Leibniz Institute for Crystal Growth
Recent publications
Ferroelectric tunnel junction (FTJ) is a promising emerging memristor for the artificial synapse in neuro-inspired computing, which has parallel data processing and low power consumption. The achievement of high-performance electronic synapses requires in-depth exploration of the correlation between the material properties and the device performances as well as the related physical mechanism, which are, however, still quite lacking. We demonstrate here a robust electronic synapse realized by epitaxial ferroelectric Hf0.5Zr0.5O2 (HZO) films with a high Curie temperature of 930 °C and a pristine highly uniform polarization. Based on the optimized ferroelectric HZO film and in-depth understanding of the FTJ mechanism, a robust and high-performance electronic synapse has been successfully realized with high ON/OFF ratio of >500, large continuous conductance regulation range of 1–250 nS and high reliability with the retention of >10⁴ s. Such electronic synapses show good multilevel conductance modulations and synaptic behaviors, such as long-term potentiation (LTP), long-term depression (LTD) and spike-timing dependent plasticity (STDP). A simulated neural network with the synaptic characteristics indicates high recognition accuracy (93.7%) for MNIST database. These results pave a pathway to apply HZO based electronic synapses as the active block in future neuromorphic computing.
A new mechanism for the excitation of impurity related terahertz radiation in semiconductors under the conditions of exciton condensation into an electron–hole liquid is reported. The interaction of impurity centers with plasmons localized on droplets of an electron–hole liquid produces ionization of the centers. The subsequent capture of nonequilibrium charge carriers by ionized impurities is followed by terahertz intracenter radiative transitions. In these processes, impurity centers play the role of antennas that convert the near electromagnetic field of plasmons on droplets of an electron–hole liquid into detected radiation. The main experiments were carried out on lithium-doped silicon crystals at helium temperatures under conditions of interband photoexcitation. A theoretical model of the excitation of impurity centers by localized plasmons is developed, which explains the main regularities observed in the experiment.
Structural martensitic transformations enable various applications, which range from high stroke actuation and sensing to energy efficient magnetocaloric refrigeration and thermomagnetic energy harvesting. All these emerging applications benefit from a fast transformation, but up to now their speed limit has not been explored. Here, we demonstrate that a thermoelastic martensite to austenite transformation can be completed within ten nanoseconds. We heat epitaxial Ni-Mn-Ga films with a nanosecond laser pulse and use synchrotron diffraction to probe the influence of initial temperature and overheating on transformation rate and ratio. We demonstrate that an increase of thermal energy drives this transformation faster. Though the observed speed limit of 2.5 x 10²⁷ (Js)¹ per unit cell leaves plenty of room for a further acceleration of applications, our analysis reveals that the practical limit will be the energy required for switching. Thus, martensitic transformations obey similar speed limits as in microelectronics, expressed by the Margolus–Levitin theorem.
Here, we investigate the effect of post-metallization anneal temperature on Ti/Au ohmic contact performance for (100)-oriented Ga2O3. A low contact resistance of ∼2.49 × 10−5 Ω·cm2 is achieved at an optimal anneal temperature of ∼420 °C for (100) Ga2O3. This is lower than the widely-used temperature of 470 °C for (010)-oriented Ga2O3. However, drastic degradation of the (100)-oriented contact resistance to ∼1.36 × 10−3 Ω·cm2 is observed when the anneal temperature was increased to 520 °C. Microscopy at the degraded ohmic contact revealed that the reacted Ti–TiOx interfacial layer has greatly expanded to 25–30 nm thickness and GaAu2 inclusions have formed between (310)-Ga2O3 planes and the Ti–TiOx layer. This degraded interface, which corresponds to the deterioration of ohmic contact properties, likely results from excess in-diffusion of Au and out-diffusion of Ga, concurrent with the expansion of the Ti–TiOx layer. These results demonstrate the critical influence of Ga2O3 anisotropy on the optimal post-metallization anneal temperature. Moreover, the observed Ti/Au contact degradation occurs for relatively moderate anneal conditions (520 °C for 1 min in N2), pointing to the urgent necessity of developing alternative metallization schemes for gallium oxide, including the use of Au-free electrodes.
Nickel Disilicide Precipitates Nickel is an important metal impurity in float‐zone silicon and has to be controlled tightly in device production. Its precipitation into NiSi2 particles is well understood except for the initial stages. It is shown that substitutional nickel species, whose concentration are steered by excess vacancies stored in nitrogen‐vacancy complexes, serve as nucleation sites, which is explained by a microscopic model in article number 2200220 by Michael Seibt and co‐workers, published as part of the GADEST 2022 Special Issue. More articles from this GADEST‐19 series can be found here: https://onlinelibrary.wiley.com/toc/18626319/2021/218/23.
