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Electrical properties of MIS capacitor using low temperature electron beam gun—evaporated HfAlO dielectrics
Microelectronics Reliability 45 (2005) 933–936 www.elsevier.com/locate/microrel
Electrical properties of MIS capacitor using low temperature electron beam gun—evaporated HfAlO dielectrics V. Mikhelashvili, B. Meyler, J. Shneider, O. Kreinin, G. Eisenstein
*
Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 3200, Israel Received 28 June 2004; received in revised form 4 October 2004 Available online 22 December 2004
Abstract A low effective oxide thickness of 1.45 nm was achieved in HfAlO films deposited by an electron beam gun evaporator on unheated p-Si substrate. A reduction of the leakage current density from 1 · 104 to 4.5 · 107 A/cm2, at an electric field 3 MV/cm, with annealing temperature and a breakdown electric field of 10 MV/cm were demonstrated for ultra thin films. 2004 Elsevier Ltd. All rights reserved.
1. Introduction Hafnium oxide is being studied extensively as a promising high permittivity insulator with properties, which are similar to Ta2O5 [1] and better than other rare-earth metal oxides [2]. The predicted equivalent oxide thickness (EOT) of HfO2 is of the order of 0.8– 1.5 nm and it also offers a low leakage. However, HfO2 has poor thermal stability, which results in an increase of leakage currents following thermal processes due to crystallization. The crystallization temperature of HfO2 (400–450 C) can be increased by incorporation of Al2O3 forming an HfAlO alloy, as was recently demonstrated [3–5] using two processing techniques: atomic layer deposition (ALD) and metal organic chemical vapor deposition. In this paper we demonstrate the use of electron beam gun (EBG) evaporation to deposit high quality HfAlO films close to room temperature. The effect of * Corresponding author. Tel.: +972 4 829 4694; fax: +972 4 832 2185/972 4 832 3041. E-mail address: [email protected] (G. Eisenstein).
insulator thickness on the relative dielectric constant, leakage current density as well as the current flow mechanism and the breakdown electric field (EBD) of MIS structures have been characterized before and after annealing.
2. Experimental procedure A p-type (1 0 0) Silicon wafer (q = 10–20 Xcm) was used as the substrate. Following a standard RCA cleaning process, we deposited the dielectric layers by EBG evaporation from a 10HfO2:1Al2O3 alloy. The substrate was unheated and no oxygen was added. Au gate electrodes with an area of 5 · 104 cm2 were evaporated on the oxide surface while Al was used as the back contact. rapid thermal annealing (RTA) process was carried out in a nitrogen environment up to 950 C for 2 min. The chemical composition was investigated using an X-ray photoelectron spectroscope (XPS). Capacitance– voltage (C–V) and current-electric field (J–E) measurements were carried out in wide intervals of temperatures, from 295 to 473 K.
0026-2714/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2004.11.022
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3. Results and discussion XPS measurement of a 4 nm thick films revealed an Al2O3 to HfO2 incorporation into of 1:3. The binding energies for O1s, C 1s, Hf4f and Al2p, are respectively 532, 18, 286 and 75 eV, close to the values obtained for similar films deposited by the ALD method [3]. The Si2p core-level XPS spectrum is shown in Fig. 1. The high intensity peak located at 99.45 eV is attributed to Si–Si bonds from the Si substrate and the peak at 103.2 eV is related to Si–O bonds from the interfacial layer (IL). Note that an IL exits for all as deposited samples independent on thickness. The peak related to Si–Si bonds is accompanied by a shoulder at 100 eV, which was associated in [4] with Silicate and Silicide layers. An RTA process at 950 C increases the peak intensity associated with Si–O bonds by a factor of two however, this rise is less than that in HfO2 films and is similar to Al2O3 films [5].
Fig. 2. keff and CET versus physical thickness of as-deposited and RT annealed HfAlO films. Curves 2, 2 0 and 3, 3 0 for films annealed respectively at 550 and 950 C were measured.
