Heterogeneity of molecular structure of Ag photo-di!usedGe30Se70 thin films

Mingji Jin a, Punit Boolchand a, Maria Mitkova b,*

aDepartment of Electrical and Computer Engineering and Computer Science, University of Cincinnati, Cincinnati, OH 45220-0013, USAbDepartment of Electrical and Computer Engineering, Boise State University, Boise ID 83725-2075, USA

Available online 13 February 2008

Abstract

Silver photodi!usion in thin Ge30Se70 films is investigated in order to understand the structure forming in the Ge–Se host after intro-duction of Ag in it. Optical micrographs point towards formation of homogeneous films after deposition of the Ge–Se material on glasssubstrate. This structure changes and occurrence of heterogeneous regions is imaged after Ag is introduced into the Ge–Se host. Ramanspectroscopy provides data about the development of the structural organization in the host in depth. It confirms that while the initialnon-doped films are organized in a structure corresponding to the bulk material with analogous composition, after Ag photodi!usion,the structure on the interface Ag/Ge30Se70 film is identified as being amorphous Ag8GeSe6 which continuously changes in depth to struc-ture of Ge-rich material. Raman spectra testify that Ag distribution reaches the bottom of the hosting Ge30Se70 films.! 2008 Elsevier B.V. All rights reserved.

PACS: 78.30.j; 61.10.Nz

Keywords: Chalcogenides; Raman spectroscopy

1. Introduction

Chalcogenide glasses have been recently recognized asthe hosting material for introduction of metals (typicallyAg or Cu) for creation of solid state electrolytes whichare the active medium for programmable metallization cell(PMC) memory devices [1]. Among the entire family ofchalcogenide glasses those, based on Ge-chalcogenides cor-respond best to the processing requirements of the semi-conductor industry and hence they draw the researchers’attention. These glasses possess a number of photoinducede!ects ranging from subtle bond rearrangement to bondbreaking which enable faster di!usion and better fillingwith metals in them. This is the reason that introductionof Ag in these chalcogenide glasses is usually accomplisheddue to photodi!usion since this results in a stable structure,

the process is fast and allows introduction of a very highamount of Ag in the particular glass [2]. The performanceof the PMC devices based on so prepared solid electrolytesdepends on many factors like electrical conductivity, struc-tural stability, photosensitivity, etc. of the solid electrolytematerial and they can be particularly met by an electrolytecomprising Ag dopant in Ge30Se70 hosting glass. Thismaterial has been investigated quite extensively but never-theless there are still many discussions and questionsrelated to the molecular structure occurring after introduc-tion of Ag in the Ge–Se backbone.

One problem which has been debated for some time is: ifthe Ag photo-di!used material is homogeneous or hetero-geneous in its structure, i.e. there are discussions about ifAg becomes a part of the chalcogenide backbone and hencereplaces part of Ge in it or whether Ag reacts with the ele-ments of the backbone and the new formed compositionphase separates from it. Based on X-ray photoelectronspectroscopy studies Zembutsu [3] proposed that photod-oping of Ag in Ge20Se80 results in formation of phase like

0022-3093/$ - see front matter ! 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.jnoncrysol.2007.10.091

* Corresponding author. Tel.: +1 208 426 3395; fax: +1 208 426 2470.E-mail address: mariamitkova@boisestate.edu (M. Mitkova).

www.elsevier.com/locate/jnoncrysol

Available online at www.sciencedirect.com

Journal of Non-Crystalline Solids 354 (2008) 2719–2723

Ag2Se, containing less Ag than the stoichiometric compo-sition. Chen and Tay [4] revealed the formation of pre-dominantly bcc-Ag2Se. While the properties of thephoto-di!used material have been discussed only on handof the di!used amount of Ag into the chalcogenide back-bone [3–5], some of us [6] have put on view the importanceof the hosting backbone and evidences have been presentedabout the dual role of Ag when introduced in Ge–Se glass;in the Se-rich Ge–Se glasses Ag is glass modifier formingAg-chalcogenides that phase separate from the Ge–Sebackbone; in the Ge-rich glasses Ag is a glass former andit replaces Ge into the network of the chalcogenide glass.Formation of Ag2Se and Ag8GeSe6 and Ge-rich chalcogen-ide backbone has been documented after Ag photodi!u-sion in Ge30Se70 thin films at illumination with lowintensity light [2]. Some recent works [7,8] confirmed alsoformation of a phase separated structure after introductionof Ag into the Se – reach Ge–Se system.

