S. Chenot

French National Centre for Scientific Research, Lutetia Parisorum, Île-de-France, France

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Publications (66)85.94 Total impact

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    ABSTRACT: AlxGa1−xN-based ultra-violet (UV) light emitting diodes (LEDs) are seen as the best solution for the replacement of traditional mercury lamp technology. By adjusting the Al concentration, a large emission spectrum range from 360 nm (GaN) down to 200 nm (AlN) can be covered. Owing to the large density of defects typically present in AlxGa1−xN materials usually grown on sapphire substrates, LED efficiencies still need to be improved. Taking advantage of the 3D carrier confinement, quantum dots (QDs) are among the solutions currently under investigation to improve the performances of UV LEDs. The objectives of this work are to present and discuss the morphological and optical properties of GaN nanostructures grown by molecular beam epitaxy on the (0 0 0 1) and the (11–22) orientations of Al0.5Ga0.5N. In particular, the dependence of the morphological properties of the nanostructures on the growth conditions and the surface orientation will be presented. The optical characteristics as a function of the nanostructure design (size, shape and dimensionality) will also be shown and discussed. The electroluminescence characteristics of a first series of QD-based GaN/Al0.5Ga0.5N LEDs grown on the polar (0 0 0 1) plane will be investigated.
    Semiconductor Science and Technology 06/2014; 29(8):084001. · 1.92 Impact Factor
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    ABSTRACT: In the present paper, we describe the development of new AlN seed layers obtained by combining molecular beam epitaxy and low temperature physical vapour deposition (magnetron sputtering). It is shown that it is possible to grow thick AlN seed layers with a good in-plane crystal ordering. GaN based structures on silicon can then be regrown with device quality active layers, as attested by the realization of high electron mobility transistors. Furthermore, the low substrate bowing achieved with these structures is of high interest for the fabrication of large GaN-on-silicon wafers. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2014;
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    ABSTRACT: (Al,Ga)N light emitting diodes (LEDs), emitting over a large spectral range from 360 nm (GaN) down to 210 nm (AlN), have been successfully fabricated over the last decade. Clear advantages compared to the traditional mercury lamp technology (e.g. compactness, low-power operation, lifetime) have been demonstrated. However, LED efficiencies still need to be improved. The main problems are related to the structural quality and the p-type doping efficiency of (Al,Ga)N. Among the current approaches, GaN nanostructures, which confine carriers along both the growth direction and the growth plane, are seen as a solution for improving the radiative recombination efficiency by strongly reducing the impact of surrounding defects. Our approach, based on a 2D - 3D growth mode transition in molecular beam epitaxy, can lead to the spontaneous formation of GaN nanostructures on (Al,Ga)N over a broad range of Al compositions. Furthermore, the versatility of the process makes it possible to fabricate nanostructures on both (0001) oriented "polar" and (11 2 2) oriented "semipolar" materials. We show that the change in the crystal orientation has a strong impact on the morphological and optical properties of the nanostructures. The influence of growth conditions are also investigated by combining microscopy (SEM, TEM) and photoluminescence techniques. Finally, their potential as UV emitters will be discussed and the performances of GaN / (Al,Ga)N nanostructure-based LED demonstrators are presented.
    02/2014;
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    ABSTRACT: Commercially available inorganic white light emitting diodes (LEDs) are essentially based on the combination of a blue InGaN based LED chip covered by a long wavelength emitting (yellow, red) phosphor. We propose to avoid this step of phosphor deposition by taking advantage of the fact that yellow to red emission can be achieved using InGaN alloys. By stacking an InGaN/GaN multiple quantum well (QW) emitting in the yellow, acting as a light converter, and a short wavelength blue-violet pump LED grown on top, white light emission can be obtained. Furthermore, if we extend the emission spectrum of the light converter into the red, a warm white light color is demonstrated when a pump LED is grown on top. However, the high In content InGaN QWs of the light converter have a low thermal stability and the QW efficiency tends to degrade during the growth of the pump LED. Three different solutions are explored to avoid the thermal degradation of the light converter. The monolithic LED structures were grown by molecular beam epitaxy (MBE), by a combination of both MBE and metal-organic chemical vapor phase epitaxy (MOCVD), or by a low temperature full-MOCVD process. The best results are obtained using a complete MOCVD growth process. The structure and the MOCVD growth conditions are specifically adapted in order to avoid the thermal degradation of the large In composition InGaN QWs emitting at long wavelength during the growth of the subsequent layers.
