Search

Benefiting from exceptional credits voted in 1833, the Muséum national d’Hstoire naturelle de Paris had the opportunity to modernize the garden that houses it and was one with it: the Jardin des Plantes. The young architect Charles Rohault de Fleury (1801-1875), appointed a year earlier chief architect of the Museum, thus inherited the responsibility of designing new buildings and decorations that reflect the scientific ambition attached to this place: to preserve and exhibit all the natural productions of the globe. [ABSTRACT FROM AUTHOR]

Les propriétés germicides des rayonnements ultraviolets (UV) suscitent un intérêt de plus en plus important dans les domaines d'applications stratégiques que sont l'environnement et la santé. Les diodes électroluminescentes (LEDs) semi-conductrices à base de nitrure d'aluminium et de gallium représentent les nouvelles sources d'émission UV qui pourront répondre à cette demande. Leurs performances, bien qu'encore modestes, progressent chaque jour et nous vivons le point de bascule entre le laboratoire et le domaine grand public. Pour définitivement parvenir à s'imposer, elles doivent associer efficacité, forte capacité de développement, ainsi que souplesse et facilité d'utilisation, performances qu'aucune source UV n'a encore atteint jusqu'à présent. [ABSTRACT FROM AUTHOR]

AlN epilayer properties (120 nm thick) grown by ammonia assisted molecular beam epitaxy on c-sapphire substrates with different low temperature AlN buffer layers (LT-BL) have been studied. The role of the LT-BL on the AlN structural and optical properties was investigated as a function of the LT-BL thickness and growth temperature. Optimum growth conditions were identified with LT-BL thickness of 3 nm and growth temperature between 480 °C and 520 °C. It was shown that by optimizing these conditions, a reduction of both mixed and edge threading dislocation densities up to 75% is achieved. The impact of the growth temperature of the AlN epilayer was also studied showing an additional improvement of the AlN crystal and morphological properties while growing at higher temperature. A correlation between the epilayer strain and the PL emission was also investigated. Finally, an Al 0.7 Ga 0.3 N:Si doped layer was grown on the top of the optimized AlN template showing a smooth surface with monoatomic steps and a roughness ∼0.2 nm, confirming the potential of such templates for the fabrication of AlGaN based heterostructures. [ABSTRACT FROM AUTHOR]

The optical properties of Al 0.1 Ga 0.9 N nanostructures grown by molecular beam epitaxy on Al 0.5 Ga 0.5 N (0001) templates are investigated. By combining morphological and photoluminescence (PL) characterizations, we have performed an in-depth analysis of the nanostructures properties as a function of the deposited amount. It is shown that: 1) the nanostructures present an asymmetrical height distribution, and a variation in their shape (i.e. both symmetric and elongated nanostructures are observed); 2) the main PL emission is found in the UV range (above 3.6 eV); and 3) a broad emission in the near UV-blue range is observed. These results allowed to attribute the main PL peak to nanostructures with properties in close agreement to the nominal Al 0.1 Ga 0.9 N concentration and deposited amount. Concerning the broad PL band at lower energy, it has been correlated to the formation of an additional type of nanostructures with larger size and lower Al composition compared to the Al 0.1 Ga 0.9 N ones. [ABSTRACT FROM AUTHOR]

(Al,Ga)N-based quantum dots (QDs) grown on Al 0.5 Ga 0.5 N by molecular beam epitaxy have been studied as the active region for the fabrication of ultra-violet (UV) light emitting diodes (LEDs). In the first part, using both “polar” (0001) and “semipolar” (112¯2) surface orientations, the structural and optical properties of different QD structures are investigated and compared. In particular, their propensity to get an emission in the UV range is analyzed in correlation with the influence of the internal electric field on their optical properties. In a second part, (0001) and (112¯2)-oriented LEDs using GaN/Al 0.5 Ga 0.5 N QD as active regions have been fabricated. Their main current-voltage characteristics and electroluminescence properties are discussed, with a focus on the LED emission wavelength range reached for both surface orientations: it is shown that a large part of the UV-A region can be covered, with longer wavelengths-from 415 to 360 nm-for the “polar” LEDs, and shorter ones-from 345 to 325 nm-for the “semipolar” LEDs. In addition, the influence of the internal electric field on the QD-LEDs working operation is shown. [ABSTRACT FROM AUTHOR]

