- Email: [email protected]
Effect of ion irradiation on the optical properties of Ag-doped Ge2Sb2Te5 (GST) thin films
Nuclear Inst. and Methods in Physics Research B 467 (2020) 40–43
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of ion irradiation on the optical properties of Ag-doped Ge2Sb2Te5 (GST) thin films
T
Neetu Kandaa, Anup Thakurb, Fouran Singhc, A.P. Singha,
⁎
a
Thin Film Lab., Physics Department, Dr B R Ambedkar National Institute of Technology, Jalandhar 144 011, India Advanced Materials Research Lab., Department of Basic and Applied Sciences, Punjabi University, Patiala 147 002, India c Material Science Group, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India b
ARTICLE INFO
ABSTRACT
Keywords: Chalcogenide alloys Swift heavy ions (SHI) Phase change materials and crystallization
Structural and optical study of pure and Ag-doped Ge2Sb2Te5 (GST) thin films after swift heavy ion irradiation, is presented. In the present work, we have used 120 MeV Ag9+ ions for irradiation using various ion fluence. Structural properties were investigated using XRD and Raman spectroscopy. Undoped films were found to crystallize at the highest fluence (1 × 1013 ions/cm2). Ag doping is found to suppress crystallization. From UVVis-NIR spectra, a decrease in optical bandgap was observed due to ion irradiation on these films. Observed results are explained on the basis of crystal nuclei, which results in crystallization and observed structural changes. Changes in defect chemistry are proposed to be responsible for the observed optical changes.
1. Introduction Chalcogenide alloys are one of the best materials for rewritable memory applications. The most widely studied material among chalcogenides is Ge2Sb2Te5 (GST), which is a glassy alloy. Because of fast crystallization time ( 100 ns) [1], high thermal stability, good endurance, scalability, reliability, and better data storage applications, this material is widely used for nonvolatile memory [2]. It has also been proposed to be used in a reversible near-infrared window [3]. This alloy can be switched between amorphous and crystalline phases by electric pulse, laser pulse, local heating, and pressure of appropriate intensity and duration [3]. The kinetics of amorphization and crystallization determines the switching times between these phases. These materials show high contrast in their optical and electrical properties while changing phase from amorphous (Eg = 0.7 eV) to crystalline (Eg = 0.5 eV) and vice-versa [4]. In the amorphous phase, Ge atoms are arranged tetrahedrally, whereas in the crystalline phase they are arranged octahedrally. Even a small change in structure is sufficient to flip the phase from amorphous to crystalline or vice-versa, which is the reason for the fast switching speed of GST [5]. The transmission contrast in amorphous and crystalline phases of GST thin films has been studied by Singh et al. [3]. They observed that transmission decreases from 92% to 2% on phase transition from amorphous to hcp. Due to this large contrast in transmission, GST becomes a feasible candidate for reversible near-infrared window. Optical and structural properties of GST thin films can be improved by doping ⁎
with different elements like Ni [6], Mg [7], Si [6,8], Se [9] and Ag [10,11]. Since Ag is more reflective and soluble in chalcogenide alloys [12] and forms bonds with defect levels present in chalcogenide alloys (GST), it helps in improving its optical and electrical properties [13]. Excellent crystallization speed and large contrast in resistance have been observed in Ag-doped GST thin films as compared to pure GST thin films [10]. Ion irradiation has been used to introduce defects in these films which results in improved properties [14]. Recently Bastiani et al. investigated 120 keV Sb+ ions irradiated GST thin films with fluence between 5 × 1012 to 1 × 1014 ions/cm2 [15]. They studied crystallization kinetics of pure and irradiated thin films and found that after ion irradiation, crystallization kinetics of the films increases. This happens due to a reduction in Ge-Te tetrahedral bonds. Privitera et al. investigated GST polycrystalline films irradiated by 150 keV Ar+ ions with fluence in the range 1 × 1011 to 5 × 1014 ions/cm2 [16]. They observed ion irradiation-induced disorder on structural and electrical properties of polycrystalline GST thin films. They also found an increase in disorder induced by ion irradiation that resulted in amorphization in rocksalt structure and transition from metallic to insulating behavior in the trigonal phase. Fedorenko et al. [17] have studied ion implantation of bismuth in lone pair chalcogenide glasses and their electrical properties after ion implantation. They observed that electrical conductivity increases with Bi ion implantation. This enhanced conductivity was found to be sensitive to the dose of implantation. Most of the reports on the effects of ion irradiation on GST are
Corresponding author. E-mail address: [email protected] (A.P. Singh).
