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The specific features of low-energy electron backscattering from different Ge surfaces
Surface Science 499 (2002) L113–L118 www.elsevier.com/locate/susc
Surface Science Letters
The specific features of low-energy electron backscattering from different Ge surfaces T.Yu. Popik a, V.M. Feyer a, O.B. Shpenik a
a,*
, Yu.V. Popik
b
Institute of Electron Physics, Ukr. Nat. Acad. Sci., Universytetska St. 21, Uzhhorod 88000, Ukraine b Uzhhorod National University, Voloshyn St. 54, Uzhhorod 88000, Ukraine Received 24 July 2001; accepted for publication 3 December 2001
Abstract Low-energy electron backscattering technique using a hypocycloidal electron spectrometer is applied to measure energy loss spectra at different incident electron energies from mirror-polished (1 1 1), (1 0 0) and (1 1 0) surfaces of Ge. The obtained results confirm and complement the available data on the energy position of the extrema of the density of both filled (below EF ) and empty (above EF ) surface and bulk electron states in the reduced Brillouin zone for different surfaces of germanium. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron energy loss spectroscopy (EELS); Surface electronic phenomena (work function, surface potential, surface states, etc.); Germanium
Germanium is characterized by the diamondlike crystal structure. A typical feature of the ideal surface of this structure is the presence of dangling hybrid orbitals directed into the vacuum [1]. Supposing that when the Ge crystal cleaves, a pair of bonding electrons belonging to an orbital in the bulk are split between the two crystal parts, leaving half filled broken orbitals. One atom in the unit cell at the (1 1 1) surface then possesses one dangling hybrid bond and two dangling bonds at the (1 0 0) surface, while each of the two atoms in the unit cell at the (1 1 0) surface possesses one dan-
*
Corresponding author. Tel.: +380-3122-43668/43650; fax: +380-3122-43650. E-mail address: [email protected] (O.B. Shpenik).
gling bond (Fig. 1). The surface density of the dangling bonds is minimal on the (1 1 1) surface and maximal on the (1 0 0) surface [1]. The (1 1 1) plane is the natural cleavage plane for germanium crystal [1] and its properties have been studied most extensively. Upon heating to T > 400 K, the clean Ge(1 1 1) surface structure is shown to convert irreversibly into the stable Ge(1 1 1)-c(2 8) lattice, where the adatoms (the topmost atoms of the surface) saturate 3/4 of the dangling bonds of the ideal first layer and donate their extra electron to the remaining first-layer atoms (rest atoms) [2,3]. 2 2-size block is the main building block in the Ge(1 1 1)-c(2 8) lattice [2]. The spectrum of surface electron states (SES) of Ge(1 1 1)-c(2 8) surface consists of three groups
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 9 3 6 - 7
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Fig. 1. Germanium surfaces under investigation and ideal bond breakdown at these surfaces.
of levels. The first group includes two states, located in the upper part of the energy gap. It has two electron-state density maxima with the threshold energies 0.4 and 0.45 eV [2,4,5]. The SES bands of the second group are located on both sides of the Fermi level: the donor band, 0.02 eV below EF and the acceptor band, 0.02 eV above EF [2,6]. The third group of SES is located in the valence band of the bulk crystal. This group consists of two 0.2 eV wide surface bands centred at 0.85 and 1.4 eV below the valence band maximum [3,7–9] as well as SES with the energies 0.4 eV [2,7,10] and 1.1 eV [7,9]. Clean Ge(1 1 0) and Ge(1 0 0) surfaces have been studied much less. Ge(1 1 0) surface, cleaned by subsequent cycles of ion sputtering and annealing to T ¼ 1150 K [11] (according to Ref. [12] – to 703 K) on cooling to room temperature (RT) shows a cð8 10Þ pattern [11–14]. The (1 1 0) surfaces are characterized by coexistence of chains of surface atoms and clusters of adatoms [15]. Based on the STM studies of Ge(1 1 0) surface at RT a model is suggested, describing a surface consisting of additional atomic species such as dimers, rebonded atoms and rest atoms [16]. Besides this, the main features of the reconstruction are shown to be periodic clusters of five atoms [12]. The valence-band and core-level photoemission studies on Ge(1 1 0)-c(8 10) at room and higher temperatures are reported in Refs. [11,12]. The normal emission angle-integrated valence band spectra of the cð8 10Þ structure at RT show a rich structure: a shoulder at the binding energy
