STRUCTURAL INVESTIGATION OF TELLURIUM BASED THIN FILMS

Transcription

1 Vladislava Ivanova, Journal Yordanka of Chemical Trifonova, Technology Vanya and Lilova, Metallurgy, Valdec Mikli, 53, 4, Angelina 2018, Stoyanova-Ivanova STRUCTURAL INVESTIGATION OF TELLURIUM BASED THIN FILMS Vladislava Ivanova 1, Yordanka Trifonova 1, Vanya Lilova 1, Valdec Mikli 2, Angelina Stoyanova-Ivanova 3 1 University of Chemical Technology and Metallurgy 8 Kliment Ohridski, 1756 Sofia, Bulgaria ivanova_vl@uctm.edu 2 Institute of Materials and Environmental Technology Received 30 March 2018 Accepted 20 April 2018 Tallinn University of Technology, Ehitajate 5 Tallinn 19086, Estonia 3 Georgi Nadjakov Institute of Solid State Physics Bulgarian Academy of Science, Sofia, Bulgaria ABSTRACT Tellurium based materials possess a low temperature phase change transition. Sometimes they cannot be obtained in an amorphous state under normal conditions. Thus they can be used as an active element in phase change memory devices. Our study is dedicated to the synthesis of tellurium based materials using the systems (GeTe 3, x = 5 mol %, 10 mol %, 15 mol % and 20 mol % and (GeTe 4, x = 5 mol %, 10 mol %, 15 mol % and 20 mol %, the deposition of thin films on their ground and the investigation of their morphology, topology and structure aiming to obtain new knowledge for evaluation of the possibility for application of these materials in phase change memory devices. Keywords: chalcogenides, thin films, structure, morphology, topology. INTRODUCTION In the 1960s Ovshinsky [1] has already observed a phase change while studying chalcogenide thin layers of the composition of Te 81 Ge 15 Sb 2 S 2. Phase change materials exist in an amorphous as well as in a single or sometimes several crystalline phases switching rapidly and repeatedly between these phases. The switching is typically induced by heating through optical or electrical pulses. The optical and electrical properties can vary significantly in amorphous and crystalline phases. This combination of optical and electrical contrast and repeated switching allows data storage. The two important characteristics of the optical phase change refer to the operating speed and memory capacity. Lucovsky [2] investigates the atomic changes in thin Ge-Sb-Te films accompanying the phase change. There are two hypotheses. The standard one assumes a phase change between a crystalline (ordered) and an amorphous (disordered) structure [3]. The second one deals with the change of Ge atoms coordination from octahedral to tetrahedral configurations associated with Te [4]. New materials showing a short crystallization time are intensively investigated [5]. The memory capacity of a disk can be increased by increasing the number of Ge-Sb-Te layers. In 2009 Yamada et al. [6] develop a three-layer disk with a capacity of 100GB. Although the main mechanism of electrical phase change is similar to that of optical phase change, it possesses inherent qualities. Electrical resistance has a great impact on the phase change [7]. Another problem is the electrical switching that occurs prior to the thermal crystallization [8, 9]. In electric phase change the size of the electrodes is essential. The minimum size at which the phase change occurs at room temperature is 10 nm -50 nm [10, 11]. The electrodes size is governed by an amorphous-crystalline interfacial energy and is important in decreasing the switching current and the device miniaturization. This ultimate size could be the biggest advantage of this kind of atomic memories over electronic and/or dipolar memories. 749

