Thomas M. Christensen

Equilibrium surface composition of diblock copolymers

The surface composition of a series of symmetric diblock copolymers of polystyrene (PS) and polymethylmethacrylate (PMMA) was studied using x‐ray photoelectron spectroscopy (XPS). Each copolymer was composed of N segments where N ranged from 270 to 5200. At large N, where the system is highly segregated, the surface is composed only of PS, the lower surface energy component. For smaller N, where the system is in the weak segregation limit, both the PS and PMMA segments coexist at the surface. The surface excess ψ1 of PS in the weak segregation limit is well described by ψ1=α−βN−1/2, in good agreement with mean field predictions. The constants α and β depend on the interactions between the unlike segments on the copolymer chain and on the surface energy differences between the components. We also show that the strength of the segmental interactions in the vicinity of the surface are weaker than from those in the bulk.

High-frequency magnetic microstrip local bandpass filters

This paper presents results for a compact, local bandpass filter. We fabricated the filter using two different ferromagnetic materials, Fe and NiFe, in a microstrip geometry. The different values of saturation magnetization of the two ferromagnets give rise to different gyromagnetic resonance frequencies and provide a local bandpass region between them. The results show that the center frequency of the filter can be tuned over a wide range by adjusting the magnitude of the bias magnetic field. The filter’s bandwidth is almost constant (3.5±0.5GHz) over the entire tuning frequency range (6–26GHz). Theoretical calculations are in good agreement with the experimental results.

Thin film properties of germanium oxide synthesized by pulsed laser sputtering in vacuum and oxygen environments

Germanium oxide thin films were deposited on Si substrates at ambient temperature using pulsed laser sputtering of GeO2 targets in both vacuum and oxygen environments. The structure and composition of these films were deduced from infrared transmittance and Auger electron spectroscopy measurements. The film properties were strongly influenced by the pressure of the background reactive gas and the laser fluence during deposition with O2. The vacuum deposited films had a low oxygen content and the film’s stoichiometry varied from metal rich at the center to oxidelike near the edges of the film. The films deposited in oxygen appeared stoichiometric and they showed no content variation with radius. Ellipsometry studies indicated, however, that the oxygen grown films were porous. The trends observed in growing these films under various conditions are discussed with respect to potential reaction kinetics in the plume and gas expansion phenomena.

Clean surface and oxidation behavior of vanadium carbide, VC0.75(100)

The clean single-crystal VC 0.75 (100) surface, its oxidation, and the stability of the resulting oxide were examined with electron spectroscopy (AES, XPS and UPS), electron diffraction (low-energy and reflection high energy electron diffraction) and atomic force microscopy (AFM). The clean VC 0.75 surface exhibits carbon vacancy-related features in valence band spectroscopy data. This surface was exposed to O 2 at 500 °C for up to 10 000 Langmuirs. The oxidation leads to the formation of vanadium oxide, VO x (100), crystallites on the VC 0.75 (100) substrate. The AFM images indicate that oxide crystallites grow with an elongated shape along type directions. When heated above 1000 °C, the oxide is unstable and a clean VC 0.75 (100) surface is produced.

Ellipsometric investigation of Be(0001) oxidation

Studies by ellipsometry of the early stages of oxidation of the Be(0001) surface in the temperature range 30–600 °C have been made in conjunction with Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). An initial decrease in the ellipsometric parameter Ψ is taken to indicate incorporation of oxygen into the beryllium surface. The magnitude of the decrease in Ψ depends on temperature and is greatest around 100 °C. It is proposed that at the minimum in Ψ the surface consists of islands of BeO in an electron depleted Be layer; an effective medium approximation, assuming nearly free electron behavior for Be, can describe the experimental results with reasonable values of the parameters. After oxide coalescence, p(2×2)‐BeO and ((3)1/2x(3)1/2)R30° ‐BeO LEED patterns are observed. They are unstable under an electron beam, but can be regenerated by short anneals at 500 °C. The ellipsometric parameter Δ is observed to be significantly different for the surface with and without this order...

Magneto-optical AlN/Fe/AlN structures optimized for operation in the violet spectral region

The magneto-optical (MO) polar Kerr response of AlN/Fe/AlN/Cu/Si structures was optimized for a wavelength of 410 nm. Prior to the growth of the structures, the optimal thicknesses of the individual layers were determined by modelling to find the maximum MO figure of merit. Then the structures with different thicknesses of AlN layers were grown by sputtering. The optical properties of the AlN layers, grown by reactive sputtering, were characterized by ellipsometry. The MO polar Kerr spectroscopy was employed to measure the performance of the structures in the field of 3 kOe. The MO signal was scaled for saturation (factor 7) and saturation azimuth rotation greater than 20° was observed. The agreement between calculation and experimental data was remarkable, verifying the correct set of optical and MO parameters used in the model. These results provide a guide for the design of a MO sensor with optimal performance at a desired wavelength.