(BE) 1:0 eV, a main peak at about 2:4 eV, a pronounced shoulder at 3:8 eV and two peaks at 7.6 and 12.8 eV [11]. The features at the energies 1:0, 2:4 and 3:8 eV are attributed to the surface states, and the peaks at 7.6 and 12.8 eV––to the bulk band transitions. In the valence-band photoemission spectra for Ge(1 1 0)-c(8 10), measured at RT, the peaks at 2.0, 4.0, 5.5, 8.0 and 9.5 eV BE are revealed, being ascribed to bulk interband transitions. A low BE shoulder near the 2.0 eV peak is related to a surface state [12]. The basic building blocks of Ge(1 0 0) surface are asymmetric dimers, which between 220 and 955 K exhibit paramagnetic (2 1) order [17]. In the photoemission spectra at RT, the features with the energies 0.4 and 1.4 eV, are revealed, assigned to the surface states, as well as a feature at 3.25 eV due to the emission from the bulk bands (in Ref. [17] the origin of the energy scale is taken at EF , its position being determined within 0.06 eV at the overall resolution of 0.16 eV). Ge(0 0 1)-(2 1) surfaces, obtained by either molecular beam epitaxy (MBE), or sputtering, followed by annealing, were studied by angleresolved photoemission technique [18]. The surface electronic structure is shown to be independent of the surface preparation procedure. Two observed features in the normal emission spectra with constant BE of about 0.5 and 1.3 eV relative to the valence band maximum (VBM) (EF EVBM ¼ 0:1 eV) are related to surface states [18]. A richer structure in the spectra was observed in similar studies of the Ge(0 0 1)-(2 1) surface in Ref. [19]. In the energy range 0–6 eV, a set of angle-resolved photoemission spectra shows five surface features at 0.4, 0.8, 3.0, 1.15 and 1.5 eV. A strong structure, revealed in these spectra at about 3.5 eV, is attributed to a direct bulk transition. (In the discussed paper the energy values are measured with respect to EF and EF EVBM ¼ 0:1 eV, overall spectral resolution being 0.18 eV.) The obtained data confirm the existence of two surface structures at 0.6 and 1.3 eV below VBM, detected in Ref. [20]. The reference data on the SES, characteristic for the reconstructed surfaces of Ge, are summarised in Table 1.
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T.Yu. Popik et al. / Surface Science 499 (2002) L113–L118 Table 1 SES of the reconstructed surfaces of Ge (the scale origin is put at the Fermi level) SES
Surface
Structure
Energy (eV)
References
S1
(1 1 1) (1 0 0) (1 1 1) (1 1 0) (1 0 0) (1 1 1) (1 0 0) (1 1 1) (1 0 0) (1 1 1) (1 1 1) (1 1 1) (1 1 1)
c(2 8) (2 1) c(2 8) c(8 10) (2 1) c(2 8) (2 1) c(2 8) (2 1) c(2 8) c(2 8) c(2 8) c(2 8)
1.4
[3,7–9] [18–20] [7,9] [11] [19] [3,7–9] [19] [2,7,10] [17–20] [2,4,5] [2,4,5] [2,6] [2,6]
S2
S3 S4 S5 S6 S S0
1.1
0.8 0.4 0.4 0.45 0.02 0.02
As shown in Refs. [21–26], the low-energy electron scattering technique is sensitive to both bulk and surface electron states of solids. This technique was used to investigate the surface and bulk electronic structure of metals [21,22] and semiconductors [23–26]. The energy positions of the features in the backscattered electron energy loss spectra are shown to correlate with the energy distances between the density-of-states maxima in the valence band and conduction band in the reduced Brillouin zone and the surface electron states, corresponding to direct and indirect transitions of the excited electrons [21–26]. We have used this technique to investigate specific features, due to both bulk and surface electron states, in the low-energy electron backscattering from the (1 1 1), (1 1 0) and (1 0 0) surfaces of Ge. The experiments were performed using a highvacuum setup, described in detail in Refs. [23,27]. We developed a hypocycloidal electron spectrometer for obtaining monoenergetic electron beam and for the analysis of the reflected (scattered) electrons. The main characteristics of the spectrometer are as follows: a primary beam current 108 A, reflected beam current 1010 A, the beam diameter 0.5 mm, full width of the electron energy spread at half-maximum (FWHM) in the primary beam 6 20 meV, the analyser energy resolution (FWHM) 50 meV. The technique and the hypocycloidal electron spectrometer design are described in Refs. [21,23,27].
