Si interfaces by 1 nm silicon nitride layer

Surface Science 602 (2008) 1948–1953

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Complete prevention of reaction at HfO2/Si interfaces by 1 nm silicon nitride layer Hikaru Kobayashi a,b,*, Kentaro Imamura a,b, Ken-ichi Fukayama a,b, Sung-Soon Im a,b, Osamu Maida a,b, Young-Bae Kim c, Hyun-Chul Kim c, Duck-Kyun Choi c a

Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan CREST, Japan Science and Technology, Kawaguchi, Japan c Division of Materials Science and Engineering, Hanyang University, Seongdong Ku, Seoul 133-791, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 21 November 2007 Accepted for publication 21 March 2008 Available online 29 March 2008 Keywords: X-ray photoelectron spectroscopy Hafnium oxide High k dielectrics Silicon nitride Metal–insulator–semiconductor (MIS) structures Semiconductor–insulator interfaces Sputtering

a b s t r a c t When hafnium oxide (HfO2) is directly deposited on Si by the RF sputtering method, Hf silicide is formed and post-deposition anneal (PDA) at 400 °C transforms Hf silicide to Si suboxide plus Hf suboxide. The leakage current density for the haluminum (Al)/HfO2/Si(1 0 0)i diodes without PDA is high due to the high density interface states near the Fermi level (0.86 eV above the Si valence band maximum, VBM) and minute conduction channels. PDA at 400 °C eliminates the interface states and the conduction channels, and improves the characteristics of the HfO2 layer, but interface states are newly formed at 0.53 eV above the VBM, resulting in still high leakage current density. Silicon nitride (SiN) layers formed by Si nitridation using N2-plasma generated by the low energy electron impact method possess a high nitrogen atomic concentration ratio, N/(N + O) of 0.65. When a 1.0 nm SiN layer is inserted between HfO2 and Si, interfacial reaction is completely prevented, resulting in a smaller effective oxide thickness, EOT of 1.4 nm. In spite of the smaller EOT, the leakage current density is nearly the same as that with no SiN layer, possibly due to the prevention of the formation of the conduction channels. PDA at 400 °C improves HfO2 characteristics without causing the interfacial reaction, leading to a decrease in the leakage current density. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Miniaturization of complementary metal–oxide–semiconductor (CMOS) devices requires use of ultrathin gate insulating layers with maintaining a low leakage current density. Hafnium dioxide (HfO2) is one of the most promising materials for gate insulating layers to replace silicon dioxide (SiO2) because of its high dielectric constant, high thermal stability, wide band-gap, etc. However, when HfO2 is directly deposited on Si, an interfacial layer is formed, resulting in high interface state densities and in an increase of the effective oxide thickness (EOT) [1–12]. The composition of the interfacial layers strongly depends on the deposition conditions of HfO2 films, i.e., SiO2 [1–6], Hf silicate [7–9], SiO2 rich hafnium silicate [4,10], and Hf silicide [11,12]. To prevent the formation of interfacial layers, SiO2 [9,13], silicon oxynitride (SiON) [13–16], and silicon nitride (SiN) [15–17] are inserted between HfO2 and Si. However, insertion of a SiO2 layer increases the effective oxide thickness, and moreover the interfacial reaction cannot be prevented completely by the ultrathin (e.g., 1 nm) SiO2 layer [18]. SiON is more favorable than SiO2 because of its denser structure and higher dielectric constant than that of SiO2. We have

recently found that the energy distribution of interface states strongly depends on the kinds of buffer layers [18]. Ultrathin SiON layers can be formed at low temperatures (e.g., 450 °C) by plasma nitridation of Si in NH3, N2O, etc. [14,19,20]. In the case of NH3 plasma, high concentration hydrogen atoms are incorporated, resulting in the formation of high density interfacial trap states [19–21]. Ultrathin SiON buffer layers formed by NH3 and N2O plasma nitridation cannot prevent the interfacial reaction completely, leading to the formation of an Hf silicate interfacial layer [14]. The incomplete prevention of the interfacial reaction is possibly due to a low nitrogen concentration in the SiON layer. We have developed a low temperature formation method of SiON (or SiN) layers by use of nitrogen (N2)-plasma generated by the low energy electron impact method [22–24]. Using this method, SiON (or SiN) layers having a nitrogen atomic concentration more than 10% can easily be formed below 400 °C. In the present study, 1 nm silicon nitride (SiN) layers formed by the low energy electron impact method are applied to buffer layers between HfO2 and Si. It has been found that the 1 nm SiN layers can completely prevent the interfacial reaction. 2. Experiments

