Molybdenum Sulfide based electronics. Gotthard Seifert Physikalische Chemie,Technische Universität Dresden, Germany

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2 Molybdenum Sulfide based electronics Gotthard Seifert Physikalische Chemie,Technische Universität Dresden, Germany

3 Early studies 1960-ies R. Fivaz, E. Mooser Mobility of Charge Carriers in semiconducting layer structures Phys. Rev. 163 (1967) 743 high purity mobilities MoS /MoSe 500 cm /Vs T 00 K 1980-ies Solar cells (Kautek, Gehrischer, Tributsch ) Heterojunctions (Bucher )

4 Layered Structure H-MoS Space group: D 4 6h P6 3 /mmc Sulfur Molybdenum Sulfur Sulfur Molybdenum Sulfur van der Waals E B (van der Waals) 0.06 ev/atom E B (van der Waals - Graphite) 0.01 ev/atom

5 MoS : formally Mo 4+ S - MoS 6 8- z Trigonal prismatic D 3h y x Ligand field splitting e d xz, d yz e d xy, d x -y a 1 d z Cluster

6 Triple layer S-Mo-S E conduction band E F d xy, d x -y d z Mo-d Valence Band Mo-d S-p d z

7 MoS Semiconductor Band gap direct ev Band gap indirect ev Work function ev Effective mass m ~0.5 * 1 1 d ε = * m dk * L.F. Mattheiss, Phys.Rev. 8 (1973) 3719

8 MoS Semiconductor strong anisotropy in conductivity σ ~ 10-3 σ σ ~ 0,6-7,9 S/cm Charge carrier mobility: 500 cm /Vs nominally undoped :n-mos sulfur defects

9 Electronic properties of MoS no defect - S defect

10 Electronic properties of MoS Density of States Density-of-States vacancy state n-doping Energy/eV - S defect

11 Periodic Table

12 Electronic Configuration Nb: d 3 s Nb 4+ : d 1 Mo: d 4 s Mo 4+ : d Re: d 5 s Re 4+ : d 3 E Rigid Band Model E E conduction band conduction band conduction band d z d xy, d x -y E F d xy, d x -y d z E F d z E F d xy, d x -y Valence Band Nb-d S-p Valence Band Mo-d S-p Valence Band Mo-d S-p

13 Periodic Table

14 Periodic Table

15 Doping of semiconducting MoS Analogy to Si: Si P n-doped Si Si B p-doped Si Mo Re n-doped MoS? Mo Nb p-doped MoS?

16 Densities of states (DOS) for MoS layer Mo doped by Nb hole creation p-doping E F E F Nb4d 10x10 supercell 363 atoms 10xMo 1xNb 4xS ~1% doping

17 Densities of states (DOS) for MoS layer Mo doped by Re donor level n-doping E F E F Re 5d 10x10 supercell 363 atoms 10xMo 1xRe 4xS ~1% doping

18 Electronic Devices? High mobility Chemical stability Low processing temperature High purity High availability Appropriate band gap Flexibility Doping

19 Appl. Phys. Lett. 84, 3301 (004) Diss. R. Zeis (005) US-patent (007) But: High doping level On/off ratio 10 4 T 150 K

20 Two-dimensional atomic crystals K. S. Novoselov et al. PNAS (005) Graphene MoS layer double-layer of MoS

21 Band structure MoS hexagonal lattice a=b=3.3å, c d xy, d x -y T.S. Li, G.L. Galli, J.Phys.Chem. C111 (007)1619 d z L.F. Mattheiss, Phys.Rev. 8 (1973)3719

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23 Mean free path ~ 10 0 nm drift diffusion model Φ= 111meV µ = 50 cm Vs onset : x10 11 cm -

24 Contacts - Schottky Barrier contact semiconductor Φ Φ Small Schottky Barrier? Au MoS

25 MoS Au contact DFT calculations Geometry Binding Energy I. Popov, G. Seifert, D. Tomanek, PRL 108, (01)

26 Electronic structure at the interface Charge density (States: E F -0.1eV < E < E F +0.1 ev) Potential Schottky- Barrier

27 Design of Contacts: Au Ti

28 Electronic structure at the interface Charge density (States: E F -0.1eV < E < E F +0.1 ev) Potential MoS - Au Schottky- Barrier MoS - Ti

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31 Molybdenumdisulfide surfaces Dangling bonds! Edge states! Inert (van der Waals) surface

32 MoS zig-zag edge

33 Infinite layer MoS zig-zag stripe

34 Infinite layer MoS zig-zag stripe gap edge states bulk states

35 MoS zig-zag stripe wave functions

36 Infinite layer MoS arm-chair stripe gap edge states bulk states

37 Transport properties of MoS nanoribbons: edge priority E. Erdogan, I.H. Popov, A.N. Enyashin and G. Seifert Eur. Phys. J. B (01) 85: 33

38 MoS Nanotubes

39 MoS Nanotubes I-V curves (Landauer formulae)

40 Conclusions MoS semiconducting with gap 1eV p- and n-type doping (Nb; Re, I) Sulfur-shell with small defect concentration resistant/robust against humidity, oxidation etc. Sulfur-shell can mediate the binding to leads (Ti) small contact resistance

41 Challenges - Outlook Understanding/Tuning Schottky barrier Influence of edge properties p- and n-type doping (Nb; Re) Doping level <-> mobility! Gate control Atomistic device simulations New devices: - Nanotube based devices no edge effects! - CD waves, superconductivity (NbS, TaS )

42 Thanks Ezgi Erdogan (Dresden) Igor Popov (Dublin) David Tomanek (E. Lansing) Aldo di Carlo (Rome) Alessandro Pecchia (Rome) Financial Support NSF-NSEC European Research Council

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