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DOI: https://doi.org/10.18721/JPM.153.140
Laser-induced periodic surface structures formation and reversible crystallization in amorphous Ge2Sb2Te5 thin films as a result of femtosecond irradiation
A. V. Kolchin 1 e, S. V. Zabotnov \ D. V. Shuleiko \ D. E. Presnov 1 2, 3, M. E. Fedyanina 4, E. V. Kuzmin 5, P. K. Kashkarov 1 1 Lomonosov Moscow State University, Faculty of Physics, Moscow, Russia
2 Lomonosov Moscow State University, Skobeltsyn Institute of Nuclear Physics, Moscow, Russia 3 Lomonosov Moscow State University, Quantum Technology Center, Moscow, Russia
4 National Research University of Electronic Technology, Institute of Advanced Materials and Technologies, Zelenograd, Russia
5 Lebedev Physical Institute of the Russian Academy of Science, Moscow, Russia
H avkolchin@physics.msu.ru
Abstract. Femtosecond laser structuring opens for chalcogenide semiconductor Ge2Sb2Te5 new perspectives in photonics applications due to wide change of its structural and optical properties in such processing. We studied laser-induced modification of amorphous Ge2Sb2Te5 thin films on silicon substrates. The investigations show that periodic relief formation is accompanied by phase transitions to the fcc crystalline phase and back. Furthermore, the irradiated Ge2Sb2Te5 samples demonstrate optical transparency in the near infrared region. The examined structures are interesting for further studies as a base of new memory devices which may possess optical anisotropy and be integrated into fiber optics applications.
Keywords: Ge2Sb2Te5, femtosecond laser processing, laser-induced periodic surface structures, reversible phase transitions
Funding: This study was funded by the Russian Foundation for Basic Research (project no. 20-32-90111).
Citation: Kolchin A. V., Zabotnov S. V., Shuleiko D. V., Presnov D. E., Fedyanina M. E., Kuzmin E. V., Kashkarov P. K., Laser-induced periodic surface structures formation and rewersible crystallization in amorphous Ge2Sb2Te5 thin films as a result of femtosecond irradiation, St. Petersburg State Polytechnical University Journal. Physics and Mathematics. 15 (3.1) (2022) 237-242. DOI: https://doi.org/10.18721/JPM.153.140
This is an open access article under the CC BY-NC 4.0 license (https://creativecommons. org/licenses/by-nc/4.0/)
Материалы конференции УДК 544.6
DOI: https://doi.org/10.18721/JPM.153.140
Лазерно-индуцированное формирование поверхностных периодических структур и обратимая кристаллизация в аморфных тонких пленках G&S^Te как результат фемтосекундного облучения
А. В. Колчин 1 е, С. В. Заботнов 1, Д. В. Шулейко 1, Д. Е. Преснов 1 2, 3, М. Е. Федянина 4, Е. В. Кузьмин 5, П. К. Кашкаров 1
1 Московский государственный университет имени М.В. Ломоносова, Физический факультет, Москва, Россия 2 Московский государственный университет имени М.В. Ломоносова, Научно-исследовательский институт ядерной физики имени Д.В. Скобельцына, Москва, Россия
© Kolchin A. V., Zabotnov S. V., Shuleiko D. V., Presnov D. E., Fedyanina M. E., Kuzmin E. V., Kashkarov P. K., 2022.
Published by Peter the Great St. Petersburg Polytechnic University.
