Matthias T. Elm

Tailoring the magnetoresistance of MnAs/GaAs:Mn granular hybrid nanostructures

The magnetoresistance properties of GaAs:Mn∕MnAs granular hybrid structures consisting of ferromagnetic MnAs clusters within a paramagnetic GaAs:Mn host differ considerably from those of paramagnetic and ferromagnetic (Ga,Mn)As alloys. We analyze the magnetoresistance effects on the basis of a resistor network model. Typical experimental findings are reproduced and their dependence on cluster density and random spatial arrangement of the clusters are revealed. Controlled spatial positioning of the MnAs clusters within the GaAs:Mn host offers interesting opportunities for optimizing the magnetoresistance properties for applications and for overcoming problems of miniaturization arising from cluster statistics.

Effects of artificially structured micrometer holes on the transport behavior of Al-doped ZnO layers

We study the transport properties of artificially structured n-type ZnO:Al thin films prepared by rf magnetron sputtering on glass substrates. The samples were patterned with an array of 4×4 and 8×8 μm2 holes. With decreasing hole size, the resistance of the samples increases. Filling the holes with Au or Al increases and decreases the resistance, respectively. All samples show a negative magnetoresistance, which becomes more pronounced with decreasing hole diameter. The filling of the holes with Au or Al reduces the effects of the artificial structuring on the magnetoresistance.

Nanoscale FeS2 (Pyrite) as a Sustainable Thermoelectric Material

We have synthesized undoped, Co-doped (up to 5%), and Se-doped (up to 4%) FeS2 materials by mechanical alloying in a planetary ball mill and investigated their thermoelectric properties from room temperature (RT) to 600 K. With decreasing particle size, the undoped FeS2 samples showed higher electrical conductivity, from 0.02 S cm−1 for particles with 70 nm grain size up to 3.1 S cm−1 for the sample with grain size of 16 nm. The Seebeck coefficient of the undoped samples showed a decrease with further grinding, from 128 μV K−1 at RT for the sample with 70-nm grains down to 101 μV K−1 for the sample with grain size of 16 nm. The thermal conductivity of the 16-nm undoped sample lay within the range from 1.3 W m−1 K−1 at RT to a minimal value of 1.2 W m−1 K−1 at 600 K. All doped samples showed improved thermoelectric behavior at 600 K compared with the undoped sample with 16 nm particle size. Cobalt doping modified the p-type semiconducting behavior to n-type and increased the thermal conductivity (2.1 W m−1 K−1) but improved the electrical conductivity (41 S cm−1) and Seebeck coefficient (-129 μV K−1). Isovalent selenium doping led to a slightly higher thermal conductivity (1.7 W m−1 K−1) as well as to an improved electrical conductivity (26 S cm−1) and Seebeck coefficient (110 μV K−1). The ZT value of FeS2 was increased by a factor of five by Co doping and by a factor of three by Se doping.

Comparison of the magnetic properties of GaInAs/MnAs and GaAs/MnAs hybrids with random and ordered arrangements of MnAs nanoclusters

Random arrangements of ferromagnetic MnAs nanoclusters were deposited on (111)B-GaInAs surfaces by standard metal-organic vapor-phase epitaxy (MOVPE). Ordered arrangements of MnAs nanoclusters and cluster chains were obtained by selective-area MOVPE on prepatterned (111)B-GaAs substrates. This new method enables one to control the arrangement of nanoclusters in the growth process offering interesting opportunities to tune the properties of individual MnAs clusters as well as the interaction between the carriers in the surrounding semiconductor matrix and the clusters. The magnetic anisotropy of the MnAs clusters was investigated by magnetic force microscopy and ferromagnetic resonance measurements. The in-plane magnetic anisotropy is mainly determined by the interplay of cluster shape and magnetocrystalline anisotropy while the hard magnetic axis of the clusters is perpendicular to the sample plane independent of cluster shape. The magnetotransport measurements demonstrate that the cluster arrangements st...

Ionic Conductivity of Mesostructured Yttria-Stabilized Zirconia Thin Films with Cubic Pore Symmetry—On the Influence of Water on the Surface Oxygen Ion Transport.

Thermally stable, ordered mesoporous thin films of 8 mol % yttria-stabilized zirconia (YSZ) were prepared by solution-phase coassembly of chloride salt precursors with an amphiphilic diblock copolymer using an evaporation-induced self-assembly process. The resulting material is of high quality and exhibits a well-defined three-dimensional network of pores averaging 24 nm in diameter after annealing at 600 °C for several hours. The wall structure is polycrystalline, with grains in the size range of 7 to 10 nm. Using impedance spectroscopy, the total electrical conductivity was measured between 200 and 500 °C under ambient atmosphere as well as in dry atmosphere for oxygen partial pressures ranging from 1 to 10(-4) bar. Similar to bulk YSZ, a constant ionic conductivity is observed over the whole oxygen partial pressure range investigated. In dry atmosphere, the sol-gel derived films have a much higher conductivity, with different activation energies for low and high temperatures. Overall, the results indicate a strong influence of the surface on the transport properties in cubic fluorite-type YSZ with high surface-to-volume ratio. A qualitative defect model which includes surface effects (annihilation of oxygen vacancies as a result of water adsorption) is proposed to explain the behavior and sensitivity of the conductivity to variations in the surrounding atmosphere.

