RSC Advances View Article Online PAPER View Journal | View Issue Optical constants acquisition and phase change properties of Ge Sb Te thin films based on Cite this: RSC Adv.,2018,8,21040 2 2 5 spectroscopy Zemin Xu, Chaonan Chen, Zhewei Wang, Ke Wu, Haining Chong and Hui Ye * In this work, phase change chalcogenide Ge2Sb2Te5 (GST) thin films were fabricated by magnetron sputtering. The optical properties, especially the optical constants (refractive index and extinction coefficient), of such alloys were systematically studied by investigating their thermally and photo- thermally induced switching between different phases. The results show that GST films are highly tunable in microstructure and optical constants, either by post-annealing at 160 C, 200 C, 250 C and 350 C, respectively, or by laser irradiation of 1 mW, 3 mW, 5 mW and 10 mW power with beam diameter of 7 mm at 532 nm, respectively. From the structural analysis, we can clearly observe different crystallinities and chemical bonding in the different post-treated GST films. The optical constants of GST Creative Commons Attribution-NonCommercial 3.0 Unported Licence. films under various phases were obtained from spectrophotometry, by fitting their transmittance data with the Tauc–Lorentz (TL) dispersion model. The refractive index and extinction coefficient exhibit Received 13th February 2018 notable change upon annealing and laser irradiation, specifically at 1550 nm, from 3.85 (amorphous) to Accepted 1st June 2018 6.5 (crystalline) in refractive index. The optical constants have been proved capable of fine tuning via the DOI: 10.1039/c8ra01382a laser irradiation method. Hence, the pronounced adjustability in optical properties due to rapid and rsc.li/rsc-advances repeatable phase change render GST suitable for tunable photonic devices. Introduction does not affect the performance.14–16 Our research focuses on This article is licensed under a tracking the GST optical constant evolution process during the Chalcogenide glasses (ChGs), a notable branch of amorphous successive stages of phase-change. As there are coexistence of semiconductors, have been widely applied as critical materials different proportion of amorphous and crystalline phases at 1 ff Open Access Article. Published on 08 June 2018. Downloaded 10/2/2021 6:34:43 AM. in phase-change memories, sensors and photonic devices. One di erent phase transformation stages. The optical constants of the most attractive properties of ChGs is their outstanding obtained from various stages are proved to be continuously phase changing characteristic, making them suitable candi- variable. It is therefore possible to realize the precise modula- dates for building rewritable storage devices, multi-color tion on optical properties of GST chalcogenide by tracking their tunable lters, and electrically- or optically- driven intelligent optical constants at different phase stages. It is also helpful for displays.2–6 In addition, the high material nonlinearity of GST, data support to the phase transformation physics. combined with the strong connement and dispersion in ber In this paper, Ge2Sb2Te5 lms with controllable composition and waveguide devices, makes it attractive for fast nonlinear and tunable optical properties were successfully fabricated. For optical devices.7,8 However, there are also some deciencies in the determination of optical constants, the transmittance data the performance of GST, especially its relatively large optical in the NIR region were tted with Tauc–Lorentz model and nonlinear absorption in the optical communication band and thus, the refractive index and extinction coefficient were effec- poor chemical and thermal stability.9,10 tively retrieved. Hence, the correspondence of post treatment Nowadays, phase change materials (PCMs) are the focus of process and optical constants of GST lms make it possible to scientic research.11–13 Ge-Sb-Te (GST) is a relatively mature and ne control optical properties of chalcogenide materials in NIR superior PCM which can be classied into three categories: range. On this basis, the utilization of GST lms in designing Ge1Sb2Te4,Ge1Sb4Te7 and Ge2Sb2Te5. Most researchers are and manufacturing a variety of photonic integrated devices in interested in Ge2Sb2Te5 due to its faster crystallization rate, the eld of optical communication, interconnection and lower power consumption, as well as the undemanding computing will be realized. It is worth mentioning that GST composition requirements that minor composition deviation lms have large absorption in the visible region resulting in a very low transmittance (almost near zero). Thus, compared with spectroscopic ellipsometry, the method based on trans- State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou, 310027, P. R. China. E-mail: mittance spectra are relatively inaccurate to obtain optical [email protected] 21040 | RSC Adv.,2018,8,21040–21046 This journal is © The Royal Society of Chemistry 2018 View Article Online Paper RSC Advances ! constants in visible region. This paper focuses on the optical 2 2 0 0 1 A L a E þ E þ aE ð Þ¼ 2 À 2 ¼ ðNÞþ ln 0 g g constants acquisition in the near-infrared band. 3TL E n k 3TL 4 ln 2 2 2 p z aE0 E þ E À aE 0 g g A a 2E þ a À2E þ a À atan p À a tan g þ a tan g p 4 Experimental and theoretical basis z E0( " L !