Physical Model and Simulation Platform for High-Level Instability-Aware Design of Amorphous Oxide Semiconductor Thin-Film Transistors

Dae Hwan Kim1, Woojoon Kim1, Yongsik Kim1, Inseok Hur1, Minkyung Bae1, Dongsik Kong1, Hyun Kwang Jeong1, Dong Myong Kim1, Byung Du Ahn2,

Gun Hee Kim2, Je-Hun Lee2, Ho-Hyun Nahm3, Yong-Sung Kim3 1School of Electrical Engineering, Kookmin University, Seoul 136-702, Republic of Korea

2LCD R&D Center, Samsung Electronics, Yongin 449-711, Republic of Korea 3Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea

Abstract The negative bias illumination stress (NBIS)-induced VT instability of amorphous InGaZnO thin-film transistors (TFTs) is quantitatively investigated and a shallow donor state-creation model is proposed as a physical mechanism. Furthermore, the difference between InGaZnO and HfInZnO TFTs in perspective of NBIS-induced instability is consistently elucidated. We expect that the proposed model and simulation platform are potentially powerful tools for high-level instability-aware design of oxide TFT devices and circuits.

Author Keywords amorphous InGaZnO; amorphous HfInZnO; thin-film transistors; shallow donor creation model; density-of-states; negative bias illumination stress; reliability characterization.

1. Introduction Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) are under active research and development as promising devices for next-generation display backplanes and high-performance stackable switch/driver devices in transparent and/or flexible displays, 3-D memory/computing systems. As the demand of AOS applications explosively increases, the bias/current/temperature/photo-induced instabilities have emerged as challenging issues for their mass production [1]. Therefore, the quantitative projection of instability and physical parameter-based instability-aware design become important and urgent for systematical design for AOS systems in near future.

In this work, we quantitatively investigate the negative bias illumination stress (NBIS)-induced threshold voltage shift (VT) in amorphous InGaZnO (a-IGZO) TFTs and establish its physical model and simulation platform. For extracting subgap density-of-states (DOS) and calculating I-V characteristics, the multi-frequency C-V spectroscopy [2], the generation-recombination current (IG-R) spectroscopy [3], and the modified DeAOTS [4] are intensively used. In addition, it is found that the proposed shallow donor states creation model reproduces the NBIS time (tNBIS)-evolution of transfer characteristics very well. Furthermore, the NBIS instability of amorphous HfInZnO (a-HIZO) TFT is comparatively studied with that of a-IGZO TFT, based on our model and simulation platform.

2. Device Fabrication and NBIS Instability Characterization

AOS TFTs with a bottom gate etch stopper structure were integrated on a glass substrate as shown in Fig. 1(a). The rf sputter-deposited Mo was used as the gate electrode and patterned through a dry etching. A 400-nm-thick SiNx/50-nm-thick SiOx bilayer was used as the gate insulator and was deposited through the plasma-enhanced chemical vapor deposition (PECVD) at

370 °C. The dc sputtered In:Ga:Zn=2:2:1 and/or Hf:In:Zn=2:2:1 thin-film was used for active layer of a-IGZO and/or a-HIZO TFT, respectively. The 50-nm-thick IGZO/HIZO active thin-film was deposited by the dc sputtering at room temperature in a gas mixture of Ar/O2=35/63 (at sccm) and patterned by the wet etch process with a diluted HF. During the sputtering, the chamber pressure was maintained at 5 mTorr and the dc power at 80 W. Then, the 50-nm-thick etch stopper SiOx layer was deposited by PECVD and patterned by dry etching. For the formation of source/drain (S/D) electrodes, Mo was dc sputter-deposited and then patterned by a dry etching. Subsequently, the passivation layers (100-nm-thick SiOx and 100-nm-thick SiNx) were deposited by PECVD at 280 °C. Finally, fabricated TFTs were annealed at 250 °C for 1 hr The channel width/length (W/L) was 200/100 m.

Figure 1. (a) A schematic illustration of integrated a-IGZO

TFTs. Measured tNBIS-evolutions of the IDS-VGS characteristic (b) in a linear scale, (c) in a log scale, and (c) the IG-R-VG characteristic of a-IGZO TFTs (symbols). The lines indicate the calculated tNBIS-evolutions by using the

extracted parameters in Table I.

