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Black phosphorus is the new graphene

Posted: 06 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Graphene? molybdenum disulphide? International Technology Roadmap for Semiconductors?

Scientists predict that two-dimensional (2D) materials like graphene will succeed silicon by 2028, the end of the International Technology Roadmap for Semiconductors (ITRS). They are also investigating other "miracle materials" including transition metal dichalcogenides (TMDs) such as molybdenum disulphide (MoS2). Based on research from the University of Minnesota, a new 2D material!black phosphorus!is seen as the solution to graphene's problems.

Black phosphorus doesn't have graphene's hang-ups: graphene lacks bandgap and is incompatible with silicon. A compatibility with silicon could hasten silicon photonics, when light rather than electronics carry digital signals on future chips.

"For the first time we have demonstrated that crystalline black phosphorus photodetectors can be transferred to silicon photonic circuits and perform as well as germanium!the gold standard in photodetectors," said Professor Mo Li, lead researcher on the project at the University of Minnesota.

Black phosphorus

High-performance photodetectors use only a few layer black phosphorus (red) to sense light in the waveguide (green). Graphene (grey atoms) is also used to tune the performance. Source: College of Science and Engineering, University of Minnesota

In nature, phosphorus is highly reactive!that's why they make match heads from it!but after being cooked in an oven at a precise temperature, it turns black and becomes not only stable, but converts to a pure crystalline form that can be exfoliated onto a silicon substrate. For their first device, the researchers used 20 monolayers of black phosphorus to demonstrate their optical circuits. They were able to achieve communications speeds of 3 billion bits per second (3Gbit/s).

Researchers integrate black phosphorus

The researchers integrate black phosphorus on an optical inteferometer of silicon waveguides (thin traces in the figure) to measure optical absorption accurately and detect the photocurrent generated in it.

The biggest advantage that black phosphorus has over graphene is that it has a bandgap, enabling it to easily detect light. What's more that bandgap is tuneable by controlling how many layers of black phosphorus are stacked atop silicon, enabling it to absorb in both the visible range and the infrared ranges used for communications. And because black phosphorus is also a direct-band semiconductor, it can also convert electrical signals into light.

"One of short term goals is to make a black-phosphorus transistor, and our long term goal is to achieve a black phosphorus laser on a silicon chip," Li told us.

Of all the 2D materials being investigated today, Li claims that black phosphorus does not have serious trade-offs between having a tuneable bandgap and a high-speed operation like the others, making black phosphorus "the best of both worlds," according to Li.

Funding was provided by the Air Force Office of Scientific Research and the National Science Foundation.

- Colin Johnson
??EE Times U.S.

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