N.G. van der Berg

Structural colours from the feathers of the bird Bostrychia hagedash

Iridescence in the bird Bostrychia hagedash is interesting in the sense that instead of the usual single, fairly narrow, reflection peak seen for many birds and insects, up to six strong peaks were found covering the spectral range from the near ultraviolet (UV) to the near infrared (IR). By recording reflectance spectra over this range and comparing it to model calculations based on a structural analysis using electron microscopy, the mechanism for iridescence as well as the optical constants of the materials comprising the coloured feathers, could be established. It was discovered that iridescence is mainly based on a relatively thick but uniform keratin outer layer on the feather barbules. In contrast to iridescence in most other birds, the melanin-containing platelets inside the barbules only serve to define the layer thickness of the keratin without contributing to the iridescence in any other way. Unlike previous work where keratin was assumed to be transparent, it was established that both keratin and melanin are absorbing and that keratin exhibits an anomalous dispersion in the green, blue and near UV.

An investigation of green iridescence on the mollusc Patella granatina

In this paper we investigate the relatively rare phenomenon of iridescence on the outer surface of seashells (not the well known pearly inner surfaces). Using reflection spectroscopy and scanning electron microscopy we show that rows of iridescent green spots on the mollusc Patella granatina are caused by a thin-film stack buried about 100 µm below the rough outer surface of the shell. The high-density layers in the stack seem to be made of crystalline aragonite, but according to Raman spectroscopy and ellipsometry measurements the low-density layers as well as the bulk of the shell wall are a mixture of porous aragonite and organic materials such as carotenoids.

Atomic force microscopy investigation of noble gas ion bombardment on InP: Effect of ion energy

Mirror-polished InP(100) samples, n-doped with S atoms to 4 x 10 18 cm -3 , were bombarded with neon, argon and krypton ions. The energy of the ions was varied from 0.5-5 keV at a constant angle of incidence at 41° with respect to the sample normal. The ion dose density for Ne + was 1.8 x 10 16 ions cm -2 and for Ar + and Kr + was 5 x 10 18 ions cm -2 . A low ion current density of 5 x 10 13 ions cm -2 s -1 was used to minimize sample heating. The resulting topography development was investigated by atomic force microscopy (AFM). The topography development was quantified in terms of RMS (root-mean-square) roughness using the software of the AFM and analysed as a function of the energy of the bombarded ions. Atomic force micrographs showing various types of topography for some specific ion energies are presented. For each ion species there is a critical energy for which the surface roughness attains a maximum value.

The fcc-hcp phase transition in electrodeposited epitaxial cobalt films☆

Abstract Thin (50–500 A) films of cobalt were electrodeposited from sulphate solutions having pH values in the range 1·9–5 onto thin (001) single-crystal copper substrates. Bicrystals and stripped deposits were examined by transmission electron microscopy and electron diffraction. Deposits from solutions with 45 g/l boric acid were essentially fcc for all pH values. Stacking faults, associated with imperfect misfit dislocations, converted only a very small fraction of the fcc cobalt into the hcp structure. Deposits from lightly-buffered (6 g/l boric acid) solutions contained small crystallites of hcp cobalt provided the solution pH ≳ 3. Growth of the hcp phase is probably associated with the presence of colloidal hydroxide at the cathode surface.

The structure of gold silicide in thin Au/Si films☆

Abstract Vacuum-deposited Au/Si double layers were heat treated to the eutectic temperature and were examined by transmission electron diffraction. Electron diffraction patterns ascribed to crystalline silicides were observed. Some of the patterns and associated d spacings were in satisfactory agreement with a primitive cubic unit cell with a = 6.75 A as proposed previously. Other diffraction patterns were consistent with the assumption of an orthorhombic unit cell with a = 6.8, b = 7.5 and c = 9.56 A .

