Lars Norin

Deposition of Transition Metal Carbides and Superlattices Using C 60 as Carbon Source

Thin films of TiC, VC, and NbC have been deposited on MgO(001) by coevaporation of the metals and C-60 It was found that these metals induced a decomposition of the C-60 molecule and that carbide f ...

Ionospheric plasma density irregularities measured by stimulated electromagnetic emission

Electromagnetic waves can be used to transmit information over long distances and are therefore often employed for communication purposes. The electromagnetic waves are reflected off material objects on their paths and interact with the medium through which they propagate. For instance, the plasma in the ionosphere can refract and even reflect radio waves propagating through it. By increasing the power of radio waves injected into the ionosphere, the waves start to modify the plasma, resulting in the generation of a wide range of nonlinear processes, including turbulence, in particular near the reflection region. By systematically varying the injected radio waves in terms of frequency, power, polarisation, duty cycle, inclination, etc. the ionosphere can be used as an outdoor laboratory for investigating fundamental properties of the near-Earth space environment as well as of plasma turbulence. In such ionospheric modification experiments, it has been discovered that the irradiation of the ionosphere by powerful radio waves leads to the formation of plasma density structures and to the emission of secondary electromagnetic radiation, a phenomenon known as stimulated electromagnetic emission. These processes are highly repeatable and have enabled systematic investigations of the nonlinear properties of the ionospheric plasma. In this thesis we investigate features of the plasma density structures and the secondary electromagnetic radiation. In a theoretical study we analyse a certain aspect of the formation of the plasma structures. The transient dynamics of the secondary radiation is investigated experimentally in a series of papers, focussing on the initial stage as well as on the decay. In one of the papers we use the transient dynamics of the secondary radiation to reveal the intimate relation between certain features of the radiation and structures of certain scales. Further, we present measurements of unprecedentedly strong secondary radiation, attributed to stimulated Brillouin scattering, and report measurements of the secondary radiation using a novel technique imposed on the transmitted radio waves.

Low-temperature epitaxial growth of metal carbides using fullerenes

Epitaxial transition metal carbides can be deposited at low temperatures by simultaneous evaporation of C60 and either metal e-beam evaporation or metal d.c. magnetron sputtering. Hitherto, epitaxial films of TiC, VC, NbC, MoC, W2C and WC have been deposited on MgO(100), MgO(111) and in some cases 6H- and 4H-SiC(0001). Epitaxial TiC films with a good quality have been deposited at temperatures as low as 100°C with metal sputtering, while somewhat higher temperatures (>200°C) are required for the other metals. In general, the plasma-assisted process allows lower deposition temperatures than the co-evaporation process. Most carbides can be deposited in a wide range of compositions within their homogeneity ranges by a fine-tuning of the Me/C60 flux. However, the results suggest that the formation of free surface carbon can be a limiting factor. The processes have also been used to deposit superlattices of TiC/NbC and TiC/VC at 400–500°C as well as epitaxial ternary TixV1−xCy films. Furthermore, epitaxial films of ternary carbides with well-controlled metal concentration profiles can be deposited at temperatures below 500°C.

Deposition of epitaxial titanium carbide films on MgO(001) and 6H-SiC(0001) by coevaporation of Ti and C 60

Epitaxial films of TiC 1- x (0.15 x 60 and Ti. Films deposited on MgO(001) were single-crystalline down to deposition temperatures of at least 250 °C as determined by x-`ray diffraction (XRD), low energy electron diffraction (LEED), and transmission electron microscopy (TEM). Films deposited on 6H–SiC(0001) were also epitaxial at 250 °C, but TEM showed a columnar microstructure due to the occurrence of twinned domains in the [111] growth direction normal to the substrate.

Deposition of transition metal carbide superlattices using C60 as a carbon source

Epitaxial films of TiC and VC can be deposited at low temperatures on Mg(001) substrates by coevaporation of the metals with C60 in a ultrahigh vacuum system. This process was used to deposit TiC/VC (001) superlattices on MgO(001) at 400 °C. The superlattice structure was characterized with low electron energy diffraction, x-ray diffraction, and transmission electron microscopy (TEM). Cross-sectional high-resolution TEM showed good registry between the different carbide layers.

Chemical vapour deposition of molybdenum carbides using C60 as a carbon source

Abstract Single-phase molybdenum carbide films were deposited on sapphire by CVD from a C 60 /MoCl 5 /H 2 gas mixture, Mo 2 C could be grown at substrate temperatures of 600 and 800°C using total pressures of 100 and 10 Torr, respectively, Metastable δ-MoC 1− x was also deposited at 800°C by lowering the total pressure to 2 Torr. For a constant mass flow of carrier gas, a high total pressure resulted in an increased growth rate of Mo 2 C at 600°C, whereas a low total pressure favoured the growth of δ-MoC 1− x at 800°C. The films were lustrous and showed good adhesion to the substrate. Resistivities of 56−140 μΩ were measured for the single-phase films.

Transient dynamics of secondary radiation from an HF pumped magnetized space plasma

In order to systematically analyze the transient wave and radiation processes that are excited when a high-frequency (HF) radio wave is injected into a magnetized space plasma, we have measured the ...

Use of oxygen-stabilized C60 films for selective chemical vapor deposition

Thin C60 films exposed to ultraviolet/visible light in the presence of oxygen were used as a selective mask for tungsten chemical vapor deposition on silicon substrates. An uptake of oxygen in the fullerene films as well as a significant increase in their thermal stability resulted from the simultaneous exposure to the radiation and oxygen. The thermal stability and inertness of these films to tungsten hexaflouride, which is readily reduced by silicon to form the metal at 350 °C, allowed selective deposition of tungsten in both ultrahigh vacuum and low pressure environments. X-ray photoelectron spectroscopy and x-ray fluorescence spectroscopy were used to characterize the tungsten deposited on the C60 mask and the unmasked silicon substrate.

Deposition of Epitaxial Carbide Films and Superlattices by Co-Evaporation of C 60 and Transition Metals

Thin films of TiC, VC and NbC have been deposited on MgO(001) by co-evaporation of C 60 and the metal. The metal induced a decomposition of the C 60 cage and a subsequent carbide formation at 100 °C. Epitaxial TiC films were easily obtained at 250 °C, while higher deposition temperatures were required for epitaxial growth of VC(400 °C) and NbC(500 °C). The films grew with the relation MeC(001)//MgO(00 1) and MeC[ 100]//MgO[ 100]. It was also possible to deposit TiC/NbC/MgO polycrystalline multilayers and TiC/VC/MgO superlattices structures by a sequential evaporation of the metals.