https://doi.org/10.1016/j.nimb.2020.01.025 Received 30 August 2019; Received in revised form 23 January 2020; Accepted 24 January 2020 0168-583X/ © 2020 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 467 (2020) 40–43
N. Kanda, et al.
focussed on the low energy regime ( 100 keV) [15–17]. In this regime the energy loss by the incident ion in the material is dominated by nuclear energy loss. For swift heavy ions (SHI), the dominant energy loss is electronic energy loss [18]. In the present work, we have studied the effect of Ag9+ ion irradiation on optical and structural properties of pure and Ag-doped GST thin films with fluence in the range of 5 × 1011 to 1 × 1013 ions/cm2. The energy of the incident ion was 120 MeV.
et al. [14] can be used. According to this model, the introduction of defects due to SHI irradiation increases the free energy of the system. Due to the increase in free energy, the thermodynamic barrier for nucleation reduces, leading to crystallization of the material. Since the introduced defects increase with increase in irradiation fluence, that is why peaks of fcc phase were observed only for the highly irradiated pure GST thin films. However, no such peak was observed for highly irradiated Ag-doped thin films. It may be due to an increase in the crystallization temperature by Ag doping, which limits the phase transformation from amorphous to crystalline. Similar suppression in the crystallization temperature of GST thin films was observed by Singh et al. [21]. Raman spectra for Ag-doped and irradiated Ge2Sb2Te5 thin films are shown in Fig. 2. The Raman peaks were fitted with Lorentzian profile. For G1Ag0 sample, two peaks were observed at 127 cm−1 and 151 cm−1. In accordance with previous reports, the first peak is assigned to corner-sharing GeTe4 tetrahedral vibration, and the second peak corresponds to Sb-Te vibration in the SbTe3 unit [19]. There was no shift in the peak at 127 cm−1 within the instrumental resolution. The peak at 151 cm−1 gradually shifted towards lower wavenumber as the irradiation fluence is increased (148 cm−1 for G1Ag1e13). With irradiation, the weakening of Sb-Te bonds may occur in SbTe3 unit, which reduces the force constant, along with the oscillation frequency of these bonds. This may result in the observed redshift in Raman peaks. This peak also gets flattened, which may be due to the reduction of Sb-Te vibration in the SbTe3 unit. These results are not in agreement with the results obtained by Bastiani et al. [15]. They observed that the 127 cm−1 peak vanished with irradiation because of the breakage of corner-sharing GeTe4 tetrahedral vibration. The difference in the observed results may be due to the different mechanisms through which energy gets deposited in the two energy regimes. Bastiani et al. used Sb+ ions to irradiate GST with 120 keV energy. The nuclear and electronic energy losses are 252 and 17.7 eV/Å, respectively. For this energy, nuclear energy loss is dominant. This means larger structural damage being caused due to elastic collisions. In the present study, the electronic energy loss is the dominant mechanism, in which the energy is transferred to the electronic system. So, lesser structural damages are expected for swift heavy ion irradiated samples. The transmission spectra of all the samples are shown in Fig. 3. The optical bandgap was calculated from this spectra using the Tauc plot [22]. The bandgap variation with SHI irradiation and Ag doping is given in Table 2 which shows that in pure and Ag-doped GST samples, bandgap, in general, decreases with irradiation. This may be due to the reason that ion irradiation causes the generation of crystal nuclei, as discussed before. Moreover, grain growth also increases because of an increase in the atomic mobility at the amorphous-crystal interface [14]. The SHI induced crystallization leads to bandgap reduction. From Table 2, it is clear that bandgap increases with Ag doping. The increase in bandgap with Ag doping may be due to the reduction in the density of localized states. When the doped neutral Ag atom is weakly bound to the trapping center, it loses one electron to become Ag+. The Ag+ ion gets trapped to D− state making these defect states neutral [13,23,24].
