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Solid-State Electronics 51 (2007) 950–954

Investigation of phase change Si2Sb2Te5 material and its applicationin chalcogenide random access memory

Ting Zhang a,b,*, Zhitang Song a, Bo Liu a, Songlin Feng a, Bomy Chen c

a Laboratory of Nanotechnology, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, Chinab Graduate School of the Chinese Academic of Sciences, Beijing 100049, China

c Silicon Storage Technology, Inc., 1171 Sonora Court, Sunnyvale, CA 94086, USA

Received 29 November 2006; received in revised form 6 March 2007; accepted 23 March 2007Available online 17 May 2007

The review of this paper was arranged by Prof. S. Cristoloveanu

Abstract

A wide band-gap phase change material Si2Sb2Te5 for chalcogenide random access memory application was investigated. Thematerial possesses a low threshold current from amorphous to polycrystalline state in voltage–current measurement, and shows a gooddata retention. Band-gap width of the amorphous and polycrystalline Si2Sb2Te5 are determined to be 0.89 and 0.62 eV by means ofFourier Transform Infrared Spectroscopy. Chalcogenide random access memory device with bottom electrode contact of 260 nm indiameter was fabricated and characterized. With a 50 ns width voltage pulse, RESET and SET voltage values of 3.2 and 1.4 V wereachieved.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Phase change; Chalcogenide random access memory; Si2Sb2Te5

1. Introduction

Chalcogenide random access memory (C-RAM) is con-sidered to be one of the most promising candidates for thenext-generation nonvolatile memory, because of its advan-tages of high density, low cost, non-volatility and radiationharden ability [1–6]. It is based on the reversible phasechange between amorphous and polycrystalline state withinthe chalcogenide alloys, by Joule heating. The amorphousstate has a high resistivity and the polycrystalline state hasa low resistivity, which correspond to ‘‘1’’ (RESET state)and ‘‘0’’ (SET state), respectively. Presently, Ge2Sb2Te5 iswidely adopted in C-RAM research and development due

0038-1101/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.sse.2007.03.016

* Corresponding author. Address: Laboratory of Nanotechnology,Shanghai Institute of Micro-system and Information Technology, ChineseAcademy of Sciences, Shanghai 200050, China. Tel.: +86 2162511070x8408; fax: +86 21 62134404.

E-mail address: tzhang@mail.sim.ac.cn (T. Zhang).

to its outstanding electrical performance and thermal stabil-ity. It is well known that low energy cost, good data reten-tion and high density are desired qualities in C-RAMdevelopment. To meet the requirement of high performanceC-RAM, great efforts have been made on the developmentof new phase change material. Various kinds of materialswere proposed, such as Sb–Te [7] Ge1Se1Te2 [8] Sb–Se [9]In–Se [10] etc. Si–Sb–Te serial materials, expected withlow energy cost and good data retention, are investigatedin our group [11,12]. Silicon, the most widely used semicon-ductor, is similar with Germanium in various properties.The atomic radii of Si and Ge are 1.46 and 1.52 A, respec-tively. Moreover, Si is with a band-gap width (Eg) of1.17 eV, which is wider than that of Ge (0.74 eV). Hence,Eg of Si2Sb2Te5 may be larger than that of Ge2Sb2Te5.According to Refs. [7,13] a sufficiently wide electronicband-gap is required in order to reduce threshold current.Thus, Si2Sb2Te5 may possess a more outstanding electricalproperty than Ge2Sb2Te5 do if Si2Sb2Te5 does have a larger

Fig. 1. Dependence of resistance on annealing temperature. Inset selectedarea electron diffraction patterns (a) as-deposited amorphous state and (b)polycrystalline hexagonal state after annealed at 400 �C for 1 min.

Fig. 2. XRD patterns of film annealed at 300 and 400 �C, respectively, for1 min. The films annealed at 300 and 400 �C are with fcc and hexagonalstructure, respectively.

T. Zhang et al. / Solid-State Electronics 51 (2007) 950–954 951

Eg. In order to prove it, new composition Si2Sb2Te5 mate-rial is prepared and investigated in this work, as well asthe Si2Sb2Te5 based C-RAM.

