arXiv subjects

Manfred Ramsteiner

Publications and source records attributed to Manfred Ramsteiner.

24 records · Page 2Linked to original sources

arXiv2019

Signatures of free carriers in Raman spectra of cubic In$_2$O$_3$

We discuss the influence of free carriers on the Raman scattering in $n$-type In$_2$O$_3$. For high-quality cubic single crystals, electronic single-particle excitations are revealed as a relatively broad Raman feature in the frequency range below 300~cm$^{-1}$. Furthermore, discrete phonon lines in the same frequency range exhibit asymmetric lineshapes characteristic for Fano resonances. The two observed spectral features contain the potential to be utilized for the quantitative determination of the free carrier concentration in $n$-type In$_2$O$_3$ using Raman spectroscopy as a contactless experimental technique.

cond-mat.mtrl-sci↗

arXiv2017

Modelling the electronic properties of GaAs polytype nanostructures: impact of strain on the conduction band character

We study the electronic properties of GaAs nanowires composed of both the zincblende and wurtzite modifications using a ten-band k.p model. In the wurtzite phase, two energetically close conduction bands are of importance for the confinement and the energy levels of the electron ground state. These bands form two intersecting potential landscapes for electrons in zincblende/wurtzite nanostructures. The energy difference between the two bands depends sensitively on strain, such that even small strains can reverse the energy ordering of the two bands. This reversal may already be induced by the non-negligible lattice mismatch between the two crystal phases in polytype GaAs nanostructures, a fact that was ignored in previous studies of these structures. We present a systematic study of the influence of intrinsic and extrinsic strain on the electron ground state for both purely zincblende and wurtzite nanowires as well as for polytype superlattices. The coexistence of the two conduction bands and their opposite strain dependence results in complex electronic and optical properties of GaAs polytype nanostructures. In particular, both the energy and the polarization of the lowest intersubband transition depends on the relative fraction of the two crystal phases in the nanowire.

cond-mat.mes-hall↗

arXiv2014

Controlled pore formation on mesoporous single crystalline silicon nanowires: threshold and mechanisms

Silicon nanowires are prepared by the method of the two-step metal-assisted wet chemical etching. We have analyzed the structure of solid, rough and porous nanowire surfaces of boron-doped silicon substrates with resistivities of ρ> 1000 Ωcm, ρ= 14-23 Ωcm, ρ< 0.01 Ωcm by scanning electron microscopy and nitrogen gas adsorption. Silicon nanowires prepared from highly-doped silicon reveal mesopores on their surface. However, we found a limit for pore formation. Pores were only formed by etching below a critical H$_2$O$_2$ concentration (c$_{\mbox{H}_2\mbox{O}_2}$ < 0.3 M). Furthermore, we have determined the pore size distribution in dependence on the etching parameters and characterized the morphology of the pores on the nanowire surface. The pores are in the regime of small mesopores with a mean diameter of 9-13 nm. Crystal and surface structure of individual mesoporous nanowires have been investigated by transmission electron microscopy. The vibrational properties of nanowire ensembles have been investigated by Raman spectroscopy. Heavily boron-doped silicon nanowires are highly porous and the remaining single crystalline silicon nanoscale mesh leads to a redshift and a strong asymmetric line broadening for Raman scattering by optical phonons at 520 cm$^{-1}$. This redshift, λ$_{\mbox{Si bulk}}$ = 520 cm$^{-1}$ $\rightarrow$ λ$_{\mbox{Si nanowire}}$ = 512 cm$^{-1}$, hints to a phonon confinement in mesoporous single crystalline silicon nanowire.

cond-mat.mes-hall↗

arXiv2013

All-electrical spin injection and detection in the Co2FeSi/GaAs hybrid system in the local and non-local configuration

We demonstrate the electrical injection and detection of spin-polarized electrons in the Co2FeSi/GaAs hybrid system using lateral transport structures. Spin valve signatures and characteristic Hanle curves are observed both in the nonlocal and the local configuration. The comparatively large magnitude of the local spin valve signal and the high signal-to-noise ratio are attributed to the large spin polarization at the Fermi energy of Co2FeSi in the well-ordered L21 phase.

cond-mat.mes-hall↗

arXiv2012

Correlation between In content and emission wavelength of InGaN/GaN nanowire heterostructures

GaN nanowire ensembles with axial InGaN multi-quantum wells (MQWs) were grown by molecular beam epitaxy. In a series of samples, we varied the In content in the MQWs from almost zero to about 20%. Within the nanowire ensemble, the MQWs fluctuate strongly in composition and size. Statistical information about the composition was obtained from x-ray diffraction and Raman spectroscopy. Photoluminescence at room temperature was obtained in the range from 2.2 eV to 2.5 eV depending on In content. Contrary to planar MQWs, the intensity increases with increasing In content. We compare the observed emission energies with transition energies obtained from a one-dimensional model, and conclude that several mechanisms for carrier localization affect the luminescence of these three-dimensional structures.

cond-mat.mes-hall↗

arXiv2012

Luminescence of GaAs nanowires consisting of wurtzite and zincblende segments

GaAs nanowires (NWs) grown by molecular-beam epitaxy may contain segments of both the zincblende (ZB) and wurtzite (WZ) phases. Depending on the growth conditions, we find that optical emission of such NWs occurs either predominantly above or below the band gap energy of ZB GaAs [E(g,ZB)]. This result is consistent with the assumption that the band gap energy of wurtzite GaAs [E(g,WZ)] is larger than E(g,ZB) and that GaAs NWs with alternating ZB and WZ segments along the wire axis establish a type II band alignment, where electrons captured within the ZB segments recombine with holes of the neighboring WZ segments. Thus, the corresponding transition energy depends on the degree of confinement of the electrons, and transition energies exceeding E(g,ZB) are possible for very thin ZB segments. At low temperatures, the incorporation of carbon acceptors plays a major role in determining the spectral profile as these can effectively bind holes in the ZB segments. From cathodoluminescence measurements of single GaAs NWs performed at room temperature, we deduce a lower bound of 55 meV for the difference E(g,WZ)-E(g,ZB).

cond-mat.mtrl-sci↗