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Electrical characterization of TiO2 gate oxides on strained-Si
Microelectronic Engineering 72 (2004) 253–256 www.elsevier.com/locate/mee
Electrical characterization of TiO2 gate oxides on strained-Si C.K. Maiti *, S.K. Samanta, G.K. Dalapati, S.K. Nandi, S. Chatterjee Department of Electronics and ECE, IIT Kharagpur, Kharagpur 721302, India
Abstract The electrical properties of low temperature (150 °C) plasma deposited TiO2 gate dielectrics on strained-Si are reported. The deposited films have been analyzed by X-ray photoelectron spectroscopy for chemical composition. The interfacial and electrical properties of the deposited films have been characterized using a metal–insulator–semiconductor (MIS) capacitor structure. The charge trapping properties in TiO2 gate dielectrics have been studied using constant current stressing. The leakage current has been found to be dominated by the Poole–Frenkel emission. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Strained-Si; High-k dielectrics; TiO2
1. Introduction Fundamental limits to CMOS scaling are rapidly approaching as devices are scaled below the 50 nm range. New methods and materials for CMOS fabrication must be investigated to allow continued device improvement. The ITRS 2002 update [1] proposes an aggressive requirement for equivalent physical oxide thickness of 1–1.4 nm (it was 1–1.6 nm) by 2007. Gate leakage reduction in ultrathin gate dielectrics is the main motivation for the search of high-k materials such as ZrO2 , Ta2 O5 , TiO2 , Al2 O3 , and HfO2 to replace conventional SiO2 . In this respect, TiO2 is being considered as a promising candidate due to its high dielectric constant (>10), compatibility with Si substrates and low leakage current [2–4]. Recently, the use of biaxial tensilely strained-Si has attracted considerable attention for advanced CMOS devices because of enhancements *
Corresponding author. Fax: +91-3222-255303. E-mail address: [email protected] (C.K. Maiti).
of in-plane mobility of both the electrons and holes compared to bulk-Si [5,6]. In this paper, we present the results of our study on the electrical properties of TiO2 films, deposited on gas source MBE grown tensilely strained-Si, as a possible candidate for future strained-Si CMOS applications and we demonstrate the feasibility of integration of high-k TiO2 gate dielectric with the strained-Si for the first time. The chemical analysis of the TiO2 thin films has been done by XPS. The electrical and interfacial quality of these films under Fowler-Nordheim (FN) constant current stressing is examined using a metal–insulator–semiconductor (MIS) structure. We have also investigated the interfacial properties and leakage current conduction mechanism of the as-deposited TiO2 films. 2. Experiment Strained-Si layers on relaxed SiGe grown using gas source molecular beam epitaxy (GSMBE),
0167-9317/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2003.12.046
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C.K. Maiti et al. / Microelectronic Engineering 72 (2004) 253–256
were chemically etched in H2 O2 :H2 SO4 solution followed by a dip in 1% HF solution to remove the native oxide layer. TiO2 films were deposited (12 nm) on strained-Si heterolayers using metallorganic compounds based titanium tetrakis isopropoxide (TTIP) [Ti(OC3 H7 )4 ] and oxygen in a microwave (1400 W, 2.45 GHz) plasma enhanced chemical vapor deposition system. The deposition pressure and time were 300 mTorr and 30 s, respectively. The bubbler containing TTIP was maintained at 45 °C. Although there was no external heating of the substrate, the discharge itself raised the substrate temperature to about 150 °C. The chemical bonding configuration of the TiO2 films were analyzed by using XPS studies. In order to study the electrical characteristics of the TiO2 films deposited on strained-Si layers, MIS structures were fabricated with Al gate (area: 1.96 103 cm2 ). The conductance–voltage (G–V ), capacitance–voltage (C–V ), current–voltage (I–V ) and constant current stressing characteristics were studied using a HP-4061A semiconductor component test system and HP-4145B DC parameter analyzer, respectively.
