Solid-State Electronics 56 (2011) 95–99

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Solid-State Electronics

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Comparative study of quasi-static and normal capacitance–voltage characteristicsin amorphous Indium-Gallium-Zinc-Oxide thin film transistors

Sangwon Lee, Yong Woo Jeon, Sungchul Kim, Dongsik Kong, Dae Hwan Kim, Dong Myong Kim ⇑School of Electrical Engineering, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 May 2010Received in revised form 6 October 2010Accepted 16 October 2010Available online 11 November 2010

The review of this paper was arranged byProf. E. Calleja

Keywords:AmorphousThin film transistorsInGaZnODensity of statesCapacitance–Voltage characteristicsOptical illuminationQuasi-static C–V

0038-1101/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.sse.2010.10.016

⇑ Corresponding author. Tel.: +82 2 910 4719; fax:E-mail address: dmkim@kookmin.ac.kr (D.M. Kim)

Capacitance–voltage (C–V) characteristics of amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) thin filmtransistors (TFTs) are comparatively investigated with two different measurement configurations. Nor-mal gate-to-source/drain (S/D) C–V and quasi-static C–V curves are employed to characterize physicalmechanisms with equivalent circuit models for a-IGZO TFTs. The difference between the normal C–Vand the quasi-static C–V (QSCV) characteristics is investigated by the dependence on the gate voltage(VG), measurement configuration, and optical illumination. The discrepancy is analyzed to be due to ahigh hole barrier in the S/D contact region and a slow response of active bulk charges (Qloc and Qfree) inthe a-IGZO active layer.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction is most probably due to the high subgap density of states

Amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) thin film tran-sistors (TFTs) have been widely investigated as promising switchingdevices for active-matrix liquid crystal displays (AMLCDs) anddriving or switching devices for active-matrix organic light-emitting diode displays (AMOLEDs) due to the advantages of largescaled integration, low cost room temperature (RT) fabrication pro-cess, high mobility, large area uniformity, and the compatibilitywith transparent and electronic paper applications [1–4]. In orderto make a-IGZO TFTs affordable for various innovative and practicalapplications, the physical mechanism of the gate capacitance–gatevoltage (CG–VG) characteristics, which is indispensable for thedesign of circuits and systems with a-IGZO TFTs, should be compre-hensively understood as is the case for TFTs with other materials[5–8].

In practical CG–VG curves of n-channel TFTs integrated in n-typea-IGZO films (measured by using an LCR meter between the gateand grounded source/drain (S/D) terminals), the gate capacitanceCG is observed to be saturated at CMIN under a large negative volt-age while it is saturated at CMAX under a large positive gate voltageas shown in Fig. 1a. Therefore, it is generally known that a-IGZOTFTs are difficult to operate in an inverted p-channel mode, which

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+82 2 910 4449..

(DOS: g(E)) near the valence band maximum (VBM: EV) in thea-IGZO film [9]. However, the reason why CMIN and CMAX are notmodulated by the gate voltage has not been completely clarifiedyet. In addition, the layout-dependence of the CG–VG curves ina-IGZO TFTs has not been clearly characterized to date.

In this work, CG–VG curves are comprehensively investigatedincluding the channel length (L) and width (W) dependencesof a-IGZO TFTs. For the characterization of physical mechanismson a-IGZO TFTs, we investigated C–V responses for the dependenceon the layout (W and L), two different measurement configurations(2- and 3-terminals), two different C–V measurement techniques(normal and quasi-static C–V responses), and optical illumination.

2. Fabrication and experimental result of amorphous InGaZOTFTs

The device under characterization in this work is an invertedstaggered gate type a-IGZO TFT, as shown in Fig. 1b–d, and themost commonly used structure for AMLCDs. Devices are fabricatedas follows: On a thermally grown SiO2/Si substrate, sputtereddeposition at RT and patterning of molybdenum (Mo) for the gateis followed by the plasma-enhanced chemical vapor deposition(PECVD) of the gate oxide (thickness of SiO2 TOX = 100 nm) at300 �C. An a-IGZO film (In2O3:Ga2O3:ZnO = 2:2:1 at %) is then sput-tered by the RF magnetron sputtering at RT in a mixed Ar/O2

Fig. 1. (a) A normal CG–VG characteristic, (b) a schematic top view, schematic cross-sections along (c) A-A0 and (d) B-B0 lines in a-IGZO TFT.

Fig. 2. Schematics for the measurement setups ((a) 3-terminal method and (b) 2-terminal method). (c) The extracted series resistance RS for the two measurementsetups (from the saturated magnitude of the impendence at high frequency [10]).

