High-temperature phase transitions at the Ge(110) surface

Surface Science 444 (2000) 156–162 www.elsevier.nl/locate/susc

High-temperature phase transitions at the Ge(110) surface A. Santoni a, *, L. Petaccia b, V.R. Dhanak c, S. Modesti b,d a ENEA-INN-FIS, C.R. Frascati, v. E. Fermi 45, I-00044 Frascati, Italy b Laboratorio TASC-INFM, Padriciano 99, I-34012 Trieste, Italy c Surface Science Centre, Liverpool University P.O. Box 147, Liverpool L69 3BX, UK d Dipartimento di Fisica, Universita` di Trieste, I-34127 Trieste, Italy Received 4 June 1999; accepted for publication 5 August 1999

Abstract The Ge(110) surface has been investigated by core-level and valence band ( VB) photoemission spectroscopy as a function of the temperature starting from the room temperature c(8×10) reconstructed surface up to 1196 K. Evidence of a phase transition is found at about 750±50 K from core-level and VB data analysis. VB photoemission shows that above 750 K the Ge(110) surface acquires an increasing metallic character up to 1110 K where a sudden, intense jump of the emission intensity at the Fermi level is observed. At higher temperatures up to 1196 K, a finite and constant density of states is observed, indicating the presence of a metallic surface layer. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Germanium; Low index single crystal surfaces; Surface melting; Surface thermodynamics; Synchrotron radiation photoelectron spectroscopy

1. Introduction Experimental and theoretical work carried out in recent years has shown that order–disorder transitions such as surface melting or surface roughening can take place on low index surfaces of solids near the bulk melting temperature T m [1–7]. The determination of the geometric and electronic structure of the disordered high temperature surface phases is of great practical and theoretical importance [8,9], as it can improve our understanding of the electronic properties of very thin liquid films and of the interatomic forces which determine the atomic structure at the surface of a solid under these conditions. * Corresponding author. Fax: +0694005400. E-mail address: [email protected] (A. Santoni)

High-temperature phase transitions have been observed on the surfaces of the elemental semiconductors Si(100) [10–13], Si(111) [14–18], Ge(100) [19–21] and Ge(111) [22]. The last surface has been the subject of extensive experimental work and was predicted to undergo incomplete surface melting by molecular dynamics simulations [8,9]. Incomplete surface melting takes place when the thickness of the disordered layer does not monotonically increase as T is approaching T . m In contrast with the popular (100) and (111) surfaces of group IV semiconductors, the electronic structure and geometry of their (110) faces above room temperature (RT ) have scarcely been investigated. On the Si(110) surface, LEED, RHEED and STM data taken at RT showed a reconstructed structure called (16×2) [23–26 ] and a phase transition (16×2)(1×1) has been observed at 1030 K [27]. To our knowledge, even less work has been

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devoted to the clean Ge(110) surface. This surface is known to show at RT a (16×2) structure or a c(8×10) pattern depending on the annealing temperature [28,29], and in a recent work [30] a possible surface phase transition of unclear nature at about 800 K has been reported. The (110) surfaces seem to be characterized by the coexistence of chains of surface atoms and of clusters of adatoms [31]. In addition, a recent STM study on RT Ge(110) has proposed a model consisting of additional atomic species such as dimers, rebonded atoms and rest atoms [32]. Because of the intimate relationship between electronic bonds and surface topology, photoemission spectroscopy (PES ) techniques such as corelevel PES and valence band ( VB) PES are powerful tools for the investigation of both the electronic and the geometric structure of surfaces. On tuning the photon energy and selecting different experimental geometries it is possible to identify contributions from different atomic species on the surface and to follow their behaviour as function of temperature [22]. In this work we present a detailed VB and bulkand surface-sensitive core-level synchrotron radiation PES study of the Ge(110) surface from room temperature to 1196 K, i.e. close to the bulk melting temperature T (1210 K ). Particular attention m has been devoted to the investigation of the VB and the Ge(110) 3d core-level lineshapes, especially in the vicinity of the 800 K transition temperature and close to the melting point.

