Performance Improvement of ZnSnO Thin-Film Transistors with Low-Temperature Self-Combustion Reaction

Abstract

Conventional sol-gel solutions have received significant attention in thin-film transistor (TFT) manufacturing because of their advantages such as simple processing, large-scale applicability, and low cost. However, conventional sol-gel processed zinc tin oxide (ZTO) TFTs have a thermal limitation in that they require high annealing temperatures of more than 500 °C, which are incompatible with most flexible plastic substrates. In this study, to overcome the thermal limitation of conventional sol-gel processed ZTO TFTs, we demonstrated a ZTO TFT that was fabricated at low annealing temperatures of 350 °C using self-combustion. The optimized device exhibited satisfactory performance, with μ_sat of 4.72 cm²/V∙s, V_th of −1.28 V, SS of 0.86 V/decade, and I_ON/_OFF of 1.70 × 10⁶ at a low annealing temperature of 350 °C for one hour. To compare a conventional sol-gel processed ZTO TFT with the optimized device, thermogravimetric and differential thermal analyses (TG-DTA) and X-ray photoelectron spectroscopy (XPS) were implemented.

1. Introduction

Indium–gallium–zinc oxide thin-film transistors (TFTs) based on amorphous oxide semiconductors (AOSs) have been widely used because of their numerous advantages, such as high electron mobility and transparency [1,2,3]. However, because indium and gallium are rare-earth metals, they are too expensive to be applied in TFT channels. Therefore, zinc tin oxide (ZTO), which is low-cost and prevalent, has been recently considered as an alternative material [4,5]. Furthermore, AOS TFTs are generally deposited through a vacuum-based process, which imposes a spatial limit and is costly. In contrast, solution processes have attracted attention owing to their advantages, such as large-scale applicability, low cost, and simplicity [6,7,8]. However, when a ZTO channel layer is fabricated using a solution process, the required annealing temperature exceeds 500 °C, which is incompatible with most flexible plastic substrates [9,10].

To overcome this limitation, methods such as high-pressure annealing and combustion synthesis have been studied [6,11,12]. In particular, a self-combustion reaction efficiently lowers the annealing temperature because it requires lower chemical activation energy when compared with the conventional solution process [13]. The general combustion reaction requires various additives to initiate a reaction. However, self-combustion generates a combustive reaction by using precursor ligands that act as oxidizers and fuels [13,14,15,16]. Therefore, we simply lowered the annealing temperature without additional materials. In this study, to overcome the aforementioned thermal limitation, we fabricated a ZTO dual active layer (DAL) TFT at low annealing temperatures by varying the annealing conditions using the self-combustion reaction.

2. Materials and Methods

A conventional sol-gel processed ZTO (con-ZTO) solution (0.2 M) was prepared by dissolving zinc acetate dihydrate [Zn(CH₃COO)₂·2H₂O; Aldrich, 99.999%] and tin chloride dihydrate [SnCl₂·2H₂O; Aldrich, ≥99.995%] in 2-methoxyethanol with acetic acid added as stabilizer. The Zn:Sn molar ratio in the solution was 7:3. The self-combustion-based ZTO (self-ZTO) solution (0.1 M) was prepared by dissolving zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O, Aldrich, 98%] and tin(II) acetylacetonate [C₁₀H₁₄O₄·Sn; Aldrich, 99.9%] in 2-methoxyethanol. The Zn:Sn molar ratio in the solution was 5:5, and no stabilizer was used. The solutions were stirred at 400 rpm at 65 °C for 2 h and aged for 24 h to form homogeneous solutions. The mole concentration and ratio of each solution were achieved under the optimized conditions proposed in advanced reports [13,17].

A p-type Si wafer with a layer of SiO₂ (150 nm), which is formed as the gate-insulating layer, was utilized. The channel layers of the TFT were all deposited as a DAL, consisting of a front channel layer and a back channel layer. The con-ZTO solution was deposited on a substrate of Si by spin coating at 4500 rpm for 30 s and annealed at 180 °C for 10 min on a hot plate to form the front channel layer. The back channel layers were deposited under the same coating conditions of the front channel layer and annealed at 350, 400, and 470 °C for 1 h, respectively. The self-ZTO solution was deposited on a silicon substrate at 4500 rpm for 30 s and annealed at 180 °C for 20 min to form the front channel layer. Next, the deposition of the back channel layers was performed under the coating condition of the front channel layer and annealed at 330, 350, and 370 °C for 1 h, respectively. The self-ZTO DAL devices were annealed for 20 min, 40 min, and 1 h, respectively, at a fixed temperature of 350 °C. All devices are shown to have an amorphous structure because they are fabricated below an annealing temperature of 500 °C [18,19]. The channel layer thickness of the con-ZTO and self-ZTO was 25 nm. After channel deposition, the source–drain electrodes were deposited with a 100-nm thick aluminum layer via thermal evaporation. The structures of the fabricated devices are presented as cross-sectional schematics in Figure 1.

