Simultaneous Measurements of Short and Long-range order in GeTe
P. Fons1,2, A. V. Kolobov1,2, M. Krbal1, J. Tominaga1, T. Matsunaga3, and N. Yamada4
1

National Institute of Advanced Industrial Science & Technology,
Tsukuba Central 4, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8562, Japan
2
SPring8, Japan Synchrotron Radiation Institute (JASRI),
Kouto 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
3
Materials Science and Analysis Technology Center, Panasonic Corporation,
3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570-8501, Japan
4
Digital & Network Technology Development Center, Panasonic Corporation, 3-1-1
Yagumo-Nakamachi, Moriguchi, Osaka 570-8501, Japan

ABSTRACT
GeTe is an endpoint of the technologically important pseudobinary tie line (GeTe)x(Sb2Te3)(1-x) that is widely used forboth re-writable optical recording as well as nonvolatile phase-change electrical memory. Until recently, it had been
believed that GeTe underwent a displacive ferroelectric to paraelectric transition at Tc~ 700 K, however recent
temperature-dependent XAFS results suggested that the first-nearest neighbors maintain a rhombohedral (3+3)
disordered octahedral coordination through Tc. In the current paper, we carry out a temperature-dependent pairdistribution function analysis using x-rays to study order in GeTe on a variety of different length scales in a single
measurement and show that the transition is of an order-disorder nature with significant levels of disorder present
even at room temperature. These results suggest that GeTe and possible other phase-change materials on thepseudobinary tie-line experience significant temperature activated dynamic disorder. These results are expected to bepresent in partially transformed amorphous phase as well and may have important consequences for electricaltransport in phase-change materials.
Keywords: order-disorder, pair-distribution function, scattering

1. INTRODUCTION
GeTe is a phase-change material and constitutes one of the endpoints of the technologically important GeTe-Sb2Te3
pseudobinary tie-line. Compounds along this tie-line exhibit unusually large changes in electronic properties
between disordered (RESET) and ordered (SET) states that have made them the materials of choice for both opticaland electrical non-volatile storage devices. The phase-change process occurs on the order of nanoseconds and fordomains on nanometer length scales. GeTe based non-volatile electrical memory has been shown to switch i on theorder of 10~ns, a writing speed on the same order as the dynamic memory of the recent past, underscoring theindustrial interest in these materials. The switching mechanism is now being debated and the details of the structure
on different length scales may well play an important role in the phase-change mechanism and its optimization incommercial devices. The extent of thermally-induced disorder is also of interest as it has been shown boththeoretically and experimentally that temperature-induced disorder may couple with photo-induced excited-state
effects to induce the formation of the so-called amorphous phase via athermal pathways ii,iii. The short temporal andsmall spatial domains make modeling of changes in electronic properties using ab-initio quantum mechanicaltechniques attractive. Quantum mechanical calculations of changes in material properties have been used to unveil theunderlying mechanisms behind the large changes in material properties undergone by phase-change materials between
SET and RESET states as well as for the design of novel artificial structures with more ideal switching attributes. iv

GeTe is a narrow band gap semiconductor and at room temperature assumes a simple rhombohedral structure (αGeTe) with two atoms in the unit cell. The α phase of GeTe can be visualized as a rocksalt structure with a small
sheer relaxation along the pseudo-cubic [111] direction. In α-GeTe, both Ge and Te atoms are nominally octahedrally
coordinated with two triplets of shorter (2.83 Å) and longer (3.15 Å) bonds, a structure that has been described as aPeierl distortion due the quasi one-dimensional nature of the p-orbital based bonding in GeTe. v