    02/2014;
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    SPIE Photonics West 2014, Moscone Center San Francisco, CA, United States; 02/2014
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    ABSTRACT: We report on the selective area growth of semipolar (11-22) GaN epilayers on wet etched r-plane patterned sapphire substrates (PSS) by metal organic chemical vapor deposition. Using a three-step growth method, planar (11-22) GaN epilayers on 2 in. wafers with significant optical and structural quality improvements have been obtained. The filtering of basal stacking faults and dislocations was achieved by overlapping adjacent crystals and forming voids between them. These voids act as a barrier to defect propagation which results in reduced defect density at the surface of the epilayer. Cathodoluminescence measurements at 80 K revealed a dislocation density of 5.1×107 cm−2 and a basal stacking fault density below 30 cm−1. Moreover, photoluminescence and X-ray diffraction measurements attested a material quality similar to conventional GaN on c-plane sapphire. Such large scale semipolar GaN templates are opening the way for efficient semipolar devices grown heteroepitaxially.
    Journal of Crystal Growth 01/2014; 404:177–183. · 1.55 Impact Factor
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    ABSTRACT: Hall effect and capacitance-voltage C(V) measurements were performed on p-type GaN:Mg layers grown on GaN templates by molecular beam epitaxy with a high range of Mg-doping concentrations. The free hole density and the effective dopant concentration NA−ND as a function of magnesium incorporation measured by secondary ion mass spectroscopy clearly reveal both a magnesium doping efficiency up to 90% and a strong dependence of the acceptor ionization energy Ea with the acceptor concentration NA. These experimental observations highlight an isolated acceptor binding energy of 245±25 meV compatible, at high acceptor concentration, with the achievement of p-type GaN:Mg layers with a hole concentration at room temperature close to 1019 cm−3.
    Applied Physics Letters 07/2013; 103(3). · 3.79 Impact Factor
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    ABSTRACT: We report magnetotransport measurements performed on AlGaN/GaN devices with different buffer layers. Standard samples with a 1 μm thick GaN buffer show a linear Hall resistance and an almost constant magnetoresistance, as expected from a single two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. Other samples, with an AlxGa1–xN buffer (x = 5%) and a buried linear aluminium gradient, have an additional three-dimensional electron slab (3DES) close to the GaN substrate. In this case, the Hall resistance is strongly non-linear and presents an incorrect hole-type carrier signature, evidenced by low field mobility spectrum analysis. This effect is strengthened when the 3D layer, parallel to the mesa-etched 2DEG, is infinite. We suggest that the misplacement of the electrical contacts in the 3DES, i.e., far from the sample edges, could explain the wrong carrier type determination.
    Journal of Applied Physics 07/2013; 114(2). · 2.21 Impact Factor
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    ABSTRACT: The vertical bulk (drain-bulk) current (I{sub db}) properties of analogous AlGaN/GaN hetero-structures molecular beam epitaxially grown on silicon, sapphire, and free-standing GaN (FS-GaN) have been evaluated in this paper. The experimental I{sub db} (25-300 Degree-Sign C) have been well reproduced with physical models based on a combination of Poole-Frenkel (trap assisted) and hopping (resistive) conduction mechanisms. The thermal activation energies (E{sub a}), the (soft or destructive) vertical breakdown voltage (V{sub B}), and the effect of inverting the drain-bulk polarity have also been comparatively investigated. GaN-on-FS-GaN appears to adhere to the resistive mechanism (E{sub a} = 0.35 eV at T = 25-300 Degree-Sign C; V{sub B} = 840 V), GaN-on-sapphire follows the trap assisted mechanism (E{sub a} = 2.5 eV at T > 265 Degree-Sign C; V{sub B} > 1100 V), and the GaN-on-Si is well reproduced with a combination of the two mechanisms (E{sub a} = 0.35 eV at T > 150 Degree-Sign C; V{sub B} = 420 V). Finally, the relationship between the vertical bulk current and the lateral AlGaN/GaN transistor leakage current is explored.
