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Electronic structure of As2Te3–GeTe crystalline compounds
Journal of Physics and Chemistry of Solids 62 (2001) 467±474
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Electronic structure of As2Te3 ±GeTe crystalline compounds P.E. Lippens a,*, E. Brousse a, J. Olivier-Fourcade a, A. Gheorghiu de la Rocque b, C. SeÂneÂmaud b a
Laboratoire des AgreÂgats MoleÂculaires et MateÂriaux Inorganiques, CNRS UMR 5072, Universite Montpellier II, Place EugeÁne Bataillon, F-34095 Montpellier CeÂdex 05, France b Laboratoire de Chimie Physique MatieÁre et Rayonnement (CNRS URA176), Universite Paris VI, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France Received 2 December 1999; accepted 18 May 2000
Abstract The electronic structures of As2Te3, GeTe and the layered compounds As2GenTe31n with n 1±5 have been experimentally studied by means of X-ray photoemission spectroscopy (XPS). The XPS valence bands show changes as a function of n, which are explained by tight-binding calculations of the total and partial densities of states. The values of the binding energy of the Ge 3d5/2, As 3p3/2, and Te 3d5/2 core-levels were measured and are related to the calculated atomic charges. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Chalcogenides; D. Crystal structure; D. Electronic structure
1. Introduction There has been considerable interest in layered chalcogenides because of their properties of anisotropy and their possibility of intercalated positive ions within inter-layer space. For example, layered sul®des or selenides have been proposed as intercalation materials [1±5]. There are less studies on layered tellurides, which in addition mainly concern transition metals [6±7]. In this paper we focus our interest on the crystalline compounds of the As±Ge±Te system. This system has been previously studied for glasses because of their infrared transmission abilities [8]. However, a great number of layered compounds have been recently found in the pseudo-binary system As2Te3 ± GeTe which form a family of As2GenTe31n compounds, where n is an integer number [9±14]. A detailed structural analysis has been proposed which shows that the compounds form a homologous series and can be described by layers containing n layers of GeTe alternating with a single layer of As2Te3 for n # 9: The local electronic structure of the Te atoms has been previously investigated [15] and we propose in the present paper a study of the relation* Corresponding author. Tel.: 133-4-67-14-4548; fax: 133-4-6714-3304. E-mail address: [email protected] (P.E. Lippens).
ships between the structure and the electronic properties of the As2GenTe31n compounds. The valence bands of As2Te3, GeTe and As2GenTe31n with n 1±5 have been determined by X-ray photoemission spectroscopy (XPS). The contributions of the different electronic states to the observed XPS peaks are discussed from a tight-binding calculation of the densities of states (DOS) within the Slater±Koster framework [16]. This approach is used here because it provides a reliable analysis of the XPS spectra for materials with rather complex structures such as chalcogenide materials [17±18]. Attention will be focused on the changes in the electronic structure from As2Te3 to GeTe and the in¯uence of the different types of atoms and their local environments upon the DOS. We also report values of the Ge 3d5/2, As 3p3/2, and Te 3d5/2 core-level binding energies that will be related to the calculated atomic charges. 2. Synthesis and structures The crystalline phases of the As2Te3 ±GeTe system were prepared in fused silica tubes evacuated to about 10 25 Torr. The binary compounds As2Te3 and GeTe were obtained from high purity elements heated to about 8008C for 3 days and slowly cooled. The ternary phases As2GenTe31n
0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(00)00188-8
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Table 1 Space groups, lattice parameters a, b, c, a , b and average As±Te and Ge±Te inter-atomic distances for the crystalline phases of the system Ê . The inter-atomic distances are averaged over the ®rst As2Te3 ±GeTe. The values of a, b, c and the inter-atomic distances are given in A neighbors. The As and Ge atoms are bonded to six Te ®rst-nearest neighbors except in As2Te3, which contains both AsTe3 trigonal pyramids and AsTe6 distorted octahedra
Groups a b c As±Te Ge±Te
As2Te3 [19]
As2GeTe4 [11]
As2 Ge2Te5 [11]
As2 Ge3Te6 [11]
As2Ge4Te7 [11]
As2Ge5Te8 [11]
GeTe [20]
C2/m 14.339 4.006 b 958 9.873 2.80
R3Åm 4.083 4.083 40.38 2.96 2.92
P3Åm1 4.084 4.084 17.01 2.97 2.93
R3Åm 4.102 4.102 61.59 2.99 2.94
R3Åm 4.106 4.106 72.17 2.99 2.95
P3Åm1 4.112 4.112 27.54 2.99 2.95
R3Åm 8.344 8.344 10.71
were obtained from As2Te3 and GeTe heated for 2 days at a temperature of about 8008C and then at 3508C for 2 weeks. All the compounds were ®nely grounded for the experimental studies and their structures were controlled by X-ray diffraction. The main structural information of the As2Te3 ±GeTe compounds studied in this paper is reported in Table 1. As2Te3 has a monoclinic structure which consists of zigzag chains containing AsTe6 distorted octahedra connected to AsTe3 trigonal pyramids [19]. There are two and three different crystallographic sites for the As and Te atoms, respectively. The two arsenic sites, denoted by As(1) and As(2), have very different local environments since they are bonded to three and six Te atoms as ®rst-nearest neighbors, respectively. The three different Te atoms are trigonally bonded to the As atoms. GeTe has a slightly distorted NaCl structure [20]. The lattice parameters of the hexagonal cell are given in Table 1. Each atom is surrounded by six atoms of the opposite type, forming a slightly distorted octahedron. The structures of the ternary compounds As2GenTe31n have been established previously and belong to the trigonal system [9±14]. The compounds can be described by layers perpendicular to the c-axis which are formed by Te±As± Te±(Ge±Te)n ±As±Te stacking patterns. The intra-layer forces are greater than the bonding between the Te atoms belonging to adjacent layers. The As and Ge atoms are octahedrally surrounded by Te atoms with average distances Ê depending on the position of varying from about 2.8 to 3.0 A the atom in the layer and on the value of n. The octahedra are found to be slightly distorted except for the Ge atoms at the center of the layers of As2GenTe31n with odd n. In the As2GenTe31n compounds there is one crystallographic site for the As atoms and there are p different crystallographic sites for the Ge atoms labeled Ge(p) where n 2p 2 1 and n 2p: For example, there is one Ge site p 1 for As2GeTe4 n 1 and As2Ge2Te5 n 2: There are p 1 2 different Te crystallographic sites where n 2p and n 2p 1 1: For example there are three Te sites p 1 for As2Ge2Te5 n 2 and As2Ge3Te6 n 3: The Te sites are numbered from the edge to the center of the layers. In all the ternary compounds the Te(1) atoms are bonded to
3.01
Ê , the Te(2) three arsenic ®rst-nearest neighbors at about 3 A atoms are bonded to three arsenic and three germanium atoms and the other Te atoms are surrounded by six Ge atoms. In summary, very similar AsTe6 octahedral environments are found in the ternary compounds and for one of the two arsenic sites of As2Te3, the other arsenic site in As2Te3 being trigonally bonded to the Te atoms. The local environments of the Ge atoms do not change noticeably (GeTe6). Finally, the Te atoms are threefold coordinated to As atoms in As2Te3 and between the layers in the ternary phases. They are octahedrally coordinated within the layers of the ternary phases and in GeTe. 3. Experimental and theoretical methods The spectra of the valence bands and of the core levels were measured by means of XPS. Because of the surface sensitivity of this technique, the samples were prepared from specimens kept and sealed under Ar atmosphere. They were controlled to be less than 5% oxide contaminated. The photoelectron spectra were excited by using a Mg anode X-ray tube hn 1253:6 eV and analyzed with an electrostatic hemispherical analyzer in the ®xed-analyzer-transmission mode. The spectrometer resolution was estimated from the Ag 3d5/2 energy to be 0.8 eV. The binding energy scale has been calibrated considering the C1s peak at 285.0 eV, which we attributed to the small super®cial hydrocarbonated contamination. In these conditions, the origin of the binding energies corresponds to the Fermi level of the samples. The XPS valence bands and the variations of the core levels have been analyzed from a tight-binding calculation. This approach is used here because of the complexity of the crystalline structures and has been found to provide correct trends in the variations of the electronic properties of complex covalent materials [17,18,21]. The tight-binding scheme used here is based on the Slater±Koster method [16], and has been described in detail elsewhere [18]. For the compounds considered in the present paper a minimal orbital basis set formed by the s and p valence orbitals is
