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Highly transparent IGZO-TFTs uses IGZO source and drain electrodes with a composite insulation layer structure
Optik - International Journal for Light and Electron Optics 204 (2020) 163654
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Highly transparent IGZO-TFTs uses IGZO source and drain electrodes with a composite insulation layer structure
T
Lei Zhang*, Jiabang Wei, Kangjian Zhou, Chen Wan, Hang Sun University of Electronic Science and Technology of China, 610054, Chengdu, China
A R T IC LE I N F O
ABS TRA CT
Keywords: IGZO-TFTs IGZO Source/Drain electrodes Composite insulation layer Oxygen partial pressure
In this paper, the characteristic that the proper oxygen partial pressure during sputtering can change IGZO from semiconductor to conductor was used to fabricate IGZO-TFTs. Firstly, in order to determine the optimum parameters for the preparation of IGZO thin films as active layer and source/drain electrode, the influence of oxygen partial pressure on the conductivity and transmittance of IGZO thin films prepared by magnetron sputtering was studied, then a highly transparent IGZO-TFT device based on IGZO source/drain electrode was fabricated, in which an ultra-thin layer of atomic layer deposited (ALD) alumina under the original PMMA insulation layer was added to make a composite insulation layer structure. The mobility of the device is 6.07 cm2/Vs, the switching ratio is 0.87 × 104, threshold voltage and sub-threshold swing are 2.6 V and 3.5 V/decade, respectively. And the transmittance of the device in the visible light range is more than 80%, showing a good optical transmittance.
1. Introduction IGZO is a typical transparent n-type oxide semiconductor material doped with In2O3, Ga2O3 and ZnO. Its band gap is about 3.5 eV. In 2004, Nomura et al. [1] prepared IGZO as a channel layer thin film transistor for the first time with a carrier mobility of 8 cm2/Vs, a threshold voltage of 1.6 V, and a switching ratio of 103. For many flat panel display devices, transparent conducting oxide (TCO) semiconductor thin films must be used as electrodes. Although ITO thin films prepared by magnetron sputtering have been applied in most transparent thin film transistors [2–7], the cost of ITO's main material, indium, is high and resources are scarce. So materials that can replace ITO should be study [8–10]. Arun Suresh [11] prepared IGZO (In2O3: Ga2O3: ZnO = 1:1:10 mol %) film by pulsed laser deposition. The transmittance of the film in visible light is about 80%, the highest mobility is about 16 cm2/Vs, and the highest carrier concentration is about 1 × 1019 cm−3. This study also shows that the partial pressure of oxygen can change the conductivity of the film. When the partial pressure of oxygen is low, the film has better conductivity. When the partial pressure of oxygen increases, the conductivity decreases. In 2013, Wu et al. [12] prepared a high-performance TFT device based on IGZO as a source/drain electrode and a channel layer. The device has a carrier mobility of 18 cm2/Vs and a subthreshold swing of 30 mV/decade. The threshold voltage is 0.3 V, the switching ratio is up to 108, and through experiments they found that rapid thermal annealing can make the IGZO film exhibit the characteristics of the conductor, and the annealing operation has little effect on the transmittance of the film, so the IGZO thin film can be used as a transparent electrode. Because of the flexibility of IGZO thin films, a-IGZO TFT is developing towards the direction of transparency and flexibility.
⁎
Corresponding author. E-mail address: [email protected] (L. Zhang).
https://doi.org/10.1016/j.ijleo.2019.163654 Received 12 July 2019; Received in revised form 15 October 2019; Accepted 15 October 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 204 (2020) 163654
L. Zhang, et al.
Fig. 1. Structure of the IGZO-TFTs.
Traditional inorganic insulation materials such as SiO2 are not enough to meet such requirements. Thus, organic insulation materials like polymethyl methacrylate (PMMA) are beginning to enter our vision. When PMMA acts as an insulation layer, the roughness and defects of the films decrease. In this paper, the effect of oxygen partial pressure on the conductivity and transmittance of IGZO films prepared by magnetron sputtering was studied. Then, IGZO-TFTs based on the IGZO source drain electrode and PMMA single insulation layer/ PMMA + ALD composite insulation layer was fabricated and tested. 2. Experimental details Firstly, several IGZO films with different processes were prepared. The conductivity of these films was tested by four-probe tester. Then the surface morphology of these films was tested by AFM. Finally, the transmittance of these devices in the wavelength range of 300–1000 nm was measured by Shimadzu UV-1700 ultraviolet spectrophotometer. The effects of different sputtering oxygen partial pressures on the photoelectric properties of IGZO thin films were investigated and the optimal parameters for active layers and source/drain electrodes of IGZO thin films were determined. After that, a highly transparent TFT based on IGZO as an active layer and a transparent electrode were prepared. The structure of the fabricated device is shown in the Fig. 1. The device used a 100 nm thick transparent ITO glass substrate as a transparent substrate for the TFT device and a transparent gate. Then, a layer of ultra-thin ALD alumina was added, with a thickness of 45 nm, after that, a PMMA transparent film with a thickness of 380 nm at a rotation speed of 2500 r/min was spin-coated. The layers of Al2O3 and PMMA formed the composite insulation layer structure of the TFT device. And a 260 nm thick transparent IGZO film was sputtered by a magnetron sputtering method as a channel layer of the TFT device. The sputtering power was 180 W, the oxygen