Principal Research Results

Charge Dynamics at the Interface of Organic/Inorganic Composites:
Best-performing Organic Field-effect Transistors Featuring Organic
Crystalline Surfaces
Background“God created the solids, the devil their surfaces” (W. Pauli). Since addressed in early 19’s, a number of surface/interface
properties still remain mysteries. On the other hand, we are being profited by technologies such as batteries and catalysts which utilize
the charge motion at/around the interfaces. Therefore, CRIEPI-MSRL believes that microscopic understanding of the interface directly
leads to development of next-generation energy technologies. Among varieties of material combinations, organic/inorganic interfaces
are particularly promising, noticing sensitivity and controllability of the former and structural stability of the latter. We extract attractive functions of the unique composite by employing advanced techniques of charge-transport measurements established in our laboratory.

Objectives
To develop a method for fabricating ideal organic/inorganic interface making use of self-organization of the organic molecules, and to construct high-performance field-effect transistors with the self-organized high-quality interface.

Principal Results
1. Development of a method to fabricate an ideal organic/inorganic interface
In the newly developed method shown in Fig.2a, organic crystals are independently grown (Fig.1) and are softly attached bynatural electrostatic force on SiO2/doped Si substrates, whose surfaces are coated with organosilane self-assembled monolayers ＊ 1.
Minimizing damages on the crystal surface, the soft lamination technique provides flat and homogenious organic/inorganic interfaces,
which were inaccessible by conventional methods such as vacuum evaporation.
2. Pioneering measurements of charge transport at the surface of organic crystals with field-effecttransistor structures
In the field-effect transistors (FETs), charges are induced at the interface as shown in Fig.2b when the gate voltage is applied
to the SiO2 dielectric (just as the mechanism of capacitors). To examine quality of the single-crystal interface, we have developed a“four-terminal” measurement of the charge conductivity induced at the crystalline surface. The pioneering measurement revealed thatamount of interface defects in the single-crystal FETs are much less than that in normal polycrystalline-based FETs. Pronounced mobility 9 cm2/Vs of the interface charge conduction is realized as a result.
3. The best switching performance as a consequence of minimized interface defects
The best FET performance is achieved with a particular organic compound called Rubrene; on-off switching property due toopening of the positive charge-conduction channel (negative charge cannot transport in this system) is improved by 3 times as compared to best organic devices ever reported, owing to the minimized defect density as small as 1011 (a patent submitted). The result indicates that the self-organization of the organic interface is a promising strategy to have high-performance FETs, as well as to develop
low-loss devices for energy conversion in the future.

Future Developments
In addition to the present study of the charge dynamics in parallel to the interface, we will extend the measuremnt to thecharge transport across the interface. Microscopic elucidation of electronic and ionic conduction through metal-organic interfaces
gives an indication for developing a new energy storage and/or conversion devices.
Main Researcher: Jun-ichi Takeya, Ph.D.,
Research Scientist, Materials Physics and Synthesis Sector, Materials Science Research Laboratory
Reference“Field-induced charge transport at the surface of pentacene single crystals: a method to study charge dynamics of two-dimensional
electron systems in organic crystals”, J. Takeya et al., Journal of Applied Physics 94, 5800 (2003);“Effects of polarized organosilane self-assembled monolayers on organic single crystal field effect transistors”, J. Takeya et al.,Applied Physics Letters 85, 5078 (2004).
＊1：organosilane self-assembled monolayer: lengthy molecules such as CH3(CH2)7(CH2)2Si(OC2H5)3, whose alkane chains are terminated with asilane group. The silane group chemically bonds the SiO2 surface to form self-ordered film with the thickness of one-molecular length. The
technique is actually applied to treat a glass surface to hydrophobic, for example.
＊2：Rubrene: An organic molecule (C42H28) used for yellow electroluminescence.

11410. Advanced Basic Technologies - Material science

Organic raw material

Organic single crystal

Argon gas flow
Temperature~ 7μm

Rubrene crystal

positionFig.1 Physical Vapor Transport: a growthtechnique of organic single crystals
In the presence of temperature gradient of 200-300℃, organic
molecules are sublimed at the higher-temperature right-handside, carried by Ar gas, and crystallize at the lowertemperature left-hand side of the ftube furnace.

~ 30μmGold electrode(a)
Organic single crystal

SiO 2

Fig.3 Top-view photo of the rubrene-single
crystalSelf-assembled monolayer
Gold electrode-6

100.25~ 0.4 V/ decade

0.2

SiO 2

0.15

Doped Si

0.1-710-810-910(b)0.05-10Charges induced by gate electric field100Gate voltage+++++-8+Fig.2 (a) Newly developed method tofabricate single-crystal field-effecttransistors. (b) Mechanism of thecharge accumulation by the gatevolatge application. Front view of thedevice depected in (a).
Charge is accumulated at the surface of the organic
crystal in proportion to gate voltage VG, so that theinterface current between the gold electrodes isamplified with V G (field-effect). The extent of thecurrent enhancement is determined by molecular-scale
flatness of the interface.

-6-4-202Gate voltage (V)Fig.4 Outstanding switching performance of
rubrene crystal/self-assembled
monolayer/SiO2 structure. Blue dots areplotted in a linear scale, and red are inalogarithmic scale.
With application of gate voltage, the transistor switches fromoff- to on-state with the slope of ~ 0.4 V/decade, demonstrating
the highest-class organic FET performance. The steepness
indicates successful fabrication of a near ideal interface of theself-organized interface. The steepness of the switch isessential in low-power applications such as for logic circuitcomponents.

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