P.E. Lippens et al. / Journal of Physics and Chemistry of Solids 62 (2001) 467±474
Fig. 1. XPS valence band (dashed line) and calculated total DOS (solid line) of As2Te3, GeTe and the ternary compounds As2GenTe31n with n 1±5:
used. The elements of the Hamiltonian matrix are written in terms of intra-atomic and inter-atomic parameters. The values of the intra-atomic terms are the free-atom energies calculated by Hermann and Skillman [22]: E4s Ge 214:38 eV; E4p Ge 26:36 eV; E4s As 217:33 eV; E4p As 27:91 eV; E5s Te 217:11 eV; E5p Te 28:59 eV: The inter-atomic terms are obtained within a two-center approximation considering the exponential scaling law proposed by Robertson [23] and the Harrison two-center parameters [24]. The XPS valence bands are analyzed from the calculated DOS broadened by a Gaussian of 1 eV width. The variations of the XPS core-level binding energy are related to those of the effective atomic charge de®ned by Qa Za 2 Ns;a 2 Npx ;a 2 Npy ;a 2 Npz ;a
1
where Za is the number of valence electrons of the free atom a : Za 4 Ge; 5(As), 6(Te). Ni,a is the number of valence
469
Fig. 2. XPS valence band and calculated DOS of As2Te3. The XPS experimental data (crosses) have been smoothed (solid line). The total and partial DOS are shown for the different As and Te crystallographic sites. For the partial DOS the dashed and solid lines correspond to the s and p valence states, respectively. The three vertical bars indicate: b(tp) the p-type bonding states of the AsTe3 trigonal pyramids; b(o) the p-type bonding states of the AsTe6 octahedra; and nb the p-type non-bonding state, within the molecular model.
electrons of type i (s, px, py, pz) for the atom a in the solid: X
Ni;a 2 wi;a jc n;k 2 2 n;k
where w i,a is the atomic orbital of type i on the atom a and C n,k is the wavefunction, solution of the SchroÈdinger equation for the band n and the wave vector k. The summation runs over the valence band for n and a grid of points in the ®rst Brillouin zone for k. 4. Results and discussion 4.1. Valence bands The XPS valence-band spectra and the calculated DOS of the binary and ternary compounds are shown in Fig. 1. The
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Fig. 3. XPS valence band and calculated DOS of GeTe. The XPS experimental data (crosses) have been smoothed (solid line). The total and partial DOS are shown for the Ge and Te atoms. For the partial DOS the dashed and solid lines correspond to the s and p valence states, respectively.
origin of energies is taken at the top of the valence bands. The XPS spectra of all the compounds can be roughly described by a broad band between 215 and 26 eV, labeled I, and a more pronounced peak between 26 and 0 eV, labeled II. Going from As2Te3 to GeTe the observed main trends in the XPS valence bands are the splitting of band I in two peaks labeled Ia ( < 211 eV) and Ib ( < 28 eV), and the broadening of the two structures IIa ( < 23 eV) and IIb ( < 21 eV), which collapse in a single peak. These changes roughly occur for As2GeTe4 and As2Ge2Te5 and the XPS spectra of As2GenTe31n are found to be rather similar for n $ 2: Fig. 1 shows that an overall agreement between these experimental spectra and the theoretical results is obtained except for some small differences. The broadening of the XPS bands compared to the rather well-resolved features in the calculated DOS is probably due to the instrumental response function and the polycrystalline nature of the samples. The energy variations of the photoionisation cross-section, which mainly modify the amplitude of the peaks, could also explain some differences between the DOS and XPS curves. However, the calculations give both the correct positions and the relative amplitudes of the main peaks without adjusting the tight-binding parameters. It is thus possible to derive the main characters of the XPS peaks and explain the observed changes from As2Te3 to GeTe from the calculated projected DOS on the orbitals of the atoms occupying the different crystallographic sites. The XPS spectrum and the total and partial DOS of
As2Te3 are plotted in Fig. 2. The Te 5s and As 4s states mainly contribute to band I. In this energy range the total DOS exhibits 5 peaks, labeled 1a±1e. The central peak, 1c, is mainly formed by Te 5s states whereas the other peaks, 1a, 1b, 1d and 1e, account for interactions between Te 5s and As 4s states. The distribution of peaks is quasi-symmetrical around peak 1c. The splitting into 5 peaks is mainly due to structural effects and not to the ionicity of the bonds, since the atomic energy levels of the As 4s (217.33 eV) and Te 5s (217.11 eV) states are very close. Band II of the XPS spectrum of As2Te3 is made of Te 5p and As 4p states. The two observed XPS structures, IIa and IIb, can be compared to the DOS peaks labeled 2a and 2b, respectively (Fig. 2). Peak 2b has a strong Te 5p character and its position is close to the Te 5p atomic energy level. It corresponds to the socalled ªlone-pairº band. There is also a small contribution of the As(2) 4s states at the top of the valence band due to interaction with the Te 5p states. Peak 2a accounts for the interactions between the p-type valence orbitals of As and Te. The asymmetry of this peak, which is also observed for XPS peak IIa, can be attributed to the existence of the two different local environments for As(1) and As(2). The partial p-DOS of As(1) and As(2) clearly show peaks at about 22 and 23 eV, respectively, indicating that the observed XPS structure IIa is mainly due to interactions involving the As(1) atoms while the low-energy side is due to the As(2) atoms. This can be easily understood in terms of the local environments of the As atoms from a simple molecular picture [17]. As2Te3 can be described as a set of As(1)Te3/ 3 and As(2)Te6/3 units linked by the Te atoms. Because the As±Te bonds are nearly orthogonal the molecular units form isolated species if only the dominant pps interactions are considered. Thus, a simple description of the electronic structure of the p bands for As2Te3 is obtained by considering the bonding/antibonding molecular states of As(1)Te3 and the bonding/non-bonding/antibonding states of As(2)Te6, which are all threefold degenerated. The bonding and non-bonding molecular states can be compared to peaks 2a and 2b, respectively (Fig. 2). The two bonding molecular states for As(1)Te3 and As(2)Te6 are found at the same energies as those of the p-DOS peaks for As(1) and As(2), respectively. Thus, the energy difference between these two states can be related to the two different local environments of the As atoms which are bonded to three and six Te atoms, respectively. This explains the asymmetry of XPS peak IIa. The experimental and calculated valence bands of GeTe are shown in Fig. 3. The XPS spectrum exhibit three peaks at about 211, 28 and 22 eV, labeled Ia, Ib and II, respectively. The partial DOS show that the main characters of XPS peaks Ia and Ib are given by the Te 5s and Ge 4s states, respectively. The positions of these two peaks can be mainly related to the values of the Te 5s (217.11 eV) and Ge 4s (214.38 eV) atomic energy levels and the shape of the peaks is due to the interactions between these two states. Peak II mainly accounts for the interactions between the Te 5p and Ge 4p orbitals. Although this band does not show
P.E. Lippens et al. / Journal of Physics and Chemistry of Solids 62 (2001) 467±474
Fig. 4. XPS valence band and calculated DOS of As2GeTe4. The XPS experimental data (crosses) have been smoothed (solid line). The total and partial DOS are shown for the different crystallographic sites of Ge, As and Te. For the partial DOS the dashed and solid lines correspond to the s and p valence states, respectively.