3.1. C–V characteristics High frequency (1 MHz) C–V measurements of as deposited structures yield a hysteresis and midgap states density (Dit) of less than 20 mV and 1 · 1011 cm2 eV1, respectively. A minimum capacitance equivalent thickness (CET) of the order of 1.69 nm (corresponding to 1.45 nm quantum mechanical corrected EOT) and an effective dielectric constant (keff) of 9–19.3 were determined from the accumulation capacitance (see curves 1 and 1 0 in Fig. 2). An IL native oxide of 0.7–1 nm and an actual dielectric constant of 21.6 were extracted from CET versus physical thickness (see curve 1 0 in Fig. 2) from respectively the intersection of the thin dashed line with the ordinate axis and its slope. The IL thickness is in the range estimated from XPS data (see Fig. 1). The positive flat band voltage (VFB = 0.4–0.5 V) slightly changes with film thickness. Similarly to Al2O3 films, the tetrahedrally coordinated Al3+ is believed to be the source of the negative charge in HfAlO. The dominant feature in C–V curves of ultra thin (<10 nm) films is a ‘‘shoulder’’ (see Fig. 3a), which is exhibited in the deple-
Fig. 1. XPS Si2p spectra of as-deposited and RT annealed at 950 C of 4 nm thick HfAlO film.
Fig. 3. Frequency dependence of C–V characteristics of MIS structure with (a) as-deposited and (b) after RT annealed at 950 C of 4 nm thick HfAlO films.
tion region near the flat band voltage. This is caused by the interface states and in particular, it is due to silicon dangling bonds [6]. The frequency dependence of C–V curves of asdeposited MIS structures shows an insignificant dispersion of the keff in the 10 KHz–1 MHz range (see Fig. 3a). This may be attributed to the series resistance associated with the device structure. The interface dangling bonds become important at low frequencies due to their long response times. It manifest itself as a flat band voltage shift to the positive direction when the frequency is reduced. The accumulation capacitance density decreases with an annealing temperature increases above 550 C. The observed decrease of keff and the consequent increase of CET (see curves 2, 3 and experimental points 2 0 , 3 0 in Fig. 2) results from the increasing of the of the IL thickness as seen in the XPS spectrum (see Fig. 1). The thickness of the IL was estimated to be about 2 nm for films annealed at 950 C (see thick dashed line intersection with ordinate axis in Fig. 2). The near doubling of the IL thickness coincides with ratio of Si–O-peak integral intensities in XPS spectra before and after anneal-
V. Mikhelashvili et al. / Microelectronics Reliability 45 (2005) 933–936
ing. Note that the reduction of the keff with annealing temperature is smaller than that for HfO2 films measured in [4]. The actual dielectric constant determined from the slope of the line (see experimental points 3 0 in Fig. 2) is reduced to 18.5 after annealing at 950 C. The flat band voltage shifts with annealing temperature in the positive direction and then saturates. This fact is related to the increase of the negative charge, which is the result of an increased concentration of tetrahedrally coordinated Al3+ with annealing temperature due to the growth of a thicker SiOx interfacial layer. Annealing reduces the hysteresis width of as deposited ultra thin films and reaches zero at Tann = 550 C, indicating a decreasing influence of charge trapping in the oxide. The annealing processes at 950 C results in an increase of Dit. For a 4 nm thick film at Tann = 950 C the value of Dit increases to 3.2 · 1011 cm2 eV1, the keff decreases to 6.7 and the IL doubles (see Figs. 1 and 2). The minimum CET increases therefore to 23.5 nm. RTA above 850 C prevents the the ‘‘shoulder’’ in the C–V curves up to 10 KHz (see Fig. 3b). Some nitrogen or oxygen atoms (there are small quantities of O2 present in the N2 gas) activated at high temperature, may diffuse towards the insulator-Si interface and saturate the dangling bonds. This process causes the reduction of the low frequency response of the interface states densities. A high temperature causes a change of the slope of the C–V curves, which is exhibited as stretching to the negative applied bias side (see curves measured at 433 and 473 K in Fig. 4). The stretch does not produce a parallel shift of the C–V similar to the case when fixed charges are present or for work functions differences. It is the result of an increase in the trap states density due to ionization processes under influence of electric field–temperature treatment. A probable explanation of the shift in the negative direction of the flat band voltage is the partial compensation of the room temperature negative interface charge by neutralization (ionization)
Fig. 4. Temperature dependence of the C–V characteristics of MIS structures with RT annealed at 950 C of 5 nm thick HfAlO film.
