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Low energy argon ion irradiation surface effects on triglycine sulfate
Applied Surface Science 280 (2013) 858–861
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Low energy argon ion irradiation surface effects on triglycine sulfate Carmen Aragó ∗ , José L. Plaza, Manuel I. Marqués, Julio A. Gonzalo Dept. Física de Materiales, Facultad de Ciencias. Universidad Autónoma, Madrid, Spain
a r t i c l e
i n f o
Article history: Received 28 February 2013 Received in revised form 25 April 2013 Accepted 16 May 2013 Available online 23 May 2013 Keywords: Ion irradiation Ferroelectrics Phase transition
a b s t r a c t An experimental study of the effects of low energy (1–2 keV) argon ion (Ar+ ) irradiation on Triglycine Sulfate (TGS) has been performed. Ferroelectric parameters, such as the Curie temperature TC determined from the dielectric constant peaks ε(T), or the remnant polarization Pr , and coercive field Ec , obtained from the hysteresis loops, show interesting differences between samples irradiated in ferroelectric and paraelectric phases, respectively. The radiation damage seems to be superficial, as observed by AFM microscope, and the surface alteration in both phases becomes eventually notorious when the radiation dosage increases. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The effect of radiation on ferroelectric materials is a subject of great interest because it may result in a permanent modification of the dielectric properties and the critical temperature of the ferro–paraelectric phase transition. Most experiments in the past have been carried out with X-ray irradiated TGS [1–3], where a deep penetration on the sample and subsequent dislocations and internal fields are expected to appear. More recent works [4–7] report the effect of different types of ions (O, Al+ , Cu+ ) irradiation at high energy but low fluence, which actually results in some kind of thermal diffusion as well as ion implantation and hence internal electric fields. However, it remains to be studied the effect of low energy ion beam sputtering (LEIBS), that is, ions which should be expected to damage just the surface of the sample with no deep penetration, and these kind of experiments are also interesting from the point of view of the formation of surface nanostructures. It has also been reported [8–10] that low energy ion sputtering at normal incidence appears to be a very promising technique for the fabrication of nanostructures and quantum dots. This technique represents an attractive alternative to the Stranski–Krastanov (SK) method due to a much more simple (one small high vacuum chamber and an ion gun) and inexpensive experimental equipment needed when compared to a MBE based technique like SK. It has been demonstrated, for example, that hexagonal arrays of uniform semiconductor quantum dots, can be developed by LEIBS using Ar+ ions [8,9]. In particular, it has been shown by Facsko et al. [10,11] that dot
∗ Corresponding author. Tel.: +34 9149764243; fax: +34 4978579. E-mail address: [email protected] (C. Aragó). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.078
patterns can be spontaneously formed on GaSb and InSb when surfaces of these materials are sputtered with low energy argon ions at normal incidence. They also demonstrated quantum confinement in quantum dots created by LEIS on GaSb layers on AlSb substrates. An additional advantage of this technique is that, by tuning the sputtering time and the energy of the incident ions, the size and the density of the dots can be independently controlled. The formation of these ordered nanostructures has been extensively studied and reviewed both for metals [12] and semiconductors [13]. The self-organization of quantum-dot periodic arrays during sputtering has been attributed to the interplay between surface roughening induced by the ion sputtering and smoothing processes of the surface. In this work, we have used low energy (up to 2 keV) Ar+ beam sputtering, with a the aim of comparing the effect of the damage induced by LEIBS on the ferroelectric state (keeping the sample during the whole process at 40 ◦ C, that is, below the phase transition temperature TC ∼ = 49 ◦ C) and the paraelectric state (irradiating the sample at a temperature of 60 ◦ C, above the critical temperature). In addition, these effects seem to be correlated to the nanostructures formed at the surface of the TGS crystals. Subsequent irradiating doses have been the same for the two samples, keeping each of them at the respective same temperature. Dielectric measurements have been performed in both samples after irradiation and then compared with another reference sample, all of them cleaved from the same crystal. 2. Experimental The TGS samples used in this work were three slabs cleaved normally to ferroelectric axis from the same crystalline block. One of them was taken as the reference M0 and its dimensions
C. Aragó et al. / Applied Surface Science 280 (2013) 858–861
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65
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Fig. 1. (a) Dielectric constant plotted vs. temperature for the three samples. (b) Hysteresis loops of the reference and irradiated samples.
