Investigation of channel width-dependent threshold voltage variation in a-InGaZnO
thin-film transistors
Kuan-Hsien Liu, Ting-Chang Chang, Ming-Siou Wu, Yi-Syuan Hung, Pei-Hua Hung, Tien-Yu Hsieh, Wu-Ching Chou, Ann-Kuo Chu, Simon M. Sze, and Bo-Liang Yeh
Citation: Applied Physics Letters 104, 133503 (2014); doi: 10.1063/1.4868430 View online: http://dx.doi.org/10.1063/1.4868430
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/13?ver=pdfcov Published by the AIP Publishing
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Investigation of channel width-dependent threshold voltage variation
in a-InGaZnO thin-film transistors
Kuan-Hsien Liu,1Ting-Chang Chang,2,3,a)Ming-Siou Wu,4Yi-Syuan Hung,4Pei-Hua Hung,5 Tien-Yu Hsieh,2Wu-Ching Chou,1Ann-Kuo Chu,5Simon M. Sze,4and Bo-Liang Yeh6
1
Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2
Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
3
Advanced Optoelectronics Technology Center, National Cheng Kung University, Taiwan
4
Department of Electronics Engineering, National Chiao Tung University, Hsinchu, Taiwan
5
Department of Photonics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
6
Advanced Display Technology Research Center, AU Optronics, No. 1, Li-Hsin Rd. 2, Hsinchu Science Park, Hsinchu 30078, Taiwan
(Received 25 November 2013; accepted 28 February 2014; published online 31 March 2014) This Letter investigates abnormal channel width-dependent threshold voltage variation in amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors. Unlike drain-induced source barrier lowering effect, threshold voltage increases with increasing drain voltage. Furthermore, the wider the channel, the larger the threshold voltage observed. Because of the surrounding oxide and other thermal insulating material and the low thermal conductivity of the IGZO layer, the self-heating effect will be pronounced in wider channel devices and those with a larger operating drain bias. To further clarify the physical mechanism, fast IV measurement is utilized to demonstrate the self-heating induced anomalous channel width-dependent threshold voltage variation.VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4868430]
Many recent consumer products require the extensive use of low power consumption IC,1non-volatile memory,2–7 and thin film transistors (TFTs).8,9 TFTs with active layers composed of transparent oxide-based semiconductors, such as ZnO and amorphous InGaZnO (a-IGZO), have attracted much attention due to their considerable potential application in flat, flexible, and transparent displays.10–12 In particular, a-IGZO thin film transistors have been widely investigated for the next generation of the display industry owing to their good uniformity, high mobility, excellent transparency to visible light, and room temperature fabrication.12–15 Therefore, they are very promising alternatives to replace amorphous silicon TFTs for application in active matrix liq-uid crystal displays (AMLCD) and organic light-emitting diode displays (AMOLED) as switching/driving devices. However, there are some difficulties which are necessary to overcome for oxide TFTs to be practical in these applica-tions, such as instability under gate bias stress or the sur-rounding ambiance.16–19 Moreover, a-IGZO TFTs can also be used for gate driver on array (GOA) technology. Conventionally, driving ICs have been fabricated through CMOS technology and mechanically attached to the sides of the panel. However, GOA technology fabricates gate driver ICs on the array itself instead of attaching them to the panel sides. As a result, GOA technology can reduce process steps and cost as well as achieving thinner panels with narrower edge.20,21 However, mobility of driving ICs fabricated by single crystal silicon is about one hundred times that of a-IGZO. As a result, in order to achieve the same driving cur-rent, it is necessary to increase channel width of a-IGZO TFTs for GOA operation. Therefore, investigating the
performance and reliability of a-IGZO TFTs with large chan-nel width is of great importance.
The n-type a-IGZO TFTs in this work were fabricated with a bottom gate and back-channel-etching structure. The double-layer Cu/Mo (500/20 nm) gate electrodes films were deposited and then patterned via photolithography on a glass substrate. Then 300-nm-thick Si3N4 and 70-nm-thick SiO2
gate dielectric films were sequentially deposited on the pat-terned gate electrode by plasma enhanced chemical vapor deposition (PECVD). An active layer of 30-nm-thick a-IGZO film was deposited by DC magnetron sputtering using a target of In2O3:Ga2O3:ZnO¼ 1:1:1 in atomic ratio at
room temperature, and then patterned. The Mo/Cu (20/500 nm) source/drain electrodes were formed by DC-sputtering and then patterned. Finally, 160-nm-thick SiO2and 50-nm-thick Si3N4were sequentially deposited as a
passivation layer by PECVD. After that, the device was annealed in an oven at 300C for 2 h in a dark environment. In this Letter, the conventional and fast I-V measurements were performed by Agilent B1500A and Agilent B1530A semiconductor analyzers, respectively. The device dimen-sions of channel width/length (W/L) were 100, 500, 1000, 5000, and 10 000 lm/5.5 lm. The threshold voltage is defined as the gate voltage when the normalized drain cur-rent (NID¼ ID L/W) reaches 1 nA, where L and W are
channel length and width, respectively. All measurements were performed in a dark environment.