The luminescence efficiency of AlxGa1−xN quantum dots (QDs) and quantum wells (QWs), buried in AlN cladding layers and emitting in the ultraviolet range between 234 and 310 nm, has been investigated. The growth and optical properties have been done using similar aluminum composition (varying from 0.4 to 0.75) for both QDs and QWs. In order to compare as much as possible the optical properties, the QWs were fabricated with a growth time tuned such that the QW width is similar to the average height of the QDs. The photoluminescence (PL) showed emission ranging from 4 to 5.4 eV, putting into evidence differences in terms of full width at half maximum, PL intensity, and asymmetry of the line shape between QDs and QWs. The results show shorter wavelengths and a slightly narrower PL linewidth for QWs. To determine the light emission dependence with the electric field direction in the crystal, the evolutions of the emission diagrams for all samples were recorded along two orthogonal directions, namely, the "in-plane" (growth) and the "on-side" directions, from which the light emission was collected. For the whole QDs and QWs samples' series, the shapes of the emission diagram indicate emission in both in-plane and on-side directions, as evidenced by intra-valence band mixings caused by strain effects combined with the anisotropic Coulomb interactions that are particularly contributing to the polarization at wavelengths below 260 nm. Furthermore, the degree of polarization, determined for each sample, showed good agreement with results from the literature. [ABSTRACT FROM AUTHOR]

A process has been validated for the deconstruction of lignocellulose on a pilot scale installation using six types of biomass selected for their sustainability, accessibility, worldwide availability, and differences of chemical composition and physical structure. The process combines thermo-mechano-chemical and bio-catalytic action in a single twin-screw extruder. Three treatment phases were sequentially performed: an alkaline pretreatment, a neutralization step coupled with an extraction–separation phase and a bioextrusion treatment. Alkaline pretreatment destructured the wall polymers after just a few minutes and allowed the initial extraction of 18–54% of the hemicelluloses and 9–41% of the lignin. The bioextrusion step induced the start of enzymatic hydrolysis and increased the proportion of soluble organic matter. Extension of saccharification for 24 h at high consistency (20%) and without the addition of new enzyme resulted in the production of 39–84% of the potential glucose. [ABSTRACT FROM AUTHOR]

A continuous process combining an alkaline thermo-mechano-chemical pretreatment neutralization step, followed by injection of enzymes into the twin-screw extruder, was developed using sweet corn co-products as a biomass model. The implementation of the continuous process is described. Particular attention is paid to the influence of the bio-catalytic action of enzymatic hydrolysis on the deconstruction of annual plant material in the twin-screw extruder (a process called “bioextrusion”). The use of a twin-screw extruder allows working with high consistency (20%), in a high shear environment, for a short time (∼2 min). In the present work, the nature of the ligno(hemi)cellulosic material transformations, covering solubilization and extraction of saccharides and modification of cellulosic fibers, were investigated. 41% of hemicelluloses and 14% of lignin are extracted by the alkaline pretreatment. Hydrolytic enzymes are not deactivated during bioextrusion, which has a destructing effect on the fiber. It leads to a change of rheological properties and induces an increase of sugars released in the form of mono and polysaccharides (up to 13%/DM of total sugars) with longer chains than in the case of a batch reactor. At the same time, the degree of polymerization decreases and a shortening of the cellulose chains occurs. [ABSTRACT FROM AUTHOR]

Highlights: [•] New lignocellulosic biomass deconstruction process was evaluated on four biomasses. [•] Process combines thermo-mechano-chemical and the bio-catalytic action in extruder. [•] Saccharification begins during bioextrusion. [•] The process is efficient despite the variability of biomass characteristics. [•] Deconstruction process does not generate toxic compounds penalizing fermentation. [Copyright &y& Elsevier]

