I04(Invited)

Phase transition characteristics of GeCu2Te3 thin film
1Y. Sutou1, Y. Saito2, J. Koike1
Dept. of Materials Science, Tohoku University, Sendai, Miyagi, Japan
2
Nanoelectronics Research Institute, AIST, Tsukuba, Ibaraki, Japan
E-mail: ysutou@material.tohoku.ac.jp
ABSTRACT

Recently, the present authors have found that a GeCu2Te3 (GCT) amorphous film has a high crystallization
temperature of over 200˚C. In this study, we investigated the optical properties and the density change upon
crystallization of the GCT film. And also, the thermal conductivities of the amorphous and crystalline GCT films wereinvestigated. It was found that the amorphous GCT shows a higher refractive index than the crystalline GCT in thewavelength range of over 500 nm. Consequently, it was confirmed that the reflectance of amorphous state is higher
than that of crystalline state in the wavelength range of 300-1000 nm. XRR measurements showed that the densities ofthe amorphous and crystalline GCT are 6.06 g/cm3 and 5.80 g/cm3, respectively, which means that the amorphous
GCT is denser than the crystalline GCT. It was found by thermoreflectance technique that the amorphous GCT has
almost the same thermal conductivity as amorphous GST, while the crystalline GCT shows about two times higher
thermal conductivity than crystalline GST.
Key words: Ge-Cu-Te, volume change, optical properties, thermal conductivity

1. INTRODUCTION
Ge2Sb2Te5 (GST) is the most widely studied for PCRAM application because it shows a fast crystallization [1,2].
However, the GST needs a high reset current which causes high power consumption because of its high melting point(~ 630˚C). Moreover, the thermal stability of GST amorphous film is relatively low because of its low crystallization
temperature of about 150˚C. Therefore, it is necessary to develop new PCMs with a low melting point, maintaining ahigh crystallization temperature.
Recently, the present authors have found that a GeCu2Te3 (GCT) amorphous film has a high crystallization
temperature of over 200˚C [3]. The GCT memory device shows memory switching behavior and exhibits lower power
consumption for the reset operation than conventional GST memory device because of its low melting point of about500˚C [4]. It is noteworthy that the reflectance change in the wavelength range of 400-1000 nm and thickness change
upon crystallization of the GCT film are opposite to those of conventional phase change materials, such as GST andGeTe [5]. Moreover, it was found that the GCT film shows a fast phase change [6]. In this study, to understand thetransition behaviors of GCT film, we investigated the density change upon crystallization and the optical properties of
the GCT film. And also, the thermal conductivities of the amorphous and crystalline GCT films were investigated.
2. EXPERIMENTS
GCT films with 200 nm in thickness were deposited on SiO2/Si substrate by co-sputtering of GeTe and CuTe targets.
The film composition was determined by inductively-coupled plasma mass spectrometry and the deviation from thestoichiometric composition GeCu2Te3 was found to be less than 1% for all elements. The densities of amorphous andcrystalline states of the GCT films were measured by X-ray reflectivity (XRR). The optical properties wereinvestigated by ellipsometry. The thermal conductivity of the amorphous and crystalline GCT film was evaluated bythermoreflectance technique.

3. RESULTS AND DISCUSSION
3-1. Density change
It was found from XRR measurements that the densities of the amorphous and crystalline GCT are 6.06 g/cm3 and5.80 g/cm3, respectively. This indicates that the amorphous GCT shows a volume expansion of about 4.3% upon
crystallization. The value of the volume change is nearly equal to the increase in thickness of about 3.4% upon
crystallization measured with atomic force microscopy (AFM). Figure 1 shows the density change (thickness change)
upon crystallization in GCT, GST and GeTe films, where the crystalline film was heated to 350ºC. It is seen that theGCT shows a smaller volume change than the conventional PCMs. And, it is noteworthy that the volume change ofthe GCT shows an opposite sign to that of conventional PCMs. Very recently, it was suggested by ab initio moleculardynamics (AIMD) simulations that the high density of amorphous GCT might be related to shorter Cu bonds
compared to crystalline GCT [7].
10After heating to 350ºC

Density change (%)

864
20GCTGSTFig. 1 Density change upon crystallization of GCT, GST andGeTe films. The density change was defined by (dcrydamo)/damo. The result of the GST was taken from reference[8]. The result of the GeTe was estimated from the film
thickness change by AFM.

