X-ray photoelectron studies of clean and oxidized α-GeTe(111) surfacesL. V. Yashina, R. Püttner, V. S. Neudachina, T. S. Zyubina, V. I. Shtanov et al. Citation: J. Appl. Phys. 103, 094909 (2008); doi: 10.1063/1.2912958 View online: http://dx.doi.org/10.1063/1.2912958 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v103/i9 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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X-ray photoelectron studies of clean and oxidized �-GeTe„111… surfacesL. V. Yashina,1,a� R. Püttner,2 V. S. Neudachina,1 T. S. Zyubina,3 V. I. Shtanov,1 andM. V. Poygin2

1Department of Chemistry, Moscow State University, Leninskiegory 1-3, 119991 Moscow,Russian Federation2Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin-Dahlem, Germany3Institute of Problems of Chemical Physics, RAS, 142432 Chernogolovka, Moscow Region,Russian Federation

�Received 24 December 2007; accepted 26 February 2008; published online 7 May 2008�

Clean and oxidized �104–1015 L of O2� surfaces of �-GeTe have been investigated with x-rayphotoelectron spectroscopy by using the synchrotron radiation facility BESSY II as well as an Al K�source. To understand the first steps of oxidation, complementary quantum chemical calculationswere performed. The cleaved surfaces of �-GeTe were found to be rumpled with �111� domains thatcan be related to the domain �twin� structure of the bulk. Both the Ge 3d and the Te 4d spectra offreshly cleaved surfaces exhibit at least three components, which are explained by a Ge or Tetermination of the surface domains with possible contributions of a surface reconstruction. Thesurface oxidation starts at exposures of 104 L and proceeds via several steps. At low exposures,only changes in the Ge spectra are observed. Consequently, the first step of the reaction is associatedwith the formation of intermediate peroxidelike structures, wherein both oxygen atoms are bondedto germanium atoms. In the range of exposures between 1010 and 1015 L, a layer of a relativelystable oxidation product with the approximate stoichiometry Ge1+�

+4 Te1−�0 O2�1+��

2− is formed, whichshows growth kinetics that obey a time-logarithmic law. At this stage, the peroxidelike structures arestill present at the oxide/crystal interface. Once the oxidized layer exceeds a thickness of �2.5 nmat �1013 L, a transformation of the Te0 state into the Te+4 state is observed at the surface of theoxide layer. The final oxidation product can be described as mGeO2�nTeO2. © 2008 AmericanInstitute of Physics. �DOI: 10.1063/1.2912958�

I. INTRODUCTION

Germanium telluride is a narrow-band-gap semiconduc-tor, which is routinely used as a thermoelectric material. An-other modern application for GeTe—as well as for the otherchalcogenide materials such as SbTe and GeSbTe—lies inthe field of optical and electrical storage devices. This is dueto its reversible crystalline to amorphous phase transitions,which can be easily detected due to huge changes in theoptical reflectivity or the electrical resistivity.1–5 Moreover,stoichiometric films of GeTe can be crystallized by usinglasers with a pulse duration of less than 100 ns.6 For thisreason, it was very recently proposed to use thin amorphousGeTe films1 and nanowires2,3 as well as stacked GeTe/SnTelayers4 to produce nonvolatile phase-change random accessmemory devices.5 These nonvolatile memory devices withfast writing and reading speeds have attracted great attentionsince they possess the potential to overcome many intrinsiclimitations of conventional memory devices, such as dy-namic random access memory, static random access memory,and flash. Finally, due to its structure, �-GeTe belongs to theferroelectric crystals, which have a multiplicity of techno-logical applications.

Up until now, most of the research on GeTe focused onaspects of application, such as design, fabrication, and per-

formance evaluation of devices. Only few recent publica-tions addressed the fundamental properties of GeTe, namely,its band structure7,8 as well as the local structure in the crys-talline and amorphous phases of the bulk.9,10 However, infor-mation on the surface properties is practically absent. To ourknowledge, a possible relaxation and reconstruction as wellas the surface core-level shift of clean surfaces of crystalline�-GeTe have not been studied in detail up until now. In arecent study, the x-ray photoelectron spectrum of theoxygen-free surface of a sputtered GeTe film was by partiallyinterpreted partially the existence of Ge–Ge bonds.11 How-ever, in these studies, the nature of the sample was notclearly specified and its structure might have been essentiallydifferent from that of an �-GeTe crystal, in particular, sincethe surface was sputtered with Ne+ ions. In addition, thephoton energy of 635 eV used in these experiments did notprovide a high surface sensitivity.

For the manufacturing of devices based on GeTe, infor-mation on the surface stability in an oxygen-containing en-vironment is of great importance. It was, e.g., shown that theoptical properties of thin amorphous GeTe films are stronglyinfluenced by oxygen impurities.12 Several years ago, theoxidation mechanism of GeTe in an ambient atmosphericenvironment was studied �see Ref. 13, and referencestherein�, and it was found that the GeTe surface does notresist oxidation; already several minutes of exposure to airresult in the oxidation of all germanium surface atoms into aGe+4 state without the surface passivation. Moreover, a two-

a�Author to whom correspondence should be addressed. Electronic mail:yashina@inorg.chem.msu.ru. Tel.: �7 495 939 4665. FAX: �7 4959390998.

JOURNAL OF APPLIED PHYSICS 103, 094909 �2008�

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step oxidation process was reported. In detail, the first stepincludes simultaneous Ge+2→Ge+4 and Te−2→Te0 transfor-mations, and the second step consists of a Te0→Te+4 trans-formation. However, for the preparation of GeTe-based ma-terials, it is important to understand the behavior at oxygenpressures substantially lower than 1 bar since production-related processes are often performed under vacuum condi-tions. Therefore, from the industrial as well as from the fun-damental point of view, investigations of fresh surfaces aswell as of the very initial steps of oxygen adsorption are offundamental interest.