* Corresponding author. Address: Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. Tel./fax: +81 6 6879 8450. E-mail address: [email protected] (H. Kobayashi). 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.03.031

Metal–insulator–semiconductor (MIS) structures were fabricated from phosphorus-doped n-type Si(1 0 0) wafers having

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1 X cm resistivity. After cleaning the Si wafers using the RCA method and etching with dilute hydrofluoric acid to remove a native oxide layer, Si was nitrided by N2-plasma generated by the low energy electron impact method. In this method, a tungsten filament was heated at 1500 °C and 30 V was applied to a grid with respect to the filament in 1 Pa N2 atmosphere. HfO2 layers of 3 nm thickness monitored by a quartz thickness monitor were deposited on SiN/Si and bare Si specimens by means of an electronbeam evaporation method using an HfO2 target. No external heating was performed during the deposition. For some specimens, post-deposition annealing (PDA) was performed at 400 °C in N2 atmosphere for 10 min. Aluminum (Al) dots of 0.3 or 0.37 mm diameter were formed on the HfO2 surfaces, leading to hAl/HfO2/ SiN/Si(1 0 0)i metal–insulator–semiconductor (MIS) structure. X-ray photoelectron spectroscopy (XPS) measurements were performed using a VG Scientific ESCALAB 220i-XL spectrometer with a monochromatic Al Ka radiation source. Photoelectrons were detected in the surface-normal direction. Current–voltage (I–V) curves were measured using a HP 4140B picoammeter. Capacitance–voltage (C–V) and conductance–voltage (G–V) curves were recorded at 1 MHz using an YHP 4192A impedance analyzer.

a

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b Si 2p Fig. 1 shows XPS spectra in the Hf 4f region (spectra a) and Si 2p region (spectra b) for the HfO2 layer directly deposited on the Si(1 0 0) substrate. The main peaks at 17.7 and 19.3 eV in the Hf 4f spectra are due to Hf 4f7/2 and 4f5/2 levels of HfO2. With no PDA (upper spectrum), weak peaks attributable to Hf silicide [25–27] were observed at 14.6 and 16.2 eV while these peaks disappeared after PDA at 400 °C (lower spectrum). In the Si 2p XPS spectrum for the HfO2/Si(1 0 0) specimens with no heat treatment (upper spectrum), sharp doublet peaks due to Si 2p3/2 and 2p1/2 levels of the substrate were present at 99.3 and 99.9 eV, respectively, but a peak due to Si oxide was not observed in the higher energy region. After the heat treatment at 400 °C in nitrogen, on the other hand, a broad peak appeared at 102.2 eV (i.e., 2.9 eV shift from the substrate Si 2p3/2 peak), which was attributable Si suboxide species [28]. Fig. 2 shows C–V (a), I–V (b), and G–V (c) curves of the hAl/HfO2/ Si(1 0 0)i MIS diodes with no buffer layer. With no PDA (solid line), the saturation capacitance was 1700 pF and the effective oxide thickness, EOT, was estimated to be 2.2 nm. A hysteresis with the magnitude of 0.3 V was present in the bias region higher than 0.7 V. With PDA at 400 °C (dotted line), the saturation capacitance decreased to 1350 pF, indicating an increase in EOT to 2.7 nm. In this case, a hysteresis with the magnitude of 0.3–0.4 V was present in all the bias regions. The flat-band voltage for the MOS diodes with no PDA was estimated to be 0.2–0.3 V, while those with PDA to be 0.03 and 0.16 V for the bias sweeps in the positive and negative voltage directions, respectively. The leakage current density for the MIS diodes without PDA (solid line) was considerably high (0.8 and 10 A/cm2 at the forward gate bias, VG, of 1 V, for the bias sweeps in the positive and negative voltage directions, respectively, and 0.015 A/cm2 at VG of 1 V). With PDA at 400 °C (dotted line), the leakage current density considerably decreased (0.3 A/cm2 at VG = 1 V, and 8  105 A/cm2 at VG = 1 V). Fig. 3 shows the equivalent circuit of the MOS diodes with the ultrathin insulating layer having capacitance, CI. CD is the capacitance of the semiconductor depletion layer, and its response time is very short. Thus, the resistance in series to CD is negligibly small. Cis is the capacitance due to interface states and Ris is the corresponding resistance which is needed to express that interface

