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Electrical characterization of HfO2 films obtained by UV assisted injection MOCVD
Microelectronics Reliability 45 (2005) 929–932 www.elsevier.com/locate/microrel
Electrical characterization of HfO2 films obtained by UV assisted injection MOCVD J.M. Decams a,*, H. Guillon a, C. Jime´nez b,*, M. Audier b, J.P. Se´nateur b, C. Dubourdieu b, O. Cadix c, B.J. OÕSullivan c, M. Modreanu c, P.K. Hurley c, S. Rusworth d, T.J. Leedham d, H. Davies d, Q. Fang e, I. Boyd e a
b
Qualiflow-Jipelec, Parc du Mille´naire, 395 rue Louis Lepine, BP7, 34935 Montpellier Cedex 9, France Laboratoire des Mate´riaux et du Ge´nie Physique, UMR CNRS 5628 ENSPG, B.P. 46, Saint Martin d’He`res, France c NMRC, University College Cork, Lee Maltings, Prospect Row, Cork, Ireland d Epichem Limited, Power Road, Bromborough, Wirral, Merseyside CH62 3QF, UK e UCL, Torrington Place, London WCIE 7JE, UK Received 28 June 2004; received in revised form 6 October 2004
Abstract HfO2 films were deposited at low temperature (400 C) by UV assisted injection metal-organic chemical vapor deposition (UVI-MOCVD). A three-step process was used for this study, consisting of (A) Pre-deposition anneal for nitridation; (B) Deposition step; (C) Post-deposition annealing in oxygen. Special attention was paid to the effect of UV exposure during these steps. Films were characterized by physical, optical and electrical techniques. Thickness was determined by different methods (X-ray Reflectrometry (XRR), spectroscopic ellipsometry and transmission electron microscopy) and a good agreement was found for all samples. The HfO2 permittivity, equivalent oxide thickness (EOT), flat-band voltage (Vfb) and total charge (Qt) were extracted from the CV response at high frequency taking into account the HfO2 and SiO2 thicknesses obtained by XRR. The calculated permittivity values were in the range 7–13, i.e. lower than theoretical values for the monoclinic phase. Explanations are suggested in the context of the other characterizations. JEeff characteristics were constructed taking into account the EOT values (Eeff = V/EOT). Effective breakdown fields range between 8.7 and 16.9 MV/cm. No dependence of Eeff with UV exposure was found. 2004 Elsevier Ltd. All rights reserved.
1. Introduction The scaling limit of SiO2 as the gate dielectric in complementary metal-oxide-semiconductor devices comes from the increase of the leakage current and also from the intrinsic reliability. High-k materials are currently
*
Corresponding authors.
being investigated to replace SiO2, allowing an increase of film thickness. Hafnium oxide (HfO2) is receiving intensive investigation as an attractive alternative gate dielectric due to its high dielectric constant (k 20–25) [1] and thermal stability in contact with silicon. In this work, we have studied HfO2 films deposited at low temperature (400 C) by UV assisted injection metal-organic chemical vapor deposition. The work has been mainly focused on the effect of the UV exposure during the process.
0026-2714/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2004.11.023
930
J.M. Decams et al. / Microelectronics Reliability 45 (2005) 929–932
2. Experimental HfO2 films were deposited at low temperature (400 C) by UV assisted injection metal-organic chemical vapor deposition (UVI-MOCVD). An industrial prototype was used equipped to work with up to 200 mm substrates and containing KrF* excimer UV lamps at 222 nm (5.58 eV) to assist the deposition process. A solution of Hf(mmp)2(OtBu)2 precursor dissolved in octane at 0.05 mol/l was used for our experiments. The precursor injector worked at 2 ms and 3 Hz for an evaporation temperature of 150 C and a working pressure in the reactor of 5 Torr. Other deposition conditions were total gas flow-rate 2000 sccm at 50% of O2. Substrates preparation consisted on a standard SC1/ SC2 clean, but not SiO2 etching. Depositions with labscale reactors [2,3] have enabled the optimized process sequences to be established. The industrial prototype reactor allows pre-deposition surface preparation, high-k deposition and post-deposition annealing in a single reaction chamber, and a three-step process was used for this study, consisting of: (A) Pre-deposition anneal for nitridation: T = 800 C for 5 min at atmospheric pressure of N2 (to stabilize the silicon surface). (B) Deposition step, 3 Torr, Ts = 400 C, number of pulses = 2400. (C) Post-deposition annealing in O2: T = 800 C for 3 min at atmospheric pressure of O2 (to generate a stoichiometric oxide). The three steps were processed consecutively. An experimental matrix was generated to investigate the effect of UV exposure, summarized in Table 1. To characterize the sample optically and physically three different techniques were used: X-ray Reflectrometry (XRR), Spectroscopic Ellipsometry (SE), and Transmission Electron Microscopy (TEM).
