- Email: [email protected]
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Surface Science 273 (1992) l-8 North-Holland
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Strain relaxation during the surfactant modified epitaxial growth of Ge/Si( 001) J.M.C. Thornton,
A.A. Williams, J.E. Macdonald
Department of Physics, University College, Cardifi P.O. Box 913, Cardiff CFl 3TH, UK
R.G. van Silfhout FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, Netherlands
M.S. Finney and C. Norris Department of Physics, University of Leicester, Leicester LEl
7RH, UK
Received 28 November 1991; accepted for publication 20 February 1992
The initial strain relaxation of Ge on Si(OO1)has been investigated during epitaxial growth modified by a “surfactant” layer of Sb. Grazing-incidence X-ray diffraction was used to measure the strain relaxation due to its high sensitivity to the in-plane distribution of lattice spacings. We have observed the critical thickness for strain relaxation in the Ge overlayer to be - 11 monolayers (ML), with further relaxation developing gradually and in stages. A meta-stable, partially relaxed layer forms up to a coverage of - 30 ML, after which a more pronounced relaxation takes place. Even at a coverage of - 55 ML, complete strain relief has not been reached, and was only achieved after a 700°C thermal anneal. Concurrent specular reflectivity measurements also reveal that the Ge overlayer grows in a layer-by-layer fashion, and not in the Stranski-Krastanow mode expected for the Ge/Si system. These data show dramatically how the modification of surface energies through the presence of a “surfactant” can affect the morphology of, and strain in, a lattice-mismatched heteroepitaxial system.
1. Introduction
The epitaxial growth of Ge on Si has been extensively studied from a wide range of viewpoints, yet despite this, it continues to raise further questions of the physics of heteroepitaxial growth. The main avenues of approach have been to develop the properties desirable for optoelectronic and other novel device structures, and also to use it as a simple isovalent system with which to study strained layer growth. The effect of strain in a lattice-mismatched system has been known for a relatively long time. Through an elastic distortion of the overlayer, the strain energy is allowed to increase as the layer grows pseudomorphically. Eventually, however, the strain energy becomes too large to sustain, 0039-6028/92/$05.00
and at some critical thickness, the layer is able to overcome a reordering energy barrier, and thereby reach a lower energy, relaxed state. In relatively lightly strained systems (where the lattice mismatch is small, e.g. In,,GaO,&/ GaAs) the critical thickness may be many pm, at which point the film becomes substantially relaxed through dislocation 111. The prediction of the critical thickness of such systems is now fairly accurate, and has been described by various models based on the existence and propagation of dislocations in the overlayer [2]. In a more highly strained system like Ge on Si, which has a 4% lattice mismatch, the initial strain relaxation mechanism has been found to be rather different. This makes an accurate prediction of critical thickness rather more difficult, since it requires
0 1992 - Elsevier Science Publishers B.V. All rights reserved
2
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge /Si(OOl)
an in-depth understanding of the overlayer growth processes. The preferred mode of strain relaxation in the Ge/Si system is through the well known Stranski-Krastanow growth mode, whereby three-dimensional (3D) growth of islands occurs after only a few complete pseudomorphic monolayers (ML) of Ge have formed. Even though this growth mode has been known for many years, the exact nature of the islanding still remains open to question. It appears that the initial islands are coherent with the substrate, and only become dislocated when they reach a critical size 131. Under specific growth conditions, it also appears as though some elastic deformation of the Si substrate takes place in association with these islands. Perhaps the most graphic display of the complexity of this surface strain/energy system is that of the facetted “hut” clusters observed using scanning tunnelling microscopy @TM) [41. Although limited to formation under specific conditions, they are an initial mechanism to strain relaxation, perhaps forming an intermediate phase prior to achieving an equilibrium state. Moreover, their shape and distribution on the growing surface emphasize how important surface kinetics and diffusion are in overlayer growth processes, which together with the thermodynamics of surface bonding and strain due to a lattice mismatch, provide us with a highly complex system to try to understand. One recent development which highlights the delicate balance of parameters controlling the Ge/Si surface is where a surfactant species may be used to modify the growth behaviour. By depositing a monolayer of As onto the Si surface prior to the Ge deposition, it has been shown that 3D islanding can be inhibited, and so the Ge grows in a layer-by-layer fashion [5]. This phenomenon is thought to occur because the As segregates to the surface of both Si and Ge very strongly. As a result, the incoming Ge is incorporated into the Ge layer before it encounters any other mobile surface Ge, thereby pre-empting cluster formation. Previously, we have shown that the formation of Ge islands is fundamental to the strain relax-
ation mechanism, whereby the creation of elastically deformed islands coherent with the substrate permits strain relief before any dislocation takes place [6]. If islanding may be inhibited by the action of a surfactant species, the question then arises as to how the Ge now relaxes. Some morphological studies have shown that the layer becomes defected after a critical thickness, greater than that required to form islands, when a surfactant is not present, but it is by no means clear exactly how strain relaxation occurs or behaves with the formation of such defects. It is by altering the balance of surface energy and diffusion by the deposition of a surfactant layer of Sb, that we explore how the Ge layer achieves strain relaxation without the formation of islands. We have observed the strain relaxation as a function of Ge coverage directly, by measuring the in-plane lattice parameter near to the sample surface. This was performed by using in-situ grazing-incidence X-ray diffraction after each Ge deposition. The specular reflectivity was also measured in order to monitor the morphology of the overlayer as the Ge coverage increased.