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of the surface states at high temperature. At 473 K, a drastic change of generation-recombination processes in the Si close to the interface causes a modification of the depletion condition and the result is a non-monotonic behavior of the C–V curve at positive applied voltages. 3.2. J–E characteristics J–E curves of as-deposited MIS structures with different HfAlO thickness, measured in the accumulation mode (with negatively biased Au electrode) are shown in Fig. 5a. The leakage current density (JL) is as small as 5 · 106 A/cm2 and 1 · 104 A/cm2 for 4 and 6 nm thick films respectively, at a total electric field of Etot = 1 and 3 MV/cm (see Fig. 5b). The EBD is 8–10 MV/cm for these films (see Fig. 5b). A sharp increase of JL up to 1.2 · 102 A/cm2 even at Etot = 1 MV/cm and reduction of EBD to 2 MV/cm (for 38 nm thick films) was observed for thick (>10 nm) films. We established that in as-deposited structures with films thicknesses of less than 10 nm and at moderate applied electric fields, the current through the insulators is limited by the Schottky effect [7] (see solid lines in Fig. 5a) with an electron barrier height (USch) of the order of 0.82–0.85 eV. For relatively thick films USch lowers to 0.61 eV and the Schottky mechanism is realized only at low electric fields, above which the current flow has Ohmic and Space Charge limited character, governed by traps with a concentration of 5.3 · 1018 cm3 and an ionization energy of 0.65 eV. JL for films thinner then 10 nm is reduces significantly with annealing temperature to 3 · 109 and 5 · 107 A/cm2, respectively at = 1 and 3 MV/cm (see Fig. 6b). The EBD, measured for various physical thicknesses
Fig. 5. (a) JL–Etot curves (symbols and solid lines are measured and calculated, respectively) and (b) EBD and JL versus physical thickness of MIS structures with as-deposited HfAlO films.
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out post deposition annealing exhibit: (a) low hysteresis and interface states density at midgap, comparable to HfO2 films, (b) high breakdown electric field for ultra thin films of about 10 MV/cm (more than HfO2) and low leakage current at applied field of 3 MV/cm, without any degradation under RTA processes. The structures withstand well high temperature annealing and temperature–electric field treatments. All these make the HfAlO films, deposited by EB technique a prospective replacement for SiO2 in complementary MIS devices.
References
Fig. 6. (a) Measured J–Etot (solid lines) and calculated JFP–Etot and JFN–Etot curves (dash lines) for structures with 5 nm thick HfAlO films after RTA at 950 C and (b) JL versus RTA temperature for two values of Etot for structures with 5 nm thick films.
reveals a weak dependence on annealing temperature. RTA process changes the current flow mechanisms at moderate as well as at high electric field regimes in the Frenkel–Poole (JFP) emission and Fowler–Nordheim (JFN) tunneling (see Fig. 6a) with a trap ionization energy and a tunneling barrier height of about 0.975 and 2.95 eV, respectively. The electron effective mass was taken to be 0.32 in the calculation of JFN. Temperatures up to 373 K do not change the current transport mechanisms. However above 373 K, the sharp rise of the leakage current density is observed for all measured structures. In conclusion we have shown that HfAlO composite films evaporated on unheated p-Si substrate even with-
[1] Wilk GD, Wallace RM, Anthony JM. High-k gate dielectrics: current status and materials properties considerations. J Appl Phys 2001;89:5243–75. [2] Mikhelashvili V, Eisenstein G, Edelman F, Brener R, Zakharov N, Werner P. Structural and electrical properties of electron beam gun evaporated Er2O3 insulator thin films. J Appl Phys 2004;95:613–20. [3] Cho MH, Chang HS, Cho YJ, Moon DW, Min KH, Sinclair R et al. Change in the chemical state and thermal stability of HfO2 by the incorporation of Al2O3. Appl Phys Lett 2004;84:571–3. [4] Cho MH, Roh YS, Whang CN, Jeong K, Nahm SW, Ko DH et al. Thermal stability and structural characteristics of HfO2 films on Si (1 0 0) grown by atomic-layer deposition. Appl Phys Lett 2002;81:472–4. [5] Yu HY, Wu N, Li MF, Zhu C, Cho BJ, Kwong DL et al. Thermal stability of (HfO2)x(Al2 O3)1x on Si. Appl Phys Lett 2002;81:3618–20. [6] Ragnarsson LA, Lundgren P. Electrical characterization of Pb centers in (1 0 0) Si–SiO2 structures: the influence of surface potential on passivation during post metallization anneal. J Appl Phys 2000;88:938–42. [7] Sze SM. Physics of semiconductors devices. 2nd ed. New York: Wiley; 1981.