The electrical characterization was performed by high-frequency Capacitance–Voltage (CV), and Current–Voltage (IV) characterisation. For the CV measurements, the meter used was a HP 4284A LCR meter and the IV characterisation was performed using a HP 4156A parameter analyzer.
3. Results and discussion The thicknesses of the HfO2 films and SiO2 layers formed at the Si interface were determined by different methods. Table 2 compares these values. The thickness results of the HfO2 layers calculated by XRR and SE are compared in Fig. 1. The influence of the UV irradiation during the deposition step is clearly noted in these results. The films grown without UV exposition are thinner in at least a factor of 2. The interaction of the lamps radiation (wavelength 222 nm) with the precursor and processing gas is demonstrated.
Table 2 ˚ ) obtained by different characterization techniques Thickness (A Sample
H1 H2 H3 H4 H5 H6 H7 H8
XRR
TEM
SiO2
HfO2
11 4 10 10 13 30 35 9
279 256 89 104 225 256 243 74
SiO2
SE HfO2
20 22
100 114
20
270
SiO2
HfO2
18 11 27 30 20 27 19 26
319 301 107 121 232 266 268 87
350 300
Sample
H1 H2 H3 H4 H5 H6 H7 H8
Silicon type
UV Exposure Nitridation annealing
Deposition Step
Oxygenation annealing
n n n n n n p p
Yes No Yes No Yes No Yes Yes
Yes Yes No No Yes Yes Yes No
No No No No Yes Yes No No
T HfO2(Å) by XRR
H1
Table 1 Substrate type and UV exposure for the grown samples
250
H5
H6 H7
H2
200 150 100
H8
H4 H3
50 50
100
150
200
250
300
350
T HfO2(Å) by SE
Fig. 1. Comparision of HfO2 thickness obtained by different techniques.
J.M. Decams et al. / Microelectronics Reliability 45 (2005) 929–932
The thickness results of the HfO2 layers calculated by XRR and SE are compared in Fig. 1. A rather good agreement is observed between the two techniques. Points measured by TEM are also included as open circles. Silicon oxide is rather thin, even with a post-deposition anneal at high temperature. TEM observations also showed that HfO2 layers are crystallized in the monoclinic phase while the SiO2 layers are 2 nm thick. A curious feature when looking in plan view is that all these crystalline layers contain small bubbles or voids ˚ diameter inside the HfO2 layer, indepenof about 50 A dent of the experimental conditions. This feature was not observed in samples obtained by the same process in lab-scale reactor, nor in samples deposited in the same prototype reactor without annealing. It seems that bubbles are produced by the concatenation of the steps, avoiding the desorption of non-reacted molecules which keep trapped in the oxide during post-deposition annealing. Electrical analysis was performed on Au/HfO2/ Si(1 0 0) MIS structures. High frequency CV analysis exhibited negligible capacitance dispersion in accumulation (1 kHz to 100 kHz, see Fig. 2), and the dependence of Cmax versus area was linear as expected, allowing extraction of the HfO2 permittivity, and equivalent oxide thickness (EOT). The HfO2 permittivity was determined from the HfO2 and interface SiO2 thickness values from XRR and TEM analysis, combined with the accumulation capacitance data. EOT, HfO2 permittivity, flat-band voltage (Vfb) and total charge (Qt) values as determined from these measurements are presented in Table 3. Qt values were calculated with the gold work function equal to 5.1 eV. H1 and H2 are the only samples presenting a low frequency response in the high frequency CV characteristics, which suggests the presence of negative charge groups on the surface. The film grown at the same conditions on p-type substrate (H7) does not present the
931
Table 3 EOT, permittivity, Vfb and Qt values obtained from CV measurements. Type of silicon is indicated in brackets Sample
EOT (nm)
er HfO2
Vfb (V)
H1 H2 H3 H4 H5 H6 H7 H8
11.2 10.3 5.9 6.6 9.2 10.8 11.1 5.8
10.9 10.2 6.9 7.2 11.2 12.8 12.3 5.8
1.1 1.4 0.67 0.56 0.62 0.58 2.36 0.61
(n) (n) (n) (n) (n) (n) (p) (p)
