H2 plasma as a reactant

Thin Solid Films 520 (2012) 6100–6105

Contents lists available at SciVerse ScienceDirect

Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Atomic layer deposition of Ru thin film using N2/H2 plasma as a reactant Tae Eun Hong a, Ki-Yeung Mun b, Sang-Kyung Choi b, Ji-Yoon Park b, Soo-Hyun Kim b,⁎, Taehoon Cheon c, Woo Kyoung Kim d, Byoung-Yong Lim e, Sunjung Kim e a

Busan Center, Korea Basic Science Institute, 1275 Jisadong, Gangseogu, Busan, 618-230, Republic of Korea School of Materials Science and Engineering Yeungnam University 214-1, Dae-dong, Gyeongsan-si, Gyeongsangbuk-do, 712-749, Republic of Korea Center for Core Research Facilities, Daegu Gyeongbuk Institute of Science & Technology, Sang-ri, Hyeonpung-myeon, Dalseong-gun, Daegu, Republic of Korea d School of Chemical Engineering, Yeungnam University, 214-1, Dae-dong, Gyeongsan-si, Gyeongsangbuk-do, 712-749, Republic of Korea e School of Materials Science and Engineering, University of Ulsan, Mugeo-dong, Nam-go, Ulsan, 680-749, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 27 November 2011 Received in revised form 22 May 2012 Accepted 24 May 2012 Available online 29 May 2012 Keywords: Ruthenium Atomic layer deposition Nitrogen/hydrogen plasma Copper metallization Seed layer N-incorporation Microstructure

a b s t r a c t Ruthenium (Ru) thin films were grown by atomic layer deposition using IMBCHRu [(η6-1-Isopropyl-4MethylBenzene)(η4-CycloHexa-1,3-diene)Ruthenium(0)] as a precursor and a nitrogen–hydrogen mixture (N2/H2) plasma as a reactant, at the substrate temperature of 270 °C. In the wide range of the ratios of N2 and total gas flow rates (fN2/N2 + H2) from 0.12 to 0.70, pure Ru films with negligible nitrogen incorporation of 0.5 at.% were obtained, with resistivities ranging from ~20 to ~30 μΩ cm. A growth rate of 0.057 nm/cycle and negligible incubation cycle for the growth on SiO2 was observed, indicating the fast nucleation of Ru. The Ru films formed polycrystalline and columnar grain structures with a hexagonal-close-packed phase. Its resistivity was dependent on the crystallinity, which could be controlled by varying the deposition parameters such as plasma power and pulsing time. Cu was electroplated on a 10-nm-thick Ru film. Interestingly, it was found that the nitrogen could be incorporated into Ru at a higher reactant gas ratio of 0.86. The N-incorporated Ru film (~20 at.% of N) formed a nanocrystalline and non-columnar grain structure with the resistivity of ~340 μΩ cm. © 2012 Elsevier B.V. All rights reserved.

1. Introduction As the dimension of Cu interconnect continuously scales down as shown by the international technology roadmap for semiconductors [1], the line resistance of Cu deposited by electroplating (EP) remarkably rises due to the size effect of the metal film on the resistivity [2]. One possible solution to address this is to maximize the portion of the EP-Cu volume in the dual damascene structure with the thin and conformal diffusion barrier and seed layer. In these respects, atomic layer deposition (ALD) is a viable solution for preparing them since ALD enables atomic-scale control of the film thickness with excellent step coverage [3,4]. The portion of Cu in the dual damascene structure can be increased further by direct plating because the EP of Cu can be achieved on a diffusion barrier without a seed layer. It has been suggested that Cu can be directly electroplated on a family of candidate materials such as Pt, Pd, Ru, Rh, Ir, and Ag [5]. Among them, Ru has excellent properties such as low resistivity (7.1 μΩ-cm), chemical/thermal stability, and immiscibility with Cu. It has also been shown that Ru adheres well to Cu [6,7] and promotes a greater Cu (111) texture than Ta [8], making Ru a most promising material in back-end-of-line Cu-