well-resolved peaks, the calculated DOS exhibits two structures at about 23 and 21 eV labeled 2a and 2b, respectively. Peak 2a is mainly due to the interactions between Ge 4p and Te 5p orbitals. Peak 2b is formed by the Ge 4p, Ge 4s and Te 5p states. The top of the valence band contains a small amount of Ge 4s character due to the GeTe6 octahedral environment. The existence of the two peaks, 2a and 2b, can be related to the structure of GeTe considering a simple molecular model. Assuming a perfect cubic structure for GeTe and considering only the pps interactions, a simple picture of the electronic structure of the p band can be roughly obtained from that of an in®nite linear chain formed by Ge 4p and Te 5p orbitals. Thus, the molecular DOS has the well-known U-shape and the effect of the other interactions is mainly to broaden the molecular levels to give the DOS shown in Fig. 3. As shown in Fig. 1, the XPS spectrum of As2GeTe4 differs from that of As2Te3 by (i) the more pronounced asymmetry of band I on the high energy side (29/26 eV) and (ii) the overlap of the two structures IIa and IIb which form a broad band. These two changes are also predicted by the calculated DOS, which show that the amplitude of the peaks at about 28 and 27 eV increases and that the two DOS structures at about 23 and 21 eV are broadened from As2Te3 to As2GeTe4. The XPS spectrum and the total and partial DOS
471
Fig. 5. Same as Fig. 4 but for As2Ge2Te5.
of As2GeTe4 are shown in Fig. 4. The partial DOS show that the main contributions to XPS peaks I and II come from the s and p valence states of the different atoms, respectively. The total DOS exhibit ®ve peaks, 1a±1e, in the energy range between 215 and 26 eV. The character of peak 1c is given by the Te 5s states, which mainly contribute to XPS peak I. On the low-energy side peaks 1a and 1b account for interactions between As 4s and Te 5s states although interactions between Ge 4s and Te 5s states also contribute to 1b. On the high energy side (29/26 eV), peak 1d is mainly formed by the Te 5s and As 4s states and peak 1e by the Ge 4s states. The observed differences between the positions of these peaks in the As 4s and Ge 4s partial DOS can be related to the difference between the values of the atomic energy levels which is smaller between As 4s and Te 5s states than between Ge 4s and Te 5s states. This also explains why interactions between the Ge 4s and Te 5s orbitals lead to more localized states than those between the As 4s and Te 5s orbitals. Thus, the asymmetry of XPS peak I is mainly due to the contribution of the Ge 4s states. It is worth noticing that the two types of Te atoms provide very different contributions to peak I. The 5s orbitals of the inter-layer Te(1) atoms interact with the 4s orbitals of their three As nearest-neighbors, leading to delocalized states in the Te(1) 5s partial DOS. The 5s orbitals of the intra-layer Te(2) atoms interact with the As 4s and Ge 4s states leading to a more pronounced peak at about 210 eV in the Te(2) 5s partial
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Fig. 6. Same as Fig. 4 but for As2Ge5Te8.
DOS, which mainly contributes to the shape of XPS peak I. For energies greater than 26 eV, the DOS is formed by two peaks, labeled 2a and 2b, which are less resolved than those of As2Te3. Structure 2a is mainly formed by Te 5p and As 4p states at about 23 eV and by Te 5p and Ge 4p states at about 22 eV. These two different contributions of the cations can be related to the values of the atomic energy levels. The difference between the As 4p and Te 5p energies is smaller than that between the Ge 4p and Te 5p states. From simple molecular arguments this means that interactions between the valence p orbitals of As and Te lead to a deeper bonding state than that due to the Ge±Te interactions. The two types of Te atoms contribute to peak 2a since they both interact with the As 4p and Ge 4p states. Peak 2b is mainly formed by Te 5p states and has a ªlone-pairº character as in the case of As2Te3. However, it is worth noticing that a small amount
of Ge 4s states is found at the top of the valence band due to the interactions with the Te 5p states as in the case of GeTe. As shown in Fig. 1, the XPS spectra of the As2GenTe31n ternary compounds with n $ 2 are rather similar, showing the same features as for GeTe. The spectra show two structures at about 211 and 28 eV, which are more pronounced for n 5 than for n 2 and a broad band II at about 22 eV. These structures can be favorably compared with those of the calculated DOS. Since they do not change noticeably for n $ 2 we only consider the analysis of the XPS spectra based on the partial DOS for As2Ge2Te5 (Fig. 5) and As2Ge5Te8 (Fig. 6). The ®ve peaks 1a±1e of the total DOS, which clearly occur for energies lower than 26 eV in the case of As2Te3 and As2GeTe4, are not found for n $ 2 and the DOS are mainly formed by two asymmetric peaks at about 211 and 28 eV labeled 1a and 1b, respectively. These two peaks are similar to those of GeTe in the same energy range and their characters are mainly given by the Te 5s (1a) and Ge 4s (1b) states, respectively. The As 4s and Te(1) 5s states are delocalized over the energy range of peak I and have a decreasing in¯uence on the total DOS of the ternary compounds as n increases because of the decreasing number of As and Te(1) atoms. The 5s partial DOS of the intra-layer Te atoms: Te(2) and Te(3) in As2Ge2Te5, Te(2), Te(3) and Te(4) in As2Ge5Te8, are similar to that of the Te atoms in GeTe. The intra-layer Te atoms are octahedrally surrounded by Ge atoms, except Te(2) which is bonded to both As and Ge atoms. As a consequence the Te 5s DOS around 212 eV is found to be greater for Te(2) than for the other Te atoms. However, the contribution of these atoms to the DOS decreases with increasing value of n. XPS peak II can be compared to broad band 2 of the total DOS, which is formed by a main peak at about 23 eV and a shoulder at 21 eV (Figs. 5 and 6). The main peak contains the p-type valence states of the three different types of atoms. Interactions between the As 4p and the 5p orbitals of Te(1) and Te(2) contribute to the DOS at about 23 eV for all the ternary compounds. This contribution decreases with increasing n but has no signi®cant in¯uence on the DOS because of the contribution of the Ge 4p and intra-layer Te 5p states, which form a broad band between 24 and 22 eV. The shoulder at 21 eV is mainly formed by the Te 5p states providing a ªlone-pairº character to the top of the valence band. Thus, the partial DOS of the ternary compounds clearly show an increasing in¯uence upon the total DOS of the interactions between the Ge 4s (4p) orbitals and the intra-layer Te 5s (5p) orbitals with increasing n.