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The measurements were made for the (1 1 1), (1 1 0) and (1 0 0) surfaces of nominally ultrapure Ge, which were mirror-polished and X-ray-oriented to within 1°, using Cu Ka radiation. The samples were cleaned by annealing, using a rear electron-beam heating arrangement, to 1000–1100 K for about 5 h in 1 107 Pa vacuum. In the course of the measurements the vacuum in the chamber was 108 Pa. The cleanness of the sample surface was checked by the presence of the features in the energy dependences of the reflected electron intensity. As we have shown earlier for Ge(1 1 1) [26], practically no fine structure is revealed in the spectra for not sufficiently cleaned surfaces because of the strong background of the backscattered electrons due to the adsorption of the residual gas molecules. The reliability and accuracy in determining the energy position of the features in the spectra was provided by high reproducibility of the results in a number of runs. The studies were carried out at various incident electron energies from 0.3 to 1.0 eV, varied in 0.l eV step, since, as shown earlier for p-Si(1 0 0) [24] and Ge(1 1 1) [26] surfaces, the processes of the electron transition excitations in this energy range are of resonant character. The origin of the scale in the electron energy-loss spectra was put at the energy position of the elastic peak. The accuracy of determination of the energy position of the features was 0.05 eV. We have used EF EVBM ¼ 0 eV when analysing all the spectra. The phonon energy for Ge is 10–50 meV [28], this being close to the analyser resolution. Phonons can hardly make a considerable contribution into the scattered electron energy loss spectra. The adsorption of the residual gas molecules led to the smearing of the fine structure in the spectra. Therefore the features at the plots under investigation can be attributed to the excitation of the electron transitions. Fig. 2 shows the electron energy loss spectra for Ge(1 1 1), Ge(1 1 0) and Ge(1 0 0) surfaces at different incident electron energies Ep . As seen from the figure, the shape of the loss spectra for the Ge(1 1 1) and Ge(1 1 0) surfaces is similar. The shape of the valence band photoemission spectra is also similar [3,11,29]. Since both surfaces are reconstructed, the results obtained here confirm the
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Fig. 2. Electron energy loss spectra form the (1 1 1), (1 1 0) and (1 0 0) surfaces of Ge for incident electron energies of: (a) Ep ¼ 0:4 eV; (b) Ep ¼ 0:6 eV; (c) Ep ¼ 0:9 eV.
conclusion of Ref. [11] on the similarity of the reconstruction of these surfaces, i.e. that both Ge(1 1 1)-c(2 8) and Ge(1 1 0)-c(8 10) surfaces contain adatoms and rest atoms as building blocks. The loss spectra for Ge(1 0 0) somewhat differ from those for Ge(1 1 1) and Ge(1 1 0) (especially at Ep ¼ 0:4 eV, see Fig. 2a), though the energy positions of the features in the spectra are close for all three surfaces.
Besides, the similarity of the spectra may result from the high step density on the probed surfaces since the orientation accuracy was within 1°. At Ep ¼ 0:4 eV (Fig. 2a) for different surfaces the features in the range 0.15–0.18 and 0.27–0.32 eV are clearly revealed in the spectra. Since the maxima in the loss spectra in the studied energy range are related to the electron excitation from the filled states to the empty ones, and in
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photoemission spectra no features at such energies have been revealed, they, in our opinion, can be attributed to the surface electron states, located in the energy gap above the Fermi level. At higher Ep these features are not revealed in the spectra (Fig. 2b,c). At Ep ¼ 0:6 eV (Fig. 2b) in the loss spectra for all the Ge surfaces under investigation the maxima at 0.35–0.40 and 0.45–0.50 eV were observed. According to Refs. [2,4,5], for Ge(1 1 1)-c(2 8) in the energy gap above the Fermi level the surface electron states exist with the density-of-states maxima at the threshold energies 0.40 eV (S5 ) and 0.45 eV (S6 ). The surface states with the 0.4 eV energy below the Fermi level have been found in the valence band both for Ge(1 1 1)-c(2 8) and Ge(1 0 0)-(2 1) [2,7–10,17–20]. In addition, as mentioned above, for Ge(1 1 1)-c(2 8) the presence of two surface bands in the vicinity of the Fermi level is specific [2,6]. These data are confirmed by the positions of the maxima in the loss spectra in Fig. 2b, taking into account the resolution of the technique (0.05 eV). The feature with the 0.35–0.40 eV can be due to the S(0.02 eV)–S5 (0.4 eV), S4 (0.4 eV)–S0 (0.02 eV) or C025 – S5 (0.4 eV) transitions, while that with the 0.45– 0.50 eV to the S(0.02)–S6 (0.45 eV) or C025 –S6 (0.45 eV) transitions. It is seen from Fig. 2c that already at Ep ¼ 0:9 eV the surface states in the 0.40–0.50 eV range are not resolved and have a form of weak broad maxima. The data of Fig. 2 confirm our earlier conclusion on the resonant character of excitation of one-electron transitions in the low-energy range [21–26]. In Fig. 2c the maxima in the energy range 0.60– 0.65 eV and 0.75–0.80 eV are distinctly revealed. The peaks at 0.75–0.80 eV can result from both direct transitions in the bulk (C025 C02 ¼ 0:8 eV) [30], and the electron transitions between the following SES: S3 (0.8 eV)–S0 (0.02 eV) and S4 (0.4 eV)–S5 (0.4 eV). The peaks at the energy 0.6–0.65 eV can be related to both indirect transitions of electrons from the bulk valence band maximum to the bulk conduction band bottom (C025 L1 ¼ 0:67 eV) [30], and to the transitions from SES S4 (0.4 eV) to the empty SES with the energy 0.27–0.32 eV in the energy gap [26].