3. Results and discussion The surface chemical state of the as-deposited films was analyzed by XPS. Non-monochromatized Mg Ka (hc ¼ 1253:6 eV) radiation was used at an angle 30° between the analyzer axis and normal to the sample to investigate the chemical structure of the films from the shift of the corelevel binding energy. To investigate the bonding chemistry, the Ti 2P spectra were deconvoluted rather than the oxygen spectra because the peak position of O 1S in TiO2 is the same as that of in TiO and Ti2 O3 [7]. Fig. 1(a) shows the Ti 2P XPS spectra of the as-deposited TiO2 films. The binding energy of Ti 2P3=2 and Ti 2P1=2 were observed at 458.3 and 463.9 eV, respectively, with a separation of 5.6 eV between the peaks which is a typical characteristic of the Ti4þ in TiO2 [8]. Binding energy of O 1S for SiO2 appears at 533.5 eV and for TiO2 appears at 530 eV. In Fig. 1(b), the O 1S spectrum shows deconvoluted two peaks at 530.1 and 529.6 eV, which also shows the formation of
Fig. 1. (a) XPS spectra of Ti 2P for the as-deposited TiO2 films on strained-Si layers. (b) XPS spectra of deconvoluted O 1S and (c) Si 2P for the as-deposited TiO2 films on strained-Si layers.
TiO2 films. The Si 2P peak assigned to Si–Si bond, is found from the strained-Si substrate at 100.4 eV, as shown in Fig. 1(c). The asymmetric nature of Si 2P peak due to the formation of small amount of Si–O bond [9]. The normalized high frequency (100 kHz) C–V and G–V characteristics of the MIS capacitors are shown in Fig. 2. The fixed insulator charge density (Qf =q) and interface trap density (Dit ) were found to be 1:38 1011 cm2 and 1:18 1012 cm2 eV1 using Fig. 2. It is found that the Qf =q and the midgap Dit values of the samples are comparable to some reported values [10]. Fig. 3 shows the C–V characteristics of the TiO2 gate oxides measured at different frequencies in the range from 100 kHz to 1 MHz. In the accumulation region, the frequency dispersion of the C–V characteristics is due to the formation of an
C.K. Maiti et al. / Microelectronic Engineering 72 (2004) 253–256
Fig. 2. High frequency C–V and G–V characteristics of TiO2 films.
Fig. 3. Frequency dispersion in the C–V characteristics.
inhomogeneous layer at the semiconductor–insulator interface. Since the capacitance of such a layer acts in series with the insulator capacitance causing a frequency dispersion in the accumulation. The C–V characteristics are found to stretch along the voltage axis. This is due to the presence of both donor and acceptor like interface traps is occupying a portion of the semiconductor band gap [11]. The total capacitance in inversion is found to increase with decrease in frequency. This is because an inversion layer beyond the gate governs the response time of the minority carriers [12]. The charge trapping behavior was studied by continuously monitoring the change in gate voltage (DVg ) required to maintain a constant current ()10 mA/cm2 ) under gate injection is shown in Fig. 4. It is observed that TiO2 films exhibit an
255
Fig. 4. Gate voltage shift against stress time under a constant current stressing ()10 mA/cm2 ).
electron trapping. The effect of Fowler-Nordheim constant current stressing ()10 mA/cm2 ) on C–V characteristics with different stressing times (300 and 500 s) is shown in Fig. 5. It is seen that there is a positive voltage shift in the high frequency C–V characteristics for both the cases, indicating the presence of electron traps in the oxide [13]. Fig. 6 shows the stress-induced leakage current (SILC) for constant current stressing of 300 and 500 s, which indicates a higher leakage current at a low field due to stressing. This is due to the ÔgenerationÕ of neutral traps in thin oxides and is responsible for the SILC phenomena. To determine the dominant leakage current mechanism in the TiO2 films, a plot of lnðI=EÞ versus E1=2 (Poole– Frenkel (PF) mechanism) is commonly used
Fig. 5. High frequency C–V characteristics for fresh, 300 and 500 s after constant current ()10 mA/cm2 ) stressing.
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(Fig. 7). TiO2 films show a linear relationship and the conduction mechanism is dominated by the PF mechanism at a high field. But at low field (<1.4 MV/cm), the logarithm of the current was plotted against E1=2 and is shown in Fig. 8, which indicates the Schottky emission. 4. Conclusion
Fig. 6. I–V characteristics of TiO2 films before and after stressing.
