Effect of Trap Density on Carrier Propagation in Organic ... - IOPscience

Japanese Journal of Applied Physics 49 (2010) 01AE14

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Effect of Trap Density on Carrier Propagation in Organic Field-Effect Transistors Investigated by Impedance Spectroscopy Jack Lin, Martin Weis, Dai Taguchi, Takaaki Manaka, and Mitsumasa Iwamotoy Department of Physical Electronics, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro, Tokyo 152-8552, Japan Received March 15, 2009; accepted August 6, 2009; published online January 20, 2010 The effect of interfacial traps at the organic semiconductor and insulating oxide interface in a pentacene organic field-effect transistor was examined by both DC and AC methods, represented by the steady-state current–voltage condition and impedance spectroscopy, respectively. A comparative technique for the observation of the effects of low and high trap densities on carrier injection and transport was proposed. An equivalent circuit based on the transmission line model was used to model the system, and the measured results across various biases showed very good fit to this model. We found that for high trap densities, the contact resistance increased markedly, and consequently affected the transport properties, which led to the disappearance of the typical space-charge-limited condition. The mobility under AC bias was also much lower than that under DC bias, which was likely due to the trapping-and-release process involved in the AC bias but not in the DC bias. # 2010 The Japan Society of Applied Physics DOI: 10.1143/JJAP.49.01AE14

1.

Introduction

Over the past couple of decades, we have seen tremendous improvements in organic electronic devices, and the effects of space charge are dominant in these devices regarding carrier injection and transport properties. On the other hand, in recent nanoparticle research, various applications based on the ability to accumulate charge as spherical capacitors1,2) were also found. In several papers, memory or singleelectron devices3,4) have been reported. These effects of nanoparticles have their origin in Coulomb blockade,5) i.e., the existence of energy wells with barriers higher than the thermal energy, and charge transport phenomena in nanoscale materials.6) These properties make nanoparticles (NPs) promising candidates for future nanoscaled architecture for fabricating microelectronic devices. Recent devices based on organic semiconductors have exhibited an injection-type behavior, and carrier transport is mostly limited by the space-charge effect.7) That is, carriers injected from the electrode dominate the device operation, and the injected excess charge markedly affects charging and transport properties. Many theoretical studies have been carried out to clarify the physics of organic semiconductors in terms of the space-charge effect. Their results encourage us to investigate the effect of traps (trapped charges) and hence the created internal electric field. Traps are known to affect the transport properties of organic field-effect transistors (OFET),8) and their memory effect9) and energy levels10) are commonly discussed as bulk phenomena. On the other hand, traps exist at organic–metal interfaces. Hence, the trapped charges may alter the local electric field and consequently affect the injection properties. In this study, we investigated the effect of designed traps on a pentacene OFET by depositing Si nanoparticles at the semiconductor-insulator interface. The term ‘‘designed traps’’ stems from the fact that we could precisely control the threshold voltage by changing the density of nanoparticles.11) The device performance was observed to change from a typical space-charge-limited behavior to an injectionlimited behavior with an increase in trap density. The fitting

On leave from Slovak Academy of Science. E-mail address: [email protected]

y

of the impedance spectroscopy (IS) results allows us to isolate the contribution of the contact and bulk of the system. Furthermore, the comparison of the current–voltage (I–V) and IS results gave us insights into the different behaviors of traps under DC and AC conditions, whose importance cannot be ignored when designing for different applications. In conclusion, trapped charges were found to have significant effects on not only the transport properties but also the injection properties of OFETs. Under AC bias, devices typically exhibit a lower mobility than under DC bias, which is most likely due to the role of trapping states. 2.