2. Experiments

200 nm-thick Si2Sb2Te5 film was deposited on oxidizedSi, W/Si and quartz substrates by co-sputtering Si, Sband Te targets. The background pressure and Ar gas pres-sure were 1 · 10�4 Pa and 0.21 Pa, respectively. Composi-tion of the film was determined by means of energydispersive spectroscopy (EDS, Oxford INCAEnergyequipped in Hitachi S4700) which suggests the materialis with stoichiometric composition of 2:2:5. Differentialscanning calorimetry (DSC), X-ray diffraction (XRD),temperature dependent resistance measurement, as well asvoltage–current (V–I) measurement, were carried out tostudy the physical and electrical properties of Si2Sb2Te5

film. Fourier Transform Infrared Spectroscopy (FTIR)was performed using Si2Sb2Te5 film sputtered on quartzsubstrate.

Data retention assessment relies on the evaluation of thetime-dependent resistance change of the amorphous phasechange film under a constant temperature annealing. Inorder to avoid oxidation in the measurement, phase changefilm was covered with 10 nm-thick SiO2 layer.

Si2Sb2Te5 based C-RAM device with a tungsten heatingelectrode of 260 nm in diameter was fabricated by 0.18-lmCMOS Technology. RESET/SET characteristics were car-ried out on a Cascade prove station employing a pulse gen-erator (Agilent 81104A) controlled by a computer. Theresistance of the memory cell was measured using Keithly2400C.

3. Results and discussion

3.1. Phase change of Si2Sb2Te5

To study the phase change property of the film, anneal-ing temperature dependent resistance was measured apply-ing four-point probe method. Samples were annealed witha rapid thermal processor in N2 atmosphere for 1 min atvarious temperatures before the resistance measurement.Dependence of resistance on annealing temperature wasshown in Fig. 1. The figure shows resistance of the filmdecreases with increase of annealing temperature withquick drops at around 200 and 270 �C. Resistance of as-deposited film is about 5 orders of magnitude higher thanthat of the one annealed at 400 �C. Selected area electrondiffraction patterns of Si2Sb2Te5 film as-deposited andannealed at 400 �C are shown in inset figure (a) and (b).The figures show the films are with amorphous and hexag-onal structure, respectively. The results can also be sup-ported by XRD measurement of the annealed film, whichis shown in Fig. 2. As shown in the figure, the filmsannealed at 300 and 400 �C for 1 min are with face-cen-tered cubic (fcc) and hexagonal state, respectively. The

before-mentioned experiments show the phase change ofSi2Sb2Te5 is a two-step process, which is similar withGe2Sb2Te5. Si2Sb2Te5 first changes from amorphous stateto fcc state at a lower temperature and further changes tohexagonal state at a higher temperature.

In order to determine thermal properties of the material,DSC measurement was performed at various heating ratesusing fresh Si2Sb2Te5 powder scratched from glass sub-strate. The two crystallization temperatures are about157.5 and 296.7 �C according to the values of the two peakson DSC curve when heating rate is 10 �C/min. While thecorresponding crystallization temperatures for Ge2Sb2Te5

are about 150 and 360 �C. Kissinger’s method is used todetermine the activation energy of the crystallization fromamorphous to fcc state. From the slope of the linear fit in

Fig. 3. Kissinger plots from which the crystallization activation energy ofSi2Sb2Te5 is determined.

952 T. Zhang et al. / Solid-State Electronics 51 (2007) 950–954

the Kissinger plot shown in Fig. 3, the activation energycan be calculated. It is determined to be 2.59 ± 0.2 eV,which is a little higher than �2.4 eV of Ge2Sb2Te5 [14,15].

