28 February 2022

9:00 - 9:35: Chemical vapor deposition of nanoporous metal-organic materials and metal-oxides

Marianne Kräuter

Solvent-free methods for fabrication of nanoporous materials have been on the rise with the aim of facile processing and accessing new application fields. To establish nanoporous metal-oxides and metal-organic thin films in crucial fields, such as microelectronics or energy conversion, an inexpensive synthesis technique is needed, which excels at scalability and controllability. These requirements are met by chemical vapor deposition methods, which allow for the synthesis of highly conformal layers with precise thickness control and excellent conformality.

This talk will focus on the vapor deposition of the prototypical metal-organic framework ZIF-8, and of nanoporous ZnO thin films.

ZIF-8 is synthesized via a two-step chemical vapor deposition process, termed “MOF-CVD”. First, an ultrathin ZnO seed layer is deposited via plasma-enhanced atomic layer deposition (PE-ALD). ZIF-8 layers are subsequently grown by subjecting the ZnO-layer to a 2-methyl imidazole vapor at elevated temperatures. To gain better control over the novel deposition technique, the impact of the conversion time in combination with different thicknesses (1 to 10 nm) and densities (4.6 g/cm3 and 5.2 g/cm3) of the ZnO precursor onto the resulting ZIF-8 layers was investigated.

To obtain nanoporous ZnO, metal-organic “zincone” thin films are deposited via molecular layer deposition (MLD). Subsequently these layers are calcinated to remove their organic contents, thus introducing cavities in the resulting ZnO thin films. The influence of the calcination temperature as well as the influence of the MLD deposition temperature onto the porosity of the ZnO thin films was explored via porosimetric ellipsometry, a technique which has already shown its usefulness for the determination of porosity in polymer-derived oxides.

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9:35 - 10:10: Strain in Oxide-Confined Vertical Surface Emitting LASERs

Michael Pusterhofer

Compared to edge-emitting LASERs, vertical cavity surface emitting LASERs have many advantages that make them preferable for fiber-based telecommuni- cation [1], and compact sensor systems [2, 3]. The introduction of oxide con- finement was able to further increase this lead by producing emitters with very high power conversion ratios. [4] This improvement, however, also comes with some drawbacks. The oxide used in the process was found to be the source of various defects, which in part can be attributed to the volume change caused by the wet oxidation process. [5, 6] Therefore investigations of the oxide in respect to its deformation were conducted. In this talk, these investigations are pre- sented, which include mechanical simulations, electro-thermal simulations and modeling of the oxidation. In addition, experimental results from Nano-beam precession electron diffraction and thermal resistance measurements are shown to support the simulation.

  1. [1]  N. Ledentsov et al. “Development of VCSELs and VCSEL-based Links for Data Communication beyond 50Gb/s”. In: Optical Fiber Communication Conference (OFC) 2020. OSA Technical Digest. San Diego, California: Op- tical Society of America, Mar. 2020, M2A.3. doi: 10.1364/OFC.2020.M2A. 3.

  2. [2]  Serdal Okur et al. “High-Power VCSEL Arrays with Customized Beam Divergence for 3D-sensing Applications”. In: Proc.SPIE. Vol. 10938. Mar. 2019.

  3. [3]  Jean-Franc ̧ois P. Seurin. “High-Power VCSEL Arrays”. In: VCSELs: Fundamentals, Technology and Applications of Vertical-Cavity Surface-Emitting Lasers. Ed. by Rainer Michalzik. Springer Series in Optical Sciences. Berlin, Heidelberg: Springer, 2013, pp. 263–290. isbn: 978-3-642-24986-0. doi: 10. 1007/978-3-642-24986-0_8.

  4. [4]  K. L. Lear et al. “Selectively Oxidised Vertical Cavity Surface Emitting Lasers with 50% Power Conversion Efficiency”. In: Electronics Letters 31.3 (Feb. 1995), pp. 208–209. issn: 0013-5194. doi: 10.1049/el:19950125.

  5. [5]  Christopher J. Helms et al. “Reliability of Oxide VCSELs at Emcore”. In: Integrated Optoelectronic Devices 2004. Ed. by Chun Lei, Kent D. Cho- quette, and Sean P. Kilcoyne. San Jose, CA, United States, June 2004, p. 183. doi:10.1117/12.539282.

  6. [6]  David T. Mathes et al. “Nanoscale Materials Characterization of Degrada- tion in VCSELs”. In: Integrated Optoelectronics Devices. Ed. by Chun Lei and Sean P. Kilcoyne. San Jose, CA, June 2003, p. 67. doi: 10.1117/12. 482858.

