J. Barjon

Surfactant effect of In for AlGaN growth by plasma-assisted molecular beam epitaxy

In this article, the surfactant capability of In for AlGaN growth by plasma-assisted molecular beam epitaxy has been assessed. We have determined the range of substrate temperatures and In fluxes to form a self-regulated 1×1 In adlayer on AlxGa1−xN(0001). The presence of this In film favors two-dimensional growth of AlGaN under stoichiometric conditions. The formation of metal droplets on the surface is inhibited. In incorporation, if any, is lower than 0.01%. The structural quality of the layers is verified by high-resolution x-ray diffraction, both in symmetric and asymmetric reflections.In this article, the surfactant capability of In for AlGaN growth by plasma-assisted molecular beam epitaxy has been assessed. We have determined the range of substrate temperatures and In fluxes to form a self-regulated 1×1 In adlayer on AlxGa1−xN(0001). The presence of this In film favors two-dimensional growth of AlGaN under stoichiometric conditions. The formation of metal droplets on the surface is inhibited. In incorporation, if any, is lower than 0.01%. The structural quality of the layers is verified by high-resolution x-ray diffraction, both in symmetric and asymmetric reflections.

Origin of the excitonic recombinations in hexagonal boron nitride by spatially resolved cathodoluminescence spectroscopy

The excitonic recombinations in hexagonal boron nitride (hBN) are investigated with spatially resolved cathodoluminescence spectroscopy in the UV range. Cathodoluminescence images of an individual hBN crystallite reveals that the 215 nm free excitonic line is quite homogeneously emitted along the crystallite whereas the 220 nm and 227 nm excitonic emissions are located in specific regions of the crystallite. Transmission electron microscopy images show that these regions contain a high density of crystalline defects. This suggests that both the 220 nm and 227 nm emissions are produced by the recombination of excitons bound to structural defects.

Excitonic recombinations in hBN: from bulk to exfoliated layers

Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices is now of intense research focus, and it becomes particularly important to evaluate the role played by crystalline defects on their properties. In this paper, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared to those of nanosheets mechanically exfoliated from them. First, the link between the presence of structural defects and the recombination intensity of trapped excitons, the so-called D series, is confirmed. Low defective h-BN regions are further evidenced by CL spectral mapping (hyperspectral imaging), allowing us to observe new features in the near-band-edge region, tentatively attributed to phonon replicas of exciton recombinations. Second, the h-BN thickness was reduced down to six atomic layers, using mechanical exfoliation, as evidenced by atomic force microscopy. Even at these low thicknesses, the luminescence remains intense and exciton recombination energies are not strongly modified with respect to the bulk, as expected from theoretical calculations, indicating extremely compact excitons in h-BN.

Metal organic chemical vapor deposition growth and luminescence of ZnO micro- and nanowires

One dimensional (1D) ZnO heterostructures were deposited on C-plane sapphire using metal organic chemical vapor deposition at atmospheric pressure. Both catalyst-assisted and catalyst-free processes were investigated. In this latter case, growth parameters such as the substrate temperature, the oxygen/zinc ratio, or the carrier gas nature were varied in order to observe the modification of the film morphology. Different 1D shapes were produced, including rods, tubes, and needles, and were observed by scanning electron microscopy. Photoluminescence spectra reveal sharp excitonic transitions, and cathodoluminescence signals recorded along a ZnO conical needle exhibit a blueshift from the base to the top. The vapor-liquid-solid growth of ZnO nanowires using gold droplets was also successful.

Determination of the phosphorus content in diamond using cathodoluminescence spectroscopy

In n-type diamond doped with phosphorus, exciton properties have been investigated by cathodoluminescence as a function of the phosphorus concentration and the temperature. The homoepitaxial diamond layers were grown by microwave plasma-assisted chemical vapor deposition and doped using a liquid organic precursor of phosphorus (tertiarybutylphosphine). The phosphorus concentration ranges from 5.2×1016 to 3.3×1018 cm−3 as measured by secondary ion mass spectrometry. It is shown that the ratio between the luminescence intensities of the neutral phosphorus-bound exciton and the free exciton follows the dopant concentration. Calibration graphs are presented to determine the phosphorus contents in diamond using cathodoluminescence spectroscopy.

