André Vantomme

Tuning Quantum Corrections and Magnetoresistance in ZnO Nanowires by Ion Implantation

Using ion implantation, the electrical as well as the magnetotransport properties of individual ZnO nanowires (NWs) can be tuned. The virgin NWs are configured as field-effect transistors which are in the enhancement mode. Al-implanted NWs reveal a three-dimensional metallic-like behavior, for which the magnetoresistance is well described by a semiempirical model that takes into account the presence of doping induced local magnetic moments and of two conduction bands. On the other hand, one-dimensional electron transport is observed in Co-implanted NWs. At low magnetic fields, the anisotropic magnetoresistance can be described in the framework of weak electron localization in the presence of strong spin-orbit scattering. From the weak localization, a large phase coherence length is inferred that reaches up to 800 nm at 2.5 K. The temperature-dependent dephasing is shown to result from a one-dimensional Nyquist noise-related mechanism. At the lowest temperatures, the phase coherence length becomes limited by magnetic scattering.

Suppression of rare-earth implantation-induced damage in GaN

We have studied the damage induced by 80 keV Er implantation in epitaxial GaN/sapphire layers at room temperature and at 450°C. The dopant distribution and lattice damage were investigated using Rutherford backscattering, channeling spectrometry and X-ray diffraction, whereas photoluminescence was used to probe the optical response. Random implantation results in substantial damage accumulation, which is difficult to recover during subsequent annealing. To reduce the ion-induced damage, we applied channeled implantation, i.e. directing the Er-beam along the nitride c-axis. Using this implantation geometry, a drastic decrease in the induced damage is observed. Channeled implantation generally results in green luminescence lines at room temperature, whereas no Er-related luminescence is observed after random implantation.

Searching for room temperature ferromagnetism in transition metal implanted ZnO and GaN

Significant progress in the field of wide-gap dilute magnetic semiconductors (DMS) depends on the discovery of a material system which not only shows high-temperature ferromagnetism but is also simple to prepare and thus easy to reproduce. In this context, ion implantation is an attractive doping method, being both relatively simple and highly reproducible. Here, we report on the search for high-temperature ferromagnetism in ZnO and GaN implanted with Mn, Fe, and Co, prepared under a wide range of implantation and post-processing conditions. We focused on the low concentration regime (∼0.3−4%) in order to avoid phase segregation and applied strict experimental procedures to avoid ferromagnetic contamination. Despite the wide range of materials, implantation and post-processing conditions, none of the DMS systems showed room-temperature ferromagnetism. These results support the view that dilute transition-metal moments do not order ferromagnetically in ZnO and GaN.

Relaxor Ferroelectricity and Magnetoelectric Coupling in ZnO–Co Nanocomposite Thin Films: Beyond Multiferroic Composites

ZnO-Co nanocomposite thin films are synthesized by combination of pulsed laser deposition of ZnO and Co ion implantation. Both superparamagnetism and relaxor ferroelectricity as well as magnetoelectric coupling in the nanocomposites have been demonstrated. The unexpected relaxor ferroelectricity is believed to be the result of the local lattice distortion induced by the incorporation of the Co nanoparticles. Magnetoelectric coupling can be attributed to the interaction between the electric dipole moments and the magnetic moments, which are both induced by the incorporation of Co. The introduced ZnO-Co nanocomposite thin films are different from conventional strain-mediated multiferroic composites.

Epitaxy of CoSi2/Si(100) : from Co/Ti/Si(100) to reactive deposition epitaxy

Abstract High-quality CoSi2(100) layers in the thickness range of 200 to 500 A are formed by reactive deposition epitaxy. A model is described in which the deposition rate and substrate temperature are crucial in determining the epitaxial nature of the silicide. It is shown that good CoSi 2 Si(100) alignment is only achieved when using low deposition rates (in the order of 0.1 A/s or less) combined with relatively high substrate temperatures during deposition (∼600°C). At higher rates and/or lower temperatures, a fraction of misoriented CoSi2 grains is formed. A study of the crystalline quality of the CoSi2 layers as a function of Co flux and deposition temperature is presented, and special attention is devoted to the thermal stability of the layers during high temperature annealing.

Lattice expansion induced by Zn channeled implantation in GaN

Abstract 140xa0keV Zn channeled implantations in GaN are performed at room temperature and in a dose range from 1×1013 to 4×1016/cm2, respectively. The lattice expansion is calculated from (0xa00xa00xa02) and (0xa00xa00xa04) X-ray diffractions of GaN after channeled implantation. Three dose dependence regimes are observed: the perpendicular lattice parameter initially increases with dose, subsequently varies slowly in an intermediate dose range, and finally diminishes at very high dose. The high-resolution XTEM observations show the different defects, such as the clustered point defect, the thread defects and loops, the broken crystals and the amorphous in nanometer sizes. The variation of lattice expansion is attributed to the changes of density and types of defects.

Tuning the ferromagnetic-antiferromagnetic interfaces of granular Co-CoO exchange bias systems by annealing

The low-temperature magnetic behavior of granular Co-CoO exchange bias systems, prepared by oxygen ion implantation in Co thin films and subsequent annealing, is addressed. The thermal activation effects lead to an O migration which results in virtually pure Co areas embedded in a structurally relaxed and nearly stoichiometric CoO phase. This yields decreased training and exchange bias shifts, while the blocking temperature significantly increases, coming close to the Neel temperature of bulk CoO for samples implanted to a fluence above 1u2009×u20091017 ions/cm2 (15% O). The dependence of the exchange bias shift on the pristine O-implanted content is analogous to that of the antiferromagnetic thickness in most ferromagnetic/antiferromagnetic systems (i.e., an increase in the exchange bias shift up to a maximum followed by a decrease until a steady state is reached), suggesting that, after annealing, the enriched Co areas might be rather similar in size for samples implanted above 1u2009×u20091017 ions/cm2, whereas the co...

(Invited) Assessment of Ge1-xSnx Alloys for Strained Ge CMOS Devices

Strained Ge CMOS Devices S. Takeuchi, Y. Shimura, T. Nishimura, B. Vincent, G. Eneman, T. Clarysse, J. Demeulemeester, K. Temst, A. Vantomme, J. Dekoster, M. Caymax, R. Loo, O. Nakatsuka, A. Sakai, and S. Zaima Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Covalent Materials Co., Ltd., Higashikou, Seirou-machi, Kitakanbara-gun, Niigata 957-0197, Japan Imec, Kapeldreef 75, B-3001 Leuven, Belgium Instituut voor Kernen Stralingsfysica, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan

Zn channeled implantation in GaN: damages investigated by using high resolution XTEM and channeling RBS

Abstract Zn (140 keV) channeled (along 〈0001〉) implantations in GaN are performed at room temperature and in a dose range from 1×1013 to 4×1016 cm−2, respectively. Channeling RBS measurements and the high-resolution XTEM investigations show the two damage regimes after implantation: one at the surface and another in the projected range. The damage level is very small at low doses and then gradually rises with increasing dose. The backscattering yield from the near surface region reaches the random level at doses higher than 2×1016 cm−2 and the broken crystals and the amorphous in nanometer size are formed in the top thin surface layer after implantation at a dose of 3×1016 cm−2. In the followed defective crystalline layer, the density of defects decreases with increasing the depth. The thread defects and loops are dominant in the region close to the surface at high dose and the clustered point defects are dominant in the deeper layer.

Formation of ultra-thin PtSi layers with a 2-step silicidation process

We propose a new technique to form ultra-thin PtSi layers based on sputter deposition of the metal and two-step silicidation by rapid thermal processing, which are production tools used in standard silicon processing technologies.