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In summary, the obtained results confirm the presence of the known SES with the 0.4, 0.45 eV energies above EF and in the vicinity of the Fermi level with the þ0:02, 0.02 eV energies for Ge(1 1 1)-c(2 8) and 0.4, 0.8 eV below EF for Ge(1 1 1)-c(2 8) and Ge(1 0 0)-(2 1) surfaces. The same states are observed for the Ge(1 1 0)c(8 10) surface. In addition, the existence of SES with the energies 0.15–0.18 and 0.27–0.32 eV in the energy gap for the investigated surfaces is shown. The similarity of the low-energy electron backscattering spectra for the reconstructed Ge(1 1 1), Ge(1 1 0) surfaces confirms the conclusion of Refs. [3,16,29] on these surfaces being built up of similar building blocks. Unambiguous assignment of the features in the loss spectra to the specific electron transitions requires additional experimental and theoretical studies.
Acknowledgements The authors would like to thank A.M. Solomon and L.G. Romanova for their assistance in sample preparation, Yu.M. Azhniuk and A.V. Snegursky for their help in manuscript improvement.
References [1] A. Zangwill, Physics at Surfaces, Cambridge University Press, Cambridge, 1988. [2] B.A. Nesterenko, O.V. Snitko, Physical Properties of Atomically Pure Surfaces of Semiconductors, Naukova dumka, Kiev, 1983. [3] A. Goldoni, A. Santoni, M. Sancrotti, V.R. Dhanak, S. Modesti, Surf. Sci. 382 (1997) 336. [4] A.A. Galaev, E.A. Vygovskaya, L.B. Suleeva, Yu.N. Parkhomenko, Izv. AN SSSR. Ser. Phys. 43 (1979) 469. [5] M. B€ uchel, H. L€ uth, Surf. Sci. 50 (1975) 451. [6] S.G. Davison, J.D. Levine, Surface States, Academic press, New York, 1970. [7] R.D. Bringans, H. H€ ochst, Phys. Rev. B. 25 (1982) 1081. [8] A.L. Wachs, T. Miller, T.C. Hsieh, A.P. Shapiro, T.-C. Chiang, Phys. Rev. B 32 (1985) 2326. [9] R.D. Bringans, R.I.G. Uhrberg, R.Z. Bachrach, Phys. Rev. B 34 (1986) 2373. [10] T. Murotani, K. Fujiwara, M. Nishijima, Phys. Rev. B 12 (1975) 2424. [11] A. Santoni, L. Petaccia, V.R. Dhanak, S. Modesti, Surf. Sci. 444 (2000) 156.