In summary, ultra-thin TiO2 films have been deposited at 150 °C on strained-Si heterolayers by microwave PECVD using TTIP and oxygen. Asdeposited films have been analyzed by XPS for chemical composition. A separation of 5.6 eV between two peaks of Ti 2P confirms the formation of TiO2 . Interfacial and electrical properties of the deposited films were characterized using capacitance–voltage and conductance–voltage techniques. The C–V characteristics exhibit the frequency dependence due to presence of interface traps. The leakage current has been found to be dominated by the Schottky emission at a low electric field, whereas Poole–Frenkel emission takes over at a higher electric field. References
Fig. 7. Poole–Frenkel plot for TiO2 films at high field.
Fig. 8. Schottky emission plot for TiO2 films at a low field.
[1] International Technology Roadmap for Semiconductors, 2002 Update. [2] G.K. Dalapati, S. Chatterjee, S.K. Samanta, C.K. Maiti, Electron. Lett. 39 (2003) 323. [3] C.K. Maiti, S. Chatterjee, S.K. Nandi, S.K. Samanta, Solid-State Electron. 47 (2003) 1995. [4] S.A. Campbell, H.S. Kim, D.C. Gilmer, B. He, T.P. Ma, W.L. Gladfelter, IBM J. Res. Dev. 43 (1999) 383. [5] C.K. Maiti, N.B. Chakrabarti, S.K. Ray, Strained silicon Heterostructures: Materials and Devices, IEE, UK, 2001. [6] C.K. Maiti, L.K. Bera, S.S. Dey, D.K. Nayak, N.B. Chakraborti, Solid-State Electron. 41 (1997) 1863. [7] C.N. Sayers, N.R. Armstrong, Surf. Sci. 77 (1978) 301. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Hand Book of X-ray Photoelectron Spectroscopy, Perkin–Elmer Corporation, Minnesota, 1978. [9] S.K. Samanta, S. Chatterjee, L.K. Bera, H.D. Banerjee, C.K. Maiti, Appl. Phys. Lett. 80 (2002) 2547. [10] G.K. Dalapati, S. Chatterjee, S.K. Samanta, C.K. Maiti, Appl. Surf. Sci. 210 (2003) 249. [11] R. Razouk, B. Deal, J. Electrochem. Soc. 126 (1979) 1573. [12] E.H. Nicollian, J.R. Brews, Metal Oxide Semiconductor Physics and Technology, Wiley, New York, 1982. [13] B.E. Deal, IEEE Trans. Electron Dev. 27 (1980) 606.
Electrical characterization of TiO2 gate oxides on strained-Si C.K. Maiti *, S.K. Samanta, G.K. Dalapati, S.K. Nandi, S. Chatterjee Department of Electronics and ECE, IIT Kharagpur, Kharagpur 721302, India
Abstract The electrical properties of low temperature (150 °C) plasma deposited TiO2 gate dielectrics on strained-Si are reported. The deposited films have been analyzed by X-ray photoelectron spectroscopy for chemical composition. The interfacial and electrical properties of the deposited films have been characterized using a metal–insulator–semiconductor (MIS) capacitor structure. The charge trapping properties in TiO2 gate dielectrics have been studied using constant current stressing. The leakage current has been found to be dominated by the Poole–Frenkel emission. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Strained-Si; High-k dielectrics; TiO2
1. Introduction Fundamental limits to CMOS scaling are rapidly approaching as devices are scaled below the 50 nm range. New methods and materials for CMOS fabrication must be investigated to allow continued device improvement. The ITRS 2002 update [1] proposes an aggressive requirement for equivalent physical oxide thickness of 1–1.4 nm (it was 1–1.6 nm) by 2007. Gate leakage reduction in ultrathin gate dielectrics is the main motivation for the search of high-k materials such as ZrO2 , Ta2 O5 , TiO2 , Al2 O3 , and HfO2 to replace conventional SiO2 . In this respect, TiO2 is being considered as a promising candidate due to its high dielectric constant (>10), compatibility with Si substrates and low leakage current [2–4]. Recently, the use of biaxial tensilely strained-Si has attracted considerable attention for advanced CMOS devices because of enhancements *
Corresponding author. Fax: +91-3222-255303. E-mail address: [email protected] (C.K. Maiti).
