La diode Schottky en diamant : le présent et le futur

International audienceDes diodes Schottky en diamant avec une structure pseudo-verticales montrant une densité de courant de 10^3A/cm2 (à 6V) avec un champ de claquage supérieur à 7.7MV/cm ont été réalisées. Ces diodes ont été obtenues par croissance homoépitaxiale du diamant avec le zirconium comme métal de contact Schottky. Ces résultats ont permis d'avoir le record mondial actuel du facteur de Baliga pour le diamant avec 244MV/cm^2. Ces travaux montrent que les potentialités du diamant ne sont pas uniquement théoriques. La maitrise actuelle des interfaces et de la croissance permettent d'imaginer des composants de puissance performant en poussant le diamant dans ses retranchements

Diamond power devices: State of the art, modelling and figures of merit

With its remarkable electro-thermal properties such as the highest known thermal conductivity (~22W/cmbold dotK at room temperature) of any material, high hole mobility (> 2000cm2/Vbold dots), high critical electric field (>10MV/cm), and large bandgap (5.47eV), diamond has overwhelming advantages over silicon and other wide bandgap semiconductors (WBG) for ultra-high- voltage and high temperature applications (>3kV and >450 K, respectively). However, despite their tremendous potential, fabricated devices based on this material have not delivered yet the expected high-performance. The main reason behind this is the absence of shallow donor and acceptor species. The second reason is the lack of consistent physical models and design approaches specific to diamond-based devices that could significantly accelerate their development. The third reason is that the best performances of diamond devices are expected only when the highest electric field in reverse bias can be achieved, something that has not been widely obtained yet. In this context, high temperature operation and unique device structures based on the 2DHG formation represent two alternatives which could alleviate the issue of the incomplete ionization of dopant species. Nevertheless, ultra-high temperature operations and device parallelization could result in severe thermal management issues and affect the overall stability and long-term reliability. Additionally, problems connected to the reproducibility and the long-term stability of 2DHG based-devices still need to be resolved. This review paper aims at addressing these issues by providing the power device research community with a detailed set of physical models, device designs and challenges associated to all the aspects of the diamond power device value chain, from the definition of figures of merits, the material growth and processing conditions, to packaging solutions and targeted applications. Finally, the paper will conclude with suggestions on how to design power converters with diamond devices and will provide the roadmap of diamond devices development for power electronics.This work was supported by the U.K. Engineering and Physical Sciences Research Council for the University of Cambridge Centre for Doctoral Training under Grant EP/M506485/1 and by the French ANR Research Agency under grant ANR-16-CE05-0023 #Diamond-HVDC. The research leading to these results has been performed within the GREENDIAMOND project and received funding from the European Community's Horizon 2020 Programme (H2020/2014–2020) under grant agreement no. 640947

Oxygen vacancy and EC − 1 eV electron trap in ZnO

International audienceFourier transform deep level transient spectroscopy has been performed between 80 and 550 Kin five n-type ZnO samples grown by different techniques. The capture cross section andionization energy of four electron traps have been deduced from Arrhenius diagrams. A trap1 eV below the conduction band edge is systematically observed in the five samples with alarge apparent capture cross section for electrons (1.6 ± 0.4 × 10−13 cm2) indicating a donorcharacter. The assignment of this deep level to the oxygen vacancy is discussed on the basis ofavailable theoretical predictions

High conductivity in Si-doped GaN wires

4 pagesInternational audienceTemperature-dependent resistivity measurements have been performed on single Si-doped GaN microwires grown by catalyst-free metal-organic vapour phase epitaxy. Metal-like conduction is observed from four-probe measurements without any temperature dependence between 10 K and 300 K. Radius-dependent resistivity measurements yield resistivity values as low as 0.37 mohm.cm. This is in agreement with the full width at half maximum (170 meV) of the near band edge luminescence obtained from low temperature cathodoluminescence study. Higher dopant incorporation during wire growth as compared to conventional epitaxial planar case is suggested to beresponsiblefortheuniqueconductivity

Zr/oxidized diamond interface for high power Schottky diodes

International audienceHigh forward current density of 103 A/cm2 (at 6 V) and a breakdown field larger than 7.7 MV/cm for diamond diodes with a pseudo-vertical architecture, are demonstrated. The power figure of merit is above 244 MW/cm2 and the relative standard deviation of the reverse current density over 83 diodes is 10% with a mean value of 10 9 A/cm2. These results are obtained with zirconium as Schottky contacts on the oxygenated (100) oriented surface of a stack comprising an optimized lightly boron doped diamond layer on a heavily boron doped one, epitaxially grown on a Ib substrate. The origin of such performances are discussed

