Observation of molecular nitrogen in N-doped Ge 2 Sb 2 Te 5Kihong Kim, Ju-Chul Park, Jae-Gwan Chung, Se Ahn Song, Min-Cherl Jung, Young Mi Lee, Hyun-Joon Shin,Bongjin Kuh, Yongho Ha, and Jin-Seo Noh Citation: Applied Physics Letters 89, 243520 (2006); doi: 10.1063/1.2408660 View online: http://dx.doi.org/10.1063/1.2408660 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effects of germanium and nitrogen incorporation on crystallization of N-doped Ge2+xSb2Te5 (x=0,1) thin filmsfor phase-change memory J. Appl. Phys. 113, 044514 (2013); 10.1063/1.4789388 Crystallization dynamics of nitrogen-doped Ge 2 Sb 2 Te 5 J. Appl. Phys. 105, 104902 (2009); 10.1063/1.3126501 Change in electrical resistance and thermal stability of nitrogen incorporated Ge 2 Sb 2 Te 5 films Appl. Phys. Lett. 90, 021908 (2007); 10.1063/1.2431462 Effects of N 2 + ion implantation on phase transition in Ge 2 Sb 2 Te 5 films J. Appl. Phys. 100, 083502 (2006); 10.1063/1.2357640 Amorphous-to-crystal transition of nitrogen- and oxygen-doped Ge 2 Sb 2 Te 5 films studied by in situ resistancemeasurements Appl. Phys. Lett. 85, 3044 (2004); 10.1063/1.1805200

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Observation of molecular nitrogen in N-doped Ge2Sb2Te5

Kihong Kim, Ju-Chul Park, Jae-Gwan Chung, and Se Ahn SongAE Center, Samsung Advanced Institute of Technology, Yongin 446-712, Korea

Min-Cherl Jung, Young Mi Lee, and Hyun-Joon Shina�

Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 790-784, Korea

Bongjin Kuh and Yongho HaMemory Division, Samsung Electronics Co., Ltd., Yongin, Gyeonggi-Do 449-900, Korea

Jin-Seo NohDevice Laboratory, Samsung Advanced Institute of Technology, Yongin 446-712, Korea

�Received 21 September 2006; accepted 11 November 2006; published online 15 December 2006�

Ge2Sb2Te5 �GST� film in the crystalline state was nitrogen doped using the reactive sputteringmethod in order to increase sheet resistance. High-resolution x-ray absorption spectroscopy revealedthat molecular nitrogen �N2� existed in the N-doped GST film. This finding implies that bothmolecular nitrogen and atomic-state nitrogen should be taken into account in understanding thestructures of N-doped GST film. The molecular nitrogen is believed to exist at interstitial andvacancy sites, and more likely at grain boundaries. © 2006 American Institute of Physics.�DOI: 10.1063/1.2408660�

Flash memory has achieved a great commercial successdue to its nonvolatile characteristics. However, flash memo-ry’s extended programming time and high programmingvoltage present obstacles to achieving next generation de-vices requiring scaled-down cells of nanometer size. Phase-change materials1,2 based on the Ge–Sb–Te system havebeen widely studied as substitutes for flash memory, giventheir fast write speed, superb scalability, and compatibilitywith current silicon-based mass production. The critical com-ponent of the Ge–Sb–Te system is Ge2Sb2Te5 film. Still, theissue that the Ge2Sb2Te5 film requires high power in order tophase change from the crystalline to amorphous state, due tothe very low resistivity of the crystalline state, remains to beresolved. Therefore, it is very essential to increase the resis-tance of the Ge2Sb2Te5 film in the crystalline state. The cur-rently demanded sheet resistance of the crystalline state is onthe order of k�sq.