Studies of clean �-GeTe surfaces are rather difficultsince the sub-�m domain �twin� structure of the bulk mate-rial leads to relatively small surface areas of well-definedorientation. As a result of this, most experimental techniquesdo not allow investigating a single surface domain. The do-main structure in the bulk material is due to the ferroelectriccubic-rhombohedral ��→�� phase transition that occurs dur-ing the postgrowth cooling process at temperature of ap-proximately 700 K.14,15 This phase transition due to an innerdisplacement of the atoms along one of the eight equivalentdirections of the type �111� goes along with a stretch of thelattice in this direction. This phase transition results in Ge–Tedouble layers that define the �111� cleavage planes for�-GeTe. Additionally, since the rhombohedral angle stronglydepends on the temperature, the postgrowth cooling processafter this phase transition leads to a stress in the lattice and toa division of each domain into a set of subdomains that arenearly parallel to each other.

This publication is devoted to investigations on the sur-faces of �-GeTe crystals freshly cleaved under UHV condi-tions, as well as to their oxidation behavior in a wide rangeof oxygen exposures. This includes pressures essentially be-low 1 bar to get further insight into the oxidation mechanismcompared to our previous study.13

II. EXPERIMENTAL DETAILS

Several samples of �-GeTe have been obtained from onecrystal that was grown by using the vapor-liquid-solidmethod, as described in Ref. 16. The sample compositionwas close to 50 at. % Te, i.e., near the Ge-rich boundary ofthe homogeneity range15 as deduced from x-ray diffractiondata.

For the interpretation of surface reactions, it is essentialto know the surface sensitivity of the method applied. In thepresent studies, the photoelectron spectra of the Ge and Tecore levels were recorded by using the photon energies h�=125 eV and h�=1486.6 eV. The first value allows obtain-ing of a high surface sensitivity for the Ge 3d and Te 4dlevels, while the second value results in a relatively highsurface sensitivity for the Ge 2p level and a high bulk sen-sitivity for all other levels. The estimated spectral contribu-tions of the two uppermost layers for the different experi-mental conditions are summarized in Table I.

The x-ray photoelectron spectra that used a photon en-ergy of 125 eV were recorded at the Russian-German beam-line of the synchrotron radiation facility BESSY II in Berlin,Germany. This beamline is equipped with a Petersen-type

plane-grating monochromator and provides a high spectralresolution.18 The spectra were taken with an experimentalsetup equipped with a CLAM 4 �Thermo VG Scientific�electron energy analyzer. The experiments were performed ata base pressure in the low 10−10 mbar range. Clean surfacesof �-GeTe were prepared by cleaving the crystals. The sur-faces of GeTe were directly studied after the cleavage, aswell as after several consecutive oxygen exposure cycles.The exposure to O2 was performed in the preparation cham-ber at room temperature by using a high purity gas�O2-99.998 vol %�. In the following, the exposures of thesurfaces to 1.4�104, 2.5�105, and �1�1010 L are labelede1 , e2, and e3, respectively. During the measurements, thesample holder was cooled with liquid nitrogen since thisleads to a decrease in the phonon broadening in the photo-emission spectra.

At h�=125 eV, the Ge 3d and Te 4d spectra were mea-sured by using an analyzer pass energy of 2.5 eV; in thisway, an overall �monochromator and analyzer� energy reso-lution of about 40 meV full width at half maximum wasobtained. The absolute energy scale is limited by the me-chanics of the monochromator and is on the order of�0.2 eV. The detection angle was �80°, i.e., close to nor-mal emission. The surface size of the crystal was �5�5 mm2, while the diameter of the x-ray spot was estimatedto be on the order of 100 �m; its exact position at thesample surface was not precisely controlled.

At h�=1486.6 eV, complementary oxidation studies onthree additional samples were performed in a wider range ofexposures �103–1015 L� by using an ESCALAB MKII spec-trometer equipped with an achromatic Al/Mg dual x-raysource and a hemispherical electron energy analyzer. Allspectra were recorded in a constant analyzer pass energymode of 20 eV, resulting in a resolution of about 1 eV. Thesamples were cleaved in the preparation chamber at a basepressure of �10−8 mbar and transferred to the analysischamber. With this experimental setup Ge 3d, Ge 2p3/2,Te 4d, Te 3d5/2, and O 1s spectra were recorded. In order tostudy the oxidation kinetics, the GeTe samples have beenexposed to oxygen for defined periods of time by using pres-sures in the range of 10−5 mbar to 1 bar. Prior to this expo-sure, the oxygen was purified by keeping it in an isolatedvolume with granulated sodium hydroxide for several days.The surface microstructure of the samples measured with the

TABLE I. The IMFP and the calculated intensity fractions for the two up-permost layers as realized under the different experimental conditions. TheIMFP is calculated by using the TPP 2m formula �Ref. 17� and the contri-bution of the uppermost layer by Hill �Eq. �3��.

Level

Photonenergy�eV�

IMFP �20%��

First layer-relatedintensity fraction

�%�

Second layer-relatedintensity fraction

�%�

Ge 3d 125.0 5.20 386 255Te 4d 125.0 5.03 396 255Ge 2p 1486.6 8.70 295Ge 3d 1486.6 29.30 102Te 3d 1486.6 20.3 143Te 4d 1486.6 28.86 102

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ESCALAB MKII spectrometer was subsequently studied byscanning electron microscopy �SEM�, by using a Leo Supra50VP device. In addition, scanning tunnel microscopy�STM� images were obtained for other surfaces freshlycleaved in air and then simultaneously transferred to an inertgas environment.

