Current Applied Physics - Yonsei Universityweb.yonsei.ac.kr/semicim/Publications/Paper/Int/88....

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Electrode metal penetration of amorphous indium gallium zinc oxide semiconductor thin lm transistors Jihyun Ka a , Edward Namkyu Cho a , Min-Jung Lee b , Jae-Min Myoung b , Ilgu Yun a, * a Department of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea b Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea article info Article history: Received 6 February 2015 Received in revised form 2 March 2015 Accepted 14 March 2015 Available online 16 March 2015 Keywords: a-IGZO Thin lm transistor Metal penetration Work function Oxygen vacancy abstract Penetration effects of various electrode materials, namely Al, Au, and Cu, on the physical and electrical characteristics of amorphous oxide semiconductor thin lm transistors (TFTs) were investigated. Amorphous indium gallium zinc oxide (a-IGZO) TFTs were fabricated with conventional staggered bot- tom gate structures on a p-type Si substrate. X-ray photoemission spectroscopy (XPS) analysis under the electrode deposition area revealed variations in the oxygen bonding states and material compositions of the a-IGZO layer. Field-emission scanning electron microscopy (FE-SEM) with the line scan of energy dispersive spectroscopy (EDS) showed lateral penetration by the electrode metal. To compare the elec- trical characteristics of the tested TFTs, the initial currentevoltage (IeV) transfer characteristics were examined. In addition, the tested TFTs fabricated using various electrode materials were tested under bias stress to verify the correlations between variations in TFT characteristics and both the metal work function and penetration-induced oxygen vacancies in the channel around the contact area. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Amorphous InGaZnO (a-IGZO) thin-lm transistors (TFTs) have attracted interest for use in next-generation display devices because of their high mobility, good uniformity, and exible and transparent characteristics [1,2]. Despite recent successes [3], however, some issues still need to be overcome. One of these issues is the inuence of the source and drain electrode materials on resistance, contact area, and parasitic capacitance [4]. Although Cu has been studied as an electrode material to reduce the RC delay because of its low sheet resistance, it has not been used in the mass production of active matrix liquid crystal displays (AMLCDs) or active matrix organic light emitting diodes (AMOLEDs). In a pre- vious study, the effect of incorporating Cu to reduce the sheet resistance of metal was investigated [5], because the electrical properties of a-IGZO TFTs are dependent on the electrode material. However, the effects of metal penetration by various electrode materials, which can impact TFT characteristics, have not previ- ously been reported. Therefore, in this paper, we analyze the effect of the lateral and vertical metal penetration around electrodes using X-ray photoemission spectroscopy (XPS) and energy disper- sive spectroscopy (EDS). In addition, the results can be veried using the positive gate/drain bias stress test by monitoring the drain current. 2. Experiments In this work, a-IGZO TFT test structures with a conventional staggered bottom gate on p-type Si substrate were fabricated as shown in Fig. 1 . The doping concentration of the Si substrate used in the gate was 8.0 10 19 cm -3 and the resistivity was 1.0 10 3 U cm. The SiO 2 gate insulating layer was deposited with a thickness of 300 nm using thermal oxidation. The a-IGZO channel layer was deposited by RF sputtering on a SiO 2 /p-Si substrate at a RF power of 150 W at room temperature and O 2 /Ar gas ow of 5/45 sccm at a working pressure of 5 10 3 Torr for 6 min. Source and drain electrodes with a thickness of 100 nm were deposited by shadow mask using thermal evaporation at room temperature at a working pressure of 5 10 6 Torr. Source and drain electrode materials with a xed channel width (W) of 1000 mm and channel length (L) of 200 mm were fabricated using three types of metals (Al, Au, and Cu). The transfer characteristics and stress instability of the a-IGZO TFTs were measured using a Keithley 236 source measurement unit (SMU) at room temperature. The oxygen contents of a-IGZO TFTs * Corresponding author. E-mail address: [email protected] (I. Yun). Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap http://dx.doi.org/10.1016/j.cap.2015.03.004 1567-1739/© 2015 Elsevier B.V. All rights reserved. Current Applied Physics 15 (2015) 675e678