3 Nitrogen doping has been used to in-crease the resistance of crystalline Ge2Sb2Te5 film,4–6 andsome results showed several k�sq sheet resistances in theirN-doped films. However, in order for the GST system to becompetitive with other systems, it is still necessary to refinethe recipe of N-doped Ge2Sb2Te5 and to understand its struc-ture as well as the role of nitrogen, which results in resis-tance changes in that structure. The crystal structure of theN-doped Ge2Sb2Te5 system, doped mostly by the reactivesputtering method, has been investigated and understoodonly in consideration of the atomic-state nitrogen.4–6 How-ever, in this letter, through a high-resolution near-edge x-rayabsorption fine structure �NEXAFS� study at the nitrogen Kedge, we show that the N-doped Ge2Sb2Te5 contains mo-lecular nitrogen also. The molecular nitrogen was observedin all of the nitrogen containing Ge2Sb2Te5 films, whichwere deposited using the reactive sputtering method. Thisfinding thus implies that one has to understand the structureand also the resulting resistance change by considering the

existence of both molecular nitrogen and atomic-statenitrogen.

Ge2Sb2Te5 film was grown by sputtering from aGe2Sb2Te5 target onto a silicon wafer at room temperature�RT� and 200 °C. The nitrogen concentration of the N-dopedGe2Sb2Te5 film was obtained in the range of 0–2 at. %�% hereafter� by controlling the nitrogen gas flow rate andfixing the argon gas flow rate. The nitrogen concentration�including both the molecular nitrogen and the atomic-statenitrogen� was measured by x-ray fluorescence analysis. Thesheet resistance of the film was measured by the four-pointprobe method. The structural state of the sample was inves-tigated by x-ray diffraction �XRD� and high-resolution trans-mission electron microscopy. The chemical structure of thenitrogen �N�, specifically the molecular nitrogen �N2�, wasinvestigated by NEXAFS spectroscopy. The NEXAFS studywas conducted at the 8A1 undulator-radiation �U7� beamlineat the Pohang Light Source using synchrotron radiation.7

Figure 1 shows the sheet resistance of the Ge2Sb2Te5films deposited at RT and 200 °C, respectively. The sheet

a�Electronic mail: shj001@postech.ac.krFIG. 1. Sheet resistances of Ge2Sb2Te5 films deposited at RT and 200 °C asa function of nitrogen concentration.

APPLIED PHYSICS LETTERS 89, 243520 �2006�

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resistance increased effectively with the nitrogen concentra-tion for both growth conditions. For the films deposited atRT, the resistances were already higher than several tens ofM�sq. The resistances of the films for 1.3% and 2.0% nitro-gen concentrations were beyond the measurement limit ofthe four-point probe and thus are not shown in the figure.According to our sample growth recipe, the undoped filmsgrown at RT and 200 °C were initially in the amorphousstate and the crystalline state, respectively, as confirmed inFig. 2. And from the sheet resistance data, it is expected thatthe N-doped films grown at RT were in the amorphous state,which is also confirmed in Fig. 2. For the Ge2Sb2Te5 filmsdeposited at 200 °C, the resistance increased from�50 �sq to �1 k�sq and to �1 M�sq when the nitrogenconcentration grew to 0.7% and 1.3%, respectively. Thedrastic change of the resistance is not believed to be relatedto the number of carriers, as impurities, generated by theamount of deposited nitrogen. Considering the resistance ofthe amorphous-state films grown at RT, it is expected fromthe resistance data that as the nitrogen concentration in-creased, the N-doped film grown at 200 °C underwent astructural change from the crystalline �0%� to the amorphousstates �2.0%�.

Figure 2 shows the x-ray diffraction data of the N-dopedGe2Sb2Te5 films, showing the crystal structure changes. Forthe films deposited at RT, each of the x-ray diffraction datashowed a broad peak near 28°, which is a characteristic pat-tern of the GST system in the amorphous state. Another in-teresting feature is the observation of a broad peak, near 48°,that indicates the existence of a short-range order in theatomic arrangement. The broad peak decreased in intensityas the nitrogen concentration increased, indicating the corre-

lation of a short-range order reduction with that increase ofnitrogen concentration.

