High-performance InGaZnO thin-film transistor incorporating a HfO₂/Er₂O₃/HfO₂ stacked gate dielectric

Abstract

In this paper, a HfO₂/Er₂O₃/HfO₂ (HEH) stacked structure was developed as a gate dielectric for amorphous InGaZnO (α-IGZO) thin-film transistor (TFT) applications. Atomic force microscopy and X-ray photoelectron spectroscopy were used to study the morphological and chemical features of Er₂O₃ and HEH films. In comparison to the Er₂O₃ dielectric, the α-IGZO TFT device incorporating a HEH stacked dielectric exhibited a lower threshold voltage of 0.7 V, a higher I_on/I_off current ratio of 2.86 × 10⁷, a larger field-effect mobility of 15.8 cm² V⁻¹ s⁻¹, and a smaller subthreshold swing of 101 mV per dec, suggesting a smooth surface at the dielectric-channel interface. Furthermore, the threshold voltage stability of α-IGZO TFT under positive gate voltage stress can be improved by using a HEH stacked structure.

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Introduction

An amorphous indium gallium zinc oxide (α-IGZO) thin-film transistor (TFT) is a promising candidate for a switching element in flat, flexible, electric papers, and transparent display applications.¹ Silicon dioxide (SiO₂) is commonly used as a gate dielectric in α-IGZO TFTs.^2,3 However, due to its poor thermal conductivity,⁴ the application of an SiO₂ insulating layer in high-power and high-temperature logic devices is limited by the self-heating effect.⁵ Therefore, the development of new insulating materials with good thermal conductivity and high dielectric constants has received much attention in recent years.^6–8 The search for suitable dielectric thin films at low temperatures has also been investigated for use in various display applications.^9–11 The most important criteria for a dielectric material are: a high band gap (>5 eV) and a favorable conduction or valence band offset (for n- and p-type semiconductors, respectively) to minimize the gate leakage current.^1,12 Furthermore, the quality of the dielectric-semiconductor interface is a very important issue because the transistor performance can be very poor if the gate insulator with a rougher surface leads to excessive interface states.¹²

Rare-earth metal oxides (REOs) have been extensively studied because of their interesting properties, such as high dielectric constant, wide bandgap energy, high transparency, and high refractive index.^13,14 Erbium oxide (Er₂O₃) is one of the interesting REOs due to its multifunctionality in various fields of technology. Er₂O₃ has attracted considerable interest for applications in photonics, telecommunications and optics due to its photoluminescence and electroluminescence properties;¹⁵ it can be used to substitute SiO₂ as the gate dielectric to suppress the gate leakage current with the scaling of complementary metal-oxide semiconductor (CMOS) devices because it possesses a combination of a large band gap (5–7 eV), a high dielectric constant (κ = ∼14) and high resistivity (10¹² to 10¹⁵ cm⁻³).¹⁶ Furthermore, its high chemical and thermal stabilities also makes an Er₂O₃ film as a protective and corrosion-resistant coating.¹⁷ Mikhelashvili et al.¹⁸ demonstrated that Er₂O₃ thin film showed a low gate leakage current and a high breakdown electric field. It also had a small hysteresis voltage and a low interface trap density in our previous report.¹⁹ However, the hygroscopic nature of REO films can degrade their permittivity and surface morphology due to the formation of low-permittivity hydroxides.²⁰ To solve this issue, a triple layer dielectric structure was proposed to improve physical and electrical properties of both the gate/dielectric and the dielectric/α-IGZO channel.^21–23 The smooth surfaces are preferable regarding materials for TFT application because they generally provide higher carrier mobility, lower leakage current and better reliability.²⁴ Lee et al.²² reported that α-IGZO TFT with Al₂O₃/HfO₂/Al₂O₃ gate dielectric had a smaller hysteresis width and lower subthreshold swing (SS) than that of HfO₂ dielectric. In this study, a novel HfO₂/Er₂O₃/HfO₂ (HEH) stacked structure was employed to improve interface properties of the dielectric/channel. We used atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) to explore the surface morphologies and chemical compositions of Er₂O₃ and HEH films. In comparison with Er₂O₃ α-IGZO TFT, the HEH α-IGZO TFT device exhibited better electrical characteristics, such as a higher field-effect mobility (μ_FE), lower threshold voltage (V_TH), smaller (SS), and higher I_on/I_off current ratio. The stability of α-IGZO TFTs under positive gate bias stress (PBS) were also examined.

