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Scripta Materialia 65 (2011) 327–330

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Nitrogen incorporated GeTe phase change thin film forhigh-temperature data retention and low-power application

Cheng Peng,a,b,⇑ Liangcai Wu,a,⇑ Feng Rao,a Zhitang Song,a Xilin Zhou,a Min Zhu,a

Bo Liu,a Dongning Yao,a Songlin Feng,a Pingxiong Yangb and Junhao Chub

aState Key Laboratory of Functional Materials for Informatics and Nanotechnology Laboratory, Shanghai Institute of

Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of ChinabKey Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronics,

East China Normal University, Shanghai 200241, People’s Republic of China

Received 9 March 2011; revised 26 April 2011; accepted 27 April 2011Available online 3 May 2011

The crystallization temperature of GeTe film increases markedly from 187 to 372 �C as a result of 9.81 at.% nitrogen doping, anda rhombohedral–rocksalt phase transition is observed in both GeTe and nitrogen-doped GeTe (GeTeN) films. Up to 105 cycles ofendurance for phase change memory (PCM) cells based on GeTeN have been achieved. Extrafine data retention (10 years at 241 �C)and relatively low power consumption suggest GeTeN as a promising alternative material to improve the performance of PCM.� 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Phase change memory; GeTeN film; Power consumption; Data retention

In the last few years, along with the developmentof microelectronics technology, phase change memory(PCM) has drawn much attention as a next-generationsubstitute for rewritable nonvolatile memories becauseof its conspicuous advantages, including high speed,high density, low power consumption, possible multiplewriting per bit and competitive cost [1,2]. Although theprinciple of a phase change memory was demonstratedas long ago as the 1960s [3], phase change materials,such as GeSbTe and Sb–Te alloys, were not utilized innonvolatile electronic memories until recently [4,5].For PCM, data is sorted by switching the phase changematerial between the amorphous (RESET state, withhigh resistance) and crystalline (SET state, with lowresistance) phases, which have very different opticaland electrical properties. The switching is performedby Joule heating: an amorphous phase is formed byusing a short pulse with high amplitude which can melt

1359-6462/$ - see front matter � 2011 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2011.04.033

⇑Corresponding authors. Address: State Key Laboratory of Func-tional Materials for Informatics and Nanotechnology Laboratory,Shanghai Institute of Micro-system and Information Technology,Chinese Academy of Sciences, Shanghai 200050, People’s Republicof China; E-mail addresses: pengcheng870128@yahoo.com.cn;52091202003@ecnu.cn; wuliangcai@mail.sim.ac.cn

and rapidly quench the storage material, while a longerduration pulse with moderate amplitude is applied tosufficiently crystallize the amorphous volume in thematerial. The amount of energy that needs to be appliedto accomplish the rewritable operations is crucial. An-other crucial characteristic for actual commercial appli-cation is the thermal stability of the amorphous state.For consumer application, the operation temperatureis 85 �C, and Ge2Sb2Te5 complies with this requirement[6]. On the other hand, questions still remain about howto ensure better data-retention performances and to ad-dress, with PCM, the embedded memory market. Forinstance, fail temperature after 10 years of 125 and150 �C are required for PC materials and automotiveapplications, respectively.

Great efforts have been made to improve the thermalstability of phase change materials, such as doping thematrix with nitrogen atoms, capping the phase changefilm with dielectric layers and developing new materialsystems [7–9]. A PCM alloy based on a GeTe alloyhas been reported to have a higher crystallization tem-perature, a higher resistance contrast and a faster SEToperation than those of the Ge2Sb2Te5 [10–12]. Fantiniet al. [10] reported the crystallization temperature ofamorphous GeTe to be 185 �C. However, it is still notsuitable for automotive applications due to its limited

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Figure 1. (a) R–T curves of Ge2Sb2Te5, GeTe and GeTeN films at aheating rate of 13 �C min–1; XRD patterns of (b) GeTe, and (c) GeTeNfilms annealed at different temperatures for 5 min in Ar atmosphere.

