Improvement of photo-induced negative bias stability of oxide thin film transistors byreducing the density of sub-gap states related to oxygen vacanciesKyoung-Seok Son, Joon Seok Park, Tae Sang Kim, Hyun-Suk Kim, Seok-Jun Seo, Sun-Jae Kim, Jong Baek

Seon, Kwang Hwan Ji, Jae Kyeong Jeong, Myung Kwan Ryu, and Sangyoon Lee

Citation: Applied Physics Letters 102, 122108 (2013); doi: 10.1063/1.4794419 View online: http://dx.doi.org/10.1063/1.4794419 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/102/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhancement of bias and illumination stability in thin-film transistors by doping InZnO with wide-band-gap Ta2O5 Appl. Phys. Lett. 102, 242102 (2013); 10.1063/1.4811416 Temperature dependence of negative bias under illumination stress and recovery in amorphous indium galliumzinc oxide thin film transistors Appl. Phys. Lett. 102, 143506 (2013); 10.1063/1.4801762 Defect reduction in photon-accelerated negative bias instability of InGaZnO thin-film transistors by high-pressurewater vapor annealing Appl. Phys. Lett. 102, 143503 (2013); 10.1063/1.4801436 Effect of hydrogen incorporation on the negative bias illumination stress instability in amorphous In-Ga-Zn-O thin-film-transistors J. Appl. Phys. 113, 063712 (2013); 10.1063/1.4792229 Effect of high-pressure oxygen annealing on negative bias illumination stress-induced instability of InGaZnO thinfilm transistors Appl. Phys. Lett. 98, 103509 (2011); 10.1063/1.3564882

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Improvement of photo-induced negative bias stability of oxide thin filmtransistors by reducing the density of sub-gap states related to oxygenvacancies

Kyoung-Seok Son,1 Joon Seok Park,1 Tae Sang Kim,1 Hyun-Suk Kim,1 Seok-Jun Seo,1

Sun-Jae Kim,1 Jong Baek Seon,1 Kwang Hwan Ji,2 Jae Kyeong Jeong,2 Myung Kwan Ryu,1

and Sangyoon Lee1,a)

1Display Laboratory, Samsung Advanced Institute of Technology, Nongseo-Dong, Giheung-Gu, Yongin-Si,Kyonggi-do 446-712, South Korea2Department of Materials Science and Engineering, Inha University, Incheon 402-751, South Korea

(Received 31 October 2012; accepted 21 February 2013; published online 26 March 2013)

The optical absorption in the sub-gap region of amorphous indium zinc oxide films and the photo-

induced negative bias stability of the resulting thin film transistors were studied. As the indium

ratio increases, optical absorption via sub-gap states increases, and the threshold voltage

degradation under negative bias temperature stress (NBTS) with light illumination becomes more

severe. By applying high pressure anneal treatments in oxygen ambient, the density of sub-gap

states is reduced by an order of magnitude compared to air-annealed devices. Consequently,

significant improvements are observed in the threshold voltage shifts and the stretched

exponential parameters under NBTS with light illumination. VC 2013 American Institute of Physics.

[http://dx.doi.org/10.1063/1.4794419]

In active matrix flat panel displays (AM-FPDs), the panel

size is becoming larger (>50 in.) and high resolution displays

such as ultra-definition (UD) and retina displays represent the

major current technological trends. In addition, the frame rate

keeps increasing in order to achieve smoother motion

images.1,2 As the display resolution and frame rate increase,

the required charging time of the switching device becomes

very short, e.g., less than 2 ls for 240 Hz UD displays. With

conventional amorphous silicon thin film transistors (TFTs)

with field effect mobility of around 1 cm2/Vs, it is not possi-

ble to charge the pixel capacitance within this charging time.

