Phase change behavior of Ge1Cu2Te3 thin films
Toshiya Kamada*, Yuta Saito, Masashi Sumiya, Yuji Sutou and Junichi Koike
Department of Materials Science, Tohoku University, 6-6-11 Aoba-yama, Sendai 980-8579*a9tm5309@s.tohoku.ac.jp
ABSTRACTThe electrical resistances on the crystallization process of co-sputtered Ge-Cu-Te films were investigated by atwo-point probe method. It was found that the amorphous Ge1Cu2Te3 film crystallized into a single Ge1Cu2Te3 phase,
which lead to a large resistance drop. The crystallization temperature of the Ge1Cu2Te3 amorphous film was higher
than the conventional Ge2Sb2Te5 amorphous film. The activation energy for the crystallization of the Ge1Cu2Te 3amorphous film was higher than that of the Ge2Sb2Te5 amorphous film. The Ge1Cu2Te3 compound with a low melting
point can be expected to be suitable as a phase change material for PCRAM.
Key words: Ge1Cu2Te3, low melting point, high crystallization temperature.
1. INTRODUCTION
Phase change random access memory (PCRAM) has been regarded as one of the promising candidates for thenext-generation nonvolatile memories because of low production cost and excellent scalability. Generally, a phasechange material (PCM) for PCRAM needs to possess a low melting point (Tm) for low power consumption, a highcrystallization temperature (Tx) for long data retention, and to show crystallization without phase decomposition for
excellent repeatability. Currently, Ge2Sb2Te5 compound (GST) is attracting considerable attention as a PCM. The
amorphous GST crystallizes first into a NaCl structure, and then transforms into a more stable hexagonal structure byfurther heating [1, 2]. However, the GST has a high melting point (~ 632 oC), and its crystallization temperature isrelatively low (~ 160 oC). According to International Technology Roadmap for Semiconductors 2007, the expected
operating temperature for PCRAM that guarantees a minimum operating data retention capability of 10-year is set to125 °C on and after 2011 for high temperature operation, e.g., automotive application. Therefore, a new PCM with alow Tm and a high Tx is expected to be developed. A Ge-Cu-Te system is known to be glassy alloys [3-6] andthermoelectric material [7]. In this system, there is a ternary calcogenide compound of Ge1Cu2Te3, which has a Tm of
about 520 °C [8]. Therefore, the Ge1Cu2Te3 compound is expected to be suitable as PCM from the viewpoint ofequilibrium phase stability. However, the amorphization and crystallization of the compound have not been studied. In
this study, phase change behavior and the corresponding electrical resistance change of the Ge-Cu-Te films depositedby RF sputtering were investigated.
2. EXPERIMENTS
Ge-Cu-Te films with 200 nm thickness were deposited on SiO2(20 nm)/Si substrates by co-sputtering of GeTe, Te andCu targets. The compositions of co-sputtered samples lie on Cu2Te– Ge33.3Te66.7 pseudobinary line. The compositionsof these films were Ge23.8Cu17.1Te59.1, Ge20.4Cu29.1Te50.5 and Ge16.7Cu33.3Te50 (hereafter, we called each sample
GCT17, GCT29 and GCT33), where the composition of the films was determined by transmission electron
microscopy (TEM) with energy dispersive spectroscopy (TEM-EDS). The composition of the GCT33 film almost
corresponds to that of Ge1Cu2Te3 compound film. In-situ electrical resistance measurement during annealing processof the film was performed by a two point probe method. X-ray diffraction (XRD) analysis was employed for thestructural identification of the film using an X-ray diffractometer with Cu-Kα radiation. TEM analysis was carried out
to investigate the microstructure and to identify crystalline structure.
3. RESULTS & DISCUSSION
Figure 1 shows XRD patterns of the as-deposited Ge-Cu-Te films. From XRD patterns, these films were confirmed tobe amorphous. Figure 2 shows the temperature dependence of the electrical resistance for the Ge-Cu-Te films at aheating rate of 10 °C/min. It is found that the electrical resistance of amorphous state decreases with increasing Cu
content and the as-deposited Ge-Cu-Te films show abrupt electrical resistance change by more than 102 ohm becauseof crystallization at about 200 oC. The temperature dependence of the electrical resistance for GST film is also shownin Fig. 2. The crystallization temperatures of the Ge-Cu-Te films are higher than that of GST film. The electricalresistances of the amorphous and the crystalline phases of the Ge-Cu-Te films are lower than those of the GST film.
And also, the difference of the electrical resistance between the amorphous and the crystalline phases of the Ge-Cu-Tefilms is smaller than that of the GST film. Figure 3 shows the crystallization temperatures of the Ge-Cu-Te films onCu2Te – Ge33.3Te66.7 pseudobinary phase diagram, where Tx was determined from the minimum value of thetemperature derivative of the electrical resistance. It is found that the crystallization temperature slightly decreases
with increasing Cu content. Figure 4 shows XRD patterns of Ge-Cu-Te films heated up to 300 ℃. It is seen fromthese patterns that phase decomposition occurred in GCT17 and GCT29. The GCT17 and GCT29 amorphous films
crystallize into three phases (Ge1Cu2Te3, GeTe and Te), and two phases (Ge1Cu2Te3 and Te), respectively, while
GCT33 crystallizes into a Ge1Cu2Te3 single crystalline phase. In this study, the activation energy Ea for crystallizationof GCT33 was estimated by Kissinger method. Figure 5 shows Arrenius plots of the dependence of heating rate oncrystallization temperature. From these plots, the Ea of the GCT33 is estimated to be 2.52 eV, which is higher thanthat of GST (Ea=2.33 eV). Moreover, it was estimated that the Ge1Cu2Te3 amorphous film shows 10-year lifetime at
the maximum temperature of 171 ºC. These results suggest that the amorphous GCT exhibits a higher thermal stability
than the amorphous GST. Therefore, the Ge1Cu2Te3 film with a low melting point and a high crystallization
temperature holds great promise as a new PCM for high temperature operation.
4. CONCLUSION
We investigated the electrical resistance change with the crystallization and the thermal stability of the Ge-Cu-Te
amorphous films. The results obtained were as follows:
1. The off-stoichiometric amorphous Ge-Cu-Te films crystallized into two or three-phase crystalline state, while theGe1Cu2Te3 amorphous film crystallized into a single Ge1Cu2Te3 crystalline phase. The crystallization temperatureof the co-sputtered Ge1Cu2Te3 film was about 40 °C higher than that of the Ge2Sb2Te5 (GST) film.
2. The electrical resistances of the amorphous and the crystalline phases of the Ge-Cu-Te films were lower than thoseof the GST film. Moreover, the difference of the electrical resistance between the amorphous and the crystalline
phases of the Ge-Cu-Te films was smaller than that of the GST film.
3. The activation energy for the crystallization of the Ge1Cu2Te3 amorphous film was higher than that of the GST
amorphous film.
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Heating rate 10ºC/min.