We examined the tNBIS-evolution of dc characteristics. The conditions of NBIS were VGS=-20 V, VDS=10 V, and tNBIS from 0 to 11000 s. The illumination was applied through a commercial LED backlight unit with a brightness of 300 nit. As shown in symbols of Figs. 1(b) and (c), with the VT defined at a constant current at IDS=1 nA, the NBIS-induced VT without a significant change of subthreshold swing (SS) is observed to be -3.7/-5.4/-5.8 V after NBIS for 500/2000/5500 s, respectively.

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3. Physical Model and Simulation Platform Our model is based on the subgap DOS [g(E)=gA(E)+gD(E)] and it may be modeled as follows:

C CA DA TA DA TA

DA TA

( ) ( ) ( ) exp expE E E E

g E g E g E N NkT kT

(1)

D TD OV

2

V OVTD OV

TD OV

( ) ( ) ( )

exp exp

g E g E g E

E E E EN N

kT kT

(2)

with gA(E)=density of acceptor-like states, gDA(E)=density of acceptor-like deep states, gTA(E)=density of acceptor-like tail states near conduction band minimum (EC), gD(E)=density of donor-like states, gTD(E)=density of donor-like tail states near valence band maximum (EV), and gOV(E)=density of shallow donor states. Both gOV(E) and gTD(E) are strongly correlated with oxygen vacancies (VOs) in AOS film. The VOs are well-known native defects in AOS materials such as ZnO and a-IGZO [5], [6]. The g(E) in the pristine state of a-IGZO TFT was extracted as shown in Fig. 2(c). Here, the gA(E) has been extracted by the multi-frequency C-V spectroscopy [2] and the gTD(E) and gOV(E) have been extracted by the IG-R spectroscopy [3]. Parameters were summarized in Table I.

Figure 2. The energy band diagrams of (a) a thermal

equilibrium and (b) NBIS conditions. The DOS under (c) a thermal equilibrium and (d) NBIS conditions, respectively.

The excitation of VO to VO2+/VO

+ followed by the stabilization of VO

2+/VO+ can be modeled as the increase of gOV(E).

In order to explain the NBIS-induced instability, the shallow donor state-creation model is proposed as schematically illustrated in Fig. 2. The VOs-related gTD(E) states, which have been occupied by electrons and therefore electrically neutral under thermal equilibrium in Figs. 2(a) and 2(b), would remain unoccupied states due to the lowered EFn as long as a large negative VGS is applied. Then, VO states can be translated into the excited VO

2+ states. These excitations become more activated with a larger negative VGS and/or the photo-illumination. This charge state transition (ionization) of VO into VO

2+/VO+ causes an outward

relaxation of the neighboring metal atoms [7]. If the outward

relaxations of neighboring metal atoms become more significant (by the increase of negative VGS, longer tNBIS, or more intensive photo-illumination) and cause a change in the positions of the atoms around them, including the ones that located quite far away from VO

2+/VO+, the total energy needed to return to their initial

positions is increased. It is followed by a stabilization of VO2+/VO

+ as denoted by the change from Fig. 2(a) to 2(b). Therefore, the shallow donor states generated by the NBIS can be modeled by the increased gOV(E) (or VO

2+s/VO+s with positive charges) as

illustrated in the change from Fig. 2(c) to 2(d). They would result in a negative shift of the transfer characteristics consistently with the measured ones.

We assume that the photon-assisted electron detrapping (or hole trapping) followed by VO ionization is a dominant mechanism. If the photo-excited sub-bandgap energy range is assumed to be (EC-Eph)~EFn as indicated in the slashed region of Fig. 3(a), it is non-uniformly distributed along the x-direction (across the depth direction of a-IGZO thin-film from the interface between the gate insulator). Then, it can be translated into the slashed region (i.e., E2~E1) in Fig. 3(b). Therefore, E1 and E2 are derived as following equations:

1 C C ph,effeff

cE E h E E

(3)

2 Fn C FBE E E E q x (4)

with Eph,eff=the effective energy of the incident photon under NBIS condition and eff=the effective wavelength. Because the slashed energy region lies within gTD(E) under the NBIS condition, the increased gOV(E) [i.e., gOV(E)] is given by Fig. 3(c) and Eq. (5):

2 C

1 FnTD OV

E E

E Eg E dE Δg E dE (5)

where Eq. (5) means that the area of the shaded region in Fig. 3(b) should be equivalent to that in Fig. 3(c).

