Electronic transport through Single Organic Crystals

Transcription

1 Electronic transport through Single Organic Crystals Alberto Morpurgo The Delft Team * R.W.I. de Boer * A. Stassen * N. Iosad Collaborations * M.E Gershenson * N.Karl * T.T.M. Palstra

2 Outline Introduction organic thin-film transistors time-of-flight on single crystals dc Transport through organic single crystals growth and characterization FET fabrication Transport through FETs Conclusions

3 Field Effect Transistors source gate electrode gate dielectric drain pentacene thin film FET Conducting layer Molecular material Schoonveld et al Nature 2000

4 Mobility of Charge Carriers Thin film FETs Polymers and oligomers Dimitrakopoulos & Malenfant 2002

5 Best organic thin-film FETs mobility µ = 1-3 cm 2 /Vs Nelson 1998 Identically prepared devices behave differently Go beyond thin-films

6 Intrinsic Transport Properties Single crystals Time-of-flight data N. Karl 85 µ ~ 1 cm 2 /Vs Zone-refined molecules µ ~ 1 cm 2 RT dµ/dt < 0 µ anisotropy

7 Molecular Crystals Anthracene Tetracene Pentacene Perylene Rubrene

8 Crystal growth Important: Growth process also purifies the molecules

9 1st Growth Purification by sublimation: Tetracene 2nd Growth Re-growing crystals => Less Impurities

10 Tetracene single crystals 1 SCLC + TOF Characterization Mobility (cm 2 /Vs) * µ ~ 1 cm 2 room T * dµ/dt < 0 metallic-like T dependence Temperature (K) Mobility (cm 2 /Vs) * Structural phase transition at K Consistent results T (K)

11 Electrostatic bonding Compatible with any insulating layer: e.g., high-k dielectrics

12 Rubrene Electrostatically bonded Rubrene single crystal FET

13 Rubrene/ Tetracene crystal FETs Drain Source 20 µm µ = 6 cm 2 /Vs I sd (ua) Delft Single Organic Crystal FETs V sd (V)

14 Mobility-anisotropy in Rubrene FETs Effective Mobility (cm 2 V -1 s -1 ) Figure 3a: Rotation experiment v0803 sample Impossible in thin films log (Peak Current) First experimental observation in 1/T (K) FETs Figure 3b: Temperature dependence in orthogonal direction Rogers/Gershenson Activation energy along b-axis = 83 K to appear in Science Activation energy along a-axis = 400

15 FET fabrication on top of crystal Gate electrode Source Gate insulator Drain Organic Crystal FET Device Interface quality?

16 Gershenson 2003

17 Rubrene FETs with Parylene Gate Insulator Gershenson 2003 RT up to 15 cm 2 /Vs

18 Insensitivity to processing FETs fabricated on top of crystals with parylene gate insulator Pentacene µ = 0.5 cm 2 /Vs Limited by purity contacts Rubrene µ = 4 cm 2 /Vs µ = 15 cm 2 /Vs Rutgers

19 Metal/Organic interface Tetracene crystal Evaporated Contact Bonded Contact 10-4 Substrate 10-6 Contact fabrication Introduces surface traps I (A) bonded contact injecting evaporated contact injecting Extrinsic Effects V (V)

20 Overview of µ(t) in single crystal FETs Pentacene Rubrene 10 µ (cm 2 /Vs) Tetracene T (K) µ (cm 2 /Vs) Non-monotonic behavior often observed in single crystals T (Kelvin)

21 High mobility in pentacene J (A/m 2 ) a L c * b µ from SCLC: no high-µ single crystal FET yet pentacene single crystal E (V/m) in-plane Space charge limited current I-V characteristics O.D. Jurchescu (Palstra group/groningen) µ (cm 2 /Vs) T (K)

22 Conclusions Technological advances * Different single crystal FET fabrication techniques * Reproducibility Measurements through single crystals * record mobilities * signatures of intrinsic properties * new relevant molecules Upcoming work * metal/organic interface * chemical purity (zone refinement) Rapid developments: Single crystal FETs seem suitable for fundamental studies of organic semiconductors

23 Rubrene vs Tetracene Rubrene Tetracene Non-planar side groups

24 π-orbital overlap Expected: Better in rubrene than in polyacenes High µ Herringbone Low µ structure Crystal structure vs Polaronic effects? Still purity limited? µ ~ 10 RT (Palstra last month)

25 How can we check? Alkyne-substituted Pentacene Similar to Rubrene Collaboration with J. Anthony Same crystal structure