Experimental Procedure

We prepared top-contact pentacene OFETs for our experiments (the sample structures shown in Fig. 1). Heavily doped Si wafers with a 100-nm-thick thermally prepared silicon dioxide (SiO2 ) insulating layer were used as the base substrates. The substrates were ultrasonically cleaned with acetone, ethanol, and water, and then UV/ozone-cleaned for 30 min before the deposition of the organic layer. The pentacene deposition (100 nm) was carried out below 2 ˚ /s, 106 Torr and the deposition rate was maintained at 0.5 A monitored using a quartz crystal microbalance. The pentacene was purchased from Tokyo Chemical Industry, and used without further purification. Au electrodes of 50 nm thickness were deposited on the pentacene surface below 4 106 Torr. The channel width and length (W and L) were 3 mm and 50 mm, respectively. The OFETs were investigated by standard I–V analysis using a Keithley 2400 SourceMeter, as well as IS using a Solartron 1260 impedance/gain-phase analyzer. In the IS measurement, the amplitude of the AC signal was 0.5 V superposed on the DC bias of 3 V. In the case of the NP sample, sodium n-dodecylbenzenesulfonic acid (DBSA)-enveloped silicon NPs with diameters of 5 nm (from Meliorum Technologies) were spin-coated on the SiO2 insulator surface, followed by subsequent pentacene deposition. The DBSA envelope was used to isolate each nanoparticle electronically to prevent direct conduction.12) The thickness of the Si nanoparticle film was evaluated to be approximately 25 nm by capacitance measurement using Bruggeman’s effective medium approximation for effective dielectric constant. The quantized

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Jpn. J. Appl. Phys. 49 (2010) 01AE14

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J. Lin et al.

Drain-Source current (μA)

Drain Pentacene SiO2

VDS

DC

Heavily doped Si VGS

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(b) Fig. 1. (Color online) Schematic view of the experimental setup for (a) reference and (b) NP samples.

-4 -3 -2 -1 0 0

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Gate-Source voltage (V)

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(b) Fig. 2. Output characteristics of (a) reference and (b) NP samples. The applied voltages (Vg Vth ) ranged from 0 to 30 V with 5 V intervals.


double-layer charging based on the approximation of a spherical capacitor, where CNP ¼ 4"0 "r rðr þ dÞ=d (r: radius of the NP core, d: thickness of the organic envelope), can be used to describe the charge storage phenomenon of nanoparticles.13) The Coulomb barrier’s corresponding energy (e2 =2CNP ¼ 102 meV in our case) must be larger than the thermal energy kB T to prevent the escape of thermally activated electrons from the nanoparticles. As a result, the nanoparticles may be introduced as trapping centers to control the trap density in an OFET.11) The evaporations of pentacene and gold electrodes for the reference sample without NPs and the NP sample were simultaneously carried out to keep sample preparation conditions uniform. Results

The reference sample showed a typical OFET behavior [Fig. 2(a)]. In the presence of the NP layer, changes in transport properties were observed [Fig. 2(b)]. Such changes were successfully analyzed on the basis of the Maxwell– Wagner effect model as a result of low-density mobile carriers in the presence of charged NPs on the semiconductor-gate insulator interface,14) where injected carriers are swept out from the channel and transported from the source to the drain as soon as they reach the semiconductorgate insulator interface. Therefore, the transport has been reported as injection-limited, as opposed to the space-charge limited conditions in typical OFETs. The transfer characteristics depict a change in threshold voltage (Fig. 3), which was expected from the change in internal field due to the designed traps. The equivalent circuit shown in Fig. 4 is used for the fitting of the impedance spectroscopy results, which are typically presented by the Nyquist plot and phase plot (Figs. 5 and 6). The Nyquist plot shows the negative imaginary part of impedance versus the real part of

Reference NP sample

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Gate-Source voltage (V) Fig. 3. (Color online) Transfer characteristics of reference and NP samples.

Source

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Fig. 4.

Equivalent circuit of the transmission line method.

impedance, with the implicit variable, frequency, increasing from right to left. The experimental data and simulation results are shown by symbols and solid lines, respectively. The phase plot in the frequency domain illustrates more

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Reference NP

-400k

Im (Z) (Ω)

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Re (Z) (Ω) Fig. 5. (Color online) Example of fitting results of the Nyquist plot at 50 V applied voltage.

Phase (degrees)

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clearly the changes at various frequencies, which indicates the speeds at which these events occur. The main relaxation process is observed as charge transfer across the channel, making the organic material more conductive, evidenced by a significant decrease in phase angle. An ideal insulator has a phase angle of 90 and a conductor has a phase angle of 0 . Hence, the starting point of the conductive current flow (deviation from 90 ) can be assigned to the timeof-flight frequency. Note that this relaxation process occurs at higher frequencies for the reference sample than for the NP sample. 4.