96 S. Lee et al. / Solid-State Electronics 56 (2011) 95–99

(100:1 at sccm) and wet-etched with diluted HF to get the patternof the 70-nm-thick active layer (active thin film thicknessTIGZO = 70 nm). In order to form the S/D electrode, a 200-nm-thicklayer of Mo is sputtered at RT and then patterned by dry-etching.After N2O plasma treatment, a SiO2 passivation layer is continu-ously deposited at 150 �C by PECVD without a vacuum break. Aschematic top view of a-IGZO TFTs is shown in Fig. 1b. Schematiccross-sections along A-A0 and B-B0 lines are shown in Fig. 1c andd, respectively.

In order to characterize physical mechanisms on the C–Vresponses, two C–V measurement techniques (normal and quasi-static C–V measurement) are employed for the frequency responseof charges in a-IGZO TFTs. The normal CG–VG curves were obtainedby using Agilent 4284A LCR meter and the quasi-static CG–VG

curves are measured by Agilent 4156C semiconductor parameteranalyzer. The geometrical parameters for the device under charac-terization are as follows: the width of the region between the gateand the a-IGZO active layer; WIGZO = 5 lm, the space between S/Dand the a-IGZO active layer; LIGZO = 15 lm, and the overlap lengthbetween the gate and S/D; LOV = 10 lm, as indicated in Fig. 1b–d.Two different measurement setups, i.e., (a) 2-terminal (voltagesweep VG = �15� + 15 V, VD = 0 V, the source is opened, and smallsignal frequency f = 10 kHz) and (b) 3-terminal (VG = �15� + 15 V,VD = VS = 0 V, and f = 10 kHz) configurations, were employed forthe C–V characterization of a-IGZO TFTs. As shown in Fig. 2a andb, current paths for the 2- and 3-teminal configurations are differ-ent from each other.

C–V characteristics through the quasi-static measurement areobtained at voltage step = 200 mV, hold time = 10 s, delay time =100 ms, integration time = 100 ms, QSCV measure voltage = 80 mV.As shown in Fig. 3(a), the quasi-static CG–VG curve is independentof 2- or 3-terminal measurement setup. We also note that a cleardiscrepancy under negative VG is observed in the quasi-static C–Vcurves from the normal CG–VG curves. While the measured imped-ance is monitored during the normal CG–VG technique through theLCR meter, VG-dependent gate capacitance CG is observed in thequasi-static CG–VG measurement through the semiconductorparameter analyzer with a high current sensitivity (like an extre-mely low-frequency C–V).

The W-dependence (W = 50, 90, and 200 lm for L = 50 lm) ofthe normal CG–VG curve through the LCR meter at frequencyf = 10 kHz and the measurement setup-dependence of the quasi-static CG–VG curve is comparatively shown in Fig. 3a and b alsoshows the L-dependence (L = 10, 20, 30, 40, and 50 lm forW = 200 lm) and the measurement setup-dependence of the nor-mal CG–VG characteristics in a-IGZO TFTs compared with calculatedcapacitances. In the normal CG–VG curves at f = 10 kHz, CMAX isdependent not on the measurement setup but on W and L. How-ever, CMIN is dependent not on the channel length L but on thewidth W and measurement setup. In particular, it should be notedthat the 2-terminal CMIN is a half value of 3-terminal CMIN, which is

Fig. 3. (a) The W-dependence (W = 50, 90, and 200 lm for L = 50 lm) of normal CG–VG curve and the measurement setup-dependence of quasi-static CG–VG curve in a-IGZOTFTs. (b) The L- (L = 10, 20, 30, 40, and 50 lm for W = 200 lm) and measurement setup-dependences of normal CG–VG curves in a-IGZO TFTs. (c) Schematic illustration ofsubgap effective donor-like and acceptor-like effective DOS in the active layer.

S. Lee et al. / Solid-State Electronics 56 (2011) 95–99 97

definitely due to the areal difference in the gate-to-S/D overlapregions.

In addition, CG–VG characteristics in the weak depletion andaccumulation regions are different for the 3-terminal configurationfrom that for the 2-terminal method. This is because the effectiveseries resistance RS is different for the 3-terminal from that for the2-terminal method. In other words, the spreading series resistanceof the 2-terminal configuration is larger than that of the 3-terminalconfiguration. Actually, the extracted RS (from the saturated magni-tude of the impendence at high frequency [10]) is clearly differentfrom each other configuration. As shown in Fig. 3c, RS from the 3-ter-minal configuration is half of RS from the 2-terminal method. It alsomakes the saturation of the gate capacitance in the 2-terminalslower than that of the 3-terminal CG–VG characteristics.