2. Experimental The experiments were carried out using synchrotron radiation on the undulator TGM6 beamline at the BESSY storage ring in Berlin. The ultrahigh vacuum chamber operating at a base pressure of 2×10−10 mbar was equipped with an angle-integrating CLAM analyser and LEED optics. In order to differentiate better the surface and bulk contributions, the Ge 3d photoelectrons were excited using photon energies of 45 eV (bulk) and 70 eV (surface). At 70 eV two different emission angles were used: normal emission (NE ) and grazing emission (GE) (25° with respect to the surface).

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All the VB spectra were taken at normal emission at 45 eV photon energy. The total experimental resolution at 70 eV photon energy was ≤0.22 eV, as determined from the Fermi edge of a clean Ta foil in good electrical contact with the sample. The sample was an n-type Ge(110) wafer (r≤10 V cm). A clean c(8×10) surface was obtained by repeated cycles of Ar+ ion etching and annealing to 1150 K. The cleanness and surface order were monitored periodically by LEED and VB photoemission at RT. During data acquisition the sample was resistively heated by current pulses. The temperature on the sample surface was measured using an infrared pyrometer with the emissivity set at 0.43 and calibrated against the Ge melting point, determined by final melting of the sample. The error on the temperature reading was estimated to be less than 20 K.

3. Results and discussion The analysis of the core-level lineshape as a function of temperature can reveal possible structural modifications occurring at the surface: these manifest themselves as variations in the intensity of some lineshape components resulting in a change of the measured spectrum. Fig. 1a shows a selection of the Ge(110) 3d core-level photoemission spectra normalized to the same height taken at GE and different temperatures starting from RT up to 1196 K and plotted as a function of the BE. In the following all the BEs are referred to the Fermi level E . The GE F high-temperature (1196 K ) Ge(110) 3d core level (dotted line) shows a shift to lower BE by about 100m eV with respect to the RT spectrum. The evolution of the Ge(110) spectral lineshape as a function of temperature is well reflected by the shift of the centroid of the Ge 3d spectrum versus temperature ( Fig. 1b). Fig. 1a shows no appreciable change of the lineshape up to about 700 K. From this temperature the centroid increases steadily up to about 930 K. Above 930 K the slope changes resulting in a slow but apparently steady increase of the shift. From Fig. 1a one can see that the observed asymmetry is due to an increased emission located in the region around 28.8 eV. An

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Fig. 1. (a) Comparison of Ge(110) 3d core-level spectra taken at 70 eV photon energy and GE at different temperatures. Full line, RT c(8×10); dashed line, 700 K; dash-dotted line, 990 K; dotted line, 1196 K. Spectra are normalized to the height. The inset shows the intensity of the tails obtained by integrating over the 33–30 eV binding energy (BE) range and normalized to the total area. (b) Plot of the shift in the centroid of the Ge 3d core levels versus temperature obtained by Ge 3d corelevel photoemission at GE.

increased emission is also observed in the 1196 K spectrum at about 29.5 eV. This contribution shifts the centroid in the opposite direction resulting in a reduced net centroid shift. The study of the inelastic tail in the high BE region of the spectra can provide additional information. As can be seen from Fig. 1a, the high temperature core-level spectra show a high inelastic tail indicating a possible metallic-like behaviour of the high temperature Ge(110) surface. The integrated intensities of the tails were also evaluated. After subtraction of a straight line fitting the tail in the low BE region, they were obtained by integrating the GE surface-sensitive Ge 3d core-level spectra normalized to the total area in the 33–30 eV BE range. Although additional noise is introduced by the background subtraction procedure, the plot of the

Fig. 2. (a) Ge(110) VB normal emission spectra taken at 45 eV photon energy. Full line, RT c(8×10) spectrum; dashed line, VB spectrum at 1180 K. Data are normalized to the area. The BE scale is referred to the Fermi level E . (b) Comparison of F the Ge(110) RT c(8×10) VB normal emission spectrum (full line) with the corresponding contaminated surface (dashed line).

