1. Introduction

The field of organic thin film transistors (OTFTs) has been vastly developed in the past two decades due to its promise for low cost solution based processing for various electronic applications, mechanical flexibility, versatility of chemical design and synthesis, and ease of processing^[1-4]. OTFTs offer great potential for applications in chemical and biological sensing for environmental monitoring, and industry manufacturing. A key challenge for realizing practical applications lies in developing gate dielectrics with low leakage current, low interface trap density, high breakdown strength, and high capacitance for low-voltage OTFTs^[5]. Typically, low-voltage devices are achieved through increasing the capacitance density of the gate dielectric ($C_{\rm i})$ by either decreasing the thickness ($d$) or increasing the dielectric constant ($k)$ ($C_{i}$ $=$ $\varepsilon_{0}k/d$). Common dielectric materials for low-voltage devices include ultra thin polymer films^[6], polyelectrolytes^[7], inorganic oxides^[8], and hybrid organic/inorganic dielectrics^{[9, 10]}. Therefore, control over the interface between the organic semiconductor and inorganic electrode/dielectric is essential. Molecular self-assembled monolayers (SAMs) have been proven to be excellent candidates for gate dielectrics in low-voltage OTFTs^[11].

In order to design electronic devices with good electrical properties, SAMs fabricated on Si/SiO$_{2}$ gate dielectrics were introduced in the process of making devices. SAMs provide a converient, flexible, and simple system with which to tailor the interfacial properties of the dielectric layer and semiconductor layers. SAMs are ordered molecular assemblies formed by the adsorption of active surfactants on a solid surface^[12]. The order in these two-dimensional systems is produced by a spontaneous chemical reaction at the interface. This simple fabrication process makes SAMs easy to manufacture and thus technologically attractive for surface and interface engineering^[13]. Generally, by tuning the surface terminal group of the SAM, such as inducing the group of electron donator or electron accepter, it is possible to modify the interface between the organic semiconductor and dielectric by exploiting compatible organic/organic interactions resulting in improved device performances^{[14, 15]}. Phosphonic acids (PA) as the SAM material are more desirable for self-assembly on metal oxides and the silicon dioxide surface. Also, the phosphonic acid monolayer shows greater hydrolytic stability than siloxane monolayers^[16]. Liakos et al. compared phosphonic acid monolayers to a variety of binding chemistries and found that phosphonic acid readily formed well-packed monolayers compared to the amine, trimethoxysilane, trichlorosilane, and epoxy-binding groups. They also noted an inability to form trichlorosilane monolayers due to self-polymerization^{[17, 18]}. PA-based SAMs were chosen taking into consideration several distinct advantages over their silane-based counterparts including (1) better stability to moisture, (2) less tendency for homo-condensation between PAs, and (3) the reaction between PA and the silicon oxide substrate is not limited by the content of surface hydroxyl groups^{[19, 20]}. PA head-groups form dense and robust SAMs suitable as resists for selective chemical etching and patterning, and functional materials for organic electronics^[21].

In this work, we mainly use a phosphonic acid derivative (ADO-phosphonic acid) as the self-assembled monolayers (SAMs) on the Si/SiO$_{2}$ surface to induce the crystallization of rubrene in vacuum deposited thin film transistors, which showed a better field-effect mobility than rubrene on a bare SiO$_{2}$ wafer. It is found that ADO-phosphonic acid SAMs play a unique role in modulating the morphology of rubrene to form a crystalline film in the thin-film transistors.

2. Experiment

2.1 Synthesis of ADO-phosphonic acid

ADO-phosphonic acid was obtained from PCI synthesis.

2.2 SiO$_{2}$ surface modification with ADO-phosphonic acid

An oxidized silicon wafer (Si is highly n-doped with a resistivity smaller than 0.005 G$\cdot$cm and the thermally grown SiO$_{2}$ is 300 nm thick) was used as the substrate for organic thin film transistors. The following surface treatments of the SiO$_{2}$ were performed on the dielectric surface before vacuum sublimation of the semiconductor film, a 10 min sonication in acetone, followed by a 70 : 30 H$_{2}$SO$_{4}$/H$_{2}$O$_{2}$ (piranha) etch for 1 h at 100 ℃, then a 1 : 1 : 5 NH$_{3}$$\cdot$H$_{2}$O/H$_{2}$O$_{2}$/deionized H$_{2}$O wash for 20 min at 70 $^{o}$C, and the silicon wafer was held vertically using a small clamp in a solution of ADO-PA (1 mM in THF) in a 50 mL beaker. The solvent was allowed to evaporate slowly over 3 h, until the level of the solution fell below the silicon wafer. Under these conditions, the concentration of the ADO-PA in the remaining solution increased by about 30%. The treated Si sample was then removed from its holder and was heated at 140 ℃ in a simple glass tube for two days to bond the SAMs to the SiO$_{2}$/Si as octadecylphosphonate. ADO-PA /SiO$_{2}$/Si substrate was sonicated in THF for 10 min. The dielectric surface was characterized using contact angle measurements, which were $<$5$^\circ$ after cleaning and 85$^\circ$-90$^\circ$ after ADO-PA layer formation.

