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We report on time-resolved optical and terahertz ultrafast spectroscopy of charge-carrier dynamics in the room-temperature antiferromagnetic semiconductor $\alpha $-MnTe. By optically pumping the system with 1.55 eV photons at room temperature, we excite charge carriers in the conduction band through the indirect band gap and investigate the dynamical response of nonequilibrium states using optical as well as terahertz transmission probes. Three relaxation processes are revealed by their characteristic relaxation times of the order of 1, 10, and 100~ps, whose exact values are functions of the pump fluence. For high pump fluences nonlinear dependence on the pump fluence is observed both in the optical and terahertz probes.

Rashba materials have appeared as an ideal playground for spin-to-charge conversion in prototype spintronics devices. Among them, α-GeTe(111) is a non-centrosymmetric ferroelectric semiconductor for which a strong spin-orbit interaction gives rise to giant Rashba coupling. Its room temperature ferroelectricity was recently demonstrated as a route towards a new type of highly energy-efficient non-volatile memory device based on switchable polarization. Currently based on the application of an electric field, the writing and reading processes could be outperformed by the use of femtosecond light pulses requiring exploration of the possible control of ferroelectricity on this timescale. Here, we probe the room temperature transient dynamics of the electronic band structure of α-GeTe(111) using time and angle-resolved photoemission spectroscopy. Our experiments reveal an ultrafast modulation of the Rashba coupling mediated on the fs timescale by a surface photovoltage, namely an increase corresponding to a 13% enhancement of the lattice distortion. This opens the route for the control of the ferroelectric polarization in α-GeTe(111) and ferroelectric semiconducting materials in quantum heterostructures.

We uncovered for the first time the spin dynamics involved in the birth and growth of giant spin polarons in a magnetic semiconductor. For this purpose, we developed a new measurement technique, which for the first time provides direct access to the spin dynamics, irrespective of phonons and carriers involved in the process. Moreover, we solved for the first time the Landau-Liftshitz equation in the specific scenario of spin polarons, which fits our data excellently, and demonstrates for the first time that the spin polaron growth slows down dramatically when the sample is cooled in the paramagnetic phase. Finally, temperature dependent Monte Carlo simulations were performed, which are in excellent agreement with the observed slowdown, which demonstrates for the first time that fluctuations in the Weiss field play a decisive role in spin coherence generation induced by light in magnetic materials. These results offer a new tool and new insight for spin dynamics investigations.

Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high-precision metrology, edge channel spintronics, and topological qubits. The stable 2+ state of Mn enables intrinsic magnetic topological insulators. MnBi

Many conductors, including recently studied Dirac materials, show saturation of coherence length on decreasing temperature. This surprising phenomenon is assigned to external noise, residual magnetic impurities, or two-level systems specific to noncrystalline solids. Here, by considering the SnTe-class of compounds as an example, we show theoretically that breaking of mirror symmetry deteriorates Berry's phase quantization, leading to additional dephasing in weak-antilocalization magnetoresistance (WAL-MR). Our experimental studies of WAL-MR corroborate these theoretical expectations in (111) Pb1-xSnxSe thin film with Sn contents x corresponding to both topological crystalline insulator and topologically trivial phases. In particular, we find the shortening of the phase coherence length in samples with intentionally broken mirror symmetry. Our results indicate that the classification of quantum transport phenomena into universality classes should encompass, in addition to time-reversal and spin-rotation invariances, spatial symmetries in specific systems.

Periodic stacking of topologically trivial and non-trivial layers with opposite symmetry of the valence and conduction bands induces topological interface states that, in the strong coupling limit, hybridize both across the topological and normal insulator layers. Using band structure engineering, such superlattices can be effectively realized using the IV-VI lead tin chalcogenides. This leads to emergent minibands with a tunable topology as demonstrated both by theory and experiments. The topological minibands are proven by magneto-optical spectroscopy, revealing Landau level transitions both at the center and edges of the artificial superlattice mini Brillouin zone. Their topological character is identified by the topological phase transitions within the minibands observed as a function of temperature. The critical temperature of this transition as well as the miniband gap and miniband width can be precisely controlled by the layer thicknesses and compositions. This witnesses the generation of a new fully tunable quasi-3D topological state that provides a template for realization of magnetic Weyl semimetals and other strongly interacting topological phases., 21 pages, 8 figures