Qt (cm 2) 5.95 · 1011 1.24 · 1012 3.76 · 1011 7.14 · 1011 3.63 · 1011 3.83 · 1011 4.1 · 1012 1.39 · 1012
same behavior, which would confirm the hypothesis of negative charge. When analyzing the effect of the pre-deposition anneal on the permittivity values, no difference are found due to UV irradiation. For the post-deposition anneal, a slight increase in er appear when using UV exposure. Nevertheless, the differences in calculated HfO2 permittivity are essentially links to EOT values. In all cases, the permittivity results are lower than theoretical values (er HfO2 = 20–25); this difference may be due to the bubbles observed in the crystalline HfO2 layer. Other possible explanations may be that the permittivity of the interfacial layer is higher than for pure SiO2 (3.9), or more likely, that another layer of low permittivity is present at the HfO2/gold interface, reducing the overall capacitance of the stack. This effect is more pronounced for the thinner films. Current–Voltage (IV) measurements were performed in accumulation mode on n-type and p-type MIS structures, and JEeff characteristics were constructed taking into account the EOT values (Eeff = V/EOT). JV responses were reproducible and independent of contact area (see Fig. 3). Effective breakdown fields range between 8.7 and 16.9 MV/cm. These values are summarized in Table 4. -3
10 8.00E-011
-4
10
7.00E-011 -5
10
C [F ]
F=1 k Hz F=10 kHz F=100 kHz
4.00E-011 3.00E-011
2
5.00E-011
J [ A/cm ]
6.00E-011
-6
10
-7
-4
2
-4
2
area=2.3x10 cm
10
0.00E+000
2
area=1.2x10 cm
-9
1.00E-011
2
-5
area=4.9x10 cm
-8
10
2.00E-011
-5
area=2.5x10 cm
10
-10
10 -2
-1
0
1
2
3
4
Vg [V] Fig. 2. CV response for sample H3 at 1, 10 and 100 kHz.
0
2
4
6
Vsub [V] Fig. 3. IV response for sample H8.
8
10
932
J.M. Decams et al. / Microelectronics Reliability 45 (2005) 929–932
Table 4 EBD values obtained from IV curves Sample
EBD MV/cm
H1 H2 H3 H4 H5 H6 H7 H8
15.8 14.5 10.8 8.7 16.9 15.3 10.8 14.2
n-type n-type n-type n-type n-type n-type p-type p-type
The different deposition conditions do not seem to affect the breakdown fields, except for samples grown without UV irradiation during deposition. Nevertheless, EOT values are also lower for this sample, which would involve an increase of tunneling contribution to current. This factor should be corroborated on samples with comparable thickness.
4. Conclusions HfO2 films with EOT thickness between 5.8 and 11.2 nm were deposited by UV assisted Injection MOCVD at low temperature. The study was focused on the influence of the UV irradiation during each step of the process (pre-deposition, deposition or post-deposition anneal). UV irradiation provokes an increase of the deposition rate, which confirms the interaction of the wave-
length used (222 nm) with the active species of the reaction. The samples deposited without UV irradiation show a lower permittivity value. Nevertheless, this effect seems rely more on the interface nature (with silicon or at the metallization level) than on the UV influence. We have not found essential differences in the other parameters when using or not UV radiation on the other steps. The permittivity values obtained for all the sample were lower than the theoretical one, this fact seems due to the presence of bubbles. As inferred in this paper, this bubbles are due to the use of the successive steps during process, which does not allow the desorption of products. Further studies have to be performed to confirm this hypothesis and to increase the permittivity values.
Acknowledgement This work has been supported by the European Commission under IST Research Project No. 10541 (TOPSII).
References [1] Zhao X, Vanderbilt D. Phys Rev B 2002;65:233106. [2] Roussel F, Roussel H, Audier M, Dubourdieu C, Se´nateur JP, Jime´nez C, et al. ECS Proceedings, EuroCVD-14 Vol. 2003-08. p. 1508–13. [3] Fang Q, Zhang J-Y, Wang ZM, Wu JX, OÕSullivan BJ, Hurley PK et al. Thin Solid Films 2003;427:391–6.
