Q74 ECS Journal of Solid State Science and Technology, 2 (4) Q74-Q76 (2013) 2162-8769/2013/2(4)/Q74/3/$31.00©The Electrochemical Society
Dependence of Light-Accelerated Instability on Bias and
Environment in Amorphous Indium-Gallium-Zinc-Oxide
Thin Film Transistors
Yu-Chun Chen,a,∗Ting-Chang Chang,a,b,zHung-Wei Li,cWan-Fang Chung,dTien-Yu Hsieh,a Yi-Hsien Chen,a,∗Wu-Wei Tsai,eWen-Jen Chiang,eand Jing-Yi Yane
aDepartment of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan
bAdvanced Optoelectronics Technology Center, National Cheng Kung University, Taiwan
cDepartment of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University,
Hsinchu, Taiwan
dDepartment of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University,
Hsinchu, Taiwan
eIndustrial Technology Research Institute, Hsinchu, 31040, Taiwan
This study examines the dependence of light-accelerated instability on bias and environment in amorphous indium-gallium-zinc-oxide thin film transistors. When device is in vacuum ambient, the threshold voltage of device after negative gate bias illumination stress (NBIS) showed bias-dependent electrical degradation behavior. However, experimental results show the degradation of electrical characteristic in a-IGZO devices does not only rely on the charge trapping mechanism for NBIS. During NBIS in oxygen, moisture-simulated and atmosphere ambient, the negative shift in electrical characteristic is suppressed when compared to that in vacuum. This implies that the adsorbent gas species in the surrounding environment dominates the electrical characteristic degradation of devices during NBIS, which leading the change of dominant mechanism from photon-created carrier trapping to adsorbed/desorbed gas phenomenon.
© 2013 The Electrochemical Society. [DOI:10.1149/2.028304jss] All rights reserved.
Manuscript submitted December 7, 2012; revised manuscript received January 14, 2013. Published February 21, 2013.
In the last decade, amorphous oxide semiconductors (AOSs) have become popular for the channel layer material in thin film transis-tors (TFTs) for use in active matrix light emitting diode (AMOLED) and other flexible displays.1Because of their large electron mobility
(>10 cm2/V.s) and uniform electrical characteristic over large area
deposition at room temperature, AOSs TFTs are expected to act as the driven-TFTs in large area displays (e.g.>55 inches) or displays with a high frame rate (e.g., 240 Hz) and to become integrated into inno-vative transparent electronics such as AOS-based systems on glass.2–4
In particular, to satisfy the requirements above, amorphous indium gallium zinc oxide (a-IGZO) has been intensively studied as an active layer for TFTs requiring superior electrical performance.
When used to drive AMOLED pixels, the a-IGZO TFTs are re-quired to be under a high gate bias operation condition, which results in an accelerated degradation in their electrical characteristics.5This
issue has been intensively studied in recent years.6–8However, when
used as the switching TFT in the AMOLED pixel circuit, most of the time they remain in the “off” state or experience a negative bias. In par-ticular, light illumination during gate bias operation is inevitable for a-IGZO TFTs in display applications. The reliability of a-IGZO TFTs under negative bias stress with light exposure has been reported to be induced by several mechanisms: photo generation of ionized oxy-gen vacancies,9hole trapping at the channel/insulator interface10,11or
environmental gas ambient effects.12–14For the first mechanism, the
photo generation of ionized oxygen vacancies usually accompanies raised off-current owing to increase of electron concentration in a-IGZO. For the latter two mechanisms, however, that there have been few attempts to establish a direct relationship between the influences which combine environmental gases and gate bias and the degradation of a-IGZO TFTs after negative gate bias illumination stress (NBIS). This work investigated the dependence of light-accelerated instability on negative bias and environmental gas ambient in a-IGZO TFTs.
