P07

Chemical composition analysis of individual chalcogenide alloy
nanostructures
M. Abea), M. Kuwahara*, J. Sakai**, H. Uratsuji***, Y. Uehara
Fundamental Technology Center of RIEC, Tohoku University
Katahira, Sendai, Japana) maho@riec.tohoku.ac.jp (corresponding author)* Electronics and Photonics Research Institute, AIST
Tsukuba, Ibaraki, Japan** Groupe de Recherche En Matériaux Microélectronique Acoustique Nanotechnologies,
Université François Rabelais de Tours
Tours, France*** SHIBAURA ELETEC Company Ltd.
Yokohama, Kanagawa, JapanABSTRACT
We have determined the chemical composition of nanometer-scale alloys made of Sb and Te by scanning electron
microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX). 5 nm-thick Sb2Te3 was first
deposited on a highly oriented pyrolytic graphite (HOPG) substrate by a radio-frequency (RF) magnetron sputtering
method. No conspicuous nanostructures were observed in the SEM images at this stage. Then the sample wasannealed at 200 ℃ in an Ar gas atmosphere for five-minutes. SEM observations informed that nanostructures withlateral sizes of around 20 nm, which are isolated from each other, were successfully produced. EDX measurements
revealed that the chemical composition of each nanostructure is Sb : Te = 1 : 3, while the gross composition of thesample surface is Sb: Te = 2: 3, even after the annealing process. This discrepancy in chemical compositionindicates that Te atoms are much more mobile than Sb on the HOPG surface for the adopted annealing condition. As
a result, the Te atoms exclusively cohered to form Te-rich nanostructures.
Key words: Chalcogenide alloys, Nanostructures, Chemical composition
1. INTRODUCTION
Chalcogenide alloys have attracted many researchers because of their unique material properties, i.e., significant
changes in electronic and optical properties upon amorphous to crystalline phase transitions. Such properties are
currently applied to optical disks and are intensively investigated towards non-volatile phase change memories. [1, 2]
In the high-storage-capacitance memories, the size of each element is reduced to nanometer scale range, where somekind of size effect may additionally appear. Therefore, exploring properties of individual nanostructures made ofthese materials are very interesting. While scanning probe microscopy (SPM) is a powerful tool for surface
structural analyses with required spatial resolution, its capability for chemical composition analyses is poor. In thispaper, we report determination of the chemical composition of individual chalcogenide-alloy-nanostructures byscanning electron microcopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX).
2. EXPERIMENTS
Highly oriented pyrolytic graphite (HOPG) was chosen for the substrates of samples because of its atomic level
flatness of surface. 5 nm-thick Sb2Te3 was first deposited on the surface in an Ar gas pressure of 0.5 Pa by aradio-frequency (RF) magnetron sputtering method. No conspicuous nanostructures were seen on the surface at thisstage. Then the sample was annealed at 200 oC in an Ar gas atmosphere for five minutes.
The surface structures and chemical compositions of the samples were simultaneously observed by SEM combinedwith EDX. The SEM was operated with an acceleration voltage of 10 kV throughout the experiments. Two typesof EDX spectra are shown in the resultant section; one is for determining element distributions in certain sample areas
and the other for determining chemical compositions at specified positions on the sample surfaces. The exposure
time was 30 and 5 minutes for the former and latter measurements, respectively.

3. RESULTS AND DISCUSSION
Figure 1(a) shows a typical SEM image ofthe post-annealed samples.
We see thatnanostructures with lateral sizes of around 20nm, being isolated from each other, aresuccessfully produced. EDX spectra weretaken from each 128 x 88-divided areas incorresponding SEM images to determine
element distributions. Figures 1(b) and 1(c)show the results for Sb and Te, respectively, inwhich the amount of each element isrepresented by densities of white dots. By
comparing Fig. 1(a) with Figs. 1(b) and 1(c),one sees that relationship between locations ofindividual nanostructures in the SEM images
and the element distributions is hardly found.
Figure 1(d) shows the EDX spectrum obtainedby integrating all EDX spectra from the 128 x
88 pixels. The ratio of Sb : Te deduced fromFig. 1 (a) SEM image of a sample surface area and element
Fig. 1(d) is approximately 2 : 3, which is
distributions of (b) Sb and (c) Te in the area. (d) EDX spectrum
reasonably consistent with that of the targetof the area.
used for deposition.
Figures 2(a) and 2(b) show the EDX spectraof (a) one of the nanostructures and of (b) a location where the flat surface
is exposed, respectively. The signals from Te are dominant in Fig. 2(a),even though smaller signals from Sb are also seen. Detailed analyses ofFig. 2(a) reveal that the chemical composition of the nanostructure isapproximately Sb : Te = 1 : 3. The EDX signals seen in Fig. 2 (b) aresignificantly weak in comparison with those in Fig. 2(a), showing that theSb and/or Te layer covering the surface of the substrate is extremely thin.
Since the chemical composition in gross determined from Fig. 1(b) is Sb :Te = 2 : 3 and one of each nanostructure is Te rich, the surface of thesubstrate must be dominantly covered by Sb.
4. CONCLUSION
We investigated chemical composition of nanostructures produced by
RF magnetron sputtering followed by the five-minutes-annealing in Ar.
The size and chemical composition of each nanostructure were determinedby simultaneous measurements of SEM and EDX. The lateral sizes ofnanostructures produced by the present method were around 20 nm andtheir chemical composition was Sb : Te = 1 : 3. On the other hand, thesurface of the substrate was dominantly covered by Sb. This discrepancy
in chemical composition strongly suggests that Te atoms deposited on theHOPG are much more mobile than Sb for the adopted annealing condition
so that the Te rich nanostructures are formed during the annealing process.

Fig. 2 EDX spectra of (a) ananostructure and (b) a flat areaon the sample surafce.

ACKNOWLEDGEMENT
One of the authors (M. A.) would like to acknowledge valuable advice from Mr. Takenori Tanno.
REFERENCES[1] M. Okuda (Ed.), Material for future optical memories, CMC Publishing Co. LTD (2009).
[2] D. Lencer, M. Salinga, and M. Wuttig, Design Rules for Phase-Change Materials in Data Storage Applications,Adv. Mater. 23, (2011) 2030-2058.