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GZO transparent MIM capacitors
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 9 (2006) 1119–1124
Structural, electrical and optical properties of GZO/HfO2/GZO transparent MIM capacitors Byung Du Ahn, Jong Hoon Kim, Hong Seong Kang, Choong Hee Lee, Sang Hoon Oh, Gun Hee Kim, Dong Hua Li, Sang Yeol Lee Department of Electrical and Electronic Engineering, Yonsei University, 134, Shinchon-dong, Seodaemoon-ku, 120-749, Seoul, Korea Available online 13 November 2006
Abstract Metal–insulator–metal (MIM) transparent capacitors were prepared by pulsed laser deposition (PLD) on glass substrates. The effect of the thickness of the dielectric layer and oxygen pressure on structural, electrical, and optical properties of these capacitors was investigated. Experimental results show that film thickness and oxygen pressure have no effect on the structural properties. It is also found that the optical properties of the HfO2 thin films depend strongly on both the film thickness and oxygen pressure. The electrical properties of transparent capacitors were investigated at various thickness of the dielectric layer. The capacitor shows an overall high performance, such as a high dielectric constant of 28 and a low leakage current of 2.03 106 A/cm2 at 75 V. Transmittance above 70% was observed in visible region. r 2006 Elsevier Ltd. All rights reserved. Keywords: Transparent capacitor; Ga-doped ZnO; HfO2
1. Introduction Transparent electronic circuits are expected to serve as the basis for new optoelectronic devices [1]. A key device for realizing transparent circuits is a transparent thin-film transistor (TTFT). The use of high dielectric constant (k) materials for the gate insulator has several potential advantages such as a compensation for the relatively low mobility in the semiconductor, a reduction of the operating voltage by increasing the gate capacitance, and an increment of the charge density at lower gate potentials [3]. Nomura et al. have reported that the use of a Corresponding author. Tel.: +82 2 2123 2776; fax: +82 2 364 9770. E-mail address: [email protected] (S.Y. Lee).
1369-8001/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2006.10.030
gate insulator with a high dielectric constant (amorphous HfO2) results in an overall improvement of the TTFT performance [2]. Several metal oxides could be the candidates due to their high transparency and a high dielectric constant. The examples are titanium oxide (TiO2), tantalum oxide (Ta2O5), zirconium oxide (ZrO2) and hafnium oxide (HfO2). Among these materials, both ZrO2 and HfO2 are good candidates once they are stable up to 900 1C [4], their bands offsets and barrier heights are suitable [5], and they have a wide band gap (higher than 5 eV) and a high k of about 20–25 [6]. The improved thermal stability and better interfacial properties after thermal annealing when compared with ZrO2 justify the option for HfO2 [7]. To improve the TTFT performance, extremely reliable and high-quality HfO2 thin films are
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desired. Several thin-film growth techniques such as atomic layer deposition [8], evaporation with ionassisted deposition [9], sol gel [10], sputtering [11], and metal–organic molecular beam epitaxy [12] have been employed to fabricate good-quality HfO2 thin films. The properties of HfO2 thin films have been reported to be strongly dependent on the fabrication method, and an understanding of process–structure–property correlation is of a great importance to understand and exploit these films for devices. Pulsed laser deposition (PLD) is increasingly being used to prepare a wide variety of materials in thin-film form, since PLD has been established as a simple, reliable, and fast technique that offers a great experimental versatility [13]. More importantly, the PLD technique is well known for the quality of the layers grown at relatively lower substrate temperatures than other thin-film