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Binding Energy (eV) Fig. 1. XPS spectra in the Hf 4f region (a) and Si 2p region (b) for the HfO2 layer deposited on bare Si(1 0 0) surfaces without (upper) and with (lower) PDA at 400 °C in nitrogen.

states possess a time constant, sis (sis = CisRis). RF expresses a leakage current flowing through the ultrathin dielectrics. The conductance, G, of the equivalent circuit is given by G¼

1 C is sis þ x2 RF 1 þ x2 s2is

ð1Þ

where x is the angular frequency. RF can be estimated from the leakage current density in Fig. 2b (from the slope of the I–V curve), and the background due to RF is shown by the dashed lines in Fig. 2c. The background is relatively high for the MOS diodes without PDA, while it is negligibly low for the MOS diodes with PDA. After subtracting 1/RF from the measured conductance, it is in proportion to Cis which is, in turn, proportional to the interface state density [29]. The conductance after background subtraction is shown by the solid–dotted line. The conductance of the MIS diodes with no PDA (solid–dotted line) was low in the gate bias region negative of 0.1 V, but it was very high near 0 V. This result shows that the density of interface states more than 0.1 eV below the Fermi level (i.e., 0.86 eV above the Si valence band maximum, VBM) is negligibly low, while that near the Fermi level is considerably high. With PDA at 400 °C (dotted line), the conductance near 0 V decreased but peaks appeared at 0.22 and 0.46 V for the bias sweeps in the negative

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and positive directions, respectively. This result indicates that the interface states near the Fermi level are eliminated by PDA, but

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Voltage (V) Fig. 2. C–V (a), I–V (b), and G–V (c) curves for the hAl/HfO2/Si(1 0 0)i MOS diodes without (solid line) and with (dotted line) PDA at 400 °C in nitrogen. The dashed and solid–dotted lines in Fig. 2c show the background due to the leakage current flowing through the ultrathin dielectrics, and the conductance after subtraction of the background, respectively.

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Fig. 3. Equivalent circuit for a MOS diode.

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Fig. 4. XPS spectra in the following energy regions for the SiN/Si(1 0 0) structure formed by N2-plasma generated by the low energy electron impact method (a) N 1s region (b) O 1s region (c) Si 2p region.

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region (spectrum a), a peak is present at 397.8 eV, and it is attributed to a nitrogen atom singly bound to three Si atoms having a planer structure [30]. In the O 1s region (spectrum b), a peak was observed at 532.7 eV. From the intensity ratio between these peaks, the nitrogen atomic concentration ratio, N/(N + O), is estimated to be 0.65. In the Si 2p region (spectrum c), a broad peak was observed at 2.8 eV with respect to the substrate Si 2p3/2 peak. This binding energy shift is in good agreement with that of SiN [31,32]. Fig. 5 shows XPS spectra for the HfO2/1 nm SiN/Si(1 0 0) structure. In the Hf 4f region (spectrum a), only peaks due to HfO2 were observed at 17.6 and 19.3 eV. This XPS spectrum was not changed by the heat treatment at 400 °C. These results demonstrate that the interfacial reaction between HfO2 and Si to form Hf silicide (cf. upper spectrum in Fig. 1a) is completely prevented by the 1 nm SiN layer. With no heat treatment, the XPS spectrum in the Si 2p region (upper spectrum in Fig. 5b) had a structure nearly identical to that without HfO2 deposition (Fig. 4c), indicating the unchanged thickness (i.e., 1.0 nm) and composition of the SiN layer. This result also verifies complete prevention of the interfacial reaction. The XPS spectrum in the Si 2p region for the HfO2/1 nm SiN/Si(1 0 0) structure was not changed by the heat treatment at 400 °C (lower spectrum in Fig. 5b), clearly showing that diffusion of oxygen and Hf atoms is completely prevented by the 1 nm SiN layer even at 400 °C. Fig. 6 shows the C–V (Fig. 6a) and I–V (Fig. 6b) curves of the hAl/ HfO2/SiN/Si(1 0 0)i MIS diodes with the SiN layer formed by N2-plasma generated by low energy electron impact. EOT for both