Table 2 Values (in eV) of the Ge 3d5/2, As 3p3/2 and Te 3d5/2 core-level binding energy for As2Te3, GeTe and As2GenTe31n with n 1±5
Ge 3d5/2 As 2p3/2 Te 3d5/2
As2Te3
As2GeTe4
As2Ge2Te5
As2Ge3Te6
As2Ge4Te7
As2Ge5Te8
141.2 573
30.1 141.3 573
30 141.3 572.9
30 141.3 572.8
30 141.4 572.8
29.9 141.2 572.7
GeTe 29.7 572.7
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473
Table 3 Values of the calculated Ge, As and Te atomic charges averaged over the different crystallographic sites
Ge As Te
As2Te3
As2GeTe4
As2Ge2Te5
As2Ge3Te6
As2Ge4Te7
As2Ge5Te8
0.48 20.33
0.34 0.49 20.33
0.36 0.49 20.34
0.36 0.50 20.36
0.36 0.51 20.35
0.37 0.52 20.37
4.2. Core levels The values of the binding energy of the Ge 3d5/2, As 3p3/2, and Te 3d5/2 core levels are reported in Table 2. The Ge 3d5/2 and As 3p3/2 core-level binding energies were measured for bulk crystalline germanium and arsenic, respectively, and are found to be 29.5 eV (Ge 3d5/2) and 140.9 eV (As 3p3/2) in good agreement with previously reported values [25,26]. The latter values are slightly lower than those of the binary and ternary compounds (Table 2), suggesting a weak cationic character for the Ge and As atoms in the As2Te3 ±GeTe compounds. The atomic charges have been evaluated following the procedure described in Section 3 and averaged over the atoms occupying the different crystallographic sites (Table 3). The positive and rather small values obtained for Ge and As are consistent with the XPS observation. The situation of the Te atoms is somewhat more complicated since they are found in two very different local environments in the ternary compounds, depending on their position in the layers. The calculated charges are found to be of about 20.5 and 0 for the intra-layer and inter-layer Te atoms, respectively. The charge difference is rather small, in agreement with the absence of well-resolved structures in the XPS spectra for these two types of Te atoms. Thus, we have considered average values of the tellurium charges in order to analyze the XPS Te 3d5/2 core-level binding energies (Table 3). As a main result, the averaged charges have small and negative values for all the compounds. This can be correlated with the values of the Te3d5/2 core-level binding energies, which are slightly lower than the value measured for the bulk crystalline tellurium: 573.1 eV, which is close to the previously published values [25]. The weak ionic character of the Ge, As and Te atoms is consistent with the values of their electronegativities, which are very close. This shows that the character of the Ge±Te and As±Te bonds in the As2Te3 ±GeTe compounds is rather covalent. Finally, neither the shape of the experimental core level curves nor the energy positions of the maxima of As 3p3/2 and Ge 3d5/2 vary signi®cantly from one sample to another. This can be ascribed to minor changes in the atomic environments of these elements and can be correlated to the calculated averaged charges, which do not vary noticeably as a function of n (,0.1). However, it is worth noticing that the small decrease of the Te 3d5/2 core level binding energy with increasing n can be correlated with the decrease of the Te averaged charge. This is mainly due to the increase of the
GeTe 0.42 20.42
number of intra-layer atoms which have a negative charge compared to the number of quasi-neutral inter-layer Te atoms. 5. Conclusions The electronic structure of the crystalline phases of the system As2Te3 ±GeTe has been experimentally determined by XPS and theoretically analyzed by a tight-binding calculation. The XPS valence bands are mainly formed by two bands between 215 and 26 eV and greater than 26 eV which have been attributed to the s and p valence states of the different atoms, respectively. From As2Te3 to GeTe the s-type band splits in two peaks because of the difference between the As 4s and Ge 4s atomic energy levels. The two structures found in the p-type band of As2Te3 collapse in one band for GeTe because of the increasing in¯uence of the Ge 4p states. These changes mainly occur for the ternary compounds As2GeTe4 and As2Ge2Te5. The values of the core-level binding energy are found to be slightly different to those of the bulk monoatomic crystals and indicate a weak cationic character for Ge and As and a weak anionic character for Te in agreement with the calculated charges. This shows that the As±Te and Ge±Te bonds in the As2GenTe31n compounds are rather covalent. Acknowledgements Computer resources provided by the Centre National Universitaire Sud de Calcul (France) under Project no. C97092 are gratefully acknowledged. References [1] J. Morales, C. Perez-Vicente, J.L. Tirado, Solid State Ionics 51 (1992) 133. [2] C. Perez-Vicente, J.L. Tirado, P.E. Lippens, J.C. Jumas, Phys. Rev. B 11 (1997) 6371. [3] T. Pietrass, F. Taulelle, P. Lavela, J. Olivier-Fourcade, J.C. Jumas, S. Steuernagel, J. Phys. Chem. B 101 (1997) 6715. [4] G.A. Wiegers, A. Meerschaut, Incommensurate Sandwiched Layered Compounds, in: A. Meerschaut (Ed.), Technol. Publication, 1992. [5] G.F. Khudorozhko, L.G. Bulusheva, L.N. Mazalov, V.E. Fedorov, J. Morales, E.A. Kravtsova, I.P. Asanov, G.K. Parygina, Yu.V. Mironov, J. Phys. Chem. Solids 59 (1998) 283.
