Lead-induced reconstructions of the Ge(001) surface

surface s c i e n c e Surface Science 372 (1997) 155-170

ELSEVIER

Lead-induced reconstructions of the Ge(001) surface G. Falkenberg, L. Seehofer *, R. Rettig, R.L. Johnson H. Institutffir Experimentalphysik, Universitat Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany

Received 7 July 1995; accepted for publication 21 August 1996

Abstract

The two-dimensional phases of Pb on Ge(001) have been studied with scanning tunneling microscopy and electron diffraction. The common feature of all the low coverage reconstructions is the coexistence of Pb and Ge dimers either in parallel or in perpendicular orientations depending on the coverage. The tunneling voltage polarity dependence in the STM images of the c(8 × 4) reconstruction and the atomic displacements in the (2 x 2) structure are explained by buckling of the Pb surface dimers. Structural models for (2 × 2), c(8 × 4), (1 x 5), and (0z 31)phases are presented which elucidate the changes taking place with increasing coverage. The high coverage c(8 × 4)i reconstruction is interpreted as an incommensurate hexagonal overlayer on the cubic substrate. Heat treatment of the higher coverage reconstructions at 300°C leads to the formation of multiple steps, which can be interpreted as facets, whereas heating of the low coverage phases induces a roughening of the surface. Keywords: Germanium; Lead; Low energy electron diffraction (LEED); Low index single crystal surfaces; Reflection high-energy

electron diffraction (RHEED); Scanning tunnehng microscopy; Surface relaxation and reconstruction; Surface structure, morphology, roughness, and topography

1. Introduction

M e t a l - s e m i c o n d u c t o r interfaces are of interest for b o t h fundamental a n d technological reasons. The closely related systems P b on Ge and P b on Si are considered as p r o t o t y p e examples of abrupt metal semiconductor interfaces [ 1 ]. Consequently, P b overlayers o n the (111) surfaces of elemental semiconductors have been studied extensively with a variety of experimental techniques over the last few years [ 2 - 1 1 ] . F o r P b coverages less than 1/3 m o n o l a y e r ( M L ) a 2-D intermixing of P b and G e (or Si) a t o m s occurs, which contrasts strongly to the low bulk solubility [4,8]. At higher coverages ( ~ 1.3 M L ) a hexagonal P b overlayer is formed

* Corresponding author.

which corresponds to a slightly distorted, closep a c k e d Pb(111) layer [2,7,8] and, depending on the exact coverage and heat treatment, a variety of different 2-D phases have been observed. These apparently simple adsorbate systems exhibit a surprising complexity, with commensurate~--~ i n c o m m e n s u r a t e phase transitions, striped and hexagonal phases, and have been suggested as candidates for studies o n 2-D melting [3,5,8]. Less w o r k has been performed on the adsorption of P b on Si(001) and Ge(001) surfaces [ 1 2 - 1 7 ] , a l t h o u g h the first low energy electron diffraction ( L E E D ) studies revealed that the phase diagram of P b on b o t h substrates is rather complex. Recently Yang et al. using scanning tunneling m i c r o s c o p y ( S T M ) observed eight different superstructures for P b on G e (001 ) [ 15,16 ]. T h e y found that the low coverage (2 × 2), (2 x 4), c(2 x 4) and c(8 x 4)a reconstruc-

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G. Falkenberg et al./Surface Science 372 (1997) 155-170

tions exhibit coexisting Ge and Pb dimers. For the high coverage "3 x 6", c(12 x 5.3), and c(8 x 4)/3 phases they proposed a cluster model with various arrangements of Pb clusters located above a completely dimerized layer. On annealed samples they observed c(10 x 4) and (5 x 2) structures which they identified with the previously reported (5 x 1) reconstruction [14]. For five of these reconstructions we present in this paper new STM and electron diffraction measurements. Our results for the low coverage (2 x 2) and c(8x4)e reconstructions are mainly in agreement with the findings of Yang et al.; however, we provide additional information about the size and shape of the domains, characteristic domain boundaries and substitutional Pb atoms. For the high coverage "3 x6" and c(8 x4)/3 phases we introduce a new structural model in which the surface is terminated with a slightly distorted Pb(111) layer. Finally, we discuss the formation of the (5 x 1) reconstruction as a function of Pb coverage and annealing temperature.

2. E x p e r i m e n t a l

The experiments were performed in an ultra high vacuum (UHV) system (base pressure < 4 x 10 -11 mbar) consisting of a molecular beam epitaxy deposition chamber equipped with a RHEED-apparatus, sample heating and a quartz film-thickness monitor, a separate preparation chamber for sputtering and annealing to clean the substrates and an analysis chamber containing a LEED-system and a commercial STM (OMICRON Vakuumphysik). Polished Ge(001) wafers (~ 7 x 4 x 0.5 ram) were rinsed in pure methanol, loaded in the UHV chamber and outgassed at temperatures up to 650°C. Clean reconstructed Ge(001) surfaces were prepared by repeated cycles of sputtering with 500 eV Ar + ions at a temperature of 500°C and annealing at 650°C, until a sharp (2 x 1) LEED pattern with a low background and modulated streaks indicating extended c(4 x 2) domains was observed. Neither the duration of the annealing nor the cooling rate was found to influence the surface quality significantly. Before depositing Pb the uniformity and quality of the

