12.1-in. WXGA AMOLED display driven by InGaZnO thin-film transistors

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1 12.1-in. WXGA AMOLED display driven by InGaZnO thin-film transistors Jae Kyeong Jeong (SID Member) Jong Han Jeong Hui Won Yang Tae Kyung Ahn Minkyu Kim Kwang Suk Kim Bon Seog Gu Hyun-Joong Chung Jin Seong Park Yeon-Gon Mo Hye Dong Kim Ho Kyoon Chung Abstract A full-color 12.1-in. WXGA active-matrix organic-light-emitting-diode (AMOLED) display was, for the first time, demonstrated using indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) as an active-matrix backplane. It was found that the fabricated AMOLED display did not suffer from the well-known pixel non-uniformity in luminance, even though the simple structure consisting of two transistors and one capacitor was adopted as the unit pixel circuit, which was attributed to the amorphous nature of IGZO semiconductors. The n-channel a-igzo TFTs exhibited a field-effect mobility of 17 cm 2 /V-sec, threshold voltage of 1.1 V, on/off ratio >10 9, and subthreshold gate swing of 0.28 V/dec. The AMOLED display with a-igzo TFT array is promising for large-sized applications such as notebook PCs and HDTVs because the a-igzo semiconductor can be deposited on large glass substrates (larger than Gen 7) using the conventional sputtering system. Keywords Oxide TFT. DOI # /JSID Introduction Active-matrix organic-light-emitting diodes (AMOLEDs) provide the best solution to achieving the ultimate display due to their fast motion-picture response time, vivid color, high contrast, and super-slim lightweight nature. Recently, the AMOLED display has been burgeoning for mid-tolarge-sized applications, such as notebook personal computers (NPCs), monitors, and high-definition television (HDTV), which makes up most of the flat-panel-display market. The most important point for AMOLED NPCs and/or HDTV is how to find a way to mass produce it in large quantities at an affordable price. The best strategy is to increase the motherglass size up to Gen 6, which corresponds to the most costeffective backplane size for NPC applications. However, scaling-up of the production line causes several technological challenges. First, amorphous-si (a-si) TFTs are well-established and is now a proven technology in the LCD industry. This technology has the advantage of good scalability up to Gen 8 and a low-cost process because it does not require crystallization and the ion-doping process. In addition, the excellent uniformity in device performance, including the mobility and threshold voltage, can be assured due to its amorphous nature. However, the mobility of a-si TFTs are quite low (~1 cm 2 /V-sec), which may not be high enough to drive large-sized and high-resolution AMOLED displays. 1 Furthermore, the device instability has been a long-time issue. 2 For example, the threshold voltage of a-si TFTs is seriously shifted under constant current stress due to either the charge trapping into the underlying gate dielectric or weak-bonding break-up of silicon and hydrogen in a-si thin films, leading to the image burn-out or serious image sticking (short lifetime) in AMOLED displays. This is the reason why a-si TFTs are rarely used as an actual backplane in AMOLED displays. On the other hand, low-temperature polycrystalline-si (LTPS) TFTs have high mobility and excellent stability, which is in contrast to their a-si TFT counterpart. The key process in fabricating LTPS TFTs is the crystallization methods that convert a-si into polycrystalline-si (poly-si). These methods can be classified to nonlaser crystallization and laser annealing. Among the non-laser crystallization processes, the simplest method is solid-phase crystallization (SPC). But SPC requires 600 C annealing for tens of hours, which makes it unsuitable for large-area glass substrates. Other non-laser methods employ metal seeds for crystallization, which may cause large leakage current in the channel area. Among the laser methods, excimer-laser annealing (ELA) has been the most widely used due to its excellent crystallinity, fast crystallization speed, and high mobility. In addition, ELA is already employed in mass-production; thus, well-developed apparatus are commercially available. However, ELA suffers from a narrow process window, high initial investment, and maintenance costs. Moreover, the limitation in laser-beam length and instability are the major obstacles in using ELA on large-sized glass: the largest equipment available is applicable to Gen 4 (motherglass size of mm).Finally,allLTPSTFTs,including the ELA technique, suffer from non-uniformity because of the existence of grain boundaries, which requires the use of a complicated compensation unit pixel circuit such as a five transistor + two capacitor circuit, leading to loss in device yield. 3,4 Therefore, an obvious question arises: Is there any new TFT that both demonstrates high mobility and excellent uniformity that is suitable for large-sized AMOLED displays? Amorphous-oxide TFTs can be an attractive alter- Extended revised version of a paper presented at Display Week 2008 (SID 08) held May 20 23, 2008 in Los Angeles, California. The authors are with Samsung SDI Co., Ltd., Corporate R&D Center, Gongse-dong, Kiheung-gu, Yongin-si, Gyeonggi-do , Korea; telephone , fax 4482, ygmo@samsung.com. Copyright 2009 Society for Information Display /09/ $1.00 Journal of the SID 17/2,

TABLE 1 Comparison of TFT technologies including a-si, poly-si, and oxide TFTs. was formed by using the photo-resist coater and developed to define the source/drain (S/D) contact region.

