Effect of Ta2O5-PMMA Compound Gate Insulator on The Performance of Organic Field Effect Transistors

SHI Xiao-dong, WANG Wei, LI Chun-jing, REN Li-peng, YIN Qiang

(School of Electronics and Information Engineering, Hebei University of Technology, Tianjin Key Laboratory of Electronic Materials and Device, Tianjin 300401, China)*Corresponding Author, E-mail: wangwei@hebut.edu.cn



Effect of Ta2O5-PMMA Compound Gate Insulator on The Performance of Organic Field Effect Transistors

SHI Xiao-dong, WANG Wei*, LI Chun-jing, REN Li-peng, YIN Qiang

(SchoolofElectronicsandInformationEngineering,HebeiUniversityofTechnology,
TianjinKeyLaboratoryofElectronicMaterialsandDevice,Tianjin300401,China)
*CorrespondingAuthor,E-mail:wangwei@hebut.edu.cn

This paper reports pentacene field effect transistors (OFETs) with a gate insulator made of compound Ta2O5-PMMA where PMMA (poly (methyl methacrylate)) is spin-coated onto the top of evaporated layer of Ta2O5. A comparison with devices with only Ta2O5is presented. The latter not only shows a high surface roughness but also exhibits very low field-effect mobility. These drawbacks can be overcome by depositing a PMMA layer on Ta2O5. The influence of PMMA thickness in the range 20-60 nm is presented. The results show that when the thickness of PMMA is approximately 40 nm, the electrical performance of OFETs is optimal. Compared with conventional OFETs, the field-effect mobility increases from 4.2×10-2to 0.31 cm2/(V·s), and the on/off current ratio increases from 2.9×102to 2.9×105when the gate voltage increases to -20 V.

Ta2O5-PMMA; insulator; OFETs; mobility; on/off current ratio

1 Introduction

Organic field effect transistors (OFETs), as one of the important organic semiconductor devices, have many advantages such as large area, low cost, lightweight, mechanical flexibility, easy fabrication and environmental friendliness[1-5]. In recent years, the performance of OFETs has been greatly improved, and OFETs have a broad market prospect in driving circuits for future all-organic OLEDs flat panel displays, plastic radio frequency identification circuits, gas sensors and chemical species sensors[5-7]. However, the mobility and on/off current ratio of OFETs are still lower than those of inorganic counterparts. Therefore, further performance improvement for the OFETs is desirable.

OFETs operate in accumulation regime and most of the modulated charge lies within the first 10 nm layer adjacent to gate insulator[8]. That does mean that the interfacial properties between semiconductor and gate insulator are of tremendous importance on the field effect mobility. Improvement of the gate insulator material and semiconductor/insulator interface would be highly beneficial to the performances of OFETs. Roughness and dielectric constant are crucial parameters. Insulator material can divide into two categories according to its nature: inorganic and organic. Inorganic insulator materials have many advantages, such as stable chemical properties, excellent electrical properties and high temperature resistance[9]. Conventional SiO2and high dielectric constant oxide material (Ta2O5, TiO2, Al2O3,etc.) are commonly used as inorganic insulator materials[10]. Thereinto, Ta2O5has a high dielectric constant (k≈26), a bandgap of around 4.6 eV, with higher refractive index and chemical stability. But the Ta2O5film has a strong polar effect which restricts the carrier transport[11]. Organic insulator materials are mainly composed of polymer insulator materials, including polyvinyl phenol (PVP), polyvinyl benzene (PS), PMMA,etc.[12-14]. The dielectric constant of PMMA is close to 3 and PMMA contains hydrophobic methyl groups which can resist humid environment. What’s more, PMMA not only provides a uniform nonporous non-polarized interface, but also improves the molecular order[11,15]. Hence, PMMA can be used as buffer layer material between organic semiconductor and inorganic gate insulator.

In order to reconcile the respective advantages of Ta2O5for high dielectric properties and PMMA for a better interface with the organic semiconductor, a compound gate insulator consisting of a Ta2O5film covered with a PMMA film is reported in this paper. The OFET devices with Ta2O5-PMMA compound insulator were prepared to achieve higher field-effect mobility, lower threshold voltage and higher on/off current ratio.