Figure 3. The energy band diagram indicating (a) the sub-bandgap energy range of the photo-excited VO ionization

and (b) the calculation of E1~E2. (c) The sub-bandgap DOS indicating the shallow donor state creation.

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On the other hands, we describe the procedure to calculate the tNBIS-evolution of I-V characteristics of a-IGZO TFTs. First of all, the measured I-V characteristics in pristine state were reproduced by the extracted g(E) and DeAOTS [4]. Then, E2 and E1 levels are calculated by using Eqs. (3) and (4). Consequently, if Eph,eff of the incident photons from the backlight unit is known, we can calculate the change of I-V characteristics from the pristine state to tNBIS=500 s. Here, we can find Eph,eff by adjusting it until the transfer and IG-R characteristics calculated from the g(E) in the pristine state reproduce the measured ones after tNBIS=500 s. In other words, we semi-empirically determine Eph,eff to guarantee the consistency between the measured NBIS-induced VT after tNBIS=500 s and the DeAOTS-calculated one. In this way, gA(E) [it was extracted from the multi-frequency C-V spectroscopy after tNBIS=500 s] and gOV(E) after tNBIS=500 s were found. Also, IDS-VGS and IG-R-VG characteristics after tNBIS=500 s can be calculated by DeAOTS. Similar procedures are also applied for tNBIS=500~2000 s, tNBIS=2000~5500 s, and tNBIS=5500~11000 s. We found that, when applying Eph,eff=2.42 eV, the calculated IDS-VGS and IG-R-VG characteristics reproduced the measured ones after tNBIS very well, as denoted by the lines in Figs. 1(b)~1(d).

Figure 4. The tNBIS-evolution of subgap DOS. The slashed, dotted, and hashed region corresponds to the photo-excited shallow donor states creation region during tNBIS=0~500 s,

500~2000 s, and 2000~5500 s, respectively.

Table I. Used model parameters

Parameter Pristine tNBIS= 500 s

tNBIS= 2000 s

tNBIS= 5500 s

Unit

NC 21018 [cm-3]

NV 11014 [cm-3]

BAND 10.7 [cm2/Vs]

NTA 11018 51017 71017 51017 [cm-3eV-1]

kTTA 0.03 0.06 0.02 0.02 [eV]

NDA 11017 11017 2.41017 2.41017 [cm-3eV-1]

kTDA 0.19 0.22 0.19 0.19 [eV]

NOV 11016 2.81017 4.11017 4.71017 [cm-3eV-1]

kTOV 0.1 0.1 0.1 0.1 [eV]

EOV 2.9 2.9 2.9 2.9 [eV]

NTD 4.51021 [cm-3eV-1]

kTTD 0.10 [eV]

W/L 200/100 [m]

Finally extracted tNBIS-evolution of gTD(E), gOV(E), and gA(E) is shown in Fig. 4 with all parameters summarized in Table I. Noticeably, they are validated by verifying that calculated IDS(VGS, VDS) and IG-R(VG, VD) reproduce the measured ones very well as shown in Figs. 1(b)~1(d). It should be noted that in Fig. 4 the change of gA(E) during the NBIS is directly extracted by the multi-frequency C-V spectroscopy. It is clearly observed that the change in gA(E) is insignificant under the NBIS. This suggests that the creation and/or redistribution of gA(E) is not a dominant mechanism on the NBIS-induced VT.

With the increase of tNBIS, the moving up of the electron quasi-Fermi level EFn (at NBIS condition) toward EC becomes retarded, which supports the quantitative simulation of the saturated VT during tNBIS. Consequently, with the emphasis on the consistency between the measured characteristics and models in Fig. 1, the increase of gOV(E), i.e., the shallow donor state-creation model, explains the observed NBIS-induced VT of a-IGZO TFTs (the left shift of transfer characteristic without a significant change of SS) very well quantitatively as well as qualitatively.

4. Comparison between a-IGZO and a-HIZO TFTs in terms of the NBIS-induced Instability

As mentioned, our physical model and simulation platform (including all of the method for extracting the model parameters, the I-V model, simulator, and shallow donor state-creation model) make the instability-aware design of AOS TFTs possible because they can project the tNBIS-evolution of electrical characteristics of a-IGZO TFTs.