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[12] A.D. Laine, C. Cepek, A. Goldoni, S. Vandre, M. DeSeta, N. Franco, J. Avila, M.C. Asensio, M. Sancrotti, Surf. Sci. 402–404 (1998) 875. [13] F.M. Leibsle, E.S. Hirschorn, A. Samsavar, T. Miller, T.-C. Chiang, Phys. Rev. B 44 (1991) 8115. [14] T. Ichikawa, T. Sucyosi, T. Sato, M. Iwatsuki, F. Udagawa, I. Sumir, Solid State Commun. 93 (1995) 541. [15] W.E. Packard, J.D. Dow, Phys. Rev. B 55 (1997) 15643. [16] Z. Gai, R.G. Zhao, W.S. Yang, Phys. Rev. B 57 (1998) R6795. [17] A.D. Laine, M. DeSeta, C. Cepek, S. Vandre, A. Goldoni, N. Franco, J. Avila, M.C. Asensio, M. Sancrotti, Surf. Sci. 402–404 (1998) 871. [18] T.C. Hsieh, T. Miller, T.-C. Chiang, Phys. Rev. B 30 (1984) 7005. [19] E. Landemark, R.I.G. Uhrberg, P. Kr€ uger, J. Pollmann, Surf. Sci. 236 (1990) 359. [20] J.G. Nelson, W.J. Gignac, R.S. Williams, S.W. Robey, J.G. Tobin, D.A. Shirley, Phys. Rev. B 27 (1983) 3924. [21] T.Yu. Popik, O.B. Shpenik, Yu.V. Popik, Phys. Solid State 43 (2001) 408.
[22] T.Yu. Popik, V.M. Feyer, M.M. Erdevdy, O.B. Shpenik, Yu.V. Popik, Ukr. Fiz. Zh. 46 (2001) 456. [23] O.B. Shpenik, N.M. Erdevdy, T.Yu. Popik, Tech. Phys. 42 (1997) 550. [24] V.M. Feyer, T.Yu. Popik, O.B. Shpenik, Yu.V. Popik, M.M. Erdevdy, Uzhhorod Univ. Scientific Herald Ser. Phys. 8 (2000) 156. [25] T.Yu. Popik, O.B. Shpenik, P.P. Puga, Yu.V. Popik, J. Optoelectron. Adv. Mater. 2 (2000) 171. [26] T.Yu. Popik, V.M. Feyer, O.B. Shpenik, Yu.V. Popik, Surf. Sci. 491 (2001) 175. [27] O.B. Shpenik, N.M. Erdevdy, N.I. Romanyuk, T.Yu. Popik, A.N. Zavilopulo, Instrum. Exp. Tech. 41 (1998) 97. [28] V.F. Kiselev, S.N. Kozlov, A.V. Zoteev, Fundamentals of Solid Surface Physics, Moskow University Press, Moscow, 1999. [29] G. Paolicelli, V.R. Dhanak, A. Goldoni, S. Modesti, M. Sancrotti, A. Santoni, J. Phys. Condens. Matter 9 (1997) 1959. [30] I.M. Tsidilkovski, Band Structure of Semiconductors, Nauka, Moscow, 1978.
Surface Science Letters
The specific features of low-energy electron backscattering from different Ge surfaces T.Yu. Popik a, V.M. Feyer a, O.B. Shpenik a
a,*
, Yu.V. Popik
b
Institute of Electron Physics, Ukr. Nat. Acad. Sci., Universytetska St. 21, Uzhhorod 88000, Ukraine b Uzhhorod National University, Voloshyn St. 54, Uzhhorod 88000, Ukraine Received 24 July 2001; accepted for publication 3 December 2001
Abstract Low-energy electron backscattering technique using a hypocycloidal electron spectrometer is applied to measure energy loss spectra at different incident electron energies from mirror-polished (1 1 1), (1 0 0) and (1 1 0) surfaces of Ge. The obtained results confirm and complement the available data on the energy position of the extrema of the density of both filled (below EF ) and empty (above EF ) surface and bulk electron states in the reduced Brillouin zone for different surfaces of germanium. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron energy loss spectroscopy (EELS); Surface electronic phenomena (work function, surface potential, surface states, etc.); Germanium
Germanium is characterized by the diamondlike crystal structure. A typical feature of the ideal surface of this structure is the presence of dangling hybrid orbitals directed into the vacuum [1]. Supposing that when the Ge crystal cleaves, a pair of bonding electrons belonging to an orbital in the bulk are split between the two crystal parts, leaving half filled broken orbitals. One atom in the unit cell at the (1 1 1) surface then possesses one dangling hybrid bond and two dangling bonds at the (1 0 0) surface, while each of the two atoms in the unit cell at the (1 1 0) surface possesses one dan-
*
Corresponding author. Tel.: +380-3122-43668/43650; fax: +380-3122-43650. E-mail address: [email protected] (O.B. Shpenik).
