Fabrication of a normally on organic thin film transistor for active sensor construction

FABRICATION OF A NORMALLY-ON ORGANIC THIN FILM TRANSISTOR FOR ACTIVE SENSOR CONSTRUCTION Khong Duc Chien 1,2* , Hoang Van Phuc 1 , Dao Thanh Toan 2 1 Le Quy Don Technical University; 2

FABRICATION OF A NORMALLY-ON ORGANIC THIN FILM TRANSISTOR FOR ACTIVE SENSOR CONSTRUCTION

Khong Duc Chien 1,2* , Hoang Van Phuc 1 , Dao Thanh Toan 2

1 Le Quy Don Technical University;

2 University of Transport and Communications

Abstract

In organic active pressure sensor, a reduction in supply voltage of transistor is an effective way to decrease the power consumption Up to now, for the development of pressure sensor based on normally-on OTFT (organic thin-film transistor), the OTFT where the conductive channel is formed without gate voltage supply, is still challenging In this paper, we propose an approach to fabricate normally-on OTFT based on floating gate, photoactive gate dielectric layer and programming process using external UV source After fabrication, measurements of OTFT characteristics, including transfer and output, were performed Estimation of the critical parameters including the threshold voltage and field effect mobility was also described Our fabricated OTFT shows good performance with a low

the OTFT changes to normally-on state with drain current of 10 -6 A at 0 V gate voltage OTFT fabrication is the essential step to construct an organic active pressure sensor in the future work

Keywords: Organic thin-film transistor; OTFT fabrication; normally on OTFT; organic active

pressure sensor

1 Introduction

In recent years, a pressure sensor using organic material has attracted a lot of interest from many researchers due to its unique advantages including low temperature processing, solution process ability, low manufacturing cost, mechanical flexibility, and large-area possibility [1-12] In terms of structure, an organic active pressure sensor device consists of passive element integrating with organic thin-film transistor [1-4] In

a standard active sensor cell, the passive device has been designed and fabricated in connection to the input gate electrode (G) of transistor as illustrated in Fig 1 [1-4] However, organic semiconductor basically does not have available carriers, thus if

a voltage (V G) is not applied to the gate electrode, the conductive channel in the semiconductor layer is not formed, as presented in Fig 1a (so-called normally-off state)

To turn the transistor element of active sensor conductive, a certain value of V G needs to

be provided to create an electric field, which will induce the accumulated carriers from

the source electrode (S) into the semiconducting channel as described in Fig 1a Even

the sensing signal is amplified, sensor power consumption is large because both the

drain voltage (V D ) and the V G are used during sensor operating [3, 4]

VD

VD

Passive

pressure

sensor

Passive pressure

S

Fig 1 Active sensor structure using (a) normally-off and (b) normally-on OTFT.

At device level, it is figured out that reduction of the supply voltage is the most

effective way to scale down the power consumption [1, 2] It is clear that V D is the

essential voltage to create I D by collecting the induced carriers in the semiconductor

layer Consequently, the V G should be the objective to study for decreasing the sensor power consumption Following that idea, in some active research groups, to construct an active sensor device, the second gate electrode of metal or electrolyte has been inserted into OTFT (called floating-gate) and directly connected to the output electrode of passive element as shown in Fig 1b [14-16] That allows the electricity from passive element directly charge the floating-gate, resulting in the enhancement of the accumulated carriers in the semiconductor layer due to built-in potential by the

floating-gate The V G could be significantly decreased, but a certain value was still required to make the transistor channel become conductive during sensor operating, for example,

V G of -2 V was utilized in recent report by Yin et al [15]

In this paper, we present a process where a normally-on OTFT using a Cytop floating-gate is fabricated Firstly, fabrication process is presented in detail, then, the basic parameters are obtained following the IEEE 1640 standard Finally, programming operation of the transistor is performed The injected electrons are trapped and released at the Cytop floating-gate layer under external programming voltage The trapped electron density is able to sufficiently induce the hole charges accumulating in the transistor channel, the fully conductive channel of OTFT will be formed without a need of an applied