Electrical properties of MIS capacitor using low temperature electron beam gun—evaporated HfAlO dielectrics V. Mikhelashvili, B. Meyler, J. Shneider, O. Kreinin, G. Eisenstein
*
Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 3200, Israel Received 28 June 2004; received in revised form 4 October 2004 Available online 22 December 2004
Abstract A low effective oxide thickness of 1.45 nm was achieved in HfAlO films deposited by an electron beam gun evaporator on unheated p-Si substrate. A reduction of the leakage current density from 1 · 104 to 4.5 · 107 A/cm2, at an electric field 3 MV/cm, with annealing temperature and a breakdown electric field of 10 MV/cm were demonstrated for ultra thin films. 2004 Elsevier Ltd. All rights reserved.
1. Introduction Hafnium oxide is being studied extensively as a promising high permittivity insulator with properties, which are similar to Ta2O5 [1] and better than other rare-earth metal oxides [2]. The predicted equivalent oxide thickness (EOT) of HfO2 is of the order of 0.8– 1.5 nm and it also offers a low leakage. However, HfO2 has poor thermal stability, which results in an increase of leakage currents following thermal processes due to crystallization. The crystallization temperature of HfO2 (400–450 C) can be increased by incorporation of Al2O3 forming an HfAlO alloy, as was recently demonstrated [3–5] using two processing techniques: atomic layer deposition (ALD) and metal organic chemical vapor deposition. In this paper we demonstrate the use of electron beam gun (EBG) evaporation to deposit high quality HfAlO films close to room temperature. The effect of * Corresponding author. Tel.: +972 4 829 4694; fax: +972 4 832 2185/972 4 832 3041. E-mail address: [email protected] (G. Eisenstein).
insulator thickness on the relative dielectric constant, leakage current density as well as the current flow mechanism and the breakdown electric field (EBD) of MIS structures have been characterized before and after annealing.
2. Experimental procedure A p-type (1 0 0) Silicon wafer (q = 10–20 Xcm) was used as the substrate. Following a standard RCA cleaning process, we deposited the dielectric layers by EBG evaporation from a 10HfO2:1Al2O3 alloy. The substrate was unheated and no oxygen was added. Au gate electrodes with an area of 5 · 104 cm2 were evaporated on the oxide surface while Al was used as the back contact. rapid thermal annealing (RTA) process was carried out in a nitrogen environment up to 950 C for 2 min. The chemical composition was investigated using an X-ray photoelectron spectroscope (XPS). Capacitance– voltage (C–V) and current-electric field (J–E) measurements were carried out in wide intervals of temperatures, from 295 to 473 K.
0026-2714/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2004.11.022
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V. Mikhelashvili et al. / Microelectronics Reliability 45 (2005) 933–936
3. Results and discussion XPS measurement of a 4 nm thick films revealed an Al2O3 to HfO2 incorporation into of 1:3. The binding energies for O1s, C 1s, Hf4f and Al2p, are respectively 532, 18, 286 and 75 eV, close to the values obtained for similar films deposited by the ALD method [3]. The Si2p core-level XPS spectrum is shown in Fig. 1. The high intensity peak located at 99.45 eV is attributed to Si–Si bonds from the Si substrate and the peak at 103.2 eV is related to Si–O bonds from the interfacial layer (IL). Note that an IL exits for all as deposited samples independent on thickness. The peak related to Si–Si bonds is accompanied by a shoulder at 100 eV, which was associated in [4] with Silicate and Silicide layers. An RTA process at 950 C increases the peak intensity associated with Si–O bonds by a factor of two however, this rise is less than that in HfO2 films and is similar to Al2O3 films [5].
Fig. 2. keff and CET versus physical thickness of as-deposited and RT annealed HfAlO films. Curves 2, 2 0 and 3, 3 0 for films annealed respectively at 550 and 950 C were measured.