Finally, the sample was subjected to thermal annealing at 250 C for 1 hour.

2 TABLE 1 Comparison of TFT technologies including a-si, poly-si, and oxide TFTs. was formed by using the photo-resist coater and developed to define the source/drain (S/D) contact region. As a S/D electrode, either Mo or Ti/Al/Ti material was formed by sputtering and defined by photolithography and then patterned by dry-etching. Finally, the sample was subjected to thermal annealing at 250 C for 1 hour. The device characteristics of the a-igzo TFTs were measured at room temperature with an Agilent 4156C precision semiconductor parameter analyzer. native solution to this question. Amorphous-oxide semiconductors (AOSs) provide unique properties that combine the advantages of a-si and LTPS TFTs. 5 8 For example, amorphous-oxide TFTs are free of the non-uniformity in mobility and threshold voltage, yet exhibit large carrier mobility (~10 cm 2 /V-sec) and excellent subthreshold gate swing (down to 0.20 V/dec). Moreover, the channel layer can be fabricated by using a simple sputtering process. Therefore, large-sized fabrication can be easily implemented up to Gen 8 without using expensive laser apparatus. The process route is essentially the same as that for a-si TFTs so that the existing production line can be used without significant change. In addition, oxide TFTs can be deposited at room temperature, which, in principle, makes it possible for the mass-production of AMOLEDs on flexible plastic substrates or cheap soda-lime glass. The technological comparisons among a-si, poly-si, and oxide TFTs are summarized in Table 1. We have developed a bottom-gate a-igzo TFT structure that has excellent device uniformities in terms of fieldeffect mobility, subthreshold gate swing, and threshold voltage. Moreover, we successfully demonstrated a 12.1-in. WXGA AMOLED display with a bottom-emission mode, which was driven by a-igzo TFTs array. 2 Experimental Lithographically patterned Mo (200 nm) on a SiO 2 /glass substrate with a surface area of mm 2 was used as thegateelectrode.sin x (120 nm) film, as the gate dielectric layer, was deposited by plasma-enhanced chemical vapor deposition (PECVD) at a substrate temperature of 390 C. The a-igzo film was grown by sputtering on the SiO 2 /glass substrate using a polycrystalline In 2 Ga 2 ZnO 7 target at room temperature. The sputtering was carried out at a gas mixing ratio of Ar/O 2 = 72/28 and an input power of 1200 W. After defining the a-igzo channel using photolithography and wet-etching, a SiO x etch stopper layer (ESL) was deposited by PECVD and then patterned. The device with photo-acryl (PA) material as the ESL was also fabricated; the PA layer 3 Oxide TFT technology 3.1 TFT structure Figure 1 shows a schematic cross section of the IGZO TFTs, which have an inverted-staggered bottom-gate architecture with an ESL. For an a-igzo TFT without an ESL, severe degradation of the subthreshold gate swing and the uniformity of threshold voltage were observed. This is why we chose an ESL-type structure rather than a back-channel etch structure, which is generally adopted for LCDs. 3.2 Device characteristics It was found that the etch stopper material and process is one of the key factors for high-performance transistors with a bottom-gate architecture. Figure 2 compares the representative transfer characteristics of the a-igzo TFTs with PA material and SiO x thin film used as the passivation layer. Table 1 lists the principal device characteristics of both a-igzo TFTs. The threshold voltage (V th,sat ) was defined by the gate voltage, which induces a drain current of L/W 10 na at a V DS of 5.1 V. It is noted that the roughly 1 µa and 1 na are needed to embody the full-on and black gray scale for OLED devices, respectively. Although this type of V th,sat definition does not have a unique physical meaning, the gate voltage to induce a drain current of 10 na can be a useful guideline for the panel design as well as the process control. That is why V th,sat is defined in this way. The apparent fieldeffect mobility induced by the transconductance at a low drain voltage (V DS 1 V) is determined by FIGURE 1 The schematic cross section of an a-igzo TFT with an inverted-staggered architecture. 96 Jeong et al. / A 12.1-in. WXGA AMOLED display driven by InGaZnO TFTs

3 TABLE 2 Comparisons of the extracted parameters for n-channel a-igzo TFTs with (a) photo-acryl and (b) SiO 2 thin film as the ESL. Lgm m FE =, WCiVDS (1) where C i and g m are the gate capacitance per unit area and the transconductance, respectively. From the transfer characteristics, the subthreshold gate swing (S) can be extracted using the equation dvgs S =. d (log IDS ) It can be seen from Table 1 that the µ FE (8.4 cm 2 /Vsec) and I on/off ratio (>10 8 ) for the organic passivated device (device A) are comparable to those (10.8 cm 2 /V-sec, >10 8 ) for the inorganic passivated device (device B). However, there is a dramatic difference in the S value of the two devices. The S value of device B (0.62 V/decade) is superior to that (0.90 V/decade) of device A, indicating that the organic coating and curing on the IGZO back surface led to the creation of an appreciable number of interfacial traps at or near the IGZO/PA. A similar deterioration of the S value was reported for a-si TFTs with an organic passivation layer, which was attributed to the presence of more fixed charges and a higher density of states in the back-channel interface. 9 Thus, we believe that the interface between the IGZO film and organic PA material is more susceptible to the formation of water- or carbon-related traps compared to the SiO x /IGZO interface, as will be discussed later. It should be FIGURE 2 The representative transfer curve of an a-igzo TFT with an W/L = 25/10 µm with an ESL. The S value and V th were very sensitively affected by the material of the ESL. (2) noted that the overall device performances of amorphous- IGZO TFTs are better than those of microcrystalline-si and a-si TFTs. Next, we investigated the effect of ESL material on the bias stability of the resulting a-igzo TFTs. The device was stressed under the following conditions: the I DS was set to 10 µaandthev DS was fixed at 5.1 V. The maximum stress duration was 36,000 sec. The switching and driving transistor in the basic unit pixel circuit of AMOLED displays act as a switcher for the data signal and analog driver (constant current source) for the OLED diode, respectively. 10 Therefore, the driving transistor in an actual pixel is under conditions of more severe stress. This is the reason why the constant I DS stress condition was chosen. Considering that an I DS of approximately 1 µa is required to manifest the full-white gray color in an AMOLED device, 10 the applied stress current (10 µa) corresponds to very severe test conditions. Figures 2(a) and 2(b) show the evolution of the transfer curves as a function of the applied stress time for devices A and B, respectively. It can be seen that while device A shows a change in the S value as well as a large positive V th shift during bias stress, a parallel V th shift to a higher gate voltage with increasing stress time without any significant change in the field-effect mobility, S, ori on/off ratio was observed for device B. Although V th for device A was strongly shifted by approximately 8.1 V, from 5.5 to 13.6 V, the positive V th shift in device B was very small (~2.0 V) after a constant drain current stress of 36,000 sec. The positive V th shift of oxide TFTs under positivegate-voltage stress has been explained by two models: charge trapping or defect creation While the parallel shift in V th without significant change in the S value during stress is attributed to simple charge trapping in the gate dielectric and/or at the channel/dielectric interface, the positive shift in V th accompanying the change in S is a result from the creation of defects within the oxide-semiconductor channel material. In addition, it has been claimed that the recovery behavior of V th without any thermal annealing excludes the possibility of charge injection into the gate dielectric. 11 In this study, because devices A and B have an identical gate dielectric and IGZO semiconductor, the charge trapping at or near the channel/dielectric interface during gate-voltage stress would be expected to be similar to each other. However, the positive V th shifts for devices A and B were 8.1 and 2.0 V, respectively. Therefore, the instability of the V th shift cannot be explained by only the chargetrapping model. We believe that this instability comes from the environmental influence (O 2,H 2 O, etc.) ratherthan chare trapping at/near the interface between the gate dielectric and a-igzo semiconductor, which will be published elsewhere because its detailed mechanism is beyond the scope of this paper. Therefore, it is suggested that a suitable ESL of the active back channel for a bottom-gate structure is believed to be one of a key factors in achieving a good subthreshold property and excellent device reliability. Journal of the SID 17/2,