The sandwiched structure for the V–I measurement wasshown in the inset figure in Fig. 4. Contact size betweentop tungsten electrode and phase change material layer isabout 20 lm in diameter. V–I measurement was performedapplying a steady DC current. According to the measure-ment, average threshold currents (Ith) for Si2Sb2Te5 andGe2Sb2Te5 films, with same thicknesses and electrode size,are about 85 and 145 lA, respectively. It is well known thattwo models were proposed for the switching in the chalco-genide material. Some researchers support the switching isessentially a thermal effect and the switching is due to theforming of a hot filament within the phase change material[7]. Some other researchers, such as Pirovano et al., suggest

Fig. 4. V–I curves for Si2Sb2Te5 and Ge2Sb2Te5 film which were measuredwith a sandwiched structure. Cross-section of the structure is shown ininset figure. Thicknesses of the Si2Sb2Te5 and Ge2Sb2Te5 film are about220 nm.

that the switching is an electronic one [16]. Controversy,however, still remains. We consider the switching is mainlythe effect of electronic one. As mentioned before, Ith ofSi2Sb2Te5 is much smaller than that of Ge2Sb2Te5 film.Reasons below are proposed for this: (1) Resistivity ofSi2Sb2Te5 is higher than that of Ge2Sb2Te5 prepared atthe same condition. Higher resistivity will lead to higherthreshold voltage but lower current [17] and (2) Si2Sb2Te5

possesses a wider band-gap comparing to Ge2Sb2Te5 does.As we know, band-gap of Si is larger than that of Ge. Inorder to determine the band-gap width of the materials,Fourier Transform Infrared Spectroscopy measurementwas performed. According to the FTIR curve shown inFig. 5, the band-gap width for the amorphous Si2Sb2Te5,polycrystalline Si2Sb2Te5 (annealed at 300 �C for 1 min)is 0.89 and 0.62 eV, respectively. While the band-gap widthfor amorphous Ge2Sb2Te5 is only 0.73 eV according to ourmeasurement. The value of amorphous Ge2Sb2Te5 mea-sured employing FTIR is similar with the references avail-able [16,18]. It has been mentioned before that a sufficientlywide electronic band-gap is required in order to reducethreshold current according to the references available.Thus, because of the wider band-gap a lower threshold cur-rent was got in the Si2Sb2Te5 film based structure.

Data retention is another important parameter forC-RAM since its principle is based on thermal inducedphase change. Both the annealing time-dependent resistanceof Ge2Sb2Te5 and Si2Sb2Te5 materials is shown in Fig. 6.Resistances as a function of time were measured at anneal-ing temperature of 130, 150 and 170 �C, respectively. A 50%failure criterion was used in the evaluation. According tothe result shown in Fig. 6a, failure times of amorphousSi2Sb2Te5 are about 1800, 250 and 16 s when the annealingtemperatures are 130, 150 and 170 �C, respectively. Whilethe corresponding failure times are 760, 77 and 7 s for theamorphous Ge2Sb2Te5 film prepared with same condition.Obviously, Si2Sb2Te5 possesses better data retention than

Fig. 5. FTIR spectrum of the amorphous Si2Sb2Te5 and polycrystallineone (annealed at 300 �C for 1 min). The band-gaps of the two films aredetermined to be 0.89 and 0.62 eV, respectively.

Fig. 6. Resistance as a function of time when annealed at varioustemperatures. The initial states for the films are amorphous. (a) Ge2Sb2Te5

and (b) Si2Sb2Te5.

T. Zhang et al. / Solid-State Electronics 51 (2007) 950–954 953

Ge2Sb2Te5 does at high temperature. This is mainly due tothe higher crystallization temperature and activation energyof Si2Sb2Te5 comparing to those of Ge2Sb2Te5.

According to our before-mentioned research, Si2Sb2Te5

is a material with low threshold current and good dataretention. It is a promising candidate for the next-genera-tion C-RAM devices.

Fig. 7. (a) Schematic cross-sectional view of the C-RAM device and (b)R–V curve for the C-RAM unit cell. Voltage pulse is provided in 50 nswidth. Inset is cross-section scanning electron micrograph of the memorydevice.

