Study of GeCu2Te3 by Hard X-ray Photoelectron Spectroscopy
K. Kobayashi1,2), M. Kobata1,3), Y. Saito4), Y. Sutou4), and J. Koike4)
1) Condenced Matter Science Devision, Japan Atomic Energy Agency, Kouto 1-1-1,
Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
2) Hiroshima Synchrotron Radiation Center, Hiroshima University, Kagamiyama
2-313, Higashi-Hiroshima City, Hiroshima 739-0046 JAPAN3) VG Scienta. Blue Building 4F, Hongo 2-19-7,Bunkyou-ku, Tokyo 11-0038, Japan4) Department of Materials Science, Graduate School of Engineering, TohokuUniversity, 6-6-11 Aoba-yama, Sendai 980-8579, JapanAbstract
The most interesting point in the novel phase change material Ge1Cu2Te3, recently
reported by Sutou et al. is that this material has 4 sp valence electrons per atom. The
material takes a sp3 bonded crystalline structure, thus very different from standard PC
materials such as GeTe-Sb2Te3 pseudo-binary alloy system conventional PC materials.
In order to clarify the PC mechanism in this material, hard X-ray photoelectron
spectroscopy studies on GCT films with Ge-rich, stoichiometric, and Cu-rich films havebeen performed using a monochromatic Cr Kα X-ray photoelectron spectrometer. The
results indicate coexistences of different charge states in Ge, Cu and Te both inamorphous and crystalline phases. The behaviors of these different components uponannealing depend on deviation from stoichiometry, suggesting controllability of the PC
in this material. A PC mechanism is proposed basing upon the present experimentalresults.

Introduction
Recently, Sutou et al. has discovered that GeCu2Te3 (GCT) exhibits reversible phasechange (PC) phenomena [1,2]. The most interesting point in this new class of PC
materials is that it has 24 sp valence electrons per molecule (4 sp electrons/atom) thus
takes a tetrahedral bonded chalcopylite related structure in the crystal phase as shownin Fig. 1. In the standard PC materials of GST and related compounds, average valence
electron number is approximately
CuTe
GeTe

5/atom, corresponding to s2p3
valence electron configuration. Thes2 electrons form lone pair band,
and p3 electrons mainly sustain

CuTe

crystal bonding, resulting in theinherent instability in NaCl typeFig. 1 Crystal structure of GeCu2Te3

cubic crystal lattice. Mechanism ofreversible PC phenomena in these

average 5 valence electron family have been discussed in relation with thischaracteristic valence electron configuration.
In case of GCT, however, tetrahedral s-p hybridized bonds are too rigid, consequently
not favorable for the PC. This implies that the PC in this material requires anotherfreedom in the crystal bonding which brings flexibility in the bond configurations.
In order to investigate the problem, we have performed hard x-ray photoelectron
spectroscopy ( HAXPES/HXPES) to observe variations in valence band and core levels ofGCT samples with various compositions (deviations from stoichiometry) and annealing
temperatures. Among those spectra, core level spectra of Ge show distinctive changes
due to deviations from stoichiometry and annealing, suggesting that Ge atoms play a
key role in the PC mechanism in GCT.

Experimental

Stoichiometric, Ge-rich, and Cu-rich films were deposited at Tohoku University byco-sputtering with GeTe, and CuTe targets onto Si substrates covered with 20 nm of
SiO2, and then C capping layers of few nm were deposited soon after the sputtering. Thecompositions of the films were controlled by microwave power ratio for the two targets.
The compositions were determined by ICP-MS in several samples as shown in Table I.
The XRD measurements indicate that all the films are in single phase, in spite ofconsiderably large deviation from stoichiometry in Ge-rich and Cu-rich films. Samples
Table I作製条件

Composition (at.%)

備考CuGeTeSingle phase filmby co-sputtering

33.6

17.4

49.0

ICP-MSGe-rich filmby co-sputtering

31.7

19.1

49.2ICP-MS

Cu-rich filmby co-sputtering

37.9

13.1

49.0

ICP-MS

were then annealed at various temperatures. Electric resistances of these films werefound to show typical phase change behaviors as shown in Fig. 2. That is, the resistancesteeply decreases at a temperature range between 200-250 C due to amorphous tocrystalline phase change. Further increase of annealing temperature results in slightdecrease of resistance. The crystalline phase samples then show metallic behavior withpositive temperature coefficients of the resistances.

Fig. 2 Temperature dependence of Electric resistanceThe hard X-ray photoelectron spectra of the samples were measured by HEARP Lab(High Energy Angle Resolved Photoelectron Spectrometer for Laboratory Use) system,
which consists of a monochromatic Cr Kα (5.4 keV) source, a wide acceptance objective lens,and a VG Scienta high energy analyzer. [3,4]. Total energy resolution is 0.55eV. The information
depths of the measurements are estimated ca. 10 nm, due to the high kinetic energy of photoelectrons.
The C capping layers give no detectable traces in the valence band and core levels spectra because ofthe smaller photoionization cross sections comparing to those of Ge, Cu and Te.