Thermogravimetric and differential thermal analyses (TG-DTA) were performed to confirm the chemical reaction of the solutions according to temperatures. X-ray photoelectron spectroscopy (XPS) was implemented to compare the binding energy states in the con-ZTO and self-ZTO channel layers annealed at 350 °C for 1 h.

3. Results

3.1. Electronical Performance

The transfer characteristic curves of the con-ZTO and self-ZTO DAL TFTs annealed at various annealing temperatures for one hour were measured, as shown in Figure 2. The drain voltage (V_D) was fixed at 20 V. The electrical parameters of the saturation mobility (μ_sat), threshold voltage (V_th), subthreshold swing (SS), and on–off-current ratio (I_ON/OFF) are listed in

Table 1. Electrical parameters of two TFT devices according to the annealing temperature.

	Temperature [°C]	μsat [cm2/V∙s]	Vth [V]	ION/OFF	SS [V/dec]
Con-ZTO DAL	350	1.37	+12.16	3.72 × 105	0.73
	400	2.80	+11.36	1.58 × 106	0.56
	470	5.67	+2.40	2.51 × 106	0.46
Self-ZTO DAL	330	2.91	+1.74	9.33 × 105	0.97
	350	4.72	−1.28	1.70 × 106	0.86
	370	5.60	−0.32	1.95 × 106	0.72

.

The electrical performance of the TFT devices was dependent on the annealing temperature. The on-current and saturation mobility increased with the temperature. Based on the list in Table 1, the con-ZTO DAL TFT deposited at 470 °C exhibited satisfactory performance. In contrast, the con-ZTO DAL TFT fabricated at 350 °C exhibited poor performance because the temperature was not sufficiently high to form a thin film. For the self-ZTO DAL TFT, the on–off-current ratios (I_ON/OFF) were 10⁵ and 10⁶ at 330 and 350 °C, respectively. The annealing temperature of 350 °C in the self-ZTO DAL TFT is associated with metal–oxygen (M–O) bonding. Therefore, the TFT device exhibited remarkable performance because of the sufficient formation of a thin film. As the temperature increased, the performance improved with the density of the thin film.

3.2. TG-DTA Analyses

The TG-DTA results of the chemical reactions of the con-ZTO and self-ZTO solutions at various temperatures are shown in Figure 3. In all the solutions, a significant weight loss occurred at temperatures approximating 100 °C owing to the evaporation of the solvent. The weight of the con-ZTO solution decreased at 350 °C because of the pyrolysis and hydrolysis via an endothermic reaction. Weight loss also occurred at 450 °C owing to condensation, and a thin film was formed because of the M–O bonding. Therefore, the con-ZTO DAL TFT exhibited the best performance at an annealing temperature of 470 °C.

There was no remarkable exothermic peak corresponding to the con-ZTO solution during the chemical reaction. However, in the case of the self-ZTO solution, the first and second exothermic peaks appeared at approximately 130 °C and 350 °C, respectively. After the occurrence of the first exothermic peak, the initial combustion caused weak endothermic M–O bonding and ligand pyrolysis; however, this was insufficient to form a thin film. After the second exothermic peak was observed, M–O bonding was completed, and a thin film was formed. The thin film densified because of the endothermic reaction at temperatures exceeding 350 °C [12,13]. Based on these TG-DTA results, the formation temperatures of the con-ZTO and self-ZTO thin films were confirmed as 450 °C and 350 °C, respectively.