2 Experiments

The OFET devices used bottom-gate top-contact structure[16-17], as shown in Fig.1. ITO substrate (50 mm×50 mm) was ultrasonically cleaned with deionized water, acetone and ethanol for 10 min, respectively. ITO gate electrode was obtained by the UV lithography. Ta2O5was deposited by e-beam evaporation on ITO gate electrode. The deposition rate and the thickness were 0.5 nm/s and 140 nm, respectively. For the compound gate insulator, a solution of PMMA in chloroform was spin-coated onto the Ta2O5. The thickness of PMMA is in the range 20-60 nm for a solution in the concentration range 10-20 mg/mL. Then, a 50 nm thick pentacene film was deposited by thermal evaporation at a rate of 0.6 nm/s on the substrate maintained at 80 ℃. The pentacene was used as received without any further purification. The devices were completed by the evaporation of silver source and drain electrodes through a shadow mask. The channel length was 170 nm and the width was 510 nm.

The surface morphology and roughness of samples were analyzed by Agilent 5600LS Atomic Force Microscopy (AFM). The surface roughness (Sq) value was defined as:

Fig.1 Schematic cross-sectional structure of OFET

(1)

where,z(x,y) represented the residual surface;lx,lyrepresented the length of the sampling region;M,Nrepresented the discrete sampling points ofxandyin the sampling region.

3 Results and Discussion

Fig. 2 shows the AFM images andSqvalues of the insulator layer. The surface roughness of Ta2O5film is larger (Sq=0.956 nm). Ta2O5deposited by e-beam evaporation at room temperature may contain several defects: multiphase material, local variation of composition, inhomogeneity, bulk and surface traps. All of these factors increase the surface roughness of Ta2O5film and degrade the electrical characteristics of the OFETs. In comparison to Ta2O5, the surface roughness of Ta2O5-PMMA compound insulator reduces significantly. PMMA, as a kind of high molecular polymer, can effectively reduce defects of Ta2O5film surface and provide a non-polarized insulator surface. What’s more, one major advantage of organic-organic interface is the noninteracting nature of this interface in most cases. That means an abrupt interface without reactive interlayer or dipoles as observed in metal/organic interfaces. The polymer plays an important role in improving OFET performance, the surface of compound insulator may influence growth behavior of pentacene thin films and enhance physical connection between gate insulator and semiconductor channel.

Fig.2 AFM images andSqvalues of different insulator structure. (a) Ta2O5(140 nm),Sq=0.956 nm. (b) Ta2O5(140 nm)-PMMA (20 nm),Sq=0.733 nm. (c) Ta2O5(140 nm)-PMMA (40 nm),Sq=0.463 nm. (d) Ta2O5(140 nm)-PMMA (60 nm),Sq=0.351 nm.

Fig.3 shows the output and transfer characteristic curves of three devices with a gate insulator made of 140 nm Ta2O5with spun on layers of PMMA with thicknesses of 20, 40 and 60 nm. Fig.3(a), (c) and (e) show that the prepared OFET devices have typical hole transport properties. When the source drain voltage (Vds) is low, the source drain current (Ids) has a linear growth trend, device operates in the linear region. With the increase ofVds,Idsalmost unchanges and reaches the saturation current. That is to say, the device operates in the saturation region. Under the saturation region, the conductive channel appears to pinch off and the source drain current can be expressed as:

(2)

Tab.1 shows electrical performances of OFET devices with different gate insulator. The OFET devices with only Ta2O5have a higher capacitance and lower threshold voltage, but its field-effect mobility and on/off current ratio are only 0.042 cm2/(V·s) and 2.9×102, respectively. The larger insulator capacitance associates to the high dielectric constant of this oxide, which makes the OFET be opened under lower threshold voltage. But surface roughness of Ta2O5film is greater (Sq=0.956 nm), which increases the probability of several defects. What’s more, high dielectric constant insulator has relatively higher interfacial polarity. These factors limit the transport of carriers, thus the field-effect mobility of OFET devices is low.

