New article: Band structure engineering in strain-free GaAs mesoscopic structures

We investigate the optical properties of strain-free mesoscopic GaAs/Al(x)Ga(1-x)As structures (MGS) coupled to thin GaAs/A(x)Ga(1-x)As quantum wells (QWs) with varying Al content (x). We demonstrate that quenching the QW emission by controlling the band crossover between AlGaAs X-point and GaAs Gamma-point gives rise to long carrier lifetimes and enhanced optical emission from the MGS. For x = 0.33, QW and MGS show typical type-I band alignment with strong QW photoluminescence emission and much weaker sharp recombination lines from the MGS localized exciton states. For x >= 0.50, the QW emission is considerably quenched due to the change from type-I to type-II structure while the MGS emission is enhanced due to carrier injection from the QW. For x >= 0.70, we observe PL quenching from the MGS higher energy states also due to the crossover of X and Gamma bands, demonstrating spectral filtering of the MGS emission. Time-resolved measurements reveal two recombination processes in the MGS emission dynamics. The fast component depends mainly on the X-Gamma mixing of the MGS states and can be increased from 0.3 to 2.5~ns by changing the Al content. The slower component, however, depends on the X-Gamma mixing of the QW states and is associated to the carrier injection rate from the QW reservoir into the MGS structure. In this way, the independent tuning of X-Gamma mixing in QW and MGS states allows us to manipulate recombination rates in the MGS as well as to make carrier injection and light extraction more efficient.

Vanessa Ors Gordo, Leonarde Nascimento Rodrigues, Floris Knopper, Ailton J Garcia, Fernando Iikawa, Odilon D. D. Couto Jr. and Christoph Deneke

Nanotechnology 31, 255202 (2020)

 

New article: Scanning Tunneling Measurements in Membrane-Based Nanostructures

Our new article: “Scanning Tunneling Measurements in Membrane-Based Nanostructures: Spatially- Resolved Quantum State Analysis in Postprocessed Epitaxial Systems for Optoelectronic Applications” is out in ACS Appl. Nano Mater.

Nanoscale heterostructure engineering is the main target for the development of optoelectronic devices. In this sense, a precise knowledge of local electronic response after materials processing is required to envisage technological applications. A number of local probe techniques that address single nanostructure signals were satisfactorily employed in semiconductor epitaxial systems. In this work we show that the use of chemically etched semiconductor nanomembranes allows carrying out scanning tunneling spectroscopy (STS) measurements in a postprocessed system which was otherwise studied mainly under in situ conditions that differ from the operational regime. We were able to acquire STS spectra with energy level resolved response on InAs quantum dots grown within a 15 nm-thick GaAs single-crystalline film transferred to an Au(111) surface. The presence of a native oxide layer does not affect the result, keeping the reliability of the usual ultra high vacuum (UHV) procedures. The use of nanomembranes also opens up the possibility of tailoring properties via additional variables such as nanomembrane thickness and surface charge depletion. Our method is applicable to a broad class of postprocessed layers extracted in nanomembrane format from epitaxial systems that are potential candidates for optoelectronic applications.

Authors: Barbara L. T. Rosa, Carlos A. Parra-Murillo, Thais Chagas, Ailton J. Garcia Junior, Paulo S. S. Guimarães, Paulo S. S. Guimarães, Ch. Deneke, Rogerio Magalhães-Paniago and Angelo Malachias

ACS Appl. Nano Mater. 2, 4655-4664,(2019)
DOI: 10.1021/acsanm.9b01124

As boas universidades no Brasil custam pouco, caro mesmo é não saber criar conhecimento

I would like to share out an interesting article (in Portuguese) thinking a bit about the cost and befits of the public founded universities of Brazil.

It is worth reading “Excelência acadêmica requer custeio público“, an articled found at the webpages of Unicamp.

 

 

After two years of refereeing: Nanoletter finally out

That must have been the longest submission in my scientific life.  After two years, with 14 referee reports from the first submission to Nature Photonics and three more rounds with Nanoletters our work: “Anisotropic Flow Control and Gate Modulation of Hybrid Phonon-Polaritons” is out.