partial pressure was 2%, the sputtering chamber pressure was 2 mTorr, and the sputtering time was 600 s. The aspect ratio (W/L) of the channel layer was 50/1, W = 10 mm and L = 0.2 mm. Then, the IGZO transparent electrode of the device was prepared by sputtering, the sputtering power was 125 W, the sputtering oxygen partial pressure was 0%, the sputtering chamber pressure was 1 mTorr, the sputtering time was 1500s, and the thickness was about 150 nm. 3. Results and discussion The conductivity test results of the thin films samples No. 1–5 that prepared with different sputtering oxygen partial pressures of 0%, 1%, 2%, 3%, and 4% are shown in the Table 1. It can be seen from the table that as the partial pressure of oxygen increases during the sputtering process, the electrical conductivity of the film gradually decreases. When the oxygen partial pressure is 0%, the resistivity of the a-IGZO film is 1.8 × 10−3 Ω m. In the process, only argon gas is introduced during the sputtering, the covalent bond between the oxygen ions and the metal ions in the oxide film is caused by the bombardment of argon ions, resulting in the cleavage of the covalent bond. In the process, oxygen atoms are segregated, many oxygen vacancies are left in the oxide to form free carriers, so the film resistivity at this time is very low. [13] When the oxygen partial pressure increased to 1%, 2%, the resistivity of the film is 1 × 10-1 Ω m, which is nearly 50 times Table 1 Conductivity test result of IGZO thin films. Sample
Resistance (MΩ)
Conductivity (S/m)
Resistivity(Ω m)
1 2 3 4 5
0.03 1.68 130 1600 3000
5.6 × 102 10 1.3 × 10−1 1 × 10−2 5.6 × 10−3
1.8 × 10−3 1 × 10−1 7.8 9.6 × 101 1.8 × 102
2
Optik - International Journal for Light and Electron Optics 204 (2020) 163654
L. Zhang, et al.
Fig. 2. Resistivity of IGZO thin films.
higher than that in the case of no oxidation. The reason is that oxygen is decomposed into oxygen atoms during sputtering, and most of the oxygen atoms fill the oxygen vacancies in the film. As a result, the number of free carriers of the film is drastically lowered, so that the resistivity of the film is increased. When the oxygen partial pressure of sputtering increased to 4%, the resistivity of the film is 1.8 × 102 Ω m, because the oxygen vacancies in the film are mostly occupied by oxygen atoms, the number of free carriers is greatly reduced and the resistivity increased, the electrical conductivity is thus reduced and nearly saturated, and the film exhibited the characteristics of an insulator. It can be seen from Fig. 2 that the conductivity of IGZO films is decreasing when the oxygen partial pressure increasing, which is consistent with the conclusion in the article by Yabuta et al [14]. Next, we performed AFM topography tests on samples 1, 3 and 5. The test results are shown in Fig. 3. The roughness of sample No. 1, No. 3 and No. 5 is 5.44 nm, 0.33 nm and 1.05 nm. It can be seen from the topography that when the partial pressure of oxygen is 0%, there is a slight island-like structure on the surface of the film. When the partial pressure of oxygen reaches 2% or more, the island surface of the film is gradually reducing, and almost no obvious bulge is observed. It is because when the oxygen partial pressure is 0%, only argon gas is introduced during the sputtering process, and the covalent bond between the oxygen ions and the metal ions on the surface of the film is broken by the high-energy argon ion bombardment, and agglomeration occurs in the released Ga ions and In ions, resulting in an island structure in the topography. As the oxygen partial pressure increases, the oxygen in the sputtering gas will meet with the metal ions bombarded by the argon ions to form a covalent bond to deposit on the surface of the film. Oxygen will fill the oxygen vacancies on the surface of the film. Therefore, the agglomeration phenomenon between metal ions is reduced, so the flatness of the surface of the film will be improved. [15]. However, when the oxygen partial pressure is higher than 2%, the surface roughness of the film increased because the oxidation of the film material is already sufficiently sufficient, thereby suppressing the movement of the metal ions, so that the defects cannot be better filled. The oxygen partial pressure during the sputtering process of a-IGZO film affected not only the electrical properties, but also the surface morphology of the film. Appropriate increase of oxygen partial pressure can increase the resistivity of the film, and can also make the film surface more smoothing, so in this paper, a-IGZO thin film with oxygen partial pressure of 2% was used as the active layer of the TFT for subsequent research. For source/drain electrodes of TFTs, conductivity and transparency are concerned. IGZO thin fims with oxygen partial pressures of 0%,1% 2% and 3% were prepared using a magnetron sputtering apparatus at a sputtering power of 150 W, a cavity pressure of 2 mTorr, a sputtering time of 600 s, a target pitch of 10 mm, and a substrate rotation speed of 15 r/min. IGZO film samples were measured using a Shimadzu UV-1700 UV spectrophotometer for transmittance at different oxygen partial pressures, where the transmittance range was 300–1100 nm. Fig. 4 shows the relationship between the transmittance and the wavelength of an IGZO film prepared at 0%, 1%, 2%, and 3% oxygen partial pressure. It can be seen that the general trend of the transmittance curves of the devices prepared under different
Fig. 3. Results of AFM topography tests of IGZO thin films. (a) Oxygen partial pressure was 0%. (b) Oxygen partial pressure was 2% (c) oxygen partial pressure was 4%. 3
Optik - International Journal for Light and Electron Optics 204 (2020) 163654
L. Zhang, et al.
Fig. 4. Transmittance of IGZO films with different oxygen partial pressure under the wavelength.from 300 nm to 1100 nm.