For the data analysis, the spectra were fitted by theGaussian–Lorentzian convolution functions with simulta-neous optimization of the background parameters. The back-ground was described by the function

U�E� = a + s � S�E� + t � T�E� , �1�

where S�E� is a Shirley background and T�E� is a Tougaardbackground with the parameter C=1643 eV2. For the spec-tra recorded with the achromatic x-ray source, a subtractionof satellite structures was performed prior to the fitting pro-cedure. Additional details on the fitting procedure are givenbelow.

III. RESULTS

A. Clean surfaces

1. The surface morphology

For a better understanding of the photoelectron spectra,we shall first discuss the surface morphology of freshlycleaved GeTe. Typical surface images obtained by SEM areshown in Fig. 1. The size of the image in Fig. 1�a� approxi-mately matches the size of the x-ray spot used in the syn-chrotron radiation experiments. Larger magnifications inFigs. 1�b� and 1�c� show further details of the surface mor-phology obtained by cleavage. A comparison to the transmis-sion electron microscope images presented in Ref. 19 allowsus to conclude that the observed surface morphology reflectsthe bulk domain structure. Figure 2�a� displays the schematicof an ideal �-GeTe crystal. The thick �thin� lines representthe Ge �Te� �111� planes and it can be seen that the distancebetween layers formed by different species is altering. Sincecleavage preferentially occurs between two layers with along distance, the surface is either Ge or Te terminated. Fig-ures 2�a�–2�f� show the schematics of the possible domainboundaries, which are indicated by the twinning planes�dashed lines�. At the domain boundaries, the �001� and �011�twinning planes cause angles of 109° and 70.5°, respectively,between the tellurium or germanium layers. These anglesalso give rise to a rumbled surface structure. Figure 2�g�shows a schematic of the �-GeTe surface in relation to thebulk domain structure with the front plane being of the �001�orientation.

With available information on the phase transformationmechanism mentioned above in combination with geometri-cal aspects, let us assume that the domain boundary corre-sponds to the �110� twinning plane and the “lamella” surface

have �111� and �1̄1̄1� orientations in the original cubic lat-tice, as visualized in Fig. 1�b�. The dashed white line indi-cates a �110� twinning plane that is oriented perpendicular tothe surface while the white parallelograms I and II indicate�111� surface domains. As a consequence, the stripes in Fig.1 correspond to the subdomain boundaries that are parallel to

atomic layers in the �111� plane. Therefore, the subdomainsurfaces are either Ge or Te terminated, as shown in Figs.2�a�–2�f�. Domain boundaries caused by the �100� or the�111� twinning planes are not fully excluded. For the samplesunder investigation, the domains are one to several micronswide and several tens of microns long; the subdomains areseveral microns wide and �0.1 �m long. The STM imagein Fig. 1�d� provides information on the surface roughnessand suggests that the subdomain surfaces are composed byatomically flat terraces.

2. The photoelectron spectra

The Ge 3d and Te 4d photoelectron spectra of freshlycleaved GeTe surfaces are shown in Fig. 3�a� and the corre-sponding fit parameters are given in Table II. The presentresolution is higher than for recent GeTe spectra by Jung etal.11 and allows to resolve the spin-orbit splitting for both theGe 3d �0.560.01 eV� and the Te 4d �1.470.03 eV� lev-els.

To obtain a good fit description for the asymmetric linesin the Ge 3d spectrum, we employed three spin-orbit splitcomponents. In contrast to this, Jung et al.11 described theirspectrum, which does not resolve the spin-orbit splitting,with only two components. These authors assigned the majorcomponent to the Ge 3d bulk state in a GeTe surroundingand the minor component at lower binding energies toGe–Ge bonds in a quasiamorphous Ge phase that separatesdifferent GeTe crystallites. This assignment is based on thelocal structure obtained for thin GeTe films by using ex-tended x-ray absorption fine structure spectra.9 In our spec-tra, the most intense component is labeled Ge I and can beassigned to the bulk state based on its behavior under oxida-tion �see below�. Its relative intensity also agrees well withthe theoretically predicted bulk intensity fraction �compareTables I and II�.

Note that contrary to Jung et al., we do not see a com-ponent at the lower binding energy side of the dominantcomponent Ge I. This might be due to a higher crystal qual-ity than in the studies of Jung et al.; in the latter case, it isinfluenced by the Ne+ sputtering. The parameters of compo-nent Ge II are reproducible for all fitted spectra; the averagevalue of its chemical shift relative to Ge I amounts to19515 meV. Note that throughout the publication, allchemical shifts are given relative to component Ge I �forGe 3d�, Te I �for Te 4d�, Ge i �for Ge 2p3/2, see below�, andTe i �for Te 3d5/2, see below�. Component Ge III has a largertotal width and the peak parameters scatter to a larger extendthan those for components Ge I and Ge II. As shown in TableII, the average shift of Ge III amounts to 51432 meV.However, the average value of 514 meV does not lead tosatisfactory fit results for all spectra. We, therefore, assumethat this component consists of a number of unresolved lineswith slightly different binding energies; their relative inten-sities depend on the position of x-ray spot on the surface.

The widths of the components in the Te 4d photoelec-tron spectrum in Fig. 3�a� exceed those recently obtained forPbTe and SnTe �100� single crystals under the same experi-mental conditions.20 Although individual states are appar-ently not resolved in the Te 4d spectra, three different spin-

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FIG. 1. SEM images of an �-GeTe surface obtained by cleaving thesamples. �a�–�c� show increasing magnifications of the sample, which areindicated by the scale below the images. �d� STM image of two subdomains.In �b�, the white dashed line indicates a �110� twinning plane oriented per-pendicular to the surface and the white parallelograms I and II surface

domains with an �111� and �1̄1̄1� orientation in the initial cubic lattice,respectively.

