Effect of Titanium Cation Doping on the Performance of In₂O₃ Thin Film Transistors Grown via Atomic Layer Deposition

Abstract

Indium oxide semiconductors, as one of the channel materials for thin film transistors (TFTs), have been extensively studied. However, the high carrier concentration and excess oxygen defects of intrinsic In₂O₃ can cause the devices to fail to work properly. We overcame this hurdle by incorporating the titanium cation (Ti⁴⁺) into In₂O₃ via atomic layer deposition (ALD). The InTiO_x TFTs with an In:Ti atomic ratio of 15:1 demonstrated excellent electrical and optical properties, such as a lower threshold voltage (V_th) of 0.17 V, a lower subthreshold swing (SS) of 0.13 V/dec., a higher I_on/I_off ratio of 10⁷, and a transmittance greater than 90% in the visible region. With the doping ratio increasing from 20:1 to 10:1, the mobility decreased from 9.38 to 1.26 cm²/Vs. The threshold voltage shift (ΔV_th) of InTiO (15:1) under 5 V positive bias stress (PBS) for 900 s is 0.93 V, which is less than other devices. The improvement in stability with increasing Ti⁴⁺ concentrations is attributed to the reduction of oxygen defects. Therefore, these InTiO (15:1) TFTs with excellent performance show great potential for future applications in transparent electronic devices.

1. Introduction

Recently, amorphous oxide semiconductors (AOS) have received extensive attention in the field of flat panel displays and wearable devices due to their excellent photoelectric properties [1,2,3]. Among the various candidate materials, indium oxide (In₂O₃) with a wide bandgap (3.6–3.7 eV) has a high transmittance to visible light and exhibits excellent chemical stability [4,5,6], making it one of the most promising thin film transistor channel materials. In-ions with an outer electronic structure of (n − 1)d¹⁰ns⁰ (n ≥ 4) [7] contribute greatly to increasing the carrier concentration and mobility of in-based TFTs. However, its practical application is limited by the instability of In₂O₃ TFTs due to excessive oxygen vacancies (V_O). Therefore, it is essential for large-scale applications to perfect the stability of In₂O₃ TFTs. To date, several doping elements have been reported to improve the stability of devices, including Al [8], Ga [9], and Mg [10], which could suppress the formation of V_O. To obtain stable indium oxide TFTs, the doping element and oxygen should have a higher bond dissociation energy than In-O (360 kJ/mol). Among the many doping elements, titanium (Ti-O) has a larger oxygen bonding dissociation energy (662 kJ/mol) [6] than indium (In-O), which can reduce the V_O concentration in the In₂O₃ film and be an effective oxygen vacancy inhibitor.

Among the methods for depositing oxide thin films, magnetron sputtering [11], electrospinning [12], sol-gel [13], etc. are not satisfactory in terms of uniformity of film formation and control of film thickness. Atomic layer deposition (ALD) technology offers many advantages over the above preparation methods, including excellent repeatability, low deposition temperatures, and high control of composition and thickness [14,15,16]. As a result of the self-limiting nature of the ALD process, thin oxide films with precise thickness and conformability can be formed. For instance, Bak et al. [17] prepared IZTO TFTs with high-k HfO₂ as the dielectric layer by atomic layer deposition. Compared with HfO₂ dielectric devices prepared by magnetron sputtering, IZTO TFT samples prepared by the ALD method have a higher I_on/I_off ratio and slightly improved field effect mobility. Yang et al. [18] fabricated 5 nm ultrathin ZrO₂ dielectrics by ALD for high-performance ZnO TFTs. The TFTs exhibit the lowest operating voltage of 1 V, field effect mobility of 36.8 cm²/Vs, and SS of 69 mV/dec., which is close to the theoretical limit. Sheng et al. [19] successfully fabricated IGZO TFTs by PEALD with mobility up to 70 cm²/Vs. As shown above, ALD processes have the potential to fabricate oxide TFTs with many advantages.

In this work, InTiO TFTs were fabricated using the ALD technique. Through the application of the proper amount of Ti⁴⁺ doping, we have demonstrated improved electrical performance and stability of InTiO TFTs due to a reduction in V_O defects and the density of interface traps.

