arXiv subjects

Zbigniew Galazka

Publications and source records attributed to Zbigniew Galazka.

27 records · Page 2Linked to original sources

arXiv2018

Current direction anisotropy of the spin Hall magnetoresistance in nickel ferrite thin films with bulk-like magnetic properties

We utilize spin Hall magnetoresistance (SMR) measurements to experimentally investigate the pure spin current transport and magnetic properties of nickel ferrite (NiFe2O4,NFO)/normal metal (NM) thin film heterostructures. We use (001)-oriented NFO thin films grown on lattice-matched magnesium gallate substrates by pulsed laser deposition, which significantly improves the magnetic and structural properties of the ferrimagnetic insulator. The NM in our experiments is either Pt or Ta. A comparison of the obtained SMR magnitude for charge currents applied in the [100]- and [110]-direction of NFO yields a change of 50% for Pt at room temperature. We also investigated the temperature dependence of this current direction anisotropy and find that it is qualitatively different for the conductivity and the SMR magnitude. From our results we conclude that the observed current direction anisotropy may originate from an anisotropy of the spin mixing conductance or of the spin Hall effect in these Pt and Ta layers, and/or additional spin-galvanic contributions from the NFO/NM interface.

cond-mat.mes-hall↗

arXiv2018

Transport Properties and Finite Size Effects in $β$-Ga$_2$O$_3$ Thin Films

Thin films of the wide band gap semiconductor $β$-Ga$_2$O$_3$ have a high potential for applications in transparent electronics and high power devices. However, the role of interfaces remains to be explored. Here, we report on fundamental limits of transport properties in thin films. The conductivities, Hall densities and mobilities in thin homoepitaxially MOVPE grown \mbox{(100)-orientated $β$-Ga$_2$O$_3$} films were measured as a function of temperature and film thickness. At room temperature, the electron mobilities ((115$\pm$10) $\mathrm{\frac{cm^2}{Vs}}$) in thicker films (> 150 nm) are comparable to the best of bulk. However, the mobility is strongly reduced by more than two orders of magnitude with decreasing film thickness ((5.5$\pm$0.5) $\mathrm{\frac{cm^2}{Vs}}$ for a 28 nm thin film). We find that the commonly applied classical Fuchs-Sondheimer model does not explain the contribution of electron scattering at the film surfaces sufficiently. Instead, by applying an electron wave model by Bergmann, a contribution to the mobility suppression due to the large de Broglie wavelength in $β$-Ga$_2$O$_3$ is proposed as a limiting quantum mechanical size effect.

cond-mat.mtrl-sci↗

arXiv2016

Atomic signatures of local environment from core-level spectroscopy in $β$-Ga$_2$O$_3$

We present a joint theoretical and experimental study on core-level excitations from the oxygen $K$ edge of $β$-Ga$_2$O$_3$. A detailed analysis of the electronic structure reveals the importance of O-Ga hybridization effects in the conduction region. The spectrum from O 1$s$ core electrons is dominated by excitonic effects, which overall redshift the absorption onset by 0.5 eV, and significantly redistribute the intensity to lower energies. Analysis of the spectra obtained within many-body perturbation theory reveals atomic fingerprints of the inequivalent O atoms. From the comparison of energy-loss near-edge fine-structure (ELNES) spectra computed with respect to different crystal planes, with measurements recorded under the corresponding diffraction conditions, we show how the spectral contributions of specific O atoms can be enhanced while quenching others. These results suggest ELNES, combined with ab initio many-body theory, as a very powerful technique to characterize complex systems, with sensitivity to individual atomic species and to their local environment.

cond-mat.mtrl-sci↗

arXiv2016

Temperature-dependent thermal conductivity and diffusivity of a Mg-doped insulating $β$-$\mathrm{Ga_2O_3}$ single crystal along [100], [010] and [001]

The monoclinic crystal structure of $β$-$\mathrm{Ga_2O_3}$ leads to significant anisotropy of the thermal properties. The 2$ω$-method is used to measure the thermal diffusivity $D$ in [010] and [001] direction respectively and to determine the thermal conductivity values $λ$ of the [100], [010] and [001] direction from the same insulating Mg doped $β$-$\mathrm{Ga_2O_3}$ single crystal. We detect a temperature independent anisotropy factor of both the thermal diffusivity and conductivity values of $D_{[010]}/D_{[001]}=λ_{[010]}/λ_{[001]}=1.4\pm 0.1$. The temperature-dependence is in accord with phonon-phonon-Umklapp scattering processes from 300 K down to 150 K. Below 150 K point-defect-scattering lowers the estimated phonon-phonon-Umklapp-scattering values.

cond-mat.mtrl-sci↗

arXiv2015

On the influence of inversion on thermal properties of magnesium gallium spinel

Measurements of thermal diffusivity \k{appa} (using laser flash analysis) for temperatures from 100 °C to 1200 °C and of the inversion parameter x (using X-ray analysis) for the single crystal spinel MgGa2O4 have been performed. X-ray analysis showed a small change in x for samples annealed at 800 °C (x = 0.838) and 1200 °C (x = 0.834). Thermal diffusivity revealed a significant rise of \k{appa} for temperatures higher than 950 °C, deviating from the typical 1/T drop of \k{appa}. An additional systematic rise of \k{appa} with time is observed for temperatures between 950 °C and 1100 °C. A connection between cation disordering processes typical for spinels at elevated temperatures and the observed deviation of \k{appa} is proposed: Data indicate that cations changing their occupancy site at equilibration processes contribute significantly to \k{appa} and therefore represent a new type of heat transport.

