Effect of oxygen partial pressure on electrical characteristics of amorphous indium gallium zinc oxide thin-film transistors fabricated by thermal annealing

Vacuum 86 (2011) 246e249

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Effect of oxygen partial pressure on electrical characteristics of amorphous indium gallium zinc oxide thin-film transistors fabricated by thermal annealing C.J. Chiu a, Z.W. Pei b, S.T. Chang b, S.P. Chang a, *, S.J. Chang a a

Institute of Microelectronics & Department of Electrical Engineering, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan b Institute of Optoelectronic Engineering, Department of Electrical Engineering, National Chung Hsing University, Taichung 40227, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 March 2011 Received in revised form 16 June 2011 Accepted 18 June 2011

We report the fabrication and electrical characteristics of high-performance amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with a polymer gate dielectric prepared by spin coating on a glass substrate at different oxygen partial pressure values. The transmittance of the deposited polymer film was greater than 90% at 600 nm a-IGZO thin films were deposited on glass substrates using RF magnetron sputtering at different oxygen partial pressure values. The a-IGZO TFTs were prepared by rapid thermal annealing at 350
C for 10 min at a 0.2% oxygen partial pressure. It was observed that aIGZO TFTs with an active channel layer exhibited enhanced mode operation, a threshold voltage of 1 V, an on-off current ratio of 103, and a field-effect mobility of 18 cm2/Vs. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.

Keywords: a-IGZO Polymer TFT High mobility

1. Introduction In conventional active-matrix liquid crystal displays (LCDs), the active layer is composed of hydrogenated amorphous or polycrystalline silicon. These Si-based thin-film transistors (TFTs) have an inherent limitation: they are opaque in the visible wavelength region, which results in reduced light transmittance and reduced brightness for the display. In recent years, transparent-oxidesemiconductor-based transistors have emerged as promising alternatives to Si-based TFTs. Indeed, the fabrications of transistors based on tin oxide (SnO), zinc oxide (ZnO), indium zinc oxide (IZO), gallium zinc oxide (GZO), and indium gallium zinc oxide (IGZO) have been demonstrated [1e5]. Among these transparent-oxidesemiconductor materials, IGZO has attracted considerable attention because of its relatively high electron mobility and good chemical stability [5]. In addition, amorphous IGZO (a-IGZO) can be coated at low temperatures with good large area uniformity and good bias-stress stability [6]. In addition to the active layer material, the insulating material plays a significant role in high-performance TFTs. To the best of our knowledge, SiNx [7], Y2O3 [8], SiOx [9], HfO2 [10], and SiON [11] deposited by plasma-enhanced chemical vapor deposition (PE-

* Corresponding author. E-mail address: [email protected] (S.P. Chang).

CVD) have all been used as gate insulators for a-IGZO TFTs prepared on glass substrates. Compared to PE-CVD, the spin coating method is much easier, faster, and cost effective. Spin coating is also suitable for the mass production of devices prepared on large substrates. It has been shown that a polymer gate dielectric prepared by spin coating can be used for fabricating organic TFTs [12,13]. Recently, Lee et al. reported the fabrication of a-ZnO TFTs using a polymer dielectric deposited by spin coating; they used these a-ZnO TFTs to realize glass-based inorganiceorganic electronics [14,15]. Poly(4-vinylphenol) (PVP), an organic dielectric, has been widely used in organic thin-film transistors (OTFTs) because it is inexpensive and can be used for the fabrication of films having a large area, making it particularly useful for applications to flexible electronics. Klauk et al. [12] reported that pentacene TFTs with spin-coated PVP gate dielectric layers had better electrical properties than TFTs with thermally grown SiO2 gate dielectric layers. The use of PVP as a polymer insulator has enabled numerous advances. In this study, we report the fabrication process and characteristics of high-performance, a-IGZO TFTs with a polymer gate dielectric prepared on glass substrates by spin coating at different oxygen partial pressures. The optical and electrical properties of the fabricated devices are also discussed. We also prepared a-IGZO TFTs using rapid thermal annealing at 350
C for 10 min at different oxygen partial pressure values. We investigated the effects of thermal annealing on the characteristics of the a-IGZO TFTs.