oxygen partial pressures are similar, and the IGZO film has a strong absorption in the ultraviolet wavelength range, resulting in a low transmittance of the film [16], but in the visible range, the average transmittance of the film is above 80%, showing high transparency. As can be seen from Table 2, the average transmittance of No.1 device reached 84.49%, and the No.3 device with the lowest average transmittance also reached 80.19%, which indicates that a-IGZO material can be applied to high transparency display screen. The a-IGZO has a high transmittance in the visible range because the material itself has a high forbidden band width and the forbidden band width is about 3.2 eV. That is to say, when the photon energy of the a-IGZO film is less than 3.2 eV (electron volts), that is, the wavelength is longer than 387.5 nm, the energy does not allow electrons to jump from the valence band to the conduction band, so the a-IGZO film in the visible range have a high transmittance. When the incident photon energy is higher than 3.2 eV, the incident light wavelength is less than 387.5 nm, the energy of the incident light is relatively large, and the kinetic energy of the valence band electron absorption in the wide band gap material of a-IGZO is enough high to jump to the conduction band. So the aIGZO film has a strong absorption in ultraviolet light. The above data shows that when the oxygen partial pressure is 0%, the obtained IGZO film has a low resistivity and a high transmittance, so when IGZO is used as a transparent source/drain electrode, the optimum process parameter of oxygen partial pressure is 0%. After determining the optimum parameters of active layer and source-drain electrode for TFT devices, we fabricated two IGZOTFT devices, one with a single insulation layer structure (PMMA) and the other with a composite insulation layer structure (PMMA + Al2O3). As shown in the Figs. 5 and 6, the transfer characteristic curves and output characteristic curves of single insulation layer device and composite insulation layer device are compared. When testing the transfer characteristic curve, the drain voltage VDS was set to 15 V, the scan range of gate voltage VGS was set to 0–40 V, and the step size was set to 0.2 V. When testing the output characteristic curve, the source-drain voltage VDS setting range was 0–10 V, the gate voltage VGS setting range was 0–20 V, and the voltage step was 5 V. It can be seen that the on-state currents of both are above 10−3 A, but the off-state current of the TFT device of a single PMMA insulation layer is relatively high, 1.99 × 10−6 A, and a layer of ALD oxidation is added, the off-state current of composite insulation layer structure TFT device is 2.33 × 10−7 A, which is an order of magnitude lower than that of a single insulation layer device. The composite insulation layer device can effectively reduce the off-state current of the device, thereby improving the switching ratio and reducing the power consumption of the device. This is mainly because the atomic layer deposited aluminum oxide film is very dense and has few defects. It can be seen from the figure that as the gate voltage rises, the leakage current first increases linearly and then tend to be saturated, indicating that both devices are prepared N type TFT. From the saturation current of the output, the composite insulation layer device has a tendency to saturate when the gate voltage was 10 V, and the drain current of the single insulation layer device does not approach saturation. Because there are many defects near the channel layer of single insulation layer device, which results in a large off-state current. When the gate voltage increases, the influence of gate leakage current becomes obvious, and there exists reverse leakage current. It is shown that the output curve does not intersect with the origin and is far from the origin with the increase of gate voltage when testing the output characteristic curve. On the contrary, the composite insulation layer device has less
Table 2 Transmittance of samples with different oxygen partial pressure. Sample
Oxygen partial pressure (%)
Average transmittance in visible range (%)
1 2 3 4
0 1 2 3
84.49 82.64 80.19 81.32
4
Optik - International Journal for Light and Electron Optics 204 (2020) 163654
L. Zhang, et al.
Fig. 5. (a) The transfer characteristic curve of single insulation layer device. (b) The output characteristic curve of single insulation layer device.
Fig. 6. (a) The transfer characteristic curve of composite insulation layer device. (b) The output characteristic curve of composite insulation layer device.
