Single-Crystal Organic Field Effect Transistors

Michael Gershenson Rutgers University V. Podzorov S. Sysoev E. Loginova V. Pudalov

Supported by Outline:

• Organic field-effect transistors (OFETs) as a tool for studying charge transport in molecular crystals.

• Performance of the thin-film OFETs

• Single-crystal OFETs: challenges and tricks

• The rubrene single-crystal OFETs: first results

• Transport of surface charge carriers in organic molecular crystals: unsolved problems Field-effect doping and organic FETs

Fundamental academic interest: • many isolators can be “field-effect doped”; the “doping” level in OFETs might be higher than that accessible with chemical methods; • potentially, OFETs could be a very important tool for studying strongly-correlated 2D systems, 2D superconductivity, etc.; • OFETs also can help to solve the puzzle of the crossover “polaronic transport Û band transport” in organic crystals. Applications of OFETs: (potentially) inexpensive, flexible, light-weight, suitable for large area applications Thin-film OFETs Uniqueness of organic molecular compounds – in an intrinsically low density of charge traps at the interface, due to a weak, van der Waals bonding of molecules.

The thin-film OFET performance depends crucially on the film structure: presence of different phases, inter- crystalline boundaries, etc. reduces SEM micrograph of a pentacene film drastically the carrier mobility, and results in the other undesirable effects

(e.g., dependence of m on Vg and VSD).

The best characteristics are realized in the devices where a thin organic film is deposited in high vacuum on an oxidized Si wafer (this compromises the whole idea of a cheap and flexible OFET). Current status of the thin-film OFETs

On one hand, a decade of the research in thin-film OFETs has produced impressive results: the performance of the best thin-film OFETs is comparable to the a:Si FETs. In particular, for the best thin-film OFETs, m ~ 1 cm2/V·s.

On the other hand, the further progress is limited by the morphology of organic films: orientational molecular disorder and inter- crystalline boundaries affect the mobility and other characteristics.

In this situation, it makes sense to explore the single-crystal OFETs, as an “ideal” system which is free of the bulk defects. The price considerations are not an issue in this case (the best thin-film OFETs are not cheap either…).

However, fabrication of the single-crystal OFETs is still a challenge... Challenges of the single-crystal OFET fabrication (or why it took almost two years…)

Epitaxial growth of single-crystal organic films has not been developed yet. At present, the only option is to form FETs on the surface of pre-fabricated organic molecular crystals. However, these crystals are very vulnerable and, very often, incompatible with the standard processes of thin-film technology (sputtering, photolithography, etc.).

Technological stages: • purification of the starting organic material and growth of suitable organic crystals: “molecularly-flat” facets with a low density of charge traps • deposition of the low-resistance source and drain contacts • formation of the disorder-free interface between the organic crystal and the gate insulator.

The OFET should be compatible with low-T measurements. Organic molecular crystals

Organic molecules carry carbon 2pz electronic wavefunctions (conjugated p-electron systems).

Usually, the van der Waals-bonded low molecular weight crystals are rubrene aromatic molecules deriving from the benzene ring (e.g., the naphthalene family).

From the viewpoint of the charge transport and optical properties, TCNQ these crystals are wide-band semiconductors PVT crystal growth and purification

belt heater (T1) cold water (T2) H2

TC Impurities affect strongly the electrical properties of organic Temperature profile 200 crystals (similar to inorganic semiconductors). 150

C) The impurity concentration has to 0

(

T 100 be reduced down to or even below the ppm level (1 ppm = 10-6 50 molecular fractional content): very 0 10 20 30 40 50 slow growth and multiple re-growth x (cm) cycles. Molecular packing in organic crystals: tetracene

The crystal structure of tetracene, c-axis the so-called herringbone packing.

Note that the overlap of p-orbitals is small

a-b plane, flat crystal facet Rubrene

The overlap of p-orbitals seems to be greater than in tetracene, and the axes of the molecules are in the plane of layers – relevance of this fact to a larger mobility in rubrene FETs remains to the a-b plane be clarified… “Bulk” charge transport in organic crystals

naphthalene a - perylene The crossover from the polaronic transport at higher temperatures to the band transport at lower temperatures: “upon increasing temperature, increased phonon scattering slows down band transport, whence the local polarization interactions increase and, hence, the effective mass grows, in conjunction with band narrowing. Finally,…the thermally activated polaron-hopping transport becomes more efficient and takes over”. N. Karl, Synt. Metals 133-134, 649 (2003) N. Karl et al., J. Vac. Sci. Technol. A 17, 2318 (1999). “Bulk” charge transport along the c-axis: the purity test

The I-V characteristic for rubrene (current flows in the direction perpendicular to the facets)

trap free regime 10-3 2 TF jTF = 9/8em(1/d)(V/d) -4 2 10 at V > V µ N d 10-5 TF traps

-5 -7 10 VTF 10 VTF

(A) 300 400

I -9 10 SCLC space-charge Ohmic regime -11 10 Ohmic limited current j = emp (V/d) V 0 W j = 9/8em(Q/d)(V/d)2 10-13 10-1 100 101 102 103 V (V)