2. Materials and methods (Ge2Sb2Te5)100-xAg x (x = 0, 1) samples were prepared by melt quenching technique. According to the atomic weight percentage, Ge, Sb, Te, and Ag were weighed and then sealed in quartz ampoules under high vacuum condition. These ampoules were then heated for two hours, each in increasing order of melting point of constituent elements. Then the mixture was heated at the highest melting point of constituent element for 24 hours. To ensure the homogeneity of the samples, they were simultaneously rocked. At the end, quenching of these mixtures were done in liquid nitrogen. Thermal evaporation using Hind HIVAC system was used for preparing thin films of 500 nm on the glass substrate at deposition rate 10 Å/s at a base pressure of 2 × 10 6 mbar. The composition of the GeSbTe alloy and the thin film has been confirmed previously [19]. These films were irradiated with 120 MeV Ag9+ ion using the 15 UD tandem Pelletron accelerator at IUAC, New Delhi at fluences of 5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2. During irradiation, the beam current was maintained at 1 particle nano Ampere and the beam was scanned over an area of 1.25 × 1.4 cm2. The damage introduced due to the interaction of the target material and ion beam was studied using SRIM software [20]. The nuclear and electronic energy loss for 120 MeV Ag9+ ions in GST is equal to 9.13 and 1.73 × 103 eV/Å, respectively. The range of the ions was calculated to be 13.01 μm. The longitudinal and lateral straggling is 9998 Å and 1.27 μm, respectively. The displacement per atom obtained for different fluences (5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2) comes out to be 1.3 × 10 5, 2.62 × 10 5 and 2.62 × 10 4 respectively. The naming of the samples with these fluences and Ag-doping is given in Table 1. The structural information of these films was obtained from XRD using PANalytical x-ray diffractometer using Cu-K radiation source. The diffraction data were obtained in the range 20° to 80° in BraggBrentano geometry. The vibrational properties were investigated through Raman spectroscopy using Renishaw inVia Raman microscope. Argon laser (514 nm) was used for excitation in the range of 100–500 cm−1. UV-Vis-NIR spectroscopy was used to study the optical properties using Cary 500 UV-Vis-NIR spectrophotometer. Transmission data were taken in the range of 500-3200 nm at room temperature. 3. Results and discussions The XRD data of all the samples are shown in Fig. 1. Two peaks were observed for the G0Ag1e13 sample. No peak was observed for other samples, which only show a broad hump. The peaks observed in G0Ag1e13 samples, indexed in Fig. 1, are of the fcc phase of GST. These observations suggest the onset of crystallization in these samples on SHI irradiation. To understand these results, the model proposed by Bastiani
D+ + e D + Ag+
Ag doping % Fluence (ions/cm2)
0
1
0
G0Ag0 G0Ag5e11
G1Ag0 G1Ag5e11
G0Ag1e13
G1Ag1e13
1 × 1012 1 × 1013
G0Ag1e12
[D Ag+]0
(1) (2)
This mechanism reduces the density of states, leading to a decrease in the width of the band tail and the Urbach energy. As a result, optical bandgap increases with Ag doping.
Table 1 Naming of the thin films with respect to fluences and Ag doping.
5 × 1011
D0
4. Conclusion Effect of 120 MeV Ag9+ ion irradiation on pure and Ag-doped Ge2Sb2Te5 (GST) thin films were probed with different fluences using XRD, Raman, and UV-Vis-NIR spectroscopy. The experimental results indicate that crystallization is observed in pure samples for the highest
G1Ag1e12
41
Nuclear Inst. and Methods in Physics Research B 467 (2020) 40–43
N. Kanda, et al.
Fig. 1. XRD spectra of (a) pure, (b) 1% Ag-doped with irradiation fluence 5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2. Table 2 Effect of ion irradiation and Ag doping on bandgap of GST thin films. Ag doping % Fluence (ions/cm2)
0
1
0
0.65 0.61
0.67 0.66
0.58
0.61
5 × 1011
1 × 1012 1 × 1013
0.60
0.65
fluence of irradiation. However, in the Ag-doped sample, only small structural changes are observed with irradiation. The decrease in optical bandgap is also observed with ion irradiation, which indicates the formation of crystal grain due to the energy deposited by the incident ions. An increase in bandgap is observed with Ag doping, which is attributed to a decrease in the density of defect states. Defects play an essential role in determining the properties of GST films. SHI irradiation can be used to control the defects in the films. The SHI irradiated films can be further studied for their crystallization kinetics. This can prove to be the superior method compared to low
Fig. 2. Raman spectra of 1% Ag-doped samples with irradiation fluence 5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2 respectively.
Fig. 3. Transmission spectra of (a) pure, (b) 1% Ag-doped samples with irradiation fluence 5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2 respectively.
42
Nuclear Inst. and Methods in Physics Research B 467 (2020) 40–43
N. Kanda, et al.
energy ion irradiation because of reduced structural damage to the samples.