3. Modeling and characterization of CG–VG characteristics in a-IGZO TFTs

3.1. Dependence on the layout and measurement setups

Based on the experimental results as a function of the layoutand measurement setup, the normal CG–VG curves can be modeledas

COX ¼eOX

TOX; CIGZO ¼

eIGZO

TIGZO; and COV ¼

COX � CIGZO

COX þ CIGZOð1Þ

CMAX ¼ COX �Weff ;G�C � Leff ;G�C ; Weff ;G�C

¼W þ 2� ðWIGZO þW0Þ and Leff ;G�C

¼ Lþ 2� ðLOV þ L0Þ ð2Þ

CMIN ¼ COV � 2�Weff OV ;G�S=D � Leff OV ;G�S=D;

Weff OV ;G�S=D ¼W þ 2�W1; Leff OV ;G�S=D ¼ LOV þ L1; ð3Þ

where Weff,G-C and Leff,G-C are the effective channel width and chan-nel length while Weff_OV,G-S/D, and Leff_OV,G-S/D are defined as the effec-tive overlap width and the effective overlap length, respectively.W0, and L0 are defined as the characteristic width and length relatedto the fringing capacitance between the gate and active channel,and W1 and L1 as those related to the fringing overlap capaci-tance(Cfringing) between the gate and S/D, respectively.

Model parameters from the experimental characterization areextracted to be W0 = 5.9 lm (=1.18 �WIGZO), L0 = 5.3 lm (=0.53 �LOV), W1 = 10 lm (=0.2 �W), and L1 = 4.3 lm (=0.43 � LOV), whichwere verified to be consistent with the device geometry as shownin Fig. 1b–d. The calculated capacitances (CMAX, CMIN) also agreedwell enough with measured capacitances for the channel widthas summarized in Table 1. Equivalent capacitance model for thea-IGZO TFT (CG(VG)) is shown in the inset of Fig. 4 and can be de-scribed as

CGðVGÞ ¼ CTðVGÞ �Weff G�C � Leff G�C ð4Þ

CTðVGÞ ¼COX � CB

COX þ CB; CB ¼

@Q loc

@VGþ@Qfree

@VG¼ Cloc þ Cfree ð5Þ

Qloc ¼ q�Z x¼TIGZO

x¼0

Z EC

EV

gðEÞ � f ðEÞ½ �dEdx ð6aÞ

Qfree ¼ q�Z x¼TIGZO

x¼0NC exp � EC � EF

kT

� �dx; ð6bÞ

where CT(VG) = the VG-dependent total gate capacitance per unitarea, CB = the capacitance per unit area due to VG-responsive totalcharge in the active film, Qloc = the localized trapped charges perunit area, Qfree = free charges per unit area, f(E)=the Fermi-Dirac dis-tribution function, NC = the effective DOS in the conduction band(CB), kT = thermal energy, and x = the position along the verticaldirection into the a-IGZO active film.

Table 1Measurement and calculation value of CMAX and CMIN in a-IGZO TFTs.

W/L = 200 lm/50 lm W/L = 90 lm/50 lm W/L = 50 lm/50 lm

CMAX CMIN CMAX CMIN CMAX CMIN

Measurement [pF] 6.18 1.74 3.11 0.88 1.99 0.55Calculation [pF] 6.17 1.75 3.11 0.88 2.00 0.56

Fig. 4. Schematic energy band diagram and equivalent capacitor model for the physical mechanisms on (a) normal CG–VG and quasi-static CG–VG curves under positive VG, and(b) the normal CG–VG curve under negative VG.

98 S. Lee et al. / Solid-State Electronics 56 (2011) 95–99

3.2. The physical mechanism in the CG–VG characteristics of a-IGZOTFTs

Fig. 4 shows the schematic energy band diagram with equiva-lent capacitance model illustrating the mechanism of CG–VG char-acteristics (a) for normal CG–VG and quasi-static CG–VG curvesunder positive VG, and (b) for normal CG–VG curve under negativeVG. As shown in Fig. 4a, electrons are injected from the S/D con-tacts into the a-IGZO active layer as the positive gate voltage(VG > 0) increases (Schottky barrier heights are from Refs. [11](0.2 eV) and [12] (0.4 eV)), and the Fermi-level (EF) becomes clo-ser to the conduction band minimum (CBM: EC). Some of the in-jected electrons are trapped in the subgap DOS in the active layer.Others would exist as free charges in the CBM close to the SiO2/active layer interface. Then, electrons (both dQfree and dQloc in theinset of Fig. 4b) respond to the small signal (f = 10 kHz) super-posed with the gate sweep voltage VG, resulting in VG-dependentCG response as indicated in Fig. 1a. As VG increases more posi-tively, free electrons in the CBM dominantly respond to the smallsignal because EF is sufficiently close to the CBM. In other words,Cfree (see in Eq. (5)) becomes dominant and CG is saturated atCMAX(=COX �Weff,G-C � Leff,G-C + CMIN) as shown in the inset ofFig. 4a and Eq. (2). Under negative gate sweep voltage VG, how-ever, the negative gate charges (dQ 0g in the inset of Fig. 4b) arecompensated by the positive charges accumulated in the over-lapped interface between the S/D and the a-IGZO active layer(dQ 0S=D in the inset of Fig. 4b). Also, the active bulk charges (Qloc

and Qfree) cannot respond to the variation of the small signal withf = 10 kHz during a normal CG–VG measurement by the LCR meter,which is due to both the electron depletion and the EF far awayfrom EC. Therefore, under negative VG, the total gate capacitanceper unit area CT(VG) becomes to zero (as seen in Eq. (5)) and CG

is consequently saturated at CMIN according to Eq. (4).