3.1. C–V characteristics High frequency (1 MHz) C–V measurements of as deposited structures yield a hysteresis and midgap states density (Dit) of less than 20 mV and 1 · 1011 cm2 eV1, respectively. A minimum capacitance equivalent thickness (CET) of the order of 1.69 nm (corresponding to 1.45 nm quantum mechanical corrected EOT) and an effective dielectric constant (keff) of 9–19.3 were determined from the accumulation capacitance (see curves 1 and 1 0 in Fig. 2). An IL native oxide of 0.7–1 nm and an actual dielectric constant of 21.6 were extracted from CET versus physical thickness (see curve 1 0 in Fig. 2) from respectively the intersection of the thin dashed line with the ordinate axis and its slope. The IL thickness is in the range estimated from XPS data (see Fig. 1). The positive flat band voltage (VFB = 0.4–0.5 V) slightly changes with film thickness. Similarly to Al2O3 films, the tetrahedrally coordinated Al3+ is believed to be the source of the negative charge in HfAlO. The dominant feature in C–V curves of ultra thin (<10 nm) films is a ‘‘shoulder’’ (see Fig. 3a), which is exhibited in the deple-
Fig. 1. XPS Si2p spectra of as-deposited and RT annealed at 950 C of 4 nm thick HfAlO film.
Fig. 3. Frequency dependence of C–V characteristics of MIS structure with (a) as-deposited and (b) after RT annealed at 950 C of 4 nm thick HfAlO films.
tion region near the flat band voltage. This is caused by the interface states and in particular, it is due to silicon dangling bonds [6]. The frequency dependence of C–V curves of asdeposited MIS structures shows an insignificant dispersion of the keff in the 10 KHz–1 MHz range (see Fig. 3a). This may be attributed to the series resistance associated with the device structure. The interface dangling bonds become important at low frequencies due to their long response times. It manifest itself as a flat band voltage shift to the positive direction when the frequency is reduced. The accumulation capacitance density decreases with an annealing temperature increases above 550 C. The observed decrease of keff and the consequent increase of CET (see curves 2, 3 and experimental points 2 0 , 3 0 in Fig. 2) results from the increasing of the of the IL thickness as seen in the XPS spectrum (see Fig. 1). The thickness of the IL was estimated to be about 2 nm for films annealed at 950 C (see thick dashed line intersection with ordinate axis in Fig. 2). The near doubling of the IL thickness coincides with ratio of Si–O-peak integral intensities in XPS spectra before and after anneal-
V. Mikhelashvili et al. / Microelectronics Reliability 45 (2005) 933–936
ing. Note that the reduction of the keff with annealing temperature is smaller than that for HfO2 films measured in [4]. The actual dielectric constant determined from the slope of the line (see experimental points 3 0 in Fig. 2) is reduced to 18.5 after annealing at 950 C. The flat band voltage shifts with annealing temperature in the positive direction and then saturates. This fact is related to the increase of the negative charge, which is the result of an increased concentration of tetrahedrally coordinated Al3+ with annealing temperature due to the growth of a thicker SiOx interfacial layer. Annealing reduces the hysteresis width of as deposited ultra thin films and reaches zero at Tann = 550 C, indicating a decreasing influence of charge trapping in the oxide. The annealing processes at 950 C results in an increase of Dit. For a 4 nm thick film at Tann = 950 C the value of Dit increases to 3.2 · 1011 cm2 eV1, the keff decreases to 6.7 and the IL doubles (see Figs. 1 and 2). The minimum CET increases therefore to 23.5 nm. RTA above 850 C prevents the the ‘‘shoulder’’ in the C–V curves up to 10 KHz (see Fig. 3b). Some nitrogen or oxygen atoms (there are small quantities of O2 present in the N2 gas) activated at high temperature, may diffuse towards the insulator-Si interface and saturate the dangling bonds. This process causes the reduction of the low frequency response of the interface states densities. A high temperature causes a change of the slope of the C–V curves, which is exhibited as stretching to the negative applied bias side (see curves measured at 433 and 473 K in Fig. 4). The stretch does not produce a parallel shift of the C–V similar to the case when fixed charges are present or for work functions differences. It is the result of an increase in the trap states density due to ionization processes under influence of electric field–temperature treatment. A probable explanation of the shift in the negative direction of the flat band voltage is the partial compensation of the room temperature negative interface charge by neutralization (ionization)
Fig. 4. Temperature dependence of the C–V characteristics of MIS structures with RT annealed at 950 C of 5 nm thick HfAlO film.