[8] S.J. Park, I.S. Kim, S.K. Kim, S.M. Yoon, B.G. Yu, S.Y. Choi, Phase transition characteristics and device performance of Si-doped Ge2Sb2Te5, Semicond. Sci. Technol. 23 (2008) 105006 . [9] E.M. Vinod, K. Ramesh, R. Ganesan, K.S. Sangunni, Direct hexagonal transition of amorphous (Ge2Sb2Te5)0.9Se0.1 thin films, Appl. Phys. Lett. 104 (2014) 063505 . [10] K.H. Song, S.W. Kim, J.H. Seo, H.Y. Lee, Characteristics of amorphous Ag0.1(Ge2Sb2Te5)0.9 thin film and its ultrafast crystallization, J. Appl. Phys. 104 (2008) 103516 . [11] B. Prasai, G. Chen, D.A. Drabold, Direct ab-initio molecular dynamic study of ultrafast phase change in Ag-alloyed Ge2Sb2Te5, Appl. Phys. Lett. 102 (2013) 041907 . [12] M. Frumer, T. Wagner, Ag doped chalcogenide glasses and their applications, Curr. Opin. Solid State Mater. Sci. 7 (2003) 117. [13] P. Singh, R. Kaur, P. Sharma, V. Sharma, M. Mishra, G. Gupta, A. Thakur, Optical band gap tuning of Ag doped Ge2Sb2Te5 thin films, J. Mater. Sci. Mater. Electron. 28 (2017) 11300. [14] R.D. Bastiani, A.M. Piro, I. Crupi, M.G. Grimaldi, E. Rimini, Effect of ion irradiation on the stability of amprphous Ge2Sb2Te5 thin films, Nucl. Instrum. Methods Phys. Res. B 266 (2008) 2511. [15] R.D. Bastiani, A.M. Piro, M.G. Grimaldi, E. Rimini, G.A. Baratta, G. Strazzulla, Ion irradiation-induced local structural changes in amorphous Ge2Sb2Te5 thin film, Appl. Phys. Lett. 92 (2008) 241925 . [16] S.M.S. Privitera, A.M. Mio, E. Smecca, A. Alberti, W. Zhang, R. Mazzarello, J. Benke, C. Persch, F. La Via, E. Rimini, Structural and electronic transitions in Ge2Sb2Te5 induced by ion irradiation damage, Phys. Rev. B 94 (2016) 094103 . [17] Y.G. Fedorenko, Electrical properties of Bi-implanted amorphous chalcogenide films, Thin Solid Films 589 (2015) 369. [18] D.K. Avasthi, Some interesting aspects of swift heavy ions in materials science, Curr. Sci. 78 (2000) 1297. [19] P. Singh, P. Sharma, V. Sharma, A. Thakur, Linear and non-linear optical properties of Ag-doped Ge2Sb2Te5 thin films estimated by single transmission spectra, Semicond. Sci. Technol. 32 (2017) 045015 . [20] J.F. Ziegler, J.P. Biresack, U. Littmark, The Stopping and the Range of Ions in Solids, Pergamon, New York, 1985. [21] Palwinder Singh, A.P. Singh, Anup Thakur, Thermal stability improvement and crystallization behavior of Ag doped Ge2Sb2Te5 phase change materials, J. Mater. Sci.: Mater. Electron. 30 (2019) 3604. [22] Anup Thakur, G.S.S. Gurdishpal Singh, Navdeep Goyal Saini, S.K. Tripathi, Optical properties of amorphous Ge20 Se80 and Ag6 (Ge0.20 Se0.80)94 thin films, Opt. Mater.Opt. Mater. 30 (2007) 565. [23] I. Chaudhary, F. Inam, D.A. Drabold, Ab initio determination of ion traps and dynamics of silver in silver-doped chalcogenide glass, Phys. Rev. B 79 (2009) 100201 . [24] S. Kumar, D. Singh, R. Thangaraj, Optical properties and phase transition in photodoped amorphous Ge-Sb-Te: Ag thin films, Thin Solid Films 540 (2013) 271.
Funding sources This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The support provided by the pelletron group at IUAC, New Delhi during irradiation experiments is highly appreciated. References [1] G. Atwood, Phase-change materials for electronic memories, Science 321 (2008) 210. [2] H. Lv, P. Zhou, Y. Lin, T. Tang, B. Qiao, Y. Lai, J. Feng, B. Cai, B. Chen, Electronic properties of GST for non-volatile memory, Microelectron. J. 37 (2006) 982. [3] P. Singh, A.P. Singh, N. Kanda, M. Mishra, G. Gupta, A. Thakur, High transmittance contrast in amorphous to hexagonal phase of Ge2Sb2Te5: Reversible NIR-window, Appl. Phys. Lett. 111 (2017) 261102 . [4] T. Kato, K. Tanaka, Electronic properties of amorphous and crystalline Ge2Sb2Te5 films, Jpn. J. Appl. Phys. 44 (2005) 7340. [5] M. Wuttig, N. Yamada, Phase change materials for rewritable data storage, Nat. Mater. 6 (2007) 824. [6] H. Kolpin, D. Music, G. Laptyeva, R. Ghadimi, F. Merget, S. Richter, R. Mykhaylonka, J. Mayer, J.M. Schneider, Influence of Si and N additions on structure and phase stability of Ge2Sb2Te5 thin films, J. Phys.: Condens. Matter 21 (2009) 435501 . [7] J. Fu, X. Shen, Q. Nie, G. Wang, L. Wu, S. Dai, T. Xu, R.P. Wang, Crystallization characteristics of Mg-doped Ge2Sb2Te5 films for phase change memory applications, Appl. Surf. Sci. 264 (2013) 269.