2. Experiments

Figure 1a shows TFT fabricated on a 0.45-mm-thick p-type silicon wafer substrate with a bottom gate and top contact. Prior to preparation, the silicon substrates were washed sequentially with acetone, alcohol, and deionized water for 15 min in a sonicator, followed by 10 min of ozone and ultraviolet light treatment to enhance hydrophilicity. Subsequently, the 50-nm Al₂O₃ dielectric was deposited by the thermal ALD (TFS-200, Beneq) method at 250 °C with trimethylaluminum (TMA) and deionized water as aluminum and oxygen precursors, respectively. The optimized ALD cycle consists of a 200-ms precursor pulse combined with a 10-s N₂ purge. The InTiO films were grown on Al₂O₃ for the active layer at a growth temperature of 200 °C. High-purity (3-dimethylaminopropyl) dimethylindium (DADI) and tetrakis (dimethylamino) titanium (TDMATi) were used as precursors for indium and titanium, respectively, and ozone and deionized water were used as oxygen sources. The DADI and TDMATi canisters were heated to 35 and 45 °C, respectively. The pulse times of DADI and O₃ were 200/300 ms, and the pulse times of TDMATi and H₂O were 500/250 ms. The nitrogen purging time is 5 s. X cycles of DADI/O₃ followed by one cycle of TDMATi/H₂O were repeated for Ti⁴⁺ incorporation into the In₂O₃ film, where X varied between 20, 18, 15, and 10, and films with approximately 17 nm thickness were obtained by adjusting the total supercycles. Thus, the grown InTiO thin films with different In₂O₃ sublayer cycles are named from InTiO (20:1) to InTiO (10:1). Figure 1b presents the detailed growth conditions, and Figure 1c shows the cross-sectional TEM image of the InTiO TFT. The pure In₂O₃ film was also prepared by ALD for contrastive analysis. After the active layer was deposited, 200-nm Al films were deposited by thermal evaporation as the source/drain electrodes. The channel width (W) and channel length (L) were 1000 and 100 µm, respectively. The devices were then annealed in the air for 10 min at 250 °C. The same process was used to grow InTiO thin films on quartz glass substrates to investigate their optical properties.

Through X-ray diffraction (XRD; Rigaku D/max-rB, Akishima, Japan) and X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-Alpha+, Waltham, MA, USA), the crystal structure and chemical composition of the thin films were characterized. Atomic force microscopy (AFM, nanonaviSPA-400 SPM, SII Nano Technology Inc. Chiba City, Japan) was used to observe surface morphology, and a double-beam spectrophotometer (Hitachi U-3900H, Tokyo, Japan) was used to measure optical transmission. The semiconductor characteristics of TFT were measured by a semiconductor parameter analyzer (Keithley, 4200, Cleveland, OH, USA) and probe station (Lakeshore, TTP4, Carson, CA, USA). The electrical properties were tested by the Hall effect (Lakeshore 8400, Toyo Corporation, Tokyo, Japan).

3. Results and Discussion

To evaluate the morphological evolution of the films with different composition ratios, the resulting InTiO films were characterized by AFM in the region of 2 µm × 2 µm, as shown in Figure 2a–d. AFM micrographs show a systematic decrease in surface roughness with decreasing number of X cycles (higher Ti⁴⁺ doping ratio), which is well displayed by the obtained root mean squared (RMS) values due to the decrease in crystallinity of the film caused by Ti⁴⁺ implantation. The values of RMS for InTiO (20:1), InTiO (18:1), InTiO (15:1), and InTiO (10:1) are 0.32, 0.28, 0.27, and 0.26 nm, respectively. The roughness of the active layer is one of the important parameters for ensuring high-performance thin-film transistor devices [20]. Its smooth surface is beneficial to source and drain electrode adhesion [21], thereby reducing the contact resistance between the electrode and the active layer. Contact resistance will be discussed later. All InTiO films have very flat surface morphology, which will help to reduce carrier scattering and promote mobility. Amorphous structures are observed in the XRD patterns of InTiO thin films, as shown in Figure 2e. TFT devices can be manufactured with high uniformity and smooth surfaces thanks to the amorphous phase [22]. The results are consistent with AFM analysis.

