High-Performance Amorphous InGaSnO Thin-Film Transistor with ZrAlO_x Gate Insulator by Spray Pyrolysis

Abstract

Here, we report the high-performance amorphous gallium indium tin oxide (a-IGTO) thin-film transistor (TFT) with zirconium aluminum oxide (ZAO) gate insulator by spray pyrolysis. The Ga ratio in the IGTO precursor solution varied up to 20%. The spray pyrolyzed a-IGTO with a high-k ZAO gate insulator (GI) exhibits the field-effect mobility (μ_FE) of 16 cm²V⁻¹s⁻¹, threshold voltage (V_TH) of −0.45 V subthreshold swing (SS) of 133 mV/dec., and ON/OFF current ratio of ~10⁸. The optimal a-IGTO TFT shows excellent stability under positive-bias-temperature stress (PBTS) with a small ΔV_TH shift of 0.35 V. The enhancements are due to the high film quality and fewer interfacial traps at the a-IGTO/ZAO interface. Therefore, the spray pyrolyzed a-IGTO TFT can be a promising candidate for flexible TFT in the next-generation display.

1. Introduction

Nowadays, there is increasing interest in amorphous metal oxide semiconductor (MOS) thin-film transistors (TFTs) to replace silicon-based TFTs [1,2,3,4,5]. Wide bandgap MOSs, such as zinc oxide (ZnO), amorphous indium gallium zinc oxide (a-IGZO), and amorphous indium tin zinc oxide (a-ITZO), are widely studied for high-performance TFT [1,2,3,4,5,6]. The metal oxide TFTs can be manufactured via vacuum [1] or solution process [7,8,9,10,11,12]. Spray coating and spin coating [2,7,8,12] are popular deposition methods in the solution process. Solution-processed oxide semiconductor shows a polycrystalline structure when the substrate temperature is high (≥200 °C) [2,6,9]. The grain boundary has a lot of defects in polycrystalline MOs [2,3,10,13].

To obtain highly stable, solution-processed multicomponent amorphous MOS TFTs, various MOS are studied by doping metal cations such as aluminum (Al), gallium (Ga), hafnium (Hf), lanthanum (La), magnesium (Mg), yttrium (Y), and zirconium (Zr) into ZnO, InO_x, or IZO [2,10,14,15,16]. The mixing of two or more metal cations with different ionic radii and ionic charges is adequate to have an amorphous phase by suppressing crystallization [2,5,10]. The doping of a carrier suppressor can improve the electrical performance of MO TFTs [2,11,17,18,19,20,21,22,23]. Note that the doping of the metal cations with a lower standard electrode potential (SEP) can reduce the trap density due to their ability to form a strong bond with oxygen [2,10,19].

In this study, we introduced the high-performance amorphous indium-gallium-tin oxide (a-IGTO) TFT by spray pyrolysis. The Ga concentration was varied up to 20% in the InSnO precursor solution. The 10% Ga mixed a-IGTO TFT exhibits the field-effect mobility (μ_FE) of 16 cm²V⁻¹s⁻¹, threshold voltage (V_TH) of −0.45 V, subthreshold swing (SS) of 133 mV/dec., and ON/OFF current (I_ON/I_OFF) ratio of ~10⁸. The a-IGTO TFT exhibits excellent stability under PBTS with a small threshold voltage shift (ΔV_TH) of ~0.35 V at 5 V for 1 h. The improvements in the electrical properties of the a-IGTO TFT are due to the high quality of the film and smaller interfacial traps at the a-IGTO and zirconium aluminum oxide (ZAO) interface. Therefore, the spray pyrolyzed a-IGTO TFT with high-k ZAO can be a favorable candidate for high-performance, low-voltage MOS TFTs.

2. Materials and Methods

A 0.1 mole (M) IGTO precursor solution was synthesized by mixing indium nitrate hydrate (In(NO₃)·xH₂O), gallium nitrate hydrate (Ga(NO₃)₃.xH₂O), and tin chloride hydrate (SnCl₂.xH₂O) into a 2-methoxyethanol. The molar ratio of indium to tin is 3:1(In:Sn). The molar ratio of indium:tin is kept at 3:1(In:Sn) for InGaSnO and the Ga ratio varies from 1% to 15%. For example, for 1%, 3%, 8%, 10%, and 15% Ga in IGTO, the molar ratios of In:Ga:Sn are 3:0.04:1, 3:0.12:1, 3:0.35:1, 3:0.44:1, and 3:0.70:1, respectively. The molar ratio of Zr:Al is 1:1(Zr:Al) for ZAO as reported elsewhere [4]. All precursor solutions were vigorously stirred to obtain a transparent/homogeneous solution. Finally, all the precursor solutions were filtered through a 0.45 μm polytetrafluoroethylene (PTFE) filter.

