- Email: [email protected]
The interface microscopy and spectroscopy on the cleavage surfaces of the In4Se3 pure and copper-intercalated layered crystals
ARTICLE IN PRESS
Physica E 35 (2006) 88–92 www.elsevier.com/locate/physe
The interface microscopy and spectroscopy on the cleavage surfaces of the In4Se3 pure and copper-intercalated layered crystals P.V. Galiya,, A.V. Musyanovychb, Ya.M. Fiyalaa a
Department of Electronics, Ivan Franko Lviv National University, Dragomanov str. 50, 79005 Lviv, Ukraine b University of Ulm, Macromolecular Chemistry, Albert-Einstein-Allee 11, 89069 Ulm, Germany Received 14 April 2006; received in revised form 9 June 2006; accepted 9 June 2006 Available online 7 August 2006
Abstract Surface properties of In4Se3, In4Se3(Cu) crystals were studied. SEM, STM surfaces micro-and nanostructure, and XPS spectra were obtained for interface on the cleavage surfaces of crystals that have been exposed to air. The intense XPS lines, viz. Se 3d, In 3d, Cu 2p, C 1s, and O 1s were recorded in an expanded binding-energy scale. In each case Gaussian line shape analysis has been done to determine the exact peak positions and peak areas. Chemical shifts have been obtained for the binding-energy values of the XPS lines for Se, In, C, and Cu. The formation of In–In metallic and In–O oxidized bindings and corresponding phases on the cleavage surfaces of In4Se3 and Cu–In–Se bindings for In4Se3(Cu) intercalated crystals have been found. Phases formation was also observed by SEM and STM. r 2006 Elsevier B.V. All rights reserved. PACS: 68.65 Keywords: In4Se3 layered single crystal; Cleavage surfaces; Interfaces microscopy and spectroscopy
1. Introduction The studies of In4Se3 semiconductor crystal cleavage surfaces interface formation are interesting from a fundamental point of view as this material crystallizes in a layered structure (Fig. 1) with weak interaction of van der Waals type between the layers and strong covalent–ionic interaction within the layer. The quasi-two-dimensionality (2D) of layered crystals and, consequently, absence of free unsaturated electron bonds on the cleavage surfaces, is the reason for their peculiarities according to reconstruction, adsorption [1,2] and interface phases formation. In4Se3 seems to be less studied among the layered chalcogenides crystals and attract attention as natural low-dimensional2D materials [3,4]. The intercalation of In4Se3 crystal by metals (Ag, Cu), i.e., doping of interlayer cleavage surfaces might cause appearance of new electron interactions involving intercalates and, respectively, new phases on cleavage surfaces. Corresponding author. Tel.: +380 322 964 678.
E-mail address: [email protected] (P.V. Galiy). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.06.003
This paper aims at their study a continuation of our previous results [1]. In this study we focus on the cleavage surfaces characterization (scanning electron microscopy— SEM, scanning tunnelling microscopy—STM) and on the quantitative analysis (X-ray photoelectron spectroscopy— XPS) of peculiarities of the interfaces and interface phases formation on the surfaces of pure In4Se3 and copperintercalated In4Se3(Cu) crystals during exposition in air at room temperature. 2. Experimental details The layered crystal structure of In4Se3 allows to obtain cleavage surfaces that are at the same time interlayer (1 0 0) planes (Fig. 1). Crystals have been grown by Czochralski method in our laboratory. Further thermo-treatment of crystal, containing copper impurity (In4Se3(Cu)), in evacuated quartz ampoules during 30 and more hours at 540 K leads to Cu intercalation. Crystal structure of pure In4Se3 has been considered previously [5] and later in Ref. [6]. The copper intercalation of In4Se3 layered crystals, i.e. introduction of the impurity atoms Cu into van der Waals
ARTICLE IN PRESS P.V. Galiy et al. / Physica E 35 (2006) 88–92
89
elemental standards [7] has been applied for the quantitative XPS analysis. XPS spectra were obtained for interface formation on the FRCs and OLCs surfaces of In4Se3, In4Se3(Cu) crystals that have been exposed in the air. A wide–scan (0–1100 eV) was recorded for each sample. The analyser was operated with a pass energy of 160 eV (wide–scan) and 20 eV on an expanded binding energy scale (Se 3d5/2, C 1s, In 3d5/2, O 1s, Cu 2p3/2 regions). 3. Results and discussion 3.1. Scanning microscopy results
Fig. 1. Interlayer cleavage plane (1 0 0) of In4Se3 crystal structure fragment according to Ref. [6] (projection of (0 0 1) plane). Triangle upper left shows the cleavage direction by stainless-steel tip. [In3]5+ is polycation of indium (In1, In2, In3); In+—cation of indium (In4) and intercalated copper impurity of Cu in interlayer space. The lattice spaces of crystal structure: a ¼ 15.296(1) A˚; b ¼ 12.308(1) A˚; c ¼ 4.0806(5) A˚; space group Pnnm [6]. Cleavage plane (1 0 0) is normal to the axis of crystal growth a.