Results
Figure 3(a) shows annealing temperature dependence of valence band spectra forstoichiometric films. A remarkable change is recognized at the topmost band, whichspans from Fermi level (Ef) to about 6 eV below Ef. Dominant contribution to this band

Normalized intensity(a) stoichiometricCu3d+Te5s

Normalized intensity

Te5s

Ge4s

(b) as-deposited
amorphous

Ge 4s
Cu-rich15105050Binding Energy (eV)

300C250C
230C

Normalized Intensity

350C(c) 350 C annealed
crystalline

Ge 4sCu-richas deposited
1515105010Binding Energy (eV)Binding Energy (eV)Fig. 3 Valence band spectral of GCT.
(a) Annealing temperature dependence for stoichiometric film. (b) and (c)dependence on deviation from stoichionetry.
comes from Cu 3d, and somewhat smaller contribution comes from Te 5d. The Ge 4p
participation is negligible. Two broad peaks are seen at 1.4 eV and 3.7 eV binding
energies (Eb) in the as-deposited amorphous film. Upon annealing at 230 K, the high Eb
peak becomes sharp and its intensity increases. The Ge 4s band which peaks at aroundstoichiometric

Ge2

Ge3

Ge1

350C

(a)

Ta=350C(b)

Cu-richNolmarized intensity

stoichiometric

300C
Ge-rich
1224

1222

250C

1220
1218
Binding Energy (eV)

1216

1214

as deposited
Cu-rich230C(c)stoichiometricas deposited
Ge-rich

1224

1222

1220

1218

Binding Energy (EV

1216

1214

1224

1222

1220

1218

Binding Energy (eV)

1216

1214

Fig. 5 (a) annealing temperature dependence of Ge2p3/2 spectra for stoichiometricfilm. Dependence on deviation from stoichiometry for (b) 350 C annealed and (c)as deposited films.
7.5 eV becomes narrower and peak height increases. The further increase of annealingtemperature does not change the spectral shape distinctively. The valence band spectraof as-deposited and 350 C annealed samples of stoichiometric, Ge-rich and Cu-rich films
are compared in Fig. 3(b) and 3(c), respectively. The Ge 4s band shows a difference due
to differences in Ge composition. However, the rest of spectral shape shows no otherdistinctive difference. The recognizable spectral shape variation seems to complete
already at lowest annealing temperature of from 217 C (for Cu-rich sample) to 234 C(for Ge-rich sample); however, to see more closely, we find the state density at around Ef
clearly increases by annealing in Fig. 4(a). Figure 4(b) shows annealing temperaturedependences of integrated density of states DOS(Ef) in the binding energy range of -0.5
eV–0.5 eV at around Ef for stoichiometric, Ge-rich and Cu-rich films. The increasing
behavior of DOS(Ef) are consistent with electric resistance decreases in Fig. 2.
Annealing temperature dependence of Ge 2p3/2 spectrum in stoichiometric film is shownin Fig. 5. The Ge 2p spectrum consists of 3 components denoted as Ge1, Ge2, and Ge3 inthis figure. Ge1 appears at 1217.9 eV, which correspond to Ge 2p3/2 binding energy ofcovalent Ge (1218.0 eV in GeTe). Ge2 shift about 2.2 eV toward higher binding energy,
thus considered to come from Ge atoms in ionic bonding states. Weaker Ge3 is inbetween. Upon annealing, Ge1 apparently decreases whereas Ge2 increases.
Figure 5(b) and 5(c) show comparisons of Ge2p3/2spectra for Ge-rich, stoichiometric, andCu-rich samples of as deposited and 350 C annealed films. The Ge2/Ge1 ratio increaseas Cu/Ge composition ratio increases. Annealing also causes increase in Ge2/Ge1 ratio.
Evolutions of Cu2p3/2 and Te3d5/2 with annealing for stoichiometric samples are shownin Fig. 6(a) and (b), respectively. Both the Cu and Te spectra consist of two components,
as denoted Cu1 and Cu2, and Te1 and Te2 in this figure. Cu2 component is very weak in
as deposited film, whereas Te2 is somewhat stronger. Annealing causes distinctive
enhancement of Te2 component. The variations of these components are summarized inFig. 7, showing the relative intensities of each component by bars.