FIG. 2. �a� The schematic of the domain structure of a �-GeTe crystal. Thebold �thin� lines indicate the Ge �Te� �111� planes. ��b�–�f�� The differentpossible twinning structures and �g� a schematic of the GeTe surface in itsrelation to bulk domain structure �g�.

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orbit split components are necessary for a good fitting �seeFig. 3�a��. Based on their behavior under oxidation, we sup-pose that component Te I can be related to bulk atoms. Al-though there is an uncertainty in the absolute binding energyscale �see Fig. 1�, which is mainly due to not carefully moni-tored changes in the work function caused by the oxygen andpartially to the mechanics of the monochromator, the energydifference between components Te I and Ge I is equal to10.610.02 eV for all spectra both before and after oxida-tion. The chemical shifts for Te II and Te III are 270 and−327 meV, respectively. Like Ge III, component Te II ischaracterized by a larger widths and a more pronounced scat-tering of the spectral parameters.

In the following, we want to discuss the origin of at leastthree components in the Ge 3d and Te 4d spectra. For this,

we will consider two different explanations. First, in an�-GeTe crystal structure, there are two different Ge–Te bondlengths �2.801 and 3.136 Å�,9 i.e., for each Ge atom, thereare three shorter and three longer Ge–Te bonds that give riseto double layers perpendicular to the �111� direction �see Fig.2�a��. Evidently, during cleavage, the longer bonds break andthe resulting polar �111� surface domains of GeTe can beeither Ge or Te terminated with a statistical distribution. Inthe case of a Ge termination, the germanium atoms at theideal surface �without relaxation and reconstruction� haveonly three shorter bonds. This should lead to a decrease inthe electron density and to an increase in the binding energyof the core level. For the Te atoms in the subsurface layer,however, the change in the electron density and, therefore,the change in the binding energy compared to the bulk stateshould be much smaller. Vice versa, for a Te-terminated sur-face, the binding energy of Ge atoms in subsurface layerdiffers less from that in the bulk of GeTe; however, Te atomsshould acquire a more negative charge and a lower bindingenergy. In this model, components Ge III and Te II belong toGe-terminated surfaces and subsurfaces, respectively, whilecomponents Ge II and Te III belong to the Te-terminatedones.

A second possible reason for several components is asurface reconstruction. Unfortunately, no information on sur-face reconstruction of GeTe is available since the surfacewidth of each lamella is only on the order of 100 nm, and thesurfaces are not parallel to the “overall” orientation of thecleavage plane. Therefore, only methods that allow the studyof very small surface areas with an edge length of less than100 nm, such as atomic resolution STM, can be used toinvestigate the surface reconstruction. However, a �2�2� re-construction was observed for a Pb-terminated PbTe �111�surface by using Rutherford backscattering.21 For GeTe, themore covalent character of the chemical bonds as well as thetendency to form Ge–Ge bonds are strong arguments for thepresence of a surface reconstruction.

In conclusion, based on the observations subsequent tothe oxidation �see below�, we assign Ge III to germanium inthe uppermost layer of a Ge-terminated surface and Ge II toa Te-terminated surface. The latter component might havesome contributions caused by the reconstruction of a Ge-terminated surface, although it is unlikely since this compo-nent is rather narrow. In contrast to this, component Ge III ismuch broader so that it is more likely that a possible surfacereconstruction leads to this broadening. This might also holdfor component Te II of a Te-terminated surface.

B. Oxygen adsorption

In Sec. III B, we present the spectra for GeTe surfacesafter different exposures to oxygen in the range from 104 to1015 L. For low exposures up to 109 L, high resolution mea-surements were performed by using synchrotron radiationand a photon energy of 125 eV. This photon energy allowssurface sensitive measurements for the Ge 3d and Te 4d lev-els and is, therefore, well suited to study adsorption. In ad-dition, higher exposures to oxygen and, therefore, thicker

FIG. 3. �Color online� The Ge 3d and Te 4d photoelectron spectra recordedwith a photon energy of 125 eV. �a� a clean GeTe surface and �b� and �c�after oxygen exposures e2 and e3, respectively.

TABLE II. The fit results for the Ge 3d and Te 4d peaks of a clean GeTe�111� surface measured at h�=125 eV �compare Fig. 3�a��. Given are theassignments, the chemical shifts, as well as the relative intensities. Thespin-orbit splitting has been fixed �0.560.01 eV for Ge 3d and1.470.03 eV for Te 4d�.

Component AssignmentWidth�meV�

Chemical shift�meV�

RelativeIntensities

�%�

Ge 3dGe I Bulk 29315 0 463Ge II Te-terminated surface 29615 19515 201Ge III Ge-terminated surface 54234 51432 343

Te 4dTe I Bulk 46015 0 532Te II Ge-terminated surface 67628 27840 284Te III Te-terminated surface 39315 −32730 192

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oxide layers were studied with an Al K� source. The follow-ing discussion will be started with the high resolution andsurface sensitive measurements.