328 C. Peng et al. / Scripta Materialia 65 (2011) 327–330

crystallization temperature. In addition, the RESETpower consumption of GeTe material will be higher be-cause of its extremely low crystalline resistance. Aneffective method to improve the performance of PCMbased on GeTe material is to introduce other elements.Kim et al. [13] reported that nitrogen-doped GeTe alloyhas a higher crystallization temperature, and an investi-gation of this material has indicated that nitrogen dop-ing can suppress the tendency for the Ge/Te ratio toincrease towards the surface. In addition, Fantini et al.[14] presented a 4 at.% nitrogen-doped GeTe materialthat had a data retention temperature of 124 �C for10 years. In this paper, we systematically study nitro-gen-doped GeTe with a high nitrogen concentration(9.81 at.%) using optical, thermal and electrical mea-surements, and compare the characteristics of GeTeand GeTeN. In particular, power consumption, endur-ance and data retention are evaluated.

GeTe and GeTeN films (200 nm) were deposited onSiO2/Si and Si substrates by the magnetron co-sputter-ing method using Ge and Te targets at room tempera-ture. The nitrogen concentration was fixed at an Ar/N2 ratio of 48:2 sccm. The composition of the GeTe filmwas determined to be 51:49 by means of energy-disper-sive spectroscopy. The nitrogen concentrations ofmolecular nitrogen and atomic-state nitrogen weredetermined by X-ray photoelectron spectroscopy(XPS). Ge2Sb2Te5 film was also prepared using the alloytarget for comparison. In situ temperature-dependentresistance measurements were performed in a vacuumchamber with a heating rate of 13 �C min–1. X-ray dif-fraction (XRD) was employed to characterize the crystalstructure of the films. The diffraction patterns were ta-ken in the 2h range of 10–68� using Cu-Ka radiation,with a scanning step of 0.02�. Raman spectroscopyand XPS were carried out to study the bonding modesand optical properties of GeTe and GeTeN. In orderto assess the data lifetime in PCM devices, thedata retention, which relies on the evaluation of thetime-dependent change in resistance of the amorphousphase film at isothermal annealing temperatures, wasmeasured.

T-type PCM devices with electrode diameters of260 nm were fabricated by 0.18 lm complementary me-tal-oxide semiconductor (CMOS) technology. A GeTe,GeTeN or Ge2Sb2Te5 thin film (phase change layer,PCL), with thickness of about 100 nm, was depositedon the W bottom electrode contact by sputtering. Subse-quently, a 10 nm TiN adhesion layer and a 300 nm thickAl electrode layer were sputter-deposited and patterned.Resistance–voltage (R–V) measurements were per-formed using a Keithley-2600 digital source meter, andan Agilent-8104A pulse generator was used to providethe voltage pulse.

Figure 1a shows the sheet resistances as a function oftemperature (R–T). For all the phase change thin films,the sheet resistance decreases as the temperature in-creases, with a quick drop at the crystallization temper-ature. The crystallization temperatures of Ge2Sb2Te5,GeTe and GeTeN were determined to be 162, 187 and372 �C, respectively, according to the derivative of sheetresistance with respect to temperature (dR/dT). Thecrystallization temperature values of Ge2Sb2Te5 and

GeTe agree well with the reported data [10]. It shouldbe noted that sheet resistance and crystallization temper-ature are significantly increased by nitrogen dopingGeTe. It is well known that a high crystallization tem-perature helps improve the stability of phase changematerial, while a large crystalline resistance decreasesthe RESET power consumption.