43
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of ion irradiation on the optical properties of Ag-doped Ge2Sb2Te5 (GST) thin films
T
Neetu Kandaa, Anup Thakurb, Fouran Singhc, A.P. Singha,
⁎
a
Thin Film Lab., Physics Department, Dr B R Ambedkar National Institute of Technology, Jalandhar 144 011, India Advanced Materials Research Lab., Department of Basic and Applied Sciences, Punjabi University, Patiala 147 002, India c Material Science Group, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India b
ARTICLE INFO
ABSTRACT
Keywords: Chalcogenide alloys Swift heavy ions (SHI) Phase change materials and crystallization
Structural and optical study of pure and Ag-doped Ge2Sb2Te5 (GST) thin films after swift heavy ion irradiation, is presented. In the present work, we have used 120 MeV Ag9+ ions for irradiation using various ion fluence. Structural properties were investigated using XRD and Raman spectroscopy. Undoped films were found to crystallize at the highest fluence (1 × 1013 ions/cm2). Ag doping is found to suppress crystallization. From UVVis-NIR spectra, a decrease in optical bandgap was observed due to ion irradiation on these films. Observed results are explained on the basis of crystal nuclei, which results in crystallization and observed structural changes. Changes in defect chemistry are proposed to be responsible for the observed optical changes.
1. Introduction Chalcogenide alloys are one of the best materials for rewritable memory applications. The most widely studied material among chalcogenides is Ge2Sb2Te5 (GST), which is a glassy alloy. Because of fast crystallization time ( 100 ns) [1], high thermal stability, good endurance, scalability, reliability, and better data storage applications, this material is widely used for nonvolatile memory [2]. It has also been proposed to be used in a reversible near-infrared window [3]. This alloy can be switched between amorphous and crystalline phases by electric pulse, laser pulse, local heating, and pressure of appropriate intensity and duration [3]. The kinetics of amorphization and crystallization determines the switching times between these phases. These materials show high contrast in their optical and electrical properties while changing phase from amorphous (Eg = 0.7 eV) to crystalline (Eg = 0.5 eV) and vice-versa [4]. In the amorphous phase, Ge atoms are arranged tetrahedrally, whereas in the crystalline phase they are arranged octahedrally. Even a small change in structure is sufficient to flip the phase from amorphous to crystalline or vice-versa, which is the reason for the fast switching speed of GST [5]. The transmission contrast in amorphous and crystalline phases of GST thin films has been studied by Singh et al. [3]. They observed that transmission decreases from 92% to 2% on phase transition from amorphous to hcp. Due to this large contrast in transmission, GST becomes a feasible candidate for reversible near-infrared window. Optical and structural properties of GST thin films can be improved by doping ⁎
with different elements like Ni [6], Mg [7], Si [6,8], Se [9] and Ag [10,11]. Since Ag is more reflective and soluble in chalcogenide alloys [12] and forms bonds with defect levels present in chalcogenide alloys (GST), it helps in improving its optical and electrical properties [13]. Excellent crystallization speed and large contrast in resistance have been observed in Ag-doped GST thin films as compared to pure GST thin films [10]. Ion irradiation has been used to introduce defects in these films which results in improved properties [14]. Recently Bastiani et al. investigated 120 keV Sb+ ions irradiated GST thin films with fluence between 5 × 1012 to 1 × 1014 ions/cm2 [15]. They studied crystallization kinetics of pure and irradiated thin films and found that after ion irradiation, crystallization kinetics of the films increases. This happens due to a reduction in Ge-Te tetrahedral bonds. Privitera et al. investigated GST polycrystalline films irradiated by 150 keV Ar+ ions with fluence in the range 1 × 1011 to 5 × 1014 ions/cm2 [16]. They observed ion irradiation-induced disorder on structural and electrical properties of polycrystalline GST thin films. They also found an increase in disorder induced by ion irradiation that resulted in amorphization in rocksalt structure and transition from metallic to insulating behavior in the trigonal phase. Fedorenko et al. [17] have studied ion implantation of bismuth in lone pair chalcogenide glasses and their electrical properties after ion implantation. They observed that electrical conductivity increases with Bi ion implantation. This enhanced conductivity was found to be sensitive to the dose of implantation. Most of the reports on the effects of ion irradiation on GST are
Corresponding author. E-mail address: [email protected] (A.P. Singh).