A large Rashba effect is essential for future applications in spintronics. Particularly attractive is understanding and controlling nonequilibrium properties of ferroelectric Rashba semiconductors. Here, time- and angle-resolved photoemission is utilized to access the ultrafast dynamics of bulk and surface transient Rashba states after femtosecond optical excitation of GeTe. A complex thermalization pathway is observed, wherein three different timescales can be clearly distinguished: intraband thermalization, interband equilibration, and electronic cooling. These dynamics exhibit an unconventional temperature dependence: while the cooling phase speeds up with increasing sample temperature, the opposite happens for interband thermalization. It is demonstrated how, due to the Rashba effect, an interdependence of these timescales on the relative strength of both electron-electron and electron-phonon interactions is responsible for the counterintuitive temperature dependence, with spin-selection constrained interband electron-electron scatterings found both to dominate dynamics away from the Fermi level, and to weaken with increasing temperature. These findings are supported by theoretical calculations within the Boltzmann approach explicitly showing the opposite behavior of all relevant electron-electron and electron-phonon scattering channels with temperature, thus confirming the microscopic mechanism of the experimental findings. The present results are important for future applications of ferroelectric Rashba semiconductors and their excitations in ultrafast spintronics. Nanyang Technological University Published version J.S.-B. acknowledges financial support from the Impuls- und Vernetzungsfonds der Helmholtz-Gemeinschaft under grant No. HRSF-0067. I.W. and M.B. acknowledge financial support from the Nanyang Technological University, NAP-SUG. G.S. acknowledges financial support by the Austrian Science Fund (FWF), Projects No. P30960-N27 and I 4493-N. Open access funding enabled and organized by Projekt DEAL.

Ferroelectric GeTe is unveiled to exhibit an intriguing multiple non-trivial topology of the electronic band structure due to the existence of triple-point and type-II Weyl fermions, which goes well beyond the giant Rashba spin splitting controlled by external fields as previously reported. Using spin- and angle-resolved photoemission spectroscopy combined with ab initio density functional theory, the unique spin texture around the triple point caused by the crossing of one spin degenerate and two spin-split bands along the ferroelectric crystal axis is derived. This consistently reveals spin winding numbers that are coupled with time reversal symmetry and Lorentz invariance, which are found to be equal for both triple-point pairs in the Brillouin zone. The rich manifold of effects opens up promising perspectives for studying non-trivial phenomena and multi-component fermions in condensed matter systems., 5 pages, 5 figures

We studied the photoelectromagnetic (PEM) effect induced by laser terahertz radiation in $\text{Pb}_{1_{-x}}\text{Sn}_{x}\text{Te}$ films in the composition range $x=(0.15-0.45)$ . A transition from the trivial phase to the topological crystalline insulator phase occurs within this composition interval at $x=0.35$ . The films had the crystallographic orientation $ $ , for which the appearance of topologically nontrivial surface states is expected. It was found that in the trivial phase, the amplitude of the PEM effect is determined by the power of the incident radiation, while in the topological phase, the amplitude is proportional to the flux of laser radiation quanta. Possible mechanism leading to the observed effect is discussed.

An interplay of conservation and breaking of local and global symmetries in topological phases of matter leads to the emergence of topological phenomena including quantum anomalous (QAH) Hall effect, topological superconductivity, and non-Abelian quantum statistics. Among the members of the family of topological materials, magnetically doped topological crystalline insulators (TCI) were foreseen to host topologically protected QAH states generating multiple dissipationless edge and surface conduction channels with Chern number C ≥1. The symmetry protected topological phase of the SnTe class of TCI is characterized by a mirror symmetry resulting in topological surface states. Theoretical and experimental studies demonstrated that four Dirac points are located at the time-reversed-invariant-momentum (TRIM) points for the (111) surface of the SnTe compounds. In this work, via low temperature magnetotransport studies, the opening of the gaps at the TRIMs is demonstrated in ferromagnetic 30 nm thick Sn1-xMnxTe (111) thin epitaxial layers grown onto BaF2(111) substrates by molecular beam epitaxy. While a spin mediated magnetoconductance is observed in layers with 𝑥 ≥ 0.03, the appearance of hysteretic magnetoconductance behaviour and square-like anomalous Hall effect indicate the onset of a hole mediated ferromagnetic ordering in epitaxial Sn1-xMnxTe (111) layers for 𝑥 ≥ 0.06. The anomalous Hall angle, 𝜃AH~0.3 estimated for Sn0.92Mn0.08Te is one of the highest recorded for magnetic topological quantum materials. The tuning of the global band topology by magnetic doping opens wide perspectives for topology driven quantum spintronic technology.