deposition methods [13]. In order to realize TTFT devices, research on several materials is needed: transparent semiconducting oxides used as the FET active channel materials, transparent conducting oxides as the source, drain and gate metals, and transparent dielectric materials as gate dielectrics. There have been few reports on transparent dielectric materials for TTFT applications. In this study, we investigated the structural, electrical, and optical properties of HfO2 film deposited on Ga-doped ZnO (GZO) transparent electrodes using the PLD method to confirm the feasibility of their application in TTFTs. The effects of the film thickness and oxygen pressure on the properties of HfO2 thin films grown at room temperature were analyzed and compared. 2. Experimental The HfO2 films were deposited at room temperature by PLD. The target used in this study was a sintered HfO2 pellet. It was placed on the target holder that was constantly rotated by an external motor in order to prevent the formation of surface craters. Glass substrates were used for HfO2 thin films at various oxygen pressures varying from vacuum to 100 mTorr and at various film thicknesses ranging from 100 to 145 nm. At first, GZO thin films were deposited at 500 1C by sputtering on glass as bottom electrodes. Following that, high-k HfO2 dielectric films were deposited in oxygen ambient of 100 mTorr. Finally, GZO films were deposited at room temperature as top electrodes by
PLD. The resistivity of bottom and top GZO layers is about 2.4 104 and 3.4 104 O cm, respectively. In our previous study we found that postdeposition annealing and rapid thermal annealing (RTA) cause the increment the leakage current and transmittance without the increment of the dielectric constant. It is because the structure of HfO2 changes from amorphous to crystalline. The crystal structures of HfO2 thin films were investigated by X-ray diffraction (XRD) where a ˚ source was used. Ni-filtered Cu Ka ðl ¼ 1:54056 AÞ The surface morphology was measured by using an Au-to-Probe CP atomic force microscope (AFM) in the contact mode. The film thickness was measured by scanning electron microscopy (SEM). The optical transmission measurements were performed using a UV-near IR grating spectrometer. For electrical measurements, the leakage current was measured using an HP4155 parameter analyzer, and the capacitance was characterized using an HP4284 precision LCR meter at 1 MHz.
3. Results and discussion HfO2 is a material forming several polymorphs. Pure HfO2 tends to appear in the monoclinic phase at room temperature and atmospheric pressure. Orthorhombic and tetragonal phases can be formed at high pressures and/or high temperatures [14,15]; these phases are generally thought to be metastable if present at room temperature and atmospheric pressure. Fig. 1 shows the XRD patterns of HfO2 films deposited at room temperature with different oxygen pressures and film thickness. As shown in Fig. 1, all films are amorphous without any crystallization peak independently of the oxygen pressures and film thickness. The broad peaks observed around 251 correspond to the glass substrate and can be attributed to the low substrate temperature. With increasing the oxygen pressure, there are no obvious effects on the crystallinity of the HfO2 films. Amorphous HfO2 films may have better dielectric properties and higher laser damage threshold than crystalline films [16]. Moon et al. suggested that HfO2 films transformed from amorphous to polycrystalline form as the film thickness increases [17]. Hang et al. suggested that the HfO2 films transform from the amorphous to the polycrystalline form when the substrate temperature was higher than 300 1C [18].
ARTICLE IN PRESS B.D. Ahn et al. / Materials Science in Semiconductor Processing 9 (2006) 1119–1124
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Fig. 1. X-ray diffraction pattern of HfO2 films deposited on glass with a different (a) oxygen pressure, and (b) film thickness.
