Catherine Bougerol

In this work, two series of AlGaN/GaN/AlN high electron mobility transistor (HEMT) heterostructures have been grown by molecular beam epitaxy on AlN bulk substrates. The effects of reduction in the GaN channel thickness as well as the AlGaN barrier thickness and composition on structural and electrical properties of the heterostructures have been studied. The material analysis involved high-resolution x-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy. In a first series of HEMT structures grown with an aluminum content of 30% in the AlGaN barrier, the channel downscaling results in a reduction in the GaN strain relaxation rate but at the expense of degradation in the mean crystal quality and in the electron mobility with a noticeable increase in the sheet resistance. An opposite trend is noticed for the three-terminal breakdown voltage of transistors, so that a trade-off is obtained for a 50 nm width GaN channel HEMT, which exhibits a sheet resistance of 1700 Ω/sq. with transistors demonstrating three-terminal breakdown voltage up to 1400 V for 40 μm gate to drain spacing with static on resistance R_on= 32 mΩ cm². On the other hand, a second series of HEMT structures with high aluminum content AlGaN barriers and sub-10 nm GaN channels have been grown perfectly strained with high sheet carrier densities allowing to preserve sheet resistances in the range of 880–1050 Ω/sq.

Recently, there has been an increased interest in germicidal ultraviolet (GUV) lamps for disinfection. Despite extensive studies on GUV LEDs, their efficiency and cost per Watt is still far from that of mercury lamps due to electrical injection issues, among others. Also, the fact that 254 nm radiation is highly carcinogenic and cataractogenic, has motivated research on radiation with shorter penetration (200-230 nm) depth, for non-invasive disinfection. In this study, we propose electron pumped UV lamps as an alternative to LEDs (to tackle electrical issues) in the spectral range 230-330 targeting both wavelength ranges of disinfection and exhibiting IQE ranging from 20%-50%.

A dual‐color emission is achieved combining two monolithic sets of core–shell multiple quantum wells (MQWs) grown on GaN microwires. The shell heterostructure is composed of 3× blue MQWs and 7× green MQWs with photoluminescence emission covering 400–450 and 450–550 nm wavelength bands, respectively. Both emissions are coming from the two MQW sets grown on m‐plane sidewall surface, as revealed by cathodoluminescence mapping. Advanced structural characterization combining transmission electron microscopy and energy‐dispersive X‐ray analysis is performed on longitudinal cross‐sectional slices of the wires. Considering asymmetrical QW interface, the In content is measured to be equal to 15.7 (±0.5)% and 23.5 (±2.0)% in the first and the second MQW set consistent with the blue and green emissions. Extended defects are formed originating from the second MQW set due to higher In content. A flexible light‐emitting diode (LED) is fabricated based on these dual core–shell MQWs, showing an electroluminescence dominated by green emission due to efficient hole injection in the In‐rich second MQW set. This work opens the way for multiple color emission from core–shell MQWs for phosphor‐free nanowire‐based LED applications.

Relaxation of tensile strain in AlGaN heterostructures grown on a GaN template can lead to the formation of cracks. These extended defects locally degrade the crystal quality, resulting in a local increase in non-radiative recombinations. The effect of such cracks on the optical and structural properties of core–shell AlGaN/AlGaN multiple quantum wells grown on GaN microwires is comprehensively characterized by means of spectrally and time-correlated cathodoluminescence (CL). We observe that the CL blueshifts near a crack. By performing 6 × 6 k.p simulations in combination with transmission electron microscopy analysis, we ascribe this shift to the strain relaxation by the free surface near cracks. By simultaneously recording the variations of both the CL lifetime and the CL intensity across the crack, we directly assess the carrier dynamics around the defect at T = 5 K. We observe that the CL lifetime is reduced typically from 500 ps to less than 300 ps and the CL intensity increases by about 40% near the crack. The effect of the crack on the optical properties is, therefore, of two natures. First, the presence of this defect locally increases non-radiative recombinations, while at the same time, it locally improves the extraction efficiency. These findings emphasize the need for time-resolved experiments to avoid experimental artifacts related to local changes of light collection.