substrates was checked carefully with RHEED, LEED and STM. The size of the terraces measured with STM indicated that the substrates were miscut by ~0.5 ° in the [100] direction. Usually the substrates exhibited only a small number of missing dimers (~ 0.1%) and extended areas of the c(4 x 2) reconstruction were free of defects. Pb was evaporated from an effusion cell with a PBN crucible at a rate of ~2.5 x 10 -3 M L / s , where one monolayer (1 ML) is equal to 6.24x 1014 atoms cm -2. The effusion cell has been calibrated with a quartz microbalance and by counting the lead adatom density observed in STM images of low coverage G e ( l l l ) c ( 2 x 8)-Pb reconstructed surfaces [8]. Above 400°C the sample temperature was measured with an optical pyrometer and below 400°C the temperature was estimated from the heater power. During deposition of Pb the pressure was maintained at less than 1 x 10 -1° mbar. We found that samples could be cleaned for reuse either by sputtering and annealing or by simply heating up to 600°C to desorb all of the Pb. For the STM measurements we used electrochemically etched tungsten tips, which were formed in situ by scanning at high bias voltage and tunneling current. All data was recorded in constant current mode and all the images shown here are unfiltered raw data with only a linear background subtracted.

3. R e s u l t s and discussion

Fig. 1 displays a schematic diagram illustrating the formation of the various phases of Pb on Ge(001) as a function of Pb coverage and anneaPb/Ge(O01) TPC

300,

L surface

J[____~{21'~11

~ c ( 8 x 4 )(-, -, -) ~ i RT- ~ 0.0

~ 0.5 0.75 1.0

c(8x4)i

k° 3 } ] l ÷ 3danlsdsi ~

O / ML

1.7

Fig. 1. Schematic diagram showing the room temperature phases of Pb/Ge(001) as a function of lead coverage and annealingtemperature.

G Falkenberg et aL/Surface Science 372 (1997) 155-170

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ling temperature as derived from our measurements. The (2 x 2) and the c(8 x 4) reconstructions are both metastable; on annealing at ~ 200°C the (2 x 2) changes irreversibly into a c(8 x 4) structure, which in turn above ,-~300°C changes irreversibly into either the (1 x 5) reconstruction or a leadinduced rough surface, depending on the initial coverage. At a coverage of ~ 5/3 M L a homogeneous (2 31) reconstruction (in the following referred to as (2103)) is formed. Adding a small amount of Pb leads to the incommensurate c(8 x 4)i reconstruction. Both the (2103) and the c(8 x 4)i reconstruction undergo a phase transition to a (2 x 1) structure on heating. However, since this transition is reversible it is not included in the schematic diagram of the room-temperature phases. Our diagram is in good agreement with the previous L E E D and STM results ['14,15] except that at low coverage we observe a (2 x 2) instead of a c(4 x 2) reconstruction. The c(4 x 2) L E E D pattern observed by Zhang et al. is possibly caused by an enhanced defect-induced buckling of the clean Ge dimer rows, since defect-induced buckling has been observed for trace amounts of various adsorbates on Ge(001) [18]. The low-coverage reconstructions of Pb on Ge(001), namely the ( 2 x 2 ) and the c ( 8 x 4 ) are both characterized by the coexistence of Ge and Pb dimers, as described in the following section. Our measurements indicate that the common feature of the two high-coverage reconstructions (described in Section 3.2) is a close-packed Pb overlayer with an almost hexagonal symmetry, which is commensurate for the (2103) reconstruction and incommensurate for the c(8 x 4)i reconstruction. The ( l x 5 ) superstructure, which is described in Section 3.3. is the only 2-D phase of Pb on Ge(001) that cannot be prepared at room temperature. The formation of an incomplete (1 x 5) reconstruction leads to considerable surface roughening, which contrasts to the fact that Pb on Ge(001) is generally considered to form an abrupt metal-semiconductor interface.

The large scale STM image Fig. 2 displays large flat terraces covered with some additional bright islands which have a remarkably large aspect ratio (on our samples typically 15:1). In the high resolution image Fig. 3 of the same sample it can be seen that terraces without adsorbate atoms exhibit the symmetrical and asymmetrical dimer rows characteristic of clean Ge(001). The symmetrical dimer rows correspond to a (2 x 1) reconstruction whereas the buckled dimers usually form locally a c(4 x 2) superstructure [19]. The bright domains are (2 x 2) reconstructed and display one protrusion per (2 x 2) unit cell. The corrugation is typically ~0.8 A along the small sides of the (2 x 2) domains. The height difference between the (2 x 2) reconstructed areas and the Ge dimer rows is ~ l . 6 A . The long side of the ( 2 x 2 ) domains always runs perpendicular to the Ge dimer rows. The arrow in Fig. 3 indicates an interesting phenomenon that can frequently be observed in STM images of the (2 x 2) reconstruction. The indicated row of protrusions is shifted with respect to the neighboring rows in the direction perpendicular to the Ge dimer rows. The magnitude of this shift corresponds to ~,0.5a where a is the (1 x 1) sub-

3.1. Low-coverage phases

Fig. 2. STM image (sample bias V = 0.6 V, tunneling current I = 1.0 hA) of Ge(001) covered with ~0.1 ML Pb deposited at room temperature. The image shows bright (2 x2) reconstructed rectangular domains with a large aspect ratio terminated by c(8 x 4) structures, on clean reconstructed Ge(001) (2 x 1) terraces.

