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Growth of Ge–Si(111) epitaxial layers: intermixing, strain relaxation and island formation
Surface Science 406 (1998) 254–263
Growth of Ge–Si(111) epitaxial layers: intermixing, strain relaxation and island formation N. Motta a,*, A. Sgarlata a, R. Calarco a, Q. Nguyen a, J. Castro Cal a,1, F. Patella a, A. Balzarotti a, M. De Crescenzi b a Istituto Nazionale di Fisica della Materia, Dip.Fisica, Universita` di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy b Istituto Nazionale di Fisica della Materia, Dip. Fisica, Universita` di Camerino, 62032 Camerino, Italy Received 28 July 1997; accepted for publication 19 January 1998
Abstract We have followed by scanning tunneling microscopy (STM ) the growth of thin Ge films obtained by reactive deposition epitaxy on Si(111) substrates kept at 500°C. For a Ge coverage less than 0.45 ML, STM images show large 7×7 flat areas without any protrusion. For increasing coverage, flat, triangular 5×5 islands start nucleating while the Si substrate retains the 7×7 reconstruction. The islands’ evolution, up to the completion of the wetting layer, is described in the framework of a statistical model of growth. At 3 ML, the composition and ordering of the wetting layer are investigated by Current Imaging Tunneling Spectroscopy (CITS ) measurements, revealing small differences in the atomic corrugations. The analysis of topographic and current images supports an elemental composition for the topmost layer. At coverages larger than 3 ML, thick Ge islands nucleate according to the Stranski–Krastanov mechanism of growth. We analyze the evolution of the lattice strain up to 15 ML coverage. A clear expansion of the lattice parameter as a function of coverage is found both on the islands’ top and on the wetting layer. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial layers; Ge–Si(111); Intermixing; Island formation; Strain relaxation
1. Introduction In the last decade, much interest has been devoted to the growth of Ge–Si alloys mainly because of their potential applications in microelectronics [1] and optoelectronics [2]. Several novel applications, such as heterojunction bipolar transistors and quantum devices, require strained struc* Corresponding author. Tel.: (+39) 6 72594438; fax: (+39) 6 2023507; e-mail: [email protected]. 1 Permanent address: Departamento de Fisica Aplicada, Universidade de Vigo, Lagoas-Marcosende, 9 36299 Vigo, Spain. 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 01 2 1 -6
tures. Since Si and Ge lattices differ by about 4%, the interfacial energy puts severe constraints on the growth and affects the morphology of the films. Therefore, the way in which the strain is relieved and the role of interdiffusion are crucial points to investigate [1–3]. Several studies of the surface properties have been carried out with different techniques [4–8], suggesting the formation of Ge Si alloys due x 1−x to substrate strain [9–11] or kinetic effects [12– 14]. Recently, the application of Scanning Tunneling Microscopy (STM ) to measure the in-plane surface lattice strain g [15] and the degree s
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of ordering of the resulting alloy has been undertaken. The observation of the height modulation of the adatoms in the 5×5 surface has led to contrasting conclusions [16,17] on the distribution of the adatoms and on the elemental composition of the wetting layer. The precise measure of the in-plane lattice constant of Ge–Si(111) islands has indicated a progressive increase of the interatomic distance as a function of the island height towards that of pure Ge [15]. This relaxation suggests an enrichment in Ge content of the islands up to about 100%. However, so far, no definite conclusions about the true composition, the lattice parameter and the ordering of the wetting layer have been reached. In this paper, we address these problems by following the epitaxial growth of the first monolayers of Ge–Si at various coverages by STM. Initially, we find an interdiffusion of Ge atoms into the 7×7 Si substrate. After 0.65 ML (1 ML = ˚ ) of Ge deposition, flat, triangular islands 3.4 A start to grow; their lateral dimension increases until a continuous layer fully 5×5 reconstructed is formed around 3 ML of Ge coverage. On this surface, we have carried out a Current Imaging Tunneling Spectroscopy (CITS) experiment. CITS spectra do not give any clear-cut evidence of Si adatoms in the top layer. However, by noting that the surface lattice constant at this coverage is that of bulk Ge, we conclude in favour of a termination layer of monoatomic composition. For Ge coverages greater than 3 ML, three-dimensional islands develop. We have acquired atomically resolved images of both the islands’ top and the substrate, evidencing a 7×7 reconstruction of the former and a 5×5 reconstruction of the latter, in agreement with previous reports [7]. Due to the tensile stress at the surface, the lattice parameter of the substrate and of the islands’ top expands as a function of coverage.
