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107 results on '"Dahal, Rajendra"'

1. Solid-state neutron detectors based on thickness scalable hexagonal boron nitride

Author

Ahmed, Kawser, Dahal, Rajendra, Weltz, Adam, Lu, James J. -Q., Danon, Yaron, and Bhat, Ishwara B.

Subjects
Physics - Instrumentation and Detectors
Abstract

This paper reports on the device processing and characterization of hexagonal boron nitride (hBN) based solid-state thermal neutron detectors, where hBN thickness varied from 2.5 to 15 microns. These natural hBN epilayers (with 19.9% B-10) were grown by a low pressure chemical vapor deposition process. Complete dry processing was adopted for the fabrication of these metal-semiconductor-metal (MSM) configuration detectors. These detectors showed intrinsic thermal neutron detection efficiency values of 0.86%, 2.4%, 3.15%, and 4.71% for natural hBN thickness values of 2.5, 7.5, 10, and 15 microns, respectively. Measured efficiencies are very close (more than 92%) to the theoretical maximum efficiencies for corresponding hBN thickness values for these detectors. This clearly shows the hBN thickness scalability of these detectors. A 15-micron thick hBN based MSM detector is expected to yield an efficiency of 21.4%, if enriched hBN (with ~100% B-10) is used instead of natural hBN. These results demonstrate that the fabrication of hBN thickness scalable highly efficient thermal neutron detectors is possible.

Published
2016
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2. Metalorganic chemical vapor deposition of hexagonal boron nitride on (001) sapphire substrates for thermal neutron detector applications

Author

Ahmed, Kawser, Dahal, Rajendra, Weltz, Adam, Lu, James J. -Q., Danon, Yaron, and Bhat, Ishwara B.

Subjects
Condensed Matter - Materials Science
Abstract

This paper reports on the growth and characterization of hexagonal boron nitride (hBN) and its use for solid-state thermal neutron detection. The hBN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates at a temperature of 1350 C. X-ray diffraction peak from the (002) hBN plane at a 2theta angle of 26.7 deg exhibited the c-lattice constant of 6.66 {\AA} for these films. A strong peak corresponding to the high frequency Raman active mode of hBN was found for the films at 1370.5/cm. hBN-based solid-state neutron detectors were fabricated and tested with a metal-semiconductor-metal configuration with an electrode spacing of 1 mm and hBN thickness of 2.5 micron. Fabricated detectors showed strong response to deep UV light as well. An intrinsic thermal neutron detection efficiency of 0.86% was measured, which is close to the theoretically expected efficiency of 0.87%. These results demonstrate that epitaxial hBN films are promising for thermal neutron detection applications.

Published
2016

3. Effects of sapphire nitridation and growth temperature on the epitaxial growth of hexagonal boron nitride on sapphire

Author

Ahmed, Kawser, Dahal, Rajendra, Weltz, Adam, Lu, James J. -Q., Danon, Yaron, and Bhat, Ishwara B.

Subjects
Condensed Matter - Materials Science
Abstract

This paper reports on the epitaxial growth of hexagonal boron nitride (hBN) films on sapphire substrates in a cold wall chemical vapor deposition (CVD) system where different sapphire nitridation and hBN growth temperatures were employed. A thin and amorphous nitridated layer was formed at a low temperature (850 C), which enabled subsequent epitaxial hBN growth at 1350 C. The influences of the sapphire nitridation temperature and the growth temperature on the film quality were analyzed by X-ray diffraction (XRD) measurements. Higher than optimum nitridation and growth temperatures improve the crystalline quality of the nitridated layer, but does not favor the epitaxial growth of hBN. hBN films grown at the optimum conditions exhibit the c-lattice constant of 6.66 {\AA} from the XRD {\theta}-2{\theta} scan, and the characteristic in plane stretching vibration at 1370.5/cm from Raman spectroscopy. X-ray photoelectron spectroscopy analysis confirmed the formation of stoichiometric hBN films with excellent uniformity., Comment: 4 pages, 4 figures

Published
2016

7. Determining Drug Dosage Using Calculus.

Author

Dahal, Rajendra and Bullard, Keith

Subjects
*DRUG dosage, *DIFFERENTIAL equations, *DRUG utilization, *CALCULUS, *DRUGS
Abstract