3.1.1. The (2 x 2) phase First, we present results for Ge(001) covered with 0.1 M L Pb deposited at room temperature.

V= - 0 . 6 V I= 1.0nA

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V = - 0 . 9 V I= 1.6nA

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~ ) C) 1st layer Pb (up/down) O 100

2nd layer Ge

Fig. 4. Schematic model of the Ge(001) (2 x 2)-Pb reconstruction. Small circles correspond to the dimerized Ge-atoms in the second layer. The shading of the large circles represents the buckling of the first layer Pb dimers. The shaded square corresponds to a (2 x 2) unit cell.

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Fig. 3. High resolution STM image of the same surface as Fig 2. The (2 x 2) reconstruction displays one protrusion per unit cell. Some rows of protrusions (e.g. indicated by the arrow) are displaced ~0.5a in the direction perpendicular to the

germanium dimer rows.

strate lattice constant. Determining the registry of the m a x i m a in the (2 x 2) relative to the neighboring Ge dimer rows reveals that all protrusions of the (2 x 2) are off-centered from the middle of the troughs of the Ge dimer rows by ~0.25a. The observation of both the row shift and the asymmetrical registry of the protrusions of the ( 2 x 2 ) reconstruction can be explained by the structural model depicted in Fig. 4, in which the P b and Ge dimers in the first two layers are parallel. The observation that the protrusions of the (2 x 2) reconstruction are displaced from the middle of the troughs of the Ge dimer rows indicates that the P b dimers are asymmetrical, like the Ge dirners in the G e ( 0 0 1 ) - c ( 4 x 2 ) reconstruction. T h e row shift visible in Fig. 3 can be interpreted as a phase shift in the buckling of the adatoms as illustrated in the model. It is instructive to compare the Ge(001)(2 x 2)Pb phase with the various closely related (2 x 2) reconstructions which are formed by different group I I I and IV elements such as A1, Ga, In, Sn,

and P b on Si(001) at a coverage of ,-~0.5 M L [12,13,20-27]. Most experimental and theoretical work for group I I I metals (A1,Ga,In) on Si(001) favors paralM orientation of the a d a t o m dimers relative to the substrate dimers, with the adsorption site in the troughs of the Si dimer rows. Interestingly, the corresponding STM images show that the protrusions of the group III-induced (2 x 2) reconstructions are exactly in-line with the middle of the troughs of the substrate dimer rows and no row shift has been reported. In comparison with the schematic model of Fig. 4 this reveals that the group I I I a d a t o m dimers on Si(001) appear symmetrical whereas the Pb adatom dimers on Ge(001) and Si(001) [13] are asymmetrical. This difference can be explained as follows. Each Pb a t o m has four valence electrons but bonds to only three neighboring atoms. In this case the total energy of the symmetrical dimers can be lowered via a Jahn-Teller distortion [28]. This leads to a symmetry-breaking geometrical rearrangement which goes along with a charge transfer between the two Pb atoms of each dimer. For the group I I I dimers, however, the symmetrical configuration is stable since it does not contain unpaired electrons. A c o m m o n feature of all the related (2 x 2) reconstructions is a strong growth anisotropy. Brooks et al. recently proposed a polymerization-

G. Falkenberg et al./ Surface Science 372 (1997) 155-i70

like growth mechanism for A1 on Si(001) [26], which explains the one-dimensional growth of the (2 x 2) reconstructed domains. The same growth mechanism may be responsible for the large aspect ratio of the (2x2) domains of Pb on Ge(001) visible in Fig. 2. It should be pointed out that the growth mechanism requires the adatom dimers to be parallel to the substrate dimers which gives additional supporting evidence for the structural model of Fig. 4. The detailed evolution of the size and shape of the domains as a function of coverage reveals some differences between the various adsorbates. Group III metals initially grow in the form of long lines perpendicular to the Si dimer row direction [20-23]. With increasing coverage the separation between the lines decreases until a wellordered (2 x 2) reconstruction is formed at 0.5 ML. For Sn on Si(001) the growth is similar except for the fact that 2a-8a wide gaps form, which destroy the long range order in the (2 x 2) regions 1-24]. For Pb on Ge we find extended (2 x 2) domains even at very low coverages, although their long range order in the direction perpendicular to the metal dimers is destroyed by the row shifts. However, the existence of the domains gives evidence for an attractive interaction between neighboring dimer rows. Interestingly the short side of the (2 x 2) domains is always terminated by either a defect, a step edge, or by a structure which has a chain-like appearance in the STM images and corresponds to the Pb induced c(8 x 4) reconstruction. This is indicative of an instability of the corresponding domain boundaries and a high mobility of "free" Pb atoms or dimers. 3.1.2. The Ge(O01)c(8 x 4) reconstruction The Pb-induced (2 x 2) reconstruction is only formed on unannealed samples at a coverage below ~0.5 ML, since at higher coverages, or upon heating, the (2 x 2) transforms into a c(8 x 4) reconstruction (see Fig. 1). Fig. 5 shows an STM image of a Ge(001) sample covered with -,~0.4 ML Pb deposited at ~200°C. The image extends over three Ge(001) terraces which are covered with a mixture of the c(4 x 2) and (2 x 2) buckled dimer reconstructions typical of clean Ge(001) (e.g. at A) together with the Pb-induced c(8 x 4) reconstruction (e.g. at B), but hardly any trace of the symmet-

V=

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Fig. 5. Filled-state STM image of 0.4 ML Pb on Ge(001) deposited at 200°C. Buckled germanium dimer rows (A) coexist with domains of the c(8 x 4) reconstruction (B). The periodicity of the c(8 x 4) is perturbed by antiphase domain boundaries (C) and irregular arrangements of the subunits (D).

