Achieving High-Performance Solution-Processed Thin-Film Transistors by Doping Strong Reducibility Element Into Indium-Zinc-Oxide

Adding an appropriate amount of suitable elements is an effective way to suppress In-Zn-O (IZO) carriers. The influence of different doping concentration Ca incorporation on In-Zn-O oxide-based thin film transistors (Ca-IZO TFTs) was investigated theoretically and experimentally. First-principles calculations show that as the doping concentration of Ca increases, the IZO bandgap gradually widens. Moreover, low-concentration doping of Ca does not significantly reduce carrier density and mobility. A novel high-performance Ca-IZO film has been experimented by a low-cost sol-gel process. And the best Ca-IZO TFT device was achieved when the mole ratio of Ca was 3%. The slight Ca-doped In-Zn-O TFT showed a high on/off current ratio ~106, a high field-effect saturated mobility of 1.4 cm2/Vs, and a positive threshold voltage $(V_{\mathrm{ th}})$ of 7.7 V.


I. INTRODUCTION
Today solid-state circuits based on thin-film transistors (TFT) are widely used in displays, large-scale sensors, and electrical skins [1], [2], [3], [4], which are difficult to be achieved with traditional Si-based chips. Recently the amorphous oxide including the IZO shown superior characteristics in0F0F TFT's application with good transparency [5]. In addition, it can be prepared by sol-gel which can be used for printable TFT fabrication. This method perfectly meets the low-cost demands of universal Internet of Things (IoT) circuits' application in the future. Since the conductivity of IZO film approximates a conductor, doping a proper element such as Ga, Ti, La, and Hf is an effective way to suppress the carriers in the conduction band of IZO [6], [7], [8], [9]. Besides, alkaline-earth metal ion (Mg, Ba, Sr) doped IZO-based oxide TFT has also attracted much attention [10], [11]. In the solgel process, the doping concentration of metal ions can vary widely and the type of dopant can be randomly changed, enabling the doping-based performance enhancement of IZO film [12], [13], [14].
According to the previously reported study [26], In atom tends to be incorporated into the substitutional sites instead of the interstitial sites in the In-doped ZnO oxide system. This enables the analysis of IZO crystals using the firstprinciple, as well as the analysis of Ca-IZO crystals doped with varying quantities of Ca.
In this paper, we calculate the bandgap and band structure  of ZnO, IZO and Ca-IZO crystal materials with different  doping concentrations of Ca. Furthermore, to clarify how Ca  doping affects the transport properties of TFTs, we calculate  the carrier distribution of IZO before and after doping and  the effective mass of electrons in Ca-IZO with different  doping concentrations. In addition, we employ theoretical calculations to help us design our experiments. High-quality IZO and different level Ca-doped IZO thin films are achieved by the sol-gel method. Corresponding TFTs are fabricated to study the doping effect of Ca element on IZO system.

II. CALCULATION METHOD AND EXPERIMENTS A. CALCULATION DETAILS
First-principles calculations based on an all-electron projected augmented wave (PAW) [15], [16] and Density Functional Theory (DFT) [17], [18] methods are performed on Ca-doped IZO crystal structures and band structures. The band structure calculations were performed using the Vienna ab initio simulation package VASP [20], [21]. The generalized gradient approximation Perdew-Burke-Ernzerhof (GGA-PBE) approximation is selected for the exchangecorrelation functional [19]. The 3s, 3p states, and 3d states were included as valence electrons for Ca and Zn respectively. Plane-wave cutoff energies of 400 eV was considered for the calculations. K-point grids of (8×6×7) were adopted for the self-consistent field and relaxation calculation. All structures were obtained by full relaxation of the atomic positions, minimizing the quantum mechanical energy via changing energy of the unit cell less than 1E-8 eV.

B. EXPERIMENTAL PROCEDURES
In this work, a low-cost process was proposed to prepare Ca-IZO TFTs with competitive electronic properties. Sol-gel technology uses the precursors to dissolve in a certain solvent to form a stable solution under a low temperature, deposited on the substrate via a spin-coating method. Consequently, it will offer conveniences for flexibly controlling components and large-size flat panel production, enhancing the possibility of realizing printable transparent electronics manufacturing.
The TFTs fabrication process is illustrated as follows: A 0.5 M Ca-IZO solution was prepared in 2-methoxyethanol using zinc acetate dihydrate, indium nitrate hydrate, and calcium chloride dihydrate as Zn, In, and Ca precursors, respectively, as shown in Fig. 1(a). A stable sol solution was formed at 50 • C for 30 min, while dropping the stabilizer monoethanolamine (MEA) slowly, as in Fig. 1(b). Ca-IZO films were obtained by spin coating the prepared sol solution on the thermally oxidized p-Si, shown in Fig. 1(c). The derived film needs to be dried under 120 • C for 10 min and then annealed under 450 • C for 1 h. For the fabrication of the top contact structure, 100-nm-thick Al electrodes were deposited via thermal evaporation, as shown in Fig. 1(d). The channel length (L) and width (W) are 200um and 2000um. Finally, the obtained TFT devices with the bottom gate top contact structure of p+-Si/SiO 2 /Ca-IZO/Al were achieved, as shown in Fig. 1, where the p+-Si substrate works as the gate, SiO 2 as the gate dielectric, Ca-IZO film as the active layer and Al as drain/source contact electrodes. The atomic force microscope (AFM) was applied to measure the surface quality of the sol-gel Ca-IZO films. And, the electrical measurements of the TFTs were performed by using Agilent-4156B.