2. Experimental
details
The experiments were performed using focussed radiation from the Wiggler beamline at the Synchrotron Radiation Source in DarFsbury (UK). The X-rays of wavelength A = 1.38 A were focussed using a toroidal mirror 20 m before the sample and monochromated using the (111) reflection from a channel cut silicon crystal. This gave a spectral width of Ah/h = 3 X lop4 and a beam divergence of 0.04”, which resulted in a slight degradation of resolution near Bragg peaks when compared to work on pure Ge/Si(OOl) without a surfactant layer [6]. Thus most scans were performed with an incident beam angle of 0.12”, well below the critical angle of 0.20” to minimize thermal diffuse scattering from the substrate, which peaks about reciprocal lattice points. The detector aperture subtended an angle of 0.10” at the sample. Radial scans were performed in a grazing-incidence geometry, which yields the
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge /Si(OOl)
strain distribution parallel to the interface. Specular reflectivity scans were also performed at some coverages to probe the electron density distribution at the surface and hence monitor the Sb distribution. The equipment consists of a large 5circle diffractometer, connected to an ultrahigh vacuum chamber with in-situ MBE growth, as well as standard surface science techniques [7]. The use of a 5circle diffractometer rather than an ordinary 4-circle diffractometer enlarges the accessible range of momentum transfer and allows the surface normal to lie in the horizontal plane during scans. The Si(OO1)substrates were cleaned by light sputtering with 800 eV Art ions for 60 s followed by an anneal for 3 min at 1060°C. This gave a sharp 2 x 1 diffraction pattern, observed with RHEED and X-ray diffraction. The full width at half maximum (FWHM) of the ($, 0) and (0, t> fractional order reflections, arising from the double domain 2 x 1 reconstruction, were N 0.02” for the clean surface, corresponding to an average reconstructed domain size of N 7000 A. The miscut of the surface relative to the (001) crystallographic axis was determined to be N 0.05”. The real unit-cell vectors, used to define the scattering vector Q = hb, + kb, + lb, in reciprocal space, may be related to the conventional bulk real cubic-cell vectors by a, = (1, 1, Ojcubic, ~2 = (I, - I, O)cubicand a3 = (0, 0, i)cubic. The Ge overlayers were deposited onto the Si surface at a rate of N 1 ML per 8 min from a Knudsen cell calibrated by Rutherford backscattering. The desired overlayer thickness could therefore be deposited to within a 6% margin. The chamber pressure during Ge deposition was < 2 X 1O-9 mbar and < lo-” mbar during measurements. The sample was kept at a temperature of 520 t- 10°C during deposition, and then allowed to cool to room temperature for the strain measurements. 3. Results
3
was made on the Si substrate. With the geometry used, the scattering vector can be in the plane of the sample surface, and so this type of scan is sensitive to the distribution of lattice spacings parallel to the interface. Any change in lattice spacing due to strain relaxation in the overlayer can therefore be probed directly. The lattice spacing of Ge is 4% larger than that of Si, and so in reciprocal space it is correspondingly smaller. Relaxed, bulk-like Ge may therefore be expected to be seen at a value of h = 1.92 reciprocal lattice units (r.1.u.) with respect to the (2, 0) Bragg peak for Si. Furthermore, any partially relaxed material, with a distribution of lattice spacings between these two extremes, will result in diffracted intensity within this range. The early depositions of both Sb and Ge onto the Si substrate resulted in no change to the profile of the radial scan, and hence in the distribution of in-plane lattice spacings. This is indicative of pseudomorphic growth, where the overlayer is completely strained to the lattice parameter of the substrate. After a deposition of N 11 monolayers (ML) of Ge, however, a small shoulder becomes apparent on the “large lattice spacing” side of the Bragg peak, which is attributable to the onset of strain relaxation of the Ge overlayer (fig. 1). It has previously been shown that an epitaxial Ge overlayer on SKOOl) will begin to relax after a coverage of N 3 ML has been deposited [6]. The effect of the Sb layer has therefore been to extend the range of pseudomorphic growth beyond this, up to the observed onset at N 10 ML. This is a large increase in critical thickness (19~) and is indicative of a quite different strain relaxation mechanism from the un-mediated Ge/Si(OOl) system. An interesting comparison may be made here between the growth of Ge on Si both with and without a surfactant species. Previously, when observing un-mediated growth of Ge, we have seen a broadening to the base of the (2, 0) Bragg peak. This was attributed to the curvature of rrvctal nlnnec hmmloht nhmlt hv the fnrmntinn nf
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge /Si(OOl)
4
0
34
0
ML
0 + +
0
1.90
1.95
2.00 h
0
2.05
1.90
1.95
(r.1.u.)
Fig. 1. Radial scans of the (2,O) Bragg peak as a function of coverage using an angle of incidence below the critical angle. They show the appearance of a shoulder (and so the onset of strain relaxation) at - 11 ML.
previously does not take place under these conditions. The absence of strain relaxation at N 3 ML coverage therefore affirms our earlier assertion that the formation of islands acts as a relaxation mechanism, and not that it follows as a consequence. With such a mechanism unavailable as a means of achieving strain relaxation, the Ge film continues to increase in thickness, with the strain energy increasing correspondingly. This must continue until another mechanism to strain relief becomes favourable over further pseudomorphic growth. Beyond the observed onset, the development of strain relaxation in the overlayer was monitored as a function of coverage. As can be seen in fig. 1, the shoulder to the Bragg peak gradually shifts further away and increases in intensity with overlayer coverage. After N 21 ML, a distinct feature becomes apparent at h = 1.975 r.1.u. (fig. 2) which suggests at least some meta-stability in the overlayer structure. Comparison with similar radial scans taken with angles of incidence above the critical angle (and hence at a lower surface nn.rn:+:..:h.\ -1._._.” rI.,c __ ,.:~~:F:~~_c J:IX^I^_^ ^
2.00 h
2.05
(r.1.u.)
Fig. 2. Radial scans as fig. 1, showing the development of strain relaxation with further Ge coverage. Note the feature at h = 1.975 r.1.u. for the 21 ML coverage.
ation mechanism involving a completed, equilibrium structure. At higher coverages, another feature gains intensity at h = 1.94 r.1.u. as further strain relaxation occurs (fig. 4). The growth of this peak in the distribution of lattice spacings seems to occur at the expense of the remainder, showing that a
3 ‘Z
500
W 2
0
1.90
1.95
2.00 h
2.05
(r.1.u.)
Fig. 3. Radial scans at comparable coverages to fig. 2, though mn,,r....nA *t “.? on”,, -6 ;..,:Aanr~ “l.,T.,.A lI.- r..:,:,..,, n..“ln
5
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge /Si(OOl)
1500
0
1.90
1.95
2.00 h
ih@m-d 2.05
(r.1.u.)
more complete relaxation is achieved with increasing coverage. If we again compare with the more bulk-sensitive, high-incident angle scans, it is now revealed that the feature at h = 1.97 r.1.u. has stabilized at a coverage of N 30 ML and beyond (figs. 3 and 5). The only change in intensity is the increase seen at h = 1.94 r.1.u. which
I
1.90
q ,
,
,
,
,
,
1.95
1.90
I
I I +*+ + +
,
1 I
I
,
(
,
,
,
,
0 520°C
1.95
2.00
2.05
h (r.l.u.)
