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1 ASIAA/CCMS/IAMS/LeCosPA/NTU Phys Joint Colloquium 30 Oct 2012, NTU, Taipei, Taiwan Novel Materials and Devices for Millimeterwave and THz Applications Integrity Service Excellence Jim Hwang Program Officer, GHz THz Electronics AFOSR/RTD

2 DoD Basic Research Enterprise Total FY12 = $2.12B

3 THz Gap D. Woolard (2005)+B. Williams (2012)

4 Introduction Compact, high power and room temperature THz source has been a dream for decades Three terminal devices are needed for wideband modulation/tuning with low noise Transistors are compact and easy to integrate New devices usually come from new material and new physics This talk will focus on new materials

5 New Semiconductors Conventional Covalent Semiconductors Si, GaAs, InP, GaN Heterostructures of 2D Layers graphene, silicene, germanene, 2D boron nitride Ionic Semiconductors Transparent Electronics: ZnO, MgO, InGa 3 Zn 5 O 5 Heterojunctions: MgZnO/ZnO, LaAlO 3 /SrTiO 3 Multiferroics: BiFeO 3, EuO Metal Insulator Transition: VO 2, SmNiO 3, NdNiO 3 Topological Insulators: Bi 2 Se 3, Bi 2 Te 3 Chalcogenides: sulfides, selenides, tellurides

6 THz Electronics X tal Reliability AFOSR Sub-millimeter-wave radar & imaging Space situation awareness Chemical/biological/nuclear sensing Ultra-wideband communications Ultra-high-speed on-board & front-end data processing DARPA ONR III-N THz ONR DEFINE DARPA (Power) Intel IBM THz (Speed) Cutoff Frequency J. Albrecht (2009)

7 Intel s High k FinFETs Production Development S Gate Stack I Qv V Q Source e Channel Drain Q V k d C 0

8 Covalent Semiconductors 3D 2D heterostructures of graphene, silicene, germanene, boron nitride NSF/AFOSR Workshop on 2D Materials and Devices Beyond Graphene, May

9 Heterostructures of 2D Layers Graphene beautiful but difficult to deal with Best to mate graphene with other 2D layers through van der Waals force Graphene conductor, 2D BN insulator, 2D MoS 2 semiconductor, 2D NbSe 2 superconductor Bandgap of MoS 2 transitions from being indirect in bulk to direct in 2D Free of epitaxial strains, 2D heterostructures can do more wonders than 3D heterostructures 3D VCSEL Heterostructure h BN/Graphene/h BN

10 Electronic Properties of 2D MoS 2 F. Schwierz (2010)

11 Challenges for 2D Heterostructures While new 2D layers continue to be synthesized, 2D heterostructure is in infancy Only graphene grown on BN substrate have comparable properties to that of exfoliated graphene Limited success for growing 2D BN on graphene without metal catalyst Little theoretical understanding/prediction of properties of 2D heterostructures

12 Funded Projects A. Geim & K. Novoselov, Manchester U., Graphene Based Heterostructures and Vertical Transistors, 9/1/2011 8/31/2016. M. Snure & Q. Paduano, AFRL/RY, Growth of Wafer Scale Hexagonal Boron Nitride and Graphene for THz Electronics, 10/1/2012 9/30/2015. A. Kiefer & B. Clafin, AFRL/RY, Strain Induced Band Gap Modification of α Sn (Grey Tin), 10/1/2012 9/30/2015.

13 Graphene BN Superlattice A. Geim & K. Novoselov (2012) Bright Field STEM Dark Field STEM 2 nm 2 nm

14 Van Der Waals Epitaxy A. Koma (1984) MBE VDWE VDWE on 3D Substrate

15 Fundamental Studies of 2D Material Synthesis/Modeling/Characterization/Prototype Innovative growth techniques for uniform and reproducible growth of 2D heterostructures with precise control of purity and stoichiometry Theoretical tools for predictive modeling/simulation of properties of 2D materials and their interactions with the environment and other 2D materials including edge effects Advanced characterization techniques and structureproperty correlation for 2D materials and interfaces Demonstration of 2D heterostructure devices with unprecedented performance 2 3 interdisciplinary teams each at ~$1M/yr

16 Other Important Challenges Growth of 2D materials on top of graphene or other 2D materials with controlled doping/edge effects and without metal catalysts Understanding/demonstration of correlated transport in 2D heterostructures Understanding/demonstration of electrical/thermal contacts to 2D layers Nanocomposite properties of 2D materials 1 3 smaller teams each at ~$300K/yr BAA AFOSR scheduled for Feb Additional NSF funding to be announced later

17 Ionic Semiconductors Transparent Electronics: ZnO, MgO, InGaZnO Heterojunctions: MgZnO/ZnO, LaAlO 3 /SrTiO 3 Multiferroics: BiFeO 3, EuO, Metal Insulator Transition: VO 2, SmNiO 3 Topological Insulators: Bi 2 Se 3, Bi 2 Te 3 Sulfides, selenides, tellurides BAA AFOSR

18 Merits of Ionic Semiconductors Less demanding on crystalline perfectness Deposition on almost any substrate at low temperature Radiation hard, fault tolerant, self healing High electron concentration with correlated transport Wide bandgap for high power and transparency Multiferroics, metal-insulator transition, topological effects SWAP-C and conforming Challenges Composition and purity control Transport not well understood Mobility Covalent Semiconductor Oxygen 2p-orbital Ionic Covalent Ionicity Ionic Semiconductor

19 High T Superconductors A. Geim (2012) Conductive CuO Planes Insulating Spacers Conductive Graphene Planes Insulating BN Spacers

20 Semiconductor Technologies Correlated Transport Polarization Engineering Polarization Engineering ITRS Roadmap (2009) Bandgap Engineering Bandgap Engineering Bandgap Engineering Field Effect Field Effect Field Effect Field Effect Si GaAs, InP GaN Oxides

21 Power Transistors Figure of Merit S. Rajan & S. Stemmer (2010)

22 Challenges for Ionic Semiconductors Composition control P doping K. Takahashi (2007)

23 Funded Projects Y. Ando, Osaka, Topological Insulators and Superconductors for Innovative Devices, 3/21/2012 3/20/2015. D. Goldhaber Gordon, Stanford, Electrolyte Gating of Correlated Electron Materials and Nanostructures in Complex Oxides, 5/1/2012 4/30/2015. S. Ramanathan, Harvard, Fabrication and Electrical Characterization of Correlated Oxide Field Effect Switching Devices for High Speed Electronics, 6/1/2012 5/31/2017. C. Ahn, Yale, Control of Interface Phenomena in Nanostructured Functional Oxide Heterostructure, 6/1/2012 5/31/2015. J. Levy, Pittsburgh, Spin Orbit Enhanced Functionality in LaAlO 3 /SrTiO 3 Nanostructures, 6/15/2012 6/14/2015. C. B. Eom, Wisconsin, Tunable Metamaterials with Two Dimensional Electron Gas Interfaces, 7/15/2012 7/14/2015. I. Schuller, UCSD, Functional Hybrid Nano Oxides, 7/15/2012 7/14/2015.

24 Conclusion While the speed of conventional covalent semiconductor transistors are approaching THz, power capacity is likely limited. Ionic semiconductor devices may offer speed, power and robustness, especially with correlated transport, metal Insulator transition, and/or topologically protected states. Future of heterostructure materials and devices of 2D layers is wide open.