of in-plane mobility of both the electrons and holes compared to bulk-Si [5,6]. In this paper, we present the results of our study on the electrical properties of TiO2 films, deposited on gas source MBE grown tensilely strained-Si, as a possible candidate for future strained-Si CMOS applications and we demonstrate the feasibility of integration of high-k TiO2 gate dielectric with the strained-Si for the first time. The chemical analysis of the TiO2 thin films has been done by XPS. The electrical and interfacial quality of these films under Fowler-Nordheim (FN) constant current stressing is examined using a metal–insulator–semiconductor (MIS) structure. We have also investigated the interfacial properties and leakage current conduction mechanism of the as-deposited TiO2 films. 2. Experiment Strained-Si layers on relaxed SiGe grown using gas source molecular beam epitaxy (GSMBE),
0167-9317/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2003.12.046
254
C.K. Maiti et al. / Microelectronic Engineering 72 (2004) 253–256
were chemically etched in H2 O2 :H2 SO4 solution followed by a dip in 1% HF solution to remove the native oxide layer. TiO2 films were deposited (12 nm) on strained-Si heterolayers using metallorganic compounds based titanium tetrakis isopropoxide (TTIP) [Ti(OC3 H7 )4 ] and oxygen in a microwave (1400 W, 2.45 GHz) plasma enhanced chemical vapor deposition system. The deposition pressure and time were 300 mTorr and 30 s, respectively. The bubbler containing TTIP was maintained at 45 °C. Although there was no external heating of the substrate, the discharge itself raised the substrate temperature to about 150 °C. The chemical bonding configuration of the TiO2 films were analyzed by using XPS studies. In order to study the electrical characteristics of the TiO2 films deposited on strained-Si layers, MIS structures were fabricated with Al gate (area: 1.96 103 cm2 ). The conductance–voltage (G–V ), capacitance–voltage (C–V ), current–voltage (I–V ) and constant current stressing characteristics were studied using a HP-4061A semiconductor component test system and HP-4145B DC parameter analyzer, respectively.
3. Results and discussion The surface chemical state of the as-deposited films was analyzed by XPS. Non-monochromatized Mg Ka (hc ¼ 1253:6 eV) radiation was used at an angle 30° between the analyzer axis and normal to the sample to investigate the chemical structure of the films from the shift of the corelevel binding energy. To investigate the bonding chemistry, the Ti 2P spectra were deconvoluted rather than the oxygen spectra because the peak position of O 1S in TiO2 is the same as that of in TiO and Ti2 O3 [7]. Fig. 1(a) shows the Ti 2P XPS spectra of the as-deposited TiO2 films. The binding energy of Ti 2P3=2 and Ti 2P1=2 were observed at 458.3 and 463.9 eV, respectively, with a separation of 5.6 eV between the peaks which is a typical characteristic of the Ti4þ in TiO2 [8]. Binding energy of O 1S for SiO2 appears at 533.5 eV and for TiO2 appears at 530 eV. In Fig. 1(b), the O 1S spectrum shows deconvoluted two peaks at 530.1 and 529.6 eV, which also shows the formation of
Fig. 1. (a) XPS spectra of Ti 2P for the as-deposited TiO2 films on strained-Si layers. (b) XPS spectra of deconvoluted O 1S and (c) Si 2P for the as-deposited TiO2 films on strained-Si layers.
TiO2 films. The Si 2P peak assigned to Si–Si bond, is found from the strained-Si substrate at 100.4 eV, as shown in Fig. 1(c). The asymmetric nature of Si 2P peak due to the formation of small amount of Si–O bond [9]. The normalized high frequency (100 kHz) C–V and G–V characteristics of the MIS capacitors are shown in Fig. 2. The fixed insulator charge density (Qf =q) and interface trap density (Dit ) were found to be 1:38 1011 cm2 and 1:18 1012 cm2 eV1 using Fig. 2. It is found that the Qf =q and the midgap Dit values of the samples are comparable to some reported values [10]. Fig. 3 shows the C–V characteristics of the TiO2 gate oxides measured at different frequencies in the range from 100 kHz to 1 MHz. In the accumulation region, the frequency dispersion of the C–V characteristics is due to the formation of an
C.K. Maiti et al. / Microelectronic Engineering 72 (2004) 253–256
Fig. 2. High frequency C–V and G–V characteristics of TiO2 films.
