Effects of Active Layer Thickness on the Electrical Characteristics and Stability of High-Mobility Amorphous Indium–Gallium–Tin Oxide Thin-Film Transistors

Abstract

Herein, we investigated the effects of active layer thickness (t_S) on the electrical characteristics and stability of high-mobility indium–gallium–tin oxide (IGTO) thin-film transistors (TFTs). IGTO TFTs, with t_S values of 7 nm, 15 nm, 25 nm, 35 nm, and 50 nm, were prepared for this analysis. The drain current was only slightly modulated by the gate-to-source voltage, in the case of the IGTO TFT with t_S = 50 nm. Under positive bias stress (PBS), the electrical stability of the IGTO TFTs with a t_S less than 35 nm improved as the t_S increased. However, the negative bias illumination stress (NBIS) stability of these IGTO TFTs deteriorated as the t_S increased. To explain these phenomena, we compared the O1s spectra of IGTO thin films with different t_S values, acquired using X-ray photoelectron spectroscopy. The characterization results revealed that the better PBS stability, and the low NBIS stability, of the IGTO TFTs with thicker active layers were mainly due to a decrease in the number of hydroxyl groups and an increase in the number of oxygen vacancies in the IGTO thin films with an increase in t_S, respectively. Among the IGTO TFTs with different t_S, the IGTO TFT with a 15-nm thick active layer exhibited the best electrical characteristics with a field-effect mobility (µ_FE) of 26.5 cm²/V·s, a subthreshold swing (SS) of 0.16 V/dec, and a threshold voltage (V_TH) of 0.3 V. Moreover, the device exhibited robust stability under PBS (ΔV_TH = 0.9 V) and NBIS (ΔV_TH = −1.87 V).

1. Introduction

Amorphous indium–gallium–zinc oxide (IGZO) thin-film transistors (TFTs) were reported for the first time by Nomura et al. in 2004; since, they have attracted significant attention because of their excellent electrical properties, low process temperature, large-area uniformity, and low fabrication cost [1,2,3,4,5]. Currently, IGZO TFTs are being widely used as a backplane for large-area active-matrix flat-panel displays, such as organic light-emitting diode (OLED) displays and liquid-crystal displays (LCD) [6,7,8]. The recent development of low-temperature polysilicon oxide technology is expected to further expand the application range of IGZO TFTs in the field of displays [7,8]. However, the relatively low field-effect mobility of IGZO TFTs (µ_FE = ~10 cm²/V·s) still hinders their application in the backplane of ultra-high-resolution and high-frame-rate displays [9,10]. This is because the OLED pixels require a high current to emit light. Therefore, the study presents a new low-voltage driving OLED pixel circuit with high-mobility amorphous oxide TFTs as the driving device with high resolution [11]. To date, various oxide semiconductors have been studied as active materials for high-mobility oxide TFTs [12,13,14]. Among them, IGTO has recently attracted considerable attention as a promising active material for next-generation high-mobility oxide TFTs. The In³⁺ and Sn⁴⁺ ions have almost similar electronic structures. They have a small effective electron mass due to the 5 s orbital overlapping structure, leading to a highly conductive path for electron carriers, and remarkably high mobility [15,16]. Furthermore, the IGTO TFTs exhibit excellent electrical characteristics, even at low annealing temperatures (below 200 °C) [17,18,19].

For oxide TFTs, active layer thickness (t_S) is an important parameter that strongly affects the electrical performance and stability of these TFTs. To date, extensive research has been performed to determine the effects of t_S on the electrical characteristics and stability of oxide TFTs comprising different active materials. However, previous studies have reported different results for the effects of t_S on the electrical characteristics and stability of oxide TFTs according to the type of active material and fabrication process conditions. For example, Cho et al. [20] and Yang et al. [21] reported that the positive bias stress (PBS) stability of indium–zinc-oxide and IGZO TFTs deteriorated with an increase in t_S, respectively. Nevertheless, Lee et al. [22] and Li et al. [23] observed an improvement in the PBS stability of IGZO TFTs with an increase in t_S. These results imply that investigating the effects of t_S on the electrical performance and stability of IGTO TFTs is extremely necessary to determine the optimal t_S for IGTO TFTs. In this study, we examined the effects of t_S on the electrical characteristics and stability of IGTO TFTs. IGTO TFTs, with the t_S values of 7 nm, 15 nm, 25 nm, 35 nm, and 50 nm, were prepared for this analysis. A systematic study was conducted to investigate the physical mechanisms responsible for the observed effects of t_S on the electrical performance and PBS/NBIS stability of IGTO TFTs.

