S42

Electronic Structure of the Novel Phase-Change Material GeCu2Te3
K. Kobayashi and M. Kobata,
Quantum Beam Science Directory, Japan Atomic Energy Agency
Kouto 1-1-1, Sayo-cho, Sayo-gun, Hyogo Japan
Koba_kei@spring8.or.jp
J. Skelton and S. Elliott,
Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, UK.
Y. Saito, Y. Sutou, and J. Koike
Department of Materials Science, Graduate School of Engineering, Tohoku University,
6-6-11 Aoba-yama, Sendai 980-8579, Japan.
Key words: Phase Change, GeCu2Te3, HXPES, Ab initio molecular-dynamics simulation.

ABSTRACT
GeCu2Te3 (GCT) has been demonstrated as a potential phase-change material (PCM) for next-generation non-volatilememories by Sutou et al.[1-3] The phase transition takes place to a tetrahedrally-bonded crystal, a very different
geometry to the octahedrally-bonded cubic structures of the conventional phase-change materials of GeTe-Sb2Te3(GST) pseudo-binary alloys and related compounds. GCT was found to show a higher density and higher opticalreflectivity in the amorphous phase than in the crystalline phase, also in contrast to the conventional PCMs. An
electronic and chemical structure study of the amorphous and crystalline phases of GCT has been carried out by bulksensitive hard X-ray photoelectron spectroscopy (XPS) using a laboratory XPS system with monochromatic Cr Kα(5.4 keV) excitation. The results are compared with a computer-simulational study of CGT, which has been conductedvery recently by the Cambridge University group. The simulation has been successful in giving consistent results in
respect of the atomic-level structure and physical properties. The results also lead to hypotheses to account for thehigher amorphous-phase density and optical reflectivity. Regarding a comparison with the HXPES results, thedominant features of the experimental valence-band spectra are well reproduced using the calculated partial density ofstates, except near the Fermi-level region. The simulation also shows the existence of two charge states of Ge atoms in
the unit cell, consistent with the observed splitting of the Ge 2p3/2 spectra into two dominant components.
Introduction
As a novel PCM, the ternary alloy GeCu2Te3 (GCT) has been shown to have much promise by Sutou and hiscoworkers[1-3]. They have shown that GCT possesses better data-retention properties and reduced amorphisation
power requirements, together with a relatively short crystallization time compared to GST (GeTe)x(Sb2Te3)1-x [3]. The
density of amorphous GCT (aGCT) is greater than the crystalline form (cGCT), and a volume change of +2–4%
between the phases compares favorably to the -6.5% reported for GST films. This anomalous density behavior isexpected to improve device cycling endurance [2,3]. Furthermore, it has recently been shown that GCT, incombination with other PCMs, can be used to form layered multi-bit devices that can be switched between any of theirresistance levels in a single write operation [4].
One of the most interesting features of GCT that it has 24 s-p electrons per molecule (4 s-p electrons per atom
on average). Actually, it was verified that cGCT adopts a “chalcopyrite-like” structure with 4-fold sp3-like bonding in
the crystalline phase. This fact means that GCT is a different family of PCM from that of GST and related compounds,
where the average valence electron numbers are approximately 5. Thus, a resonance-bonding picture, which isconsidered to be effective in the GST family [5], is not applicable to an understanding of the PC mechanism in GCT.
In order to elucidate the PC mechanism and the origins of the above-mentioned anomalous properties, hard Xray photoelectron spectroscopy (HXPES) investigations of the as-grown and annealed GCT films have been
conducted using a laboratory HXPES system with Cr Kα (5.4 keV) excitation [6,7] by K. K and M. K at SPring-8.
DFT-based theoretical calculations to elucidate the relation between the atomic structure and electronic and opticalproperties of both phases have also been conducted by J. S and S. E at Cambridge University [8].
In this paper, we present experimental and theoretical results to discuss the electronic structure and chemicalbonding of cGCT and aGCT.

