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Study on the Effects of Zr-Incorporated InZnO Thin-Film Transistors Using a Solution Process

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2011 Jpn. J. Appl. Phys. 50 070202

(http://iopscience.iop.org/1347-4065/50/7R/070202)

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Study on the Effects of Zr-Incorporated InZnO Thin-Film Transistors

Using a Solution Process

Tae Hoon Jeong, Si Joon Kim, Doo Hyun Yoon, Woong Hee Jeong,

Dong Lim Kim, Hyun Soo Lim, and Hyun Jae Kim�

School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Republic of Korea

Received March 7, 2011; accepted April 18, 2011; published online July 5, 2011

The effects of adding Zr into solution-processed InZnO (IZO) thin-film transistors (TFTs) were investigated with varying Zr content. ZrInZnO

(ZIZO) TFTs showed a lower off-current and a sharper subthreshold swing than IZO TFTs. The addition of Zr plays a role in reducing the oxygen

vacancies, and thus effectively decreases the carrier concentrations of the ZIZO active layer. The characteristics of various DC bias stresses are

provided and discussed to evaluate the reliability of ZIZO TFTs. After 1000 s of various DC bias stresses, although the threshold voltage may shift

dramatically, variations in subthreshold swing are negligible. # 2011 The Japan Society of Applied Physics

Transparent amorphous oxide semiconductors(TAOSs) have been investigated as an alternativeto thin-film transistors (TFTs) in next-generation

displays because they possess better electrical characteristicsthan amorphous Si. Several oxide material combinationshave been explored in recent studies such as InZnO (IZO),1)

SnZnO (TZO),2) GaInZnO (GIZO),3) and GaSnZnO(GTZO)4) for high field-effect mobility and stability.Recently, other materials including Al, Zr, Hf, and Mg,which have high oxygen affinity, have been added to IZOand ZTO systems as substitutes for Ga.5–8) In particular,ZrInZnO (ZIZO) formed an amorphous phase similar tothat of GIZO, and Zr suppressed carrier generation moreeffectively than Ga due to the low standard electrodepotential (SEP) of Zr, which reflects a tendency towardmetal oxidation. Consequently, the ZIZO TFTs showedhigher on/off ratios than the GIZO TFTs due to the lowerSEP of Zr than that of Ga.6,7) The simultaneous optimizationof both mobility and stability of TAOS TFTs has been a keyfactor for the successful development of next-generationdisplays. However, mobility and stability are difficult tooptimize simultaneously because they inherently conflict.Thus, TAOS TFTs fabricated by pulsed laser depositionand sputtering require minute oxygen pressure control forthe optimization of electrical characteristics.9,10) However,previous research was related to manufacturing of devices ina vacuum chamber. Since solution-processed techniquesallow for a dramatic reduction in process costs (unlikethe vacuum chamber process), there has been tremendousinterest in these techniques.

In this study, we investigated the effects of Zr incorpora-tion in IZO transistors using a solution process as a functionof Zr-incorporated concentration. Additionally, we at-tempted to control the electrical characteristics and stabilityof ZIZO TFTs.

A 0.5M ZIZO solution was synthesized by the sol–gelmethod using indium nitrate hydrate, zinc acetate dihydrate,and zirconium chloride dissolved in 2-methoxyethanol. Theatomic ratio of Zr varied from 0 to 40 at.% of Zn (at. %/Zn),and that of In : Zn was fixed at 3 : 2 (i.e., atomic ratiosof Zr : In : Zn ¼ 0{0:8 : 3 : 2). The mixed solutions werestirred at 300 rpm at 70 �C for 40min to prepare the ZIZOsolutions. The TFTs had a bottom gate and top contactstructure with a width of 1000 �m and length of 150 �m. The

200-nm-thick MoW gate was deposited onto a SiO2/glasssubstrate by sputtering. Plasma-enhanced chemical vapordeposition was then used to form a 200-nm-thick SiNx layeras a gate insulator. A 45-nm-thick ZIZO active layer wasdeposited by spin coating. After postannealing at 500 �Cfor 2 h in air ambient, 200 nm of Ta was sputter-depositedthrough a shadow mask as the source and drain electrodes.The same method was performed to obtain GIZO TFTsexcept for the active layer process. To analyze the electricalcharacteristics and various DC bias stresses, ZIZO TFTswere measured at Vds ¼ 10:1V in the dark at room tem-perature with an HP 4156C semiconductor parameteranalyzer.

Figure 1 shows the transfer characteristics of ZIZOTFTs according to the Zr concentration and GIZO TFT.To confirm the oxygen suppressing effect for speciesof materials, the characteristics of TFTs based on ZIZO(Zr ¼ 10 at. %/Zn), GIZO (Ga ¼ 10 at. %/Zn), and IZO(without both Ga and Zr) were compared. The ZIZO TFTdemonstrates a higher performance than the GIZO TFT,especially showing a relatively high on/off ratio, and a lowoff-current. Moreover, as the Zr concentration was increas-ed, the on- and off-currents were decreased.

Park et al. proposed that the substitution of Zr4þ ionsinto In3þ ion sites may work as oxygen binders for oxygenout-diffusion, thereby possibly suppressing the formationof oxygen vacancies.6) The Zr used in ZIZO is thoughtto be a superior oxygen binder owing to its lower SEP

Fig. 1. (Color online) Transfer characteristics of ZIZO TFTs according to

the Zr concentration and GIZO TFT.

