Due to their exciting properties, 2D crystals like graphene and transition metal dichalcogenides promise to become the material of the future.
BY STEFAN DE GENDT, CEDRIC HUYGHEBAERT, IULIANA RADU and AARON THEAN, imec, Leuven, Belgium.
As we enter into the era of functional scaling where the cross-roads of More-Moore and More-Than-Moore meet, the search for new devices and their enabling material comes to the forefront of technology research. 2D crystals provide very interesting form-factors with respect to traditional 3D crystals (bulk, Si, and III-V semiconductors). In this elegant 2D form, electronic structure, mechanical flexibility, defect formation, and electronic and optical sensitivity become dramatically different. Aaron Thean: “As researchers at imec explore the physics and applications of such material, it is now becoming important to find a wafer-scale path towards technology implementation and integration of these novel materials.” Working closely with research teams across universities and industry partners, the first important step for imec is to enable the flake-to-wafer transition, while concurrently exploring the material, and device-to-circuit applications. The work will build new infrastructure (e.g. epitaxy, metrology, patterning, and electrical characterizations, etc.) around it.
Graphene and beyond
A 2D material is basically formed as a regular network in two dimensions, not extending in the third dimension. It is a monolayer-type of material, where monolayer should be understood as ‘up to a few monolayers’. The most known 2D material is graphene, a crystalline monolayer of carbon atoms arranged in a hexagonal honeycomb lattice structure. Recently, the exploration of 2D materials has moved beyond graphene. Stefan De Gendt: “2D materials cover all classes of materials, from semiconductors to insulators to metals. Graphene is a prominent example of a (semi-)metal. Transition metal dichalcogenides (or MX2 with M a transition metal and X a chalcogen such as sulfur or selenium) and hexagonal boron nitrides are well known examples of 2D semiconductors and insulators, respectively.”
2D materials: the new silicon?
Many of these materials exhibit remarkable properties that can be exploited in a range of applications. Cedric Huyghebaert: “Graphene, for example, is a fantastic electronic and thermal conductor. It has a record thermal conductivity, a very high intrinsic mobility, a high current density and long mean free path of electrons. Its surface is chemically inert, it has a low surface energy and no out-of-plane dangling bonds. MX2 have versatile properties that complement those of graphene. For example, they have a wide range of bandgaps as opposed to graphene, where the bandgap is absent. In case of graphene, we have to open the bandgap by using e.g. graphene nanoribbons or bilayer graphene.”
2D materials represent interesting alternatives to Si-based transistors. Iuliana Radu: “When scaling the gate length of a traditional Si-based MOSFET, overlapping junctions lead to short channel effects which degrade transistor performance. By introducing 2D materials in the channel of the MOSFET, they could show superior immunity to short channel effects. 2D materials could therefore extend traditional CMOS scaling beyond its current limits. They are also being considered for tunnel-FET (or TFET) applications, where carrier transport happens through band-to-band tunneling. In principle, 2D materials have no dangling bonds at the interfaces. These dangling bonds are one of the main limiters for TFETs with conventional semiconductors and limit strongly their performance.”
2D materials hold promises in other domains as well. Cedric Huyghebaert: “Many applications become possible by integrating these exciting materials in a monolithic way on top of CMOS. In (bio)sensing applications, for example, owing to their ability to adsorb and desorb various atoms and molecules. Or in optoelectronics, where the combination of a low absorption and high carrier mobility turns out very beneficial. Researchers are also assessing the potential of 2D materials to replace copper wires in back-end-of-line interconnects. Finally, 2D materials have been considered for appli- cation in domains such as plasmonics, photovoltaics and energy storage, and as transparent electrodes. In the latter applications, the requirements for graphene are less stringent than in aggressive transistor scaling. Therefore, the first graphene-based commercial products will most likely be introduced in one of these domains.”
The hamburger experience
Stefan De Gendt: “Ultimately, they potentially enable the engineering of new nano-based stacks: sandwich structures that are composed of various 2D materials, including semiconductors, metals and insulators. This view was nicely described at the 2013 IEDM conference, by the plenary speaker Andrea Ferrari. If you take a hamburger, it’s a layered combination of various ingredients, each with a specific flavor. But it’s the combination of all these layers that makes the hamburger a unique experience. The same will potentially hold for stacks made up of different 2D materials.”