By measuring the spin polarization of GeTe films as a function of light polarization we observed that the bulk states are fully spin polarized in the initial state, in strong contrast with observations for other systems with a strong spin orbit interaction and the surface derived states in the same system. In agreement with state of the art theory, our experimental results show that fully spin polarized bulk states are an intrinsic property of the ferroelectric Rashba semiconductor amp; 945; GeTe 111 . The fact that the measured spin polarization vector does not change with light polarization can be explained by the absence of a mixing of states with a different total angular momentum J

Kürzlich wurde entdeckt, wie sich zwei Klassen topologischer Isolatoren ineinander überführen lassen. Die Art und Weise, wie das geschieht, verspricht neue funktionelle Eigenschaften in den Materialien. So lie en sich elektrische oder spintronische Kanäle mit Spannungspulsen ein und ausschalten

Imaging with surface- and bulk-sensitive electron and X-ray diffraction based microscopic techniques enabled identification of the twin domain distribution of Bi2Te3 and Bi2Se3 thin films. Correlations between the surface topography and crystal orientation are established., The twin distribution in topological insulators Bi2Te3 and Bi2Se3 was imaged by electron backscatter diffraction (EBSD) and scanning X-ray diffraction microscopy (SXRM). The crystal orientation at the surface, determined by EBSD, is correlated with the surface topography, which shows triangular pyramidal features with edges oriented in two different orientations rotated in the surface plane by 60°. The bulk crystal orientation is mapped out using SXRM by measuring the diffracted X-ray intensity of an asymmetric Bragg peak using a nano-focused X-ray beam scanned over the sample. By comparing bulk- and surface-sensitive measurements of the same area, buried twin domains not visible on the surface are identified. The lateral twin domain size is found to increase with the film thickness.

Structure inversion asymmetry is an inherent feature of quantum confined heterostructures with non equivalent interfaces. It leads to a spin splitting of the electron states and strongly affects the electronic band structure. The effect is particularly large in topological insulators because the topological surface states are extremely sensitive to the interfaces. Here, the first experimental observation and theoretical explication of this effect are reported for topological crystalline insulator quantum wells made of Pb1 amp; 8722;xSnxSe confined by Pb1 amp; 8722;yEuySe barriers on one side and by vacuum on the other. This provides a well defined structure asymmetry controlled by the surface condition. The electronic structure is mapped out by angle resolved photoemission spectroscopy and tight binding calculations, evidencing that the spin splitting decisively depends on hybridization and, thus, quantum well width. Most importantly, the topological boundary states are not only split in energy but also separated in space unlike conventional Rashba bands that are splitted only in momentum. The splitting can be strongly enhanced to very large values by control of the surface termination due to the charge imbalance at the polar quantum well surface. The findings thus, open up a wide parameter space for tuning of such systems for device applications

A thorough investigation is presented of the magnetic and structural properties of Mn-doped Bi2Se3 topological insulators grown by molecular beam epitaxy on top of insulating BaF2 (111) substrates. The magnetic properties have been studied in the temperature range from 2 K to 300 K in magnetic fields up to 7 T. The systems were further characterized by means of high-resolution X-ray diffraction, electron-microprobe analysis, and X-ray photoemission spectroscopy. Samples with the atomic concentration of Mn up to about 0.06 exhibit an almost perfect crystalline structure while, for higher Mn concentrations, diffuse scattering from defects is observed. Photoemission results suggest a localized non-metallic Mn 3d5 ground state which is weakly or intermediately coupled to the Bi2Se3 environment. The exchange interaction between the Mn moments leads to a ferromagnetic phase at low temperatures with a roughly linear relation between the Curie temperature and the atomic concentration of Mn.

