Bias Stability Enhancement in Thin-Film Transistor with a Solution-Processed ZrO2 Dielectric as Gate Insulator

Abstract

The gate dielectric plays an important role in TFTs because it manipulates the conductance of the semiconducting channel by accumulating charge carriers. [...]its electrical insulation to minimize a leakage current is another critical requirement for minimal static dissipation [5], which simultaneously affects the transfer performance and the stability and lifetime of TFT devices. High-k dielectrics can increase the capacitive coupling between the gate and the active channel layer, which not only increases the driving current, but also reduces the operating voltage. [...]high-k materials are highly desirable for improving the electrical performance, reducing the size of the device, as well as reducing energy consumption [6]. [...]the Al source/drain electrodes with 100-nm-thickness were deposited by direct current (DC) sputtering at room temperature. [...]ZrO2 films fabricated by spin-coating with 0.3 M precursor for six times had lower leakage current density and better electrical uniformity than the film fabricated by spin coating with 0.6 M precursor for three times, thus rendering excellent electrical properties and bias stability of the TFT device.

1. Introduction

Due to their high mobility, good transparency to visible light, good uniformity and reasonable electrical stability, metal-oxide thin-film transistors (TFTs) have attracted great attention in the field of active matrix devices such as liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs) [1,2,3,4]. The gate dielectric plays an important role in TFTs because it manipulates the conductance of the semiconducting channel by accumulating charge carriers. Moreover, its electrical insulation to minimize a leakage current is another critical requirement for minimal static dissipation [5], which simultaneously affects the transfer performance and the stability and lifetime of TFT devices.

Recently, considerable efforts have been devoted to fabricating metal-oxide-based TFTs with high dielectric constant (high-k) gate dielectrics. High-k dielectrics can increase the capacitive coupling between the gate and the active channel layer, which not only increases the driving current, but also reduces the operating voltage. Moreover, high-k materials are highly desirable for improving the electrical performance, reducing the size of the device, as well as reducing energy consumption [6]. Nowadays, a number of high-k metal-oxide dielectrics (such as Al₂O₃ [7], ZrO₂ [8], HfO₂ [9], TiO₂, Y₂O₃ [10], Ta₂O₅ and CeO₂) have been widely studied and used as an alternative to conventional SiO₂ dielectric layers (k = 3.9) with low leakage current density. Among these gate oxides, zirconium oxide (ZrO₂) is one of the attractive materials because of its excellent physical properties, for example, high dielectric constant (23–29), good thermal stability, wide band gap (5–7 eV), and large transparency in the visible and infrared ranges [11,12].

Various vacuum preparation technologies (such as sputter deposition) have been used to prepare metal-oxide films. However, these methods require high-vacuum conditions and a photolithography patterning process that leads to high costs and cumbersome fabrication procedures [2,5,13,14]. To overcome these problems, alternative film-deposition methods have been proposed [15]. The solution-phase processes (such as spin-coating, spray-coating and ink-jet printing) possess many advantages of scalability, roll-to-roll manufacturing and low-cost fabrication processability. For instance, solution-processed ZrO₂ films has attracted much interest, since they have the advantages of low cost, easy chemical composition control and compatibility for large-scale roll-to-roll production [16]. Gong et al. [17] fabricated high-quality ZrO₂ films by a combination of a solution-phase process and ultraviolet (UV) irradiation. The ZrO₂ films with 1 h UV curing showed a leakage current of 1.7 × 10⁻⁶ A/cm² at −3 V, a bandgap of 6.13 eV, and a high dielectric constant of 17.8. Naik et al. [18] reported on solution-processed bottom-gate, bottom-contact indium–zinc–tin oxide (IZTO) TFTs with a mobility of >2 cm² V⁻¹ s⁻¹. The solution-processed ZrO_x film, with a thickness of 40 nm, showed a leakage current less than 10⁻¹⁰ A/μm.

