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Hofstetter, Daniel

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Hofstetter, Daniel
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Ancien.ne collaborateur.trice
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https://libra.unine.ch/handle/123456789/472

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  • Publication
    Accès libre
    GaN/AlN short-period superlattices for intersubband optoelectronics: A systematic study of their epitaxial growth, design, and performance
    (2008)
    Kandaswamy, Prem K.
    ;
    Guillot, Fabien
    ;
    Bellet-Amalric, Edith
    ;
    Monroy, Eva
    ;
    Nevou, Laurent
    ;
    Tchernycheva, Maria
    ;
    Michon, A.
    ;
    Julien, François H.
    ;
    Baumann, Esther
    ;
    Giorgetta, Fabrizio R.
    ;
    Hofstetter, Daniel 
    ;
    Remmele, T.
    ;
    Albrecht, M.
    ;
    Birner, S.
    ;
    Dang, Le Si
    We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well (MQW) structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3–1.9 μm wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures (>720 °C) due to the thermal activation of the AlN etching of GaN. From the point of view of strain, GaN/AlN MQWs evolve rapidly to an equilibrium average lattice parameter, which is independent of the substrate. As a result, we do not observe any significant effect of the underlayers on the optical performance of the MQW structure. The average lattice parameter is different from the expected value from elastic energy minimization, which points out the presence of periodic misfit dislocations in the structure. The structural quality of the samples is independent of Si doping up to 1020 cm−3. By contrast, the intersubband absorption spectrum broadens and blueshifts with doping as a result of electron-electron interactions. This behavior is independent of the Si doping location in the structure, either in the QWs or in the barriers. It is found that the magnitude of the intersubband absorption is not directly determined by the Si concentration in the wells. Instead, depending on the Al mole fraction of the cap layer, the internal electric field due to piezoelectric and spontaneous polarization can deplete or induce charge accumulation in the QWs. In fact, this polarization-induced doping can result in a significant and even dominant contribution to the infrared absorption in GaN/AlN MQW structures.
  • Publication
    Accès libre
    Latest developments in GaN-based quantum devices for infrared optoelectronics
    (2008)
    Monroy, Eva
    ;
    Guillot, Fabien
    ;
    Leconte, Sylvain
    ;
    Nevou, Laurent
    ;
    Doyennette, Laeticia
    ;
    Tchernycheva, Maria
    ;
    Julien, François H.
    ;
    Baumann, Esther
    ;
    Giorgetta, Fabrizio R.
    ;
    Hofstetter, Daniel 
    In this work, we summarize the latest progress in intersubband devices based on GaN/AlN nanostructures for operation in the near-infrared. We first discuss the growth and characterization of ultra-thin GaN/AlN quantum well and quantum dot superlattices by plasma-assisted molecular-beam epitaxy. Then, we present the performance of nitride-based infrared photodetectors and electro-optical modulators operating at 1.55 μm. Finally, we discuss the progress towards intersubband light emitters, including the first experimental observation of intersubband photoluminescence in nitride nanostructures.
  • Publication
    Accès libre
    Lattice-Matched GaN–InAlN Waveguides at λ = 1.55 μm Grown by Metal–Organic Vapor Phase Epitaxy
    (2008)
    Lupu, A.
    ;
    Julien, François H.
    ;
    Golka, Sebastian
    ;
    Pozzovivo, G.
    ;
    Strasser, Gottfried
    ;
    Baumann, Esther
    ;
    Giorgetta, Fabrizio R.
    ;
    Hofstetter, Daniel 
    ;
    Nicolay, S.
    ;
    Mosca, M.
    ;
    Feltin, E.
    ;
    Carlin, J.-F.
    ;
    Grandjean, N.
