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St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.3 Научно-технические ведомости СПбГПУ. Физико-математические науки. 15 (3.3) 2022
Conference materials
UDC 535.417, 544.015.4, 536.331, 538.958, 538.975 DOI: https://doi.org/10.18721/JPM.153.342
Investigation of crystallinity degree for Ge2Sb2Te5 films by reflection and transmission photometry
N. M. Tolkach ,e, N. V. Vishnyakov 2, V. G. Litvinov 2, A. O. Yakubov \ E. S. Trofimov 2, A. A. Sherchenkov 1
1 National Research University of Electronic Technology (MIET), Zelenograd, Moscow, Russia;
2 Ryazan State Radio Engineering University named after V. F. Utkin, Ryazan, Russia
Abstract. To assess the crystallinity degree for Ge2Sb2Te5 (GST) films the method of reflection and transmission photometry and the approximation of an effective medium of the Brueggemann type were used. It was found that the changes in the energy and duration of the laser pulse lead to the change in the ratio of crystalline and amorphous fractions and change in crystallinity degree. It was found that an increase of crystalline fraction leads to an increase of refractive index and extinction coefficient at telecommunication wavelength of 1550 nm, which also leads to changes of reflectivity and transmissivity of the GST film and can be used to perform modulation and switching of signals.
Keywords: crystallinity degree, photometry, Ge2Sb2Te5, phase change material, phase transition, phase state, amorphous, crystalline
Funding: The reported study was funded by RFBR according to the research project No. 19-37-60023 "A study of optical switching processes in alternating structures based on crystal-amorphous materials of Ge-Sb-Te system for optical switches".
Citation: Tolkach N. M., Vishnyakov N. V., Litvinov V. G., Yakubov A. O., Trofimov E.S., Sherchenkov A. A., Investigation of crystallinity degree for Ge2Sb2Te5 films by reflection and transmission photometry. St. Petersburg State Polytechnical University Journal. Physics and Mathematics, 15 (3.3) (2022) 219-222. DOI: https://doi.org/10.18721/JPM.153.342
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 535.417, 544.015.4, 536.331, 538.958, 538.975 DOI: https://doi.org/10.18721/JPM.153.342
Исследование степени кристалличности пленок Ge2Sb2Te5 методами фотометрии отражения и пропускания
Н. М. Толкач ,н, Н. В. Вишняков 2, В. Г. Литвинов 2, А. О. Якубов ,, Е. С. Трофимов 2, А. А. Шерченков 2
1 Национальный исследовательский университет «МИЭТ», Зеленоград, Москва, Россия;
2 Рязанский государственный радиотехнический университет им. В. Ф. Уткина, Рязань, Россия
Аннотация. Для оценки степени кристалличности Ge2Sb2Te5 (GST) в данной работе мы использовали метод фотометрии отражения и пропускания и приближение эффективной среды типа Брюйггемана. Установлено, что изменение энергии и длительности лазерного импульса приводит к изменению соотношения кристаллической и аморфной фракций, что наблюдается по изменению степени кристалличности. Также установлено, что увеличение доли кристаллитов приводит к увеличению показателя преломления и коэффициента экстинкции для телекоммуникационной длины волны 1550 нм, что также приводит к изменению отражательной и пропускательной способностей пленки GST и будет использовано для осуществления модуляции и переключения сигналов.
© Tolkach N. M., Vishnyakov N. V., Litvinov V. G., Yakubov A. O., Trofimov E.S., Sherchenkov A. A., 2022. Published by Peter the Great St.Petersburg Polytechnic University.
^St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.3
Ключевые слова: степень кристалличности, фотометрия, Ge2Sb2Te5, материал c фазовым переходом, фазовый переход, фазовое состояние, аморфный, кристаллический
Финансирование: Работа выполнена при финансовой поддержке РФФИ в рамках научно-исследовательского проекта № 19-37-60023.