The Special Issue on “Artificial Intelligence for Crystal Growth and Characterization” comprises six original articles in this emerging field of research [...]
We outline a method to synthesize ( ATiO 3 ) n AO Ruddlesden–Popper phases with high- n, where the A-site is a mixture of barium and strontium, by molecular-beam epitaxy. The precision and consistency of the method described is demonstrated by the growth of an unprecedented (SrTiO 3 ) 50 SrO epitaxial film. We proceed to investigate barium incorporation into the Ruddlesden–Popper structure, which is limited to a few percent in bulk, and we find that the amount of barium that can be incorporated depends on both the substrate temperature and the strain state of the film. At the optimal growth temperature, we demonstrate that as much as 33% barium can homogeneously populate the A-site when films are grown on SrTiO 3 (001) substrates, whereas up to 60% barium can be accommodated in films grown on TbScO 3 (110) substrates, which we attribute to the difference in strain. This detailed synthetic study of high n, metastable Ruddlesden–Popper phases is pertinent to a variety of fields from quantum materials to tunable dielectrics.
The research interest to deep donors in silicon is due in particular to the quantum structure of such centers; these are promising for application in silicon photonics in the mid‐IR range and quantum technologies. At interstitial lattice positions, magnesium atoms create double‐charge deep donors. The review is an attempt to summarize the accumulated knowledge on properties of magnesium impurity in silicon. Among methods to obtain Si:Mg samples, there has been focus on the impurity diffusion from the solid phase using the so‐called sandwich method. Techniques to investigate samples include, among others, Hall effect measurements, optical absorption, luminescence spectroscopy and DLTS. The diffusivity of magnesium in silicon and stability of parameters of Si:Mg samples under post‐diffusion heat treatment is discussed. The energy spectrum of the helium‐like magnesium donor is considered in detail. Owing to the interaction with other impurities, magnesium forms a variety of donor levels in silicon. These include complexes of Mg with substitutional acceptor atoms B, Al, Ga, In, interstitial lithium, and oxygen, as well as numerous donors with a currently unknown nature. Some defects are similar to so‐called thermal donors in silicon. The pairing of Mg atoms is proved in samples prepared from the isotope 28Si enriched silicon. This article is protected by copyright. All rights reserved.
Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO 2 . Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO 2 have been made by replacing cobalt with iron in PdCoO 2 films grown by MBE. Using ARPES, we observe similar bulk bands in these PdCoO 2 films with Pd-, CoO 2 -, and FeO 2 -termination. Nevertheless, Pd- and CoO 2 -terminated films show a reduced intensity of surface states. Additionally, we are able to epitaxially stabilize PdFe x Co 1− x O 2 films that show an anomaly in the derivative of the electrical resistance with respect to temperature at 20 K, but do not display pronounced magnetic order.
We study the critical thickness for the plastic relaxation of the Si quantum well layer embedded in a SiGe/Si/SiGe heterostructure for qubits by plan-view transmission electron microscopy and electron channeling contrast imaging. Misfit dislocation segments form due to the glide of pre-existing threading dislocations at the interface of the Si quantum well layer beyond a critical thickness given by the Matthews–Blakeslee criterion. Misfit dislocations are mostly [Formula: see text] dislocations (b=a/2 <110>) that are split into Shockely partials (b=a/6 <112>) due to the tensile strain field of the Si quantum well layer. By reducing the quantum well thickness below critical thickness, misfit dislocations can be suppressed. A simple model is applied to simulate the misfit dislocation formation and the blocking process. We discuss consequences of our findings for the layer stack design of SiGe/Si/SiGe heterostructures for usage in quantum computing hardware.
Ga2O3 and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga2O3 offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. γ‐Ga2O3 presents a particular challenge across synthesis, characterisation, and theory due to its inherent disorder and resulting complex structure – electronic structure relationship. Here, density functional theory is used in combination with a machine learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the γ‐phase. Theoretical results are compared with surface and bulk sensitive soft and hard X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of γ‐Ga2O3. The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by PLE spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step towards exploring how their electronic structure can be understood in terms of local coordination and overall structure. This article is protected by copyright. All rights reserved
Here we demonstrate a dramatic improvement in Ti/Au ohmic contact performance by utilizing the anisotropic nature of β-Ga2O3. Under a similar doping concentration, Ti/Au metallization on (100) Ga2O3 shows a specific contact resistivity 5.11 × 10-5 Ω·cm2, while that on (010) Ga2O3 is as high as 3.29 × 10-3 Ω·cm2. Temperature-dependent contact performance and analyses suggest that field emission or thermionic field emission is the dominant charge transport mechanism across the Ti/Au-(100) Ga2O3 junction, depending on whether reactive ion etching was used prior to metallization. Cross-sectional high-resolution microscopy and elemental mapping analysis show that the in situ-formed Ti-TiOx layer on (100) Ga2O3 is relatively thin (2-2.5 nm) and homogeneous, whereas that on (010) substrates is much thicker (3-5 nm) and shows nanoscale facet-like features at the interface. The anisotropic nature of monoclinic Ga2O3, including anisotropic surface energy and mass diffusivity, is likely to be the main cause of the differences observed under microscopy and in electrical properties. The findings here provide direct evidence and insights into the dependence of device performance on the atomic-scale structural anisotropy of β-Ga2O3. Moreover, the investigative strategy here─combining comprehensive electrical and materials characterization of interfaces on different semiconductor orientations─can be applied to assess a variety of other anisotropic oxide junctions.