�E-mail address: hjk3@yonsei.ac.kr

Japanese Journal of Applied Physics 50 (2011) 070202

070202-1 # 2011 The Japan Society of Applied Physics

RAPID COMMUNICATIONDOI: 10.1143/JJAP.50.070202

(Zr ¼ �1:4V) than Ga (�0:5V), and therefore it effectivelylowers the carrier concentration and conductivity of theZIZO TFT by reducing the number of oxygen-related donorsites. Coincidently, the localized state in the band gap relatedto oxygen vacancies was suppressed,11) which resulted in thedecrease in subthreshold swing (SS) with the elevation of Zrat. %/Zn in the ZIZO TFT. However, for a ZIZO TFT(Zr ¼ 40 at. %/Zn), the SS is increased to 1.23V/decade.This effect can be explained by ZrO2 segregation, morphol-ogy deterioration of ZIZO films, or trap creations caused byZr interstitials due to excess Zr.

The ZIZO TFT (Zr ¼ 20 at. %/Zn) has optimal perfor-mance, and exhibited a �sat of 0.77 cm

2 V�1 s�1, a thresholdvoltage (Vth) of 2.1 V, an on/off ratio of �106, and a SS of0.73V/decade.

Using this optimized Zr concentration, the degradationof a ZIZO TFT by various DC bias stresses was investigated,as shown in Fig. 2. The negative gate bias (Vgs ¼ �30V,Vds ¼ 0V), positive gate bias (Vgs ¼ 30V, Vds ¼ 0V), andcurrent stress (Id ¼ 20 �A, Vgs ¼ 20V, Vds ¼ 10V) wereapplied to the ZIZO TFTs for 1, 10, 100, and 1000 s,respectively. The off-current portion of transfer curvesin Fig. 2 exhibits an abrupt decrease. This anomalisticphenomenon is ascribed to trap emission and subsequent trapfilling in the interface between the channel and the gateinsulator.12)

Several degradation mechanisms of ZnO-based TFTshave already been reported along with (1) charge trappingdefect formation in the bulk material and (2) charge trappingin oxide bulk and the interface between the channel and thegate insulator.

As the applied time of the positive gate bias increases, apositive shift of the Vth was observed, while the negativegate bias makes the Vth move in the negative direction,as shown in Fig. 3. These characteristics indicate that duringthe periods of stress, electrons or holes are temporarilytrapped/detrapped in the channel, the gate insulator, or theinterface at the existing traps depending on the polarityof the applied bias. The absolute amount of positive Vth

shift is more than that of negative Vth shift because the

electron trappings are more likely to occur than holetrappings.

Figure 4 shows the variation of the SS according tovarious stress tests. Since the SS is related to the maximuminterface trap (Nmax) site formation in the TFTs, the

Fig. 2. (Color online) Transfer characteristics of ZIZO TFTs according to various DC bias stresses for 1, 10, 100, and 1000 s.

Fig. 3. (Color online) Variation of the Vth according to various DC bias

stresses for 1, 10, 100, and 1000 s.

Fig. 4. (Color online) Variation of the SS according to various DC bias

stresses for 1, 10, 100, and 1000 s.

T. H. Jeong et al.Jpn. J. Appl. Phys. 50 (2011) 070202

070202-2 # 2011 The Japan Society of Applied Physics

variation in the SS serves as an index of the increase ordecrease in the trap density of the interface. The variationsof Nmax were calculated using the following equation;13)

Nmax ¼ SS � log ekT=q

� 1

� �Ci

q; ð1Þ

where k is the Boltzmann constant, T is room temperature,Ci is the gate insulator capacitance per unit area, and q is theelementary charge. From this calculation, the Nmax almostnever changed under various DC bias stresses due to theSS. After 1000 s stress, the Nmax slightly changed from1:93� 1012 to 1:62� 1012 cm�2 for the negative gate bias,1:88� 1012 to 1:76� 1012 cm�2 for the positive gate bias,and 1:95� 1012 to 2:03� 1012 cm�2 for the current stress.

The dramatic Vth shifts with the slightly changed SS after1000 s DC bias stresses indicate that electron or holetrapping effects are dominant in ZIZO TFT DC bias stresses.

The current stress test was performed under a constantcurrent. This test could be a key factor for applicability inan active-matrix organic light-emitting diode (AMOLED),which is operated with current driving. As the applied timeof current stress increases, the Vth moves forward, whichshows a similar tendency to the positive gate bias stress,except the amount of the Vth shift is less than that for thepositive gate stress. Suresh and Muth explained that thelarger current flow increased the trapping probability, andtherefore caused a more significant shift in Vth.

14) However,the bias voltage is a rather more important factor than thecurrent flow in ZIZO, as shown in Fig. 3. These differencesmay be explained by the suppression of defect sites by theadded Zr.

In summary, the effects of adding Zr into solution-processed IZO TFTs were investigated with varying Zrconcentration. The ZIZO TFTs show a lower off-current anda sharper SS than the IZO TFTs. The addition of Zr plays arole in reducing the number of oxygen vacancies, and thuseffectively decreases the carrier concentrations of the ZIZOactive layer. For comparison, GIZO TFTs were fabricated

with the same (Ga/Zn) at.% as that of (Zr/Zn) in ZIZO. TheZIZO TFTs perform better than the GIZO TFTs, and have arelatively high on/off ratio and a low off-current. Theseresults suggest that Zr is superior to Ga as an oxygen binderin IZO systems. To evaluate reliability, the characteristics ofvarious DC bias stresses are provided and discussed. Whilethe Vth shifts dramatically after 1000 s of various electricalbias stresses, variations in the SS are negligible.Acknowledgement This work was supported by a National Research

Foundation of Korea (NRF) grant funded by the Korean Ministry of Education,

Science and Technology (MEST, No. 2007-0055837).

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