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different evaporation sources are available [18]. The scanning tunneling microscope ( Tops System, WA Technology) consists of a UHV attachment with an antivibration stacking and a piezoelectric tube with 2 mm maximum scanning area for the tip movement. The lateral resolution of the micro˚ , and the accuracy in the lateral scope is ±1 A ˚ . Tungsten tips were displacement2 is ±0.05 A chemically etched in a solution of NaOH and glycerol. The Si substrates consisted of P-doped Si(111) wafers (0.6×1 cm) of resistivity r≈0.003 V · cm. For each deposition, we started from a reconstructed Si 7×7 surface obtained by repeatedly flashing the sample at 1250°C by joule heating with a current of ≈9 A. The temperature of the sample surface was checked by an optical pyrometer. Surface reconstruction and cleanliness were monitored by RHEED. The deposition of Ge on the Si substrate kept at 500°C was made from an effusion cell (Reactive Deposition Epitaxy) with ˚ min−1. The low evaporation rates of 0.5 to 1.0 A Ge coverage was measured with a quartz thickness ˚ . After Ge monitor with an accuracy of ±0.2 A deposition, the samples were kept at the deposition temperature for several minutes in order to allow the system to relax. Each Ge deposition was made on a new Si sample in order to minimize the uncertainties of successive evaporations, and analyzed in situ by STM after waiting 1–2 h for adequate cooling.
3. Results 3.1. Nucleation and growth of the wetting layer
2. Experimental
STM images of the clean Si(111) surface (not shown here) show large flat regions with 7×7 reconstruction [19]. The images of the Ge–Si(111) surface after deposition of 0.45 ML of Ge at 500°C are shown in Fig. 1. The 7×7 reconstruction is maintained, as confirmed by the RHEED pattern that provides a probe of a larger and deeper region
The experiments were carried out in a UHV chamber (base pressure <2×10−10 Torr) where RHEED, STM, CMA-Auger spectrometer and
2 The Si(111) 7×7 was used as reference surface to calibrate the piezoelectric tube.
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N. Motta et al. / Surface Science 406 (1998) 254–263 Table 1 Reconstructions observed by RHEED and STM on Ge–Si(111) interfaces grown at 500°C Reconstructions
˚ 2 of a Ge–Si (111) surface Fig. 1. STM image (260×260) A after deposition of 0.45 ML of Ge at 500°C. The 7×7 reconstruction on a larger and deeper region of the surface is confirmed by the RHEED pattern shown in the inset.
of the surface. The presence of Ge on the sample was checked by Auger spectroscopy. The absence of islands and of reconstructed (2×n) areas of Ge suggest a process of in-diffusion of Ge adatoms into the Si substrate, in agreement with X-ray photoelectron diffraction and RHEED studies [6,20,21]. As proposed in the case of submonolayer deposition of Ge on Si (001) surfaces [22], Ge might change place with Si, and the displaced Si atoms can diffuse towards the step edges of the substrate (displacive adsorption). At 0.65 ML of deposited Ge, 1 ML thick triangular islands appear (Fig. 2a). Their lateral dimension increases progressively with coverage until they merge to form a full layer (Fig. 2b and c). Small domains with different reconstructions (7×7, E3×E3 R30°) are occasionally included in the 5×5 areas while the substrate retains its 7×7 structure. It is likely that the (E3×E3 R30°) phase, which is not typical of Ge, is caused by the doping atoms of the substrate. STM images show the coexistence of 7×7 and 5×5 phases on the top of the islands adding important microscopic details to the phase diagram of the alloy [6 ]. The mixed phases could depend on the local content
Ge coverage (ML)
RHEED
STM
0.45 0.75 1.5 3
7×7 7×7 5×5+7×7 5×5
7×7 5×5+7×7 5×5+7×7 5×5
of Ge and disappear as soon as the wetting layer is completed. The triangular shape of the islands follows the threefold symmetry of the substrate with the edges of triangles aligned along the 110 symmetry lines of the 7×7 reconstruction. Where the density of the steps is high and the distance between two nearby steps is small enough, island growth is greatly favoured along the step borders at the expenses of the terraces. This situation evolves until, at 3 ML coverage, the surface is flat and fully 5×5 reconstructed with very few defects ( Fig. 3). The whole evolution is confirmed by the RHEED analysis except at 0.75 ML where the mixed phase is revealed only by STM (see Table 1). 3.2. Growth above the wetting layer When the coverage exceeds 3 ML, several threedimensional islands appear at random locations over the surface. At the beginning, they have a triangular shape with average lateral dimensions ˚ and are about 1 ML high. It has of 500–1000 A been suggested [23] that this morphological transition occurs when the free energy of the islands is lower than that of the strained layer. At 9 ML coverage (Fig. 4a), two kinds of islands are visible: ˚ (1) small and high islands typically 1800 A ˚ high; and (2) large and ripened flat wide×100 A ˚ wide×100 A ˚ islands of average dimensions 3500 A high. We suppose that, after nucleation, the islands grow vertically up to a critical value to which the strain energy induces a morphological transition with the introduction of dislocations [24]. Then, they grow laterally, apparently drawing material from the top or collapsing with other islands. This process is termed ‘‘ripening’’ and leads to flat and
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˚ 2. Right panels: corresponding Fig. 2. Evolution of the Ge–Si layer for increasing Ge coverage. Left panels: surface area (2600×2600) A ˚ 2 across the substrate–island boundary. (a) and (d) refer to 0.65 ML; (b) and (e) 1.35 ML; (c) and (f ) 2 ML of zooms (235×235) A Ge coverage. Note the 5×5 reconstruction of the islands and the 7×7 reconstruction of the substrate. In (d ), a (E3×E3)R30° domain is visible on the top left corner of the island; in (e), two contiguous domains with different reconstructions are evidenced.