Ссылка при цитировании: Толкач Н. М., Вишняков Н. В., Литвинов В. Г., Якубов А. О., Трофимов Е. С., Шерченков А. А. Исследование степени кристалличности пленок Ge2Sb2Te5 методами фотометрии отражения и пропускания // Научно-технические ведомости СПбГПУ. Физико-математические науки. 2022. Т. 15. № 3.3. C. 219—222. DOI: https://doi.org/10.18721/JPM.153.342
Статья открытого доступа, распространяемая по лицензии CC BY-NC 4.0 (https:// creativecommons.org/licenses/by-nc/4.0/)
Introduction
Semiconductor materials with first kind phase transition (Phase Change Material — PCM), in particular, chalcogenide glassy semiconductors, have found their application in non-volatile radiation-resistant memory elements for creating dynamic storage devices. Currently, PCM material Ge2Sb2Te5 (GST) is actively used for this purpose. GST has a fast and low-energy phase transition, which can be excited by both an electric and a laser pulse [1—3]. In addition to the main phase states (amorphous and crystalline) of GST, there can be additional intermediate states due to the presence of nanosized fractions of the material in both these states. Controlling the ratio of the amorphous and crystalline fractional composition (crystallinity degree) of GST opens up wide possibilities in fine-tuning its optical parameters, but at the same time, it requires studying the processes and peculiarities that occur when laser radiation is applied to the GST material. Therefore, the aim of the work is to investigate in the GST thin films the amorphous and crystalline fraction ratio, which is characterized by the crystallinity degree.
Materials and Methods
To estimate the crystallinity degree of GST films in this work, we used the method of reflection and transmission photometry and the approximation of an effective medium of the Brueggemann type. Such approximation represents a medium in the form of amorphous matrix with growing quantity of spherical crystallites. Fraction of crystals in the matrix (crystallinity degree) is equal to [4]:
C = (n2-na 2) (2n2 + nc2) / (3n (nc 2-na 2)), (1)
where in, na and n are the complex refractive index of GST film in the current, amorphous and crystalline state respectively. The complex refractive index n is found from the model expressions determined by the Airy formulas for reflectivity ^ and transmissivity 3 of the GST structure taking into account processes of multiple re-reflection of electromagnetic waves in the film and their interference.
System of nonlinear equations is solved to search the complex refractive index:
f f =Ше -ЩП, k) = 0 (2
f = Зе -3(И,k) = 0 ,
where n and k are a refractive index and an extinction ratio of the researched medium; ^ and 3
? e e
are experimental data on reflectivity and transmissivity; Щп, к) and 3(n, k) — model expressions for reflectivity and transmissivity of the thin film structure taking into account processes of multiple re-reflection of electromagnetic waves in the film and their interference, are determined according to the Airy and Fresnel formulas [5].
Solution of simultaneous equations (2) is performed numerically according to the nonlinear system optimization method. This solution is the global minimum of the objective function of F = (f / Af )2 + (f / A/2 )2. Here Af and Af2 are scale coefficients offj and f2 function calculated as the average of experimental and model data (Afj = (^e + ^(n, к)) / 2, Af = (3e + 3(n, к)) / 2).
© Толкач Н. М., Вишняков Н. В., Литвинов В. Г., Якубов А. О., Трофимов Е. С., Шерченков А. А., 2022. Издатель: Санкт-Петербургский политехнический университет Петра Великого.
4
Physical optics
More generally, the objective function is the following:
F(me,,n,k) = i(me -Wn,k)) /(me +W(n,k))2+(pe -3(n,k)) /pe+3(n,k))2.
The GST thin film with the thickness of 24 nm were deposited by magnetron sputtering. The measurement setup based on the NTEGRA Spectra (NT-MDT SI, Russia) was used and allowed to carry out investigations using methods of Raman spectroscopy, atomic force microscopy, optical microscopy, photometry, laser modification.
Laser modification of the GST film (see square regions in Figure lab) has been performed by radiation with the 405 nm wavelength, with the focused spot diameter of about 1 ^m and with various duration values (from 4 ns to 1 microseconds), repetition intervals T (from 20 ns to 1 microseconds) and radiant exposures (from 0.02 nJ/^m2 to 20 nJ/^m2) of the pulse.
