PORTLAND, Ore. — Two-dimensional (2D) materials like graphene are predicted to succeed silicon at the end of the International Technology Roadmap for Semiconductors (ITRS) circa 2028. Graphene is the most popular, but scientists are also investigating other "miracle materials" including transition metal dichalcogenides (TMDs) such as molybdenum disulphide (MoS2). Now a new 2D material — black phosphorus — is being hawked as solving graphene's problems.
Black phosphorus doesn't have graphene's hangups: graphene lacks bandgap and is incompatibile 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," professor Mo Li, lead researcher on the project at the University of Minnesota told EE Times.
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 three billion bits per second (3-Gbps).
High performance photodetectors use only a few layer black phosphorus (red) to sense light in the waveguide (green). Graphene (gray atoms) is also used to tune the performance.
Photo Credit: College of Science and Engineering
(Source: University of Minnesota)
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 tunable 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.
The researchers integrate black phosphorus on an optical inteferometer of silicon waveguides (thin traces in the figure) to measure its optical absorption accurately and detect the photocurrent generated in it.
Of all the 2D materials being investigated today, Li claims that black phosphorus does not have serious trade-offs between having a tunable 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.
— R. Colin Johnson, Advanced Technology Editor, EE Times