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one of the latter. This is valid for both strained and ripened islands and was checked on several samples having different coverages.
4. Discussion Much work has been done on Ge–Si interface epitaxial growth, regarding the intermixing of the wetting layer. Here, we address by STM the following points that deserve further clarification: (1) the kinetics of growth of the wetting layer; and (2) the composition and the surface lattice parameter of the topmost layer. 4.1. Kinetics of growth of the wetting layer As evident from Fig. 2, the growth of the wetting layer starts with the nucleation of triangular islands, and evolves towards a complete layer by coalescence. The general problem of the nucleation of an atomic species over a different one has been studied as a function of temperature and evaporation rate [28]. Here, we investigate the nucleation and time evolution of the total perimeter per unit area P of Ge–Si islands as a function of the fraction of covered surface S up to 3 ML. To this purpose, we follow the approach taken by Tomellini and Fanfoni [29] to model the cluster impingement on a surface [30,31]. The model assumes that the clusters are uniformly and randomly distributed over the surface and the nucleation rate of all clusters is d-like. The latter assumption implies that P(S) is independent of the growth law. In the case of triangular clusters, the perimeter is given by: Fig. 3. STM images of a Ge–Si (111) surface after deposition ˚ 3; I=1 nA, of 3 ML of Ge at 500°C. (a) (1250×1250×4.2) A ˚ 2. V=+3 V; (b) zoom of (a) (230×230) A
round islands. Interestingly, some of these islands display a hole at their center (Fig. 5) caused probably by the more rapid depaupering of the Ge layer at the center, where the strain is higher than at the border [25,26 ]. Zooms on the top of the islands (Fig. 4b) and on the substrate ( Fig. 4c) display the 7×7 structure of the former [27] and the 5×5
6 EN (1−S) P(S)= o 4 E3
S
ln
1 1−S
,
(1)
where N is the number of nuclei per unit surface. o The measured distribution of the cluster perimeters extracted from the experimental data of Fig. 2 is described by Eq. (1) using N =3.8×1010 o nuclei cm−2 (Fig. 6). The order of magnitude of N is intermediate between that of metallic clusters o on metals [28] and of diamonds clusters on semiconductors [32].
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Fig. 4. STM images of a Ge–Si (111) surface (I=1 nA and V=6 V ) after deposition of 9 ML of Ge. (a) STM image ˚ 3. Two different kind of islands are visible: tall and triangular (1800 A ˚ wide×100 A ˚ high); low and rounded (20 000×10 000×120) A ˚ wide×100 A ˚ high). Zooms (80×80) A ˚ 2 (b) on island’s top and (c) on the substrate display the 7×7 structure of the former (3500 A and the 5×5 of the latter.
4.2. Elemental composition and lattice parameter of the wetting layer An interesting open point about surface composition of the 5×5 phase is the possibility of distinguishing Ge from Si adatoms. Some authors [33–35] answered positively a similar question on GaAs and its ternary alloys because the change of the tunneling polarity makes it possible to image atoms of either electronegativity. Unfortunately, the covalent bonding of Ge and Si [36 ] does not allow for a simple distinction. A first attempt was made by Becker et al. [16 ] on the basis of the
observed height modulation of the adatoms along selected lines of the topographic images. Later, Fukuda [17] made a statistical analysis of the height differences between neighboring adatoms in the same half unit and combined it with spectroscopic characterization. Within the limitations posed by the different sample preparation, the conclusions of the two authors are the opposite of each other: Becker et al. [16 ] find an ordered distribution of adatoms, whereas Fukuda [17] suggests a random substitution of Ge atoms by Si. In order to contribute in clarifying this point, we have acquired topographic images of the 5×5
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˚ 3. Fig. 5. STM image of a ripened island (8000×5000×27) A ˚ (2 ML). The depth of the central hole is ~6 A
Fig. 6. Total cluster perimeter per unit area, P, vs. the covering fraction, S, extracted from STM images. The points are the experimental values and the solid line is the fit made using Eq. (1).
Ge–Si(111) surface. In Fig. 7a, a topography ˚ 2, V=−2.0 V ) is shown. We notice that (46×46 A in some of the faulted units, one adatom appears darker than the other two. In several line profiles taken on this image (Fig. 7b) we measure differences in the adatoms heights of each subunit of ˚ , which is comparable to the difference about 0.2 A ˚ ) and Ge (1.2 A ˚ ). in the covalent radii of Si (1.1 A This suggests either a random replacement of some Ge adatoms by Si or a bond length relaxation caused by the presence of Si in the underlying layers. However, the Si atoms mixed in the subsurface layers could also modify the local electronic
Fig. 7. CITS images of the Ge–Si(111) surface after deposition ˚ 2. Top frame: (a) conof 3 ML of Ge. Image size is (46×46) A stant current image (I=1 nA) measured with a sample bias of −2.0 V; (b) profile taken along the black line indicated in the topography on the left side. Other frames: CITS images corresponding to: −3.0 V, −2.5 V, −1.5 V, +3.0 V, +2.5 V and +1.5 V.