Fig.3 Output and transfer characteristics of OFETs with Ta2O5-PMMA as compound gate insulator. PMMA thickness is 20 nm ((a) and (b)), 40 nm ((c) and (d)) and 60 nm ((e) and (f)). Ta2O5thickness is 140 nm.

Tab.1 Electrical performances of OFETs with Ta2O5 and Ta2O5-PMMA gate insulator

In comparison to devices with only Ta2O5, the equivalent gate capacitance reduces due to the low dielectric constant of the PMMA layer. The gate capacitance (Ci) decreases to 18 nF/cm2for the 60 nm thick PMMA. From the transfer characteristics, the measured threshold voltage (Vth) is about -3.5 V for the two thinnest PMMA layers andVthincreases up to -8.2 V for the 60 nm thick PMMA. The major improvement brought by PMMA is the strong increase of field effect mobility which reaches 0.31 cm2/(V·s) for 40 nm PMMA, which is about seven times as much as that of the device without PMMA insulator. The on/off current ratio reaches 2.9×105for 40 nm PMMA, which is better than the device with only one Ta2O5insulator layer. All these data point to the beneficial influence of a thin PMMA film on Ta2O5on device performances.

Based on the above analysis, when the PMMA film thickness is 40 nm, the electrical performance of the device is optimal. Improvement is to be associated with a pentacene/PMMA interface of much better quality than the pentacene/Ta2O5interface in terms of electronic states but also in terms of roughness.

4 Conclusion

In this paper, we report a compound gate insulator of Ta2O5-PMMA for OFETs. This insulator design combines the respective advantages of the two materials, say, the high dielectric constant of Ta2O5and an improved PMMA/pentacene interface. It is found that the OFETs with a PMMA modified compound insulator have higher performance than that of the devices without PMMA insulator. When the PMMA film thickness is approximately 40 nm, the electrical performance of the device is optimal, and we obtain the largest field effect mobility, small threshold voltage and large on/off current ratio. They are 0.31 cm2/(V·s), -3.8 V and 2.9×105, respectively. This way can be performed with other organic buffer layer materials, and high-performance OFETs will be realized for the flexible OFETs.

615 Quality of life of inpatients with lung cancer and related influencing factors

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石晓东 (1990-)，男，河北晋州人，硕士研究生，2014年于河北工业大学获得学士学位，主要从事新型电子材料及器件的研究。

E-mail: shixiaodong103@163.com王伟(1976-)，男，天津人，博士，副教授，2005年于天津大学获得博士学位，主要从事半导体器件与物理、电子功能材料与元器件的研究。

E-mail: wangwei@hebut.edu.cn

2016-07-02；

2016-08-28

河北省自然科学基金(F2012202075)； 天津市自然科学基金(15JCYBJC52100)资助项目 Supported by Natural Science Fundation of Hebei Province(F2012202075); Tianjin Natural Science Fundation(15JCYBJC52100)

Ta2O5-PMMA复合栅绝缘层对OFETs性能的影响

石晓东， 王 伟*， 李春静， 任利鹏， 尹 强

(河北工业大学 电子信息工程学院， 天津市电子材料与器件重点实验室， 天津 300401)

选用五氧化二钽(Ta2O5)-聚甲基丙烯酸甲酯(PMMA)复合材料作为栅绝缘层制备了并五苯有机场效应晶体管(OFETs)。通过在Ta2O5表面旋涂一层PMMA可以降低栅绝缘层的表面粗糙度，增大其场效应晶体管的迁移率。研究了厚度在20～60 nm范围内的PMMA对复合绝缘层表面形貌、粗糙度以及器件电学性能的影响。结果表明，当PMMA厚度为40 nm时，器件的电学性能最佳。与单一的Ta2O5栅绝缘层器件相比，其场效迁移率由4.2×10-2cm2/(V·s)提高到0.31 cm2/(V·s)；栅电压增加到-20 V时，开关电流比由2.9×102增大到2.9×105。

Ta2O5-PMMA； 绝缘层； OFETs； 迁移率； 开关电流比

1000-7032(2017)01-0070-06

O47; TN321+.5 Document code： A

10.3788/fgxb20173801.0070