wiring as a seed layer or adhesion layer, and even as a Cu directplateable diffusion barrier. The investigations on Ru ALD that have been reported generally use molecular oxygen (O2) as a reactant [9–16]. These films show excellent properties, like high conductivity and ~ 100% step coverage in high-aspect ratio structures. However, an O2-based Ru ALD process would lead to oxidation of the underlying Ta-based diffusion barrier when used as a seed layer, or of the Cu line when used as a Cu direct-plateable diffusion barrier. This might result in an increase in the structure's resistivity and poor electromigration resistance. To avoid these problems, Ru ALD processes in non-oxidizing ambient was introduced. NH3 molecules [17,18] or NH3 plasma [19–21] instead of O2 molecules as reactants were reported. In this study, we investigated Ru ALD using nitrogen–hydrogen mixture (N2/H2) plasma, instead of NH3 plasma as a potential non-oxidizing reactant, and systematically studied the changes in ALD-Ru film properties depending on the ratio of gas flow rates. The deposited film was also evaluated as a seed layer for Cu electroplating. 2. Experiments

⁎ Corresponding author. Tel.: + 82 53 810 2472. E-mail address: [email protected] (S.-H. Kim). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.05.069

Ru films were deposited on thermally grown SiO2-covered or TiN/ SiO2-covered Si wafers using a showerhead-type plasma-enhanced ALD (PEALD) reactor (Lucida-M100, NCD Technology) with IMBCHRu(0)

T.E. Hong et al. / Thin Solid Films 520 (2012) 6100–6105

[IMBCH = (η6-1-Isopropyl-4-MethylBenzene)(η4-CycloHexa-1,3diene)] as a precursor. The typical deposition temperature was around 270 °C. Ru precursor was vaporized in a bubbler at a temperature of 100 °C and carried into the process chamber by Ar gas at 50 standard cubic centimeters per minute (sccm). As a reactant, a mixture of N2 and H2 gas was introduced into the chamber with a radio frequency plasma power of 100 W. The ratio of N2 versus total gas (N2 + H2) flow rates (fN2/N2 + H2) was varied from 0 to 0.86, keeping the amount of total gas flow rate at 50 sccm. Between the precursor pulsing and the reactant pulsing, a purging process was done with 200 sccm of Ar. A sequence of precursor pulsing, purging, reactant pulsing, and then purging was performed in one ALD cycle. The thickness of the Ru film was determined from X-ray reflectance (XRR, PANanalytical X'-pert MRD with Cu-Kα radiation at 1.5 kW). Its resistivity was determined by combining the sheet resistance of the film measured by four-point probe and its thickness. The compositions of the films were characterized by Rutherford backscattering spectrometry (RBS) with the incident He++ energy of 3.695 MeV and secondary ion mass spectrometry (SIMS, CAMECA IMS-6f) depth profiling, whose results were calibrated by RBS. SIMS analysis was performed using Cs + gun with the impact energy of 5 keV. Glazing incidence angle (θ = 3°) X-ray diffraction (XRD, 2θ scan, PANanalytical X'-pert MRD with Cu-Kα radiation at 1.5 kW) analysis was performed for phase and crystallinity identification. Plan-view transmission electron microscopy (TEM, Tecnai F20 equipped with a 200 kV accelerating voltage and a field emission gun) and cross-sectional view TEM (XTEM) were used for the analysis of the microstructures of the films depending on the deposition conditions. We prepared the TEM samples using conventional 360

3. Results and discussion 3.1. Effect of reactant gas composition on the film properties Fig. 1 shows the resistivity of the film as a function of the ratios of N2 versus total gas (N2 + H2) flow rates. There was a total of 400 ALD cycles. The film was not deposited with only H2 plasma (fN2/N2 + H2 = 0), and the metallic film started to be deposited after adding N2 gas to the reactant. In the gas ratio range between 0.12 and 0.5, the resistivity was maintained under 23 μΩ cm. Interestingly, when the gas ratio was increased to 0.7, the resistivity started to increase to around 30 μΩ cm. Upon further increasing the gas ratio, the resistivity rapidly increased. The resistivity of the film deposited with a gas ratio of 0.86 (the highest ratio of N2 and total gas flow rates in this study) was around 340 μΩ cm. The lowest resistivity (~ 19 μΩ cm) was obtained at the gas ratio of 0.24 (N2 flow rate: 12 sccm and H2 flow rate: 38 sccm), which is close to the ratio of nitrogen and hydrogen in NH3 gas. 3000

a raw data simulation

Deposition temperature : 270OC Number of cycles : 400

70

Intensity (counts/s)