The polarized photoluminescence emitted on the edge of a series of aluminum‐rich (Al,Ga)N‐AlN quantum wells (QWs) grown by molecular beam epitaxy on AlN templates deposited by metal organic chemical vapor deposition on c‐plane sapphire is measured. The contrast and the principal axis of the emission diagrams for 2 nm‐thick (Al,Ga)N QWs grown for aluminum compositions between 40% and 90% are studied. The light is emitted on the edge of the QWs at wavelengths going from 280 nm down to 209 nm. The emission diagram, a change from oblate to prolate with respect to the in‐plane orientation, for an aluminum composition is found to occur around 72%, that is, at an emission wavelength of about 235 nm. The orientations and shapes of the edge‐emission diagrams indicate that the fluctuations of the composition of the (Al,Ga)N confining layer are deep enough for producing intravalence band mixings. This property, that acts in concert with the built‐in strain and quantum‐confined Stark effect, contributes to the anisotropy of the light emission when the aluminum composition reaches 60–70%, that is, for an emission wavelength of 260–235 nm. [ABSTRACT FROM AUTHOR]

Aluminium Gallium Nitride (AlyGa1-yN) quantum dots (QDs) with thin sub-µm AlxGa1-xN layers (with x > y) were grown by molecular beam epitaxy on 3 nm and 6 nm thick hexagonal boron nitride (h-BN) initially deposited on c-sapphire substrates. An AlN layer was grown on h-BN and the surface roughness was investigated by atomic force microscopy for different deposited thicknesses. It was shown that for thicker AlN layers (i.e., 200 nm), the surface roughness can be reduced and hence a better surface morphology is obtained. Next, AlyGa1-yN QDs embedded in Al0.7Ga0.3N cladding layers were grown on the AlN and investigated by atomic force microscopy. Furthermore, X-ray diffraction measurements were conducted to assess the crystalline quality of the AlGaN/AlN layers and examine the impact of h-BN on the subsequent layers. Next, the QDs emission properties were studied by photoluminescence and an emission in the deep ultra-violet, i.e., in the 275–280 nm range was obtained at room temperature. Finally, temperature-dependent photoluminescence was performed. A limited decrease in the emission intensity of the QDs with increasing temperatures was observed as a result of the three-dimensional confinement of carriers in the QDs. [ABSTRACT FROM AUTHOR]

In the case of molecular beam epitaxy (MBE), the Mg acceptors are electrically active in the as‐grown material and a priori no additional annealing procedure is necessary. However, there are still some peculiarities in the electrical properties of ammonia‐process grown GaN:Mg and some annealing effect can be observed. Additionally, the character of weak temperature dependence in the vicinity of room temperature suggests that to describe the conduction process an additional conduction channel not related to the free carriers in the valence band must be taken into account. For these reasons, this article presents the results of low‐temperature resistivity and Hall Effect studies of Mg‐doped, ammonia‐process‐grown GaN. The studied samples are grown on low‐temperature buffers of GaN deposited on a sapphire substrate. High‐temperature annealing process (≈800 K) is carried out for all of them. The temperature dependences of the electrical transport properties before and after the annealing procedure are especially investigated at temperatures ranging from 10 up to 300 K. It is found that the low temperatures transport properties are sensitive to the annealing procedure and to describe the observed effects the hopping phenomena must be taken into account. [ABSTRACT FROM AUTHOR]

A correction to the article "Dependence of the Mg-related acceptor ionization energy with the acceptor concentration in p-type GaN layers grown by molecular beam epitaxy" that was published in the previous issue is presented.

Hall effect and capacitance-voltage C(V) measurements were performed on p-type GaN:Mg layers grown on GaN templates by molecular beam epitaxy with a high range of Mg-doping concentrations. The free hole density and the effective dopant concentration NA-ND as a function of magnesium incorporation measured by secondary ion mass spectroscopy clearly reveal both a magnesium doping efficiency up to 90% and a strong dependence of the acceptor ionization energy Ea with the acceptor concentration NA. These experimental observations highlight an isolated acceptor binding energy of 245±25 meV compatible, at high acceptor concentration, with the achievement of p-type GaN:Mg layers with a hole concentration at room temperature close to 1019 cm-3. [ABSTRACT FROM AUTHOR]