Fig. 3. Frequency dispersion in the C–V characteristics.
inhomogeneous layer at the semiconductor–insulator interface. Since the capacitance of such a layer acts in series with the insulator capacitance causing a frequency dispersion in the accumulation. The C–V characteristics are found to stretch along the voltage axis. This is due to the presence of both donor and acceptor like interface traps is occupying a portion of the semiconductor band gap [11]. The total capacitance in inversion is found to increase with decrease in frequency. This is because an inversion layer beyond the gate governs the response time of the minority carriers [12]. The charge trapping behavior was studied by continuously monitoring the change in gate voltage (DVg ) required to maintain a constant current ()10 mA/cm2 ) under gate injection is shown in Fig. 4. It is observed that TiO2 films exhibit an
255
Fig. 4. Gate voltage shift against stress time under a constant current stressing ()10 mA/cm2 ).
electron trapping. The effect of Fowler-Nordheim constant current stressing ()10 mA/cm2 ) on C–V characteristics with different stressing times (300 and 500 s) is shown in Fig. 5. It is seen that there is a positive voltage shift in the high frequency C–V characteristics for both the cases, indicating the presence of electron traps in the oxide [13]. Fig. 6 shows the stress-induced leakage current (SILC) for constant current stressing of 300 and 500 s, which indicates a higher leakage current at a low field due to stressing. This is due to the ÔgenerationÕ of neutral traps in thin oxides and is responsible for the SILC phenomena. To determine the dominant leakage current mechanism in the TiO2 films, a plot of lnðI=EÞ versus E1=2 (Poole– Frenkel (PF) mechanism) is commonly used
Fig. 5. High frequency C–V characteristics for fresh, 300 and 500 s after constant current ()10 mA/cm2 ) stressing.
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C.K. Maiti et al. / Microelectronic Engineering 72 (2004) 253–256
(Fig. 7). TiO2 films show a linear relationship and the conduction mechanism is dominated by the PF mechanism at a high field. But at low field (<1.4 MV/cm), the logarithm of the current was plotted against E1=2 and is shown in Fig. 8, which indicates the Schottky emission. 4. Conclusion
Fig. 6. I–V characteristics of TiO2 films before and after stressing.
In summary, ultra-thin TiO2 films have been deposited at 150 °C on strained-Si heterolayers by microwave PECVD using TTIP and oxygen. Asdeposited films have been analyzed by XPS for chemical composition. A separation of 5.6 eV between two peaks of Ti 2P confirms the formation of TiO2 . Interfacial and electrical properties of the deposited films were characterized using capacitance–voltage and conductance–voltage techniques. The C–V characteristics exhibit the frequency dependence due to presence of interface traps. The leakage current has been found to be dominated by the Schottky emission at a low electric field, whereas Poole–Frenkel emission takes over at a higher electric field. References
Fig. 7. Poole–Frenkel plot for TiO2 films at high field.
Fig. 8. Schottky emission plot for TiO2 films at a low field.
[1] International Technology Roadmap for Semiconductors, 2002 Update. [2] G.K. Dalapati, S. Chatterjee, S.K. Samanta, C.K. Maiti, Electron. Lett. 39 (2003) 323. [3] C.K. Maiti, S. Chatterjee, S.K. Nandi, S.K. Samanta, Solid-State Electron. 47 (2003) 1995. [4] S.A. Campbell, H.S. Kim, D.C. Gilmer, B. He, T.P. Ma, W.L. Gladfelter, IBM J. Res. Dev. 43 (1999) 383. [5] C.K. Maiti, N.B. Chakrabarti, S.K. Ray, Strained silicon Heterostructures: Materials and Devices, IEE, UK, 2001. [6] C.K. Maiti, L.K. Bera, S.S. Dey, D.K. Nayak, N.B. Chakraborti, Solid-State Electron. 41 (1997) 1863. [7] C.N. Sayers, N.R. Armstrong, Surf. Sci. 77 (1978) 301. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Hand Book of X-ray Photoelectron Spectroscopy, Perkin–Elmer Corporation, Minnesota, 1978. [9] S.K. Samanta, S. Chatterjee, L.K. Bera, H.D. Banerjee, C.K. Maiti, Appl. Phys. Lett. 80 (2002) 2547. [10] G.K. Dalapati, S. Chatterjee, S.K. Samanta, C.K. Maiti, Appl. Surf. Sci. 210 (2003) 249. [11] R. Razouk, B. Deal, J. Electrochem. Soc. 126 (1979) 1573. [12] E.H. Nicollian, J.R. Brews, Metal Oxide Semiconductor Physics and Technology, Wiley, New York, 1982. [13] B.E. Deal, IEEE Trans. Electron Dev. 27 (1980) 606.