Environment-dependent thermal instability of sol-gel derived amorphous indium-gallium-zinc-oxide thin film transistors

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Environment-dependent thermal instability of sol-gel derived amorphous

indium-gallium-zinc-oxide thin film transistors

Wan-Fang Chung, Ting-Chang Chang, Hung-Wei Li, Shih-Ching Chen, Yu-Chun Chen, Tseung-Yuen Tseng, and Ya-Hsiang Tai

Citation: Applied Physics Letters 98, 152109 (2011); doi: 10.1063/1.3580614 View online: http://dx.doi.org/10.1063/1.3580614

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/98/15?ver=pdfcov Published by the AIP Publishing

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Environment-dependent thermal instability of sol-gel derived amorphous

indium-gallium-zinc-oxide thin film transistors

Wan-Fang Chung,1Ting-Chang Chang,2,3,a兲Hung-Wei Li,4Shih-Ching Chen,2 Yu-Chun Chen,2Tseung-Yuen Tseng,1and Ya-Hsiang Tai5

1

Department of Electronics Engineering, Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan

2

Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan 3

Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan 4

Department of Photonics, Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

5

Department of Photonics, Display Institute, National Chiao Tung University, Hsinchu 300, Taiwan

共Received 2 February 2011; accepted 31 March 2011; published online 14 April 2011兲

The environment-dependent electrical performances as a function of temperature for sol-gel derived amorphous indium-gallium-zinc-oxide共a-IGZO兲 thin film transistors are investigated in this letter. In the ambients without oxygen, thermal activation dominates and enhances device performance. In oxygen-containing environments, mobility and drain current degrades and the threshold slightly increase as temperature increases. We develop a porous model for a-IGZO film relating to the drain current and mobility lowering due to film porosity and oxygen adsorption/penetration. It also relates to the threshold voltage recovery at high temperature owing to the varying form of adsorbed oxygen and the combination of oxygen and vacancies. © 2011 American Institute of Physics.

关doi:10.1063/1.3580614兴

Zinc oxide-based materials have been extensively inves-tigated as active layers in thin film transistors 共TFTs兲 for optoelectronic electronics such as active matrix liquid crystal displays共AMLCDs兲 due to the potential applications of low cost and large area deposition.1,2 Compared to conventional amorphous silicon共a-Si兲 TFTs, amorphous indium-gallium-zinc-oxide共a-IGZO兲 TFTs have similar device performance, while they have more advantages of low manufacturing cost and a process flow compatible with conventional a-Si TFTs.3,4

Having uniform and stable performance in AMLCD, stable transfer characteristics such as threshold voltage共Vt兲

and field effect mobility共␮eff兲 are essential. In real working

mode, the TFT array might experience a temperature stress, generated by prolonged operation. It is important to investi-gate the sensitivity of temperature, which affects adsorbing modes in a-IGZO TFTs. The a-IGZO film loses oxygen eas-ily owing to their low oxygen-vacancy formation energies, and causes a change in device characteristics with increasing temperature.5However, there appears to be a lack of

contri-bution to the operating temperature characteristics of a-IGZO TFTs. Takechi et al.6 ascribed the lower threshold voltage at higher temperatures to the generation of oxygen-vacancies. Hoshino et al.7explained this observation attrib-uted to the more populated conduction band states with tem-perature elevation. The similar trend is also observed in this study. The above two literatures, however, did not consider the ambient effect, which may affect the electrical character-istics of a-IGZO TFTs significantly.8 Thus, the work pre-sented herein is to report on the temperature dependence of electrical characteristics for sol-gel derived a-IGZO TFTs in atmospheric and vacuum ambients. We also develop a model of porous film, relating to the incorporation of charged oxy-gen ions into the sol-gel derived a-IGZO film as temperature increases.