interlayer gaps leads to doping of interlayer cleavage surfaces (Fig. 1). The XPS quantitative analysis shows that Cu surface averaged concentration is 0.22–0.25 at%. According to Xray structural analysis, the availability of such Cu concentrations allows to obtain In4Se3(Cu) crystals with well-layered structure. The cleavage surfaces of In4Se3 and In4Se3(Cu) layered crystals have been obtained and exposed in air. Those exposed for approximately 2–15 min are called ‘‘fresh’’ cleavages (FRCs), and those exposed for a longer time (more than 24 h) are called ‘‘old’’ cleavages (OLCs). SEM images of the In4Se3, In4Se3(Cu) crystal cleavage surfaces were obtained using a DSM 982 Gemini (ZEISS) instrument. The cleavage surfaces were not coated with a thin layer of gold/palladium before measurements because of their good conductivity. The residual pressure in SEM chamber was 1.3 103 Pa. STM measurements of the In4Se3 crystal cleavage surfaces were performed in air at room temperature with a STM RHK Technology Inc. microscope. The STM tip was 0.25 mm Pt/Ir (80%/20%) wire and the STM images were acquired at constant-current mode. The tunnelling current was 80 pA at the bias voltage 200 mV. XPS has been carried out with an AXIS ULTRA (Kratos Analytical) equipment, employing monochromatic Al Ka X-rays (hn ¼ 1486.6 eV) and full-width of halfmaximum of 0.3 eV at an electron take-off angle of 901. The X-ray beam size was 0.4 0.7 mm2 in area with separated points on the sample surface. The energy resolution of the hemispherical mirror analyser was 1.5%. The residual pressure in UHV chamber was 1.3 107 Pa during the spectra recording. XPS experiments are described elsewhere [1]. The method of pure
Cleavage of the layered crystals by stainless-steel tip (Fig. 1) allows obtaining excellent cleavage surfaces with a relatively small quantity of defects (Fig. 2). Fig. 2 shows SEM images of surface microstructures of the FRCs and OLCs surfaces of In4Se3, In4Se3(Cu) - layered crystals. The typical images obtained for FRCs surfaces are shown in Fig. 2a,c. Their surface microstructures are regular and smooth. The microrelief becomes complicated and some induced micro- and nanophases are observed (see XPSresults below) for OLCs surfaces of In4Se3, In4Se3(Cu) crystals (Fig. 2b). The OLCs surfaces of crystals are covered with some new microphases—less for the samples with small exposure (some days) and more for longer exposed samples (some weeks).
Fig. 2. Microstructure of the cleavage surfaces of the In4Se3, In4Se3(Cu) crystals: a,c ‘‘fresh’’ cleavages (FRCs) surfaces; b ‘‘old’’ cleavages (OLCs) surfaces; d,e—layered crystals microfragments.
ARTICLE IN PRESS 90
P.V. Galiy et al. / Physica E 35 (2006) 88–92
Fig. 3. STM image of the 50 50 nm2 area of the In4Se3 crystal ‘‘fresh’’ cleavage (FRC) surface.
Figs. 2c and 2e show the layered microfragments of the crystal. These fragments (Figs. 2d,e) have been obtained during cleavage, because the crystal is fragile and was destroyed at the place of cleavage tip–crystal contact. We can see the layered structure of crystal on these fragments: the fine steps similar to the steps of edge dislocation (Fig. 2e) – really, obviously, there are destroyed layers and excellent surface, but we must remember that surfaces are not smooth and are rough in atomic scale (see Figs. 1, 3); Fig. 2d shows the deformed and discontinuous layers. The accumulation and surface nanophase formation (these processes are much more effective for OLCs surfaces) are observed on the edge of dislocations and discontinuous layers of the crystal cleavage surface. Some induced nanophases are observed for surfaces of In4Se3, In4Se3(Cu) crystals not only by SEM (Fig. 2b) but also by STM method (Fig. 3). STM-results of the In4Se3 crystal cleavage surfaces (Fig. 3) show part of the surface (50 50 nm2 STM image) with deviation from the flat. The formed surface nanophase is observed in the centre of the STM image (quasi-flat terraces separated by the step edges). Arrow shows the appeared quasi-periodic rumpling. 3.2. X-ray photoelectron spectroscopy (XPS) results The presence of carbon on the FRCs and OLCs surfaces of In4Se3 crystals was confirmed by XPS analysis and it may be the result of hydrocarbon contamination [1,2], that arises from availability of other adsorbed gases. The exact peak position of the intense lines, viz. Se 3d, In 3d, Cu 2p and C 1s have been subsequently determined from the