rical (2 x 1) Ge dimer reconstruction remains. The outlined area in Fig. 5 corresponds to an (8 x 4) unit cell. The corrugation along the chains of the c(8 x 4) structure is typically ,-~1.0 A for negative sample bias (occupied sample states) and ~0.6 for positive sample bias (unoccupied sample states). The height difference between the c(8 x 4) and the Ge dimer rows equals ~ 1.8 A. Despite the fact that the sample was prepared at elevated temperature the c(8 x 4) domain size is rather small, and many defects such as antiphase domain boundaries (e.g. at C), or stacking faults of the subunits of the c(8 x4) reconstruction (e.g. at D), can be found. On approaching the saturation coverage of the c(8 x 4) reconstruction an increase in the domain size was observed. However, the maximum domain size of a few thousands of A2 was still mainly limited by antiphase domain boundaries. Fig. 6 displays four close-ups of the sample area shown in Fig. 5. Piezo drift corrected double polar-

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(d) 0

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Fig. 6. Enlarged regions,from Fig. 5 near (A) and (B): (a) filled states, (b) empty states, (c) filled states, (d) empty sample states. The outlined c(8 x 4) unit cells in (a) and (b) as well as the highlighted circles in (c) and (d) indicate in each case the same location on the sample. ity images have been used to ensure that the outlined (8 x 4) unit cells in Figs. 6a and 6b are located at the same positions on the sample surface. In Fig. 6a the chain-like structure of the c(8 x 4) reconstruction is clearly resolved and an irregular stacking of the chain subunits is visible in the lower right corner. The corresponding empty state image Fig. 6b looks quite similar. However, the protrusions are shifted 2a in the direction parallel to the chains and the ends of the chains look different, e.g. the chain element in the lower left corner appears closed in Fig. 6a and open in Fig. 6b. These observations can be explained by the structural model shown in Fig. 7 which has been derived from STM measurements for Sn on Si(001)

and Pb on Ge(001) 1-24,17]. The surface is terminated by dimerized Pb atoms corresponding to a coverage of 0.75 ML. The Pb atoms are arranged in triple rows separated by trenches which allow the structure to accommodate the surface stress caused by the larger covalent radii of the Pb-atoms compared to the Ge-atoms. The position of the Pb dimers relative to the substrate was determined by triangulation with respect to adjacent Ge dimer rows. As in the Ge(001)c(4 x 2) reconstruction the dimers are buckled and there is a charge transfer from the lower dimer a t o m to the upper one [19,29]. Thus, we expect the filled (empty) states to be located at the position of the higher (lower) Pb atoms which agrees with the bias polarity dependence in Figs. 6a and 6b. The buckling can

G. Falkenberget al./ Surface Science 372 (1997) 155-170

oQ)o~lo--oQ) (8x4>C ) o - ~ o (~)o+

0

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161

dimers determine the registry of the two phases. This interpretation is in agreement with the structural model of Fig. 7 in which half of the Ge dimer bonds in the second layer are severed while the other half remain intact. The disruption of the Ge dimer bonds to form the c(8 x 4) reconstruction is possibly the reason for the existence of the metastable (2 x 2) reconstruction in a similar coverage range. According to the structural model of Fig. 4 all the Ge dimer bonds remain intact in the (2 x 2) reconstruction. Apparently a significant activation energy is needed to cut the Ge dimer bonds. A short anneal of the (2 x 2 ) phase at ~200°C is sufficient to provide enough energy to cut half of the Ge-dimer bonds so that the surface transforms irreversibly from the (2 x 2) into the c(8 x 4) reconstruction.

lstlayerPb (down/up) 3.1.3. Substitutional Pb aroms

o--o

o

2nd layerGe (dimerized/notdimerized)

Fig. 7. Schematicmodel of the Ge( 111)c(8 x 4)-Pb reconstruction. The outlined area corresponds to an (8 x4) unit cell. Shading of the circles symbolizes buckling of the first layer Pb-atoms. be described as a Jahn-Teller distortion which stabilizes the Ge(001)(8 x 4)-Pb reconstruction. As described previously this Jahn-Teller distortion is not possible for group III atoms in the same geometrical configuration which explains the absence of a group III induced c(8 x 4) reconstruction on Si(001) or Ge(001). Electron counting indicates that the Ge dimer bonds under the Pb triple rows must be disrupted, while the Ge dimers in the trenches remain intact. Accordingly, half of the Ge atoms of the second layer are dimerized. This conclusion can be supported by determining the registry of the c(8 x 4) relative to neighboring clean Ge dimer rows. Generally the substrate provides a (1 x 1) translational periodicity. However, when we observed the c ( 8 x 4 ) reconstruction in coexistence with the clean Ge dimer rows, the c(8 x 4 ) chains were always exactly centered on the dimer rows and not between them. This indicates that at least some of the Ge dimer bonds under the Pb layer of the c(8 x 4 ) reconstruction remain intact during the formation of the c (8 x 4) phase and these remaining

Figs. 6c and 6d show two enlarged STM images of buckled Ge dimer rows which contain some characteristic bright point defects. The highlighted circles surrounding two of the defects indicate the same locations on the sample. The maxima on the buckled Ge dimer rows correspond to the positions of the "up" atoms for negative and "down" atoms for positive sample bias. The bright point defects exhibit a similar bias polarity dependence which indicates that they are also dimers. Since we observed such defects only after Pb deposition we conclude that they are either Pb-dimers or mixed Ge/Pb-dimers. Because of the small corrugation difference of ~0.4 A for both bias polarities it seems unlikely that the defect dimers are located above the first layer Ge atoms. Instead we suggest that Pb atoms occupy substitutional sites in the Ge dimer rows. Apparently the activation barrier for substitution is quite high because such defects are only observed on annealed samples.: It seems likely that the substitutional Pb dimers are responsible for the suppression of the symmetrical (2 x 1) Ge dimer reconstruction, since they induce a buckling of the neighboring Ge dimers which leads locally to c ( 4 x 2 ) and ( 2 x 2 ) reconstructions, depending on the phase relation of the buckling of adjacent dimer rows. Similar defect-induced buckling has been observed for trace amounts of various adsorbates on Ge(001) and Si(001) [18,30].

