S. Godey

Gold-free growth of GaAs nanowires on silicon: arrays and polytypism

We report growth by molecular beam epitaxy and structural characterization of gallium-nucleated GaAs nanowires on silicon. The influences of growth temperature and V/III ratio are investigated and compared in the case of oxide-covered and oxide-free substrates. We demonstrate a precise positioning process for Ga-nucleated GaAs nanowires using a hole array in a dielectric layer thermally grown on silicon. Crystal quality is analyzed by high resolution transmission electron microscopy. Crystal structure evolves from pure zinc blende to pure wurtzite along a single nanowire, with a transition region.

Graphene growth by molecular beam epitaxy on the carbon-face of SiC

Graphene layers have been grown by molecular beam epitaxy (MBE) on the (0001¯) C-face of SiC and have been characterized by atomic force microscopy, low energy electron diffraction (LEED), and UV photoelectron spectroscopy. Contrary to the graphitization process, the step-terrace structure of SiC is fully preserved during the MBE growth. LEED patterns show multiple orientation domains which are characteristic of graphene on SiC (0001¯), indicating non-Bernal rotated graphene planes. Well-defined Dirac cones, typical of single-layer graphene, have been observed in the valence band for few graphene layers by synchrotron spectroscopy, confirming the electronic decoupling of graphene layers.

Helium desorption from cavities induced by high energy 3He and 4He implantation in silicon

A detailed study has been made of helium release from silicon wafers implanted with MeV helium ions at fluences of 5 × 10 16 cm -2 and 10 17 cm -2 . Thermal desorption spectrometry (TDS), neutron depth profiling (NDP), non-Rutherford elastic backscattering (NREBS) and nuclear reaction analysis (NRA) have been employed to measure the helium content and release rate during isothermal annealing at annealing temperatures of 800 and 1000°C. TDS has also been used for isochronal annealing. Transmission electron microscopy (TEM) is used to monitor changes in morphology in the formed bubble layer. The helium release results can be modeled rather well when it is assumed that the helium initially is present in overpressurized bubbles. The present study reveals a single activation energy for helium release of 1.83 (0.05) eV.

Cavities and dislocations induced in silicon by MeV He implantation

We implanted n-type silicon with 1.6 MeV helium at fluences ranging from 1×1016 to 1×1017 He/cm2 while keeping a constant dose rate. These samples were then subjected to 800 °C annealing for 30 min. The results obtained by means of cross-sectional transmission electron microscopy indicate that the density of cavities is fluence dependent with homogeneous distribution of cavity sizes when fluences of 5×1016 and 1017 He/cm2 are used. The threshold fluence required to form cavities is found to be between 1 and 2×1016 He/cm2. For the 2×1016 He/cm2 dose, we observed loop punching induced by a concerted action of overpressurized bubbles, whereas He implants at doses of 5×1016 and 1×1017/cm2 lead to the formation of {311} defects. At the same time, non Rutherford elastic backscattering (NREBS) experiments using 2.5 MeV H+ provide the fraction of helium remaining in cavities after different annealing times at 800 °C. The NREBS data show a fast He release process for short annealing times (<2000 s). Then, the He a...

PROXIMITY GETTERING OF PLATINUM IN PROTON IRRADIATED SILICON

Epitaxial silicon samples of n type have been implanted with 850 keV protons at doses of 5.8×1011 to 5×1013 H+ cm−2. Subsequent indiffusion of platinum at 700 °C for 30 min resulted in the presence of a single deep level, which is attributed to the platinum acceptor level, at 0.23 eV below the conduction band edge. Depth profiling of this level shows that the substitutional platinum is following the vacancy profile in the peak region around the projected range for the protons. In addition, at more shallow depths, a strong increase of the platinum concentration is also observed. Without ion implantation, no deep levels are detected after in-diffusion at 700 °C, while at 800 °C, the Pt deep level concentration is inferior to the one reached after preimplantation of hydrogen with a dose of at least 5×1012 H+ cm−2. In-diffusion at 600 °C into 5×1013 H+ cm−2 implanted samples did not lead to an enhanced platinum accumulation. A tentative explanation of this proximity gettering of Pt is proposed, which is for t...

