ResearchLabII_OFET_SS13

7/28/2019 ResearchLabII_OFET_SS13

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Research Lab II

Fabrication and characterization

of organic field-effect transistors

Contact persons:

Paul Mundt Elmar Kersting

Room 156, Tel: 06151/ 16-6331 Room 151, Tel: 06151/ 16-6358

[email protected] [email protected]

Emanuelle Reis Simas Andrea Gassmann

Room 158, Tel.: 06151/ 16-6689 Room 158, Tel.: 06151/ [email protected] [email protected]

Meetingpoint: in front of room 158

1. IntroductionIn the last decade organic field-effect transistors (OFETs) have attracted a lot of interest due totheir potential applications in the field of low cost and/or large area flexible electronic devices.One of the most investigated organic semiconductors for organic transistors is pentacene. Theperformance of pentacene-based transistors is greatly influenced by the morphology and thequality of the pentacene thin film and the trap states at the interface between pentacene and thegate insulator.In this practical course pentacene thin film transistors are fabricated and their electricalcharacteristics are measured. A surface treatment of the gate dielectric is carried out to modifythe surface properties essentially codetermining the channel conductivity.

2. Organic semiconductorsOrganic semiconductors are carbon-based molecules that allow for the injection and transport ofcharge carriers. Due to their conjugated -electron system they feature an optical energy gap inthe range of about 2-3 eV, spanning the energy spectrum from the near ultraviolet to the nearinfrared energy range.Organic semiconductors can be classified into two material classes: small molecules andpolymers. While the electronic properties of both materials are more or less the same, they differwith respect to their processability. Small molecules are typically deposited by physical vapordeposition while evaporation of polymers leads to their decomposition. This is the result of theirhigh molecular weight. That is why polymers are mostly processed from solution using usuallyspin coating or drop casting or common printing techniques.

In a conjugated -system the carbon atoms are sp2-hybridized. The sp2-hybrid orbitals ofneighboring C-atoms form - (bonding) and *-molecular states (anti-bonding), while the


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overlapping residual pz-orbitals lead to the formation of rather weakly bound - (bonding) and*-bonds (anti-bonding). The former result in strong sigma bonding while the latter form ratherweak-bonds. Due to the weak exchange energy of the pz-orbitals the p-states split weakly andthus also the ,*-states determining the highest occupied molecular orbital (HOMO) and the

lowest unoccupied molecular orbital (LUMO). In ideal small molecules electrons in -orbitals aredelocalized over the entire mesomeric system. Yet, the conjugation length can be shortened dueto impurities and molecular imperfections.Due to small intermolecular interactions the energetic splitting occurring during the formation ofan organic solid is low, leading to a band width of less than 300meV. Thus, the energeticdistance between HOMO and LUMO in a molecular solid is very similar to the energetic splittingbetween the- and *-orbitals in a molecule. In each case the -electrons are responsible for theelectronic properties of the respective organic semiconductor. Figure 1 sums up the evolution ofenergetic levels for a molecule build from C-atoms and for a molecular solid.

Figure 1 Schematic illustration of the energy diagram of an atom (left), a molecule (middle) and amolecular solid (right).

2.1.Charge carrier transportDue to their relatively high energy gap the intrinsic charge carrier density in organicsemiconductors is very low (only about 1cm-). Therefore, charge carrier transport has to besupported by excess charge carriers injected into the organic semiconductor from electrodes.

These charge carriers are localized on individual molecules leading to an electronic polarizationof the neighboring molecules. Depending on the distance between the molecules the degree ofpolarization can be different. Consequently, depending on the related polarization the energeticpositions of the HOMO and LUMO states compared to the electronic states of an unchargedmolecule change. Assuming a statistical distribution of the intermolecular interactions thepolarization energies are also distributed statistically leading to a Gaussian distribution of theHOMO and LUMO states. This so-called Gaussian density of states (DOS) distribution isschematically depicted in Figure 2.In organic molecular crystals where the localization of an excess charge carrier on a molecule

prevails, the charge carrier transport is a hopping process from one molecule to another. Inprinciple, hopping transport can be described as successive tunneling processes. In the inset of


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Figure2 three possible hopping occurrences are illustrated according to the Miller-Abrahamsmodel in order to visualize the transport of electrons along the LUMO levels in an organic solid.For transitions1 (isoenergetic tunneling) and 2 (energy dissipation after the isoenergetictunneling), no thermal activation is necessary prior to the tunneling process. Such a thermal

activation, however, is needed for transition3, where a hop upwards in energy is required forthe transport.