were thickness d = 1.85 mm; and surface area s = 16 mm2 , and it was characterized, after been electroded with silver electro conductive paint, by hysteresis loop at RT and by dielectric constant along a slow temperature sweep up to 70 ◦ C. A second sample M40 (d = 2.55 mm) was kept at T ∼ = 40 ◦ C through all the irradiation process in order to avoid the phase transition, and the third sample M60 (d = 1.35 mm) was heated at T ∼ = 60 ◦ C before each irradiation step. The same dielectric characterization was performed for each sample each time. The hysteresis loops were obtained at RT by means of a DDP bridge, applying a sinusoidal signal of 50 Hz provided by a power source HP-33120A and amplified by a Kepco bipolar voltage amplifier. They were observed and automatically recorded from a Nicolet-310 oscilloscope. The dielectric constant measurements were performed with LCRmeter HP-4284A, under the usual conditions, 1 V of applied voltage and a frequency of 1 KHz. The temperature was controlled by means of an Eurotherm 425A and the heating rate was 0.5 ◦ C/min. The LEIBS experiments were performed in a homemade high vacuum chamber (base pressure 10−6 torr) with and SPECS IQE 11 ion gun attached to it. The ion gun provides ions with energies up to 5 kV and the working pressure of the high purity Ar gas was 10−4 torr. A resistive heater (temperature range up to 350 ◦ C) with a controller was used to keep the samples temperature above and below the Curie point respectively, and a tantalum mask provided a damaged surface area of 50 mm2 . A Faraday cup was used to monitorize the sputtering ion current reaching the surface. The first irradiation of samples M40 and M60 was under a beam of 1keV and fluence 1.1 × 1016 cm−2 for a short interval of 17 min. In a second run, we have kept the ions energy being the same but the fluence increases to 8.9 × 1016 cm−2 and the irradiation time was 135 min. Finally the third run to the ion beam lasts the same time but the energy has been doubled up to 2 keV and the fluence was 2 × 1017 cm−2 . It must be noted that after each dielectric characterization, the silver electrodes were carefully removed before the next irradiation step. It is possible that this removal can alter the surface of the sample and then no cumulative effects should be considered. The surface of the samples has been observed by means of a Park Systems XE 100 AFM microscope working in non-contact mode, and a comparative measure of the roughness of the different samples has been obtained.
3. Results and discussion Fig. 1 shows the dielectric constant ε(T) and the hysteresis cycles of both samples irradiated in identical conditions and compared with the reference sample. Fig. 1(a) on the left plots the dielectric constant vs. temperature. It can be observed that in spite of the slightly shifted maximum peak in M40 (the phase transition temperature diminishes around 0.5 ◦ C from the reference TC ), the behavior of this sample that has been irradiated at ferroelectric phase is rather similar to that of reference sample M0. The effect of this short irradiation time is much more notorious in M60, the sample irradiated at 60 ◦ C, that is, in a paraelectric phase: the maximum lowers and the critical temperature is shifted more than 1 ◦ C. These same differences can be observed on Fig. 1(b) that displays the corresponding hysteresis loops, all them recorded at RT and applying a 50 Hz sinusoidal field of 2 kV/cm of amplitude. M0 and M40 show almost the same high remnant polarization Pr ∼ 3 C/cm2 , while in M60 lowers to 2.4 C/cm2 . Moreover, the coercive field is practically the same in both M0 and M60, but smaller in M40, shaping a more narrow loop and hence suggesting more easiness to switch the sense of the polarization. It must be noted that the hysteresis loops of all the three samples are rather asymmetric. Their asymmetry must be obviously due to some defects on the whole crystal that originate an internal field and then, different coercive field values on the two senses of the ferroelectric axis. Figs. 2 and 3 show the behavior of the samples M40 and M60 respectively under progressive irradiations. The label (+) stands for the second run (1 keV, 8.9 × 1016 cm−2 , 135 min) and (++) correspond to the third and last one (2 keV, 2 × 1017 cm−2 ,135 min). We can see in Figs. 2(a) and 3(a) that the critical temperature is monotonously descending after each irradiation for both samples. In the case of the M40, the peak maximum value is even smaller in the second step but it recovers after the third. The hysteresis loops display similar behavior: in both samples the spontaneous polarization after the second irradiation diminishes and the cycle shrinks, but after the third irradiation the hysteresis loop looks much like as the first one (Table 1). Regarding the ferroelectric parameters, we can see that all coercive fields at RT for the sample that has been irradiated in ferroelectric phase (M40) are smaller than the non-irradiated reference sample (M0) while in the sample irradiated in paraelectric phase (M60) their values keep approximately in the same range. Moreover the symmetry of the hysteresis loops (as evaluated from
860
C. Aragó et al. / Applied Surface Science 280 (2013) 858–861
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Fig. 2. (a) Temperature dependence of dielectric constant for M40 after irradiation processes. (b) Hysteresis loops at RT for M40 sample after each irradiation step.