cond-mat.mtrl-sci↗

arXiv2014

Temperature-dependent thermal conductivity in Mg-doped and undoped $β$-$\mathrm{Ga_2O_3}$ bulk-crystals

For $β$-$\mathrm{Ga_2O_3}$ only little information exist concerning the thermal properties, especially the thermal conductivity $λ$. Here, the thermal conductivity is measured by applying the electrical 3$ω$-method on Czochralski-grown $β$-$\mathrm{Ga_2O_3}$ bulk crystals, which have a thickness of $200~\mathrm{μm}$ and $800~\mathrm{μm}$. At room temperature the thermal conductivity along the [100]-direction in Mg-doped electrical insulating and undoped semiconducting $β$-$\mathrm{Ga_2O_3}$ is confirmed as $13\pm 1~\mathrm{Wm^{-1}K^{-1}}$ for both crystals. The thermal conductivity increases for decreasing temperature down to $25~\mathrm{K}$ to $λ(25~\mathrm{K})=(5.3\pm 0.6)\cdot 10^2~\mathrm{Wm^{-1}K^{-1}}$. The phonon contribution of $λ$ dominates over the electron contribution below room temperature. The observed function $λ(T)$ is in accord with phonon-phonon-Umklapp scattering and the Debye-model for the specific heat at $T\gtrsim 90~\mathrm{K}$ which is about $0.1$ fold of the Debye-temperature $θ_\mathrm{D}$. Here a detailed discussion of the phonon-phonon-Umklapp scattering for $T< θ_\mathrm{D}$ is carried out. The influence of point defect scattering is considered for $T<100~\mathrm{K}$.

cond-mat.mtrl-sci↗

arXiv2013

Top-seeded solution growth of SrTiO3 crystals and phase diagram studies in the SrO-TiO2 system

The TiO2 rich part of the (1-x) SrO + x TiO2 phase diagram 0.5 <= x <= 1.0 was redetermined and the eutectic point between SrTiO3 and TiO2 was found at x_eut=0.7700+/-0.0001; T_eut=(1449+/-3) deg C. From TiO2 rich melt solutions, x=0.75 centimeter-sized single crystals could be grown. The best crystals with etch pit density <2 x 10^4/cm^2 were obtained for growth directions <110> and <100>. AFM investigation of the interface reveals layer-by-layer growth.

cond-mat.mtrl-sci↗

arXiv2013

High-Voltage Field Effect Transistors with Wide-Bandgap β-Ga2O3 Nanomembranes

Nanoscale semiconductor materials have been extensively investigated as the channel materials of transistors for energy-efficient low-power logic switches to enable scaling to smaller dimensions. On the opposite end of transistor applications is power electronics for which transistors capable of switching very high voltages are necessary. Miniaturization of energy-efficient power switches can enable the integration with various electronic systems and lead to substantial boosts in energy efficiency. Nanotechnology is yet to have an impact in this arena. In this work, it is demonstrated that nanomembranes of the wide-bandgap semiconductor gallium oxide can be used as channels of transistors capable of switching high voltages, and at the same time can be integrated on any platform. The findings mark a step towards using lessons learnt in nanomaterials and nanotechnology to address a challenge that yet remains untouched by the field.

cond-mat.mes-hall↗

arXiv2011

Growth Techniques for Bulk ZnO and Related Compounds

ZnO bulk crystals can be grown by several methods. 1) From the gas phase, usually by chemical vapor transport. Such CVT crystals may have high chemical purity, as the growth is performed without contact to foreign material. The crystallographic quality is often very high (free growth). 2) From melt fluxes such as alkaline hydroxides or other oxides (MoO3, V2O5, P2O5, PbO) and salts (PbCl2, PbF2). Melt fluxes offer the possibility to grow bulk ZnO under mild conditions (<1000 deg. C, atmospheric pressure), but the crystals always contain traces of solvent. The limited purity is a severe drawback, especially for electronic applications. 3) From hydrothermal fluxes, usually alkaline (KOH, LiOH) aqueous solutions beyond the critical point. Due to the amphoteric character of ZnO, the supercritical bases can dissolve it up to several per cent of mass. The technical requirements for this growth technology are generally hard, but this did not hinder its development as the basic technique for the growth of α-quartz, and meanwhile also of zinc oxide, during the last decades. 4) From pure melts, which is the preferred technology for numerous substances applied whenever possible, e.g. for the growth of silicon, gallium arsenide, sapphire, YAG. The benefits of melt growth are (i) the high growth rate and (ii) the absence of solvent related impurities. In the case of ZnO, however, it is difficult to find container materials that are compatible from the thermal (fusion point Tf = 1975 deg. C) and chemical (required oxygen partial pressure) point of view. Either cold crucible (skull melting) or Bridgman (with reactive atmosphere) techniques were shown to overcome the problems that are inherent to melt growth. Reactive atmospheres allow to grow not only bulk ZnO single crystals, but also other TCOs such as β-Ga2O3 and In2O3.

cond-mat.mtrl-sci↗