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In this study, the effect of implementing island growth in an AlN/Al0.1Ga0.9N superlattice on the structural properties of vertical GaN‐on‐Si(111) PIN‐structures is investigated. It is demonstrated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) that islands are formed on‐top of V‐pits present in the AlN nucleation layer and that the island coalescence height can be controlled by the growth temperature. Defect selective etching analyses confirm a noteworthy reduction in the threading dislocation density (TDD), which diminishes from 1.2 × 109 cm−2 ± 7.5 × 107 cm−2 to 8.5 × 108 cm−2 ± 7.3 ×107 cm−2 as the island coalescence height increases from ≈160 nm to ≈450 nm, achieved by increasing the growth temperature. Cross‐sectional transmission electron microscopy (TEM) shows that island growth is particularly favorable for the reduction of a‐type dislocations. As a consequence of the significant reduction of a‐type dislocations in the buffer, stress relaxation during the GaN film growth is reduced, which is supported by in situ wafer curvature measurements and high‐resolution X‐ray diffraction (XRD). Owing to the optimized island growth conditions, thick and crack‐free GaN layers on Si(111) substrates are obtained with an absolute wafer bow of <50 μm. [ABSTRACT FROM AUTHOR]

We report on the electrical and microstructural characterization of an Au-free V/Al/Ti/TiN ohmic contact for AlGaN/GaN heterostructures. Ultra-low contact resistance and specific contact resistivity of Rc < 0.1 Ω mm and ρc < 2.4 × 10−7 Ω cm2 have been achieved with very low RMS surface roughness. This was accomplished at a comparably low annealing temperature of 800 °C and without applying any contact recess, regrowth, or implantation process. High electron mobility transistors were fabricated and a comparison of the electrical performance with state-of-the-art Ti/Al/Ti/TiN and Ti/Al/Ni/Au contacts was made. The contact formation mechanism is discussed on the basis of microstructural features. [ABSTRACT FROM AUTHOR]

Threading dislocations in the AlGaN-barrier of four pairwise differently grown AlGaN/GaN high electron mobility transistor structures on Si were investigated with respect to their structural and electrical properties in direct comparison simultaneously ensuring statistical significance of the results. Portions of pure screw and mixed type dislocations were observed to serve as leakage current paths and to be clearly dependent on growth conditions like the AlN nucleation layer growth temperature. The role of impurity segregation at dislocation cores due to growth-dependent locally characteristic strain fields as for example induced by specific dislocation reactions at the AlGaN/GaN interface is discussed as the origin. [ABSTRACT FROM AUTHOR]

Analytical transmission electron microscopy (ATEM) was used to quantitatively measure sub-micrometer compositional profiles produced experimentally by Fe-Mg interdiffusion experiments in olivine. Although analysis of minerals by ATEM is common, compositional profiles suitable for quantitative modeling of diffusion are not generally measured in minerals with ATEM. To demonstrate the suitability of ATEM for diffusion studies in minerals, we have investigated Fe-Mg interdiffusion in experimentally annealed olivine. Because the compositional gradients were induced under well-controlled laboratory conditions, the accuracy of the measurements could be tested by comparing compositional profiles measured by both ATEM and EMPA as well as by retrieving diffusion coefficients from both TEM and EMPA data. The agreement in diffusion coefficients shows that point defect equilibration in the interfacial region of the diffusion couple occurs extremely rapidly at 1200 [degrees] C. The ability to obtain diffusion data from such short anneals enables various experiments that were not previously possible - for example, to study diffusion rates at high pressures where long [[f.sub.O].sub.2] buffered anneals are not generally feasible. ATEM profile measurement is compared with other techniques such as SIMS and RBS and some limitations and applications of ATEM profile measurements are also discussed.