0.8
0.6
0.4
0.2
0Ge1
Ge2
Ge3

0.8
0.6
0.4
0.2
0

a_Ge-rich a_stoichio a_Cu-rich

Ge1
Ge2
Ge3
c_Ge-rich c_stoichio c_Cu-rich

1.5

11

Cu1
Cu2

0.5
0Cu1
Cu2

0.5
0a_Ge-rich a_stoichio a_Cu-richC_Ge-rich c_stoichio c_Cu-rich

1Te1
Te2

0.5

0a_Ge-richa_stoichioa_Cu-rich

0.8
0.6
0.40.20Te1
Te2c_Ge-rich_ c_stoicheo c_Cu-rich

As deposited

Annealed at 350CFig. 7 Relative intensity variations due to annealing.
Discussion
The present results clarify followings
1) Variation of valence band spectral shape is already completes at 217–230 C, where
the electric resistances are on the half way of steep decreases. As valence bandDOSs are considered to be determined dominantly by local bonding characteristics,
the present results suggest annealing at rather low temperature already results inbond reconstructions, which is connected to PC.

2) The DOS(Ef) changes due to annealing are consistent with the resistance changes,
however, amount of DOS(Ef) increase is apparently not enough large to explain theresistance drops. Thus main cause of the high resistance in amorphous phase isconsidered to be due to randomness of atomic arrangements.
3) Ge 2p spectra evidence three kinds of Ge atoms. In amorphous (as deposited) film,
Ge2 concentration is less in stoichiometric films, whereas it is almost equal to Ge1in Cu-rich film. In the crystalline film annealed at 350 C, Ge1 and Ge2
concentration becomes almost equal, and Te2 increases up to about 40% instoichiometric film.
4) The relative compositions and their variations with annealing depends on
deviations from stoichiometry. This implies that PC behaviors in GCT can becontrolled by compositions of films.
5) As shown in Fig. 1, GCT crystalline structure consists of tacking of CuTe-GeTe-CuTe

CuTe

covalent

ionic

2sp3

(CuTe)1-2sp3(Ge4+Te2-)2+2sp3(CuTe)1-

s1+s2p4-eGeTes2p2+s2p4-eCuTe

s1+s2p4

Fig. 8 Schematic explanation of coexistence og
two charge state in Ge..
layered unit. In the formal charge framework, we can consider coexistence of two
different charge states for Ge atoms as schematically shown in Fig. 8. That is, sp3
like covalent bonding state and (Ge4+Te2-)2+ states. The 2+ charges are compensatedby 1+ charge in two CuTe layers. If we assume these + charges are delocalized inCuTe layers, we have Ge4+ and Te2- of the same concentrations in ionic states. This isconsistent with the present results. The Te2 appears a little higher binding energy
than Te1, although the binding energy difference is relatively small. Te1 isconsidered to come from covalent whereas Te2 come from ionic Te (negativelycharged). This discrepancy may be due to difference in relaxation energies.
6) Existing XRD measurements [5] seem not to support coexistence of the two differentGe charge states in crystalline GCT. This may be explained by assuming random
distribution of the two charge states or dynamical alteration between the two chargestates.
7) The coexistence of the two charge state in Ge could offer a kind of freedom in therearrangements of atoms without introducing broken sp3 bonds, thus enables
reversible PC in this material.

Summary
We have investigated novel PC materials GCT by bulk sensitive hard X-ray
photoelectron spectroscopy using laboratory HXPES system at 5.4 keV excitationenergy with0.55 eV resolutions. Present results indicate that valence band change takes
place by annealing at low temperatures of 217–234 C. The behaviors of valence bandand core level spectra were found to depend on deviations from stoichiometry. Density ofstates at Fermi level increases upon annealing indicating increases in metallic nature
due to crystallization, however, amount of DOS increase alone does not explain the dropof electric resistances due to annealing. Ge 2p spectra indicate coexistence of neutral
and ionic Ge both in amorphous and crystalline phases. Te and Cu spectra also exhibit
coexistence of two different states in both Te and Cu. These coexistences of different
charge states may introduce a kind of freedom in atomic arrangements in GCT, whichenables fast reversible phase change.

References[1] Y. Sutou, T. Kamada, M. Sumiya, Y. Saito, J. Koike, Acta Mater. (2012) 872.
[2] T. Kamada, Y. Sutou, M. Sumiya, Y. Saito, J. Koike, Thin Solid Films, 520 (2012) 4389.

[3] M. Kobata, I. Piš, H. Iwai, H. Yamazui,H. Takahashi, M. Suzuki H. Matsuda, H. Daimon

and K. Kobayashi, ANALYTICAL SCIENCES, 26 (2010) 1.
[4] Kobata, M., Píš, I., Nohira, H., Iwai, H., Kobayashi, K., Surface and Interface Analysis 43 (2011),
pp. 1632-1635[5] G. E. Delgado, A. J. Mora, M. Pirela, A.Velasquez-Velasquez, M. Villareal, B.J.Fernandez,,
Physica Status Solidi, Sectio A: Applied Research 201(13), (2004), 2900-2904.

r Parth6* und Jorge Garin