A. THE AB-INITIO SIMULATION
In this work, the substitutional supercell models of ZnO, IZO, and various concentration Ca-doped IZO (Ca-IZO) oxide materials are shown in Fig. 2.
The band gap values with various Ca doping concentrations Ca-IZO were discussed in Fig. 3. The Fermi level is shifted to valence band maximum (VBM). It can be found that the band gap approximates 0.9 eV for the pure IZO. As the doping level of Ca increases from 0% to 20%, the band gap shows an obvious growth trend, meaning that the conductivity is effectively suppressed. Moreover, all Ca concentrations Ca-IZO approach direct band gap semiconductors, because of conduction band minimum (CBM) and VBM are all at the G point.
Besides, both the carrier concentration and mobility are figured out to evaluate the influence of Ca doping on IZO oxide-based TFTs.
Firstly, we calculated the carrier distribution of IZO before and after doping which is shown in Fig. 4(a)(b) respectively. The result is shown in Fig. 4(c)(d). It is obvious that if the Ca atom replaces the In atom which is shown as  place P1 in Fig. 4, the carrier density change is not obvious. But if the Ca atom replaces the Zn atom which is shown as place P2, the carrier density decreases significantly.  Because it is impossible to determine which type of atom Ca replaces, in general, the doping of Ca will reduce the carrier concentration.
Because the IZO oxide-based TFTs behavior N-type the electron effective mass to clarify the mobility of the above TFTs. The effective mass consists of a longitudinal effective mass (m l ) and two transverse effective masses 1,2 (m t1 and m t2 ), as shown in (1). Table 1 presents the mobilities of Ca-IZO TFTs with various Ca doping concentrations. It's apparent that the effective mass tends to increase due to the rise of the Ca doping mount. According to (2). mobility is roughly inversely related to effective mass. Therefore, the calculated results suggest that mobility decreases rapidly. As a result, low-level Ca doping is preferred. (2)

B. THE FABRICATED DEVICES
The IZO and Ca-IZO films with various doping contents of 3%, 10%, and 20% were deposited by the developed solgel process. The achieved Ca-IZO films behaved with good surface quality, with the surface root mean square roughness (Rrms) as low as 0.33 nm, as shown in Fig. 5. Fig. 6 presented the comparison of I D -V G curves measured in Ca-IZO TFTs and pure IZO TFT at V D = 10 V   and V D = 40 V respectively. The pure IZO TFT exhibits much poor transfer performance with a lower on-off current ratio and larger negative threshold voltage (V th ) compared with Ca-doped IZO. And as Ca content increased from 3% to 20%, on-current and off-current decreased, and the threshold voltage shifted to a positive bias region unexpected for various applications. It indicates that Ca has played a significant role in the characteristic optimization of Ca-IZO TFTs. In addition, Fig. 7(a) depicted the exact change tendency of I on /I off and V th of sol-gel prepared Ca-IZO TFTs as the variation of doping concentration. The relationship of field effect mobility with the Ca doping amount at V D = 40 V is shown in Fig. 7(b). Evidently, Ca-IZO TFT of Ca% = 3% showed the largest I on /I off ratio, the highest mobility, and the lowest positive threshold voltage V rmth .  According to the drain current formula (3) in the linear region, the threshold voltage was extracted by linear extrapolation method using the I d -V G plot for low V D in Fig. 6(a).
where C ox is gate dielectric capacity, whose thickness is 200 nm. The Field-Effect Mobility in the saturation region (μ sat ) of the achieved devices were calculated from (4).
The extracted mobility gradually increases with carrier density. It indicated that the mobility in the IZO material system is dominated by Coulomb scattering. Slightly doping Ca effectively tunes the behavior of IZO, with an acceptable reduction of mobility.
The best device characteristics with 3% doped Ca-IZO TFT were observed in Fig. 8, with the on-off current ratio of about 10 6 , the field-effect mobility of 1.4 cm 2 /Vs, and saturation current of 109 µA at V G = 40 V. In addition, compared with widely studied oxide semiconductor material system such as indium-gallium-zinc-oxide, calcium metal element shows price advantage obviously and is a kind of more common resource on earth.
Furthermore, the previously reported metals doping IZO TFTs characteristics were summarized in Table 2. The comparison shows that doping Ca can achieve a large threshold voltage, which helps to reduce noise interference. At the same time, a very small off-state current at V g = 0V and a large window are achieved, which helps to reduce the power VOLUME 11, 2023 409 Authorized licensed use limited to the terms of the applicable license agreement with IEEE. Restrictions apply.
consumption of the TFT. More importantly, the implementation of these advantages did not result in a significant decrease in mobility.

IV. CONCLUSION
In summary, we have developed a new precursor metal Ca to be introduced to the IZO semiconductor material system for TFTs application. First principle calculation results show that carrier density will fall and electron mobility will not decrease significantly for low doping levels of Ca-IZO.
Through systematic study about Ca-IZO TFTs, the incorporation of Ca metal demonstrated sharp improvements including clear pinch-off, a positive shift of V th , and lower I off , compared with the inactive state shown in pure IZO TFT. Experimentally, with Ca content as low as 3%, Ca-IZO TFTs exhibited outstanding properties which is consistent with theoretical calculations, with V th of 7.7 V, μ sat of 1.4 cm 2 /Vs, and I on /I off of ∼10 6 . This overall performance of Ca-IZO oxide semiconducting films is so remarkable that it will allow for the feasibility of its applications in high-speed, good stability, and low-cost printable TFTs.

ACKNOWLEDGMENT
Jifang Cao thanks Xiaowen Shi (from HZWTECH) for help and discussions regarding this study.