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[15] P.E. Lippens, E. Brousse, J.C. Jumas, J. Phys. Chem. Solids 60 (1999) 1663. [16] J.C. Slater, G.F. Koster, Phys. Rev. 94 (1954) 1494. [17] P.E. Lippens, L. Aldon, Solid State Commun. 108 (1998) 913. [18] P.E. Lippens, J. Olivier-Fourcade, J.C. Jumas, S. Dupont, A. Gheorghiu, C. SeÂneÂmaud, Phys. Rev. B 56 (1997) 13 054. [19] G.J. Carron, Acta Crystallogr. 16 (1963) 338. [20] J. Goldak, C.S. Barrett, D. Innes, W. Youdelis, J. Chem. Phys. 44 (1966) 3323. [21] J. Robertson, Phys. Rev. B 28 (1983) 4671. [22] F. Herman, S. Skillman, Atomic Structure Calculations, Prentice-Hall, Englewood Cliffs, NJ, 1963. [23] J. Robertson, J. Phys. C: Solid State Phys. 12 (1979) 4753. [24] W.A. Harrison, Phys. Rev. B 24 (1981) 5835. [25] D. Briggs, M.P. Seah, Practical Surface Analysis, 2nd ed., Wiley, New York, 1996. [26] M.K. Bahl, R.O. Woodall, R.L. Watson, K.J. Irgolic, J. Chem. Phys. 64 (1976) 1210.
www.elsevier.nl/locate/jpcs
Electronic structure of As2Te3 ±GeTe crystalline compounds P.E. Lippens a,*, E. Brousse a, J. Olivier-Fourcade a, A. Gheorghiu de la Rocque b, C. SeÂneÂmaud b a
Laboratoire des AgreÂgats MoleÂculaires et MateÂriaux Inorganiques, CNRS UMR 5072, Universite Montpellier II, Place EugeÁne Bataillon, F-34095 Montpellier CeÂdex 05, France b Laboratoire de Chimie Physique MatieÁre et Rayonnement (CNRS URA176), Universite Paris VI, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France Received 2 December 1999; accepted 18 May 2000
Abstract The electronic structures of As2Te3, GeTe and the layered compounds As2GenTe31n with n 1±5 have been experimentally studied by means of X-ray photoemission spectroscopy (XPS). The XPS valence bands show changes as a function of n, which are explained by tight-binding calculations of the total and partial densities of states. The values of the binding energy of the Ge 3d5/2, As 3p3/2, and Te 3d5/2 core-levels were measured and are related to the calculated atomic charges. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Chalcogenides; D. Crystal structure; D. Electronic structure
1. Introduction There has been considerable interest in layered chalcogenides because of their properties of anisotropy and their possibility of intercalated positive ions within inter-layer space. For example, layered sul®des or selenides have been proposed as intercalation materials [1±5]. There are less studies on layered tellurides, which in addition mainly concern transition metals [6±7]. In this paper we focus our interest on the crystalline compounds of the As±Ge±Te system. This system has been previously studied for glasses because of their infrared transmission abilities [8]. However, a great number of layered compounds have been recently found in the pseudo-binary system As2Te3 ± GeTe which form a family of As2GenTe31n compounds, where n is an integer number [9±14]. A detailed structural analysis has been proposed which shows that the compounds form a homologous series and can be described by layers containing n layers of GeTe alternating with a single layer of As2Te3 for n # 9: The local electronic structure of the Te atoms has been previously investigated [15] and we propose in the present paper a study of the relation* Corresponding author. Tel.: 133-4-67-14-4548; fax: 133-4-6714-3304. E-mail address: [email protected] (P.E. Lippens).
ships between the structure and the electronic properties of the As2GenTe31n compounds. The valence bands of As2Te3, GeTe and As2GenTe31n with n 1±5 have been determined by X-ray photoemission spectroscopy (XPS). The contributions of the different electronic states to the observed XPS peaks are discussed from a tight-binding calculation of the densities of states (DOS) within the Slater±Koster framework [16]. This approach is used here because it provides a reliable analysis of the XPS spectra for materials with rather complex structures such as chalcogenide materials [17±18]. Attention will be focused on the changes in the electronic structure from As2Te3 to GeTe and the in¯uence of the different types of atoms and their local environments upon the DOS. We also report values of the Ge 3d5/2, As 3p3/2, and Te 3d5/2 core-level binding energies that will be related to the calculated atomic charges. 2. Synthesis and structures The crystalline phases of the As2Te3 ±GeTe system were prepared in fused silica tubes evacuated to about 10 25 Torr. The binary compounds As2Te3 and GeTe were obtained from high purity elements heated to about 8008C for 3 days and slowly cooled. The ternary phases As2GenTe31n
0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(00)00188-8
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Table 1 Space groups, lattice parameters a, b, c, a , b and average As±Te and Ge±Te inter-atomic distances for the crystalline phases of the system Ê . The inter-atomic distances are averaged over the ®rst As2Te3 ±GeTe. The values of a, b, c and the inter-atomic distances are given in A neighbors. The As and Ge atoms are bonded to six Te ®rst-nearest neighbors except in As2Te3, which contains both AsTe3 trigonal pyramids and AsTe6 distorted octahedra
Groups a b c As±Te Ge±Te
As2Te3 [19]
As2GeTe4 [11]
As2 Ge2Te5 [11]
As2 Ge3Te6 [11]
As2Ge4Te7 [11]
As2Ge5Te8 [11]
GeTe [20]
C2/m 14.339 4.006 b 958 9.873 2.80
R3Åm 4.083 4.083 40.38 2.96 2.92
P3Åm1 4.084 4.084 17.01 2.97 2.93
R3Åm 4.102 4.102 61.59 2.99 2.94
R3Åm 4.106 4.106 72.17 2.99 2.95
P3Åm1 4.112 4.112 27.54 2.99 2.95
R3Åm 8.344 8.344 10.71
were obtained from As2Te3 and GeTe heated for 2 days at a temperature of about 8008C and then at 3508C for 2 weeks. All the compounds were ®nely grounded for the experimental studies and their structures were controlled by X-ray diffraction. The main structural information of the As2Te3 ±GeTe compounds studied in this paper is reported in Table 1. As2Te3 has a monoclinic structure which consists of zigzag chains containing AsTe6 distorted octahedra connected to AsTe3 trigonal pyramids [19]. There are two and three different crystallographic sites for the As and Te atoms, respectively. The two arsenic sites, denoted by As(1) and As(2), have very different local environments since they are bonded to three and six Te atoms as ®rst-nearest neighbors, respectively. The three different Te atoms are trigonally bonded to the As atoms. GeTe has a slightly distorted NaCl structure [20]. The lattice parameters of the hexagonal cell are given in Table 1. Each atom is surrounded by six atoms of the opposite type, forming a slightly distorted octahedron. The structures of the ternary compounds As2GenTe31n have been established previously and belong to the trigonal system [9±14]. The compounds can be described by layers perpendicular to the c-axis which are formed by Te±As± Te±(Ge±Te)n ±As±Te stacking patterns. The intra-layer forces are greater than the bonding between the Te atoms belonging to adjacent layers. The As and Ge atoms are octahedrally surrounded by Te atoms with average distances Ê depending on the position of varying from about 2.8 to 3.0 A the atom in the layer and on the value of n. The octahedra are found to be slightly distorted except for the Ge atoms at the center of the layers of As2GenTe31n with odd n. In the As2GenTe31n compounds there is one crystallographic site for the As atoms and there are p different crystallographic sites for the Ge atoms labeled Ge(p) where n 2p 2 1 and n 2p: For example, there is one Ge site p 1 for As2GeTe4 n 1 and As2Ge2Te5 n 2: There are p 1 2 different Te crystallographic sites where n 2p and n 2p 1 1: For example there are three Te sites p 1 for As2Ge2Te5 n 2 and As2Ge3Te6 n 3: The Te sites are numbered from the edge to the center of the layers. In all the ternary compounds the Te(1) atoms are bonded to
3.01