Experimental
TFT fabrication.— The bottom gate and bottom contact structure of the a-IGZO TFTs are fabricated. The DC sputtered Ti/Al/Ti gate and source/drain electrodes with thickness of 50/200/50 nm were
∗Electrochemical Society Student Member. zE-mail:[email protected]
deposited and patterned by standard photolithography on a glass sub-strate, respectively. The 30-nm a-IGZO (In2O3:Ga2O3:ZnO= 1:1:1)
was deposited on a 300-nm plasma enhanced chemical vapor de-posited SiOx to serve as the channel layer by DC magnetron sputter-ing system at room temperature in an ambient gas mixture ratio of O2/Ar= 6.7% with a working pressure of 5 mTorr. Finally, the TFTs
were annealed at 150◦C for 30 minutes under atmosphere ambient.
TFT characterization.— All of the current-voltage (I-V) and capacitance-voltage (C-V) characteristic measurements were mea-sured at room temperature by using an Agilent B1500 semiconductor parameter analyzer. The probe station was set in a vacuum cham-ber with gas flow control and moisture feeding equipment.12 The
threshold voltage (VT) was determined by using the constant current
method as the gate voltage which induced a drain current of L/W × 1 nA of normalized drain current (NID), and subthreshold swing
(SS) was determined by the equation SS= dVGS/d(log IDS) (V/dec)
while the current was between 10−10and 10−9A. The light illumina-tion of 10000 lux intensity in this work was obtained by a halogen lamp whose spectrum is shown in the inset of Fig.1a.
Experimental Results and Discussion
Before to understand the effect of NBIS on a-IGZO TFTs, the illumination on devices without stress bias in different gas am-bience are performed to investigate the impact of photo-induced
Figure 1. Transfer NID-VGcharacteristics of a-IGZO TFTs during and after illumination for 2000s (a) in atmospheric ambient. (b) in vacuum ambient. The inset of (a) shows the illumination spectrum of the halogen lamp.
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ECS Journal of Solid State Science and Technology, 2 (4) Q74-Q76 (2013) Q75
Figure 2. The C-V characteristics of a-IGZO TFTs during and after illumina-tion for 2000s (a) in atmospheric ambient. The inset shows the result in vacuum ambient. (b) Time dependence of delta VTduring and after illumination for 2000s in atmospheric, vacuum, oxygen and moisture-simulated ambient.
instability of a-IGZO TFTs. Figure1aand1bshow the NID-VG
electri-cal characteristics of a-IGZO TFTs in atmospheric or vacuum ambient, respectively. By comparison, the electrical characteristics of a-IGZO TFTs shift in a more negative direction when the environment varies from atmospheric to vacuum ambient. The suppressed negative shifts of a-IGZO TFTs in atmospheric ambient with only minute variations in SS after illumination can be attributed to two mechanisms. First, the subgap photon excitation occurs from the deep-subgap density of state (DOS) to the conduction band. The photon generation of electron-hole pairs during illumination is generated from subgap pho-ton excitation due to the large deep-subgap DOS in a-IGZO.13Then
the photon generation of holes discharges the negatively charged ad-sorbed oxygen ions into the active backchannel as a form of O2−(ads) + h+ → O2 (solid), with no change in drain off-current and SS, as
shown in Figure1.15The second mechanism is that the surrounding
oxygen molecules from the atmospheric ambient can capture electrons from the conduction band and then cause the adsorption of oxygen ions (O−, O2−) on the active layer, resulting in a depletion layer on the backchannel and an increase in the VTof AOS-based TFTs.15–17
Because this second oxygen adsorption mechanism will not occur in vacuum, the negative shift of a-IGZO TFTs in vacuum after illumina-tion is more significant than for in atmospheric ambient. This result reveals the important role of ambient gases, which affect the electrical characteristics in passivation-free devices.