https://doi.org/10.1016/j.nimb.2020.01.025 Received 30 August 2019; Received in revised form 23 January 2020; Accepted 24 January 2020 0168-583X/ © 2020 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 467 (2020) 40–43
N. Kanda, et al.
focussed on the low energy regime ( 100 keV) [15–17]. In this regime the energy loss by the incident ion in the material is dominated by nuclear energy loss. For swift heavy ions (SHI), the dominant energy loss is electronic energy loss [18]. In the present work, we have studied the effect of Ag9+ ion irradiation on optical and structural properties of pure and Ag-doped GST thin films with fluence in the range of 5 × 1011 to 1 × 1013 ions/cm2. The energy of the incident ion was 120 MeV.
et al. [14] can be used. According to this model, the introduction of defects due to SHI irradiation increases the free energy of the system. Due to the increase in free energy, the thermodynamic barrier for nucleation reduces, leading to crystallization of the material. Since the introduced defects increase with increase in irradiation fluence, that is why peaks of fcc phase were observed only for the highly irradiated pure GST thin films. However, no such peak was observed for highly irradiated Ag-doped thin films. It may be due to an increase in the crystallization temperature by Ag doping, which limits the phase transformation from amorphous to crystalline. Similar suppression in the crystallization temperature of GST thin films was observed by Singh et al. [21]. Raman spectra for Ag-doped and irradiated Ge2Sb2Te5 thin films are shown in Fig. 2. The Raman peaks were fitted with Lorentzian profile. For G1Ag0 sample, two peaks were observed at 127 cm−1 and 151 cm−1. In accordance with previous reports, the first peak is assigned to corner-sharing GeTe4 tetrahedral vibration, and the second peak corresponds to Sb-Te vibration in the SbTe3 unit [19]. There was no shift in the peak at 127 cm−1 within the instrumental resolution. The peak at 151 cm−1 gradually shifted towards lower wavenumber as the irradiation fluence is increased (148 cm−1 for G1Ag1e13). With irradiation, the weakening of Sb-Te bonds may occur in SbTe3 unit, which reduces the force constant, along with the oscillation frequency of these bonds. This may result in the observed redshift in Raman peaks. This peak also gets flattened, which may be due to the reduction of Sb-Te vibration in the SbTe3 unit. These results are not in agreement with the results obtained by Bastiani et al. [15]. They observed that the 127 cm−1 peak vanished with irradiation because of the breakage of corner-sharing GeTe4 tetrahedral vibration. The difference in the observed results may be due to the different mechanisms through which energy gets deposited in the two energy regimes. Bastiani et al. used Sb+ ions to irradiate GST with 120 keV energy. The nuclear and electronic energy losses are 252 and 17.7 eV/Å, respectively. For this energy, nuclear energy loss is dominant. This means larger structural damage being caused due to elastic collisions. In the present study, the electronic energy loss is the dominant mechanism, in which the energy is transferred to the electronic system. So, lesser structural damages are expected for swift heavy ion irradiated samples. The transmission spectra of all the samples are shown in Fig. 3. The optical bandgap was calculated from this spectra using the Tauc plot [22]. The bandgap variation with SHI irradiation and Ag doping is given in Table 2 which shows that in pure and Ag-doped GST samples, bandgap, in general, decreases with irradiation. This may be due to the reason that ion irradiation causes the generation of crystal nuclei, as discussed before. Moreover, grain growth also increases because of an increase in the atomic mobility at the amorphous-crystal interface [14]. The SHI induced crystallization leads to bandgap reduction. From Table 2, it is clear that bandgap increases with Ag doping. The increase in bandgap with Ag doping may be due to the reduction in the density of localized states. When the doped neutral Ag atom is weakly bound to the trapping center, it loses one electron to become Ag+. The Ag+ ion gets trapped to D− state making these defect states neutral [13,23,24].