corresponding profiles, recorded on an expanded binding energy scale and for Se 3d, In 3d5/2 and Cu 2p3/2 are shown in Fig. 4, and for C 1s—in Fig. 5. In each case Gausian line shape analysis has been done for the XPS expanded profiles after making the background correction using Shirley’s method [8] to determine the exact peak position and peak area. The peak areas were applied for elemental quantitative XPS analysis [1] and to determine the composition of the interface formation on the surface (Fig. 4b). C 1s peak at 284.39 eV and O 1s peak at 530.99 eV give the main contribution to the XPS spectrum of the interface for the OLCs In4Se3 surface, but there are also signals at 443.99 (3d5/2) eV and 408.0 (M4N4,5N4s) eV corresponding to the In 3d5/2 core level and In M4N4,5N4s-Auger transition and the signal at 53.69 eV corresponding to the Se (3d5/2+3d3/2) levels. XPS wide-scan of the OLCs surface interface indicates on missing of nitrogen N 1s peak at 400.0 eV thus confirming the presence of slightly physical adsorbed nitrogen on the OLCs surface exposed in air. The nitrogen that is slightly adsorbed from air desorbs from surface in UHV conditions. This indicates, that in UHV XPSchamber the adsorbed nitrogen (N2) is absent and carbon monoxide is the main adsorbate on the In4Se3 surface, and it forms In4Se3–C interface during CO adsorption with its dissociation and carbon chemisorption, as it was obtained by mass-spectroscopy and AES studies of In4Se3 UHV cleavages in situ [1,2]. On the expanded-scan XPS spectra for FRCs surfaces the binding energy peak of the indium atoms In 3d5/2 444.0 eV indicates that indium atoms form metal-like binding (for metallic indium In 3d5/2 443.84–443.9 eV [7]) slightly shifting to In 3d5/2 443.99 eV for the OLCs surfaces (Fig. 4(a,b)). The formation of In–In metallic binding and metallic phase on the free-cleavage surfaces of In4Se3 is possible in consequence of the slight In+(In4) cation bindings with the layer-packets (Fig. 1). The mean binding length of In–In binding is 2.77 A˚ for the three In1, In2, In3 atoms, that is smaller, than one for the metallic indium— 3.24–3.36 A˚. Thus these structures contain polycation, which consists of three atoms [In3]5+ and bindings for these structures are ionic–covalent, but non-metallic ones [5,6]. The atoms of Se are sp3-hybridization state and each is bonded with three indium atoms and the selenium atoms are in the twice ionised state Se2 [5,6]. Fig. 4b with XPS spectra expanded-scan for the OLCs surfaces shows the formation of metallic and In2O3oxidized phase for which the binding energy peak of the In 3d5/2 is observed at 444.4 eV [7]. The obtained binding energy Se 3d5/2 peaks for FRC at 54.3 eV and for OLC Se (3d5/2+3d3/2) at 53.69 eV surfaces of In4Se3 (Fig. 4(a,b)) also indicate on the oxygen interaction with the indium atoms and formation of oxidized In2O3 [7] and on selenium atoms interaction with carbon during molecule CO adsorption with its decomposition—oxygen desorption and carbon 5s-binding with the surface of layered crystals (Se–C bindings [2]). Its well known that under SeO2 oxide
ARTICLE IN PRESS P.V. Galiy et al. / Physica E 35 (2006) 88–92
91
Fig. 4. Expanded-scan XPS spectra of the matrix components (Se 3d5/2; Se 3d3/2; and In 3d5/2; In 3d3/2) for the cleavage surfaces of In4Se3 (a, b) crystals: a—‘‘fresh’’ cleavage surfaces (FRCs); b—‘‘old’’ cleavage surfaces (OLCs); c—OLCs of intercalated In4Se3(Cu) (Se 3d; Cu 2p3/2 and Cu 2p1/2).