In Figure2a, the C-V measurement of a-IGZO TFTs in atmo-spheric or vacuum ambient after illumination of visible light are shown, respectively. That the C-V characteristics of a-IGZO TFTs show results similar to I-V characteristics means that there is no de-fect creation at the GI/ semiconductor interface under different gas ambient. To investigate more about the influence of gas ambient on the electrical characteristics of a-IGZO TFTs during illumination, the Figure2bshows the progressive VTshift of I-V after illumination in
atmospheric, in oxygen, in moisture-simulated and in vacuum ambi-ent, respectively. The shift of VT after illumination in atmospheric
and oxygen ambient exhibits the same tendency. This result can be attributed to the fact that both adsorption and desorption of oxygen may occur on the back channel of a-IGZO TFTs during illumination.
Figure 3. (a) Transfer NID-VGcharacteristics (b) C-V characteristics of a-IGZO TFTs before illumination in atmospheric ambient, moisture-simulated ambient and after illumination for 2000s in moisture-simulated ambient.
Figure 4. Transfer NID-VGcharacteristics of a-IGZO TFTs during and after NBIS for 2000s in (a) atmospheric ambient (b) vacuum ambient (c) oxygen ambient (d) moisture-simulated ambient.
Figure3aand3bshow the NID-VG electrical characteristics and
C-V measurement of a-IGZO TFTs in moisture-simulated ambient after illumination, respectively. The I-V shows a negative shift of VT
and a less SS deterioration after illumination. The increase of SS in electrical characteristics can be attributed to the formation of defects in the a-IGZO. Meanwhile, the created states influence the C-V char-acteristics before flatband voltage. In literature, the created states can be associated to moisture-induced donor-type states in the a-IGZO.10
After illumination in moisture-simulated ambient, surprisingly, the VTof a-IGZO TFTs shows less shift than that in vacuum, as shown in
Figure2b. This implies that there is another mechanism that affects the phenomenon, which will be discussed later.
Next, we investigated the stability of a-IGZO TFTs under NBIS in different environmental gas ambient. Figure4a–4dshow the trans-fer NID-VGcharacteristics of a-IGZO TFTs in atmospheric, vacuum,
oxygen and moisture-simulated ambient under NBIS for the same negative gate biases of –15 V with grounded drain and source for 2000 seconds. In atmospheric, vacuum and oxygen ambient, parallel shifts in I-V characteristics with a negligible change in SS after NBIS are observed. However, the smaller negative shifts in the TFT I-V characteristics after NBIS in atmospheric or oxygen ambient indicate that the charge trapping model cannot entirely account for this result. Figure5shows the progressive shift of VTin atmospheric,
vac-uum, oxygen and moisture-simulated ambient under NBIS of−15 V.
Figure 5. Time dependence of delta VT during and after NBIS in atmo-spheric, vacuum, oxygen and moisture-simulated ambient. The inset shows the schematic diagram of instability mechanism under NBIS in different en-vironmental gases and the variation of the bent energy band of a-IGZO as the negative VTshift reaches stress gate voltage, which lead photo generated holes to drift toward the front channel or back channel of the device.
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Q76 ECS Journal of Solid State Science and Technology, 2 (4) Q74-Q76 (2013)
Figure 6. The evolution of I-V characteristics of a-IGZO TFTs after NBIS of (a)−5 V (b) −30 V. The inset shows time dependence of delta VTduring and after NBIS of−5 or −30 V in vacuum, which are well fit to the stretched-exponential equation.
The VT shift seems to be closely connected to the stress gate bias.
