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Spin Logic Switch

Collaborators: Appenzeller, Datta's groups

Spin coupled nanomagnet devices and circuits are proposed as one of the next generation information processing solutions. Compared with the conventional charge based CMOS logic, using spin as the state variable offers advantages such as low voltage operation, non-volatility, and the possibility of combining analog and digital computation. Based on the recent developments in spintronics and nanomagnetics, it is argued that a novel spintronic switch and associated circuits could help continue Moore¨s law, especially if novel functionalities could be incorporated to expand the ^user value ̄ of the IC chips. Two spin switch ideas, All Spin Logic (ASL) and Charge-coupled Spin Logic (CSL), proposed by Prof. Datta's group are experimentally investigated here.


All Spin Logic (ASL)            

Different from conventional spin-based devices in which spin is only used as an internal variable and spin-to-charge conversion is still required at terminals, a novel spintronic device concept that uses spin in every stage of its operation has been proposed by Prof. Datta's group. The key idea is to translate information from nano-magnets into a distinct spin state and then to use this spin state to impact the magnetization of a second nano-magnet. In this way information gets transferred and stored, making this All-Spin approach suitable for both logic and memory applications. We try to experimentally implement All Spin Logic/Memory devices using multiple layer graphene. Room temperature non-local spin valve and magnetic field assisted spin transfer torque have been demonstrated in graphene. (Images from: ACS Nano, 8, 3584 (2014) & Nano Letters, 13, 5177 (2013))


      

Charge-Coupled Spin Logic (CSL)            

In this new generation spin logic devices proposed by Prof. Datta's group, giant spin Hall effect (GSHE) is used to generate a large spin current in the Write unit, and an electrically isolated but magnetically coupled magnetic tunnel junction (MTJ) serves as the Read unit. Various GSHE materials are being investigated to achieve a large charge to spin current conversion. To couple the Write and Read units, we have demonstrated directional magnetic dipolar coupling in vertical magnet stacks and developed theoretical models for ultra-scaled nanomagnet systems. (Images from IEEE Trans. Mag., 52, 3400207 (2016))


      

2-Dimensional Layered Materials and Devices

Bandgap Engineering

Bandgap engineering is a powerful technique for the design of new electronic and optoelectronic devices. Different from traditional approaches that rely on sophisticated material synthesis systems, we demonstrate that bandgap engineering is feasible in 2D layered materials through electric field control. We show that a bandgap of ~200meV can be opened in bilayer graphene (left figure), while the bandgap of bilayer MoS2 can be reduced at a rate of ~275meV per 1V/nm displacement field (right figure), to the extent that a semiconductor to metal transition can be achieved before the dielectric breakdown limit. More importantly, this spontaneous field-controlled bandgap tuning occurs during device operation, which creates a new platform to design novel electronic devices with dynamic bandwidth. (Images from Nano Research, 8, 3228 (2015) & Nano Letters, 15, 8000 (2015))


    

High Performance Cu-Graphene Hybrid Interconnects

Low Temperature PECVD Growth of Graphene Directly on Scaled Cu Nanowires

Due to sidewall and grain boundary scattering, the resistivity of Cu at small dimensions increases rapidly, which leads to an increase in RC delay for aggressively scaled Cu interconnects and starts to negatively impact the overall system performance. We have demonstrated a novel Cu/graphene hybrid nanowire system that outperforms Cu interconnects in terms of electrical and thermal conductivity. Passivation of the Cu surface states with a 2D layered material like graphene gives rise to partially elastic surface scattering and results in enhancement in electron transport through the nanowire. We achieved 15% higher electrical conductivity, 27% higher thermal conductivity and 40% larger breakdown currents in Cu/graphene hybrid nanowires compared to pure Cu wires with the same dimensions. More importantly, the perfect sp2 bonded graphene layer serves as an ultra-thin Cu diffusion barrier that outperforms conventional Ta based barriers. (Images from Nano Letters, 15, 2024 (2015))


    

Graphene Optical Meta-materials

            Collaborator: Webb¨s group

Metamaterials are artificial materials engineered to have properties that may not be found in nature. We are particularly interested in fabricating graphene based metamaterials without sophisticated lithography. The Webb group presented the idea of a graphene stack metamaterial, and analysis suggests that it is the blackest broadband material. The Chen group experimentally realizes such a stack and measures its optical properties. (Images from APL, 106, 061102 (2015))