Ohm (unité) | EPFL Graph Search

L’ohm, de symbole Ω (la lettre capitale grecque oméga), est l'unité de résistance électrique du Système international (SI). Il a été nommé ainsi en l’honneur de Georg Ohm, physicien allemand à l'origine notamment de la loi d'Ohm. Pour caractériser les isolants électriques on utilise le kiloohm (ou kilo-ohm) et surtout le mégaohm, généralement renommés, par euphonie, kilohm et mégohm : = , = . Définition La résistance électrique entre deux points d'un conducteur est d'un ohm lorsqu'une différence de potentiel constante de , appliquée entre ces deux points, produit dans ce conducteur un courant de , ce conducteur n'étant le siège d'aucune force électromotrice. Un ohm correspond donc à un volt par ampère ( ou ), donc dans les unités de base du Système international à un kilogramme mètre carré par seconde cube et par ampère carré : Caractère Unicode Unicode a un caractère spécifique, le « caractère Ohm » Ω, codé par U+2126, bien que son aspect

Sondheimer oscillations as a probe of non-ohmic flow in WP2 crystals

Jonas De Jesus Diaz Gomez, Chunyu Guo, Philip Johannes Walter Moll, Jacopo Oswald, Matthias Carsten Putzke, Yi-Chiang Sun, Maarten Ruud van Delft

As conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm's law. Depending on the length scales of momentum conserving (l(MC)) and relaxing (l(MR)) electron scattering, and the device size (d), current flows may shift from ohmic to ballistic to hydrodynamic regimes. So far, an in situ methodology to obtain these parameters within a micro/nanodevice is critically lacking. In this context, we exploit Sondheimer oscillations, semi-classical magnetoresistance oscillations due to helical electronic motion, as a method to obtain l(MR) even when l(MR) >> d. We extract l(MR) from the Sondheimer amplitude in WP2, at temperatures up to T similar to 40 K, a range most relevant for hydrodynamic transport phenomena. Our data on mu m-sized devices are in excellent agreement with experimental reports of the bulk l(MR) and confirm that WP2 can be microfabricated without degradation. These results conclusively establish Sondheimer oscillations as a quantitative probe of l(MR) in micro-devices.

NATURE PORTFOLIO2021

Optimization of NH₃-MBE grown p-doped (Al)GaN layers and their implementation in long wavelength laser diodes and tunnel junctions

Marco Malinverni

Over the last two decades III-nitride optoelectronic devices have experienced an impressive evolution in terms of performance. However, their potential is far from being fully exploited. Although they offer bandgaps from the deep UV to the infrared spectral range, lasing is currently restricted to the 336¿536 nm wavelength region. Several challenges still need to be overcome. For example, the thermal stability of the InGaN active region becomes an issue in long-wavelength (¿ > 500 nm) laser diodes: high indium content (In > 25%) quantum wells (QWs) have been shown to markedly degrade when kept above 900 °C. For this reason, the growth temperature of the subsequent p-type waveguide and cladding layers must be sufficiently low to avoid QW degradation, which is accompanied by a reduction in the internal quantum efficiency. Thanks to the high vacuum environment, molecular beam epitaxy (MBE) allows growing these layers at a much lower temperature compared to metalorganic vapor phase epitaxy (MOVPE), and could be used to reduce the thermal budget on the active region, further extending the wavelength range. Thus the aim of this thesis is the realization of long wavelength laser diodes by combining high efficiency active regions grown by MOVPE with p-doped layers grown by ammonia-MBE. Hence, the p-type doping by ammonia-MBE is first scrupulously studied and optimized on 500 nm thick (Al)GaN layers. In order to comprehensively address the physical processes governing the doping efficiency, the role of the growth temperature, the V-III ratio, and the growth rate is investigated. In addition, the Mg cell configuration resulting in the best doping profile control and reproducibility is studied as well. Electrochemical capacitance-voltage profiling is extensively used to study the compensation mechanisms. The optimization leads to the achievement of layers exhibiting low dopant compensation (below 5%) up to high doping levels and resistivity values as low as 0.4 Ohm cm. These layers are then implemented on top of active regions grown by MOVPE and lasing is obtained for the first time in hybrid structures exhibiting state-of-the-art electrical characteristics. The threshold voltage is as low as 4.3 V in an index guided structure with 800x2 ¿m² ridge dimension designed for violet emission (¿ = 400 nm). These excellent current-voltage characteristics are achieved owing to the doping profile control in the cladding and contact layers which is confirmed by the low contact resistance value of 5E-4 cm². Lasing is also demonstrated beyond 500 nm, keeping state-of-the-art electrical characteristics, despite the reduced doping in the cladding layer. These devices also exhibit values for internal optical losses as low as 6 cm¿1. The growth of tunnel junctions is also attempted. Low specific resistance values are demonstrated in ammonia-MBE grown GaN tunnel homojunctions. The absence of hydrogen passivation and the reduced memory effects of MBE are considered to be crucial for the realization of these devices together with the stability of the doping at reduced growth temperatures. n-p-n structures featuring a tunnel junction in series with a p-n junction are then used to spread the current across light-emitting diodes. A device architecture featuring a buried tunnel junction to be used for current injection inmicro-LEDs or vertical cavity surface emitting lasers is also demonstrated and excellent current confinement properties are obtained.

EPFL2015

Full tungsten and platinum nanoprobes for electrically conducting scanning probe methods

Jürgen Brugger, Johannes Steen

We have developed hybrid AFM probes with SU-8 polymer body and full platinum and tungsten cantilever and tip. The fabrication process uses surface micromachining of a (100) silicon wafer. The metal (platinum or tungsten) is sputter deposited in oxidation sharpened moulds, giving a final tip sharpness smaller than 20 nm. A SU-8 body is subsequently formed. The release is either done by a wet back-side or dry top-side process. The measured hardness of the tungsten and platinum film values 14.8 GPa and 7.3 GPa, respectively. Contact resistances on a gold sample show 25 Ohm and 10 Ohm for the tungsten and platinum probes, respectively. The results are promising for the development of a robust electrically conducting probe.

2006

Optimization of NH₃-MBE grown p-doped (Al)GaN layers and their implementation in long wavelength laser diodes and tunnel junctions

Marco Malinverni

EPFL2015