use of photodiodes fabricated in planar technologies to address the detection problem in these applications. A number of solutions exist, optimized for these wavelengths, based on Germanium (Ge) and other III-V materials. In this thesis we focused on Ge thanks to its versatility and ease to use in the clean room. The main advantage of this material is in fact a good compatibility with Silicon and standard CMOS processes. Note that the proposed technology is not based on Silicon/Germanium (SiGe), whereby Ge is used to strain Si to achieve higher bandwidth in Si, not higher sensitivity. In our pure Ge approach, Ge is grafted onto Si (Ge-on-Si), achieving high responsivity at wavelengths of 900nm and higher. The proposed devices can operate in avalanche mode (avalanche photodiodes - APDs), and in Geiger mode (Geiger mode APDs (GAPDs) or single-photon avalanche diodes (SPADs)). To combine the advantages of Ge with single-photon sensitivity and excellent timing resolution of Si-based SPADs, this thesis proposes a new generation of SPADs, achieved in collaboration with Prof. Nanver at TUDelft, aimed at near-infrared range. The fabrication process of the Ge-on-Si SPAD approach, which we are investigating together with the TUDelft group, consists of a standard CMOS process combined with post-processing steps to grow Ge on top of a Si/SiO2 layer. In our study we have investigated the potential for a new generation of massively parallel, Ge-on-Si sensors fabricated in fully CMOS compatible technology. The objective was to address the next challenges of super-parallel pixel arrays, while exploiting the advantages of Ge substrate. The key technology developed in the thesis is a selective chemical-vapor deposition (CVD) epitaxial growth. A novel processing procedure was developed for the p+ Ge surface doping by a sequence of pure-Ga and pure-B depositions (PureGaB). The resulting p+n diodes have exceptionally good I-V characteristics with ideality factor of ~1.1 and low saturation currents. They can be operated both in proportional and in Geiger mode, and exhibit relatively low dark counts. We also looked at techniques to improve red and infrared sensitivity in conventional deep-submicron CMOS processes, by careful selection of standard layers at high depths in the Si substrate. Using the proposed approach, 12 µm-diameter SPADs were fabricated in 0.18µm CMOS technology showing low dark count rates (363 cps) at room temperature and considerably lower rates at cryogenic temperatures (77 K), while the FWHM timing jitter is as low as 76 ps. That of cryogenic SPADs is a novel research direction and in this thesis it was advocated as a significant trend for the future of optical sensing, especially in mid-infrared wavelengths. Low temperature characterizations reported in this thesis exposed how the relevant properties of fabrication materials, such as strength, thermal conductivity, ductility, and electrical resistance are changing. One of the most important properties is superconductivity in materials cooled to extreme temperatures: this is an important trend that will be pursued in the future activities of our group.

EPFL2014