Sverre Vegard Pettersen | Norwegian University of Science and Technology (original) (raw)

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Papers by Sverre Vegard Pettersen

A Bianchi type -I metric of Kasner form is considered, when the space is filled with a viscous fl... more A Bianchi type -I metric of Kasner form is considered, when the space is filled with a viscous fluid. Whereas an ideal (nonviscous) fluid permits the Kasner metric to be anisotropic provided that the fluid satisfies the Zel'dovich equation of state, the viscous fluid does not permit the Kasner metric to be anisotropic at all. In the latter case, we calculate the Kasner (isotropic) metric expressed by the fluid's density, pressure, and bulk viscosity, at some chosen instant t=t_0. The equation of state is also calculated. The present paper is related to a recent Comment of Cataldo and del Campo [Phys. Rev. D, scheduled to April 15, 2000], on a previous work of the present authors [Phys. Rev. D 56, 3322 (1997)].

A Bianchi type-I metric of Kasner form is considered, when the space is filled with a viscous flu... more A Bianchi type-I metric of Kasner form is considered, when the space is filled with a viscous fluid. Whereas an ideal (nonviscous) fluid permits the Kasner metric to be anisotropic provided that the fluid satisfies the Zel’dovich equation of state, the viscous fluid does not permit the Kasner metric to be anisotropic at all. In the latter case, we calculate the Kasner (isotropic) metric expressed by the fluid’s density, pressure, and bulk viscosity, at some chosen instant t = t0. The equation of state is also calculated. The present paper is related to a recent Comment of Cataldo and del Campo [Phys. Rev. D, scheduled to April 15, 2000], on a previous work of the present authors

Nanotechnology, Sep 20, 2017

We report on plasma-assisted molecular beam epitaxial growth of almost randomly oriented, uniform... more We report on plasma-assisted molecular beam epitaxial growth of almost randomly oriented, uniformly tilted, and vertically aligned self-assembled GaN nanowires, respectively, on different types of polycrystalline Ti foils. The nanowire orientation, which is affected by an in-situ treatment of the foil surface before nanowire growth, depends on the crystallinity of the native oxide. Direct growth on the as-received foils results in the formation of ensembles of nearly randomly oriented nanowires due to the strong roughening of the surface induced by chemical reactions between the impinging elements and Ti. Surface nitridation preceding the nanowire growth is found to reduce this roughening by transformation of the uppermost layers into TiN and TiO x N y species. These compounds are more stable against chemical reactions and facilitate the growth of ensembles of uniformly oriented GaN nanowires within the individual grains of the polycrystalline Ti foils. If an amorphous oxide layer is present at the foil surface, vertically oriented nanowires are obtained all across the substrate because this layer blocks the transfering of the epitaxial information from the underlying grains. The control of nanowire orientation and the understanding behind the achievement of vertically oriented nanowires obtained in this study represent an important step towards the realization of GaN nanowire-based bendable devices on polycrystalline metal foils.

ABSTRACTWe report on fabrication and measurement of a surface acoustic wave resonator prepared on... more ABSTRACTWe report on fabrication and measurement of a surface acoustic wave resonator prepared on ∼10m thick GaN(0001) films. The films were grown by metal-organic vapor phase epitaxy on a c-plane sapphire substrate. The surface morphology of the films were examined with scanning electron and atomic force microscopy. A metallic bilayer of Al/Ti was subsequently evaporated on the nitride film surface. Definition of the resonator interdigital transducers, designed for a wavelength of λ=7.76m, was accomplished with standard UV lithography and lift-off. S-parameter measurements showed a resonator center frequency f0=495MHz at room temperature, corresponding to a surface acoustic wave velocity of 3844m/s. The insertion loss at center frequency was measured at 8.2dB, and the loaded Q-factor was estimated at 2200. Finally, measurements of the resonator center frequency for temperatures in the range 25–155°C showed a temperature coefficient of -18ppm/°C. The intrinsic GaN SAW velocity and electromechanical coupling coefficient were estimated at νSAW=383 1m/s and K2=1.8±0.4·10−3.