Figures1(a)and1(b)show the normalized ID-VGcurve
at VD¼ 1, 5, 10, and 20 V, with W/L ¼ 100/5.5 lm for
Figure 1(a) and W/L¼ 10 000/5.5 lm for Figure 1(b). Obviously, threshold voltage increases with increasing drain voltage. Furthermore, the larger the channel width, the larger threshold voltage that can be observed. Conventionally, channel width and drain voltage do not affect threshold a)
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voltage in long channel devices. However, this anomalous threshold voltage variation depends on channel width and drain voltage that are observed in Figures1(a)and1(b).
In order to inspect this phenomenon further, Figure 2
illustrates the threshold voltage shift versus various channel width and drain voltages, with threshold voltage shift defined as Vth(measurement) Vth(@VD¼ 1 V). Note that at low
drain voltage (VD¼ 5 V), threshold voltage shift is negligible
and unapparent, with the same being true for relatively small channel widths (W¼ 100 lm) at all drain voltages. As W⭌ 500 lm and VD⭌ 10 V, a significant threshold voltage
shift can be observed with increasing channel width and increasing drain voltage. Accordingly, the abnormal channel width-dependent threshold voltage variation may in fact be induced by the self-heating effect.22It is well known that the self-heating effect arises in silicon-on-insulator (SOI) MOSFETs and low-temperature-polycrystalline silicon (LTPS) TFTs, a situation quite similar to that in the IGZO channel layer.23 In addition, the thermal conductivity of IGZO is much lower than Si and is comparable to SiO2.
Therefore, heat dissipation in IGZO TFTs is relatively more difficult than in Si-based TFTs.24 Because the larger drain voltage will form a higher drain current, resulting in higher power (P¼ IV), the heat in channel will be higher, resulting in a more severe self-heating effect. Furthermore, because the heat will more likely accumulate at the center of the channel region and dissipate to the surrounding materials along the channel width direction, larger channel widths make heat dissipation in channel more difficult, again result-ing in a more pronounced self-heatresult-ing effect.25The inset of Figure 2 shows the energy band diagram. When the large channel width TFT is operated at high drain voltage, signifi-cant self-heating effect will occur, and channel electrons will
be trapped at the IGZO/SiO2 interface or in SiO2 bulk
through the thermionic-field emission process, resulting in a larger observed threshold voltage.22,26,27 In addition, from Figure 1(b), note that threshold voltage shifts without obvious variation of the slope in transfer characteristics. This indicates that no additional trapping states are created at the IGZO active layer/gate dielectric interface during the trap-ping process, resulting in unobvious mobility and subthres-hold swing degradation.28–30
In order to confirm that the abnormal channel width-dependent threshold voltage variation is indeed induced by self-heating effect, the ID-VD output characteristic is
per-formed. Figures 3(a) and 3(b) show the ID-VD curve at a
fixed channel length (5.5 lm) but different channel widths (100 lm and 10 000 lm, respectively). Compared to the W¼ 100 lm device, the W ¼ 10 000 lm one exhibits the anomalous output characteristic. When the measurement drain voltage exceeds approximately 15 V, drain current decreases instead of saturating with an increase in drain volt-age. The heat dissipation in channel will be rather difficult for the W¼ 10 000 lm device because of the considerably large channel width. As the large channel width device is operated at high drain voltage conditions (VD⭌ 15 V here),
a severe self-heating effect-induced charge trapping phenom-enon will occur, resulting in a considerable threshold voltage shift. Because of this large threshold voltage shift, the abnor-mal drain current decreases as drain voltage increases when VD⭌ 15 V.
To further confirm the proposed self-heating effect induced anomalous channel-width dependent threshold volt-age variation, fast IV measurement is performed. The inset of Figure 4(a) illustrates the waveform of conventional ID-VG
measurement in which drain voltage is fixed with gate volt-age performed stepwise. Note that the time scale of each gate voltage step is on the order of milliseconds (ms). For compar-ison, the inset of Figure 4(b) shows the waveform of fast ID-VGmeasurement in which drain voltage is fixed with gate
voltage performed in a pulse form. Significantly, the time scale of peak/base time is rather short, approximately on the order of microseconds (ls). From previous research,21 suffi-cient heating time is necessary for Joule heating to take place within the channel, resulting in a pronounced self-heating effect-induced charge trapping phenomenon. This sufficient heating time is approximately on the order of ms. Because the gate pulse peak/base time in fast I-V measurement is on the order of ls, the short heating time is insufficient for Joule heating to occur. Therefore, use of the fast I-V measurement will exclude the self-heating effect induced-charge trapping
FIG. 1. ID-VGtransfer characteristic of a-IGZO TFT operated at VD¼ 1, 5,
10, and 20 V for (a) W/L¼ 100/5.5 lm. (b) W/L ¼ 10 000/5.5 lm.