Low Schottky barrier height for ErSi2−x/n-Si contacts formed with a Ti cap

In this paper, the formation of Er disilicide (ErSi2−x) with a Ti cap on low doping n-type Si(100) is investigated. After deposition in ultrahigh vacuum, the solid-state reaction between Er and Si is performed ex situ by rapid thermal annealing between 450 and 600 °C in a forming gas ambience with a 10 nm thick Ti capping layer to protect Er from oxidation. X-ray diffraction analyses have confirmed the formation of ErSi2−x for all annealing temperatures. The formed films are found to be free of pinholes or pits and present a sharp and smooth interface with the Si bulk substrate. The extracted Schottky barrier height (SBH) corresponds to the state-of-the-art value of 0.28 eV if the annealing temperature is lower than or equal to 500 °C. This result demonstrates the possibility to form low SBH ErSi2−x/n-Si contacts with a protective Ti cap. However, when the annealing temperature is set to a higher value, the SBH concomitantly rises. Based on our experiments, this SBH increase can be mainly related to an enhanced diffusion of oxygen through the stack during the annealing, which degrades the quality of the ErSi2−x film.

The influence of diffusion temperature and ion dose on proximity gettering of platinum in silicon implanted with alpha particles at low doses

Platinum has been diffused into epitaxial n-type silicon at 600, 650, and 700 °C for 30 min following implantation with 3.3 MeV alpha particles. The doses employed were between 1×1011 and 1×1014 He+ cm−2. Thereafter the samples were characterized using deep level transient spectroscopy (DLTS). The samples diffused at 700 °C show only the deep level at 0.23 eV below the conduction band that is attributed to substitutional platinum. DLTS profiling reveals a decoration of the region of maximal damage by the platinum for lower doses while for higher ones the platinum concentration is observed to decrease or vanish in this region. In addition, other deep levels may appear (so-called K lines). As the implantation dose increases, so does the platinum concentration following diffusion at 700 °C at the shallow end of the DLTS working region. It is shown that, by controlling the amount of implantation induced defects and the diffusion temperature, one can steer the amount of platinum that arrives in the region of m...

Schottky barrier lowering with the formation of crystalline Er silicide on n-Si upon thermal annealing

The evolution of the Schottky barrier height (SBH) of Er silicide contacts to n-Si is investigated as a function of the annealing temperature. The SBH is found to drop substantially from 0.43 eV for the as-deposited sample to reach 0.28 eV, its lowest value, at 450 degrees C. By x-ray diffraction, high resolution transmission electron microscopy, and x-ray photoelectron spectroscopy, the decrease in the SBH is shown to be associated with the progressive formation of crystalline ErSi2-x.

Growth mechanism of cavities in MeV helium implanted silicon

A study of silicon implanted with 1.55 MeV helium 3 and thermally annealed to generate a subsurface cavity region was performed using neutron depth profiling and transmission electron microscopy (TEM). Results show that about 30% of the initial implanted helium is still present in cavities even after a 900 °C-1 h anneal. In addition, TEM measurement of cavity size on anneal temperature yields an activation energy of 1.65 eV for the growth of cavities. This value is very close to the activation energy (1.7 eV) reported for helium diffusion in silicon. Cavity growth hence results essentially from exchange of helium atoms between cavities.

Initial stages of graphitization on SiC(000-1), as studied by phase atomic force microscopy

The initial stages of graphitization on 4H- and 6H-SiC (000-1) under ultrahigh vacuum at temperatures of 1125–1175°C have been studied by atomic force microscopy (AFM), X-ray photoemission spectroscopy and reflected high energy electron diffraction. A progressive coverage of the surface by graphene has been observed depending on the time and temperature of annealing. Graphene growth mainly starts from the step edges, although it sometimes nucleates in the middle of a SiC terrace. Comparison of the topographic and phase AFM images shows that the latter are the most efficient for identifying graphene before complete coverage of the surface.