density of states on the adatoms. To analyze the electronic properties of the top layer, we have collected a number of CITS spectra at different bias voltages. In the CITS mode [37–39], topography and spectroscopy are performed simulta-
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neously. During constant current imaging, the tip is periodically stopped, the feedback is turned off, and the current is measured at several predefined bias voltages and stored in a three-dimensional matrix. The data are displayed as a series of current images each for a different bias voltage. The quality of our tip was checked by measuring constant current images before and after the CITS experiment. We present in Fig. 7 two sets of current images at 3 ML coverage, collected with negative and positive sample bias. Filled (negative bias) and empty (positive bias) states are imaged with atomic resolution. We describe these data with reference to the schematic DAS [40] diagram of the 5×5 units shown in Fig. 8. At −3.0 V, we see brighter spots at the corner holes of the 5×5 reconstruction and in correspondence of intermediate holes delimited by the backbonds between adatoms and rest atoms (see Fig. 8b); in the −2.5 V image, such bonds form a hexagonal structure surrounding the corner hole. The current from the dimers and from the bottom corner adatom in the faulted unit is small, particularly at −1.5 V bias, where the tripod-shaped bonds pointing from the central rest atom (see Fig. 8a) to the dimer layer atoms are imaged. This feature was also found on the Si(7×7) surface [37] where, at −1.7 V, the current from the three rest atoms is clearly imaged, and it is assigned to their dangling bonds [41,42]. At +2.5 and +3.0 V, the adatoms in the unfaulted unit provide a higher tunneling current and are in phase contrast with the topography taken at −2.0 V where the faulted units are brighter than the unfaulted ones. This inversion exists also in Si 7×7 surface [41,42]. From the current data of the CITS images, we have constructed the conductance curves for those atoms that have different brightnesses in both faulted and unfaulted units in the topography. For each subunit, the darker adatoms in the topography have a higher conductance at a positive bias; for a negative bias, no significant changes have been measured. This asymmetry could result from the fact that in the reference topography ( Fig. 7a), the tip was stabilized at a negative bias. Hence, the combined information drawn from topography
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Fig. 8. 5×5 reconstruction of the (111) surface according to the DAS model: (a) position of atoms in the last four layers (top view); (b) corner and intermediate holes formed by dimer layer atoms and rest atoms. The atom sizes are scaled down with respect to the distance from the top layer (see legend).
and spectroscopy does not allow us to rule out the mixing of Ge and Si in the top-layer adatoms. To fix this point, we have analyzed the lattice
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urements averaged over a penetration depth of ˚ . Furthermore, we notice that the expansion 10 A of the in-plane lattice constant can take place due to the presence of edge steps on the surface. We conclude that the surface layer grown at T≈500°C consists primarily of Ge atoms. This does not preclude random mixing of Ge and Si atoms in lower subsurface layers, [20,21,43–45], which is expected on the basis of energy considerations discussed for the Ge–Si (001) surface [46,47]. Above the wetting layer, the lattice parameter on top of the three-dimensional islands has been measured, and its value agrees with that of Theiss et al. [15] (Fig. 9b).
5. Conclusions
Fig. 9. Surface lattice mismatch g =(a −a )/(a ): (a) meas Ge/Si Si Si sured on the 5×5 regions of the wetting layer as a function of coverage (the curve is a guide for the eye); (b) measured on the 7×7 regions on the islands top as a function of the island height.
mismatch g =(a −a )/(a ) as a function of s Ge/Si Si Si coverage. We measured the lattice constant of the 5×5 unit cell by averaging the line profiles of several images acquired at the same nominal deposition3. We find a continuous increase of g s up to 4% when the wetting layer is completed (3 ML) ( Fig. 9a). This indicates a progressive enrichment of Ge up to about 100%. Recent RHEED data of the lattice constant [14] agree satisfactorily up to 3 ML coverage with our meas3 Data were collected along atomic lines as close as possible to the fast scanning direction (see Ref. [15]).
We have applied STM and RHEED techniques to study the initial stages of Ge–Si(111) epitaxy at 500°C. Below 3 ML, we observe the formation of fully reconstructed triangular islands starting from 0.65 ML of Ge coverage. The evolution of these islands is described by a model based on the Avrami theory [30,31]. At 3 ML coverage, we have acquired for the first time CITS images of the 5×5 Ge–Si surface. The analysis indicates a termination layer of monoatomic composition and does not preclude mixing in the lower subsurface layers. The extent to which small differences in sample preparation could alter the degree of mixing and therefore the composition of the top layer remains an interesting point to investigate.
Acknowledgements We acknowledge useful discussions with M. Fanfoni. This work has been supported by the Istituto Nazionale di Fisica della Materia (INFM ) and by the CEE HCM contract CHRXCT93-0355. J.C.C., University of Vigo, Spain, thanks the Physics Department of the University of Rome ‘‘Tor Vergata’’ for the hospitality.
