Stacking 2-dimensional materials may lower cost of semiconductor devices
A team of researchers led by North Carolina State University has found that stacking materials that are only one atom thick can create semiconductor junctions that transfer charge efficiently, regardless of whether the crystalline structure of the materials is mismatched – lowering the manufacturing cost for a wide variety of semiconductor devices such as solar cells, lasers and LEDs.
“This work demonstrates that by stacking multiple two-dimensional (2-D) materials in random ways we can create semiconductor junctions that are as functional as those with perfect alignment” says Dr. Linyou Cao, senior author of a paper on the work and an assistant professor of materials science and engineering at NC State.
“This could make the manufacture of semiconductor devices an order of magnitude less expensive.”
Schematic illustration of monolayer MoS2 and WS2 stacked vertically. Image: Linyou Cao.
For most semiconductor electronic or photonic devices to work, they need to have a junction, which is where two semiconductor materials are bound together. For example, in photonic devices like solar cells, lasers and LEDs, the junction is where photons are converted into electrons, or vice versa.
All semiconductor junctions rely on efficient charge transfer between materials, to ensure that current flows smoothly and that a minimum of energy is lost during the transfer. To do that in conventional semiconductor junctions, the crystalline structures of both materials need to match. However, that limits the materials that can be used, because you need to make sure the crystalline structures are compatible. And that limited number of material matches restricts the complexity and range of possible functions for semiconductor junctions.
“But we found that the crystalline structure doesn’t matter if you use atomically thin, 2-D materials,” Cao says. “We used molybdenum sulfide and tungsten sulfide for this experiment, but this is a fundamental discovery that we think applies to any 2-D semiconductor material. That means you can use any combination of two or more semiconductor materials, and you can stack them randomly but still get efficient charge transfer between the materials.”
Currently, creating semiconductor junctions means perfectly matching crystalline structures between materials – which requires expensive equipment, sophisticated processing methods and user expertise. This manufacturing cost is a major reason why semiconductor devices such as solar cells, lasers and LEDs remain very expensive. But stacking 2-D materials doesn’t require the crystalline structures to match.
“It’s as simple as stacking pieces of paper on top of each other – it doesn’t even matter if the edges of the paper line up,” Cao says.
Scientists measure speedy electrons in silicon
The entire semiconductor industry, not to mention Silicon Valley, is built on the propensity of electrons in silicon to get kicked out of their atomic shells and become free. These mobile electrons are routed and switched though transistors, carrying the digital information that characterizes our age.
An international team of physicists and chemists based at the University of California, Berkeley, has for the first time taken snapshots of this ephemeral event using attosecond pulses of soft x-ray light lasting only a few billionths of a billionth of a second.
While earlier femtosecond lasers were unable to resolve the jump from the valence shell of the silicon atom across the band-gap into the conduction electron region, the new experiments now show that this transition takes less than 450 attoseconds.
“Though this excitation step is too fast for traditional experiments, our novel technique allowed us to record individual snapshots that can be composed into a ‘movie’ revealing the timing sequence of the process,” explained Stephen Leone, UC Berkeley professor of chemistry and physics.
Leone, his UC Berkeley colleagues and collaborators from the Ludwig-Maximilians Universität in Munich, Germany, the University of Tsukuba, Japan, and the Molecular Foundry at the Department of Energy’s Lawrence Berkeley National Laboratory report their achievement in the Dec. 12 issue of the journal Science.
Century-old discovery observed
Leone notes that more than a century has elapsed since the discovery that light can make certain materials conductive. The first movie of this transition follows the excitation of electrons across the band-gap in silicon with the help of attosecond extreme ultraviolet (XUV) spectroscopy, developed in the Attosecond Physics Laboratory run by Leone and Daniel Neumark, UC Berkeley professor of chemistry.
In semiconducting materials, electrons are initially localized around the individual atoms forming the crystal and thus cannot move or contribute to electrical currents. When light hits these materials or a voltage is applied, some of the electrons absorb energy and get excited into mobile states in which the electrons can move through the material. The localized electrons take a “quantum jump” into the conduction band, tunneling through the barrier that normally keeps them bound to atoms.
These mobile electrons make the semiconductor material conductive so that an applied voltage results in a flowing current. This behavior allows engineers to make silicon switches, known as transistors, which have become the basis of all digital electronics.