Figure2Distribution of the transport states in a molecular solid. Three different hopping transitions areillustrated in the inset. Only for transition3 activation energy is required prior to the tunneling process.

The charge carrier transport in molecular crystals as well as in disordered molecular solids isdominated by a hopping transport for temperatures > 150K. A good structural order of thesolid, as it is found in molecular crystals, is beneficial for charge carrier transport. In particularfor organic field-effect transistors the overlap of the -orbitals has to take place in the directionof charge carrier transport in order to lead to an increase in charge carrier mobility.The charge carrier mobility depends also on the amount of charges contributing to the hoppingprocess: The higher the density of states is filled, the easier the charge carriers can hop from oneHOMO or LUMO level to the next HOMO or LUMO level and the higher is the charge carriermobility.

2.2.Charge carrier trapsEnergetic states in the energy gap above the HOMO or below the LUMO transport levels act ascharge carrier trap states. Once a charge carrier is localized on these states it no longercontributes to charge carrier transport unless it is thermally activated to escape its trap.Nevertheless, trapped charge carriers influence other charge carriers by their electric field.The distribution of trap states can range from monoenergetic states to a random trapdistribution. The origins of semiconductor trap states are of morphological or chemical nature.Morphological traps are usually due to grain boundaries or a local order / disorder of themolecular solid. On the other hand, chemical traps result from neutral doping or impurities inthe organic semiconductor as well as from defects in the monomer units or chain irregularities.Furthermore, functional groups containing oxygen can significantly influence the charge carriertransport by the localization of negative charge [A. Kadashchuk et al., Journal of Applied Physics

98, 2 (2005)].


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3. The organic field-effect transistor3.1.Operational principle and possible transistor geometries

The organic field-effect transistor is a type of thin film transistor (TFT) that differs from the

standard FET structure by its operational principle as well as by its processability. Inorganic FETsare widespread in today's electronic applications since their structure is compatible with commonthin film techniques such as physical or chemical vapor deposition as well as solution-basedprocesses such as dip or spin coating. Therefore, one of the main advantages of FETs is that theyare not limited to a specific substrate.The working principle of an OFET is based on the field-effect that is schematically shown inFigure3(a) for a metal-insulator-semiconductor (MIS)-diode. By applying a gate voltage VGmobile charge carriers, either thermally generated or injected from the source / drain electrodes,are accumulated at the insulator/semiconductor interface to compensate VG. This accumulationof charge carriers at the interface is called field-effect.

Figure3 (a) MIS-diode with hole accumulation at the semiconductor / dielectric layer interface.(b) Bottom-contact bottom-gate transistor in hole accumulation (negative voltages applied).

As a result, the total amount of accumulated charge n and therefore the conductivityat theinsulator/semiconductor interface can be controlled by the applied gate potential. Theconductivity is defined as = ne with the charge carrier mobility and the elemental chargee. The dependency of the conductivityon the applied gate voltage is exploited in the transistorby dividing the upper electrode into two electrodes: They are called source and drain contact anddefine the transistor channel in between them. The setup of the resulting field-effect transistor isillustrated in Figure 3(b). A current can flow through the transistor channel by applying avoltage VD between the sourceanddrain electrodes as long as the conductivity of thesemiconductor layer is sufficiently high. Depending on the sign of the applied voltages either

electrons or holes are accumulated in the transistor channel.Organic field-effect transistors represent an interesting extension to the transistor family. This isdue to the promise of cheap role-to-role processability or the implementation of transistors onplastic substrates for applications such as flexible displays or RFID (radio frequencyidentification) tags. In Figure4(a) and (b) two typical standard OFET designs are illustrated:Figure4(a) depicts a top-gate configuration comprising bottom source / drain contacts, while inFigure4(b) a bottom-gate configuration with bottom source / drain contacts is illustrated. Bothtop- and bottom-gate configurations are also commonly implemented with a respective top orbottom source/drain architecture (not shown). The resulting typical four transistor structure

combinations can be chosen in dependence of the application or material requirements.