Fig. 4. Radial scans as fig. 2 showing the development of strain relaxation with further Ge coverage. Note the peak forming at h = 1.94 r.l.u., which does not correspond to a complete strain relaxation, and the reducing intensity at h = 1.975 i.1.u..
1500
I
,
, , , , , + + 43 ML _
2.00
x 47 ML
_
o55ML
-
2.05
h (r.1.u.) Fig. 5. Radial scans as fig. 4, but measured at an angle of incidence above the critical angle. The intensity at h = 1.970 r.1.u. remains constant, while the development in relaxation is manifested by an increase at h = 1.94 r.1.u..
Fig. 6. Above critical angle, radial scans at a coverage of - 55 ML, showing the effect of a thermal anneal at 700°C (compared to the 520°C growth temperature). The anneal has driven a more complete strain relaxation to near bulk-like Ge (h = 1.92 r.1.u.) and the intensity due to partially relaxed material has diminished.
continues with increasing coverage. The material near to the surface of the Ge film is clearly more relaxed than that closer to the interface, even at our highest coverages. The overall picture, therefore, is one where a second stage of strain relaxation takes place over a stable, partially relaxed layer of - 30 ML in thickness. This is an interesting result, since there is no evidence here for a mechanism where strain relaxation takes place throughout the entire overlayer. It is interesting to note the effect of a thermal anneal at this stage. A 10 min anneal at 700°C resulted in the lattice spacing distribution seen in fig. 6. A clear jump to complete strain relaxation (at h = 1.92 r.1.u.) has occurred for most of the material, as though the higher temperature has allowed any defects or dislocations to become complete throughout the Ge overlayer. The previously stable layer close to the Si: Ge interface has been annihilated, together with any residual strain in the overlayer. The observed strain relaxation behaviour is clearly very different from the case where Ge is grown on the Si(OO1)without a surfactant species. The onset of strain relaxation has been delayed from -3 to - 11 ML, and the relaxation has
.I.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge / SifOOii
ti
occurred much more gradually, over a range of > 55 ML, rather than the N 11 ML seen previously [6].
Concurrent with the radial scans were measurements of the specular reflectivity as a function of incident angle. This technique is sensitive to the electron density near to the sample surface, and so may be used as a measure of surface roughness, morphology and also of overlayer intermixing. The reflectivity curve obtained from the clean Si(OO1) 2 x 1 surface can be seen to be smooth and homogeneous, as expected from such a surface (fig. 7). Fitting this curve using an electron density continuum model yields a surface roughness of 0.4 ML [8]. A very similar curve was obtained from the surface after the deposition, to saturation coverage, of N 0.7 ML Sb. The main difference between the curves is an increase in the reflectivity over the entire range observed. This is to be expected if a near complete overlayer of a higher electron density material is present on the surface. Using the above continuum model, a surface layer of 0.73 ML of Sb was
2
10’
i
0.00
0.05
0.10
0.15 I (r.1.u.)
0.20
0.25
0.30
Fig. 7. Specular reflectivity curves showing the effect of an overlayer of Sb (0.7 ML) on the Si surface. A subsequent 13 ML deposition of Ge results in oscillations in intensity as a function of incident angle, which are due to the overlayer forming in a layer-by-layer fashion. The continuous line is a curve fitted using an electron density continuum model.
0.00
0.05
0.10
0.15
0.20
0.25
0.30
1 (r.1.u.) Fig. 8. Specular reflectivity curves as a showing how the oscillations, and so mode, continues even to a coverage of how the period of oscillation decreases layer thickness.
function of coverage, the uniform growth - 55 ML. Notice too with increasing over-
found to give the best fit to the curve, in good agreement with the known saturation coverage [9]. Hiowever, an increase in the RMS surface roughness to N 1.5 ML had to be introduced, showing some disordering of the Si surface by the Sb. With the deposition of Ge, a dramatic change was observed in the reflectivity. Very clear, periodic oscillations in intensity with incident angle were seen, which are indicative of layer-by-layer growth of the Ge (figs. 7 and 8). The oscillations are due to interference between reflected intensity from the complete layers, with the period being determined by the number of unit cells perpendicular to the Si/Ge interface. The period can be seen to become smaller as coverage increases (fig. 8). If uncertainty in the number of cells is introduced by roughening or islanding, a superposition of different periods would create a more homogeneous curve [6]. As seen in figs. 7 and 8, these curves can be fitted very closely using the same continuum model as before, which assumes a complete overlayer without islanding. Interestingly, the Si/Ge, Ge/Sb and Sb/vacuum interfaces are all found to require only small roughnesses to obtain a best fit. The Ge/Sb interface, for example, has an RMS roughness of - l-l.5 ML for all the Ge
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge/Si(OOl)
coverages. This is not surprising if one considers the strong ordering seen recently for the Sb : Ge(OO1) 2 x 1 surface [lo]. Furthermore, the fit reveals that the Sb coverage remains constant at 0.73 ML for all Ge coverages, showing that incorporation of Sb into the Ge overlayer can only be negligible. The effect of a thermal anneal also manifests itself in the reflectivity as well as in the radial scan data. It was observed that a 10 min, 700°C anneal hardly affected the reflectivity from the 55 ML overlayer. A slight decrease in the strength of oscillation was observed, commensurate with some roughening or disorder at the surface. When combined with the radial scans, this shows that the anneal merely develops the relaxation mechanism further in the Ge, leaving a complete overlayer without any significant 3D islanding. The clear result, therefore, is one where the Sb surface segregates very strongly, driving the Ge sub-surface. This inhibits 3D islanding, driving the Ge overlayer to grow in a layer-by-layer fashion, and independently of overlayer thickness.