Topological materials promise new functionalities, which are revealed with the help of angle resolved photoemission. Herein, the search for the magnetic bandgap at the Dirac point as a precondition for the quantum anomalous Hall effect is reviewed and its opening for the topological insulator heterostructure MnBi2Te4 Bi2Te3 is demonstrated. Essential preconditions are explained and the reasons why nonmagnetic gaps occur when Se replaces Te. Angle resolved photoelectron spectroscopy ARPES probes the quantum mechanical final state, and this allows investigation of spin manipulation by light using spin resolved ARPES and the dependence of the charge carrier lifetime on the peculiar spin texture of topological states. It is shown that ARPES data do not support SmB6 as the first strongly correlated topological insulator and an alternative, trivial explanation for the results of ARPES and electrical transport experiments is formulated. Epitaxially grown topological crystalline insulators are, due to their dependence on crystal symmetries, more versatile in the control of individual bulk band inversions. It is shown that this leads to topological quantum phase transitions and associated novel functionalities. Finally, the surface and bulk band connectivity of a type II 3D Weyl semimetal is investigated and an outlook is given for the scientific field

Metal‐halide‐perovskites revolutionized the field of thin‐film semiconductor technology, due to their favorable optoelectronic properties and facile solution processing. Further improvements of perovskite thin‐film devices require structural coherence on the atomic scale. Such perfection is achieved by epitaxial growth, a method that is based on the use of high‐end deposition chambers. Here epitaxial growth is enabled via a ≈1000 times cheaper device, a single nozzle inkjet printer. By printing, single‐crystal micro‐ and nanostructure arrays and crystalline coherent thin films are obtained on selected substrates. The hetero‐epitaxial structures of methylammonium PbBr3 grown on lattice matching substrates exhibit similar luminescence as bulk single crystals, but the crystals phase transitions are shifted to lower temperatures, indicating a structural stabilization due to interfacial lattice anchoring by the substrates. Thus, the inkjet‐printing of metal‐halide perovskites provides improved material characteristics in a highly economical way, as a future cheap competitor to the high‐end semiconductor growth technologies.

We demonstrate that light resonant with the band gap forces the antiferromagnetic semiconductor EuSe to enter ferromagnetic alignment in the picosecond timescale. A photon generates an electron-hole pair, whose electron forms a supergiant spin polaron of magnetic moment of nearly 6000 Bohr magnetons. By increasing the light intensity, the whole of the illuminated region can be fully magnetized. The key to the novel large photoinduced magnetization mechanism is the huge enhancement of the magnetic susceptibility when both antiferromagnetic and ferromagnetic interactions are present in the material and are of nearly equal magnitude, as is the case in EuSe.

Molecular beam epitaxy of IV–VI semiconductor multilayers, quantum dots and device applications is described. The properties of the IV–VI compounds differ in several respects from zinc blende III–V or II–VI semiconductors. This has significant consequences on the growth processes of molecular beam epitaxy as well as for the electronic properties of quantum wells and superlattices. Quantum dots can be obtained by the Stranski–Krastanov growth mode as well as by epitaxial phase separation and nanoprecipitation. In the first case, excellent three-dimensional ordering and stacking of quantum dots in superlattices can be achieved due to the high elastic anisotropy, leading to the formation of three-dimensional quantum dot crystals. In the second case, strong mid-infrared emission of quantum dots at room temperature has been obtained that is tunable over a wide spectral region. The main application of IV–VI or lead salt compounds is in mid-infrared optoelectronic devices, and examples for diode lasers, vertical cavity surface-emitting laser (VCSELs) and microdisc lasers are described.

The evolution of local ferroelectric lattice distortions in multiferroic Ge1−xMnxTe is studied by x-ray diffraction, x-ray absorption spectroscopy and density functional theory. We show that the anion/cation displacements smoothly decrease with increasing Mn content, thereby reducing the ferroelectric transition from 700 to 100 K at x = 0.5, where the ferromagnetic Curie temperature reaches its maximum. First principles calculations explain this quenching by different local bond contributions of the Mn 3d shell compared to the Ge 4s shell in excellent quantitative agreement with the experiments.