3 Московский государственный университет имени М.В. Ломоносова, Центр квантовых технологий, Москва, Россия
4 Национальный исследовательский университет «Московский институт электронной техники», Институт перспективных материалов и технологий, Зеленоград, Россия
5 Физический институт имени П.Н. Лебедева РАН, Москва, Россия н avkolchin@physics.msu.ru
Аннотация. Фемтосекундное лазерное структурирование открывает для халькогенидного полупроводника Ое2БЪ2Те5 новые перспективы использования в приложениях фотоники благодаря широкому изменению его структурных и оптических свойств при такой обработке. Нами исследована лазерно-индуцированная модификация тонких аморфных пленок Ое2БЪ2Те5 на кремниевых подложках. Исследования показывают, что периодическое формирование рельефа сопровождается фазовыми переходами в ГЦК-кристаллическую фазу и обратно. Кроме того, облученные образцы Ое2БЪ2Те5 демонстрируют оптическую прозрачность в ближней инфракрасной области. Рассмотренные структуры представляют интерес для дальнейших исследований в качестве основы новых запоминающих устройств, которые могут обладать оптической анизотропией и быть интегрированы в волоконно-оптические приложения.
Ключевые слова: Ое2БЪ2Те5, фемтосекундная лазерная обработка, лазерно-индуцированные поверхностные периодические структуры, обратимые фазовые переходы
Финансирование: Работа выполнена в рамках гранта Российского фонда фундаментальных исследований № 20-32-90111.
Ссылка при цитировании: Колчин А. В., Заботнов С. В., Шулейко Д. В., Преснов Д. Е., Федянина М. Е., Кузьмин Е. В., Кашкаров П. К. Лазерно-индуцированное формирование поверхностных периодических структур и обратимая кристаллизация в аморфных тонких пленках Ое2БЪ2Те5 как результат фемтосекундного облучения // Научно-технические ведомости СпбгПУ. Физико-математические науки. 2022. Т. 15. № 3.1. С. 237—242. БО1: https://doi.org/10.18721/JPM.153.140
Статья открытого доступа, распространяемая по лицензии СС БУ-МС 4.0 (И^:// creativecomшons.org/licenses/Ъy-nc/4.0/)
Introduction
The laser-induced periodic surface structures (LIPSS or "ripples") based on Ge2Sb2Te5 (GST225) promise large perspectives in the development of novel memory technologies and reconfigurable nanophotonic devices [1]. Usually, LIPSS have the period which is comparable to the wavelength of acted radiation and is significantly less than the laser spot diameter. Optical anisotropy in the visible and near-infrared ranges may emerge in such structures [2]. This is a promising way to encode information, especially in the case of GST225 processing via femtosecond laser irradiation when rewritable phase transitions to the crystalline phase are possible [3]. The significant difference between the optical properties of amorphous and crystalline phases of this material [4] makes a considerable contribution to optical anisotropy. Therefore, currently of great interest is the simultaneous LIPSS formation and phase transitions in amorphous GST225 thin films induced by femtosecond laser irradiation, including reversible phase transitions.
Thus, in this work we have irradiated thin GST225 films by femtosecond laser pulses to simultaneously fabricate LIPSS on the surface and achieve crystallization of GST225 and then investigated structural and optical properties of the modified samples.
Materials and Methods
Initial thin amorphous GST225 films with the thicknesses of 130 nm were deposited on dielectric substrates by magnetron sputtering of a crystalline target (ACI Alloys). Subsequent irradiation was provided by femtosecond laser system Avesta (wavelength X = 1250 nm, pulse duration T = 135 fs, repetition rate и = 10 Hz). The laser fluence F varied from 0.05 to 0.15 J/cm2. The
© Колчин А. В., Заботнов С. В., Шулейко Д. В., Преснов Д. Е., Федянина М. Е., Кузьмин Е. В., Кашкаров П. К., 2022. Издатель: Санкт-Петербургский политехнический университет Петра Великого.
focused laser spot diameter was D = 120±10 ^m. The samples were irradiated at normal incidence while moving continuously in one direction along the laser polarization at various speeds. As a result, single scanlines with the number of overlapping laser spots Ns from 3 to 750 were formed on the sample surface. Additionally, based on the results obtained for the single scanlines, a square 3x3 mm2 area was fabricated in a raster mode for the pulse number Ns = 150 and step between scanlines r =100 ^m, for optical properties investigation.
