Device Geometry Engineering for Controlling Organic Antiambipolar

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Device Geometry Engineering for Controlling Organic Anti-Ambipolar Transistor Properties Kazuyoshi Kobashi, Ryoma Hayakawa, Toyohiro Chikyow, and Yutaka Wakayama J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00015 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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The Journal of Physical Chemistry

Device Geometry Engineering for Controlling Organic Anti-Ambipolar Transistor Properties

Kazuyoshi Kobashi,†,‡ Ryoma Hayakawa,† Toyohiro Chikyow,† and Yutaka Wakayama*,†,‡

†

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for

Materials Science (NIMS), ‡

Department of Chemistry and Biochemistry, Faculty of Engineering, Kyushu University

1-1 Namiki, Tsukuba 305-0044, Japan

ACS Paragon Plus Environment

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Abstract The key concept behind this study is the use of “geometry engineering” to elucidate the carrier transport path and to control the device properties of organic anti-ambipolar transistors. Investigations of carrier transport properties with different device geometries, such as the pnheterojunction length and the channel layer thickness, revealed that charge carriers transported through the lateral edge junction between p- and n-type channels. We also found that the peak voltage was effectively reduced from -49 V to -39 V in a device with asymmetric channel lengths. These results suggest that device performance can be enhanced by taking advantage of the designability of the device geometry, which can employ the strong features of organic antiambipolar transistors.


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Introduction Recently, anti-ambipolar transistors have attracted a lot of attention due to their nonlinear carrier transport properties. A typical device structure is illustrated in Fig. 1(a). The uniqueness of this device can be ascribed to a partially overlapped pin-heterointerface around the centre of a transistor channel. In this device configuration, the drain current shows sharp increases and decreases only within a certain gate bias range as shown in Fig. 1(b). The device properties are characterized by certain parameters, namely peak voltage (Vpeak), onset (Von) and offset (Voff) voltage, peak current (Ipeak) and peak-to-valley ratio (PVR). This nonlinear property is similar to negative differential resistance (NDR), particularly in terms of the huge decrease in electrical current in response to an increase in gate bias voltage. NDR devices have been recognized as promising candidate elements of future very-large-scale integration (VLSI) architectures because their unique carrier transport property can provide conventional circuits with outstanding functionalities.1–6 For example, multi-valued logic circuits can be designed by using NDR devices.7–10 However, conventional NDR devices have the fatal disadvantages of a poor PVR and a low operation temperature.11-16 Anti-ambipolar transistors can provide an alternative solution in this regard because high PVRs of more than 104 have already been realized at room temperature, and these are suitable for practical applications.17 Most reported anti-ambipolar transistors have employed thin films made of transition metal dichalcogenides (TMDCs) as the channel layers.19-22 These devices had been fabricated by a mechanical exfoliation method. Therefore, such device geometries as the pn-heterojunction length (∆L), and the thickness and length of the channel layers cannot be controlled. On the other hand, our device utilizes organic semiconductor films, which are formed by vacuum deposition through shadow masks18. This process allows us to design the device geometries precisely. This flexibility in device geometry will offer new opportunities for nusing anti-ambipolar transistors.


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In this study, we investigated the geometry dependence of the device properties of organic anti-ambipolar transistors. We had two main purposes. The first was to obtain further insight into the carrier transport path in the channel layers. The other was to explore a way of controlling device performance by optimizing device geometries. For these purposes, the pn-heterojunction area, thickness and length of channel layers were systematically varied.

Experimental methods Our devices are composed of α-sexithiophene (α-6T) and N,N′-dioctyl-3,4,9,10perylenedicarboximide (PTCDI-C8) as p-type and n-type organic semiconductors, respectively. The device fabrication process is detailed in our previous work18. Here, the thickness of α-6T film (t6T) is 3 monolayers (MLs), which corresponds to the typical carrier accumulation layer thickness (2~3 MLs) in an organic field-effect transistor (OFET).23,24 Therefore, charge carriers possibly transport across the vertical heterointerface and/or lateral edge junction as shown in Fig. 1(a).

