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Vertical PFM amplitude images of PbTiO3/ SrRuO3 films deposited on DyScO3 (a) and TbScO3 (b) substrates. Lateral PFM amplitude images of PbTiO3/SrRuO3 films on GdScO3 (c) and SmScO3 (d) substrates. // Source: Ferroelectric Domain Walls in PbTiO3 Are Effective Regulators of Heat Flow at Room Temperature (DOI: 10.1021/acs.nanolett.9b02991)

Experiments confirm theoretical research: SiGe alloys for optoelectronics and ferroelectric phononic switches

ICMAB Researcher Riccardo Rurali, from the Materials Simulation and Theory (MST) group, has found two of his theoretical predictions to be confirmed through experimental approaches, regarding the use of hexagonal SiGe alloys for optoelectronic applications and the use of domain walls as switches for phononic devices.

Optical Emission in Hexagonal SiGe Nanowires

The first case refers to the article Optical Emission in Hexagonal SiGe Nanowires, published in Nano Letters (2017, 17, 4753−4758). The article, based on research by Xavier Cartoixà, Maurizia Palummo, Håkon Ikaros T. Hauge, and Erik P. A. M. Bakkers, besides Riccardo Rurali, explored the possibility of using SiGe hexagonal alloys for the creation of optoelectronic devices. The interest in this alloy comes from the fact that Silicon, the main material used in electronics, has very inefficient light emission, due to its indirect bandgap. The combined use of Ge and Si could allow for a direct bandgap, as long as the alloy is synthesized in a hexagonal crystal structure, and thus efficient light emission.

Through the use of many body calculations beyond single-particle density-functional theory, researchers found that a Si1−xGex  alloy that had at least 0.3-0.4 Ge could allow for direct bandgap optical transition. They also found it would have 3 times faster radiative reconfiguration than Si bulk options, and that its wavelength would be close to the infrared. 

These theoretical results were recently confirmed experimentally in an article published in Nature by Elham M. T. Fadaly, Alain Dijkstra, Jens Renè Suckert, Dorian Ziss, Marvin A. J. van Tilburg, Chenyang Mao, Yizhen Ren, Victor T. van Lange1, Ksenia Korzun, Sebastian Kölling, Marcel A. Verheijen, David Busse, Claudia Rödl, Jürgen Furthmüller, Friedhelm Bechstedt, Julian Stangl, Jonathan J. Finley, Silvana Botti, Jos E. M. Haverkort and Erik P. A. M. Bakkers: “Direct-bandgap emission from hexagonal Ge and SiGe alloys.

Their article used the work of ICMAB and other center's researchers as the theoretical base to further investigate this alloy, and the results are consistent with the predictions. The experiment shows that the particular hexagonal structure turns this alloy into a direct-bandgap semiconductor with a strong emission. The direct-bandgap is predicted at the Γ-point for x > 0.65, with a wavelength that can be tuned in the energy range between 0.3 and 0.7 eV. 

There are many applications for this alloy in optoelectronics, like silicon quantum photonic circuits, optical sensing, or for optical interconnection in computing.

A phononic switch based on ferroelectric domain walls

Another theoretical prediction appeared on the article A phononic switch based on ferroelectric domain walls, published on Physical Review B 96, 140101 (2017), and penned by Juan Antonio Seijas-Bellido, Carlos Escorihuela-Sayalero, Miquel Royo, Mathias P. Ljungberg, Jacek C. Wojdeł, and Jorge Íñiguez as well as Riccardo Rurali. 

In this article, molecular dynamics simulations are used in order to showcase how ferroelectric perovskite domain walls can be used as barriers for lattice thermal conduction. This could be key for the development of devices that operate with heat currents, rather than charge carriers or electromagnetic waves. These thin films are made of PbTiO3, a ferroelectric material that has a reorientable polarization through the use of an external electric field.

Also in this case, these theoretical predictions were confirmed by an experimental study reported in the paper Ferroelectric Domain Walls in PbTiO3 Are Effective Regulators of Heat Flow at Room Temperature, published in Nano Letters (2019, 19, 7901−7907) last year and authored by Eric Langenberg, Dipanjan Saha, Megan E. Holtz, Jian-Jun Wang, David Bugallo, Elias Ferreiro-Vila, Hanjong Paik, Isabelle Hanke, Steffen Ganschow, David A. Muller, Long-Qing Chen, Gustau Catalan, Neus Domingo, Jonathan Malen, Darrell G. Schlom, and Francisco Rivadulla.

In their experiment, the theoretical model was replicated depositing a crystal thin film of PbTiO3 on a (001) SrTiO3 substrate with a 10 nm-thick buffer layer of conducting SrRuO3The results were measured through different methods like piezoresponse force microscopy (PFM) and X-ray reciprocal space maps (RSM), and they showed a similar response to the one obtained through the calculations. 

Their results show that the ferroelectric domain walls affect heat flow in a single material, and they can be used as efficient regulators in a simpler way than other multilayered systems. This property will allow to modulate thermal flux to encode logic functions in phononic devices.

Congratulations Riccardo Rurali, and coworkers! It is always a pleasure when experiments confirm theoretical research! Eureka!

Referenced articles: 

Optical Emission in Hexagonal SiGe Nanowires
Xavier Cartoixà, Maurizia Palummo, Håkon Ikaros T. Hauge, Erik P. A. M. Bakkers, and Riccardo Rurali
Nano Letters 17, 8, 4753–4758, 2017
DOI: 10.1021/acs.nanolett.7b01441

Direct-bandgap emission from hexagonal Ge and SiGe alloys
Elham M. T. Fadaly, Alain Dijkstra, Jens Renè Suckert, Dorian Ziss, Marvin A. J. van Tilburg, Chenyang Mao, Yizhen Ren, Victor T. van Lange, Ksenia Korzun, Sebastian Kölling, Marcel A. Verheijen, David Busse, Claudia Rödl, Jürgen Furthmüller, Friedhelm Bechstedt, Julian Stangl, Jonathan J. Finley, Silvana Botti, Jos E. M. Haverkort & Erik P. A. M. Bakkers
Nature  580, 205–209, 2020
DOI: 10.1038/s41586-020-2150-y

A phononic switch based on ferroelectric domain walls
Juan Antonio Seijas-Bellido, Carlos Escorihuela-Sayalero, Miquel Royo, Mathias P. Ljungberg, Jacek C. Wojdeł, Jorge Íñiguez, and Riccardo Rurali
Phys. Rev. B  96, 140101(R),  2017
DOI: 10.1103/PhysRevB.96.140101

Ferroelectric Domain Walls in PbTiO3 Are Effective Regulators of Heat Flow at Room Temperature
Eric Langenberg, Dipanjan Saha, Megan E. Holtz, Jian-Jun Wang, David Bugallo, Elias Ferreiro-Vila, Hanjong Paik, Isabelle Hanke, Steffen Ganschow, David A. Muller, Long-Qing Chen, Gustau Catalan, Neus Domingo, Jonathan Malen, Darrell G. Schlom, and Francisco Rivadulla
Nano Letters 19 (11), 7901-7907, 2019
DOI: 10.1021/acs.nanolett.9b02991

Cover image: Vertical PFM amplitude images of PbTiO3/ SrRuO3 films deposited on DyScO3 (a) and TbScO3 (b) substrates. Lateral PFM amplitude images of PbTiO3/SrRuO3 films on GdScO3 (c) and SmScO3 (d) substrates. // Source: Ferroelectric Domain Walls in PbTiO3 Are Effective Regulators of Heat Flow at Room Temperature (DOI: 10.1021/acs.nanolett.9b02991)

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