Nils Lundberg

Thermally stable low ohmic contacts to p-type 6HSiC using cobalt silicides

Abstract Cobalt silicide (CoSi 2 ) ohmic contacts possessing low specific contact resistivity ( p c −6 μ cm 2 ) to p -type 6HSiC are reported. The contacts were fabricated through sequential electron-beam evaporation of Co and Si layers forming a Si/Co/SiC structure, followed by a two-step vacuum annealing process at 500 and 900°C, respectively. Specific contact resistivities were extracted from transmission line model (TLM) structures at temperatures ranging from 22 to 200°C. p c is investigated as a function of current density, temperature and ageing in a vacuum furnace at 1100°C. Furthermore, comparison with a Co SiC contact structure subjected to an identical annealing process revealed higher p c and a modified sheet resistance requiring a different method of contact parameter extraction.

Multivariate analysis method for energy calibration and improved mass assignment in recoil spectrometry

Abstract Heavy ion recoil spectrometry is rapidly becoming a well established analysis method, but the associated data analysis processing is still not well developed. The pronounced nonlinear response of silicon detectors for heavy ions leads to serious limitation and complication in mass gating, which is the principal factor in obtaining energy spectra with minimal cross talk between elements. To overcome the above limitation, a simple empirical formula with an associated multiple regression method is proposed for the absolute energy calibration of the time of flight-energy dispersive detector telescope used in recoil spectrometry. A radical improvement in mass assignment was realized, which allows a more accurate and improved depth profiling with the important feature of making the data processing much easier.

Temperature stability of cobalt Schottky contacts on n- and p-type 6H silicon carbide

Abstract Rectifying Schottky contacts have been manufactured on n- and p-type 6H silicon carbide using e-beam evaporation of cobalt. Heat treatments in the 300 to 1100°C temperature range have been made to study the feasibility of high temperature contacts in this material system. Rutherford backscattering spectrometry and X-ray diffraction have revealed the formation of different cobalt silicides (Co2Si, CoSi, and CoSi2) at higher temperatures than for the Co/Si system. No evidence of silicidation was found below 600°C and SEM micrographs revealed carbon agglomerates at the surface after silicidation. Electrical properties have been examined using I–V and C–V measurements, and the barrier heights of cobalt and Co2Si were evaluated. The contacts displayed excellent forward I–V characteristics with good linearity over 3–6 decades and were rectifying even after heat treatments at 800°C.

CoSi2 ohmic contacts to n-type 6HSiC

Cobalt disilicide (CoSi2) ohmic contacts possessing low specific contact resistivity (ϱc < 3.0 ± 0.4 × 10−5 ωcm2) to n-type 6HSiC are reported. The contacts were fabricated via sequential electron-beam evaporation of Co and Si layers followed by a two-step vacuum anealing process at 500 and 900°C. Stochiometry of the contact so formed was confirmed by Rutherford backscattering spectrometry and X-ray diffraction. Specific contact resistivities were obtained via current-voltage (I-V) analysis at temperatures ranging from 25 to 500°C. ϱc is compared as a function of carrier concentration, current density, temperature and time at elevated temperature.

RBS and recoil spectrometry analysis of CoSi2 formation on GaAs

Abstract Mass and energy-dispersive recoil spectrometry has recently reached the state of development where it is possible to separately characterise Ga and As in GaAs samples. Since it is possible to simultaneously characterise several elements (light as well as heavy), e.g. C, O, Si, Co, Ga and As, the technique is suited for examining the depth distribution of metallisation contacts on GaAs. In a Swedish-Australian collaboration a recoil detector telescope was attached to a beamline of the FN tandem accelerator “ANTARES”, at Lucas Heights Research Laboratories, Australia. In the measurements presented here, 127 I 10+ at an energy of 77 MeV was employed to analyse GaAs samples with thin film overlayers — Si(220 nm)/Co(50 nm)/〈100〉-GaAs. A reference sample and samples annealed at 300 to 600°C were analysed. The measurements showed that CoSi 2 is formed during annealing at and above 500°C with no detectable reaction between the GaAs-substrate and the CoSi 2 overlayer.