2. Materials and Methods

Figure 1a,b shows the schematic and top-view optical image of the fabricated IGTO TFTs, respectively. The IGTO TFTs were constructed on a heavily doped p-type Si wafer (resistivity < 0.005 Ω$·$cm) covered by 100-nm thick thermally grown SiO₂. The heavily doped p-type Si wafer was used as a substrate and a gate electrode. Thermally grown SiO₂ was used as a gate dielectric. IGTO active layers with the thicknesses of 7 nm, 15 nm, 25 nm, 35 nm, and 52 nm were deposited on the substrate by direct current magnetron sputtering under the following conditions: working pressure, 3.0 mTorr, Ar/O₂; gas mixing ratio, 21/9 sccm; sputtering power, 150 W; and chuck temperature, room temperature (RT). Then, a 100-nm thick indium–tin-oxide layer was deposited on the IGTO active layer-coated substrate to prepare source and drain electrodes of the TFTs. Subsequently, a 30-nm thick Al₂O₃ thin film was deposited as a passivation layer on top of the resulting substrate using radio frequency magnetron sputtering at RT. Finally, the IGTO TFTs were thermally annealed on a hot plate at 200 °C for 2 h in ambient air.

The active, source/drain electrodes, and passivation layers were patterned using photolithography and a lift-off process. Electrical characteristics and stability of the IGTO TFTs were measured using a semiconductor parameter analyzer (4156C, Agilent Technologies, Santa Clara, CA, USA) at RT in ambient air. Crystalline structure of the IGTO thin films was analyzed using X-ray diffraction (XRD, New D8-Advance, Bruker-AXS, Wisconsin, USA) with CuKα radiation (λ = 0.15406 nm). Chemical properties of the IGTO thin films with different t_S were examined using X-ray photoelectron spectroscopy (XPS, K-alpha+, Thermo Fisher Scientific-KR, Seoul, Korea). An Ar ion beam was employed to sputter the Al₂O₃ and IGTO thin films before XPS characterization.

3. Results and Discussion

Figure 2 shows the XRD patterns of a 35-nm thick IGTO thin film fabricated on an aluminosilicate glass substrate. The XRD pattern demonstrates only halo peaks at approximately 23° and 45°, originating from the glass substrate [24]. The results shown in Figure 2 indicate that the IGTO thin film has an amorphous phase, which is consistent with the results of previous studies [17].

Figure 3 shows a semi-logarithmic scale plot of transfer curves for the IGTO TFTs (width/length (W/L) = 500 μm/500 μm), with the t_S of 7, 15, 25, 35, and 50 nm. Herein, the drain current (I_D) of all of the IGTO TFTs was evaluated by varying the gate-to-source voltage (V_GS) from −20 to 20 V at a fixed drain-to-source voltage (V_DS) of 1 V. Figure 3 shows that the I_D of the IGTO TFT with a t_S of 50 nm is slightly modulated by V_GS; therefore, the electrical characteristics and stability of only the IGTO TFTs with the t_S of 7, 15, 25, and 35 nm were examined in this study.