VG This research is an important step for the construction of active sensor

2 Proposed fabrication process of OTFT

Figure 2 shows the cross-section of our designed normally-on OTFT which uses the same layer structure in our recent report [16] A thin film from 6-[4'-(N,N-diphenylamino)phenyl]-3-ethoxycarbonylcoumarin (DPA-CM) and Polymethyl methacrylate (PMMA) is used as the gate dielectric layer while Cytop layer is the floating layer An active layer of the OTFT is chosen as Pentacene material since it is the most common organic p-channel semiconductor and the source/drain electrodes are made from Cu thanks to its low-cost electrode material

DPA-CM:PMMA CYTOP Pentacene

Source/Drain

Gate Glass Substrate

Fig 2 Cross-sectional structure and symbol of normally-on OTFT

In order to save fabrication cost, we have designed the mask for 4 OTFTs on substrate (25 mm × 25 mm) The fabrication of the OTFTs basically includes 7 steps schematically described in Fig 3 The all processes were done in a clean room with a class of 1000

 For the first step, glass substrates coated with a 150 nm gate electrode layer of indium tin oxide (ITO) were cleaned in acetone and isopropanol in sequence for

10 min each using ultrasonication, followed by UV-O3 treatment

 For the second step, photoactive molecules of DPA-CM and PMMA (Aldrich, USA, Mw = 94,600) were dissolved in a chloroform at a 1:10 molar ratio of a monomer unit of PMMA (2 wt%) to CM A gate dielectric layer of

DPA-CM and PMMA was prepared by spin-coating of the solution on the ITO/glass substrate at 4000 rpm for 60 s, and heated on a hot plate at 100C for 60 min to remove residual solvent

 For the third step, the CYTOP layer (CTX-809AP2, Asahi Glass, Japan) was spin-coated onto the DPA-CM:PMMA gate dielectric layer at 2000 rpm for 60 s using a 0.5 wt% fluoro carbon solution and dried at 100C for 1 h

 For the fourth step, dielectric DPA-CM:PMMA and Cytop layers were partly removed from the substrate at the position of the source and drain electrodes

 For the fifth step, a 30-nm-thick film of pentacene was formed on the CYTOP layer by conventional vacuum deposition method at a deposition rate of 0.02 nm s-1 The vacuum deposition processes were performed at a pressure of 2×10-6 Torr

 For the sixth step, the fabrication of the OTFT was completed by deposition of

Cu source-drain electrodes at a deposition rate of 0.1 nm s-1 through a shadow mask The channel length and the channel width of the OTFT were 50 μm and

2000 μm, respectively The vacuum deposition processes were performed at a pressure of 5×10-6 Torr

 For the seventh step, the OTFT substrate was finally encapsulated using the glass cap in dry nitrogen glove box

Floating gate: Cytop (30-50 nm)

P-channel: Pentacene (30-50 nm) Package: Glass cap

S/D electrodes : Cu (50-100 nm) Gate dielectric:PMMA:DPACM (300-400 nm )

Gate electrode

Clean substrate

Gate

PMMA:DPACM Cytop (30-50 nm)

Pentacene (30-50 nm)

partly-removal

of Cyttop

S/D

S/D

S/D

S/D S/D

S/D

S/D

S/D

Gate

Gate Gate

S/D electrodes

Spin-coating

Evaporation Package Gate

Gate

Fig 3 OTFT fabrication process

3 Electrical characterization of OTFT

Fig 4a and 4b show a photo and an equivalent circuit of OFET array after fabricating, respectively In our design, the 4 OTFTs share the gate electrode For

electrical characteristics, a typical OTFT was selected to perform the electrical