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Growth of Ge–Si(111) epitaxial layers: intermixing, strain relaxation and island formation N. Motta a,*, A. Sgarlata a, R. Calarco a, Q. Nguyen a, J. Castro Cal a,1, F. Patella a, A. Balzarotti a, M. De Crescenzi b a Istituto Nazionale di Fisica della Materia, Dip.Fisica, Universita` di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy b Istituto Nazionale di Fisica della Materia, Dip. Fisica, Universita` di Camerino, 62032 Camerino, Italy Received 28 July 1997; accepted for publication 19 January 1998
Abstract We have followed by scanning tunneling microscopy (STM ) the growth of thin Ge films obtained by reactive deposition epitaxy on Si(111) substrates kept at 500°C. For a Ge coverage less than 0.45 ML, STM images show large 7×7 flat areas without any protrusion. For increasing coverage, flat, triangular 5×5 islands start nucleating while the Si substrate retains the 7×7 reconstruction. The islands’ evolution, up to the completion of the wetting layer, is described in the framework of a statistical model of growth. At 3 ML, the composition and ordering of the wetting layer are investigated by Current Imaging Tunneling Spectroscopy (CITS ) measurements, revealing small differences in the atomic corrugations. The analysis of topographic and current images supports an elemental composition for the topmost layer. At coverages larger than 3 ML, thick Ge islands nucleate according to the Stranski–Krastanov mechanism of growth. We analyze the evolution of the lattice strain up to 15 ML coverage. A clear expansion of the lattice parameter as a function of coverage is found both on the islands’ top and on the wetting layer. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial layers; Ge–Si(111); Intermixing; Island formation; Strain relaxation
1. Introduction In the last decade, much interest has been devoted to the growth of Ge–Si alloys mainly because of their potential applications in microelectronics [1] and optoelectronics [2]. Several novel applications, such as heterojunction bipolar transistors and quantum devices, require strained struc* Corresponding author. Tel.: (+39) 6 72594438; fax: (+39) 6 2023507; e-mail: [email protected]. 1 Permanent address: Departamento de Fisica Aplicada, Universidade de Vigo, Lagoas-Marcosende, 9 36299 Vigo, Spain. 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 01 2 1 -6
tures. Since Si and Ge lattices differ by about 4%, the interfacial energy puts severe constraints on the growth and affects the morphology of the films. Therefore, the way in which the strain is relieved and the role of interdiffusion are crucial points to investigate [1–3]. Several studies of the surface properties have been carried out with different techniques [4–8], suggesting the formation of Ge Si alloys due x 1−x to substrate strain [9–11] or kinetic effects [12– 14]. Recently, the application of Scanning Tunneling Microscopy (STM ) to measure the in-plane surface lattice strain g [15] and the degree s
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of ordering of the resulting alloy has been undertaken. The observation of the height modulation of the adatoms in the 5×5 surface has led to contrasting conclusions [16,17] on the distribution of the adatoms and on the elemental composition of the wetting layer. The precise measure of the in-plane lattice constant of Ge–Si(111) islands has indicated a progressive increase of the interatomic distance as a function of the island height towards that of pure Ge [15]. This relaxation suggests an enrichment in Ge content of the islands up to about 100%. However, so far, no definite conclusions about the true composition, the lattice parameter and the ordering of the wetting layer have been reached. In this paper, we address these problems by following the epitaxial growth of the first monolayers of Ge–Si at various coverages by STM. Initially, we find an interdiffusion of Ge atoms into the 7×7 Si substrate. After 0.65 ML (1 ML = ˚ ) of Ge deposition, flat, triangular islands 3.4 A start to grow; their lateral dimension increases until a continuous layer fully 5×5 reconstructed is formed around 3 ML of Ge coverage. On this surface, we have carried out a Current Imaging Tunneling Spectroscopy (CITS) experiment. CITS spectra do not give any clear-cut evidence of Si adatoms in the top layer. However, by noting that the surface lattice constant at this coverage is that of bulk Ge, we conclude in favour of a termination layer of monoatomic composition. For Ge coverages greater than 3 ML, three-dimensional islands develop. We have acquired atomically resolved images of both the islands’ top and the substrate, evidencing a 7×7 reconstruction of the former and a 5×5 reconstruction of the latter, in agreement with previous reports [7]. Due to the tensile stress at the surface, the lattice parameter of the substrate and of the islands’ top expands as a function of coverage.