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Fig. 3. (a) Temperature dependence of dielectric constant for M60 after irradiation processes. (b) Hysteresis loops at RT for M60 sample after each irradiation step.
Ec + − Ec − ) is generally greater than in the reference sample (Fig. 1b) probably due to some kind of compensation of the internal bias already existing in the crystal [8]. It seems that, in any case, the hysteresis cycles are better for the sample M40, higher remnant polarization and lower coercive field, than the corresponding M60. This could indicate that the effects of this irradiation process are mainly superficial and do not alter properly the crystal domain structure. We have also observed that after the second run the ferroelectric response decreases dramatically in both samples but it recovers after the third dose. This rather surprising behavior could Table 1 Ferroelectric parameters of TGS samples before and after Ar+ ion irradiation.
M40 M40 (+) M40 (++) M60 M60 (+) M60 (++) M0 (reference)
Pr (C/cm2 )
Ec + (kV/cm)
2.98 2.24 2.77 2.41 1.66 2.41 3
0.64 0.73 0.70 0.88 0.66 0.97 0.88
Ec − (kV/cm) 0.28 0.36 0.40 0,40 0.56 0.78 0.44
Ec + − Ec −
TC (◦ C)
+0.36 +0.37 +0.30 +0.48 +0.11 +0.19 +0.44
49.04 48.08 47.50 48.25 47.66 46.83 49.53
be explained in terms of free charges generated on the surface. In fact, irradiating with rather heavy ions like Ar+ implies little penetration in the crystal bulk as compared with X-ray irradiation, so more free charge is located in the surface layer where the low energy ions are halted. These free charges would be electrostatically attracted to the domains (in the ferroelectric phase) or the domain seeds (in the paraelectric phase), compensating the surface charge density and then, diminishing the spontaneous polarization as well as the dielectric constant. Moreover, after the third run we have observed (see Fig. 4) the formation of very small crystals that might explain the improvement of the ferroelectric response. In order to check out the effect that the irradiation process has produced on the samples surface, we have used AFM imaging as it is shown in Fig. 4. After a shorter irradiation time (17 min) there is a slight smoothening effect of both surfaces, but after the second irradiation run (135 min) we can observe a sort of spherical nanoparticles with sizes close to 100 nm. It must also be noted that the different temperatures of the substrate do not noticeably modify the geometry of such nanoparticles, at least for the fluences used in this work. The two first doses produce then an amorphous surface in both samples but the third irradiation run (135 min, higher energy and fluence) results in a notorious crystalline aspect,
C. Aragó et al. / Applied Surface Science 280 (2013) 858–861
861
Fig. 4. AFM analysis of the different samples irradiated above and below the Curie temperature. On the left, labeled “outside”, the images of the non-irradiated surfaces. On the right, labeled “inside”, the irradiated surface of both samples after each irradiation step.