These results indicate that the crystal structure of HfO2 films varies with the deposition temperature rather than with the oxygen pressures and film thicknesses. We thus conclude that the crystallization of the HfO2 layer depends on the deposition temperature. In order to investigate the electrical properties of HfO2 films with different oxygen pressures and film thicknesses, GZO dots with an area of about 2.8 103 cm2 were deposited on HfO2 films and GZO/HfO2/GZO capacitor structure was fabricated. C–V measurements were performed at 1 MHz frequency by sweeping gate voltage from 5 to +5 V. The resulting C–V characteristics for films with different oxygen pressures and thicknesses are shown in Fig. 2. In order to investigate the effect of oxygen pressure on the capacitance, the
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film thickness was fixed. The film thickness was monitored by cross-section SEM. All films were grown by PLD on GZO and had the thickness of 140 nm. It is observed that the capacitance density decreases with increasing the oxygen pressure and HfO2 film thickness, ranging from 1.19 103 to 2.49 103 F/m2. Using the capacitance measured at 0 V and the physical thickness of the films, the dielectric constant (k) of the HfO2 layer was evaluated. The dielectric constant of HfO2 thin films depends on the oxygen pressure decreases with increasing oxygen pressure (Fig. 2(b)). A wide range of oxygen pressures was selected in order to establish the optimum oxygen pressure necessary for the achievement of a high dielectric constant, a high optical transparency, and a low leakage current. From this investigation we conclude that the oxygen pressure of 100 mTorr is the most favorable condition to achieve an optimal combination of the above-mentioned parameters. Therefore, in order to investigate the effect of the film thickness on the dielectric constant, the oxygen pressure was fixed under 100 mTorr. It is also observed that as the HfO2 film thickness increases, the effective dielectric constant of HfO2 decreases, as shown in Fig. 2(c). At 1 MHz, the dielectric constant is around 28, which is larger than the value of the dielectric constant for HfO2 films prepared by another techniques, reported by other groups [19]. Fig. 3 shows the leakage characteristics of HfO2 films grown with various oxygen pressures and having various thickness. The leakage current density of the MIM capacitors (measured at a gate voltage of +5 V) decrease with increasing oxygen pressure. The reduction of leakage for the HfO2 prepared under a high oxygen pressure is attributed to the improvement in the quality of the HfO2 film. In general, the leakage current density is increased by increasing the density of leakage paths such as grain boundaries and dislocations created during the crystallization of the film. As shown in Fig. 1(b), the structure of all films is amorphous, independently of the film thickness. Therefore, the decrease in leakage current is attributed solely to the increase of the physical thickness of the film. This result is in agreement with that of Moon et al. [17]. Fig. 4 shows the effects of oxygen pressure and film thickness on the optical transmission spectra of HfO2 thin films. High transmittance (above 85%) in the visible region is exhibited by the films prepared
ARTICLE IN PRESS B.D. Ahn et al. / Materials Science in Semiconductor Processing 9 (2006) 1119–1124
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Fig. 2. (a) C–V characteristics of MIM capacitors for various oxygen pressures and film thickness. (b) Calculated dielectric constant of MIM capacitors for various oxygen pressures (all films have the thickness of 140 nm) and (c) film thicknesses (all films have been deposited at 100 mTorr).
above the oxygen pressure of 50 mTorr, as shown in Fig. 4(a). The transmittance of HfO2 films increases with increasing oxygen pressure. This effect can be attributed to a smaller concentration of oxygen vacancies in the films grown at a higher oxygen pressure. As the film thickness decreases, transmittance of the HfO2 films increases, and all of the HfO2 films deposited under the oxygen pressure of 100 mTorr were of high transparency (above 85%) in the visible region. It is known that the increase in optical transmittance can be attributed to the improvement of crystallinity and stoichiometry [20]. The stoichiometry of HfO2 films has a significant effect on the optical properties. Therefore, the research on the stoichiometry of HfO2 films is of a particular interest. It has been reported that substrate temperature has little effect on the stoichiometry, whereas the oxygen pressure plays an important role in determining the ratio of Hf and O [18]. The results indicate that the stoichiometry of
HfO2 films depends more on the oxygen pressure than on the film thickness. Fig. 5 shows the optical transmittance spectra of the entire MIM capacitor (excluding the glass substrate). The average optical transmission in the visible region of the spectrum is 70.6%, which points to transmission losses due to the top electrode. The top GZO electrode exhibits a transmittance of below 80% in the visible region when it is deposited on glass at room temperature, although without the top electrode layer the general transmittance reaches more than 88% in the visible range. The bottom electrode layer was deposited at 500 1C and the top electrode layer was deposited at room temperature. The deposition temperature is restricted in order to suppress crystallization of the HfO2, i.e., in order to obtain satisfactory leakage characteristics. We conclude that it is necessary to improve the transmittance of GZO electrode for MIM capacitors.