We present a simple semiclassical model to sustain that in europium chalcogenides (EuX), Faraday rotation (FR) in the transparency gap is proportional to the magnetization of the sample, irrespective of the material’s magnetic phase, temperature, or applied magnetic field. The model is validated by FR and magnetization measurements in EuSe in the temperature interval 1.7–300 K, covering all EuSe magnetic phases (paramagnetic, antiferromagnetic type I or type II, ferrimagnetic, and ferromagnetic). Furthermore, by combining the semiclassical model with the explicit electronic energy structure of EuX, the proportionality coefficient between magnetization and FR is shown to be dependent only on the wavelength and the bandgap. Due to its simplicity, the model has didactic value; moreover, it provides a working tool for converting FR into magnetization in EuX. The possible extension of the model to other intrinsic magnetic semiconductors is discussed.We present a simple semiclassical model to sustain that in europium chalcogenides (EuX), Faraday rotation (FR) in the transparency gap is proportional to the magnetization of the sample, irrespective of the material’s magnetic phase, temperature, or applied magnetic field. The model is validated by FR and magnetization measurements in EuSe in the temperature interval 1.7–300 K, covering all EuSe magnetic phases (paramagnetic, antiferromagnetic type I or type II, ferrimagnetic, and ferromagnetic). Furthermore, by combining the semiclassical model with the explicit electronic energy structure of EuX, the proportionality coefficient between magnetization and FR is shown to be dependent only on the wavelength and the bandgap. Due to its simplicity, the model has didactic value; moreover, it provides a working tool for converting FR into magnetization in EuX. The possible extension of the model to other intrinsic magnetic semiconductors is discussed.

We employ infrared transmission spectroscopy to explore the temperature-dependent absorption edge and electron-phonon (e-ph) interaction in topological insulator Bi2Se3 and band insulator (Bi0.89In0.11)2Se3 films. Upon heating from 5 K to 300 K, the absorption edge shifts from 262 to 249 meV for Bi2Se3 and from 367 to 343 meV for (Bi0.89In0.11)2Se3. By analyzing the temperature dependence of the Urbach tail, the significant role of Raman-active phonon mode E g 2 in e-ph interaction is identified, which agrees well with the ab initio calculation.

Identifikace bulkových a povrchových Rashba stavů v ferroelektrickém GeTe Macroscopic ferroelectric order in α-GeTe with its noncentrosymmetric lattice structure leads to a giant Rashba spin splitting in the bulk bands due to strong spin-orbit interaction. Direct measurements of the bulk band structure using soft x-ray angle-resolved photoemission (ARPES) reveals the three-dimensional electronic structure with spindle torus shape. By combining high-resolution and spin-resolved ARPES as well as photoemission calculations, the bulk electronic structure is disentangled from the two-dimensional surface electronic structure by means of surface capping, which quenches the complex surface electronic structure. This unravels the bulk Rashba-split states in the ferroelectric Rashba α-GeTe(111) semiconductor exhibiting a giant spin splitting with Rashba parameter αR around 4.2 eV A°, the highest of so-far known materials.

The interplay between electronic eigenstates, spin, and orbital degrees of freedom, combined with fundamental breaking of symmetries is currently one of the most exciting fields of research. Multiferroics such as (GeMn)Te fulfill these requirements providing unusual physical properties due to the coexistence and coupling between ferromagnetic and ferroelectric order in one and the same system. Here we show that multiferroic (GeMn)Te inherits from its parent ferroelectric {\alpha}-GeTe compound a giant Rashba splitting of three-dimensional bulk states which competes with the Zeeman spin splitting induced by the magnetic exchange interactions. The collinear alignment of ferroelectric and ferromagnetic polarization leads to an opening of a tunable Zeeman gap of up to 100 meV around the Dirac point of the Rashba bands, coupled with a change in spin texture by entanglement of magnetic and spin-orbit order. Through applications of magnetic fields, we demonstrate manipulation of spin- texture by spin resolved photoemission experiments, which is also expected for electric fields based on the multiferroic coupling. The control of spin helicity of the bands and its locking to ferromagnetic and ferroelectric order opens fascinating new avenues for highly multifunctional multiferroic Rashba devices suited for reprogrammable logic and/or nonvolatile memory applications., Comment: 10 pages, 4 figures

The evolution of local ferroelectric lattice distortions in multiferroic Ge1-x Mn x Te is studied by x-ray diffraction, x-ray absorption spectroscopy and density functional theory. We show that the anion/cation displacements smoothly decrease with increasing Mn content, thereby reducing the ferroelectric transition from 700 to 100 K at x = 0.5, where the ferromagnetic Curie temperature reaches its maximum. First principles calculations explain this quenching by different local bond contributions of the Mn 3d shell compared to the Ge 4s shell in excellent quantitative agreement with the experiments.