Figure 1b and c shows the changes of crystal struc-ture with annealing temperature by XRD measurement.The films were annealed at different temperatures in Aratmosphere for 5 min. For the as-deposited films, thereis no diffraction peak in the XRD pattern, indicatingamorphous structures (not shown here). In Figure 1b,the GeTe film crystallizes into a rhombohedral structurewith no separate Te or Ge observed when the annealingtemperature reaches 250 �C. A similar crystallizationprocess is found in the GeTeN film at 400 �C (Fig. 1c).It is apparent that the annealing temperatures have ex-ceeded the crystallization temperatures of GeTe and Ge-TeN, as determined in Figure 1a. In addition, all thediffraction peaks are suppressed as a result of nitrogendoping. For the main peak at around 29.8�, the grainsizes of the 400 �C annealed GeTe and GeTeN filmsare 42.8 and 33.4 nm, respectively, calculated using theScherrer equation (Dhkl ¼ 0:89k=b cos h). The decreaseof grain size indicates that the reversible phase changecan be realized more easily, which may be helpful inreducing the power consumption of PCM cells. It canbe seen in Figure 1b and c that cubic phases graduallyappear with subsequent annealing for GeTe and GeTeNfilms, and GeTeN starts to change its structure at a high-er temperature (up to 450 �C). Previously, Rabe andJoannopoulos [15] had calculated the rhombohedral–rocksalt phase transition for GeTe by using the self-consistent ab initio method. Their calculated value oftransition temperature agrees with our XRD data. Theyalso demonstrated that the rhombohedral phase can be

C. Peng et al. / Scripta Materialia 65 (2011) 327–330 329

described as a rocksalt structure distorted along the[1 1 1] direction and that the rocksalt structure is morestable than the rhombohedral structure. The phaseswitching between the amorphous and the distortedrocksalt structure is just what is utilized in practicalPCM applications [15,16].

In order to further reveal the binding mode in crystal-line GeTeN films, Raman measurements were carriedout. In Figure 2a, all the peaks observed below200 cm�1 agree with the Raman bands previously re-ported in the literature for GeTe compounds [17,18].Amorphous Ge–Te bonds and crystalline Ge–Te bondsare marked in the figure as “a” and “c”, respectively.For GeTeN films, an amorphous Ge–Te band, whichis not found in GeTe, is seen at 160 cm�1. This resultindicates that GeTe has sufficiently crystallized at250 �C, while GeTeN retains some amorphous Ge–Tecontent even at 450 �C. This can be explained as follows:in the GeTeN film, N atoms bond with the Ge atoms toform nitrides [13]. These nitrides may precipitate to thegrain boundaries and stay in the amorphous phase, sup-pressing GeTe grain growth. Thus, amorphous GeTebonds will remain in the district around the nitrides.In addition, the 124.3 cm�1 peak observed at 250 �Cdramatically disappears with increasing temperaturefor GeTe. Meanwhile, two new peaks gradually emergeat around 116 and 137 cm�1 for the 300 and 400 �C an-nealed GeTe films. A similar transformation can also bedetected in the 400 and 450 �C annealed GeTeN films.This may be attributable to the greater degree of crystal-lization of the films under higher annealing tempera-tures. Moreover, for the highest temperature annealedsamples of GeTe and GeTeN, the 116 and 137 cm�1

peaks shift to 119.6 and 139 cm�1, respectively. The lar-ger wavenumber in the sample after nitrogen doping isascribed to Ge–Te bonds in a somewhat different envi-ronment, where N atoms, which are much lighter than

Figure 2. (a) Raman spectra of the annealed GeTe and GeTeN films;(b) the N1s spectra for as-deposited GeTe and GeTeN films.

Te atoms, replace some of the Te atoms. The N1s spec-tra for as-deposited GeTe and GeTeN films obtained byXPS are shown in Figure 2b. The binding energy of theN1s peak for as-deposited GeTeN is found to beapproximately 396.75 eV, which is a typical binding en-ergy for nitrides. Considering previous reports, the N1snitride peak may be attributed to GeNx [19].

To evaluate the electrical properties of the GeTeNfilms, the pulse-mode R–V characteristics of the PCMdevices are shown in Figure 3a, for which a voltage pulseof 1000 ns width was provided. The inset in Figure 3ashows a cross-sectional diagram of the cell structure.A reversible phase-change process can been observed.The SET and RESET voltages for the GeTeN-based cellare 1.6 and 3.3 V, respectively. Both values are lowerthan those of the Ge2Sb2Te5-based ones and higher thanthose of the GeTe-based ones. However, the crystallineresistance of GeTeN is much larger than those of GeTeand Ge2Sb2Te5. The required dissipated energy for theRESET operation can be estimated using the equationðV 2

