High-Speed Phase Change in GeTe-Sb 2 Te 3 Pseudobinary Compound, a Superior DVD-RAM Material In a re writable phase-change optical disk such as a DVD-RAM (digital versatile disc - random access memory), information is recorded using the change in optical property, such as reflectivity or transmissivity, that results from the phase transformation of the material between the crystal (erase) and amorphous (record) states. Currently, the best-known phase-change material is the GeTe (1- x )- S b 2 Te 3 ( x ( ( ) pseudobinary compound, in which the reversible structural change can be induced in a very brief time of 1 0 ns. The crystalli z ation speed of this compound is very fast, which contributes to the shortening of the erasing time, and e x tremely high-speed rewriting can be reali z ed . To clarify this high-speed phase-change mechanism, the crystal and amorphous structures of this pseudobinary compound has been studied for several years by powder diffraction, e x te nded X -ray absorption fine structure ( EX A FS ) , and gra z i ng incidence X -ray scattering (GI XS ) m easu rements using synchrotron radiation at beamlines BL02B2 a nd BL19B2 . X -ray diffraction measurements revealed that this compound crystalli z es into a single phase with a simple N aCl-type structure ( Fm3 m 3 3 ) m as the metastable state, upon laser annealing (erasing operation) over a wide composition range of GeTe from 1 00 mol % to at least 50 mol % ( F ig . 1). Te occupies the 4 ( a ) sites (C l sites) and Ge and S b locate randomly at the 4 ( b ) site s ( N a sites) [ 1 ] . EX A FS meas u rements of the amorphous compound ( x = 0 , 1 /7 , 1 /5 , 1 /3 , a nd 1 /2 ) sho wed that Ge, S b, and Te are about four-, three-, and two-fold coordinated, respectively, and these coordination numbers were almost constant and independent of the pseudosbinary Fig. 2. Determined coordination numbers of the atoms surrounding Te (top), Sb (middle) and Ge (bottom). n a n nd x in the figures indicate coordination number and mol fraction of Sb 2 Te 3 in the GeTe-Sb 2 Te 3 pseudobinary system, respectively. composition ( x ( ( ). In addition, the amorphous compound had a specific coordination feature. The analy z ed coordination numbers ( n ( x )) of Ge, S b, and Te to each x other are shown in F ig. 2 . F rom this figure, the following relations can be derived. (i) Te-Te pairs are rarely present in this pseudobinary amorphous compound. In other words, Te is surrounded by only Ge or S b. (ii) S b bonds more easily to Te than to Ge or S b. F or e x ample, it is estimated that when one S b atom is doped into amorphous GeTe ( x = x 0 ) , the S b atom, which is three-fold coordinated, is surrounded by about two Te atoms but by only one Ge. (iii) Ge bonds to the three kinds of atoms with almost e q ual probabilities. These suggest that two kinds of atomic pairs, Ge-Te and S b-Te, e x ist i n this pseudobinary amorphous Fig. 1. The crystal structure of laser-crystallized G e T e - S b 2 T e 3 m e t a s t a b l e p h a s e s h o w n schematically in perspective. Green circles show atomic positions for Te. Gray circles show those for Ge or Sb. The lattice constant, a , is about 6 Å. z y x 4.0 3.0 2.0 1.0 0.0 0. 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 x (mol fraction) n (coordination number) n (coordination number) 2.0 1.5 1.0 0.5 0.0 0. 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 x (mol fraction) 2.0 1.5 1.0 0.5 0.0 0. 