ECE 492 Future Electronic Devices from Condensed Matter Physics Topics Prof. Stephen Wu [email protected] OH: F 9-10 AM HPN 340 http://labsites.rochester.edu/swulab/teaching/ece-492-course-materials/ Lecture 5:New 2D materials for single layer electronics Last time… We explored carbon based electronics • Graphene • Carbon Nanotubes Last time… Some problems: Graphene Carbon Nanotubes • No bandgap • Cannot easily synthesize one type of CNT • Low on-off ratio • Cannot orient them on a • Can’t compete substrate against HEMTs Slowly making progress as evidenced by our presentation papers New 2D materials revolution Transition metal Phosphorene Silicene dichalcogenides Hexagonal Boron Nitride Bi2Se3 Borophene Outline • Transition metal dichalcogenides • Introduction to family • Transistors • Optoelectronic properties • Valleytronics • Heterostructures • Xenes • Introduction to family • Growth • Properties Transition metal dichalcogenides (TMDCs) Chalcogens Transition metals M=Mo, W, Ta, Nb, … Early successes: MoS2 TMDC= MX2 X=S, Se, Te, … Transition metal dichalcogenides (TMDCs) 2H 1T Source: Wang, et al., Nature Nanotechnology (2012) Kappera, et al. Nature Materials (2014) Lin, et al. Nature Nanotechnology (2014) TMDC Library and Phases Metallic/Superconducting/Charge density wave Metallic Semiconducting NbS2, NbSe2, TaS2, TaSe2 TaTe2, NbTe2 MoS2, WS2, MoSe2, WSe2, MoSe2, MoTe Source: Wang, et al., Nature Nanotechnology (2012) 2 Isolation and growth Exfoliation Wet processing Chemical Vapor Deposition Transfer Process Source: Shanmugam, et al., Nanoscale (2012) Van der Zande, et al., Nature Materials (2013) TMDC Transistors Source: B. Radisavljevic, et al., “Single Layer MoS2 Transistors”, Nature Nanotechnology 6, 147 (2010) TMDC Transistors pros and cons Advantages Disadvantages • Flexible • Trade off is poor mobility • Transparent • Can’t compete with Si mobility • Ultra scalable due to 2D nature • Sensitive to contacts and adsorbates • Higher on-off ratio than graphene Powerful method to create flexible transparent electronics TMDC optical properties Indirect to direct gap transition TMDC optical properties Indirect to direct gap transition Source: Ellis, et al., Applied Physics Letters (2016) TMDC optical properties Photoluminescence spectroscopy Source: Mak, et al., Physical Review Letters (2010) Valleytronics TMDC valleys and pseudospin Similar band structure to graphene except gapped and Spin split due to spin orbit coupling Valleytronics Exploit the valley degree of freedom for electronics 1 0 Originally an idea from 3D materials: Valleytronics Optical valley access Can pump one K or K’ valley and generate finite valley polarization Source: Zeng, et al., Nature Nanotechnology (2012) Van der waals Heterostructures Semiconductor heterostructures TEM Example Heterostructure Epitaxially grown using MBE Van der waals Heterostructures VdW heterostructures Van der waals Heterostructures Assembly Source: Liu et al., Nature Reviews Materials (2016) Van der waals Heterostructures devices Encapsulated 2D materials & graphene contacts Source: Cui, et al., Nature Nanotechnology (2015) Van der waals Heterostructures devices 2D diodes Details saved for presentation Source: Lee et al., Nature Nanotechnology (2014) What else is out there? 1T-TaS2 analogy to VO2 transistor VO2 1T-TaS2 Electron interactions dominate behavior 1T-TaS2 FET Make another Mott transistor? Details also saved for presentation. Source: Yu et al., Nature Nanotechnology (2015) Xenes 2D X-ene, X= Group IV materials Xenes Group IV Non Group IV Silicene, Germanene, Stanene,… Borophene, Phosphorene,… Ge P Si B Source: Molle, et al., Nature Materials (2017) Epitaxy and device integration Molecular beam epitaxy is the method of choice for growing these new materials Typically not grown on insulating substrates Scanning Tunneling Microscopy (STM) 2D materials grown my MBE are examined using STM Tunneling current depends on distance to sample Tip is scanned across area Graphite STM image STM images of different Xene materials Silicene Germanene Borophene Silicene Transistor Details saved for presentation Source: Tao, et al., Nature Nanotechnology (2015) Phosphorene Transistors Higher mobility exfoliated material Phosphorene Degradation • Looking for air stable analogs: SnS, SnSe, GeS, and GeSe • Work on encapsulation Source: Kim et al., Scientific Reports (2015) 2D electronics outlooks • Vibrant field with huge potential • Number of materials systems • Ease of experiments • Amount of interesting new phases and exotic behaviors • Flexible transparent electronics • Fast moving field • Different challenges for different systems but lots of ongoing oppurtunity Next time: Spintronics.