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B.D. Ahn et al. / Materials Science in Semiconductor Processing 9 (2006) 1119–1124
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4. Conclusions 100 80 Transminttance (%)
Fabrication and analysis of transparent GZO/ HfO2/GZO MIM capacitors is reported. The influence of the oxygen pressure and film thickness on the structural, electrical, and optical properties of the HfO2 films was investigated and analyzed. It is concluded that oxygen pressure and film thickness have little effect on the crystal structure; this may be attributed to the low substrate temperature during deposition. The results show that the optical transmittance of HfO2 thin films strongly depends on the oxygen pressure and on the film thickness. Thus, the appropriate choice of oxygen pressure and film thickness in PLD processing of HfO2 thin films is of a great importance. It was found that the HfO2 MIM capacitor prepared at room temperature under oxygen pressure of 100 mTorr has a high
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overall performance, such as a high dielectric constant of 28, a low leakage current of 2.03 106 A/cm2 at 75 V, and a transmittance of 70% in the visible region. All of these indicate that transparent HfO2 MIM capacitors are very suitable for use in transparent electronics. However, it is still necessary to improve the transmittance of MIM capacitors. Acknowledgment This work was supported by KOSEF through National Core Research Center for Nanomedical Technology (R15-2004-024- 0000-0). References [1] Wager JF. Transparent electronics. Science 2003;300(5623): 1245–6. [2] Wang G, Moses D, Heeger AJ, Zhang HM, Narasimhan M, Demaray RE. Poly(3-hexylthiophene) field-effect transistors with high dielectric constant gate insulator. J Appl Phys 2004;95(1):316–22. [3] Nomura K, Ohta H, Ueda K, Kamiya T, Hirano M, Hosono H. Thin-film transistor fabricated in single-crystalline transparent oxide semiconductor. Science 2003; 300(5623):1269–72. [4] Wallace RM, Wilk G. High-k gate dielectric materials. MRS Bull 2002;27(3):192–7. [5] Robertson J. Electronic structure and band offsets of highdielectric-constant gate oxides. MRS Bull 2002;27(3):217–21. [6] Ng KL, Zhan N, Kok CW, Poon MC, Wong H. Electrical characterization of the hafnium oxide prepared by direct sputtering of Hf in oxygen with rapid thermal annealing. Microelectron Reliab 2003;43(8):1289–93. [7] Callegari A, Cartier E, Gribelyuk M, Schmidt HFO, Zabel T. Physical and electrical characterization of Hafnium oxide and Hafnium silicate sputtered films. J Appl Phys 2001; 90(12):6466–75.
[8] Aarik J, Aidla A, Man¨dar H, Uustare T, Kukli K, Schuisky M. Phase transformations in hafnium dioxide thin films grown by atomic layer deposition at high temperatures. Appl Surf Sci 2001;173(1–2):15–21. [9] Gilo M, Croitoru N. Study of HfO2 films prepared by ionassisted deposition using a gridless end-hall ion source. Thin Solid Films 1999;350(1–2):203–8. [10] Nishide T, Honda S, Matsuura M, Ide M. Thin Solid Films 2000;371(1–2):61–5. [11] Lee BH, Kang L, Nieh R, Qi WJ, Lee JC. Thermal stability and electrical characteristics of ultrathin hafnium oxide gate dielectric reoxidized with rapid thermal annealing. Appl Phys Lett 2000;76(14):1926–8. [12] Moon TH, Ham MH, Kim MS, Yun I, Myung JM. Growth and characterization of MOMBE grown HfO2. Appl Surf Sci 