Doping-induced metal-insulator transition in aluminum-doped 4H silicon carbide

International audienceWe report an experimental determination of the doping-induced metal-insulator transition in aluminum-doped 4H silicon carbide. Low temperature transport measurements down to 360 mK and temperature dependent Raman experiments down to 5 K, together with secondary ion mass spectroscopy profiling, suggest a critical aluminum concentration lying between 6.4 and 8.7 1020 cm−3 for the metal-insulator transition in these epilayers grown by the vapor-liquid-solid technique. Preliminary indications of a superconducting transition in the metallic sample are presented

Hole injection contribution to transport mechanisms in metal/p− /p++and metal/oxide/p− /p++ diamond structures

International audienceHeterostructures such as Schottky diodes and metal/oxide/semiconductor structures are the building blocks of diamond electronic devices. They are able to carry large current densities, up to several kA/cm$^2$, if a heavily boron doped layer (p$^{++}$) is included in the semiconducting stack, thus affording a metallic reservoir of mobile holes close to the lightly doped layer (p$^{-}$). In this work, hole injection effects are evidenced experimentally in the two previously mentioned devices and also simulated numerically. Although the potential barrier height at metal/semiconductor interfaces is a fundamental parameter, a more general approach consists in defining the current density from the product of an effective velocity and carrier concentration at interface. In accordance with experimental results, such a view permits to describe both depletion and accumulation regimes, which indeed can exist at the metallic or oxide interface, and to take into account the increase of the hole concentration above the thermal equilibrium one in the p$^{-}$ layer. The lower the temperature, the larger is this second effect. For sufficiently thin p$^{-}$ layers, typically below 2~$\mu$m, this effect frees device operation from the limitation due to incomplete ionization of acceptors and allows a strong decrease of the specific resistance and forward losses while preserving breakdown voltages in the range 1.4 to 2 kV