formation the binding energy peak Se 3d5/2 is observed at 58.8 eV [7], and it is absent for FRC and OLC surfaces of In4Se3. At the same time the In–O binding formation on the expanded-scan XPS spectra could be observed clearly, and Se–C binding formation also takes places as one can see from XPS (Fig. 4(a,b), Fig. 5(a,b)) and our AES results [1,2]. It must be noted that for OLCs shifts in Cu 2p3/2 peak position from 931.8 eV for metallic Cu–Cu binding to Cu 2p3/2(930.59 eV), In 3d5/2(443.99 eV) and Se 3d(53.71 eV) confirm possibilities of Cu–In–Se bindings. The Cu 2p3/2, In 3d5/2, Se 3d binding energies are close to ones for CuInSe2 [7]. The presence of non-simple interaction in the In4Se3–C interfaces of OLCs and FRCs surfaces can also be confirmed by a closer inspection of the C 1s core level of XPS spectra shown in Fig. 5, when analyser was operated with a pass energy of 20 eV in C 1s region. Shirley background subtraction in the least-squares fittings is used for inspection of the C 1s core level. Peak positions and widths are determined from least-squares fitting using the standard for this equipment software. The deconvolution for C 1s carbon peak is obtained with application of existing database and software for organic compounds. The deconvoluted C 1s (284.7 eV) XPS peak for FRCs of In4Se3–C,O surface interface (Fig. 5a) shows the presence of no less than three types of carbon. They could be
attributed to: C–Se (284.7 eV); C–O (286.0 eV) and CQO (287.0 eV) interactions. The deconvoluted carbon C 1s peak, for the OLCs of In4Se3 surface interface (Fig. 5b) shows peaks at 284.39 eV attributed to C–Se; C–C interaction in ‘‘graphite phase’’ or C–C/C–H (285.0 eV); C–O; CQO and possible C–N (285.7 eV). The last one is confirmed by the presence of N 1s (400.0 eV) peak in the spectra and its negligible quantity on the OLCs of In4Se3 surface. It must be noted, that CQO or C–N type bindings obtained from C 1 s peak deconvolution are not necessarily implemented in the adsorbed coatings at formation of the interface In4Se3–C,O layer. CQO and C–N bindings (Fig. 5) are only the result of C 1 s peak deconvolution. At the same time In–O and Se–C bindings exist really in In4Se3–C,O surface interface layer with the formation of ‘‘graphite’’ phase (C–C interactions) under carbon coverage degree yX1 [1,2]. The surface elemental compositions for the series of the ‘‘fresh’’ and ‘‘old’’ cleavages of In4Se3 surface interface that have been calculated from the experimental XPS results are presented in Ref. [1]. 4. Conclusion In consequence of our studies it may be assumed that, in spite of crystal structure and electronic structure of the
ARTICLE IN PRESS 92
P.V. Galiy et al. / Physica E 35 (2006) 88–92
were obtained for cleavage surfaces of the In4Se3, In4Se3(Cu) crystals. The new electron interactions have been observed on cleavage surfaces. The formation of In–In metallic (on FRCs and OLCs) and In–O oxidized (OLCs) bindings and consequently metallic and oxidized phases on the cleavage surfaces of In4Se3, and Cu–In–Se bindings for In4Se3(Cu) intercalated crystals (Cu 2p3/2, In 3d5/2, Se 3d binding energies are close to ones for CuInSe2) have been found. The presence of carbon in the interface layers, formed on the surfaces, obtained by cleavage both in the air and in UHV [1], has been shown. Bindings formed by carbon are attributed to C–Se and C–C electronic interactions. Acknowledgements The authors are grateful to Dr. F. Simon and Mrs. E. Kern from Dresden University of Technology for providing assistance in XPS and SEM measurements. References
Fig. 5. Expanded-scan XPS smooth spectra of the deconvoluted C 1 s peak with background correction for In4Se3–C,O interface system: a— ‘‘fresh’’ (FRCs) and b—old cleavage surfaces (OLCs). The peaks intensity is multiplied by two for the expanded-scan XPS spectra of the FRCs surfaces.
layer, the cleavage surfaces of the layered semiconductor crystals In4Se3, In4Se3(Cu), are not stable and are not inert to gases adsorption. Interfaces and interface phases formation on the cleavage surfaces are dependent on the duration of the exposition in air. The images of micro- and nanostructures together with formed surface micro- (SEM) and nanophases (STM), whose existence was proved by XPS-results,
[1] P.V. Galiy, A.V. Musyanovych, T. Nenchuk, J. Electron. Spectrosc. and Relat. Phenom. 142 (2005) 121. [2] P.V. Galiy, T.M. Nenchuk, J.M. Stakhira, J. Phys. D 34 (2001) 18. [3] W. Jaegermann, A. Klein, C. Pettenkofer, in: H.P. Hughes, H.I. Starnberg (Eds.), Physics and Chemistry of Materials with LowDimensional Structures, vol. 24, Kluwer Academic Publishers, Dordrecht, 2000, pp. 317–402. [4] O.A. Balitskii, V.P. Savchyn, B. Jaeckel, W. Jaegermann, Phys. E 22 (2004) 921. [5] L.I. Man, R.M. Imamov, S.A. Semiletov, Crystalographija 21 (1976) 628. [6] U. Schwarz, H. Hillebrecht, H.J. Deiseroth, R. Walther, Z.Kristallogr 210 (1995) 342. [7] D. Briggs, M.P. Seah, in: C.D. Wagner (Eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, App. 5, vol. 1, second ed., Wiley New York, 1990, p. 511. [8] D.A. Shirley, Phys. Rev. B 5 (1972) 4709.