For the undoped Ge2Sb2Te5 film grown at 200 °C, hex-agonal and cubic structures coexisted. As the nitrogen con-centration increased to 0.7%, the diffraction peak of the hex-agonal structure diminished by a significant amount. Bycontrast, the cubic structure increased relatively to the hex-agonal structure. Thus, the increase of sheet resistance in thisnitrogen concentration regime is partly related to the de-crease of the hexagonal structure in the crystalline state.6 Inthe 1.3% N-doped Ge2Sb2Te5 film, the �111� and �200� peaksof the cubic structure became broader and started to overlap,implying a mixture of the amorphous and crystalline cubicstates. This mixture was also confirmed by transmission elec-tron microscope �data not shown here�. The 2.0% nitrogen-doped film showed an XRD pattern representing the amor-phous state. The drastic increase of the sheet resistance ofthis N-doped film, from �50 �sq to �20 M�sq for 2.0%nitrogen doping, is believed to be related to the generation ofthe amorphous state and the refinement of grains, where thegrain boundaries scatter the electron propagation resulting inan increase of resistance.4

Figure 3 shows nitrogen K-edge NEXAFS spectra,which provide information on the chemical structure of thenitrogen. Each spectrum in Fig. 3�a� can be represented bystructure A on top of the broader spectral feature B. Spectralfeature B is a result of atomic-state nitrogen, bonding withthe GST structure. The exact nature of the chemical bondingof N is not yet clear. It has been suggested that N bondsdominantly with Ge.4,6 The photoemission spectra of the air-contaminated GST film showed oxidized Te and Sb peaks,8

which implies that N might also bond with Te and Sb. In thecase of N bonding with either Ge, Te, or Sb, the transitionenergy of the B feature becomes effectively lower than thatof the N bonding with nitrogen �A in the figure�. One plau-sible explanation for the lower transition energy is the effec-tive charge transfer from the higher Z element towards N,upon hybridization, as N chemically bonds with the GSTstructure and as the electronegativity of the N is lower thanthose of the GST elements. The effectively shifted electronsshield the core levels of the N atom, and this results in theeffectively lower energies of the unoccupied levels relativeto the core levels. Structure A appeared at the energy positionwhere molecular nitrogen was observed.9–11 The detailedstructure of the A feature is shown in Figs. 3�b� and 3�c�. Theclear splitting of the subpeaks, arising from the vibrationmode of the molecule, is a characteristic feature of the 1s→�* transition of molecular nitrogen �N2�.9 This observa-tion indicates that N exists in free-molecular state that allowsvibration along the triple-bonding direction, implying thatthe molecule is physically trapped inside the film. The x-rayabsorption spectroscopy is surface sensitive at the N K-edgex-ray energy. Since the GST film is easily contaminated byoxygen when exposed to air,8 each spectrum was obtainedafter sputtering the surface of the sample to remove the sur-face oxidation. Thus, our results imply that N2 as well as Nexist inside bulk GST.

The claimed structures of Ge2Sb2Te5 film are of twotypes. One is the NaCl-type cubic structure in the Fm-3mspace group, where the Na site is occupied with Te andwhere the Cl site is randomly occupied with Ge, Sb, andsome vacancies.12,13 The other is the hexagonal �P-3m1�structure with all sites occupied.14,15 The proposed stacking

FIG. 2. X-ray diffraction data for Ge2Sb2Te5 films deposited at RT and200 °C. In the figure, H and C stand for hexagonal structure and cubicstructure, respectively.