10:30 - 11:05: From Macro to Nano: AFM and Optical Spectroscopy

Elisabeth Schöffmann

At Wood K plus in St. Veit an der Glan the main research focus lies on wood and paper surface technologies. This includes surface characterization, determination of correlations and interactions between technological properties and surface appearance and development of new surface characterization methods. Therefore, the overall goal of this PhD research project is to investigate the correlation of macroscopic, microscopic and nanoscopic properties for coated wood-based panels, paper and composites by combining Atomic Force Microscopy (AFM) with infrared spectroscopy and microscopy. Different coated wood-based panels, paper and composite samples with different numbers of coating layers and coating material curing-grades were examined and their influence on the samples were investigated through experimental validation. The used coated wood- based panels consisted of particle boards or MDF boards and were coated with impregnates as well as with lacquer layers. The investigated papers were raw, as well as impregnated and coated. The research project is separated into three main research topics, where the invention of an appropriate sample preparation strategy for the investigated coated wood-based materials, papers and impregnates was the first important step. Challenging was the possibility to extract representative sample pieces and to get them into appropriate sample sizes for ultra microtomy. Also, how all layers could be determined, was challenging. A major problem was that the substrates with wood were soaked with water and that parts of particles broke out of the particle boards. With paper and impregnate samples also swelling and cracking between embedding media and samples were problems to solve. The second part aims at investigating which characteristic chemical and physical information of these materials can be obtained with AFM and IR-microscopy. Therefore, the main goal was to identify how the ideal sample area could be found. Due to the different measurement area size of AFM and IR-microscopy it was also important to investigate how a representative overview of the observed cross- section or over the surface part could be obtained. The otherwise difficult evaluation of the large dataset of about 4000 spectra for one IR-microscopy image could be overcome with multivariate data analysis. Afterwards, the last topic is to examine systematically produced samples and investigate what significant differences on the macroscopic to nanoscopic level could be found. Finally, the correlation between the current state-of-the-art destructive testing methods on macroscopic level, like scratch resistance and acid test, and the proposed testing procedure with AFM and IR-microscopy is started in 2021 and will be further continued in 2022. In this part, the main challenges are to produce macroscopic differing samples and to get representative parts for the microscopic to nanoscopic investigation, as well as the further step to obtain more information with AFM from force distance curve recording.

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11:05 - 11:40: Electron-phonon interaction under extreme pressures: Effects on materials properties from first-principles calculations

Roman Lucrezi

In microscopic descriptions of solids, phonons and electron-phonon interactions (EPI) are often ignored in a first approach. Yet, the thermodynamics of a real system, structural and relative stability of different phases, electrical and thermal conductivity are significantly affected by vibrational energy contributions and strong EPI. These quantities are inevitably influenced by external pressure, which alters directly the lattice and its vibrations, as well as the thermodynamics of the system through a non-negligible enthalpic contribution.
We developed a density-functional-theory-based workflow combining several fully ab initio and state-of-the-art methods in order to predict accurate high-pressure phase diagrams of binary systems, focusing on EPI-related phenomena and taking care of anharmonic phonon potentials as well as many-body contributions.

In particular, this talk includes results for two different EPI environments.
[1]: Transition metal chalcogenides (TMC) tend to crystallize into layered structures that can promote EPI. A few TMC phases are known that exhibit EPI-related phenomena such as charge-density wave (CDW) formation or superconductivity (SC), but complete phase diagrams with respect to pressure for the full TMxCy systems are largely unexplored. Here, we apply our workflow in order to search for new high-pressure structures in the binary Mo-S, Mo-Se, Nb-Se and Nb-S systems from ambient pressures up to 250 GPa. We discuss the electronic and vibrational properties of stable structures, as well as their EPI-related behaviour with focus on superconductivity.

[2]: Superhydrides are materials that incorporate a high amount of hydrogen in their crystal structure, and are the most promising materials in the hunt for room-temperature SC. Their lightweight nuclei lead to strong EPI, but also to strong anharmonic phonon modes that tend to cause lattice instabilities in many hydride systems. Here, we apply our methods to the two novel ternary compounds BaSiH8 and SrSiH8, with special focus on the determination of their low critical stability pressures (3GPa, 27GPa) and high critical SC temperature (71K, 126K), as well as on an advanced metastability analysis.

Corresponding publications:

[1] R. Lucrezi and C. Heil, J. Phys.: Condens. Matter 33, 174001 (2021)
[2] R. Lucrezi et al., arXiv:2112.02131 [cond-mat.supr-con] (2021)

This work is supported by the Austrian Science Fund (FWF) Project No. P 32144-N36.

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14:00 - 14:35: Correlated materials modelling: The example of magnetism in Ba2YIrO6

Hermann Schnait

(Strongly) correlated materials such as the big group of transition-metal oxides show a large variety of fascinating effects in experiment: Rich phase diagrams, topological effects and novel magnetic phases just to name a few. When modelling such materials 'in-silico', the single-particle description of Density Functional Theory (DFT) breaks down and different methods have to be used.

Here, I will present the treatment of such materials using DFT + Dynamical Mean Field Theory (DFT+DMFT) on the example of Ba2YIrO6. In this double perovskite the presence of both strong correlations and spin-orbit coupling leads to an unexpected (para-) magnetic ground state. However, there is contradicting experimental evidence, whether those magnetic moments order (antiferromagnetically) at low temperatures (~1 K). As standard Monte-Carlo impurity solvers are limited to higher temperatures, a T=0 solver based on Tensor Product States was used. While both at finite as well as zero temperature we find a finite magnetic moment comparable to experiments, no long range ordering is observed even at zero temperature.

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14:35 - 15:10: Failure analysis and reliability study for high power short pulsed vertical cavity surface emitting lasers

Robert Fabbro

In recent years, vertical cavity surface emitting lasers (VCSELs) are on the rise and are being utilized increasingly in high power applications like 3D-sensing in consumer electronics or light detection and ranging (LIDAR) in the automotive market. For these applications, VCSELs are typically arranged in 2D-arrays, with hundreds or even thousands of emitters, collectively reaching peak powers of a few watts at pulsed operation. In this work, new VCSEL failure analysis methodologies were developed and the impact of short pulsed high power operation on the VCSEL lifetime was studies. Failure analysis methods like reverse biased emission microscopy and high temperature in-situ scanning transmission electron microscopy proofed to be valuable additions to the VCSEL failure analysis portfolio by adding improved defect detection and the possibility to directly observe the formation of common VCSEL defects, respectively. Furthermore, lifetime studies showed high expected mean time to failures of about 90 years under standard operating conditions. Results also showed no dependency of the duty cycle on the observed lifetime. Consequently, until further investigations, the current VCSEL lifetime model seem to be suitable also for short pulsed high power devices.