Analysis of temperature-dependent and time-resolved ellipsometry spectra of Ge. - In: IEEE Xplore digital library. - New York, NY : IEEE, ISSN 2473-2001, (2021), , insges. 2 S.
Determination of piezo-resistive coefficient π44 in p-type silicon by comparing simulation and measurement of pressure sensors. - In: AIP Advances. - New York, NY : American Inst. of Physics, ISSN 2158-3226, Bd. 11 (2021), 8, S. 085005-1-085005-6
Chemical changes of float glass surfaces induced by different sand particles and mineralogical phases. - In: Journal of non-crystalline solids : a journal on the chemical, electronic, optical and mechanical properties of glasses, amorphous semiconductors and metals, sol-gel materials, the liquid state of these solids and the processes by which they are formed.. - Amsterdam [u.a.] : Elsevier Science, ISSN 0022-3093, Bd. 566 (2021), 120868
Particles play an important role in the storage, transportation and natural weathering of glasses, but their influence on glass degradation is little studied. In this work, the influence of main sand components is investigated. Feldspar exhibits the strongest leaching rate for the network former Na, while quartz has the lowest. The leaching rate of natural sands is in between. Based on these findings, a model describing the leaching mechanism was developed: Hereby, hydroxyl groups adhering on sand grains adsorb network modifiers by substituting their hydrogen by network formers from the glass surface. The amount of available hydroxyl groups determines the leaching rate. This model is supported by loss on ignition performed for the sands, which might be a suitable method to roughly estimate their leaching rates. The adsorption of network modifiers suppresses carbonate formation, dendritic growth and Mg diffusion in the glass surface region. Pimple-like crystal growth is observed.
Broadband femtosecond spectroscopic ellipsometry. - In: Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques.. - [S.l.] : American Institute of Physics, ISSN 1089-7623, Bd. 92 (2021), 3, S. 033104-1-033104-14
We present a setup for time-resolved spectroscopic ellipsometry in a pump-probe scheme using femtosecond laser pulses. As a probe, the system deploys supercontinuum white light pulses that are delayed with respect to single-wavelength pump pulses. A polarizer-sample-compensator-analyzer configuration allows ellipsometric measurements by scanning the compensator azimuthal angle. The transient ellipsometric parameters are obtained from a series of reflectance-difference spectra that are measured for various pump-probe delays and polarization (compensator) settings. The setup is capable of performing time-resolved spectroscopic ellipsometry from the near-infrared through the visible to the near-ultraviolet spectral range at 1.3 eV-3.6 eV. The temporal resolution is on the order of 100 fs within a delay range of more than 5 ns. We analyze and discuss critical aspects such as fluctuations of the probe pulses and imperfections of the polarization optics and present strategies deployed for circumventing related issues.
Enhanced cycling performance of binder free silicon-based anode by application of electrochemically formed microporous substrate. - In: Electrochimica acta : the journal of the International Society of Electrochemistry (ISE).. - New York, NY [u.a.] : Elsevier, Bd. 380 (2021), 138216, S. 1-9
In this work, an electrochemically formed porous Cu current collector (p-Cu) is utilized for the development of a high-performance binder-free silicon anode. Two electrolyte compositions based on sulfolane (SL) and [BMP][TFSI] ionic liquid (IL) are implemented for silicon deposition. The electrochemical experiments confirm the advantages of applying the p-Cu structure in terms of specific capacity, rate capability, and long-term cycling, where the best electrochemical properties have been observed for the Si deposited from SL electrolyte. The Si-based p-Cu anodes formed in SL display stable 2500 mAh g^-1 reversible capacity during the first 250 cycles and promising capacity retention. Compared to this result, the cycling performance of the same type of material deposited on flat Cu foil (f-Cu) showed significantly reduced capacity (1400 mAh g^-1) and inferior cycling performance. The positive effect can be attributed to the improved mechanical stability of the active material and accelerated ionic transport in the porous structure of the anode. The improved functional properties of the electrochemically deposited Si from SL electrolyte in p-Cu samples compared to those obtained in IL can be ascribed to differences in the chemical composition. While the layers deposited in SL electrolyte involve Si domains incorporated in a matrix containing C and O that offer high mechanical stability, the Si material obtained in IL is additionally influenced by N and F chemical species, resulting from active IL decomposition. These differences in the chemical surrounding of the Si domains are the primary reason for the inferior electrochemical performance of the material deposited from [BMP][TFSI] electrolyte. XPS analysis shows that the initial composition of the as deposited layers, containing a considerable amount of elemental Si, is changed after lithiation and that the electrochemical activity of the anode is governed by switching between the intermediate redox states of Si, where the carbon-oxygen matrix is also involved.