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3.2. High-coverage phases 3.2.1. The Ge(001)(2103) reconstruction A (2103) L E E D pattern is obserVed for Pb on Ge(001) at a coverage of ,-~1.6 ML. On our samples the L E E D images of the (2103) phase often displayed a symmetry which was lower than that of the substrate. This indicates that the possible rotational subdomains of the (2103) reconstruction were not equally populated. A similar observation also has been reported in an earlier L E E D study by Zhang et al. 1-14] and it seems to be a characteristic feature of this reconstruction. Fig, 8 is a STM image from a Pb-covered Ge(001) sample which shows two (2103) reconstructed terraces. The (2103) domain in the upper half of the image is on the same Ge terrace as the c(8 ×4) reconstructed domain in the upper left corner. The appearance of the (2103) domains is dominated by one protrusion per unit cell. The protrusions are located on bright lines (see arrows) which have a separation of 2a. From the STM V= -1,7V I= 10.0 nA 250

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IO0

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Fig. 8. Filled-state S T M image of Ge(001) with 1.5 M L Pb deposited at r o o m temperature without annealing. Two (2103) reconstructed terraces are shown. In the upper left corner the (2103) reconstruction is in coexistence with a c(8 x 4) domain. The outlined area corresponds to a (2103) unit cell. The arrows indicate parallel bright lines with a separation of 2a.

images we found that our samples were slightly miscut ( ~ 0.5 °) and that the resulting steps had on average an angle of 45 ° to the dimer rows of the clean Ge. We observed that the sides of the primitive (2103) unit cells tend to be parallel to the step edges and believe that the alignment of the reconstruction along the step edges is the reason for the reduced symmetry of the L E E D pattern. Compared to the c(8 x 4) and (1 x 5) phases the (2103) reconstruction exhibits a much lower defect density and usually one terrace is covered by a single domain. We very rarely observed domain boundaries. It is a characteristic feature of the (2103) reconstruction that near step edges, or defects, the otherwise predominating protrusions are absent and only the bright lines with a separation of 2a can be seen. Fig. 9a shows a close-up of a (2103) reconstructed domain near a step edge. Despite the low resolution of this image it is clear that the bright lines near the step edge exhibit a (2 × 1) pattern. It is diificult to draw conclusions about the detailed atomic structure of the Ge(001)(2103)-Pb reconstruction from the STM data alone. Recently Jahns et al. have used surface X-ray diffraction (SXRD) to derive a structural model for the Ge(001)(2103)-Pb reconstruction in which the surface is terminated by a slightly distorted closely packed Pb(111) layer [31]. This model corresponds to a saturation coverage of 5/3 ML. To check this value we performed an in situ R H E E D experiment. For this experiment we used a sensitive CCD video camera to record the development of the R H E E D pattern during Pb adsorption at room temperature. We observed a maximum of the (1/4) streak, which we attribute to a complete c(8 × 4) reconstruction, after 380 s and a maximum of the (1/2) streak, which we attribute to a complete (2103) reconstruction, after 610 s. Assuming that our structural model for the c (8 x 4) corresponding to an ideal coverage of 0.75 M L is correct, we determine that the ideal coverage of the (2103) phase is (1.6__+0.1) ML, which is in good agreement with the model of Jahns et al. [-31]. A comparison of the structural model of the (2103) reconstruction with corresponding STM images reveals that it is not possible to identify

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G. Falkenberg et al./ Surface Science 372 (1997) 155-170 V=

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Fig. 9. (a) Detail from Fig. 8 in which the dimer row like substructure of the (2103) reconstructionis resolved.A (2103) and a (2 x 1) unit cell are indicated. (b) Structural model for the Ge(001) (2103)-Pb reconstruction.A (2103) and a (2 x 11 unit cell are indicated. Small circlessymbolizethe dimerized Ge substrate. Large circlescorrespond to the close-packedPb adlayer. Some of the Pb atoms, indicated by filled circles, are located on highly symmetricpedestal sites.