2 Journal of Chemical Technology and Metallurgy, 53, 4, 2018 Ge-Te-Sb based materials are currently considered to be leading candidates for active materials in phase change memory devices preparation. However, they have low crystallization temperatures T c [12]. A higher T c means that unintentional crystallization is less likely and thus a longer data retention time can prevail. Other disadvantages refer to the high reset current (about 800 μa) and the insufficient retention. Alternate chalcogenide materials are currently under investigation in attempts to solve these problems. The aim of this study is the synthesis of materials from Ge-Te-In system, the deposition of thin films from the bulk samples and investigation of their structure and composition to obtain new knowledge for evaluation of the possibility for application of these materials in phase change memory devices. EXPERIMENTAL Bulk samples were synthesized by the meltquenched technique. The initial elements Ge, Te and In were with 4N purity. The respective amounts of the initial elements were evacuated in quartz ampules under a residual pressure of 1, Pa. The synthesis was carried out by heating with a constant rate of K/s up to the final temperature of 1300 K. The glasses were obtained after quenching in a mixture of water and ice with a quenching rate of K/s. Thin films were deposited from the corresponding bulk samples with the application of the thermal vacuum evaporation method on glass and monocrystalline Si substrates. A tantalum evaporator with an evaporating surface of m 2 was used for the substances evaporation. The conditions of the process were as follows: the distance between the source and the substrate was equal to 0,12 m, the residual gas pressure was equal to 1, Pa, while the evaporation temperature ranged from 800K to 900K. The deposited films were characterized with respect to their morphology and topography in order to explore the effect of the preparation method on the films behavior. The microstructure, the thickness and the elemental composition of the films were studied at room temperature using ZEISS HR FESEM Ultra 55 scanning electron microscope (SEM) with Bruker EDS system ESPRIT 1.8. The acceleration voltage for SEM measurements was 4.0 kv, while that for EDS was 20 kv. The topography and roughness information was collected by atomic force microscopy using AFM CP II. RESULTS AND DISCUSSION Fig. 1 shows SEM images of the surface of (GeTe 3 thin films. A smooth film surface containing some globular structures of an approximate size from 50 nm to 100 nm is observed for the sample of 5 % In content. Aggregations of a size of about 50 nm are uniformly distributed on the surface of the sample containing 20 % In. Their presence is increased. SEM micrographs of (GeTe 4 thin films are presented in Fig. 2. The top-view pictures of both systems films reveal uniform and homogeneous surfaces. The obviously finer crystalline structure of (GeTe 4 a) b) Fig. 1. SEM micrographs of thin films surface from the system (GeTe 3 : a) (GeTe 3 ) 95 In 5 ; b) (GeTe 3 ) 80 In

3 Vladislava Ivanova, Yordanka Trifonova, Vanya Lilova, Valdec Mikli, Angelina Stoyanova-Ivanova a) b) Fig. 2. SEM micrographs of thin films surface from the system (GeTe4)100-xInx: a) (GeTe4)95In5; b) (GeTe4)80In20. a) b) Fig. 3. SEM micrographs of thin films cross section from the system (GeTe3)100-xInx: a) (GeTe3)95In5; b) (GeTe3)80In20. a) b) Fig. 4. SEM micrographs of thin films cross section from the system (GeTe4)100-xInx: a) (GeTe4)95In5; b) (GeTe4)80In20. series implies an easier phase change under influence. Therefore, the materials in this section are more suitable for application in phase change memory devices. In our previous studies [13] on thin films of the investigated system, we found that the (GeTe4)100-xInx series have greater photoinduced changes in the refractive index, the absorption coefficient and the optical band gap compared to those of (GeTe3)100-xInx series. 751

4 Journal of Chemical Technology and Metallurgy, 53, 4, 2018 Fig. 5. SEM image on the surface of a thin film with the composition (GeTe3)95In5. Fig. 6. SEM image on the surface of a thin film with composition (GeTe4)95In5. Fig. 7. AFM micrograph of a thin film with composition (GeTe3)90In10. The cross section SEM micrographs of the investigated films presented in Figs. 3 and 4, exhibit a compactness of the film structure in depth as well as lack of voids in the film and at the film-substrate interface. It can be seen that a process of micro-crystallization occurs in the films depth. This is probably due to the great ability of tellurium crystallization and the low evaporation rate in the course of thin layers formation. Recrystallization follows the heating from the evaporator (the evaporation temperature is 1273K measured by PtPt/Ro thermocouple) and the existing metastable GeTe2 decomposes to GeTe and extra tellurium [14, 15]. This causes the inhomogeneities observed and the eutectic 752 melting conditions in the areas of greater tellurium content. This causes extra tellurium crystallization and well-ordered crystals formation. There is also a very thin surface layer, of 10 nm thickness, which is vitreous because it is not continuously heated by the evaporator. The thickness of each film can be determined on the ground of the cross section SEM micrographs. The images show that the films have a thickness ranging from 1.6 µm to 2.0 µm. X-ray microanalysis using energy dispersive spectroscopy (EDS) is applied to provide further information on the composition of the surface. This method allows quantitative identification of the elemental chemical

