Abstract The photoconduction in Ag 0.5 Sb 0.5 S films changes anomalously with the excitation energy. Although the usual instantaneous generation and recombination of photocarriers appear in the ...resistance of the films for the illumination at a wavelength of 633 nm, the photoresponse becomes slow with time scales of minutes when the illumination is performed at a wavelength of 280 nm. The rapid and slow phototransients are mixed for an intermediate excitation wavelength of 375 nm. In the simultaneous photoexcitation at multiple wavelengths, the response is complex instead of a superposition of the rapid and slow behaviors, indicating the mutual interaction in the photocarrier transport. The ultraviolet (UV) illumination can thereby block the rapid response that should be caused by the visible light. Moreover, the resistance can even increase during the illumination. Although the adsorption of molecules at the film surface plays an important role for the resistance, the anomalous properties are unaffected by the surface condition. They are thus suggested to be the bulk properties of the films, plausibly caused by the defects generated in the UV irradiation.
The self-assembly of heteroepitaxial GaN nanowires using either molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE) mostly results in wafer-scale ensembles with ultrahigh (>10
m
...) or ultralow (<1
m
) densities, respectively. A simple means to tune the density of well-developed nanowire ensembles between these two extremes is generally lacking. Here, we examine the self-assembly of SiN
patches on TiN(111) substrates which are eventually acting as seeds for the growth of GaN nanowires. We first found that if prepared by reactive sputtering, the TiN surface is characterized by {100} facets for which the GaN incubation time is extremely long. Fast GaN nucleation is only obtained after deposition of a sub-monolayer of SiN
atoms prior to the GaN growth. By varying the amount of pre-deposited SiN
, the GaN nanowire density could be tuned by three orders of magnitude with excellent uniformity over the entire wafer, bridging the density regimes conventionally attainable by direct self-assembly with MBE or MOVPE. The analysis of the nanowire morphology agrees with a nucleation of the GaN nanowires on nanometric SiN
patches. The photoluminescence analysis of single freestanding GaN nanowires reveals a band edge luminescence dominated by excitonic transitions that are broad and blue shifted compared to bulk GaN, an effect that is related to the small nanowire diameter and to the presence of a thick native oxide. The approach developed here can be principally used for tuning the density of most III-V semiconductors nucleus grown on inert surfaces like 2D materials.
We investigate the structural and optical properties of spontaneously formed GaN nanowires with different degrees of coalescence. This quantity is determined by an analysis of the cross-sectional ...area and perimeter of the nanowires obtained by plan-view scanning electron microscopy. X-ray diffraction experiments are used to measure the inhomogeneous strain in the nanowire ensembles as well as the orientational distribution of the nanowires. The comparison of the results obtained for GaN nanowire ensembles prepared on bare Si and AlN buffered 6H-SiC reveals that the main source of the inhomogeneous strain is the random distortions caused by the coalescence of adjacent nanowires. The magnitude of the strain inhomogeneity induced by nanowire coalescence is found not to be determined solely by the coalescence degree, but also by the mutual misorientation of the coalesced nanowires. The linewidth of the donor-bound exciton transition in photoluminescence spectra does not exhibit a monotonic increase with the coalescence degree. In contrast, the comparison of the root mean square strain with the linewidth of the donor-bound exciton transition reveals a clear correlation: the higher the strain inhomogeneity, the larger the linewidth.