#)L À Á 2 À 2 AE0L 2 2 g Eg Fabrication þ 2 Eg E À g p þ 2a tan pz4 aL The Ge2Sb2Te5 thin lms were fabricated with magnetron E2 þ E 2 sputtering method. A Ge2Sb2Te5 ceramic target with diameter of AE L g E À E À 2 0 ln g 3 inch was located 15 cm away from the rotatable sample stage. p 4 E E þ E z 0 g 1 150 nm-thick Ge2Sb2Te5 composite lms were deposited on BK7 À À ÁÁ glass substrates. The radiofrequency (RF) sputtering power was B C AE L B E À E E þ E C xed at 80 W and the substrates temperature was set at room þ 2 0 E lnBsffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffig g C 4 g B C pz @ 2 A temperature (RT). Some as-deposited samples were annealed in 2 2 2 2 E0 À Eg þ Eg L vacuum atmosphere for 20 minutes under four temperatures: 160 C, 200 C, 250 C and 350 C, respectively. Some others (3) were irradiated by 532 nm continuous wave (CW) laser (diam- m where eter size of the laser spot is 7 m) with beam power of 1 mW, 3 mW, 5 mW and 10 mW, respectively. 2 2 2 2 2 2 2 2 aln ¼ Eg À E0 E þ Eg L À E0 E0 þ 3Eg (4a) À Á 2 2 2 2 2 2 Characterization aatan ¼ E À E0 E0 þ Eg þ Eg L (4b) The composition of as-prepared Ge2Sb2Te5 thin lms deter- Creative Commons Attribution-NonCommercial 3.0 Unported Licence. À Á 2 2 mined by energy dispersive spectrometer (EDS, X-Max 20) was 4 2 2 2 a L z ¼ E À g þ (4c) 22.00 at% Ge, 24.75 at% Sb, and 53.25 at% Te, which is very 4 close to a nominal composition of bulk GST material (Ge22.2- qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 Sb22.2Te55.6). The lm thickness was in situ controlled by quartz a ¼ 4E0 À L (4d) crystal monitor. Transmission electron microscopy (TEM, Tec- rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi nai G2) and Raman spectroscopy (Horiba Jobin Yvon, LabRAM L2 g ¼ E2 À (4e) HR Evolution) were applied to analyze the microstructures of 0 2 GST lms. The crystallinity of each sample was retrieved from X- This article is licensed under a ray diffractometer (XRD, Panalytical B.V. X-pert Powder). Spec- trophotometer (Agilent Cary-7000) was used to obtain the For the sake of convenience, eqn (2) and (3) are marked as f2 ffi transmittance for GST lms on glasses. and f3 respectively. Refractive index (n) and extinction coe - cient (k) can therefore be given by Open Access Article. Published on 08 June 2018. Downloaded 10/2/2021 6:34:43 AM. ¼ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffif2 Optical constant acquisition n qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (5) 2 þ 2 À The dispersion behavior of narrow gap semiconductor chalco- 2 f1 f2 f1 genide lms can be well described by the classic Tauc–Lorentz vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi model.17,18 This oscillation function (eqn (1)) describes the u qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 17,19 2 2 complex dielectric permittivity as: tÀf1 þ f1 þ f2 k ¼ (6) ¼ 0 þ 00 2 3TL 3TL i3TL (1) À Á 2 E À E ALE E 00 ð Þ¼ ¼ g  À Á0 According to the characteristic matrix formalism of the thin 3TL E 2nk 2 E2 2 2 2 2 21,22 E À E0 þ L E lm available, equation of transmittance is given by À Á 2 ALE E À E 1 B cos d i sin d=ðn À ikÞ 1 ¼ À 0 Á g ¼  (7) 2 (2) ð À Þ À 2 2 2 2 E C i n ik sin d cos d ns iks E À E0 þ L E 0 00 where 3 and 3 are the real part and imaging part of complex 4ns TL TL T ¼ (8) permittvity, respectively. A is the amplitude (oscillator strength), ðB þ CÞ2 L is the broadening term, Eg is the optical bandgap energy and where d ¼ 2pd(n À ik)/l is the phase angle of the lm, E0 is the peak transition energy. The eqn (2) is workable when pffiffiffiffiffi pffiffiffiffiffi 00 ns ¼ð1 þ Rs Þ=ð1 À Rs Þ is the refractive index of substrate, Rs photon energy E > Eg, otherwise 3TL is 0. 0 ¼ 1 À T is the reectivity index of substrate (in the wavelength The real part of the permittvity 3 can be obtained by s TL range between 700–2000 nm, the extinction coefficient of BK7 Kramers–Kronig integration, which can be solved in closed form, and is given by20 This journal is © The Royal Society of Chemistry 2018 RSC Adv.,2018,8,21040–21046 | 21041 View Article Online RSC Advances Paper glass is approximately zero), Ts is the transmittance of substrate and d is the physical thickness of lm.