Hans Werner Becker

Investigations on InN whiskers grown by chemical vapour deposition

Abstract Nanostructures of compound semiconductors of group-III nitrides are ideal building blocks for nanoscale optoelectronic devices. InN has a low decomposition temperature and the growth of nanoscale crystalline InN material at low temperatures is difficult. One of the approaches is to design single molecule precursors that decompose at low temperatures. Single molecule precursors of the type N 3 In[(CH 2 ) 3 NMe 2 ] 2 were developed and the growth of dense crystalline InN layers with preferred orientation was achieved using this compound. However, employing specific CVD process parameters we were able to grow InN whiskers consistently by CVD using a cold wall CVD reactor on bare sapphire substrates at a growth temperature of 500°C. These whiskers were characterised by XRD, SEM, EDX, XPS, RBS, TEM and SAED measurements.

12C + 12C Fusion Reactions near the Gamow Energy

The fusion reactions 12C(12C,a)20Ne and 12C(12C,p)23Na have been studied from E = 2.10 to 4.75 MeV by gamma-ray spectroscopy using a C target with ultra-low hydrogen contamination. The deduced astrophysical S(E)* factor exhibits new resonances at E<= 3.0 MeV, in particular a strong resonance at E = 2.14 MeV, which lies at the high-energy tail of the Gamow peak. The resonance increases the present non-resonant reaction rate of the alpha channel by a factor of 5 near T = 8x10^8 K. Due to the resonance structure, extrapolation to the Gamow energy E_G = 1.5 MeV is quite uncertain. An experimental approach based on an underground accelerator placed in a salt mine in combination with a high efficiency detection setup could provide data over the full E_G energy range.

Fe-Mg interdiffusion rates in clinopyroxene: Experimental data and implications for Fe-Mg exchange geothermometers

AbstractnChemical interdiffusion of Fe–Mg along the c-axis [001] in natural diopside crystals (XDixa0=xa00.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200xa0°C and oxygen fugacities from 10−11 to 10−17 bar. Diffusion couples were prepared by ablating an olivine (XFoxa0=xa00.3) target to deposit a thin film (20–100xa0nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400xa0nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of DFe–Mg on composition of clinopyroxene (Fe/Mg ratio between Di93–Di65) or oxygen fugacity could be detected within the resolution of the study. The lack of fO2-dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, DFe–Mg, can be described by a single Arrhenius relation with

Development of graded Ni―YSZ composite coating on Alloy 690 by Pulsed Laser Deposition technique to reduce hazardous metallic nuclear waste inventory

D^{{{text{Fe}} - {text{Mg}}}} = 2. 7 7pm 4. 2 7times 10^{ - 7} {text{exp(}}-3 20. 7pm 1 6.0{text{ kJ}}/{text{mol}}/{text{RT)m}}^{ 2} /{text{s}}.

Ligand stabilised dialkyl aluminium amides as new precursors for aluminium nitride thin films

DFe-Mg=2.77±4.27×10-7exp(-320.7±16.0kJ/mol/RT)m2/s.DFe–Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., DCa–Mg) while it is slower than DFe–Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100xa0°C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks).

Hydrogen Interaction with Defects in ZnO

The use of alternative nitrogen sources for growing GaN materials by organometallic vapor phase epitaxy (OMVPE) is being continuously investigated in the hope of achieving device-quality films under moderate conditions, in comparison to conventional methods. Employing the single molecule precursor (N 3 ) 2 Ga[(CH 2 ) 3 NMe 2 ], and using a cold-wall CVD reactor, epitaxial films of GaN, transparent in appearance and stoichiometric in composition, were deposited on c-plane sapphire, in the absence of ammonia, above 1073 K, under low pressures (between 0.080 and 100.0 mbar). Dense, amorphous, and very smooth films were grown at temperatures as low as 773 K. The influence of substrate temperature, reactor pressure, and the effect of small quantities of additional ammonia, on the growth rate and the film properties, were studied in some detail. The films were characterized by high-resolution X-ray diffraction (XRD) (e.g., full width at half maximum (FWHM) of the 0002 GaN rocking curve of 130 arcsec), X-ray reflectometry, scanning electron microscopy (SEM), atomic force microscopy (AFM) (root mean square roughness of 1.9 nm), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), Rutherford backscattering (RBS) (Ga/N = 1:1 ± 0.05), and photoluminescence (PL) measurements (band edge luminescence at 3.45 eV and FWHM of 0.22 eV at 300 K).

Diffusivity of Hydrogen in ZnO Single Crystal

Alloy 690 based nuclear waste vitrification furnace components degrade prematurely due to molten glass-alloy interactions at high temperatures and thereby increase the volume of metallic nuclear waste. In order to reduce the waste inventory, compositionally graded Ni-YSZ (Y(2)O(3) stabilized ZrO(2)) composite coating has been developed on Alloy 690 using Pulsed Laser Deposition technique. Five different thin-films starting with Ni80YSZ20 (Ni 80 wt%+YSZ 20 wt%), through Ni60YSZ40 (Ni 60 wt%+YSZ 40 wt%), Ni40YSZ60 (Ni 40 wt%+YSZ 60 wt%), Ni20YSZ80 (Ni 20 wt%+YSZ 80 wt%) and Ni0YSZ100 (Ni 0 wt%+YSZ 100 wt%), were deposited successively on Alloy 690 coupons. Detailed analyses of the thin-films identify them as homogeneous, uniform, pore free and crystalline in nature. A comparative study of coated and uncoated Alloy 690 coupons, exposed to sodium borosilicate melt at 1000°C for 1-6h suggests that the graded composite coating could substantially reduced the chemical interactions between Alloy 690 and borosilicate melt.

Anisotropy of Hydrogen Diffusivity in ZnO

The nuclear spins of ionized donors in silicon have become an interesting quantum resource due to their very long coherence times. Their perfect isolation, however, comes at a price, since the absence of the donor electron makes the nuclear spin difficult to control. We demonstrate that the quadrupolar interaction allows us to effectively tune the nuclear magnetic resonance of ionized arsenic donors in silicon via strain and determine the two nonzero elements of the S tensor linking strain and electric field gradients in this material to S(11)=1.5×10(22)u2009u2009V/m2 and S(44)=6×10(22)u2009u2009V/m2. We find a stronger benefit of dynamical decoupling on the coherence properties of transitions subject to first-order quadrupole shifts than on those subject to only second-order shifts and discuss applications of quadrupole physics including mechanical driving of magnetic resonance, cooling of mechanical resonators, and strain-mediated spin coupling.