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Figure4Possible device structures: (a) bottom contact top-gate and(b) bottom-gate bottom-contact transistor.

3.2.Current-voltage characteristics of an OFETThe so called drain current IDflowing in the channel between the source and drain electrode independence on the applied voltages is defined by the Shockley equations:

Eq. 1with W channel width, C areal capacitance, L channel length,

mobility, VG gate voltage, VD drain voltage

In a FET the current transport occurs in the channel developing at the insulator/semiconductorinterface. This channel is spatially limited to the charge carrier accumulation zone typicallyextending over the first few monolayers of the organic semiconductor.

According to the Shockley equations the drain current depends both on the gate voltage as wellas the drain voltage. Therefore, ID can be plotted as a function of the drain voltage at constantgate voltage (output characteristics) or as a function of the gate voltage at constant drain voltage(transfer characteristics). Ideal transistor characteristics are schematically illustrated in Figure5.

Figure5Ideal output and transfer characteristics of a unipolar transistor.In the output characteristics the drain current increases linearly at small drain voltages|VD||VG| (linear regime) and is saturated at voltages |VD|>|VG| (saturation regime) as

depicted in Figure 6 (a) and (b). In the transfer characteristics the drain current is increasingquadratically with the gate voltage for |VG|< |VD| and linearly for |VG||VD|.


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Figure 6 Schematic illustration of the charge carrier accumulation in the(a) linear and (b) saturation regime of an OFET.

The above-mentioned considerations ignored trap states in the transistor channel or at thedielectric/semiconductor interface which may localize charge carriers otherwise available forcurrent transport. Furthermore, it was neglected that mobile charge carriers might already be

available in the transistor channel at zero gate bias. To account for these effects resulting in aneffective gate voltage, a threshold voltage Vth is introduced into the Shockley equations:

Eq. 2

3.3.Effects of the dielectric / semiconductor interface on transistor performanceIn field-effect transistors charge carrier transport takes place at the dielectric/ semiconductorinterface within the first monolayers. Thus, electronic interface states may influence the chargecarrier transport distinctly.For example it has been discovered that certain functional groups present at the dielectricsurface hinder or even fully inhibit electron transport as they act as electron traps. The mostprominent example of this effect is hydroxyl groups that are also present on SiO2 surfaces.Figure 7 illustrates the electron trapping mechanism of hydroxyl groups as proposed by Chua etal. [L. L. Chua et al.,Nature 434, 7030 (2005)]. It is suggested that the trapping of an electron

occurs alongside the dissociation of a hydrogen atom leading to the formation of a negativelycharged oxygen ion at the dielectric interface. While hydroxyl groups act as electron traps theydo not seem to influence the charge carrier transport of holes.

Figure7Electron trapping mechanism of hydroxyl groups as suggested by Chua and coworkers[L. L. Chua et al.,Nature 434, 7030 (2005)].


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To overcome this problem and allow for electron transport a surface treatment to functionalizethe hydroxyl groups (see chapter 4.2) or a second dielectric layer to cover them can be applied.Another advantage of surface treatments is that they can influence the thin film formation of theorganic semiconductor and therefore impact the charge carrier transport properties. Further

parameters like the pressure during the deposition, the growth rate and the temperature havealso an effect on the development and as result the properties of the thin film.Figure8 shows scanning electron micrographs of pentacene (a small molecule semiconductor)thin films deposited on differently treated substrates at different temperatures and differentdeposition rates. Depending on the deposition parameters and the treatment, the crystallite sizeand consequently the charge carrier mobility changed.

SiO2 OTS-SiO2

Figure8Scanning electron micrographs of pentacene thin films deposited by organic vapor phasedeposition onto (left) pure SiO2 and (right) SiO2 pre-treated with octadecyltrichlorosilane (OTS).

Substrate temperature, deposition pressure, deposition rate, and resulting saturation hole mobilitiesare given [M. Shtein et al.,Appl. Phys. Lett. 81, 2 (2002)].