By differential thermal analysis (DTA) is was shown that the pseudo-binary system Tb2O3–SiO2 contains intermediate compounds Tb2SiO5, Tb4Si3O12 and Tb2Si2O7. All three compounds melt congruently, a phase diagram was proposed, and from the latter compound single crystals could be grown for the first time. This was possible from stoichiometric feed rods by the optical floating-zone (OFZ) technique. X-ray powder analysis proved the Pna21 phase of the crystals. Melting temperature and heat of fusion were measured by DTA to be 1769◦C and ≈150 kJ/mol, respectively. The specific heat capacity of Tb2Si2O7 was measured by dynamic DSC to be cp = 149.4982 + 0.2242 ⋅ T − 7.3913 × 10⁻⁵ ⋅ T² + 558449.3624 ⋅ T⁻².
The integration of both optical and electronic components on a single chip, despite the challenge, holds the promise of compatibility with CMOS technology and high scalability. Among all candidate materials, III-V semiconductor nanostructures are key ingredients for opto-electronics and quantum optics devices, such as light emitters and harvesters. The control over geometry, and dimensionality of the nanostructures, enables one to modify the band structures, and hence provide a powerful tool for tailoring the opto-electronic properties of III-V compounds. One of the most creditable approaches towards such growth control is the combination of using patterned wafer and the self-assembled epitaxy. This work presents monolithically integrated catalyst-free InP nanowires grown selectively on nanotip-patterned (001)Si substrates using gas-source molecular-beam epitaxy. The substrates are fabricated using CMOS nanotechnology. The dimensionality of the InP structures can be switched between two-dimensional nanowires and three-dimensional bulk-like InP islands by thermally modifying the shape of Silicon nanotips, surrounded by the SiO 2 layer during the oxide-off process. The structural and optical characterization of nanowires indicate the coexistence of both zincblende and wurtzite InP crystal phases in nanowires. The two different crystal structures were aligned with a type-II heterointerface.
A Langmuir adsorption model of the Si incorporation mechanism into metalorganic vapor-phase epitaxy grown (100) β-Ga 2 O 3 thin films is proposed in terms of the competitive surface adsorption process between Si and Ga atoms. The outcome of the model can describe the major feature of the doping process and indicate a growth rate-dependent doping behavior, which is validated experimentally and further generalized to different growth conditions and different substrate orientations.
Silicon, a ubiquitous material in modern computing, is an emerging platform for realizing a source of indistinguishable single photons on demand. The integration of recently discovered single-photon emitters in silicon into photonic structures is advantageous to exploit their full potential for integrated photonic quantum technologies. Here, we show the integration of an ensemble of telecom photon emitters in a two-dimensional array of silicon nanopillars. We developed a top-down nanofabrication method, enabling the production of thousands of nanopillars per square millimeter with state-of-the-art photonic-circuit pitch, all the while being free of fabrication-related radiation damage defects. We found a waveguiding effect of the 1278 nm-G center emission along individual pillars accompanied by improved brightness compared to that of bulk silicon. These results unlock clear pathways to monolithically integrating single-photon emitters into a photonic platform at a scale that matches the required pitch of quantum photonic circuits. Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0094715
The recent successes in the isolation and characterization of several bismuth radicals inspire the development of new spectroscopic approaches for the in-depth analysis of their electronic structure. Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for the characterization of main group radicals. However, the large electron-nuclear hyperfine interactions of Bi (209Bi, I = 9/2) have presented difficult challenges to fully interpret the spectral properties for some of these radicals. Parallel-mode EPR (B1∥B0) is almost exclusively employed for the study of S > 1/2 systems but becomes feasible for S = 1/2 systems with large hyperfine couplings, offering a distinct EPR spectroscopic approach. Herein, we demonstrate the application of conventional X-band parallel-mode EPR for S = 1/2, I = 9/2 spin systems: Bi-doped crystalline silicon (Si:Bi) and the molecular Bi radicals [L(X)Ga]2Bi• (X = Cl or I) and [L(Cl)GaBi(MecAAC)]•+ (L = HC[MeCN(2,6-iPr2C6H3)]2). In combination with multifrequency perpendicular-mode EPR (X-, Q-, and W-band frequencies), we were able to fully refine both the anisotropic g- and A-tensors of these molecular radicals. The parallel-mode EPR experiments demonstrated and discussed here have the potential to enable the characterization of other S = 1/2 systems with large hyperfine couplings, which is often challenging by conventional perpendicular-mode EPR techniques. Considerations pertaining to the choice of microwave frequency are discussed for relevant spin-systems.