On the other hand, low temperature poly-silicon (LTPS)

TFTs show superior TFT performance with high mobility

and stability. However, because of the non-uniformity caused

by grain boundary and high cost issues from laser anneal and

implantation process, their applications are restricted to small

size displays. Recently, amorphous oxide semiconductor

TFTs using materials such as indium-gallium-zinc oxide

(IGZO) and indium-zinc-tin oxide (IZTO) are drawing atten-

tion because of their high mobility, low fabrication cost, and

large area uniformity.3–5 However, the photo-induced stabil-

ity, especially under negative gate bias, still remains to be

improved for practical applications. There are basically two

generally well-accepted models that describe the threshold

voltage (VT) degradation under light illumination. One is the

electron generation model based on persistent photo conduc-

tivity (PPC).6–8 This model can be applied when the photo-

generated electron carriers are dominant, for example, when

large intensities of light are applied directly on top of the

devices. It is speculated that localization of photo-generated

holes at oxygen vacancy sites results in a slow recombination

process and causes the PPC effect. The other model involves

trapping of photo-generated holes near the gate insulator and

semiconductor interface.9–11 This model is suitable when the

photo current is negligible, i.e., when using backlight illumi-

nation for bottom gate TFT structures. Since most oxide

semiconductors have an optical band-gap larger than the pho-

ton energy of light emitting diode (LED) backlight, the

electron-hole generation via sub-gap states is considered.

From 1st principles calculations, it was reported that neutral

oxygen vacancy levels of zinc oxide (ZnO) and IGZO appear

near the valence band maximum in the band-gap.12,13 So it

can be assumed in practice that the deep sub-gap states

mainly arise from oxygen vacancies.

In the present work, the effects of sub-gap states on the

photo induced stability of indium zinc oxide (IZO) TFTs are

investigated. IZO is chosen as the semiconductor material

because it is reported that IZO TFTs are very sensitive to

light and indium oxide (In2O3) and ZnO have relatively lower

oxygen vacancy formation energies of 3.06 and 3.75 eV,

respectively,14 than other oxide semiconductors. This means

that a reasonably high amount of oxygen vacancy is already

present in the as-deposited film, which allows us to control

and observe the variation in oxygen vacancy concentration

with respect to external processes. The reduction of density

of sub-gap states (DOS) and the resultant improvement in

photo-induced stability through high pressure anneal (HPA)

process in oxygen ambient are observed. Consequently, it is

experimentally verified that the control of oxygen vacancy

related sub-gap states is crucial in order to improve the

photo-induced stability of TFTs.

Bottom gate TFTs with an etch stop layer were fabri-

cated, as shown in Fig. 1(a). The channel width and length

were 75 and 15 lm, respectively. Both gate and source-drain

electrodes were formed using sputtered 200 nm thick Mo

film. A 350 nm thick silicon nitride film and a 50 nm thick

silicon oxide film were sequentially deposited by plasma

enhanced chemical vapor deposition (PECVD) at 350 �C as

the gate insulator. Then 50 nm thick IZO films were

a)Author to whom correspondence should be addressed. Electronic mail:

sangyoon.lee@samsung.com.

0003-6951/2013/102(12)/122108/4/$30.00 VC 2013 American Institute of Physics102, 122108-1

APPLIED PHYSICS LETTERS 102, 122108 (2013)

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deposited using a radio frequency (RF) magnetron co-

sputtering system at room temperature with two ceramic tar-

gets of In2O3 and ZnO. Single IZO films with a thickness of

250 nm were also prepared for optical measurements. A

100 nm thick silicon oxide film deposited by PECVD at

200 �C was applied as an etch stop layer. Before I-V mea-

surement, all devices were annealed in air at 250 �C for 1 h,

except the sample to be treated with oxygen HPA. During

the HPA process, the pressure was maintained at 5 atm in ox-

ygen ambient. The device characterization was carried out

using a Keithley 4200 semiconductor parameter analyzer.