Resistivity (μΩcm)

procedures, such as backside grinding and Ar-ion milling. For Ar-ion milling for electron transparency, Gatan precision ion polishing system was used. Finally, Cu was electrodeposited on 10-nm thick Ru film to confirm its feasibility as a seed layer for Cu electroplating. The detailed description of the Cu electroplating condition was given in a previous report [22]. The morphology of electroplated Cu was examined by cross-sectional view scanning electron microscopy (XSEM, Hitachi S-4800 with 5 kV accelerating voltage).

a

300

6101

60 50 40

Ru

He++ : 3.695MeV

2000

O in SiO2 layer

1000

O

30 20 0.0

0.2

0.4

0.6

0.8

0 200

1.0

400

600

800

1000

Channel

fN2/N2+ H2 Ru(103)

0.86 0.82

2000

0.70 0.50 1000

0.32

b O Concentration (at.%)

Ru(002) Ru(101)

Ru(110)

Intensity (arb. units)

3000

Ru(100)

100

b

80

Ru 60

40

N 20

0.24

C

0.12

0 30

40

50

60

70

80

2θ (degree) Fig. 1. (a) The resistivity of the films deposited with different gas ratios of N2 versus total gas flow rates and (b) corresponding grazing incidence angle (θ = 3°) XRD results.

0

10

20

30

40

Sputter depth (nm) Fig. 2. (a) N-resonance RBS result of the film deposited with a gas ratio of 0.24 and (b) SIMS depth profile of the film deposited with a gas ratio of 0.86.

6102

T.E. Hong et al. / Thin Solid Films 520 (2012) 6100–6105

Grazing incident angle (θ = 3°) XRD results as a function of the gas ratio are shown in Fig. 1(b). In the wide range of gas ratio between 0.12 and 0.7, crystalline peaks from hexagonal close-packed (HCP) phase Ru were shown. Interestingly, when the gas ratio was increased to 0.82, the peak intensities from Ru were significantly decreased, indicating that its crystallinity was degraded. When the gas ratio was increased to 0.86 (the highest gas ratio in this study), all the peaks from Ru except for Ru (100) disappeared and a broad hump appeared. This indicated that the film was almost amorphized, or grain sizes were very small. It was also noted that the center of Ru (100) peak was moved to lower 2θ, suggesting that the lattice parameter might be increased. To investigate compositions and impurities in films, the films were analyzed using RBS and SIMS depth profile. Two films were selected, one deposited with a gas ratio of 0.24 (having the lowest resistivity) and the other deposited with a gas ratio of 0.86 (having the highest resistivity in this study). Fig. 2(a) shows the RBS spectra of the film with a gas ratio of 0.24. We used an N-resonance RBS method, with an incident He ++ energy of 3.695 MeV, to sensitively detect a possible incorporation of N into Ru. Fig. 2(a) clearly shows that N incorporation into Ru was negligible when the fraction of N2 in the total gas was low and pure Ru film was deposited with N2/H2 plasma as a reactant. The composition of the film was determined by RUMP simulation. The N content in the film was under the detection limit of RBS (~ 0.5 at.%), which is the same result using the same Ru precursor and NH3 plasma as a reactant [21]. The C content in the film was also very low at ~ 1.5 at.%. Fig. 2(b) shows the SIMS depth profile of the film deposited with a gas ratio of 0.86. In this case, the nitrogen was incorporated into the film, and carbon was also detected as an impurity. Thus, the current investigation showed that N could be incorporated into Ru during