The available results posed the question of how does thestructure of the di!used films develop in depth. Based onoptical micrographs as well as Raman scattering and con-focal microscope set up for surface- and depth- profilestudies, this work gives data about the changes occurringin the structure of the Ge30Se70 films after Ag photodi!u-sion in them on the front, the back sides and in depth ofthe sandwich Ag–Ge30Se70.

2. Experimental

Thin (250 nm) Ge30Se70 films were deposited with a rateof 1 nm/s using thermal evaporation onto glassy substratefrom a previously synthesized material. To keep the stoi-chiometry of the films close to this of the source material,an evaporator with a construction of a semi-Knudsen cellwas used. On some of these films 80 nm Ag was evaporatedand then photo-di!used into the Ge30Se70 by illuminationwith UV light from a mercury lamp with a light intensity200 mW/cm2 for 10 min. In some cases the residual Ag filmwas dissolved in 1 M solution of Fe(NO3)3 .

The composition of the films before (only the Ge–Seglass films) and after the di!usion process was studiedusing Rutherford Backscattering Spectrometry (RBS) anal-ysis, performed with 2 MeV 4He+ with the beam at normalincidence to the sample and a backscattering angle of 65" ata reduced charge of around 0.25 lC/mm2.

The basic appearance and structure of the films formedin this manner were demonstrated using an opticalmicroscope.

Raman spectra were obtained to provide information onthe short range order occurring in the hosting materialbefore and after the di!usion process. Because of the lightsensitivity of the investigated materials, the need to exciteRaman scattering at low energy is paramount. For this rea-son, resonant enhancement of the scattering by tuning thelaser energy closer to the optical band gap of the glass isparticularly desirable. Bearing this is mind, the Ramanstudies were performed in the micro-Raman mode with

the following conditions: 15 s at 15 accumulations by illu-mination with 1.5 mW of light power on a sample with647.1 nm wavelength of a Kr+ ion laser that is supposed notto produce photoinduced changes during the measurement.

Three series of samples were studied: Ge30Se70 films;sandwiches of Ag and Ge30Se70 in which Ag was photo-dif-fused and sandwiches of Ag and Ge30Se70 in which Ag wasphoto-di!used and the residual Ag was dissolved. Ramanspectra were collected from the surface of the samples,from their back side as well as by changing the focal dis-tance of the scattered light from the depth of the filmsachieving di!erent depth penetration.

Ag8GeSe6 crystalline ternary was synthesized and itsRaman spectrum was measured. Rapid thermal annealingwas performed increasing the power of the scattering laserlight up to 20 mW after which the bulk material was airquenched into the Raman system, became amorphousand its Raman spectrum was excited and measured.

3. Results

The RBS data confirmed the initial composition of thehosting material to contain about 2 at.% Se less than thesource material, i.e. the films composition was Ge32Se68.We have established analogous results in earlier studies[2], where the RBS data are discussed in detail. Ag di!usionresults in 37 at.% average amount in the films. This compo-sition corresponds well to the saturation limit of Ag intoGe30Se70 glass as recognized earlier [2].

The results of the studies are presented in the three pan-els shown in Fig. 1. Sample A presents the data forGe30Se70 film. The optical micrographs of the front andback side of the film show quite good homogeneity of thefilm. The Raman spectra of both sides of the films are quiteidentical and they correspond well to the Raman spectrumof bulk Ge32Se68 [9]. The specific feature of the films spec-tra is that they show some tendency of formation of ethanelike units which we depict at the shoulders forming around172 cm!1.

Sample B in Fig. 1 presents the data related to a sand-wich of Ge30Se70 films with Ag on top of it. The opticalmicrographs of these films prove formation of heteroge-neous structure, visible on the front as well on the back sideof the films. Raman spectroscopy has been performed onlyon the back side of the films at which the Ge–Se glass canbe reached since the front side is covered with Ag and isconducting. The Raman spectra demonstrate seriouschanges in the structural organization of the Ge–Se back-bone after introduction of Ag in the films. There are indi-cations for formation of Ge-rich backbone which arerelated to increased scattering from ethane like structuralunits and lack of evidence for edge sharing structural units.

