Studying saturation mobility, threshold voltage, and stability of PMMA-SiO₂-TMSPM nano-hybrid as OFET gate dielectric

Highlights

• The PMMA-SiO₂-TMSPM nano-hybrid samples were prepared using sol-gel technique.

• PMMA-SiO₂-TMSPM thin films were fabricated by spin coating technique on p-type Si.

• Influence of surface morphology devices on the leakage current density, charge-carrier mobility and output current were studied.

• At V_DS = –10 V, the OFET device exhibited good transfer characteristics such as μ_FET,S of 0.0749 cm² s⁻¹ V⁻¹ and V_th of 32 V.

• By applying high voltages to the gate dielectric (V_GS = –80 V), the OFET devices had good stability.

Abstract

PMMA-SiO₂-TMSPM (polymethyl methacrylate- silicon oxide- (3-trimethoxysilyl)propyl methacrylate) nano-hybrid solutions were synthesized using sol-gel process with the constant weight ratio of PMMA-SiO₂ and different weight ratios of TMSPM. Hybrid solutions were deposited on p-type Si (111) substrate using the spin coating technique. Surface morphology was studied using scanning electron microscopy (SEM) technique and the size of the nanoparticles was about 6 nm to 13 nm. Capacitance-voltage (C-f) measurements and current-voltage (I–V) curves were also studied in metal-insulator-semiconductor (MIS) structures. According to the results presented from J_GS curves in terms of V_GS, the gate leakage current densities were small enough to be used as gate dielectric material in OFETs. At V_DS = −10 V, in the saturation region, it is considered (transfer characteristic curves), maximum mobility μ_S,FET was related to the sample without TMSPM because of its minimum dielectric constant. However, the surface morphology of this sample shows discontinuous nanoparticles with maximum traps on the path, which led to decreased I_DS in the channel. To overcome this challenge, other samples were produced in the presence of TMSPM as the coupling agent. The sample with 0.15 wt ratio of TMSPM has flatter surface morphology and more continuous dispersion than that of other samples so that the number of traps on the path decrease, nanoparticles distribute continuously on the surface of the thin films, and I_DS increased in the channel. By applying high gate voltages (V_GS = −80 V), according to I_DS curves in terms of V_DS, the OFETs show good stability.

Introduction

The interface layer between (silion oxide) and silicon in the gate dielectric are highly important, because the operation of the field-effect transistors and reliability of devices are controlled by the transistor’s gate [1], [2], [3], [4], [5], [6]. Additionally, the thinner the thickness of the oxide at interface layer, the more important its role for compatibility with higher rates in thin electrical complex circuits would be; also, challenges such as increased (the tunneling and leakage currents through the silicon oxide thin film.) [1], [2], [3], [4], [5], [6].

To overcome this challenge, devices are suggested which used the oxides with high dielectric constant as the gate dielectric that decreased the leakage current [1], [2], [3], [4], [5], [6]. Despite the mentioned advantage, thermal instability and increased equivalent oxide thickness are among the disadvantages of these thin film devices [1].

Researchers have proposed a new approach based on using organic materials as gate dielectric [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Organic filed-effect transistors (OFETs) are often produced by two structures of metal-insulator-semiconductor (MIS) and metal-polymer-semiconductor (MPS) [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. The advantages of using polymers as organic gate dielectric materials are decreased leakage current, low weight, as well as high flexibility and capability of preparation thin films at ambient temperature and pressure [9], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. In recent years, flexible OFETs have attracted great attention as a suitable option in order to construct flexible monitors, electronic coatings, smart cards [21], [22], [23]. One of the unique advantages of present polymers as organic materials is that, by making a layer with flat and smooth surface morphology, the electrical current of transistors increases [21]; also, by decreasing dielectric loss, the thermodynamic stability of transistors improve during their operation at high frequencies [10].

Previous studies have shown that using a suitable polymeric dielectric in the interface layer lead to modify gate dielectric (organic and inorganic hybrids) and also composites to increase the field-effect mobility [7], [13], [14], [15]. Because the band gap of the polymer is smaller than the oxide band gap (PMMA band gap is equal to 4.4 eV between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) compared with SiO₂ band gap of 9.3 eV between oxide conduct band and valence band), the charge carriers mobility increases, which can be because of the phenomena such as tunneling, field emission, and conductance due to the production and recombination of charge carriers in a double electrical potential barrier with different shapes [7], [9], [12], [16]. Their main objects are to decrease leakage currents as well as increasing the mobility of charge-carriers in substances which are used as a gate dielectric in OFETs. Also, at high frequency, the presence of polaron carriers in the polymer can be mentioned as another worthy characters of OFETs [7], [9], [12], [16].

In this study, PMMA-SiO₂-TMSPM nano-hybrid samples with constant weight ratio of polymer and oxide and different weight ratios of TMSPM are synthesized by sol-gel method at room temperature and ambient pressure. Then, they are deposited using spin coating technique on p-type Si (111). Quantities such as the field-effect charge carrier mobility μ_S,FET, threshold voltage V_th, and on/off current ratio I_on/I_off are obtained by current-voltage (I–V) measurements and transfer characteristic curves. In addition, the electrical stability effects are investigated by applying high voltages to the gate.