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different evaporation sources are available [18]. The scanning tunneling microscope ( Tops System, WA Technology) consists of a UHV attachment with an antivibration stacking and a piezoelectric tube with 2 mm maximum scanning area for the tip movement. The lateral resolution of the micro˚ , and the accuracy in the lateral scope is ±1 A ˚ . Tungsten tips were displacement2 is ±0.05 A chemically etched in a solution of NaOH and glycerol. The Si substrates consisted of P-doped Si(111) wafers (0.6×1 cm) of resistivity r≈0.003 V · cm. For each deposition, we started from a reconstructed Si 7×7 surface obtained by repeatedly flashing the sample at 1250°C by joule heating with a current of ≈9 A. The temperature of the sample surface was checked by an optical pyrometer. Surface reconstruction and cleanliness were monitored by RHEED. The deposition of Ge on the Si substrate kept at 500°C was made from an effusion cell (Reactive Deposition Epitaxy) with ˚ min−1. The low evaporation rates of 0.5 to 1.0 A Ge coverage was measured with a quartz thickness ˚ . After Ge monitor with an accuracy of ±0.2 A deposition, the samples were kept at the deposition temperature for several minutes in order to allow the system to relax. Each Ge deposition was made on a new Si sample in order to minimize the uncertainties of successive evaporations, and analyzed in situ by STM after waiting 1–2 h for adequate cooling.
3. Results 3.1. Nucleation and growth of the wetting layer
2. Experimental
STM images of the clean Si(111) surface (not shown here) show large flat regions with 7×7 reconstruction [19]. The images of the Ge–Si(111) surface after deposition of 0.45 ML of Ge at 500°C are shown in Fig. 1. The 7×7 reconstruction is maintained, as confirmed by the RHEED pattern that provides a probe of a larger and deeper region
The experiments were carried out in a UHV chamber (base pressure <2×10−10 Torr) where RHEED, STM, CMA-Auger spectrometer and
2 The Si(111) 7×7 was used as reference surface to calibrate the piezoelectric tube.
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N. Motta et al. / Surface Science 406 (1998) 254–263 Table 1 Reconstructions observed by RHEED and STM on Ge–Si(111) interfaces grown at 500°C Reconstructions
˚ 2 of a Ge–Si (111) surface Fig. 1. STM image (260×260) A after deposition of 0.45 ML of Ge at 500°C. The 7×7 reconstruction on a larger and deeper region of the surface is confirmed by the RHEED pattern shown in the inset.
of the surface. The presence of Ge on the sample was checked by Auger spectroscopy. The absence of islands and of reconstructed (2×n) areas of Ge suggest a process of in-diffusion of Ge adatoms into the Si substrate, in agreement with X-ray photoelectron diffraction and RHEED studies [6,20,21]. As proposed in the case of submonolayer deposition of Ge on Si (001) surfaces [22], Ge might change place with Si, and the displaced Si atoms can diffuse towards the step edges of the substrate (displacive adsorption). At 0.65 ML of deposited Ge, 1 ML thick triangular islands appear (Fig. 2a). Their lateral dimension increases progressively with coverage until they merge to form a full layer (Fig. 2b and c). Small domains with different reconstructions (7×7, E3×E3 R30°) are occasionally included in the 5×5 areas while the substrate retains its 7×7 structure. It is likely that the (E3×E3 R30°) phase, which is not typical of Ge, is caused by the doping atoms of the substrate. STM images show the coexistence of 7×7 and 5×5 phases on the top of the islands adding important microscopic details to the phase diagram of the alloy [6 ]. The mixed phases could depend on the local content
Ge coverage (ML)
RHEED
STM
0.45 0.75 1.5 3
7×7 7×7 5×5+7×7 5×5
7×7 5×5+7×7 5×5+7×7 5×5
of Ge and disappear as soon as the wetting layer is completed. The triangular shape of the islands follows the threefold symmetry of the substrate with the edges of triangles aligned along the 110 symmetry lines of the 7×7 reconstruction. Where the density of the steps is high and the distance between two nearby steps is small enough, island growth is greatly favoured along the step borders at the expenses of the terraces. This situation evolves until, at 3 ML coverage, the surface is flat and fully 5×5 reconstructed with very few defects ( Fig. 3). The whole evolution is confirmed by the RHEED analysis except at 0.75 ML where the mixed phase is revealed only by STM (see Table 1). 3.2. Growth above the wetting layer When the coverage exceeds 3 ML, several threedimensional islands appear at random locations over the surface. At the beginning, they have a triangular shape with average lateral dimensions ˚ and are about 1 ML high. It has of 500–1000 A been suggested [23] that this morphological transition occurs when the free energy of the islands is lower than that of the strained layer. At 9 ML coverage (Fig. 4a), two kinds of islands are visible: ˚ (1) small and high islands typically 1800 A ˚ high; and (2) large and ripened flat wide×100 A ˚ wide×100 A ˚ islands of average dimensions 3500 A high. We suppose that, after nucleation, the islands grow vertically up to a critical value to which the strain energy induces a morphological transition with the introduction of dislocations [24]. Then, they grow laterally, apparently drawing material from the top or collapsing with other islands. This process is termed ‘‘ripening’’ and leads to flat and
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˚ 2. Right panels: corresponding Fig. 2. Evolution of the Ge–Si layer for increasing Ge coverage. Left panels: surface area (2600×2600) A ˚ 2 across the substrate–island boundary. (a) and (d) refer to 0.65 ML; (b) and (e) 1.35 ML; (c) and (f ) 2 ML of zooms (235×235) A Ge coverage. Note the 5×5 reconstruction of the islands and the 7×7 reconstruction of the substrate. In (d ), a (E3×E3)R30° domain is visible on the top left corner of the island; in (e), two contiguous domains with different reconstructions are evidenced.
