In this work, the Ti/Al Ohmic contact quality on n-type gallium nitride (GaN) films has been studied as a function of different process parameters such as surface cleaning procedure, etching, thickness of the deposited layers or annealing conditions. GaN epilayers, with uniform doping concentration from 1 x 10(16) to 5.8 x 10(18)?at./cm3 were grown on sapphire or silicon substrates using AlN and/or AlGaN buffer layers. Electrical characterizations were made using circular transfer length method (cTLM) patterns with a four-probe equipment. Specific contact resistance (SCR) was then extracted from currentvoltage (IV) characteristics, for all the process conditions. Contact structures depending on experiment parameters were studied by means of (scanning) transmission electronic microscopy (STEM-TEM). Our results reveal that process parameters such as surface treatment have a lower impact than annealing temperature or metal thickness and annealing duration. Finally, SCR values of 1 x 10(-6)?O?cm2 can be reproducibly achieved. Moreover, good Ohmic contacts have been obtained on etched surfaces or on low-doped layers implanted with Si. This low value demonstrates a good Ohmic contact and this large parameter process window is of high interest for future device fabrication based on GaN (planar or mesa structures).

This work reports of investigation on the origin of the ohmic behavior for Ti/Al based contacts on n-GaN. Indeed, last publications tend to prove that the "nitrogen vacancy" theory is not confirmed by the experimental ground available. To find new answers, we first made electrical characterizations on some Al/Ti/n(+)-GaN contacts with different annealing conditions. Then, we performed SIMS depth profiles of annealed TiN/Ti/n(+)-GaN samples. Results developed in this paper tend to demonstrate that the ohmic behavior of Ti/Al based contact is mainly due to an increase of the doping concentration. Moreover, the high doping level can be explained by an in-diffusion of Ti and an exo-diffusion of Si elements close to the metal / semiconductor interface.

In this work, both “Schottky to Schottky” structure (STS) and pseudo-vertical Schottky barrier diode (pv-SBD) have been processed on GaN heteroepitaxially grown on sapphire or silicon by metal organic chemical vapor deposition (MOCVD) and characterized physically and electrically. Ni and Ti/Al were used to obtain respectively Schottky and Ohmic contacts using rapid thermal annealing (RTA). Adequate passivation steps and insertion of a resistive guard ring were also implemented in pv-SBD. The STS results, presented in this paper, evidence the impact of the substrate on the growth as well as all the progresses that have been done on GaN material quality. Furthermore, we demonstrate that high quality Schottky diodes can be obtained on GaN grown on sapphire by MOCVD. Indeed, ideality factors of 1.09 as well as a barrier height of 1.06 eV were obtained on pv-SBD devices that have a breakdown voltage over 600 V.

Ohmic contacts represent a major technological brick for the development of high power devices on Gallium Nitride. Al(200 nm) Ti(70 nm) metallization on n+-GaN, annealed at 650 °C, provides a “Specific Contact Resistivity” (SCR) in the range mid 10-5 Ω.cm², which is low enough for the main switching power applications. However, the Al-Ti metallic compound phases formed during the annealing step result from solid-solid reactions, which may lead to high stress and / or poor cohesion, possibly deleterious to contact reliability. In this work, we have investigated several configurations of Ti-Al-Si based contacts, aiming at favoring liquid-solid reactions and / or Si element diffusion, in order to get better SCR and / or morphology and cohesion of the metallic phase. Surprisingly, only contacts annealed at low temperature (450 °C) provide low contact SCR, comparable to that of Ti-Al only contact, but systematically higher.