1. Chemical transformations at submonolayercoverages

The photoelectron spectra of oxidized surfaces subse-quent to exposures e2 and e3, which were recorded with aphoton energy of 125 eV, are shown in Figs. 3�b� and 3�c�,respectively. Since no sign of chemisorption was observedfor the lowest exposure e1, the spectra are not shown here.Exposure e2 resulted in three broad components on the high-energy side of the Ge 3d spectrum, which are labeled Ge Vto Ge VII, but the corresponding Te 4d spectrum shows nochanges. This indicates that a Ge-terminated surface is morereactive to oxygen than a Te-terminated one. Exposure e3leads to an increase in the intensities of the oxidation-relatedcomponents Ge V to Ge VII and to a shoulder on the higherbinding energy side of the Te 4d spectrum. The broad com-ponents Ge V to Ge VII consist of several unresolved com-ponents, which form a “band” with a width of �3 eV. Forthis reason, we used the smallest possible number of peaksfor a good description of all data by taking into account theresults obtained for the clean surface described above; thesefit results are summarized in Table III. In our description,four components appear as a result of surface oxidation.Components Ge V to Ge VII already appear after exposuree2, i.e., at lower exposures to oxygen, while component GeIV is observed only after exposure e3. In the latter case, thesurface-related components Ge II and Ge III vanished; thisindicates a complete oxidation of the uppermost double layerand supports the assignment of these components to surfaceor subsurface atoms. After exposure e3, the component lo-cated at the highest binding energy, Ge VII, becomes the

most intense one in the spectrum. The intensity of the com-ponent Te V indicates that the first surface layer is com-pletely modified by the oxidation process. The initialsurface-related structures Te II and Te III disappeared. Wewant to point out that an additional narrow and less intensestructure Te IV with a negative chemical shift relative to thebulk state can be seen in the spectrum. We suppose thatcomponents Ge IV and Te IV correspond to Ge and Te atomsin slightly different electron density surroundings in the sub-surface layer of Te- and Ge-terminated surfaces, respectively.

In order to explain the observed components in terms ofthe arising chemical bonds and to propose a reaction mecha-nism on an atomic scale, we performed quantum chemicalcalculations by using a simple model. This model describesthe interaction of one oxygen molecule with the �GeTe�4

cluster displayed as structure 1 in Fig. 4. We recently appliedthis approach successfully to qualitatively explain the surfacemodifications of the A4B6 semiconductors PbTe and PbS dur-ing oxidation.22 In these studies, it turned out that this ap-proach is useful to predict the tendencies in the reactivitywithin a sequence of compounds. We have also shown thatthe results of this approach are in reasonable agreement withthose obtained for larger clusters �including 68 and 102atoms20�, although the adsorption enthalpy is overestimatedin the case of small clusters.

In the calculations, the �GeTe�4 cluster was in a singletstate and for an infinite distance between both compounds,the O2 molecule was assumed to be in a triplet state. Thegeometry optimization and vibrational frequency calcula-tions were performed by using the hybrid density functionalB3LYP method �with the exponents equal to 0.237 �d� for Teand 0.246 �d� for Ge �Ref. 23�� and by using the pseudopo-tential LanL2 for the Te and the Ge atoms and using a6-31G* basis for the O atom �B3LYP /LanL2DZ*�. All cal-

TABLE III. The fit results for the Ge 3d and Te 4d peaks of a GeTe �111� surface after exposures e1 , e2, ande3 measured at h�=125 eV �compare Figs. 2�b� and 2�c��. Given are the assignments, the chemical shifts, aswell as the relative intensities. The spin-orbit splitting has been fixed �0.560.01 eV for Ge 3d and1.470.03 eV for Te 4d�.

Relative intensities�%�

Component AssignmentWidth�meV�

Chemical shift�meV� Exposure e1 Exposure e2 Exposure e3

Ge 3dGe I Bulk 29315 0 45 38 28Ge II Te-

terminated surface29615 29615 20 14 17

Ge III Ge-terminated surface 54234 54234 35 0 0Ge IV Ge2+–O2

− 20630 32150 0 0 0Ge V Ge2+–O2

2− 91350 738100 0 29 20Ge VI Ge2+–O2− 95250 1854100 0 11 19Ge VII Ge4+–O2− 101850 2850100 0 8 32

Te 4dTe I Bulk 46015 0 55 49 29Te II Te-terminated surface 67628 27840 29 33 0Te III Ge-terminated surface 39315 −32730 16 18 0Te IV Te–O2−–Ge2+ 36630 −27650 0 0 10Te V Te–O2−–Ge4+ 104930 48550 0 0 61

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culations were performed with the GAUSSIAN-03 �Ref. 24�program package and all structures were completely opti-mized. The minima of the potential surface were confirmedby real vibrational frequencies obtained in a complete vibra-tional analysis in a harmonic approximation. The relativeenergies of the calculated isomers are given with zero-pointenergy �ZPE� corrections. In addition, we derived the effec-tive atomic charges in the approximation given by Mulliken.The reaction enthalpy has been estimated by using

H = Estructure − �Ecluster + EO2� , �2�

where Estructure corresponds to the total energy of the obtainedstructure after optimization, Ecluster to the energy of the initial�GeTe�4 cluster, and EO2

to the energy of one oxygen mol-ecule.

Figure 4 illustrates all of the obtained structures. In thecalculations, three types of structures have been considered:peroxidelike structures �type 2�, structures with both oxygenatoms bonded to the same Te atom �type 3�, and structureswith both oxygen atoms bonded to the same Ge atom �type4�.

It was found that the formation of peroxidelike structures�type 2� is energetically favorable. In these structures, theO–O bond lengths are 1.46–1.55 Å, which is close to thevalue for H2O2 �1.45 Å�. Structure 2a is characterized bythe presence of two Ge–O and one Te–O bond, while instructure 2d, both oxygen atoms are bonded to the same Geatom. The energetically most favorable structures 2b ,c donot contain Te–O bonds but possess a relatively high�0.22–0.27e� Ge–O bond population. In these structures, theelectron density is considerably transferred from the Ge tothe O atoms.