a

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interface states are newly generated below the Fermi level. Using the flat-band voltage and the conductance peak positions, the energy levels of the interface states of the MOS diodes with PDA was estimated to be 0.53 eV above the Si VBM. The time constant, sis, can be obtained from the plot of the conductance peak intensity, Gpeak, divided by x vs. the frequency, since Gis/x in Eq. (1) has the maximum value when xsis is equal to one. Gis/x values measured in the frequency range between 10 k and 1 MHz for the MOS diodes with PDA decreased only gradually with x. This result is probably due to the presence of interface states with various time constants. A conductance value for MOS diodes includes only contribution from interface states (after subtracting the effect due to a leakage current) (cf. Eq. (1)) while capacitance of MOS diodes consists of semiconductor capacitance, oxide capacitance, and interface state capacitance. Since the semiconductor capacitance and oxide capacitance do not show frequency dispersion, it is highly probable that conductance of MOS diodes is much more frequency-dependent than capacitance. Detailed study of the frequency dispersion of the C–V and G–V characteristics will be reported elsewhere. Fig. 4 shows XPS spectra for the SiN layers formed by N2-plasma generated by the low energy electron impact method. In the N 1s

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Binding Energy (eV) Fig. 5. XPS spectra for the HfO2/SiN/Si(1 0 0) structure a) in the Hf 4f region with no PDA b) in the Si 2p region with no PDA (upper spectrum) and with PDA at 400 °C in nitrogen (lower spectrum).

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0.5

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Voltage (V) Fig. 6. C–V (a) and I–V (b) curves for the hAl/HfO2/1 nm SiN/Si(1 0 0)i MOS diodes with the SiN layer formed by the low energy electron impact method without (solid line) and with (dotted line) PDA at 400 °C in nitrogen.

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the diodes without and with PDA at 400 °C was estimated to be 1.4 nm from the saturation capacitance. Without PDA (solid line), the magnitude of the hysteresis in the C–V curves was 0.05 V, while it increased to 0.3 V after PDA (dotted line). The flat-band voltages estimated from the center of the two curves measured in the different bias sweep directions were 0.05 and 0.48 V, for the MOS diodes without and with PDA, respectively. The leakage current density was decreased by PDA at 400 °C for 10 min in spite of unchanged EOT. 4. Discussion When HfO2 is deposited on bare Si, Hf silicide is formed (cf. Fig. 1a). There are two possibilities for the mechanism of the Hf silicide formation 1) Hf atoms formed during electron-beam evaporation directly react with Si, and 2) deposited HfO2 reacts with Si. It is reported in the previous literature that in the case of HfO2 deposition by means of reactive magnetron sputtering, Hf silicide is formed only at temperatures above 700 °C [33]. This indicates that deposited HfO2 does not react with Si at temperatures below 700 °C, and thus mechanism 1) is more probable. In the case of reactive sputtering, the oxygen pressure is sufficiently high to form HfO2 in the gas phase, and consequently, no Hf atoms impinge on Si surfaces, while in the case of the evaporation method, Hf atoms are likely to be incident to the Si surfaces, resulting in the formation of Hf silicide. Hf silicide induces interface states near the Fermi level (i.e., 0.86 eV above the Si VBM). A leakage current flows via interface states, resulting in an increase in its density [34,35]. Another reason for the high leakage current density may be the formation of Hf silicide islands which form thin HfO2 regions and/or minute electrical contacts between Si and Al. When PDA at 400 °C is performed, Hf silicide reacts with HfO2, leading to the formation of Si suboxide (cf. Fig. 1b) and Hf suboxide. No spectral feature due to Hf suboxide was observed in Fig. 1a. This is probably because of i) a decrease in the intensity of the suboxide peak by the HfO2 overlayer, ii) small energy shift of the suboxide peak from the HfO2 peaks [36], and iii) low suboxide concentration. PDA eliminates the conduction channels due to Hf silicide, resulting in a decrease in the leakage current density. However, the formation of suboxides induces interface states below the Fermi level (0.53 eV above the Si VBM) and slow states which cause the hysteresis in the C–V curve (cf. dotted line in Fig. 2a) [37]. The leakage current is likely to flow via interface states, resulting in still high leakage current density. Considering that each oxygen atom bound to a Si atom causes 1 eV energy shift of the Si 2p peak from the substrate peak [28], the 2.9 eV shift (lower spectrum in Fig. 1b) indicates that Si3+ (i.e., Si2O3) is the most probable interfacial species formed by PDA at 400 °C. Assuming that the Si suboxide layer is present directly on the Si substrate, its thickness is estimated to be 1.2 nm from the intensity ratio between the Si 2p peak due to Si suboxide, Isuboxide , and the Si substrate peak, ISi [28,38]   Isuboxide C Si rSi kSi dsuboxide ¼ ksuboxide ln þ1 ð2Þ ISi csuboxide rsuboxide ksuboxide where k is the mean free path of photoelectrons, c is the concentration of Si atoms, r is the photoionization cross section, and subscripts suboxide and Si denote the values for the suboxide layer and the Si substrate, respectively. In the estimation, 3.2 and 2.7 nm were used for ksuboxide and kSi, respectively, 1.9 for cSi,/csuboxide and 1.1 for rsuboxide/rSi [28]. The values for a SiO2 layer are adopted as ksuboxide and rsuboxide. Hf silicide forms interface states near the Fermi level (interfacial Fermi level: 0.86 eV above the Si VBM). PDA at 400 °C converts Hf silicide to Si suboxide plus Hf suboxide, and consequently the