The topological properties of lead-tin chalcogenide topological crystalline insulators can be widely tuned by temperature and composition. It is shown that bulk Bi doping of epitaxial Pb

Kriegner, Dominik et al., Commercial magnetic memories rely on the bistability of ordered spins in ferromagnetic materials. Recently, experimental bistable memories have been realized using fully compensated antiferromagnetic metals. Here we demonstrate a multiple-stable memory device in epitaxial MnTe, an antiferromagnetic counterpart of common II–VI semiconductors. Favourable micromagnetic characteristics of MnTe allow us to demonstrate a smoothly varying zero-field antiferromagnetic anisotropic magnetoresistance (AMR) with a harmonic angular dependence on the writing magnetic field angle, analogous to ferromagnets. The continuously varying AMR provides means for the electrical read-out of multiple-stable antiferromagnetic memory states, which we set by heat-assisted magneto-recording and by changing the writing field direction. The multiple stability in our memory is ascribed to different distributions of domains with the Néel vector aligned along one of the three magnetic easy axes. The robustness against strong magnetic field perturbations combined with the multiple stability of the magnetic memory states are unique properties of antiferromagnets., We thank A.B. Shick and M. Veis for fruitful discussions and M. Marysko for technical support. This work is supported from the Austrian Science Fund (FWF): J3523-N27 and SFB-025 IRON, from the European Research Council (ERC) Advanced Grant No. 268066, from the Ministry of Education of the Czech Republic Grant No. LM2015087, and from the Grant Agency of the Czech Republic Grant No. 14-37427G and 15-13436S. J.G. acknowledges the Ramon y Cajal program (RYC-2012-11709). Scanning transmission electron microscopy research (J.G.) was supported through a user project at Oak Ridge National Laboratorys Center for Nanophase Materials Sciences (CNMS), Scientific User Facilities Division, BES-DOE. J.G. acknowledges the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragon—Universidad de Zaragoza for offering access to their instruments and expertise.

The self-assembly process of Ge islands on patterned Si (0 0 1) substrates is investigated using scanning tunneling microscopy. The substrate patterns consist of one-dimensional stripes with “V”-shaped geometry and sidewalls inclined by an angle of 9° to the (0 0 1) surface. Onto these stripes, Ge is deposited in a step-wise manner at different temperatures from 520 °C to 650 °C. At low temperature, the Ge first grows nearly conformally over the patterned surface but at about 3 monolayers a strong surface roughening due to reconstruction of the surface ridges as well as side wall ripple formation occurs. At 600 °C, a similar roughening takes place, but Ge accumulates within the grooves such that at a critical thickness of 4.5 monolayers, 3D islands are formed at the bottom of the grooves. This accumulation process is enhanced at 650 °C growth, so that the island formation starts about 1 monolayers earlier. At 600 and 650 °C, all islands are all aligned at the bottom of the stripes, whereas at 550 °C Ge island form preferentially on top of ridges. The experimental observations are explained by the strong temperature dependence of Ge diffusion over the patterned surface.

Self-assembly of colloidal nanocrystals and other nanosized building blocks has led to numerous large-scale and well-ordered superstructures. To quantify the superlattice quality we present a simple and efficient method, based on analysis of the autocorrelation function to determine characteristic order parameters for short-range and long-range ordering. This provides a feedback for further improvements of deposition techniques and self-assembly processes. To show the power of this method, it is applied to various two-dimensional ordered single component and binary nanocrystal assemblies. A quantitative comparison of the normalized long-range order parameter for various colloidal or epitaxially grown superlattice structures evidences that the long-range ordering in monodisperse colloidal superlattices by far supersedes that obtained at best by epitaxially grown quantum dots. Astonishingly, for selected binary nanocrystal superlattices the long-range ordering parameter reaches almost the same values as for single component superlattices. Besides the high sensitivity of the introduced quantification method to lattice imperfections our analysis also reveals any anisotropy in the ordering of the superlattices, which again can be quantified, for example, to identify the areas of highest quality within one specific sample.