Recoil spectrometry : ion accelerator based elemental characterisation of surface layers

Recoil Spectrometry covers a group of techniques that are very similar to the well known Rutherford backscattering Spectrometry technique, but with the important difference that one measures the recoiling target atom rather than the projectile ion. This makes it possible to determine both the identity of the recoil and its depth of origin from its energy and velocity, using a suitable detector system. The incident ion is typically high-energy (30–100MeV)35C1,81Br or127I. Low concentrations of light elements such as C, O and N can be profiled in a heavy matrix such as Fe or GaAs. Here we present an overview of mass and energy dispersive recoil Spectrometry and illustrate its successful use in some typical applications.

Chromium germanides: formation, structure and properties

Abstract The reaction of Cr and Ge films leads to the successive formation of Cr5Ge3, CrGe (FeSi structure), and Cr11Ge19 (Mn11Si19 structure). The first one of the phases could not be identified unambiguously because of difficulties relating to the proper crystalline structure of this compound. The formation of both Cr5Ge3 and CrGe (respectively at 400–500°C, and 600°C) leave the surface of the sample absolutely smooth strongly implying that the rate-limiting mechanism is diffusion. On the contrary, the formation of Cr11Ge19 occurs in island-like fashion, islands that spread laterally to consume the remaining CrGe, in a process that is quite characteristic of nucleation-controlled reactions. The growth of Cr5Ge3 proceeds as t1/n with n greater than 2. It is possible for this to occur if the rate-controlling mechanism is grain-boundary diffusion, with grains growing simultaneously with the thickness of the compound. The upper limit for the resistivities of Cr5Ge3, Cr11Ge8, CrGe an Cr11Ge19 were found to be 200, 160, 150 and 430 μω · cm, respectively. The last one of these phases is ferromagnetic with a Curie temperature of 91 K.

Formation of thin films of CoSi2 on GaAs

CoSi2 exhibits the features of low resistivity and stability at elevated temperatures which make it interesting to employ for metallization on GaAs. The interfacial reactions in GaAs samples with thin film overlayers of Si and Co [Si(220 nm)/Co(50 nm)/(〈100〉‐GaAs)] were studied using x‐ray diffraction, scanning electron microscopy, x‐ray photoelectron spectroscopy, and mass and energy dispersive recoil spectrometry. Samples were vacuum furnace annealed for time periods between 1 and 8 h at temperatures ranging from 300 to 700 °C. It was found that a CoSi2 layer formed without observable reaction with the substrate at 500 °C and above. The excess Si (Si/Co atomic ratio of 2.41) remained near the surface as elemental Si and as SiO2 for the 500 and 600 °C annealings. For the 700 °C annealing the excess near‐surface Si was not observed.

Oxidation of Semiconducting Iron Disilicide (β-FeSi 2 )

Iron disilicide (β-FeSi 2 ) has gained significant importance in recent years because of its semiconducting properties. The bandgap is reported to be direct with an energy of 0.85–0.89 eV, thus making the suicide a potential candidate for optical communications and detector applications. Compatibility with standard VLSI processing might involve a suicide thermal oxidation step. This work concerns the kinetics of both dry and wet oxidations of β-FeSi 2 . The oxide quality was characterized with respect to the electrical breakdown voltage. The results indicate an oxidation temperature dependence of the oxide quality and that dry oxidation yield higher breakdown voltage than wet oxidation. Structural and semiconducting suicide properties were investigated before and after oxidation. High energy implantation of xenon was used in a marker experiment to investigate a possible change of oxidation mechanism between dry and wet oxidation.

Metal/InP thin film reactions: Studies using mass and energy dispersive recoil spectrometry

Abstract Preliminary results are presented from a study of transition metal/lnP reactions using mass and energy dispersive recoil spectrometry. 50 nm films of Ni, Pd and Pt were evaporated on InP substrates, heat treated in vacuum and subsequently analysed with X-ray diffraction and mass and energy dispersive recoil spectrometry using 77 MeV 127 I 10+ ions as projectiles. The unique information on the elemental distributions in the surface layers which can be obtained using the latter technique has allowed us to follow the progress of the solid state reactions. These reactions are quite different for the three metals studied, with significant P loss in the Pd/InP case.