No abstract

Articles you may be interested inInductively coupled plasma-reactive ion etching of c-and a-plane AlGaN over the entire Al composition range: Effect of BCl3 pretreatment in Cl2/Ar plasma chemistry J. Vac. Sci. Technol. A 31, 061305 (2013); 10.1116/1.4818871 Highly selective zero-bias plasma etching of GaN over AlGaN Evolution of surface roughness of AlN and GaN induced by inductively coupled Cl 2 / Ar plasma etching J. Appl. Phys. 95, 4635 (2004); 10.1063/1.1688993 Effect of additive noble gases in chlorine-based inductively coupled plasma etching of GaN, InN, and AlN J.

No abstract

Thick GaN bars with ͓112 គ 0͔ orientation have been sliced from GaN boules grown on freestanding films by hydride vapor phase epitaxy ͑HVPE͒ in the ͓0001͔ direction. High-resolution x-ray diffraction and transmission electron microscopy have been used to study the structural quality and defect distribution in the material in comparison to heteroepitaxially grown thick HVPE-GaN films grown in the ͓112 គ 0͔ direction on ͑11 គ 02͒-plane sapphire. It is demonstrated that while the heteroepitaxial material possesses a high density of stacking faults and partial dislocations, leading to anisotropic structural characteristics, the ͑112 គ 0͒-plane bulk GaN, sliced from boules, exhibits low dislocation density and narrow rocking curves with isotropic in-plane character.

Laser-induced fluorescence spectroscopy in a collimated atomic beam has been applied to determine isotope shifts and the hyperfine structure of an isotopic chain of the radioactive element polonium (' Po, 202Po, '0 ' Po). The relative isotope shifts show a striking similarity with results for other elements in the vicinity of Pb, even reproducing details of the odd-even staggering.Optical spectroscopy, in particular, highly sensitive laser-spectroscopic methods, have proven to be a powerful tool for investigations of nuclear ground-state properties through measurements of isotope shifts and hyperfine structure in atomic spectra [1]. In heavy elements, isotope shifts directly reflect, to a large extent, the variations in the mean-square radius (r ) of the nuclear charge distribution. From the point of view of nuclear structure interest, this is advantageous and is in contrast to the situation found for medium-mass nuclei where the uncertainties of mass-dependent effects (due to many-electron correlations) may obscure the nuclear information. Thus, systematic studies of relative isotope shifts in neighboring heavy elements may provide a basis for isotopic and isotonic comparisons, i.e. , of studies of the nucleon rearrangement when neutrons and protons are added or removed. In this aspect the region in the vicinity of the doubly magic nucleus Pb (Z =82, N=126) has attracted considerable attention following the observation of a sudden change of the nuclear deformation in light Hg isotopes [2] with subsequent investigation of Au (Ref. [3]) and Pt (Ref. [4]) nuclei.Nevertheless, there appears to be a conspicuous lack of experimental information for elements with Z )82, such as Bi, Po, and At, which would allow interesting comparisons of the nuclear structure before and after closing of shells. This situation has prompted the present laser-spectroscopic studies of polonium isotopes for which experimental data on isotope shifts and nuclear radii were completely missing, and the knowledge on electromagnetic moments of the ground state is scarce.The measurements are based on the observation of resonance fluorescence after excitation of the atomic transition 6p P2 6p 7s S2 of Po I in a collimated atomic beam. The required ultraviolet light (A, =255.8 nm) was generated in a Coherent-type 699 cw ring dye laser with intracavity frequency doubling (Fig. 1). The dye laser was operated with Coumarin 498 at k =511. 6 nm; it was optically pumped by the 457.9-nm line of an argon-ion laser at a power of about 1.5-2.2 W. Because of the low pump power available, the internal losses of the dye laser cavity had to be reduced. For this purpose the mode selecting intracavity assembly was removed from the laser and replaced by two almost "lossless" uncoated etalons of 2 mm and 0.5 mm thickness [5].A P-barium borate crystal [6] (5 mm length) was used for frequency doubling; its entrance and exit faces were laser fluorescence & computerf requency intensity iodine j spectrum