integrated intensities of the tails as a function of the temperature (see inset to Fig. 1a), shows, corresponding to the centroid behaviour, a strong increase of the intensity starting from about 800 K. Fig. 2a shows the Ge(110) NE angle-integrated VB spectra measured at RT ( line) and at 1180 K (dots). Both spectra are normalized to the total area. The RT c(8×10) spectrum shows a rich variety of structures: a shoulder at −1 eV BE, a main peak at about −2.4 eV, a pronounced shoulder at −3.8 eV and two further peaks at −7.6 and −12.8 eV. Fig. 2b shows the contaminated surface, obtained after many hours of exposure to the residual gases of the UHV chamber. As the features at −1 and −3.8 eV are quenched and an additional loss of intensity at about −3 eV is

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Fig. 3. Evolution of the near-Fermi level region as a function of temperature. Circles, RT c(8×10); full line, 660 K; squares, 707 K; dotted line, 1196 K.

observed; these structures are probably associated with surface-related states. According to previous work on Ge(111) [33,34], the peaks at −7.6 and −12.8 eV can be assigned to bulk bands transitions. It is worth noting that the shape of the Ge(110) VB spectrum shows a similarity with the VB data taken on the Ge(111) c(2×8) surface [35,36 ]. This could be explained by observing that both Ge(111)c(2×8) and Ge(110)c(8×10) surfaces contain adatoms and restatoms as building blocks. On this basis, it is possible tentatively to assign the feature at −1 eV to the emission from the restatoms’ and/or adatoms’ dangling bonds. Previous work on Ge(111) [37,38] and Si(110) [39] has tentatively assigned VB features at and below 3 eV BE to backbonds. In this view, it is possible tentatively to make a similar assignment for the features at −2.4 and −3.8 eV in the Ge(110) VB spectrum. In the high-temperature Ge(110) VB spectrum ( Fig. 2a, dots, 1180 K ), only the peak at about −2.4 eV can be observed while the features at −3.8 eV have disappeared. In addition, appreciable emission from the Fermi edge region is observed. At higher BEs the peak at −7.6 eV is unchanged while the peak appearing at −12.8 eV in the RT data seems to have lost

intensity and to have shifted by about −0.5 eV towards E . Fig. 3 shows a series of spectra taken F by temperature-dependent VB photoemission and normalized to the total area, measured at RT (circles), 660 K ( line), 707 K (open squares) and 1196 K (dotted line). Between 606 and 707 K the structure at −1 eV becomes hardly distinguishable and the bump at −3.8 eV is found to vanish. At 1196 K, the highest temperature measured, the main peak is found to shift from −2.4 eV to about −2.1 eV. Fig. 4 shows the emission intensity near E as F

Fig. 4. Plot of the values of the integrals of the VB spectra calculated between −0.5 and 0.5 eV BE as a function of the surface temperature.