Normalized Intensity

Normalized Intensity

Resistance (Ohm)Methods
Stoichiometric, Ge-rich, and Cu-rich GCT films were deposited at Tohoku University by co-sputtering using
GeTe, and CuTe targets onto Si substrates covered with 20 nm of SiO2. The compositions of the films werecontrolled by the microwave-power ratio for the two targets, and were determined by ICP and SEM-EDXmeasurements. The electric resistance of the films was measured in the course of the annealing process in order tosee the phase changes from as-deposited amorphous to crystalline phases, which accompany a sharp drop of theresistance. Crystalline structures were determined by XRD measurements, and optical-reflectance measurements
were performed in a wavelength region between 250 and 1000 nm.
1.E+07
For HXPES measurements, samples were capped by C layers
Ge-rich 234
ofafew
nm soon after the GCT film deposition by sputtering.
Ge-rich 250
1.E+06
HXPES
measurements
were done using a laboratory system, with
Ge-rich 300
CrKα(5.4keV)
monochromatized
X-rays for excitation (HEARP
1.E+05
Ge-rich 350
Lab)
with
anenergy
resolutionof0.55
eV [6,7]. The information
Ge-rich 400
1.E+04depth
isestimatedtobeca.
10
nm.
Thus,
surface-oxidation-freeSingle
phase 230
spectraoftheGCTfilms
wereable
tobeobserved
through the C
1.E+03Singlephase 250capping layer. Valence-band Ge 2p3/2. Cu 2p3/2, and Te 3d5/2
1.E+02
spectra were measured. The Fermi edge of an Au plate wasmeasured as an energy reference.
1.E+010100
200300400
Ab initio molecular-dynamics (AIMD) simulations wereAnnealing temperature (C°)
carried out using the VASP code. A 96-atom model of cGCT, witha composition Ge16Cu32Te48, was created by tiling the unit cellFig. 1 Resistance vs. annealing temperature for GCT
reportedin [10], and the atomic positions were energy-relaxedfilms.
within the fixed-size simulation cell. Models of aGcT weregenerated via an MD melt-quench procedure previously used with
0.7
Ge2Sb2Te5 [9]. Initial configurations in cubic supercells containing(a) Ge2p3/2
0.696, 144, and 192 atoms, at the experimental crystalline density ofcrystalline
0.5
6.13 g cm-3 [10], were randomised by heating at 3000 K for 20 ps.
0.4
Following this, the models were maintained in liquid states at 1200
0.3
K for 40 ps, before being quenched to 300K at a rate of -15 K ps-1,0.2
which invariably led to amorphisation. The PBE functional [11],0.1
amorphous
0.0with PAW pseudopotentials, was used [12,13], including the semi1224 1222 1220 1218 1216 1214 1212
core Cu 3p and Ge 3d electrons. Spin-polarized calculations wereBinding Energy (eV)performed to ensure that any unpaired electrons on the Cu atoms(b) Cu2p3/2
1.4
would be treated correctly.
1.2
1.0
0.8
0.6
0.4

crystalline

Normalized Intensity

Results and Discussion
As shown in Fig. 1, the electric resistance of the as-depositedfilms steeply decreases in a narrow temperature region between
amorphous
210 C° and 240 C°, due to crystallization. The Cu-rich films and0.2
0.0
Ge-rich films show lower and higher phase-change temperatures
936
934
932
930
928
compared to the stoichiometric films, respectively. The resistivity
Binding Energy (eV)values at room temperature before and after annealing are higher in
1.2(c)Te3d5/2
Ge-rich films and lower in Cu-rich films, and those of the1.0
stoichiometric films are in-between. In the following, we mainly
crystalline
0.8discuss the HXPES and theoretical calculation results on
0.6
stoichiometric films.
0.4
In Fig. 2 (a), Ge 2p3/2, (b) Te 3d5/2, and (c) Cu 2p3/2 coreamorphous
0.2level spectra of stoichiometric cGCT and aGCT films are shown.
0.0Open circles are experimental data points. Curve fit results using
578
576
574
572
570
568
Voigt functions are shown by solid (total) and dotted curves
Binding Energy (eV)(components) for each spectrum. The Ge 2p3/2 spectrum consists oftwo dominant components with a trivial third component inFig. 2 Ge2p3/2, Cu2p3/2, and Te3d5/2 spectra ofbetween. The high binding-energy component is slightly weaker,cGCT and aGCT.
whereas the low binding- energy component is stronger in theamorphous phase (aGCT) compared to the crystalline phase (cGCT). The Te 3d5/2 spectrum of cGCT also shows
two component features, with a higher intensity in the lower binding-energy component with an intensity ratio of