15 -3 The density of charge traps can be estimated from VTF: Nt ~ 10 cm Metallization of contacts Two major issues: (a) A metal should be a good injector of charge carriers (e.g., in the case of hole conductivity in OFETs on tetracene, rubrene, etc., gold and silver work well, while aluminum does not work at all) (b) The resistance of the Schottky barrier at the organic/metal interface depends strongly on a method of contact fabrication. The rule of thumb: the less “invasive” the method, the better…

Examples of fabrication techniques, which result in a low contact resistance: • “rubber-stamp” deposition of gold • water-based solution of colloidal graphite forms low-resistance contacts

Thermal deposition of metals requires extreme care for two reasons: • the thermal load on a crystal should be minimized (large crystal-source distance, crystal cooling) • even minor contamination of the channel surface by deposited metal reduces drastically the field effect. Gate insulator problem

Schon claimed that the conventional sputtering of Al2O3 worked well…. Sputtering “kills” the field effect almost instantaneously, because of bombardment of the surface by charged particles. The field effect vanishes in plasma even if the crystal is placed in the region of shadow, where no material is deposited on the surface… All our attempts to protect the organic surface by applying a bias voltage, experimenting with the deflecting electrodes, etc. failed, and, after a year, we gave up. Thermal deposition of SiO also has not produced any promising results…

As a “quick and dirty” test for the field effect (e.g., for characterization of organic crystals), we used a 1-mm-thick aluminized mylar film – a loop made of such a film was pressed gently against the surface of a molecular crystal. Parylene solution ( pin-hole free conformal coating)

Parylene is the common name for a unique family of thermoplastic polymers used in the conformal coating process.

This vapor deposition process: • a raw material (dimer) is heated in a vacuum system until the material sublimes to form a gaseous monomer. • The gas of monomers flows into a deposition chamber containing the substrate to be coated. •At the substrate surface, the gas converts into a solid polymer state, and forms of a completely conformal transparent polymer film. Parylene deposition

Sublimation Pyrolysis Polymerization 100 oC 700 oC 25 oC

To LN2 trap and pump

P ~ 0.1 mbar Breakdown characteristics

10-7

10-8

10-9

-10

(A) 10

leak -11 I 10 SiO

10-12 Al2O3 Parylene 10-13 103 104 105 106 107 108 E (V/cm) OFET on tetracene

The OFET is “dead on arrival” – because of

the Al2O3 gate insulator, but the photo shows all essential elements. Double-gate OFET on rubrene

TOP VIEW

SIDE VIEW 2-probe and 4-probe configurations

The resistance of the Schottky barriers at the metal/organics interface is large, and it depends

on both Vg and VSD. To eliminate the contact effects, we used the 4-probe configuration:

V probes

Conventional 2-probe measurements Transistor characteristics of rubrene OFETs

1.5

d ISD/dV g 1.0 A) m

(

SD 0.5 I

0.0 -100 -75 -50 -25 0

V g (V)

s2D = en2Dm

en2D µ CVg

m µ (¶ISD/¶Vg)/VSD

Unipolar FET with the hole conductivity Can we trust the 2-probe measurements?

The resistance of Schottky barriers at the interface metal/organics diminishes with the increase of the

carrier density (large |Vg|).

At room temperature, the contact resistance can be made negligibly small, provided a sufficiently large

Vg is applied.

With cooling, however, the contact resistance increases significantly… Contact effect in 2-probe measurements

The contact resistance is 1.0 also a function of the 0.5 4-probe source-drain voltage: it decreases with increasing 0.8 max V . For this reason, in

m SD ·cm)

0.4 /

W the low-T measurements, m 0.6 2-probe a large VSD is usually

(M 0.3 applied.

W 0.4 10 20 30 40 50 The “apparent” carrier 0.2 V (V) contacts SD mobility in the 2-probe R measurements depends 0.1 on VSD, while the “4- 10 20 30 40 50 probe” mobility is almost VSD-independent. VSD (V) Threshold-free operation of rubrene OFETs

Application of VSD in the presence of the gate electrode -4 2 only 1 mm away from the 10 m = 7.5 cm /Vs interface creates a strong -5 10 electric field normal to the 10-6 crystal surface, and induces propagation of the conducting -7 10 channel from source to drain.

2 -8 (A) m = 3.55 cm /Vs The onset of the field-effect 10 50 80 V SD conductivity – at I -9 25 10 V onset = V . 10 g SD -10 10 5 threshold-free operation Û V = 1 V 10-11 SD the density of charge traps is -75 -50 -25 0 25 50 75 100 very low for rubrene facets, at least at T = 300K (for tetracene and TCNQ, the threshold is non- Vg (V) zero even at room temperature).