The researchers used attosecond XUV spectroscopy like an attosecond stop watch to follow the electron’s transition. They exposed a silicon crystal to ultrashort flashes of visible light emitted by a laser source. The subsequent illumination with x-ray-pulses of only a few tens of attoseconds (10-18 seconds) in duration allowed the researchers to take snapshots of the evolution of the excitation process triggered by the laser pulses.
Unambiguous interpretation of the experimental data was facilitated by a series of supercomputer simulations carried out by researchers at the University of Tsukuba and the Molecular Foundry. The simulations modeled both the excitation process and the subsequent interaction of x-ray pulses with the silicon crystal.
Electron jump makes atoms rebound
The excitation of a semiconductor with light is traditionally conceived as a process involving two distinct events. First, the electrons absorb light and get excited. Afterwards, the lattice, composed of the individual atoms in the crystal, rearranges in response to this redistribution of electrons, turning part of the absorbed energy into heat carried by vibrational waves called phonons.
In analyzing their data, the team found clear indications that this hypothesis is true. They showed that initially, only the electrons react to the impinging light while the atomic lattice remains unaffected. Long after the excitation laser pulse has left the sample – some 60 femtoseconds later – they observed the onset of a collective movement of the atoms, that is, phonons. This is near the 64 femtosecond period of the fastest lattice vibrations.
Based on current theory, the researchers calculated that the lattice spacing rebounded about 6 picometers (10-12 meters) as a result of the electron jump, consistent with other estimates.
“These results represent a clean example of attosecond science applied to a complex and fundamentally important system,” Neumark said.
The unprecedented temporal resolution of this attosecond technology will allow scientists to resolve extremely brief electronic processes in solids that to date seemed too fast to be approached experimentally, says Martin Schultze, who was a guest researcher in Leone’s lab last year, visiting from the Ludwig-Maximilians Universität München. This poses new challenges to the theory of light-matter interactions, including the excitation step, its timescale and the interpretation of experimental x-ray spectra.
“But here is also an advantage,” Schultze added. “With our ultrashort excitation and probing pulses, the atoms in the crystal can be considered frozen during the interaction. That eases the theoretical treatment a lot.”
Holst Centre and imec develop thin-film hybrid oxide-organic microprocessor
Holst Centre, imec and their partner Evonik have realized a general-purpose 8-bit microprocessor, manufactured using complementary thin-film transistors (TFTs) processed at temperatures compatible with plastic foil substrates (250°C). The new “hybrid” technology integrates two types of semiconductors—metal-oxide for n-type TFTs (iXsenic, Evonik) and organic molecules for p-type TFTs—in a CMOS microprocessor circuit, operating at unprecedented for TFT technologies speed—clock frequency 2.1kHz. The breakthrough results were published online in Scientific Reports, an open access journal from the publisher of Nature.
Low temperature thin-film electronics are based on organic and metal-oxide semiconductors. They have the potential to be produced in a cost effective way using large-area manufacturing processes on plastic foils. Thin-film electronics are, therefore, attractive alternatives for silicon chips in simple IC applications, such as radio frequency identification (RFID) and near field communication (NFC) tags and sensors for smart food packaging, and in large-area electronic applications, such as flexible displays, sensor arrays and OLED lamps. Holst Centre’s (imec and TNO) research into thin-film electronics aims at developing a robust, foil-compatible, high performance technology platform, which is key to making these new applications become a reality.
The novel 8-bit microprocessor performs at a clock frequency of 2.1 kHz. It consists of two separate chips: a processor core chip and a general-purpose instruction generator (P2ROM). For the processor core chip, a complementary hybrid organic-oxide technology was used (p:n ratio 3:1). The n-type transistors are 250°C solution-processed metal-oxide TFTs with typically high charge carrier mobility (2 cm2/Vs). The p-type transistors are small molecule organic TFTs with mobility of up to 1 cm2/Vs. The complementary logic allows for a more complex and complete standard cell library, including additional buffering in the core and the implementation of a mirror adder in the critical path. These optimizations have resulted in a high maximum clock frequency of 2.1kHz. The general-purpose instruction generator or P2ROM is a one-time programmable ROM memory configured by means of inkjet printing, using a conductive silver ink. The chip is divided into a hybrid complementary part and a unipolar n-TFT part and is capable of operating at frequencies up to 650 Hz, at an operational voltage of Vdd=10V.
Interested companies can join Holst Centre’s R&D program on organic and oxide transistors, exploring and developing new technologies for producing thin-film transistors (TFTs) on plastic foils.