3.2. Oxidation of Si2Sb2Te5

It has been found crystallization temperature ofGe2Sb2Te5 was reduced by oxidation of the film [19]. Toinvestigate the effect of oxidation on Si2Sb2Te5, Si2Sb2Te5

powder scratched from glass substrate was kept in dry airfor five days before DSC performance. DSC results suggestthe oxidation of Si2Sb2Te5 results in both the lower crystal-lization temperature and activation energy. When the heat-ing rate is 10 �C/min, the first crystallization temperaturefor the oxided material is only about 90 �C. It reduces by

�70 �C comparing to that of the fresh one. One the otherhand, crystallization activation energy reduced to 2.4 eVas well. From the three elements within the film, Si hasthe highest oxygen affinity and is preferentially oxidized.Hence, the drop of crystallization temperature is mainlydue to oxidation of Si atom. The remaining film will thusbecome enriched in Sb and Te. It is well known that crys-tallization temperature decrease with increase of Sb2Te3 inpseudo-binary GeTe–Sb2Te3 system. It has already beencalculated that crystallization temperature for ‘‘Sb2Te5’’is only about 35 �C [19,20]. Consequently, it is reasonablethat Si2Sb2Te5 oxide has such a low crystallization temper-ature comparing to fresh Si2Sb2Te5.

Performance of data retention of the material shows thatoxidation results not only in the drop of crystallization tem-perature but also in the drop of data retention. For theSi2Sb2Te5 film kept in air for five days, the failure times(resistance 50% criterion) are about 25, 13 and 7 s, respec-tively, when annealing temperatures are 130, 150 and170 �C. The failure time is much shorter than that of freshSi2Sb2Te5. This is due to the drop of crystallization temper-ature and activation energy. Although, lower crystallizationtemperature may result in smaller programming power, amuch shorter failure time is fatal to C-RAM devices. Theshorter failure time leads to poor data retention.

3.3. C-RAM device

C-RAM device based on the new composition Si2Sb2Te5,which is outstanding in data retention and electrical

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performance, was fabricated by 0.18-lm CMOS Technol-ogy. Schematic cross-sectional view of the device was shownin Fig. 7a. The C-RAM device is with a tungsten heatingelectrode of 260 nm in diameter. The thicknesses ofSi2Sb2Te5, TiN and Al layer in the work are about 100,30 and 200 nm, respectively. Fig. 7b shows typical resis-tance–voltage (R–V) curve for the fabricated C-RAM cell,in which voltage pulse is provided in 50 ns width. The insetfigure is the cross-section scanning electron micrograph ofthe memory cell. As shown in the R–V curve, the re-amor-phized memory cell changes from high resistance state tolow resistance state when voltage pulse value reaches1.4 V by which Si2Sb2Te5 around the tungsten electrode isheated up to the crystallization temperature. The cellchanges from low resistance state to high resistance statewhen voltage pulse exceeds 3.2 V. Voltage pulse of 3.2 V50 ns is enough to heat the material around the electrodeabove the melting temperature and RESET it to high resis-tance state. Resistances for RESET state and SET state areabout 2 · 106 and 3 · 104 X, respectively. The ratio betweenRESET state and SET state is about two orders. Both theSET and RESET voltage values are smaller than theGe2Sb2Te5 based one with same structure and thickness.Endurance up to 106 has been achieved without obviousreduction of resistance ratio.

4. Conclusions

In summary, a new phase change material with a wideband-gap was investigated. The Si2Sb2Te5 material pos-sesses a low threshold current from amorphous to poly-crystalline state in the V–I measurement, and shows agood data retention. The crystallization temperature ofthe material is about 157.5 and 296.7 �C, respectively. Oxi-dation of the material results in the sharply drop of crystal-lization temperature, which further result in the poor dataretention. C-RAM device based on Si2Sb2Te5 was fabri-cated and characterized. The device is with a tungsten heat-ing electrode of 260 nm in diameter. SET and RESETvoltage values are 1.4 and 3.2 V when pulse is providedin 50 nm width. Endurance up to 106 with a resistance ratioof 100 has been achieved.

Acknowledgements

The authors wish to thank Prof. J. Shao and Dr. F.Y.Yue of CAS for FTIR measurement. This work is sup-ported by the special funds for Major State Basic ResearchProject of China (2006CB302700), Chinese Academy ofSciences (Y2005027), Science and Technology Council ofShanghai (05JC14076, 0552nm043, AM0517, 06QA14060,06XD14025, 0652nm003).

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