The structural properties of initial and modified samples were studied by scanning electron microscopy (SEM; Carl Zeiss Supra 40) and Raman spectroscopy (Horiba Jobin Yvon HR800) at 488 nm excitation, as well as energy-dispersive X-ray (EDX) spectroscopy (Tescan Vega 3). Optical properties were defined via ellipsometry data analysis in the range 300—2000 nm (Horiba Uvisel 2). The type of phase transitions observed in the modified film was analyzed via the two-temperature model (TTM) calculations [5].
a) b)
Fig. 1. SEM images of GST225 thin film, irradiated by the N=150 (a) and Ns=750 (b) laser pulses
Results and Discussion
According to SEM data the LIPSS formation on the samples irradiated with F = 0.10 ± 0.02 J/cm2 occurs at Ns=150 and higher values. Observed ripples period A varied from 1150 to 1350 nm, which is close to the wavelength of modifying laser pulses 1 (Fig. 1,a). In all cases the gratings are directed orthogonally to the laser polarization. Such characteristics of the surface relief commonly indicate its formation mechanism as a result of interference between surface plasmon-po-laritons and incident laser radiation [6]. In case of semiconductors such as GST225, the surface electromagnetic waves can be excited due to the transition of the near-surface layer to metal-like state, which means the dielectric permittivity of the irradiated material changes the sign from the positive to the negative one due to the intensive photoinduced generation of free charge carriers.
With the further increase of the Ns > 300 another LIPSS type was observed on the irradiated surface, in a form of ordered elongated clusters with the period A = 130 ± 30 nm (Fig. 1,b). The orientation of obtained clusters is also perpendicular to the laser polarization. The clusters possess the longitudinal size in the range of 50—200 nm and the transversal size about 50 nm. The formation of such LIPPS is most likely explained by the thermo-capillar effects at the times which exceed pulse duration [7].
Raman spectroscopy indicated that surface modification of amorphous GST225 thin films induced by femtosecond laser pulses is accompanied by reversible phase transitions (Fig. 2). The Raman spectra of non-irradiated GST225 areas (Ns=0) shows the wide broadband at the range of 110—200 cm1 due to the short-range crystallographic order of the amorphous phase (Fig. 2,a) [8]. The band consists of several lines near 125, 140, 158, 190 and 270 cm-1 which correspond to the A1 GeTe4-nGen (n = 1,2) corner-sharing tetrahedra, Sb2Te3 pyramids, A1g hexagonal GeTe4-nGen (n = 1,2), A1 GeTe4-nGen (n = 1,2) edge-sharing tetrahedra and F2 GeTe4 mode vibrations [8].
After femtosecond laser irradiation, the Raman spectra demonstrate the next transformations. The intensity of narrow peak near 125 cm-1, which corresponds to GenTe4-nGen (n=1,2), increases with the growth of Ns from 3 to 15 (Fig. 2, a, c) while the line at 158 cm-1 decreases in intensity (Fig. 2,d). Such behavior may indicate transition to the face-centered cubic (fcc) crystalline phase [8]. The TTM calculations for the F = 0.1 J/cm2 confirm this hypothesis (Fig. 3,a). According to the calculations the temperature of electron gas achieves ~1.4-104 K during single
Pnlae Number Piplic Numbe r
Fig. 2. Raman spectra of non-irradiated GST225 thin film (N=0) and samples for Ns=6 and Ns=15 (a). Raman spectra of non-irradiated GST225 thin film (N=0) and samples for Ns=150 and Ns=750 (b). Dependencies of integrated intensities of 125 cm1 (c) and 158 cm1 (d) line for center and edge of the scanlines on Ns
pulse treatment. The lattice temperature then increases due to the electron-phonon relaxation, and 2 ps after the laser pulse end exceeds the temperature of transition to fcc crystalline phase, which is 410 K. The peak temperature the GST225 lattice achieves during such heating is 480 K.