2005;240(1–4):105–11. [13] Chrisey DB, Hubler GK. Pulsed laser deposition of thin films. New York: Wiley; 1994. [14] Le´ger JM, Haines J, Blanzat B. Materials potentially harder than diamond: quenchable high-pressure phases of transition metal dioxides. J Mater Sci Lett 1994;13(23):1688–90. [15] Le´ger JM, Atouf A, Tomaszewski PE, Pereira AS. Pressureinduced phase transitions and volume changes in HfO2 up to 50 GPa. Phys Rev B 1993;48(1):93–8. [16] Hasegawa K, Ahmet P, Okazaki N, Hasegawaa T, Fujimoto K, Watanabe M. Amorphous stability of HfO2 based ternary and binary composition spread oxide films as alternative gate dielectrics. Appl Surf Sci 2004;223(1–3): 229–32. [17] Moon TH, Ham MH, KIM MS, Yun Ig, Myung JM. Growth and characterization of MOMBE grown HfO2. Appl Surf Sci 2005;240(1–4):105–11. [18] Hu H, Zhu C, Lu YF, Wu YH, Liew T, Li MF, et al. Physical and electrical characterization of HfO2 metal–insulator–metal capacitors for Si analog circuit applications. J Appl Phys 2003;94(1):551–7. [19] Zhu J, Li YR, Liu ZG. Fabrication and characterization of pulsed laser deposited HfO2 films for high-k gate dielectric applications. J Phys D 2004;37(L29–L33):2896–900. [20] Djaoued Y, Phong VH, Badilescu S, Ashrit PV, Girouard FE, Truong VV. Sol–gel-prepared ITO films for electrochromic systems. Thin Solid Films 1997;293(1–2):108–12.
Materials Science in Semiconductor Processing 9 (2006) 1119–1124
Structural, electrical and optical properties of GZO/HfO2/GZO transparent MIM capacitors Byung Du Ahn, Jong Hoon Kim, Hong Seong Kang, Choong Hee Lee, Sang Hoon Oh, Gun Hee Kim, Dong Hua Li, Sang Yeol Lee Department of Electrical and Electronic Engineering, Yonsei University, 134, Shinchon-dong, Seodaemoon-ku, 120-749, Seoul, Korea Available online 13 November 2006
Abstract Metal–insulator–metal (MIM) transparent capacitors were prepared by pulsed laser deposition (PLD) on glass substrates. The effect of the thickness of the dielectric layer and oxygen pressure on structural, electrical, and optical properties of these capacitors was investigated. Experimental results show that film thickness and oxygen pressure have no effect on the structural properties. It is also found that the optical properties of the HfO2 thin films depend strongly on both the film thickness and oxygen pressure. The electrical properties of transparent capacitors were investigated at various thickness of the dielectric layer. The capacitor shows an overall high performance, such as a high dielectric constant of 28 and a low leakage current of 2.03 106 A/cm2 at 75 V. Transmittance above 70% was observed in visible region. r 2006 Elsevier Ltd. All rights reserved. Keywords: Transparent capacitor; Ga-doped ZnO; HfO2
1. Introduction Transparent electronic circuits are expected to serve as the basis for new optoelectronic devices [1]. A key device for realizing transparent circuits is a transparent thin-film transistor (TTFT). The use of high dielectric constant (k) materials for the gate insulator has several potential advantages such as a compensation for the relatively low mobility in the semiconductor, a reduction of the operating voltage by increasing the gate capacitance, and an increment of the charge density at lower gate potentials [3]. Nomura et al. have reported that the use of a Corresponding author. Tel.: +82 2 2123 2776; fax: +82 2 364 9770. E-mail address: [email protected] (S.Y. Lee).