108
Si substrate

GCT17
GCT29

2030402θ(degree)

Resistance (Ω)

Intensity (a.u.)

107
106

Tx

105 GCT29104 GCT33

GST

103

GCT33

102

60

101

50

GCT1760140
220300
Heating Temperature (ºC)

380

Figure 2. Temperature dependence of the electricalresistance of GCT17, GCT29 and GCT33.

Figure 1. XRD patterns of as-deposited GCT17 , GCT29and GCT33.

Tx

Si substrate

Ge1Cu2Te3
GeTe321103602
202021

312
110102110

Te202400

310100600012

LiquidIntensity (a.u.)

Temperature (ºC)

800

GCT17
GCT29200
GCT33

0Cu2Te20

Ge33.3Te66.7

at.% Ge

Figure 3. Crystallization temperatures of GCT17,GCT29 and GCT33 on Cu2Te-Ge33.3Te66.7 pseudobinary
phase diagram.

ln{(dT/dt)/Tｘ2}-12-13-14-15-16

1.98

2

2.02

2.04

2.06

2.08

2.1

1/Tｘ × 1000
Figure 5. Kissinger plot for crystallization of GCT. The activationenergy Ea is estimated to be 2.52 eV.

3040
2θ (degree)

50

Figure 4. XRD patterns of GCT17, GCT29 and GCT33
heated up to 300 ºC.

60