The present work investigates the use of the refractory metal alloy TiW as a possible candidate for the realization of ohmic contacts to the ultrawide bandgap semiconductor β-Ga2O3. Ohmic contact properties were analyzed by transfer length measurements of TiW contacts annealed at temperatures between 400 and 900 °C. Optimum contact properties with a contact resistance down to 1.5 × 10−5 Ω cm2 were achieved after annealing at 700 °C in nitrogen on highly doped β-Ga2O3. However, a significant contact resistance increase was observed at annealing temperatures above 700 °C. Cross-sectional analyses of the contacts using scanning transmission electron microscopy revealed the formation of a TiOx interfacial layer of 3–5 nm between TiW and β-Ga2O3. This interlayer features an amorphous structure and most probably possesses a high amount of vacancies and/or Ga impurities supporting charge carrier injection. Upon annealing at temperatures of 900 °C, the interlayer increases in thickness up to 15 nm, featuring crystalline-like properties, suggesting the formation of rutile TiO2. Although severe morphological changes at higher annealing temperatures were also verified by atomic force microscopy, the root cause for the contact resistance increase is attributed to the structural changes in thickness and crystallinity of the interfacial layer.
Visible lasers are sought for in a variety of applications. They are required in fields as diverse as medicine, materials processing, display and entertainment technology and many others. Moreover, in contrast to infrared lasers, they enable very simple and efficient access to the UV spectral range by a single frequency doubling step. Currently, the choice of direct visibly emitting lasers is limited: The ‘green gap’ prohibits the development of semiconductor lasers with emission in the green and yellow spectral range and only few laser active ions allow for efficient visible lasing. In particular trivalent praseodymium (Pr³⁺) and terbium (Tb³⁺) ions have been shown to be the most successful candidates for efficient high power visible solid-state lasers. Compared to semiconductor lasers, solid-state lasers also provide other advantages, e.g., in terms of energy storage in Q-switched operation as well as beam quality at high output power. In recent years, visibly emitting solid-state lasers have seen a revival enabled by the increasing commercial availability of GaN-based blue emitting pump diodes and an ever-increasing number of publications evidences the vivid research activities in this field. Still, due to the relatively short history of diode-pumped visible solid-state lasers, these are still in an early stage of their development and up to now only few direct visibly emitting solid-state lasers with comparably low output power are commercially available. However, we are convinced that visibly emitting solid-state lasers based on Pr³⁺ and Tb³⁺ have the potential for 100-W-class continuous wave output power levels as well as sub-ns pulse durations in Q-switched and sub-ps-pulse durations in mode-locked operation, which would qualify them to fulfil the requirements of most of the applications named above. In this work, we review the state of the art of continuous wave and pulsed visibly emitting solid-state lasers and amplifiers based on Pr³⁺ and Tb³⁺ as the active ion. After an introduction, we briefly review the spectroscopic properties of these two ions and their particularities for laser operation as well as the requirements for suitable host materials. In the third chapter, we present the state of the art in the field of continuous wave Pr³⁺-lasers emitting in the cyan-blue, green, orange, red, and deep-red spectral range based on fluoride, glass, and oxide host materials and discuss prospects for further power scaling. The fourth chapter is devoted to the current state of Tb³⁺-based continuous wave green and yellow emitting solid-state lasers. In the fifth and sixth chapter we give an overview over existing pulsed visibly emitting solid-state lasers in Q-switched and mode-locked operation mode, respectively. Finally, the seventh chapter is devoted to pulse amplifiers for ultrafast visible lasers before this review closes with a short conclusion.
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74 members
Frank M. Kiessling
  • Classical Semiconductors
Detlef Klimm
  • Simulation & Characterization
H. Wilke
  • Dielectric Crystals
A. Dittmar
  • AlN crystal growth
Max-Born-Str. 2, 12489, Berlin, Germany