GeTe-2-4-6

3-2. Optical properties
Fig. 2(a) shows the refractive index n and extinction coefficient k for the amorphous (as-deposited) and crystalline(heated to 350ºC) GCT films. It is seen that the amorphous GCT shows a higher refractive index than the crystalline
GCT in the wavelength range of over 500 nm. Meanwhile, amorphous GST has been reported to show a lower n thancrystalline GST [1]. Based on the results in Fig. 2(a), the vertical reflectance of amorphous and crystalline GCT films
was calculated as shown in Fig. 2(b). It is seen that the reflectance of amorphous state is higher than that of crystalline
state in the wavelength range of 300-1000 nm. The reflectance of amorphous and crystalline state obtained by AIMD
simulations are also shown in Figure 2(b) [7]. It is seen that the simulated reflectance spectra are almost the same

443.5

3.5

crystal32.52amorphous1.5
10.5021.5crystal

10.5200

400600800 1000 1200 1400 1600
Wavelength (nm)

60

3n2.5

70

4.50Refl ectance (%)

amorphousk4.5

amo.-GCT(cal.)50

cry.-GCT(cal.)4030amo.-GCT(exp.)

cry.-GCT(exp.)

20
10300

400

500

600
700
800
Wavelength (nm)

900

1000

Fig. 2 (a) Refractive index n and extinction coefficient k for the amorphous (as-deposited) andcrystalline (heated to 350ºC) GCT films. (b) Reflectance of amorphous and crystalline GCT films
calculated based on the results of (a). The dotted lines indicate the reflectance simulated by AIMD [7].

tendency with the experimentally observed spectra, that is, the amorphous phase shows a higher reflectivity than thecrystalline phase across a 300–1000 nm wavelength range.
Figure 3 shows the relationship between reflectance change (optical contrast = ΔR/R = (Rcry-Ramo)/Rcry) and densitychange in various phase change materials [6]. It is seen that there is a clear correlation between optical contrast anddensity change. Generally, it has been suggested that a PCM showing a large optical contrast has a crystal structurewith a high coordination number Z, such as NaCl and cubic structures (Z=6) [9]. Meanwhile, a PCM like AgInTe2
with a small coordination number, such as chalcopyrite and wurtzite structures (Z=4) shows almost no optical contrast.
The GCT crystal is known to have a chalcopyrite structure (Z=4), but the GCT shows a relatively large opticalcontrast. Therefore, the present results indicate that rather than the coordination number, the density change shows abetter correlation with the optical contrast. It has been also suggested from AIMD simulation that the higher
reflectance of amorphous CGT may be due to a higher degree of electron delocalisation, in particular of the Cu delectrons [7].
Wavelength = 630 nm

0.6

Ge4Sb1Te5

0.5(Z=6)

Ge2Sb2Te5

0.4

Ag5.5In6.5Sb59Te29

0.3(Z=6)

Ge2Sb2Te5(Z=6)

0.2
⊿R/R

GeTe

(Z=6)

(Z=6)

AgSbTe2 (Z=6)

0.1
0Fig. 3. Relationship between reflectance change(optical contrast) and density change in various
phase change materials, where the optical contrast
was defined as ΔR/R = (Rcry-Ramo)/Rcry.

AgInTe2

-0.1

(Z=4)

-0.2

GCT-0.3

(Z=4)

-0.4
-5-4-3-2-1012 345Density change (%)

6789103-3. Thermal conductivity
Table 1 indicates the thermal conductivity of GCT and GST film obtained in this study. It is seen that the amorphous
phase shows a lower thermal conductivity than the crystalline phase in both films. The amorphous GCT has almost
the same thermal conductivity as the amorphous GST, while the crystalline GCT shows about two times higherthermal conductivity than the crystalline GST.
Table 1. Thermal conductivity of GCT and GST.
phase
GCT
GST

amorphous
crystalamorphous
crystal(hcp)

Annealing
temperature (ºC)
350
400

Thermal conductivity(W×m-1×K-1)
0.14
1.9
0.12
1.1

4. CONCLUSION
The amorphous GCT showed a larger density than the crystalline GCT and consequently, the GCT showed thevolume increase upon crystallization, which was the opposite behavior of the conventional PCMs, such as GST andGeTe. The GCT also showed an opposite sign in reflectance change upon crystallization to the conventional PCMs. Itwas found that there is a clear correlation between optical contrast and density change in various PCMs. Moreover, itwas found that the crystalline GCT showed a higher thermal conductivity than the crystalline GST.

ACKOWLEADGMENT
This work was supported by “Japan Science and Technology Agency” and KAKENHI (23360297).
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