4. Conclusions These experiments have shown that the presence of a “surfactant” species can dramatically alter the morphology, growth-mode and the strain relaxation mechanism of a heteroepitaxial growth system. Using Sb to alter the growth surface energy, the Ge/Si(OOl) system has been found to grow under some conditions in a uniform, layerby-layer fashion, as opposed to the StranskiKrastanow mode seen when it is not present. The change in growth mode has also forced the strain relaxation mechanism to change, thereby revealing how closely the surface morphology and growth kinetics are linked with strain relaxation. From a critical thickness of - 3 ML when a surfactant is not present, the onset of strain relaxation has been increased to - 11 ML. This is direct evidence that the initial strain relaxation during unmodified growth is due to the formation of islands. When islanding is inhibited, such a
I
strain relaxation mechanism is no longer possible, so the layer must continue to grow under increasing strain until another mechanism becomes more favourable than further pseudomorphic growth. The precise mechanism for strain relaxation in this system could be one of a number of possibilities. It may be argued that the system has adopted the more familiar misfit dislocation mechanism to strain relaxation. Indeed, the predicted critical thickness for the dislocation mechanism is approximately 10 ML, as observed [ll]. If one considers the results of some similar modified-growth studies of this system, where As has been used as a surfactant, a rather different possibility from dislocation emerges. Ion-scattering and electron microscopy studies performed by Le Goues et al. have shown strain relief to occur in the Ge when “V-shaped”, defects form throughout the overlayer, again approximately at our observed critical thickness 151.These defects seem to develop with Ge coverage, leading to the injection of dislocations, finally relieving all strain after - 60 ML. This too is in rough agreement with our data, which shows near complete relaxation at the Ge surface after a deposition of - 55 ML. More recent STM studies by Jusko et al. point to a variation on this theme however [12]. They observe the formation of the defects at the critical thickness, but rather than developing with the thickness of the overlayer, the defects are completely stable. Furthermore, no Ge growth is observed over these defects after their creation, with Ge accumulation only taking place between them. The resulting morphology is one where rectangular islands of Ge form, with trenches separating them. High Ge mobility along the trenches causes the infilling to initiate at the trench junctions, from where the growth develops, finally leading to a relaxed Ge overlayer. It is reasonable to assume that Sb acts as a surfactant in the same way as As, and so a relaxation mechanism close to those described above may well be the case. One must consider the problem at a more fundamental level, however, if a complete understanding is to be achieved. It has been assumed that the primary action of a surfactant species has been to inhibit 3D island growth on the substrate surface. This
8
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge/Si(OOl)
occurs due to very active surface segregation of the surfactant, which drives the incoming Ge to grow sub-surface, thus preventing surface clustering. Surface clustering can also be inhibited thermally, as shown by Eaglesham and Cerullo, yet they do not observe any novel defect formation, only misfit dislocation [13]. This may suggest a defect nucleation role for the surfactant species, which could occur when its inco~oration into the Ge overlayer takes place. If this were the case, one might expect different critical thicknesses and general defect development for the As and Sb cases due to their different inco~oration rates, and yet this does not appear to be so. Indeed, a similar result has recently been reported where Te was used as a surfactant [14], which is more supportive of the defect structure being intrinsic in nature, and not nucleated by surfactant atoms incorporated into the overlayer. It is tempting to assign the observed strain relaxation with an Sb surfactant to a defect mechanism, due to the similar critical thickness and relaxation in stages as seen with As. However, we also observe a stable near-interfacial region, where relaxation does not increase with overlayer thickness, such as one would expect if the “dislocation injection” mechanism of Le Goues were in operation. Despite the similarities between this and several other independent studies, it would seem premature to suggest the observed behaviour is applicable to all growth conditions. Clearly, however, this system can exhibit dramatic changes in behaviour for only subtle alterations in conditions, and so continues to be of great interest. In order that a proper conclusion be drawn as to the detailed strain relaxation mechanism, however, it still requires a full growth and morphological study to be undertaken.
We wish to thank J. Flapper and Th. Michielsen from the Philips Research Laboratories (Eindhoven) for providing us with accurately polished Si(OO1)substrates. We would also like to thank C.C. Matthai for most helpful discussions. This work is supported by the United Kingdom Science and Engineering Research Council (SERC) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). References [I] D.A. Woolf, D.I. Westwood and R.H. Williams, J. Cryst. Growth 100 (1990) 635. [2] J.W. Matthews and A.E. Blakeslee, J. Vat. Sci. Technol. 12 (1975) 126. [3] D.J. Eaglesham and M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. f4] Y.-W. MO, D.E. Savage, B.S. Swartzentruber and M.G. Lagally, Phys. Rev. Lett. 65 (1990) 1020. [5] F.K. LeGoues, M. Cope1 and R.M. Tromp, Phys. Rev. B 42 (1990) 11690. 161 A.A. Williams, J.M.C. Thornton, J.E. Macdonald, R.G. van S&l-rout, J.F. van der Veen, M.S. Finney, A.D. Johnson and C. Norris, Phys. Rev. B 43 (1991) 5001. [7] E. Vlieg, A. van’t Ent, A.P. de Jong, H. Neerings and J.F. van der Veen, Nucl. Instrum. Methods A 262 (1987) 522. [8] A. Braslau, P.S. Pershan, G. Swislow, B.M. Ocko and J. Als-Nielsen, Phys. Rev. A 38 (1988) 2457. [9] W.F.J. Slijkerman, J.M. Gay, P.M. Zagwijn, J.F. van der Veen, J.E. Macdonald, A.A. Williams, D.J. Gravesteijn and G.F.A. van der Walle, J. Appl. Phys. 68 (1990) 5105. [lo] M. Lohmeier, E. van der Vegt, E. Vlieg and J.M.C. Thornton, in preparation. 1111 C.C. Matthai and P. Ashu, Colloq. Phys. 51 (19901 Cl873. [12] 0. Jusko, U. Kohler, G.J. Pietsch, B. Miiller and M. Henzler, unpublished. [13] D.J. Eaglesham and M. Cerullo, Appl. Phys. Lett 58 (1991) 2276. 1141 S. Higuchi and Y. Nakanishi, Surf. Sci. Lett. 254 (1991) J-465.