Two different structures of type 3 with structure 3b be-ing somewhat lower in energy are also shown in Fig. 4. Inthis case, the tellurium atoms acquire highly positive effec-tive charges of +0.76–1.00e, which essentially differ fromthe initial charge of −0.19e in structure 1. The charge statevalues for the germanium atoms are slightly less than forperoxidelike structures 2. A transition from the energeticallymost favorable structure of type 2 to any structure of type 3

FIG. 4. �Color online� Theoreticalstructures and relative energies �inkcal/mol� for some stationary points ofthe potential surface for the interactionof an oxygen molecule with a �GeTe�4

cluster. The energies were obtained atthe B3LYP /LanL2DZ� level with theZPE correction. The effective chargesby Mulliken are indicated with brack-ets, and all bond lengths are given inangstroms.

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leads to an increase in the Te–O bond population by 0.175 e.In contrast to this, the population of Ge–O bond slightlydecreases.

The enthalpy for the formation of structures of type 4 isgenerally higher than for both structures of type 3. Threepossible structures of type 4a–c with similar reaction enthal-pies have been found, with structure 4c being the most stableone. In structures 4a, 4b, and 4c, the germanium atoms pos-sess effective atomic charges of +0.64–0.67e; these valuesare generally higher than for peroxidelike structures. Thecharges of the Te atoms are +0.17–0.20e, i.e., they are closeto zero. Structure 4d differs from the other structures since inthis case, each oxygen atom is bonded to two Ge and one Te.Here, for this structure, we also calculated a slightly positivecharge of Te atoms bonded with oxygen �+0.20e�. The en-thalpy for the formation of this structure is less favorablethan for structures 4a–c.

Based on the theoretical results, it can be expected thatthe adsorption of oxygen starts with the formation of perox-idelike structures of type 2. These structures posses a nega-tive formation enthalpy and are more stable than similarstructures calculated for the oxidation of PbTe.20 In the per-oxidelike structures of PbTe, the O–O fragment is alwaysbonded with both tellurium and lead atoms, whereas inGeTe, the most stable peroxide structures 2b–c exhibitGe–O bonds only. The effective charges of the germaniumatoms in these structures are by +0.21–0.49e higher than forthe cluster �GeTe�4 and they strongly depend on the geom-etry of the different adsorption structures of type 2, althoughthe structures are close in energy. In contrast to this, theeffective charge of the Te atoms increase only slightly by+0.05–0.1e, so that the values remain negative. These resultsallow us to explain the components observed after exposurese2 and e3, namely, a number of Ge 3d components withhigher binding energies, but only minor changes in the Te 4dspectrum �see Figs. 3�b� and 3�c��.

Once the O–O bonds are broken, the structures with bothoxygen atoms bonded to the same germanium atom �type 4�are preferred on the basis of energy and bond geometry ar-guments rather than those in which both oxygen atoms arebonded to the same tellurium atom. In the structures of type4, the germanium atoms possess the maximal effectivecharge of +0.67e, whereas the effective charge of the tellu-rium atoms becomes close to zero.

2. Oxide layer growth kinetics

The oxidation kinetics was also studied in a wider rangeof exposures that used an Al K�-source. The obtained seriesof spectra are illustrated in Fig. 5. Since the low-energy res-olution and surface sensitivity does not allow to resolve in-dividual components in the spectra of clean surfaces, we de-scribe the corresponding Ge 2p3/2 and Te 3d5/2 spectra withonly one component each, namely, Ge i and Te i. Due to thelow surface sensitivity under the given experimental condi-tions, we assume that components Ge i and Te i are related tobulk atoms, i.e., they correspond to Te I and Ge I. For theoxidized surfaces, the Ge 2p3/2 spectrum is fitted with threecomponents, namely, Ge i, Ge ii with a chemical shift of 1.41eV relative to Ge i, and Ge iii with a chemical shift of 2.85

eV. A comparison of the binding energy for the componentGe iii �1220.6 eV� to the value for the Ge 2p3/2 core level ofa reference sample �GeO2·TeO2� suggests assignment of thecomponent to germanium in a formal +4 state. The signal-to-noise ratio and the surface sensitivity for the Ge 3d spec-tra measured at h�=1486.6 eV is much lower than for thespectra measured at h�=125 eV. We therefore fitted theGe 3d spectra with only two spin-orbit split doublets andrelate the low-energy doublet to Ge in the bulk of GeTe �GeI�.

Both the Te 3d5/2 and the Te 4d spectra of the oxidizedsurface are described by using three components. The com-ponents in the Te 3d5/2 spectra are labeled Te i, Te ii, and Teiii in the order of increasing binding energies with chemicalshifts of +0.5 and +4.24 eV for Te ii and Te iii, respectively.In the Te 4d spectrum, a line broadening of the strongestpeaks was observed with increasing oxidation. This might bedue to contributions of additional unresolved componentswith slightly different binding energies. In addition, a weakcontribution with a chemical shift of +3.9 eV was found andlabeled Te VI. Comparing the energy positions of the com-ponents to the reference data for different samples �see Ref.13, and references therein� allows us to conclude that Te Vand Te ii correspond to a Te0 state, whereas Te VI and Te iiicorrespond to a Te+4 state. The results for the oxidation ki-netics are summarized in Fig. 6. Figure 6�a�, for differentcomponents, depicts the relative intensities as a function ofthe oxygen exposure. The calculated thicknesses h of theoxidized layers are shown in Fig. 6�b�. Note that the data onthe oxidation kinetics extracted from the Ge 2p3/2 andTe 3d5/2 spectra are more reliable than those from Ge 3d andTe 4d spectra due to the low intensity of the latter two struc-tures. These values were derived in the simple uniform layerapproximation by using the Hill equation,

h = � cos � ln�1 +Isurface

Ibulk , �3�

where � is the inelastic mean free path �IMFP� �see Table I�,� is the detection angle, Isurface is the intensity related to theoxidized layer, and Ibulk is the intensity related to the unoxi-dized bulk. Due to the complicated surface morphology dis-cussed above, the uniform layer model gives only approxi-mate values for the thickness. However, the STM image of astrongly oxidized surface presented in Fig. 7 shows that thismodel is still a reasonable approximation since there are evi-dently no islands related to the oxidation product, althoughthe oxide layers are slightly thicker in the vicinity of thesteps. This uniformity of the oxide layer thickness is alsodirectly confirmed by transmission electron microscopymeasurements.3

