Black Phosphorus Takes a Step Toward CMOS

Prototype transistors of both the types needed for CMOS can be made by adjusting the material's thickness

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Black Phosphorus Takes a Step Toward CMOS
Illustration: Institute for Basic Science

Researchers working at the Institute for Basic Science Center for Integrated Nanostructure Physics at Sungkyunkwan University (SKKU) in South Korea have discovered that they can manipulate black phosphorus to behave as an n-type (excess electrons) semiconductor, a p-type (excess holes), or as if it were ambipolar (both n- or p-type) simply by changing its thickness and its bandgap or by using a different metal to contact it with. (Today’s digital logic, CMOS, requires both n-type and p-type transistors.)

With this knowledge, the Korean researchers were able to fabricate a transistor from the material that can operate at lower voltages than a silicon-based transistor. (Though it wasn’t the first black phosphorus transistor ever made.)

Black phosphorus—or, as it sometimes called, phosphorene—is a fairly recent arrival in the two-dimensional landscape. After languishing in chemical catalogs for nearly a century, the material took on new life last year when researchers were able to exfoliate films of the substance having just 10 to 20 atomic layers.

Excitement about the material has grown quickly since then because unlike its older cousin, graphene, black phosphorus possesses an inherent band gap. Also, the range of that band gap is appealing because it falls between those of graphene and transition metal dichalcogenides—covering the visible to infrared range needed for optoelectronics.

However, the Korea-based researchers have skipped passed optoelectronics and gone right into the jaws of the dragon—testing to see if the material could compete with silicon in digital logic. In research published in the journal Nature Communications, the SKKU team discovered that by simply adjusting the thickness of the material they could change its band gap, which should prove to be a valuable tool in the struggle against mighty silicon.

In their research, the team discovered that when aluminum was used as a contact, 13-nanometer-thick black phosphorus sheets displayed ambipolar properties similar to graphene. Meanwhile, thinner, 3-nm flakes are unipolar n-type with on/off current ratios greater than 105.  The thinner they can make the material, the better its switching performance. 

“The driving force in black phosphorus is the carrier mobility,” said David J. Perello, one of the researchers and authors of the article, in a press release. Carrier mobility is the speed with which charge can move through a material. While the mobility of silicon can compete with the today’s experimental devices made from black phosphorous, the potential carrier mobility of black phosphorus is much higher than silicon. “Everything centers around that,” said Perello. “The fact that the band gap changes with thickness also gives us flexibility in circuit design. As a researcher it gives me a lot of things to play with.” 

But the material has a long way to go. “I don’t think it can compete with silicon at the moment, that’s a dream everybody has,” said Perello in the release. “Silicon is cheap and plentiful and the best silicon transistors we can make have mobilities that are similar to what I was able to make in these black phosphorus devices.”

Perello added: “The fact that it was so simple to make such an excellent transistor without having access to state of the art commercial growth, fabrication, and lithography facilities means that we could make it significantly better.”

Of course, any dreams of a post-silicon, black-phosphorus world are soon brought to earth when one considers the near impossibility of producing the substance in bulk. However, scientists are looking into chemical vapor deposition (CVD) as a possibility for ramping up production of the material.

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