We investigated the structural evolution of the Ni/Au contact on GaN(000l) during annealing in N2, using in-situ x-ray diffraction, anomalous x-ray scattering, and high resolution electron microscopy. GaN decomposition occurred mostly along GaN dislocations at temperature higher than 500°C. The decomposed Ga diffused into Au and Ni substitutional positions, and the decomposed nitrogen reacted with Ni, forming Ni4N. Interestingly, Ni4N was grown epitaxially. The epitaxial relationship of the Ni4N, Au, and Ni was identified as M(111)//GaN(0002) and M[1 −1 0]//GaN[1 1 −2 0] (M= Ni4N, Au, and Ni). At dislocation free regions, however, the atomically smooth interface remained intact up to 700 °C. Remarkable improvement of device reliability is expected in the contact on dislocation free regions compared with the contact on dislocations.
Quantifying humidity has long been an unavoidable task in science, industry, and society. Recent developments of nanoscience and technology that deal with ultrasmall droplets have aroused interest in microscopic moisture. Utilization of nanomaterials has been emerging as a promising strategy to miniaturize hygrometers for high‐sensitive, ultrasmall‐area sensing. However, a lack of high‐precision, on‐demand position control of sensing nanomaterials makes it difficult to explore spatial distribution of humidity at the micro‐ and nanoscale. Here, a scanning probe hygrometry (SPH) is developed that enables not only micro/nanoresolution but also scalable spatial mapping of humidity distribution. The SPH is realized with an unprecedented scanning nanowire probe interferometer (NPI) that is produced by direct 3D nanoprinting of a moisture‐sensitive polymer on a tapered optical fiber. Notably, the interferometric response of the NPI probe in ultrasmall areas quantitatively depends on humidity, arising from its refractive index change and volumetric swelling. By scanning the NPI probe and reading out the interferometric signals, multiscale spatial mapping of humidity distribution with versatile scanning steps from ≈102 nm to a few mm is demonstrated. The NPI is expected to provide a new nanoscale metrology that can answer fundamental questions about evaporation‐related science and engineering.
The structural evolution of GaN films during the initial growth process of metalorganic chemical vapor deposition (MOCVD) - low temperature nucleation layer growth, annealing, and high temperature epitaxial growth - was investigated in a synchrotron x-ray scattering experiment. The nucleation layer grown at 560°C that was predominantly cubic GaN consisted of tensile-strained aligned domains and relaxed misaligned domains. The hexagonal GaN, transformed from the cubic GaN during annealing to 1100 °C, showed disordered stacking. The atomic layer spacing decreased as the fraction of the hexagonal domains increased. Subsequent growth of epitaxial GaN at 1100 °C resulted in the formation of ordered hexagonal GaN domains with rather broad mosaicity.
We investigated the effects of surface treatments by aqua regia and (NH 4 ) 2 S x on the electrical and the microstructural changes of Pd contact on p-type GaN during annealing. The formation of a surface oxide was suppressed by the (NH 4 ) 2 S x treatment, and S-Ga and S-N bonds with binding energy of 162.1 eV and 163.6 eV were formed, degrading the structural ordering of Pd. After 300°C annealing, the contact resistivity in the aqua regia-treated sample increased significantly. This could be attributed to the outdiffusion of N atoms leaving N vacancies below the contact, as confirmed by the increase of the Pd (111) plane spacing probably due to the dissolution of N atoms in Pd interstitial sites. Meanwhile, the contact resistivity in the (NH 4 ) 2 S x -treated sample was not degraded and no change was observed in the Pd (111) plane spacing. These results suggest that S-Ga and S-N bonds formed on (NH 4 ) 2 S x -treated GaN could act as a diffusion barrier for the outdiffusion of N atoms. The contact resistivity for the aqua regia-treated sample decreased again, probably due to the outdiffusion of Ga as well as N atoms at 500°C.
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