PEALD-Ru process using N2/H2 plasma mixture if one controls the ratio of N2 and H2 gas. Considering the fact that the nitrogen incorporation into sputter-deposited Ru enabled the change of its microstructure from columnar to nanocrystalline [23], the present investigation is worth to note though its resistivity was increased to ~ 340 μΩ cm with the nitrogen incorporation in the film. To check the microstructures of pure Ru and N-incorporated Ru, we performed TEM analysis. The plan-view TEM bright-field (BF) image of Ru film deposited with a gas ratio of 0.24 [Fig. 3(a)] showed crystalline Ru grains with an average size of around 17 nm. XTEM BF analysis [Fig. 3(b)] revealed that Ru film was grown with columnar grain structure, which is the same result using NH3 plasma as a reactant with the same Ru precursor [21]. In the case of N-incorporated Ru film deposited by increasing the gas ratio to 0.86, the grain size was reduced to around 8 nm [Fig. 3(c)], and a bright region appeared between grains. We assume that this bright region is nitrogen-rich considering the mass contrast of TEM. The corresponding XTEM image [Fig. 3(d)] showed that the columnar grain structure was broken. We think that this microstructure has advantages in acting as a diffusion barrier against Cu, because either the Cu diffusion length will be increased or diffusion of Cu will be more difficult in these microstructures. A detailed study on ALD of N-incorporated Ru film and its application as a Cu diffusion barrier will be reported as a separate paper. These results indicated that by controlling the N2/H2 gas ratio in the plasma, we could deposit a nitrogen-incorporated Ru film as well as a pure Ru film. They also indicated that it was critical to have both hydrogen and nitrogen in the plasma to obtain high quality (low resistivity) Ru thin films. Previously, it was reported that the density of NH3 radicals synthesized in the N2/H2 mixture plasma depend on the N2/H2

Fig. 3. The microstructure of the films; (a) plan-view TEM BF image, (b) cross-sectional view TEM BF image of the film deposited with a gas ratio of 0.24, (c) plan-view TEM BF image, and (d) cross-sectional view TEM BF image of the film deposited with a gas ratio of 0.86.

T.E. Hong et al. / Thin Solid Films 520 (2012) 6100–6105

gas flow rate, and becoming maximum at fN2/H2 = 1/3, which is same ratio for the nitrogen and hydrogen in NH3 gas [24]. As Fig. 1 shows, the resistivity of Ru film becomes lowest at the N2 flow rate of 12 sccm and H2 flow rate of 38 sccm, which is close to the ratio of nitrogen and hydrogen in NH3 gas. Thus, we assume that the NH3 radicals, not N and H radicals, play an important role in depositing low resistivity Ru film during Ru ALD using N2/H2 mixture plasma as well as NH3 plasma. Now, we will focus on the deposition condition where we can obtain the optimized Ru film with the lowest resistivity. 28

a

Thickness (nm)

24

20

16

12

Reactant pulsing time : 8s

8 0

3

6

9

12

15

18

Precursor pulsing time (s)

6103

3.2. PEALD-Ru deposited using N2/H2 plasma Fig. 4(a) shows the thickness of Ru film grown on thermally grown SiO2 with a gas ratio of 0.24 (producing the lowest resistivity) after 400 reaction cycles. It seemed to become saturated with precursor pulsing time above 7 s. This indicates that no thermal selfdecomposition of the precursor takes place at this temperature and the deposition is accomplished by the self-limiting surface reaction between adsorbed precursors and reactants. A quite similar growth behavior was observed as N2/H2 plasma pulsing time was varied from 3 to 20 s, as shown by Fig. 4(b). We considered that N2/H2 plasma pulsing for 12 s was enough to guarantee self-limited film growth in the current ALD–Ru process. Thus, from Fig. 4(a) and (b), the basic gas pulsing conditions were set as follows: precursor pulsing of 7 s, precursor purging of 10 s, reactant pulsing of 12 s with 100 W of RF power, and reactant purging of 10 s. Under the basic pulsing conditions, the reaction cycles were varied from 100 to 800 cycles [Fig. 4(c)]. The film thickness depended linearly on the number of reaction cycles. From Fig. 4(c), the growth rate, which could be determined by fitting the line through the origin, was 0.057 nm/cycle, which is lower than that of ALD-Ru with NH3 plasma as a reactant using the same precursor [21]. The extrapolated line shows that negligible incubation (~7 cycles) cycles are needed for the growth of Ru films, demonstrating a high adsorption of precursors and a high reactivity of precursors and N2/H2 plasma, leading to the fast nucleation of Ru on SiO2 substrate. Because the main application of Ru in this study is a seed layer for Cu electroplating, the resistivity needs to be lowered as possible. Thus,

28

b

40

a

24

Resistivity (μΩcm)

Thickness (nm)

35 20

16

12

Precursor pulsing time : 7s

30

25

20 Temperature : 270oC Precursor pulsing time : 7s Reactant pulsing time : 12s

15 8 0

4

8

12

16

20

24

10

Reactant pulsing time (s)

50

100

150

Plasma power (watt) 50

30

20

10

Precursor pulsing time : 7s Reactant pulsing time : 12s

b

Ru(103)

Ru(110)

Ru102)

Ru(002) Ru(101)

1500

Intensity (arb. units)

Thickness (nm)

40

Ru(100)

c

1000

150W

500

100W

50W

0 0

200

400

600

800

Number of cycles

0 30

40

50

60

70

80

2θ (degree) Fig. 4. The thickness of ALD-Ru film deposited on SiO2 substrates as a function of (a) the Ru precursor pulsing time, (b) the reactant (N2/H2 plasma) pulsing time, and (c) the number of reaction cycles. Here, the gas ratio was 0.24.

