Ge-Cu-Te films for phase change random access memory
Y. Sutou, Y. Saito, T. Kamada, M. Sumiya and J. Koike
Department of Materials Science, Graduate School of Engineering, Tohoku University, 6-6-11 Aoba-yama,
Sendai 980-8579, Japan.
Tel&Fax: 022-795-7338 E-mail:ysutou@material.tohoku.ac.jp
ABSTRACT
Simple memory devices with a Ge1Cu2Te3 (GCT) were fabricated to evaluate the set-reset switching
behaviors of the GCT film. Moreover, the crystallization processes of the amorphous GCT film wereinvestigated by differential scanning calorimetric measurements under non-isothermal conditions. It wasconfirmed that the GCT device showed typical memory switching behaviors as well as GST device.
Moreover, it was found that the local activation energy of the crystallization of the GCT amorphous film
decreases with increasing crystallization fraction, which indicates that the crystallization process of theamorphous GCT film has a multi-step mechanism.
Keywords: Ge1Cu2Te3, switching behavior, local activation energy, crystallization mechanism
INTRODUCTION
Phase change random access memory (PCRAM) has attracted much attention as a new class of non-volatilememories because of its low production cost and high scalability. Ge2Sb2Te5 (GST) film has been widely
studied as a phase change material for PCRAM because GST shows a fast crystallization speed and a goodreversibility between the amorphous and crystalline states [1,2]. However, GST has a low crystallization
temperature Tx of about 150°C and a high melting point Tm of over 600°C [1,3], which limits data retention
capability and causes high power consumption for reset operation, respectively.
Very recently, the present authors have found that a Ge1Cu2Te3 (GCT) amorphous film has a highcrystallization temperature of about 250˚C [4]. Since GCT compound has a low melting point of about 500˚C[5], the GCT is expected to be suitable for PCRAM applications. In the present study, we investigated theelectrical properties of the GCT films and the crystallization behaviors of the sputter-deposited GCTamorphous films.
EXPERIMENTS
Simple phase change memory devices of the GCT films as shown in Fig. 1 were fabricated using aconventional photolithography technique to measure the I-V characteristics and switching behaviors, where
those of the GST films were also fabricated for comparison. The GCT films were prepared by sputtering of a
Ge-Cu-Te alloy target. Differential scanning calorimetric (DSC) measurements were employed to investigate
the crystallization kinetics of the GCT films on the SiO2/Si substrate at various heating rates of 12 50ºC/min in Ar atmosphere. Film thickness for the DSC measurements was set to be 1500 nm in order toclearly detect an exothermic peak for the crystallization.
RESULTS AND DISCUSSIONFigures 2(a) and (b) show the plots of device resistance vs. voltage applied to the device for the GCT andGST devices, respectively. It is seen that the GCT device shows set-reset switching behaviors as well as theGST device. Figure 2(b) indicates that the Vreset for the GCT device are 5.7 V which is lower than that for theGST device of 8.1 V. The power consumption Wreset for the reset operation can be estimated by the relation of(Vreset)2/RPCM, and the Wreset for the GCT and GST devices are estimated to be 0.55 W and 0.61 W, where theelectrical resistance RPCM was obtained by averaging the electrical resistance of the set state of the devices.
The power consumption ratio WresetGCT/GST of the GCT to GST devices is calculated to be 0.9, which meansthat the GCT device achieves a 10% reduction in power consumption for the reset operation compared with
the GST device, which is mainly due to a lower melting point of GCT compound.
Figure 3 shows the fraction of crystallization x as a function of temperature at different heating rates, where
the crystallization fraction x was determined from the DSC curves [6]. Based on the results of Fig. 3, thelocal activation energy Ea(x) of the crystallization for different crystallization fractions x is estimated fromthe following equation, suggested by Ozawa for non-isothermal crystallization [7,8]:E ( x)log β = const. − 0.4567 aRT(1).

Here, β is heating rate, R is gas constant and T is temperature for a given value of x. Figure 4 shows the Ea(x)plots as a function of the crystallization fraction x. The results indicate that the Ea(x) decreases withincreasing crystallization fraction x. This variation of the Ea(x) may be due to the variation of the nucleationand growth processes during crystallization. The overall average activation energy Ea of the crystallization is2.78 eV, which is very close to the activation energy of 2.81 eV obtained from Kissinger plots using theresults of two-point probe resistance measurements [4]. The GST film also shows a similar variation of thelocal activation energy and the Ea was 2.08 eV. Consequently, the activation energy of the GCT film forcrystallization is seen to be higher than that of the GST film.
CONCLUSION
We investigated the electrical switching and the crystallization behaviors of the GCT film. The GCT film
showed a typical switching behavior and the GCT device has a 10% lower power consumption for the reset
operation than the GST device. The local activation energy Ea(x) of the GCT film decreased with increasing
crystallization fraction x, indicating a multi-step crystallization mechanism and the overall activation energyof the crystallization of the GCT amorphous film was higher than that of the GST film. Therefore, the GCTfilm with a low melting point, high thermal stability of the amorphous phase is a promising PCM for
PCRAM application.
ACKNOWLEDGMENT
This work was supported by “Japan Science and Technology Agency” and KAKENHI(23360297).
REFERENCES

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(a) GCT

Al (300 nm)
PCM (200 nm)

Device resistance / Ω104

SiO2 (100 nm)
TiN (100 nm)
SiO2 (20 nm)
Si substrate

Figure 1
Schematic cross-sectional view of fabricateddevice. PCM indicates phase change material.

Vreset103
102
set : 50 µs
reset : 5 µs

101 0123 4 5 6 7Applied voltage / V89(b) GST

Figure 2
Set-reset switching behaviors of (a) GCTand (b) GST devices as a function of pulse
voltage, the voltage pulse width for the set
and reset operations being set to 50 µs and5 µs, respectively.

Device resistance / Ω104
103

Vreset102
set : 50 µs
reset : 5 µs

1100123 4 5 6 7Applied voltage / V

810.8
0.7
0.6

0.30.2246

n242

30°C/min
40°
C/m
in
50°
C/m
in

0.4

20°
C/m
i0.5
12°
C/m
in

Fraction of crystallization x0.9

Figure 3
Plots of crystallization fraction x as afunction of temperature obtained from
DSC curves at different heating rates.

0.1
0234

238

250
254
Temperature / °C

258

262

266

Local activation energy Ea(x) / eV

4
3.8
3.6
3.4
3.2
3
2.8
2.6

Figure 4
Dependence of the local activationenergy Ea(x) on the crystallization
fraction x.

2.4
2.2
20

0.2
0.4
0.6
0.8
Crystallization fraction x19