F. H. Julien

Institute of Fundamental Electronics, Orsay, Île-de-France, France

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Publications (229)472.59 Total impact

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    ABSTRACT: We report a systematic experimental and theoretical investigation of core-shell InGaN/GaN single wire light-emitting diodes (LEDs) using electron beam induced current (EBIC) microscopy. The wires were grown by catalyst-free MOVPE and processed into single wire LEDs using electron beam lithography on dispersed wires. The influence of the acceleration voltage and of the applied bias on the EBIC maps was investigated. We show that the EBIC maps provide information both on the minority carrier effects (i.e. on the local p-n junction collection efficiency) and on the majority carrier effects (i.e. the transport efficiency from the excited region toward the contacts). Because of a finite core and shell resistance a non-negligible current redistribution into the p-n junction takes place during the majority carrier transport. A theoretical model for transport in a core-shell wire is developed, allowing to explain the dependence of the EBIC profiles on the experimental parameters (the electron beam acceleration voltage and the bias applied on the device) and on the structural parameters of the wire (core and shell resistance, shunt resistance, etc). Comparison between simulated and experimental profiles provides valuable information concerning the structure inhomogeneities and gives insight into the wire electrical parameters.
    Nanotechnology 06/2014; 25(25):255201. · 3.67 Impact Factor
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    ABSTRACT: We demonstrate quantum cascade lasers in the InAs/AlSb material system emitting at wavelengths of λ = 19 μm and λ = 21 μm. The maximum operating temperatures are 291 K and 250 K, and the threshold current densities at 78 K are as low as 0.6 kA/cm2 and 1.3 kA/cm2 for the lasers emitting at λ = 19 μm and λ = 21 μm, respectively. These values represent the best performance to date for quantum cascade lasers operating above λ = 16 μm. Although the devices employ metal-metal waveguide geometries, the diffraction effects which typically hinder the output beam of THz devices are not observed.
    Applied Physics Letters 01/2014; 104(2):021106-021106-5. · 3.52 Impact Factor
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    ABSTRACT: We report on the mixed photovoltaic/photoconductive operation mode of a visible blind photodetector based on GaN nanowires containing a p-i-n junction. The photodetector operates as a photovoltaic device close to zero bias and exhibits a photoconductive gain (>100) for biases above |V| > 2 V. We show that this unusual behavior of a p-i-n photodiode is specific to the case of nanowires. The gain is attributed to the illumination-induced modulation of the width of the depleted region at the nanowire lateral surface allowed because of the specific nanowire geometry with the p-i-n junction orthogonal to the lateral nanowire surface.
    Applied Physics Letters 01/2014; 104(2):023116-023116-5. · 3.52 Impact Factor
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    ABSTRACT: A novel GaN/AlN quantum cascade detector with simplified alloy extractor is integrated with metallic holes' array. Rotation of polarization by surface plasmons under normal incidence results in ×10 enhancement of responsivity at room temperature.
    CLEO: Science and Innovations; 06/2013
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    ABSTRACT: The optical properties of wurtzite GaN nanowires containing single AlGaN/GaN quantum discs of different thickness have been investigated. The dependence of the photoluminescence (PL) transition energy on the quantum disc thickness and the thickness of a lateral AlGaN shell has been simulated in the framework of a three-dimensional effective mass model, accounting for the presence of a lateral AlGaN shell, strain state and the piezoelectric and spontaneous polarization. The predicted transition energies are in good agreement with the statistics realized on more than 40 single nanowire emission spectra and PL spectra of ensembles of nanowires. The emission spectra of the single quantum discs exhibit a Lorentzian shape with a homogeneous line width as low as 3 meV. Finally, we discuss the dependence of the interband transition energy on diameter.
    Nanotechnology 03/2013; 24(12):125201. · 3.67 Impact Factor
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    ABSTRACT: Optical and structural properties of asymmetric coupled cubic-GaN/-AlN quantum wells (QW) are studied. The samples are grown by molecular beam epitaxy on a 50 nm c-GaN buffer on 3C-SiC substrate. The active region contains 100 periods of a 2.3 nm AlN barrier, a 1.9–2.1 nm silicon doped GaN QW and a 1.0–1.2 nm undoped GaN QW coupled by a 0.9–1.1 nm AlN tunnelling barrier. Phase purity and partial relaxation of the superlattice is observed in reciprocal space maps measured by high resolution X-ray diffraction. Optical properties of coupled QWs are investigated using cathodoluminescence spectroscopy. A clear shift in the emission energy associated with the thickness of the QWs can be observed. Furthermore clear TM-polarized infrared absorption in the 0.55–0.87 eV range is observed at room temperature using Fourier transform infrared spectroscopy. The asymmetric shape of the infrared absorptions reveals the existence of a three level system in the QWs and is explained by contributions of the e1–e3 and e2–e3 intersubband transitions. Measured transition energies are compared to model calculations using a Schrödinger–Poisson solver based on an effective mass model (nextnano3).
    physica status solidi (a) 03/2013; 210(3):455-458. · 1.21 Impact Factor