FIG. 2. Dependence of threshold voltage shift on the channel width and drain voltage. The inset illustrates the thermionic-field emission process of electron trapping.
FIG. 3. ID-VDoutput characteristic of a-IGZO TFT operated at VG¼ 10, 15,
and 20 V for (a) W/L¼ 100/5.5 lm. (b) W/L ¼ 10 000/5.5 lm.
phenomenon, and therefore threshold voltage shift can also be excluded. The measurement sequences of Figures4(a)and
4(b)are as follows. First, ID-VGcurve is measured by
con-ventional ID-VG measurement at low drain voltage
(VD¼ 1 V) to avoid the self-heating effect and act as the
ini-tial state. Second, ID-VGcurve is measured at high drain
volt-age (VD¼ 10 V) by conventional ID-VG measurement for
Figure4(a)and by fast ID-VGmeasurement for Figure 4(b).
Finally, ID-VG curve is measured by conventional ID-VG
measurement at low drain voltage (VD¼ 1 V) to serve as the
final state. Clearly, there is a positive threshold voltage shift between initial and final states after conventional ID-VG
measurements at high drain voltage (VD¼ 10 V), as shown in
Figure 4(a). During the conventional ID-VG measurement
(VD¼ 10 V), there was sufficient heating time for Joule
heat-ing to occur, leadheat-ing to the self-heatheat-ing effect-induced charge trapping phenomenon and resulting in the threshold voltage shift between initial and final states. Conversely, during the fast ID-VGmeasurement (VD¼ 10 V), the insufficient heating
time required for Joule heating results in no threshold voltage shift being observed, as shown in Figure4(b). The fast ID-VG
measurement further corroborates that the abnormal channel-width dependent threshold voltage variation does in fact result from the self-heating effect-induced charge trap-ping phenomenon. In addition, no matter the second measure-ment step, which is with VD¼ 10 V, is carried out with
conventional or fast ID-VGmeasurement, there is no
thresh-old voltage shift between initial and final states, as shown in Figures5(a)and5(b). This indicates that channel width is an important factor in the self-heating effect-induced charge
trapping phenomenon because larger channel widths make heat dissipation in channel more difficult.
This paper has investigated the anomalous channel width-dependent threshold voltage variation in a-IGZO TFTs. Devices with larger channel widths and which are operated at higher drain voltages will produce larger thresh-old voltages, with the effect becoming even more pro-nounced as channel width or drain voltage increases. This is due to the surrounding oxide and other thermal insulating material and the low thermal conductivity of the IGZO layer. The more pronounced self-heating effect is a product of both the more difficult heat dissipation in wider channels as well as the higher drain current in devices operated at higher drain voltages. Because sufficient heating time (approximately on the order of ms) is necessary for Joule heating to take place within the channel, the fast ID-VGmeasurement is performed
to confirm the proposed mechanism. Because the time scale of the peak/base time in the fast ID-VG measurement is
shorter, on the order of ls, the heating time is insufficient for Joule heating, resulting in no observed threshold voltage shift. The fast ID-VG measurement confirms that the
abnor-mal channel-width dependent threshold voltage variation is due to the self-heating effect induced-charge trapping phenomenon.
This work was performed at the National Science Council Core Facilities Laboratory for Nano-Science and Nano-Technology in the Kaohsiung-Pingtung area and was supported by the National Science Council of the Republic of China under Contract No. NSC 102-2120-M-110-001.
FIG. 4. The transfer characteristic of a-IGZO TFT with W/L¼ 10 000/ 5.5 lm before and after measuring at high drain voltage (VD¼ 10 V) by (a)
conventional ID-VGmeasurement. (b) Fast ID-VGmeasurement. The inset of
Figure4(a)and4(b)illustrate the waveform of conventional and fast ID-VG
measurement, respectively.
FIG. 5. The transfer characteristic of a-IGZO TFT with W/L¼ 100/5.5 lm before and after measuring at high drain voltage (VD¼ 10 V) by (a)
conven-tional ID-VGmeasurement. (b) Fast ID-VGmeasurement. The inset of Figure
5(a)and5(b)illustrate the waveform of conventional and fast ID-VG
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