This work reports on Differential Scanning Calorimetry (DSC) measurements performed on Ti-Al metallic layers stacks deposited on n+-GaN. The aim is to get better understanding of the mechanisms leading to ohmic contact formation during the annealing stage. Two exothermic peaks were found, one below 500 °C and the other one around 660 °C. They can be respectively attributed to Al3Ti and Al2Ti compounds formation. The locations of these peaks provide clear evidence of solid-solid reactions. Lowest contact resistance is well correlated with the presence of Al3Ti compound, corresponding to Al(200nm)/Ti(50nm) stoichiometric ratio. Subsequently, Al(200 nm)Ti(50 nm) stacks on n+-GaN were annealed from 400 °C to 650 °C. Specific Contact Resistivity (SCR) values stay in the mid 10-5 Ω.cm² range for annealing temperatures between 450 °C and 650 °C. Such low-temperature annealed contacts on n+-GaN may open new device processing routes, simpler and cheaper, in which Ohmic and Schottky contacts are annealed together. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Gallium nitride (GaN) is a promising material for power electronic devices. Due to GaN sensitivity to high temperature treatments, dopant activation, after ion implant, is one of the major critical steps to be overcome. An annealing cap layer is then mandatory during high temperature treatment to avoid degradations. In this work, cap layers, such as AlN and SiOx, were deposited on Si-implanted N-type GaN. Samples were annealed using both classical (FA) and rapid thermal (RTA) annealing for times ranging from 30s to 8h and temperatures from 1000 to 1150°C. Transmission Electron Microscopy has been done to observe the implanted layer structure. After cap layer removal, samples surface has been investigated through Atomic Force Microscopy measurements. Dopant activity was indirectly evaluated by Specific Contact Resistance (SCR) measurements. This work demonstrates that low SCR value (8.2×10−5Ωcm2) with low surface roughness (∼1nm) can be reached using RTA and an oxide cap layer. However, presence of hexagonal pits in GaN layer is difficult to avoid. Compromise between low SCR with low roughness value and low hexagonal pits density on the GaN surface must be found.

Ce travail de these porte sur la realisation d’une diode Schottky de puissance sur GaN epitaxie sur substrat saphir.La majeure partie du travail a ete consacree au developpement du contact ohmique. Differentes conditions de fabrication ont ete etudiees autour d’un contact base sur l’empilement titane-aluminium. De bonnes resistances specifiques de contact ont ete obtenues sur du GaN de type n+ dope in-situ (autour de 1x10-5 Ω.cm-2) ainsi que sur du materiau implante silicium et sur des couches dopees in-situ mises a jour apres gravure seche. Nous avons egalement etudie le contact Schottky, au travers d’une structure simple : « Schottky To Schottky ».Les resultats issus de ces etudes sur les contacts nous ont permis de realiser des diodes Schottky laterales et pseudo-verticales. Des hauteurs de barriere de plus de 1eV et des facteurs d’idealite de 1,05 ont pu etre mesures sur les diodes pseudo-verticales avec, pour certaines, une tension de claquage de 600V.

The main rectifier device structures for power electronics based on SiC and on GaN are compared and the main issues for each structure are evaluated in terms of performance and manufacturability. The driving volume markets for power electronics devices correspond to the systems working on 127, 240 and 400 V energy supply networks, setting the device voltage handling to 300, 600, and 1200V respectively. We have limited the scope hereafter to the 600 V typical target, for which SiC Schottky rectifiers are now commercially available from at least 3 sources. The key physical properties for any semiconductor material used as the active layer of a unipolar device for power electronics are the breakdown field and carriers mobility. The bulk values are very similar for SiC and GaN. Two main other key issues are related to quality of the ohmic and Schottky contacts. For the ohmic contacts, adequate solutions have been found for both SiC and GaN. Surprisingly, on hetero-epitaxial GaN layers on sapphire despite of the very high crystal defects density ( ≥ 109cm-2 ), the ideality factor of the best Schottky contacts seems very promising. On the other hand, improving this ideality factor and the reverse leakage current for Schottky contacts on GaN layers grown on silicon substrate remains a fierce challenge. For the SiC Schottky rectifiers, cost and availability of the SiC substrates appear as the main residual limiting factors today. For GaN based rectifiers, although engineering device prototypes have already been published [1], there are both basic issues to be validated regarding reverse leakage current and reliability, and also difficult manufacturing issues to be solved in relation with device reliability, directly resulting from the nature of the possible substrates: mainly sapphire and silicon.