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lable at ScienceDirect

Current Applied Physics 15 (2015) 675e678

Contents lists avai

Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Electrode metal penetration of amorphous indium gallium zinc oxidesemiconductor thin film transistors

Jihyun Ka a, Edward Namkyu Cho a, Min-Jung Lee b, Jae-Min Myoung b, Ilgu Yun a, *

a Department of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Koreab Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea

a r t i c l e i n f o

Article history:Received 6 February 2015Received in revised form2 March 2015Accepted 14 March 2015Available online 16 March 2015

Keywords:a-IGZOThin film transistorMetal penetrationWork functionOxygen vacancy

* Corresponding author.E-mail address: [email protected] (I. Yun).

http://dx.doi.org/10.1016/j.cap.2015.03.0041567-1739/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

Penetration effects of various electrode materials, namely Al, Au, and Cu, on the physical and electricalcharacteristics of amorphous oxide semiconductor thin film transistors (TFTs) were investigated.Amorphous indium gallium zinc oxide (a-IGZO) TFTs were fabricated with conventional staggered bot-tom gate structures on a p-type Si substrate. X-ray photoemission spectroscopy (XPS) analysis under theelectrode deposition area revealed variations in the oxygen bonding states and material compositions ofthe a-IGZO layer. Field-emission scanning electron microscopy (FE-SEM) with the line scan of energydispersive spectroscopy (EDS) showed lateral penetration by the electrode metal. To compare the elec-trical characteristics of the tested TFTs, the initial currentevoltage (IeV) transfer characteristics wereexamined. In addition, the tested TFTs fabricated using various electrode materials were tested underbias stress to verify the correlations between variations in TFT characteristics and both the metal workfunction and penetration-induced oxygen vacancies in the channel around the contact area.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) haveattracted interest for use in next-generation display devicesbecause of their high mobility, good uniformity, and flexible andtransparent characteristics [1,2]. Despite recent successes [3],however, some issues still need to be overcome. One of these issuesis the influence of the source and drain electrode materials onresistance, contact area, and parasitic capacitance [4]. Although Cuhas been studied as an electrode material to reduce the RC delaybecause of its low sheet resistance, it has not been used in the massproduction of active matrix liquid crystal displays (AMLCDs) oractive matrix organic light emitting diodes (AMOLEDs). In a pre-vious study, the effect of incorporating Cu to reduce the sheetresistance of metal was investigated [5], because the electricalproperties of a-IGZO TFTs are dependent on the electrode material.However, the effects of metal penetration by various electrodematerials, which can impact TFT characteristics, have not previ-ously been reported. Therefore, in this paper, we analyze the effectof the lateral and vertical metal penetration around electrodes

using X-ray photoemission spectroscopy (XPS) and energy disper-sive spectroscopy (EDS). In addition, the results can be verifiedusing the positive gate/drain bias stress test by monitoring thedrain current.

2. Experiments

In this work, a-IGZO TFT test structures with a conventionalstaggered bottom gate on p-type Si substrate were fabricated asshown in Fig. 1. The doping concentration of the Si substrate used inthe gatewas 8.0� 1019 cm-3 and the resistivitywas 1.0� 10�3U cm.The SiO2 gate insulating layer was deposited with a thickness of300 nm using thermal oxidation. The a-IGZO channel layer wasdeposited by RF sputtering on a SiO2/p-Si substrate at a RF power of150 W at room temperature and O2/Ar gas flow of 5/45 sccm at aworking pressure of 5 � 10�3 Torr for 6 min. Source and drainelectrodes with a thickness of 100 nm were deposited by shadowmask using thermal evaporation at room temperature at a workingpressure of 5� 10�6 Torr. Source and drain electrodematerials witha fixed channel width (W) of 1000 mm and channel length (L) of200 mmwere fabricated using three types of metals (Al, Au, and Cu).The transfer characteristics and stress instability of the a-IGZO TFTswere measured using a Keithley 236 source measurement unit(SMU) at room temperature. The oxygen contents of a-IGZO TFTs

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Fig. 1. Schematic diagram of a-IGZO TFTs.