Molecular beam epitaxy of Si and Ge on the stripe-patterned Si (0 0 1) substrates is studied by scanning tunneling microscopy. The stripe patterns consist of one-dimensional etch grooves oriented along different azimuth directions within the Si (0 0 1) surface. During Si buffer growth, the stripe morphology rapidly changes and the side walls of the grooves are transformed into differently inclined low-energy Si facets, depending on the stripe orientation and growth conditions. Subsequent Ge growth drastically changes the surface topography to form energetically favorable {1 0 5} sidewall facets. Depending on the sidewall geometry, Ge nanoislands formed at a coverage of around 5 monolayers nucleate either in the middle of the sidewalls or in the center of the grooves as a result of material accumulation induced by capillary forces and strain relaxation.

Si growth on pit-patterned Si (001) substrates fabricated by electron beam lithography and reactive ion etching was studied by in situ scanning tunnelling microscopy. After reactive ion etching, the pits have cylindrical shape with vertical side walls, which begins to change from the bottom already after annealing at 740 °C. During Si overgrowth, the pit shape evolves into inverted pyramid-like or into multi-facetted inverted dome-like geometries depending on the growth conditions as well as thickness of the deposited Si layer. High-resolution STM images clearly show that the walls of the pits after overgrowth are composed predominantly of DB-type of steps.

In this review, molecular beam epitaxy of IV–VI semiconductor hetero- and nanostructures is described. The properties of the IV–VI compounds differ in several respects from zinc-blende III–V or II–VI semiconductors. This has significant consequences on the growth processes of molecular beam epitaxy. The main application of IV–VI or lead salt compounds is in mid-infrared optoelectronic devices. As example, the growth of lead salt lasers is described with special emphasis on vertical cavity surface emitting lasers. Furthermore, it is shown that due to the large elastic anisotropy of IV–VI compounds, self-organized quantum dot superlattices show remarkable ordering and vertical correlations. As a result, 3D ordered quantum dot crystals can be obtained. Thus, these materials are ideal model systems for the investigation of lateral, vertical and staggered dot ordering in multilayer structures. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This article provides a review of our results on nanostructurization of lead telluride, PbTe. This IV–VI group narrow-gap semiconductor exhibits paraelectric behaviour leading to a huge dielectric constant e > 1000 at helium temperatures. Because the Coulomb potential fluctuations produced by charged defects are strongly suppressed in PbTe nanostructures, one can reach the quantum ballistic regime at significantly relaxed conditions in comparison with other systems. In particular, we observe precise zero-field conductance quantization in the wires made of modulation doped PbTe/PbEuTe quantum wells where the heavily doped layer is separated from the conducting channel only by a 2 nm thick spacer layer. The second important property is the very large Zeeman splitting. It reaches 4 meV/T. Accordingly, significant spin splitting of the conductance plateaux is observed already at fields below 1 T. Therefore, the system is attractive for the construction of local spin filters. We show that the presence of metal layers does not impair the quantum ballistic properties. Furthermore, we have developed a new method of tuning the PbTe nanostructures, using laterally placed metallic electrodes. We have found that this method is more effective than previous schemes using used p–n junctions and it provides better stability of the nanostructures.

Lattice-type mismatched heteroepitaxy is demonstrated as a novel concept for the fabrication of almost ideal, highly luminescent nanocrystal quantum dots that are coherently embedded in a single-crystalline matrix. In this approach, the formation of quantum dots is induced by transformation of a metastable epitaxial 2D quantum well into an array of isolated nanocrystals with-highly symmetric shape. This process is driven by the lattice-type mismatch between the constituent materials and the resulting miscibility gap. The investigated PbTe/CdTe heterosystem has a model character because it combines two compounds with different cubic lattice types but almost identical lattice constants. The obtained epitaxial nanocrystals exhibit outstanding properties such as a well-defined symmetric shape, the absence of strain, intermixing and a wetting layer, which is in contrast to the conventional Stranski–Krastanow quantum dots. The small-rhomboedric-cubo-octahedron PbTe/CdTe nanocrystals on GaAs substrates display intense room temperature mid-infrared luminescence as is crucial for device applications. Ab initio density functional theory is used to clarify the interface structure, indicating that the covalent and ionic bonding character of CdTe and PbTe is maintained across the interface.

Anomalous X-ray diffraction was used for the investigation of shape and chemical composition of self-organized PbSe quantum dots covered by PbEuTe capping layers. From reciprocal-space maps of diffracted intensities measured at two energies of the primary radiation, we discriminated the contributions of the dot volumes and the surrounding crystal lattice to the diffracted intensity. We have found that the presence of Eu atoms suppresses the flattening of the dots during their overgrowth.