Effect of nanostructuring on the band structure and the galvanomagnetic properties in Bi1−xSbx alloys

Magnetotransport measurements were performed on a series of nanostructured Bi1−xSbx alloy samples with an Sb content in the range between 0% and 60%. The samples were prepared by cold pressing and annealing of crystalline Bi1−xSbx nanoparticles, which were synthesized by mechanical alloying. The incorporation of Sb changes the band structure of these nanotextured alloys as well as their transport behavior. With increasing Sb content the band gap increases and reaches a maximum band gap of 42 meV at an Sb concentration of about 14% determined from temperature dependent resistivity measurements. For even higher Sb content, the band gap decreases again. The bands and thus the band gaps are shifted with respect to bulk material due to quantum confinement effects in the nanostructures. The change of the band structure with varying Sb content strongly affects the magnetoresistance behavior as well as the magnetic field dependence of the Hall-coefficient. Using a three band model in order to describe both proper...

Nonferromagnetic nanocrystalline ZnO:Co thin films doped with Zn interstitials

ZnCoO thin films were synthesized via a wet chemical route and subsequently annealed in Zn vapor to increase the conductivity by introducing Zn interstitials. All samples show small hysteresis loops close to the detection limit of the magnetometer. Thus the samples were thoroughly investigated to obtain evidence for further ferromagneticlike behavior. Optical and magneto-optical experiments show the crystal field transitions of Co2+ in the near infrared and visible spectral range. At energies above 2.8 eV a charge transfer transition of Co2+ is observed. The results of magnetotransport measurements are explained by the formation of an impurity band situated below the conduction band. No further evidence for ferromagnetism is obtained.

Quantitative modeling of the annealing-induced changes of the magnetotransport in Ga1−xMnxAs alloys

We study the changes of magnetoresistance induced by controlled thermal annealing at temperatures ranging from 300to600°C of a Ga0.98Mn0.02As alloy grown by low-temperature molecular beam epitaxy. We use a resistor-network model for describing the electrical transport as a function of temperature and external magnetic field. The model is founded on classical semiconductor band transport and neglects many-body interactions. The peculiarities of dilute magnetic semiconductors, in particular, the magnetic-field induced changes of the density of states and the potential fluctuations due to the giant Zeeman splitting in the paramagnetic phase as well as spontaneous magnetization effects in the ferromagnetic phase, are accounted for in a mean-field fashion. This empirical transport model based on reasonable assumptions and realistic material parameters yields a satisfactory quantitative description of the experimentally obtained temperature and magnetic-field dependence of the resistivity of the entire series o...

Selective-area growth and magnetic characterization of MnAs/AlGaAs nanoclusters on insulating Al2O3 layers crystallized on Si(111) substrates

We report on selective-area metal-organic vapor phase epitaxy and magnetic characterization of coupled MnAs/AlGaAs nanoclusters formed on thin Al2O3 insulating layers crystallized on Si(111) substrates. Cross-sectional transmission electron microscopy reveals that poly-crystalline γ-Al2O3 grains are formed after an annealing treatment of the amorphous Al2O3 layers deposited by atomic layer deposition on Si(111) substrates. The ⟨111⟩ direction of the γ-Al2O3 grains tends to be oriented approximately parallel to the ⟨111⟩ direction of the Si substrate. We observe that hexagonal MnAs nanoclusters on AlGaAs buffer layers grown by selective-area metal-organic vapor phase epitaxy on partially SiO2-masked Al2O3 insulator crystallized on Si(111) substrates are oriented with the c-axis along the ⟨111⟩ direction of the substrates, but exhibit a random in-plane orientation. A likely reason is the random orientation of the poly-crystalline γ-Al2O3 grains in the Al2O3 layer plane. Magnetic force microscopy studies at ...

Doping-Induced Universal Conductance Fluctuations in GaN Nanowires

The transport properties of Ge-doped single GaN nanowires are investigated, which exhibit a weak localization effect as well as universal conductance fluctuations at low temperatures. By analyzing these quantum interference effects, the electron phase coherence length was determined. Its temperature dependence indicates that in the case of highly doped nanowires electron-electron scattering is the dominant dephasing mechanism, while for the slightly doped nanowires dephasing originates from Nyquist-scattering. The change of the dominant scattering mechanism is attributed to a modification of the carrier confinement caused by the Ge-doping. The results demonstrate that the phase coherence length can be tuned by the donor concentration making Ge-doped GaN nanowires an ideal model system for studying the influence of impurities on quantum-interference effects in mesoscopic and nanoscale systems.