Fig. 5. (a) Resistivity of ALD-Ru films deposited at 270 °C as a function of the N2/H2 plasma power and (b) corresponding glazing incident angle (θ = 3°) XRD results.

6104

T.E. Hong et al. / Thin Solid Films 520 (2012) 6100–6105

deposition parameters that affect the film resistivity were evaluated. We carried out the deposition with increasing plasma power. A low resistivity, around 18 μΩ cm [Fig. 5(a)], was obtained as the plasma power of the PEALD-Ru process was increased to 150 W, although the resistivity of the film deposited with 50 W of plasma power was as high as 37 μΩ cm. Grazing incidence angle XRD analysis [Fig. 5(b)] showed the improvement of crystallinity as supported from the increase in the peak intensities from the HCP phase crystalline Ru with the increasing plasma power. We also evaluated the resistivities of Ru films with reactant pulsing time from 3 to 20 s at the fixed plasma power of 100 W. Its result was similar with the plasma power effect. As the reactant pulsing time increased, the resistivity of the Ru film decreased and a resistivity as low as ~ 16.5 μΩ cm was obtained when the plasma pulsing time was increased up to 20 s [Fig. 6(a)]. The same argument on the decrease in the resistivity with the increasing plasma pulsing could be raised based on the XRD analysis with increasing reactant pulsing time as shown by Fig. 6(b). Finally, the electroplating of Cu on PEALD-Ru film deposited at basic pulsing conditions was investigated. The potentiostatic deposition method in the Cu plating bath was used to grow Cu films directly on the surface of a 10-nm-thick PEALD-Ru film. The cross-sectional view SEM image (Fig. 7) showed that ~ 100 nm-thick Cu film was deposited directly on the 10-nm-thick Ru film. This means that the PEALD-Ru film developed in this study can be used as a seed layer for Cu interconnects.

70

a Resistivity (μΩcm)

60 50 40 30 20

EP-Cu PEALD-Ru SiO2

Si wafer

150 nm

Fig. 7. Cross-sectional view SEM image of the Cu film electrodeposited on the ALD-Ru film deposited with a gas ratio of 0.24.

4. Summary and conclusions Ru and N-incorporated Ru films were deposited by the ALD process using an IMBCHRu and N2/H2 mixture plasma as a reactant at a deposition temperature of 270 °C. The results showed that by controlling the N2/H2 gas ratio in the plasma, one could deposit a nitrogenincorporated Ru film as well as a pure Ru film. The Ru film with the lowest resistivity (~ 19 μΩ cm) was deposited at a N2 flow rate of 12 sccm and H2 flow rate of 38 sccm (close to the ratio of nitrogen and hydrogen in NH3 gas). At this condition, self-limiting film growth was confirmed with the growth rate of 0.057 nm/cycle and negligible incubation (~7 cycles). Cu was electroplated on a 10-nm-thick Ru film, indicating that it is a viable candidate for a Cu direct-plateable diffusion barrier. N-incorporated Ru film (~20 at.% N) prepared at the gas ratio of 0.86 formed a nanocrystalline and non-columnar grain structures, unlike a pure Ru film with polycrystalline columnar grain structure. In conclusion, we consider that the Ru ALD process developed in this study has many advantages as compared to NH3 plasma-based one in view of the process controllability on composition, resistivity and microstructure. Acknowledgments

Temperature : 270OC Precursor pulsing time : 7s Plasma power: 100 W

10 0 0

4

8

12

16

20

24

This work was supported by a KBSI Grant (T31601) to T.E. Hong and also partially supported by a National Research Foundation of Korea Grant funded by the Korean Government (2010-0011187).

