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J. Sadowski

Publications and source records attributed to J. Sadowski.

At least 19 recordsLinked to original sources
arXiv2020

Hard X-ray Photoelectron Momentum Microscopy and Kikuchi Diffraction on (In,Ga,Mn)As Thin Films

Recent advances in the brilliance of hard-X-ray beamlines and photoelectron momentum microscopy facilitate bulk valence-band mapping and core-level-resolved hard-X-ray photoelectron diffraction (hXPD) for structural analysis in the same setup. High-quality MBE-grown (In,Ga,Mn)As films represent an ideal testing ground, because of the non-centrosymmetric GaAs crystal structure itself and In and Mn doping concentrations of few percent. Here we present results of k-mapping and hXPD for the title compound with 3% In and 2.5 or 5.6% Mn using hard X-ray photons (3 to 5 keV) at beamline P22 at PETRA III (DESY, Hamburg). Numerical processing (difference or ratio images) emphasizes subtle differences of hXPD patterns like the fingerprint-like hXPD-signatures of As and Ga sites. XPD calculations using the Bloch-wave method show a one-to-one correspondence with the measurements. The hXPD results reveal a predominant Ga substitutional site for Mn. Valence band mapping shows that the Fermi energy lies within the valence band and decreases as the Mn concentration increases. The results support the p-d Zener model of ferromagnetism in the title compound. In addition to the shift of the Fermi energy, the band splitting increases with increasing Mn content, which we attribute to an increase of many-body correlations with increasing metallicity of the sample.

physics.ins-det↗
arXiv2018

Defect-free SnTe topological crystalline insulator nanowires grown by molecular beam epitaxy on graphene

SnTe topological crystalline insulator nanowires have been grown by molecular beam epitaxy on graphene/SiC substrates. The nanowires have cubic rock-salt structure, they grow along [001] crystallographic direction and have four sidewalls consisting of {100} crystal planes known to host metallic surface states with Dirac dispersion. Thorough high resolution transmission electron microscopy investigations show that the nanowires grow on graphene in the van der Walls epitaxy mode induced when the catalyzing Au nanoparticle mixes with Sn delivered from SnTe flux, providing liquid Au-Sn alloy. The nanowires are totally free from structural defects, but their {001} sidewalls are prone to oxidation, which points out on necessity of depositing protective capping in view of exploiting the magneto-electric transport phenomena involving charge carriers occupying topologically protected states.

cond-mat.mtrl-sci↗
arXiv2018

Bi monocrystal formation on InAs(111)A and B substrates

The growth of Bi films deposited on both A and B faces of InAs(111) has been investigated by low-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy using synchrotron radiation. The changes upon Bi deposition of the In 4d and Bi 5d5/2 photoelectron signals allow to get a comprehensive picture of the Bi/InAs(1 1 1) interface. From the initial stages the Bi growth on the A face (In-terminated InAs) is epitaxial, contrary to that on the B face (As- terminated InAs) that proceeds via the formation of islands. Angle-resolved photoelectron spectra show that the electronic structure of a $\approx 10$~BL deposit on the A face is identical to that of bulk Bi, while more than $\approx 30$ BL are needed for the B face. Both bulk and surface states are well accounted for by fully relativistic ab initio spin-resolved photoemission calculations.

