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Register a new userLocal structure of Fe impurity atoms in ZnO: bulk versus surface
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By studying Fe-doped ZnO pellets and thin films with various x-ray spectroscopic techniques, and complementing this with density functional theory calculations, we find that Fe-doping in bulk ZnO induces isovalent (and isostructural) cation substitution (Fe2+ -> Zn2+). In contrast to this, Fe-doping near the surface produces both isovalent and heterovalent substitution (Fe3+ -> Zn2+). The calculations performed herein suggest that the most likely defect structure is the single or double substitution of Zn with Fe, although, if additional oxygen is available, then Fe substitution with interstitial oxygen is even more energetically favourable. Furthermore, it is found that ferromagnetic states are energetically unfavourable, and ferromagnetic ordering is likely to be realized only through the formation of a secondary phase (i.e. ZnFe2O4), or codoping with Cu.
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Surface acoustic wave (SAW) devices based on thin films of ZnO are a well established technology. However, SAW devices on bulk ZnO crystals are not practical at room temperature due to the significant damping caused by finite electrical conductivity of the crystal. Here, by operating at low temperatures, we demonstrate effective SAW devices on the (0001) surface of bulk ZnO crystals, including a delay line operating at SAW wavelengths of {lambda} = 4 and 6 {mu}m and a one-port resonator at a wavelength of {lambda} = 1.6 {mu}m. We find that the SAW velocity is temperature dependent, reaching $v simeq 2.68$ km/s at 10mK. Our resonator reaches a maximum quality factor of $Q_i simeq 1.5times 10^5$, demonstrating that bulk ZnO is highly viable for low temperature SAW applications. The performance of the devices is strongly correlated with the bulk conductivity, which quenches SAW transmission above about 200 K.
In this paper, the reported experimental data in [Sci. Rep., 2012, 2, 533] related to electrical transport properties in bulk ZnO, ZnMgO/ZnO, and ZnMgO/ZnO/ZnMgO single and double heterostructures were analyzed quantitatively and the most important scattering parameters for controlling electron concentration and electron mobility were obtained. Treatment of intrinsic mechanisms included polar-optical phonon scattering, piezoelectric scattering and acoustic deformation potential scattering. For extrinsic mechanisms, ionized impurity, dislocation scattering, and strain-induced fields were included. For bulk ZnO, the reported experimental data were corrected for removing the effects of a degenerate layer at the ZnO/sapphire interface via a two layer Hall effect model. Also, donor density, acceptor density and donor activation energy were determined via the charge balance equation. This sample exhibited hopping conduction below 50K and dislocation scattering closely controlled electron mobility closely. The obtained results indicated that the enhancement of electron mobility in double sample, compared with the single one, can be attributed to the reduction of dislocation density, two dimensional impurity density in the potential well due to background impurities, and/or interface charge and strain-induced fields, which can be related to better electron confinement in the channel and enhancement in the sheet carrier concentration of 2DEG in this sample.
Topological insulators (TIs) are predicted to be composed of an insulating bulk state along with conducting channels on the boundary of the material. In Bi2Se3, however, the Fermi level naturally resides in the conduction band due to intrinsic doping by selenium vacancies, leading to metallic bulk states. In such non-ideal TIs it is not well understood how the surface and bulk states behave under environmental disorder. In this letter, based on transport measurements of Bi2Se3 thin films, we show that the bulk states are sensitive to environmental disorder but the surface states remain robust.
Current-voltage dependence is measured in (Ga,Sb)-doped ZnO up to 150 kV/cm electric fields. A channel temperature is controlled by applying relatively short (few ns) voltage pulses to two-terminal samples. The dependence of electron drift velocity on electron density ranging from 1.42$times$10$^{17}$ cm$^{-3}$ to 1.3$times$10$^{20}$ cm$^{-3}$ at a given electric field is deduced after estimation of the sample contact resistance and the Hall electron mobility. Manifestation of the highest electron drift velocity up to $sim$1.5$times$10$^{7}$ cm/s is estimated for electron density of 1.42$times$10$^{17}$ cm$^{-3}$ and is in agreement with Monte Carlo simulation when hot-phonon lifetime is below 1 ps. A local drift velocity maximum is observed at $sim$1.1$times$10$^{19}$ cm$^{-3}$ and is in agreement with ultra-fast hot phonon decay.
We prove that scanning tunneling microscopy (STM) images of sub-surface Mn atoms in GaAs are formed by hybridization of the impurity state with intrinsic surface states. They cannot be interpreted in terms of bulk-impurity wavefunction imaging. High atomic resolution images obtained using a low-temperature apparatus are compared with advanced, parameter-free tight-binding simulations accounting for both the buckled (110) surface and vacuum electronic properties.
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