Optical characteristics of hexagonal GaN self-assembled quantum dots: Strong influence of built-in electric field and carrier localization

Optical characteristics of hexagonal GaN self-assembled quantum dots (QDs) were systematically studied by photoluminescence (PL), PL excitation (PLE), time-resolved PL, and cathodoluminescence (CL). We observed a Stokes-like shift between PLE absorption edge and PL emission from the GaN QDs as well as from the Al(Ga)N base layer. With decreasing emission energy, the measured lifetime of the hexagonal GaN QDs emission increased, while that of the cubic GaN QDs kept almost constant. The optical emission from the GaN QDs was measured as a function of temperature from 10 to 300 K, and their properties were compared with GaN quantum-well structures. With increasing temperature, the PL intensity of Al(Ga)N base layer or GaN quantum wells was dramatically decreased, while that of GaN QDs was not changed much. We observed CL images showing strong carrier localization in GaN QDs. Therefore, we conclude that the GaN QD emissions are strongly influenced by built-in electric field as well as by carrier localization i...

Self-Assembled GaN Quantum Dots Grown by Plasma-Assisted Molecular Beam Epitaxy

It is shown that under specific growth conditions the strain relaxation of GaN deposited on AlN obeys the Stranski-Krastanov mechanism. As a consequence, it is demonstrated that, for both cubic and hexagonal phases, the growth of self-organized three-dimensional islands can be achieved. These islands behave as quantum dots, exhibiting optical properties dominated by localization effects.

Hexagonal boron nitride nanowalls: physical vapour deposition, 2D/3D morphology and spectroscopic analysis

Hexagonal boron nitride nanowalls were synthesized using reactive radio-frequency magnetron sputtering in combination with a hexagonal BN target. The nanowall formation is purely governed by addition of hydrogen to the nitrogen/argon gas mixture, and leads to a decreased incorporation of carbon and oxygen impurities. The surface morphology is assessed with scanning electron microscopy, while stoichiometry and reduced impurity content of the material was evidenced using Rutherford backscattering spectroscopy. Transmission electron microscopy confirms the hexagonal nature of the nanowalls, whose luminescent properties are studied with cathodoluminescence spectroscopy, shedding more light on the location and nature of the excitonic emission and crystalline quality of the h-BN nanowalls.

Growth and in situ annealing conditions for long-wavelength (Ga, In)(N, As)/GaAs lasers

The conjugated effect of growth temperature and in situ thermal annealing on the photoluminescence properties of In0.4Ga0.6As0.985N0.015/GaAs quantum wells (QWs) grown by molecular-beam epitaxy has been investigated. The interplay between growth temperature and annealing effects is such as the optimum growth temperature is not the same for as-grown and annealed samples. By using the combination of a low growth temperature and a high in situ annealing temperature, separate confinement heterostructure laser diodes with a single In0.4Ga0.6As1−xNx (x=0.015–0.021)/GaAs QW have been grown. The broad area devices emit from 1.34 to 1.44 μm at room temperature with a threshold current density of 1500–1755A∕cm2.

Evaluation of freestanding boron-doped diamond grown by chemical vapour deposition as substrates for vertical power electronic devices

In this study, 4 × 4 mm2 freestanding boron-doped diamond single crystals with thickness up to 260 μm have been fabricated by plasma assisted chemical vapour deposition. The boron concentrations measured by secondary ion mass spectroscopy were 1018 to 1020 cm−3 which is in a good agreement with the values calculated from Fourier transform infrared spectroscopy analysis, thus indicating that almost all incorporated boron is electrically active. The dependence of lattice parameters and crystal mosaicity on boron concentrations have also been extracted from high resolution x-ray diffraction experiments on (004) planes. The widths of x-ray rocking curves have globally shown the high quality of the material despite a substantial broadening of the peak, indicating a decrease of structural quality with increasing boron doping levels. Finally, the suitability of these crystals for the development of vertical power electronic devices has been confirmed by four-point probe measurements from which electrical resisti...