cond-mat.mtrl-sci↗
arXiv2018

Cubic anisotropy in high homogeneity thin (Ga,Mn)As layers

Historically, comprehensive studies of dilute ferromagnetic semiconductors, e.g., $p$-type (Cd,Mn)Te and (Ga,Mn)As, paved the way for a quantitative theoretical description of effects associated with spin-orbit interactions in solids, such as crystalline magnetic anisotropy. In particular, the theory was successful in explaining {\em uniaxial} magnetic anisotropies associated with biaxial strain and non-random formation of magnetic dimers in epitaxial (Ga,Mn)As layers. However, the situation appears much less settled in the case of the {\em cubic} term: the theory predicts switchings of the easy axis between in-plane $\langle 100\rangle$ and $\langle 110\rangle$ directions as a function of the hole concentration, whereas only the $\langle 100\rangle$ orientation has been found experimentally. Here, we report on the observation of such switchings by magnetization and ferromagnetic resonance studies on a series of high-crystalline quality (Ga,Mn)As films. We describe our findings by the mean-field $p$-$d$ Zener model augmented with three new ingredients. The first one is a scattering broadening of the hole density of states, which reduces significantly the amplitude of the alternating carrier-induced contribution. This opens the way for the two other ingredients, namely the so-far disregarded single-ion magnetic anisotropy and disorder-driven non-uniformities of the carrier density, both favoring the $\langle 100\rangle$ direction of the apparent easy axis. However, according to our results, when the disorder gets reduced a switching to the $\langle 110\rangle$ orientation is possible in a certain temperature and hole concentration range.

cond-mat.mtrl-sci↗
arXiv2017

Wurtzite (Ga,Mn)As nanowire shells with ferromagnetic properties

(Ga,Mn)As in wurtzite crystal structure, is coherently grown by molecular beam epitaxy on the {1100} side facets of wurtizte (Ga,In)As nanowires and further encapsulated by (Ga,Al)As and low temperature GaAs. For the first time a true long-range ferromagnetic magnetic order is observed in non-planar (Ga,Mn)As, which is attributed to a more effective hole confinement in the shell containing Mn by a proper selection/choice of both the core and outer shell materials.

cond-mat.mtrl-sci↗
arXiv2017

On the origin of magnetism in (Ga,Mn)As: from paramagnetic through superparamagnetic to ferromagnetic phase

The high-spectral-resolution spectroscopic studies of the energy gap evolution, supplemented with electronic, magnetic and structural characterization, show that the modification of the GaAs valence band caused by Mn incorporation occurs already for a very low Mn content, much lower than that required to support ferromagnetic spin - spin coupling in (Ga,Mn)As. Only for n-type (Ga,Mn)As with the Mn content below about 0.3% the Mn-related extended states are visible as a feature detached from the valence-band edge and partly occupied with electrons. The combined magnetic and low-temperature photoreflectance studies presented here indicate that the paramagnetic - ferromagnetic transformation in p-type (Ga,Mn)As takes place without imposing changes of the unitary character of the valence band with the Fermi level located therein. The whole process is rooted in the nanoscale fluctuations of the local (hole) density of states and the formation of a superparamagnetic-like state. The Fermi level in (Ga,Mn)As is coarsened by the carrier concentration of the itinerant valence band holes and further fine-tuned by the many-body interactions.

cond-mat.mtrl-sci↗
arXiv2016

Electronic structure of (Ga,Mn)As revisited

The detailed nature of electronic states mediating ferromagnetic coupling in dilute magnetic semiconductors, specifically (Ga,Mn)As, has been an issue of long debate. Two confronting models have been discussed emphasizing host band vs. impurity band carriers. Using angle resolved photoemission we are for the first time able to identify a highly dispersive Mn-induced energy band in (Ga,Mn)As. Our results show that the electronic structure of the (Ga,Mn)As system is significantly modified from that of GaAs throughout the valence band. Close to the Fermi energy, the presence of Mn induces a strong mixing of the bulk bands of GaAs, which results in the appearance of a highly dispersive band in the gap region of GaAs. For Mn concentrations above 1% the band reaches the Fermi level, and can thus host the delocalized holes needed for ferromagnetic coupling. Overall, our data provide a firm evidence of delocalized carriers belonging to the modified host valence band.

cond-mat.mtrl-sci↗
arXiv2016

Novel Quaternary Dilute Magnetic Semiconductor (Ga,Mn)(Bi,As): Magnetic and Magneto-Transport Investigations