1369-8001/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2006.10.030
gate insulator with a high dielectric constant (amorphous HfO2) results in an overall improvement of the TTFT performance [2]. Several metal oxides could be the candidates due to their high transparency and a high dielectric constant. The examples are titanium oxide (TiO2), tantalum oxide (Ta2O5), zirconium oxide (ZrO2) and hafnium oxide (HfO2). Among these materials, both ZrO2 and HfO2 are good candidates once they are stable up to 900 1C [4], their bands offsets and barrier heights are suitable [5], and they have a wide band gap (higher than 5 eV) and a high k of about 20–25 [6]. The improved thermal stability and better interfacial properties after thermal annealing when compared with ZrO2 justify the option for HfO2 [7]. To improve the TTFT performance, extremely reliable and high-quality HfO2 thin films are
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desired. Several thin-film growth techniques such as atomic layer deposition [8], evaporation with ionassisted deposition [9], sol gel [10], sputtering [11], and metal–organic molecular beam epitaxy [12] have been employed to fabricate good-quality HfO2 thin films. The properties of HfO2 thin films have been reported to be strongly dependent on the fabrication method, and an understanding of process–structure–property correlation is of a great importance to understand and exploit these films for devices. Pulsed laser deposition (PLD) is increasingly being used to prepare a wide variety of materials in thin-film form, since PLD has been established as a simple, reliable, and fast technique that offers a great experimental versatility [13]. More importantly, the PLD technique is well known for the quality of the layers grown at relatively lower substrate temperatures than other thin-film deposition methods [13]. In order to realize TTFT devices, research on several materials is needed: transparent semiconducting oxides used as the FET active channel materials, transparent conducting oxides as the source, drain and gate metals, and transparent dielectric materials as gate dielectrics. There have been few reports on transparent dielectric materials for TTFT applications. In this study, we investigated the structural, electrical, and optical properties of HfO2 film deposited on Ga-doped ZnO (GZO) transparent electrodes using the PLD method to confirm the feasibility of their application in TTFTs. The effects of the film thickness and oxygen pressure on the properties of HfO2 thin films grown at room temperature were analyzed and compared. 2. Experimental The HfO2 films were deposited at room temperature by PLD. The target used in this study was a sintered HfO2 pellet. It was placed on the target holder that was constantly rotated by an external motor in order to prevent the formation of surface craters. Glass substrates were used for HfO2 thin films at various oxygen pressures varying from vacuum to 100 mTorr and at various film thicknesses ranging from 100 to 145 nm. At first, GZO thin films were deposited at 500 1C by sputtering on glass as bottom electrodes. Following that, high-k HfO2 dielectric films were deposited in oxygen ambient of 100 mTorr. Finally, GZO films were deposited at room temperature as top electrodes by
PLD. The resistivity of bottom and top GZO layers is about 2.4 104 and 3.4 104 O cm, respectively. In our previous study we found that postdeposition annealing and rapid thermal annealing (RTA) cause the increment the leakage current and transmittance without the increment of the dielectric constant. It is because the structure of HfO2 changes from amorphous to crystalline. The crystal structures of HfO2 thin films were investigated by X-ray diffraction (XRD) where a ˚ source was used. Ni-filtered Cu Ka ðl ¼ 1:54056 AÞ The surface morphology was measured by using an Au-to-Probe CP atomic force microscope (AFM) in the contact mode. The film thickness was measured by scanning electron microscopy (SEM). The optical transmission measurements were performed using a UV-near IR grating spectrometer. For electrical measurements, the leakage current was measured using an HP4155 parameter analyzer, and the capacitance was characterized using an HP4284 precision LCR meter at 1 MHz.
3. Results and discussion HfO2 is a material forming several polymorphs. Pure HfO2 tends to appear in the monoclinic phase at room temperature and atmospheric pressure. Orthorhombic and tetragonal phases can be formed at high pressures and/or high temperatures [14,15]; these phases are generally thought to be metastable if present at room temperature and atmospheric pressure. Fig. 1 shows the XRD patterns of HfO2 films deposited at room temperature with different oxygen pressures and film thickness. As shown in Fig. 1, all films are amorphous without any crystallization peak independently of the oxygen pressures and film thickness. The broad peaks observed around 251 correspond to the glass substrate and can be attributed to the low substrate temperature. With increasing the oxygen pressure, there are no obvious effects on the crystallinity of the HfO2 films. Amorphous HfO2 films may have better dielectric properties and higher laser damage threshold than crystalline films [16]. Moon et al. suggested that HfO2 films transformed from amorphous to polycrystalline form as the film thickness increases [17]. Hang et al. suggested that the HfO2 films transform from the amorphous to the polycrystalline form when the substrate temperature was higher than 300 1C [18].