However, solution-processed TFTs suffer from severe bias-stress instability for device operation because of inherently rich and undefined defect states, such as pores and organic impurities [19]. For instance, Kim et al. [20] fabricated a zinc–tin oxide (ZTO)/indium–gallium–zinc oxide (IGZO) dual-active-layered ZTO/IGZO TFT (DALZI TFT) and an unpassivated IGZO TFT, which exhibited voltage shifts of −1.86 V and −19.59 V, respectively, under negative bias illumination stress conditions (stress time = 1000 s). Lee et al. [21] reported on an IGZO TFT with a solution-processed Al₂O₃ gate dielectric, and the threshold voltage shift of the IGZO TFT was +3.66 V under a gate bias of +20 V for 3600 s. Some methods, such as increasing annealing temperature or using a UV/ozone treatment, have been used to solve this problem [22,23,24]. Liu et al. [23] reported an In₂O₃ thin-film transistor using a solution-processed ZrO₂ dielectric. The ZrO₂ film, with a leakage current density of 10⁻⁹ A/cm² at 2 MV/cm, was processed by a UV/ozone treatment and annealed at 500 °C. The threshold voltage shift was 0.22 V for In₂O₃ TFTs under PBS with a V_GS value of 1.5 V for 7200 s. Solution-processed amorphous indium–zinc–tin oxide (a-IZTO) thin-film transistors (TFTs) with spin-coated zirconium oxide (ZrO_x) as the gate insulator were presented by Naik et al. [18]. The ZrO_x gate insulator was processed without and with UV/O₃ treatment. The threshold voltage shift was 1.13 V for the untreated ZrO_x-based TFT and was 0.64 V for the UV/O₃-treated ZrO_x-based TFTs. Ha et al. [24] reported on a solution-processed zinc–tin–oxide (ZTO)/ZrO₂ TFT with a 90-nm-thick ZrO2 dielectric annealed at 500 °C for 1 h. The TFT exhibited very small hysteresis windows in both dark and illuminated conditions, and the shift in V_th was 0.4 V of negative bias-stress under illumination over 5000 s.

In this paper, solution-processed ZrO₂ dielectric films were demonstrated at a deposition temperature of 400 °C. The electrical properties of ZrO₂ films were characterized by using metal–insulator–metal (MIM) structures, and the physical properties were measured by X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM). IGZO-TFTs with solution-processed ZrO₂ dielectrics were fabricated on glass substrates. By decreasing the precursor concentration and increasing the spin-coating times, an optimal thickness of ZrO₂ film was achieved, which showed a smooth surface and reduced internal defects. Consequently, the resulting TFT devices had not only good electrical properties but also improved bias stability. When under positive bias and negative bias stress over 3600 s, the offset of threshold voltage was less than 0.6 V. Compared with the previous reports of other groups, our work proposes a simple and feasible way to reduce the defect state of the insulating thin films without a special thermal annealing process such as UV treatment. Therefore, it will help improve the bias stability of TFTs and promote the development of solution-method TFTs in practical applications.

2. Materials and Methods 2.1. Preparation of ZrO₂ Film

ZrO₂ solution was synthesized by dissolving ZrCl₂O·8H₂O (Richjoint, Shanghai, China) in a 2-methoxyethanol (2ME) (Fuyu Fine Chemical, Tianjin, China) solvent. The solution was stirred at 300 r/min at room temperature for 2 h, and was then aged for at least one day [25]. The precursor solution was spin-coated on the indium tin oxide (ITO) substrate at 5000 rpm for 40 s. If multiple spin-coating was used, the wet films were pre-annealed at 300 °C for 5 min after each spin. Finally, the films were post-annealed at 400 °C on the hotplate for 1 h to drive off the solvent and promote the oxidation reaction.

2.2. Fabrication of TFTs

A 150-nm-thick ITO deposited by direct current (DC) sputtering on glass substrates was used as a bottom electrode. ZrO₂ films were formed by spin-coating several times to acquire a certain thickness. 10-nm-thick IGZO was deposited by direct current (DC) pulse sputtering with a pressure of 1 mTorr (O:Ar = 5%) and patterned by shadow mask. Then, the IGZO film was annealed at 300 °C for 1 h. The IGZO target is composed of the atomic ratio of In:Ga:Zn:O = 1:1:1:4. Finally, the Al source/drain electrodes with 100-nm-thickness were deposited by direct current (DC) sputtering at room temperature. The channel width (W) and length (L) of TFTs were 530 μm and 270 μm, thus the W/L ratio was 1.96.