measurement at a probe station with a SCS4200 system (Keithley, USA) The

equivalent circuits of the measurements are drawn in Fig 4c and 4d

Typical output and transfer curves of the OTFT are described on Fig 5 For

the output characteristics, as can be seen from the Fig 5a, the drain current (ID) of the

transistors increased at a negative gate voltage (VG) In the linear region of the graph, ID

showed a linear increase at a low drain voltage (VD), implying a good Ohmic contact

between the pentacene semiconductor and Cu drain electrode Then, ID

saturated at a high VD because the conducting channel was pinched-off These curves

indicated that the transistor devices showed standard p-channel field-effect operation The squared curve in Fig 5b represents transfer characteristic of the OTFT At VG = 0 V, OTFT is at the Off state where the drain current value is -10-13 A To get into an On state current of about -10-6 A, OTFT needs a gate voltage supply, which is above -20 V

(a)

G2 G1

G3

G4

A

G

S D

(c) (b)

A

A

G

S D

(d)

S/D S/D S/D

S/D S/D

S/D

G

Fig 4 Photo of OTFT array after fabrication (a), equivalent circuit diagram of OTFT array (b), equivalent circuits for (c) initial output and (d) transfer characterizations

1E-15 1E-14 1E-13 1E-12 1E-11 1E-10 1E-9 1E-8 1E-7 1E-6

Initial Root Curent Linear fit data

Gate Voltage (V)

-2,0E-04 0,0E+00 2,0E-04 4,0E-04 6,0E-04 8,0E-04 1,0E-03 1,2E-03 1,4E-03 1,6E-03

R-Square(COD) 0,99806

-2,5E-06

-2,0E-06

-1,5E-06

-1,0E-06

-5,0E-07

0,0E+00

-12 V -9 V

-18 V

Drain Voltage (V)

-20 V

-(36) V

Fig 5 Initial (a) output and (b) transfer characteristics of OTFT

On the other hand, following to the IEEE 1620 standard [1, 17, 18], besides the electrical characteristics, the fabricated OTFT should be provided the critical factors

including the threshold voltage (V T), field effect mobility (), those are extracted from

the transfer characteristics as follows

Theoretically, the relation between the saturated drain current and gate voltage can be presented [15, 18]:

  2 1

2

W

L

where W and L are the channel width and channel length; C i is the capacitance per unit

area of the gate dielectric.TheC i was measured to be 140 nF/cm2 using a LCR meter Hioki 3522-50

By taking square root, equation (1) can be re-written as:

Equation (2) is in the standard form of:

y   a x b (3)

in which

D

y I ; xV G; 1

W

L

 ;

1

W

L

The relation between square-root of ID and VG (Eq (2)) obtained from the experiment is shown by the circled curve in Fig 5b, that is well fitted to equation (3) with an R-Square of 0.9981 Thus, it is obvious that equation (3) can be represented as:

y    x   (5)

From equation (4) and (5), by using designed parameters W, L and C i in Tab 1, the  and the V T can be estimated to be 0.893 cm2/Vs and -4 V, respectively

Table 1 shows the basic parameters of OTFT fabricated in this study which are typical among OTFTs for pressure sensors [3, 12]

Tab 1 Basic parameters of OTFT

Designed parameters

experimental data

4 Achievement of the normally-on state

As mentioned above, the initial state of OTFT is normally at the Off state In order

to turn OTFT to the On state, a programming process was carried out [16] Fig 6 shows the equivalent circuit and photo of experimental setup for programming A negative voltage pulse of -20 V is applied to the gate electrode under UV light irradiation The UV-light ( = 365 nm) was generated from an Omron ZUV UV irradiator and the irradiation power was set at appropriate level during the programming process

G

S

D

1 s

- 20 V Programming pulse

UV source

SCS 4200

Gate electrode to SCS 4200

Fig 6 Schematic diagram (a) and photo (b) of programming process

Photoelectrons are generated from DPA-CM molecular then injected and trapped at the Cytop thin-film layer The trapped electron density is able to sufficiently induce the hole charges accumulating in the transistor channel, the fully

conductive channel of OTFT will be formed without a need of an applied VG To support that explanation, the transfer characteristic was measured after programming and plotted in Fig 7