Figure 6�c� shows the Ge/Te ratio in the analyzed layeras a function of the oxygen exposure calculated on the basisof the observed Ge 2p /Te 3d and Ge 3d /Te 4d intensity ra-tios. The curves indicate that the oxidized layer is enriched ingermanium. This effect is strongest for the curve based onthe intensity ratio Ge 2pox /Te 3dox �diamonds�; here, the in-dex “ox” summarizes all components that are due to the oxi-dation. We can exclude a depletion of Te due to losses intothe vacuum �e.g., TeO2 evaporation� since the changes in

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composition correlates with the time of the oxygen exposurebut not with the time of storage in vacuum. In addition, theGe/Te ratio in the GeTe suboxide layer �Ge i/Te i shown inFig. 6�c� with squares� diminishes with increasing exposure.We therefore suppose that the Ge atoms tend to migrate fromthe bulk to the interface due to their strong affinity to oxy-gen. A depletion of Te in an oxidized surface layer was also

observed in Ref. 2. We estimated the amount of oxygen inthe oxidized layer by using atomic sensitivity factors and

areference sample �GeO2� and found that the layer containsapproximately two oxygen atoms per oxidized Ge atom, i.e.,the Ge-enriched oxidized layer can be formally describedwith Ge1+�Te1−�O2�1+��, where � ranges from 0 to 0.5.

FIG. 5. �Color online� The Ge 2p3/2,Ge 3d, Te 3d5/2, and Te 4d photoelec-tron spectra recorded at 1486.6 eV af-ter sequential exposures to oxygen.The exposure increases from top tobottom.

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In order to study the final oxidation products, we alsoinvestigated one sample that was stored in ambient air for 5.5months. For this case, we found that the surface of thissample contains Ge4+, Te4+, and O2− and can be described asmGeO2·nTeO2.

IV. DISCUSSION AND CONCLUSIONS

The entire set of experimental data for the oxidation ki-netics presented in Fig. 6 can be divided into four differentregions, which will be discussed in the following.

A. Region A

In the range of exposures between 105 and 107 L, anumber of new Ge states are present in the surface sensitiveGe 3d photoelectron spectrum �exposure e2�. They are notindividually resolved, and let us assume that their relativeintensities originate from changes in the uppermost mono-layer. In contrast to this, the tellurium spectrum shows nochanges �Fig. 3�b��. On the basis of the experimental andtheoretical data, we conclude that this region corresponds tothe formation of different peroxidelike structures with oxy-gen atoms being bonded to germanium only. The calculatedatomic charges for the Ge atoms in these peroxidelike struc-

FIG. 6. �Color online� Oxidation ki-netics: �a� the proportional intensitiesof individual components as a functionof exposure, �b� oxide layer thicknesscalculated from different groups ofspectral data, and �c� GeTe ratios ascalculated from the different peaks.

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tures are much higher than the value for the �GeTe�4 clusterand scatters in the range of +0.44–0.57e; this result can ex-plain the presence of a large number of unresolved compo-nents with higher binding energies in the Ge 3d spectrum inFig. 3�b�. The usage of an Al K� photon source results in ahigh surface sensitivity only for the Ge 2p electrons. Thisexplains the intense increase of the oxygen-induced compo-nent Ge ii for exposures up to 107 L while the photoelectronspectra taken at all other core levels remain almost un-changed at such low exposures.

B. Region B

In the range of exposures between 107 and 2�1010 L,the oxide layer is approximately 2 ML thick. The spectrumof the Ge 3d level recorded at hv=125 eV displays an in-tense component with a chemical shift of +2.85 eV relativeto the bulk state �Ge VII after exposure e3 in Fig. 3� and theGe 2p spectra show at least two components �Ge ii and Geiii� in addition to Ge i. We observed that the intensity forcomponent Ge ii decreases and for component Ge iii in-creases with increasing exposure. At the end of this region,all tellurium atoms at the surface are converted into the Te0

state; this follows from the relative intensities of componentsTe ii and Te V. From these results, we conclude that theoxidation product consists of Ge4+, Te0, and O2−. Theseproducts correlate with the formation of structures of type 4and contain approximately two oxygen atom per Ge atom,i.e., the composition of the Ge-enriched layer can be for-mally expressed by Ge1+�Te1−�O2�1+��. The intensity of com-ponent Ge ii, which is assigned to an intermediate peroxide-like structure, starts to decrease in region B �Fig. 6�a��. Thisobservation suggests that peroxides exist only in an interfacelayer and we believe that they are not a time-dependent phe-nomenon since �1� our experiments are ex situ measurementsperformed a relatively long time after exposure, �2� thesurface-sensitive Ge 3d spectra show that peroxide-related

components exist at least for several hours, and �3� the datafor the kinetics are well reproduced for three differentsamples by using different time-pressure combinations. Notethat the observed components in this region correspond tothe first step of oxidation described in Ref. 13 since regionsA and B correspond to an oxygen pressure essentially lowerthan 1 bar.