Semiconductor Science and Technology, 2007

The formation of Au/Ni and Pt/Ni contacts to p-GaN has been examined with Auger depth profiling a... more The formation of Au/Ni and Pt/Ni contacts to p-GaN has been examined with Auger depth profiling and x-ray photoelectron spectroscopy analysis and compared to circular transmission line measurements of their respective transport properties. The metal bilayers were deposited by radio frequency argon ion sputtering, and the samples were annealed at different temperatures in a 10 −3 mbar oxygen ambient. Both contacts were distinctly non-ohmic as deposited. Ohmic behaviour was achieved after oxidation at temperatures above 400 • C, and the two contacts were found to have comparable performance, with a lowest specific contact resistance ρ c = 1.0 × 10 −2 cm 2 for the Pt/Ni contact after annealing at 400 • C and ρ c = 1.3 × 10 −2 cm 2 for the best Au/Ni contact annealed at 600 • C. Au and Pt both serve to ensure a highly conducting contact layer after annealing in oxygen and help lower the Schottky barrier at the metal/p-GaN interface by virtue of their large work functions. Distinct differences were observed in the alloying properties of the Pt/Ni and the Au/Ni contact layers, however. While Pt and Ni interdiffuse to form a uniform alloy upon annealing in oxygen at 600 • C, Ni diffuses outwards to form surface nickel oxide for the Au/Ni metallization. Trace amounts of Ga were observed at the surface after annealing at 600 • C for the Pt/Ni and the Au/Ni bilayers alike, albeit in considerably larger measure for Pt/Ni. We believe that this surface Ga derives from a thermally activated reaction between Ni and GaN, which adversely affects the contact performance. The outward diffusion of Ni upon thermal oxidation of the Au/Ni contact appears to inhibit the rate of this interface reaction, which speaks in favour of using Au over Pt in ohmic contacts to p-GaN. Preconditioning of the p-GaN surface by deposition and subsequent removal of a thin coating of Ni are found to improve the contact performance, provided the coating is not annealed before removal.

Physical Review D, 2000

A Bianchi type-I metric of Kasner form is considered, when the space is filled with a viscous flu... more A Bianchi type-I metric of Kasner form is considered, when the space is filled with a viscous fluid. Whereas an ideal (nonviscous) fluid permits the Kasner metric to be anisotropic provided that the fluid satisfies the Zel'dovich equation of state, the viscous fluid does not permit the Kasner metric to be anisotropic at all. In the latter case, we calculate the Kasner (isotropic) metric expressed by the fluid's density, pressure, and bulk viscosity, at some chosen instant t = t 0. The equation of state is also calculated. The present paper is related to a recent Comment of Cataldo and del Campo [Phys. Rev. D, scheduled to April 15, 2000], on a previous work of the present authors [Phys. Rev. D 56, 3322 (1997)].

In this study, we examine the growth conditions for Pb0.92La0.08Zr0.4Ti0.6O3 thin films with a li... more In this study, we examine the growth conditions for Pb0.92La0.08Zr0.4Ti0.6O3 thin films with a linear electro-optical response. Films grown on SrTiO3(100) substrate show good crystalline quality and surface smoothness, if deposited within a narrow range of substrate temperatures around 530 °C. Such films had a (002) rocking curve full width at half maximum less than 0.2°, and a surface roughness

Physical Review D, 1997

ABSTRACT

A Bianchi type -I metric of Kasner form is considered, when the space is filled with a viscous fl... more A Bianchi type -I metric of Kasner form is considered, when the space is filled with a viscous fluid. Whereas an ideal (nonviscous) fluid permits the Kasner metric to be anisotropic provided that the fluid satisfies the Zel'dovich equation of state, the viscous fluid does not permit the Kasner metric to be anisotropic at all. In the latter case, we calculate the Kasner (isotropic) metric expressed by the fluid's density, pressure, and bulk viscosity, at some chosen instant t=t_0. The equation of state is also calculated. The present paper is related to a recent Comment of Cataldo and del Campo [Phys. Rev. D, scheduled to April 15, 2000], on a previous work of the present authors [Phys. Rev. D 56, 3322 (1997)].

A Bianchi type-I metric of Kasner form is considered, when the space is filled with a viscous flu... more A Bianchi type-I metric of Kasner form is considered, when the space is filled with a viscous fluid. Whereas an ideal (nonviscous) fluid permits the Kasner metric to be anisotropic provided that the fluid satisfies the Zel’dovich equation of state, the viscous fluid does not permit the Kasner metric to be anisotropic at all. In the latter case, we calculate the Kasner (isotropic) metric expressed by the fluid’s density, pressure, and bulk viscosity, at some chosen instant t = t0. The equation of state is also calculated. The present paper is related to a recent Comment of Cataldo and del Campo [Phys. Rev. D, scheduled to April 15, 2000], on a previous work of the present authors

Nanotechnology, Sep 20, 2017

We report on plasma-assisted molecular beam epitaxial growth of almost randomly oriented, uniform... more We report on plasma-assisted molecular beam epitaxial growth of almost randomly oriented, uniformly tilted, and vertically aligned self-assembled GaN nanowires, respectively, on different types of polycrystalline Ti foils. The nanowire orientation, which is affected by an in-situ treatment of the foil surface before nanowire growth, depends on the crystallinity of the native oxide. Direct growth on the as-received foils results in the formation of ensembles of nearly randomly oriented nanowires due to the strong roughening of the surface induced by chemical reactions between the impinging elements and Ti. Surface nitridation preceding the nanowire growth is found to reduce this roughening by transformation of the uppermost layers into TiN and TiO x N y species. These compounds are more stable against chemical reactions and facilitate the growth of ensembles of uniformly oriented GaN nanowires within the individual grains of the polycrystalline Ti foils. If an amorphous oxide layer is present at the foil surface, vertically oriented nanowires are obtained all across the substrate because this layer blocks the transfering of the epitaxial information from the underlying grains. The control of nanowire orientation and the understanding behind the achievement of vertically oriented nanowires obtained in this study represent an important step towards the realization of GaN nanowire-based bendable devices on polycrystalline metal foils.