5 Vladislava Ivanova, Yordanka Trifonova, Vanya Lilova, Valdec Mikli, Angelina Stoyanova-Ivanova Fig. 8. AFM microphotograph of a thin film with composition (GeTe 4 ) 90 In 10. composition. This is done by an interactive PB-ZAF standardless method. For higher certainty, EDS is done at 4 points of each sample investigated (Figs. 5, 6). The analysis confirms that the thin films are homogeneous by composition (Tables 1, 2). Their composition is close to that of the starting bulk material with deviations of the order of the method accuracy. The accuracy of the measurement is ± 2 % for tellurium content and less than ± 1 % for that of the other elements. AFM study of thin films deposited on monocrystalline silicon substrates is performed. The AFM images reveal that the films under investigation are homogeneous with smooth surfaces as evident from the Figs. 7, 8. The results referring to the morphology of the thin layers for the two studied series show that Ge:Te = 1:3 thin film layers are more rough than those of Ge:Te = 1:4 ratio. The deposited chalcogenide thin films from Ge-Te-In system are characterized by surface irregularities of less than 3% of the total film thickness. This gives us reason to assert that the chosen geometry of the experimental apparatus provides deposition of thin layers of a relatively smooth surface. CONCLUSIONS Bulk samples from the systems (GeTe 3 and (GeTe 4, x = 5 mol %, 10 mol %, 15 mol % and 20 mol % are synthesized by melt-quenching technique and thin films are deposited from them by the thermal vacuum evaporation method. The morphology, topology and structure of the prepared thin films are investigated using AFM and SEM, while their composition - by EDS. The EDS results show that the thin films are homogenous by composition Table 1. Elements content in thin film (GeTe 3 ) 95 In 5. Table 2. Elements content in thin film (GeTe 4 ) 95 In 5. Spectrum Spectrum Elements At. % At. % At. % At. % Elements At. % At. % At. % At. % Germanium Germanium Tellurium Tellurium Indium Indium

6 Journal of Chemical Technology and Metallurgy, 53, 4, 2018 which is identical with that of the initial bulk samples. The chalcogenide thin films are also homogeneous in thickness and relatively smooth. These properties make them suitable for application in optics. The surface of (GeTe 3 thin films is glassy, while that of (GeTe 4 thin films is characterized by uniform distribution of aggregations areas, whose number increases with In content increase. The results referring to the thin films cross section show that a process of microcrystallization proceeds in the depth of these films. REFERENCES 1. S.R. Оvshinsky, Reversible electrical switching phenomena in disordered structure, Phys. Rev. Lett., 21, 1968, G. Lucovsky, J.C. Phillips, Reversible chemical phase separation in on-state of art rewritable Ge 2 Sb 2 Te 5 optical phase change memories, J. Non-Cryst. Solids, 354, 2008, M. Wutting, N. Yamada, Phase-change materials for picoseconds pulse nonlinear propagation in chalcogenide As 2 S 3 fiber, Appl. Opt., 48, 2009, A.V. Kolobov, P. Fons, A.I. Frenkel, A.L. Ankudinov, J. Tominaga, T. Uruga, Understanding the phasechange mechanism of rewritable optical media, Nat. Mater., 3, 2004, D. Lencer, M. Salinga, B. Grabowski, T. Hickel, J. Neugebauer, M. Witting, A map for phase-change materials, Nat. Matter., 7, 2008, N. Yamada, R. Kojima, T. Nishihara, A. Tsuchino, Y. Tomekawa, H. Kusada, 100 GB rewritable triplelayer optical disk having Ge-Sb-Te films, Proc. E*PCOS 2009, 2009, M.H. Jang, S.J. Park, M.H. Cho, E.Z. Kurmaev, L.D. Finkelstein, G.S. Chang, The origin of the resistance change in GeSbTe films, Appl. Phys. Lett., 97, 15, 2010, A. Madam, M.P. Shaw, The physics and applications of amorphous semiconductors, Chap. 5, Academic, Boston, MA, M. Nardone, V.G. Karpov, D.C.S. Jackson, I.V. Karpov, A unified model of nucleation of swiching, Appl. Phys. Lett., 94, 2009, J.C. Scott, Is there an immortal memory?, Science, 304, 2004, Y. Fujisaki, Current status of nonvolatile semiconductor memory technology, Jpn. J. Appl. Phys., 49, 2010, M.H. Jang, S.J. Park, D.H. Lim, S.J. Park, M.-H. Cho, D.-H. Ko, M.Y. Heo, H.C. Sohn, S.-O. Kim, Effect of In incorporated into SbTe on phase change characteristics resulting from changes in electronic structure, Appl. Phys. Lett., 96, 2010, V. Ivanova, Y. Trifonova, P. Petkov, T. Petkova, The influence of In on photo-induced properties of Ge-Te-In chalcogenide thin films, J. Optoel. Adv. Mat. - R. Comm., 8, 1-2, 2014, D.I. Bletskan, Glass-formation and crystallization in the system Ge-Te, Chalcogenide Letters, 2, 12, 2005, D.I. Bletskan, Phase equilibrium in the systems A IV - B VI, Journal of Ovonic Research, 1, 5, 2005,