    Journal of Applied Physics 05/2013; 113(17). · 2.21 Impact Factor
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    ABSTRACT: GaN-based power switches are expected to play a key role in uncooled electronics at elevated temperatures. In this paper we explore the thermal activation mechanisms taking place in analogous AlGaN/GaN high electron mobility transistors grown on silicon and sapphire. The on-resistance (α = 1.4/1.8 [Si/sapphire]) and saturation current (α = −1.5/−1.8) temperature coefficients, the thermal activation energies (Ea = 0.02–0.30/0.30 eV), the drain current on/off ratio (α = −1.5–9.1/−9.4), or the thermal impedances (Rth = 76.9/125.8 K/W) were determined and comparatively analyzed by means of physical-based models which include polar-optical phonon scattering, Poole-Frenkel trap assisted and Schottky emission, and the channel self-heating.
    Applied Physics Letters 03/2013; 102(9). · 3.79 Impact Factor
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    ABSTRACT: We report on the study of AlGaN/GaN high electron mobility transistors (HEMTs) incorporating an InGaN back-barrier grown by ammonia-assisted molecular beam epitaxy (NH3-MBE). The structural characterizations confirm the good crystalline quality of the heterostructure and the pseudomorphic growth of InGaN. This is also verified by the Hall effect mobility of about 2130 cm2 V−1 s−1. The associated two-dimensional electron gas carrier concentration is in the range of 8 × 1012 cm−2 for heterostructures grown on GaN:Fe-on-sapphire templates. Normal DC transistor operation is observed. Similar results have been obtained on silicon substrates. To our knowledge, this is the first demonstration of AlGaN/GaN HEMTs with an InGaN back-barrier grown by NH3-MBE.
    physica status solidi (a) 03/2013; 210(3):480-483. · 1.21 Impact Factor
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    ABSTRACT: Emission of terahertz (THz) radiations from interdigitated GaN quantum-wells structures under DC-bias has been measured at room temperature. This measurements has been performed by a 4K Si-Bolometer associated with a Fourier Transform Spectrometer. Using an analytical model, we have shown that the observed peak at approximately 3 THz due to 2D ungated plasma-waves oscillations in the quantum well, is emitted by the metallic contacts of our device acting as antennas.
    Proc SPIE 03/2013;
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    ABSTRACT: The potential of GaN for X-ray detection in the range from 5 to 40 keV has been assessed. The absorption coefficient has been measured as a fonction of photon energy. Various detectors have been fabricated including MSM and Schottky diodes. They were tested under polychromatic X-ray illumination and under monochromatic irradiation from 6 to 22 keV in the Soleil synchrotron facility. The vertical Schottky diodes perform better as their geometry is better suited to the thick layers required by the low absorption coefficient. The operation mode is discussed in terms of photoconductive and photovoltaic behaviors. Some parasitic effects related to the electrical activation of defects by high energy photons and to the tunnel effect in lightly doped Schottky diodes have been evidenced. These effects disappear in diodes where the doping profile has been optimized. The spectral response is found to be very consistent with the spectral absorption coefficient. The sensitivity of GaN Schottky diodes is evaluated and found to be on the order of 40 photons per second. The response is fast nd linear.
    Proc SPIE 03/2013;
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    ABSTRACT: The stress distribution on crack free thick continuous GaN film (12 µm) grown by Metal organic chemical vapour deposition (MOCVD) on the arrays of different sizes of the patterned silicon substrate is investigated by micro-Raman (µRaman) spectroscopy. On the largest crack free mesa (400 μm) both µ-photoluminescence (µPL) at low temperature and µRaman measurements are performed. The µRaman shift of the GaN E2 mode shows the U-shape in-plane stress distribution across the mesa. The center of the mesa is under tensile stress and it relaxes near the corner and edges. A similar trend is observed also from the µPL spectra. The size of the mesa, the trench height and the trench width of the patterned silicon are varied to study the stress of the thick epitaxial crack free GaN layer. The size and trench height of the mesa have a large influence on the GaN film stress but the trench width does not show any significant effect. The maximum stress is saturated for the large sizes of mesas. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2013; 10(3):425-428.