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Surface Science 273 (1992) l-8 North-Holland
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Strain relaxation during the surfactant modified epitaxial growth of Ge/Si( 001) J.M.C. Thornton,
A.A. Williams, J.E. Macdonald
Department of Physics, University College, Cardifi P.O. Box 913, Cardiff CFl 3TH, UK
R.G. van Silfhout FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, Netherlands
M.S. Finney and C. Norris Department of Physics, University of Leicester, Leicester LEl
7RH, UK
Received 28 November 1991; accepted for publication 20 February 1992
The initial strain relaxation of Ge on Si(OO1)has been investigated during epitaxial growth modified by a “surfactant” layer of Sb. Grazing-incidence X-ray diffraction was used to measure the strain relaxation due to its high sensitivity to the in-plane distribution of lattice spacings. We have observed the critical thickness for strain relaxation in the Ge overlayer to be - 11 monolayers (ML), with further relaxation developing gradually and in stages. A meta-stable, partially relaxed layer forms up to a coverage of - 30 ML, after which a more pronounced relaxation takes place. Even at a coverage of - 55 ML, complete strain relief has not been reached, and was only achieved after a 700°C thermal anneal. Concurrent specular reflectivity measurements also reveal that the Ge overlayer grows in a layer-by-layer fashion, and not in the Stranski-Krastanow mode expected for the Ge/Si system. These data show dramatically how the modification of surface energies through the presence of a “surfactant” can affect the morphology of, and strain in, a lattice-mismatched heteroepitaxial system.
1. Introduction
The epitaxial growth of Ge on Si has been extensively studied from a wide range of viewpoints, yet despite this, it continues to raise further questions of the physics of heteroepitaxial growth. The main avenues of approach have been to develop the properties desirable for optoelectronic and other novel device structures, and also to use it as a simple isovalent system with which to study strained layer growth. The effect of strain in a lattice-mismatched system has been known for a relatively long time. Through an elastic distortion of the overlayer, the strain energy is allowed to increase as the layer grows pseudomorphically. Eventually, however, the strain energy becomes too large to sustain, 0039-6028/92/$05.00
and at some critical thickness, the layer is able to overcome a reordering energy barrier, and thereby reach a lower energy, relaxed state. In relatively lightly strained systems (where the lattice mismatch is small, e.g. In,,GaO,&/ GaAs) the critical thickness may be many pm, at which point the film becomes substantially relaxed through dislocation 111. The prediction of the critical thickness of such systems is now fairly accurate, and has been described by various models based on the existence and propagation of dislocations in the overlayer [2]. In a more highly strained system like Ge on Si, which has a 4% lattice mismatch, the initial strain relaxation mechanism has been found to be rather different. This makes an accurate prediction of critical thickness rather more difficult, since it requires
0 1992 - Elsevier Science Publishers B.V. All rights reserved
2
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge /Si(OOl)
an in-depth understanding of the overlayer growth processes. The preferred mode of strain relaxation in the Ge/Si system is through the well known Stranski-Krastanow growth mode, whereby three-dimensional (3D) growth of islands occurs after only a few complete pseudomorphic monolayers (ML) of Ge have formed. Even though this growth mode has been known for many years, the exact nature of the islanding still remains open to question. It appears that the initial islands are coherent with the substrate, and only become dislocated when they reach a critical size 131. Under specific growth conditions, it also appears as though some elastic deformation of the Si substrate takes place in association with these islands. Perhaps the most graphic display of the complexity of this surface strain/energy system is that of the facetted “hut” clusters observed using scanning tunnelling microscopy @TM) [41. Although limited to formation under specific conditions, they are an initial mechanism to strain relaxation, perhaps forming an intermediate phase prior to achieving an equilibrium state. Moreover, their shape and distribution on the growing surface emphasize how important surface kinetics and diffusion are in overlayer growth processes, which together with the thermodynamics of surface bonding and strain due to a lattice mismatch, provide us with a highly complex system to try to understand. One recent development which highlights the delicate balance of parameters controlling the Ge/Si surface is where a surfactant species may be used to modify the growth behaviour. By depositing a monolayer of As onto the Si surface prior to the Ge deposition, it has been shown that 3D islanding can be inhibited, and so the Ge grows in a layer-by-layer fashion [5]. This phenomenon is thought to occur because the As segregates to the surface of both Si and Ge very strongly. As a result, the incoming Ge is incorporated into the Ge layer before it encounters any other mobile surface Ge, thereby pre-empting cluster formation. Previously, we have shown that the formation of Ge islands is fundamental to the strain relax-
ation mechanism, whereby the creation of elastically deformed islands coherent with the substrate permits strain relief before any dislocation takes place [6]. If islanding may be inhibited by the action of a surfactant species, the question then arises as to how the Ge now relaxes. Some morphological studies have shown that the layer becomes defected after a critical thickness, greater than that required to form islands, when a surfactant is not present, but it is by no means clear exactly how strain relaxation occurs or behaves with the formation of such defects. It is by altering the balance of surface energy and diffusion by the deposition of a surfactant layer of Sb, that we explore how the Ge layer achieves strain relaxation without the formation of islands. We have observed the strain relaxation as a function of Ge coverage directly, by measuring the in-plane lattice parameter near to the sample surface. This was performed by using in-situ grazing-incidence X-ray diffraction after each Ge deposition. The specular reflectivity was also measured in order to monitor the morphology of the overlayer as the Ge coverage increased.