Figure 5. The measured tNBIS-evolution of transfer characteristics. (a) a-IGZO and (b) a-HIZO TFTs.

To make the feasibility of our model and simulation platform more general for AOS TFTs, it was also applied to a-HIZO TFTs. The measured tNBIS-evolution of transfer characteristics are shown in Fig. 5. After tNBIS=11000 s, the negative VT of a-IGZO (-6.35 V) is larger than that of a-HIZO TFT (-0.56 V). This result is qualitatively consistent with that the Hf ions may play a key role in improving the instability of TFTs, due to their high oxygen bonding ability.

The subgap DOSs of two different materials are comparatively shown in Figs. 6(a) and 6(b). Here, the gA(E) was extracted by the multi-frequency C-V spectroscopy [Fig. 6(d)], and the gTD(E) was derived based on the first-principle calculation [Fig. 6(c)]. The bandgap difference was also taken into account based on the first-principle calculation. Larger bandgap in HIZO is consistent with experimental reports [8]. The model and extracted DOS in Fig. 6 reproduced the measured electrical characteristics very well (not shown here). Furthermore, the photo-excited energy range (E2~E1) indicated by the shaded region on gTD(E) in Fig. 6(c) elucidates why IGZO TFT is more unstable under NBIS than

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HIZO TFT. Based on the shallow donor states creation model, in case of HIZO TFT, the donor states creation would be negligible compared with IGZO TFT because a very smaller amount of states in gTD(E) is located in the photo-excited energy range. It

suggests that our physical model is consistent with the measured material effects and applicable to quantitative analysis on the material- and process-dependent NBIS instability of AOS TFTs.

Figure 6. The g(E) over subgap range of (a) a-IGZO and (b) a-HIZO TFTs. Enlarged DOS (c) near EV and (d) EC. The shaded region on gTD(E) corresponds to the photo-excited energy range (E2~E1) in Fig. 3.

5. Conclusion NBIS-induced VT in a-IGZO TFTs was quantitatively investigated and the measured tNBIS-evolution of IG-R-VG as well as IDS-VGS characteristic was reproduced by our model and simulation platform. Shallow donor states creation is suggested to be a dominant mechanism on the NBIS-induced VT. Furthermore, it is found that the effect of material on NBIS instability can be reasonably understood by calculating the position of Fermi level under NBIS condition, which becomes possible by using our DOS-extraction methodology, physical instability model, and the simulation platform.

Conclusively, our physical model with simulation platform is expected to be potentially useful for the sophisticated instability-aware design of the AOS material, process, and devices.

6. Acknowledgements This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Ministry of Education, Science and Technology(MEST) (Grant no. 2011-0000313).

7. References [1] T. Kamiya et al., “Present status of amorphous In-Ga-Zn-O

thin-film transistors,” Sci. Technol. Adv. Mater. 11, 044305 (2010).

[2] S. Lee et al., “Extraction of Subgap Density of States in

Amorphous InGaZnO Thin Film Transistors by Using Multi-Frequency Capacitance-Voltage Characteristics,” IEEE Electron Device Lett. 31, 231-233 (2010).

[3] M. Bae et al., “Extraction of Subgap Donor States in a-IGZO TFTs by Generation-Recombination Current Spectroscopy,” IEEE Electron Device Lett. 32, 1248-1250 (2011).

[4] Y.W. Jeon et al., “Subgap Density-of-States-Based-Amorphous Oxide Thin Film Transistor Simulator (DeAOTS),” IEEE Trans. Electron Devices 57, 2988-3000 (2010).

[5] A. Janotti and C.G.V. de Walle, “Oxygen vacancies in ZnO,” Appl. Phys. Lett. 87, 122102 (2005).

[6] H.-K. Noh et al., “Electronic structure of oxygen-vacancy defects in amorphous In-Ga-Zn-O semiconductors,” Phys. Rev. B 84, 115205 (2011).

[7] T. Kamiya et al., “Electronic structure of oxygen deficient amorphous oxide semiconductor a-InGaZnO4-x: Optical analyses and first-principle calculations,” Phys. Status Solidi C 5(9), 3098-3100 (2008).

[8] Y.R. Denny et al., “Electronic and optical properties of hafnium indium zinc oxide thin film by XPS and REELS,” Journal of Electron Spectroscopy and Related Phenomena 185, 18- 22 (2012).

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