2. Materials and methods (Ge2Sb2Te5)100-xAg x (x = 0, 1) samples were prepared by melt quenching technique. According to the atomic weight percentage, Ge, Sb, Te, and Ag were weighed and then sealed in quartz ampoules under high vacuum condition. These ampoules were then heated for two hours, each in increasing order of melting point of constituent elements. Then the mixture was heated at the highest melting point of constituent element for 24 hours. To ensure the homogeneity of the samples, they were simultaneously rocked. At the end, quenching of these mixtures were done in liquid nitrogen. Thermal evaporation using Hind HIVAC system was used for preparing thin films of 500 nm on the glass substrate at deposition rate 10 Å/s at a base pressure of 2 × 10 6 mbar. The composition of the GeSbTe alloy and the thin film has been confirmed previously [19]. These films were irradiated with 120 MeV Ag9+ ion using the 15 UD tandem Pelletron accelerator at IUAC, New Delhi at fluences of 5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2. During irradiation, the beam current was maintained at 1 particle nano Ampere and the beam was scanned over an area of 1.25 × 1.4 cm2. The damage introduced due to the interaction of the target material and ion beam was studied using SRIM software [20]. The nuclear and electronic energy loss for 120 MeV Ag9+ ions in GST is equal to 9.13 and 1.73 × 103 eV/Å, respectively. The range of the ions was calculated to be 13.01 μm. The longitudinal and lateral straggling is 9998 Å and 1.27 μm, respectively. The displacement per atom obtained for different fluences (5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2) comes out to be 1.3 × 10 5, 2.62 × 10 5 and 2.62 × 10 4 respectively. The naming of the samples with these fluences and Ag-doping is given in Table 1. The structural information of these films was obtained from XRD using PANalytical x-ray diffractometer using Cu-K radiation source. The diffraction data were obtained in the range 20° to 80° in BraggBrentano geometry. The vibrational properties were investigated through Raman spectroscopy using Renishaw inVia Raman microscope. Argon laser (514 nm) was used for excitation in the range of 100–500 cm−1. UV-Vis-NIR spectroscopy was used to study the optical properties using Cary 500 UV-Vis-NIR spectrophotometer. Transmission data were taken in the range of 500-3200 nm at room temperature. 3. Results and discussions The XRD data of all the samples are shown in Fig. 1. Two peaks were observed for the G0Ag1e13 sample. No peak was observed for other samples, which only show a broad hump. The peaks observed in G0Ag1e13 samples, indexed in Fig. 1, are of the fcc phase of GST. These observations suggest the onset of crystallization in these samples on SHI irradiation. To understand these results, the model proposed by Bastiani
D+ + e D + Ag+
Ag doping % Fluence (ions/cm2)
0
1
0
G0Ag0 G0Ag5e11
G1Ag0 G1Ag5e11
G0Ag1e13
G1Ag1e13
1 × 1012 1 × 1013
G0Ag1e12
[D Ag+]0
(1) (2)
This mechanism reduces the density of states, leading to a decrease in the width of the band tail and the Urbach energy. As a result, optical bandgap increases with Ag doping.
Table 1 Naming of the thin films with respect to fluences and Ag doping.
5 × 1011
D0
4. Conclusion Effect of 120 MeV Ag9+ ion irradiation on pure and Ag-doped Ge2Sb2Te5 (GST) thin films were probed with different fluences using XRD, Raman, and UV-Vis-NIR spectroscopy. The experimental results indicate that crystallization is observed in pure samples for the highest
G1Ag1e12
41
Nuclear Inst. and Methods in Physics Research B 467 (2020) 40–43
N. Kanda, et al.
Fig. 1. XRD spectra of (a) pure, (b) 1% Ag-doped with irradiation fluence 5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2. Table 2 Effect of ion irradiation and Ag doping on bandgap of GST thin films. Ag doping % Fluence (ions/cm2)
0
1
0
0.65 0.61
0.67 0.66
0.58
0.61
5 × 1011
1 × 1012 1 × 1013
0.60
0.65
fluence of irradiation. However, in the Ag-doped sample, only small structural changes are observed with irradiation. The decrease in optical bandgap is also observed with ion irradiation, which indicates the formation of crystal grain due to the energy deposited by the incident ions. An increase in bandgap is observed with Ag doping, which is attributed to a decrease in the density of defect states. Defects play an essential role in determining the properties of GST films. SHI irradiation can be used to control the defects in the films. The SHI irradiated films can be further studied for their crystallization kinetics. This can prove to be the superior method compared to low
Fig. 2. Raman spectra of 1% Ag-doped samples with irradiation fluence 5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2 respectively.
Fig. 3. Transmission spectra of (a) pure, (b) 1% Ag-doped samples with irradiation fluence 5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2 respectively.
42
Nuclear Inst. and Methods in Physics Research B 467 (2020) 40–43
N. Kanda, et al.
energy ion irradiation because of reduced structural damage to the samples.