but slightly different depending on the substrate temperature: the structures formed are larger for the sample irradiated at 60 ◦ C than at 40 ◦ C. As suggested above, this apparent recrystallization could explain the improvement of ferroelectric behavior compared with the same long lasting but less energetic earlier process. In summary, we have checked that the Ar+ ions irradiation, at low energies but relatively high fluences, on TGS modifies the crystal surface and alters their ferroelectric parameters. The response to the irradiation process depends on the phase of the sample, that is, if it has been irradiated keeping the temperature over or under the corresponding to the phase transition. As the ferroelectric behavior is always better in the sample M40 (ferroelectric phase) than in M60 (paraelectric phase) we should conclude that irradiation process does not cause significantly modification in crystal bulk ferroelectric domains. Finally it seems that a higher energy and fluence beam could alter the surface, enhancing eventually the ferroelectric response. It would be a way to explore the surface patterning of ferroelectric materials in order to improve their response by means of a simple and inexpensive technique as LEIBS. In any case this hypothesis should be confirmed by new experiments, in order to
evaluate the energy and fluence values adequated to achieve an optimum nanopattern for eventual applications. References [1] K. Okada, J.A. Gonzalo, J.M. Rivera:, Journal of Physics and Chemistry of Solids 28 (1967) 689–695. [2] C. Alemany, J. Mendiola, B. Jimenez, E. Maurer:, Ferroelectrics 13 (1976) 487–489. [3] N. Nakatani:, Japanese Journal of Applied Physics 24 (1985) 583–585. [4] Z. Xie, E.Z. Luo, J.B. Xu, et al., Physics Letters A 309 (2003) 121–125. [5] P.K. Bajpai, D. Shah, R. Kumar:, Nuclear Instruments & Methods in Physics Research B 244 (2006). [6] K. Peithmam, P.D. Eversheim, J. Goetze, et al., Applied Physics B 105 (2011) 113–127. [7] P.K. Bajpai, D. Shah, R. Kumar:, Nuclear Instruments & Methods in Physics Research B 270 (2012). [8] S. Facsko, H. Kurz, T. Dekorsy:, Physical Review B 63 (2001) (165329-1-5). [9] S. Facsko, T. Dekorsy, C. Koerdt, et al., Science 285 (1999) 1551. [10] S. Facsko, T. Bobek, T. Dekorsy, H. Kur:, Physica Status Solidi B-Basic Research 224 (2001) 537. [11] V. Valbusa, C. Boragno, F. Buatier de Mongeot, Journal of Physics and Condensation Materials 14 (2002). [12] G. Carter:, Journal of Physics D: Applied Physics 34 (2001). [13] A. Mansingh, E. Prasad:, Journal of Physics D: Applied Physics 9 (1976) 1379–1386.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Low energy argon ion irradiation surface effects on triglycine sulfate Carmen Aragó ∗ , José L. Plaza, Manuel I. Marqués, Julio A. Gonzalo Dept. Física de Materiales, Facultad de Ciencias. Universidad Autónoma, Madrid, Spain
a r t i c l e
i n f o
Article history: Received 28 February 2013 Received in revised form 25 April 2013 Accepted 16 May 2013 Available online 23 May 2013 Keywords: Ion irradiation Ferroelectrics Phase transition
a b s t r a c t An experimental study of the effects of low energy (1–2 keV) argon ion (Ar+ ) irradiation on Triglycine Sulfate (TGS) has been performed. Ferroelectric parameters, such as the Curie temperature TC determined from the dielectric constant peaks ε(T), or the remnant polarization Pr , and coercive field Ec , obtained from the hysteresis loops, show interesting differences between samples irradiated in ferroelectric and paraelectric phases, respectively. The radiation damage seems to be superficial, as observed by AFM microscope, and the surface alteration in both phases becomes eventually notorious when the radiation dosage increases. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The effect of radiation on ferroelectric materials is a subject of great interest because it may result in a permanent modification of the dielectric properties and the critical temperature of the ferro–paraelectric phase transition. Most experiments in the past have been carried out with X-ray irradiated TGS [1–3], where a deep penetration on the sample and subsequent dislocations and internal fields are expected to appear. More recent works [4–7] report the effect of different types of ions (O, Al+ , Cu+ ) irradiation at high energy but low fluence, which actually results in some kind of thermal diffusion as well as ion implantation and hence internal electric fields. However, it remains to be studied the effect of low energy ion beam sputtering (LEIBS), that is, ions which should be expected to damage just the surface of the sample with no deep penetration, and these kind of experiments are also interesting from the point of view of the formation of surface nanostructures. It has also been reported [8–10] that low energy ion sputtering at normal incidence appears to be a very promising technique for the fabrication of nanostructures and quantum dots. This technique represents an attractive alternative to the Stranski–Krastanov (SK) method due to a much more simple (one small high vacuum chamber and an ion gun) and inexpensive experimental equipment needed when compared to a MBE based technique like SK. It has been demonstrated, for example, that hexagonal arrays of uniform semiconductor quantum dots, can be developed by LEIBS using Ar+ ions [8,9]. In particular, it has been shown by Facsko et al. [10,11] that dot
∗ Corresponding author. Tel.: +34 9149764243; fax: +34 4978579. E-mail address: [email protected] (C. Aragó). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.078
patterns can be spontaneously formed on GaSb and InSb when surfaces of these materials are sputtered with low energy argon ions at normal incidence. They also demonstrated quantum confinement in quantum dots created by LEIS on GaSb layers on AlSb substrates. An additional advantage of this technique is that, by tuning the sputtering time and the energy of the incident ions, the size and the density of the dots can be independently controlled. The formation of these ordered nanostructures has been extensively studied and reviewed both for metals [12] and semiconductors [13]. The self-organization of quantum-dot periodic arrays during sputtering has been attributed to the interplay between surface roughening induced by the ion sputtering and smoothing processes of the surface. In this work, we have used low energy (up to 2 keV) Ar+ beam sputtering, with a the aim of comparing the effect of the damage induced by LEIBS on the ferroelectric state (keeping the sample during the whole process at 40 ◦ C, that is, below the phase transition temperature TC ∼ = 49 ◦ C) and the paraelectric state (irradiating the sample at a temperature of 60 ◦ C, above the critical temperature). In addition, these effects seem to be correlated to the nanostructures formed at the surface of the TGS crystals. Subsequent irradiating doses have been the same for the two samples, keeping each of them at the respective same temperature. Dielectric measurements have been performed in both samples after irradiation and then compared with another reference sample, all of them cleaved from the same crystal. 2. Experimental The TGS samples used in this work were three slabs cleaved normally to ferroelectric axis from the same crystalline block. One of them was taken as the reference M0 and its dimensions
C. Aragó et al. / Applied Surface Science 280 (2013) 858–861
a
12000
b
First irradiation (1 keV, 17 min.)
4 3
M0 M40 M60
10000
M0 M40 M60
2
8000
859
2
P (μC/cm )
1
ε
6000
4000
0 -1 -2
2000
-3 -4
0 30
35
40
45
50
55
60
65
70
-2
-1
T(ºC)
0
1
2
E(kV/cm)
Fig. 1. (a) Dielectric constant plotted vs. temperature for the three samples. (b) Hysteresis loops of the reference and irradiated samples.