Results and Discussion To identify the carrier transport path, drain currents (ID) were measured as a function of ∆L (50, 150 and 250 µm). Optical images and drain current-gate voltage (ID-VG) curves are shown in Fig. 2(a) and (b), respectively. In these measurements, a Au electrode on the α-6T film was grounded to work as a source electrode, while the counterpart on the PTCDI-C8 film acted as a drain electrode. A negative gate bias voltage was applied in the 0 to – 60 V range at a constant drain voltage of – 60 V. Importantly, Ipeak was almost constant at 117 ± 12 nA in spite of the fivefold difference in ∆L. If the vertical interface is the dominant carrier transport path, the amount of ID will change with ∆L. Meanwhile, our results suggest that the dominant carrier


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transport path is not the vertical interface but the lateral edge junction. The slight increase in Ipeak from 105 nA (∆L = 50 µm) to 118 nA (∆L = 250 µm) can be attributed to the shift in the threshold voltage (Vth) of the PTCDI-C8 channel, which involved shifts in Vpeak and Voff. For example, Vpeak was shifted from – 47.6 V (∆L = 50 µm) to – 50.4 V (∆L = 250 µm). The relationship between Vth and Vpeak (Voff) will be described in detail later. To confirm the above discussion, we performed a controlled experiment, where we increased t6T to 20 MLs. Then, the PTCDI-C8 thickness and ∆L were fixed at 12 MLs and 150 µm, respectively. The 20 MLs of α-6T were 50 nm thick, which is more than double the thickness of the PTCDI-C8 film (24 nm with 12 MLs). Hence, it is reasonable to consider the PTCDI-C8 channel to be disconnected at the edge of the α-6T film as illustrated on the right in Fig. 3(a). Figures 3(b) show an AFM image and cross-sectional profiles of the PTCDI-C8 film on the α-6T film and that on the PMMA underlayer. The profiles showed distinct difference in the surface roughness: root mean square (RMS) of 12.2 nm on the α-6T film and that of 1.9 nm on the PMMA. These are resulted from the rough morphology of underlying thick α-6T film, suggesting that the PTCDI-C8 on the α-6T has discontinuous film. This is another evidence to prove that the PTCDI film doesn’t work as a transport channel. Despite this fact, the device operated properly as shown by the blue line in Fig. 3(c). This result clearly proved that the charge carriers transport through the lateral edge junction. Here, it should be noted that Von shifted significantly from – 30.6 to – 9.4 V with increasing t6T, while the Voff remained constant at around – 55 V. The shift in Von accompanied a decrease in Vpeak from – 49. 4 to – 41.2 V and an increase in Ipeak from 130 to 450 nA. These variations in device properties (∆Von, ∆Vpeak and ∆Ipeak) are illustrated in Fig. 3(d) and can be ascribed to the decreased Vth of the α-6T channel. This is because the Vth value in thicker channel layers is less affected by the carrier traps located near the gate insulators to reduce Vth. In fact, we confirmed


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the decrease in Vth in the neat α-6T film transistors with increasing t6T from 3ML to 6ML (not shown here). The Vpeak shift observed in Fig. 2(b) can also be interpreted in terms of the Vth shift. In the device with ∆L = 50 µm, the lateral edge junction is located at the channel centre. Meanwhile, those in the other devices (∆L = 150 and 250 µm) are not aligned with the centre. These variations in the channel lengths caused the change in Vth and, as a result, Vpeak was shifted. These experimental results provide an important hint as regards reducing the operation voltage of anti-ambipolar transistors; the device parameters can be controlled by optimizing the device geometry. With anti-ambipolar transistors, Von and Voff are primary factors determining the operation voltage (Vpeak), and both are determined by the Vth values of α-6T and the PTCDIC8 channels, respectively. Additionally, it is generally known that in p-type transistors the Vth value shifts toward a positive gate voltage as the channel length decreases when there is a potential drop at the electrode/organic semiconductor interface. As a result, the operation voltage is reduced in anti-ambipolar transistors.25,26 Based on these facts, we fabricated a device whose lengths were 100 µm (L6T of α-6T) and 300 µm (LPTCDI of PTCDI-C8), respectively, as shown in Fig. 4(a) and (b). This asymmetric geometry was designed to reduce the Vth value of the α-6T channel (p-type) and to increase that of the PTCDI-C8 channel (n-type). The blue line in Fig. 4(c) shows the ID-VG curve of the asymmetric device (L6T/LPTCDI =1/3). A reference curve (a red line) obtained from the symmetric geometry (L6T/LPTCDI =1) is duplicated from Fig. 2(b). As we expected, Vpeak was successfully reduced from – 49 to – 39 V. This was brought about by the simultaneous shifts of Von and Voff with an almost constant Ipeak. These variations in the device properties (∆Von, ∆Voff and ∆Vpeak) are summarized in Fig. 4(d).