Experimental

Fig. 1 shows the cross section view of the HEH α-IGZO TFT device. The bottom gate staggered HEH α-IGZO TFTs were produced on the SiO₂/Si substrates. A 50 nm TaN film was deposited as a bottom gate through a reactive sputtering system. Then, a 6 nm HfO₂ film was grown by atomic layer deposition using tetrakis(ethylmethylamino)hafnium as the precursor and O₂ as the oxidizer at a temperature of 250 °C. A ∼38 nm Er₂O₃ was deposited with sputtering from an Er target at room temperature, followed by furnace in O₂ ambient for 10 min at 300 °C. After the deposition of Er₂O₃ film, a 6 nm HfO₂ film was then grown to form a HEH sandwich stacked structure. The α-IGZO channel layer (∼20 nm) was deposited on the HEH by using sputtering from a ceramic IGZO target (In₂O₃ : Ga₂O₃ : ZnO = 1 : 1 : 1) at room temperature. Subsequently, a 50-nm Al film is deposited by a thermal evaporation system and patterned for the source/drain (S/D) electrodes. The channel width/length (W/L) of TFT device was 100/10 μm. In comparison with HEH α-IGZO TFTs, a ∼50 nm Er₂O₃ gate dielectric was also fabricated to make α-IGZO TFTs.

Fig. 1 Schematic diagram of the HEH stacked gate dielectric IGZO TFT device structure.

The film composition of the dielectric films was investigated using XPS. The surface morphology and roughness of the films were analysed by AFM. The TFT characteristics and the PBS tests were performed at room temperature and in the dark using an Agilent semiconductor parameter 4156C. The V_TH is defined as the gate voltage (V_GS) when the normalized drain current (NI_DS = I_DS × L/W) reaches 1 nA The SS was considered as the minimum value of [d log(I_DS)/dV_GS]⁻¹, where I_DS is the drain-source current The μ_FE was extracted from the linear operation regime using the following formula¹

where g_m is the transconductance, V_DS is the drain-source voltage and C_ox is the gate capacitance per unit area.

Results and Discussion

Fig. 2 shows AFM surface images of the Er₂O₃, HfO₂ and HEH films on the TaN/SiO₂. The surface roughness of the Er₂O₃ film (2.21 nm) is higher compared with the HfO₂ film (0.18 nm). The surface roughness enhancement is a nonuniform moisture absorption of Er₂O₃, resulting in the nonuniform volume expansion of the film.²⁰ The cause of volume expansion is the density difference between Er(OH)_x and Er₂O₃. The moisture absorption phenomenon is the reaction between the solid oxide (Er₂O₃) film and gaseous state water (H₂O) in air, which can be expressed as.²⁵

Er₂O₃ + nH₂O_(g) ↔ Er₂(OH)_2n (2)

Fig. 2 AFM surface images of (a) Er₂O₃ (50 nm), (b) HfO₂ (6 nm), and (c) HEH (6 nm/38 nm/6 nm) dielectric films on the TaN/SiO₂.

The Er₂O₃ film with a negative Gibbs free-energy value can cause a large moisture-absorption-reaction rate. In contrast, HfO₂ film possesses a positive Gibbs free-energy value to reduce the moisture-absorption-reaction rate. Therefore, the HEH film exhibits a smoother surface than the Er₂O₃ film, indicating that the HEH sandwich structure can suppress the moisture absorption of Er₂O₃ film.

Fig. 3(a) shows the Er 4d_5/2 and Hf 4f XPS spectra of Er₂O₃ and HEH dielectric films. The peak position of the Er 4d_5/2 at 168.7 eV represented Er₂O₃,²⁶ while Hf 4f_5/2 and 4f_7/2 double peaks at 18.2 and 16.7 eV were assigned to HfO₂,²⁶ respectively. The O 1s spectra of Er₂O₃ sample in Fig. 3(b) can be deconvoluted to two chemical states: the low binding energy state at 530.1 eV can be related to O atoms in Er₂O₃ (ref. 26) and the high binding energy state at 532.5 eV to O atoms in the Er(OH)_x. For HEH sample, the O 1s peaks at 530.4 and 531.7 eV represent the HfO₂ (ref. 27) and non-lattice, respectively. The O 1s signal corresponding to Er(OH)_x was a larger peak intensity than that of Er₂O₃, suggesting a high degree of the reaction between the water and Er₂O₃ causing hydroxide units to form in the Er₂O₃ film. On the contrary, no hydroxide peak was observed in the HEH sample. In addition, the O 1s peak corresponding to HfO₂ is larger intensity than that of non-lattice oxygen, possibly suggesting the formation of a stoichiometric HfO₂ film.

Fig. 3 (a) Er 4d_5/2 and Hf 4f, and (b) O 1s energy levels in XPS spectra of Er₂O₃ and HEH dielectric films.