The ZAO gate insulator layer was deposited by spray pyrolysis on a glass substrate at the substrate temperature of 400 °C for 20 cycles in an air environment, and a detailed process flow for ZAO appears elsewhere. [4] The spray-pyrolyzed ZAO exhibits the surface RMS roughness of 0.32 nm, film density of 4.16 g cm⁻³, leakage current density of 2.26 × 10⁻⁶ Acm⁻², breakdown electric field of 5.15 MVcm⁻¹, and negligible frequency-dependent capacitance [4].

We fabricated a bottom gate, top contact a-IGTO TFTs. First, a 40 nm molybdenum (Mo) film was deposited by sputtering and patterned through wet etching to form the gate electrodes (Mask #1). After the deposition of the ZAO film, the sample was kept on a hotplate for 3 min. The IGTO solution was deposited at the substrate temperature of 350 °C by spray pyrolysis. An Alpha-Step D-500 Stylus Profiler was used to analyze the thickness of the IGTO films. The thickness of all the IGTO films is ~9.2 nm. The a-IGTO layer was patterned through wet etching to have an active island by conventional photolithography (Mask #2). Then, immediately, a ZAO layer was patterned through wet etching to form via contacts (Mask #3). Finally, a 40 nm thick IZO layer was deposited by sputter and patterned through wet etching for source/drain electrodes (Mask #4). The Agilent 2902C semiconductor device parameter analyzer was used to measure the I-V curves for the electrical properties of a-IGTO TFTs.

2.1. Results and Discussions

Figure 1a displays the fabrication process flow on a glass substrate and schematic representation for the cross-sectional view of an a-IGTO TFT as shown at the bottom of the figure. The channel width (W) and channel length (L) of the TFTs are 50 and 10 μm, respectively. Figure 1b shows the cross-sectional TEM image of spray pyrolyzed IGTO/ZAO stacked film, where a clear interface can be seen between IGTO (~10 nm) and ZAO (~40 nm). Figure 2 shows the XRD spectra of IGTO films, where the Ga ratio in the IGTO precursor solution varied from 1 to 15%. The XRD spectra of all the IGTO films show no crystalline peaks indicating an amorphous structure. The peak at 25° is related to glass. Ga has a low standard electrode potential (SEP) of −0.54 V, low electronegativity of 1.81, and high bonding energy with oxygen (354 kJ/mole), which leads to a good nominee as a structure transformer of ITO.

The optical bandgaps of IGTO films (Ga: 1 to 10%) are shown in Figure 3a. For UV-Vis measurement, the a-IGTO films were deposited on the glass (Corning eagle), where the glass absorption can be negligible [2,22,23,24]. The increase in bandgap with an increase in Ga ratio in IGTO from 3.62 (1% Ga) to 3.69 eV(10% Ga). The bandgap of IGTO is higher than that of ITO (3.19 eV) due to a higher bandgap of GaO (4.8 eV) [3,6,10]. The optical bandgap, E_g, was extracted using the relationship: αhν = A (hν − E_g)^1/n, where α, hν, A, and n are the absorption coefficient, photon energy, a proportionality constant, and optical transition exponent in the absorption process, respectively. XPS valance band spectra of IGTO ((Ga: 0 to 15%)) films measured are shown in Figure 3b. The Fermi energy level (E_F) shifts from 2.44 (1% Ga) to 2.27 eV (15% Ga) above the valence band edge, as shown in Figure 3b. To achieve the accurate Fermi-level position, the Ultra-violet photoelectron spectroscopic (UPS) experiment was conducted for the IGTO thin films with different Ga fractions. The work function (Φ) values for IGTO films with the different (1, 10, and 15%) Ga ratios are 4.49, 4.56, and 4.59 eV, respectively. With increasing Ga incorporation Fermi level shifts away from the conduction band (CB) as shown in Figure 3c. This indicates the shift of E_F toward the valance band (VB) edge, as shown in Figure 3d. The energy difference between the Fermi energy level (E_F) and the conduction band (CB) edge for IGTO is 1.18 eV (1% Ga) and 1.44 eV(15% Ga). Note that the general trend for the electron concentration in the oxide semiconductor decreases with an increase in Ga doping ratio in IGTO [12,13,14,25]. The surface morphology of spray pyrolyzed IGTO film can be seen in Figure 3e. The root means square roughness (R_RMS) for 1, 3, 10, and 15% Ga-doped IGTO films are 2.31, 1.34, 0.74, and 0.81 nm, respectively. With the increase in the Ga ratio in the IGTO film, the surface roughness became smoother. The smooth surface morphology of the IGTO film could provide a good interface with the gate insulator. Therefore, these results confirm that an appropriate Ga ratio in IGTO film could enhance the electrical characteristics of IGTO TFT.