Effect of residual gas pressure on the epitaxial growth of vacuum-deposited chromium on Cu{111} substrates

Abstract Transmission electron microscopy, electron diffraction and Auger electron spectroscopy were used to study the effect of vacuum conditions on the epitaxial growth of vacuum-deposited chromium films of thicknesses less than 40 nm on Cu{111} substrates. The chromium films were condensed (i) in ultrahigh vacuum at 10 −9 Torr, (ii) at total pressures of 10 −5 , 10 −6 and 5 × 10 −8 Torr and (iii) at oxygen partial pressures of 5 × 10 −9 , 10 −7 and 10 −6 Torr. The presence of contaminants during the chromium evaporation suppressed the formation of Kurdjumov-Sachs orientations. A low oxygen partial pressure was most effective in altering the orientation and morphology of the deposits.

Study of implantation damage ranges in metals at temperatures ranging from 5 to 300 K

Abstract Cu〈111〉 single crystals were implanted with 150 keV argon ions at temperatures between 5 and 300 K and analysed in-situ by α-particle channeling in a backscattering geometry. At all temperatures similar damage ranges were observed, which exceed the TRIM-estimate approximately four times. Ranges did not change during warming up, but the defect density changed significantly in the case of the 5 K implantation. The observations are in disagreement with a simple point defect migration process as interstitial atoms become mobile only above 45 K. Possible mechanisms to explain these results are discussed. Furthermore some room temperature measurements of damage ranges in gold and silver single crystals are also given.

Epitaxial growth of chromium on Cu{111} substrates

Abstract Deposits of chromium (1–40 nm thick) condensed in ultrahigh vacuum (about 10 −9 Torr) onto Cu{111} films at room temperature (about 300 K) were examined by transmission electron microscopy, electron diffraction and Auger electron spectroscopy (AES). Diffraction patterns consistent with Nishiyama-Wassermann (NW) and Kurdjumov-Sachs (KS) orientations were observed. Arcing of the chromium reflections indicated a continuous distribution of alignments within a ±6° spread about the exact NW orientation. Dark field and defocusing phase contrast techniques revealed small (about 28 nm) irregularly shaped chromium grains within which smaller parallel-oriented subcrystallites, elongated approximately in the Cr〈001〉 direction, exist. AES confirmed the existence of discontinuos chromium deposits for thicknesses of about 6 nm or less and indicated the presence of a thin layer of Cr 2 O 3 .

Argon bombardment-induced topography and sputter yields on Si0.84Ge0.16

Abstract The argon ion sputter yields of epitaxial Si 0.84 Ge 0.16 films on a Si substrate are presented for different ion energies (0.5–5 keV) and angles of incidence (0–70°). The results were obtained by using atomic force microscopy (AFM) to determine the sputter depths of shielded samples. There was reasonable agreement between the experimental sputter yields and the predictions from the Sigmund theory. The results showed the applicability of this method using AFM to determine sputter yields of thin films. The surface topographies of the unbombarded and sputtered surfaces were characterised by AFM and were quantified in terms of the rms roughness. The results showed that very little topography developed on the surfaces due to the ion bombardment. The rms roughness exhibited no significant dependence on the ion energy or the angle of incidence. There was only a slight correlation with the ion dose density.

The orientation and morphology of chromium deposits on hot Cu{111} substrates

Abstract Chromium (1.5–20 nm thick) was vapour deposited in ultrahigh vacuum (2×10 -9 Torr) onto single-crystal Cu{111} films. The copper substrate temperatures T s were in the range 75–256 °C. Transmission electron microscopy and electron diffraction were used to study the orientation and morphology of the deposits. For T s ≲ °C the electron diffraction patterns showed arced chromium reflections consistent with a distribution of orientations between Nishiyama- Wassermann (NW) and Kurdjumov-Sachs (KS) at ±5° 16′ from NW. A greater tendency towards KS was observed as T s increased to about 130 °C. For T s = 190 and 256 °C relatively well-defined KS orientations occured. These results are interpreted in terms of various geometric models of f.c.c.-b.c.c. interfaces. At T s ≲ 85 °C the morphology was qualitatively similar to that observed for room temperature substrates: flat irregularly shaped crystals within which a substructure of parallel-oriented crystallites with elongated shape existed. For T s ≲ 130 °C the morphology changed from the flat irregularly shaped crystals to isolated three- dimensional crystals with an elongated shape. The long direction of the crystals was approximately parallel to the direction of least misfit, i.e. Cr〈111〉  Cu〈110〉.