The optical transmission spectra of all films on quartz glass substrates were measured in order to determine the effect of different Ti⁴⁺ doping ratios on the optical band gap (E_g) of films. As shown in Figure 3a, all samples in the visible range have a transmittance of about 90%, which makes them suitable for use in transparent electronic devices. The formula for E_g and absorption coefficient (α) is as follows:

$$\alpha h\nu ={(h-E_{\mathrm{g}})}^{\frac{1}{2}}$$

(1)

where α, h, ν, and E_g are the optical absorption coefficient, the Planck constant, the incident photon frequency, and the optical energy bandgap, respectively. The absorption coefficient (α) can be calculated using the following equation:

$$\alpha =(\frac{1}{d})\ln (\frac{1}{T})$$

(2)

where d is the film thickness and T is the transmittance. Figure 3b shows the relationship between (αhv)² and photon energy (eV). The E_g is calculated to be 3.71, 3.72, 3.72, and 3.75 eV of InTiO (20:1), InTiO (18:1), InTiO (15:1), and InTiO (10:1), respectively. It is worth mentioning that the E_g of pure In₂O₃ is 3.69 eV, which is mentioned in the introduction. As the Ti⁴⁺ doping ratio increases, the optical band gap of thin film increases gradually. The narrower the band gap of a semiconductor, the easier valence band electrons transition to the conduction band to form electron-hole pairs, which causes higher carrier concentrations and results in devices that do not work properly [23]. With increasing Ti⁴⁺ doping, the broadening of E_g.

The chemical compositions of the InTiO thin films were investigated by the XPS. Figure 4 shows the O 1s peaks of the InTiO thin films with various doping ratios. All of these have been corrected by C 1s (284.5 eV). The O 1s of the XPS spectra were divided into three peaks centered at 529.8 (O_I), 531.3 (O_II, and 532.3 (O_Ⅲ) eV, respectively. The O_I sub-peak is attributed to the oxygen in oxide lattices (M-O), the O_II sub-peak is associated with the oxygen bonds near V_O in the lattice matrix of metal oxide, and the O_Ⅲ sub-peak can be assigned to hydroxyl species (M-OH) [24]. The O_II / O_I + O_II + O_Ⅲ values of InTiO (20:1), InTiO (18:1), InTiO (15:1), and InTiO (10:1) are 39.09%, 37.26%, 36.15%, and 33.21%, respectively. It was found that the proportion of V_O decreased with increasing Ti⁴⁺ doping concentrations, which indicates that Ti⁴⁺ doping can effectively reduce the amount of V_O, thus enhancing stability. Under ambient conditions after annealing at 250 °C for 10 min, the transportation characteristics of InTiO TFTs were measured to investigate the effect of Ti⁴⁺ doping.

As shown in Figure 5a, the source-drain voltage (V_D) is fixed at 5 V, while the gate-source voltage (V_G) increases from −5 to 15 V. As V_G increases, the drain current (I_D) increases, indicating that InTiO semiconductors are n-type. Our previous work describes the method of determining μ and subthreshold swing (SS) [25].

The key electrical parameters, including V_th, mobility (μ), I_on/I_off, and sub-threshold swing (SS), were presented in

Table 1. Electrical parameters of InTiO TFTs with different Ti⁴⁺ doping ratios.

Devices	Vth (V)	μ (cm2/Vs)	Ion/Ioff	SS (V/dec.)
InTiO (20:1)	−1.12	9.38	103	1.43
InTiO (18:1)	−0.52	8.73	105	0.74
InTiO (15:1)	0.17	7.69	107	0.13
InTiO (10:1)	0.29	1.26	105	0.25

. Pure In₂O₃ TFTs did not exhibit semiconductor transfer characteristics and showed an on-conducting state. With the Ti⁴⁺ doping ratio dropping from 20:1 to 10:1, V_th increases from −1.12 to 0.29 V and μ decreases from 9.38 to 1.26 cm²/Vs. The μ is affected by shallow traps near the conduction band and interfacial oxygen vacancies [26]. In addition, the value of SS drops sharply from 1.43 to 0.13 V/dec. when the Ti⁴⁺ doping rate continues to increase to 10:1, the SS increases to 0.25 V/dec. The value of SS is related to trapping states (N_trap); N_trap is extracted using the following equation [27]:

$$N_{\mathrm{trap}}=\left[\frac{SS\log (e)}{kT/q}-1\right]\frac{C_{\mathrm{i}}}{q}$$

(3)

where k is Boltzmann’s constant, T is the temperature in Kelvin, and q is the electron charge. It can be seen from the equation that the reason for the increase in SS is attributed to the generation of more trap states due to excessive Ti⁴⁺ doping.

The output characteristics of InTiO (15:1) TFTs are shown in Figure 5b. It can be seen from the figure that I_D saturates at high source/drain bias (V_D). The pinch-off behavior and the saturation of the I_D indicate that the contact between the source/drain electrodes and the active layer is excellent.