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one of the latter. This is valid for both strained and ripened islands and was checked on several samples having different coverages.
4. Discussion Much work has been done on Ge–Si interface epitaxial growth, regarding the intermixing of the wetting layer. Here, we address by STM the following points that deserve further clarification: (1) the kinetics of growth of the wetting layer; and (2) the composition and the surface lattice parameter of the topmost layer. 4.1. Kinetics of growth of the wetting layer As evident from Fig. 2, the growth of the wetting layer starts with the nucleation of triangular islands, and evolves towards a complete layer by coalescence. The general problem of the nucleation of an atomic species over a different one has been studied as a function of temperature and evaporation rate [28]. Here, we investigate the nucleation and time evolution of the total perimeter per unit area P of Ge–Si islands as a function of the fraction of covered surface S up to 3 ML. To this purpose, we follow the approach taken by Tomellini and Fanfoni [29] to model the cluster impingement on a surface [30,31]. The model assumes that the clusters are uniformly and randomly distributed over the surface and the nucleation rate of all clusters is d-like. The latter assumption implies that P(S) is independent of the growth law. In the case of triangular clusters, the perimeter is given by: Fig. 3. STM images of a Ge–Si (111) surface after deposition ˚ 3; I=1 nA, of 3 ML of Ge at 500°C. (a) (1250×1250×4.2) A ˚ 2. V=+3 V; (b) zoom of (a) (230×230) A
round islands. Interestingly, some of these islands display a hole at their center (Fig. 5) caused probably by the more rapid depaupering of the Ge layer at the center, where the strain is higher than at the border [25,26 ]. Zooms on the top of the islands (Fig. 4b) and on the substrate ( Fig. 4c) display the 7×7 structure of the former [27] and the 5×5
6 EN (1−S) P(S)= o 4 E3
S
ln
1 1−S
,
(1)
where N is the number of nuclei per unit surface. o The measured distribution of the cluster perimeters extracted from the experimental data of Fig. 2 is described by Eq. (1) using N =3.8×1010 o nuclei cm−2 (Fig. 6). The order of magnitude of N is intermediate between that of metallic clusters o on metals [28] and of diamonds clusters on semiconductors [32].
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Fig. 4. STM images of a Ge–Si (111) surface (I=1 nA and V=6 V ) after deposition of 9 ML of Ge. (a) STM image ˚ 3. Two different kind of islands are visible: tall and triangular (1800 A ˚ wide×100 A ˚ high); low and rounded (20 000×10 000×120) A ˚ wide×100 A ˚ high). Zooms (80×80) A ˚ 2 (b) on island’s top and (c) on the substrate display the 7×7 structure of the former (3500 A and the 5×5 of the latter.
4.2. Elemental composition and lattice parameter of the wetting layer An interesting open point about surface composition of the 5×5 phase is the possibility of distinguishing Ge from Si adatoms. Some authors [33–35] answered positively a similar question on GaAs and its ternary alloys because the change of the tunneling polarity makes it possible to image atoms of either electronegativity. Unfortunately, the covalent bonding of Ge and Si [36 ] does not allow for a simple distinction. A first attempt was made by Becker et al. [16 ] on the basis of the
observed height modulation of the adatoms along selected lines of the topographic images. Later, Fukuda [17] made a statistical analysis of the height differences between neighboring adatoms in the same half unit and combined it with spectroscopic characterization. Within the limitations posed by the different sample preparation, the conclusions of the two authors are the opposite of each other: Becker et al. [16 ] find an ordered distribution of adatoms, whereas Fukuda [17] suggests a random substitution of Ge atoms by Si. In order to contribute in clarifying this point, we have acquired topographic images of the 5×5
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˚ 3. Fig. 5. STM image of a ripened island (8000×5000×27) A ˚ (2 ML). The depth of the central hole is ~6 A
Fig. 6. Total cluster perimeter per unit area, P, vs. the covering fraction, S, extracted from STM images. The points are the experimental values and the solid line is the fit made using Eq. (1).
Ge–Si(111) surface. In Fig. 7a, a topography ˚ 2, V=−2.0 V ) is shown. We notice that (46×46 A in some of the faulted units, one adatom appears darker than the other two. In several line profiles taken on this image (Fig. 7b) we measure differences in the adatoms heights of each subunit of ˚ , which is comparable to the difference about 0.2 A ˚ ) and Ge (1.2 A ˚ ). in the covalent radii of Si (1.1 A This suggests either a random replacement of some Ge adatoms by Si or a bond length relaxation caused by the presence of Si in the underlying layers. However, the Si atoms mixed in the subsurface layers could also modify the local electronic
Fig. 7. CITS images of the Ge–Si(111) surface after deposition ˚ 2. Top frame: (a) conof 3 ML of Ge. Image size is (46×46) A stant current image (I=1 nA) measured with a sample bias of −2.0 V; (b) profile taken along the black line indicated in the topography on the left side. Other frames: CITS images corresponding to: −3.0 V, −2.5 V, −1.5 V, +3.0 V, +2.5 V and +1.5 V.
