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

• • 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 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 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: