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Effect of hafnium addition on the electrical properties of indium zinc oxide thin film transistors
Thin Solid Films 519 (2011) 6815–6819
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of hafnium addition on the electrical properties of indium zinc oxide thin film transistors Dae-Ho Son, Dae-Hwan Kim ⁎, Jung-Hye Kim, Shi-Joon Sung, Eun-Ae Jung, Jin-Kyu Kang Public & Original Technology Research Center, Daegu Gyeongbuk Institute of Science & Technology, Daegu Technopark Venture 1, 711 Hosan-dong, Dalseo-gu 704-230, Republic of Korea
a r t i c l e
i n f o
Available online 21 April 2011 Keywords: Metal oxide thin film transistor HfInZnO High-k material
a b s t r a c t This study reports the performance and stability of hafnium–indium zinc oxide (HfInZnO) thin film transistors (TFTs) with thermally grown SiO2. The HfInZnO channel layer was deposited at room temperature by a cosputtering system. We examined the effects of hafnium addition on the X-ray photoelectron spectroscopy properties and on the electrical characteristics of the TFTs varying the concentration of the added hafnium. We found that the transistor on–off currents were greatly influenced by the composition of hafnium addition, which suppressed the formation of oxygen vacancies. The field-effect mobility of optimized HfInZnO TFT was 1.34 cm2 V−1 s−1, along with an on–off current ratio of 108 and a threshold voltage of 4.54 V. We also investigated the effects of bias stress on HfInZnO TFTs with passivated and non-passivated layers. The threshold voltage change in the passivated device after positive gate bias stress was lower than that in the non-passivated device. This result indicates that HfInZnO TFTs are sensitive to the ambient conditions of the back surface. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide semiconductors have attracted considerable attention as semiconductor channels in thin film transistors (TFTs) because of their relatively high scalability, uniform structure, low-temperature processing, high mobility, and reasonably high on/off ratio [1,2]. TFTs with a ZnO-based semiconductor as the active layer have already been developed for use in active-matrix organic light-emitting diodes (AMOLEDs) [3,4]. In particular, indium–gallium–zinc oxide (InGaZnO) TFTs have been suggested for deriving future display devices with high performance characteristics in AMOLED [5–7]. Furthermore, a few research groups have reported TFTs substituting different metal materials for Ga including ZrInZnO, HfInZnO, and AlZnSnO [8–11]. The ZrInZnO and HfInZnO thin films were fabricated using zirconium and hafnium, respectively, both of which are metals with high dielectric constants (high-k). Generally, high-k metal oxide materials such as ZrO2, Y2O3, and HfO2 have been widely investigated and employed as gate insulators of transistors because a thicker film can be utilized to reduce the gate leakage current while maintaining the same gate capacitance. The high-k metal oxide materials are fabricated at low temperature and have many desirable properties such as high dielectric constant, and a relatively large band gap is fabricated [12–14].
⁎ Corresponding author. Tel.: + 82 53 430 8429; fax: + 82 53 430 8475. E-mail address: [email protected] (D.-H. Kim). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.079
Recently, high-k oxide materials have been used in other applications. Specifically, they can be used for the fabrication process of metal oxide TFTs because they suppress carrier generation via oxygen vacancy formation [8,9]. The metal oxide TFTs, by using high-k material, have many advantages such as low cost of fabrication and good properties of devices. However, currently there have been only few reports on the stability of new metal oxide TFTs that contain highk oxide materials in the active layer. In this study, we have fabricated TFTs using a hafnium indium zinc oxide (HfInZnO) thin film as the active channel layer and have investigated their stability. We found that the ambient effect on HfInZnO surfaces significantly influenced the shift in the threshold voltage of the TFTs. 2. Experimental details Fig. 1(a) shows a schematic cross-sectional view of the HfInZnO TFT structure in the coplanar geometry on a thermally grown SiO2 gate dielectric. As shown in Fig. 1(b), an HfInZnO active channel layer approximately 20 nm thick was deposited by co-sputtering onto a SiO2 film (100 nm in thickness) used as the gate insulating layer. The HfInZnO thin film was deposited on the SiO2/Si substrate using a magnetron co-sputtering system with two cathodes (DC/RF), during which the substrate was not heated. InZnO (In2O3:ZnO = 90:10 wt.%, 4 in) and HfO2 targets (99.99%, 4 in) were mounted on the DC and RF cathodes, respectively, and the composition of the active layer film was controlled by the power ratio of the cathodes. The DC cathode was maintained at a voltage of 400 V and a current of 0.5 A, and the power
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Fig. 1. (a) Schematic structure of HfInZnO TFT and (b) a cross-sectional TEM image of co-sputtered HfInZnO thin film annealed at 300 °C. The upper inset shows the FFT of HfInZnO layer in amorphous region. The lower left and right inset shows enlarged image and the FFT of HfInZnO layer in crystal region.
of the RF cathode was increased from 200 to 400 W. Co-sputtering was carried out at a gas mixing ratio of Ar/O2 = 120/25 sccm and a total sputtering pressure of 5 mT. The post-annealing process was performed in a vacuum furnace at 300 °C. Al source and drain contacts 100 nm thick were vapor-deposited on the active layer through a shadow mask on the fabricated non-passivated HfInZnO TFTs. We also applied a photolithography process to measure the stability of the HfInZnO TFTs. The active area of the TFTs was patterned by conventional photolithography and a wet etching process. After forming the active layer, aluminum deposition was carried out using a thermal evaporator followed by a lift-off process to form the source and drain electrodes. The length and width of the channel were 40 and 1000 μm, respectively. Finally, the TFT devices were passivated with a photoresist (AZ-7210). The electrical characteristics of the HfInZnO TFTs were measured using a Keithley 4200-SCS semiconductor parameter analyzer. 3. Results and discussion We fabricated non-passivated HfInZnO TFTs and measured their electrical characteristics. Fig. 2 shows the drain current–gate voltage (IDS–VGS) characteristics of HfInZnO TFTs with VDS = 20 V. The RF power applied to HfInZnO TFTs whose active layers were of the same thickness was varied between 200 and 350 W. Except at an RF power of 200 W, all co-sputtered HfInZnO TFTs were operated in an n-channel enhancement-mode. The device characteristics of the HfInZnO TFTs,
extracted from the results shown in Fig. 2, are summarized in Table 1. Electrical parameters including the saturation field-effect mobility and the threshold voltage, were derived from a linear fit to a plot of the square root of the drain current versus the gate voltage. The following equation is the general expression for the operation of a field-effect transistor in the saturation region: [15] IDS =
WCi μ sat 2 ðVGS −Vth Þ 2L
for VDS N VGS −Vth
ð1Þ
where W is the channel width, L is the channel length, Ci is the capacitance per unit area of the gate insulator, VG is the gate voltage, ID is the drain-to-source current, and Vth is the threshold voltage of the TFT. When the RF power applied to the HfO2 target is 200 W, the HfInZnO TFT show large on-currents, but off-current are also large, which makes on–off current ratio small. With decreasing power of the HfO2 target, the off-current in the transfer characteristics of the HfInZnO TFT increases. Although the on-current decreases with increasing RF power to the HfO2 target, the on–off current ratio of HfInZnO TFTs shows a distinct improvement and the turn-on-voltage, Von, is shifted to a positive voltage from −1.4 to 5.7 V. Concerning the variation of the on–off current ratio and Von, these surely are related with the carrier density of the active layer. These results show that the free electron concentration in the HfInZnO channel layer changed with the RF power applied to the target. This variation was due to the decreased electron carrier concentration in HfInZnO thin films with increasing power of the HfO2 target because elemental Hf characteristically results in the suppression of carrier generation by reducing the oxygen vacancy formation in the HfInZnO matrix system. In other words, the free carrier concentration of HfInZnO thin film can be controlled by the Hf composition ratio. The Hf atomic concentration in the HfInZnO thin film is minimized when the RF power applied to the HfO2 target is 200 W, which is supported by X-ray photoelectron spectroscopy (XPS) measurements. After the carbon contamination of the HfInZnO thin film was removed by Ar+ sputtering, XPS analysis Table 1 Electrical properties of HfInZnO TFTs with various levels of RF power applied to HfO2.
Fig. 2. Transfer characteristics of HfInZnO TFTs for different values of RF power applied to the HfO2 target at a VDS of 20 V.
RF Power [W]
μsat (cm2/Vs)
Von (V)
On–off current ratio
Vth
200 250 300 350
– 4.4 2.1 0.6
– −1.4 0.1 5.5
– 1.9 × 107 4.7 × 107 8.3 × 106
3.3 6.4 15.2
D.-H. Son et al. / Thin Solid Films 519 (2011) 6815–6819
was carried out to quantitatively establish the chemical properties of the HfInZnO thin films, the detailed descriptions and the atomic concentration of which are described in a previous publication [9]. Fig. 3 shows (a) Hf 4f, (b) In 3d, (c) Zn 2p, and (d) O 1s peaks of the XPS spectra for Hf–In–Zn–O films deposited and vacuum-annealed at 300 °C when the RF power applied to the HfO2 target was 300 W. The Hf 4f doublet appears at 16.3 (Hf 4f7/2) and 17.8 (Hf 4f5/2) eV and originates from Hf–O bonds [13]. The In 3d5/2 peak centered at 444.6 eV, and the Zn 2p3/2 peak centered at 1021.8 eV, originate from oxygen-bound In and Zn, respectively [16]. The O 1s peak of HfInZnO can be fitted by two nearly Gaussian distributions, the details of which are also explained in a previous publication [9]. In previous work, we examined the O 1s peaks of HfInZnO that were obtained for the HfInZnO thin film as a function of RF power. The results show that the increase in the area ratio higher binding peaks in the O 1s peak indicates that the oxygen vacancies increased with decreasing RF power. In addition, the HfInZnO O 1s main peak was slightly shifted from 529.9 eV to 530.2 eV with the increasing power of the HfO2 target. We also investigated the Hf, In and Zn core levels of HfInZnO thin film with RF powers of the HfO2 target. The inset in Fig. 3 shows the XPS peaks for core levels obtained for the HfInZnO thin film as a function of RF power. The central peak of the binding energy for Hf 4f slightly shifted to a higher energy level as the RF power applied to the HfO2 target increased. These results indicate an increase in the binding energy with oxygen due to the effect of element Hf as carrier suppressor. This phenomenon is similar to other reported results [17]. In other words, because the added element Hf can bond with oxygen, the oxygen deficiency of the HfInZnO thin film decreases. These XPS results and TFT properties indicate a role for Hf in the HfInZnO thin film. In other words, Hf ions form stronger chemical bonds with oxygen than do In or Zn ions because of their high electronegativity
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(Hf = 1.3) in HfInZnO thin films. Hence, Hf ion play a role in suppressing the formation of oxygen vacancies [18,19]. Fig. 4(a) shows the transfer characteristics of the non-passivated HfInZnO TFT after annealing treatment at 300 °C in ambient vacuum for 1 h. The active layer was deposited when the RF power applied to the HfO2 target was 300 W; the power applied to the InZnO target was fixed at 400 V/0.5 A. The saturation field-effect mobility was 1.34 cm2 V−1 s−1; the on–off current ratio was ~ 1.1 × 108; the threshold voltage was 4.54 V; and the subthreshold swing was about 980 mV/dec. These values are better than those for a-Si TFTs. Fig. 4(b) shows the output characteristics of the non-passivated HfInZnO TFT after annealing treatment at 300 °C in ambient vacuum for 1 h. The output characteristics exhibited strong saturation and a clear pinchoff. We also observed the state of the co-sputtered HfInZnO thin film through TEM and fast Fourier transformation (FFT) analysis, as shown in Fig. 1(b). The structural characterization of HfInZnO thin film shows a partially crystalline state in an amorphous phase. The locally crystalline feature of the thin film was verified in the magnified part image of the right rectangular region and FFT analysis of the slightly luminescent spots in the bright halo in Fig. 1(b) inset. We also check the InZnO thin film as compared to the HfInZnO thin film. There are different states between HfInZnO and InZnO thin films. The approximately 10 nm InZnO thin film shows an amorphous state in the TEM analysis (not shown here). We think that one of the possible reasons for this is the high RF power of the HfO2 target during the deposition of HfInZnO thin film. The increase in RF power creates a crystalline state probably because of ion bombardment. To evaluate the stability of HfInZnO TFTs that were fabricated at 300 W, the power of the applied HfO2 target and the DC cathode were maintained at a voltage of 400 V and a current of 0.5 A respectively; we compared the non-passivated HfInZnO TFT with a conventional
Fig. 3. The (a) Hf 4f, (b) In 3d, (c) Zn 2p, and (d) O 1s peaks of core level XPS spectra at the indicated RF power of HfO2 for co-sputtered HfinZnO film.
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Fig. 4. (a) Transfer and (b) output characteristics of HfInZnO TFTs annealed at 300 °C. The active layer was deposited when the RF power applied to the HfO2 target was 300 W.
photo-resist passivated HfInZnO TFT. Fig. 5(a) shows a microscope image of a photo-resist passivated HfInZnO TFT viewed from above. The photo-resist was coated by the spin-coating method, and the passivated HfInZnO TFT was cured at 120 °C for 1 h in air. The stability of the HfInZnO TFT was measured under harsh conditions; a gate bias stress of VGS = 30 V for varying time periods in 50% ambient relative humidity. Fig. 5(b) and (c) show changes in the transfer characteristics of HfInZnO TFT in the absence and presence, respectively, of a passivation layer during gate bias stress. The Von of the nonpassivated HfInZnO TFT shifted significantly in the positive direction from −0.5 to 9.3 V after 3996 s of stress. There are many potential causes for this device's instability, such as charge trapping inside the dielectric or at the interface, defect state creation within the channel layer, or ambient interactions [20,21]. Among these reasons, the backchannel effect by ambient is considered as one of the most critical problem in our device structure. When we measured the stability of the photo-resist passivated HfInZnO TFT in blocking the ambient effect, the Von of the passivated HfInZnO TFT shifted from −0.6 V to 4 V after 29970 s of stress. Although photo-resist passivation definitely improved the gate bias stability, the bias stress properties of the photo-resist passivated HfInZnO TFT were still unstable. The enhanced stability of the device with photo-resist passivation may be due to a barrier material that cannot perfectly protect against ambient from the back-channel surface of the HfInZnO TFT. It is therefore necessary to find a material for the passivation layer, a material that will protect from the ambient such as oxygen and water vapor in metal oxide TFTs. 4. Conclusions In this study, we have fabricated ZnO-based TFTs using SiO2/p-Si substrates on which a HfInZnO channel layer was deposited by a
Fig. 5. (a) Optical microscope image (top view) of a photo-resist passivated HfInZnO TFT, and transfer curve shifts of non-passivated (b) and photo-resist passivated (c) HfInZnO TFTs under a 30 V gate bias voltage in 50% ambient relative humidity.
co-sputtering system at different RF powers applied to HfO2. The on–off current ratio of the HfInZnO TFTs was improved because the free carrier concentration in HfInZnO channel layer decreased as the Hf composition increased. We found that when the RF power applied to the HfO2 target was 300 W, the HfInZnO TFT showed good electrical properties. We also investigated the stability of the device under positive bias stress in 50% ambient relative humidity and compared non-passivated and passivated HfInZnO TFTs. The stability was much better in passivated HfInZnO TFTs than in nonpassivated TFTs. These results can be understood by considering the effects of the back-channel surface properties. This suggests that a passivation layer for the back-channel surface, one that provides good barrier properties, is essential for the electrically stable HfInZnO TFTs.
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Acknowledgements This work was supported by MEST and DGIST. (Renewable Energy and Intelligent Robot Convergence Technology Development). We thank Dr. Bae of the Korea Basic Science Institute Busan Center for obtaining the XPS spectra. References [1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature 432 (2004) 488. [2] H.-H. Hsieh, T. Kamiya, K. Nomura, C.-C. Wu, H. Hosono, Appl. Phys. Lett. 92 (2008) 133503. [3] J.-S. Park, T.-W. Kim, D. Stryakhilev, J.-S. Lee, S.-G. An, Y.-S. Pyo, D.-B. Lee, Y.G. Mo, D.-U. Jin, H.K. Chung, Appl. Phys. Lett. 95 (2009) 013503. [4] S.-H.K. Park, C.-S. Hwang, M. Ryu, S. Yang, C. Byun, J. Shin, J.-I. Lee, K. Lee, M.S. Oh, S. Im, Adv. Mater. 21 (2009) 678. [5] J.-K. Jeong, J.-H. Jeong, H.-W. Yang, J.S. Park, Y.-G. Mo, H.-D. Kim, Appl. Phys. Lett. 91 (2007) 113505. [6] J.-S. Park, J.-K. Jeong, Y.-G. Mo, H.-D. Kim, S.-I. Kim, Appl. Phys. Lett. 90 (2007) 262106. [7] H.-H. Hsieh, C.-C. Wu, Appl. Phys. Lett. 91 (2007) 013502.
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[8] J.-S. Park, K.-S. Kim, Y.-G. Park, Y.-G. Mo, H.-D. Kim, J.-K. Jeong, Adv. Mater. 21 (2008) 329. [9] D.-H. Son, D.-H. Kim, J.-H. Kim, S.-J. Sung, E.-A. Jung, J.-K. Kang, Electrochem. Solid State Lett. 13 (2010) H274. [10] H.-S. Kim, K.-B. Park, K.-Seok Son, J.-S. Park, W.-J. Maeng, T.-S. Kim, K.-H. Lee, E.-S. Kim, J. Lee, J. Suh, J.-B. Seon, M.-K. Ryu, S.-Y. Lee, K. Lee, S. Im, Appl. Phys. Lett. 97 (2010) 102103. [11] D.-H. Cho, S. Yang, C. Byun, J. Shin, M.-K. Ryu, S.-H.K. Park, C.-S. Hwang, S.-M. Chung, W.-S. Cheong, S.-M. Yoon, H.-Y. Chu, Appl. Phys. Lett. 93 (2008) 142111. [12] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243. [13] T. Tan, Z. Liu, H. Lu, W. Liu, F. Yan, Vacuum 83 (2009) 1155. [14] B.-H. Lee, L. Kang, R. Nieh, W.-J. Qi, J.C. Lee, Appl. Phys. Lett. 96 (2000) 1926. [15] D.K. Schroder, Semiconductor Material and Device Characterization, third ed. Wiley, New York, 1998. [16] G.-H. Kim, H.-S. Kim, H.-S. Shin, B.-D. Ahn, K.-H. Kim, H.-J. Kim, Thin Solid Films 517 (2009) 4007. [17] E. Chong, K.-C. Jo, S.-Y. Lee, Appl. Phys. Lett. 96 (2010) 152102. [18] W.-H. Jeong, G.-H. Kim, H.-S. Shin, B.-D. Ahn, H.-J. Kim, M.-K. Ryu, K.-B. Park, J.-B. Seon, S.-Y. Lee, Appl. Phys. Lett. 96 (2010) 093503. [19] E. Chong, K.-C. Jo, S.-Y. Lee, Appl. Phys. Lett. 96 (2010) 152102. [20] R.B.M. Cross, M.M. De Souza, Appl. Phys. Lett. 89 (2006) 263513. [21] J.-K. Jeong, H.-W. Yang, J.-H. Jeong, Y.-G. Mo, H.-D. Kim, Appl. Phys. Lett. 93 (2008) 123508.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of hafnium addition on the electrical properties of indium zinc oxide thin film transistors Dae-Ho Son, Dae-Hwan Kim ⁎, Jung-Hye Kim, Shi-Joon Sung, Eun-Ae Jung, Jin-Kyu Kang Public & Original Technology Research Center, Daegu Gyeongbuk Institute of Science & Technology, Daegu Technopark Venture 1, 711 Hosan-dong, Dalseo-gu 704-230, Republic of Korea
a r t i c l e
i n f o
Available online 21 April 2011 Keywords: Metal oxide thin film transistor HfInZnO High-k material
a b s t r a c t This study reports the performance and stability of hafnium–indium zinc oxide (HfInZnO) thin film transistors (TFTs) with thermally grown SiO2. The HfInZnO channel layer was deposited at room temperature by a cosputtering system. We examined the effects of hafnium addition on the X-ray photoelectron spectroscopy properties and on the electrical characteristics of the TFTs varying the concentration of the added hafnium. We found that the transistor on–off currents were greatly influenced by the composition of hafnium addition, which suppressed the formation of oxygen vacancies. The field-effect mobility of optimized HfInZnO TFT was 1.34 cm2 V−1 s−1, along with an on–off current ratio of 108 and a threshold voltage of 4.54 V. We also investigated the effects of bias stress on HfInZnO TFTs with passivated and non-passivated layers. The threshold voltage change in the passivated device after positive gate bias stress was lower than that in the non-passivated device. This result indicates that HfInZnO TFTs are sensitive to the ambient conditions of the back surface. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide semiconductors have attracted considerable attention as semiconductor channels in thin film transistors (TFTs) because of their relatively high scalability, uniform structure, low-temperature processing, high mobility, and reasonably high on/off ratio [1,2]. TFTs with a ZnO-based semiconductor as the active layer have already been developed for use in active-matrix organic light-emitting diodes (AMOLEDs) [3,4]. In particular, indium–gallium–zinc oxide (InGaZnO) TFTs have been suggested for deriving future display devices with high performance characteristics in AMOLED [5–7]. Furthermore, a few research groups have reported TFTs substituting different metal materials for Ga including ZrInZnO, HfInZnO, and AlZnSnO [8–11]. The ZrInZnO and HfInZnO thin films were fabricated using zirconium and hafnium, respectively, both of which are metals with high dielectric constants (high-k). Generally, high-k metal oxide materials such as ZrO2, Y2O3, and HfO2 have been widely investigated and employed as gate insulators of transistors because a thicker film can be utilized to reduce the gate leakage current while maintaining the same gate capacitance. The high-k metal oxide materials are fabricated at low temperature and have many desirable properties such as high dielectric constant, and a relatively large band gap is fabricated [12–14].
⁎ Corresponding author. Tel.: + 82 53 430 8429; fax: + 82 53 430 8475. E-mail address: [email protected] (D.-H. Kim). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.079
Recently, high-k oxide materials have been used in other applications. Specifically, they can be used for the fabrication process of metal oxide TFTs because they suppress carrier generation via oxygen vacancy formation [8,9]. The metal oxide TFTs, by using high-k material, have many advantages such as low cost of fabrication and good properties of devices. However, currently there have been only few reports on the stability of new metal oxide TFTs that contain highk oxide materials in the active layer. In this study, we have fabricated TFTs using a hafnium indium zinc oxide (HfInZnO) thin film as the active channel layer and have investigated their stability. We found that the ambient effect on HfInZnO surfaces significantly influenced the shift in the threshold voltage of the TFTs. 2. Experimental details Fig. 1(a) shows a schematic cross-sectional view of the HfInZnO TFT structure in the coplanar geometry on a thermally grown SiO2 gate dielectric. As shown in Fig. 1(b), an HfInZnO active channel layer approximately 20 nm thick was deposited by co-sputtering onto a SiO2 film (100 nm in thickness) used as the gate insulating layer. The HfInZnO thin film was deposited on the SiO2/Si substrate using a magnetron co-sputtering system with two cathodes (DC/RF), during which the substrate was not heated. InZnO (In2O3:ZnO = 90:10 wt.%, 4 in) and HfO2 targets (99.99%, 4 in) were mounted on the DC and RF cathodes, respectively, and the composition of the active layer film was controlled by the power ratio of the cathodes. The DC cathode was maintained at a voltage of 400 V and a current of 0.5 A, and the power
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Fig. 1. (a) Schematic structure of HfInZnO TFT and (b) a cross-sectional TEM image of co-sputtered HfInZnO thin film annealed at 300 °C. The upper inset shows the FFT of HfInZnO layer in amorphous region. The lower left and right inset shows enlarged image and the FFT of HfInZnO layer in crystal region.
of the RF cathode was increased from 200 to 400 W. Co-sputtering was carried out at a gas mixing ratio of Ar/O2 = 120/25 sccm and a total sputtering pressure of 5 mT. The post-annealing process was performed in a vacuum furnace at 300 °C. Al source and drain contacts 100 nm thick were vapor-deposited on the active layer through a shadow mask on the fabricated non-passivated HfInZnO TFTs. We also applied a photolithography process to measure the stability of the HfInZnO TFTs. The active area of the TFTs was patterned by conventional photolithography and a wet etching process. After forming the active layer, aluminum deposition was carried out using a thermal evaporator followed by a lift-off process to form the source and drain electrodes. The length and width of the channel were 40 and 1000 μm, respectively. Finally, the TFT devices were passivated with a photoresist (AZ-7210). The electrical characteristics of the HfInZnO TFTs were measured using a Keithley 4200-SCS semiconductor parameter analyzer. 3. Results and discussion We fabricated non-passivated HfInZnO TFTs and measured their electrical characteristics. Fig. 2 shows the drain current–gate voltage (IDS–VGS) characteristics of HfInZnO TFTs with VDS = 20 V. The RF power applied to HfInZnO TFTs whose active layers were of the same thickness was varied between 200 and 350 W. Except at an RF power of 200 W, all co-sputtered HfInZnO TFTs were operated in an n-channel enhancement-mode. The device characteristics of the HfInZnO TFTs,
extracted from the results shown in Fig. 2, are summarized in Table 1. Electrical parameters including the saturation field-effect mobility and the threshold voltage, were derived from a linear fit to a plot of the square root of the drain current versus the gate voltage. The following equation is the general expression for the operation of a field-effect transistor in the saturation region: [15] IDS =
WCi μ sat 2 ðVGS −Vth Þ 2L
for VDS N VGS −Vth
ð1Þ
where W is the channel width, L is the channel length, Ci is the capacitance per unit area of the gate insulator, VG is the gate voltage, ID is the drain-to-source current, and Vth is the threshold voltage of the TFT. When the RF power applied to the HfO2 target is 200 W, the HfInZnO TFT show large on-currents, but off-current are also large, which makes on–off current ratio small. With decreasing power of the HfO2 target, the off-current in the transfer characteristics of the HfInZnO TFT increases. Although the on-current decreases with increasing RF power to the HfO2 target, the on–off current ratio of HfInZnO TFTs shows a distinct improvement and the turn-on-voltage, Von, is shifted to a positive voltage from −1.4 to 5.7 V. Concerning the variation of the on–off current ratio and Von, these surely are related with the carrier density of the active layer. These results show that the free electron concentration in the HfInZnO channel layer changed with the RF power applied to the target. This variation was due to the decreased electron carrier concentration in HfInZnO thin films with increasing power of the HfO2 target because elemental Hf characteristically results in the suppression of carrier generation by reducing the oxygen vacancy formation in the HfInZnO matrix system. In other words, the free carrier concentration of HfInZnO thin film can be controlled by the Hf composition ratio. The Hf atomic concentration in the HfInZnO thin film is minimized when the RF power applied to the HfO2 target is 200 W, which is supported by X-ray photoelectron spectroscopy (XPS) measurements. After the carbon contamination of the HfInZnO thin film was removed by Ar+ sputtering, XPS analysis Table 1 Electrical properties of HfInZnO TFTs with various levels of RF power applied to HfO2.
Fig. 2. Transfer characteristics of HfInZnO TFTs for different values of RF power applied to the HfO2 target at a VDS of 20 V.
RF Power [W]
μsat (cm2/Vs)
Von (V)
On–off current ratio
Vth
200 250 300 350
– 4.4 2.1 0.6
– −1.4 0.1 5.5
– 1.9 × 107 4.7 × 107 8.3 × 106
3.3 6.4 15.2
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was carried out to quantitatively establish the chemical properties of the HfInZnO thin films, the detailed descriptions and the atomic concentration of which are described in a previous publication [9]. Fig. 3 shows (a) Hf 4f, (b) In 3d, (c) Zn 2p, and (d) O 1s peaks of the XPS spectra for Hf–In–Zn–O films deposited and vacuum-annealed at 300 °C when the RF power applied to the HfO2 target was 300 W. The Hf 4f doublet appears at 16.3 (Hf 4f7/2) and 17.8 (Hf 4f5/2) eV and originates from Hf–O bonds [13]. The In 3d5/2 peak centered at 444.6 eV, and the Zn 2p3/2 peak centered at 1021.8 eV, originate from oxygen-bound In and Zn, respectively [16]. The O 1s peak of HfInZnO can be fitted by two nearly Gaussian distributions, the details of which are also explained in a previous publication [9]. In previous work, we examined the O 1s peaks of HfInZnO that were obtained for the HfInZnO thin film as a function of RF power. The results show that the increase in the area ratio higher binding peaks in the O 1s peak indicates that the oxygen vacancies increased with decreasing RF power. In addition, the HfInZnO O 1s main peak was slightly shifted from 529.9 eV to 530.2 eV with the increasing power of the HfO2 target. We also investigated the Hf, In and Zn core levels of HfInZnO thin film with RF powers of the HfO2 target. The inset in Fig. 3 shows the XPS peaks for core levels obtained for the HfInZnO thin film as a function of RF power. The central peak of the binding energy for Hf 4f slightly shifted to a higher energy level as the RF power applied to the HfO2 target increased. These results indicate an increase in the binding energy with oxygen due to the effect of element Hf as carrier suppressor. This phenomenon is similar to other reported results [17]. In other words, because the added element Hf can bond with oxygen, the oxygen deficiency of the HfInZnO thin film decreases. These XPS results and TFT properties indicate a role for Hf in the HfInZnO thin film. In other words, Hf ions form stronger chemical bonds with oxygen than do In or Zn ions because of their high electronegativity
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(Hf = 1.3) in HfInZnO thin films. Hence, Hf ion play a role in suppressing the formation of oxygen vacancies [18,19]. Fig. 4(a) shows the transfer characteristics of the non-passivated HfInZnO TFT after annealing treatment at 300 °C in ambient vacuum for 1 h. The active layer was deposited when the RF power applied to the HfO2 target was 300 W; the power applied to the InZnO target was fixed at 400 V/0.5 A. The saturation field-effect mobility was 1.34 cm2 V−1 s−1; the on–off current ratio was ~ 1.1 × 108; the threshold voltage was 4.54 V; and the subthreshold swing was about 980 mV/dec. These values are better than those for a-Si TFTs. Fig. 4(b) shows the output characteristics of the non-passivated HfInZnO TFT after annealing treatment at 300 °C in ambient vacuum for 1 h. The output characteristics exhibited strong saturation and a clear pinchoff. We also observed the state