To elucidate the physical mechanism, we studied and compared O1s XPS spectra for the chemical bonding analysis of the a-IGTO films by varying Ga ratios as shown in Figure 4a–f. The O1s spectra are deconvoluted into three regions with the binding energies centered around ~529 eV (M-O-M), ~531 eV (V_o), and ~532 eV (−OH), respectively. The M-O m percentages in IGTO films with different Ga ratios (1, 3, 8, 10, and 15%) are 56.67, 61.16, 63.01, 69.22, and 61.86%, V_o ratios are 32.34, 28.42, 28.12, 23.59, and 28.11%, and the −OH ratios are 10.99, 10.42, 8.87, 7.19, and 10.03%, respectively. M-O m bonds, fewer oxygen-related defects (V_O + OH) were obtained in Ga (10%) case compared to the others. The decrease in V_o is compensated by a higher M-O-M ratio [12]. The percentages of M-O, Vo, and -OH of O1s peak summarized in Figure 4f and Table S1.

The charge transport properties in a-IGTO as a variation of the Ga ratio were studied by measuring the transfer curves of the a-IGTO TFTs. Figure 5a shows the transfer curves of IGTO TFTs with different Ga (1, 3, 8, 10, and 15%) ratios, and the corresponding electrical properties such as μ_FE (7.93, 6.76, 12.05, 16.0, and 4.0) cm²V⁻¹s⁻¹, V_TH (−2.35, −1.55, −0.75, −0.45, and 0.0) V, and SS (196.6, 148.5, 138.2, 133, and 123) mV/dec can be seen in Figure 5b. The transconductance (gm, ΔI_D/ΔV_G) of the IGTO TFTs with different Ga ratios (1, 3, 8, 10, and 15%) are (1.91, 1.75, 3.02, 3.77, and 3.08) 10⁻⁷S, respectively, as can be seen in Figure S1. The electronic configuration (4d¹⁰5s⁰) of In³⁺ is similar to Sn⁴⁺, leading to a highly conductive nature with a very depletion mode of TFT characteristics [5]. In an amorphous metal oxide semiconductor, Ga acts as a good carrier killer owing to its higher binding energy of Ga-O (374 kJ/mole) and higher bandgap of GaO_x (4.78 eV). In this work, we studied 1, 3, 8, 10, and 15% Ga doping in IGTO TFT performances. From the XPS analysis of IGTO film, upon the increase in the Ga ratio in IGTO, the M-O ratio increases and consequently, the oxygen-related defects (V_o and –OH) are reduced [25]. This change is favorable for forming a good interface with the underlayer (i.e., gate insulator). Higher M-O m bonds lead to an increase in the overlapping of the metal s orbital. Therefore, the increase in overlapping can also explain the higher mobility of the optimum Ga-doped IGTO TFT. The selection of an appropriate ratio of carrier suppressor is an important parameter to control the carrier concentration. However, an excess doping ratio can modify the material structure, which leads to deteriorating mobility. The optimum Ga doping concentration is found to be 10% from the TFT data, this is due to the 10% Ga doped IGTO film giving a good interface with the gate insulator. Due to fever traps at the interface between GI/semiconductor, the TFT exhibits lower SS and a higher I_ON, as shown in Figure 5b. Upon the Ga doping, the V_TH shifts towards positive V_GS due to gallium acting as a carrier killer since Ga⁺ has a higher binding energy with oxygen [2,10,13,14,15,26]. The decrease in the SS of IGTO TFT indicates the reduction in traps at the IGTO/ZAO interface. Figure 5c shows the output curves of IGTO TFT with a different V_GS showing clear pinch-off and saturation behavior.

To test the electrical stability of the IGTO TFT, the PBTS (at V_GS = 5 V for 1 h) on the TFT performances was investigated. The evaluation of the transfer curve of IGTO TFT under PBTS for 1 h, shows the threshold voltage shift (ΔV_TH) of less than 0.4 V, as can be seen in Figure 5d. Small V_TH shift indicates fewer electron traps at the interface between the gate dielectric and the semiconductor channel layers [5,10,13,14,16]. The interfacial trap density was calculated for the IGTO TFT at different stress times from 0 to 3600 s; negligible change in trap density obtained after 1 h of bias stress is shown in Figure 4d. Note the insignificant change in SS even after bias stress because of fewer trap states at the interface [2,3,10,11,12,13,24,25,26,27]. Comparison of the electrical properties of oxide TFTs using spray pyrolysis appears in the literature [17,18,19,20,21,26] (

Table 1. Comparison of the electrical properties of oxide TFTs fabricated by spray pyrolysis reported in the literature.