For most drugs, there is a concentration below for which the drug is ineffective and a concentration above for which the drug is dangerous. In this paper, we will study how the dosage and dosing intervals can be modified to ensure a safe, yet efficient level of the drug in the bloodstream using calculus. In particular, we will study the concentration of a drug over time and its dosing schedule. [ABSTRACT FROM AUTHOR]

Published
2024

16. Analytical and numerical negative boundedness of fractional differences with Mittag-Leffler kernel.

Author

Mohammed, Pshtiwan Othman, Dahal, Rajendra, Goodrich, Christopher S., Hamed, Y. S., and Baleanu, Dumitru

Subjects
MATHEMATICS, FUNCTIONAL equations, DISCRETE choice models, KERNEL (Mathematics), KERNEL functions
Abstract

We show that a class of fractional differences with Mittag-Leffler kernel can be negative and yet monotonicity information can still be deduced. Our results are complemented by numerical approximations. This adds to a growing body of literature illustrating that the sign of a fractional difference has a very complicated and subtle relationship to the underlying behavior of the function on which the fractional difference acts, regardless of the particular kernel used. [ABSTRACT FROM AUTHOR]

Published
2023
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31. Mixed order monotonicity results for sequential fractional nabla differences.

Author

Dahal, Rajendra and Goodrich, Christopher S.

Subjects
*FRACTIONAL calculus, *ORDER
Abstract

We investigate sequential nabla fractional differences of the form in the case where , , and , as well as differences of the form in the case where , , and. We demonstrate connections between the sign of these fractional sequential differences and the monotonicity of the function f. [ABSTRACT FROM AUTHOR]

Published
2019
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41. Electrophoretic Deposition of 10B Nano/Micro Particles in Deep Silicon Trenches for the Fabrication of Solid State Thermal Neutron Detectors.

Author

Koirala, Machhindra, Wu, Jia Woei, Weltz, Adam, Dahal, Rajendra, Danon, Yaron, and Bhat, Ishwara

Subjects
ELECTROPHORETIC deposition, SILICON, SOLID state chemistry, THERMAL neutrons, NEUTRON counters, NANOPARTICLES
Abstract

We present a cost effective and scalable approach to fabricate solid state thermal neutron detectors. Electrophoretic deposition technique is used to fill deep silicon trenches with 10B nanoparticles instead of conventional chemical vapor deposition process. Deep silicon trenches with width of 5-6 μm and depth of 60-65 μm were fabricated in a p-type Si (110) wafer using wet chemical etching method instead of DRIE method. These silicon trenches were converted into continuous p-n junction by the standard phosphorus diffusion process. 10B micro/nano particle suspension in ethyl alcohol was used for electrophoretic deposition of particles in deep trenches and iodine was used to change the zeta potential of the particles. The measured effective boron nanoparticles density inside the trenches was estimated to be 0.7 gm cm-3. Under the self-biased condition, the fabricated device showed the intrinsic thermal neutron detection efficiency of 20.9% for a 2.5 × 2.5 mm2 device area. [ABSTRACT FROM AUTHOR]

Published
2018
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43. Boron-10 nanoparticles filled silicon trenches for thermal neutron detection Application.

Author

Jia-Woei Wu, Weltz, Adam, Koirala, Machhindra, Lu, James J.-Q., Dahal, Rajendra, Danon, Yaron, and Bhat, Ishwara B.

Subjects
BORON, NANOPARTICLES, THERMAL neutrons, DIODES, STATIC relays
Abstract

This paper reports on the use of 10B nano/microparticles in order to fill microstructures of deep trenches fabricated in n-type Si (110) bulk wafers for the development of solid-state thermal neutron detectors. The high aspect-ratio trenches were fabricated in the wafer by wet etching, with a trench width of 3.5 to 6 µm and a maximum depth of 120 µm. Boron was diffused at a temperature of ~1000 °C in order to convert the entirety of the delicate Si microstructures into a p+-n junction diode. The deep trenches of the diode were completely filled with 10B nanoparticles using a simple room-temperature process involving the pumping and venting of a vacuum chamber containing the etched wafer with 10B nanoparticles atop. The simple filling process was reproduced consistently, and the best 2.5 x 2.5 mm² device demonstrated an intrinsic thermal neutron (En <0.5 eV) detection efficiency of 32.2 ± 1.5% under a self-biased condition. This result is promising as it demonstrates a complete, low-cost fabrication process for the development of efficient thermal neutron detectors. [ABSTRACT FROM AUTHOR]