ARTICLE IN PRESS B.D. Ahn et al. / Materials Science in Semiconductor Processing 9 (2006) 1119–1124
Intensity (arb.unit)
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Fig. 1. X-ray diffraction pattern of HfO2 films deposited on glass with a different (a) oxygen pressure, and (b) film thickness.
These results indicate that the crystal structure of HfO2 films varies with the deposition temperature rather than with the oxygen pressures and film thicknesses. We thus conclude that the crystallization of the HfO2 layer depends on the deposition temperature. In order to investigate the electrical properties of HfO2 films with different oxygen pressures and film thicknesses, GZO dots with an area of about 2.8 103 cm2 were deposited on HfO2 films and GZO/HfO2/GZO capacitor structure was fabricated. C–V measurements were performed at 1 MHz frequency by sweeping gate voltage from 5 to +5 V. The resulting C–V characteristics for films with different oxygen pressures and thicknesses are shown in Fig. 2. In order to investigate the effect of oxygen pressure on the capacitance, the
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film thickness was fixed. The film thickness was monitored by cross-section SEM. All films were grown by PLD on GZO and had the thickness of 140 nm. It is observed that the capacitance density decreases with increasing the oxygen pressure and HfO2 film thickness, ranging from 1.19 103 to 2.49 103 F/m2. Using the capacitance measured at 0 V and the physical thickness of the films, the dielectric constant (k) of the HfO2 layer was evaluated. The dielectric constant of HfO2 thin films depends on the oxygen pressure decreases with increasing oxygen pressure (Fig. 2(b)). A wide range of oxygen pressures was selected in order to establish the optimum oxygen pressure necessary for the achievement of a high dielectric constant, a high optical transparency, and a low leakage current. From this investigation we conclude that the oxygen pressure of 100 mTorr is the most favorable condition to achieve an optimal combination of the above-mentioned parameters. Therefore, in order to investigate the effect of the film thickness on the dielectric constant, the oxygen pressure was fixed under 100 mTorr. It is also observed that as the HfO2 film thickness increases, the effective dielectric constant of HfO2 decreases, as shown in Fig. 2(c). At 1 MHz, the dielectric constant is around 28, which is larger than the value of the dielectric constant for HfO2 films prepared by another techniques, reported by other groups [19]. Fig. 3 shows the leakage characteristics of HfO2 films grown with various oxygen pressures and having various thickness. The leakage current density of the MIM capacitors (measured at a gate voltage of +5 V) decrease with increasing oxygen pressure. The reduction of leakage for the HfO2 prepared under a high oxygen pressure is attributed to the improvement in the quality of the HfO2 film. In general, the leakage current density is increased by increasing the density of leakage paths such as grain boundaries and dislocations created during the crystallization of the film. As shown in Fig. 1(b), the structure of all films is amorphous, independently of the film thickness. Therefore, the decrease in leakage current is attributed solely to the increase of the physical thickness of the film. This result is in agreement with that of Moon et al. [17]. Fig. 4 shows the effects of oxygen pressure and film thickness on the optical transmission spectra of HfO2 thin films. High transmittance (above 85%) in the visible region is exhibited by the films prepared
ARTICLE IN PRESS B.D. Ahn et al. / Materials Science in Semiconductor Processing 9 (2006) 1119–1124
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Fig. 2. (a) C–V characteristics of MIM capacitors for various oxygen pressures and film thickness. (b) Calculated dielectric constant of MIM capacitors for various oxygen pressures (all films have the thickness of 140 nm) and (c) film thicknesses (all films have been deposited at 100 mTorr).