0 0 0.1 0.2 0.3 0.4 0.5 0.6 x (mol fraction) n (coordination number) 4 4 4 4 4 4 4 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 0 1200 200 400 600 800 1000 0 1 2 3 4 5 6 7 8 r (Å) ERDF (electron number/Å) Fig. 3. The electron radial distribution functions (RDFs) of the amorphous films of GeSb 2 Te 4 (solid line) and Ge 2 Sb 2 Te 5 (dashed line). T oshiyuki Matsunaga a , Noboru Yamada b and Masugu Sato c (a) Characterization Technology Group, Matsushita Technoresearch, Inc. (b) Storage Media Systems Development Center, Matsushita Electric Industrial Co. (c) SPring-8 / JASRI E-mail: matsunaga.toshiyuki@jp.panasonic.com References [ 1] T. Matsunaga and N. Yamada: Jpn. J. Appl. Phys. 41 (2002) 1674. g [2] T. Matsunaga, N. Yamada and M. Sato: submitted in Phys. Rev. B [3] M. Sato et al. : 2 003 M RS F all M eeting Proc. (Boston in U S A, 2003). Fig. 4. The electron radial distribution function (R D F ) o f t h e a m o r p h o u s f i l m o f G e S b 2 T e 4 (indicated by thick hi k line). p li ) The bars indicate the calculated RDF of the crystal structure of GeSb 2 Te 4 . compound to a greater e x te nt than i f they w er e coo rdinated completely at random. These t w o p airs constitute the structural f rame in the crystal phase o f this compound ; as s een in F ig . 1, only t w o k i nds o f bond pairs, Ge-Te and Sb-Te, e x ist in it. It is conse q uently presumed that this amorphous compound already has atomic arrangements w ith the c ha racteristics seen in its crystalline state. W hen thermal energy is added to the amorphous (recording) mark by laser irradiation, these pairs act as crystallization nuclei and trigger the arrangement o f atoms into an NaCl-type structure. F igure 3 sho w s t he elect ron radial distribution f unctions (RD F ) f or GeSb 2 Te 4 ( x = 1/2) and Ge 2 Sb 2 Te 5 (1/3) amorphous f ilms, w hich w ere de rived by using the GI X S i ntensities up to the w a ve number o f about 12 Å -1 . The patterns o f these RD F s are very similar to each other and clearly indicatea medium-range order ; in the patterns w e can see three distinct peaks indicating atomic pair distribution s at the distances o f r = 2 .8, 4.2, and 6.2 Å . F igure 4 sho w s t he RD F c alculated f o r the crystal structure o f GeSb 2 Te 4 (indicated by bars) w ith that observed f or its amorphous state. It can be seen that the positions and the amplitudes o f the three RD F peaks o f the amorphous phase are near those o f the 1st ( r = 3 Å a nd CN = 6), 2nd (4.2 Å and 12) and 5 th (6.7 Å and 24) atomic pairs in the crystalline phase. H ere, CN m N eans the number o f ato ms at the i th coordination shell in the crystal (see F ig . 1). It w as, h o w ever, di ff icult to f i nd the peaks corresponding to the radial distances o f th e 3 rd ( 5 .2 Å and 8) and 4th (6.0 Å and 6) atomic pairs in the crystalline phase. W e the n divided the RD F o f the amorphous f ilm into si x single Gaussian peaks (sho w n by dashed curves in F ig . 4) by least s q uares f i tting. The synthesized curve (thin solid curve) sho w ed g ood agreement w ith the observed RD F p ro f il e. The third observed peak consists o f three single peaks at r = 5 .7, 6.2 and 6. 9 Å , w hich are p resumed to correspond to the 3rd, 4th and 5 th atomic pairs o f t he crystal structure. These results suggest that the local structure o f this pseudobinary amorphous compound bears the structural characteristics o f its crystalline phase to a large e x tent [3]. W e ha ve seen that the crystal and amorphous phases o f the GeTe-Sb 2 Te 3 pseudo-binary phase- change material have very similar atomic arrangements to each other over a w ide composition range. It is believed that w hen an amorphous mark is given energy by laser irradiation to erase a record, this structural similarity bet w ee n the t w o ph ases enables the phase change to occur w ith only a slight atomic movement, w hi ch results in rapid phase trans f ormation. 0 1200 200 400 600 800 1000 0 1 2 3 4 5 6 7 8 r (Å) ERDF (electron number/Å) 5 5 5 5 5 5 5 5 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
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