0042-207X/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.06.014

C.J. Chiu et al. / Vacuum 86 (2011) 246e249

2. Experimental

3. Results and discussion Fig. 2 shows the transmission spectra of the polymer layer and polymer/a-IGZO double-layer prepared at an oxygen partial pressure of 0.2%. The transmittance of each sample was normalized with respect to that of the glass substrate. When the wavelength of the incident light was 600 nm, the transmittance of the polymer film was greater than 90%. At the same wavelength as the incident light, the transmittance of the polymer/a-IGZO double-layer was greater than 80% at 600 nm. These values indicate that the polymer film used in this study is optically usable as the gate dielectric of aIGZO TFTs. Fig. 3 shows the XPS spectra of the O 1s peak of the a-IGZO film at different oxygen partial pressures. The typical O 1s peak on the

Transmittance (%)

100 80 60 40 20 0 300

PVP/glass PVP/IGZO(50 nm)/glass

400

500 600 700 Wavelength (nm)

800

Fig. 2. Normalized transmission spectra of polymer layer and polymer/a-IGZO doublelayer prepared at oxygen partial pressure of 0.2%.

surface could be consistently fitted by near Gaussians, centered at 530.06  0.03. Ahn et al. [16] reported that the low binding energy peak (OL) was attributed to the O2 ions surrounded by Zn, Ga, and In atoms with their full complement of nearest-neighbor O2 ions. At oxygen partial pressures of 0.1%, 0.14%, and 0.2%, the binding energies were 529.43, 529.22, and 529.18 eV, respectively. It was observed that the binding energy shift of the related oxygen vacancy decreased with an increase in the oxygen partial pressure. On the basis of these results, we could confirm the role of the oxygen partial pressure during the processing of the a-IGZO thin film. Lee et al. [17] reported that all polymeric dielectrics that exhibit a large hysteresis contain a significant number of hydroxyl groups. Although the number of eOH groups is reduced after cross-linking, the remaining eOH groups should be able to inside the surface of aIGZO effectively. Furthermore, when hydroxyl groups are present in a polymer dielectric, the electron trappings related to the hydroxyl groups increase, resulting in a large gate leakage current. The above results are evidently caused by the mobile charge in the gate dielectric or low trap density in the interface between the PVP and a-IGZO layers. However, these characteristics are attractive for nonvolatile thin-film transistor memory applications [18]. Fig. 4(aec) show the output IeV characteristics (i.e., IDSeVDS) of a-IGZO TFTs fabricated at oxygen partial pressures of 0.1%, 0.14%, and 0.2%, respectively, without annealing. It can be observed that all of the devices exhibit typical transistor characteristics with a clear pinch-off and current saturation. However, Hansen et al. [19]

Intensity (arb. units)