defects, indicating that the surface morphology of the PMMA film spin-coated on the atomic layer deposited film is better, so that the surface morphology is better. According to the transfer characteristic curve and output characteristic curve of the two devices, various performance parameters of the device were calculated. The calculation results are shown in Table 3. The Saturated carrier mobility of a single PMMA insulation device and a composite insulation device is 4.10 cm2/Vs and 6.07 cm2/Vs, indicating that the channel quality of a composite insulation device is better than that of a single insulation device. The PMMA film spin-coated on the surface of the alumina has fewer defects, which reduces the probability of carriers being trapped by traps in the active layer film, so the mobility of the device is improved. From the switch ratio point of view, since the composite insulation layer effectively reduces the off-state current of the device, the switching ratio of the device increased by an order of magnitude. From the threshold voltage, the composite insulation layer device is reduced by 0.5 V compared to the single insulation layer device. The composite insulation layer device has better channel layer quality and a larger number of carriers. In comparison. The smaller gate voltage induces the same free electrons inside the channel, causing the transfer characteristic curve of the composite insulation layer device to shift in the negative direction of VGS with respect to the single insulation layer device transfer characteristic curve, and thus the threshold voltage is lower. From the subthreshold swing, the composite insulation device has a lower subthreshold swing, indicating that the current response speed of the device is faster and the stability is higher. The transmittance of the device was measured by a visible-ultraviolet spectrophotometer, and the wavelength range was set to 300–1000 nm, and the transmittance curve is shown in the Fig. 7. The absorption of ultraviolet light in the device is very strong, but the transmittance in the visible light range is more than 80%, showing a good optical transmittance, indicating that the prepared IGZO electrode TFT has high transparency and is suitable for transparent display field. The photograph of the fabricated IGZO-TFT is shown in the inset of Fig. 7. In summary, IGZO, as a commonly used semiconductor material, can reduce the resistivity of the film by changing the process parameters in the preparation process, and the transmittance of the film is high, so that the transparent electrode can be realized instead of the conventional metal electrode. The composite insulation layer device can effectively reduce the off-state current of the Table 3 The corresponding performance parameter values of single/composite insulation layer device. Insulation type PMMA Al2O3 + PMMA
Mobility (cm2/Vs) 4.10 6.07
Witch ratio 0.696 × 10 0.87 × 104
3
5
Threshold voltage (V)
Sub-threshold swing (V/decade)
3.1 2.6
6.4 3.5
Optik - International Journal for Light and Electron Optics 204 (2020) 163654
L. Zhang, et al.
Fig. 7. The transmittance of the IGZO-TFT with composite insulation layer, inset is the photograph of the fabricated IGZO-TFT.
device and improve the switching ratio of the device. At the same time, the surface defects of the composite insulation layer are less, the quality of the subsequently deposited active layer is better, the saturated carrier mobility of the device is higher, and the reduction of the threshold voltage and the subthreshold swing also prove that the composite insulation layer device can be improved. The current response speed increases the stability of the device and reduces the power consumption of the device. References [1] K. Nomura, H. Ohta, A. Takagi, et al., Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors, Nature 432 (7016) (2004) 488–492. [2] P.F. Carcia, R.S. Mclean, M.H. Reilly, et al., Transparent ZnO thin-film transistor fabricated by rf magnetron sputtering, Appl. Phys. Lett. 82 (7) (2003) 1117–1119. [3] E.M.C. Fortunato, P.M.C. Barquinha, A.M.F. Gonçalves, et al., Fully Transparent ZnO Thin-film transistor produced at room temperature, Adv. Mater. 17 (5) (2010) 590–594. [4] Y.J. Tung, R. Hewitt, A. Chwang, et al., 49.3: a 200-dpi transparent a-Si TFT active-matrix phosphorescent OLED display, Sid Symp. Dig. Tech. Pap. 36 (1) (2012) 1546–1549. [5] K.L. Chopra, S. Major, D.K. Pandya, Transparent conductors-A status review, Thin Solid Films 102 (1) (1983) 1–46. [6] A.L. Da War, J.C. Joshi, Semiconducting transparent thin films: their properties and application, J. Mater. Sci. 19 (1) (1984) 1–23. [7] H.L. Hartnagel, A.L. Dawar, Semiconducting transparent thin films, Semiconduct. Transparent Thin Films (1995). [8] H. Sato, Highly transparent and conductive Zn2In2O5 thin films prepared by RF magnetron sputtering, J. Appl. Phys. 23 (9) (1986) L280–L282. [9] T. Minami, Highly conductive and transparent zinc oxide films prepared by RF magnetron sputtering under an applied external magnetic field, Appl. Phys. Lett. (1982) 41. [10] T. Minami, Transparent and conductive multicomponent oxide films prepared by magnetron sputtering, J. Vac. Sci. Technol. A Vac. Surf. Films 17 (4) (1999) 1765. [11] A. Suresh, P. Gollakota, P. Wellenius, et al., Transparent, high mobility InGaZnO thin films deposited by PLD, Thin Solid Films 516 (7) (2008) 1326–1329. [12] H.C. Wu, Highly Transparent, High-Performance IGZO-TFTs Using the Selective Formation of IGZO Source and Drain Electrodes, IEEE Electron Device Lett. 35 (6) (2014) 645–647. [13] H.C. Wu, C.H. Chien, High performance InGaZnO thin film transistor with InGaZnO source and drain electrodes, Appl. Phys. Lett. 102 (6) (2013) 062103. [14] H. Yabuta, M. Sano, K. Abe, et al., High-mobility thin-film transistor with amorphous InGaZnO4 channel fabricated by room temperature rf-magnetron sputtering, Appl. Phys. Lett. 89 (11) (2006) 112123-112123-3. [15] C.H. Jung, D.J. Kim, Y.K. Kang, et al., Transparent amorphous In-Ga-Zn-O thin film as function of various gas flows for TFT applications, Thin Solid Films 517 (14) (2009) 4078–4081. [16] Y. Wang, S.W. Liu, X.W. Sun, et al., Highly transparent solution processed In-Ga-Zn oxide thin films and thin film transistors, J. Solgel Sci. Technol. 55 (3) (2010) 322–327.