Electrical characterization of HfO2 films obtained by UV assisted injection MOCVD J.M. Decams a,*, H. Guillon a, C. Jime´nez b,*, M. Audier b, J.P. Se´nateur b, C. Dubourdieu b, O. Cadix c, B.J. OÕSullivan c, M. Modreanu c, P.K. Hurley c, S. Rusworth d, T.J. Leedham d, H. Davies d, Q. Fang e, I. Boyd e a
b
Qualiflow-Jipelec, Parc du Mille´naire, 395 rue Louis Lepine, BP7, 34935 Montpellier Cedex 9, France Laboratoire des Mate´riaux et du Ge´nie Physique, UMR CNRS 5628 ENSPG, B.P. 46, Saint Martin d’He`res, France c NMRC, University College Cork, Lee Maltings, Prospect Row, Cork, Ireland d Epichem Limited, Power Road, Bromborough, Wirral, Merseyside CH62 3QF, UK e UCL, Torrington Place, London WCIE 7JE, UK Received 28 June 2004; received in revised form 6 October 2004
Abstract HfO2 films were deposited at low temperature (400 C) by UV assisted injection metal-organic chemical vapor deposition (UVI-MOCVD). A three-step process was used for this study, consisting of (A) Pre-deposition anneal for nitridation; (B) Deposition step; (C) Post-deposition annealing in oxygen. Special attention was paid to the effect of UV exposure during these steps. Films were characterized by physical, optical and electrical techniques. Thickness was determined by different methods (X-ray Reflectrometry (XRR), spectroscopic ellipsometry and transmission electron microscopy) and a good agreement was found for all samples. The HfO2 permittivity, equivalent oxide thickness (EOT), flat-band voltage (Vfb) and total charge (Qt) were extracted from the CV response at high frequency taking into account the HfO2 and SiO2 thicknesses obtained by XRR. The calculated permittivity values were in the range 7–13, i.e. lower than theoretical values for the monoclinic phase. Explanations are suggested in the context of the other characterizations. JEeff characteristics were constructed taking into account the EOT values (Eeff = V/EOT). Effective breakdown fields range between 8.7 and 16.9 MV/cm. No dependence of Eeff with UV exposure was found. 2004 Elsevier Ltd. All rights reserved.
1. Introduction The scaling limit of SiO2 as the gate dielectric in complementary metal-oxide-semiconductor devices comes from the increase of the leakage current and also from the intrinsic reliability. High-k materials are currently
*
Corresponding authors.
being investigated to replace SiO2, allowing an increase of film thickness. Hafnium oxide (HfO2) is receiving intensive investigation as an attractive alternative gate dielectric due to its high dielectric constant (k 20–25) [1] and thermal stability in contact with silicon. In this work, we have studied HfO2 films deposited at low temperature (400 C) by UV assisted injection metal-organic chemical vapor deposition. The work has been mainly focused on the effect of the UV exposure during the process.
0026-2714/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2004.11.023
930
J.M. Decams et al. / Microelectronics Reliability 45 (2005) 929–932
2. Experimental HfO2 films were deposited at low temperature (400 C) by UV assisted injection metal-organic chemical vapor deposition (UVI-MOCVD). An industrial prototype was used equipped to work with up to 200 mm substrates and containing KrF* excimer UV lamps at 222 nm (5.58 eV) to assist the deposition process. A solution of Hf(mmp)2(OtBu)2 precursor dissolved in octane at 0.05 mol/l was used for our experiments. The precursor injector worked at 2 ms and 3 Hz for an evaporation temperature of 150 C and a working pressure in the reactor of 5 Torr. Other deposition conditions were total gas flow-rate 2000 sccm at 50% of O2. Substrates preparation consisted on a standard SC1/ SC2 clean, but not SiO2 etching. Depositions with labscale reactors [2,3] have enabled the optimized process sequences to be established. The industrial prototype reactor allows pre-deposition surface preparation, high-k deposition and post-deposition annealing in a single reaction chamber, and a three-step process was used for this study, consisting of: (A) Pre-deposition anneal for nitridation: T = 800 C for 5 min at atmospheric pressure of N2 (to stabilize the silicon surface). (B) Deposition step, 3 Torr, Ts = 400 C, number of pulses = 2400. (C) Post-deposition annealing in O2: T = 800 C for 3 min at atmospheric pressure of O2 (to generate a stoichiometric oxide). The three steps were processed consecutively. An experimental matrix was generated to investigate the effect of UV exposure, summarized in Table 1. To characterize the sample optically and physically three different techniques were used: X-ray Reflectrometry (XRR), Spectroscopic Ellipsometry (SE), and Transmission Electron Microscopy (TEM).