2. Experimental
details
The experiments were performed using focussed radiation from the Wiggler beamline at the Synchrotron Radiation Source in DarFsbury (UK). The X-rays of wavelength A = 1.38 A were focussed using a toroidal mirror 20 m before the sample and monochromated using the (111) reflection from a channel cut silicon crystal. This gave a spectral width of Ah/h = 3 X lop4 and a beam divergence of 0.04”, which resulted in a slight degradation of resolution near Bragg peaks when compared to work on pure Ge/Si(OOl) without a surfactant layer [6]. Thus most scans were performed with an incident beam angle of 0.12”, well below the critical angle of 0.20” to minimize thermal diffuse scattering from the substrate, which peaks about reciprocal lattice points. The detector aperture subtended an angle of 0.10” at the sample. Radial scans were performed in a grazing-incidence geometry, which yields the
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge /Si(OOl)
strain distribution parallel to the interface. Specular reflectivity scans were also performed at some coverages to probe the electron density distribution at the surface and hence monitor the Sb distribution. The equipment consists of a large 5circle diffractometer, connected to an ultrahigh vacuum chamber with in-situ MBE growth, as well as standard surface science techniques [7]. The use of a 5circle diffractometer rather than an ordinary 4-circle diffractometer enlarges the accessible range of momentum transfer and allows the surface normal to lie in the horizontal plane during scans. The Si(OO1)substrates were cleaned by light sputtering with 800 eV Art ions for 60 s followed by an anneal for 3 min at 1060°C. This gave a sharp 2 x 1 diffraction pattern, observed with RHEED and X-ray diffraction. The full width at half maximum (FWHM) of the ($, 0) and (0, t> fractional order reflections, arising from the double domain 2 x 1 reconstruction, were N 0.02” for the clean surface, corresponding to an average reconstructed domain size of N 7000 A. The miscut of the surface relative to the (001) crystallographic axis was determined to be N 0.05”. The real unit-cell vectors, used to define the scattering vector Q = hb, + kb, + lb, in reciprocal space, may be related to the conventional bulk real cubic-cell vectors by a, = (1, 1, Ojcubic, ~2 = (I, - I, O)cubicand a3 = (0, 0, i)cubic. The Ge overlayers were deposited onto the Si surface at a rate of N 1 ML per 8 min from a Knudsen cell calibrated by Rutherford backscattering. The desired overlayer thickness could therefore be deposited to within a 6% margin. The chamber pressure during Ge deposition was < 2 X 1O-9 mbar and < lo-” mbar during measurements. The sample was kept at a temperature of 520 t- 10°C during deposition, and then allowed to cool to room temperature for the strain measurements. 3. Results
3
was made on the Si substrate. With the geometry used, the scattering vector can be in the plane of the sample surface, and so this type of scan is sensitive to the distribution of lattice spacings parallel to the interface. Any change in lattice spacing due to strain relaxation in the overlayer can therefore be probed directly. The lattice spacing of Ge is 4% larger than that of Si, and so in reciprocal space it is correspondingly smaller. Relaxed, bulk-like Ge may therefore be expected to be seen at a value of h = 1.92 reciprocal lattice units (r.1.u.) with respect to the (2, 0) Bragg peak for Si. Furthermore, any partially relaxed material, with a distribution of lattice spacings between these two extremes, will result in diffracted intensity within this range. The early depositions of both Sb and Ge onto the Si substrate resulted in no change to the profile of the radial scan, and hence in the distribution of in-plane lattice spacings. This is indicative of pseudomorphic growth, where the overlayer is completely strained to the lattice parameter of the substrate. After a deposition of N 11 monolayers (ML) of Ge, however, a small shoulder becomes apparent on the “large lattice spacing” side of the Bragg peak, which is attributable to the onset of strain relaxation of the Ge overlayer (fig. 1). It has previously been shown that an epitaxial Ge overlayer on SKOOl) will begin to relax after a coverage of N 3 ML has been deposited [6]. The effect of the Sb layer has therefore been to extend the range of pseudomorphic growth beyond this, up to the observed onset at N 10 ML. This is a large increase in critical thickness (19~) and is indicative of a quite different strain relaxation mechanism from the un-mediated Ge/Si(OOl) system. An interesting comparison may be made here between the growth of Ge on Si both with and without a surfactant species. Previously, when observing un-mediated growth of Ge, we have seen a broadening to the base of the (2, 0) Bragg peak. This was attributed to the curvature of rrvctal nlnnec hmmloht nhmlt hv the fnrmntinn nf
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge /Si(OOl)
4
0
34
0
ML
0 + +
0
1.90
1.95
2.00 h
0
2.05
1.90
1.95
(r.1.u.)
Fig. 1. Radial scans of the (2,O) Bragg peak as a function of coverage using an angle of incidence below the critical angle. They show the appearance of a shoulder (and so the onset of strain relaxation) at - 11 ML.
previously does not take place under these conditions. The absence of strain relaxation at N 3 ML coverage therefore affirms our earlier assertion that the formation of islands acts as a relaxation mechanism, and not that it follows as a consequence. With such a mechanism unavailable as a means of achieving strain relaxation, the Ge film continues to increase in thickness, with the strain energy increasing correspondingly. This must continue until another mechanism to strain relief becomes favourable over further pseudomorphic growth. Beyond the observed onset, the development of strain relaxation in the overlayer was monitored as a function of coverage. As can be seen in fig. 1, the shoulder to the Bragg peak gradually shifts further away and increases in intensity with overlayer coverage. After N 21 ML, a distinct feature becomes apparent at h = 1.975 r.1.u. (fig. 2) which suggests at least some meta-stability in the overlayer structure. Comparison with similar radial scans taken with angles of incidence above the critical angle (and hence at a lower surface nn.rn:+:..:h.\ -1._._.” rI.,c __ ,.:~~:F:~~_c J:IX^I^_^ ^
2.00 h
2.05
(r.1.u.)
Fig. 2. Radial scans as fig. 1, showing the development of strain relaxation with further Ge coverage. Note the feature at h = 1.975 r.1.u. for the 21 ML coverage.
ation mechanism involving a completed, equilibrium structure. At higher coverages, another feature gains intensity at h = 1.94 r.1.u. as further strain relaxation occurs (fig. 4). The growth of this peak in the distribution of lattice spacings seems to occur at the expense of the remainder, showing that a
3 ‘Z
500
W 2
0
1.90
1.95
2.00 h
2.05
(r.1.u.)
Fig. 3. Radial scans at comparable coverages to fig. 2, though mn,,r....nA *t “.? on”,, -6 ;..,:Aanr~ “l.,T.,.A lI.- r..:,:,..,, n..“ln
5
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge /Si(OOl)
1500
0
1.90
1.95
2.00 h
ih@m-d 2.05
(r.1.u.)
more complete relaxation is achieved with increasing coverage. If we again compare with the more bulk-sensitive, high-incident angle scans, it is now revealed that the feature at h = 1.97 r.1.u. has stabilized at a coverage of N 30 ML and beyond (figs. 3 and 5). The only change in intensity is the increase seen at h = 1.94 r.1.u. which
I
1.90
q ,
,
,
,
,
,
1.95
1.90
I
I I +*+ + +
,
1 I
I
,
(
,
,
,
,
0 520°C
1.95
2.00
2.05
h (r.l.u.)