In this work, Schottky barrier diodes (SBD), made using lift-off process, were realized on low doped n-type GaN grown by MOCVD. Schottky to Schottky structures were first realized, allowing to select convenient process parameters that reduce the leakage current, such as surface cleaning, thickness of the metallic contact and annealing time or temperature. Then, planar Schottky diodes were patterned and characterized to extract barrier height and ideality factor. Results show that good rectifying behaviour can be obtained with a 300nm thick Ni Schottky contact annealed in RTA at 450 degrees C during 3 min under Argon. (c) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In this work, Ti/Al bilayer sputtered ohmic contacts on n-type Gallium Nitride films were studied as a function of process parameters such as Ti thickness, surface cleaning procedure or annealing temperature. Epilayers, with doping concentration of 5.8x1018 at.cm-3, were grown on sapphire using AlN buffer layer. Electrical characterizations were made using circular Transfer Length Method (cTLM) patterns with a four probes equipment. Specific Contact Resistance (SCR) was then extracted for all the process conditions. Our results show that surface treatment is not a critical step in the ohmic contact process while annealing temperature has a larger impact. Finally, SCR values of 1x10-5 Ω.cm2 can be reproducibly achieved, which is of high interest for future devices fabrication using this material.

In this paper, a new competition mechanism for neural fields is proposed, as well as first experimental studies of its robustness. The computational properties of this algorithm are discussed, arguing that such properties are suitable for neural architectures, where some restrictions of the usual neural fields competition methods are not acceptable.

In this paper (An abbreviated version of some portions of this article appeared in reference Menard and Frezza-Buet (Menard, O., & Frezza-Buet, H. (2004). Rewarded multi-modal neuronal self-organization: Example of the arm reaching movement. In: Proceedings of international conference on advances in intelligent systems theory and application.), as part of the IJCNN 2005 conference proceedings, published under the IEEE copyright.), an original self-organizing model is presented, with experiments highlighting its ability to be used in different frameworks, as phonetic coding dependent on semantics and arm-reaching. The model relies on the coupling of the learning processes that stand at different self-organizing modules, and exhibits dynamics that can be discussed in terms of the binding of different modalities, scattered over the different modules. Such a binding property is based on an emerging constraint of keeping consistency between the modules. This process is induced by partial connectivity and appropriate neural field competition mechanisms.

This paper presents a computational self-organizing model of multi-modal information, inspired from cortical maps. It shows how the organization in a map can be influenced by the same process occurring in other maps. We illustrate this approach on a phonetic – motor association, that shows that the organization of words can integrate motor constraints, as observed in humans.

This paper presents a computational model inspired from the cortex organization that defines coherent learning in multiple cortical maps, in order to perform multi-modal coordination. The joint self-organization of the maps is described as a process allowing dealing with multi-modal information within cooperative distributed modules. The very purpose of this approach is the design of a generic distributed architecture for robotic servo-control from a functional view of the cortical information processing.

This paper presents a computational self-organizing model of multi-modal information, inspired from cortical maps. It shows how the organization in a map can be influenced by the same process occurring in other maps. We illustrate this approach on a phonetic - motor association, that shows that the organization of words can integrate motor constraints, as observed in humans.

Résumé. — Si la Bretagne a longtemps connu des particularismes en matière de répression des crimes de fausse monnaie par la pratique du « bouilli puis pendu », au XVIIIe siècle, l'ancienne spécificité bretonne n'est plus une réalité dans un royaume unifié. Toutefois, malgré la pluralité des juridictions compétentes et l'atteinte au principe de la peine fixe, la sévérité des peines reste de rigueur, même si la peine de mort n'est plus systématique.

We present a cortically-inspired self-organizing model designed to deal with sensory-motor applications, and very first experimental results. This model contains several maps and produces resonance effects between these maps in order to reach an organization compromise.