Ê , the Te(2) three arsenic ®rst-nearest neighbors at about 3 A atoms are bonded to three arsenic and three germanium atoms and the other Te atoms are surrounded by six Ge atoms. In summary, very similar AsTe6 octahedral environments are found in the ternary compounds and for one of the two arsenic sites of As2Te3, the other arsenic site in As2Te3 being trigonally bonded to the Te atoms. The local environments of the Ge atoms do not change noticeably (GeTe6). Finally, the Te atoms are threefold coordinated to As atoms in As2Te3 and between the layers in the ternary phases. They are octahedrally coordinated within the layers of the ternary phases and in GeTe. 3. Experimental and theoretical methods The spectra of the valence bands and of the core levels were measured by means of XPS. Because of the surface sensitivity of this technique, the samples were prepared from specimens kept and sealed under Ar atmosphere. They were controlled to be less than 5% oxide contaminated. The photoelectron spectra were excited by using a Mg anode X-ray tube hn 1253:6 eV and analyzed with an electrostatic hemispherical analyzer in the ®xed-analyzer-transmission mode. The spectrometer resolution was estimated from the Ag 3d5/2 energy to be 0.8 eV. The binding energy scale has been calibrated considering the C1s peak at 285.0 eV, which we attributed to the small super®cial hydrocarbonated contamination. In these conditions, the origin of the binding energies corresponds to the Fermi level of the samples. The XPS valence bands and the variations of the core levels have been analyzed from a tight-binding calculation. This approach is used here because of the complexity of the crystalline structures and has been found to provide correct trends in the variations of the electronic properties of complex covalent materials [17,18,21]. The tight-binding scheme used here is based on the Slater±Koster method [16], and has been described in detail elsewhere [18]. For the compounds considered in the present paper a minimal orbital basis set formed by the s and p valence orbitals is
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Fig. 1. XPS valence band (dashed line) and calculated total DOS (solid line) of As2Te3, GeTe and the ternary compounds As2GenTe31n with n 1±5:
used. The elements of the Hamiltonian matrix are written in terms of intra-atomic and inter-atomic parameters. The values of the intra-atomic terms are the free-atom energies calculated by Hermann and Skillman [22]: E4s Ge 214:38 eV; E4p Ge 26:36 eV; E4s As 217:33 eV; E4p As 27:91 eV; E5s Te 217:11 eV; E5p Te 28:59 eV: The inter-atomic terms are obtained within a two-center approximation considering the exponential scaling law proposed by Robertson [23] and the Harrison two-center parameters [24]. The XPS valence bands are analyzed from the calculated DOS broadened by a Gaussian of 1 eV width. The variations of the XPS core-level binding energy are related to those of the effective atomic charge de®ned by Qa Za 2 Ns;a 2 Npx ;a 2 Npy ;a 2 Npz ;a
1
where Za is the number of valence electrons of the free atom a : Za 4 Ge; 5(As), 6(Te). Ni,a is the number of valence
469
Fig. 2. XPS valence band and calculated DOS of As2Te3. The XPS experimental data (crosses) have been smoothed (solid line). The total and partial DOS are shown for the different As and Te crystallographic sites. For the partial DOS the dashed and solid lines correspond to the s and p valence states, respectively. The three vertical bars indicate: b(tp) the p-type bonding states of the AsTe3 trigonal pyramids; b(o) the p-type bonding states of the AsTe6 octahedra; and nb the p-type non-bonding state, within the molecular model.
electrons of type i (s, px, py, pz) for the atom a in the solid: X
Ni;a 2 wi;a jc n;k 2 2 n;k
where w i,a is the atomic orbital of type i on the atom a and C n,k is the wavefunction, solution of the SchroÈdinger equation for the band n and the wave vector k. The summation runs over the valence band for n and a grid of points in the ®rst Brillouin zone for k. 4. Results and discussion 4.1. Valence bands The XPS valence-band spectra and the calculated DOS of the binary and ternary compounds are shown in Fig. 1. The
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Fig. 3. XPS valence band and calculated DOS of GeTe. The XPS experimental data (crosses) have been smoothed (solid line). The total and partial DOS are shown for the Ge and Te atoms. For the partial DOS the dashed and solid lines correspond to the s and p valence states, respectively.
origin of energies is taken at the top of the valence bands. The XPS spectra of all the compounds can be roughly described by a broad band between 215 and 26 eV, labeled I, and a more pronounced peak between 26 and 0 eV, labeled II. Going from As2Te3 to GeTe the observed main trends in the XPS valence bands are the splitting of band I in two peaks labeled Ia ( < 211 eV) and Ib ( < 28 eV), and the broadening of the two structures IIa ( < 23 eV) and IIb ( < 21 eV), which collapse in a single peak. These changes roughly occur for As2GeTe4 and As2Ge2Te5 and the XPS spectra of As2GenTe31n are found to be rather similar for n $ 2: Fig. 1 shows that an overall agreement between these experimental spectra and the theoretical results is obtained except for some small differences. The broadening of the XPS bands compared to the rather well-resolved features in the calculated DOS is probably due to the instrumental response function and the polycrystalline nature of the samples. The energy variations of the photoionisation cross-section, which mainly modify the amplitude of the peaks, could also explain some differences between the DOS and XPS curves. However, the calculations give both the correct positions and the relative amplitudes of the main peaks without adjusting the tight-binding parameters. It is thus possible to derive the main characters of the XPS peaks and explain the observed changes from As2Te3 to GeTe from the calculated projected DOS on the orbitals of the atoms occupying the different crystallographic sites. The XPS spectrum and the total and partial DOS of
As2Te3 are plotted in Fig. 2. The Te 5s and As 4s states mainly contribute to band I. In this energy range the total DOS exhibits 5 peaks, labeled 1a±1e. The central peak, 1c, is mainly formed by Te 5s states whereas the other peaks, 1a, 1b, 1d and 1e, account for interactions between Te 5s and As 4s states. The distribution of peaks is quasi-symmetrical around peak 1c. The splitting into 5 peaks is mainly due to structural effects and not to the ionicity of the bonds, since the atomic energy levels of the As 4s (217.33 eV) and Te 5s (217.11 eV) states are very close. Band II of the XPS spectrum of As2Te3 is made of Te 5p and As 4p states. The two observed XPS structures, IIa and IIb, can be compared to the DOS peaks labeled 2a and 2b, respectively (Fig. 2). Peak 2b has a strong Te 5p character and its position is close to the Te 5p atomic energy level. It corresponds to the socalled ªlone-pairº band. There is also a small contribution of the As(2) 4s states at the top of the valence band due to interaction with the Te 5p states. Peak 2a accounts for the interactions between the p-type valence orbitals of As and Te. The asymmetry of this peak, which is also observed for XPS peak IIa, can be attributed to the existence of the two different local environments for As(1) and As(2). The partial p-DOS of As(1) and As(2) clearly show peaks at about 22 and 23 eV, respectively, indicating that the observed XPS structure IIa is mainly due to interactions involving the As(1) atoms while the low-energy side is due to the As(2) atoms. This can be easily understood in terms of the local environments of the As atoms from a simple molecular picture [17]. As2Te3 can be described as a set of As(1)Te3/ 3 and As(2)Te6/3 units linked by the Te atoms. Because the As±Te bonds are nearly orthogonal the molecular units form isolated species if only the dominant pps interactions are considered. Thus, a simple description of the electronic structure of the p bands for As2Te3 is obtained by considering the bonding/antibonding molecular states of As(1)Te3 and the bonding/non-bonding/antibonding states of As(2)Te6, which are all threefold degenerated. The bonding and non-bonding molecular states can be compared to peaks 2a and 2b, respectively (Fig. 2). The two bonding molecular states for As(1)Te3 and As(2)Te6 are found at the same energies as those of the p-DOS peaks for As(1) and As(2), respectively. Thus, the energy difference between these two states can be related to the two different local environments of the As atoms which are bonded to three and six Te atoms, respectively. This explains the asymmetry of XPS peak IIa. The experimental and calculated valence bands of GeTe are shown in Fig. 3. The XPS spectrum exhibit three peaks at about 211, 28 and 22 eV, labeled Ia, Ib and II, respectively. The partial DOS show that the main characters of XPS peaks Ia and Ib are given by the Te 5s and Ge 4s states, respectively. The positions of these two peaks can be mainly related to the values of the Te 5s (217.11 eV) and Ge 4s (214.38 eV) atomic energy levels and the shape of the peaks is due to the interactions between these two states. Peak II mainly accounts for the interactions between the Te 5p and Ge 4p orbitals. Although this band does not show