6
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Highly transparent IGZO-TFTs uses IGZO source and drain electrodes with a composite insulation layer structure
T
Lei Zhang*, Jiabang Wei, Kangjian Zhou, Chen Wan, Hang Sun University of Electronic Science and Technology of China, 610054, Chengdu, China
A R T IC LE I N F O
ABS TRA CT
Keywords: IGZO-TFTs IGZO Source/Drain electrodes Composite insulation layer Oxygen partial pressure
In this paper, the characteristic that the proper oxygen partial pressure during sputtering can change IGZO from semiconductor to conductor was used to fabricate IGZO-TFTs. Firstly, in order to determine the optimum parameters for the preparation of IGZO thin films as active layer and source/drain electrode, the influence of oxygen partial pressure on the conductivity and transmittance of IGZO thin films prepared by magnetron sputtering was studied, then a highly transparent IGZO-TFT device based on IGZO source/drain electrode was fabricated, in which an ultra-thin layer of atomic layer deposited (ALD) alumina under the original PMMA insulation layer was added to make a composite insulation layer structure. The mobility of the device is 6.07 cm2/Vs, the switching ratio is 0.87 × 104, threshold voltage and sub-threshold swing are 2.6 V and 3.5 V/decade, respectively. And the transmittance of the device in the visible light range is more than 80%, showing a good optical transmittance.
1. Introduction IGZO is a typical transparent n-type oxide semiconductor material doped with In2O3, Ga2O3 and ZnO. Its band gap is about 3.5 eV. In 2004, Nomura et al. [1] prepared IGZO as a channel layer thin film transistor for the first time with a carrier mobility of 8 cm2/Vs, a threshold voltage of 1.6 V, and a switching ratio of 103. For many flat panel display devices, transparent conducting oxide (TCO) semiconductor thin films must be used as electrodes. Although ITO thin films prepared by magnetron sputtering have been applied in most transparent thin film transistors [2–7], the cost of ITO's main material, indium, is high and resources are scarce. So materials that can replace ITO should be study [8–10]. Arun Suresh [11] prepared IGZO (In2O3: Ga2O3: ZnO = 1:1:10 mol %) film by pulsed laser deposition. The transmittance of the film in visible light is about 80%, the highest mobility is about 16 cm2/Vs, and the highest carrier concentration is about 1 × 1019 cm−3. This study also shows that the partial pressure of oxygen can change the conductivity of the film. When the partial pressure of oxygen is low, the film has better conductivity. When the partial pressure of oxygen increases, the conductivity decreases. In 2013, Wu et al. [12] prepared a high-performance TFT device based on IGZO as a source/drain electrode and a channel layer. The device has a carrier mobility of 18 cm2/Vs and a subthreshold swing of 30 mV/decade. The threshold voltage is 0.3 V, the switching ratio is up to 108, and through experiments they found that rapid thermal annealing can make the IGZO film exhibit the characteristics of the conductor, and the annealing operation has little effect on the transmittance of the film, so the IGZO thin film can be used as a transparent electrode. Because of the flexibility of IGZO thin films, a-IGZO TFT is developing towards the direction of transparency and flexibility.
⁎
Corresponding author. E-mail address: [email protected] (L. Zhang).
https://doi.org/10.1016/j.ijleo.2019.163654 Received 12 July 2019; Received in revised form 15 October 2019; Accepted 15 October 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 204 (2020) 163654
L. Zhang, et al.
Fig. 1. Structure of the IGZO-TFTs.
Traditional inorganic insulation materials such as SiO2 are not enough to meet such requirements. Thus, organic insulation materials like polymethyl methacrylate (PMMA) are beginning to enter our vision. When PMMA acts as an insulation layer, the roughness and defects of the films decrease. In this paper, the effect of oxygen partial pressure on the conductivity and transmittance of IGZO films prepared by magnetron sputtering was studied. Then, IGZO-TFTs based on the IGZO source drain electrode and PMMA single insulation layer/ PMMA + ALD composite insulation layer was fabricated and tested. 2. Experimental details Firstly, several IGZO films with different processes were prepared. The conductivity of these films was tested by four-probe tester. Then the surface morphology of these films was tested by AFM. Finally, the transmittance of these devices in the wavelength range of 300–1000 nm was measured by Shimadzu UV-1700 ultraviolet spectrophotometer. The effects of different sputtering oxygen partial pressures on the photoelectric properties of IGZO thin films were investigated and the optimal parameters for active layers and source/drain electrodes of IGZO thin films were determined. After that, a highly transparent TFT based on IGZO as an active layer and a transparent electrode were prepared. The structure of the fabricated device is shown in the Fig. 1. The device used a 100 nm thick transparent ITO glass substrate as a transparent substrate for the TFT device and a transparent gate. Then, a layer of ultra-thin ALD alumina was added, with a thickness of 45 nm, after that, a PMMA transparent film with a thickness of 380 nm at a rotation speed of 2500 r/min was spin-coated. The layers of Al2O3 and PMMA formed the composite insulation layer structure of the TFT device. And a 260 nm thick transparent IGZO film was sputtered by a magnetron sputtering method as a channel layer of the TFT device. The sputtering power was 180 W, the oxygen partial pressure was 2%, the sputtering chamber pressure was 2 mTorr, and the sputtering time was 600 s. The aspect