The electrical characterization was performed by high-frequency Capacitance–Voltage (CV), and Current–Voltage (IV) characterisation. For the CV measurements, the meter used was a HP 4284A LCR meter and the IV characterisation was performed using a HP 4156A parameter analyzer.
3. Results and discussion The thicknesses of the HfO2 films and SiO2 layers formed at the Si interface were determined by different methods. Table 2 compares these values. The thickness results of the HfO2 layers calculated by XRR and SE are compared in Fig. 1. The influence of the UV irradiation during the deposition step is clearly noted in these results. The films grown without UV exposition are thinner in at least a factor of 2. The interaction of the lamps radiation (wavelength 222 nm) with the precursor and processing gas is demonstrated.
Table 2 ˚ ) obtained by different characterization techniques Thickness (A Sample
H1 H2 H3 H4 H5 H6 H7 H8
XRR
TEM
SiO2
HfO2
11 4 10 10 13 30 35 9
279 256 89 104 225 256 243 74
SiO2
SE HfO2
20 22
100 114
20
270
SiO2
HfO2
18 11 27 30 20 27 19 26
319 301 107 121 232 266 268 87
350 300
Sample
H1 H2 H3 H4 H5 H6 H7 H8
Silicon type
UV Exposure Nitridation annealing
Deposition Step
Oxygenation annealing
n n n n n n p p
Yes No Yes No Yes No Yes Yes
Yes Yes No No Yes Yes Yes No
No No No No Yes Yes No No
T HfO2(Å) by XRR
H1
Table 1 Substrate type and UV exposure for the grown samples
250
H5
H6 H7
H2
200 150 100
H8
H4 H3
50 50
100
150
200
250
300
350
T HfO2(Å) by SE
Fig. 1. Comparision of HfO2 thickness obtained by different techniques.
J.M. Decams et al. / Microelectronics Reliability 45 (2005) 929–932
The thickness results of the HfO2 layers calculated by XRR and SE are compared in Fig. 1. A rather good agreement is observed between the two techniques. Points measured by TEM are also included as open circles. Silicon oxide is rather thin, even with a post-deposition anneal at high temperature. TEM observations also showed that HfO2 layers are crystallized in the monoclinic phase while the SiO2 layers are 2 nm thick. A curious feature when looking in plan view is that all these crystalline layers contain small bubbles or voids ˚ diameter inside the HfO2 layer, indepenof about 50 A dent of the experimental conditions. This feature was not observed in samples obtained by the same process in lab-scale reactor, nor in samples deposited in the same prototype reactor without annealing. It seems that bubbles are produced by the concatenation of the steps, avoiding the desorption of non-reacted molecules which keep trapped in the oxide during post-deposition annealing. Electrical analysis was performed on Au/HfO2/ Si(1 0 0) MIS structures. High frequency CV analysis exhibited negligible capacitance dispersion in accumulation (1 kHz to 100 kHz, see Fig. 2), and the dependence of Cmax versus area was linear as expected, allowing extraction of the HfO2 permittivity, and equivalent oxide thickness (EOT). The HfO2 permittivity was determined from the HfO2 and interface SiO2 thickness values from XRR and TEM analysis, combined with the accumulation capacitance data. EOT, HfO2 permittivity, flat-band voltage (Vfb) and total charge (Qt) values as determined from these measurements are presented in Table 3. Qt values were calculated with the gold work function equal to 5.1 eV. H1 and H2 are the only samples presenting a low frequency response in the high frequency CV characteristics, which suggests the presence of negative charge groups on the surface. The film grown at the same conditions on p-type substrate (H7) does not present the
931
Table 3 EOT, permittivity, Vfb and Qt values obtained from CV measurements. Type of silicon is indicated in brackets Sample
EOT (nm)
er HfO2
Vfb (V)
H1 H2 H3 H4 H5 H6 H7 H8
11.2 10.3 5.9 6.6 9.2 10.8 11.1 5.8
10.9 10.2 6.9 7.2 11.2 12.8 12.3 5.8
1.1 1.4 0.67 0.56 0.62 0.58 2.36 0.61
(n) (n) (n) (n) (n) (n) (p) (p)