Published
2017
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45. Dynamic equations on time scales

Author

Dahal, Rajendra and Dahal, Rajendra

Abstract

The model given purely by differential equations works well for continuous behavior such as population growth and other biological phenomena, and the models purely given by difference equations work well for discrete behaviors such as traffic flow with finite number of entrances and exits. But we can find many applications which are mixtures of discrete and continuous processes that are very difficult to model with differential equations or difference equations alone. The strength of time scales theory is that it is not restricted to purely continuous or regularly discrete applications and thus it can model any application that requires simultaneous modeling of continuous and discrete data. For example time scales theory can model insect populations that are continuous while in season, die out in, say winter, while their eggs are still incubating or dormant, and then hatch in a new season, giving rise to a non overlapping population. In this work, I will be concerned with second order dynamic equations on time scales. I start with a brief introduction to the time scale calculus and some theory necessary for the new results. I will give sufficient conditions under which certain semipositone boundary value problems on time scales have at least a positive solution by constructing a special cone and applying fixed point theorems. I will also give conditions under which certain semipositone boundary value systems have at least a positive solution. Finally I will give conditions under which a certain boundary value system on time scales has multiple positive solutions.

Published
2009

46. Fabrication and characterization of III-nitride nanophotonic devices

Author

Dahal, Rajendra Prasad. and Dahal, Rajendra Prasad.

Abstract

III-nitride photonic devices such as photodetectors (PDs), light emitting diode (LEDs), solar cells and optical waveguide amplifiers were designed, fabricated and characterized. High quality AlN epilayers were grown on sapphire and n-SiC substrates by metal organic chemical vapor deposition and utilized as active DUV photonic materials for the demonstration of metal-semiconductor-metal (MSM) detectors, Schottky barrier detectors, and avalanche photodetectors (APDs). AlN DUV PDs exhibited peak responsivity at 200 nm with a very sharp cutoff wavelength at 207 nm and extremely low dark current (<10 fA), very high breakdown voltages, high responsivity, and more than four orders of DUV to UV/visible rejection ratio. AlN Schottky PDs grown on n-SiC substrates exhibited high zero bias responsivity and a thermal energy limited detectivity of about 1.0 x 1015 cm Hz1/2 W-1. The linear mode operation of AlN APDs with the shortest cutoff wavelength (210 nm) and a photocurrent multiplication of 1200 was demonstrated. A linear relationship between device size and breakdown field was observed for AlN APDs. Photovoltaic operation of InGaN solar cells in wavelengths longer than that of previous attainments was demonstrated by utilizing InxGa1-xN/GaN MQWs as the active layer. InxGa1-xN/GaN MQWs solar cells with x =0.3 exhibited open circuit voltage of about 2 V, a fill factor of about 60% and external quantum efficiency of 40% at 420 nm and 10% at 450 nm. The performance of InxGa1-xN/GaN MQWs solar cell was found to be highly correlated with the crystalline quality of the InxGa1-xN active layer. The possible causes of poorer PV characteristics for higher In content in InGaN active layer were explained. Photoluminescence excitation studies of GaN:Er and In0.06Ga0.94N:Er epilayers showed that Er emission intensity at 1.54 ℗æm increases significantly as the excitation energy is tuned from below to above the energy bandgap of these epilayers. Current-injected 1.54 [Mu] m LEDs based on heterogeneous integration of Er-doped III-nitride epilayers with III-nitride UV LEDs were demonstrated. Optical waveguide amplifiers based on AlGaN/GaN:Er/AlGaN heterostructures was designed, fabricated, and characterized. The measured optical loss of the devices was ~3.5 cm-1 at 1.54 [Mu] m. A relative signal enhancement of about 8 dB/cm under the excitation of a broadband 365 nm nitride LED was achieved. The advantages and possible applications of 1.54 [Mu]m emitters and optical amplifiers based on Er doped III-nitrides in optical communications have been discussed.

Published
2009

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