In this study, AlN epilayers were grown by ammonia-assisted molecular beam epitaxy on 3 nm h-BN grown on c-sapphire substrates. Their structural properties were investigated by comparing as-grown and postgrowth annealed layers. The role of annealing on the crystalline quality and surface morphology was studied as a function of AlN thickness and the annealing duration and temperature. Optimum annealing conditions were identified. The results of X-ray diffraction showed that optimization of the annealing recipe led to a significant reduction in the symmetric (0 0 0 2) and skew symmetric (1 0 −1 1) reflections, which was associated with a reduction in edge and mixed threading dislocation densities (TDDs). Furthermore, the impact on the crystalline structure of AlN and its surface was studied, and the results showed a transition from a surface with high roughness to a smoother surface morphology with a significant reduction in roughness. In addition, the annealing duration was increased at 1650 °C to further understand the impact on both AlN and h-BN, and the results showed a diffusion interplay between AlN and h-BN. Finally, an AlN layer was regrown on the top of an annealed template, which led to large terraces with atomic steps and low roughness. [ABSTRACT FROM AUTHOR]

(Ga,In)N/GaN multiple quantum well blue light emitting diodes (LEDs) grown on mesa-patterned silicon substrates with improved electro-optic characteristics are demonstrated. The active regions are grown on top of high-reflectivity AlN/(Al,Ga)N distributed Bragg reflectors (DBRs). Due to efficient stress relaxation at the mesa edges, crack formation during growth or upon the post-growth cooling-down of the samples can be avoided. A large number of AlN/(Al,Ga)N bilayers in the DBR can be then included in the LED structures leading to strong enhancement of the LED device output power in spite of the presence of the absorbing silicon substrate at the LED emission wavelength. Photograph of a blue light emitting diode ( I = 20 mA) grown on top of a high reflectivity distributed Bragg reflector on a mesa-patterned silicon substrate. [ABSTRACT FROM AUTHOR]

High quality single crystalline n-type ZnO films, with a thickness of 200 nm, were grown on top of AlN/GaN layers under 25 W, 50 W and 100 W sputtering powers. The AlN and p-type GaN layers were deposited by metalorganic chemical vapor deposition (MOCVD). From X-ray diffraction measurements, the samples exhibited the (000 l) peaks corresponding to both ZnO, GaN and AlN monocrystalline layers. As the sputtering power was increased, the ZnO grain size increased and the dislocation density decreased. This result is supported by a reduction of the rms values obtained on the ZnO layers from Atomic Force Microscopy (AFM) results. In addition, photoluminescence peaks of ZnO at 372 nm, 375 nm and 380 nm were seen as dependent on the sputtering power. We have used an AlN electron blocking layer between ZnO and GaN films to improve the electroluminescence from the n-ZnO side. Room temperature electroluminescence (EL) of the LEDs demonstrated near UV-blue emission consisting of predominating peaks centred at 405 nm, 390 nm and 380 nm for the device with ZnO deposited at 25 W, 50 W and 100 W sputtering powers, respectively. Moreover, the I-V curves of the LEDs showed a rectifying behavior with 6.8 V, 6.4 V, 5.2 V threshold voltages for 25 W, 50 W and 100 W values. AlN/GaN/AlN/ZnO Tandem LED [Display omitted] • We presented a comprehensive and fine-tuned measurements and calculations of the n-ZnO/AlN/p-GaN/AlN hybrid type LEDs fabricated with 25 W, 50 W and 100 W sputtering powers of ZnO films. We have shown how a strong sputtering power such as 100 W has a curative effect on the crystallization of ZnO, • The intentionally used AlN electron blocking layer sandwiched between GaN and ZnO served as both a better nucleation of GaN and passivation layer, which can lead to a superior current spreading over the entire active area, and this combined with ZnO produced at high sputtering power was performed to further improve the efficiency of LED with higher and stronger light emission intensity than a conventional LED. • In this design, by depositing the AlN nucleation layer, we have also prevented the interphase, which has many insulating roles like GaOx. Thus, a better current transmissio- mechanism has been formed. • We fabricated the ZnO/GaN heterojunction with the AlN EBL layer by utilizing the continuous innovations in device fabrication techniques and analyzed in detail different application parameters such as crystallite sizes, dislocation densities and tensile stress including surface mechanisms all derived with XRD analysis. The Electroluminescence results were examined that light emission shifted from the blue region to near UV when switching from 25 W to 100 W. • The conclusions were supported by the energy band diagram by using the Anderson model, the energy barrier for electrons ΔEC, and holes ΔEV, at the interface of ZnO/AlN and GaN/AlN were obtained. Hall effect and secondary ion mass spectroscopy (SIMS) measurements were taken before and after AlN growth were obtained to explain the effects of possible H- and C- impurities, and our results show a remarkable difference in hole concentration and resistivity values. [ABSTRACT FROM AUTHOR]