From flakes to large-area synthesis
Applications based on graphene and other 2D materials have become very popular. Many of the above concepts have been successfully demonstrated and have been comprehensively described in scientific journals. However, so far, most of the demonstrations are limited to the lab, using 2D materials in the form of small exfoliated flakes. Iuliana Radu: “The real challenge today is maturing these concepts from flake-based devices towards real products that can be mass produced; only then, can they revolutionize multiple industries. And this has become a key goal at imec. Our goal is to demonstrate the manufacturability of these devices in a 300mm CMOS environment. And we house the expertise to run process flows on these materials (FIGURE 1). At imec, we work on all the unit process steps and on the sequence of steps towards an end application (e.g. TFETs, optical I/O, interconnects), and combine this with modeling and device benchmarking. We also take part in the Graphene Flagship, Europe’s 1 billion euro program that covers the whole value chain from materials production to components and systems.”
The road towards manufacturability
Due to the nature of the 2D materials, almost every unit process step such as contacting, doping, gate engineering, patterning and etch, etc, is a challenge. These steps, combined with the ability to integrate them into a cleanroom compatible process flow, are however essential to progress towards applications.
Cedric Huyghebaert: “A first challenge is related to the growth of these materials on large area templates, and their subsequent transfer to the final substrate. Graphene, for example, is typically grown on a metal template at high temperatures, up to 1000°C. The template is crucial, since the quality of graphene is very much dependent on the quality of the underlying template. Usually, the better the quality of graphene, the more difficult the transfer process becomes. At imec, we are actively working on the growth and defect-free transfer of graphene (FIGURES 2 and 3). In collaboration with AIXTRON, we focus on the synthesis of large area graphene using AIXTRON BM technology, compatible with 200 and 300mm processes. For the transfer, we rely on our knowledge on 3D Si integration processes. We also work on growth of MX2 materials by a direct sulfurization process or by atomic layer deposition in the 200 and 300mm imec fabs.”
Another hurdle is doping of the 2D semiconducting materials, which is needed to tune their energy levels and control their properties. Cedric Huyghebaert: “In the classical way, doping a semiconductor material means replacing an atom in the 3D structure. If you replace an atom in a 2D structure, you have a defect. So we have to consider different ways of doping these materials. At imec, we do this in collaboration with universities. We explore the possibility of achieving for example a semi-permanent doping by interaction with chemical molecules. Besides doping, contacting is also a challenge. The contribution of the electrodes to the total resistance of the device needs to be as low as possible. We therefore look into materials and architectures that allow for the lowest possible contact resistance.”
Several applications require a dielectric to be grown on top of the active semiconducting material. Stefan De Gendt: “In case of 2D materials, you have an almost perfectly passivated material, with no anchoring sites for the dielectric to nucleate. Consequently, the more perfect the 2D material, the more defective the dielectric on top may be.” Aaron Thean: “This is completely unlike 3D semiconductor processing, where a large part of material functionalization is achieved by surface and bulk material bond breaking and forming reactions, like dopant activation, oxidation, etc. This potential almost dangling-bond free weakly-interacting Van-Der-Waals nano-sheet system gives rise to new process challenges, as well as new opportunities like surface molecular doping and multi-layer channel stacking. One such approach is to transfer a 2D dielectric material to the 2D semiconducting material – like the hamburger experience described before.” Imec is working on understanding how to passivate, dope and grow dielectrics on various 2D materials. And there is patterning and etch, litho, and finally, characterization. Iuliana Radu:
“We are used to work with 3D bulk materials. But when you need to characterize only one or a few monolayers, there is hardly any material that can take part in the measurement. Therefore, the signals obtained with any classical characterization technique are extremely weak. And this requires new characterization strategies. At imec, we have established procedures that rely in a first phase on the physical characterization of the initial material properties. As the quality of the materials improve, we will cross-correlate physical characterization and electrical behavior of the layers.”
Demonstrating the potential
At IEDM 2014, imec and its associated lab at Ghent University have demonstrated an integrated graphene optical modulator on silicon. Cedric Huyghebaert:
“Integrated optical modulators with high modulation speed, small footprint and broadband a-thermal operation are highly desired for future chip-level optical interconnects. Due to its fast tunable absorption over a wide spectral range, graphene is well suited to achieve this. We could demonstrate a hybrid graphene-silicon modulator at bit rates up to 10Gb/s. This shows that it is possible to introduce CVD-grown graphene in a high quality Si platform and obtain a performance that can compete with traditional SiGe-based modulators. Moreover, if CVD graphene quality becomes more mature and can be brought into production, we will most probably end up with a device that is far less expensive than today’s optical components.”