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of the surface states at high temperature. At 473 K, a drastic change of generation-recombination processes in the Si close to the interface causes a modification of the depletion condition and the result is a non-monotonic behavior of the C–V curve at positive applied voltages. 3.2. J–E characteristics J–E curves of as-deposited MIS structures with different HfAlO thickness, measured in the accumulation mode (with negatively biased Au electrode) are shown in Fig. 5a. The leakage current density (JL) is as small as 5 · 106 A/cm2 and 1 · 104 A/cm2 for 4 and 6 nm thick films respectively, at a total electric field of Etot = 1 and 3 MV/cm (see Fig. 5b). The EBD is 8–10 MV/cm for these films (see Fig. 5b). A sharp increase of JL up to 1.2 · 102 A/cm2 even at Etot = 1 MV/cm and reduction of EBD to 2 MV/cm (for 38 nm thick films) was observed for thick (>10 nm) films. We established that in as-deposited structures with films thicknesses of less than 10 nm and at moderate applied electric fields, the current through the insulators is limited by the Schottky effect [7] (see solid lines in Fig. 5a) with an electron barrier height (USch) of the order of 0.82–0.85 eV. For relatively thick films USch lowers to 0.61 eV and the Schottky mechanism is realized only at low electric fields, above which the current flow has Ohmic and Space Charge limited character, governed by traps with a concentration of 5.3 · 1018 cm3 and an ionization energy of 0.65 eV. JL for films thinner then 10 nm is reduces significantly with annealing temperature to 3 · 109 and 5 · 107 A/cm2, respectively at = 1 and 3 MV/cm (see Fig. 6b). The EBD, measured for various physical thicknesses
Fig. 5. (a) JL–Etot curves (symbols and solid lines are measured and calculated, respectively) and (b) EBD and JL versus physical thickness of MIS structures with as-deposited HfAlO films.
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out post deposition annealing exhibit: (a) low hysteresis and interface states density at midgap, comparable to HfO2 films, (b) high breakdown electric field for ultra thin films of about 10 MV/cm (more than HfO2) and low leakage current at applied field of 3 MV/cm, without any degradation under RTA processes. The structures withstand well high temperature annealing and temperature–electric field treatments. All these make the HfAlO films, deposited by EB technique a prospective replacement for SiO2 in complementary MIS devices.
References
Fig. 6. (a) Measured J–Etot (solid lines) and calculated JFP–Etot and JFN–Etot curves (dash lines) for structures with 5 nm thick HfAlO films after RTA at 950 C and (b) JL versus RTA temperature for two values of Etot for structures with 5 nm thick films.
reveals a weak dependence on annealing temperature. RTA process changes the current flow mechanisms at moderate as well as at high electric field regimes in the Frenkel–Poole (JFP) emission and Fowler–Nordheim (JFN) tunneling (see Fig. 6a) with a trap ionization energy and a tunneling barrier height of about 0.975 and 2.95 eV, respectively. The electron effective mass was taken to be 0.32 in the calculation of JFN. Temperatures up to 373 K do not change the current transport mechanisms. However above 373 K, the sharp rise of the leakage current density is observed for all measured structures. In conclusion we have shown that HfAlO composite films evaporated on unheated p-Si substrate even with-
[1] Wilk GD, Wallace RM, Anthony JM. High-k gate dielectrics: current status and materials properties considerations. J Appl Phys 2001;89:5243–75. [2] Mikhelashvili V, Eisenstein G, Edelman F, Brener R, Zakharov N, Werner P. Structural and electrical properties of electron beam gun evaporated Er2O3 insulator thin films. J Appl Phys 2004;95:613–20. [3] Cho MH, Chang HS, Cho YJ, Moon DW, Min KH, Sinclair R et al. Change in the chemical state and thermal stability of HfO2 by the incorporation of Al2O3. Appl Phys Lett 2004;84:571–3. [4] Cho MH, Roh YS, Whang CN, Jeong K, Nahm SW, Ko DH et al. Thermal stability and structural characteristics of HfO2 films on Si (1 0 0) grown by atomic-layer deposition. Appl Phys Lett 2002;81:472–4. [5] Yu HY, Wu N, Li MF, Zhu C, Cho BJ, Kwong DL et al. Thermal stability of (HfO2)x(Al2 O3)1x on Si. Appl Phys Lett 2002;81:3618–20. [6] Ragnarsson LA, Lundgren P. Electrical characterization of Pb centers in (1 0 0) Si–SiO2 structures: the influence of surface potential on passivation during post metallization anneal. J Appl Phys 2000;88:938–42. [7] Sze SM. Physics of semiconductors devices. 2nd ed. New York: Wiley; 1981.