AFM Study for Electric Characteristics ofPhase-Change Superlattice Film
Takahiro ODAKA1, Toshimichi SHINTANI1, Takahiro MORIKAWA1, Junji TOMINAGA1,Hiroshi ITOH2
Green Nanoelectronics Center1
National Institute of Advanced Industrial Science and Technology
Research Institute of Instrumentation Frontier2
National Institute of Advanced Industrial Science and Technology

ABSTRACTThe phase-change property for the GeTe/Sb2Te3 phase-change superlattice film was measured byusing conductive AFM. The superlattice film was successfully set and reset after applying constantvoltage pulse with the metal-coated AFM tip. The superlattice film, however, was reset by a slow
cooling pulse, which is not the case for Ge2Sb2Te5. It implies that the mechanism of phase change
in a superlattice film is different from that in Ge2Sb2Te5. We also measured the volume changes
after applying a set programming pulse by obtaining topographic images. The volume changes
after reset of Ge2Sb2Te5 and the superlattice film were 20% and <5%, respectively. It implies that aphase-change superlattice device is damaged less by stress in recording because of less volume
change in phase change.
Key words: phase-change, superlattice, conductive AFM, write properties, volume change
1. INTRODUCTION
Phase-change materials have already been installed as recording films in rewritable optical disks
and semiconductor memory devices. Recently it was found that the atomic switching phenomenonof Ge atoms by changing coordination number of Te around a Ge atom in the phase-change film ofGeTe/Sb2Te3 pseudo-binary system[1], and have been promoting research of phase-change
superlattice material for developing low-power-recording memory devices[2][3].
The microscopic phenomena of the phase-change superlattice film in phase change are not wellknown. We have constructed the conductive AFM system to investigate the detailed properties ofphase-change superlattice materials. Usage of conductive AFM brings us the advantage that it
enables us to obtain some macroscopic and microscopic parameters simultaneously andnondestructively[4].
In this work, we used conductive AFM for measuring the electric properties and the volume
change of the GeTe/Sb2Te3 phase-change superlattice film. We also measured those parameters of
Ge2Sb2Te5 for reference. The properties of the phase-change superlattice film will be discussed bycomparing the experimental results of these materials.
2. EXPERIMENTS
Fig. 1 shows the experimental setup for measuring electric properties of phase-change films byusing AFM. A Rh coated AFM tip was equipped and contacted on a phase-change film. The tip