Analysis and Discussion

It is important to first verify the increase in trap density by the addition of nanoparticles. From the change in threshold voltage, it is possible to first verify the change in trap density. The shift in threshold voltage between the reference and NP samples can be used for the evaluation of trapped charge density change using Qt ¼ Cox Vth ;

ð1Þ

where the oxide capacitance Cox ¼ 3:4 108 F/cm2 and the threshold voltage shift Vth ¼ 19 V, assuming that all charges are trapped at the interface. The total gate insulator capacitance represents the Si NP layer and SiO2 capacitances connected in series, i.e., CNP Cox =ðCNP þ Cox Þ, where

CNP and Cox represent the capacitances of the NP and SiO2 layers, respectively. However, although a single NP layer has a very low capacitance, owing to a high density of NPs, the NP layer capacitance is much larger than the SiO2 contribution. Hence, the effect of the NP layer on gate insulator capacitance can possibly be neglected (deviation is 4%). By capacitance measurement, we found that approximately 1– 2 layers of nanoparticles were deposited. The number of nanoparticles deposited can then be evaluated by dividing the unit area by the projected area of nanoparticles. The threshold voltage shift indicates that there is an increase of approximately 3:9 1012 traps/cm2 , which corresponds to 1 trapped charge per nanoparticle. Typically, the number of carriers is on the order of 1012 –1013 /cm2 in the applied voltage range of 10 to 60 V. As a result, the designed traps are expected to have a significant effect on the carrier propagation, and it is reasonable to assume that a great majority of traps exist at the pentacene–SiO2 interface. Here, it should be mentioned that measurements with an NP layer only in the channel region showed no drain–source current for identical applied voltages. In light of these results, the conduction current passing through the NP layer was neglected. In impedance spectroscopy, the applied voltage is a superposition of both DC and AC voltages, VðtÞ ¼ VDC þ VAC sinð!tÞ, and impedance is defined by Zð!Þ ¼ VAC ð!Þ= IAC ð!Þ. Note that only the AC signal contributes to the measurement of impedance spectroscopy. On the other hand, in the case of a very low frequency, lim!!0 VðtÞ ¼ VDC . The carrier propagation in an OFET is characterized by the following steps: injection, accumulation, and transport. On the basis of the fact that carriers propagate along a long conduction channel, an equivalent circuit containing the transmission line was used for the simulation. Ideally, such an equivalent circuit would consist of infinitesimally small resistors and capacitors connected in series as a ladder. In practice, the capacitance and resistance per unit length (Cg and R) are connected in series as a ladder, with transverse resistors (Rg ) representing the leakage current. In our analysis, N ¼ 100 repeating units of R, Cg , and Rg is arbitrarily determined to represent the channel region, and Rc and Cc are used to represent the metal–organic contact. The total channel resistance and capacitance (sum of R, Cg , and Rg , respectively) do not depend on the repeating units, i.e., R ¼ Rsum ðx=LÞ, where x ¼ L=N is the increment in length. In fact, in our analysis, the overall results with N ¼ 200, etc. were identical to that with N ¼ 100 (not shown), i.e., each element only differs by the factor of the chosen N. Therefore, the resistance and capacitance per unit of length are conserved. The fitting of the measured data by this equivalent circuit showed good correspondence across various applied gate biases for both reference and NP samples. This suggests the validity of the equivalent circuit model for modeling the electrical properties of an OFET. Additionally, the capacitance Cg 0:168 nF well corresponds to the theoretical value. From the phase angle, as shown in Fig. 6, the response of the reference sample occurs at higher frequencies than that of the NP sample, i.e., the phase decrease occurs earlier in the reference sample. The phase angle decrease gives us insight into the mobility of the device.