2.3. Characterization

An MIM structure (ITO/ZrO₂/Al) was used to measure the leakage current density and capacitance of the ZrO₂ films. X-ray diffraction (XRD) (EMPYREAN, PANalytical, Almelo, The Netherlands) was used to investigate the crystalline phase of the ZrO₂ film fabricated on the glass substrate. The thickness and roughness of the ZrO₂ films were measured by X-ray reflectivity (XRR) using the same equipment. The thickness can be obtained by fitting the interference fringe of the X-ray. The surface topography of the films was evaluated by atomic force microscopy (AFM) (BY3000, Being Nano-Instruments, Beijing, China) in noncontact mode. The electrical characteristics of TFTs were measured using a semiconductor parameter analyzer (Agilent4155C, Agilent, Santa Clara, CA, USA) under an ambient atmosphere. The current–voltage (I–V) and capacitance–frequency (C–f) characteristics of the MIM capacitor were measured by the Keithley4200 (Tektronix, Beaverton, OR, USA) parameter analyzer under an ambient condition.

3. Results and Discussion

In our experiment, we spun the 0.3 M zirconia precursor on glass substrates one, two, three, four, five and six times. It was found that the thickness of ZrO₂ films increased almost linearly with increasing spin-coating times. Figure 1 is the XRR fitting results of the ZrO₂ films with varying thicknesses. From the fitting results, the thickness of the single-spin-coating film was about 22 nm, and a 130 nm ZrO₂ film can be obtained by six-times spin-coating. Obviously, a thicker film would render a smaller leakage current density. Also, the 0.6 M ZrO₂ precursor was swirled for one, two and three times. The thickness of the ZrO₂ film obtained by using the 0.6 M precursor is about two-fold thicker than that of the ZrO₂ film obtained by using 0.3 M precursor. Additional details are listed in Table 1.

The crystalline phase of the ZrO₂ films with dissimilar thicknesses (67 nm, 130 nm) annealed at different temperatures was investigated by XRD. Figure 2 is the XRD spectra of the ZrO₂ films. The 67-nm-thick film annealed at 400 °C had a clear crystal peak, while the 130-nm-film was still amorphous. We presumed that the thicker film may inhibit the transfer of heat and the crystallization of ZrO₂ film in the post-annealing processing.

To increase the thickness of ZrO₂ film, increasing the concentration of precursor or spin-coating times is an effective method. From the XRR fitting results, the ZrO₂ films obtained by spin-coating 0.6 M precursor for three times, and 0.3 M precursor for three and six times, show a corresponding thickness of 132 nm, 67 nm and 130 nm, respectively. Figure 3 shows the leakage current density of the above ZrO₂ films. The ZrO₂ film with 67 nm thickness demonstrated the largest leakage current density. For ZrO₂ films with 132 nm and 130 nm thicknesses, they exhibited comparable leakage current density despite the difference in the precursor concentration and times. A thick ZrO₂ film may inhibit the crystallization of the film that led to the reduction in leakage current density.

Figure 4 shows the device architecture of IGZO-TFTs. IGZO-TFTs were prepared based on the dielectric films obtained by spin-coating with 0.6 M precursor for three times and 0.3 M precursor for six times. The corresponding bias stability is illustrated in Figure 5 and Figure 6. The field effect mobility in the saturation region (V_ds ≥ V_gs − V_th) and subthreshold swing (SS) were separately obtained by using the following equations:

_Ids=12WLμ_Ci ^{(_Vgs−_Vth)2},

SS=d_Vgsdlog_Ids.

W, L, μ_sat, C_i, V_th and I_ds are the channel width, channel length, saturation mobility, capacitance, threshold voltage and drain current, respectively, and V_ds and V_gs separately represent the source-drain voltage and gate-source voltage. The regions from which SS has been extracted are noted on the figures. The capacitance of the ZrO₂ film is around 130 nF/cm². The channel width (W) is 530 μm and the length (L) is 270 μm (W/L ratio is 1.96).

The TFT with the ZrO₂ film achieved by spin-coating with 0.3 M precursor for six times (device A) had a saturation mobility of 12.7 cm² V⁻¹ s⁻¹ and an on/off ratio of 7.6 × 10⁵, while the TFT with the ZrO₂ film achieved by spin-coating with 0.6 M precursor for three times (device B) exhibited a saturation mobility of 10 cm² V⁻¹ s⁻¹ and an on/off ratio of 6.4 × 10⁵. Also, the device A had excellent bias stability, with an offset of the threshold voltage less than 0.6 V under positive bias, and negative bias stress over 3600 s. For comparison, Ding et al. [26] fabricated the IGZO-TFT with bottom gate and staggered electrodes using atomic-layer-deposited Al₂O₃ as gate insulator. The TFT device showed a threshold voltage shift of 1.5 V under 10 V gate voltage for 1 h. Li et al. [27] reported on the SiNx/IGZO-TFT with reactive sputtered SiOx as passivation layer, and the device exhibited a V_th shift of 1.3 V after applying positive bias stress of 20 V for 10,000 s. However, in our study, the device B not only had a lower saturation mobility, but exhibited a larger threshold voltage shifting under the bias stress test, with a voltage shifting of 1.6 V under positive bias stress over 3600 s. Additional details are listed in Table 2. The trend of parameters in Table 2 is shown in Figure 7 and Figure 8.