Table 1. Electrical parameters of IGTO TFTs with the t_S of 7, 15, 25, and 35 nm.

tS [nm]	VTH [V]	μFE [cm2·V−1·S−1]	SS [V/decade]	IOFF [A]
7	1.3	6.5	0.16	$5.75\times {10}^{-14}$
15	0.3	26.5	0.16	$2.16\times {10}^{-13}$
25	−0.7	26.9	0.20	$1.94\times {10}^{-13}$
35	−3.1	27.8	0.31	$2.91\times {10}^{-12}$

presents the electrical parameters of the IGTO TFTs with the t_S of 7, 15, 25, and 35 nm. The µ_FE was calculated using the maximum value of transconductance, and the V_TH was determined as the V_GS value causing I_D = W/L × 10⁻⁸ (A) at a V_DS of 1 V. The subthreshold swing (SS) was extracted as the d(V_GS)/d(logI_D) value in the range of 10⁻¹⁰ A < I_D < 10⁻⁹ A. The V_TH decreased and the SS and the off-current (I_OFF) increased with an increase in t_S (Figure 3 and Table 1). The IGTO TFT with the t_S of 7 nm exhibited a significantly smaller µ_FE than those of the IGTO TFTs with thicker active layers. However, the µ_FE of the IGTO TFTs with a t_S larger than 15 nm only slightly increased with an increase in t_S. The results shown in Figure 3 and Table 1 are consistent with those reported in previous studies for oxide TFTs, and they demonstrate that t_S significantly affects the transfer characteristics of IGTO TFTs [25]. The negatively shifted V_TH and the large SS for the thick active oxide TFTs have been mainly attributed to the large number of free electrons within the oxide semiconductor and the higher sheet trap density in the active layer, respectively [26,27,28]. The increase in I_OFF with an increase in t_S was considered to be due to enhanced bulk conduction through the back-active layer [27]. The increase in μ_FE with an increase in t_S has been primarily ascribed to the reduced surface roughness scattering in the oxide semiconductor. As the carrier transport layer is farther from the surface of the thick film, the effect of the surface roughness on the carrier mobility is weaker in the thick film, compared with the thin film [29,30].

Figure 4a–d shows the time dependence of the transfer curves for the IGTO TFTs with the t_S of 7, 15, 25, and 35 nm, respectively, at a V_OV (V_OV = V_GS − V_TH) of 20 V and a V_DS of 0 V. For every IGTO TFT, the transfer curves shifted in the positive direction with an increase in the stress time. The largest shift in V_TH (ΔV_TH) was observed for the IGTO TFT with the t_S of 7 nm (ΔV_TH = 3.1 V after stress for 3000 s), whereas the smallest ΔV_TH was noticed for the IGTO TFT with the t_S of 35 nm (ΔV_TH = 0.5 V after stress for 3000 s). Figure 4e depicts the ΔV_TH for the IGTO TFTs with different t_S under PBS at different stress times. Clearly, the magnitude of ΔV_TH for the fabricated IGTO TFTs decreased with an increase in t_S under PBS (Figure 4).

Figure 5a–d shows the time dependence of the transfer curves for the IGTO TFTs with the t_S of 7, 15, 25, and 35 nm, respectively, at a V_OV of −20 V and a V_DS of 0 V, under illumination using a white light-emitting diode (LED, wavelength 420–780 nm) backplane unit with a luminance of 2000 lux. For every IGTO TFT, the transfer curves shifted in the negative direction with an increase in the stress time. The largest magnitude of ΔV_TH was obtained for the IGTO TFT with the t_S of 35 nm (ΔV_TH = −11.2 V after stress for 3000 s), whereas the smallest magnitude of ΔV_TH was achieved for the IGTO TFT with the t_S of 7 nm (ΔV_TH = −0.6 V after stress for 3000 s). Figure 5e depicts the ΔV_TH for the IGTO TFTs with different t_S under negative bias illumination stress (NBIS) at different stress times. The magnitude of ΔV_TH for the fabricated IGTO TFTs increased with an increase in t_S under NBIS (Figure 5).