4 FIGURE 4 The collected transfer curves of an a-igzo TFT with W/L = 25/10 µm with an ESL. SRU data from a nine-points measurement is included in the inset of Fig. 4, where the adjacent distance between transistors was approximately 150 µm, which is quite similar to the pixel-to-pixel distance. FIGURE 3 (a) The evolution of the transfer curve of an a-igzo TFT with (a) photo-acryl and (b) SiO x during severe drain current stress of the a-igzo. The V DS was fixed at 5.1 V and the stressed V GS voltage was set to the gate voltage, which induces a drain current of 10 µa. 3.3 Uniformity of IGZO TFT performance Because the luminance of an OLED device is determined by the drain current of the driving transistor, which is serially connected to an OLED diode, the long-range uniformityoftftsisofimportancecomparedtoitstft-lcd counterpart. The standard deviation of the electrical parameter for the transistor performance was determined from the evaluation of nine points on one panel. Figure 4 shows the representative transfer characteristics of optimized IGZO TFTs with W/L =25/10µm, which was taken for a full-array panel device rather than an individual test device without any suitable ESL or passivation layer. The enhancement in the performance of the IGZO TFT was related to the structural properties of the semiconductor material and the deposition condition of the ESL. In particular, the densification of IGZO thin film via reducing the chamber pressure during channel deposition was found to be an effective way to improve the fieldeffect mobility and S value of the oxide device. We noted that the film density ( g/cm 3 ) of IGZO active films used in this transistor larger than that compared to that ( g/cm 3 ) of the devices shown in Fig. 2. Also, the device performance was greatly dependent on the deposition temperatureof thesio x ESL. The S value increased by increasing the deposition temperature, and the ESL of the optimized device was deposited by PECVD at 200 C. Thus, the high µ FE of 17 cm 2 /V-sec, an excellent SS of 0.28 V/decade, and a good I on/off ratio of >10 9 was achieved, which is the state-of-the-art characteristics for ZnO-based oxide TFTs. We previously reported that the denser IGZO film has a lower interfacial trap density (D it )betweenthegate dielectric and active channel layer, leading to an improvement in the SS value. 15 Therefore, the improvement in device performance is attributed to the lower D it due to the electronic high quality of the denser active film. The longrange standard deviations for threshold voltage and subthreshold gate swing for oxide TFTs were 0.1 and 0.02 V/decade, respectively, indicating that the excellent statistics can be guaranteed because of the amorphous phase nature of the IGZO channel layer. Next, we investigated the short-range uniformity (SRU) of an oxide TFT: The adjacent distance between transistors was approximately 150 µm, which is quite similar to the pixel-to-pixel distance. The standard deviation of the field-effect mobility for nine-point evaluation was cm 2 /V-sec. Surprisingly, the SRU of the threshold voltage was dramatically improved, which is essential for high-image 98 Jeong et al. / A 12.1-in. WXGA AMOLED display driven by InGaZnO TFTs

5 quality displays without pixel non-uniformity. The achieved standard deviation of the SRU was less than 0.01 V as shown in Fig. 4. The simple calculation predicts the non-uniformity in luminance to be less than 2%. This result suggests the use of an ultra-simple pixel circuit of 2Tr + 1Cap can be used for the design of AMOLED displays, which will impact the resulting device yield and cost very positively in. AMOLED protopyte In this work, we have successfully developed a full-color 12.1-in. WXGA AMOLED display, which was driven by an a-igzo TFT backplane. The specifications of an 12.1-in. AMOLED display is summarized in Table 3. The display has apixelcountof1280 RGB 768 with a resolution of 123 ppi. Its subpixel pitch is µm 2 and the pixel element is 2Tr + 1Cap. The channel length for the driving transistor was designed to be 10 µm, but the kink effect, which appears in poly-si TFTs with the same channel length, was not observed in the output characteristics. The scan driver was integrated on the panel and its functionality was successfully demonstrated. The bottom-emission structure was adopted. The structure has a transparent anode, organic layers, and a cathode on the TFT backplane. The OLED device structure consisted of a hole-injection layer (HIL), hole-transport layer (HTL), RGB-emitting layer (EML), electron-transfer layer (ETL), electron-injection layer (EIL), and cathode. The phosphorescent red and fluorescent green and blue were used as emitting materials. Figure 5 shows an image of a 12.1-in. WXGA AMOLED display for NPC application. The luminance and contrast ratio were >300 nit and 20,000:1, respectively. And the NTSC of the fabricated panel was more than 80%. These features are comparable to the specifications of a NPC attainable using TFT-LCDs. Recently, LG Electronic reported a 3.5-in. TABLE 3 The specifications of a 12.1-in. WXGA AMOLED display. FIGURE 5 The display image of a 12.1-in. WXGA AMOLED display, driven by an IGZO TFT array. QCIF + AMOLED prototype, which was driven by an a-igzo TFT backplane. 15 We have also reported a 4.1-in. transparent QCIF AMOLED display using a-igzo TFTs at IMID Thus far, the research and development for a-igzo TFTs has been focused on the demonstration of rather small-sized (<5 in.) AMOLED displays. It should be emphasized that our prototype is the largest and highestresolution AMOLED displays that uses a a-igzo-tft backplane. 5 Conclusion In summary, our fabricated IGZO TFT exhibited a high µ FE of 17 cm 2 /V-sec, an excellent SS of 0.28 V/decade, and a good I on/off ratio of >10 9. Also, the impact of ESL on the stability of IGZO TFTs was investigated in detail. It was shown that the device with SiO x ESL had better stability than that with PA ESL. Furthermore, the full-color 12.1-in. WXGA AMOLED display was demonstrated using a-igzo TFT backplane. Considering that a-igzo TFTs can be easily expanded into a large-sized substrate (greater than Gen. 7) because an a-igzo semiconductor is deposited by a conventional sputtering technique, our demonstration of a 12.1-in. WXGA indicates that a-igzo backplane technology is an ideal candidate for large-sized AMOLED displays. Acknowledgments The authors appreciate K. N. Kim who partially designed the layout and simulated circuit for the 12.1-in. panel. The author would like to thank all the engineers related to the oxide TFT project at Samsung SDI. Reference 1 M. Stewart, R. S. Howell, L. Pires, and M. K. Hatalis, IEEE Trans. Electron. Dev. 48, 846 (2001). Journal of the SID 17/2,