radius of Rh coated AFM tips used for this work was 100 nm. Electric pulses from the pulse
generator were applied to sample surface by the AFM tip. Electric currents in a phase-change film
were measured through I-V converter.
The scan area of the AFM tip on phase-change films during applying voltage pulse was 1 nmsquare. The contact force of the tip to a sample was fixed to 300 nN. The recording voltage pulse
width was varied from 200 to 400 nsec. The rise time and fall time of this system were both 100
nsec. The read-pulse of 0.5 V and 500 nsec was applied to a sample after 1 sec of the recordingpulse. The maximum voltage of the pulse was 8 V in this system.
The structure of the phase-change superlattice film sample used for these experiments is asfollows; 10 cycles of [GeTe_1 nm / Sb2Te3_2 nm] films are deposited on Sb2Te3_10 nm / TiN film
(electrode) by sputtering process. A Ge2Sb2Te5 film was also used for reference.
3. RESULTS & DISCUSSION
Set and reset properties are measured for both Ge2Sb2Te5 and superlattice films. The Ge2Sb2Te5film was set at 5 V (not shown), but could not be reset by applying from 200 nsec to 400 nsec
pulse. We can consider two assumptions for this reason. The first one is the recrystallization aftermelting because of long fall time of reset pulse, that is to say, the slow cooling condition. The
second one is that the power was insufficient to heat the film above the melting point. In the phasechange device,
On the other hand, the superlattice film was successfully set and reset both at 200 nsec and 400
nsec pulses. Fig. 2 shows set (Fig. 2 (a)) and reset (Fig. 2 (b)) voltage signals for phase–change
superlattice film measured by applying 200 nsec pulse. Almost the same result was obtained also incase of 400 nsec pulse (not shown). It implies that different mechanisms work in recording
between Ge2Sb2Te5 and phase-change superlattice material.
Volume changes of the films in recording were also measured for Ge2Sb2Te5 and GeTe/Sb2Te3
superlattice film by AFM. Fig. 3 and Fig. 4 show topographic images of Ge2Sb2Te5 (Fig. 3 (a), (b))
and the GeTe / Sb2Te3 superlattice film (Fig. 4 (a) and (b)) before and after applying set pulse. The
positions of the tip when applying set pulse are indicated by the arrow-marks in each figure. Noconvex is seen on the Ge2Sb2Te5 film surface in Fig.3 (a) whereas a convex appears in Fig.3 (b).
The depth of the convex is 10 nm (volume change of 20%) for Ge2Sb2Te5. As shown in Fig.4 (b),however, no convex is seen even after the resistance change for the superlattice film. We can put
two assumptions for this result. One is that phase change in the superlattice film induces no volumechange. Another is that the volume change in below the measurement limit. We can see the surface
roughness of about 2 nm in Figs. 4 (a) and (b). Although the reason for this roughness is not clear,
there is a possibility that the AFM tip roughened the surface because these AFM topographies wereobtained in the contact mode. In this case, we can insist that the volume change in phase change ofthe superlattice film is, at least, < 2 nm (volume change of 5%). Anyway, these experimental
results show that the volume change in phase change of the superlattice film is much smaller thanthat of Ge2Sb2Te5.
This small volume change is a great advantage of the superlattice film. It was pointed out that thefilm thickness of a phase-change material is reduced in phase change [5] and that the change of thestress around the phase-change memory recording film can cause significant reliability issues suchas endurance [6]. From these discussions, we can expect that adopting of the phase-change
superlattice materials will improve the endurance of phase-change devices.
4. CONCLUSION
Electric characteristics of the GeTe/Sb2Te3 phase-change superlattice film were measured byusing conductive AFM. Volume changes of the Ge2Sb2Te5 and the superlattice film accompaniedby the resistance change were also measured by obtaining topographic images before and after theresistance change. Ge2Sb2Te5 was not reset probably because of the slow cooling condition due tothe limited rise and fall time of the pulse. On the other hand, the superlattice film was successfully
set and reset even in the slow cooling condition. It implies that the mechanism of the resistancechange in the superlattice is different from that in Ge2Sb2Te5. Volume changes of the Ge2Sb2Te5
and GeTe/Sb2Te3 superlattice films measured by obtaining topographic images and 20%, less than
5%, respectively. These results imply that the stress of phase change memory of GeTe/Sb 2Te3
superlattice film is small in recording.
ACKNOWLEDGEMENT
This research is granted by the Japan Society for the Promotion of Science (JSPS) through the“Funding Program for World-Leading Innovative R&D on Science and Technology (FIRSTProgram),” initiated by the Council for Science and Technology Policy (CSTP).
REFERENCE[1] A.V. Kolobov, P. Fons, A.I. Frenkel, A.L. Ankudinov, J. Tominaga and T. Uruga,“Understanding the phase change mechanism of rewritable optical media”, Nature Materials
3,703(2004).
[2] J. Tominaga et al., Role of Ge switch in phase transition: An approach using atomicallycontrolled GeTe/Sb2Te3 superlattice. Jpn. J. Appl. Phys. 47, 5763(2008).
[3] R.E. Simpson, P.Fons, and A.Kolobov, T.Fukaya, M.Krbal, T.Yagi andJ.Tominaga:”Interfacial phase-change memory”, Nature Nanotechnology DOI: 10.1038/NNANO
2011.96(2011)[4]T. Gotoh, “Smallest (~10nm) phase-change marks in amorphous and crystalline Ge2Sb2Te5 films” J.
Noncrystalline Sorids 353, 1899(2007)[5]T.P. Leervad, J.Kalb, W.K. Njoroge, D. Wamwangi, and M. Wuttig, F.Spaepen, “Mechanicalstresses upon crystallization in phase change material”, J.Apl.Phys.Lett 79, 3597(2001).
[6]Y. S. Park, S. O. Ryu, K. J. Choi, S. Y. Lee, S. M. Yoon, B. and G. Yi, “Stress reduction duringphase change in Ge2Sb2Te5 by capping TiN film” J. Mater. Sci, 18, 1079(2007)

AFM tip

I-V converter

phase-change film

Oscilloscope+V-

Metal electrode (TiN)
Substrate: SiO2

Fig.1 Diagram of the experimental setup to measure current of phase-change films in recording byConductive AFM.
(a)set3(b)reset

3
2.5

2.5

write

22
1.5

1.5

read110.50.500-1012345-1012345-101234520
100-10-20-30-40-50-60-70

20100-10-20-30-40-50-60-70-10

12Time(μsec.)

345Time(μsec.)

Fig. 2 Time dependent signals of set (Fig. 2 (a)) and reset (Fig. 2 (b)) applied voltage andmeasured current for phase–change superlattice film.

1um(a)Before set
Fig.3

1um(b)After set

Topographic images of Ge2Sb2Te5 film measured by contact AFM before and after applying
set pulse.

1um(a)Before set

1um(b)After set

Fig.4 Topographic images of GeTe/Sb2Te3 superlattice film before and after applying set pulse.