Ge-Sb-Te alloys along the pseudobinary (GeTe)x(Sb2Te3)1-x tie-line are well known phase-change alloys that exhibit
large changes in electrical and optical properties when transformed between amorphous and crystalline phases.
Ge2Sb2Te5 (GST225) for example is considered a prototypical phase-change alloy in that it is used both in opticalstorage devices (DVD-RAM) as well as in phase-change electical memory (PC-RAM). Research on the GeTe richside of the tie-line has recently increased due to the higher crystallization temperatures potentially allowing PC-RAM
operation in high-temperature environments. Additionally, the enhancement in the reflectivity difference between
crystalline and amorphous phases near the 408 nm operating wavelength of Blu-Ray discs have made the Ge8Sb2Te11(GST8211) phase the material of choice over GST225 for Blu-Ray re-writeable optical disc applications.
The structures of phase-change compounds crystallized from as-deposited material with compositions lying along thetie-line ranging from 1 to 0.85 were observed by temperature dependent BD (Bragg diffraction) to initially crystallize
upon laser-irradiation into a rhombohedral (s.g. R3m) structure. vi The rhombohedral structure results in a distorted
octahedral local environment in which can be described as 3+3 or three shorter and three longer bonds as typified by
the A7 structure of elemental Sb. Upon further heating, a ferroelectric to paraelectric transformation to cubic
symmetry (s.g. Fm̅3m) was then observed. From this change in symmetry it was concluded that the local 3+3distorted octahedral bonding configuration transformed at TC to a ideal octahedral coordination in which all six bonds
were identical in length. In these BD measurements, a gradual decrease in the transition (Curie) temperature Tc wasobserved with increasing x leading to the prediction that for tie-line compositions above the critical composition
xc~0.86, no trigonal-cubic phase transition would exist and the average structure would be cubic upon lasercrystallization. It should be noted in passing that the bulk phase of GST8211 is reported to consist of a 21-layer stackthat is inaccessible kinetically for the temperatures discussed here. vii
Changes in local structure associated with the ferro- to para-electric phase transition in GeTe have also recently been
observed by x-ray absorption fine-structure measurements (XAFS). viii This experiment which observed changes inlocal structure about Ge atoms, concluded that there was no change in the local rhombohedral symmetry evenbehyond the Curie temperature. The change from rhomobohedral to cubic symmetry observed by BD was attributed
to averaging effects and that in fact, the transition was not displacive in nature, but of an order-disorder character.
Here we simultaneously investigate changes in atom correlation over a wide range of length scales in a single
measurement using a pair-distribution function analysis by utilizing both diffuse and coherhent diffraction
contributions.
2. EXPERIMENTS
Thin film samples of GeTe of 1 μm thickness were grown by RF sputtering in an argon atmosphere from composite
targets and deposited onto high-purity quartz substrates at room temperature. The samples were subsequently
crystallized by annealing in a nitrogen ambient for two hours at 180°C. For the GeTe total scattering measurements,
the GeTe sample was scraped off the substrate and loaded into a 200 micron diameter quartz capillary tube that wasthen sealed to prevent oxidation. The samples were then measured at beamline BL02B2 at the Japan Synchrotron
Radiation Research Institute (SPring-8); details of the beamline setup have been described elsewhere.[4] The
incident beam energy was 29.5 keV and the diffractometer imaging plate distance afforded an angular resolution of0.02°. The sample temperature was varied from 300 to 800K by immersing the sample in a N2 jet of the requiredtemperature. The incident beam energy was calibrated using a standard CeO2 powder sample at room temperature. Abackground reference spectra was taken at room temperature without the GeTe sample present. The reducedstructure factor G(r) was obtained from the raw diffraction data using the program PDFgetX2 and modeling of theunderlying pair distribution was carried out using the program PDFgui.[5] The reduced structure factor G(r) isdirectly related to the observed structure function S(Q) byThe reduced structure factor is related to the pair-density distribution function g(r) by G(r)=2π r ρ0 (g(r)-1). The

measured G(r) is modeled by summing over all possible atom pairs in the model or
where the sum is over all atom pairs i and j, bi and bj are the scattering lengths of atoms i and j, and <b> is the average
scattering length of the model. The details of the model used are described in the Results and Discussion section.
3. RESULTS & DISCUSSION
As a first approximation a simple “small cell” model based upon the earlier fitting results of the temperaturedependent GeTe XAFS measurements was constructed.[3] The basic premise of the model was the relative “dipolemoments” of the Ge-Te dipoles should average to zero along each of the pseudocubic <111> directions. A key pointof this simple model is that it is consistent with the short-range rhombohedral symmetry as observed by the XAFS
measurements. A key question the fit of the model to experiment answers is whether or not such a simple model can
properly describe the experimentally observed changes in not only short, but also intermediate range pair-correlations
with temperature. Two competing models, the aforementioned “small cell model” as well as a displacive transition
model in which the rhombohedral primitive cell is allowed to gradually transform to a cubic symmetry cell withtemperature were constructed. For each temperature, the relative weight of each model, the lattice parameters, andisotropic Debye-Waller factors were allowed to vary to match both the full G(r) experimental spectra. Figures 1 and2 show plots of the experimental G(r) data for 300 and (above the Curie temperature) 800 K respectively. The circles
represent experimental data while the continuous line represents the “small cell” model described above. As can beseen in the plots, the experimental model describes both the short and intermediate-range order quite well suggestingthat the transformation of GeTe from a rhombohedral to a cubic bonding arrangement is, in fact, a reversible orderdisorder transition.

Figure 1. The experimental 300 K reduced radial distribution function G(r) obtained by a Fourier transform of theexperimental S(Q) scattering function is shown in red (circles). The best fit to the experimental data using thesimplified model described in the text (continuous line) is shown superimposed on the experimental data.

Figure 2. The experimental (above the Curie temperature) 800 K reduced radial distribution function G(r) obtained bya Fourier transform of the experimental S(Q) scattering function is shown in red (circles). The best fit to theexperimental data using the simplified model described in the text (continuous line) is shown superimposed on theexperimental data.

Figure 3. A plot of the relative volume fraction of the displacive versus “small cell” model as described in the text.
The results clearly show the presence of a order-disorder transition.

4. CONCLUSION

The results of total scattering measurements that probe both short and intermediate range order in the crystalline phaseof GeTe as a function of temperature are consistent with earlier XAFS results indicating the presence of significant
thermally activated disorder. The simplest possible model consistent with both XAFS and BD results was constructedin which the average “dipole moment” of the rhombohedrally distorted Ge-Te bond was zero at the Curie temperature.
The model was found to be in excellent agreement not only with experimentally observed short-range order, butintermediate-range order as well.
Collectively these observations suggest that the local structure in(GeTe)x(Sb2Te3)1-x alloys is dynamic and constantly in a state of rearrangement. This conclusion also has
ramifications for the application of ab initio methods for the prediction of the properties of phase-change materials inshowing that the heretofore utilization of well ordered GeTe structures is insufficient. These observations oftemperature induced variations in local order with temperature are also likely to play a role in electrical transport. It
is also tempting to speculate that similar effects may be present in the disordered phase which in turn may play a rolein the localization of charge carriers and possibly in the drift observed in the resistivity of the RESET state inelectrical devices.
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