Additionally, at the values of N > 75, the 125 cm-1 Raman line decreases in intensity compared to the scan line edge (Fig. 2, b, c). Consequently, in the scan line center the Raman spectra become similar to non-irradiated GST225 thin films for Ns ~ 300 and higher. Such behavior can indicate re-amorphization of GST225. Such reversible phase transition is possible due to metastability of fcc crystalline GST225 [9] or rapid quenching of the melted material [10]. The cooling velocity more 1010 K/s and heat removal to the surface substrate contribute to the second mechanism of re-amorphization. It should be noted that the stoichiometry of GST225 film is not changed by femtosecond laser irradiation, which is confirmed by EDX spectroscopy data (Table 1).
Table 1
Concentration of Ge, Sb and Te atoms in initial amorphous GST225 thin film N=0), as well as irradiated for the N =3, N =150 and N =750, defined by EDX spectroscopys
N s Atomic concentration, %
Ge Sb Te
0 18.9±1.5 25±3 56±3
3 17.1±1.0 25±1 58±5
150 19.5±1.2 26±2 55±2
750 18.9±0.7 27±1 54±3
Fig. 3. Simulated surface temperature of GST225 during pulse treatment (a); reflectance and absorption spectra of irradiated GST225 sample, defined by ellipsometry data (b)
The reflectance and absorption spectra of irradiated GST225 sample are presented in Fig. 3,b. The reflectance coefficient grows from the 0.1 to 0.35 with the wavelength increase from 300 to 800 nm. Additionally, it demonstrates oscillations in the range of 800—2000 nm. Such behavior can be caused by the interference in GST225 thin film. On the other hand, the absorption coefficient decreases from 3.5-105 cm-1 to 105 cm-1 with the wavelength increase from 400 to 1800 nm. Thus, the studied GST225 films are transparent in the near-infrared range after femtosecond laser treatment and look promising for further studies and the development of new memory devices which may be compatible with the fiber optics applications, among other things.
Conclusion
We observed that surface modification of amorphous GST225 thin films induced by femtosecond laser pulses is accompanied by phase transitions to the fcc crystalline phase and back which is confirmed by Raman analysis and TTM simulations. Additionally, LIPSS with the period close to the laser wavelength were formed on GST225 surface at such treatment and the irradiated films are transparent in the near-infrared range. Therefore, the examined structures are interesting for further studies as a base of new memory devices which may possess optical anisotropy and be integrated into fiber optics applications.
Acknowledgments
The initial amorphous GST225 thin films fabrication, as well as ellipsometry measurements of the modified samples were provided at the MEMS and Electronic Components Core facilities centers, National Research University of Electronic Technology. SEM analysis was performed in the Educational and Methodical Center of Lithography and Microscopy, Lomonosov Moscow State University. Authors are grateful to L.A. Golovan, P.I. Lazarenko and S.A. Kozyukhin for fruitful discussions.
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THE AUTHORS
KOLCHIN Aleksandr V.
avkolchin@physics.msu.ru ORCID: 0000-0003-3033-9083
ZABOTNOV Stanislav V.
zabotnov@physics.msu.ru ORCID: 0000-0002-2528-4869
SHULEIKO Dmitrii V.
shuleyko.dmitriy@physics.msu.ru ORCID: 0000-0003-3555-6693
PRESNOV Denis E.
denis.presnov@phys.msu.ru ORCID: 0000-0002-9213-0165
FEDYANINA Maria E.
mfh.miet@gmail.com ORCID: 0000-0001-9779-2574
KUZMIN Evgenii V.
e.kuzmin@lebedev.ru ORCID: 0000-0002-6322-6838
KASHKAROV Pavel K.
kashkarov@physics.msu.ru
Received 22.05.2022. Approved after reviewing 25.07.2022. Accepted 25.07.2022.
© Peter the Great St. Petersburg Polytechnic University, 2022