Fig. 1 shows a schematic diagram of the cross-section of the bottom-gated a-IGZO TFTs fabricated in this study. We first cleaned the glass substrates with acetone, ethanol, and deionized water. A 100-nm-thick Al gate electrode was then deposited by thermal evaporation through a shadow mask at room temperature; selective chemical etching was performed thereafter. Next, we fabricated the polymer gate dielectric. We prepared a solution containing 8 wt.% poly(4-vinylphenol) (PVP) and 4 wt.% methylated poly(melamine-co-formaldehyde) (PMF) dissolved in propylene glycol monomethyl ether acetate (PGMEA). The polymer solution was then coated on the device by spin coating at 2000 rpm for 60 s. Cross-linking was then performed by placing the samples on a hot plate for 1 h at 200
C to form a 220-nm-thick polymer gate dielectric. The a-IGZO layer was subsequently deposited onto the polymer layer using RF sputtering through another shadow mask at room temperature. The target was a pure IGZO layer (atomic ratio In:Ga:Zn ¼ 1:1:1, 99.99% pure) with a diameter of 5.05 cm. During sputtering, the RF power and chamber pressure were maintained at 50 W and 10 mTorr, respectively. Samples were prepared using IGZO sputtering at three different oxygen partial pressures (0.1%, 0.14%, and 0.2%). To ensure that the device was transparent, we prepared a sample with only one polymer layer on the glass substrate and a sample with a polymer/a-IGZO double-layer on the glass substrate. The transmission spectra of the polymer layer and polymer/a-IGZO double-layer, deposited at an oxygen partial pressure of 0.2%, were then measured using a Hitachi U3010 spectrophotometer (Hitachi, Tokyo, Japan). After deposition of the a-IGZO layer, a 100-nm-thick Au layer was thermally evaporated onto the a-IGZO film through a third shadow mask to serve as the source and drain electrodes. The gate length (L) and gate width (W) of the fabricated a-IGZO TFTs were 100 mm and 1000 mm, respectively. Finally, rapid thermal annealing was carried out at 350
C for 10 min in order to improve the performances of these devices. The currentevoltage (IeV) characteristics of the TFTs fabricated with and without annealing were then measured at room temperature using a HP 4156 semiconductor parameter analyzer in the dark.

247

0.1% 0.14% 0.2% OL

527 528 529 530 531 532 533 Binding energy (eV) Fig. 1. Schematic cross-section of bottom-gated a-IGZO TFTs fabricated in this study.

Fig. 3. XPS spectra of O 1s peak of a-IGZO films at different oxygen partial pressures.

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a 30

V G =2V

V G =4V

V G =6V

V G =8V

-5

10

IDS(A)

20

IDS(

)

V G =10V

10

0

10

-8

2

4

6

8

60

IDS(

)

50

V G =2V

V G =4V

V G =6V

V G =8V

30 20 10 0 6

8

10

8

10

V DS (V) 40

V G =2V

V G =4V

V G =6V

V G =8V

IDS(

)

V G =10V

20

0 0

2

4

6

4

6

8

10

VGS(V)

IDS ¼

4

2

operating in the saturation region, the drain current, IDS, can be expressed as

40

2

0

Fig. 5. Transfer characteristics (i.e., IDSeVGS) of a-IGZO TFTs fabricated at different oxygen partial pressures without thermal annealing.

V G =10V

0

-2

10

V DS (V)

c

0.1% 0.14% 0.2%

-7

10

0

b

-6

10

V D S (V ) Fig. 4. Output IeV characteristics (i.e., IDSeVDS) of a-IGZO TFTs fabricated at oxygen partial pressures of (a) 0.1%, (b) 0.14%, and (c) 0.2%, without annealing.

reported that when O2 plasma was used to treat polymer materials, they found that the oxygen broke down into oxygen free-radicals. These would react with the surface of the polymer materials and change the chemical structure. In Fig. 4, severe current leakages can be observed for the a-IGZO TFTs with the PVP gate dielectric because of damage to the PVP films from Ar plasma bombardment. With an increase in the oxygen partial pressure, the turn-on voltage (VON) decreased and the drain current decreased considerably. Fig. 5 shows the transfer characteristics (i.e., IDSeVGS) of a-IGZO TFTs fabricated without thermal annealing. For a transistor

W mC ðV  Vt Þ2 2L i GS

(1)

