High Performance of InGaZnO TFTs Using Hf_xAl_yO_z Nanolaminates as Gate Insulators Prepared by ALD

Abstract

In this study, Hf_xAl_yO_z nanolaminate, single-layer Al₂O₃, and HfO₂ gate insulators were fabricated by atomic layer deposition (ALD) to successfully integrate the InGaZnO (IGZO) thin-film transistors (TFTs). Compared with single-layer HfO₂-based TFTs, the Hf_xAl_yO_z-based IGZO TFTs showed a larger field-effect mobility of 10.31 cm²/Vs and a smaller subthreshold swing of 0.12 V/decade. Moreover, it showed a smaller threshold voltage shift of 0.5 V than that of HfO₂-based TFTs under gate-bias stress at +5 V for 900 s due to the smooth surface. Moreover, the high dielectric Hf_xAl_yO_z nanolaminate had a larger equivalent SiO₂ thinness than that of Al₂O₃ gate insulators, which are beneficial in applications of high-resolution display. Thus, the high mobility and high stability TFTs could be regarded as good candidates for active-matrix flat panel displays.

1. Introduction

Thin film transistors using transparent amorphous oxide semiconductors such as amorphous indium gallium zinc oxide (a-IGZO) and zinc oxide (ZnO) have drawn great attention as back-plane devices for high-resolution active-matrix flat panel displays [1,2,3]. Since Hosono et al. first used a-IGZO as a channel layer in 2004 [4,5], many groups have paid attention to optimization of the channel layer fabrication process [6]. In fact, both the channel layer and gate insulators play crucial roles in TFT performance. There are many materials used for the gate insulators, such as SiO₂, Al₂O₃, ZrO₂, HfO₂, and so on [7,8,9,10]. Among those materials, the HfO₂ materials are suitable gate dielectrics in TFTs because of their high-k and reasonable bandgap [11], whereas the HfO₂ materials are easier to crystallize and produce grain boundaries, resulting in increasing leakage currents compared to SiO₂ and Al₂O₃ materials [12,13]. Because the proper dopant will destroy the original bond structure, it will hinder the transition from an amorphous to a crystalline structure. Therefore, the dielectric properties of a high k binary gate insulator can be improved by adding other elements [14,15,16]. Moreover, atomic layer deposition (ALD) is a highly conformal and uniform technique with inherent atomic-scale control of thin film composition, making it ideally suited for deposition of nanolaminates film [17,18,19]. Therefore, compounded Hf_xAl_yO_z nanolaminate films were developed by ALD as the gate dielectric. However, only the electrical properties of the associated TFTs have been the focus of other reports, but the stability of TFT-based Hf_xAl_yO_z nanolaminate films rarely was reported, especially the comparison of Al₂O₃, Hf_xAl_yO_z nanolaminate, and HfO₂ film.

In this study, Al₂O₃, Hf_xAl_yO_z nanolaminate, and HfO₂ film were grown by thermal ALD. The structure, interface characteristics, surface roughness, and dielectric properties were studied. Moreover, the Al₂O₃-, Hf_xAl_yO_z nanolaminate-, and HfO₂ film-based IGZO TFTs were deposited via RF-magnetron sputtering to compare the electrical properties and stability of TFTs. The IGZO-based TFTs with Hf_xAl_yO_z as gate insulators showed a field-effect mobility of 10.31 cm²/Vs and a threshold voltage shift (∆V_th) of 0.5 V after gate-bias stress at +5 V for 900 s, which was superior to other TFTs with single Al₂O₃ or HfO₂ dielectrics. The high field-effect mobility and stability were attributed to the smooth surface, high dielectric, low current leakage, and amorphous structure of ALD Hf_xAl_yO_z nanolaminate film. The fabricated TFTs with Al₂O₃, Hf_xAl_yO_z nanolaminate film, and HfO₂ as gate insulators were named ‘device A’, ‘device B’, and ‘device C’, respectively.