individual Pb atoms from the STM data. A similar phenomenon has been observed for the closely packed phases of Pb on Si(111) and G e ( l l l ) I-5,6,9]. For the latter system it has been demonstrated with X-ray standing waves measurements [ 11] and first principles calculations [32] that the Pb atoms in the closely packed layer have a high lateral mobility. Since the measuring process of an STM is slow compared to the timescale of atomic motions within the Pb layer it has been suggested that the STM is not measuring the positions of individual Pb atoms, but rather measuring an almost homogeneous Pb layer which is modulated by the substrate potential [6]. Therefore, it seems reasonable to try to explain the STM images of the (2103) reconstruction in a similar way as being caused by a superposition of substrate and adsorbate signals. Under this assumption the ( 2 x 1) reconstructed rows obviously have to be attributed to the substrate. From a comparison with the neighboring c(8 x 4) domain in Fig. 8 the orientation of the (2 x 1) rows in the (2103) reconstruction was determined to be parallel to the Ge dimer rows. The height contrast between the two phases of 1.6 seems reasonable and is e q u a l to that of the corresponding phases of Pb/Ge(111), namely the

dilute x/-3-e adatom phase and the densely packed x/-3-fl phase [-8]. The assumption that the underlying Ge substrate is dimerized is supported by the existence of a reversible (2103)+-~(2xl) phase transition at 200°C observed with R H E E D [ 12]. The single protrusion per (2103) unit cell located on the bright rows has a height of only ~ 0.5 which is rather small for an adatom and even more so for a cluster of Pb atoms as it has been suggested by Yang et al. [16]. By comparison with the structural model of Jahns et al. 1-31] we assume that the protrusions correspond to Pb atoms within the closely packed layer which are located on average on a high symmetry site. According to the STM image of Fig. 9a this location is a pedestal site above the Ge dimer rows. The corresponding structural model is shown in Fig. 9b. The small circles symbolize the Ge atoms of the second layer which are completely dimerized. The large circles correspond to the first layer Pb atoms. The shaded Pb atoms are located on the highly symmetric pedestal site. Near step edges, or defects, the phase relationship between the substrate and the adsorbate may be lost. In such cases none of the Pb atoms are located on high symmetry sites which explains the absence of protrusions near step edges in our STM images.

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At this point it is interesting to look back to the previous section and to follow the dimerization of the topmost Ge layer as a function of the Pb coverage. The (2 x 2) reconstruction corresponding to a saturation coverage of 0.5 M L contains a complete layer of dimerized Ge atoms while in the c(8 x 4) phase, corresponding to 0.75 ML, half of the Ge dimer bonds are disrupted. For the (2103) phase, with a saturation coverage of 1.667 ML, the bright stripes with a (2 x 1) pattern (see Fig. 9) and the temperature dependent (2103)+--~(2 x 1) phase transition provide evidence for the existence of a complete dimer layer. Finally, for the c(8 x 4 ) i phase at higher coverage the temperature dependent c(8 x4)i+-~(2 x 1) phase transition also supports the existence of a completely dimerized layer of Ge atoms.

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3.2.2. The c(8 x 4)i phase Adding more Pb to a saturated (2103) phase results in the formation of a new type of reconstruction which can be easily distinguished from the (2103) by both L E E D and STM. There has been some confusion in the literature about the correct designation of this reconstruction. Zhang et al. [14] observed with L E E D a (4 x 1) pattern with only dim additional eight-order spots and labeled the reconstruction accordingly (4 x 1), while Yang et al. [15,16] using STM observed a new type of c(8 x 4) reconstruction which they named c(8 x 4)ft. In the following we will designate the new reconstruction c(8 x 4)i since it almost corresponds to a c ( 8 x 4 ) reconstruction, but is incommensurate. Fig. 10 shows a high resolution STM image of the Ge(001)c(8 x 4)i-Pb reconstruction. The most striking feature of this image are the regular bright lines with a separation of 4a. Additional protrusions can be seen both on the stripes and in the trenches between the stripes. The arrows indicate that the distance between the protrusions on the stripes may be either 4a or 5a. A regular distance of 4a would correspond to a commensurate c (8 x 4) reconstruction as designated by the highlighted c(8 x 4) unit cell. However, we determined from three large scale STM images from different samples that the average distance between two protrusions on the bright rows is equal to

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Fig. 10. STM image of the Ge(001)c(8 × 4)l-Pb reconstruction.

The distance between the bright parallel stripes is 4a. The distance between the protrusions on the stripes varies between 4a and 5a as indicated by the arrows. The outlined area corresponds to a c(8 x 4) unit cell. (4.65__+0.06)a. Interestingly, this value can be reproduced by the simple geometrical construction shown in Fig. 11, which displays a hexagonal unit cell on a square substrate with the constraint that

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. . . . . . . . . . . . . . . . . . . . . . . . . . . .

~..

i i

30° / / e

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4a Fig. 11. Hexagonal unit cell on the square (1 x 1) substrate lattice (denoted by small filled circles) under the condition of commensurabilityin one direction.

G. Falkenberg et al./Surface Science 372 (1997) 155-170

the distance between the parallel sides of the hexagon is 4a. In this case the size of the ideal hexagonal unit cell is simply 8a tan 30°=4.62a. Since this theoretical value agrees well with our experimental value of (4.65_0.06)a, we propose that the Ge(001)c(8 x 4 ) i - P b reconstruction consists of a hexagonal Pb overlayer on the square substrate. The hexagonal character can be deduced directly from the L E E D patterns of the c(8 x 4)i reconstruction shown in Figs. 12a and 12b. The brightest fractional-order spots in both images are the fourth-order spots along the high symmetry lines. The pattern formed by the weaker additional fractional-order spots indicated as sl, s2, and s3 is quite complex and depends on the energy of the diffracted electrons. It can be easily ascertained by comparison with the L E E D pattern of the commensurate low coverage c(8 x 4 ) reconstruction shown in Fig. 12c that the weak fractional-order spots denoted s2 and s3 in Figs. 12a and b do not correspond to ( - ~ , -~) eight-order spots. Instead, the superstructure spots sl, s2, and s3 display an almost twelve-fold symmetry which corresponds to two domains of the quasihexagonal superstructure on different 90 ° rotated substrate terraces. The location of the sl spots, which are closest to the (0,0)-spot in Fig. 12a, is equivalent to a real space periodicity of (4.55 _+0.20)a, i.e. the corresponding unit cell agrees with both the periodicity in the STM images and the schematic model of Fig. 11. The 4a periodicity of the structure in one high symmetry direction of the substrate surface can be deduced from the quarter order spots. Although we systematically tried various preparation conditions, we were not able to confirm the existence of the commensurate closely packed c(8 x 4)/~ reconstruction described by Yang et al. [15,16]. Recent X-ray diffraction experiments [31] and the (2103)+-~c(8 x 4)i phase transition observed in L E E D [ 14] indicate that the two phases are closely related. As a starting point for finding a possible structural model for the c(8 x 4 ) i reconstruction