where m is the carrier mobility, Ci is the capacitance of the gate dielectric, and Vt is the threshold voltage. If the values of W, L, and Ci, and the I1/2 DS eVGS relationship are known, we can extrapolate the values of Vt, the sub-threshold swing (SS), on-off current ratio (Ion/Ioff), and m for the a-IGZO TFTs. Table 1 lists the values of Vt, SS, Ion/Ioff, and m that were extrapolated from the corresponding values of the a-IGZO transistors fabricated without thermal annealing. Fig. 6 shows the transfer characteristics (i.e., IDSeVGS) of a-IGZO TFTs fabricated by thermal annealing at 350
C for 10 min at different oxygen partial pressures. Table 2 lists the extrapolated values of Vt, SS, Ion/Ioff, and m for the a-IGZO TFTs fabricated by thermal annealing. From Tables 1 and 2, it can be seen that the performance of the fabricated a-IGZO TFTs improved after thermal annealing; in addition, the a-IGZO TFT prepared at a 0.2% oxygen partial pressure and subjected to annealing provided the highest mobility (18 cm2/Vs) while maintaining the SS (3 V/dec) and Ion/ Ioff (103). It should be noted that the mobility of 18 cm2/Vs observed in this study is greater than the reported mobilities of similar a-IGZO TFTs [20e22] with a polymer gate dielectric. These results, which were caused by the thermal annealing and a rearrangement of the surface morphology, led to a much improved interface condition in the active layer [23]. This enhancement in the field-effect mobility was the effect of the thermal annealinginduced decrease in trap density in the amorphous channel layer. It is well known for oxide semiconductors that free carriers in the channel are mainly caused by the generation of oxygen vacancies. The lowest VON and highest drain current were observed in the TFT produced at an oxygen partial pressure of 0%. The high-performance of the a-IGZO TFTs produced by controlling

Table 1 Values of Vt, SS, Ion/Ioff, and m extrapolated from a-IGZO TFTs fabricated at different oxygen partial pressures without thermal annealing. O2/Ar þ O2

Vt (V)

SS (V/dec)

Ion/Ioff

m (cm2/Vs)

0.1% 0.14% 0.2%

1 1.8 0.8

2.56 1.25 3.29

103 102 101

0.92 4 3.4

C.J. Chiu et al. / Vacuum 86 (2011) 246e249

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hope that these results will contribute significantly to nonvolatile thin-film transistor memory applications.

-4

10

-5

IDS(A)

10

Acknowledgments

-6

10

-7

10

0.1% 0.14% 0.2%

-8

10

-9

10

-2

0

2

4

6

8

10

VGS(V)

References

Fig. 6. Transfer characteristics (i.e., IDSeVGS) of a-IGZO TFTs fabricated by thermal annealing at 350
C for 10 min at different oxygen partial pressures.

Table 2 Values of Vt, SS, Ion/Ioff, and m extrapolated from a-IGZO TFTs fabricated by thermal annealing at different oxygen partial pressures. O2/Ar þ O2 0.1% 0.14% 0.2%

Vt (V) 3.8 0.5 1

SS (V/dec) 1.93 1.76 3

This work was partly supported by the Center for Frontier Materials and Micro/Nano Science and Technology, and partly by the Advanced Optoelectronic Technology Center, National Cheng Kung University (NCKU), under projects from the Ministry of Education. The authors would also like to thank the Bureau of Energy, Ministry of Economic Affairs of Taiwan, for financially supporting this research under Contract No. 98-D0204-6, and the LED Lighting and Research Center, NCKU, for their assistance with the measurements.

Ion/Ioff 4

10 104 103

m (cm2/Vs) 1.31 17.5 18

the oxygen partial pressure in the channel layer was interesting. From these results, we could conclude that the sensitivity of the channel layers to the appropriate oxygen contents could be used for oxygen sensor applications. 4. Conclusion We fabricated high-performance a-IGZO thin-film transistors with a polymer gate dielectric prepared by spin coating on glass substrates at different oxygen partial pressures. The transmittance of the deposited polymer film was greater than 90% at 600 nm. The polymer film used in this study is optically usable as the gate dielectric of a-IGZO TFTs. a-IGZO TFTs were prepared by rapid thermal annealing at 350
C for 10 min at a 0.2% oxygen partial pressure. a-IGZO TFTs with an active channel layer exhibited enhanced mode operation, a threshold voltage of 1 V, an on-off current ratio of 103, and a field-effect mobility of 18 cm2/Vs. We

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