We have grown (11-22)-oriented GaN/Al0.5Ga0.5N quantum wells (QWs) using molecular beam epitaxy on GaN (11-22)-oriented templates grown by metal-organic vapor phase epitaxy on m-plane oriented sapphire substrates. The performance of epitaxial growth of GaN/Al0.5Ga0.5N heterostructures on the semi-polar orientation (11-22) in terms of surface roughness and structural properties, i.e., strain relaxation mechanisms is discussed. In addition, high resolution transmission electron microscopy reveals very smooth QW interfaces. The photoluminescence of such samples are strictly originating from radiative recombination of free excitons for temperatures above 100 K. At high temperature, the population of localized excitons, moderately trapped (5 meV) at low temperature, is negligible. [ABSTRACT FROM AUTHOR]

In this paper, we report on the fabrication of a normally-off Al(Ga)N/GaN high electron mobility transistor with selective area sublimation under vacuum of the p type doped GaN cap layer. This soft method makes it possible to avoid damages otherwise induced by post processing with reactive ion etching techniques. The GaN evaporation selectivity is demonstrated on AlN as well as on AlGaN barrier layers. Furthermore, by properly choosing the AlGaN barrier thickness and composition it is possible to co-integrate a normally-off with a normally-on device on the same substrate. Finally, a local area regrowth of AlGaN can complement this process to increase the maximum drain current in the transistors. [ABSTRACT FROM AUTHOR]

AlN thin films, grown on (0001) sapphire substrates by molecular beam epitaxy (MBE), were annealed at high temperature (up to 1650 °C) in flowing N 2 . X-ray diffraction (XRD) studies, combined with Williamson-Hall and Srikant plots, have shown that annealing leads to a strong reduction of both edge and mixed threading dislocation densities, as confirmed by transmission electron microscopy (TEM) images, up to 75%. Moreover, it is found that annealing at high temperatures allows the relaxation of the tensile strain in the AlN film due to the growth process. In addition, the morphological properties of the films were determined by atomic force microscopy (AFM) and show that the annealing conditions have a strong impact on the surface morphology and roughness. Finally, an annealing at 1550 °C for 20 min appears as an ideal tradeoff to enhance the structural properties while preserving the initial AlN surface morphology. [ABSTRACT FROM AUTHOR]

Electrical transport (resistivity and Hall Effect) have been studied in silicon doped aluminum nitride (AlN) thick epitaxial layers from 250 K up to 1000 K. The investigated samples, grown by molecular beam epitaxy were characterized by n-type conduction with an ambient temperature free carrier concentration of about ∼ 1 × 10 15 cm −3 . The donor level, situated about 250 meV below the conduction band edge, was found to be responsible for the experimentally observed increase of free carrier concentration with temperature. The temperature dependence of carrier mobility has been analyzed in the framework of a multimode scattering model. In the investigated samples the main scattering mechanism is supposed to be dislocation scattering. [ABSTRACT FROM AUTHOR]

• Selective sublimation of p-GaN is developed to fabricate enhancement-mode HEMTs. • AlGaN regrowth drastically reduces access resistances in enhancement-mode HEMTs. • Sublimation and regrowth are combined to co-integrate E/D-mode GaN HEMTs. In the present study, the selective sublimation of the p-GaN cap layer of Al(Ga)N/GaN HEMTs is developed to replace the commonly used dry etching with no risk of damage in the barrier layer in order to fabricate enhanced mode transistors. Thanks to this approach, enhancement-mode transistors are fabricated with a threshold voltage between 0 V and +1.5 V depending on the barrier layer aluminum molar fraction and thickness. Furthermore, we show the benefit of the combination of selective sublimation with the regrowth of AlGaN to reduce access resistance in these transistors which can be co-integrated with depletion-mode devices fabricated in the same process in areas where p-GaN has been totally evaporated. [ABSTRACT FROM AUTHOR]