Physica E 35 (2006) 88–92 www.elsevier.com/locate/physe
The interface microscopy and spectroscopy on the cleavage surfaces of the In4Se3 pure and copper-intercalated layered crystals P.V. Galiya,, A.V. Musyanovychb, Ya.M. Fiyalaa a
Department of Electronics, Ivan Franko Lviv National University, Dragomanov str. 50, 79005 Lviv, Ukraine b University of Ulm, Macromolecular Chemistry, Albert-Einstein-Allee 11, 89069 Ulm, Germany Received 14 April 2006; received in revised form 9 June 2006; accepted 9 June 2006 Available online 7 August 2006
Abstract Surface properties of In4Se3, In4Se3(Cu) crystals were studied. SEM, STM surfaces micro-and nanostructure, and XPS spectra were obtained for interface on the cleavage surfaces of crystals that have been exposed to air. The intense XPS lines, viz. Se 3d, In 3d, Cu 2p, C 1s, and O 1s were recorded in an expanded binding-energy scale. In each case Gaussian line shape analysis has been done to determine the exact peak positions and peak areas. Chemical shifts have been obtained for the binding-energy values of the XPS lines for Se, In, C, and Cu. The formation of In–In metallic and In–O oxidized bindings and corresponding phases on the cleavage surfaces of In4Se3 and Cu–In–Se bindings for In4Se3(Cu) intercalated crystals have been found. Phases formation was also observed by SEM and STM. r 2006 Elsevier B.V. All rights reserved. PACS: 68.65 Keywords: In4Se3 layered single crystal; Cleavage surfaces; Interfaces microscopy and spectroscopy
1. Introduction The studies of In4Se3 semiconductor crystal cleavage surfaces interface formation are interesting from a fundamental point of view as this material crystallizes in a layered structure (Fig. 1) with weak interaction of van der Waals type between the layers and strong covalent–ionic interaction within the layer. The quasi-two-dimensionality (2D) of layered crystals and, consequently, absence of free unsaturated electron bonds on the cleavage surfaces, is the reason for their peculiarities according to reconstruction, adsorption [1,2] and interface phases formation. In4Se3 seems to be less studied among the layered chalcogenides crystals and attract attention as natural low-dimensional2D materials [3,4]. The intercalation of In4Se3 crystal by metals (Ag, Cu), i.e., doping of interlayer cleavage surfaces might cause appearance of new electron interactions involving intercalates and, respectively, new phases on cleavage surfaces. Corresponding author. Tel.: +380 322 964 678.
E-mail address: [email protected] (P.V. Galiy). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.06.003
This paper aims at their study a continuation of our previous results [1]. In this study we focus on the cleavage surfaces characterization (scanning electron microscopy— SEM, scanning tunnelling microscopy—STM) and on the quantitative analysis (X-ray photoelectron spectroscopy— XPS) of peculiarities of the interfaces and interface phases formation on the surfaces of pure In4Se3 and copperintercalated In4Se3(Cu) crystals during exposition in air at room temperature. 2. Experimental details The layered crystal structure of In4Se3 allows to obtain cleavage surfaces that are at the same time interlayer (1 0 0) planes (Fig. 1). Crystals have been grown by Czochralski method in our laboratory. Further thermo-treatment of crystal, containing copper impurity (In4Se3(Cu)), in evacuated quartz ampoules during 30 and more hours at 540 K leads to Cu intercalation. Crystal structure of pure In4Se3 has been considered previously [5] and later in Ref. [6]. The copper intercalation of In4Se3 layered crystals, i.e. introduction of the impurity atoms Cu into van der Waals
ARTICLE IN PRESS P.V. Galiy et al. / Physica E 35 (2006) 88–92
89
elemental standards [7] has been applied for the quantitative XPS analysis. XPS spectra were obtained for interface formation on the FRCs and OLCs surfaces of In4Se3, In4Se3(Cu) crystals that have been exposed in the air. A wide–scan (0–1100 eV) was recorded for each sample. The analyser was operated with a pass energy of 160 eV (wide–scan) and 20 eV on an expanded binding energy scale (Se 3d5/2, C 1s, In 3d5/2, O 1s, Cu 2p3/2 regions). 3. Results and discussion 3.1. Scanning microscopy results
Fig. 1. Interlayer cleavage plane (1 0 0) of In4Se3 crystal structure fragment according to Ref. [6] (projection of (0 0 1) plane). Triangle upper left shows the cleavage direction by stainless-steel tip. [In3]5+ is polycation of indium (In1, In2, In3); In+—cation of indium (In4) and intercalated copper impurity of Cu in interlayer space. The lattice spaces of crystal structure: a ¼ 15.296(1) A˚; b ¼ 12.308(1) A˚; c ¼ 4.0806(5) A˚; space group Pnnm [6]. Cleavage plane (1 0 0) is normal to the axis of crystal growth a.