ratio (W/L) of the channel layer was 50/1, W = 10 mm and L = 0.2 mm. Then, the IGZO transparent electrode of the device was prepared by sputtering, the sputtering power was 125 W, the sputtering oxygen partial pressure was 0%, the sputtering chamber pressure was 1 mTorr, the sputtering time was 1500s, and the thickness was about 150 nm. 3. Results and discussion The conductivity test results of the thin films samples No. 1–5 that prepared with different sputtering oxygen partial pressures of 0%, 1%, 2%, 3%, and 4% are shown in the Table 1. It can be seen from the table that as the partial pressure of oxygen increases during the sputtering process, the electrical conductivity of the film gradually decreases. When the oxygen partial pressure is 0%, the resistivity of the a-IGZO film is 1.8 × 10−3 Ω m. In the process, only argon gas is introduced during the sputtering, the covalent bond between the oxygen ions and the metal ions in the oxide film is caused by the bombardment of argon ions, resulting in the cleavage of the covalent bond. In the process, oxygen atoms are segregated, many oxygen vacancies are left in the oxide to form free carriers, so the film resistivity at this time is very low. [13] When the oxygen partial pressure increased to 1%, 2%, the resistivity of the film is 1 × 10-1 Ω m, which is nearly 50 times Table 1 Conductivity test result of IGZO thin films. Sample
Resistance (MΩ)
Conductivity (S/m)
Resistivity(Ω m)
1 2 3 4 5
0.03 1.68 130 1600 3000
5.6 × 102 10 1.3 × 10−1 1 × 10−2 5.6 × 10−3
1.8 × 10−3 1 × 10−1 7.8 9.6 × 101 1.8 × 102
2
Optik - International Journal for Light and Electron Optics 204 (2020) 163654
L. Zhang, et al.
Fig. 2. Resistivity of IGZO thin films.
higher than that in the case of no oxidation. The reason is that oxygen is decomposed into oxygen atoms during sputtering, and most of the oxygen atoms fill the oxygen vacancies in the film. As a result, the number of free carriers of the film is drastically lowered, so that the resistivity of the film is increased. When the oxygen partial pressure of sputtering increased to 4%, the resistivity of the film is 1.8 × 102 Ω m, because the oxygen vacancies in the film are mostly occupied by oxygen atoms, the number of free carriers is greatly reduced and the resistivity increased, the electrical conductivity is thus reduced and nearly saturated, and the film exhibited the characteristics of an insulator. It can be seen from Fig. 2 that the conductivity of IGZO films is decreasing when the oxygen partial pressure increasing, which is consistent with the conclusion in the article by Yabuta et al [14]. Next, we performed AFM topography tests on samples 1, 3 and 5. The test results are shown in Fig. 3. The roughness of sample No. 1, No. 3 and No. 5 is 5.44 nm, 0.33 nm and 1.05 nm. It can be seen from the topography that when the partial pressure of oxygen is 0%, there is a slight island-like structure on the surface of the film. When the partial pressure of oxygen reaches 2% or more, the island surface of the film is gradually reducing, and almost no obvious bulge is observed. It is because when the oxygen partial pressure is 0%, only argon gas is introduced during the sputtering process, and the covalent bond between the oxygen ions and the metal ions on the surface of the film is broken by the high-energy argon ion bombardment, and agglomeration occurs in the released Ga ions and In ions, resulting in an island structure in the topography. As the oxygen partial pressure increases, the oxygen in the sputtering gas will meet with the metal ions bombarded by the argon ions to form a covalent bond to deposit on the surface of the film. Oxygen will fill the oxygen vacancies on the surface of the film. Therefore, the agglomeration phenomenon between metal ions is reduced, so the flatness of the surface of the film will be improved. [15]. However, when the oxygen partial pressure is higher than 2%, the surface roughness of the film increased because the oxidation of the film material is already sufficiently sufficient, thereby suppressing the movement of the metal ions, so that the defects cannot be better filled. The oxygen partial pressure during the sputtering process of a-IGZO film affected not only the electrical properties, but also the surface morphology of the film. Appropriate increase of oxygen partial pressure can increase the resistivity of the film, and can also make the film surface more smoothing, so in this paper, a-IGZO thin film with oxygen partial pressure of 2% was used as the active layer of the TFT for subsequent research. For source/drain electrodes of TFTs, conductivity and transparency are concerned. IGZO thin fims with oxygen partial pressures of 0%,1% 2% and 3% were prepared using a magnetron sputtering apparatus at a sputtering power of 150 W, a cavity pressure of 2 mTorr, a sputtering time of 600 s, a target pitch of 10 mm, and a substrate rotation speed of 15 r/min. IGZO film samples were measured using a Shimadzu UV-1700 UV spectrophotometer for transmittance at different oxygen partial pressures, where the transmittance range was 300–1100 nm. Fig. 4 shows the relationship between the transmittance and the wavelength of an IGZO film prepared at 0%, 1%, 2%, and 3% oxygen partial pressure. It can be seen that the general trend of the transmittance curves of the devices prepared under different
Fig. 3. Results of AFM topography tests of IGZO thin films. (a) Oxygen partial pressure was 0%. (b) Oxygen partial pressure was 2% (c) oxygen partial pressure was 4%. 3
Optik - International Journal for Light and Electron Optics 204 (2020) 163654
L. Zhang, et al.
Fig. 4. Transmittance of IGZO films with different oxygen partial pressure under the wavelength.from 300 nm to 1100 nm.