gling bond (Fig. 1). The surface density of the dangling bonds is minimal on the (1 1 1) surface and maximal on the (1 0 0) surface [1]. The (1 1 1) plane is the natural cleavage plane for germanium crystal [1] and its properties have been studied most extensively. Upon heating to T > 400 K, the clean Ge(1 1 1) surface structure is shown to convert irreversibly into the stable Ge(1 1 1)-c(2 8) lattice, where the adatoms (the topmost atoms of the surface) saturate 3/4 of the dangling bonds of the ideal first layer and donate their extra electron to the remaining first-layer atoms (rest atoms) [2,3]. 2 2-size block is the main building block in the Ge(1 1 1)-c(2 8) lattice [2]. The spectrum of surface electron states (SES) of Ge(1 1 1)-c(2 8) surface consists of three groups
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 9 3 6 - 7
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Fig. 1. Germanium surfaces under investigation and ideal bond breakdown at these surfaces.
of levels. The first group includes two states, located in the upper part of the energy gap. It has two electron-state density maxima with the threshold energies 0.4 and 0.45 eV [2,4,5]. The SES bands of the second group are located on both sides of the Fermi level: the donor band, 0.02 eV below EF and the acceptor band, 0.02 eV above EF [2,6]. The third group of SES is located in the valence band of the bulk crystal. This group consists of two 0.2 eV wide surface bands centred at 0.85 and 1.4 eV below the valence band maximum [3,7–9] as well as SES with the energies 0.4 eV [2,7,10] and 1.1 eV [7,9]. Clean Ge(1 1 0) and Ge(1 0 0) surfaces have been studied much less. Ge(1 1 0) surface, cleaned by subsequent cycles of ion sputtering and annealing to T ¼ 1150 K [11] (according to Ref. [12] – to 703 K) on cooling to room temperature (RT) shows a cð8 10Þ pattern [11–14]. The (1 1 0) surfaces are characterized by coexistence of chains of surface atoms and clusters of adatoms [15]. Based on the STM studies of Ge(1 1 0) surface at RT a model is suggested, describing a surface consisting of additional atomic species such as dimers, rebonded atoms and rest atoms [16]. Besides this, the main features of the reconstruction are shown to be periodic clusters of five atoms [12]. The valence-band and core-level photoemission studies on Ge(1 1 0)-c(8 10) at room and higher temperatures are reported in Refs. [11,12]. The normal emission angle-integrated valence band spectra of the cð8 10Þ structure at RT show a rich structure: a shoulder at the binding energy
(BE) 1:0 eV, a main peak at about 2:4 eV, a pronounced shoulder at 3:8 eV and two peaks at 7.6 and 12.8 eV [11]. The features at the energies 1:0, 2:4 and 3:8 eV are attributed to the surface states, and the peaks at 7.6 and 12.8 eV––to the bulk band transitions. In the valence-band photoemission spectra for Ge(1 1 0)-c(8 10), measured at RT, the peaks at 2.0, 4.0, 5.5, 8.0 and 9.5 eV BE are revealed, being ascribed to bulk interband transitions. A low BE shoulder near the 2.0 eV peak is related to a surface state [12]. The basic building blocks of Ge(1 0 0) surface are asymmetric dimers, which between 220 and 955 K exhibit paramagnetic (2 1) order [17]. In the photoemission spectra at RT, the features with the energies 0.4 and 1.4 eV, are revealed, assigned to the surface states, as well as a feature at 3.25 eV due to the emission from the bulk bands (in Ref. [17] the origin of the energy scale is taken at EF , its position being determined within 0.06 eV at the overall resolution of 0.16 eV). Ge(0 0 1)-(2 1) surfaces, obtained by either molecular beam epitaxy (MBE), or sputtering, followed by annealing, were studied by angleresolved photoemission technique [18]. The surface electronic structure is shown to be independent of the surface preparation procedure. Two observed features in the normal emission spectra with constant BE of about 0.5 and 1.3 eV relative to the valence band maximum (VBM) (EF EVBM ¼ 0:1 eV) are related to surface states [18]. A richer structure in the spectra was observed in similar studies of the Ge(0 0 1)-(2 1) surface in Ref. [19]. In the energy range 0–6 eV, a set of angle-resolved photoemission spectra shows five surface features at 0.4, 0.8, 3.0, 1.15 and 1.5 eV. A strong structure, revealed in these spectra at about 3.5 eV, is attributed to a direct bulk transition. (In the discussed paper the energy values are measured with respect to EF and EF EVBM ¼ 0:1 eV, overall spectral resolution being 0.18 eV.) The obtained data confirm the existence of two surface structures at 0.6 and 1.3 eV below VBM, detected in Ref. [20]. The reference data on the SES, characteristic for the reconstructed surfaces of Ge, are summarised in Table 1.
