E. Giraud

Two-color GaN/AlGaN quantum cascade detector at short infrared wavelengths of 1 and 1.7 μm

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.

Statistical correction of atom probe tomography data of semiconductor alloys combined with optical spectroscopy: The case of Al0.25Ga0.75N

The ternary semiconductor alloy Al0.25Ga0.75N has been analyzed by means of correlated photoluminescence spectroscopy and atom probe tomography (APT). We find that the composition measured by APT is strongly dependent on the surface electric field, leading to erroneous measurements of the alloy composition at high field, due to the different evaporation behaviors of Al and Ga atoms. After showing how a biased measurement of the alloy content leads to inaccurate predictions on the optical properties of the material, we develop a correction procedure which yields consistent transition and localization energies for the alloy photoluminescence.

A simplified GaN/AlGaN quantum cascade detector with an alloy extractor

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.

Statistical nanoscale study of localised radiative transitions in GaN/AlGaN quantum wells and AlGaN epitaxial layers

Compositional disorder has important consequences on the optical properties of III-nitride ternary alloys. In AlGaN epilayers and AlGaN-based quantum heterostructures, the potential fluctuations induced by such disorder lead to the localisation of carriers at low temperature, which affects their transition energies. Using the correlations between micro-photoluminescence, scanning transmission electron microscopy and atom probe tomography we have analysed the optical behaviour of Al0.25Ga0.75N epilayers and that of GaN/AlGaN quantum wells, and reconstructed in three dimensions the distribution of chemical species with sub-nanometre spatial resolution. These composition maps served as the basis for the effective mass calculation of electrons and holes involved in radiative transitions. Good statistical predictions were subsequently obtained for the above-mentioned transition and localisation energies by establishing a link with their microstructural properties.

Temperature-Dependence of Exciton Radiative Recombination in (Al,Ga)N/GaN Quantum Wells Grown on a-Plane GaN Substrates

This article presents the dynamics of excitons in a-plane (Al,Ga)N/GaN single quantum wells of various thicknesses grown on bulk GaN substrates. For all quantum well samples, recombination is observed to be predominantly radiative in the low-temperature range. At higher temperatures, the escape of charge carriers from the quantum well to the (Al,Ga)N barriers is accompanied by a reduction in internal quantum efficiency. Based on the temperature-dependence of time-resolved photoluminescence experiments, we also show how the local disorder affects the exciton radiative lifetime at low temperature and the exciton non-radiative lifetime at high temperature.

n(+)-GaN grown by ammonia molecular beam epitaxy: Application to regrown contacts

We report on the low-temperature growth of heavily Si-doped (>1020 cm−3) n+-type GaN by N-rich ammonia molecular beam epitaxy (MBE) with very low bulk resistivity (<4 × 10−4 Ω·cm). This is applied to the realization of regrown ohmic contacts on InAlN/GaN high electron mobility transistors. A low n+-GaN/2 dimensional electron gas contact resistivity of 0.11 Ω·mm is measured, provided an optimized surface preparation procedure, which is shown to be critical. This proves the great potentials of ammonia MBE for the realization of high performance electronic devices.

First demonstration of plasmonic GaN quantum cascade detectors with enhanced efficiency at normal incidence

We have designed, fabricated and measured the first plasmon-assisted normal incidence GaN/AlN quantum cascade detector (QCD) making use of the surface plasmon resonance of a two-dimensional nanohole Au array integrated on top of the detector absorption region. The spectral response of the detector at room temperature is peaked at the plasmon resonance of 1.82 μm. We show that the presence of the nanohole array induces an absolute enhancement of the responsivity by a factor of ~30 over that of the bare device at normal incidence and by a factor of 3 with respect to illumination by the 45° polished side facet. We show that this significant improvement arises from two phenomena, namely, the polarization rotation of the impinging light from tangential to normal induced by the plasmonic structure and from the enhancement of the absorption cross-section per quantum well due to the near-field optical intensity of the plasmonic wave.

Thermal carrier emission and nonradiative recombinations in nonpolar (Al,Ga)N/GaN quantum wells grown on bulk GaN

We investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al content on the dynamics of excitons in the 10-320 K range. We first show that the effective lifetime of QW excitons τ increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing τ depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our ...

Measurement of polarization-induced electric fields in GaN/AlInN quantum wells

GaN(Si)/AlInN multiple quantum wells were grown on GaN/Al2O3 (0001) templates by metalorganic vapor-phase epitaxy. Transmission electron microscopy observations showed well-defined GaN quantum wells and AlInN barrier layers. Electrostatic potential profiles across the heterostructure have been measured using off-axis electron holography. A polarization-induced electric field with magnitude of ∼2.2 ± 0.1 MV/cm was measured across the GaN quantum wells, in reasonable agreement with simulated values. However, the measured fields across the AlInN barriers were considerably less than predicted from simulations: possible reasons are briefly discussed.

Assessing the Composition of Wide Bandgap Compound Semiconductors by Atom Probe Tomography: A Metrological Problem

1. Groupe de Physique des Matériaux, UMR 6634 CNRS, University and INSA of Rouen, Normandie University, 76800 St. Etienne du Rouvray, France 2. Institute of Physics (IPhys), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Lausanne, Switzerland 3. Laboratoire de Photonique et Nanostructures CNRS, Université Paris-Saclay, 91460 Marcoussis, France. 4. Institut d’Electronique Fondamentale, UMR 8622 CNRS, University Paris Saclay, 91405 Orsay, France 5. Centre de Recherche sur l’Hétéro-Epitaxie et ses Applications, UPR10 CNRS, 06560 Valbonne, France