Qt (cm 2) 5.95 · 1011 1.24 · 1012 3.76 · 1011 7.14 · 1011 3.63 · 1011 3.83 · 1011 4.1 · 1012 1.39 · 1012
same behavior, which would confirm the hypothesis of negative charge. When analyzing the effect of the pre-deposition anneal on the permittivity values, no difference are found due to UV irradiation. For the post-deposition anneal, a slight increase in er appear when using UV exposure. Nevertheless, the differences in calculated HfO2 permittivity are essentially links to EOT values. In all cases, the permittivity results are lower than theoretical values (er HfO2 = 20–25); this difference may be due to the bubbles observed in the crystalline HfO2 layer. Other possible explanations may be that the permittivity of the interfacial layer is higher than for pure SiO2 (3.9), or more likely, that another layer of low permittivity is present at the HfO2/gold interface, reducing the overall capacitance of the stack. This effect is more pronounced for the thinner films. Current–Voltage (IV) measurements were performed in accumulation mode on n-type and p-type MIS structures, and JEeff characteristics were constructed taking into account the EOT values (Eeff = V/EOT). JV responses were reproducible and independent of contact area (see Fig. 3). Effective breakdown fields range between 8.7 and 16.9 MV/cm. These values are summarized in Table 4. -3
10 8.00E-011
-4
10
7.00E-011 -5
10
C [F ]
F=1 k Hz F=10 kHz F=100 kHz
4.00E-011 3.00E-011
2
5.00E-011
J [ A/cm ]
6.00E-011
-6
10
-7
-4
2
-4
2
area=2.3x10 cm
10
0.00E+000
2
area=1.2x10 cm
-9
1.00E-011
2
-5
area=4.9x10 cm
-8
10
2.00E-011
-5
area=2.5x10 cm
10
-10
10 -2
-1
0
1
2
3
4
Vg [V] Fig. 2. CV response for sample H3 at 1, 10 and 100 kHz.
0
2
4
6
Vsub [V] Fig. 3. IV response for sample H8.
8
10
932
J.M. Decams et al. / Microelectronics Reliability 45 (2005) 929–932
Table 4 EBD values obtained from IV curves Sample
EBD MV/cm
H1 H2 H3 H4 H5 H6 H7 H8
15.8 14.5 10.8 8.7 16.9 15.3 10.8 14.2
n-type n-type n-type n-type n-type n-type p-type p-type
The different deposition conditions do not seem to affect the breakdown fields, except for samples grown without UV irradiation during deposition. Nevertheless, EOT values are also lower for this sample, which would involve an increase of tunneling contribution to current. This factor should be corroborated on samples with comparable thickness.
4. Conclusions HfO2 films with EOT thickness between 5.8 and 11.2 nm were deposited by UV assisted Injection MOCVD at low temperature. The study was focused on the influence of the UV irradiation during each step of the process (pre-deposition, deposition or post-deposition anneal). UV irradiation provokes an increase of the deposition rate, which confirms the interaction of the wave-
length used (222 nm) with the active species of the reaction. The samples deposited without UV irradiation show a lower permittivity value. Nevertheless, this effect seems rely more on the interface nature (with silicon or at the metallization level) than on the UV influence. We have not found essential differences in the other parameters when using or not UV radiation on the other steps. The permittivity values obtained for all the sample were lower than the theoretical one, this fact seems due to the presence of bubbles. As inferred in this paper, this bubbles are due to the use of the successive steps during process, which does not allow the desorption of products. Further studies have to be performed to confirm this hypothesis and to increase the permittivity values.
Acknowledgement This work has been supported by the European Commission under IST Research Project No. 10541 (TOPSII).
References [1] Zhao X, Vanderbilt D. Phys Rev B 2002;65:233106. [2] Roussel F, Roussel H, Audier M, Dubourdieu C, Se´nateur JP, Jime´nez C, et al. ECS Proceedings, EuroCVD-14 Vol. 2003-08. p. 1508–13. [3] Fang Q, Zhang J-Y, Wang ZM, Wu JX, OÕSullivan BJ, Hurley PK et al. Thin Solid Films 2003;427:391–6.