interface states near the Fermi level are eliminated. However, the formation of suboxides induces another interface states at 0.53 eV above the Si VBM (interfacial Fermi level: 0.94 eV above the Si VBM). Si can easily be nitrided at low temperatures (400 °C in the present study) by N2-plasma generated by low energy electron impact. Considering that the atomic ratio, N/(N + O), of 0.65, and assuming that the SiN layer consists of mixture of Si3N4 and SiO2, the Si atomic concentration ratio, DSiN/DSi, is estimated to be 0.65. Assuming the mean free path of photoelectrons and photoionization cross section for the SiN layer are the same as those for SiO2 layers [28], the SiN thickness is estimated to be 1.0 nm from the intensity ratio between the SiN peak and the substrate Si 2p3/2 peak. The physical thickness of the HfO2 layer was roughly determined to be 3 nm using a quartz thickness monitor. Using the physical thickness and the relative dielectric constants (i.e., 7.5 for SiN and 20 for HfO2), EOT for the hAl/HfO2/SiN/Si(1 0 0)i MOS diodes is estimated to be 1.2 nm, in reasonable agreement with that obtained from the C–V measurements (i.e., 1.4 nm). In the estimation, eSiN is estimated to be 6.2, assuming a linear composition dependence of eSiN and considering the atomic concentration ratio, N/(N + C), of 0.65. The nitrogen concentration of the SiN layer (i.e., N/(N + O) = 0.65) formed in the present study is much higher than that for SiON layers formed by NH3-plasma and N2O-plasma [13,14,19]. The complete prevention of the interfacial reaction even at 400 °C (cf. Fig. 5b) probably results from the dense structure of the SiN layer with a high nitrogen atomic concentration. It should be noted that a Hf silicate interfacial layer is formed after the RF sputtering of HfO2 on a SiON layer with a low nitrogen concentration of 3% [14], indicating that diffusion of Hf atoms cannot be prevented. It is also noted that when HfO2 layers are deposited on 1.0 nm SiO2 layers, the SiO2 thickness increases to 1.6 nm [18], indicating that oxygen diffusion proceeds. We think that the capability of the formation of the SiN layers with the high nitrogen atomic concentration and thus with the great effect on preventing diffusion is an advantage of the present SiN formation method using N2-plasma generated by low energy electron impact. Insertion of the 1.0 nm SiN layer can decrease the leakage current density due to avoidance of the formation of minute conduction channels and high density interface states resulting from Hf silicide or suboxides. On the other hand, the 1.0 nm SiN layer completely prevents the interfacial reaction, resulting in a smaller EOT than that without the SiN layer. On the other hand, the thickness of the interfacial Si2O3 layer for the MOS diodes with no SiN layer is estimated to be 1.2 nm from the XPS spectrum (cf. lower spectrum in Fig. 1b). The tunneling probability, PT, through an insulating layer is given by [39] PT ¼ exp½ð4p=hÞð2mvÞ1=2 d where v is the mean barrier height, d is the thickness of the insulating layer, and m is the effective mass of a charge carrier. The similar leakage current densities for the hAl/HfO2/SiN/Si(1 0 0)i and hAl/ HfO2/Si2O3/Si(1 0 0)i MOS diodes are probably because the barrier height for the Si2O3/Si structure is slightly lower (i.e., 30% lower) than that for the SiN/Si structure. The leakage current densities in the forward bias region for the hAl/HfO2/Si(1 0 0)i and hAl/HfO2/SiN/Si(1 0 0)i MOS diodes are decreased to 1/8–1/100 and 1/4–1/10, respectively, by PDA. The decrease for the latter diode is attributable to the improvement of the HfO2 layer because no interfacial reaction proceeds. For the former diode, on the other hand, the interfacial reaction occurs, resulting in the removal of Hf silicide. Considering that the PDA-induced improvement of HfO2 decreases the leakage current density to 1/4–1/10, it is roughly estimated that the interfacial reaction decreases it to 1/2–1/10.