A schematic cross-section of coplanar-type bottom-gate a-IGZO TFTs examined here is depicted in the inset of Fig. 1. Details of the device fabrication have been reported elsewhere.9An 80-nm-thick a-IGZO thin film was deposited as active layer by spin-coating at room temperature and then

a兲_{Author to whom correspondence should be addressed. Electronic mail: [email protected].}

FIG. 1. 共Color online兲共a兲 Transfer characteristics at VDS= 10 V and 共b兲 output characteristics at VG− Vt= 5 V at different temperatures for a-IGZO TFTs in atmosphere. The arrow shows the drain current trend as the tempera-ture increases. The insets of共a兲 dem-onstrated the device structure of a-IGZO TFTs.

APPLIED PHYSICS LETTERS 98, 152109共2011兲

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baked in a furnace at 450 ° C for 1hr to improve the film quality, and the molar ratio is In/Ga/Zn=1:1:2. The post-deposition annealing was conducted at 120 ° C to remove the absorbent water due to the wet etching process. The channel width and length of devices were 500/50 ␮m. All measure-ments of current-voltage characteristics were performed at various temperatures 共from 30 to 120 °C兲 in atmosphere 共760 Torr兲 and vacuum 共1⫻10−4 _{Torr兲. According to the}

saturation current equation, ␮eff and Vt can be calculated

from the slope and the abscissa intercept from the plot of I_DS1/2 versus VG.10

Figure1共a兲shows the transfer characteristics of a-IGZO TFTs, obtained at VDS= 10 V, at different temperatures in

atmosphere. As seen in this figure, except the finding at 120 ° C, the drain current 共IDS兲 in the entire gate voltage

共VG兲 region increases and the transfer characteristics curve

shifts to more negative gate voltage with the increasing tem-perature, which seems to be thermally activated.11 The V_t lowering with increasing temperature can be attributed to the free electrons generated along with oxygen-vacancies, be-cause the thermally excited oxygen atoms leave their original sites. An alternative explanation for this finding is ascribed to the more populated conduction band states 共the conduction band electron density兲 as the temperature increases.6,7

How-ever, an intrigued trend of IDS occurred which we observed

from the output characteristics, as shown in Fig. 1共b兲. The I_DS decreases gradually with increasing temperature, at V_G − Vt= 5 V. The arrow indicates the IDS trend as the ambient

temperature increases. This unusual phenomenon may be at-tributed to the ambient effect of a-IGZO TFTs, especially oxygen adsorption, because with the temperature of 120 ° C it is supposed that water molecules can be desorbed. To eliminate ambient effect, following measurement was per-formed in vacuum. Figure 2共a兲 shows the same trend of transfer characteristics as that in atmosphere, due to the

ther-mally activated vacancy formation. In contrast, IDS

appar-ently increases with the increasing temperature, different from the results in atmosphere. Hence, the strange trend of IDS in atmosphere can be ascribed to the ambient effect of

a-IGZO TFTs.

Previous research indicated that the conductivity of ox-ide semiconductors is affected by surrounding environment, causing the conductance activation energy 共EA兲 variation.12

The EAcan be calculated as a function of VGfrom the fitting

of the temperature-dependent log共I_DS兲 versus 1/T curve, where E_A= E_C− E_F.13,14 Figure 3共a兲shows E_Aas a function of VGof two surrounding environments. The EAof a-IGZO

TFTs extracted is 1.44 eV in atmosphere, and 0.58 eV in vacuum. The higher EA in atmosphere is due to the high

barrier-height induced by charged oxygen molecules 共O₂−兲, while the low EAis ascribed to the lack of oxygen adsorption

on a-IGZO film. Thus, the Vtin atmosphere would be larger

than in vacuum because of the higher barrier-height for elec-trons to overcome. The temperature dependence on Vt is

shown in Fig. 3共b兲. The results in two environments both display the Vtreduction due to the temperature activated

pro-cess of subthreshold drain current, and are consistent with the speculation that the Vt in atmosphere is larger than in

vacuum. Nevertheless, the Vt exhibits slight recovery at

120 ° C, which may be ascribed to oxygen effect.

