H. Hofsäss

Conduction processes in boron- and nitrogen-doped diamond-like carbon films prepared by mass-separated ion beam deposition

Abstract Boron- and nitrogen-doped diamond-like amorphous carbon (DLC) films were prepared by alternating direct deposition of low energy mass-separated 12C+ and dopant ions. Concentration vs. depth profiles for N and B dopants were determined by neutron depth profiling. The measured current-voltage characteristics of these films, which were deposited on polished stainless steel, are explained best by Frenkel-Poole emission for high electric fields. Two different trap states Φ1 and Φ2 were found to contribute to the conduction process. At low electric fields our results suggest that conduction is due to variable-range hopping via localized states at the Fermi level. The doped DLC films show a higher electrical conductivity, indicative of an increased density of localized states, rather than a shift in the Fermi level. A diode-like device was prepared, but the measured I-V curves did not indicate that a p-n junction had formed. DLC/Si heterojunctions were also prepared and their current-voltage characteristics are presented and discussed.

Doping and growth of diamond-like carbon films by ion beam deposition

Abstract Hydrogen-free amorphous carbon films produced by direct deposition of low energy carbon ions exhibit diamond-like properties such as extreme hardness, large band gap and high index of refraction. Using mass separated ion beam deposition, high purity diamond-like carbon (DLC) films were grown under vacuum conditions better than 10−5 Pa on Si substrates kept at room temperature. The deposition parameters such as the ion energy, vacuum conditions, substrate temperature and ion species can be controlled independently, and thus used to modify the film properties and composition. In this paper we present the first results of doping of DLC films during growth by alternating deposition of 100 eV 12C+ ions and dopant ions such as 63Cu+, 27Al+, 11B+ or 14N+. For beam currents of up to 100 μA cm−2, DLC films with a thickness of several micrometers were produced. Undoped films were characterized by Raman spectrometry, ellipsometry, electrical measurements, Rutherford backscattering spectrometry (RBS) and proton induced X-ray emission. For these films an energy gap of 2.6 eV, a Vickers hardness of 4500 kg mm−2, an index of refraction n > 2, a resistivity of 109 Ω cm and an electrical breakdown strength greater than 109 V m−1 were measured. Dopant concentration profiles of copper-doped films were analyzed by RBS. Homogeneous dopant concentrations of several atomic per cent are easily achieved.

Emission channeling and blocking

Abstract Emission channeling allows the lattice location of radioactive probe atoms in crystalline solids by measuring channeling effects of electrons, positrons or α particles emitted in the nuclear decay. The development of this experimental technique is reviewed and experimental aspects as well as theoretical approaches to emission channeling are discussed. Applications of emission channeling like defect formation studies in fcc metals Ni and Cu and lattice location and defect recovery studies in ion implanted semiconductors Si and GaAs are presented. In some examples the complementary character of the emission channeling technique and hyperfine interaction techniques is emphasized.

Intense ultraviolet cathodoluminescence at 318 nm from Gd3+-doped AlN

We present investigations of Gd-implanted aluminum nitride, studied with cathodoluminescence (CL) as well as time-resolved CL in the temperature range 12–300 K. Luminescence due to intra-4f electron transitions of Gd3+ is dominated by the 6P7/2→8S7/2 transition between the first excited state and the ground state of Gd3+ detected at around 318 nm. Time-resolved CL of the 6P7/2 level monitoring the 6P7/2→8S7/2 transition shows a temperature-dependent lifetime which decreases from 0.76 ms at 12 K to 0.69 ms at 300 K, in contrast to an increasing intensity of the 6P7/2→8S7/2 transition by a factor of more than 3.5 in the same temperature range. The decay is of the Inokuti–Hirayama-type indicating energy transfer between Gd3+ ions. Due to the overall weak splitting of the 6P7/2 and 8S7/2 multiplets phonon replica with energies of 100 and 588 cm−1 can be assigned.

