Tag Archives: van der Waals epitaxy

Memristive forming strategy

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This is highly technical and it’s here since I’m informally collecting all the research that I stumble across concerning memristors and neuromorphic engineering.

From a Sept. 5, 2022 news item on Nanowerk, Note: A link has been removed,

The silicon-based CMOS [complementary metal-oxide-semiconductor] technology is fast approaching its physical limits, and the electronics industry is urgently calling for new techniques to keep the long-term development. Two-dimensional (2D) semiconductors, like transition-metal dichalcogenides (TMDs), have become a competitive alternative to traditional semiconducting materials in the post-Moore era, and caused worldwide interest. However, before they can be used in practical applications, some key obstacles must be resolved. One of them is the large electrical contact resistances at the metal-semiconductor interfaces.

The large contact resistances mainly come from two aspects: the high tunneling barrier caused by the wide van der Waals (vdW) gap between the 2D material and the metal electrode; the high Schottky barrier accompanied by strong Fermi level pinning at the metal-semiconductor interface.

Four strategies including edge contact, doping TMDs, phase engineering, and using special metals, have been developed to address this problem. However, they all have shortcomings.

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In a new work (Nano Letters, “Van der Waals Epitaxy and Photoresponse of Hexagonal Tellurium Nanoplates on Flexible Mica Sheets”) coming out of Zhenxing Wang’s group at the National Center for Nanoscience and Technology [located in Beijing, China], the researchers have proposed a brand-new contact resistance lowering strategy of 2D semiconductors with a good feasibility, a wide generality and a high stability.

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You can fill in the blanks at Nanowerk or there’s this link and citation for the paper

Van der Waals Epitaxy and Photoresponse of Hexagonal Tellurium Nanoplates on Flexible Mica Sheets by Qisheng Wang, Muhammad Safdar, Kai Xu, Misbah Mirza, Zhenxing Wang, and Jun He. ACS Nano 2014, 8, 7, 7497–7505 DOI: https://doi.org/10.1021/nn5028104 Publication Date:July 2, 2014 Copyright © 2014 American Chemical Society

This paper is behind a paywall.

Pousse-café nanowires

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If you grow a nanowire made up of three elements on a graphene substrate, you get a surprise. At least, that’s what the research team at the University of Illinois received. From the Apr. 23, 2013 news release on EurekAlert,

Nanowires, tiny strings of semiconductor material, have great potential for applications in transistors, solar cells, lasers, sensors and more.

“Nanowires are really the major building blocks of future nano-devices,” said postdoctoral researcher Parsian Mohseni, first author of the study. “Nanowires are components that can be used, based on what material you grow them out of, for any functional electronics application.”

Li’s group uses a method called van der Waals epitaxy to grow nanowires from the bottom up on a flat substrate of semiconductor materials, such as silicon. The nanowires are made of a class of materials called III-V (three-five), compound semiconductors that hold particular promise for applications involving light, such as solar cells or lasers.

The group previously reported growing III-V nanowires on silicon. While silicon is the most widely used material in devices, it has a number of shortcomings. Now, the group has grown nanowires of the material indium gallium arsenide (InGaAs) on a sheet of graphene, a 1-atom-thick sheet of carbon with exceptional physical and conductive properties.

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“One of the reasons we want to grow on graphene is to stay away from thick and expensive substrates,” Mohseni said. “About 80 percent of the manufacturing cost of a conventional solar cell comes from the substrate itself. We’ve done away with that by just using graphene. Not only are there inherent cost benefits, we’re also introducing functionality that a typical substrate doesn’t have.”

The researchers pump gases containing gallium, indium and arsenic into a chamber with a graphene sheet. The nanowires self-assemble, growing by themselves into a dense carpet of vertical wires across the surface of the graphene. Other groups have grown nanowires on graphene with compound semiconductors that only have two elements, but by using three elements, the Illinois group made a unique finding: The InGaAs wires grown on graphene spontaneously segregate into an indium arsenide (InAs) core with an InGaAs shell around the outside of the wire. [emphasis mine]

“This is unexpected,” Li [professor Xiuling Li] said. “A lot of devices require a core-shell architecture. Normally you grow the core in one growth condition and change conditions to grow the shell on the outside. This is spontaneous, done in one step. The other good thing is that since it’s a spontaneous segregation, it produces a perfect interface.”

The group plans to make solar cells amongst other items with this new type of nanowire. You can find the whole story (Apr. 23, 2013 news item) on ScienceDaily along with a link to and citation for the researchers’ paper.

This story reminded me of a cocktail that’s fascinated me for years, a pousse-café,

Downloaded from http://www.scienceofdrink.com/2010/10/18/pousse-cafe-and-some-modern-derivatives/langswitch_lang/en/

Downloaded from http://www.scienceofdrink.com/2010/10/18/pousse-cafe-and-some-modern-derivatives/langswitch_lang/en/

The layers are not self-assembling as are the nanowires. Making this drink requires knowledge of the various weights of the liqueurs you are using and some care. You can find some recipes for modern pousse-cafés at the Science of Drink here. I believe this site has been translated from another language so you may find some unusual grammatical structures.

Have a lovely weekend.