SU N IE SC RS CE E A TT R F LE CE
T.Yu. Popik et al. / Surface Science 499 (2002) L113–L118 Table 1 SES of the reconstructed surfaces of Ge (the scale origin is put at the Fermi level) SES
Surface
Structure
Energy (eV)
References
S1
(1 1 1) (1 0 0) (1 1 1) (1 1 0) (1 0 0) (1 1 1) (1 0 0) (1 1 1) (1 0 0) (1 1 1) (1 1 1) (1 1 1) (1 1 1)
c(2 8) (2 1) c(2 8) c(8 10) (2 1) c(2 8) (2 1) c(2 8) (2 1) c(2 8) c(2 8) c(2 8) c(2 8)
1.4
[3,7–9] [18–20] [7,9] [11] [19] [3,7–9] [19] [2,7,10] [17–20] [2,4,5] [2,4,5] [2,6] [2,6]
S2
S3 S4 S5 S6 S S0
1.1
0.8 0.4 0.4 0.45 0.02 0.02
As shown in Refs. [21–26], the low-energy electron scattering technique is sensitive to both bulk and surface electron states of solids. This technique was used to investigate the surface and bulk electronic structure of metals [21,22] and semiconductors [23–26]. The energy positions of the features in the backscattered electron energy loss spectra are shown to correlate with the energy distances between the density-of-states maxima in the valence band and conduction band in the reduced Brillouin zone and the surface electron states, corresponding to direct and indirect transitions of the excited electrons [21–26]. We have used this technique to investigate specific features, due to both bulk and surface electron states, in the low-energy electron backscattering from the (1 1 1), (1 1 0) and (1 0 0) surfaces of Ge. The experiments were performed using a highvacuum setup, described in detail in Refs. [23,27]. We developed a hypocycloidal electron spectrometer for obtaining monoenergetic electron beam and for the analysis of the reflected (scattered) electrons. The main characteristics of the spectrometer are as follows: a primary beam current 108 A, reflected beam current 1010 A, the beam diameter 0.5 mm, full width of the electron energy spread at half-maximum (FWHM) in the primary beam 6 20 meV, the analyser energy resolution (FWHM) 50 meV. The technique and the hypocycloidal electron spectrometer design are described in Refs. [21,23,27].
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The measurements were made for the (1 1 1), (1 1 0) and (1 0 0) surfaces of nominally ultrapure Ge, which were mirror-polished and X-ray-oriented to within 1°, using Cu Ka radiation. The samples were cleaned by annealing, using a rear electron-beam heating arrangement, to 1000–1100 K for about 5 h in 1 107 Pa vacuum. In the course of the measurements the vacuum in the chamber was 108 Pa. The cleanness of the sample surface was checked by the presence of the features in the energy dependences of the reflected electron intensity. As we have shown earlier for Ge(1 1 1) [26], practically no fine structure is revealed in the spectra for not sufficiently cleaned surfaces because of the strong background of the backscattered electrons due to the adsorption of the residual gas molecules. The reliability and accuracy in determining the energy position of the features in the spectra was provided by high reproducibility of the results in a number of runs. The studies were carried out at various incident electron energies from 0.3 to 1.0 eV, varied in 0.l eV step, since, as shown earlier for p-Si(1 0 0) [24] and Ge(1 1 1) [26] surfaces, the processes of the electron transition excitations in this energy range are of resonant character. The origin of the scale in the electron energy-loss spectra was put at the energy position of the elastic peak. The accuracy of determination of the energy position of the features was 0.05 eV. We have used EF EVBM ¼ 0 eV when analysing all the spectra. The phonon energy for Ge is 10–50 meV [28], this being close to the analyser resolution. Phonons can hardly make a considerable contribution into the scattered electron energy loss spectra. The adsorption of the residual gas molecules led to the smearing of the fine structure in the spectra. Therefore the features at the plots under investigation can be attributed to the excitation of the electron transitions. Fig. 2 shows the electron energy loss spectra for Ge(1 1 1), Ge(1 1 0) and Ge(1 0 0) surfaces at different incident electron energies Ep . As seen from the figure, the shape of the loss spectra for the Ge(1 1 1) and Ge(1 1 0) surfaces is similar. The shape of the valence band photoemission spectra is also similar [3,11,29]. Since both surfaces are reconstructed, the results obtained here confirm the
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Fig. 2. Electron energy loss spectra form the (1 1 1), (1 1 0) and (1 0 0) surfaces of Ge for incident electron energies of: (a) Ep ¼ 0:4 eV; (b) Ep ¼ 0:6 eV; (c) Ep ¼ 0:9 eV.
conclusion of Ref. [11] on the similarity of the reconstruction of these surfaces, i.e. that both Ge(1 1 1)-c(2 8) and Ge(1 1 0)-c(8 10) surfaces contain adatoms and rest atoms as building blocks. The loss spectra for Ge(1 0 0) somewhat differ from those for Ge(1 1 1) and Ge(1 1 0) (especially at Ep ¼ 0:4 eV, see Fig. 2a), though the energy positions of the features in the spectra are close for all three surfaces.