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Conclusions In conclusion, we have investigated the geometry dependence of the device properties of organic anti-ambipolar transistors. Charge carriers were found to flow though the lateral edge junction, which means that various kinds of organic semiconductors can be employed because the roughness of the vertical heterointerface no longer impacts on the carrier transport. Meanwhile, molecular orientations of the first few monolayers and those at the heterojunction in the lateral interface must be well arranged. These are next challenge for further improvement of device performance. We have also proposed the use of geometry engineering to control the device parameters. The fine tuning of Von and Voff by designing asymmetric channels realized the effective reduction of Vpeak. As demonstrated in this study, device performance can be improved by optimizing device structures. These features, including a wide range of material selectivity and geometry designability, are strongpoints of organic anti-ambipolar transistors.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the World Premier International Center (WPI) for Materials Nanoarchitectonics (MANA) of the National Institute for Materials Science (NIMS), Tsukuba, Japan, JSPS KAKENHI Grant Numbers JP15K13819 and JP23686051.


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Figure captions

Figure 1. (a) Device and molecular structures of organic anti-ambipolar transistor. The enlarged illustration shows possible carrier transport paths: vertical interface and lateral edge junction. The thickness of α-6T and the overlapped length of the heterointerface are denoted as t6T and ∆L, respectively. (b) Carrier transport parameters of organic anti-ambipolar transistors. The peak-tovalley ratio (PVR) is calculated as the ratio between the maximum and minimum drain currents (Ipeak/Ivalley). Here, the onset (Von) and offset (Voff) gate bias voltages are determined by extrapolating the slopes of the √ID-VG curves.


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Figure 2. (a) Optical images and (b) ID-VG curves of the devices with different pn-heterojunction lengths (∆L). (b) with different ∆Ls. These curves were measured under a constant drain voltage (VD = – 60 V). No distinct variation in curves was observed in spite of a fivefold difference in ∆L.


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Figure 3. (a) Illustrations of devices with different α-6T film (t6T) thicknesses. The 20 ML of α6T disconnect the PTCDI-C8 film at the edge as shown by the blue dotted ellipse. (b) AFM image and cross-sectional profiles of the PTCDI-C8 film around the edge, showing distinct difference in the surface roughness depending on the underlying layers, i.e., thick α-6T (blue line) or the smooth PMMA (green line). (c) ID-VG curves of devices with different t6T values. (d) Variations in device properties (∆Vpeak, ∆Ipeak and ∆Von) are induced by the shift in the threshold voltage of the α-6T channel as indicated by a red arrow.


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Figure 4. (a) Schematic illustration of an organic anti-ambipolar transistor. L6T and LPTCDI indicate the channel length of α-6T and PTCDI-C8, respectively. (b) Optical image of the device with an asymmetric channel length (L6T/LPTCDI=1/3). (c) ID-VG curves of the device with the symmetric (red) and asymmetric (blue) channels. A clear reduction in Vpeak was observed in the asymmetric device. (d) Variations in device properties (∆Vpeak, ∆Voff and ∆Von) are induced by the shift in the threshold voltages of both the α-6T and PTCDI-C8 channels as indicated by red and blue arrows.


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Device Geometry Engineering for Controlling Organic Antiambipolar

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