243520-2 Kim et al. Appl. Phys. Lett. 89, 243520 �2006�

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sequence was Te–Ge–Te–Sb–Te–Te–Sb–Te–Ge–. It was sug-gested that the N atom is more likely located in the intersti-tial site, because the size, �0.07 nm diameter, is within areasonable range compared to the size of the interstitial siteof GST structures, and because N is believed to bond pref-erably to Ge.4,12,13 The size of the molecular N2 is about0.25 nm. If we consider the NaCl-type cubic structure, theN2 size can be fitted interstitially in the diagonal direction.This means that N2 can exist at the interstitial site of the GSTstructure. The other and more probable positions of the mo-lecular N2 are vacancies of the cubic structure and grainboundaries. It is not likely that N2 physically bonds with theGST surface or structure at RT or 200 °C. It was noticedfrom the NEXAFS data that N2 was observed for all of thesamples and that the N2 intensity increased as the nitrogengas flow rate was increased. This implies that N2 was trappedinside the film during the film growth process. A N2 trappingmechanism would be as follows. First, when an atomic N,bonding with GST on the film surface, impinges with a N+

ion or a cluster that contains a N atom, the atomic N attractsthe incoming N to form a stable N2. The thus-formed N2 hasthe possibility of being trapped by the surrounding and in-coming clusters. Second, the N2+ ion bonds to the GST filmsurface after either the N2+ ion or a cluster containing theN2+ ion impinges on the surface, and then the N2+ ion re-mains until the surrounding and incoming clusters trap theN2+ ion. The N2+ ion then becomes neutralized after theN-GST system stabilizes. Since the N2 size is comparativelylarger than that of N, this N2 formation effectively results insuppression of grain growth. Therefore it is expected that theN2 formed from the above mechanism would be found morein the grain boundaries. For SiOxNy film containing molecu-lar N2, the N2 could be released from the film by high-temperature annealing.11 However, in this GST film, sincethe growth temperature should be lower than the transitiontemperature for the crystalline hexagonal structure, existenceof N2 inside the film is an unavoidable feature of the currentsputtering condition.

In summary, we investigated N-doped Ge2Sb2Te5 filmsdeposited at both room temperature and 200 °C. The sheetresistance increased with the nitrogen doping concentration.In the Ge2Sb2Te5 film deposited at 200 °C, the phase trans-formation went through three states, hexagonal→cubic→amorphous, with the increase of nitrogen concentration.These phase transformations are believed to correlate tosheet resistance changes.5,6 In addition to atomic-state N, theexistence of molecular N2 was confirmed from the high-resolution nitrogen K-edge absorption spectra. Molecular N2

is believed to exist at interstitial and vacancy sites and morelikely at grain boundaries, which effectively results in grainrefinement. Overall, the more N2 and N exist, the more thefilm has refined grains and an amorphous state, resulting ingreater increases of sheet resistance.

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3W. Y. Cho, B. H. Cho, B. G. Choi, Y. R. Oh, S. Kang, K. S. Kim, K. H.Kim, D. E. Kim, C. K. Kwak, H. G. Byun, Y. Hwang, S. J. Ahn, G. H.Koh, G. Jeong, H. Jeong, and K. Kim, IEEE J. Solid-State Circuits 40,293 �2005�.

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89, 043503 �2006�.9J. Stöhr, NEXAFS Spectroscopy �Springer, Berlin, 1992�.

10C. T. Chen, Y. Ma, and F. Sette, Phys. Rev. A 40, 6737 �1989�.11Y. Chung, J. C. Lee, and H. J. Shin, Appl. Phys. Lett. 86, 022901 �2005�.12T. Nonaka, G. Ohbayashi, Y. Toriumi, Y. Mori, and H. Hashimoto, Thin

Solid Films 370, 258 �2000�.13N. Yamada and T. Matsunaga, J. Appl. Phys. 88, 7020 �2000�.14B. J. Kooi and H. Th. M. De Hosson, J. Appl. Phys. 92, 3584 �2002�.15Z. Sun, J. Zhou, and R. Ahuja, Phys. Rev. Lett. 96, 055507 �2006�.

FIG. 3. �Color online� NEXAFS data for the Ge2Sb2Te5

films deposited at RT and 200 °C. Each spectrum isshifted in the vertical direction for a clear view. �b� and�c� show detailed features of peak A indicated in �a�.The splitting feature is due to the vibration mode of thefree-molecular nitrogen, N2.

243520-3 Kim et al. Appl. Phys. Lett. 89, 243520 �2006�

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