New transistors: An alternative to silicon and better than graphene

New transistors: An alternative to silicon and better than graphene

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This is a digital model showing how molybdenite can be integrated into a transistor. Credit: Credit: EPFL

Smaller and more energy-efficient electronic chips could be made using molybdenite. In an article appearing online January 30 in the journal Nature Nanotechnology, EPFL's Laboratory of Nanoscale Electronics and Structures (LANES) publishes a study showing that this material has distinct advantages over traditional silicon or graphene for use in electronics applications.

A discovery made at EPFL could play an important role in electronics, allowing us to make transistors that are smaller and more energy efficient. Research carried out in the Laboratory of Nanoscale Electronics and Structures (LANES) has revealed that molybdenite, or MoS₂, is a very effective semiconductor. This mineral, which is abundant in nature, is often used as an element in steel alloys or as an additive in lubricants. But it had not yet been extensively studied for use in electronics.

100,000 times less energy

"It's a two-dimensional material, very thin and easy to use in nanotechnology. It has real potential in the fabrication of very small transistors, light-emitting diodes (LEDs) and solar cells," says EPFL Professor Andras Kis, whose LANES colleagues M. Radisavljevic, Prof. Radenovic et M. Brivio worked with him on the study. He compares its advantages with two other materials: silicon, currently the primary component used in electronic and computer chips, and graphene, whose discovery in 2004 earned University of Manchester physicists Andre Geim and Konstantin Novoselov the 2010 Nobel Prize in Physics.

One of molybdenite's advantages is that it is less voluminous than silicon, which is a three-dimensional material. "In a 0.65-nanometer-thick sheet of MoS₂, the electrons can move around as easily as in a 2-nanometer-thick sheet of silicon," explains Kis. "But it's not currently possible to fabricate a sheet of silicon as thin as a monolayer sheet of MoS₂." Another advantage of molybdenite is that it can be used to make transistors that consume 100,000 times less energy in standby state than traditional silicon transistors. A semi-conductor with a "gap" must be used to turn a transistor on and off, and molybdenite's 1.8 electron-volt gap is ideal for this purpose.

Better than graphene

In solid-state physics, band theory is a way of representing the energy of electrons in a given material. In semi-conductors, electron-free spaces exist between these bands, the so-called "band gaps." If the gap is not too small or too large, certain electrons can hop across the gap. It thus offers a greater level of control over the electrical behavior of the material, which can be turned on and off easily.

The existence of this gap in molybdenite also gives it an advantage over graphene. Considered today by many scientists as the electronics material of the future, the "semi-metal" graphene doesn't have a gap, and it is very difficult to artificially reproduce one in the material.

Ultrathin alternative to silicon for future electronics

Ultrathin alternative to silicon for future electronics

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Fabricating an indium oxide (InAs) device starts with a) epitaxially growing and etching InAs into nanoribbon arrays that are get stamped onto a silicon/silica (Si/SiO2 ) substrate; b) and c) InAs nanoribbon arrays on Si/SiO2; d) and e) InAs nanoribbon superstructures on Si/SiO2. Credit: courtesy of Ali Javey, UC Berkeley

There's good news in the search for the next generation of semiconductors. Researchers with the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California Berkeley, have successfully integrated ultra-thin layers of the semiconductor indium arsenide onto a silicon substrate to create a nanoscale transistor with excellent electronic properties. A member of the III–V family of semiconductors, indium arsenide offers several advantages as an alternative to silicon including superior electron mobility and velocity, which makes it an oustanding candidate for future low-power, high-speed electronic devices.

"We've shown a simple route for the heterogeneous integration of indium arsenide layers down to a thickness of 10 nanometers on silicon substrates," says Ali Javey, a faculty scientist in Berkeley Lab's Materials Sciences Division and a professor of electrical engineering and computer science at UC Berkeley, who led this research.

"The devices we subsequently fabricated were shown to operate near the projected performance limits of III-V devices with minimal leakage current. Our devices also exhibited superior performance in terms of current density and transconductance as compared to silicon transistors of similar dimensions."

For all its wondrous electronic properties, silicon has limitations that have prompted an intense search for alternative semiconductors to be used in future devices. Javey and his research group have focused on compound III–V semiconductors, which feature superb electron transport properties. The challenge has been to find a way of plugging these compound semiconductors into the well- established, low-cost processing technology used to produce today's silicon-based devices. Given the large lattice mismatch between silicon and III-V compound semiconductors, direct hetero-epitaxial growth of III-V on silicon substrates is challenging and complex, and often results in a high volume of defects.

"We've demonstrated what we are calling an 'XOI,' or compound semiconductor-on-insulator technology platform, that is parallel to today's 'SOI,' or silicon-on-insulator platform," says Javey. "Using an epitaxial transfer method, we transferred ultrathin layers of single-crystal indium- arsenide on silicon/silica substrates, then fabricated devices using conventional processing techniques in order to characterize the XOI material and device properties."

The results of this research have been published in the journal Nature, in a paper titled, "Ultrathin compound semiconductor on insulator layers for high-performance nanoscale transistors." Co-authoring the report with Javey were Hyunhyub Ko, Kuniharu Takei, Rehan Kapadia, Steven Chuang, Hui Fang, Paul Leu, Kartik Ganapathi, Elena Plis, Ha Sul Kim, Szu-Ying Chen, Morten Madsen, Alexandra Ford, Yu-Lun Chueh, Sanjay Krishna and Sayeef Salahuddin.

To make their XOI platforms, Javey and his collaborators grew single-crystal indium arsenide thin films (10 to 100 nanometers thick) on a preliminary source substrate then lithographically patterned the films into ordered arrays of nanoribbons. After being removed from the source substrate through a selective wet-etching of an underlying sacrificial layer, the nanoribbon arrays were transferred to the silicon/silica substrate via a stamping process.

Javey attributed the excellent electronic performance of the XOI transistors to the small dimensions of the active "X" layer and the critical role played by quantum confinement, which served to tune the material's band structure and transport properties. Although he and his group only used indium arsenide as their compound semiconductor, the technology should readily accommodate other compound III/V semiconductors as well.

"Future research on the scalability of our process for 8-inch and 12-inch wafer processing is needed," Javey said.

"Moving forward we believe that the XOI substrates can be obtained through a wafer bonding process, but our technique should make it possible to fabricate both p- and n- type transistors on the same chip for complementary electronics based on optimal III–V semiconductors.

"Furthermore, this concept can be used to directly integrate high performance photodiodes, lasers, and light emitting diodes on conventional silicon substrates. Uniquely, this technique could enable us to study the basic material properties of inorganic semiconductors when the thickness is scaled down to only a few atomic layers."