Besides, the similarity of the spectra may result from the high step density on the probed surfaces since the orientation accuracy was within 1°. At Ep ¼ 0:4 eV (Fig. 2a) for different surfaces the features in the range 0.15–0.18 and 0.27–0.32 eV are clearly revealed in the spectra. Since the maxima in the loss spectra in the studied energy range are related to the electron excitation from the filled states to the empty ones, and in
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photoemission spectra no features at such energies have been revealed, they, in our opinion, can be attributed to the surface electron states, located in the energy gap above the Fermi level. At higher Ep these features are not revealed in the spectra (Fig. 2b,c). At Ep ¼ 0:6 eV (Fig. 2b) in the loss spectra for all the Ge surfaces under investigation the maxima at 0.35–0.40 and 0.45–0.50 eV were observed. According to Refs. [2,4,5], for Ge(1 1 1)-c(2 8) in the energy gap above the Fermi level the surface electron states exist with the density-of-states maxima at the threshold energies 0.40 eV (S5 ) and 0.45 eV (S6 ). The surface states with the 0.4 eV energy below the Fermi level have been found in the valence band both for Ge(1 1 1)-c(2 8) and Ge(1 0 0)-(2 1) [2,7–10,17–20]. In addition, as mentioned above, for Ge(1 1 1)-c(2 8) the presence of two surface bands in the vicinity of the Fermi level is specific [2,6]. These data are confirmed by the positions of the maxima in the loss spectra in Fig. 2b, taking into account the resolution of the technique (0.05 eV). The feature with the 0.35–0.40 eV can be due to the S(0.02 eV)–S5 (0.4 eV), S4 (0.4 eV)–S0 (0.02 eV) or C025 – S5 (0.4 eV) transitions, while that with the 0.45– 0.50 eV to the S(0.02)–S6 (0.45 eV) or C025 –S6 (0.45 eV) transitions. It is seen from Fig. 2c that already at Ep ¼ 0:9 eV the surface states in the 0.40–0.50 eV range are not resolved and have a form of weak broad maxima. The data of Fig. 2 confirm our earlier conclusion on the resonant character of excitation of one-electron transitions in the low-energy range [21–26]. In Fig. 2c the maxima in the energy range 0.60– 0.65 eV and 0.75–0.80 eV are distinctly revealed. The peaks at 0.75–0.80 eV can result from both direct transitions in the bulk (C025 C02 ¼ 0:8 eV) [30], and the electron transitions between the following SES: S3 (0.8 eV)–S0 (0.02 eV) and S4 (0.4 eV)–S5 (0.4 eV). The peaks at the energy 0.6–0.65 eV can be related to both indirect transitions of electrons from the bulk valence band maximum to the bulk conduction band bottom (C025 L1 ¼ 0:67 eV) [30], and to the transitions from SES S4 (0.4 eV) to the empty SES with the energy 0.27–0.32 eV in the energy gap [26].
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In summary, the obtained results confirm the presence of the known SES with the 0.4, 0.45 eV energies above EF and in the vicinity of the Fermi level with the þ0:02, 0.02 eV energies for Ge(1 1 1)-c(2 8) and 0.4, 0.8 eV below EF for Ge(1 1 1)-c(2 8) and Ge(1 0 0)-(2 1) surfaces. The same states are observed for the Ge(1 1 0)c(8 10) surface. In addition, the existence of SES with the energies 0.15–0.18 and 0.27–0.32 eV in the energy gap for the investigated surfaces is shown. The similarity of the low-energy electron backscattering spectra for the reconstructed Ge(1 1 1), Ge(1 1 0) surfaces confirms the conclusion of Refs. [3,16,29] on these surfaces being built up of similar building blocks. Unambiguous assignment of the features in the loss spectra to the specific electron transitions requires additional experimental and theoretical studies.
Acknowledgements The authors would like to thank A.M. Solomon and L.G. Romanova for their assistance in sample preparation, Yu.M. Azhniuk and A.V. Snegursky for their help in manuscript improvement.
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