Oxidation of amorphous Ni–Nb films: Surface spectroscopy studies

Sputter deposited amorphous Ni–Nb alloy films have been proposed as thin‐film metallization layers for high‐temperature (>300 °C) compound semiconductor applications. We have used x‐ray photoelectron spectroscopy (XPS) to investigate the oxidation behavior of 1‐μm‐thick, sputter deposited, amorphous Ni60 Nb40 films exposed to oxygen or room air at temperatures in the range 30–500 °C for times of 5 min to 45 h. The initial stages of oxidation were examined in situ at oxygen partial pressures below 1×10−6 Torr. At all temperatures, Nb2 O5 formed on the surface; this remained the dominant corrosion product for specimens oxidized in air for 45 h at temperatures up to 200 °C. Angle‐resolved XPS measurements indicated that a thin (5–10 A) NiO layer formed on the surface of the Nb2 O5 after long exposures at these temperatures. Specimens oxidized at temperatures above 240 °C, however, quickly developed a thick (>100 A), crystalline NiO layer on top of the Nb2O5.Sputter deposited amorphous Ni–Nb alloy films have been proposed as thin‐film metallization layers for high‐temperature (>300 °C) compound semiconductor applications. We have used x‐ray photoelectron spectroscopy (XPS) to investigate the oxidation behavior of 1‐μm‐thick, sputter deposited, amorphous Ni60 Nb40 films exposed to oxygen or room air at temperatures in the range 30–500 °C for times of 5 min to 45 h. The initial stages of oxidation were examined in situ at oxygen partial pressures below 1×10−6 Torr. At all temperatures, Nb2 O5 formed on the surface; this remained the dominant corrosion product for specimens oxidized in air for 45 h at temperatures up to 200 °C. Angle‐resolved XPS measurements indicated that a thin (5–10 A) NiO layer formed on the surface of the Nb2 O5 after long exposures at these temperatures. Specimens oxidized at temperatures above 240 °C, however, quickly developed a thick (>100 A), crystalline NiO layer on top of the Nb2O5.

Bismuth film coalescence determined by ellipsometry

Abstract The growth and coalescence of thin (1.5–100.0 nm) filmsof bismuth were examined with ellipsometry and atomic force microscopy. The Bi films were thermally evaporated onto oxidized, heated (343 K) Si substrates. An effective medium approximation was used with the ellipsometry data to determine that the films grew in an island growth mode out to a thickness of about 8.0 nm before coalescing into a continuous film. These results were confirmed usingatomic force microscopy which also indicated that the average grain size increased in an almost linear manner with film thickness up to thicknesses around 50.0 nm before levelling off. Surface steps of 0.4 and 0.8 nm suggested that the grains were mainly oriented with the (111) rhombohedral planes parallel to the surface.

Surface studies of amorphous W75Si25 oxidation

The surface composition and oxide growth kinetics of amorphous W75Si25 sputter‐deposited thin films were examined using x‐ray photoelectron spectroscopy (XPS), ellipsometry, and thermogravimetric analysis (TGA). These films have been proposed as metallization layers on compound semiconductors for high‐temperature applications. The 1‐μm‐thick films were oxidized in air at temperatures of 30 to 600 °C for times of 10 min to 30 h. Angle resolved XPS measurements indicated that the surface region consisted of a thin layer (∼16 A) of primarily SiO2 above a layer of mainly WO3 for oxidation temperatures up to 425 °C. At oxidation temperatures between 150 and 425 °C the WO3 signal from the surface region was enhanced. In this range the SiO2 layer was thinner resulting in a larger detected signal from the underlying WO3 layer. For oxidation temperatures above 450 °C, the two oxides appeared to coexist although the surface region was enhanced in SiO2. Ellipsometry and TGA measurements did not indicate a change in ...

Laser Ablation Deposition of Germanium Oxide Thin Films

Laser pulsed sputtering of GeO 2 targets in both vacuum and oxygen environments is us;ed to produce germanium oxide thin films. Infrared transmission and Auger electron spectroscopies are employed to infer compositional information on the films. The films grown in vacuum are oxygen deficient and they show a radial variation in the metal to oxide content. In contrast, the films grown in an oxygen atmosphere appear stoichiometric along the entire film.