For negative bias temperature stress (NBTS), the gate and

drain voltage were set to �20 V and þ10 V, respectively,

and the substrate temperature was maintained at 60 �C. For

the photo-induced stability measurement, a white LED was

used as the light source. During the stress, the devices were

exposed to light with a brightness of 20 000 cd/m2. The cat-

ion ratio in IZO films was measured by inductively coupled

plasma atomic emission spectroscopy (ICP-AES). The

microstructure and optical absorption of the IZO films were

examined by X-ray diffraction (XRD) and ultraviolet-visible

(UV-VIS) spectroscopy, respectively.

IZO films with various indium ratio ([In]/[InþZn]) were

deposited by changing the plasma power of each target of

In2O3 and ZnO. XRD patterns shown in Fig. 2 reveal that

amorphous IZO films are obtained between indium ratios of

52% and 77%. This result is consistent with a previous

report.15 To exclude the effect of grain boundaries on the elec-

trical properties, three kinds of IZO films in the amorphous

phase were prepared. Fig. 1(b) shows the optical spectrum of

LED backlight and the absorption spectrum of a representative

IZO film. The absorption spectrum can be divided into band-

to-band absorption and sub-gap absorption. Since the LED

backlight mainly consists of visible light with energy smaller

than the IZO optical band-gap (Eog) of 3.2 eV, the absorption

through sub-gap states, which are composed of tail states and

gap states, is expected to have a dominant effect on the photo-

induced stability. In Fig. 1(b), it is shown that the absorption

from tail states (aTail) and gap states (aGap) follows the Urbach

model and Gaussian model, respectively, expressed as

aTail ¼ a0 expðEph=EUÞ; (1)

aGap ¼ aGM exp �ðEph � ECÞ2

2r2

!; (2)

where a0 is constant, Eph is the photon energy, EU is Urbach

energy, aGM is the height of the Gaussian peak, EC is the

position of the center of the peak, and r is the standard devi-

ation of Gaussian distribution. The optical parameters

including the full width at half maximum (FWHM) of the

Gaussian peak for the films and electrical characterization of

TFTs with different indium ratio are summarized in Table I.

Since a0 for high indium ratio is much larger than for low in-

dium ratio, the tail state absorption in the range of backlight

energy below 2.9 eV increases with the indium concentra-

tion. Also, the absorption from gap states slightly increases

with indium ratio. This can be attributed to the increase of

FIG. 1. (a) Schematic cross section of

the IZO TFT and (b) the optical spec-

trum of LED backlight and the absorp-

tion spectrum of a representative IZO

film.

FIG. 2. X-ray diffraction patterns for IZO films with different indium ratio.

122108-2 Son et al. Appl. Phys. Lett. 102, 122108 (2013)

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oxygen vacancy concentration with increasing In2O3 content

since In2O3 has relatively low oxygen vacancy formation

energy than ZnO. In Table I, the VT shifts (DVT) after NBTS

of 3 h under dark and light illumination were also compared.

The amount of DVT without light illumination is very small,

with absolute values smaller than 0.5 V, regardless of indium

content. However, when the devices are exposed to light, the

VT degradation is dramatically accelerated as a function of

indium content. Considering the sub-gap absorption, it can

be inferred that the DOS plays a critical role in the photo-

induced stability.

In order to reduce the oxygen vacancy concentration

through the supply of oxygen into an IZO film, HPA process

was performed in oxygen ambient. Fig. 3 shows the transfer

characteristics of as-fabricated, air annealed, and HPA treated

TFT, respectively. All devices were fabricated using the

same IZO film with an indium content of 60%. Taking the

air-annealed device as a reference, the HPA treatment results

in a decrease in field effect mobility in the saturation region

from 43.2 to 29.6 cm2/Vs and an increase in VT from �1.4 to

�0.9 V. This is probably caused by the decrease of carrier

concentration, which is related to the number of ionized oxy-

gen vacancies that donate free electrons. The absorption spec-

tra of as-deposited, air annealed, and HPA treated IZO films

are compared in Fig. 4(a). It is clearly shown that the absorp-

tion via sub-gap states below Eog of 3.2 eV is considerably

reduced by applying oxygen HPA. The decrease of sub-gap

state density is also observed from photo-excited charge col-

lection spectroscopy (PECCS) measurements. PECCS utilizes

the photo-induced VT response of a TFT device through pho-

ton energy scanning16 and is very useful for extracting the

deep level DOS of wide band-gap materials. In Fig. 4(b), the

areal DOS for the air annealed device is higher than

1� 1012 cm�2 eV�1. On the other hand, for the oxygen HPA

treated device, it is one order of magnitude smaller.