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    ABSTRACT: The optical properties of wurtzite GaN nanowires containing single Al 0.14 Ga 0.86 N/GaN quantum discs of different thickness have been investigated. The dependence of the photoluminescence (PL) transition energy on the quantum disc thickness and the thickness of a lateral AlGaN shell has been simulated in the framework of a three-dimensional effective mass model, accounting for the presence of a lateral AlGaN shell, strain state and the piezoelectric and spontaneous polarization. The predicted transition energies are in good agreement with the statistics realized on more than 40 single nanowire emission spectra and PL spectra of ensembles of nanowires. The emission spectra of the single quantum discs exhibit a Lorentzian shape with a homogeneous line width as low as 3 meV. Finally, we discuss the dependence of the interband transition energy on diameter. (Some figures may appear in colour only in the online journal)
    Nanotechnology 01/2013; 24(24):125201-7. · 3.67 Impact Factor
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    ABSTRACT: We report on the fabrication of graphene contact to GaN nanowire ensemble and on the demonstration of photodetectors using chemical vapor deposition-grown few-layered graphene as a transparent electrode. The optimization of the transfer method allowed to form a continuous contact to the nanowires over a large area. The adhesion energy of the graphene sheet to the nanowire ensemble is estimated to be 0.3–0.7 J/m2. Ultraviolet photodetectors with a room-temperature responsivity of ∼25 A/W at 357 nm were fabricated. The photocurrent spectrum shows that the device has a strong response up to 4.15 eV confirming a good transparency of the top graphene contact.
    Applied Physics Letters 01/2013; 103(20):201103-201103-4. · 3.52 Impact Factor
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    ABSTRACT: We report on the observation of intersubband absorption in GaN/AlN quantum well superlattices grown on (1122)-oriented GaN. The absorption is tuned in the 1.5-4.5 {mu}m wavelength range by adjusting the well thickness. The semipolar samples are compared with polar samples with identical well thickness grown during the same run. The intersubband absorption of semipolar samples shows a significant red shift with respect to the polar ones due to the reduction of the internal electric field in the quantum wells. The experimental results are compared with simulations and confirm the reduction of the polarization discontinuity along the growth axis in the semipolar case. The absorption spectral shape depends on the sample growth direction: for polar quantum wells the intersubband spectrum is a sum of Lorentzian resonances, whereas a Gaussian shape is observed in the semipolar case. This dissimilarity is explained by different carrier localization in these two cases.
    Journal of Applied Physics. 01/2013; 113(14):143109.
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    ABSTRACT: We present the determination of the index variation in the GaN/AlN heterostructures related to the population/depletion of the quantum wells fundamental state leading to the intersubband (ISB) absorption variation in the spectral domain around 1.5μm. The experiments were performed using wide-strip waveguide structure. It is shown that the determination of the refraction index in a wide-strip structure is possible when the waveguide is multimode in the vertical direction with a small number of higher order modes. The variation of the refractive index is then deduced from the shift of the position of the beating interference maxima of different order modes. The obtained index variation with bias from complete depletion to full population of the quantum wells is around -5×10-3. This value is similar to the typical index variation achieved in InP and is an order of magnitude higher than the index variation obtained in silicon. The remarkable feature is that maximum index variation is obtained at the wings of the ISB transition line where absorption is reduced with respect to the peak value. This index variation mechanism opens prospects for the realization of ISB phase modulators by inserting the active region in a Mach-Zehnder interferometer.
    Proc SPIE 01/2013; 8625:86251X.
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    ABSTRACT: We have demonstrated a GaN/AlGaN quantum cascade detector based on a simplified design of the extractor region relying on an AlGaN thick layer. The device grown by molecular beam epitaxy exhibits both TM-polarized intersubband absorption and photocurrent at room temperature at a peak wavelength of 1.87 μm. Based on the measured absorption and responsivity, we estimate the transfer efficiency of photoelectrons to the next period to be around 62%. This simplified design is robust against thickness fluctuations in the extractor region and offers prospects for ultrafast detectors.
    Applied Physics Letters 12/2012; 101(25). · 3.52 Impact Factor
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    ABSTRACT: We study the carrier localization in InN/In0.9Ga0.1N multiple-quantum-wells (MQWs) and bulk InN by means of temperature-dependent photoluminescence and pump-probe measurements at 1.55 μm. The S-shaped thermal evolution of the emission energy of the InN film is attributed to carrier localization at structural defects with an average localization energy of ∼12 meV. Carrier localization is enhanced in the MQWs due to well/barrier thickness and ternary alloy composition fluctuations, leading to a localization energy above 35 meV and longer carrier relaxation time. As a result, the luminescence efficiency in the MQWs is improved by a factor of five over bulk InN.