• Selective sublimation of p-GaN is developed to fabricate enhancement-mode HEMTs. • AlGaN regrowth drastically reduces access resistances in enhancement-mode HEMTs. • Sublimation and regrowth are combined to co-integrate E/D-mode GaN HEMTs. We report on the fabrication of an enhancement mode p-GaN/AlN/GaN high electron mobility transistor with selective area sublimation under vacuum of the p-GaN cap layer. The GaN evaporation selectivity is demonstrated on the thin 2 nm AlN barrier layer. Furthermore, the regrowth of AlGaN is a major key to increase the maximum drain current in the transistors and enables the co-integration with depletion mode devices. [ABSTRACT FROM AUTHOR]

We have investigated the optical properties of multi-stacked GaN/AlGaN self-assembled quantum dots (QDs) grown by molecular beam epitaxy. The QDs that emit visible light have a broad spectral range without incorporation of indium alloy because of the quantum-confined Stark effect. We found differences in the structural and optical properties between the layers of multi-stacked QDs. The carriers are more effectively transferred from the AlGaN barrier to the low energy side of the GaN QD emission than to the high energy side. We also observed long-lived carrier recombination dynamics for the visible range emission from QDs. [ABSTRACT FROM AUTHOR]

Inductively coupled plasma (ICP) is widely used in dry etching of III‐nitride materials, wherein the etching parameters of GaN and AlN are very different. Herein, the ICP dry etching process parameters of GaN/AlN periodically stacked structure (PSS) for avalanche photodiode (APD) fabrication are intensively studied and optimized. The flow rate ratio of Cl2/BCl3/Ar plasma, bias voltage, and the GaN‐to‐SiNx selectivity of ICP etching are optimized to achieve excellent surface morphology and nearly vertical sidewalls. It is found that the etching rate and the etched surface roughness of GaN/AlN material are significantly influenced by the flow rate of Cl2. After optimizing the etching procedure, the root‐mean‐square roughness of the etched surface is measured to be 1.46 nm, which is close to the as grown surface. By using the optimized ICP dry etching in the fabrication of the GaN/AlN PSS APD, the dark current is suppressed from 3.6 to 8.2 × 10−3 A cm−2 at −90 V. [ABSTRACT FROM AUTHOR]

Hybrid heterostructures based on bulk GaN and two-dimensional (2D) materials offer novel paths toward nanoelectronic devices with engineered features. Here, we study the electronic properties of a mixed-dimensional heterostructure composed of intrinsic n-doped MoS2 flakes transferred on p-doped GaN(0001) layers. Based on angle-resolved photoemission spectroscopy (ARPES) and high resolution x-ray photoemission spectroscopy (HR-XPS), we investigate the electronic structure modification induced by the interlayer interactions in MoS2/GaN heterostructure. In particular, a shift of the valence band with respect to the Fermi level for MoS2/GaN heterostructure is observed, which is the signature of a charge transfer from the 2D monolayer MoS2 to GaN. The ARPES and HR-XPS revealed an interface dipole associated with local charge transfer from the GaN layer to the MoS2 monolayer. Valence and conduction band offsets between MoS2 and GaN are determined to be 0.77 and -0.51eV, respectively. Based on the measured work functions and band bendings, we establish the formation of an interface dipole between GaN and MoS2 of 0.2 eV. [ABSTRACT FROM AUTHOR]

We report on the influence of a capping layer on the photoluminescence properties of self-assembled GaN quantum dots grown on an Al0.5Ga0.5N template. Self-assembled GaN quantum dots show a large quantum confined Stark shift and long carrier recombination time due to strong built-in spontaneous and piezoelectric polarization fields. Nevertheless, owing to strong carrier localization and suppressed nonradiative processes, these quantum dots have a high-quantum efficiency even at room temperature. Here, we show that the capping thickness has an important role on the optical properties of the GaN quantum dots. The radiative and nonradiative recombination processes of quantum dots are strongly affected by adjusting the capping thickness, and the GaN quantum dots with 12 monolayers-thick Al0.5Ga0.5N capping layer show a remarkably high internal quantum efficiency of more than 80% at room temperature. We also studied photoluminescence quenching and enhancement for surface (uncapped) quantum dots caused by photoadsorption and photodesorption of oxygen. [ABSTRACT FROM AUTHOR]