Two-probe measurements at room temperature (almost) Vg-independent mobility

-4 2 10 m = 7.5 cm /Vs 10-5 10-6 8 V = 80 V -7 SD 10

2 V = 50 V

(A) -8 m = 3.55 cm /Vs SD 10 80 V 50 SD

I 25 10-9 VSD = 25 V 10 -10 6 10 5 VSD = 10 V V = 1 V 10-11 SD V = 5 V -75 -50 -25 0 25 50 75 100 SD /Vs)

V (V) 2

g 4

Roughly, m does not (cm m 2 depend on the hole density. 0 For comparison, in the -75 -50 -25 0 25 50 75 100 thin-film OFETs, m V (V) typically varies by a g

factor of 10 with Vg. Room-temperature performance of rubrene FETs

• Mobility – approx. 10 times greater than in thin-film OFETs and a:Si MOSFETs,

does not depend on n and VSD.

• Threshold – none

• On/off ratio – can be > 107.

• Sub-threshold slope – an order of magnitude better than in thin- film OFETs and a:Si MOSFETs. What happens with cooling? Apparent increase of the threshold – mostly due to the increase of the 50 2 T = 300 K contact resistance at m = 5.87 cm /Vs T = 275 K 40 T = 250 K low T. T = 225 K T = 200 K The mobility changes 30 T = 175 K insignificantly with T, if A) T = 150 K m m is large enough at 20 T = 125 K ( T=300 K (³ 1 cm2/Vs). SD I

10 VSD = 50 V Note that a weakly 0 temperature-dependent mobility in a 2D system -100 -80 -60 -40 -20 0 20 40 60 80 100 with Rp >1 MW is a non- trivial fact – still Vg (V) “classical” transport…

2-probe measurements m(T) in thin-film OFETs

The field-effect mobility in OFETs based on highly- oriented pentacene films.

Nelson et al., APL 72, 1854 (1998). Double-gate FET vs. single-gate FET

0.4 50 V = 5V, 2 T = 300 K SD m = 5.87 cm/Vs T = 275 K

0.3 VGC = -50 V 300 K 40 T = 250 K 275 K T = 225 K T = 200 K 250 K 30 T = 175 K

0.2 A) 225 K T = 150 K A)

m m 200 K 20 T = 125 K (

( 175 K SD SD I I 0.1 150 K 10 VSD = 50 V

0 0.0 -100 -80 -60 -40 -20 0 20 40 60 80 100 -40 -30 -20 -10 0 10 20

Vg (V) Vg (V) What happens with cooling (cont.)

10 The results of more careful, 4- probe measurements: 9 /Vs) 2 • the dependence m(T) is weak 8 and non-monotonic

(cm (competition between two m

150 200 250 300 mechanisms?) 20

0 • the threshold does increase

with cooling (depopulation of

(V) -20 th the charge traps?) V VSD = 80 V -40

150 200 250 300 T (K) Summary

• We have developed a technique for fabrication of single- crystal organic FETs (crystal growth, low-resistance contact fabrication, gate insulator deposition). The devices are suitable for low-temperature experiments.

• The single-crystal OFETs outperform both the thin-film OFETs and a:Si MOSFETs (mobility, sub-threshold slope, etc.). Comparison between single-crystal and thin-film OFETs helps to identify the characteristics of the latter devices, associated with the structural defects in thin organic films.

• The physics of the charge transport in the OFETs is still a “terra incognita”, which, despite some technical problems, might be a very exciting field in the future… Open issues

• The mechanism of conductivity in OFETs at high temperatures (polarons?), - characterization of the charge carriers (m*, etc.) - the role of molecular packing in organic crystals.

• Realization of a high mobility of surface carriers at low T - observation of a crossover to a “band” transport - quantum effects in the conductivity.

• Non-trivial photoconductivity Electron-injecting contact

metal semiconductor Vac. level j ~ 5 eV (Au) E c E f traps

E v Hole-injecting contact

metal semiconductor Vac. level

j ~ 5 eV E c (Au)

E f traps E v Contacts “metal-organics” metal crystal vac. level j ~ 5 eV (Au)

hole-injecting contact vac. level j ~ 2 eV

electron-injecting contact Commercial Si MOSFET

100 10-2 Si MOSFET 10-4 -6 (A) 10 10 -8 SD

I 10 On/Off = 10 VSD = 1 V V = 0.1 V 10-10 SD 2 -12 10 m > 100 cm /Vs 10-14 -2 -1 0 1 2 3

VG (V)

-4 2.0x10 (A)

SD -4 I 1.0x10 VG = 2 V

0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

VSD (V) Photoconductivity

-8 10 Light ON

Light OFF 10-9

(A) SD I 10-10

Au-SD / TC, VSD=20V, in air 10-11 0 5 10 15 20 25 time (min) n-type conductivity in TCNQ-FET

-2 2 -7 m ~ 5x10 cm /Vs 10

-8 (the best previously 10 reported result for -5 10-9 TCNQ - m ~ 3x10 2

(A) cm /Vs)

SD -10

I 10 VSD = 10 V

10-11

0 50 100 150 200

Vg (V)