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The phase angle of ideal insulators is 90 , while that of a conductor is 0 . When applied to measure the output of an OFET, the decrease in phase angle signifies the opening of the conduction channel. In other words, the characteristic frequency at which the carrier accumulation and transport processes are completed can be obtained from the phase plot, and the average effective mobility can then be extracted by15) 1 L2 ; 2ðVg Vth Þ ttr

ð2Þ

where Vg Vth is the applied gate bias, L is the channel length, and ttr ¼ 1=2 f is the transit time. The IS mobility IS is the intrinsic mobility with the effects of injection and extraction times at the source and drain electrodes. For comparison, the effective mobility obtained from the I–V characteristics in the linear regime was evaluated as W Vd2 I ¼ I{V Cox ðVg Vth ÞVd ; ð3Þ L 2 where Cox is the gate insulator capacitance and Vd is the potential applied to the drain electrode. In this case, the I–V mobility I{V represents IS with the additional effect of the carrier density change. The mobilities calculated by both methods are relatively constant throughout various applied voltages, but the IS results are significantly lower than the I–V results, owing to the difference in carrier behavior under DC and AC conditions. The mobilities of the reference and NP samples obtained from the I–V (IS) results were 2:4 102 (2:6 103 ) and 1:3 102 (1:9 103 ) cm2 V1 s1 , respectively. In detail, the carrier density behavior can be described by the drift-diffusion equations16) for free and trapped carriers: @pf ¼ r ðpf rV þ Drpf Þ þ r pt cðnt pt Þpf ; ð4Þ @t @pt ð5Þ ¼ r pt þ cðnt pt Þpf ; @t where pf and pt are free and trapped carrier densities, V is the local potential, D is the diffusion constant, and c and r are the trapping and release coefficients of traps with the density nt , respectively. Neglecting the diffusion term, we can obtain the form @pf pt pf ¼ r ðpf EÞ þ ðnt pt Þ ; @t r t

ð6Þ

where the trapping and release coefficients were approximated by the trapping and release times t and r , respectively. As was already reported,17) the release time is on the order of 102 s, which is longer than the transit time at which mobilities were estimated in IS. In other words, carriers were trapped and released in I–V measurements, but not released in IS measurements. Therefore, for IS, the carrier density behavior is modified to @pf pf ¼ r ðpf EÞ ðnt pt Þ : @t t

Contact resistance (Ω)

IS ¼

1G

ð7Þ

In conclusion, without the release of carriers, the effective mobility from IS measurements appears lower than the I–V mobility.

Reference NP 100M

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Applied voltage (V) Fig. 7. (Color online) Contact resistance obtained from fitting by the equivalent circuit.

It is difficult to discuss the effect of traps on transport alone without discussing their effect on injection, as these are closely linked. As the trapping density increases in pentacene, the space-charge field generated by the trapped charges becomes a major contribution to the internal electric field. From the fitting of the equivalent circuit model, the contact resistance Rc can be obtained and is plotted in Fig. 7. The voltage dependence of contact resistance agrees well with those indicated in many previous reports.18) The lower output of the NP sample and the linear dependence of its saturated current on voltage can also be explained by the contact resistance difference. The high contact resistance has its origin in the internal field altered by trapped charges.19) The existence of a large number of trapped charges results in high internal fields that make further carrier injection more difficult. As a result, the OFET behavior also changes accordingly, from the typical spacecharge-limited condition to the injection-limited condition. While SCLC is related to the presence of a large number of carriers, in the NP case, the lower number of mobile carriers leads to the linear dependence of the saturated current on the applied voltage.20) 5.

Conclusions

The effects of traps were investigated by both steady-state DC (I–V) and AC (IS) methods. The I–V results indicated the disappearance of the space-charge-limited conditions, and suggested injection-limited transport from the output characteristics. The difference between the DC and AC mobilities was discussed with respect to the trapping-andrelease model. The IS results further separated the injection and transport processes, and indicated that the contact resistance was increased but carrier propagation was decreased by the inclusion of nanoparticles. Therefore, traps were considered to have simultaneous effects on injection and transport. Additionally, it is also important to note the different behaviors of traps under DC and AC conditions, especially for many AC applications, such as RFID. Acknowledgement

This work is supported by Grants-in-Aid for Scientific Research (20656052 and Priority Area 19206034) from the Japan Society for the Promotion of Science (JSPS).

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