interlayer gaps leads to doping of interlayer cleavage surfaces (Fig. 1). The XPS quantitative analysis shows that Cu surface averaged concentration is 0.22–0.25 at%. According to Xray structural analysis, the availability of such Cu concentrations allows to obtain In4Se3(Cu) crystals with well-layered structure. The cleavage surfaces of In4Se3 and In4Se3(Cu) layered crystals have been obtained and exposed in air. Those exposed for approximately 2–15 min are called ‘‘fresh’’ cleavages (FRCs), and those exposed for a longer time (more than 24 h) are called ‘‘old’’ cleavages (OLCs). SEM images of the In4Se3, In4Se3(Cu) crystal cleavage surfaces were obtained using a DSM 982 Gemini (ZEISS) instrument. The cleavage surfaces were not coated with a thin layer of gold/palladium before measurements because of their good conductivity. The residual pressure in SEM chamber was 1.3 103 Pa. STM measurements of the In4Se3 crystal cleavage surfaces were performed in air at room temperature with a STM RHK Technology Inc. microscope. The STM tip was 0.25 mm Pt/Ir (80%/20%) wire and the STM images were acquired at constant-current mode. The tunnelling current was 80 pA at the bias voltage 200 mV. XPS has been carried out with an AXIS ULTRA (Kratos Analytical) equipment, employing monochromatic Al Ka X-rays (hn ¼ 1486.6 eV) and full-width of halfmaximum of 0.3 eV at an electron take-off angle of 901. The X-ray beam size was 0.4 0.7 mm2 in area with separated points on the sample surface. The energy resolution of the hemispherical mirror analyser was 1.5%. The residual pressure in UHV chamber was 1.3 107 Pa during the spectra recording. XPS experiments are described elsewhere [1]. The method of pure
Cleavage of the layered crystals by stainless-steel tip (Fig. 1) allows obtaining excellent cleavage surfaces with a relatively small quantity of defects (Fig. 2). Fig. 2 shows SEM images of surface microstructures of the FRCs and OLCs surfaces of In4Se3, In4Se3(Cu) - layered crystals. The typical images obtained for FRCs surfaces are shown in Fig. 2a,c. Their surface microstructures are regular and smooth. The microrelief becomes complicated and some induced micro- and nanophases are observed (see XPSresults below) for OLCs surfaces of In4Se3, In4Se3(Cu) crystals (Fig. 2b). The OLCs surfaces of crystals are covered with some new microphases—less for the samples with small exposure (some days) and more for longer exposed samples (some weeks).
Fig. 2. Microstructure of the cleavage surfaces of the In4Se3, In4Se3(Cu) crystals: a,c ‘‘fresh’’ cleavages (FRCs) surfaces; b ‘‘old’’ cleavages (OLCs) surfaces; d,e—layered crystals microfragments.
ARTICLE IN PRESS 90
P.V. Galiy et al. / Physica E 35 (2006) 88–92
Fig. 3. STM image of the 50 50 nm2 area of the In4Se3 crystal ‘‘fresh’’ cleavage (FRC) surface.
Figs. 2c and 2e show the layered microfragments of the crystal. These fragments (Figs. 2d,e) have been obtained during cleavage, because the crystal is fragile and was destroyed at the place of cleavage tip–crystal contact. We can see the layered structure of crystal on these fragments: the fine steps similar to the steps of edge dislocation (Fig. 2e) – really, obviously, there are destroyed layers and excellent surface, but we must remember that surfaces are not smooth and are rough in atomic scale (see Figs. 1, 3); Fig. 2d shows the deformed and discontinuous layers. The accumulation and surface nanophase formation (these processes are much more effective for OLCs surfaces) are observed on the edge of dislocations and discontinuous layers of the crystal cleavage surface. Some induced nanophases are observed for surfaces of In4Se3, In4Se3(Cu) crystals not only by SEM (Fig. 2b) but also by STM method (Fig. 3). STM-results of the In4Se3 crystal cleavage surfaces (Fig. 3) show part of the surface (50 50 nm2 STM image) with deviation from the flat. The formed surface nanophase is observed in the centre of the STM image (quasi-flat terraces separated by the step edges). Arrow shows the appeared quasi-periodic rumpling. 3.2. X-ray photoelectron spectroscopy (XPS) results The presence of carbon on the FRCs and OLCs surfaces of In4Se3 crystals was confirmed by XPS analysis and it may be the result of hydrocarbon contamination [1,2], that arises from availability of other adsorbed gases. The exact peak position of the intense lines, viz. Se 3d, In 3d, Cu 2p and C 1s have been subsequently determined from the