    We report on the demonstration of low-loss, single-mode GaN-InAlN ridge waveguides (WGs) at fiber-optics telecommunication wavelengths. The structure grown by metal-organic vapor phase epitaxy contains AlInN cladding layers lattice-matched to GaN. For slab-like WGs propagation losses are below 3 dB/mm and independent of light polarization. For 2.6-μm-wide WGs the propagation losses in the 1.5- to 1.58-μm spectral region are as low as 1.8 and 4.9 dB/mm for transverse-electric- and transverse-magnetic-polarization, respectively. The losses are attributed to the sidewall roughness and can be further reduced by the optimization of the etching process.
  • Publication
    Accès libre
    MBE growth of nitride-based photovoltaic intersubband detectors
    (2006)
    Monroy, Eva
    ;
    Guillot, Fabien
    ;
    Leconte, Sylvain
    ;
    Bellet-Amalric, Edith
    ;
    Baumann, Esther
    ;
    Giorgetta, Fabrizio R.
    ;
    Hofstetter, Daniel 
    ;
    Nevou, Laurent
    ;
    Tchernycheva, Maria
    ;
    Doyennette, Laeticia
    ;
    Julien, François H.
    ;
    Remmele, T.
    ;
    Albrecht, M.
    In this work, we present the plasma-assisted molecular-beam epitaxial growth of quantum well infrared photodetector (QWIP) structures, including the Si-doped GaN/AlN short-period superlattice of the active region, conductive AlGaN claddings and integration of the final device. The growth of Si-doped GaN/AlN multiple quantum well (QW) structures is optimized by controlling substrate temperature, metal excess and growth interruptions. Structural characterization confirms a reduction of the interface roughness to the monolayer scale. P-polarized intersubband absorption peaks covering the 1.33–1.91 μm wavelength range are measured on samples with QW thickness varying from 1 to 2.5 nm. The absorption exhibits Lorentzian shape with a line width around 100 meV in QWs doped 5×1019 cm−3. To prevent partial depletion of the QWs owing to the internal electric field, we have developed highly-conductive Si-doped AlGaN cladding layers using In as a surfactant during growth. Complete ISB photodetectors with 40 periods of 1 nm-thick Si-doped GaN QWs with 2 nm-thick AlN barriers have been grown on conductive AlGaN claddings, the Al mole fraction of the cladding matching the average Al content of the active region. Temperature-dependent photovoltage measurements reveal a narrow (90 meV) detection peak at 1.39 μm.
  • Publication
    Accès libre
    III-Nitride Nanostructures for Infrared Optoelectronics
    (2006)
    Monroy, Eva
    ;
    Guillot, Fabien
    ;
    Leconte, Sylvain
    ;
    Bellet-Amalric, Edith
    ;
    Nevou, Laurent
    ;
    Doyennette, Laeticia
    ;
    Tchernycheva, Maria
    ;
    Julien, François H.
    ;
    Baumann, Esther
    ;
    Giorgetta, Fabrizio R.
    ;
    Hofstetter, Daniel 
    ;
    Dang, Le Si
    Thanks to their large conduction band offset (~1.8 eV for the GaN/AlN system) and subpicosecond intersubband scattering rates, III-nitride heterostructures in the form of quantum wells or quantum dots are excellent candidates for high-speed unipolar devices operating at optical-fiber telecommunication wavelengths, and relying on the quantum confinement of electrons. In this work, we present the plasma-assisted molecular-beam epitaxial growth of quantum well infrared photodetector structures. The growth of Si-doped GaN/AlN multiple quantum well structures is optimized by controlling substrate temperature, metal excess and growth interruptions. Structural characterization confirms a reduction of the interface roughness to the monolayer scale. P-polarized intersubband absorption peaks covering the 1.33-1.91 μm wavelength range are measured on samples with quantum well thickness varying from 1 to 2.5 nm. Complete intersubband photodetectors have been grown on conductive AlGaN claddings, the Al mole fraction of the cladding matching the average Al content of the active region. Photovoltage measurements reveal a narrow (~90 meV) detection peak at 1.39 μm at room temperature.