C. Region C

In the range of exposures between 2�1010 and 2�1013 L, the thickness of the oxidation product, namely, theGe1+�Te1−�O2�1+�� layer, linearly increases with the logarithmof the exposure. This behavior can be described by the func-tion

N�t� =1

bln�ab� +

1

bln�t + t0� , �4�

which is the solution of the Elovich equation25,26

dN

dt= a exp�− b � N�t�� , �5�

and is useful for the analysis of kinetic curves. Here, a and bare constants, N is the amount of oxygen attached to thesurface, t0 is a constant indicating the pre-Elovichian regionthat often precedes the Elovichian rate process, and t is theexposure time. The Elovich equation describes many surfacephenomena, ranging from adsorption and desorption to oxi-dation, and can be derived in different ways �surface rough-ening, progressive diffuse interface,25 tunneling-limitedoxidation,26 etc�. In the present case, we assume that thelayer thickness h is proportional to N. We also assume thatthe oxygen exposure is proportional to the exposure timesince a constant partial pressure PO2

=1 bar was used in re-gion C. An alternative recent model for the oxidation of sili-con at room temperature given by Cerofolini et al.26 consid-ers a mechanism that may be similar to the present case sinceit is also based on a layer of an intermediate product near theinterface. The dotted line in Fig. 6�b� is derived from thismodel and shows that the agreement with the data is worsecompared to the solution of Eq. �5�.

The experimental data at low exposures are not sufficientto draw detailed conclusions for the oxidation mechanismincluding adsorption, surface reaction, interface structure,and oxygen �or germanium� transport. Nevertheless, theElovichian parameters allow the estimation of the oxidethickness, which may be important for industrial applica-tions. Besides, these parameters can be compared to the cor-responding values for similar substances. For the interactionof GeTe with O2, we obtained the parameters a=0.4�10−9,b=0.33, and t0=2�1010 L. We would like to point out thatthe oxidation rate parameter 1 /b is about two times higherthan for the IV-VI semiconductors PbTe and SnTe,20,27 i.e.,GeTe is more reactive toward oxygen than the other twosemiconductors with respect to the oxide layer growth rate.

In region C, the Ge ii intensity gradually decreases sinceit corresponds most probably to a peroxide-related state thatis stable only at the interface. The values for the oxide layerthickness calculated from Te 3d and Te 4d spectra are lower

FIG. 7. STM image of an oxidized GeTe surface after �1011 L of oxygenexposure.

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than those calculated on the basis of Ge data; this observa-tion can be explained with the enrichment of the surface withGe.

D. Region D

At exposures above 2�1013 L, the tellurium atoms atthe surface of the oxidized layer slowly transform from aneutral Te0 charge state to a Te+4 state. This is accompaniedby a further increase in the total thickness of the oxidizedlayer. In order to accurately model the surface oxidation inregion D, the adsorption of more than one oxygen moleculehas to be taken into account so that the presented theoreticalresults cannot explain the surface behavior in this region. Itis interesting to note that this oxidation step was more dis-tinctly observed in Ref. 13, in which the total oxidation timeat ambient pressure was essentially longer.

Finally, the calculated structures of type 3 �with bothoxygen atoms bonded to the same Te atom� do not correlatewith any experimental data, although their formation ener-gies are close to those of type 4.

V. SUMMARY AND OUTLOOK

We presented x-ray photoelectron spectra of clean andoxidized �104–1015 L of O2� �-GeTe�111� surfaces. Thehigh-resolution Ge 3d and Te 4d spectra exhibit at least threedifferent components, which are explained by a Ge or Tetermination of the surface domains, with possible contribu-tions of a surface reconstruction.

The surface oxidation in the range from 104 to 1015 Lcan be divided into four regions. For low exposures�105–107 L�, a formation of a peroxide structure with theoxygen molecule being bonded to germanium only is ob-served. In the next region �1�107 and 2�1010 L�, a Ge-enriched thin layer of Ge1+�

+4 Te1−�0 O2�1+��

2− is formed. Between2�1010 and 2�1013 L, this layer grows following the so-lution of the Elovich equation and we derived the Elovichianparameters, which allow the estimation of the layer thicknessand may be of interest for industrial applications. In thisregion, the peroxide is still present at the interface of theoxidized layer and the bulk. Above 2�1013 L, the final oxi-dation product m�GeO2��n�TeO2� is observed.

To interpret the spectra for the oxidized layers, we per-formed quantum-chemical calculations that describe the in-teraction of a �GeTe�4 cluster with one oxygen molecule.These calculations, in particular, support the formation of theperoxide structures at the interface; however, they are only ofqualitative nature, i.e., they allow to find explanations forexperimental photoelectron spectra in terms of the effectivecharge as a result of certain geometry/bond changes at thesurface during oxygen adsorption. A more quantitative de-scription can be achieved by comparing the experimentalchemical shifts to the theoretical results obtained by model-ing the O2 adsorption at the GeTe �111� surface by an essen-

tially larger clusters or a slab. This would be a further step tounderstand the interaction between oxygen and the GeTe sur-face.

ACKNOWLEDGMENTS

The surface-sensitive photoelectron studies were per-formed as a part of the bilateral Program “Russian-GermanLaboratory at BESSY II.” We are indebted to the BESSY stafffor technical support. The calculations were performed at thecalculation center of IPCP RAS. Partial financial support ofRFBR is acknowledged. The authors would like to thank Dr.A. V. Knot’ko for invaluable help with the SEM work andGIREDMET for providing the ESCALAB facility and per-sonally Dr. S. P. Kobeleva and E. V. Tikhonov for the supportin experiments. We also appreciate Dr. A. S. Zyubin for valu-able comments and fruitful discussion.

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