Cubic boron nitride films grown by low energy B+ and N+ ion beam deposition

We have studied the growth and the properties of BN films prepared by exclusive deposition of mass separated 11B+ and 14N+ ions. BN films grown with ion energies of 500 eV and at substrate temperatures of 350 °C show the IR absorption peak at 1080 cm−1, characteristic for c‐BN. These films are nearly stoichiometric and, with transmission electron diffraction, the presence of c‐BN nanocrystals was revealed. We compare the growth conditions for ion beam deposition on BN, CN, and diamondlike carbon and propose that the nucleation of nanocrystalline c‐BN is related to the ionicity of the BN bond.

Nucleation mechanism of the seed of tetrapod ZnO nanostructures

Tetrapod zinc oxide (T-ZnO) nanorods have been synthesized by evaporation and recondensation of metallic Zn under ambient conditions. The total sizes of the T-ZnO nanostructures range from 300nmto15μm with leg diameters of about 30to650nm, depending on the deposition temperature. A detailed high-resolution electron microscopy analysis showed that the center core of T-ZnO nanorods consists of four hexagonal grains with a twinlike relation. The nucleation and growth mechanism has been generated on the basis of energy considerations during a phase transition from a fullerenelike ZnO cluster to a nanometer-sized tetrahedron, which is directly visible in our high-resolution transmission electron microscopy investigations.

Doping of Graphene by Low-Energy Ion Beam Implantation: Structural, Electronic, and Transport Properties

We investigate the structural, electronic, and transport properties of substitutional defects in SiC-graphene by means of scanning tunneling microscopy and magnetotransport experiments. Using ion incorporation via ultralow energy ion implantation, the influence of different ion species (boron, nitrogen, and carbon) can directly be compared. While boron and nitrogen atoms lead to an effective doping of the graphene sheet and can reduce or raise the position of the Fermi level, respectively, (12)C(+) carbon ions are used to study possible defect creation by the bombardment. For low-temperature transport, the implantation leads to an increase in resistance and a decrease in mobility in contrast to undoped samples. For undoped samples, we observe in high magnetic fields a positive magnetoresistance that changes to negative for the doped samples, especially for (11)B(+)- and (12)C(+)-ions. We conclude that the conductivity of the graphene sheet is lowered by impurity atoms and especially by lattice defects, because they result in weak localization effects at low temperatures.

Room temperature growth of cubic boron nitride

Boron nitride thin films were deposited at room temperature with various ion energies by mass selected ion beam deposition on cubic boron nitride (c-BN) previously nucleated on Si (100) substrates at a higher temperature. Selective area diffraction, electron energy loss, and infrared spectroscopy results reveal continued growth of the cubic phase. The reported temperature threshold of about 150 °C for c-BN film formation is therefore unmistakably related to the initial nucleation of c-BN, whereas the growth of c-BN appears to be temperature independent. The latter is in accordance with predictions of the cylindrical thermal spike growth model recently proposed by our group.

Chernobyl: an early report

An overview and assessment of the nuclear accident at Chernobyl is presented. The authors have assembled data from throughout Europe to estimate upper bounds for the possible radiation releases from the accident, the exposures these may produce in humans, and the health consequences that may follow. Measurements of radioactivity in air, fallout, and milk and other food are included. Doses from the accident are compared with those from other sources of radiation exposure and a comparison is made of the annual risk of cancer from this and other causes.

Ion beam synthesis of boron carbide thin films

Abstract We have grown boron carbide (B x C) thin films via direct ion beam deposition using mass selected 11 B + and 12 C + ions. The films were deposited on silicon and ITO-coated quartz glass substrates with an ion energy of 100 eV at room temperature. The B + :C + ion ratio during deposition was varied between 0:1 (pure carbon) and 1:0 (pure boron), and the resulting composition of the films matched this ratio, as observed by X-ray photoelectron spectroscopy (XPS). A detailed analysis of the XPS-spectra revealed that the deposited films undergo a transition from sp 3 -bonded diamond-like carbon to a boron carbide phase with a lower density with increasing B concentration. The formation of carbide bonds has been observed by means of XPS, and the valence band spectra showed a strong transition from the amorphous semiconductor ta-C to metallic boron. This transition has also been observed by optical and electrical measurements.