Jun 04

New article: Lattice stability and elastic evolution of CdTe membranes fabrication using a III-V heterostructures as a substrate

A new article from our collaboration with the UFV is out in Nanoscale:

CdTe is a key binary compound for II-VI semiconductor systems since the precise control of its growth over different semiconductor materials of different orientations provides a general roadmap for telluride compounds, ranging from optically active layers to diluted magnetic semiconductors and topological insulators. The precise understanding of its epitaxy, film orientation and built-in strain is crucial for II-VI layer integration with commercial hosting substrates used for the latest semiconductor process nodes such as Si and GaAs. In this work, we show that it is feasible to use CdTe:GaAs/InGaAs/GaAs released membranes, yielding high-quality crystalline layers. A combination of Raman scattering and X-ray diffraction results provide a concise scenario of evolution along different growth stages. Surface roughness and contact potential are evaluated by atomic force and Kelvin-probe microscopy, respectively. The coexistence of faceting types (001) and (111) becomes clear using AFM and KPFM near the edge of a CdTe membrane. At such edges the local cleavage of the membrane is probed, exposing several layer steps and reflecting the growth history of CdTe. In this condition, KPFM clearly differentiates faceting (throughout contact potential difference) more accurately than height profiles, allowing a qualitative explanation of the nucleation evolution in our system. Finally, the occurrence of a 4% in-plane interfacial compressive strain is observed by nanomembrane release and modelling with finite element methods. The results showing the flexibility of high-quality CdTe layers here can improve optoelectronic integration of II-VI semiconductors.

Wesley Fiorio Inoch, Eduarda Policarpo, Misael Cesar Isaac Muniz, Angelo Malachias, Gilberto Rodrigues-Junior, Sukarno Olavo Ferreira, Christoph Deneke, Bráulio Soares Archanjo, Erika Peixoto Pimenta Peixoto Pimenta, Luciano Moura, Eduardo Nery Duarte Araujo and Leonarde do Nascimento N. Rodrigues

Nanoscale, 2025, 17, 15279 – 15288
DOI: https://doi.org/10.1039/D4NR05029K

Nov 09

Electronic Labbook (ELN) software

Our ELN software has hit version 1.4 and is now complete. It can manage your research data by Samples, has a Wiki for documentation of processes, a dedicated instrumentation journal (for things not related to samples but to equipment) and finally an item inventory to track your stuff. It is written in Python using the Flask framework and HTMX as well as as _hyperscript for the front end.

It can be run on your own server or used for an institute as it supports different groups and data sharing between groups. Have a look at gitlab or Read the docs.

Jul 28

New article in Nanotechnology

Our paper “Review: using rolled-up tubes for strain-tuning the optical properties of quantum emitters” is out.

Rolled-up tubes based on released III–V heterostructures have been extensively studied and established as optical resonators in the last two decades. In this review, we discuss how light emitters (quantum wells and quantum dots) are influenced by the inherently asymmetric strain state of these tubes. Therefore, we briefly review whispering gallery mode resonators built from rolled-up III–V heterostructures. The curvature and its influence over the diameter of the rolled-up micro- and nanotubes are discussed, with emphasis on the different possible strain states that can be produced. Experimental techniques that access structural parameters are essential to obtain a complete and correct image of the strain state for the emitters inside the tube wall. In order to unambiguously extract such strain state, we discuss x-ray diffraction results in these systems, providing a much clearer scenario compared to a sole tube diameter analysis, which provides only a first indication of the lattice relaxation in a given tube. Further, the influence of the overall strain lattice state on the band structure is examined via numerical calculations. Finally, experimental results for the wavelength shift of emissions due to the tube strain state are presented and compared with theoretical calculations available in literature, showing that the possibility to use rolled-up tubes to permanently strain engineer the optical properties of build-in emitters is a consistent method to induce the appearance of electronic states unachievable by direct growth methods.

Gabriel Gomes, Marcos L F Gomes, Saimon F Covre da Silva, Ailton Garcia Jr, Armando Rastelli, Odilon D D Couto Jr, Angelo Malachias and Christoph Deneke

Nanotechnology 34, 412001 (2023)
DOI: 10.1088/1361-6528/ace4d1

Feb 23

New article in Physical Review Materials

Direct observation of large-area strain propagation on free-standing nanomembranes

Investigations on epitaxial nanostructures with size of tens of nanometers have been a challenging issue for techniques that present high strain sensitivity but restricted spatial resolution. This is the case of recently developed x-ray nanoprobe techniques. Despite its inherent nondestructive character, submicron x-ray spots have only been successfully applied to the study of individual nanostructures which are either strain free or present extremely mild spatial lattice parameter gradients. Such limitation, with an uttermost barrier given by the diffraction limit, leads to voxel or pixel sizes between 5 and 10 nm obtained in coherent diffraction imaging or ptychographic reconstructions of real-space objects. Whenever the strain field of a nanostructure is successfully reconstructed from reciprocal space measurements, it cannot vary considerably in short distances since this would induce diffraction peak broadening and cause abrupt phase variations, leading to convergence issues on reconstruction algorithms. Here we show how epitaxial systems with large lattice mismatch and appreciable interfacial strain can be identified and directly analyzed throughout their strain field propagation in nanometer-thin crystalline membrane platforms, using the InGaAs/GaAs Stranski-Krastanov system as a model. The strain-induced footprint becomes observable along a few microns if the membrane thickness is comparable to the nanostructure size. It is possible to retrieve both interfacial strain and nanostructure size by probing individual objects.

