François Henri Julien

We report on the systematic study of two main scattering mechanisms on intersubband transitions, namely ionized impurity scattering and interface roughness scattering. The former mechanism has been investigated as a function of the dopants position within a multiple GaAs/AlGaAs quantum well structure and compared to the transition of an undoped sample. The study on the latter scattering mechanism has been conducted using the growth interruption technique. We report an improvement of the intersubband (ISB) transition linewidth up to 11% by interrupting growth at GaAs-on-AlGaAs interfaces. As a result, the lifetime of intersubband polaritons could be improved up to 9%. This leads to a reduction of 17% of the theoretical threshold intensity for polaritonic coherent emission. This work brings a useful contribution towards the realization of polariton-based devices.

We demonstrate for the first time the efficient mechanical-electrical conversion properties of InGaN/GaN nanowires (NWs). Using an atomic force microscope equipped with a modified Resiscope module, we analyse the piezoelectric energy generation of GaN NWs and demonstrate an important enhancement when integrating in their volume a thick In-rich InGaN insertion. The piezoelectric response of InGaN/GaN NWs can be tuned as a function of the InGaN insertion thickness and position in the NW volume. The energy harvesting is favoured by the presence of a PtSi/GaN Schottky diode which allows to efficiently collect the piezo-charges generated by InGaN/GaN NWs. Average output voltages up to 330 ± 70 mV and a maximum value of 470 mV per NW has been measured for nanostructures integrating 70 nm-thick InGaN insertion capped with a thin GaN top layer. This latter value establishes an increase of about 35% of the piezo-conversion capacity in comparison with binary p-doped GaN NWs. Based on the measured output signals, we estimate that one layer of dense InGaN/GaN-based NW can generate a maximum output power density of about 3.3 W/cm². These results settle the new state-of-the-art for piezo-generation from GaN-based NWs and offer a promising perspective for extending the performances of the piezoelectric sources.

We report on a prototype of piezogenerator based on vertically-aligned GaN nanowires grown by Plasma-Assisted Molecule Beam Epitaxy (PAMBE). Our device generates a peak-to-peak voltage from 20 mV to 60 mV depending on the applied force. The output power density reaches 0.76 mW/cm³ which is of the same order of magnitude as the state-of-art value. Furthermore, the pressure applied to deform the nanowires is of the order of kPa, much less than that used in previous reports (which were of the order of MPa) utilizing ZnO nanowires for the power generation. Thus, an enhanced mechanic-electrical conversion efficiency has been achieved. Our GaN nanowire based devices are promising both for pressure sensing applications and for mechanic energy harvesting.

The optical properties of wurtzite GaN nanowires containing single Al0.14Ga0.86N/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.

Gallium nitride [GaN] nanorods grown on a Si(111) substrate at 720°C via plasma-assisted molecular beam epitaxy were studied by field-emission electron microscopy and cathodoluminescence [CL]. The surface topography and optical properties of the GaN nanorod cluster and single GaN nanorod were measured and discussed. The defect-related CL spectra of GaN nanorods and their dependence on temperature were investigated. The CL spectra along the length of the individual GaN nanorod were also studied. The results reveal that the 3.2-eV peak comes from the structural defect at the interface between the GaN nanorod and Si substrate. The surface state emission of the single GaN nanorod is stronger as the diameter of the GaN nanorod becomes smaller due to an increased surface-to-volume ratio.

In this article, the authors report on growth of InGaN/AlInN multiquantum wells (MQWs) by rf-plasma-assisted molecular beam epitaxy on (0001) GaN substrates. Intersubband transitions in InGaN/AlInN MQWs with widths varying from 1.3 to 1.8 nm has been studied experimentally and analyzed theoretically within the electron effective mass approximation. The intersubband absorption between the first and the second electron levels was observed at the wavelength of ∼ 1.3 – 1.6 μ m . The broadening of intersubband absorption for this system has been found to be much larger than for indium free system (i.e., GaN/AlN MQWs). In addition, it has been found that the broadening increases from ∼ 280 to ∼ 390 meV when the nominal quantum well(QW) width decreased from 1.8 to 1.3 nm. On the basis of theoretical calculations combined with the transmission electron microscopy analysis, it has been concluded that the observed broadening of intersubband absorption is due to the QW width fluctuations. It has been estimated that the maximal width fluctuations in these MQWs are equal to ∼ 2 monolayers. Such a significant QW width fluctuation appears in this system because of the incorporation of indium atoms.

Extending the intersubband transitions in III-nitride nanostructures from near-infrared to longer wavelengths might have significant consequences for critical applications like imaging, remote sensing and mine detection. In this work, we analyze the potential of polar and semipolar AlGaN/GaN technologies for this relevant spectral range.

We demonstrate room-temperature, high-speed operation of GaN/AlGaN quantum cascade detectors. The devices are processed as square mesas with 50 Ω coplanar access lines. Frequency response measurements were performed under illumination by a modulated laser diode emitting at λ=1.55 μm. The electrical response exhibits a first-order filter frequency response. For 17×17 μm2 (25×25 μm2) detectors the −3 dB cutoff frequency is 11.4 GHz (6.5 GHz). S-parameter analysis confirms that the cutoff frequency is extrinsically limited and that the speed of the device can be further increased by reducing the device size.