Photometry was used to measure the reflectivity and transmissivity of the GST film at 405, 532, 638 and 1550 nm wavelengths. Reflection and transmission photometry maps (see Fig. 1) obtained by scanning the GST film with the focused laser with a wavelength of 532 nm and a power of 5.3 ^W (energy exposure 1.44 nJ/^m2).
a) 0|jm 20 40 60 80 100 b) 0 Jjm
Fig. 1. Reflection (a) and transmission (b) photometry maps Results and Discussion
Crystallinity degree of the local film area was determined using the obtained values of the complex refractive index at the wavelengths of 405, 638, and 1550 nm. As can be seen from Fig. 2, a, when films are exposed to laser pulses with wavelength of 405 nm at energy exposure of pulse from 2.92 to 3.26 nJ/^m2, the crystallinity degree changes from 1 to 0.
The evaluation of crystallinity degree showed that amorphization of the film is observed when exposed to laser pulses with wavelength of 405 nm at energy exposure of pulse from 3.26 to 3.37 nJ/^m2, with pulse durations up to 12 ns (see Fig. 2, b).
2.983.01 3.04 3.07 3.1 163^193.223^5 3.28
Radianl Exposures .7
10 15 20
Pulse duration - ~ ns
Fig. 2. Changes in the crystallinity degree of the local region in the GST film: depending on the energy exposure of the 405 nm laser pulse (a); depending on the laser pulse duration (b)
Partial crystallization of amorphous GST at these energy exposures is observed at the pulse durations from 12 to 25 ns, while full crystallization is observed at the pulse durations from 25 to 78 ns. An increase in the proportion of crystalline fractions leads to an increase of refractive index and extinction coefficient at telecommunication wavelength of 1550 nm, which also leads
^St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.3 ^
to changes of reflectivity and transmissivity of the GST film and can be used to perform multilevel recording, and amplitude modulation or routing of signals.
Conclusion
Thus, it was found that a change in the energy and duration of a laser pulse leads to a change in the crystallinity degree and, so, to a change in the ratio of crystalline and amorphous fractions.
Acknowledgments
The study was performed with the equipment of Region Centre of probe microscopy of Ryazan State Radio Engineering University named after V. F. Utkin.
REFERENCES
1. Wen S., Meng Y., Jiang M. Wang Y., Multi-level coding-recoding by ultrafast phase transition on Ge2Sb2Te5 thin films. Scientific Reports. 8 (1) (2018) 4979.
2. Kozyukhin S. A., Lazarenko P. I., Vorobyov Y. V., Savelyev M. S., Polokhin A. A., Glukhenkaya V. B., Sherchenkov A. A., Gerasimenko A. Y., Laser-induced modification of amorphous GST225 phase change materials. Matmiaux & Techniques. 180070 (2018) 1—6.
3. Tolkach N. M., Vishnyakov N. V., Lazarenko P. I., Sherchenkov A. A., Sudakova A. U., Nazimov D. R., Optical switching in multilayer structures based on Ge2Sb2Te5. Journal of Physics: Conference Series, 1695 (2020) 012075.
4. Kim D.-H., Merget F., Laurenzis M., Bolivar P. H., Kurz H., Electrical percolation characteristics of Ge2Sb2Te5 and Sn doped Ge2Sb2Te5 thin films during the amorphous to crystalline phase transition. Journal of Applied Physics. 97 (8) (2005) 83538.
5. Born M., Wolf E., Principles of Optics. Cambridge University Press. (2003) 753-754.
THE AUTHORS
TOLKACH Nikita M.
[email protected] ORCID: 0000-0001-9183-7348
YAKUBOV Alexey O.
[email protected] ORCID: 0000-0003-0530-7186
VISHNYAKOV Nikolay V.
[email protected] ORCID: 0000-0002-1270-3446
TROFIMOV Egor S.
[email protected] ORCID: 0000-0002-1516-9154
LITVINOV Vladimir G.
ORCID: 0000-0001-6122-8525
SHERCHENKOV Alexey A.
[email protected] ORCID: 0000-0002-5596-4363
Received 15.08.2022. Approved after reviewing 24.08.2022. Accepted 15.09.2022.
© Peter the Great St. Petersburg Polytechnic University, 2022