Fig. 12. (a) LEED image of the c(8 x4)i phase at 41.0 eV. (b) LEED image of the c(8 x 4)i phase at 30.8 eV. (c) LEED image of the commensurate c(8 x 4) reconstruction at 32.7 eV.

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we assume that the surface is terminated with a closely packed Pb layer like the (2103) structure. From the phase diagram (Fig. 1) we know that the saturation coverage of the c ( 8 x 4 ) i is slightly higher than the 5/3 M L of the (2103) reconstruction. Furthermore, since the c(8 × 4)i reconstruction displays a quasihexagonal symmetry it seems reasonable to assume that it contains hexagonal subunits, i.e. that the Pb layer corresponds to a slightly distorted P b ( l l l ) layer. A possible arrangement of the Pb atoms which satisfies these boundary conditions is shown in Fig. 13a. The shaded lines correspond to the bright lines with a distance of 4a which are visible in most of our STM images of this reconstruction while the circles symbolize the closely packed Pb layer. The detailed appearance of the c(8 x 4 ) i phase in the STM images depended both on the tunneling parameters and the condition of the tunneling tip. However, some of our images (see Fig. 13b) display a substructure on the bright lines which is in remarkable agreement with the model of Fig. 13a. The arrangement of the Pb atoms in the model corresponds to a 9.0 ° rotated P b ( l l l ) layer which has been compressed by 5.2% in the surface plane. The corresponding saturation coverage is 1.678 ML, which is slightly above the 1.667 ML for the (2103) reconstruction and in good agreement with the experimental observations.

(a) V= +0.1V f= 27.1nA

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3.2.3. The large-scale morphology An interesting aspect of the high coverage phases are the changes in the large scale morphology which occur on annealing. Both the (2103) and the c(8 x 4)i behave in an identical way. Fig. 14a shows an STM image of Ge(001)c(8 x 4 ) i - P b prepared at room temperature. The surface exhibits wellordered domains with a typical size of ( ~ 2 0 0 ~,)2 together with some defects which show up as dark holes with a depth of ~ 2.0 ,~, and frequent antiphase domain boundaries running either parallel or perpendicular to the bright lines of the c(8 x 4)i reconstruction. The Pb-covered surfaces exhibit single atomic height steps with a distribution similar to the clean substrates before Pb deposition. The appearance changed drastically after annealing at ,-~300°C for 5 min (see Fig. 14b). During the annealing procedure all the antiphase domain

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(b) Fig. 13. (a) Possible atomic arrangement of the first Pb layer in the Ge(001)c(8×4)i reconstruction. The outlined area

corresponds to the hexagonal c(8 × 4)i unit shown in Fig. 11. The fight gray areas symbolize the parallel bright fines in Fig. 10 with a distance of 4a. The shaded circlescorrespond to the brightest protrusions in (b). (b) High resolution STM image of the Ge(001)c(8x 4)i reconstruction.Outlined are a hexagonal c(8 x 4)i unit cell and the rectangulararea which is depicted in (a). boundaries were removed so that the resulting domains were limited only by the size of the terraces. In addition, the size and shape of the terraces also changed. On the annealed samples we frequently observed double, triple and higher

G. Falkenberg et aL / Surface Science 372 (1997) 155-170

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straighter. The slope and direction of the multiple steps indicated that they correspond to (111) micro facets. Impurity-induced faceting of vicinal surfaces is a well known phenomenon [33-35]. Two mechanisms may promote the Pb-induced faceting of Ge(001). Firstly, Pb may alter the equilibrium crystal shape since (111) facets provide more favorable bonding sites for the adsorbate, which leads to a lowering of the surface free energy [-34] giving rise to Pb-stabilized facets. Secondly, the Pb probably increases the surface self-diffusion rate, as is the case for Pb on G e ( l l l ) [36], which facilitates the considerable Ge mass transport which is necessary for faceting to take place.

V= + 2 . 0 V I = 2.2 nA

3.3. The (1 x 5) reconstruction

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As shown in Fig. 1, annealing of Ge(001)-Pb with coverages between 1.0 and 1.7 ML at ,-~300°C results in an irreversible phase transition to a (1 x 5) reconstruction. Fig. 15 shows a high reso-

= 1.0 nA V = + 7 2 m Y I= 0 . 1 8 nA

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Co) Fig. 14. Two large scale images of the c(8 x 4)i phase (a) before and (b) after annealing at N300°C. In (a) the surface shows single steps and many antiphase domain boundaries (arrows). In (b) the surface exhibits multiple steps but no antiphase domain boundaries.

multiple steps running in (110) directions. In contrast to the strongly meandering step edges on the clean substrate these multiple steps were always

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Fig. 15. STM image of the Ge(001)(1 x 5)-Pb reconstruction at a coverage of 1.0 M L Pb deposited at N 300°C. The image shows three terraces covered with the (1 x 5) phase which is characterized by bright double rows of protrusions on top of a weak substructure. The arrows indicate five small protrusions of the substructure at the edge of a (1 x 5 ) domain. The rectangle corresponds to a (1 x 5) unit cell.