1.85:1. The Cu 2p3/2 spectrum essentially consists of a single
component, although an asymmetry in the lineshape increases
slightly in the cGCT film compared to that of the aGCT fim.
There are two Te environments in cGCT, that is, the Tecentered tetrahedra TeGeCu3and TeGe2Cu2. The population
Ge2
ratio of the two types of tetrahedra is 2:1. Thus, the twoGe1
component feature of the Te 3d5/2 spectrum in cGCT is naturally
understood due to the differences in the Te environment. Actually,(a)(b)
the observed intensity ratio is estimated as 1.85:1.1, very close tothe expected ratio. However, the appearance of the dominant twocomponents in the Ge 2p3/2 spectrum in cGCT is unexpected.
Cu
Fig. 3 shows calculated Bader charges of Ge, Te and Cu
Te2atoms in the model of cGCT. It is very striking that, while all
sixteen Ge atoms have the same (positive) charge in the unrelaxed
crystal model, energy relaxation reduces the average charge andTe1(c)(d)
causes the charges to split equally into two groups (Figure 3a). The
Fig. 3 Bader charge of (a)Ge, (c)Cu, and (d)Te.,
two types of Ge atom undergo small displacements from their idealand (b) site distribution of two different Ge charge
crystallographic sites in a very similar manner to a Peierls transition,states in unit cell.
and the distortion lowers the energy by 90 meV/atom. The unit cell
contains one of each type of Ge atom, which in the bulk systemform layered arrangements coincident with the crystallographic axesof the unit cell, as schematically shown in Figure 3b. It should be
StoichiometricaGCT
noted that this charge splitting was only observed when a “hard”
StoichiometriccGCTpseudopotential including the Ge 3d electrons was used, suggestingthat it is related to a variation in the Ge core electronic
Ge 4s
configuration.
GeTeNo similar splitting of the Cu or Te charges was observed onrelaxation. The Cu atoms have a lower positive charge than that of15
1050
either type of Ge, and the Te charges form two groups,Binding Energy (eV)corresponding to the two Te environments, with the TeCu3Ge sites
having a more negative charge than TeCu2Ge2 sites, consistent withthe experimentally observed splitting of the Te 3d5/2 spectra
Fig. 4 Valence band spectra of cGCT, aGCT, and GeTe
mentioned above.
Fig. 4 shows valence-band spectra of as-deposited (aGCT)
and 350 C° annealed (cGCT) films. A valence-band spectrum of
crystalline GeTe is also shown as a reference. Three dominant(a)(c)
bands, that is, Te 5s at around 12 eV, Ge 4s band at around 8 eV,
and the topmost band in the range 0-7 eV, of which dominant
contributions come from Cu 3d and Te 5p states, are clearly
resolved. Comparing with the GeTe spectrum, the Cu 3d
contribution is recognizable at around 4 eV. The relative intensity of
the Ge 4s band to the Te 5s band is weaker in GCT, due to the(b)(d)
smaller relative Ge concentration. Considerable broadening isdistinguishable in aGCT compared to cGCT. This is considered tobe due to increased disorder in the a-phase.
The cGCT and aGCT spectra are compared with the calculatedDOSs in Fig. 5(a) and (b). Sharp doublet peaks are manifest withbroad tails towards higher and lower binding energies. The higher
Fig. 5 Comparison between calculated DOS and experimental
binding-energy sharp peak with the tail corresponds to Te 5s-Cu 3dvalence bans spectra of (a) cGCT and (b) aGCT. Curve fit resultsbonding states. The tail towards lower binding energy is assigned toof valence band spectra using PDOSs introducing broadening andTe 5s-Cu 3d antibonding states. The sharp peak in between is thepeak shifts for (b) cGCT and (d) aGCT.
nonbonding state of Cu 3d, which inevitably appears due to anorientational mismatch of the Cu 3d orbitals relative to the tetrahedral sp3 bonds of Te atoms. The experimental spectra
show a much broader shape and the peak positions of these Cu 3d-Te 5s related states do not well coincide. The
disagreements of the peak positions are mainly due to the effect of the final holes in a narrow band, in which the final holeis strongly localized; thus, the outgoing photoelectrons are strongly affected by the Coulomb attraction, giving a deeper
binding energy in the observation than the calculated DOS. We corrected the positions of each peak in the partial density ofstates to get a better coincidence, as shown in Fig.5 (c) and (d) for cGCT and aGCT, respectively. We also introduced
broadening to each PDOS in this fitting procedure. The Cu 3d band shows strong broadening, which suggests that the local
disorder around the Cu atoms is appreciable.
Ge2Cu