Transient birefringence and dichroism in ZnO studied with fs-time-resolved spectroscopic ellipsometry. - In: Physical review research. - College Park, MD : APS, ISSN 2643-1564, Bd. 3 (2021), 1, S. 013246-1-013246-12
The full transient dielectric-function (DF) tensor of ZnO after UV-laser excitation in the spectral range 1.4-3.6 eV is obtained by measuring an m-plane-oriented ZnO thin film with femtosecond (fs)-time-resolved spectroscopic ellipsometry. From the merits of the method, we can distinguish between changes in the real and the imaginary part of the DF as well as changes in birefringence and dichroism, respectively. We find pump-induced switching from positive to negative birefringence in almost the entire measured spectral range for about 1 ps. Simultaneously, weak dichroism in the spectral range below 3.0 eV hints at contributions of inter-valence-band transitions. Line-shape analysis of the DF above the band gap based on discrete exciton, exciton-continuum, and exciton-phonon-complex contributions shows a maximal dynamic increase in the transient exciton energy by 80 meV. The absorption coefficient below the band gap reveals an exponential line shape attributed to Urbach-rule absorption mediated by exciton-longitudinal-optic-phonon interaction. The transient DF is supported by first-principles calculations for 1020cm^-3 excited electron-hole pairs in ideal bulk ZnO.
Electrochemical nucleation of silicon in ionic liquid-based electrolytes. - In: Meeting abstracts. - Pennington, NJ : Soc., ISSN 2151-2043, Bd. MA2020-01 (2020), 19
Impact of defects on magnetic properties of spinel zinc ferrite thin films. - In: Physica status solidi. - Weinheim : Wiley-VCH, ISSN 1521-3951, Volume 257 (2020), issue 7, 1900630, Seite 1-11
The recent developments in the study of magnetic properties in the spinel zinc ferrite system are explored. Engineering of ionic valence and site distribution allows tailoring of magnetic interactions. Recent literature is reviewed, and own investigations are presented for a conclusive understanding of the mechanisms responsible for the magnetic behavior in this material system. By varying the Zn-to-Fe ratio, the deposition, as well as thermal annealing conditions, ZnFe2O4 thin films with a wide range of crystalline quality are produced. In particular, the focus is on the magnetic structure in relation to spectroscopic properties of disordered ZnFe2O4 thin films. Comparing the cation distribution in film bulk (optical transitions in the dielectric function) and near-surface region (X-ray absorption), it is found that an inhomogeneous cation distribution leads to a weaker magnetic response in films of inverse configuration, whereas defects in the normal spinel are likely to be found at the film surface. The results show that it is possible to engineer the defect distribution in the magnetic spinel ferrite film structure and tailor their magnetic properties on demand. It is demonstrated that these properties can be read out optically, which allows controlled growth of the material and applications in future magneto-optical devices.
Atomic surface structure of MOVPE-prepared GaP(111)B. - In: Applied surface science : a journal devoted to applied physics and chemistry of surfaces and interfaces.. - Amsterdam : Elsevier, Bd. 534 (2020), 147346
Controlling the surface formation of the group-V face of (111)-oriented III-V semiconductors is crucial for subsequent successful growth of III-V nanowires for electronic and optoelectronic applications. With a view to preparing GaP/Si(111) virtual substrates, we investigate the atomic structure of the MOVPE (metalorganic vapor phase epitaxy)-prepared GaP(111)B surface (phosphorus face). We find that upon high-temperature annealing in the H2-based MOVPE process ambience, the surface is phosphorus-depleted, as evidenced by X-ray photoemission spectroscopy (XPS). However, a combination of density functional theory calculations and scanning tunneling microscopy (STM) suggests the formation of a partially H-terminated phosphorus surface, where the STM contrast is due to electrons tunneling from non-terminated dangling bonds of the phosphorus face. Atomic force microscopy (AFM) reveals that a high proportion of the surface is covered by islands, which are confirmed as Ga-rich by Auger electron spectroscopy (AES). We conclude that the STM images of the samples after high-temperature annealing only reflect the flat regions of the partially H-terminated phosphorus face, whereas an increasing coverage with Ga-rich islands, as detected by AFM and AES, forms upon annealing and underlies the higher proportion of Ga in the XPS measurements.
Control of magnetic properties in spinel ZnFe2O4 thin films through intrinsic defect manipulation. - In: Journal of applied physics : AIP's archival journal for significant new results in applied physics.. - Melville, NY : American Inst. of Physics, ISSN 1089-7550, Bd. 128 (2020), 16, S. 165702-1-165702-7
- Im Titel sind "2" und "4" tiefgestellt
We present a systematic study of the magnetic properties of ZnFe2O4 thin films fabricated by pulsed laser deposition at low and high oxygen partial pressure and annealed in oxygen and argon atmosphere, respectively. The as-grown films show strong magnetization, closely related to a non-equilibrium distribution of defects, namely, Fe cations among tetrahedral and octahedral lattice sites. While the concentration of tetrahedral Fe cations declines after argon treatment at 250 ˚C, the magnetic response is enhanced by the formation of oxygen vacancies, evident by the increase in near-infrared absorption due to the Fe2+-Fe3+ exchange. After annealing at temperatures above 300 ˚C, the weakened magnetic response is related to a decline in disorder with a partial recrystallization toward a less defective spinel configuration.