To investigate the physical mechanisms responsible for the effects of t_S on the PBS and NBIS stability of IGTO TFTs (Figure 4 and Figure 5), we characterized the O1s spectra of the IGTO thin films with different t_S obtained using XPS. Figure 6a–d shows the XPS O1s spectra of 7, 15, 25, and 35-nm thick IGTO thin films located near the IGTO/SiO₂ interface. The XPS O1s spectra were resolved into three sub-peaks originating from fully coordinated metal ions (metal–oxygen lattice (O_Latt)), oxygen vacancies (O_Vac), and impurity-related oxygen (O_Imp). The XPS peak positions assigned to these sub-peaks were 530.0 eV (O_Latt), 531.0 eV (O_Vac), and 532.5 eV (O_Imp), respectively [14,17]. Figure 7 shows the relative peak area ratios of O_Latt, O_Vac, and O_Imp for the IGTO thin films with different t_S, located near the IGTO/SiO₂ interface. The relative peak area ratio of O_Imp was highest for the 7-nm thick IGTO thin film, and decreased with an increase in t_S (Figure 7). In previously reported studies on IGZO or IGTO TFTs, O_Imp was mainly ascribed to the O bond in the hydroxyl group (OH) [17]. The OH creates acceptor-like trap states close to the conduction band (CB) edge and accelerates the electron trapping process under PBS because of its polar nature [17,31,32,33,34]. Considering this, the lower PBS stability of the IGTO TFTs with thinner active layers can be ascribed to the high concentration of OH in the channel layer. It has been reported that a thicker active layer can isolate the channel from ambient factors, including H₂O molecules, even after the passivation layer is deposited on the active layer [35]. Effective self-passivation from H₂O molecules in the air is the most probable reason for the better PBS stability of the IGTO TFTs with thicker active layers (Figure 7).

Figure 7 also shows that the relative peak area ratio of O_Vac is the smallest for the 7-nm thick IGTO thin film, and it increases with an increase in t_S. O_Vac are easily created by sputtering-induced damage in oxide semiconductors [27,36]. Therefore, the concentration of O_Vac in IGTO increases with an increase in t_S because of the longer sputtering time. O_Vac generates shallow and deep donor states in oxide semiconductors such as IGZO and IGTO [37,38]. Under NBIS, O_Vac is ionized to O_Vac²⁺, and O_Vac²⁺ drifts toward the oxide semiconductor/gate dielectric interface in oxide TFTs [39]. The increased concentration of electrons, and the formation of an O_Vac²⁺ accumulation layer near the gate, dielectric shifts the transfer curves of the TFTs in the negative direction [40]. The obtained results show that the lower NBIS stability of the IGTO TFTs with thicker active layers can possibly be attributed to the higher concentration of O_Vac in the IGTO thin film, caused by the more severe sputtering-induced damage. The experimental results shown in Figure 4 and Figure 5 indicate that it is necessary to consider the trade-off between the PBS and NBIS stability of the devices, while determining the optimum t_S for IGTO TFTs. From the transfer characteristics and PBS/NBIS stability of the IGTO TFTs with different t_S, it can be concluded that 15 nm is the optimal t_S for IGTO TFTs, leading to the best electrical characteristics (µ_FE: 26.5 cm²/V·s; SS: 0.16 V/dec; and V_TH: 0.3 V), and decent PBS and NBIS stability of IGTO TFTs.

4. Conclusions

In this study, we examined the effects of t_S on the transfer characteristics and stability of high-mobility amorphous IGTO TFTs with t_S values of 7, 15, 25, 35, and 50 nm. The obtained results showed that with an increase in t_S, V_TH shifted in the negative direction, and the SS and µ_FE of the fabricated IGTO TFTs increased. Clearly, the PBS stability of the IGTO TFTs improved as the t_S increased. However, the NBIS stability of the IGTO TFTs deteriorated with an increase in t_S. The XPS characterization results revealed that the better PBS stability and the low NBIS stability of the IGTO TFTs with thicker active layers were mainly owing to the decrease in the concentration of OH and the increase in the number of O_Vac in the IGTO, with an increase in t_S. We found that the optimum thickness of the active layer for the IGTO TFTs is approximately 15 nm, which results in a positive V_TH, an acceptably high µ_FE, and decent PBS and NBIS stability of the IGTO TFTs.