Table 1Comparison of various parameters of a-IGZO TFTs fabricated with three electrodematerials. (W/L ¼ 1000/200 mm, VDS ¼ 5.1 V).

Parameters Type of metal electrode

Al Au Cu

Vth [V] �0.8 8.2 12uFE [cm2/V s] 8.5 6.7 1.3SSUB [V/dec] 0.73 1.19 2.32

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were analyzed by X-ray photoemission spectroscopy (XPS; ThermoVG, U.K.) with Ar ion etching (ion beam: 1 kV, 2.0 mA) using afiltered Al X-ray source (hv ¼ 1486.6 eV). Here, the spectrometerwas calibrated by the photoemission line of Si. The chemicalcomposition of a-IGZO TFTs was also analyzed by the line scan ofenergy dispersive spectroscopy (EDS) using a field-emission scan-ning electron microscope (SEM; JEOL-7001F) with a Schottky-typefield-emission gun.

3. Results and discussion

Fig. 2 shows the transfer characteristics of a-IGZO TFTs with Al,Au, or Cu source and drain electrodes. Parameters used to assess theperformance of TFTs are the threshold voltage (Vth), field effectmobility (mFE), and subthreshold gate swing (SSUB). Vth is deter-mined by the gate voltage (VGS), which is defined as the value whenthe drain current (ID) is 10 nAⅹL/W at a drain voltage (VDS) of 5.1 V.Because the threshold voltages for the Al, Au, and Cu cases weredifferent, the applied VGS was determined differently for the same(VGS � Vth) sweep range, i.e.,�30 Ve30 V. Field effect mobility (mFE)was determined by

mFE ¼ Gm�WL

�CiVDS

(1)

where Gm and Ci are the transconductance and gate capacitance perunit area, respectively [6]. The subthreshold gate swing (SSUB) wasdetermined by

SSUB ¼ dVGS

dðlog IDÞ(2)

Fig. 2. Transfer characteristics (W/L ¼ 1000/200 mm, VDS ¼ 5.1 V) of the tested a-IGZOTFTs.

The electrical properties of the a-IGZO TFTs with differentsource and drain electrodes are summarized in Table 1. TFT per-formance was better when SSUB was smaller and mFE was larger.Thus, Al exhibited better performance than Cu and Au.

Previous studies reported that the TFT characteristics of a-IGZOwere related to the work functions of the source and drain metalelectrodes [2,5]. As shown in Fig. 2, the TFT with an Al electrode hadthe highest ID due to the lowest work function. Thework function ofthe Au electrode was higher than that of the Cu electrode; however,the Au electrode had a higher ID than the Cu electrode. It waspreviously reported that the metal work functions of Al, Au, and Cuare 4.06e4.26 eV, 5.1e5.47 eV and 4.53e5.1 eV, respectively. Inother words, it is found that thework function of Al is much smallerthan thework functions of Au and Cu and thework function of Au isslightly larger than the work function of Cu. Thus, in order toanalyze the relationship between the work function and devicecharacteristics of Au and Cu, we performed additional physicalexperimental analysis of the metal contact.

At first, XPS results for the electrode materials were analyzed toexamine the physical characteristics. XPS spectra of the O 1s peak inthe a-IGZO active area under the contact area with the electrodes

Fig. 3. XPS curve-fits of the O 1s peak of a-IGZO film under the contact area.

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Fig. 4. SEM images and EDS results of a-IGZO between the source and drain for three electrode materials.

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are shown in Fig. 3. The XPS spectrum could be separated into threesub peaks by Gaussian fitting. The three peaks of O 1s weredecomposed into lattice oxygen, oxygen vacancies, and eOH hy-droxide oxygen atoms with binding energies of 530.3, 531.7, and

Fig. 5. DVth and current shift at ID ¼ 10 mA for a-IGZO TFTs fabricated with threeelectrode materials exposed to a positive gate/drain bias stress.