2. Experimental

The Al₂O₃, Hf_xAl_yO_z nanolaminate film, and HfO₂ films were deposited via ALD (Beneq, TFS200, Espoo, Finland) at 250 °C on doped P-Si wafers as substrates for TFTs. Before being loaded into the reactor, the surface of the Si wafer was cleaned with consecutive rinses of acetone, isopropyl alcohol, and de-ionized water, and dehydrated in an oven for 30 min. Tetrakis dimethyl amino hafnium (TEMAHf), with a heating temperature of 60 °C, and water were used as the precursor to form HfO₂ films. Al(CH₃)₃ (TMA) and H₂O were used as the sources of Al and O to deposit Al₂O₃ film. The thickness of the film was controlled by the number of cycles, and the growth rate per cycle of Al₂O₃ and HfO₂ thin films was about 0.1 and 0.75 nm, respectively. The purging gas was nitrogen at 300 sccm. The opening time of the precursor valve was 100 ms. The purge/pulse time for TMA/TEMAHf or H₂O was 7/0.1 s. Then, the 40 nm IGZO film was deposited by RF-magnetron sputtering (KYKY Technology co., Ltd., Beijing, China) at 25 °C with an Ar:O₂ gas mixing ratio of 30:1, total pressure of 0.5 Pa, sputtering power of 60 W, target spacing of 7 cm, and sputtering time of 10 min using an IGZO target (99.99%, In₂O₃, Ga₂O₃, ZnO = 1:1:1 mol%) After deposition of the IGZO layer, about 200 nm of the Al source and drain electrodes were deposited by thermal evaporation through a shadow-mask with the channel width and length (L) of 1000 and 200 µm. Thermal annealing was carried out at 300 °C for 20 min in atmosphere. The schematic cross section of the IGZO-TFTs is shown in Figure 1. The characteristics of TFTs were measured by Keithley, 4200, Solon, OH, USA and the surface roughness and thickness of the films were measured by AFM (Aslum Research Cypher S, Oxford, UK) with a Veeco Dimension 3100 Scanning Probe Microscope (Veeco, Aschheim, Germany) operated in tapping mode and SEM (Zeiss SUPRA 55, Jena, Germany) at an operating voltage of 30 kV. The structural properties of the films were determined using X-ray diffraction (Shimadzu XRD7000, Kyoto, Japan) measurements with Cu-Kα radiation.

3. Results and Discussion

The glancing angle incidence X-ray diffraction (GAXRD) patterns were used to characterize the structure of the film. As seen in Figure 2, only HfO₂ thin film showed obvious peaks, and the Al₂O₃ film and Hf_xAl_yO_z nanolaminate film were all amorphous. The crystallization of HfO₂ could be hindered by Al atom doping in HfO₂ material due to the distorted bond structure order, which resulted in the amorphous structure of Hf_xAl_yO_z nanolaminate film. Amorphous phases gate insulators were preferred, because an amorphous structure has some advantages for TFT application, such as a lower defect state density and a smoother surface, and crystalline-structured gate dielectrics may experience large current leakage due to grain boundaries serving as efficient leakage paths [17,18,19]. Figure 2b shows the transmission spectra of Hf_xAl_yO_z thin film in the visible region. The Hf_xAl_yO_z film was fabricated on a glass, and it showed a high average transmittance value (over 90%). In addition, the bandgap of Hf_xAl_yO_z thin film was determined by the transmission spectra as follows [20,21,22]:

$$\alpha =\frac{1}{t}\ln \left(\frac{1}{T}\right)$$

(1)

$$\alpha \mathrm{h}\upsilon =A{(\mathrm{h}\upsilon {-\mathrm{E}}_{\mathrm{g}})}^{1/2}$$

(2)

where T, t, h, v, and A are the transmittance, film thickness, Planck’s constant, frequency of light, and the constant, respectively. As seen in the inset of Figure 2b, the calculated bandgap of Hf_xAl_yO_z thin film was about 7.7 eV, which was between the bandgap of HfO₂ (~5.7 eV) and Al₂O₃ (~8.8 eV), indicating a reasonable result for the Hf_xAl_yO_z thin film. This result also could be proved by XPS measurement, as seen in Figure 2c. As seen in Figure 2c, the Hf 4f and Al 2p appeared in our XPS spectra, and the real atomic concentrations of Hf, Al, and O were 22.3, 70.6, and 7.1%, respectively, which indicated that the Hf_xAl_yO_z nanolaminate film was constructed successfully.

SEM was performed to characterize the microstructure and thickness of gate insulators, as shown in Figure 3a–c. The thicknesses of Al₂O₃, Hf_xAl_yO_z nanolaminate, and HfO₂ were about 51, 47, and 59 nm, respectively. The imagine also indicated the uniform thickness and clear contacting line with silicon for all three films. The interface near the silicon wafer showed no visible defect such as cracking or twisting. The root mean square (RMS) surface roughness values for Al₂O₃, Hf_xAl_yO_z, and HfO₂ were 0.3, 0.4, and 2.1 nm, respectively, which agreed with the film morphologies observed with SEM. The smooth surface of Al₂O₃ and Hf_xAl_yO_z is beneficial for carrier transmission, leading to improved mobility and stability of their associated IGZO TFTs.

Figure 4 shows the leakage current density of 51 nm-thick Al₂O₃, 47 nm-thick Hf_xAl_yO_z, and 59 nm-thick HfO₂ thin films. Although the Hf_xAl_yO_z nanolaminate film showed a lesser thickness than single HfO₂, it exhibited a lower leakage current density. This result indicated that the introduction of Al₂O₃ successfully enhanced the dielectric property of the films. The magnitude of the leakage current density of Hf_xAl_yO_z nanolaminate was less than 10⁻⁷ A/cm², which is in favor of the preparation of TFT with a high on-state current to off-state current (I_on/I_off). Capacitance vs. frequency curves for all cases were measured with frequencies from 20 to 2 × 10⁶ Hz. As seen in Figure 4b, the capacitance values of 51 nm-thick Al₂O₃, 47 nm-thick Hf_xAl_yO_z, and 59 nm-thick HfO₂ thin films were 85, 110, and 185 nF/cm², respectively. The capacitance decreased slightly with increasing frequency, and it showed a stable capacitance for TFT applications.