Light–matter interaction in two-dimensional photonic or phononic materials allows for the confinement and manipulation of free-space radiation at sub-wavelength scales. Most notably, the van der Waals heterostructure composed of graphene (G) and hexagonal boron nitride (hBN) provides for gate-tunable hybrid hyperbolic plasmon phonon-polaritons (HP3). Here, we present the anisotropic flow control and gate-voltage modulation of HP3 modes in G-hBN on an air–Au microstructured substrate. Using broadband infrared synchrotron radiation coupled to a scattering-type near-field optical microscope, we launch HP3 waves in both hBN Reststrahlen bands and observe directional propagation across in-plane heterointerfaces created at the air–Au junction. The HP3 hybridization is modulated by varying the gate voltage between graphene and Au. This modifies the coupling of continuum graphene plasmons with the discrete hBN hyperbolic phonon polaritons, which is described by an extended Fano model. This work represents the first demonstration of the control of polariton propagation, introducing a theoretical approach to describe the breaking of the reflection and transmission symmetry for HP3 modes. Our findings augment the degree of control of polaritons in G-hBN and related hyperbolic metamaterial nanostructures, bringing new opportunities for on-chip nano-optics communication and computing.

Authors: Francisco C. B. Maia, Brian T. O’Callahan, Alisson R. Cadore , Ingrid D. Barcelos , Leonardo C. Campos, Kenji Watanabe , Takashi Taniguchi, Christoph Deneke , Alexey Belyanin, Markus B. Raschke , and Raul O. Freitas

Nano Lett.  19, 2, 708-715 ( 2019)
DOI: 10.1021/acs.nanolett.8b03732

Destaque em Física (SBF)

Article accepted in Nanoscale

In-place bonded semiconductor membranes as compliant substrates for IIIV compound devices

Overcoming the critical thickness limit in pseudomorphic growth of lattice mismatched heterostructures is a fundamental challenge in heteroepitaxy. On-demand transfer of light-emitting structures to arbitrary host substrates is an important technological path for optoelectronics and photonics devices implementation.  The use of freestanding membranes as compliant substrates is a promising approach to address both issues.

In this work, the feasibility of using released GaAs/InGaAs/GaAs membranes as virtual substrates to thin films of InGaAs alloy is investigated as a function of the Indium content in the films. Growth of flat epitaxial films is demonstrated with critical thickness beyond typical values regarding growth on bulk substrates.  Optically active structures are also grown on these membranes with strong photoluminescence signal and clear red shift for an InAlGaAs/InGaAs/InAlGaAs quantum well. The red shift is ascribed to strain reduction in the quantum well due to the use of completely relaxed membrane as substrate. Our results demonstrate that such membranes constitute a virtual substrate allowing further heterostructure strain engineering not possible by other post-growth methods.

Authors:  Ailton Jose Garcia Junior,  Leonarde N. Rodrigues,  Saimon Filipe Covre da Silva,  Sergio L.  Morelhao,  Odilon D. D. Couto Jr.,  Fernando Iikawa  and  Christoph Deneke  

Journal: Nanoscale 11, 3748-3756 (2019)

As featured on the page of the IFGW

Conference right before 1st semester 2019

I will be at the EuroMBE 2019 in February taking the chance to see, what’s going on in the European (and International) community. The conference will be in the week before the 1st semester starts from February, 17th to 20th, 2019. 

I will also present our work over membrane overgrowth in a poster session on Monday.

CU there.

 

Why the State of São Paulo is good in science and why it is important

The is a very nice article in Nature  discussing, why the state of SP is so good in science. Indeed, the state alone is responsible for more than 50% of Brazilian science production and  the leading place in South America.

That this is of great importance for the country and the society is shown on the example of the Zika virus. As we have an excellent existing infra-structure, the scientist here were one of the leading groups figuring out, what was going on and how to fight it!