RESET=RSETÞ � tRESET. Thus, the GeTeN-based cellexhibits a lower RESET power consumption. Thereate two possible reasons why the RESET power con-sumption is reduced by the addition of nitrogen atomsto GeTe film. First, it has been reported that GeTeand GeTeN have nearly the same melting point [14].Thus, equivalent Joule heat is needed in order to meltthem. Secondly, GeTeN may have a lower thermal con-ductivity compared with GeTe, extrapolating the resultsof other nitrogen-doped phase change materials [20].Therefore, heat diffusion from the phase change materialto the electrodes will be suppressed. In other words, themore effectively the Joule heating remains on the phasechange material for the RESET operation, the lower thepower consumption.

The endurance characteristics of the GeTeN-basedcell are plotted in Figure 3b. An endurance measure-

Figure 3. (a) SET and RESET characteristics of PCM cells based onGe2Sb2Te5, GeTe and GeTeN. The inset is a cross-sectional focusedion beam image of a PCM cell; (b) endurance characteristics of theGeTeN-based cell.

Figure 4. Arrhenius plots of failure time vs. 1/kT of GeTeN films, andthe extrapolated data retention time at specified temperatures. Theinset shows the change of resistance with time for the GeTeN filmmeasured at certain fixed temperatures.

330 C. Peng et al. / Scripta Materialia 65 (2011) 327–330

ment of 105 cycles with a resistance ratio of 3000 wasachieved with a RESET pulse of 4 V @ 100 ns and aSET pulse of 2.7 V @ 2000 ns.

Data retention is another important parameter forPCM since its principle is based on thermally inducedphase change. The values of data retention can be calcu-lated from extrapolation of the isothermal Arrheniusplot, which is obtained by the logarithm of failure timevs. the reciprocal of isothermal temperature [21]. The in-set of Figure 4 shows the change in resistance with timefor the GeTeN film, which was isothermally measured atcertain fixed temperatures (339, 329, 319 and 309 �C).The best fit for time to failure vs. reciprocal temperature(1/kT) is shown in Figure 4. A 10% failure criterion isused in the evaluation. The Ec is determined to be4.86 eV according to the Arrhenius equation, t = sexp (Ec/kBT), where t is the time to failure and s is a pro-portional time constant. This value is much higher thanthe �2.4 eV of Ge2Sb2Te5 [22]. The extrapolated fittingline estimates that GeTeN has an extremely high dataretention temperature (241 �C) for 10 years. Meanwhile,the data retention times at 150 and 100 �C are up to1.82 � 1011 and 1.99 � 1017 years, respectively. Thesevalues are believed to be higher when a 50% failure cri-terion is employed. By comparison, the temperature forthe 10 year archival life of Ge2Sb2Te5 thin film is only90 �C [6]. Such good thermal stability of the GeTeN filmis probably attributable to its high crystallization tem-perature and Ec. From this point of view, PCM basedon GeTeN film can be safely used in some adverse envi-ronments, such as those faced by automotive and outerspace applications.

In summary, nitrogen-doped GeTe with a nitrogenconcentration of 9.81 at.% was systematically studied.Two-step crystallization (amorphous–rhombohedral–cubic) was observed in both GeTe and GeTeN films.A reversible phase change between the SET and RESETstates was achieved in the PCM cells made of GeTe andGeTeN. For the GeTeN-based PCM cell, 105 cycles of

endurance with a resistance ratio larger than 3000 wasobtained, with a lower RESET power consumption thanthose of the GeTe- or Ge2Sb2Te5-based ones. The extre-mely good amorphous thermal stability of GeTeN filmensures better data retention for high-temperaturePCM use.

This work was supported by the NationalIntegrated Circuit Research Program of China(2009ZX02023-003), the National Basic ResearchProgram of China (2007CB935400, 2010CB934300,2011CB309602, and 2011CB932800), the NationalNatural Science Foundation of China (60906004,60906003, 61006087, and 61076121), and the Scienceand Technology Council of Shanghai (09QH1402600and 1052nm07000).

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