Ge1

Te

The atomic-level structural calculation on aGCT in the same
framework revealed a sharp peak at around 60° appearing in the Cu bond(a)angle distribution curve, indicating the presence of unusual threefold ringconfigurations. The calculated coordination numbers suggest that Ge and350 annealed
Te are fourfold-coordinated. Cu has a rather higher 6/7-fold coordination,around 35% of which, on average, comprises other Cu atoms. The
experimentally observed higher density of aGCT [2] was reproduced in thesimulations. The high density of aGCT may be related to the short Cu
Efbonds observed in the simulations. The tetrahedral coordination in cGCT
forces an “open” structure in which the lengths of Cu-Te and Ge-Te bonds
must be accommodated. Shorter Cu bonding can occur in aGCT, allowing
as-depositedcompact structural units, such as threefold rings, to form and leading to ahigher density.
1.5
1.0
0.5
0.0
-0.5
-1.0
The optical reflectance was calculated through the dielectric function. TheBinding Energy (EV)results well reproduced the experimental results in which the reflectance is
25x10
higher in aGCT than in cGCT in the range from 250 nm to 1000 nm.
20(b)There is a disagreement in the DOS behavior near Fermi level. Fig. 6(a)15Ge‐richstoichiometricshowsthe annealing temperature
10dependencesof the valence-band spectral shape near the Fermi level ofCu‐rich
5stoichiometricfilms. It is notable that the density of states, DOS(Ef), in a0narrow region around the Fermi level increases as the annealing temperature
05101520
25x10DOS(Ef) (a.u.)increases. This is not reproduced in the theoretical calculations, as seen in Fig.
5(a) and (b), in which DOS(Ef) in the amorphous film is larger thanin theFig. 6 (a) valence band spectra near Fermi level.
crystalline film.
(b) Conductance vs DOS(Ef)
Finally, we plotted the conductance at room temperature (Ca) for Curich, stoichiometric, and Ge-rich annealed films as functions of DOS(Ef), as shown in Fig. 6(b). In Cu-rich films, Ca is
proportional to DOS(Ef), suggesting that the mobility is annealing-temperature independent. In the stoichiometric and Gerich films, clear mobility edges are recognized. This result may be related to the differences in the local disorder in thesefilms.
Normalized intensity(a) stoichiometric

Conductance (1/Ohm)-3-3Conclusions
HXPES experiments and ab initio calculations have been conducted to elucidate electronic structure and chemicalbonding states of the novel tetrahedrally bonded PC material, GeCu2Te3 (GCT). The results show that Ge atoms take twodifferent charge states in the crystalline phase, in a manner reminiscent of a Peierls transition. The whole valence-band
spectral shapes of cGCT and aGCT are well reproduced by calculated PDOSs when correcting the peak-position shift due
to final–hole effects. The ab initio calculation gives a higher density of states at the Fermi level in aGCT than in cGCT.
This disagrees with the experimental results, in which the spectral weight of aGCT at the Fermi level is less than that incGCT. Plotting the electric conductance after annealing as functions of the density of states at the Fermi level determinedby HXPES measurements revealed a mobility gap in stoichiometric and Ge-rich GCT films, whereas no gap is observed inCu-rich films. In order fully to understand the PC phenomenon in materials with tetrahedral bonding, further
investigations are needed.

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