3.4.Determination of the charge carrier mobility and the threshold voltageFor OFETs two possibilities to calculate the charge carrier mobility based on the Shockleyequations use either the output characteristics or the transfer characteristics.

a) Calculation of mobility from the output characteristics

The mobility is defined from the linear region (|VD|


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In order to calculate the charge carrier mobility the drain current is plotted versus the gatevoltage at a constant drain voltage from the linear region. In the ideal case, meaning that themobility is not dependent on VG, the slope m of the resulting curve is proportional to themobility.

b) Calculation of mobility and threshold voltage from the transfer characteristics

From the transfer characteristic both the threshold voltage and the mobility can be extracted ifthe square root ofID is plotted versus VG. The result is a monotonously increasing curve thatincreases first exponentially and then linearly. According to equation 4 the slope in the linearregime is proportional to the mobility. As is derived for voltages |VD| > |VG-Vth| it is calledsaturation mobility.

Eq. 4

Plotting the square root ofID versus VG also the threshold voltage can be determined. It is the axisintercept of the fitting curve for the linear regime.

4. Experimental4.1. The substrates

In the practical course the employed substrates consist of n-doped silicon serving as the gateelectrode with a 230nm thick thermally oxidized SiO2 dielectric layer on top (dielectric constant

of SiO2 =3.9). Already pre-structured interdigitated source/drain electrodes are available.The contact structure consists of a 30nm thick gold layer deposited on a 10nm thick ITO(indium tin oxide) adhesion layer and defines transistors with a channel length L of20/10/5/2.5m and a channel width of 10mm, respectively. The structure is shownexemplarily in Figure11.Employing these substrates the preparation of working devices is very easy, as only the organicsemiconductor is left to be deposited.

(a) (b)

Figure11(a) Schematic contact structure of the employed substrates that have already pre-structuredsource/drain electrodes. The resulting transistor exhibits L=20m and W=10mm. (b) Microscope

picture of the transistor channel.

4.2. The organic semiconductor pentaceneThe acene pentacene is one of the most investigated organic semiconductors in organictransistors. Its chemical structure consists of a planar arrangement of five benzene rings as


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depicted in Figure12(a). Figure 12(b) illustrates schematically the energy diagram ofpentacene. The electron affinityEA of 2.9eV and the ionization energyIP of 5.0eV define anenergy gap of 2.1eV [J. Simon et al., Molecular semiconductors, Springer Verlag, 1985].Pentacene can be processed via physical vapor deposition under high vacuum. The morphology

and the electronic properties of the resulting layer are dependent on the surface of the usedsubstrate or dielectric, the purity of the material and deposition parameters like evaporation rate,substrate temperature and pressure.

Figure12(a)Chemical structure and (b)energy diagram of pentacene.In polycrystalline pentacene transistors a hole mobility of up to 1cm2/Vs can be achieved atoptimized deposition conditions [C. D Dimitrakopoulos at al., Advanced Materials 14, 2 (2002)].This mobility is competitive to the charge carrier mobility in amorphous silicon. Therefore,pentacene is an interesting candidate for the application in organic circuits.

In order to influence the pentacene growth on the dielectric layer and to reduce the traps at theSiO2/semiconductor interface, the hydrophilic surface of SiO2 is modified by a surface treatmentwith a surfactant. In this practical HMDS (hexamethyldisilazane) (chemical structure illustratedin Figure13(a)) will be used. It forms a self-assembled monolayer (SAM) with alkyl chainstowards the channel to modify the surface energy of the SiO2 gate insulator (see Figure 13(b))and decrease the traps induced by SiOH groups [H. Ohnuki at al., Thin Solid Films 516,2747(2008)]. A HMDS-modification of SiO2 is a cheap and simple surface treatment method toinvestigate its impact on OFET performance.

(a)

Si

CH3

N

CH3

CH3

Si

CH3

CH3CH3 (b)

Figure13 (a) Chemical structure of HMDS (hexamethyldisilazane) and (b) SiO2 surface that has beenmodified by a HMDS treatment; Me- denotes the methyl group.