    Applied Physics Letters 08/2012; 101(6):062109-062109-4. · 3.52 Impact Factor
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    ABSTRACT: The optical polarization properties of GaN/AlGaN core/shell nanowire (NW) heterostructures have been investigated using polarization resolved micro-photoluminescence (μ-PL) and interpreted in terms of a strain-dependent 6 × 6 k·p theoretical model. The NW heterostructures were fabricated in two steps: the Si-doped n-type c-axis GaN NW cores were grown by molecular beam epitaxy (MBE) and then epitaxially overgrown using halide vapor phase epitaxy (HVPE) to form Mg-doped AlGaN shells. The emission of the uncoated strain-free GaN NW core is found to be polarized perpendicular to the c-axis, while the GaN core compressively strained by the AlGaN shell exhibits a polarization parallel to the NW c-axis. The luminescence of the AlGaN shell is weakly polarized perpendicular to the c-axis due to the tensile axial strain in the shell.
    Nanotechnology 07/2012; 23(32):325701. · 3.67 Impact Factor
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    ABSTRACT: We report on the growth and electro-optical studies of photovoltaic properties of GaAsP nanowires. Low density GaAsP nanowires were grown by Au assisted MOVPE on Si(001) substrates using a two step procedure to form a radial p-n junction. The STEM analyses show that the nanowires have cubic structure with the alloy composition GaAs₀.₈₈P₀.₁₂ in the nanowire core and GaAs₀.₇₆P₀.₂₄ in the shell. The nanowire ensembles were processed in the form of sub-millimeter size mesas. The photovoltaic properties were characterized by optical beam induced current (OBIC) and electronic beam induced current (EBIC) maps. Both OBIC and EBIC maps show that the photovoltage is generated by the nanowires; however, a strong signal variation from wire to wire is observed. Only one out of six connected nanowires produce a measurable signal. These strong fluctuations can be tentatively explained by the variation of the resistance of the nanowire-to-substrate connection, which is highly sensitive to the quality of the Si-GaAsP interface. This study demonstrates the importance of the spatially resolved charge collection microscopy techniques for the diagnosis of failures in nanowire photovoltaic devices.
    Nanotechnology 07/2012; 23(26):265402. · 3.67 Impact Factor
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    ABSTRACT: The aim of the present paper was to determine the index variation in the GaN/AlN heterostructures related to the population/depletion of the quantum well fundamental state leading to the absorption variation in the spectral domain around 1.5 µm. The variation of the refractive index was deduced from the shift of the position of the beating interference maxima of different order modes in a guided wave configuration. The obtained index variation with bias from complete depletion to full population of the quantum wells is around -5 × 10(-3). This value is similar to the typical index variation achieved in InP and is an order of magnitude higher than the index variation obtained in silicon.
    Optics Express 05/2012; 20(11):12541-9. · 3.53 Impact Factor
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    Applied Physics Express 05/2012; 5(5):052203. · 2.73 Impact Factor
  • Elias Warde, Salam Sakr, Maria Tchernycheva, Francois Henry Julien
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    ABSTRACT: Using the transfer matrix formalism, we have theoretically studied the vertical ballistic transport in GaN/AlGaN resonant tunneling diodes (RTDs) and superlattices with a small number of periods. We have calculated the transmission probability versus the longitudinal electron energy (T–E) and the current density–voltage (J–V) characteristics. Calculations of both T–E and J–V characteristics have been performed for different Al contents in the barriers. The asymmetry effects due to the internal electric field in the barriers are discussed. Applied to the RTD structure, our calculations demonstrate: (i) the increase of the peak-to-valley ratio of the negative differential resistance (NDR) with increasing Al content in the barriers, (ii) the dependence of the J–V resonance values on the current direction, and (iii) the asymmetry of the NDR with respect to the current direction due to the huge internal electric field in the structure. In the case of multiple quantum well structure (MQWS), the calculation results confirm the same trends as in the RTD case when the Al content is varied. In spite of the fact that it is more difficult to analyze the results in the case of MQWS, the obtained calculations demonstrate the applicability of the used model and of the numerical method to study GaN/AlGaN devices based on quantum well (QW) heterostructures. Furthermore, a design of an optimized 7QW structure operating symmetrically whatever the direction of the applied voltage is presented.
    Journal of Electronic Materials 05/2012; 41(5). · 1.68 Impact Factor
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    ABSTRACT: A two-color GaN-based quantum cascade detector is demonstrated. This photodetector operates simultaneously at a peak wavelength of 1.7 and 1 μm at room temperature without any external voltage. These peaks correspond, respectively, to the e1e2 and e1e3 intersubband absorption of the active GaN quantum well. The extractor has been designed to allow for efficient transfer of electrons from both the e2 and e3 states to the next period. The 1 μm detected wavelength is the shortest value reported for an intersubband semiconductor based detector.
    Applied Physics Letters 04/2012; 100(18). · 3.52 Impact Factor
  • Proceedings of SPIE. 01/2012; 8262:82621Q.
  • International Workshop on Nitride semiconductors (IWN2012); 01/2012