of the co-sputtered HfInZnO thin film through TEM and fast Fourier transformation (FFT) analysis, as shown in Fig. 1(b). The structural characterization of HfInZnO thin film shows a partially crystalline state in an amorphous phase. The locally crystalline feature of the thin film was verified in the magnified part image of the right rectangular region and FFT analysis of the slightly luminescent spots in the bright halo in Fig. 1(b) inset. We also check the InZnO thin film as compared to the HfInZnO thin film. There are different states between HfInZnO and InZnO thin films. The approximately 10 nm InZnO thin film shows an amorphous state in the TEM analysis (not shown here). We think that one of the possible reasons for this is the high RF power of the HfO2 target during the deposition of HfInZnO thin film. The increase in RF power creates a crystalline state probably because of ion bombardment. To evaluate the stability of HfInZnO TFTs that were fabricated at 300 W, the power of the applied HfO2 target and the DC cathode were maintained at a voltage of 400 V and a current of 0.5 A respectively; we compared the non-passivated HfInZnO TFT with a conventional
Fig. 3. The (a) Hf 4f, (b) In 3d, (c) Zn 2p, and (d) O 1s peaks of core level XPS spectra at the indicated RF power of HfO2 for co-sputtered HfinZnO film.
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Fig. 4. (a) Transfer and (b) output characteristics of HfInZnO TFTs annealed at 300 °C. The active layer was deposited when the RF power applied to the HfO2 target was 300 W.
photo-resist passivated HfInZnO TFT. Fig. 5(a) shows a microscope image of a photo-resist passivated HfInZnO TFT viewed from above. The photo-resist was coated by the spin-coating method, and the passivated HfInZnO TFT was cured at 120 °C for 1 h in air. The stability of the HfInZnO TFT was measured under harsh conditions; a gate bias stress of VGS = 30 V for varying time periods in 50% ambient relative humidity. Fig. 5(b) and (c) show changes in the transfer characteristics of HfInZnO TFT in the absence and presence, respectively, of a passivation layer during gate bias stress. The Von of the nonpassivated HfInZnO TFT shifted significantly in the positive direction from −0.5 to 9.3 V after 3996 s of stress. There are many potential causes for this device's instability, such as charge trapping inside the dielectric or at the interface, defect state creation within the channel layer, or ambient interactions [20,21]. Among these reasons, the backchannel effect by ambient is considered as one of the most critical problem in our device structure. When we measured the stability of the photo-resist passivated HfInZnO TFT in blocking the ambient effect, the Von of the passivated HfInZnO TFT shifted from −0.6 V to 4 V after 29970 s of stress. Although photo-resist passivation definitely improved the gate bias stability, the bias stress properties of the photo-resist passivated HfInZnO TFT were still unstable. The enhanced stability of the device with photo-resist passivation may be due to a barrier material that cannot perfectly protect against ambient from the back-channel surface of the HfInZnO TFT. It is therefore necessary to find a material for the passivation layer, a material that will protect from the ambient such as oxygen and water vapor in metal oxide TFTs. 4. Conclusions In this study, we have fabricated ZnO-based TFTs using SiO2/p-Si substrates on which a HfInZnO channel layer was deposited by a
Fig. 5. (a) Optical microscope image (top view) of a photo-resist passivated HfInZnO TFT, and transfer curve shifts of non-passivated (b) and photo-resist passivated (c) HfInZnO TFTs under a 30 V gate bias voltage in 50% ambient relative humidity.
co-sputtering system at different RF powers applied to HfO2. The on–off current ratio of the HfInZnO TFTs was improved because the free carrier concentration in HfInZnO channel layer decreased as the Hf composition increased. We found that when the RF power applied to the HfO2 target was 300 W, the HfInZnO TFT showed good electrical properties. We also investigated the stability of the device under positive bias stress in 50% ambient relative humidity and compared non-passivated and passivated HfInZnO TFTs. The stability was much better in passivated HfInZnO TFTs than in nonpassivated TFTs. These results can be understood by considering the effects of the back-channel surface properties. This suggests that a passivation layer for the back-channel surface, one that provides good barrier properties, is essential for the electrically stable HfInZnO TFTs.
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