In this paper, we describe the growth and characterization of ≈530 nm thick superlattices (100 periods) of Al _x Ga _1-x N/AlN (0 ⩽ x ⩽ 0.1) Stranski–Krastanov quantum dots (QDs) for application as the active region of electron-beam pumped ultraviolet lamps. Highly dense (>10 ¹¹ cm ⁻² ) QD layers are deposited by molecular beam epitaxy, and we explore the effect of the III/V ratio during the growth process on their optical performance. The study considers structures emitting in the 244–335 nm range at room temperature, with a relative linewidth in the 6%–11% range, mainly due to the QD diameter dispersion inherent in self-assembled growth. Under electron pumping, the emission efficiency remains constant for acceleration voltages below ≈9 kV. The correlation of this threshold with the total thickness of the SL and the penetration depth of the electron beam confirms the homogeneity of the nanostructures along the growth axis. Below the threshold, the emission intensity scales linearly with the injected current. The internal quantum efficiency (IQE) is characterized at low injection, which reveals the material properties in terms of non-radiative processes, and high injection, which emulates carrier injection in operation conditions. In QDs synthesized with III/V ratio −2 , being the first kind of nanostructure that present such stable behaviour.

The present work reports high quality non-polar GaN/Al_0.6Ga_0.4N multiple quantum wells (MQWs) grown in core-shell geometry by metalorganic vapor phase epitaxy on the m-plane sidewalls of c ̅-oriented hexagonal GaN wires. Optical and structural studies reveal UV emission originating from the core-shell GaN/AlGaN MQWs. Tuning the m-plane GaN QW thickness from 4.3 to 0.7 nm leads to a shift of the emission from 347 to 292 nm, consistent with Schrödinger-Poisson calculations. The evolution of the luminescence with temperature displays signs of strong localization, especially for samples with thinner GaN QWs and no evidence of quantum confined Stark effect, as expected for non-polar m-plane surfaces. The internal quantum efficiency derived from the photoluminescence intensity ratio at low and room temperature is maximum (~7.3 %) for 2.6 nm-thick quantum wells, emitting at 325 nm and shows a large drop for thicker QWs. An extensive study of the PL quenching with temperature is presented. Two non-radiative recombination paths are activated at different temperatures. The low temperature path is found to be intrinsic to the heterostructure, whereas the process that dominates at high temperature depends on the QW thickness and is strongly enhanced for QWs larger than 2.6 nm, causing a drop of the internal quantum efficiency.

The propagation of sidewall steps during the growth of nanowires is calculated in the frame of the Burton-Cabrera-Frank model. The stable shape of the nanowire comprises a cylinder section on top of a cone section: their characteristics are obtained as a function of the radius of the catalyst-nanowire area, the desorption-limited diffusion length of adatoms on the terraces, and the sticking of adatoms at step edges. The comparison with experimental data allows us to evaluate these last two parameters for InP and ZnTe nanowires; it reveals a different behavior for the two materials, related to a difference by an order of magnitude of the desorption-limited diffusion length.

The distribution and electrical activity of p-type doping (Mg) in gallium nitride (GaN) grown by metal organic chemical vapor deposition was investigated by correlating atom probe tomography (APT) and off-axis electron holography. APT results revealed that high Mg concentrations promote the formation of Mg-rich clusters. This is associated with the formation of pyramidal inversion domains (PIDs). The direct measurement of the doping concentration outside the clusters provided by APT suggests a saturation in the p-type electrical activity for Mg concentrations above 7 × 10¹⁹ cm⁻³. Maps of the electrostatic potential provided by off-axis electron holography confirm that the highest carrier concentration was achieved in the regions with the highest dopant concentration of 2 × 10²⁰ cm⁻³, despite the presence of a high density of Mg-rich clusters revealed by APT. The correlation of these techniques suggests that PIDs are not the major cause of the reduction in electrostatic potential.

The growth of GaN on graphene by molecular beam epitaxy was investigated. The most stable epitaxial relationship, i.e. [00.1]-oriented grains, is obtained at high temperature and N-rich conditions, which match those for nanowire growth. Alternatively, at moderate temperature and Ga-rich conditions, several metastable orientations are observed at the nucleation stage, which evolve preferentially towards [00.1]-oriented grains. The dependence of the nucleation regime on growth conditions was assigned to Ga adatom kinetics. This statement is consistent with the calculated graphene/GaN in-plane lattice coincidence and supported by a combination of transmission electron microscopy, x-ray diffraction, photoluminescence, and Raman spectroscopy experiments.