above the oxygen pressure of 50 mTorr, as shown in Fig. 4(a). The transmittance of HfO2 films increases with increasing oxygen pressure. This effect can be attributed to a smaller concentration of oxygen vacancies in the films grown at a higher oxygen pressure. As the film thickness decreases, transmittance of the HfO2 films increases, and all of the HfO2 films deposited under the oxygen pressure of 100 mTorr were of high transparency (above 85%) in the visible region. It is known that the increase in optical transmittance can be attributed to the improvement of crystallinity and stoichiometry [20]. The stoichiometry of HfO2 films has a significant effect on the optical properties. Therefore, the research on the stoichiometry of HfO2 films is of a particular interest. It has been reported that substrate temperature has little effect on the stoichiometry, whereas the oxygen pressure plays an important role in determining the ratio of Hf and O [18]. The results indicate that the stoichiometry of
HfO2 films depends more on the oxygen pressure than on the film thickness. Fig. 5 shows the optical transmittance spectra of the entire MIM capacitor (excluding the glass substrate). The average optical transmission in the visible region of the spectrum is 70.6%, which points to transmission losses due to the top electrode. The top GZO electrode exhibits a transmittance of below 80% in the visible region when it is deposited on glass at room temperature, although without the top electrode layer the general transmittance reaches more than 88% in the visible range. The bottom electrode layer was deposited at 500 1C and the top electrode layer was deposited at room temperature. The deposition temperature is restricted in order to suppress crystallization of the HfO2, i.e., in order to obtain satisfactory leakage characteristics. We conclude that it is necessary to improve the transmittance of GZO electrode for MIM capacitors.
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Fig. 4. The optical transmittance as a function of wavelength for HfO2 films deposited at room temperature (a) under different oxygen pressure and (b) having various thickness.
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Fabrication and analysis of transparent GZO/ HfO2/GZO MIM capacitors is reported. The influence of the oxygen pressure and film thickness on the structural, electrical, and optical properties of the HfO2 films was investigated and analyzed. It is concluded that oxygen pressure and film thickness have little effect on the crystal structure; this may be attributed to the low substrate temperature during deposition. The results show that the optical transmittance of HfO2 thin films strongly depends on the oxygen pressure and on the film thickness. Thus, the appropriate choice of oxygen pressure and film thickness in PLD processing of HfO2 thin films is of a great importance. It was found that the HfO2 MIM capacitor prepared at room temperature under oxygen pressure of 100 mTorr has a high
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overall performance, such as a high dielectric constant of 28, a low leakage current of 2.03 106 A/cm2 at 75 V, and a transmittance of 70% in the visible region. All of these indicate that transparent HfO2 MIM capacitors are very suitable for use in transparent electronics. However, it is still necessary to improve the transmittance of MIM capacitors. Acknowledgment This work was supported by KOSEF through National Core Research Center for Nanomedical Technology (R15-2004-024- 0000-0). References [1] Wager JF. Transparent electronics. Science 2003;300(5623): 1245–6. [2] Wang G, Moses D, Heeger AJ, Zhang HM, Narasimhan M, Demaray RE. Poly(3-hexylthiophene) field-effect transistors with high dielectric constant gate insulator. J Appl Phys 2004;95(1):316–22. [3] Nomura K, Ohta H, Ueda K, Kamiya T, Hirano M, Hosono H. Thin-film transistor fabricated in single-crystalline transparent oxide semiconductor. Science 2003; 300(5623):1269–72. [4] Wallace RM, Wilk G. High-k gate dielectric materials. MRS Bull 2002;27(3):192–7. [5] Robertson J. Electronic structure and band offsets of highdielectric-constant gate oxides. MRS Bull 2002;27(3):217–21. [6] Ng KL, Zhan N, Kok CW, Poon MC, Wong H. Electrical characterization of the hafnium oxide prepared by direct sputtering of Hf in oxygen with rapid thermal annealing. Microelectron Reliab 2003;43(8):1289–93. [7] Callegari A, Cartier E, Gribelyuk M, Schmidt HFO, Zabel T. Physical and electrical characterization of Hafnium oxide and Hafnium silicate sputtered films. J Appl Phys 2001; 90(12):6466–75.
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