Sample C in Fig. 1 shows the situation when photodi!u-sion has been performed and the residual Ag film is dis-solved to open up the Ge–Se surface for characterization.Like in the previous case, the micrographs show extendedheterogeneity in the structure of the films on both sides.

2720 M. Jin et al. / Journal of Non-Crystalline Solids 354 (2008) 2719–2723

The Raman spectra of the front and back side di!er withthe back side as shown in the case B demonstrating pres-ence of ethane like structural units and absence of edgesharing units while the front side displays the breathingmodes of the GeSe4 tetrahedra shifted to 192 cm!1.Detailed study in depth of the Raman activity of the frontside shows systematical blue shift of the breathing mode ofthe GeSe4 tetrahedra while the breathing mode of these tet-rahedra in the case of pure Ge30Se70 films (shown for com-

parison) does not change in depth – Fig. 2. The inset in thisfigure presents the Raman spectra from which the data forthe position of the GeSe4 scattering modes have beenextracted.

Fig. 3 presents the spectra of crystalline Ag8GeSe6 aswell this of glassy Ag8GeSe6 and the front side of sampleC. One can compare on this figure the spectra appearingon the front of the photodoped film with those of the ter-nary composition.

Fig. 1. Schematic of the studied samples and structural results: (a) pure Ge30Se70 film on glass substrate sample sketch; (b–c) optical micrographs from thefront and back of the Ge30Se70 film; (d) Raman scattering from the front and back sides of the Ge30Se70 film compared to this of bulk material; (e) sketchof the sandwich of Ag and Ge30Se70 films on a glass substrate; (f–g) optical micrographs of the front and back of the sandwich of Ag and Ge30Se70 film; (h)Raman spectra development in depth from the back of the sandwich of Ag and Ge30Se70 films on a glass substrate –through di!erent steps in depth of thefilm; (i) sketch of the photo-di!used Ge30Se70 film with the residual Ag film dissolved; (j–k) optical micrographs of the front and back of the Ag photo-di!used Ge30Se70 film; (l) Raman spectra form the front and back of the Ag photo-di!used Ge30Se70 film nad the Raman spectrum of Ge30Se70 glass.

M. Jin et al. / Journal of Non-Crystalline Solids 354 (2008) 2719–2723 2721

4. Discussion

The good homogeneity of the pure Ge–Se films demon-strated by the microscopic study and the analogy in theRaman scattering from the front and back sides of the filmsprove the suitability of the evaporation conditions/rateused. This is a good precondition for a uniform distributionof the di!used Ag. The variety of building blocks formingthis film is indication for the specific structure that developsin the Ge–Se system. It has been demonstrated [9] that theformation of ethane like structural units containing Ge–Ge bonds starts at composition Ge32Se68 which is the realcomposition of the studied films. Hence the Raman spectracorrespond well with the RBS data about films composition.On grounds of stoichiometry an equivalent number of Se–Se

bonds are available Fig. 1(d). This is a very rare case in chal-cogenide glasses in which all possible building blocks emergein one composition. The implication of this e!ect is thatmetastable states [10] occur on Se atoms with di!erentsurroundings upon illumination. The photo-electro-ionicphenomenon presenting in essence the Ag photodi!usionin chalcogenide glasses evolves in the followingmanner: firstthe lone pair electron of the chalcogen atom is excited andthen the unpaired electronic orbital pulls the Ag+ and formsa bonding orbital between Ag and Se [11]. Therefore, thereare chances that this e!ect occurs on Se from the chains aswell as on Se that is part of some other structural units. Thisis the main reason that phases formed after Ag photodi!u-sion di!er from those obtained by quenching of a bulk glasswith analogous composition.

