Manfred Helm

Room temperature ferromagnetism in ZnO films due to defects

ZnO films were prepared by pulsed laser deposition on a-plane sapphire substrates under N2 atmosphere. Ferromagnetic loops were obtained with the superconducting quantum interference device at room temperature, which indicate a Curie temperature much above room temperature. No clear ferromagnetism was observed in intentionally Cu-doped ZnO films. This excludes that Cu doping into ZnO plays a key role in tuning the ferromagnetism in ZnO. 8.8% negative magnetoresistance probed at 5K at 60kOe on ferromagnetic ZnO proves the lack of s-d exchange interaction. Anomalous Hall effect (AHE) was observed in ferromagnetic ZnO as well as in nonferromagnetic Cu-doped ZnO films, indicating that AHE does not uniquely prove ferromagnetism. The observed ferromagnetism in ZnO is attributed to intrinsic defects.

High-intensity terahertz radiation from a microstructured large-area photoconductor

We present a planar large-area photoconducting emitter for impulsive generation of terahertz (THz) radiation. The device consists of an interdigitated electrode metal-semiconductor-metal (MSM) structure which is masked by a second metallization layer isolated from the MSM electrodes. The second layer blocks optical excitation in every second period of the MSM finger structure. Hence charge carriers are excited only in those periods of the MSM structure which exhibit a unidirectional electric field. Constructive interference of the THz emission from accelerated carriers leads to THz electric field amplitudes up to 85V∕cm when excited with fs optical pulses from a Ti:sapphire oscillator with an average power of 100mW at a bias voltage of 65V applied to the MSM structure. The proposed device structure has a large potential for large-area high-power THz emitters.

Carrier relaxation in epitaxial graphene photoexcited near the Dirac point.

We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 to 250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical-phonon frequency; however, owing to the presence of hot carriers, optical-phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.

Room temperature ferromagnetism in carbon-implanted ZnO

Unexpected ferromagnetism has been observed in carbon doped ZnO films grown by pulsed laser deposition [H. Pan et al., Phys. Rev. Lett. 99, 127201 (2007)]. In this letter, we introduce carbon into ZnO films by ion implantation. Room temperature ferromagnetism has been observed. Our analysis demonstrates that (1) C-doped ferromagnetic ZnO can be achieved by an alternative method, i.e., ion implantation, and (2) the chemical involvement of carbon in the ferromagnetism is indirectly proven.

Crystallographically oriented Co and Ni nanocrystals inside ZnO formed by ion implantation and postannealing

In the last decade, transition-metal-doped ZnO has been intensively investigated as a route to room-temperature diluted magnetic semiconductors (DMSs). However, the origin for the reported ferromagnetism in ZnO-based DMS remains questionable. Possible options are diluted magnetic semiconductors, spinodal decomposition, or secondary phases. In order to clarify this question, we have performed a thorough characterization of the structural and magnetic properties of Co- and Ni-implanted ZnO single crystals. Our measurements reveal that Co or Ni nanocrystals (NCs) are the major contribution of the measured ferromagnetism. Already in the as-implanted samples, Co or Ni NCs have formed and they exhibit superparamagnetic properties. The Co or Ni NCs are crystallographically oriented with respect to the ZnO matrix. Their magnetic properties, e.g., the anisotropy and the superparamagnetic blocking temperature, can be tuned by annealing. We discuss the magnetic anisotropy of Ni NCs embedded in ZnO concerning the strain anisotropy.

Impulsive terahertz radiation with high electric fields from an amplifier-driven large-area photoconductive antenna

We report on the generation of impulsive terahertz (THz) radiation with 36 kV/cm vacuum electric field (1.5 mW average thermal power) at 250 kHz repetition rate and a high NIR-to-THz conversion efficiency of 2 x 10(-3). This is achieved by photoexciting biased large-area photoconductive emitter with NIR fs pulses of microJ pulse energy. We demonstrate focussing of the THz beam by tailoring the pulse front of the exciting laser beam without any focussing element for the THz beam. A high dynamic range of 10(4) signal-to-noise is obtained with an amplifier based system.

Fe implanted ferromagnetic ZnO

Room-temperature ferromagnetism has been induced within ZnO single crystals by implant-doping with Fe ions.The four samples implanted initially have been analyzed using SQUID (superconducting quantum interference device) magne-tometry. It was found that only two of them, i.e. the HFHT and the LFLT samples exhibit a pronounced hysteresis loop upon magnetization reversal at 5 K and 300 K. As was found using synchrotron X-ray diffraction, transmission electron microscopy and conversion electron Mossbauer spectroscopy (CEMS) the origin of the ferromagnetic properties of the HFHT-sample are tiny alpha-Fe-nanoparticles with a mean diameter of 8 nm. At 180 keV Fe implanted ZnO single crystals can develop ferromagnetic properties that are either caused by alpha-Fe nanoparticles or an indirect coupling of the Fe ions in a DMS system, depending on the details of ion fluence and implantation temperature. In any case sophisticated structural characterization is required in order to rule out secondary phases.

Bright green electroluminescence from Tb3+ in silicon metal-oxide-semiconductor devices

Bright green electroluminescence with luminance up to 2800cd∕m2 is reported from indium-tin-oxide∕SiO2:Tb∕Si metal-oxide-semiconductor devices. The SiO2:Tb3+ gate oxide was prepared by thermal oxidation followed by Tb+ implantation. Electroluminescence and photoluminescence properties were studied with variations of the Tb3+ ion concentration and the annealing temperature. The optimized device shows a high external quantum efficiency of 16% and a luminous efficiency of 2.1lm∕W. The excitation processes of the strong green electroluminescence are attributed to the impact excitation of the Tb3+ luminescent centers by hot electrons and the subsequent crossrelaxation from D35 to D45 energy levels. Light-emitting devices with micrometer size fabricated by the standard metal-oxide-semiconductor technology are demonstrated.

Nonvolatile bipolar resistive switching in Au/BiFeO3/Pt

Nonvolatile bipolar resistive switching has been observed in an Au/BiFeO3/Pt structure, where a Schottky contact and a quasi-Ohmic contact were formed at the Au/BiFeO3 and BiFeO3/Pt interface, respectively. By changing the polarity of the external voltage, the Au/BiFeO3/Pt is switched between two stable resistance states without an electroforming process. The resistance ratio is larger than two orders of magnitude. The resistive switching is understood by the electric field–induced carrier trapping and detrapping, which changes the depletion layer thickness at the Au/BiFeO3 interface.

Ferromagnetic Gd-implanted ZnO single crystals

In order to introduce ferromagnetic properties, ZnO single crystals have been implanted with Gd ions at 180keV ion energy and two different fluences. Magnetization reversal hysteresis loops have been recorded for as-implanted as well as annealed samples using a superconducting quantum interference device. It was found that for a fluence of 5×1015Gd+∕cm2, postimplantation annealing leads to an increase of the saturation moment up to 1.8μB∕Gd at 300K. Structural investigations revealed no secondary phase formation.