We report first results from our effort to couple a high resolution photoemission electron microscope (PEEM) to the OK-4 ultraviolet free electron laser at Duke University (OK-4/Duke UV FEL). The OK-4/Duke UV FEL is a high intensity source of tunable monochromatic photons in the 3–10 eV energy range. This tunability is unique and allows us to operate near the photoemission threshold of any samples and thus maximize sample contrast while keeping chromatic berrations in the PEEM minimal. We have recorded first images from a variety of samples using spontaneous radiation from the OK-4/ Duke UV FEL in the photon energy range of 4.0–6.5 eV. Due to different photothreshold emission from different sample areas, emission from these areas could be turned on (or off) selectively. We have also observed relative intensity reversal with changes in photon energy which are interpreted as density-of-state contrast. Usable image quality has been achieved, even though the output power of the FEL in spontaneous emission mode was several orders of magnitude lower than the anticipated full laser power. The PEEM has achieved a spatial resolution of 12 nm.
Selective growth of arrays of silicon-doped GaN (Si:GaN) pyramids for field emitter applications has been achieved. The electron emission characteristics of these arrays has been measured using techniques such as field emission, field emission energy distribution analysis (FEED), photoemission electron microscopy (PEEM), and field emission electron microscopy (FEEM). The field emission current–voltage (I–V) results indicate an average threshold field as low as 7 V/μm for an emission current of 10 nA. It is suggested that the low threshold field value is a consequence of both the low work function of Si:GaN and the field enhancement of the pyramids. The results of the FEEM and FEED measurements indicate agreement with the field emission I–V characteristics. The FEED results indicate that the Si:GaN pyramids are conducting, and that no significant ohmic losses are present between the top contact to the array and the field emitting pyramids. The PEEM and FEEM images show that the emission from the arrays is uniform over a 150 μm field of view.
Electrical and chemical characterization of the Schottky barrier formed between clean n-GaN (0001) surfaces and Pt, Au, and AgApproximately 100 or 1000 Å of AlN was deposited on the ͑0001͒Si-face of on-axis n-type 6H-SiC. The surfaces were examined by ultraviolet photoemission spectroscopy ͑UPS͒ utilizing the He I ␣ ͑21.2 eV͒ and the He II ␣ ͑40.8 eV͒ excitation. Experimental difficulties are discussed. Titanium was deposited on the clean surface of in situ grown AlN. The titanium-AlN interface was also characterized with UPS. Two approaches are presented to identify the valence band maximum ͑VBM͒ and the electron affinity of the clean surface of AlN was found to be either 0 to 1 eV depending upon the position of the valence band edge. The same assumptions were applied to the analysis of the Ti/AlN interface and, for the case of ϭ0 eV, the position of the valence band maximum is 3.4 eV below the position of the Fermi level. For the case of ϭ1 eV, the position of the valence band maximum is 4.4 eV below the position of the Fermi level. Therefore, the p-type Schottky barrier height of titanium on AlN is measured to be 3.4Ϯ0.2 or 4.4Ϯ0.2 eV for ϭ0 eV and ϭ1 eV, respectively. Independent of the selection of the valence band maximum, the observed Schottky barrier differed from that predicted by the Schottky-Mott model by 1.5Ϯ0.2 eV.
International service and learning are growing phenomena in physical therapist education (PTE) and programs are developing new opportunities in all corners of the world. With this growth comes risks of duplicative efforts and harmful consequences. Purpose. This study is seeking to describe: 1) the proportion of US-based PTE programs sponsoring international experiences; 2) the number of physical therapist students participating in these activities annually; 3) the location of these programs; and 4) whether outcomes are formally collected. Methods. A 36-item survey was distributed to the American Physical Therapy Association's list of 243 accredited programs. A total of 109 programs responded (44.9%). Results. Seventy-three percent of respondent programs reported offering international experiences in 60 different countries during one academic year. It is estimated that 2,316 students participated in these experiences. Most international experiences were offered in low- and middle-income countries, with 24 different experiences being hosted in Guatemala alone. Student outcomes were collected for nearly all experiences, whereas host community outcomes were collected by less than 20% of programs. Discussion and Conclusion. This study confirms a growing trend of physical therapist students traveling the world. The high concentration of experiences in specific countries, regions, and low-resourced communities magnifies the importance of working alongside community and academic partners to share ideas and resources. Although the collection of student outcomes is critically important to academic programs, the collection of community outcomes should be considered equally, if not more important in assessing the potential benefit of international experiences.
Wafers of n-type, 6H-SiC͑0001͒ with (N D-N A)ϭ(5.1-7.5)ϫ10 17 and 2.5ϫ10 18 were etched in a flowing 25%H 2 /75%He mixture within the range of 1500-1640°C at 1 atm. Equilibrium thermodynamic calculations indicated that the presence of atomic hydrogen is necessary to achieve etching of SiC. Atomic force microscopy, optical microscopy, and low energy electron diffraction of the etched surface revealed a faceted surface morphology with unit cell and half unit cell high steps and a 1ϫ1 reconstruction. The latter sample also exhibited a much larger number of hexagonal pits on the surface. Annealing the etched samples under ultrahigh vacuum ͑UHV͒ at 1030°C for 15 min resulted in ͑1͒ a reduction of the surface oxygen and adventitious hydrocarbons below the detection limit of Auger electron spectroscopy, ͑2͒ a (ͱ3ϫͱ3)R30°reconstructed surface and ͑3͒ a Si-to-C peak-to-peak height ratio of 1.2. By contrast, using a chemical vapor cleaning ͑CVC͒ process consisting of an exposure to 3000 Langmuir ͑L͒ of silane at 1030°C for 10 min under UHV conditions resulted in a (3ϫ3) surface reconstruction, a Si-to-C ratio of 3.9, and islands of excess silicon. Continued annealing of the latter material for an additional 10 min at 1030°C resulted in a (1ϫ1) LEED pattern with a diffuse ring. Films of AlN grown via MOCVD at a sample platter temperature of 1274°C for 15 min on hydrogen etched wafers having a doping concentration of 8.7ϫ10 17 cm Ϫ3 and cleaned via annealing had a rms roughness value of Ϸ0.4 nm.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.