The leakage current densities (shown in Figure 3) of ZrO₂ films obtained by spin-coating with 0.6 M and 0.3 M precursor for three and six times, respectively, were quite similar. However, the bias stability of the TFTs based on them were greatly different. Figure 9 shows the top view and polarizing graphs of the MIM structure. In order to compare the electrical homogeneity of the insulating films mentioned above, we select nine feature points on the insulating film to measure the leakage current density, as shown in Figure 9. Figure 10 shows the leakage current density of each feature point. Table 3 shows the statistics of electrical properties of each feature point. It can be seen that the ZrO₂ film achieved by spin-coating with 0.3 M precursor for six times had better electrical uniformity.

Figure 11 is the AFM diagram of ZrO₂ films fabricated by spin-coating with 0.6 M precursor for three times and 0.3 M precursor for six times. Both of them had low roughness, and the root mean square of ZrO₂ films in devices A and B were 0.16 nm and 0.18 nm, respectively. After the precursor was spin-coated, a wet film was formed containing a large number of ions. The high-concentration precursor (0.6 M) might lead to high surface roughness due to the irregular arrangement of atoms during the annealing process, during which Zr and O elements moved and recombined to form Zr–O bonding. From the results above, the ZrO₂ films in device A showed smoother surface, lower leakage current density and better electrical uniformity. A smooth surface morphology may result in the alleviation of electron trapping at the channel–insulator interface, which is a great contribution to improve the electrical properties and bias stability of TFTs [28,29,30].

From the XRR fitting results, the densities of ZrO₂ films fabricated by spin-coating with 0.3 M precursor for six times and 0.6 M precursor for three times were 4.77 and 4.70 g/cm³, respectively. The relative porosity volume can be calculated from the densities of ZrO₂ films using the following equation:

q=_ρ1−ρ_ρ1−_ρ2,

where q, ρ, ρ1 and ρ2 represent the relative porosity volume, film density, bulk ZrO₂ density (5.68 g/cm³) and void density (air), respectively. The calculated relative porosity volume of 0.3 M ZrO₂ dielectric layer and 0.6 M ZrO₂ dielectric layer were 16.02% and 17.26%, respectively. Figure 12 shows a schematic diagram describing the internal defects of ZrO₂ films. The solution-processed films may have many pores and impurities, which were possibly formed during vaporization, decomposition and condensation processes [31]. The advantage of multilayered stacks of ZrO₂ films is that the pinholes and pore regions of the sub layers may be filled by the subsequent solution process. Another advantage is that the multilayer structure can make the direct penetration of each layer more random, and it is not easy to form a direct penetration channel that runs through all layers. Furthermore, the wet film obtained by low-concentration precursor is thinner during each spin-coating. The conduction of heat is more efficient in a thinner wet film during the pre-annealing process, which is helpful to reduce the impurities and defects.

4. Conclusions

In this paper, we explored the influence of the spin-coating times and the precursor concentration on the dielectrics of solution-processed ZrO₂ films. It is interesting that a thicker film may inhibit the crystallization of the film during the post-annealing process, thus improving the dielectric properties. Moreover, ZrO₂ films fabricated by spin-coating with 0.3 M precursor for six times had lower leakage current density and better electrical uniformity than the film fabricated by spin coating with 0.6 M precursor for three times, thus rendering excellent electrical properties and bias stability of the TFT device. Two possible mechanisms for the improvement of device characteristics have been concluded. (1) The pinholes or pore sites of the sublayer may be filled by a subsequent solution process in the multilayered structure, so there are fewer impurities and defects in the films. (2) The conduction of heat is more efficient in a thinner wet film during vaporization, decomposition and condensation processes, which is beneficial to the reduction of impurities and defects.