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lution STM image of three different terraces covered with this reconstruction. The most prominent features in this image are bright double rows of protrusions with a periodicity of la in the direction of the rows and a regular distance of 5a between two parallel double rows. Double rows on terraces separated by an odd number of single steps are rotated by 90 ° with respect to each other. The outlined area in Fig. 15 corresponds to a (1 x 5) unit cell. On the (1 x 5) we usually observed a comparatively high density of defects. The protrusions of the double rows in Fig. 15 appear ~ 1.5 higher than the substructure in the trenches between the double rows. The substructure in the trenches also displays a periodicity of la in the direction of the rows. At the end of one of the double rows (A) the two arrows indicate a row of five small protrusions which is oriented perpendicular to the double row. These five protrusions indicate that the substructure under the bright double rows has a (1 x 1) periodicity. Recent X-ray diffraction experiments confirm this observation and indicate that the (1 x 1) layer consists of Pb while the bright double rows correspond to double rows of Ge atoms [-31,37]. Therefore, a homogeneous (1 x 5) reconstruction corresponds to a coverage of 1.0 ML.

3.4. Development of the (1 x 5) phase A particular interesting aspect of the (1 x 5 ) reconstruction is the development of the structure at coverages below 1 M L at elevated temperature. Instead of exhibiting coexisting domains of the lead induced (1 x 5) phase and the (2 x 1) phase of the clean substrate, the surface breaks up and roughens. Fig. 16 shows a large scale image of a Ge(001) surface which was prepared by depositing 0.2 M L Pb at a temperature of ~300°C. The image displays a disordered network of bright stripes with a typical width of 25 A and length of (50-250) it. High resolution STM images revealed that the stripes correspond to small (001) terraces which mostly display a buckled dimer structure similar to clean Ge(001). In addition we also observed some small (1 x 1) reconstructed domains. The (001) terraces are separated by ,,-20/k wide trenches with a typical depth of a few ~t (see the cross-section Fig. 16b). The STM

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(b) Fig. 16. (a) STM image of 0.2 ML Pb on Ge(001) deposited at ~300°C. (b) Profile of the surface along the highlighted line in (a).

images revealed that the roughness of this sample extended to a depth corresponding to more than five substrate atomic layers. With increasing coverage the average distance between the stripes decreased, but the roughness of five layers still remained. The terraces mostly exhibited a (1 x 1) pattern decorated with different arrangements of adatoms. Finally, at a coverage around 1 M L the surface became fiat again, with a step density only determined by the miscut of the sample, and a regular (1 x 5) reconstruction was formed on the fiat terraces. The (1 × 1) pattern which we observed on the rough phase at a coverage slightly below 1 M L is possibly a precursor

G. Falkenberg et al. /Surface Science 372 (1997) 155-170

state to the complete (1 x 1) Pb-layer in the (1 x 5) reconstruction [ 31 ]. The "rough-surface phase" has no equivalent in the P b / G e ( l l l ) system. It seems surprising that Pb is able to change the morphology of the Ge surface so drastically in this manner. Previously it was thought that Pb on Ge would form abrupt nonreacted interfaces.

4. Conclusions The various 2-D phases of Pb on Ge(001) have been studied in detail with STM and electron diffraction. At very low coverage on slightly annealed samples the existence of substitutional Pb atoms in the G e ( 0 0 1 ) c ( 4 x 2 ) reconstruction was observed. At 0.5 M L coverage a (2 x 2) reconstruction is formed which consists of dimerized Pb atoms oriented parallel to the underlying unperturbed Ge dimers. The geometrical structure of the c(8 x 4) reconstruction can be described by a structural model with 0.75 M L coverage consisting of triple rows of dimerized Pb atoms which are oriented perpendicular to the Ge dimers and half of the Ge dimers in the second layer are disrupted. By a comparison with Si(001)-group III adsorbate systems it has been shown that the buckling of the Pb dimers in the (2 x 2) and c(8 x 4) reconstructions can be described as a Jalin-Teller effect. On the closely packed (2103) reconstruction evidence was found for dimerization of the topmost Ge layer. With in situ R H E E D the saturation coverage of this reconstruction was found to be (1.6___0.1)ML. L E E D and STM measurements of the high coverage c(8 x 4)i phase indicate that the reconstruction is incommensurate in one direction and forms a hexagonal overlayer on the cubic substrate. The effect of annealing at ,-~300°C is highly dependent on the Pb coverage. The antiphase domain boundaries are removed in the high coverage (2103) and c(8 x 4)i phases and, instead of single steps, multiple steps are formed. Between 1.0 M L and 1.7 M L annealing leads to the (1 x 5) phase which can be described as a (1 x 1) Pb layer decorated by double rows of Ge adatoms. For Pb coverages below one monolayer the surface rough-

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ens to a depth of ,-~5 atomic layers after annealing at ~ 300°C.

Acknowledgements The authors wish to thank V. Jahns for stimulating discussions. The financial support of the Volkswagen Stiftung under project no. 1/65 092 and the B M B F under project no. 05 5GUABI are gratefully acknowledged.

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