In:
ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2014-01, No. 41 ( 2014-04-01), p. 1551-1551
Abstract:
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 Si 3 N 4 /SiO 2 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 Al 2 O 3 , HfO 2 , Ta 2 O 5 , ZrO 2 and Y 2 O 3 have been intensively investigated as gate dielectric [1-3]. Hence, we paid attention to the Al 2 O 3 as gate dielectrics because of its low leakage current. However, it is well known that Al 2 O 3 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 Al 2 O 3 dielectrics grown by PE-ALD process. We especially discuss about influence of quality of Al 2 O 3 films on transistor properties. Experimental Bottom-gate-type IGZO TFT with Al 2 O 3 dielectric was fabricated as follows. A 30-nm-thick SiO 2 layer was fabricated on p ++ -Si as a back gate electrode. Next, the Al 2 O 3 layer was deposited on SiO 2 /Si substrate by the PE-ALD method at 300 °C using TMA precursor and plasma oxygen gas. The thicknesses of the Al 2 O 3 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 (In 2 O 3 :Ga 2 O 3 :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/Al 2 O 3 /SiO 2 MOS capacitor was prepared to examine quality of Al 2 O 3 film. A 4-nm-thick SiO 2 layer was fabricated on p-Si substrate, and subsequently Al 2 O 3 films with 5-25 nm thicknesses were deposited by PE-ALD at 300 °C. Finally, Pt electrode (150 nm) was deposited on Al 2 O 3 films by DC-sputtering. Results and Discussion Figure 1 shows transfer characteristics (Id-Vg) of the IGZO TFTs with Al 2 O 3 dielectrics. Note that saturated mobility of TFT without Al 2 O 3 is slightly larger than those of TFTs with Al 2 O 3 dielectrics. Furthermore, the TFTs with Al 2 O 3 dielectrics show almost same saturated mobility regardless of Al 2 O 3 thickness. The positive Vth shift (~1V) in the TFTs with Al 2 O 3 dielectrics significantly appears compared to that of TFT without Al 2 O 3 dielectric. No thickness dependence of Al 2 O 3 on Vth shift is clear. The Vfb change as a function of the equivalent oxide thickness (EOT) of the Al 2 O 3 films for Pt/Al 2 O 3 /SiO 2 MOS capacitors is shown in Fig. 2 . It is clear that these data satisfy the linear equation. This indicates that the fixed charge in Al 2 O 3 film, which deposited by PE-ALD process, is a negligible small. The fixed charge (Q IL ) at Al 2 O 3 /SiO 2 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 Al 2 O 3 , the Fermi-level of silicon substrate, the dielectric constant of SiO 2 and EOT of Al 2 O 3 film, respectively. The Q IL value was estimated about -2.0 × 10 12 /cm 2 . Hence, when the fixed charge is assumed to occur at SiO 2 /Si interface for Metal/SiO 2 (30nm)/Si capacitor, the Q IL value of -2.0 × 10 12 /cm 2 leads to positive Vfb shift of 1.5V. We notice that the positive Vth shift of ~1V in the TFTs with Al 2 O 3 dielectrics might be due to the fixed charge at Al 2 O 3 /SiO 2 interface. Conclusions We have systematically investigated transistor properties of Bottom-gate-type IGZO TFTs with Al 2 O 3 dielectric grown by PE-ALD process. We found that mobility of TFTs with Al 2 O 3 dielectrics decreases slightly irrespective of the Al 2 O 3 thickness. Note that the positive Vth shift of TFTs with Al 2 O 3 dielectrics is dominantly due to the fixed charge at Al 2 O 3 /SiO 2 interface from data of Pt/Al 2 O 3 /SiO 2 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 Al 2 O 3 gate dielectrics. Fig. 2 The Vfb change as a function of the EOT of the Al 2 O 3 films for Pt/Al 2 O 3 /SiO 2 MOS capacitors.
Type of Medium:
Online Resource
ISSN:
2151-2043
DOI:
10.1149/MA2014-01/41/1551
Language:
Unknown
Publisher:
The Electrochemical Society
Publication Date:
2014
detail.hit.zdb_id:
2438749-6
Permalink
