Influence of Al2O3 Gate Dielectric on Transistor Properties for IGZO Thin Film Transistor

Introduction

Recently, indium-gallium-zinc oxide (IGZO) thin film transistor (TFT) has attracted attention for their applications to active matrix liquid crystal display and active matrix organic light-emitting diode. As a typical IGZO TFT, the Si₃N₄/SiO₂ structure has been widely developed. To archive the requirements of a low leakage current and a high k value, several high-k dielectrics, such as Al₂O₃, HfO₂, Ta₂O₅, ZrO₂ and Y₂O₃ have been intensively investigated as gate dielectric [1-3].

Hence, we paid attention to the Al₂O₃ as gate dielectrics because of its low leakage current. However, it is well known that Al₂O₃ film, which fabricated by conventional ALD at low temperature, contains some defects [4] and results in mobility degradation and abnormal threshold voltage (Vth) shift. Plasma-enhanced ALD (PE-ALD) is expected to reduce impurities and increases the film density because of oxidation reaction.

In this paper, we report on electrical performance of IGZO TFTs with Al₂O₃ dielectrics grown by PE-ALD process. We especially discuss about influence of quality of Al₂O₃ films on transistor properties.

Experimental

Bottom-gate-type IGZO TFT with Al₂O₃ dielectric was fabricated as follows. A 30-nm-thick SiO₂ layer was fabricated on p⁺⁺-Si as a back gate electrode. Next, the Al₂O₃ layer was deposited on SiO₂/Si substrate by the PE-ALD method at 300 °C using TMA precursor and plasma oxygen gas. The thicknesses of the Al₂O₃ were varied to 5, 10, 15 and 25 nm by changing ALD cycles. A 50-nm-thick IGZO layer was subsequently deposited at room temperature by DC-sputtering using an IGZO target (In₂O₃:Ga₂O₃:ZnO = 1:1:1 mol%). Finally, the Au (100 nm) layers as source/drain electrodes were deposited by thermal evaporation method. The channel length and width of the TFTs are 50 and 1200 mm, respectively. Post annealing was performed at 300 °C for 1 h.

                        In addition, Pt/Al₂O₃/SiO₂ MOS capacitor was prepared to examine quality of Al₂O₃ film. A 4-nm-thick SiO₂ layer was fabricated on p-Si substrate, and subsequently Al₂O₃ films with 5-25 nm thicknesses were deposited by PE-ALD at 300 °C. Finally, Pt electrode (150 nm) was deposited on Al₂O₃ films by DC-sputtering.

Results and Discussion

Figure 1 shows transfer characteristics (Id-Vg) of the IGZO TFTs with Al₂O₃ dielectrics. Note that saturated mobility of TFT without Al₂O₃ is slightly larger than those of TFTs with Al₂O₃ dielectrics. Furthermore, the TFTs with Al₂O₃ dielectrics show almost same saturated mobility regardless of Al₂O₃ thickness. The positive Vth shift (~1V) in the TFTs with Al₂O₃ dielectrics significantly appears compared to that of TFT without Al₂O₃ dielectric. No thickness dependence of Al₂O₃ on Vth shift is clear.

The Vfb change as a function of the equivalent oxide thickness (EOT) of the Al₂O₃ films for Pt/Al₂O₃/SiO₂ MOS capacitors is shown in Fig. 2. It is clear that these data satisfy the linear equation. This indicates that the fixed charge in Al₂O₃ film, which deposited by PE-ALD process, is a negligible small. The fixed charge (Q_IL) at Al₂O₃/SiO₂ interface can be obtained the following equation:

Vfb = (Φ_m,eff – Φ_Si) – Q_IL / ε_sio2 × EOT_Al2O3          (1)

where f_m,eff, f_Si, e_SiO2 and EOT_Al2O3 are the effective work function of the gate electrode on Al₂O₃, the Fermi-level of silicon substrate, the dielectric constant of SiO₂ and EOT of Al₂O₃ film, respectively. The Q_IL value was estimated about -2.0 × 10¹²/cm².

Hence, when the fixed charge is assumed to occur at SiO₂/Si interface for Metal/SiO₂(30nm)/Si capacitor, the Q_IL value of -2.0 × 10¹²/cm² leads to positive Vfb shift of 1.5V. We notice that the positive Vth shift of ~1V in the TFTs with Al₂O₃ dielectrics might be due to the fixed charge at Al₂O₃/SiO₂ interface.

Conclusions

We have systematically investigated transistor properties of Bottom-gate-type IGZO TFTs with Al₂O₃ dielectric grown by PE-ALD process. We found that mobility of TFTs with Al₂O₃ dielectrics decreases slightly irrespective of the Al₂O₃ thickness. Note that the positive Vth shift of TFTs with Al₂O₃ dielectrics is dominantly due to the fixed charge at Al₂O₃/SiO₂ interface from data of Pt/Al₂O₃/SiO₂ capacitors.

References

[1] H. Nomura et al., Nature (London) 432 (2004) 488.

[2] S-Y. Lee et al., Thin Solid Films 518 (2010) 3030..

[3] I-K. Lee et al., Jpn. J. Appl. Phys. 52 (2010) 06GE05.

[4] T. Nabatame et al., Abst. 12^th Int. Conf. on ALD (2012) p.267.

Fig. 1 Id-Vg characteristics of the IGZO TFTs with and without Al₂O₃ gate dielectrics.

Fig. 2 The Vfb change as a function of the EOT of the Al₂O₃ films for Pt/Al₂O₃/SiO₂ MOS capacitors.