Magnetic and magneto-transport properties of thin layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor grown by the low-temperature molecular-beam epitaxy technique on GaAs substrates have been investigated. Ferromagnetic Curie temperature and magneto-crystalline anisotropy of the layers have been examined by using magneto-optical Kerr effect magnetometry and low-temperature magneto-transport measurements. Postgrowth annealing treatment has been shown to enhance the hole concentration and Curie temperature in the layers. Significant increase in the magnitude of magnetotransport effects caused by incorporation of a small amount of Bi into the (Ga,Mn)As layers revealed in the planar Hall effect (PHE) measurements, is interpreted as a result of enhanced spin-orbit coupling in the (Ga,Mn)(Bi,As) layers. Two-state behaviour of the planar Hall resistance at zero magnetic field provides its usefulness for applications in nonvolatile memory devices.

cond-mat.mtrl-sci↗
arXiv2015

Effect of Misfit Strain in (Ga,Mn)(Bi,As) Epitaxial Layers on their Magnetic and Magneto-Transport Properties

Effect of misfit strain in the layers of (Ga,Mn)(Bi,As) quaternary diluted magnetic semiconductor, epitaxially grown on either GaAs substrate or (In,Ga)As buffer, on their magnetic and magneto-transport properties has been investigated. High-resolution X-ray diffraction, applied to characterize the structural quality and misfit strain in the layers, proved that the layers were fully strained to the GaAs substrate or (In,Ga)As buffer under compressive or tensile strain, respectively. Ferromagnetic Curie temperature and magnetocrystalline anisotropy of the layers have been examined by using magneto-optical Kerr effect magnetometry and low-temperature magneto-transport measurements. Post-growth annealing treatment of the layers has been shown to enhance the hole concentration and Curie temperature in the layers.

cond-mat.mtrl-sci↗
arXiv2014

Electronic structure of (Ga,Mn)As revisited: an alternative view on the "Battle of the bands"

New detailed angle-resolved photoemission data are presented, revealing the existence of an Mn-induced state that extends into the band gap of GaAs. In sharp contrast to recent reports we observe that the state is highly dispersive. Spin resolved photoemission shows that the band is spin polarized even at room temperature. The results are not consistent with any of the currently discussed band models for ferromagnetism.

cond-mat.mtrl-sci↗
arXiv2014

Electron correlations in Mn$_x$Ga$_{1-x}$As as seen by resonant electron spectroscopy and dynamical mean field theory

After two decades from the discovery of ferromagnetism in Mn-doped GaAs, its origin is still debated, and many doubts are related to the electronic structure. Here we report an experimental and theoretical study of the valence electron spectrum of Mn-doped GaAs. The experimental data are obtained through the differences between off- and on-resonance photo-emission data. The theoretical spectrum is calculated by means of a combination of density-functional theory in the local density approximation and dynamical mean-field theory (LDA+DMFT), using exact diagonalisation as impurity solver. Theory is found to accurately reproduce measured data, and illustrates the importance of correlation effects. Our results demonstrate that the Mn states extend over a broad range of energy, including the top of the valence band, and that no impurity band splits off from the valence band edge, while the induced holes seem located primarily around the Mn impurity.

cond-mat.str-el↗
arXiv2014

Magnetic and magneto-transport characterization of (Ga,Mn)(Bi,As) epitaxial layers

High-quality layers of the (Ga,Mn)(Bi,As) quaternary compound semiconductor have been grown by the low-temperature molecular-beam epitaxy technique. An effect of Bi incorporation into the (Ga,Mn)As ferromagnetic semiconductor and the post-growth annealing treatment of the layers have been investigated through examination of their magnetic and magneto-transport properties. Significant enhancement of the planar Hall effect magnitude upon addition of Bi into the layers is interpreted as a result of increased spin-orbit coupling in the (Ga,Mn)(Bi,As) layers.

cond-mat.mtrl-sci↗
arXiv2013

Analysis of the $E_{1}$ and $E_{1}$ +$Δ_{1}$ optical transitions in (Ga,Mn)As epitaxial layers