Reactant pulsing time (s)

Ru(103)

Ru(110)

Ru(102)

b

Ru(002) Ru(101)

Intensity (arb. units)

2500

Ru(100)

References

2000

1500

20s 12s

1000

8s 500

5s 3s

0 30

40

50

60

70

80

2θ (degree) Fig. 6. (a) Resistivity of ALD-Ru films deposited at 270 °C as a function of the reactant (N2/H2 plasma) pulsing time (b) corresponding glazing incident angle (θ = 3°) XRD results.

[1] International technology roadmap for semiconductors—2009 edition, http://www. itrs.net/Links/2009ITRS/2009Chapters_2009Tables/2009_Interconnect.pdf, accessed November 7, 2011. [2] S.M. Rossnagel, T.S. Kuan, J. Vac. Sci. Technol. B 22 (2004) 240. [3] H. Kim, J. Vac. Sci. Technol. B 21 (2003) 2231. [4] H. Kim, H.-B.-R. Lee, W.-J. Maeng, Thin Solid Films 517 (2009) 2563. [5] M.W. Lane, C.E. Murray, F.R. McFeely, P.M. Vereecken, R. Rosenberg, Appl. Phys. Lett. 83 (2003) 2330. [6] R. Chan, T.N. Arunagiri, Y. Zhang, O. Chyan, R.M. Wallace, M.J. Kim, T.Q. Hurd, Electrochem. Solid-State Lett. 7 (2004) G154. [7] H. Kim, T. Koseki, T. Ohba, T. Ohta, Y. Kojima, H. Sato, J. Electrochem. Soc. 152 (2005) G594. [8] M. Damayanti, T. Sritharan, Z.H. Gan, S.G. Mhaisalkar, N. Jiang, L. Chan, J. Electrochem. Soc. 153 (2006) J41. [9] T. Aaltonen, P. Alën, M. Ritala, M. Leskelä, Chem. Vap. Deposition 9 (2003) 45. [10] O.-K. Kwon, J.-H. Kim, H.-S. Park, S.-W. Kang, J. Electrochem. Soc. 151 (2004) G109. [11] T. Aaltonen, M. Ritala, K. Arstila, J. Keinonen, M. Leskelä, Chem. Vap. Deposition 10 (2004) 217. [12] S.-S. Yim, D.-J. Lee, K.-S. Kim, S.-H. Kim, T.-S. Yoon, K.-B. Kim, J. Appl. Phys. 103 (2008) 113509. [13] W.-H. Kim, S.-J. Park, J.-Y. Son, H. Kim, Nanotechnology 19 (2008) 045302. [14] D.-J. Lee, S.-S. Yim, K.-S. Kim, S.-H. Kim, K.-B. Kim, J. Appl. Phys. 107 (2010) 013707. [15] S.K. Kim, J.H. Han, G.H. Kim, C.S. Hwang, Chem. Mater. 22 (2010) 2850.

T.E. Hong et al. / Thin Solid Films 520 (2012) 6100–6105 [16] S.-H. Choi, T. Cheon, S.-H. Kim, D.-H. Kang, G.-S. Park, S. Kim, J. Electrochem. Soc. 158 (2011) D351. [17] H. Li, D.B. Farmer, R.G. Gordon, Y. Lin, J. Vlassak, J. Electrochem. Soc. 154 (2007) D642. [18] D.B. Farmer, R.G. Gordon, J. Appl. Phys. 101 (2007) 124503. [19] O.-K. Kwon, S.-H. Kwon, H.-S. Park, S.-W. Kang, J. Electrochem. Soc. 151 (2004) C753. [20] S.-S. Yim, D.-J. Lee, K.-S. Kim, M.-S. Lee, S.-H. Kim, K.-B. Kim, Electrochem. Solid-state Lett. 11 (2008) K89.

6105

[21] W. Sari, T.-K. Eom, S.-H. Kim, H. Kim, J. Electrochem. Soc. 158 (2011) D42. [22] T.-K. Eom, S.-H. Kim, K.-S. Park, S. Kim, H. Kim, Electrochem. Solid-State Lett. 14 (2011) D10. [23] M. Damayantia, T. Sritharan, S.G. Mhaisalkar, Z.H. Gan, Appl. Phys. Lett. 88 (2006) 044101. [24] J. Amorim, G. Baravian, G. Sultan, Appl. Phys. Lett. 68 (1996) 1915.