The crystal quality of bulk GaN crystals is continuously improving due to advances in GaN growth techniques. Defect characterization of the GaN substrates by conventional methods is impeded by the very low dislocation density and a large scale defect analysis method is needed. White beam synchrotron radiation x-ray topography (SR-XRT) is a rapid and non-destructive technique for dislocation analysis on a large scale. In this study, the defect structure of an ammonothermal c-plane GaN substrate was recorded using SR-XRT and the image contrast caused by the dislocation induced microstrain was simulated. The simulations and experimental observations agree excellently and the SR-XRT image contrasts of mixed and screw dislocations were determined. Apart from a few exceptions, defect selective etching measurements were shown to correspond one to one with the SR-XRT results. [ABSTRACT FROM AUTHOR]

Metastable cobalt nitrides Co4N, Co3N, Co2N, and cobalt oxide nitride CoO0.74N0.24 were synthesized by ammonolysis of one single metastable azide precursor, [Co(NH3)5N3]Cl2, in high purity by controlled variation of the reaction conditions (temperature, duration, gas atmosphere, and flow rate). The alternative route towards cobalt nitrides by ammonolysis of CoBr2 proceeds via [Co(NH3)6]Br2 and Co(NH3)2Br2 to exclusively yield Co3N. For Co3N1.1 a complex magnetic behavior is observed: dominating ferromagnetic ordering at low temperatures and a small coercive field of 10 mT. Co2N orders antiferromagnetically below 10 K. Electronic structure calculations [DFT, generalized-gradient approximation (GGA)+ U] on CoO0.75N0.25 reveal that the experimentally observed rock-salt structure is less stable than the sphalerite-type polymorph by about 30 kJ mol-1 and possesses an antiferromagnetically ordered ground state. [ABSTRACT FROM AUTHOR]

Three different synthetic routes to chromium nitrides were investigated using in situ neutron diffraction (SPODI, FRM-II) and thermal analysis in ammonia: (i) The direct conversion of chromium powder in ammonia, (ii) the ammonolysis of chromium(III) chloride CrCl3, and (iii) the ammonolysis of chromium(III) sulfide Cr2S3. During the direct conversion of chromium first γ-CrN was formed above 385 °C, above 567 °C additionally β-Cr2N was detected. Finally, a complete conversion of γ-CrN to β-Cr2N was observed at 1080 °C. The ammonolysis of chromium(III) compounds (CrCl3 and Cr2S3) revealed a so far in 3d metal nitride synthesis unprecedented redox sequence: The compounds are reduced to chromium(II) chloride CrCl2 and chromium(II, III) sulfide Cr3S4, respectively, prior to the reoxidation of those intermediates to γ-CrN. In contrast to the ammonolysis of 3d metal(II) halides, for chromium(III) chloride no temporary formation of ammoniates was observed. Since formation of γ-CrN was detected only after annealing chromium(III) sulfide in flowing ammonia for 4 h to 5 h at maximum temperature, the formation of sufficient amounts of chromium(II, III) sulfide as the rate-determining step for the nitride formation appears likely. [ABSTRACT FROM AUTHOR]

Metal‐organic chemical vapor deposition growth of vertical GaN PN structures on 6″ Si(111) substrates enabling a 1200 V breakdown voltage is demonstrated. Thanks to an optimized buffer structure utilizing island growth in an AlN/Al0.1Ga0.9N superlattice, the threading dislocation density is drastically reduced, and sufficient compressive stress is incorporated in active GaN layers to compensate for the thermal mismatch. Crack‐free PN structures with drift layer thicknesses up to 7.4 μm are realized with a threading dislocation density of ≈5 × 108 cm−2 and an absolute wafer bow <50 μm. Quasi‐vertical PN diodes reveal a linear increase in the breakdown voltage with the drift layer thickness with an average breakdown field of ≈1.6 MV cm−1. Additionally, the leakage current is shown to decrease monotonically as the drift layer thickness increases. For a 7.4 μm thick drift layer with a net ionized donor concentration of 0.9 × 1016 cm−3, a high breakdown voltage of 1200 V, a low specific on‐resistance of 0.4 mΩ cm−2, and a low leakage current of 10−4 A cm−2 (at a reverse bias of 650 V) are obtained. These results demonstrate the great potential of cost‐effective vertical GaN‐on‐Si power devices operating in the kilovolt range. [ABSTRACT FROM AUTHOR]