In order to explore the electrical properties of the films, we measured the electrical properties of InTiO_x films on glass substrates. As presented in Figure 6a, all films exhibit a carrier concentration greater than 10¹⁸ cm⁻³. The Hall mobilities of InTiO (20:1), InTiO (18:1), InTiO (15:1), and InTiO (10:1) are 9.2, 5.6, 3.9, and 1.2 cm²/Vs, respectively. Notably, the Hall mobility of oxide semiconductors is partially proportional to carrier concentration [28]. Further, the resistivity of the film increases from 0.02 to 0.9 Ω·cm. We attribute this to Ti⁴⁺ doping filling the oxygen vacancy defects, leading to reduced film conductivity.

The contact resistance between the source-drain electrode and the active layer can be extracted by the transmission line method (TLM) [29,30]. In this method, the total resistance is obtained by adding the resistance of the active layer and the contact resistance between the source and drain electrodes, as shown in the following equation:

$$R_T=\frac{V_D}{I_D}=R_{SD}+\frac{L}{\mu C_{ox}W(V_G-V_{th})}$$

(4)

R_T represents the total resistance of the InTiO_x TFT, while R_SD represents the contact resistance between the source and drain electrodes and the active layer. Using the equation above, we calculate the contact resistances of InTiO (20:1), InTiO (18:1), InTiO (15:1), and InTiO (10:1) at a gate voltage of 15 V, which are 64, 54, 51, and 45 Ω·m, respectively. The relationship between contact resistance and doping ratio is shown in Figure 6b. With the increase in doping ratio, contact resistance gradually decreases. InTiO (10:1) has the smallest contact resistance, which is consistent with the analysis of AFM results.

As shown in Figure 7a–c, the latter three devices were selected for forward bias stability testing. At room temperature and dark conditions, the transfer curves of 900 s were measured under +5 V positive bias stress (PBS). The results show that the transfer characteristic curves of the three devices have a certain degree of positive shift. The stability of thin film transistors is related to the interface state of the device’s dielectric layer and active layer and the V_O inside the active layer [31]. The previous XPS results show that Ti⁴⁺ doping leads to a decrease in oxygen-related vacancies in the film, thereby improving the stability of the TFT device. The results were consistent with the XPS analysis.

Under various PBS time intervals, Figure 7d displays the threshold voltage shift of InTiO TFTs. As mentioned above, the ΔV_th of InTiO (15:1) is only 0.93 V. Compared to this device, the ΔV_th of InTiO (18:1) and InTiO (10:1) are 1.29 and 1.04 V. According to the result, Ti⁴⁺ doping can improve transistor stability with the appropriate amount. ΔV_th can be calculated by the following equation:

$$\Delta V_{\mathrm{th}}=\left(V_{\mathrm{G}}-V_{\mathrm{th}}\right)\left\{1-\exp \left[-{\left(\frac{t}{\tau }\right)}^{\beta }\right]\right\}$$

(5)

where τ is the characteristic of carrier trapping time and β is the stretching exponent. The V_th shift is proportional to the value of V_G-V_th, implying that ΔV_th is related to V_th, but τ and β hardly depend on the bias stress amplitude. The values of τ and β extracted from the equation are 6.27 × 10⁴, 4.6 × 10⁵, and 8.6 × 10⁴, and 0.235, 0.451, and 0.458 for InTiO TFTs with Ti⁴⁺ doping of 18:1, 15:1, and 10:1, respectively. The fitted curve described by Equation (5) is shown in Figure 7e. This demonstrates that under extended positive bias testing, InTiO (15:1) TFTs can resist greater performance drift, and it can be obtained that InTiO TFTs can be effectively with appropriate amounts of Ti⁴⁺ doping.

4. Conclusions

In this paper, we report the successful preparation of InTiO TFTs using the ALD method. The microstructure and defect characteristics of InTiO films were investigated through XRD and XPS analyses. All films exhibit a low RMS value of less than 1 nm and are amorphous. Additionally, all InTiO films display an average transmittance of over 90% in the visible range. Notably, the InTiO (15:1) TFTs exhibit exceptional stability due to a reduction in oxygen vacancy defects to 33.21%, resulting in a minimum ΔV_th of 0.93 V. Our results indicate that an appropriate amount of Ti⁴⁺ doping effectively inhibits oxygen defects in the active layer. Consequently, these InTiO TFTs with superior electrical properties and high transparency are promising candidates for use in the field of transparent displays.