Propriétés électriques du ZnO monocristallin

L oxyde de zinc ZnO, est un semiconducteur II-VI très prometteur pour les applications en opto-électronique dans le domaine UV, notamment pour la réalisation de dispositifs électroluminescents (LED). Les potentialités majeures du ZnO pour ces applications résident notamment dans sa forte liaison excitonique (60 meV), sa large bande interdite directe (3.4 eV), la disponibilité de substrats massifs de grand diamètre ainsi que la possibilité de réaliser des croissances épitaxiales de très bonne qualité en couches minces ou nano structurées (nanofils). Néanmoins, le développement de ces applications est entravé par la difficulté de doper le matériau de type p. L'impureté permettant d'obtenir une conductivité électrique associée à des porteurs de charges positifs (trous), et donc la réalisation de jonctions pn à base de ZnO, n'a pas encore été réellement identifiée. C'est pourquoi une des étapes préliminaires et nécessaires à l'obtention d'un dopage de type p fiable et efficace, réside dans la compréhension du dopage résiduel de type n, ainsi que des phénomènes de compensation et de passivation qui sont mis en jeu au sein du matériau. La maîtrise de la nature des contacts (ohmique ou Schottky) sur différentes surfaces d'échantillons de ZnO nous a permis dans ce but de mettre en œuvre à la fois des mesures de transport (résistivité et effet Hall) et des mesures capacitives (capacité-tension C(V), Deep Level Transient Spectroscopy (DLTS) et Spectroscopie d'admittance).Dans un premier temps, nous avons donc cherché à comprendre de manière approfondie les propriétés électriques du ZnO massif. Nous avons ainsi étudié le rôle des défauts profonds et peu profonds sur la conductivité des échantillons, aux travers de différents échantillons massifs obtenus par synthèse hydrothermale ou par croissance chimique en phase vapeur. Nous avons également étudié l'impact de la température de recuits post-croissance, sur les propriétés de transport des échantillons. A la lumière des résultats obtenus sur le dopage résiduel de type n des échantillons de ZnO massifs, nous avons ensuite procédé à différents essais de dopage de type p du ZnO par implantation ionique d'azote et par diffusion en ampoule scellée d arsenic. L'impureté azote a été choisie dans le cadre d'une substitution simple de l'oxygène qui devrait permettre de créer des niveaux accepteurs dans la bande interdite du ZnO. Nous avons également étudié l'impureté arsenic, qui selon un modèle théorique peut former un complexe qui permet d'obtenir un niveau accepteur plus proche de la bande de valence que le niveau. Outres les études réalisées sur les échantillons de ZnO massif et les essais de dopage de type p, nous avons également étudié les propriétés électriques d'échantillons de ZnO monocristallins sous forme de couches minces obtenues par croissance en phase vapeur d organométalliques, dopées intentionnellement ou non. Les corrélations entres les mesures SIMS et C(V) nous ont permis notamment de mettre en évidence une diffusion et un rôle très importante de l'aluminium sur les propriétés électriques des couches minces de ZnO épitaxiées sur substrat saphir.Dans le cadre de cette thèse nous avons réussi à clarifier les mécanismes du dopage de type n, intentionnel ou non intentionnel, dans le ZnO monocristallin. Nous avons également identifié les impuretés et les paramètres de croissance importants permettant d'obtenir un dopage résiduel de type n le plus faible possible dans les couches épitaxiées. Cette maitrise du dopage résiduel de type n est une étape préliminaire indispensable aux études de dopage de type p car elle permet de minimiser la compensation des accepteurs introduits intentionnellement. Cette approche du dopage sur des couches minces de ZnO dont le dopage résiduel de type n est très faible apparait comme une voie très prometteuse pour surmonter les problèmes d'obtention du dopage de type p.Zinc oxide (ZnO) is a II-VI semiconductor which appears as a very promising material for UV opto-electronic applications, in particular for the production of light emitting devices (LED). For these applications, ZnO presents strong advantages as a high exciton binding energy (60 meV ), a wide direct band gap (3.4 eV), the availability of large diameter bulk substrates for homoepitaxial growth of high quality thin films or nanostructures. However, the development of these applications is hampered by the difficulty to dope ZnO p-type. The impurity leading to an electrical conductivity associated with positive charge carriers (holes), and therefore the production of ZnO pn junctions have not yet been really identified.In this thesis we have studied the physical mechanisms that govern the electrical properties of ZnO single crystal and epilayers. The control of contacts (ohmic or Schottky) on different ZnO surfaces allowed us to carry out both transport measurements (resistivity and Hall effect) and capacitance measurements (C(V), Deep Level Transient Spectroscopy (DLTS) and admittance spectroscopy).At first, we have studied the role of deep and shallow defects on the n-type conductivity of bulk ZnO samples obtained by Hydrothermal synthesis (HT) or by Chemical Vapor Transport (CVT). We also investigated the impact of post-growth annealing at high temperature under oxygen atmospheres on the transport properties of samples. Thanks to the previous results on the residual n-type doping, we have reported on several attempts to obtain p-type ZnO. We have discussed the potential of different candidates for the achievement of p-type doping and present our tentative experiments to try and demonstrate the reality, the ability and the stability of p-type doping by nitrogen implantation and arsenic diffusion. The nitrogen impurity has been chosen for oxygen substitution, which should allow the creation of acceptor levels in the ZnO band gap. We also studied arsenic as a potential p-type dopant, according to a model whereby arsenic substitutes for oxygen and, if associated with two zinc vacancies, forms a complex with a shallower ionization energy than in the case of direct oxygen substitution.In addition to the studies on bulk ZnO samples and attempts on p-type doping, we have also studied the electrical properties of thin film ZnO samples obtained by Metal Organic Vapor Phase Epitaxy, either intentionally or unintentionally doped. Correlations between SIMS and C(V) measurements allowed us to highlight especially the importance of aluminum as a residual impurity in epitaxial layers grown on sapphire substrates.In this thesis we have clarified intentional or unintentional n-type doping mechanisms in ZnO single crystal samples. We have also identified impurities and growth parameters responsible for the residual n-type doping. This understanding is a crucial and preliminary step for understanding the doping mechanisms at stake in this material and is also necessary to achieve stable p-type conductivity, which is still the main challenge for the realization of optoelectronic devices based on ZnO.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Strain relaxation in GaN grown on vicinal 4H-SiC(0001) substrates

The strain of GaN layers grown by Metal Organic Chemical Vapor Deposition (MOCVD) on three vicinal 4H-SiC substrates (0, 3.4 and 8 offcut from [0001] towards [11-20] axis) is investigated by X-ray Diffraction (XRD), Raman Scattering and Cathodoluminescence (CL). The strain relaxation mechanisms are analyzed for each miscut angle. At a microscopic scale, the GaN layer grown on on-axis substrate has a slight and homogeneous tensile in-plane stress due to a uniform distribution of threading dislocations over the whole surface. The GaN layers grown on miscut substrates presented cracks, separating areas which have a stronger tensile in-plane stress but a more elastic strain. The plastic relaxation mechanisms involved in these layers are attributed to the step flow growth on misoriented surfaces (dislocations and stacking faults) and to the macroscopical plastic release of additional thermoelastic stress upon cooling down (cracks)

Thermoelectric and micro-Raman measurements of carrier density and mobility in heavily Si-doped GaN wires

International audienceCombined thermoelectric-resistivity measurements and micro-Raman experiments have been performed on single heavily Si-doped GaN wires. In both approaches, similar carrier concentration and mobility were determined taking into account the non-parabolicity of the conduction band. The unique high conductivity of Si-doped GaN wires is explained by a mobility µ=56 cm2 /V s at a carrier concentration n = 2.6 10^20 /cm 3. This is attributed to a more efficient dopant incorporation in Si-doped GaN microwires as compared to Si-doped GaN planar layers. (c) 2013 AIP Publishing LLC