density of states on the adatoms. To analyze the electronic properties of the top layer, we have collected a number of CITS spectra at different bias voltages. In the CITS mode [37–39], topography and spectroscopy are performed simulta-
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neously. During constant current imaging, the tip is periodically stopped, the feedback is turned off, and the current is measured at several predefined bias voltages and stored in a three-dimensional matrix. The data are displayed as a series of current images each for a different bias voltage. The quality of our tip was checked by measuring constant current images before and after the CITS experiment. We present in Fig. 7 two sets of current images at 3 ML coverage, collected with negative and positive sample bias. Filled (negative bias) and empty (positive bias) states are imaged with atomic resolution. We describe these data with reference to the schematic DAS [40] diagram of the 5×5 units shown in Fig. 8. At −3.0 V, we see brighter spots at the corner holes of the 5×5 reconstruction and in correspondence of intermediate holes delimited by the backbonds between adatoms and rest atoms (see Fig. 8b); in the −2.5 V image, such bonds form a hexagonal structure surrounding the corner hole. The current from the dimers and from the bottom corner adatom in the faulted unit is small, particularly at −1.5 V bias, where the tripod-shaped bonds pointing from the central rest atom (see Fig. 8a) to the dimer layer atoms are imaged. This feature was also found on the Si(7×7) surface [37] where, at −1.7 V, the current from the three rest atoms is clearly imaged, and it is assigned to their dangling bonds [41,42]. At +2.5 and +3.0 V, the adatoms in the unfaulted unit provide a higher tunneling current and are in phase contrast with the topography taken at −2.0 V where the faulted units are brighter than the unfaulted ones. This inversion exists also in Si 7×7 surface [41,42]. From the current data of the CITS images, we have constructed the conductance curves for those atoms that have different brightnesses in both faulted and unfaulted units in the topography. For each subunit, the darker adatoms in the topography have a higher conductance at a positive bias; for a negative bias, no significant changes have been measured. This asymmetry could result from the fact that in the reference topography ( Fig. 7a), the tip was stabilized at a negative bias. Hence, the combined information drawn from topography
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Fig. 8. 5×5 reconstruction of the (111) surface according to the DAS model: (a) position of atoms in the last four layers (top view); (b) corner and intermediate holes formed by dimer layer atoms and rest atoms. The atom sizes are scaled down with respect to the distance from the top layer (see legend).
and spectroscopy does not allow us to rule out the mixing of Ge and Si in the top-layer adatoms. To fix this point, we have analyzed the lattice
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urements averaged over a penetration depth of ˚ . Furthermore, we notice that the expansion 10 A of the in-plane lattice constant can take place due to the presence of edge steps on the surface. We conclude that the surface layer grown at T≈500°C consists primarily of Ge atoms. This does not preclude random mixing of Ge and Si atoms in lower subsurface layers, [20,21,43–45], which is expected on the basis of energy considerations discussed for the Ge–Si (001) surface [46,47]. Above the wetting layer, the lattice parameter on top of the three-dimensional islands has been measured, and its value agrees with that of Theiss et al. [15] (Fig. 9b).
5. Conclusions
Fig. 9. Surface lattice mismatch g =(a −a )/(a ): (a) meas Ge/Si Si Si sured on the 5×5 regions of the wetting layer as a function of coverage (the curve is a guide for the eye); (b) measured on the 7×7 regions on the islands top as a function of the island height.
mismatch g =(a −a )/(a ) as a function of s Ge/Si Si Si coverage. We measured the lattice constant of the 5×5 unit cell by averaging the line profiles of several images acquired at the same nominal deposition3. We find a continuous increase of g s up to 4% when the wetting layer is completed (3 ML) ( Fig. 9a). This indicates a progressive enrichment of Ge up to about 100%. Recent RHEED data of the lattice constant [14] agree satisfactorily up to 3 ML coverage with our meas3 Data were collected along atomic lines as close as possible to the fast scanning direction (see Ref. [15]).
We have applied STM and RHEED techniques to study the initial stages of Ge–Si(111) epitaxy at 500°C. Below 3 ML, we observe the formation of fully reconstructed triangular islands starting from 0.65 ML of Ge coverage. The evolution of these islands is described by a model based on the Avrami theory [30,31]. At 3 ML coverage, we have acquired for the first time CITS images of the 5×5 Ge–Si surface. The analysis indicates a termination layer of monoatomic composition and does not preclude mixing in the lower subsurface layers. The extent to which small differences in sample preparation could alter the degree of mixing and therefore the composition of the top layer remains an interesting point to investigate.
Acknowledgements We acknowledge useful discussions with M. Fanfoni. This work has been supported by the Istituto Nazionale di Fisica della Materia (INFM ) and by the CEE HCM contract CHRXCT93-0355. J.C.C., University of Vigo, Spain, thanks the Physics Department of the University of Rome ‘‘Tor Vergata’’ for the hospitality.
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