Fig. 4. Radial scans as fig. 2 showing the development of strain relaxation with further Ge coverage. Note the peak forming at h = 1.94 r.l.u., which does not correspond to a complete strain relaxation, and the reducing intensity at h = 1.975 i.1.u..
1500
I
,
, , , , , + + 43 ML _
2.00
x 47 ML
_
o55ML
-
2.05
h (r.1.u.) Fig. 5. Radial scans as fig. 4, but measured at an angle of incidence above the critical angle. The intensity at h = 1.970 r.1.u. remains constant, while the development in relaxation is manifested by an increase at h = 1.94 r.1.u..
Fig. 6. Above critical angle, radial scans at a coverage of - 55 ML, showing the effect of a thermal anneal at 700°C (compared to the 520°C growth temperature). The anneal has driven a more complete strain relaxation to near bulk-like Ge (h = 1.92 r.1.u.) and the intensity due to partially relaxed material has diminished.
continues with increasing coverage. The material near to the surface of the Ge film is clearly more relaxed than that closer to the interface, even at our highest coverages. The overall picture, therefore, is one where a second stage of strain relaxation takes place over a stable, partially relaxed layer of - 30 ML in thickness. This is an interesting result, since there is no evidence here for a mechanism where strain relaxation takes place throughout the entire overlayer. It is interesting to note the effect of a thermal anneal at this stage. A 10 min anneal at 700°C resulted in the lattice spacing distribution seen in fig. 6. A clear jump to complete strain relaxation (at h = 1.92 r.1.u.) has occurred for most of the material, as though the higher temperature has allowed any defects or dislocations to become complete throughout the Ge overlayer. The previously stable layer close to the Si: Ge interface has been annihilated, together with any residual strain in the overlayer. The observed strain relaxation behaviour is clearly very different from the case where Ge is grown on the Si(OO1)without a surfactant species. The onset of strain relaxation has been delayed from -3 to - 11 ML, and the relaxation has
.I.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge / SifOOii
ti
occurred much more gradually, over a range of > 55 ML, rather than the N 11 ML seen previously [6].
Concurrent with the radial scans were measurements of the specular reflectivity as a function of incident angle. This technique is sensitive to the electron density near to the sample surface, and so may be used as a measure of surface roughness, morphology and also of overlayer intermixing. The reflectivity curve obtained from the clean Si(OO1) 2 x 1 surface can be seen to be smooth and homogeneous, as expected from such a surface (fig. 7). Fitting this curve using an electron density continuum model yields a surface roughness of 0.4 ML [8]. A very similar curve was obtained from the surface after the deposition, to saturation coverage, of N 0.7 ML Sb. The main difference between the curves is an increase in the reflectivity over the entire range observed. This is to be expected if a near complete overlayer of a higher electron density material is present on the surface. Using the above continuum model, a surface layer of 0.73 ML of Sb was
2
10’
i
0.00
0.05
0.10
0.15 I (r.1.u.)
0.20
0.25
0.30
Fig. 7. Specular reflectivity curves showing the effect of an overlayer of Sb (0.7 ML) on the Si surface. A subsequent 13 ML deposition of Ge results in oscillations in intensity as a function of incident angle, which are due to the overlayer forming in a layer-by-layer fashion. The continuous line is a curve fitted using an electron density continuum model.
0.00
0.05
0.10
0.15
0.20
0.25
0.30
1 (r.1.u.) Fig. 8. Specular reflectivity curves as a showing how the oscillations, and so mode, continues even to a coverage of how the period of oscillation decreases layer thickness.
function of coverage, the uniform growth - 55 ML. Notice too with increasing over-
found to give the best fit to the curve, in good agreement with the known saturation coverage [9]. Hiowever, an increase in the RMS surface roughness to N 1.5 ML had to be introduced, showing some disordering of the Si surface by the Sb. With the deposition of Ge, a dramatic change was observed in the reflectivity. Very clear, periodic oscillations in intensity with incident angle were seen, which are indicative of layer-by-layer growth of the Ge (figs. 7 and 8). The oscillations are due to interference between reflected intensity from the complete layers, with the period being determined by the number of unit cells perpendicular to the Si/Ge interface. The period can be seen to become smaller as coverage increases (fig. 8). If uncertainty in the number of cells is introduced by roughening or islanding, a superposition of different periods would create a more homogeneous curve [6]. As seen in figs. 7 and 8, these curves can be fitted very closely using the same continuum model as before, which assumes a complete overlayer without islanding. Interestingly, the Si/Ge, Ge/Sb and Sb/vacuum interfaces are all found to require only small roughnesses to obtain a best fit. The Ge/Sb interface, for example, has an RMS roughness of - l-l.5 ML for all the Ge
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge/Si(OOl)
coverages. This is not surprising if one considers the strong ordering seen recently for the Sb : Ge(OO1) 2 x 1 surface [lo]. Furthermore, the fit reveals that the Sb coverage remains constant at 0.73 ML for all Ge coverages, showing that incorporation of Sb into the Ge overlayer can only be negligible. The effect of a thermal anneal also manifests itself in the reflectivity as well as in the radial scan data. It was observed that a 10 min, 700°C anneal hardly affected the reflectivity from the 55 ML overlayer. A slight decrease in the strength of oscillation was observed, commensurate with some roughening or disorder at the surface. When combined with the radial scans, this shows that the anneal merely develops the relaxation mechanism further in the Ge, leaving a complete overlayer without any significant 3D islanding. The clear result, therefore, is one where the Sb surface segregates very strongly, driving the Ge sub-surface. This inhibits 3D islanding, driving the Ge overlayer to grow in a layer-by-layer fashion, and independently of overlayer thickness.