corresponding profiles, recorded on an expanded binding energy scale and for Se 3d, In 3d5/2 and Cu 2p3/2 are shown in Fig. 4, and for C 1s—in Fig. 5. In each case Gausian line shape analysis has been done for the XPS expanded profiles after making the background correction using Shirley’s method [8] to determine the exact peak position and peak area. The peak areas were applied for elemental quantitative XPS analysis [1] and to determine the composition of the interface formation on the surface (Fig. 4b). C 1s peak at 284.39 eV and O 1s peak at 530.99 eV give the main contribution to the XPS spectrum of the interface for the OLCs In4Se3 surface, but there are also signals at 443.99 (3d5/2) eV and 408.0 (M4N4,5N4s) eV corresponding to the In 3d5/2 core level and In M4N4,5N4s-Auger transition and the signal at 53.69 eV corresponding to the Se (3d5/2+3d3/2) levels. XPS wide-scan of the OLCs surface interface indicates on missing of nitrogen N 1s peak at 400.0 eV thus confirming the presence of slightly physical adsorbed nitrogen on the OLCs surface exposed in air. The nitrogen that is slightly adsorbed from air desorbs from surface in UHV conditions. This indicates, that in UHV XPSchamber the adsorbed nitrogen (N2) is absent and carbon monoxide is the main adsorbate on the In4Se3 surface, and it forms In4Se3–C interface during CO adsorption with its dissociation and carbon chemisorption, as it was obtained by mass-spectroscopy and AES studies of In4Se3 UHV cleavages in situ [1,2]. On the expanded-scan XPS spectra for FRCs surfaces the binding energy peak of the indium atoms In 3d5/2 444.0 eV indicates that indium atoms form metal-like binding (for metallic indium In 3d5/2 443.84–443.9 eV [7]) slightly shifting to In 3d5/2 443.99 eV for the OLCs surfaces (Fig. 4(a,b)). The formation of In–In metallic binding and metallic phase on the free-cleavage surfaces of In4Se3 is possible in consequence of the slight In+(In4) cation bindings with the layer-packets (Fig. 1). The mean binding length of In–In binding is 2.77 A˚ for the three In1, In2, In3 atoms, that is smaller, than one for the metallic indium— 3.24–3.36 A˚. Thus these structures contain polycation, which consists of three atoms [In3]5+ and bindings for these structures are ionic–covalent, but non-metallic ones [5,6]. The atoms of Se are sp3-hybridization state and each is bonded with three indium atoms and the selenium atoms are in the twice ionised state Se2 [5,6]. Fig. 4b with XPS spectra expanded-scan for the OLCs surfaces shows the formation of metallic and In2O3oxidized phase for which the binding energy peak of the In 3d5/2 is observed at 444.4 eV [7]. The obtained binding energy Se 3d5/2 peaks for FRC at 54.3 eV and for OLC Se (3d5/2+3d3/2) at 53.69 eV surfaces of In4Se3 (Fig. 4(a,b)) also indicate on the oxygen interaction with the indium atoms and formation of oxidized In2O3 [7] and on selenium atoms interaction with carbon during molecule CO adsorption with its decomposition—oxygen desorption and carbon 5s-binding with the surface of layered crystals (Se–C bindings [2]). Its well known that under SeO2 oxide
ARTICLE IN PRESS P.V. Galiy et al. / Physica E 35 (2006) 88–92
91
Fig. 4. Expanded-scan XPS spectra of the matrix components (Se 3d5/2; Se 3d3/2; and In 3d5/2; In 3d3/2) for the cleavage surfaces of In4Se3 (a, b) crystals: a—‘‘fresh’’ cleavage surfaces (FRCs); b—‘‘old’’ cleavage surfaces (OLCs); c—OLCs of intercalated In4Se3(Cu) (Se 3d; Cu 2p3/2 and Cu 2p1/2).