Publication Stats

2k Citations
472.59 Total Impact Points

Institutions

  • 1995–2014
    • Institute of Fundamental Electronics
      Orsay, Île-de-France, France
  • 1990–2014
    • Université Paris-Sud 11
      • Institut d'Electronique Fondamentale
      Orsay, Île-de-France, France
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 2013
    • Justus-Liebig-Universität Gießen
      • I. Physikalisches Institut
      Gieben, Hesse, Germany
  • 2011
    • Universität Paderborn
      • Department of Physics
      Paderborn, North Rhine-Westphalia, Germany
  • 2008
    • University of Alcalá
      • Department of Electronics
      Alcalá de Henares, Madrid, Spain
  • 2006–2008
    • Technion - Israel Institute of Technology
      • Electrical Engineering Group
      Haifa, Haifa District, Israel
    • Chung-Ang University
      Sŏul, Seoul, South Korea
  • 2001
    • Ecole Centrale de Lyon
      • Institut des Nanotechnologies de Lyon
      Rhône-Alpes, France
    • Thales Group
      Cannes, Provence-Alpes-Côte d'Azur, France
  • 1995–1997
    • Johannes Kepler University Linz
      • Institut für Halbleiter- und Festkörperphysik
      Linz, Upper Austria, Austria
  • 1983–1988
    • Institut Supérieur d’Electronique de Paris
      Lutetia Parisorum, Île-de-France, France
  • 1987
    • University of Illinois, Urbana-Champaign
      • Department of Electrical and Computer Engineering
      Urbana, IL, United States