We study the molecular beam epitaxy of AlN nanowires between 950 and 1215 °C, well above the usual growth temperatures, to identify optimal growth conditions. The nanowires are grown by self-assembly ...on TiN(111) films sputtered onto Al
O
. Above 1100 °C, the TiN film is seen to undergo grain growth and its surface exhibits {111} facets where AlN nucleation preferentially occurs. Modeling of the nanowire elongation rate measured at different temperatures shows that the Al adatom diffusion length maximizes at 1150 °C, which appears to be the optimum growth temperature. However, analysis of the nanowire luminescence shows a steep increase in the deep-level signal already above 1050 °C, associated with O incorporation from the Al
O
substrate. Comparison with AlN nanowires grown on Si, MgO and SiC substrates suggests that heavy doping of Si and O by interdiffusion from the TiN/substrate interface increases the nanowire internal quantum efficiency, presumably due to the formation of a SiN
or AlO
passivation shell. The outdiffusion of Si and O would also cause the formation of the inversion domains observed in the nanowires. It follows that for optoelectronic and piezoelectric applications, optimal AlN nanowire ensembles should be prepared at 1150 °C on TiN/SiC substrates and will require an
surface passivation.
Background
In the past two centuries, generations of dermatologists around the world have created an enormous number of publications. To our knowledge, no bibliometric analysis of these publications ...has been performed so far, nor have registered trials been analysed to anticipate future publication trends.
Objectives
To determine the global distribution of national publication productivity, most published topics, institutions and funding sources contributing most to publications and to anticipate future trends based on registered clinical trials.
Methods
Following pre‐assessment on PubMed, Embase, Web of Science and Scopus, the number of publications for ‘dermatology’ was determined for each of 195 countries, normalized per 1 Mio inhabitants and bibliometrically analysed. Dermatology‐related trials registered at clinicaltrials.gov were specified by the top‐10 diagnoses for the top‐10 countries.
Results
The search yielded 1 071 518 publications between 1832 and 2019 with the top‐5 diagnoses being melanoma, basal cell carcinoma, psoriasis, pruritus/itch and atopic dermatitis. The top‐3 countries with highest absolute numbers of publications were the USA (30.6%), Germany (8.1%) and the UK (8.1%), whereas Switzerland, Denmark and Sweden had the highest publication rates when normalized by inhabitants. The most productive affiliation was the Harvard Medical School, the leading funding source the National Institutes of Health. Currently, maximum number of trials are registered in the USA (8111), France (1543) and Canada (1368). The highest percentage of all dermatology‐related trials in a specific country were as follows: Melanoma in the Netherlands (24.8%), psoriasis in Germany (21.7%) and atopic dermatitis in Japan (15.9%).
Conclusion
The top‐10 countries including the USA, Canada, a few European and Asian countries contributed more than 3/4 of all publications. The USA hold the dominant leader position both in past publication productivity and currently registered trials. While most Western countries continue to focus their research on the top‐10 topics, China and India appear to prioritize their scope towards other topics.
We introduce a facile route for the top-down fabrication of ordered arrays of GaN nanowires with aspect ratios exceeding 10 and diameters below 20 nm. Highly uniform thin GaN nanowires are first ...obtained by lithographic patterning a bilayer Ni/SiN
hard mask, followed by a combination of dry and wet etching in KOH. The SiN
is found to work as an etch stop during wet etching, which eases reproducibility. Arrays with nanowire diameters down to (33 ± 5) nm can be achieved with a uniformity suitable for photonic applications. Next, a scheme for digital etching is demonstrated to further reduce the nanowire diameter down to 5 nm. However, nanowire breaking or bundling is observed for diameters below ≈20 nm, an effect that is associated to capillary forces acting on the nanowires during sample drying in air. Explicit calculations of the nanowire buckling states under capillary forces indicate that nanowire breaking is favored by the incomplete wetting of water on the substrate surface during drying. The observation of intense nanowire photoluminescence at room-temperature indicates good compatibility of the fabrication route with optoelectronic applications. The process can be principally applied to any GaN/SiN
nanostructures and allows regrowth after removal of the SiN
mask.
We analyze the strain state of GaN nanowire ensembles by x-ray diffraction. The nanowires are grown by molecular beam epitaxy on a Si(111) substrate in a self-organized manner. On a macroscopic ...scale, the nanowires are found to be free of strain. However, coalescence of the nanowires results in micro-strain with a magnitude from ± (0.015)% to ± (0.03)%. This micro-strain contributes to the linewidth observed in low-temperature photoluminescence spectra.
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