P.E. Lippens et al. / Journal of Physics and Chemistry of Solids 62 (2001) 467±474
Fig. 4. XPS valence band and calculated DOS of As2GeTe4. The XPS experimental data (crosses) have been smoothed (solid line). The total and partial DOS are shown for the different crystallographic sites of Ge, As and Te. For the partial DOS the dashed and solid lines correspond to the s and p valence states, respectively.
well-resolved peaks, the calculated DOS exhibits two structures at about 23 and 21 eV labeled 2a and 2b, respectively. Peak 2a is mainly due to the interactions between Ge 4p and Te 5p orbitals. Peak 2b is formed by the Ge 4p, Ge 4s and Te 5p states. The top of the valence band contains a small amount of Ge 4s character due to the GeTe6 octahedral environment. The existence of the two peaks, 2a and 2b, can be related to the structure of GeTe considering a simple molecular model. Assuming a perfect cubic structure for GeTe and considering only the pps interactions, a simple picture of the electronic structure of the p band can be roughly obtained from that of an in®nite linear chain formed by Ge 4p and Te 5p orbitals. Thus, the molecular DOS has the well-known U-shape and the effect of the other interactions is mainly to broaden the molecular levels to give the DOS shown in Fig. 3. As shown in Fig. 1, the XPS spectrum of As2GeTe4 differs from that of As2Te3 by (i) the more pronounced asymmetry of band I on the high energy side (29/26 eV) and (ii) the overlap of the two structures IIa and IIb which form a broad band. These two changes are also predicted by the calculated DOS, which show that the amplitude of the peaks at about 28 and 27 eV increases and that the two DOS structures at about 23 and 21 eV are broadened from As2Te3 to As2GeTe4. The XPS spectrum and the total and partial DOS
471
Fig. 5. Same as Fig. 4 but for As2Ge2Te5.
of As2GeTe4 are shown in Fig. 4. The partial DOS show that the main contributions to XPS peaks I and II come from the s and p valence states of the different atoms, respectively. The total DOS exhibit ®ve peaks, 1a±1e, in the energy range between 215 and 26 eV. The character of peak 1c is given by the Te 5s states, which mainly contribute to XPS peak I. On the low-energy side peaks 1a and 1b account for interactions between As 4s and Te 5s states although interactions between Ge 4s and Te 5s states also contribute to 1b. On the high energy side (29/26 eV), peak 1d is mainly formed by the Te 5s and As 4s states and peak 1e by the Ge 4s states. The observed differences between the positions of these peaks in the As 4s and Ge 4s partial DOS can be related to the difference between the values of the atomic energy levels which is smaller between As 4s and Te 5s states than between Ge 4s and Te 5s states. This also explains why interactions between the Ge 4s and Te 5s orbitals lead to more localized states than those between the As 4s and Te 5s orbitals. Thus, the asymmetry of XPS peak I is mainly due to the contribution of the Ge 4s states. It is worth noticing that the two types of Te atoms provide very different contributions to peak I. The 5s orbitals of the inter-layer Te(1) atoms interact with the 4s orbitals of their three As nearest-neighbors, leading to delocalized states in the Te(1) 5s partial DOS. The 5s orbitals of the intra-layer Te(2) atoms interact with the As 4s and Ge 4s states leading to a more pronounced peak at about 210 eV in the Te(2) 5s partial
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Fig. 6. Same as Fig. 4 but for As2Ge5Te8.
DOS, which mainly contributes to the shape of XPS peak I. For energies greater than 26 eV, the DOS is formed by two peaks, labeled 2a and 2b, which are less resolved than those of As2Te3. Structure 2a is mainly formed by Te 5p and As 4p states at about 23 eV and by Te 5p and Ge 4p states at about 22 eV. These two different contributions of the cations can be related to the values of the atomic energy levels. The difference between the As 4p and Te 5p energies is smaller than that between the Ge 4p and Te 5p states. From simple molecular arguments this means that interactions between the valence p orbitals of As and Te lead to a deeper bonding state than that due to the Ge±Te interactions. The two types of Te atoms contribute to peak 2a since they both interact with the As 4p and Ge 4p states. Peak 2b is mainly formed by Te 5p states and has a ªlone-pairº character as in the case of As2Te3. However, it is worth noticing that a small amount
of Ge 4s states is found at the top of the valence band due to the interactions with the Te 5p states as in the case of GeTe. As shown in Fig. 1, the XPS spectra of the As2GenTe31n ternary compounds with n $ 2 are rather similar, showing the same features as for GeTe. The spectra show two structures at about 211 and 28 eV, which are more pronounced for n 5 than for n 2 and a broad band II at about 22 eV. These structures can be favorably compared with those of the calculated DOS. Since they do not change noticeably for n $ 2 we only consider the analysis of the XPS spectra based on the partial DOS for As2Ge2Te5 (Fig. 5) and As2Ge5Te8 (Fig. 6). The ®ve peaks 1a±1e of the total DOS, which clearly occur for energies lower than 26 eV in the case of As2Te3 and As2GeTe4, are not found for n $ 2 and the DOS are mainly formed by two asymmetric peaks at about 211 and 28 eV labeled 1a and 1b, respectively. These two peaks are similar to those of GeTe in the same energy range and their characters are mainly given by the Te 5s (1a) and Ge 4s (1b) states, respectively. The As 4s and Te(1) 5s states are delocalized over the energy range of peak I and have a decreasing in¯uence on the total DOS of the ternary compounds as n increases because of the decreasing number of As and Te(1) atoms. The 5s partial DOS of the intra-layer Te atoms: Te(2) and Te(3) in As2Ge2Te5, Te(2), Te(3) and Te(4) in As2Ge5Te8, are similar to that of the Te atoms in GeTe. The intra-layer Te atoms are octahedrally surrounded by Ge atoms, except Te(2) which is bonded to both As and Ge atoms. As a consequence the Te 5s DOS around 212 eV is found to be greater for Te(2) than for the other Te atoms. However, the contribution of these atoms to the DOS decreases with increasing value of n. XPS peak II can be compared to broad band 2 of the total DOS, which is formed by a main peak at about 23 eV and a shoulder at 21 eV (Figs. 5 and 6). The main peak contains the p-type valence states of the three different types of atoms. Interactions between the As 4p and the 5p orbitals of Te(1) and Te(2) contribute to the DOS at about 23 eV for all the ternary compounds. This contribution decreases with increasing n but has no signi®cant in¯uence on the DOS because of the contribution of the Ge 4p and intra-layer Te 5p states, which form a broad band between 24 and 22 eV. The shoulder at 21 eV is mainly formed by the Te 5p states providing a ªlone-pairº character to the top of the valence band. Thus, the partial DOS of the ternary compounds clearly show an increasing in¯uence upon the total DOS of the interactions between the Ge 4s (4p) orbitals and the intra-layer Te 5s (5p) orbitals with increasing n.