were thickness d = 1.85 mm; and surface area s = 16 mm2 , and it was characterized, after been electroded with silver electro conductive paint, by hysteresis loop at RT and by dielectric constant along a slow temperature sweep up to 70 ◦ C. A second sample M40 (d = 2.55 mm) was kept at T ∼ = 40 ◦ C through all the irradiation process in order to avoid the phase transition, and the third sample M60 (d = 1.35 mm) was heated at T ∼ = 60 ◦ C before each irradiation step. The same dielectric characterization was performed for each sample each time. The hysteresis loops were obtained at RT by means of a DDP bridge, applying a sinusoidal signal of 50 Hz provided by a power source HP-33120A and amplified by a Kepco bipolar voltage amplifier. They were observed and automatically recorded from a Nicolet-310 oscilloscope. The dielectric constant measurements were performed with LCRmeter HP-4284A, under the usual conditions, 1 V of applied voltage and a frequency of 1 KHz. The temperature was controlled by means of an Eurotherm 425A and the heating rate was 0.5 ◦ C/min. The LEIBS experiments were performed in a homemade high vacuum chamber (base pressure 10−6 torr) with and SPECS IQE 11 ion gun attached to it. The ion gun provides ions with energies up to 5 kV and the working pressure of the high purity Ar gas was 10−4 torr. A resistive heater (temperature range up to 350 ◦ C) with a controller was used to keep the samples temperature above and below the Curie point respectively, and a tantalum mask provided a damaged surface area of 50 mm2 . A Faraday cup was used to monitorize the sputtering ion current reaching the surface. The first irradiation of samples M40 and M60 was under a beam of 1keV and fluence 1.1 × 1016 cm−2 for a short interval of 17 min. In a second run, we have kept the ions energy being the same but the fluence increases to 8.9 × 1016 cm−2 and the irradiation time was 135 min. Finally the third run to the ion beam lasts the same time but the energy has been doubled up to 2 keV and the fluence was 2 × 1017 cm−2 . It must be noted that after each dielectric characterization, the silver electrodes were carefully removed before the next irradiation step. It is possible that this removal can alter the surface of the sample and then no cumulative effects should be considered. The surface of the samples has been observed by means of a Park Systems XE 100 AFM microscope working in non-contact mode, and a comparative measure of the roughness of the different samples has been obtained.
3. Results and discussion Fig. 1 shows the dielectric constant ε(T) and the hysteresis cycles of both samples irradiated in identical conditions and compared with the reference sample. Fig. 1(a) on the left plots the dielectric constant vs. temperature. It can be observed that in spite of the slightly shifted maximum peak in M40 (the phase transition temperature diminishes around 0.5 ◦ C from the reference TC ), the behavior of this sample that has been irradiated at ferroelectric phase is rather similar to that of reference sample M0. The effect of this short irradiation time is much more notorious in M60, the sample irradiated at 60 ◦ C, that is, in a paraelectric phase: the maximum lowers and the critical temperature is shifted more than 1 ◦ C. These same differences can be observed on Fig. 1(b) that displays the corresponding hysteresis loops, all them recorded at RT and applying a 50 Hz sinusoidal field of 2 kV/cm of amplitude. M0 and M40 show almost the same high remnant polarization Pr ∼ 3 C/cm2 , while in M60 lowers to 2.4 C/cm2 . Moreover, the coercive field is practically the same in both M0 and M60, but smaller in M40, shaping a more narrow loop and hence suggesting more easiness to switch the sense of the polarization. It must be noted that the hysteresis loops of all the three samples are rather asymmetric. Their asymmetry must be obviously due to some defects on the whole crystal that originate an internal field and then, different coercive field values on the two senses of the ferroelectric axis. Figs. 2 and 3 show the behavior of the samples M40 and M60 respectively under progressive irradiations. The label (+) stands for the second run (1 keV, 8.9 × 1016 cm−2 , 135 min) and (++) correspond to the third and last one (2 keV, 2 × 1017 cm−2 ,135 min). We can see in Figs. 2(a) and 3(a) that the critical temperature is monotonously descending after each irradiation for both samples. In the case of the M40, the peak maximum value is even smaller in the second step but it recovers after the third. The hysteresis loops display similar behavior: in both samples the spontaneous polarization after the second irradiation diminishes and the cycle shrinks, but after the third irradiation the hysteresis loop looks much like as the first one (Table 1). Regarding the ferroelectric parameters, we can see that all coercive fields at RT for the sample that has been irradiated in ferroelectric phase (M40) are smaller than the non-irradiated reference sample (M0) while in the sample irradiated in paraelectric phase (M60) their values keep approximately in the same range. Moreover the symmetry of the hysteresis loops (as evaluated from
860
C. Aragó et al. / Applied Surface Science 280 (2013) 858–861
a
b 10000
4 3
M40 M40(+) M40(++)
8000
M40 M40(+) M40(++)
2
2
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1 6000
ε 4000
0 -1 -2
2000 -3 -4
0 25
30
35
40
45
50
55
60
65
-2
70
-1
0
1
2
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T(ºC)
Fig. 2. (a) Temperature dependence of dielectric constant for M40 after irradiation processes. (b) Hysteresis loops at RT for M40 sample after each irradiation step.
a
b
7000
M60 M60(+) M60(++)
6000
M60 M60(+) M60(++)
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5000 2
P (μC/cm )
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T(ºC)
Fig. 3. (a) Temperature dependence of dielectric constant for M60 after irradiation processes. (b) Hysteresis loops at RT for M60 sample after each irradiation step.