Fig. 4(a) shows the transfer characteristics (I_DS–V_GS) of the α-IGZO TFT devices incorporating Er₂O₃ and HEH gate dielectrics. The V_TH of the Er₂O₃ and HEH IGZO TFT devices is evaluated to be 1.16 and 0.7 V, respectively. The κ of the Er₂O₃ and HfO₂ dielectric films is determined to be ∼14.1 and 10.7 from capacitance–voltage curves, respectively. The μ_FE of IGZO TFT devices incorporating Er₂O₃ and HEH gate dielectrics is 4.3 and 15.8 cm² V⁻¹ s⁻¹, respectively. This high mobility is due to the smooth surface between the dielectric layer and IGZO channel, thus leading to less surface scattering at the dielectric-channel interface. Furthermore, the HEH TFT device has very low gate leakage current (I_GS ∼ 10⁻¹²A) in the linear regime, while Er₂O₃ TFT is deteriorated by the increased I_GS. The I_on/I_off ratio of α-IGZO TFT devices fabricating Er₂O₃ and HEH gate dielectrics is 2.31 × 10⁵ and 2.86 × 10⁷, respectively. The SS value of the Er₂O₃ and HEH TFT devices is evaluated to be 268 and 101 mV per dec, respectively. The maximum states density (N_it) at the dielectric/channel interface of 5.46 × 10¹² cm⁻³ eV⁻¹ for Er₂O₃ TFT and 1.01 × 10¹² cm⁻³ eV⁻¹ for HEH TFT, were calculated by making the assumption that the interface states are independent of energy, using the following eqn (3):²⁸

where q is the electronic charge, k is Boltzmann's constant, and T is the absolute temperature. The higher N_it of the Er₂O₃ TFT device suggests a larger interface trap densities at or near the Er₂O₃/IGZO interface. The output characteristics of typical α-IGZO TFT devices using Er₂O₃ and HEH gate dielectrics are presented in Fig. 4(b). It is clear that the driving current increases significantly for TFT using the HEH sandwich stacked structure. This result is attributed to the smooth surface at the dielectric/channel interface resulting in the higher mobility.

Fig. 4 (a) Transfer (I_DS–V_GS) and (b) out put characteristics (I_DS–V_DS) of the Er₂O₃ and HEHIGZO TFT devices.

The device stability under operation is an important issue for the commercialization of α-IGZO TFT. PBS is one of the main factors which degrade the device performance as time passes. Fig. 5 shows the threshold voltage shift (ΔV_TH) as a function of stress time for the α-IGZO TFT devices fabricating Er₂O₃ and HEH gate dielectrics. The gate voltage stress was performed at V_GS = 8 V for 10⁴ s. The evolution of V_TH shift is found to fit the stretched-exponential equation as follows:²⁹

where ΔV_TH0 is the ΔV_TH at infinite time, t is stress time, β is the stretched exponential exponent, and τ is the characteristic trapping time constant. This suggests that the V_TH instability is dominated by charge-trapping in the gate dielectric or at the interface of gate dielectric and IGZO channel.³⁰ The evolutions of threshold voltage under PBS well fit the stretched-exponential equation. The shift in V_TH is attributed to accumulated electrons near the dielectric-channel interface trapped by shallow acceptor-like trap states³¹ or the oxygen molecular (negatively charged-oxygen) absorption in the IGZO channel caused the charge capture at the interface of dielectric and channel.³² The high V_TH shift (2.41 V) of the Er₂O₃ TFT indicates that more electrons trapping near/at the Er₂O₃–IGZO interface, whereas the low V_TH shift (1.38 V) of the HEH TFT device suggests suppressed the trapped charge in the film because of a smooth surface at the dielectric-channel interface. In addition, the β value of the α-IGZO TFT device with Er₂O₃ and HEH gate dielectrics was extracted to be 0.25 and 0.29, whereas the τ value was 2.8 × 10⁴ and 3.6 × 10⁴ s, respectively.

Fig. 5 Threshold voltage shift as a function of stress time for the Er₂O₃ and HEH IGZO TFT devices under PBS.

Conclusions

In this study, we proposed a novel HEH stacked dielectric α-IGZO TFT device. The HEH stacked dielectric exhibited a smooth surface (from AFM) and no erbium hydroxide (from XPS). The α-IGZO TFT using a HEH stacked structure exhibited a lower V_TH of 0.7 V, a higher I_on/I_off ratio of 2.86 × 10⁷, a larger μ_FE of 15.8 cm² V⁻¹ s⁻¹, and a smaller subthreshold swing of 101 mV per dec, compared with that of Er₂O₃ dielectric. These results are attributed to the formation of a smooth surface at the dielectric-channel interface. In the case of α-IGZO TFT with HEH stacked structure, the device stability under PBS could be improved. It is expected that the HEH stacked structure is a promising gate dielectric material to fabricate α-IGZO TFTs for future display applications.

Acknowledgements

This work was supported by the National Science Council, Taiwan, Republic of China, under contract no. NSC-102-2221-E-182-072-MY3.

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