  • Applied Physics Letters 01/2013; 103:269904. · 3.79 Impact Factor
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    ABSTRACT: Taking advantage of the strain-induced 2-dimensional (2D)–3D “Stransky–Krastanov type” growth mode of GaN on AlxGa1−xN, we report on the fabrication of ultraviolet (UV) light emitting diodes (LEDs) using GaN quantum dots (QDs) as emitters. The structures have been grown by molecular beam epitaxy on sapphire (0001) substrates. GaN QDs, with density ∼8×1010 cm−2, are formed on Al0.5Ga0.5N layers. The electroluminescence (EL) spectrum is dominated by a blue–violet emission (400–430 nm) at very low injection currents (≤2 A/cm2). At currents above 10 A/cm2, a UV emission (<390 nm) is observed. An additional peak, at wavelength ∼314 nm, originates from the EL emission from the GaN wetting layer. For increasing current, we observe a large shift (∼300 meV) towards higher energies and a reduction by one third of the full width at half maximum of the EL peak. Furthermore, the appearance of an additional peak on the EL high energy side is observed. These properties are governed by the quantum confined Stark effect and band-filling in the QDs. AlxGa1−xN-based QD-LED performances (optical power, external quantum efficiency) are presented and discussed in correlation to the specific 3D localization of excitons in the QDs.
    Journal of Crystal Growth 01/2013; 363:282–286. · 1.55 Impact Factor
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    ABSTRACT: The objective of this paper is to analyze the impact of the high temperature on the on-state and the reverse leakage currents (gate, drain and bulk) of AlGaN/GaN HEMTs grown on Si(1 1 1) in the temperature range of 25–310 °C. After intensive testing, it has been observed that the (drain and gate–to-source) leakage currents exhibit a weak increase with the temperature up to 150 °C. The leakage through the silicon substrate has a primordial role, as the drain reverse leakage appears to be due to electron injection from the Si substrate/AlN/GaN buffer layers into the GaN buffer. At higher temperatures, the activation energy (Ea), extracted from Arrhenius plots, is much higher being Ea = 0.20 eV and Ea = 0.40 eV for drain and gate leakage currents, respectively. We suggest that this relevant thermal activation is related to temperature-dependent thermionic currents over different barriers.
    Microelectronics Reliability 11/2012; 52(11):2547–2550. · 1.14 Impact Factor
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    ABSTRACT: The saturation drain current and the gate saturation transconductance for AlGaN/GaN on silicon (1 1 1) high-electron mobility transistors (HEMTs) have been experimentally investigated in the temperature range of 25–300 °C. An analytical physical-based closed-form is proposed for modeling the gate transconductance taking into account the polar-optical phonon scattering of the electrons in the two-dimensional electron gas HEMT channel. It is suggested that the experimental temperature dependence of T−1.1 is due to the electron channel mobility dependence coupled with the effect of the access resistances and the channel self-heating due to power dissipation.
    Semiconductor Science and Technology 10/2012; 27(12):125010. · 1.92 Impact Factor
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    ABSTRACT: AlGaN/GaN HEMTs are devices which are strongly influenced by surface properties such as donor states, roughness or any kind of inhomogeneity. The electron gas is only a few nanometers away from the surface and the transistor forward and reverse currents are considerably affected by any variation of surface property within the atomic scale. Consequently, we have used the technique known as conductive AFM (CAFM) to perform electrical characterization at the nanoscale. The AlGaN/GaN HEMT ohmic (drain and source) and Schottky (gate) contacts were investigated by the CAFM technique. The estimated area of these highly conductive pillars (each of them of approximately 20-50 nm radius) represents around 5% of the total contact area. Analogously, the reverse leakage of the gate Schottky contact at the nanoscale seems to correlate somehow with the topography of the narrow AlGaN barrier regions producing larger currents.
    Nanotechnology 09/2012; 23(39):395204. · 3.84 Impact Factor