4.3.How to build the OFETsAs the used layers in a transistor are rather thin (ca. 50nm) it is important to clean thesubstrates very carefully. The first step is cleaning two substrates twice in an ultrasonic bath for15min in acetone and isopropanol. Between these cleaning steps the substrates are washed with

distilled water and dried with nitrogen, respectively. To make the SiO2 surface more hydrophilicand to remove organic adsorbates left on the surface the substrates are exposed to a 15 min


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ozone treatment. One of the two substrates is then treated with HMDS. For this, drip 3-4 dropsof the liquid HMDS under the fume hood with a pipette on the ozone-treated substrate. After30s the substrate is dried with nitrogen.To deposit the semiconducting layer the substrates are first embedded in a sample holder and

then inserted into the evaporation chamber by a shuttle system. At a pressure of ca. 10 -7mbar a50nm thick pentacene layer is deposited via physical vapor deposition with a rate of 2 /s. InFigure14 the resulting transistor setup is illustrated.As the devices are sensitive to oxygen and water they have to be measured in an inertatmosphere. Thus, the OFETs are transferred into a nitrogen filled glove box.

Figure14Transistor structure of the prepared devices.

4.4.How to characterize the OFETsa. Output and transfer characteristics

Measure the output and the transfer characteristics for a HMDS treated and an untreatedtransistor with a channel length of 20m. Apply for the output characteristics a constant voltageVG of 0 V/-20V/-40V/-60V and varyVDbetween 0-60V. For the transfer characteristicsapply a constant voltage VD of 0 V/-20V/-40V/-60V and varyVGbetween 0-60V.

b. ConditioningMeasure the transfer curve (VD=-60V, VG=0-60V) of a HMDS treated and an untreatedtransistor (curve1). Condition the transistor by measuring a second transfer characteristic atVD=-60V and VG=+100-60V (curve2). Measure the transfer curve at VD=-60V andVG=0-60V of a HMDS treated and an untreated transistor again (curve3).

5. Analysis of the measured data: The protocolIntroduce the OFET by briefly (!) describing its operation principle.

Output and transfer characteristics

Plot the output and transfer characteristics for an untreated and an HMDS-treated OFET (fourgraphs). Mark the linear and the saturation regime for the different output curves. Discuss whythe drain currents recorded for the same measurement conditions, i.e. the same set ofVDand VG,are different for the untreated and the HMDS-treated transistors. What are the highest measuredgate currentsIG? Make sure thatIG is not in the range of the drain currentsID before you proceedwith the data analysis.

(1) Charge carrier mobility

Calculate the hole mobility of the HMDS treated transistor:a) Use the transfer curve (forward sweep) for VD=-60V and fitID1/2 in the saturation regime.

b) Use the output characteristics (forward sweeps).Sketch the curves needed for the calculation, show the fitting curves and explain how youcalculated the mobility. Discuss why the calculated mobilities differ from each other.


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(2) Conditioning of OFETs

Plot the measured transfer curves 1 and 3 of the conditioned untreated and HMDS-treatedOFETs. Calculate the respective threshold voltages from the forward sweeps, respectively.Compare the values before and after the conditioning and discuss the difference. What could be

the reason for the observed threshold voltage shift?(3) Charge carrier density in the channel

a) Calculate the accumulated charge carrier density in the transistor channel due to the field-effect for VG=|-60V| and VG=|-20V| and discuss the difference. Assume VD to be zero.Which parameters have to be changed to increase the charge carrier density at the samevoltage VG in the transistor? Based on these requirements which gate dielectrics can be used inOFETs? Name two possible materials.

b) Estimate the intrinsic charge carrier density in pentacene(i) from the off-current of the transfer characteristic measured during the conditioning

experiment (curve 1, OFET with untreated SiO2-dielectric) utilizing the equation for adrift current. Assume that the field-effect mobility calculated in (2) also holds in the lowcarrier density regime.

(ii)taking the energy gap of the semiconductor into account. Assume an effective density ofstates of 1021cm-3 and a temperature of 300K.

Give an explanation why these values differ.c) Explain how the areal capacitance of a device with a bilayer dielectric (layer 1: d1 and 1 and

layer 2: d2 and 2) is calculated.

6. Questions to be answered before (!)the experiment What does HOMO and LUMO mean and how do they develop? Why do organic semiconductors show such low charge carrier mobility compared to

inorganic semiconductors? Name and sketch possible transistor structures. Which contact materials are expected to provide a good charge carrier injection? Sketch a typical output and a typical transfer curve of a transistor. What do we learn from

the transistor curves? Name the Shockley equations. Why is the threshold voltage introduced?

What determines the accumulated charge carrier density in the channel? How is the capacitance Cdefined? Calculate the areal capacitance for the transistor that will

be prepared in the practical course.

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ResearchLabII_OFET_SS13