Table 2 Values (in eV) of the Ge 3d5/2, As 3p3/2 and Te 3d5/2 core-level binding energy for As2Te3, GeTe and As2GenTe31n with n 1±5
Ge 3d5/2 As 2p3/2 Te 3d5/2
As2Te3
As2GeTe4
As2Ge2Te5
As2Ge3Te6
As2Ge4Te7
As2Ge5Te8
141.2 573
30.1 141.3 573
30 141.3 572.9
30 141.3 572.8
30 141.4 572.8
29.9 141.2 572.7
GeTe 29.7 572.7
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473
Table 3 Values of the calculated Ge, As and Te atomic charges averaged over the different crystallographic sites
Ge As Te
As2Te3
As2GeTe4
As2Ge2Te5
As2Ge3Te6
As2Ge4Te7
As2Ge5Te8
0.48 20.33
0.34 0.49 20.33
0.36 0.49 20.34
0.36 0.50 20.36
0.36 0.51 20.35
0.37 0.52 20.37
4.2. Core levels The values of the binding energy of the Ge 3d5/2, As 3p3/2, and Te 3d5/2 core levels are reported in Table 2. The Ge 3d5/2 and As 3p3/2 core-level binding energies were measured for bulk crystalline germanium and arsenic, respectively, and are found to be 29.5 eV (Ge 3d5/2) and 140.9 eV (As 3p3/2) in good agreement with previously reported values [25,26]. The latter values are slightly lower than those of the binary and ternary compounds (Table 2), suggesting a weak cationic character for the Ge and As atoms in the As2Te3 ±GeTe compounds. The atomic charges have been evaluated following the procedure described in Section 3 and averaged over the atoms occupying the different crystallographic sites (Table 3). The positive and rather small values obtained for Ge and As are consistent with the XPS observation. The situation of the Te atoms is somewhat more complicated since they are found in two very different local environments in the ternary compounds, depending on their position in the layers. The calculated charges are found to be of about 20.5 and 0 for the intra-layer and inter-layer Te atoms, respectively. The charge difference is rather small, in agreement with the absence of well-resolved structures in the XPS spectra for these two types of Te atoms. Thus, we have considered average values of the tellurium charges in order to analyze the XPS Te 3d5/2 core-level binding energies (Table 3). As a main result, the averaged charges have small and negative values for all the compounds. This can be correlated with the values of the Te3d5/2 core-level binding energies, which are slightly lower than the value measured for the bulk crystalline tellurium: 573.1 eV, which is close to the previously published values [25]. The weak ionic character of the Ge, As and Te atoms is consistent with the values of their electronegativities, which are very close. This shows that the character of the Ge±Te and As±Te bonds in the As2Te3 ±GeTe compounds is rather covalent. Finally, neither the shape of the experimental core level curves nor the energy positions of the maxima of As 3p3/2 and Ge 3d5/2 vary signi®cantly from one sample to another. This can be ascribed to minor changes in the atomic environments of these elements and can be correlated to the calculated averaged charges, which do not vary noticeably as a function of n (,0.1). However, it is worth noticing that the small decrease of the Te 3d5/2 core level binding energy with increasing n can be correlated with the decrease of the Te averaged charge. This is mainly due to the increase of the
GeTe 0.42 20.42
number of intra-layer atoms which have a negative charge compared to the number of quasi-neutral inter-layer Te atoms. 5. Conclusions The electronic structure of the crystalline phases of the system As2Te3 ±GeTe has been experimentally determined by XPS and theoretically analyzed by a tight-binding calculation. The XPS valence bands are mainly formed by two bands between 215 and 26 eV and greater than 26 eV which have been attributed to the s and p valence states of the different atoms, respectively. From As2Te3 to GeTe the s-type band splits in two peaks because of the difference between the As 4s and Ge 4s atomic energy levels. The two structures found in the p-type band of As2Te3 collapse in one band for GeTe because of the increasing in¯uence of the Ge 4p states. These changes mainly occur for the ternary compounds As2GeTe4 and As2Ge2Te5. The values of the core-level binding energy are found to be slightly different to those of the bulk monoatomic crystals and indicate a weak cationic character for Ge and As and a weak anionic character for Te in agreement with the calculated charges. This shows that the As±Te and Ge±Te bonds in the As2GenTe31n compounds are rather covalent. Acknowledgements Computer resources provided by the Centre National Universitaire Sud de Calcul (France) under Project no. C97092 are gratefully acknowledged. References [1] J. Morales, C. Perez-Vicente, J.L. Tirado, Solid State Ionics 51 (1992) 133. [2] C. Perez-Vicente, J.L. Tirado, P.E. Lippens, J.C. Jumas, Phys. Rev. B 11 (1997) 6371. [3] T. Pietrass, F. Taulelle, P. Lavela, J. Olivier-Fourcade, J.C. Jumas, S. Steuernagel, J. Phys. Chem. B 101 (1997) 6715. [4] G.A. Wiegers, A. Meerschaut, Incommensurate Sandwiched Layered Compounds, in: A. Meerschaut (Ed.), Technol. Publication, 1992. [5] G.F. Khudorozhko, L.G. Bulusheva, L.N. Mazalov, V.E. Fedorov, J. Morales, E.A. Kravtsova, I.P. Asanov, G.K. Parygina, Yu.V. Mironov, J. Phys. Chem. Solids 59 (1998) 283.
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