6 2 H. L. Gomes, P. Stallinga, F. Dinelli, M. Murgia, F. Biscarini, D. M. de Leeuw, T. Muck, J. Geurts, L. W. Molenkamp, and V. Wagner, Appl. Phys. Lett. 84, 3184 (2004). 3 J. H. Lee, W. Nam, B. K. Kim, H. S. Choi, Y. M. Ha, and M. K. Han, IEEE Electron Dev. Lett. 27, 830 (2006). 4 S. H. Jung, W. J. Nam, and M. K. Han, IEEE Electron Dev. Lett. 25, 690 (2004). 5 K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature (London) 432, 488 (2004). 6 H. Yabuta, M. Sano, K. Abe, T. Aiba, T. Den, H. Kumomi, K. Nomura, T. Kamiya, and H. Hosono, Appl. Phys. Lett. 89, (2006). 7 M. Kim, J. H. Jeong, H. J. Lee, T. K. Ahn, H. S. Shin, J.-S. Park, J. K. Jeong, Y.-G. Mo, and H. D. Kim, Appl. Phys. Lett. 90, (2007). 8 J.-S. Park, J. K. Jeong, Y.-G. Mo, H. D. Kim, and S.-I. Kim, Appl. Phys. Lett. 90, (2007). 9 J. K. Yoon and J. H. Kim, IEEE Electron. Dev. Lett. 19, 335 (1998). 10 J. K. Jeong, D. U. Jin, H. S. Shin, H. J. Lee, M. Kim, T. K. Ahn, J. Lee, Y. G. Mo, and H. K. Chung, IEEE Electron. Dev. Lett. 28, 389 (2007). 11 R. B. M. Cross and M. M. De. Souza, Appl. Phys. Lett. 89, (2006). 12 P. Gorrn, P. Holzer, T. Riedl, W. Kowalsky, J. Wang, T. Weimann, P. Hinze, and S. Kipp, Appl. Phys. Lett. 90, (2007). 13 Y. Vygranenko, K. Wang, and A. Nathan, Appl. Phys. Lett. 91, (2007). 14 A. Suresh and J. F. Muth, Appl. Phys. Lett. 92, (2008). 15 J. H. Jeong, H. W. Yang, J.-S. Park, J. K. Jeong, Y.-G. Mo, H. D. Kim, J. Song, and C. S. Hwang, Electrochem. Solid-State Lett. 11, H157 (2008). 16 H. N. Lee, J. W. Kyung, S. K. Kang, D. Y. Kim, M. C. Sung, S. J. Kim, C. N. Kim, H. G. Kim, and S. T. Kim, Proc. IDW 07, (2007). 17 J. K. Jeong, M. Kim, J. H. Jeong, H. J. Lee, T. K. Ahn, H. S. Shin, K. Y. Kang, H. Seo, J. S. Park, H. Yang, H. J. Chung, Y. G. Mo, and H. D. Kim, Proc. IMID 07, (2007). 100 Jeong et al. / A 12.1-in. WXGA AMOLED display driven by InGaZnO TFTs

The Technological Trends of Future AMOLED

Invited Paper The Technological Trends of Future AMOLED Jong hyuk Lee*, Hye Dong Kim, Chang Ho Lee, Hyun-Joong Chung, Sung Chul Kim, and Sang Soo Kim Technology Center, Samsung Mobile Display Co., LTD