formation the binding energy peak Se 3d5/2 is observed at 58.8 eV [7], and it is absent for FRC and OLC surfaces of In4Se3. At the same time the In–O binding formation on the expanded-scan XPS spectra could be observed clearly, and Se–C binding formation also takes places as one can see from XPS (Fig. 4(a,b), Fig. 5(a,b)) and our AES results [1,2]. It must be noted that for OLCs shifts in Cu 2p3/2 peak position from 931.8 eV for metallic Cu–Cu binding to Cu 2p3/2(930.59 eV), In 3d5/2(443.99 eV) and Se 3d(53.71 eV) confirm possibilities of Cu–In–Se bindings. The Cu 2p3/2, In 3d5/2, Se 3d binding energies are close to ones for CuInSe2 [7]. The presence of non-simple interaction in the In4Se3–C interfaces of OLCs and FRCs surfaces can also be confirmed by a closer inspection of the C 1s core level of XPS spectra shown in Fig. 5, when analyser was operated with a pass energy of 20 eV in C 1s region. Shirley background subtraction in the least-squares fittings is used for inspection of the C 1s core level. Peak positions and widths are determined from least-squares fitting using the standard for this equipment software. The deconvolution for C 1s carbon peak is obtained with application of existing database and software for organic compounds. The deconvoluted C 1s (284.7 eV) XPS peak for FRCs of In4Se3–C,O surface interface (Fig. 5a) shows the presence of no less than three types of carbon. They could be
attributed to: C–Se (284.7 eV); C–O (286.0 eV) and CQO (287.0 eV) interactions. The deconvoluted carbon C 1s peak, for the OLCs of In4Se3 surface interface (Fig. 5b) shows peaks at 284.39 eV attributed to C–Se; C–C interaction in ‘‘graphite phase’’ or C–C/C–H (285.0 eV); C–O; CQO and possible C–N (285.7 eV). The last one is confirmed by the presence of N 1s (400.0 eV) peak in the spectra and its negligible quantity on the OLCs of In4Se3 surface. It must be noted, that CQO or C–N type bindings obtained from C 1 s peak deconvolution are not necessarily implemented in the adsorbed coatings at formation of the interface In4Se3–C,O layer. CQO and C–N bindings (Fig. 5) are only the result of C 1 s peak deconvolution. At the same time In–O and Se–C bindings exist really in In4Se3–C,O surface interface layer with the formation of ‘‘graphite’’ phase (C–C interactions) under carbon coverage degree yX1 [1,2]. The surface elemental compositions for the series of the ‘‘fresh’’ and ‘‘old’’ cleavages of In4Se3 surface interface that have been calculated from the experimental XPS results are presented in Ref. [1]. 4. Conclusion In consequence of our studies it may be assumed that, in spite of crystal structure and electronic structure of the
ARTICLE IN PRESS 92
P.V. Galiy et al. / Physica E 35 (2006) 88–92
were obtained for cleavage surfaces of the In4Se3, In4Se3(Cu) crystals. The new electron interactions have been observed on cleavage surfaces. The formation of In–In metallic (on FRCs and OLCs) and In–O oxidized (OLCs) bindings and consequently metallic and oxidized phases on the cleavage surfaces of In4Se3, and Cu–In–Se bindings for In4Se3(Cu) intercalated crystals (Cu 2p3/2, In 3d5/2, Se 3d binding energies are close to ones for CuInSe2) have been found. The presence of carbon in the interface layers, formed on the surfaces, obtained by cleavage both in the air and in UHV [1], has been shown. Bindings formed by carbon are attributed to C–Se and C–C electronic interactions. Acknowledgements The authors are grateful to Dr. F. Simon and Mrs. E. Kern from Dresden University of Technology for providing assistance in XPS and SEM measurements. References
Fig. 5. Expanded-scan XPS smooth spectra of the deconvoluted C 1 s peak with background correction for In4Se3–C,O interface system: a— ‘‘fresh’’ (FRCs) and b—old cleavage surfaces (OLCs). The peaks intensity is multiplied by two for the expanded-scan XPS spectra of the FRCs surfaces.
layer, the cleavage surfaces of the layered semiconductor crystals In4Se3, In4Se3(Cu), are not stable and are not inert to gases adsorption. Interfaces and interface phases formation on the cleavage surfaces are dependent on the duration of the exposition in air. The images of micro- and nanostructures together with formed surface micro- (SEM) and nanophases (STM), whose existence was proved by XPS-results,
[1] P.V. Galiy, A.V. Musyanovych, T. Nenchuk, J. Electron. Spectrosc. and Relat. Phenom. 142 (2005) 121. [2] P.V. Galiy, T.M. Nenchuk, J.M. Stakhira, J. Phys. D 34 (2001) 18. [3] W. Jaegermann, A. Klein, C. Pettenkofer, in: H.P. Hughes, H.I. Starnberg (Eds.), Physics and Chemistry of Materials with LowDimensional Structures, vol. 24, Kluwer Academic Publishers, Dordrecht, 2000, pp. 317–402. [4] O.A. Balitskii, V.P. Savchyn, B. Jaeckel, W. Jaegermann, Phys. E 22 (2004) 921. [5] L.I. Man, R.M. Imamov, S.A. Semiletov, Crystalographija 21 (1976) 628. [6] U. Schwarz, H. Hillebrecht, H.J. Deiseroth, R. Walther, Z.Kristallogr 210 (1995) 342. [7] D. Briggs, M.P. Seah, in: C.D. Wagner (Eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, App. 5, vol. 1, second ed., Wiley New York, 1990, p. 511. [8] D.A. Shirley, Phys. Rev. B 5 (1972) 4709.