The origin of the slow response of Qloc and Qfree could be fromthe Fermi level (EF)-pinning due to high DOS near the VBM [4].However, it is expected that the inversion p-channel operation ofa-IGZO TFTs would be still very challenging although there is noEF-pinning (at least, in comparison with hydrogenated amorphousSi TFTs), because the channel potential would not be well modu-lated by the negative VG due to the much higher hole barrier height(UP) than that against electrons (Un) in the S/D contact as illus-trated in Fig. 4. Considering that the saturation of CG to CMIN isnot observable in the quasi-static CG–VG curve as shown inFig. 3a, this hypothesis seems to be reasonable. Fig. 5 shows the en-ergy band diagram with equivalent capacitance model illustratingthe physical mechanism of the quasi-static CG–VG curves undernegative VG and (b) quasi-static CG–VG responses under negativeVG with optical illumination. The optical illumination was providedby a halogen lamp for a flood exposure over the active region. Mostof the optical power of the illumination was distributed at thewavelength k = 400–700 nm. In the quasi-static CG–VG measure-ment without optical illumination, the ionized localized charges(Qloc) trapped in the effective acceptor-like states or effective do-nor-like states in the active a-IGZO layer are assumed to have en-ough time to respond to the modulation of the gate voltage VG.Therefore, the quasi-static CG–VG curve is observed to be indepen-dent of the measurement setup (2- and 3-terminal measurementsetup) as shown in Fig. 3a.

On the other hand, as shown in Fig. 3a, there is a minimumcapacitance value in the quasi-static response at VG = �2.5 V. ForVG > �2.5 V, as VG increases in the positive polarity, CG increaseswith VG due to the effective acceptor-like state-assisted freeelectron generation (electrons de-trapped to the CBM would beswept out to the S/D contact as illustrated in Fig. 5a) and increasedfree electrons. As VG negatively increases under VG < �2.5 V, onthe other hand, CG increases again and saturated later. Under

Fig. 5. Schematic energy band diagram and equivalent capacitor circuit for the physical mechanisms on the quasi-static CG–VG curves under negative VG (a) without, and (b)with an optical illumination.

S. Lee et al. / Solid-State Electronics 56 (2011) 95–99 99

VG < �2.5 V, only the effective donor-like charges respond to thevariation of VG because free holes are unable to be supplied fromthe S/D contact due to the higher hole barrier UP. Then, CG in-creases as VG decreases slightly more negative, because the do-nor-like states increase as getting closer to the VBM as shown ing(E) in Fig. 3c. However, it is clearly observed that as VG becomesnegatively larger, CG converges into a saturated value lower thanCMAX even in the quasi-static CG–VG curve. We expect that it isdue to the increased energy difference between EC and EF. In otherwords, the subgap DOS-assisted electron generation is much lesslikely to occur as VG is going negatively larger. Eventually, CG be-comes saturated to the value lower than CMAX as VG is negativelylarger, as schematically illustrated in Fig. 3c.

In the quasi-static CG–VG curve under optical illumination (vis-ible light range), on the other hand, CG is saturated into CMAX undernegatively larger VG (as seen in Fig. 3a) because the subgap DOS-as-sisted electron generation is accelerated by the photo-excitation asillustrated in Fig. 5b.

4. Conclusion

In conclusion, capacitance–voltage (C–V) characteristics ofamorphous Indium-Gallium-Zinc-Oxide (a-IGZO) thin film transis-tors (TFTs) were comparatively investigated with two differentmeasurement configurations, Normal gate-to-source/drain (S/D)C–V and quasi-static C–V curves were employed to provide physi-cal mechanisms with equivalent circuit models. They are charac-terized and modeled as a function of W-, L-, and measurementsetup. We also investigated physical mechanisms on the differencebetween the normal CG–VG and the quasi-static CG–VG curves based

on the dependences on VG, measurement setup, and optical illumi-nation. It was found that the difference in the normal C–V andQSCV characteristics was explained to be due to a high hole barrierin the S/D contact region and a slow response of active bulkcharges (Qloc and Qfree) in the a-IGZO active layer.

Acknowledgment

This work was supported by the National Research Foundation(NRF) grants funded by the Korean government (MEST) (Nos. 2009-0080344 and 2010-0013883) and the CAD software was supportedby IC Design Education Center (IDEC).

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