3.3. Self-ZTO Properties at Various Annealing Time

The transfer characteristic curves of the self-ZTO DAL TFT at various annealing times at a fixed temperature of 350 °C are shown in Figure 4. When the self-ZTO channel layer was annealed for 20 min, 40 min, and one hour, the SS values were 1.42, 1.12, and 0.86, respectively. In addition, as the annealing time increased, the off-current value decreased. In general, the solution process based on the endothermic reaction formed a thin film by continuously transmitting external energy based on the temperature. In the combustion synthesis, the driving force required to form the thin film was derived from chemical energy. Therefore, the required temperature for the combustion reaction was considered as that required to initiate the reaction [12]. However, when the annealing time increased, the SS value improved, and the off-current decreased. It is deduced that an increase in the annealing time leads to a decrease in the defects—such as residues, impurities related to precursors, and pores—in the channel layer by the endothermic reaction. This indicates that the solution process based on the combustion reaction needs the continuous injection of thermal energy to improve the electric performance [20]. In other words, even if the combustion reaction is initiated, an endothermic reaction is involved in the annealing process. Hence, we conclude that a self-combustion reaction accompanies the endothermic reaction during the annealing process. Accordingly, the SS and off-current values are controlled by the annealing time. The self-ZTO DAL TFT exhibited optimal performance when the thin film was annealed at 350 °C for one hour.

The transfer characteristic curves of the con-ZTO and self-ZTO DAL TFTs fabricated at an annealing temperature of 350 °C for one hour are presented in Figure 5. It is shown that the V_th of the self-ZTO DAL TFT shifted in a more negative direction than V_th of the con-ZTO DAL TFT. The con-ZTO DAL TFT and self-ZTO DAL TFT showed optimal performance at a 7:3 and 5:5 mole ratio [13,17]. The self-ZTO DAL TFT has a higher Sn content than the con-ZTO DAL TFT, which leads to an increase of the electron carrier in thin film. Therefore, the V_th is shown to have shifted further in the negative direction than the con-ZTO DAL TFT. A positive V_th can be obtained by controlling the mole ratio.

The con-ZTO DAL TFT exhibited inadequate performance with a saturation mobility of 1.37 cm²/V∙s, V_th of +12.16 V, SS of 0.73 V/decade, and I_ON/_OFF ratio of 3.72 × 10⁵. When compared to the con-ZTO DAL TFT, the self-ZTO DAL TFT exhibited improved electric performance with a saturation mobility of 4.72 cm²/V∙s, V_th of −1.28 V, SS of 0.86 V/decade, and I_ON/_OFF ratio of 1.70 × 10⁶.

3.4. XPS Analysis

The O 1s peak of the XPS spectra shows the binding energy states in the (a) con-ZTO and (b) self-ZTO films in Figure 6. The typical O 1s peak consists of three energy level peaks: O_L, O_M, and O_H at 530.21, 531.40, and 532.35 eV, respectively; O_TOT is the total O 1s region. The O_L peak is a red region in the O 1s region and corresponds to the M–O lattice species due to the binding of O₂ ions with Zn and Sn. The O_M peak is the blue region in the O 1s region; it corresponds to the oxygen-deficient regions in the film. The O_H peak is the green region in the O 1s region, corresponding to the metal cation and hydroxyl group (M–OH) lattice species within the film.

The O_L/O_TOT ratios of the con-ZTO and self-ZTO films were 76.13% and 79.37%, respectively. The O_M/O_TOT ratio of the con-ZTO film was 15.65%, and that of the self-ZTO film was 18.28%. Lastly, The O_H/O_TOT ratio of the con-ZTO film was 8.22%, and that of the self-ZTO film was 2.35%. The M–O bond serves the channel to move electrons in the channel layer via the overlap of the s orbitals of the metal cations. The M–OH bonds act as trap sites that hinder the movement of electrons as defect states in the thin film; these bonds tend to depend on the annealing temperature [7,21]. Compared with the con-ZTO film, the self-ZTO film exhibited that an O_M area was larger and an O_H area was smaller. Therefore, we could fabricate a TFT device with a superior performance to that of the conventional sol-gel processed ZTO TFT by applying the self-combustion reaction at a low annealing temperature of 350 °C.

4. Conclusions

We fabricated ZTO DAL TFT using the self-combustion reaction. The annealing temperature and time of the self-ZTO DAL were varied. When the annealing temperature was increased, the on-current also increased. The formation temperature of the self-ZTO film was confirmed as 350 °C. In addition, when the annealing time was increased, the SS and off-current improved. When the ZTO thin film was fabricated using the self-combustion reaction, M–O bonding occurred at an annealing temperature lower than that of the conventional solution process. Furthermore, based on the self-combustion reaction, an endothermic reaction was found to reduce the residues and pores in the channel layer during annealing. The self-ZTO DAL TFT device exhibited better performance compared to the con-ZTO DAL TFT when both were annealed at 350 °C for one hour. The XPS analysis demonstrated that the self-ZTO film had a higher M–O ratio and lower M–OH ratio than those of the con-ZTO film, thereby reducing the number of trap sites in the film.