Conductance quantization was measured in submicron mesa constrictions, patterned into PbTe/Pb 0.92 Eu 0.08 Te quantum wells of thickness 12, 25 and 50 nm. The main observation is almost unitary transmission of the one-dimensional quantum modes, despite of significant amount of charged defects in the vicinity of the constrictions. Such an extraordinary result is a consequence of the paraelectric properties of PbTe, namely, the suppression of long-range tails of Coulomb potentials due to the huge dielectric constant.

Deformation potentials ( D u and D d ) for PbSe where analyzed using transmission data of PbSe/PbEuSeTe multi-quantum wells (MQWs). We use calculations based on a k · p model to obtain the strain induced intervalley splitting in the quantum wells. For the reduction of the Fabri–Perot interference fringes of the multilayer structures we design a PbEuSeTe / BaF 2 anti-reflex coating which is deposited on top of the MQWs. At low temperature we found PbSe deformation potentials D u = - 2.36 and 5.88. The results of the transmission measurements are compared with photo-current spectra measured with self-assembled PbSe/PbEuTe quantum dot superlattices.

Anomalous X-ray scattering from self-assembled PbSe quantum dots embedded in Pb 1− x Eu x Te was used for the study of their structure. The measured reciprocal-space distributions of diffracted intensity were compared with simulations based on kinematical scattering theory and continuum elasticity. From the comparison, the mean chemical composition of the dots and their aspect ratio (height/width) were estimated.

Molecular beam epitaxial growth of continuous-wave (cw) midinfrared IV–VI vertical-cavity surface-emitting laser structures is presented. From the structures, based on high-finesse microcavities and containing PbSe as active medium, optically pumped cw laser emission is observed at very long wavelengths between 7.9 and 6.7 μ m . Stimulated emission is found up to temperatures of 100 K . We achieved internal threshold pump intensities below 25 W / cm 2 , which is two orders of magnitude smaller than those reported so far. The observed line width of the laser emission is only 0.6 nm , with a strong line width narrowing above threshold as compared to the line width of the cavity resonance. Cw output powers are up to 4.8 mW .

Three-dimensional stacking of semiconductor nano-islands in multilayers or superlattice structures provides a powerful tool for controlling the properties of self-assembled quantum dots. These stackings can be caused by several different mechanisms based on: (i) elastic interactions due to the strain fields of the buried dots; (ii) morphological interactions due to nonplanarized spacer topographies; or (iii) interactions based on chemical composition modulations within the spacer material. All three interactions may give rise to a vertical dot alignment in columns as well as to oblique or staggered dot stackings, depending on the details of the interaction mechanisms. For the interlayer correlations mediated by the elastic strain fields, the elastic anisotropy and surface orientation, but also the dot sizes and spacer layer thicknesses play a crucial role. As a result, transitions between different types of dot stackings can be induced as a function of spacer layer thicknesses and growth parameters. The large range of parameters involved in interlayer correlation formation may allow the controlled synthesis of new types of ordered structures with novel properties. To cite this article: G. Springholz, C. R. Physique 6 (2005).

Continuous-wave (CW) as well as pulsed-laser emission from a midinfrared (/spl lambda/=7.92 /spl mu/m) IV-VI vertical-cavity surface-emitting laser at 1.8 K is presented. The high-finesse microcavity, containing PbSe as an active medium, was optically pumped with a carbon monoxide laser at a wavelength of 5.28 /spl mu/m (1894 cm/sup -1/) in either CW or Q-switched mode. The maximum achieved CW power was 4.8 mW and pulsed peak powers were up to 23 W. Linewidths are considerably narrower than 0.10 cm/sup -1/, corresponding to 0.6 nm.

Self-organized vertical and lateral ordering in PbSe/PbEuTe quantum dot superlattices grown by molecular beam epitaxy is investigated by synchrotron X-ray diffraction and atomic force microscopy. For small PbEuTe spacer thicknesses, the dots are ordered hexagonally in the lateral direction and vertically aligned along the growth direction. This yields an overall 3D hexagonal lattice of dots. For larger spacer thicknesses an FCC-like dot stacking is formed, resulting in the formation of a 3D trigonal dot lattice. From the analysis of the X-ray diffraction spectra the ordering is significantly better for the trigonally ordered dot samples.