In oxygen ambient, the VT shift after NBIS is smallest. A possible
instability mechanism of a−IGZO TFTs under NBIS is depicted in the inset of Figure5.18During NBIS testing, the subgap-excited holes
under illumination are attracted to drift toward the channel/insulator interface and trap in the preexisting traps located at the interface or in the gate dielectric. As the negative VTshift reaches stress gate voltage,
the variation of the bent energy band of a-IGZO leads photo gener-ated holes to drift toward the back channel of the device, so no hole trapping phenomenon occurs. Therefore, NBIS changes to become a sort of illuminated positive gate bias stress.19The V
Tvariation of
de-vices in oxygen ambient displays a saturated VTshift of devices near
–15 V, which indicates that bias-induced oxygen adsorption will al-ternate with the hole-trapping phenomenon during NBIS.16
In literature, the moisture-related defect state in the a-IGZO acts as a trapping center during NBIS and enhances the hole trapping phenomenon.12However, for the V
Tshift during NBIS in
moisture-simulated ambient, the result shows less negative shift when compared to that in vacuum. Moreover, in the Figure2b, the negative shift after illumination in moisture-simulated ambient is also suppressed when compared to the results in vacuum. One possible explanation for the findings is that the residual moisture-related defect states in the a-IGZO play the role of recombination centers. When device is under illumination only, the photo-generation of electron-hole pairs during illumination can recombine in residual moisture-related defect states, decreasing the desorption of adsorbed oxygen on the backchannel. In Figure4d, the hump-like I-V curves can be observed in the early 1000 seconds of NBIS, which indicates that extra moisture-induced states form in the a-IGZO. Therefore, the moisture-induced defect states, i.e., recombination centers, decrease the desorption of adsorbed oxygen and hole trapping phenomenon during NBIS, which results in less negative shift than that in vacuum.
To assess the model above, the dependence of instability on gate bias of NBIS was studied in vacuum ambient. Figure6aand
6b show the transfer NID-VG characteristics of a-IGZO TFTs in
vacuum ambient under NBIS for negative gate biases of −5 and −30 V, respectively. Clearly, the NID-VG curve only shifts in the
negative direction with negligible variations in SS and off current. Because of absent adsorption of ambient gas, these results in vac-uum have been suggested to be desorption of adsorbed oxygen and photon-generated hole trapping in the preexisting traps located at the insulator/semiconductor interface, with the time dependence ofVT
under stress following a stretched exponential equation, as follows:
V
T= VTO{1 − exp[ − (t/τ)β]}
whereVT0is theVTat infinite time,β is the stretched-exponential
exponent, andτ is the constant characteristic trapping time.16 The
inset of Figure6shows the experimental data and well fitting curves
for the devices under NBIS for negative gate biases of−5 and −30 V, respectively. Thus, the charge trapping mechanism is dominant for the results shown in Figure6. In the inset of Figure6, the shift of VT saturates rapidly near the stress gate voltage for the NBIS with
smaller gate bias. Under NBIS with larger gate bias, the delta VT of
a-IGZO TFTs shows a continuous negative shift. This demonstrates that the energy band bending of a-IGZO, which depends on gate bias, leads the different degradation behavior of device under NBIS.
In conclusion, dependence of a-IGZO TFT instability on bias and environment under NBIS operation are addressed in this work. A physical model is presented to demonstrate the transition of dominant mechanisms from photon-generated hole trapping mechanism to the adsorbed/desorbed gas phenomenon. As the influences of environ-mental gases being excluded, charge trapping becomes the dominant degradation mechanism during NBIS in vacuum ambient. For the oxy-gen ambient, an additional mechanism of the adsorption/desorption of oxygen on the back channel occurs. When the environmental moisture is present, it exhibits a less negative VTshift than that of NBIS and
il-lumination stress in the vacuum. This result indicates that the residual moisture-related defect states in the a-IGZO acts as a recombination centers during NBIS and decreases the hole trapping phenomenon and desorption of adsorbed oxygen. This study then contributes to a general understanding of the illuminated and gas ambient-related gate bias instability of a-IGZO TFTs.
Acknowledgment
This work was performed at the National Science Council Core Facilities Laboratory for Nano-Science and Nano-Technology in Kaohsiung-Pingtung area. The authors acknowledge the financial sup-port of the National Science Council of the Republic of China under Contract Nos. NSC-100-2120-M-110-003.
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