[8] S.J. Park, I.S. Kim, S.K. Kim, S.M. Yoon, B.G. Yu, S.Y. Choi, Phase transition characteristics and device performance of Si-doped Ge2Sb2Te5, Semicond. Sci. Technol. 23 (2008) 105006 . [9] E.M. Vinod, K. Ramesh, R. Ganesan, K.S. Sangunni, Direct hexagonal transition of amorphous (Ge2Sb2Te5)0.9Se0.1 thin films, Appl. Phys. Lett. 104 (2014) 063505 . [10] K.H. Song, S.W. Kim, J.H. Seo, H.Y. Lee, Characteristics of amorphous Ag0.1(Ge2Sb2Te5)0.9 thin film and its ultrafast crystallization, J. Appl. Phys. 104 (2008) 103516 . [11] B. Prasai, G. Chen, D.A. Drabold, Direct ab-initio molecular dynamic study of ultrafast phase change in Ag-alloyed Ge2Sb2Te5, Appl. Phys. Lett. 102 (2013) 041907 . [12] M. Frumer, T. Wagner, Ag doped chalcogenide glasses and their applications, Curr. Opin. Solid State Mater. Sci. 7 (2003) 117. [13] P. Singh, R. Kaur, P. Sharma, V. Sharma, M. Mishra, G. Gupta, A. Thakur, Optical band gap tuning of Ag doped Ge2Sb2Te5 thin films, J. Mater. Sci. Mater. Electron. 28 (2017) 11300. [14] R.D. Bastiani, A.M. Piro, I. Crupi, M.G. Grimaldi, E. Rimini, Effect of ion irradiation on the stability of amprphous Ge2Sb2Te5 thin films, Nucl. Instrum. Methods Phys. Res. B 266 (2008) 2511. [15] R.D. Bastiani, A.M. Piro, M.G. Grimaldi, E. Rimini, G.A. Baratta, G. Strazzulla, Ion irradiation-induced local structural changes in amorphous Ge2Sb2Te5 thin film, Appl. Phys. Lett. 92 (2008) 241925 . [16] S.M.S. Privitera, A.M. Mio, E. Smecca, A. Alberti, W. Zhang, R. Mazzarello, J. Benke, C. Persch, F. La Via, E. Rimini, Structural and electronic transitions in Ge2Sb2Te5 induced by ion irradiation damage, Phys. Rev. B 94 (2016) 094103 . [17] Y.G. Fedorenko, Electrical properties of Bi-implanted amorphous chalcogenide films, Thin Solid Films 589 (2015) 369. [18] D.K. Avasthi, Some interesting aspects of swift heavy ions in materials science, Curr. Sci. 78 (2000) 1297. [19] P. Singh, P. Sharma, V. Sharma, A. Thakur, Linear and non-linear optical properties of Ag-doped Ge2Sb2Te5 thin films estimated by single transmission spectra, Semicond. Sci. Technol. 32 (2017) 045015 . [20] J.F. Ziegler, J.P. Biresack, U. Littmark, The Stopping and the Range of Ions in Solids, Pergamon, New York, 1985. [21] Palwinder Singh, A.P. Singh, Anup Thakur, Thermal stability improvement and crystallization behavior of Ag doped Ge2Sb2Te5 phase change materials, J. Mater. Sci.: Mater. Electron. 30 (2019) 3604. [22] Anup Thakur, G.S.S. Gurdishpal Singh, Navdeep Goyal Saini, S.K. Tripathi, Optical properties of amorphous Ge20 Se80 and Ag6 (Ge0.20 Se0.80)94 thin films, Opt. Mater.Opt. Mater. 30 (2007) 565. [23] I. Chaudhary, F. Inam, D.A. Drabold, Ab initio determination of ion traps and dynamics of silver in silver-doped chalcogenide glass, Phys. Rev. B 79 (2009) 100201 . [24] S. Kumar, D. Singh, R. Thangaraj, Optical properties and phase transition in photodoped amorphous Ge-Sb-Te: Ag thin films, Thin Solid Films 540 (2013) 271.
Funding sources This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The support provided by the pelletron group at IUAC, New Delhi during irradiation experiments is highly appreciated. References [1] G. Atwood, Phase-change materials for electronic memories, Science 321 (2008) 210. [2] H. Lv, P. Zhou, Y. Lin, T. Tang, B. Qiao, Y. Lai, J. Feng, B. Cai, B. Chen, Electronic properties of GST for non-volatile memory, Microelectron. J. 37 (2006) 982. [3] P. Singh, A.P. Singh, N. Kanda, M. Mishra, G. Gupta, A. Thakur, High transmittance contrast in amorphous to hexagonal phase of Ge2Sb2Te5: Reversible NIR-window, Appl. Phys. Lett. 111 (2017) 261102 . [4] T. Kato, K. Tanaka, Electronic properties of amorphous and crystalline Ge2Sb2Te5 films, Jpn. J. Appl. Phys. 44 (2005) 7340. [5] M. Wuttig, N. Yamada, Phase change materials for rewritable data storage, Nat. Mater. 6 (2007) 824. [6] H. Kolpin, D. Music, G. Laptyeva, R. Ghadimi, F. Merget, S. Richter, R. Mykhaylonka, J. Mayer, J.M. Schneider, Influence of Si and N additions on structure and phase stability of Ge2Sb2Te5 thin films, J. Phys.: Condens. Matter 21 (2009) 435501 . [7] J. Fu, X. Shen, Q. Nie, G. Wang, L. Wu, S. Dai, T. Xu, R.P. Wang, Crystallization characteristics of Mg-doped Ge2Sb2Te5 films for phase change memory applications, Appl. Surf. Sci. 264 (2013) 269.
43