oxygen partial pressures are similar, and the IGZO film has a strong absorption in the ultraviolet wavelength range, resulting in a low transmittance of the film [16], but in the visible range, the average transmittance of the film is above 80%, showing high transparency. As can be seen from Table 2, the average transmittance of No.1 device reached 84.49%, and the No.3 device with the lowest average transmittance also reached 80.19%, which indicates that a-IGZO material can be applied to high transparency display screen. The a-IGZO has a high transmittance in the visible range because the material itself has a high forbidden band width and the forbidden band width is about 3.2 eV. That is to say, when the photon energy of the a-IGZO film is less than 3.2 eV (electron volts), that is, the wavelength is longer than 387.5 nm, the energy does not allow electrons to jump from the valence band to the conduction band, so the a-IGZO film in the visible range have a high transmittance. When the incident photon energy is higher than 3.2 eV, the incident light wavelength is less than 387.5 nm, the energy of the incident light is relatively large, and the kinetic energy of the valence band electron absorption in the wide band gap material of a-IGZO is enough high to jump to the conduction band. So the aIGZO film has a strong absorption in ultraviolet light. The above data shows that when the oxygen partial pressure is 0%, the obtained IGZO film has a low resistivity and a high transmittance, so when IGZO is used as a transparent source/drain electrode, the optimum process parameter of oxygen partial pressure is 0%. After determining the optimum parameters of active layer and source-drain electrode for TFT devices, we fabricated two IGZOTFT devices, one with a single insulation layer structure (PMMA) and the other with a composite insulation layer structure (PMMA + Al2O3). As shown in the Figs. 5 and 6, the transfer characteristic curves and output characteristic curves of single insulation layer device and composite insulation layer device are compared. When testing the transfer characteristic curve, the drain voltage VDS was set to 15 V, the scan range of gate voltage VGS was set to 0–40 V, and the step size was set to 0.2 V. When testing the output characteristic curve, the source-drain voltage VDS setting range was 0–10 V, the gate voltage VGS setting range was 0–20 V, and the voltage step was 5 V. It can be seen that the on-state currents of both are above 10−3 A, but the off-state current of the TFT device of a single PMMA insulation layer is relatively high, 1.99 × 10−6 A, and a layer of ALD oxidation is added, the off-state current of composite insulation layer structure TFT device is 2.33 × 10−7 A, which is an order of magnitude lower than that of a single insulation layer device. The composite insulation layer device can effectively reduce the off-state current of the device, thereby improving the switching ratio and reducing the power consumption of the device. This is mainly because the atomic layer deposited aluminum oxide film is very dense and has few defects. It can be seen from the figure that as the gate voltage rises, the leakage current first increases linearly and then tend to be saturated, indicating that both devices are prepared N type TFT. From the saturation current of the output, the composite insulation layer device has a tendency to saturate when the gate voltage was 10 V, and the drain current of the single insulation layer device does not approach saturation. Because there are many defects near the channel layer of single insulation layer device, which results in a large off-state current. When the gate voltage increases, the influence of gate leakage current becomes obvious, and there exists reverse leakage current. It is shown that the output curve does not intersect with the origin and is far from the origin with the increase of gate voltage when testing the output characteristic curve. On the contrary, the composite insulation layer device has less
Table 2 Transmittance of samples with different oxygen partial pressure. Sample
Oxygen partial pressure (%)
Average transmittance in visible range (%)
1 2 3 4
0 1 2 3
84.49 82.64 80.19 81.32
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Optik - International Journal for Light and Electron Optics 204 (2020) 163654
L. Zhang, et al.
Fig. 5. (a) The transfer characteristic curve of single insulation layer device. (b) The output characteristic curve of single insulation layer device.
Fig. 6. (a) The transfer characteristic curve of composite insulation layer device. (b) The output characteristic curve of composite insulation layer device.