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5. Conclusion We have performed XPS and electrical measurements of HfO2/ Si(1 0 0) and HfO2/SiN/Si(1 0 0) structures and reached the following conclusions: 1) With no buffer layer, Hf silicide is formed at the HfO2/Si structure, resulting in the high leakage current density due to minute channels and/or high density interface states near the Fermi level. 2) Heat treatment of the HfO2/Si structure transforms Hf silicide to Si suboxide plus Hf suboxide, while the leakage current density is still high due to the formation of interface states at 0.53 eV above the Si VBM. 3) A 1 nm SiN layer formed by N2-plasma generated by low energy electron impact possesses a high nitrogen atomic concentration, N/(N + O), of 65%. 4) Insertion of the 1 nm SiN layer decreases EOT because of the complete prevention of the interfacial reaction. 5) The 1 nm SiN layer can completely prevent diffusion of Hf and oxygen atoms, and consequently, PDA at 400 °C does not change EOT. In spite of the unchanged EOT, the leakage current density for the hAl/HfO2/SiN/Si(1 0 0)i MOS diode is greatly decreased by PDA due to the improvement in the characteristics of the HfO2 layer. References [1] R. Puthenkovilakam, Y.-S. Lin, J. Choi, J. Lu, H.-O. Blom, P. Pianetta, D. Devine, M. Sendler, J.P. Chang, J. Appl. Phys. 97 (2005) 023704. [2] Y. Hoshino, Y. Kido, K. Yamamoto, S. Hayashi, M. Niwa, Appl. Phys. Lett. 81 (2002) 2650. [3] J. Lu, J. Aarik, J. Sundqvist, K. Kukli, A. Hårsta, J.-O. Carlsson, J. Crystal Growth 273 (2005) 510. [4] P.K. Park, J.-S. Roh, B.H. Choi, S.-W. Kang, Electrochem. Solid-State Lett. 9 (2006) F34. [5] Y. Senzaki, S. Park, H. Chatham, L. Bartholomew, W. Nieveen, J. Vac. Sci. Technol. A 22 (2004) 1175. [6] R.P. Pezzi, J. Morais, S.R. Dahmen, K.P. Bastos, L. Miotti, G.V. Soares, F.L. Freire Jr, J. Vac. Sci. Technol. A 21 (2003) 1424. [7] R. Tan, Y. Azuma, I. Kojima, Appl. Surf. Sci. 241 (2005) 135. [8] G. He, M. Liu, L.Q. Zhu, M. Chang, Q. Fang, L.D. Zhang, Surf. Sci. 576 (2005) 67.

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