Active/GI	TFT W/L [μm]	Mobility [cm2 V−1 s−1]	SS [mV dec−1]	Stability (ΔVTH, [V])	Ref.
(a)IGTO/(a)AlOx	200/10	3.00	210	PBS (0.90)	[17]
(b)IGTO/(c)SiO2	1000/50	2.13	2440	PBS (2.10)	[18]
(d)ZTO/(e)ZAO	1000/100	12.50	150	PBS (2.90)	[19]
(b)In2O3/(b)ZAO	1000/100	10.14	100	PBS (0.47)	[20]
(f)ZnO/(b)ZrOx	50/10	12.76	260	PBS (0.0)	[21]
(f)IGTO/(f)ZAO	50/10	16.00	133	PBTS (0.40)	This Work

^(a) Ink-jet Printing; ^(b) Spin Coating; ^(c) Thermal Growth; ^(d) Sputtering; ^(e) ALD; ^(f) Spray Pyrolysis.

). The improvement in the electrical properties is due to its better interface quality with the GI because of fewer interfacial defects [25,26]. Therefore, it is concluded that the better electrical properties of IGTO TFTs can be achieved by optimizing the Ga ratio.

A higher M-O m ratio leads to an increase in the overlapping of the metal s orbitals [5,9,13,14,15,16]. The increase in overlapping can clarify the higher field-effect mobility and excellent bias stability of the a-IGTO TFT. The electrical properties of IGTO TFT exhibit relatively high mobility and superior stability under positive bias temperature stress (PBTS) at 60 °C, compared to the nanocrystalline ZnO and doped ZnO TFTs. Therefore, it is believed that the electrical properties of optimum IGTO TFTs can be achieved by altering the Ga ratio [25]. Hence, amorphous ZAO and IGTO are promising candidates for high-performance flexible TFT, and their application in the next-generation low-cost display industry.

2.2. Materials and Thin-Film Characterization

Transmittance and absorbance were measured by UV–visible spectroscopy (Scinco, S-4100). An alpha-step analyzer was used to measure the film’s thickness. The structural properties of IGTO films were measured by X-ray diffraction (XRD). Atomic force microscopy (AFM) for surface morphology. X-ray photoelectron spectroscopy (XPS) measurement was carried out to examine the elemental and chemical composition in IGTO films using PHI 5000 Versa Probe (PHI 5000 Versa Probe, Ulvac-PHI) at a basic pressure of 7.5 × 10⁻⁵ mTorr.

Electrical characterization of IGTO TFTs was performed by Agilent 4156C semiconductor parameter analyzer at room temperature in dark. The output curve was measured at the V_GS of 5 V with a step of 1 V by sweeping the V_DS from 0 to +5 V. The field-effect mobility in the saturation region (V_DS ≥ V_GS–V_TH) was obtained by using Equation (1):

$${\mathrm{I}}_{\mathrm{DS}}=\frac{1}{2}\frac{\mathrm{W}}{\mathrm{L}}{\mathsf{\mu }}_{\mathrm{sat}}{\mathrm{C}}_{\mathrm{ox}}{({\mathrm{V}}_{\mathrm{GS}}-{\mathrm{V}}_{\mathrm{TH}})}^2$$

(1)

Here, the threshold voltage (V_TH) and saturation mobility were determined from the x-axis intercept and linear part of the (I_DS)^1/2 vs V_GS plot by linear extrapolation method. The subthreshold swing (SS) was obtained from the linear region of the log (I_DS) versus V_GS fit by using the equation:

$$\mathrm{SS}=\frac{{\mathrm{dV}}_{\mathrm{GS}}}{\mathrm{d}(\log {\mathrm{I}}_{\mathrm{DS})}}$$

(2)

Here, interfacial trap density (N_it) was obtained from the following Equation (3):

$${\mathrm{N}}_{\mathrm{it}}=\left[\frac{\left(\mathrm{SS}\log \left(\mathrm{e}\right)\right)}{\left(\frac{\mathrm{KT}}{\mathrm{q}}\right)}-1\right]\left({\mathrm{C}}_{\mathrm{ox}}/q\right)$$

(3)

where I_DS, μ_sat, W/L, C_ox, V_TH, and V_GS are the drain current, saturation mobility, channel width/length, capacitance of the gate oxide, threshold voltage, and gate voltage, respectively.

3. Conclusions

In summary, we report the high performance of a-IGTO TFT by spray pyrolysis. The optimal Ga ratio is found to be 10% in the a-IGTO precursor. The spray pyrolyzed a-IGTO and ZAO layers were integrated with TFT as channel and gate insulator layers, respectively. The TFT with 10% Ga exhibits the μ_FE of 16 cm²V⁻¹ s⁻¹, SS of 133 mV/dec., V_TH of −0.45 V, and I_ON/I_OFF of ~10⁸. The improvement is due to the high film quality with fewer trap states at the interface between a-IGTO and ZAO. Hence, the solution-processed a-IGTO is a promising candidate for the high performance of amorphous metal-oxide TFT.