NBTS measurements under light illumination were per-

formed for air annealed and oxygen HPA treated IZO TFTs.

Fig. 5 exhibits the variation of DVT as a function of the stress

time. All the measurements fit the stretched exponential

equation, which is defined as

TABLE I. Optical parameters for the IZO films with different indium ratio and electrical characteristics of resulting IZO TFTs.

In

[%]

EOG

[eV]

a0

[cm�1]

EU

[eV]

FWHM

[eV]

aGM

[cm�1]

lFE

[cm2/Vs]

VT

[V]

DVT(Dark)

[V]

DVT(Photo)

[V]

52 3.2 0.007 0.195 0.696 10 552 27.8 �0.7 �0.1 �3.8

60 3.2 0.017 0.207 0.700 10 793 43.2 �1.4 �0.4 �6.1

77 3.3 0.842 0.287 0.719 10 969 54.2 �2.1 �0.4 �12.8

FIG. 3. Transfer characteristics of as-fabricated, air annealed, and oxygen

HPA treated IZO TFT, respectively.

FIG. 4. (a) Absorption spectra of as-

deposited, air annealed, and HPA treated

IZO films and (b) DOS plot extracted

from PECCS measurement for resulting

TFTs.

122108-3 Son et al. Appl. Phys. Lett. 102, 122108 (2013)

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DVT ¼ DVT0f1� exp½�ðt=sÞb�g; (3)

where DVT0 is the VT shift at infinite time, s represents the

characteristic trapping time, and b is the stretched exponen-

tial exponent. The stretched exponential parameters for each

sample are listed in Table II. The DVT values after 3 h in air

annealed and oxygen HPA treated TFT are �6.1 V and

�3.5 V, respectively. It should be noted that the characteristic

trapping time (s) increases significantly from 1.0� 105 s to

5.3� 1013 s and the stretched exponential exponent (b)

decreases from 0.47 to 0.07 after oxygen HPA treatment.

Compared with other values reported previously,17,18 s and bobtained from oxygen HPA are very encouraging. Especially,

since the VT degradation according to the stress time is

governed by b, it is expected that long term stability of oxide

TFTs is dramatically improved by oxygen HPA.

In this work, the sub-gap absorption of IZO films with

various indium contents and the photo-induced negative bias

stability of the resulting TFTs were investigated. Higher in-

dium content in IZO films resulted in increases in optical

absorption via sub-gap states and deterioration of photo-

induced device stability. This is attributed to the increase of

oxygen vacancy concentration forming deep level defects.

To reduce the DOS by the supply of oxygen, oxygen HPA

process was applied. The optical absorption spectrum and

PECCS measurements showed that the density of oxygen va-

cancy related sub-gap states was effectively suppressed. As a

result, the photo-induced NBTS of IZO TFTs was remark-

ably improved. Therefore the effect of oxygen vacancy on

the photo-induced stability was experimentally verified.

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FIG. 5. VT shift (jDVTj) as a function of the applied NBTS time with light

illumination for air annealed and oxygen HPA treated TFT.

TABLE II. VT shifts under NBTS with light illumination and the stretched

exponential parameters for air annealed and oxygen HPA treated TFTs.

DVT(Photo) [V] s [s] b

Air anneal �6.1 1.0� 105 0.47

O2 HPA �3.5 5.3� 1013 0.07

122108-4 Son et al. Appl. Phys. Lett. 102, 122108 (2013)

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