2.3 Transistor fabrication

he thin films composed of rubrene were vacuum-deposited by a TECHNOL ZHD-300 high vacuum evaporation machine through a shadow mask from a resistively heated Mo boat with the turbo-molecular pump at a pressure of 1.0 $\times$ 10$^{-6}$ Torr or lower, with a deposition rate of ca. 0.2 Å/s to the desired thickness. The temperature of the substrate was 90 ℃ for rubrene. During vacuum deposition, the distance between the source and the substrate was 15 cm. Different substrate temperatures for deposition were achieved using a radiant heater and measured with a thermocouple. Top-contact drain and source gold electrodes (50 nm) were vacuum-deposited through a shadow mask onto the films of rubrene in the same vacuum chamber, and the resulting semiconducting channels were 150 $\mu$m (L) $\times$ 2 mm (W), 100 $\mu$m (L) $\times$ 2 mm (W). In these transistors, highly n-doped silicon functioned as the gate electrode and 300 nm thick SiO$_{2}$ (untreated or treated with ADO-PA) functioned as the dielectrics.

2.4 Characterization and apparatus

The current-voltage measurement for thin-film transistors was carried out on a probe station using a Keithley 4200-SCS semiconductor parameter analyzer. During the measurement, the samples were kept at room temperature in the ambient atmosphere. The topographic images were obtained using a Nanoscope IIIa Multimode Microscope from digital instruments. AFM images were collected using the tapping mode in air under ambient conditions. The topographic images were collected from multiple samples, and for each sample, different regions were scanned to ensure the reproducibility. Polarized optical images of the devices were obtained from a Nikon 50IPOL microscope.

3. Results and discussion

Thin film transistors of rubrene on SiO$_{2}$ with the ADO-PA as SAMs were fabricated by thermal evaporation under a vacuum. With the formation of the ADO-PA SAM on SiO$_{2}$, the capacitance decreases due to a larger total dielectric thickness. However, even though ADO-PA possess chains longer than n-Octadecylphosphonic acid (ODPA) by 6-atom units, its p-conjugated anthryl surface groups are more polarizable than the methyl-terminated ODPA, likely leading to the higher capacitive coupling of the corresponding SAMs. Figure 1(a) shows the chemical structures of the rubrene and ADO-PA SAMs used in this work, and Figure 1(b) shows the device structure of the thin-film transistors.

As displayed in Fig. 2, the X-ray diffraction patterns from the rubrene film deposited on ADO-PA-treated SiO$_{2}$ show a lone diffraction peak at 2$\theta$ $=$ 6.55$^\circ$ ($d$ spacing of 13.50 Å). This peak agrees with the (2, 0, 0) diffraction derived from the single crystal structure of rubrene^[22]. Shown in Fig. 3(a) are the typical $I$-$V$ curves for such devices, from which field-effect mobilities of 0.07 to 0.18 cm$^{2}$/(V$\cdot$s) are measured in the saturation regime using the equation: $I_{\rm DS}$ $=$ ($\mu WC_{i}$/2$L)(V_{\rm G}$-$V_{\rm T})^{2}$ and $C_{i}$ $=$ 11 nF/cm$^{2}$ for 300 nm SiO$_{2}$. The on/off ratio of the drain current obtained between 0 and -50 V gate bias from the transfer $I$-$V$ curves (shown in Fig. 3(a)) is greater than 2.0 $\times$ 10$^{4}$ (Max 2.2 $\times$ 10$^{5})$.

The systematic improvements in the device characteristics include on-off current ratio ($I_{\rm on}$/$I_{\rm off})$, and field-effect mobilities ($\mu)$ using the ADO-PA SAM compared to bare SiO$_{2}$, OTS and ODPA modified SiO$_{2}$ are evident (Table 1). It is notable that the improvement of the $I_{\rm on}$/$I_{\rm off}$ using ADO-PA SAM compared to bare SiO$_{2}$ or OTS correlates to the observed decreased leakage current density using the anthryl-terminated SAMs. The improvement in the charge carrier mobility using the SAMs compared to the bare SiO$_{2}$ can be explained in part from a combination of the surface energy and chemical functionality at the rubrene/dielectric interface^[23].

To understand the unique role of ADO-PA in the induced crystallization of rubrene, the ADO-PA -treated SiO$_{2}$ with rubrene film (80 nm) was investigated using AFM. Shown in Fig. 4(a) is the AFM amplitude image of rubrene film (80 nm) on a SiO$_{2}$/ADO-PA surface. Rubrene forms flat crystals as long as 10 $\mu$m in the film deposited on the ADO-PA-modified SiO$_{2}$. The height image (Fig. 4(b)) shows that the average thickness of rubrene film is about 40-80 nm by thermal evaporation under a vacuum.

Figure 5 shows that the smoothness and uniformity of the dielectric surface does not significantly change after SAM preparation. After deposition of 80 nm rubrene on this SAM surface, we studied the reflection polarized light micrograph of the rubrene surface. In most polarization pictures (Fig. 6), rubrene is displayed as crystal film. The interlayer molecular is packing orderly, in which regions, the corresponding channels show a higher hole mobility. However, the black area is the disorderly rubrene film, the corresponding field-effect mobilities are relatively low, so the coming work will focus on optimizing the SAMs, making rubrene film crystallization and achieve stable devices properties.

4. Conclusion

In conclusion, we have unequivocally demonstrated a new strategy of using ADO-PA as the SAMs to induce the crystallization of rubrene in vacuum deposited thin film transistors. When the ADO-PA is replaced by a self-assembled monolayer of OTS or ODPA, rubrene films have much poorer crystallinity. As noted above, interface engineering, such as SAMs, is a great strategy to achieve a highly efficient semiconductor device. The molecular structure and morphology of the organic semiconductor at the interface are crucial to the organic thin film transistor performance. So the coming work will focus on finding some new SAMs, and new semiconductor materials to achieve better device performance.

Acknowledgment: We thank the Chongqing Key Laboratory of Chemical Process for Clean Energy and Resource Utilization for their testing apparatus.