Ec + − Ec − ) is generally greater than in the reference sample (Fig. 1b) probably due to some kind of compensation of the internal bias already existing in the crystal [8]. It seems that, in any case, the hysteresis cycles are better for the sample M40, higher remnant polarization and lower coercive field, than the corresponding M60. This could indicate that the effects of this irradiation process are mainly superficial and do not alter properly the crystal domain structure. We have also observed that after the second run the ferroelectric response decreases dramatically in both samples but it recovers after the third dose. This rather surprising behavior could Table 1 Ferroelectric parameters of TGS samples before and after Ar+ ion irradiation.
M40 M40 (+) M40 (++) M60 M60 (+) M60 (++) M0 (reference)
Pr (C/cm2 )
Ec + (kV/cm)
2.98 2.24 2.77 2.41 1.66 2.41 3
0.64 0.73 0.70 0.88 0.66 0.97 0.88
Ec − (kV/cm) 0.28 0.36 0.40 0,40 0.56 0.78 0.44
Ec + − Ec −
TC (◦ C)
+0.36 +0.37 +0.30 +0.48 +0.11 +0.19 +0.44
49.04 48.08 47.50 48.25 47.66 46.83 49.53
be explained in terms of free charges generated on the surface. In fact, irradiating with rather heavy ions like Ar+ implies little penetration in the crystal bulk as compared with X-ray irradiation, so more free charge is located in the surface layer where the low energy ions are halted. These free charges would be electrostatically attracted to the domains (in the ferroelectric phase) or the domain seeds (in the paraelectric phase), compensating the surface charge density and then, diminishing the spontaneous polarization as well as the dielectric constant. Moreover, after the third run we have observed (see Fig. 4) the formation of very small crystals that might explain the improvement of the ferroelectric response. In order to check out the effect that the irradiation process has produced on the samples surface, we have used AFM imaging as it is shown in Fig. 4. After a shorter irradiation time (17 min) there is a slight smoothening effect of both surfaces, but after the second irradiation run (135 min) we can observe a sort of spherical nanoparticles with sizes close to 100 nm. It must also be noted that the different temperatures of the substrate do not noticeably modify the geometry of such nanoparticles, at least for the fluences used in this work. The two first doses produce then an amorphous surface in both samples but the third irradiation run (135 min, higher energy and fluence) results in a notorious crystalline aspect,
C. Aragó et al. / Applied Surface Science 280 (2013) 858–861
861
Fig. 4. AFM analysis of the different samples irradiated above and below the Curie temperature. On the left, labeled “outside”, the images of the non-irradiated surfaces. On the right, labeled “inside”, the irradiated surface of both samples after each irradiation step.
but slightly different depending on the substrate temperature: the structures formed are larger for the sample irradiated at 60 ◦ C than at 40 ◦ C. As suggested above, this apparent recrystallization could explain the improvement of ferroelectric behavior compared with the same long lasting but less energetic earlier process. In summary, we have checked that the Ar+ ions irradiation, at low energies but relatively high fluences, on TGS modifies the crystal surface and alters their ferroelectric parameters. The response to the irradiation process depends on the phase of the sample, that is, if it has been irradiated keeping the temperature over or under the corresponding to the phase transition. As the ferroelectric behavior is always better in the sample M40 (ferroelectric phase) than in M60 (paraelectric phase) we should conclude that irradiation process does not cause significantly modification in crystal bulk ferroelectric domains. Finally it seems that a higher energy and fluence beam could alter the surface, enhancing eventually the ferroelectric response. It would be a way to explore the surface patterning of ferroelectric materials in order to improve their response by means of a simple and inexpensive technique as LEIBS. In any case this hypothesis should be confirmed by new experiments, in order to
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