Yuri Bernardes, Lucas A. B. Marçal, Barbara L. T. Rosa, Ailton Garcia, Jr., Christoph Deneke, Tobias U. Schülli, Marie-Ingrid Richard, and Angelo Malachias

Phys. Rev. Materials 7, 026002 (2023)

DOI: 10.1103/PhysRevMaterials.7.026002

Dec 14

New publication in “Journal of Physical Chemistry C”

Strain Tuning in Graded SiGe on Insulator: Interplay between Local Concentration and Nonmonotonic Lattice Evolution after Ge Condensation

TOC

Germanium condensation has proven to be a reliable route for obtaining smoothly graded composition SiGe layers with good reproducibility and reduced defect density. The process is known as a crucial tool to induce well-defined strain on Si or SiGe layers with potential use in semiconductor devices. In this work, we show that starting from a low concentration Si0.92Ge0.08 layer grown on top of a crystalline Si(001) on SOI substrates, we can reach desirable concentration with a nonmonotonic interplay on in-plane and out-of-plane strain. The Ge concentration is evaluated by a combination of ultralow energy secondary ion mass spectroscopy (ULE-SIMS) and synchrotron X-ray measurements (diffraction and reflectivity). After the evaluation of Ge content, the strain-sensitive process of rolling up tubes from the flat layers is used and combined with X-ray diffraction to provide a concise scenario of the strain evolution along an in-growth oxidation series, pointing out the conditions that maximize strain, as well as its fading, as the Ge content rises.

Gilberto Rodrigues-Junior, Francesca Cavallo, Christoph Deneke, and Angelo Malachias

J. Phys. Chem. C 126, 21368–21374 (2022)

DOI: 10.1021/acs.jpcc.2c05702

Feb 02

New article in “Nanotechnology” is out: Imaging the electrostatic landscape of unstrained self-assemble GaAs quantum dots

Unstrained GaAs quantum dots are promising candidates for quantum information devices due to their optical properties, but their electronic properties have remained relatively unexplored until now. In this work, we systematically investigate the electronic structure and natural charging of GaAs quantum dots at room temperature using Kelvin probe force microscopy (KPFM). We observe a clear electrical signal from these structures demonstrating a lower surface potential in the middle of the dot. We ascribe this to charge accumulation and confinement inside these structures. Our systematical investigation reveals that the change in surface potential is larger for a nominal dot filling of 2 nm and then starts to decrease for thicker GaAs layers. Using k · p calculation, we show that the confinement comes from the band bending due to the surface Fermi level pinning. We find a correlation between the calculated charge density and the KPFM signal indicating that k · p calculations could be used to estimate the KPFM signal for a given structure. Our results suggest that these self-assembled structures could be used to study physical phenomena connected to charged quantum dots like Coulomb blockade or Kondo effect.

Evandro Martin Lanzoni, Saimon F Covre da Silva, Matthijn Floris Knopper, Ailton J Garcia, Carlos Alberto Rodrigues Costa and Christoph Deneke

Nanotechnology 33, 165701 (2022)

DOI: 10.1088/1361-6528/ac47ce 

Nov 19

New article in “Small” out

Reconfiguration of Amorphous Complex Oxides: A Route to a Broad Range of Assembly Phenomena, Hybrid Materials, and Novel Functionalities

Reconfiguration of amorphous complex oxides provides a readily controllable source of stress that can be leveraged in nanoscale assembly to access a broad range of 3D geometries and hybrid materials. An amorphous SrTiO3 layer on a Si:B/Si1−xGex:B heterostructure is reconfigured at the atomic scale upon heating, exhibiting a change in volume of ≈2% and accompanying biaxial stress. The Si:B/Si1−xGex:B bilayer is fabricated by molecular beam epitaxy, followed by sputter deposition of SrTiO3 at room temperature. The processes yield a hybrid oxide/semiconductor nanomembrane. Upon release from the substrate, the nanomembrane rolls up and has a curvature determined by the stress in the epitaxially grown Si:B/Si1−xGex:B heterostructure. Heating to 600 °C leads to a decrease of the radius of curvature consistent with the development of a large compressive biaxial stress during the reconfiguration of SrTiO3. The control of stresses via post-deposition processing provides a new route to the assembly of complex-oxide-based heterostructures in 3D geometry. The reconfiguration of metastable mechanical stressors enables i) synthesis of various types of strained superlattice structures that cannot be fabricated by direct growth and ii) technologies based on strain engineering of complex oxides via highly scalable lithographic processes and on large-area semiconductor substrates.