The foundation for this is laid by the excellent founding conditions – mainly FAPESP – and with this allows to run two of the best universities of South America (Unicamp and USP), which provide highly qualified people for industry, governmental research institutes like the CNPEM and independent research institutes in the private sector.

We should always remember that all solutions for our society problems are based on a good and rigorous  understanding enabled by organized and well founded academic research – independent if done in academic institutions or outside in the private sector.

Membrane based solar cell paper

The membrane based solar paper in APL is out in which I was happy to contribute a bit: “Pixelated GaSb solar cells on silicon by membrane bonding”

We demonstrate thin-film GaSb solar cells which are isolated from a GaSb substrate and transferred to a Si substrate. We epitaxially grow ∼3.3 μm thick GaSb P on N diode structures on a GaSb substrate. Upon patterning in 2D arrays of pixels, the GaSb films are released via epitaxial lift-off and they are transferred to Si substrates. Encapsulation of each pixel preserves the structural integrity of the GaSb film during lift-off. Using this technique, we consistently transfer ∼4 × 4 mm2 array of pixelated GaSb membranes to a Si substrate with a ∼ 80%–100% yield. The area of individual pixels ranges from ∼90 × 90 μm2 to ∼340 × 340 μm2. Further processing to fabricate photovoltaic devices is performed after the transfer. GaSb solar cells with lateral sizes of ∼340 × 340 μm2 under illumination exhibit efficiencies of ∼3%, which compares favorably with extracted values for large-area (i.e., 5 × 5 mm2) homoepitaxial GaSb solar cells on GaSb substrates.

Authors: Vijay S. Mangu, Emma J. Renteria, Sadhvikas J. Addamane, Ahmad Mansoori, Adrian Armendáriz, Christoph F. Deneke, Sukarno O. Ferreira, Marziyeh Zamiri, Ganesh Balakrishnan, and Francesca Cavallo

Appl. Phys. Lett. 113, 123502 (2018); https://doi.org/10.1063/1.5037800

 

Scientific publishing

A true comment from XKCD:

Your manuscript “Don’t Pay $25 to Access Any of the Articles in this Journal: A Review of Preprint Repositories and Author Willingness to Email PDF Copies for Free” has also been rejected, but nice try.

XKCD is published under Creative Commons Attribution-NonCommercial 2.5 License  at xkcd.org.

Do grades really serve the propose we want?

I read these days an interesting German article, questioning the future of grades as we give them today. Basically, he repeats a critic that we already discussed in our course in the EA2. The main points are:

  • Grades are just a time snap of the moment. They do not tell you, what happened afterwards – like did the student study hard and overcome his deficits. The grade does not tell you, but last forever – something, we see every day when applying for “bolsas” and the referee points out a “bad” grade of the PostDoc candidate in his undergrad studies. By now, those grades do not tell anything about the ability of the candidate, because he had years of studying afterwards to fill the gaps – and most of the time did.
  • Grades given without the possibility and the feedback to improve are demotivating and hinder the learning process. They tell the person repeatingly, she/he is bad without the chance to attack the deficit or to improve at the area in question.  This leads to the flourishing of wrong learning, were we concentrate to get a good grade and not to dominate the content and concepts being taught – basically, we support surface learning against deep learning.
  • Grades are a general statement without telling you, what is good or bad in detail. Does a student with a low grade in a lab course, e.g.  not know how to write a report, but is great at the experiments or has he just learned the wrong part for the test, but would be able to do all the experiments and report them in a great way? Or – even worse – a student with a good grade did good in the reports and the test, but never carried out the experiment and is not able to do it as experiments been done in groups.

 

In a conclusion, the grade is very limited in their current form to fulfill the thing, we expect from them: monitoring the learning progress and act as feedback for student and teacher, if the teaching is good; telling the outside that a person managed to dominate the area of question as well as indicate the person for grants and “bolsas” inside the academic system. Which means, we have to converge to a better way to evaluate our teaching and learning.