With temperature increasing, ␮_eff increases in vacuum but decreases in atmosphere, as shown in Fig. 3共c兲. Since mobility is a strong function of temperature, ␮eff

⬀exp共−EA/kT兲, the increase in mobility under vacuum can

be ascribed to the populated conduction band states enhance-ment at higher temperatures.7 However, the mobility de-crease in atmosphere can be attributed to the repulsion of charged oxygen ions as the temperature gradually increases.

FIG. 2. 共Color online兲共a兲 Transfer characteristics at VDS= 10 V and 共b兲 output characteristics at VG− Vt= 5 V at different temperatures for a-IGZO TFTs in vacuum. The arrow shows the drain current trend as the temperature increases.

FIG. 3. 共Color online兲 Variations in 共a兲 conductance activation energy, 共b兲 threshold voltage, and 共c兲 mobility with temperatures of a-IGZO TFTs in atmospheric and vacuum ambiences.

152109-2 Chung et al. Appl. Phys. Lett. 98, 152109共2011兲

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From SEM image of a-IGZO film关Fig.4共a兲兴, the result-ing film with high surface roughness is rather porous, which may allow the adsorbed oxygen ions to easily penetrate a-IGZO film instead of only adsorbing on oxide surface. Takata et al. observed that in an air environment, the stable adsorbed oxygen ions on a-IGZO surface are O₂− below 100 ° C 关Eq.共1兲兴 and O−_{between 100 and 300 ° C}_关Eq._共2兲兴

by capturing electrons from a-IGZO films.15–17 The oxygen adsorption on a-IGZO surface can be explained by the fol-lowing equations:

O_2共ads兲+ e− O_2共ads兲− , 共1兲

O_2共ads兲+ 2e− 2O_共ads兲− . 共2兲

The free electrons transporting in a-IGZO layer will suffer from scattering by the repulsion of charged oxygen ions, causing mobility decrease with temperature elevation. The proposed model of charged oxygen ions in porous a-IGZO film is shown in Fig.4共b兲. Here, we suggest that the mobility variation in atmosphere is dominated by scattering effect, while the mobility in vacuum is mainly determined by tem-perature effect. From the simple relation between mobility and current density, J = ne␮effE, IDSundoubtedly decreases as

observed from IDS-VDS curve. As the temperature increases

above 100 ° C, the adsorption form of oxygen ions changes from O₂− to O−, causing the volume of oxygen ions to de-crease. Because of film porosity and oxygen volume reduc-tion, O− _{can penetrate the a-IGZO film more deeply and}

have a tendency to bond with oxygen-vacancies. This bond-ing behavior between oxygen ions and vacancies can be re-garded as vacancy repairment. Then, owing to the structure integrity improvement, the conductivity of the a-IGZO film decreases, leading to the slight increase in Vt, as shown in

Fig.3共b兲.

In conclusion, the temperature-related oxygen adsorption in a-IGZO film has significant influences on device charac-teristics. As temperature increases, the enhancement of threshold voltage, drain current, and mobility in vacuum is mainly due to thermal activation, inducing oxygen-vacancies and free electrons. However, in an oxygen-containing envi-ronment, the mobility and drain current lowering obtained from output characteristics and the slight increase in thresh-old voltage are mainly determined by the adsorption of charged oxygen ions. As the oxygen gas adsorbs in porous a-IGZO films, scattering of charged oxygen molecules and combination of oxygen and vacancies occur with increasing temperature, owing to the film porosity and the change in oxygen adsorption form. Moreover, the EAin atmosphere is

1.44 eV and about 0.58 eV in vacuum, owing to the adsorbed charged oxygen ions.

This work was performed at National Science Council Core Facilities Laboratory for Science and Nano-Technology in Kaohsiung-Pingtung area and was supported by the National Science Council of the Republic of China under Contract Nos. NSC-98-3114-M-110-001 and NSC 97-2112-M-110-009-MY3.

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FIG. 4.共Color online兲共a兲 SEM image of spin-coated a-IGZO film on wafer. The image is a top view with a scale of 100 nm and shows the film porosity. 共b兲 Schematic penetration model of the adsorbed oxygen ions in a porous a-IGZO film.

152109-3 Chung et al. Appl. Phys. Lett. 98, 152109共2011兲