Hard X-ray Photoelectron Spectroscopy of Crystalline andAmorphous Ge1Cu2Te3
Keisuke Kobayashi, Masaaki Kobata, Igor Pis*, and Shigenori Ueda
NIMS Beamline Station at SPring-8
Yuji Sutou, Yuta Saitou, Toshiya Kamata, Junichi Koike
Tohoku University, Department of Materials Science* Present address: Charles University, Czech.

Abstract
Valence band structures and chemical bonding states of amorphous (A) and crystalline(C) phases of an average- four- valence- electrons- material Ge1Cu2Te3 wereinvestigated by hard X-ray photoelectron spectroscopy. Prominent spectral shapedifferences between the two phases which correspond to the electric resistivity
difference between A- and C-phases were observed. The results strongly suggest thatGe1Cu2Te3 is expected be a new class of phase change memory materials, in which thechange in bonding coordination of Ge may play an essential role in the phase changemechanism.

Introduction
Crystalline (C) Ge1Cu2Te3 (GCT) has 24 s-p electrons per molecule, i. e., 4 electrons per
atom. It takes chalcopyrite related structure with 4 fold coordinated bonds. Amorphous(A) phase of this compound is possible to be obtained by quenching. Recently, Sutou etal. reported that the electrical resistivity of the A-phase is 2-3 orders of magnitudelarger than that of C-phase [1]. Being interested in the A-C phase change of thismaterial, we have conducted hard X-ray photoelectron spectroscopy (HXPS)
observation[2].

Experimental
Samples were prepared by sputtering of a Ge-Cu-Te alloy target on SiO2 (20 nm)/Si
substrates at Tohoku University. In order to avoid oxidation of the sample surfaces,
carbon layers of few nm in thickness were deposited immediately after the deposition.
As-deposited GCT films were verified to be amorphous by the X-ray diffraction pattern.
The crystalline samples were prepared by annealing the as-deposited amorphous films
at 250 °C. The electrical resistance drastically decreases by this annealing procedure,
indicating crystallization of the films. HXPS measurements were done at BL15XU ofSPring-8, NIMS contract beamline, using horizontally polarized undulator X-ray
photons of 6 keV with band width of ca. 55 meV. The total energy resolution of theHXPS measurements was set to 230 meV. Due to the large information depth of HXPS
in this photon energy region, electronic as well as chemical states of the samples wereobservable down to as deep as ca. 20 nm from the sample surface. Both the valence bandand core level spectra were measured in C- and A-phases of the thin film samples. The
carbon capping layers did not manifest in the observed spectra due to its smaller
photo-ionization cross sections comparing to those of Ge, Cu and Te.
ResultsandDiscussions

Figure 1 shows whole valence band spectra (a) and near Fermi edge spectra (b) of Aand C-phase films. The whole valence band spectra exhibit similar shapes each other,with Te 5s, Ge 4s, and mixed bands among Te 5p,Ge 4p, and Cu 3d, in the binding
energy regions of 15-10, 10-7.5, and 7.5-0 eV, respectively. However, several apparent
differences are recognized as mentioned below. The Ge 4s band becomes more
distinguishable in C-phase than in A-phase. The topmost band (7.5-0 eV) shape isdifferent between A- and C-phases. The width is broader in A-phase than that inC-phase. Intensity distribution of three peaks and a shoulder is also different. Figure 1(b) verifies that C-phase samples show finite density of states at the Fermi level,
whereas the Fermi level locates just above the onset of the valence band in A-phasesamples. This coincides with the resistivity behavior mentioned above.
A distinguishable difference was found in the Ge 2p spectra. In C-phase, the Ge 2pspectrum showed a double-peak structure with almost the same intensities, while thehigher binding energy component became stronger than the lower binding energy one inA-phase. On the other hand, the Cu and Te core spectra did not show significant
changes (not shown in the figure).
The differences between A- and C-phases observed in the present experiments areapparently dissimilar from that in Si, which is a typical tetrahedral bonding material.
It is strongly suggested that the change in s-p hybridization of the Ge-Te bonds is toplay a key role in the A-C phase change in Ge1Cu2Te3.
We are going to verify the reversible phase change in this material, expecting thatpossibility as a new candidate of materials for the phase change memory devices.

Conclusion
The present HXPS results on the Ge1Cu2Te3 sputtered films revealed that a sudden
electrical resistivity decrease due to annealing of as-deposited films at 250 C is caused

by the valence band structure change. The change also accompanied by change in Ge
chemical bonding states, which suggests a kind of Ge bonding coordination transition
takes place at the amorphous to crystalline phase change in this material.

Acknowledgements
The authors are grateful to HiSOR, Hiroshima Univ. and JAEA/SPring-8 for thedevelopment of HX-PES at BL15XU of SPring-8.

References[1] Y. Sutou et al., Proc. of 2010 Spring Meeting, MRS.
[2] K. Kobayashi, Nuclear Instruments and Methods in Physics Research A 601 32-47.

'As-depo_VB'(a)Anneal_VB

(b)

(b)

(a)

Anneal_VB_top'as-depo_VB_top'

2000

12Photoelectron Intensity (a.u.)x103 Photoelectron Intensity (a.u.)

14108641500

1000

500
200-15-10-50-1.0

Binding Energy (eV)Fig. 1-0.5

0.00.5

1.0Binding Energy (eV)(a) Whole valence band spectra of as-deposited (red) and annealed (blue)

GeCu2Te3. (b) Spectra near the Fermi level. X-ray energy: 5.95 keV.

Photoelectron intensity (a.u.)

1.4x10'as-depo_Ge2p'
Anneal_Ge2p6

1.2

1.0

0.8

Fig. 20.6

Ge 2p3/2 spectra of as-deposited (red) and

0.4

annealed (blue) samples.
-1245

-1240

-1235-1230-1225

Binding Energy (eV)

-1220

-1215

X-ray energy: 5.95 keV.