To verify the application of Hf_xAl_yO_z nanolaminates as the gate insulators in TFTs, the IGZO TFTs were fabricated. Figure 5a–c show the output characteristics of devices. The TFT was measured under the V_D from 0 to 20 V and under the V_G from 0 to 15 V at room temperature at atmosphere. All TFTs exhibited an n-type enhanced mode channel, and the output currents (I_D) were 1.25 × 10⁻⁴, 5.83 × 10⁻⁵, and 2.3 × 10⁻⁵ A for devices A, B, and C, respectively. The output curves showed no obvious current crowding at low source-drain biases, which indicates good Ohmic contact at the Al/IGZO interface.

The transfer curves are shown in Figure 5d–f. The TFT was measured under the V_D = 5 V, and V_G from −5 to 20 V at room temperature at atmosphere. The field-effect mobility (μ) and subthreshold swing (SS) were extracted by the following equations:

$$I_D=\left(\frac{W}{2L}{C}_i\mu \right){(V_G-V_{th})}^2$$

(3)

$$SS=\frac{d{V}_G}{d\left(Log{I}_D\right)}$$

(4)

Considering the capacitance per unit area (C_i) of 51 nm-thick Al₂O₃, 47 nm-thick Hf_xAl_yO_z, and 59 nm-thick HfO₂ thin films, the characteristics of TFTs were calculated statistically, as seen in Figure 6. As seen in Figure 6, device B presented a higher average mobility of 10.31 cm²/Vs and a reasonable threshold voltage of 0.12 V/decade. The average I_on/I_off ratios were about 2.0 × 10⁷, 4.0 × 10⁷, and 6.0 × 10⁶ for device A, B, and C, respectively. Moreover, an obvious saturation at high drain biases was found, indicating depletion of free electrons in all IGZO layers. The SS is usually extracted from a semi-log plot of the transfer curve taken at high V_D. Here we obtained average values of 0.14, 0.12, and 0.180 V/dec for devices A, B, and C, respectively.

According to SS, the maximum interface density of surface states (D_it) can be derived by the following equation:

$$D_{\mathrm{it}}=\left[\frac{SSLog\left(e\right)}{kT/q}-1\right]\frac{C_i}{q}$$

(5)

where k is the Boltzmann constant, T is the temperature, and q is the electric quantity of the electron. Based on the given value of C_i, the average D_it values of 3.75 × 10¹¹, 5.44 × 10¹¹, and 1.39 × 10¹² eV⁻¹ eV⁻¹cm⁻² were calculated for devices A, B, and C, respectively. Device B showed a smaller D_it than device C, indicating that the addition of Al₂O₃ had a positive effect on decreasing the trap states at the interface between the channel layer and insulator layer, which could improve the stability of its associated TFT.

In order to further analyze the device stability, bias stressing tests were conducted at room temperature in atmosphere. Figure 7a shows the transfer curves of device B, and Figure 7b shows the threshold voltage shift (∆V_th = final threshold voltage at various time-initial threshold voltages) for different devices. The gate-bias stress was set to +5 V, and the bias stress times were fixed at 100, 300, 600, and 900 s, respectively. As seen in Figure 7b, the transfer curves of all devices shifted to the positive direction, and the slopes of the transfer curves had little changes despite the increases of the bias stress time, indicating that there was no change in mobility and SS. This suggests that the dominant factor is simple charge trapping in the gate insulator and/or at the channel–insulator interface rather than the creation of defects within the oxide semiconductor channel material [23]. Moreover, ∆V_th values for device A, device B, and device C were 0.3, 0.5, and 0.8 V, respectively. Because Al₂O₃ had a smoother surface than HfO₂ and Hf_xAl_yO_z nanolaminate with the least interface trap states at the channel–insulator, resulting in the smallest ∆V_th shift for device A, adding Al₂O₃ into HfO₂ can efficiently improve the stability of Hf_xAl_yO_z nanolaminate TFT.

4. Conclusions

In conclusion, in order to achieve low-voltage-drive, high-mobility, and high stability IGZO-TFTs, the high k, fit bandgap, smooth surface, amorphous structure, and low current leakage Hf_xAl_yO_z nanolaminate gate insulator was deposited by ALD. For comparison, we also prepared Al₂O₃ and HfO₂ thin film to fabricate the associated TFTs at the same time. The Hf_xAl_yO_z nanolaminate-based IGZO TFT shows a high average field effect mobility of 10.31cm²/Vs, an average threshold voltage of 0.15 V, an I_on/I_off ratio of over 10⁷, and a very small average sub-threshold swing of 0.12 V/dec. Moreover, the ∆V_th is only 0.5V under the gate-bias stress at +5 V for 900 s. The superior bias stress stability originates from the smooth surface of Hf_xAl_yO_z nanolaminate, resulting in the few interface trap states at the channel–insulator. The proposed ALD Hf_xAl_yO_z nanolaminate film-based IGZO TFTs in this paper show broad application prospects in next-generation flat panel displays.