defects, indicating that the surface morphology of the PMMA film spin-coated on the atomic layer deposited film is better, so that the surface morphology is better. According to the transfer characteristic curve and output characteristic curve of the two devices, various performance parameters of the device were calculated. The calculation results are shown in Table 3. The Saturated carrier mobility of a single PMMA insulation device and a composite insulation device is 4.10 cm2/Vs and 6.07 cm2/Vs, indicating that the channel quality of a composite insulation device is better than that of a single insulation device. The PMMA film spin-coated on the surface of the alumina has fewer defects, which reduces the probability of carriers being trapped by traps in the active layer film, so the mobility of the device is improved. From the switch ratio point of view, since the composite insulation layer effectively reduces the off-state current of the device, the switching ratio of the device increased by an order of magnitude. From the threshold voltage, the composite insulation layer device is reduced by 0.5 V compared to the single insulation layer device. The composite insulation layer device has better channel layer quality and a larger number of carriers. In comparison. The smaller gate voltage induces the same free electrons inside the channel, causing the transfer characteristic curve of the composite insulation layer device to shift in the negative direction of VGS with respect to the single insulation layer device transfer characteristic curve, and thus the threshold voltage is lower. From the subthreshold swing, the composite insulation device has a lower subthreshold swing, indicating that the current response speed of the device is faster and the stability is higher. The transmittance of the device was measured by a visible-ultraviolet spectrophotometer, and the wavelength range was set to 300–1000 nm, and the transmittance curve is shown in the Fig. 7. The absorption of ultraviolet light in the device is very strong, but the transmittance in the visible light range is more than 80%, showing a good optical transmittance, indicating that the prepared IGZO electrode TFT has high transparency and is suitable for transparent display field. The photograph of the fabricated IGZO-TFT is shown in the inset of Fig. 7. In summary, IGZO, as a commonly used semiconductor material, can reduce the resistivity of the film by changing the process parameters in the preparation process, and the transmittance of the film is high, so that the transparent electrode can be realized instead of the conventional metal electrode. The composite insulation layer device can effectively reduce the off-state current of the Table 3 The corresponding performance parameter values of single/composite insulation layer device. Insulation type PMMA Al2O3 + PMMA
Mobility (cm2/Vs) 4.10 6.07
Witch ratio 0.696 × 10 0.87 × 104
3
5
Threshold voltage (V)
Sub-threshold swing (V/decade)
3.1 2.6
6.4 3.5
Optik - International Journal for Light and Electron Optics 204 (2020) 163654
L. Zhang, et al.
Fig. 7. The transmittance of the IGZO-TFT with composite insulation layer, inset is the photograph of the fabricated IGZO-TFT.
device and improve the switching ratio of the device. At the same time, the surface defects of the composite insulation layer are less, the quality of the subsequently deposited active layer is better, the saturated carrier mobility of the device is higher, and the reduction of the threshold voltage and the subthreshold swing also prove that the composite insulation layer device can be improved. The current response speed increases the stability of the device and reduces the power consumption of the device. References [1] K. Nomura, H. Ohta, A. Takagi, et al., Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors, Nature 432 (7016) (2004) 488–492. [2] P.F. Carcia, R.S. Mclean, M.H. Reilly, et al., Transparent ZnO thin-film transistor fabricated by rf magnetron sputtering, Appl. Phys. Lett. 82 (7) (2003) 1117–1119. [3] E.M.C. Fortunato, P.M.C. Barquinha, A.M.F. Gonçalves, et al., Fully Transparent ZnO Thin-film transistor produced at room temperature, Adv. Mater. 17 (5) (2010) 590–594. [4] Y.J. Tung, R. Hewitt, A. Chwang, et al., 49.3: a 200-dpi transparent a-Si TFT active-matrix phosphorescent OLED display, Sid Symp. Dig. Tech. Pap. 36 (1) (2012) 1546–1549. [5] K.L. Chopra, S. Major, D.K. Pandya, Transparent conductors-A status review, Thin Solid Films 102 (1) (1983) 1–46. [6] A.L. Da War, J.C. Joshi, Semiconducting transparent thin films: their properties and application, J. Mater. Sci. 19 (1) (1984) 1–23. [7] H.L. Hartnagel, A.L. Dawar, Semiconducting transparent thin films, Semiconduct. Transparent Thin Films (1995). [8] H. Sato, Highly transparent and conductive Zn2In2O5 thin films prepared by RF magnetron sputtering, J. Appl. Phys. 23 (9) (1986) L280–L282. [9] T. Minami, Highly conductive and transparent zinc oxide films prepared by RF magnetron sputtering under an applied external magnetic field, Appl. Phys. Lett. (1982) 41. [10] T. Minami, Transparent and conductive multicomponent oxide films prepared by magnetron sputtering, J. Vac. Sci. Technol. A Vac. Surf. Films 17 (4) (1999) 1765. [11] A. Suresh, P. Gollakota, P. Wellenius, et al., Transparent, high mobility InGaZnO thin films deposited by PLD, Thin Solid Films 516 (7) (2008) 1326–1329. [12] H.C. Wu, Highly Transparent, High-Performance IGZO-TFTs Using the Selective Formation of IGZO Source and Drain Electrodes, IEEE Electron Device Lett. 35 (6) (2014) 645–647. [13] H.C. Wu, C.H. Chien, High performance InGaZnO thin film transistor with InGaZnO source and drain electrodes, Appl. Phys. Lett. 102 (6) (2013) 062103. [14] H. Yabuta, M. Sano, K. Abe, et al., High-mobility thin-film transistor with amorphous InGaZnO4 channel fabricated by room temperature rf-magnetron sputtering, Appl. Phys. Lett. 89 (11) (2006) 112123-112123-3. [15] C.H. Jung, D.J. Kim, Y.K. Kang, et al., Transparent amorphous In-Ga-Zn-O thin film as function of various gas flows for TFT applications, Thin Solid Films 517 (14) (2009) 4078–4081. [16] Y. Wang, S.W. Liu, X.W. Sun, et al., Highly transparent solution processed In-Ga-Zn oxide thin films and thin film transistors, J. Solgel Sci. Technol. 55 (3) (2010) 322–327.
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