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Concentration	Layer	Density (g/cm3)	Thickness (nm)	Roughness (nm)
0.3 M	1	4.86	19.98	0.46
0.3 M	2	4.70	43.53	0.46
0.3 M	3	4.83	66.98	0.26
0.3 M	4	4.78	91.61	0.49
0.3 M	5	4.76	118.89	0.55
0.3 M	6	4.77	129.29	0.41
0.6 M	3	4.70	132.62	0.43

Device	Stress Time (s)	Negative Bias Stress (NBS)				Positive Bias Stress (PBS)
		μsat (cm2 V−1 s−1)	Ion/Ioff (×105)	SS (V/dec)	Vth (V)	μsat (cm2 V−1 s−1)	Ion/Ioff (× 105)	SS (V/dec)	Vth (V)
Device A	0	12.7 ± 0.34	7.6 ± 0.51	0.34 ± 0.06	3.34 ± 0.05	11.9 ± 0.32	5.6 ± 0.42	0.37 ± 0.05	3.81 ± 0.07
	900	12.4 ± 0.35	7.6 ± 0.55	0.35 ± 0.07	3.55 ± 0.08	8.5 ± 0.21	2.6 ± 0.22	0.45 ± 0.05	4.28 ± 0.06
	1800	12.3 ± 0.30	7.5 ± 0.55	0.34 ± 0.04	3.63 ± 0.06	7.7 ± 0.20	2.2 ± 0.19	0.46 ± 0.04	4.40 ± 0.04
	2700	12.2 ± 0.31	7.0 ± 0.52	0.35 ± 0.05	3.70 ± 0.04	7.5 ± 0.20	2.2 ± 0.19	0.47 ± 0.02	4.40 ± 0.02
	3600	12.1 ± 0.31	6.8 ± 0.50	0.35 ± 0.06	3.67 ± 0.05	7.4 ± 0.20	2.1 ± 0.18	0.48 ± 0.02	4.43 ± 0.02
Device B	0	9.8 ± 0.10	0.92 ± 0.058	0.55 ± 0.06	3.49 ± 0.16	9.5 ± 0.28	0.090 ± 0.0062	0.70 ± 0.05	3.62 ± 0.03
	900	9.7 ± 0.02	1.5 ± 0.098	0.49 ± 0.03	3.52 ± 0.06	8.6 ± 0.42	0.067 ± 0.0051	0.74 ± 0.02	5.10 ± 0.06
	1800	9.9 ± 0.02	1.3 ± 0.084	0.51 ± 0.04	3.42 ± 0.03	8.5 ± 0.37	0.13 ± 0.0098	0.70 ± 0.03	4.96 ± 0.01
	2700	9.9 ± 0.96	1.8 ± 0.15	0.44 ± 0.14	3.27 ± 0.04	7.8 ± 0.41	0.12 ± 0.0098	0.69 ± 0.03	5.11 ± 0.03
	3600	9.5 ± 0.48	0.58 ± 0.056	0.58 ± 0.04	3.17 ± 0.05	7.1 ± 0.04	0.13 ± 0.011	0.69 ± 0.03	5.18 ± 0.03

ZrO2 Films	Leakage Current Density (A/cm2) (10 V)
	Average Value	Maximum Value	Minimum Value	Standard Deviation
0.6 M precursor spin-coating 3 times	6.9 × 10−6	2.1 × 10−5	5.1 × 10−7	7.8 × 10−6
0.3 M precursor spin-coating 6 times	1.6 × 10−6	3.4 × 10−6	4.0 × 10−7	9.39 × 10−7

Author Contributions

S.Z. and H.N. conceived the project. S.Z., W.C., J.W. and Z.Z. carried out the experiment. X.L., R.Y. and J.P. analyzed the test data. With the help of Z.F. and W.Y., S.Z. wrote and completed the paper. All authors read and approved the final manuscript.

Acknowledgments

This work was supported by National Key R&D Program of China (Nos. 2016YFB0401504 and 2016YFF0203600), National Natural Science Foundation of China (Grant. 51771074, 51521002 and U1601651), National Key Basic Research and Development Program of China (973 program, Grant No. 2015CB655004) Founded by MOST, Guangdong Natural Science Foundation (No. 2016A030313459 and 2017A030310028), Guangdong Science and Technology Project (Nos. 2016B090907001, 2016A040403037, 2017A050503002 and 2016B090906002), Guangzhou Science and Technology Project (201804020033), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016).

Conflicts of Interest

The authors declare no conflict of interest.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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