Generation of neutrons in a nanosecond low-pressure discharge in deuterium

The neutron yield is measured in a high-voltage Townsend discharge in deuterium with a hollow cylinder made of tungsten or steel used as a polarizing anode of electrons. A flat metallic plate covered by a layer of deuterated zirconium is applied as a grounded cathode. The highest yield of neutrons in the reaction 2H(d,n)3He, ∼1.2 × 104 neutrons per pulse, is observed in the case of the tungsten anode at a deuterium pressure on the order of 100 Pa. The pulsed neutron flux duration estimated with data obtained from a scintillation detector is roughly equal to 1.5 ns

Ultraviolet stimulated emission in AlGaN layers grown on sapphire substrates using ammonia and plasma-assisted molecular beam epitaxy

Ammonia and plasma‐assisted (PA) molecular beam epitaxy modes are used to grow AlN and AlGaN epitaxial layers on sapphire substrates. It is determined that the increase of thickness of AlN buffer layer grown by ammonia‐MBE from 0.32 μm to 1.25 μm results in the narrowing of 101 X‐Ray rocking curves whereas no clear effect on 002 X‐Ray rocking curve width is observed. It is shown that strong GaN decomposition during growth by ammonia‐MBE causes AlGaN surface roughening and compositional inhomogeneity, which leads to deterioration of its lasing properties. AlGaN layers grown by ammonia‐MBE at optimized temperature demonstrate stimulated emission (SE) peaked at λ = 330 nm, 323 nm, 303 nm and 297 nm with the SE threshold values of 0.7 MW cm−2, 1.1 MW cm−2, 1.4 MW cm−2 and 1.4 MW cm−2, respectively. In comparison to these, AlGaN layer grown using PA‐MBE pulsed modes (migration‐enhanced epitaxy, metal‐modulated epitaxy, and droplet elimination by thermal annealing) shows a SE with a relatively low threshold (0.8 MW cm−2) at the considerably shorter wavelength of λ = 267 nm

Generation of neutrons in a nanosecond low-pressure discharge in deuterium

The neutron yield is measured in a high-voltage Townsend discharge in deuterium with a hollow cylinder made of tungsten or steel used as a polarizing anode of electrons. A flat metallic plate covered by a layer of deuterated zirconium is applied as a grounded cathode. The highest yield of neutrons in the reaction 2H(d,n)3He, ∼1.2 × 104 neutrons per pulse, is observed in the case of the tungsten anode at a deuterium pressure on the order of 100 Pa. The pulsed neutron flux duration estimated with data obtained from a scintillation detector is roughly equal to 1.5 ns

Defect-related photoluminescence and photoluminescence excitation as a method to study the excitonic bandgap of AlN epitaxial layers : Experimental and ab initio analysis

We report defect-related photoluminescence (PL) and its vacuum ultraviolet photoluminescence excitation (PLE) spectra of aluminum nitride layers with various layer thicknesses and dislocation densities grown on two different substrates: sapphire and silicon. The defect-related transitions have been distinguished and examined in the emission and excitation spectra investigated under synchrotron radiation. The broad PL bands of two defect levels in the AlN were detected at around 3 eV and 4 eV. In the PLE spectra of these bands, a sharp excitonic peak originating most probably from the A-exciton of AlN was clearly visible. Taking into account the exciton binding energy, the measurements allow determination of the bandgaps of the investigated AlN samples and their temperature dependencies. Next, they are compared with the literature data obtained by other experimental techniques for bulk AlN crystals and layers grown on different substrates. The obtained results revealed that the AlN bandgap depends on the substrate. The theoretical analysis using density functional theory calculations showed that the effect is induced by the tetragonal strain related to the lattice mismatch between the substrate and the AlN layer, which has a strong influence on the spectral positions of the intrinsic excitons, and consequently on the bandgap of AlN layers