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a function of the temperature. The intensity values were obtained by integrating the area-normalized VB spectra on a 1 eV wide region around E . The F data show a steady increase in the emission from about 700 K to about 1110 K. Above 1110 K an abrupt and strong (~50%) jump in intensity is found. The intensity remains constant at the higher temperatures. The intensity at E is a measure of the metF allicity of the surface. At about 700 K the VB surface-related features are found to vanish and the beginning of a pronounced metallic-like behaviour is registered (Fig. 2b). According to earlier work [8,9,40,41], this temperature could correspond to the onset of a disordering transition on the Ge(111) surface where the initial long-range order is lost as a result of fast diffusion of the thermally excited adatoms. Adatom jumping is expected to produce a non-zero average concentration of surface free carriers which increases on increasing the mobility of the adatoms (i.e. the temperature) [41]. Another possible mechanism is the proliferation of surface defects, such as antiphase domain walls, with temperature. The similar gradual increase in surface conductivity observed on Ge(111) can be quantitatively explained in terms of increasing density of antiphase domain boundaries at high temperature [41–43]. In agreement with the VB results, the centroid shift of the 3d core levels plotted versus the temperature reflects a possible modification of the surface starting from about 750±50 K. The probable disordering of building blocks above this temperature induces a rearrangement of the intensities and positions of the lineshape components contained in the Ge 3d core-level which are then reflected in the centroid shift. In contrast, the centroids do not show any strong variation above 1110 K. The analysis of the bulk-sensitive Ge(110) 3d corelevel lineshapes has shown that the positions of the bulk centroids (not shown) vary by less than 60 meV (about two energy steps in the spectra) over the whole temperature range in contrast to the temperature dependence of the GE data (see Fig. 1). Therefore, the centroid shift found in the GE core-level spectra is caused by changes in the surface components of the core levels and not by changes in surface band bending. A shift of the

Fig. 5. (a) Best fit (full line) of the VB data at 1180 K (circles) obtained assuming a constant DOS N ≠0. E and a are the s M fitting parameters of the power function used in the fitting procedure. The bet fit was obtained with a=0.5 and E =−0.20 eV. The Fermi level is set at 0 eV BE. (b) Best fit M (full line) of the VB data at 1180 K (circles) assuming N =0. s In this case a=0.4 and E =0. M

centroid starting at about 1000 K associated with a parallel increase of the emission intensity at E F at the same temperature was recently also observed on the Si(110) surface [44] where a disordering transition 16×2(1×1) at 1000 K has been reported [27]. The sudden jump in intensity found above 1110 K shows the presence of a further phase transition. As the metallicity of the surface is found to be constant above 1110 K to 1196 K, i.e. only 14 K below the bulk melting point, and owing to similarity with the behaviour of the Ge(111) surface, it is possible that either further building blocks break down or the Ge(110) surface undergoes incomplete surface melting above 1110 K. Theoretical simulations (e.g. molecular dynamics) would be very useful in helping to verify this hypothesis and additional experimental work performed with different techniques is needed. In order to verify that the measured intensity at E is due to a finite density of states (DOS) F and not to the experimental broadening of a photoemission peak close to E but below it, we F have fitted the VB photoemission data. The fitting function was chosen as follows: a power function I(E )=(E −E )a (for a>0), with fitting parameters M

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E and a, was added to a constant N . The two M S functions I(E) and N simulate the fast DOS S decrease below E and a nearly constant DOS F near E , respectively. The resulting function was F then multiplied by the Fermi–Dirac distribution evaluated at 1180 K and convoluted by a Gaussian function of 0.2 eV full width at half-maximum to account for the experimental broadening. Fig. 5 shows the results of the fitting. At 1180 K the VB spectrum can be satisfactorily fitted only by allowing a finite DOS at E (Fig. 5a) whereas it is F apparent that a simple temperature-induced broadening of the VB cannot account for the observed spectral shape ( Fig. 5b). The intensity of the finite DOS required to fit the data ( Fig. 5a) was found about 80% of the intensity of the band at −0.75 eV BE.

4. Conclusions The analysis of the emission at E obtained F from the Ge(110) VB data and 3d core-level photoemission data show a modification of the surface taking place at about 750±50 K. This temperature is indicative of a disordering transition. At about 750 K the surface develops a metallic character which is found to increase continuously up to 1110 K where an abrupt, intense jump of the emission intensity at E is observed. F This discontinuity could be due either to a further breakdown of surface atomic species, or, according to the similarity of the reported behaviour of the adatom–restatom system Ge(111), to the onset of an incomplete melting phase transition at 1110 K.

Acknowledgements This work was supported by BESSY (Berliner Elektronenspeicherring-Gesellschaft fu¨r Synchrotronstrahlung m.b.H.) in the framework of the European Community TMR Program, contract No. ERBFMGE CT 950031.

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