The Raman data from the backside studies of the sam-ples – Fig. 1(h) support the idea for formation of Ge-richmaterial due to the absence of the edge sharing and Sechain building blocks. The only explanation of this e!ectis that the Ge–Se backbone is depleted in Se due to its con-sumption for formation of the di!usion products with Agas established at earlier studies [12]. Indeed the data dis-cussed so far correspond in full to the data obtained at ear-lier studies and they can be considered as good evidenceabout the fact that Ag distributes in full depth of the dif-fused films as established for example by Auger spectros-copy for the case of Ag di!usion in Ge–S films [13].However the results obtained from the front surface ofthe films after dissolution of the Ag films are quite untradi-tional. The Raman data related to the Ag8GeSe6 ternaryare very indicative about the structure forming in this case.We identify the Raman scattering from the front surface ofsample C with the disordered structure of the glassy Ag8G-eSe6 because of the great similarity between the two typesof spectra (Fig. 3). This composition is very rich in Ag(53.33 at.%) and Se is fourfold, fivefold, sixfold and eight-fold coordinated in it [14]. As one can follow from the datain Fig. 2, the structure of the host continuously changes indepth with the GeSe4 mode undergoing blue shift andproving formation of Ge-rich network. The inset ofFig. 2 gives more evidence for this.

Formation of structure on the interface Ag/Ge–Se glasswhich is di!erent than the structure in the bulk of the filmsis the most interesting result of this work. We suggest thatthis shows relationship between the structure forming atAg photodi!usion and the intensity of light applied forthis. Illumination with light with much lower intensity thanthe one used in this experiment leads to occurrence pre-dominantly of Ag2Se and immediate formation of thestructural organization of the films, characteristic for thedepth of the studied films in this work [2] which we under-stand as a milder e!ect over the structure of the host.

5. Conclusions

In this work we performed structural studies of thinGe30Se70 films which were photodoped with Ag. In depth

Fig. 2. Vibrational mode frequency of corner-sharing GeSe4 tetrahedrafrom the front sides of samples A and C; Insertion: Development of theRaman mode scattering in depth from the front side of sample C.

Fig. 3. Raman scattering characteristic for Ag8GeSe6 crystal, Ag8GeSe6glass and sample C front surface.

2722 M. Jin et al. / Journal of Non-Crystalline Solids 354 (2008) 2719–2723

Raman profiles show that at high intensity light irradiationamorphous Ag8GeSe6 forms on the interface Ag/Ge–Sefilm whose structure gradually changes leading to creationof Ge–Ge bonds and formation of ethane like structuralunits within the hosting glass. Ag penetrates the entiredepth of the films depleting the Ge–Se backbone in Sedue to the chemical reaction between the two. Formationof heterogeneous structure after Ag di!usion into theGe–Se films is confirmed by optical microscope images.

References

[1] M.N. Kozicki, M. Mitkova, US Patent 6,998,312.[2] M. Mitkova, M.N. Kozicki, H.C. Kim, T.L. Alford, J. Non-Cryst.

Solids 338&340 (2004) 552.[3] S. Zembutsu, Appl. Phys. Lett. 39 (1981) 969.

[4] C.H. Chen, K.L. Tay, Appl. Phys. Lett. 37 (1980) 605.[5] E. Bychkov, Solid State Ionics 136&137 (2000) 1111.[6] M. Mitkova, Yu Wang, P. Boolchand, Phys. Rev. Lett. 83 (1999)

3848.[7] A. Piarristeguy, M. Ramonda, A. Urena, A. Pradel, M. Ribes, J.

Non-Cryst. Solids 353 (2007) 1261.[8] A. Piarristeguy, G.J. Cuello, B. Arcondo, A. Pradel, M. Ribes, J.

Non-Cryst. Solids 353 (2007) 1243.[9] P. Boolchand, D.G. Georgiev, T. Qu, F. Wang, L. Cai, S. Chakrav-

arty, C. R. Chim. 5 (2002) 713.[10] K. Shimakawa, A. Kolobov, S.R. Elliott, Adv. Phys. 44 (1995) 475.[11] G. Kluge, Phys. Stat. Sol. (a) 101 (1987) 105.[12] M. Mitkova, M.N. Kozicki, H.C. Kim, T.L. Alford, J. Non-Cryst.

Solids 352 (2006) 1986.[13] M. Mitkova, M.N. Kozicki, H.C. Kim, T.L. Alford, Thin solid Films

449 (2004) 248.[14] P.D. Carre, R. Ollitrault-Fichet, J. Flahaut, Acta Crystallogr. B36

(1980) 245.

M. Jin et al. / Journal of Non-Crystalline Solids 354 (2008) 2719–2723 2723