532.4 eV, respectively [7e9]. Each decomposed curve of the threeelectrode materials had different atomic ratios of oxygen vacancies.This can be due to dissociation of oxygen atoms frommetal reactionunder the contact area, while the carrier concentration is related tooxygen deficiencies. An increased oxygen vacancy atomic ratio re-flects an increase in carrier concentration [7,10]. These XPS resultsagree with the following ordered sequence of larger carrier con-centration in the active area under the contact: Au, Al, and Cu.

The cross-sectional SEM images of the a-IGZO active area be-tween the source and drain electrodes and the line scan of EDSresults are shown in Fig. 4. Component analysis of the horizontaldirection of the a-IGZO channel layer was performed by the linescan of EDS. We found that electrode materials penetrated into thechannel from the edge of the source and drain. Penetration length(LP) depended on the electrode material and increased as the

Table 2Summary of the properties and performance metrics of the tested a-IGZO TFTsfabricated with three electrode materials.

Properties or performance metrics Ranking order

Work function Al ≪ Cu < AuDrain Current Cu < Au < AlOxygen vacancy Cu < Al < AuPenetration length Au < Al < CuTFT Stability Cu < Au < Al

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number of oxygen vacancies decreased. This indicates that thepenetrated electrode materials reacted with oxygen in a-IGZO,resulting in the formation of compounds like AlOx and CuOx [4].

In order to use TFTs as the backplane of an AMOLED display, it isnecessary to determine the charge of TFTs when ID flows [11,12].The measured DVth and the ID ratio (ID/ID_initial) for 3000 s withrespect to different electrode materials, where ID_initial was definedas the initial value of ID when positive gate/drain bias was induced,are shown in Fig. 5. VDS can also be controlled to flow a constant IDwhen VGS with a positive bias condition compensates for variationin the transfer characteristics of electrode materials. The values ofVDS were 0.6, 0.9, and 5.1 V for the electrode materials of Al, Au, andCu, respectively. The DVth values at 3000 s under a positive gate/drain bias stress were 2.8, 5.4, and 6.2 V for Al, Au, and Cu elec-trodes, respectively. The ID/ID_inital values at 3000 s for a positivegate/drain bias stress were 91.3, 69.9, and 51.1% for the Al, Au, andCu electrodes, respectively. These test results are in good agree-ment with Al having the smallest ID variation, followed by Au andthen Cu. Tested TFTs were manufactured to have the same struc-tures except for the electrode materials. However, TFTs haddifferent initial characteristics and their reliability was dependenton their initial characteristics.

The comprehensive analysis results, which are summarized inTable 2, indicate that the initial characteristics and reliability arerelated to the metal work function as well as metal penetration,which affect how the channel reacts to oxygen defects. Thus, wecan conclude that the metal work function is a dominant effect onthe TFT characteristics shown in the case of Al electrode and themetal penetration effect can be a secondary effect to explain thebehavior between Au and Cu electrodes associated with the chan-nel reaction of oxygen vacancies.

4. Conclusion

In this paper, the effect of electrode metal penetration on theelectrical and physical characteristics of a-IGZO TFTs was investi-gated. We found that TFT characteristics were mainly affected bythe work function of electrode materials, and that TFTs with betterperformance had more stable positive gate/drain bias stress. Inparticular, although the work function of Cu was smaller than thatof Au, the TFT performance of Cu was worse than that of Au due to

the lower carrier concentration (i.e., smaller oxygen vacancies) inthe a-IGZO around contact area induced bymetal penetration of theelectrode, as verified by the line scan of EDS and XPS. Therefore, theelectric performance and stability of a-IGZO TFTs were closelycorrelated with the work function of the electrode metal as adominant effect, and the reaction of oxygen vacancies with thepenetrated electrode metal as a secondary effect.

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