ABSTRACTWe report on fabrication and measurement of a surface acoustic wave resonator prepared on... more ABSTRACTWe report on fabrication and measurement of a surface acoustic wave resonator prepared on ∼10m thick GaN(0001) films. The films were grown by metal-organic vapor phase epitaxy on a c-plane sapphire substrate. The surface morphology of the films were examined with scanning electron and atomic force microscopy. A metallic bilayer of Al/Ti was subsequently evaporated on the nitride film surface. Definition of the resonator interdigital transducers, designed for a wavelength of λ=7.76m, was accomplished with standard UV lithography and lift-off. S-parameter measurements showed a resonator center frequency f0=495MHz at room temperature, corresponding to a surface acoustic wave velocity of 3844m/s. The insertion loss at center frequency was measured at 8.2dB, and the loaded Q-factor was estimated at 2200. Finally, measurements of the resonator center frequency for temperatures in the range 25–155°C showed a temperature coefficient of -18ppm/°C. The intrinsic GaN SAW velocity and electromechanical coupling coefficient were estimated at νSAW=383 1m/s and K2=1.8±0.4·10−3.

Semiconductor Science and Technology, 2007

The formation of Au/Ni and Pt/Ni contacts to p-GaN has been examined with Auger depth profiling a... more The formation of Au/Ni and Pt/Ni contacts to p-GaN has been examined with Auger depth profiling and x-ray photoelectron spectroscopy analysis and compared to circular transmission line measurements of their respective transport properties. The metal bilayers were deposited by radio frequency argon ion sputtering, and the samples were annealed at different temperatures in a 10 −3 mbar oxygen ambient. Both contacts were distinctly non-ohmic as deposited. Ohmic behaviour was achieved after oxidation at temperatures above 400 • C, and the two contacts were found to have comparable performance, with a lowest specific contact resistance ρ c = 1.0 × 10 −2 cm 2 for the Pt/Ni contact after annealing at 400 • C and ρ c = 1.3 × 10 −2 cm 2 for the best Au/Ni contact annealed at 600 • C. Au and Pt both serve to ensure a highly conducting contact layer after annealing in oxygen and help lower the Schottky barrier at the metal/p-GaN interface by virtue of their large work functions. Distinct differences were observed in the alloying properties of the Pt/Ni and the Au/Ni contact layers, however. While Pt and Ni interdiffuse to form a uniform alloy upon annealing in oxygen at 600 • C, Ni diffuses outwards to form surface nickel oxide for the Au/Ni metallization. Trace amounts of Ga were observed at the surface after annealing at 600 • C for the Pt/Ni and the Au/Ni bilayers alike, albeit in considerably larger measure for Pt/Ni. We believe that this surface Ga derives from a thermally activated reaction between Ni and GaN, which adversely affects the contact performance. The outward diffusion of Ni upon thermal oxidation of the Au/Ni contact appears to inhibit the rate of this interface reaction, which speaks in favour of using Au over Pt in ohmic contacts to p-GaN. Preconditioning of the p-GaN surface by deposition and subsequent removal of a thin coating of Ni are found to improve the contact performance, provided the coating is not annealed before removal.

Physical Review D, 2000

A Bianchi type-I metric of Kasner form is considered, when the space is filled with a viscous flu... more A Bianchi type-I metric of Kasner form is considered, when the space is filled with a viscous fluid. Whereas an ideal (nonviscous) fluid permits the Kasner metric to be anisotropic provided that the fluid satisfies the Zel'dovich equation of state, the viscous fluid does not permit the Kasner metric to be anisotropic at all. In the latter case, we calculate the Kasner (isotropic) metric expressed by the fluid's density, pressure, and bulk viscosity, at some chosen instant t = t 0. The equation of state is also calculated. The present paper is related to a recent Comment of Cataldo and del Campo [Phys. Rev. D, scheduled to April 15, 2000], on a previous work of the present authors [Phys. Rev. D 56, 3322 (1997)].

In this study, we examine the growth conditions for Pb0.92La0.08Zr0.4Ti0.6O3 thin films with a li... more In this study, we examine the growth conditions for Pb0.92La0.08Zr0.4Ti0.6O3 thin films with a linear electro-optical response. Films grown on SrTiO3(100) substrate show good crystalline quality and surface smoothness, if deposited within a narrow range of substrate temperatures around 530 °C. Such films had a (002) rocking curve full width at half maximum less than 0.2°, and a surface roughness

Physical Review D, 1997

ABSTRACT