The diluted (Ga,Mn)As became a model ferromagnetic semiconductor, however there is still a disagreement on the source of its magnetism. The divergences arise from the results indicating that the holes mediated ferromagnetism reside in the valence band or the impurity band. Full understanding of character of the Mn states in GaAs can bring the increase of (Ga,Mn)As Curie temperature. In this paper we verify the ellipsometric results and compare with more precise photoreflectance method which gives a new insight into the interactions of Mn impurity states with GaAs valence band. Indeed, $E_{1}$ and $E_{1}$+$Δ_{1}$ inter-band transition energies for highly doped and annealed (Ga,Mn)As epitaxial layers have not confirmed the interaction between detached Mn impurity band and the valence band. Thus, the description with merged Mn states and GaAs valence band is in agreement with our results. Our findings are supported by the high resolution transmission microscopy and magnetization measurements.

cond-mat.mtrl-sci↗
arXiv2013

Effects of non-uniform Mn distribution in (Ga,Mn)As

Resonant in situ photoemission from Mn 3d states in Ga_{1-x}Mn_{x}As is reported for Mn concentrations down to very dilute limit of 0.1 at %. The properties of the peak at the valence-band maximum reveal an effective interaction between Mn 3d states for concentration as low as 2.5 %. Concentration-dependent spectral features are analyzed on the basis of first-principles calculations for systems with selected impurity positions as well as for random alloys.

cond-mat.str-el↗
arXiv2013

Electronic- and band-structure evolution in low-doped (Ga,Mn)As

Modulation photoreflectance spectroscopy and Raman spectroscopy have been applied to study the electronic- and band-structure evolution in (Ga,Mn)As epitaxial layers with increasing Mn doping in the range of low Mn content, up to 1.2%. Structural and magnetic properties of the layers were characterized with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. The revealed results of decrease in the band-gap transition energy with increasing Mn content in very low-doped (Ga,Mn)As layers with n-type conductivity are interpreted as a result of merging the Mn-related impurity band with the host GaAs valence band. On the other hand, an increase in the band-gap-transition energy with increasing Mn content in (Ga,Mn)As layers with higher Mn content and p-type conductivity indicates the Moss-Burstein shift of the absorption edge due to the Fermi level location within the valence band, determined by the free-hole concentration. The experimental results are consistent with the valence-band origin of mobile holes mediated ferromagnetic ordering in the (Ga,Mn)As diluted ferromagnetic semiconductor.

cond-mat.mtrl-sci↗
arXiv2013

Effect of low-temperature annealing on the electronic- and band-structure of (Ga,Mn)As epitaxial layers

The effect of outdiffusion of Mn interstitials from (Ga,Mn)As epitaxial layers, caused by post-growth low-temperature annealing, on their electronic- and band-structure properties has been investigated by modulation photoreflectance (PR) spectroscopy. The annealing-induced changes in structural and magnetic properties of the layers were examined with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. They confirmed an outdiffusion of Mn interstitials from the layers and an enhancement in their hole concentration, which were more efficient for the layer covered with a Sb cap acting as a sink for diffusing Mn interstitials. The PR results revealing a decrease in the band-gap-transition energy in the as-grown (Ga,Mn)As layers, with respect to that in the reference GaAs one, are interpreted by assuming a merging of the Mn-related impurity band with the host GaAs valence band. On the other hand, an increase in the band-gap-transition energy in the annealed (Ga,Mn)As layers is interpreted as a result of the Moss-Burstein shift of the absorption edge due to the Fermi level location within the valence band, determined by the enhanced free-hole concentration. The experimental results are consistent with the valence-band origin of mobile holes mediating ferromagnetic ordering in (Ga,Mn)As, in agreement with the Zener model for ferromagnetic semiconductors.

cond-mat.mtrl-sci↗
arXiv2012

Mn induced modifications of Ga 3d photoemission from (Ga, Mn)As: evidence for long range effects

Using synchrotron based photoemission, we have investigated the Mn-induced changes in Ga 3d core level spectra from as-grown ${\rm Ga}_{1-x}{\rm Mn}_{x}{\rm As}$. Although Mn is located in Ga substitutional sites, and does therefore not have any Ga nearest neighbours, the impact of Mn on the Ga core level spectra is pronounced even at Mn concentrations in the range of 0.5%. The analysis shows that each Mn atom affects a volume corresponding to a sphere with around 1.4 nm diameter.

cond-mat.mtrl-sci↗