The drain current of the OTFT is nearly -10-6 A at gate voltage of 0 V We would like to confirm that the state current of the OTFT is unchanged after programming, indicating that the OTFT is at a normally-on state It notes here that in other OTFT

sensor systems, the VG still requires to make the transistor channel become conductive,

for example, VG of -2 V was utilized in recent report by Yin et al [15]; therefore,

in comparison, our OTFT will surely help to reduce the power consumption of the active sensor

-35 -30 -25 -20 -15 -10 -5 0 5 10

1E-14 1E-13 1E-12 1E-11 1E-10 1E-09 1E-08 1E-07 1E-06 1E-05

Initial Drain Curent Programed Drain Current

Gate Voltage (V)

Fig 7 Transfer characteristics of OTFT before and after programing

5 Conclusions

In this paper, we have demonstrated an experimental study on fabricating and programming a normally-on OTFT The basic transfer and output characteristics of OTFT

were measured and presented In addition, initial VT and  were extracted in detail

according to IEEE 1620 standard Our fabricated OTFT shows good performance with a low threshold votage of -4 V and high mobility of 0.893 cm2/Vs The mobility can be compared with Yuan’s study [19] in which  ranges from 0.49 to 1.15 cm2/Vs As

normally-on state of out OTFT, the ID was obtained to be about -10-6 A without VG

supply The successful step in normally-on OTFTs fabrication is crucial for us targeting our research on creating organic active pressure sensor elements

Acknowledgment

This work was supported by the Domestic Master/PhD Scholarship Programme of Vingroup Innovation Foundation Authors would like to thank Prof H Sakai, JAIST, Japan for supporting the clean room and experimental facilities

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CHẾ TẠO TRANZITO MÀNG MỎNG HỮU CƠ THƯỜNG MỞ PHỤC VỤ VIỆC XÂY DỰNG CẢM BIẾN ÁP LỰC HỮU CƠ TÍCH CỰC

Tóm tắt: Đối với cảm biến áp lực tích cực hữu cơ, giảm điện áp cung cấp cho tranzito là

một cách hiệu quả để giảm công suất tiêu thụ Cho đến nay, phát triển được cảm biến áp lực trên cơ sở tranzito màng mỏng hữu cơ (OTFT: Organic thin-film transistor) thường mở, tranzito mà kênh dẫn được hình thành mà không cần cung cấp điện áp cho cực cửa, vẫn là một thách thức lớn Trong bài báo này, chúng tôi đề xuất phương pháp chế tạo OTFT thường mở dựa trên cấu trúc cực cửa thả nổi, lớp điện môi cực cửa nhạy sáng và quá trình lập trình sử dụng ánh sáng UV bên ngoài Sau khi chế tạo, nhóm tác giả đã tiến hành kiểm tra đặc tính truyền đạt và đặc tính ra của tranzito Thêm vào đó, các bước ước lượng điện áp ngưỡng và độ linh động điện tử từ các số liệu thí nghiệm cũng được mô tả chi tiết OTFT sau chế tạo có chất lượng tốt với điện áp ngưỡng thấp ở mức -4 V và độ linh động điện tử cao 0,893 cm 2 /Vs Trước khi lập trình OTFT ở trạng thái thường đóng với dòng điện cực máng bằng 10 -13 A ứng với điện

áp 0 V cực cửa Sau khi lập trình OTFT chuyển sang trạng thái thường mở với dòng điện cực máng bằng 10 -6 A với điện áp 0 V cực cửa

Từ khóa: Tranzito màng mỏng hữu cơ; chế tạo OTFT; OTFT thường mở; cảm biến áp lực hữu cơ

tích cực

Received: 15/02/2019; Revised: 13/11/2019; Accepted for publication: 22/11/2019

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