Divya J. Prakash, Yajin Chen, Mengistie L. Debasu, Donald E. Savage, Chaiyapat Tangpatjaroen, Christoph Deneke, Angelo Malachias, Adam D. Alfieri, Omar Elleuch, Kaddour Lekhal, Izabela Szlufarska, Paul G. Evans, Francesca Cavallo

Small 18,  2105424 (2022)
DOI: 10.1002/smll.202105424

Apr 04

Featured by SPBMat

Our last article has been featured by the Brazilian Material Research Society: See their post “Engenharia de estrutura de banda usando a geometria de tubos enrolados.” Thanks for this.

 

Feb 24

New article in ACS Applied Nano Materials

After seeing the effect the first time 10 years ago in old samples, we finally figured out what was happening. See out latest results in ACS Appl. Nano Mater. with the title “Rolled-Up Quantum Wells Composed of Nanolayered InGaAs/GaAs Heterostructures as Optical Materials for Quantum Information Technology”: 

Strain-based band structure engineering is a powerful tool to tune the optical and electronic properties of semiconductor nanostructures. We show that we can tune the band structure of InGaAs semiconductor quantum wells and modify the helicity of the emitted light by integrating them into rolled-up heterostructures and changing their geometrical configuration. Experimental results from photoluminescence and photoluminescence excitation spectroscopy demonstrate a strong energy shift of the valence-band states in comparison to flat structures, as a consequence of an inversion of the heavy-hole with the light-hole states in a rolled-up InGaAs quantum well. The inversion and mixing of the band states lead to a strong change in the optical selection rules for the rolled-up quantum wells, which show vanishing spin polarization in the conduction band even under near-resonant excitation conditions. Band structure calculations are carried out to understand the changes in the electronic transitions and to predict the emission and absorption spectra for a given geometrical configuration. Comparison between experiment and theory shows an excellent agreement. These observed profound changes in the fundamental properties can be applied as a strategic route to develop novel optical devices for quantum information technology.

Leonarde N. Rodrigues, Diego Scolfaro, Lucas da Conceição, Angelo Malachias, Odilon D. D. Couto, Jr, Fernando Iikawa, and Christoph Deneke

ACS Appl. Nano Mater., 4 (3), 3140–3147 (2021)
DOI: 10.1021/acsanm.1c00354

Research was supported by FAPESP and CNPq.

As featured by the SBPMat.

 

 

Feb 01

Nice book for people interested in Science

I finished reading a very good philosophy book trying to figure out, what makes science in its way to acquire knowledge different from other approaches. “The Scientific Attitude: Defending Science from Denial, Fraud, and Pseudoscience” by Lee McIntyre is interesting making the point the difference is not the method, but the attitude how evidence is gathered and used to progress the knowledge of the world. He tells us that this attitude separates scientist from non-scientiest and lead to the strength of the scientific obtained understanding of the world making it special against all other knowledge.

Whereas I like the idea to separate the scientific endeavor by its attitude and values involved rather then by a scientific method, I have to admit, his science idea is to much an empirical one influenced by logical positivism, Popper and Kuhn. I personally see a strong component that acquiring of  knowledge and therefor also science has its base in empirical phenomena, but also a synthesis part as pointed out by Kant. In some way, McIntyre might agree with this as he points out that science not only reports the empirical finding, but the knowledge must come with a warrant.

Also it is interesting to see, how Galileo Galilei is mentioned to be a victim of the science believe of his time. There should be mentioned: No one believed in this time that the earth is flat – the solar system just was assumed to be heliocentric; the catholic church is not really the science community – Galileo’s science colleagues of the time like Copernicus (who suggested the heliocentric defended by Galileo Galilei), Johannes Kepler  and Tycho Brahe  already gave up on the geocentric world idea – it were people outside the sciences that did not want to believe this.

Anyway, the book is very interesting and I enjoyed reading it. I can just recommend it for people doing science as well as people not doing science. It is an important contribution in these times, where scientific knowledge is seen as a opinion among others  and forgotten that generations of scientist worked to arrive on the current point of knowledge. Therefore, the current state of scientific knowledge contains the experience of many tries (and failures) to understand the world. If the knowledge is gathered with the right, scientific attitude, it is our best chance to be the correct one.

P.S.: Peer review is not as good as hoped in the book and the important fact checking is done afterwards by the community; and scientific progress does not happened in Paradigmatic shifts, but in a hard creeping way of small continuous steps (sometimes with some impressive jumps from people at the right time and place, which would not possible, if the previous generations of scientist had not collected all the data).