Making Contact With the Quantum Realm

A significant step in making silicon such a ubiquitous component of modern electronics was figuring out how to make electrical contact with the semiconductor in the first place. That problem has been solved for silicon for some time, but interfacing the necessary metals with other kinds of semiconductors remains a challenge—particularly for researchers interested in dropping dimensions to study the unique properties of atom-thin quantum materials. 

In a new paper in Nature Nanotechnology, researchers at Columbia detail their relatively simple solution for creating electrical contacts with a 2D semiconductor: add another layer. “This work is usually hidden, but it’s fundamental if we want to study the electronic properties of new quantum materials,” said senior author and Columbia physicist Cory Dean. 

Leading the research, which was included in a focus issue of the journal dedicated to 2D semiconductors, was Columbia physics PhD student Jordan Pack. Notably, the technique he and his colleagues developed doesn’t just work at room temperature, where most research has been focused, but can survive the extreme conditions where quantum mechanics manifest. Here, Pack explains why the new technique is needed and what he and other physicists hope to learn now that they can make electrical contact with the quantum realm. 

Why the interest in 2D semiconductors?

There are two perspectives. Technologically, silicon can only scale down so much before its intrinsic properties make it hard to build more transistors per unit area on a chip. These two-dimensional quantum materials might be a way to beat that limit.

From a science perspective, 2D semiconductors offer many possibilities that don’t exist in bulk semiconductors, like silicon. For example, you can stack and twist them, which lets us engineer new states. As a field, we’ve already seen a lot of exciting results, like Wigner crystals in moiré materials, the observation of topological insulators, including the first discovery of a fractional topological insulator without a magnetic field, and, more recently, superconductivity

But a lot of these properties are optical and skate around the issue that it’s just hard to make electrical contact with these materials in order to flow a current through them. 

We are really interested in the transport properties of quantum materials. A critical step to get there is to interface a metal with them, but the techniques developed for bulk materials like silicon just don’t translate in two dimensions. 

What’s your solution?

Instead of just trying to bring a metal into contact with our semiconductor, a transition metal dichalcogenide (TMD) called tungsten diselenide, we included a third layer made from a material called ruthenium trichloride. That extra material, which we can stack selectively, makes tungsten diselenide act very metallic, so we can move electrons from the metal layer into the semiconductor wherever it is in contact with the ruthenium trichloride. 

How did you come up with the idea of adding this third material?

Others around Columbia, including Dmitri Basov’s group, had been interfacing ruthenium trichloride with another quantum material, graphene, for totally different reasons than us, and they found that combining the two can redistribute the electrical charge between the layers. This new ability to change where the charges are turns out to be really useful for working with TMDs. 

It was really a bit of serendipity. I had been trying other approaches to engineer electrical contacts, but the results just weren’t consistent, and we couldn’t figure out why. Since others were looking at ruthenium trichloride, we decided to just give it a try. Everything worked, and we ran with it.

What are some of the interesting results so far?

We can now flow electrical current through this semiconductor at milliKelvin temperatures, which is where you really can start to see quantum properties. In the current paper, we observed a metal-insulator transition around the temperature and electron densities where physicists expect to see the formation of Wigner crystals, which are solids made of electrons. We also observed the fractional quantum Hall effect, a collective state of interacting electrons that is a real benchmark of quality for these materials. 

We’ve now started using the new contact technique with two layers of twisted tungsten diselenide. In the past few years, several correlated states have been observed in twisted TMDs, but one that has been notably absent in the literature is superconductivity. With our more reliable and higher-performance contacts, we can now see evidence for it in twisted tungsten diselenide, which we reported last month on arXiv. 

You are a 5th-year graduate student—how did you end up studying these materials at Columbia, and what’s next?

I worked in a 2D material group during my undergrad at Washington University in St. Louis, and what really captured my interest was how flexible these materials are—they let you study such a wide range of physics in a simple and easily modifiable way. There are so many directions to pursue in this field, and Columbia is an exciting place for 2D material research, so I decided to come to New York for my PhD. 

I am interested in using 2D semiconductors to study the properties that depend on correlations between electrons. We’ve developed the foundation to probe transport in a different way, so now it’s all about applying that to other systems where we expect to see unique physics. I hope to continue in this vein in academia after I graduate. 

Do you have any words of advice?

Don’t be afraid to explore completely new ways to approach a problem. We were originally trying to solve this issue with electrical contacts while focused on studying twisted materials, and we didn't want to explore completely new approaches to making the contacts. It wasn’t until we stepped back in frustration that we saw a new approach—one that ended up working quite well.