Phosphorene: graphene with a bandgap
US and Chinese scientist have made transistors from a graphene-like substance called black phosphorus, which is being dubbed phosphorene in its few-layer form.
Just like graphene, phosphorus can form a flat sheet molecule. Although up-close it is heavily corrugated while graphene is flat – the difference being a result of phosphorus’ three covalent bonds and carbon’s four.
“The three bonds take up all three valence electrons of phosphorus so, unlike graphene, monolayer black phosphorus is a semiconductor,” said the Chinese team in its paper ‘Black Phosphorus Field-effect Transistors‘, published on arxiv.org. “For few-layer phosphorene, interlayer interactions reduce the band gap for each layer added, and eventually reach ~0.3eV for bulk black phosphorus.”
In the bulk form, layers are held together by Van der Waals – also like graphene. For research, flakes of both materials can be removed from lumps of the raw material using stick tape.
The Chinese team, from Fudan University, Shanghai, and University of Science and Technology of China, Hefei, made back-gate transitors by putting flakes on an oxide-coated silicon wafer, then depositing metal source and drain contacts on the flake.
With a 5nm channel on a 7.5nm thick flake, and on/off current ratio of around 105 was measured.
“The measured drain current modulation is four orders of magnitude larger than that in graphene due to its [graphene's] lack of bandgap,” said the team in its paper.
The maximum ratio could be higher, but limitations in the wafer oxide layer prevented the transistors from being pushed into saturation.
Phosphorene has p-type tendencies. Mobility was thickness dependent, and hole mobility peaked around 1,000cm2/Vs on 10nm thick flakes.
The US team also found a direct bandgap, describing a mobility hole mobility of 286cm2/Vs and on/off ratio up to 104 from 1µm channel fets in a paper ‘Phosphorene: A New 2D Material with High Carrier Mobility‘, also published on arxiv.org.
Molybdenum disulphide is another material that forms 2D structures with graphene-like behaviour, this time exhibiting n-type behaviour.
Also described in its paper, the US team, from Purdue University and Michigan State University, went on to make a CMOS inverter from 2D materials – phosphorene for PMOS and MoS2 for NMOS transistors, using top-gate structures with Al2O3 high-k gate dielectric.