4. Conclusions These experiments have shown that the presence of a “surfactant” species can dramatically alter the morphology, growth-mode and the strain relaxation mechanism of a heteroepitaxial growth system. Using Sb to alter the growth surface energy, the Ge/Si(OOl) system has been found to grow under some conditions in a uniform, layerby-layer fashion, as opposed to the StranskiKrastanow mode seen when it is not present. The change in growth mode has also forced the strain relaxation mechanism to change, thereby revealing how closely the surface morphology and growth kinetics are linked with strain relaxation. From a critical thickness of - 3 ML when a surfactant is not present, the onset of strain relaxation has been increased to - 11 ML. This is direct evidence that the initial strain relaxation during unmodified growth is due to the formation of islands. When islanding is inhibited, such a
I
strain relaxation mechanism is no longer possible, so the layer must continue to grow under increasing strain until another mechanism becomes more favourable than further pseudomorphic growth. The precise mechanism for strain relaxation in this system could be one of a number of possibilities. It may be argued that the system has adopted the more familiar misfit dislocation mechanism to strain relaxation. Indeed, the predicted critical thickness for the dislocation mechanism is approximately 10 ML, as observed [ll]. If one considers the results of some similar modified-growth studies of this system, where As has been used as a surfactant, a rather different possibility from dislocation emerges. Ion-scattering and electron microscopy studies performed by Le Goues et al. have shown strain relief to occur in the Ge when “V-shaped”, defects form throughout the overlayer, again approximately at our observed critical thickness 151.These defects seem to develop with Ge coverage, leading to the injection of dislocations, finally relieving all strain after - 60 ML. This too is in rough agreement with our data, which shows near complete relaxation at the Ge surface after a deposition of - 55 ML. More recent STM studies by Jusko et al. point to a variation on this theme however [12]. They observe the formation of the defects at the critical thickness, but rather than developing with the thickness of the overlayer, the defects are completely stable. Furthermore, no Ge growth is observed over these defects after their creation, with Ge accumulation only taking place between them. The resulting morphology is one where rectangular islands of Ge form, with trenches separating them. High Ge mobility along the trenches causes the infilling to initiate at the trench junctions, from where the growth develops, finally leading to a relaxed Ge overlayer. It is reasonable to assume that Sb acts as a surfactant in the same way as As, and so a relaxation mechanism close to those described above may well be the case. One must consider the problem at a more fundamental level, however, if a complete understanding is to be achieved. It has been assumed that the primary action of a surfactant species has been to inhibit 3D island growth on the substrate surface. This
8
J.M.C. Thornton et al. / Strain relaxation during surfactant modified epitaxial growth of Ge/Si(OOl)
occurs due to very active surface segregation of the surfactant, which drives the incoming Ge to grow sub-surface, thus preventing surface clustering. Surface clustering can also be inhibited thermally, as shown by Eaglesham and Cerullo, yet they do not observe any novel defect formation, only misfit dislocation [13]. This may suggest a defect nucleation role for the surfactant species, which could occur when its inco~oration into the Ge overlayer takes place. If this were the case, one might expect different critical thicknesses and general defect development for the As and Sb cases due to their different inco~oration rates, and yet this does not appear to be so. Indeed, a similar result has recently been reported where Te was used as a surfactant [14], which is more supportive of the defect structure being intrinsic in nature, and not nucleated by surfactant atoms incorporated into the overlayer. It is tempting to assign the observed strain relaxation with an Sb surfactant to a defect mechanism, due to the similar critical thickness and relaxation in stages as seen with As. However, we also observe a stable near-interfacial region, where relaxation does not increase with overlayer thickness, such as one would expect if the “dislocation injection” mechanism of Le Goues were in operation. Despite the similarities between this and several other independent studies, it would seem premature to suggest the observed behaviour is applicable to all growth conditions. Clearly, however, this system can exhibit dramatic changes in behaviour for only subtle alterations in conditions, and so continues to be of great interest. In order that a proper conclusion be drawn as to the detailed strain relaxation mechanism, however, it still requires a full growth and morphological study to be undertaken.
We wish to thank J. Flapper and Th. Michielsen from the Philips Research Laboratories (Eindhoven) for providing us with accurately polished Si(OO1)substrates. We would also like to thank C.C. Matthai for most helpful discussions. This work is supported by the United Kingdom Science and Engineering Research Council (SERC) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). References [I] D.A. Woolf, D.I. Westwood and R.H. Williams, J. Cryst. Growth 100 (1990) 635. [2] J.W. Matthews and A.E. Blakeslee, J. Vat. Sci. Technol. 12 (1975) 126. [3] D.J. Eaglesham and M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. f4] Y.-W. MO, D.E. Savage, B.S. Swartzentruber and M.G. Lagally, Phys. Rev. Lett. 65 (1990) 1020. [5] F.K. LeGoues, M. Cope1 and R.M. Tromp, Phys. Rev. B 42 (1990) 11690. 161 A.A. Williams, J.M.C. Thornton, J.E. Macdonald, R.G. van S&l-rout, J.F. van der Veen, M.S. Finney, A.D. Johnson and C. Norris, Phys. Rev. B 43 (1991) 5001. [7] E. Vlieg, A. van’t Ent, A.P. de Jong, H. Neerings and J.F. van der Veen, Nucl. Instrum. Methods A 262 (1987) 522. [8] A. Braslau, P.S. Pershan, G. Swislow, B.M. Ocko and J. Als-Nielsen, Phys. Rev. A 38 (1988) 2457. [9] W.F.J. Slijkerman, J.M. Gay, P.M. Zagwijn, J.F. van der Veen, J.E. Macdonald, A.A. Williams, D.J. Gravesteijn and G.F.A. van der Walle, J. Appl. Phys. 68 (1990) 5105. [lo] M. Lohmeier, E. van der Vegt, E. Vlieg and J.M.C. Thornton, in preparation. 1111 C.C. Matthai and P. Ashu, Colloq. Phys. 51 (19901 Cl873. [12] 0. Jusko, U. Kohler, G.J. Pietsch, B. Miiller and M. Henzler, unpublished. [13] D.J. Eaglesham and M. Cerullo, Appl. Phys. Lett 58 (1991) 2276. 1141 S. Higuchi and Y. Nakanishi, Surf. Sci. Lett. 254 (1991) J-465.