- Email: [email protected]
MBE growth and structural characterization of Si1 − yCySi1 − xGex superlattices
Journal of Crystal Growth 175/176 (1997)4599464
ELSEVIER
MBE growth and structural characterization of Si 1 _ ,C,/Sil _ xGex superlattices S. Zerlauth”, Institut
J. Stangl, A.A. Darhuber,
ftir Halbleiterphysik,
V. Holfl,
G. Bauer, F. Schiiffler
Johannes Kepler University Linz, Altenbergerstrasse
69, A-4040 Linz, Austria
Abstract
We report on the MBE growth and X-ray characterization of Sil _,C,/Si, _xGe, superlattices (SLs). The concentrations and thicknesses of the layers were chosen such as to get strain-symmetrized superlattices, lattice-matched to the Si (0 0 1) substrates. In-situ RHEED investigations showed increasing roughness during the growth of the Sii _YC, layers and a smoothing effect of the subsequent Sir _xGex layers. Further characterizations comprised double and triple axis X-ray diffraction, X-ray refraction (XRR), and atomic force microscopy (AFM). Dynamical simulations of the various X-ray configurations yielded the structural parameters of the SLs as well as information on the morphological and replication properties of the interfaces. With increasing carbon content we found an overall interface roughening concomitant with a significant decrease of the replication length. Lateral correlation length fits were compared with AFM-measurements, which give a more detailed picture of the interface morphology as long as the replication lengths are large. PAC.9
68.55.Bd; 68.65. + g; 68.35.B~; 61.1O.L~
Keywords;
MBE; Silicon; Germanium;
Carbon; Superlattice; Interface roughness; X-ray defraction; AFM
The search for higher performance levels within a silicon-dominated world of electronic circuits has led to widespread interest in heterostructures based on group-IV semiconductors. Besides the Si/SiGe heterobipolar transistor, which is at the very brink of commercial availability [l], mobility enhancement by modulation doping [2], and band-struc*Corresponding author. Fax: + 43 732 2468 650; e-mail: [email protected]. 1Permanent address: Faculty of Science, Masaryk University, Kotlarski 2, 61137 Brno. 0022-0248/97/$17.00 Copyright PII SOO22-0248(96)00931-l
ture engineering for optical applications [3] are major research targets. Modulation doped n- and p-type Si/SiGe and Si/SiGe/Ge heterostructures with drastically increased carrier mobilities have been reported [4, 51. For proper adjustment of the strain-dependent band offsets [6] concomitant with the use of Si substrates, strain-relaxed Sii _xGex buffer layers are usually introduced between the substrate and the active layers. Although very successful on a laboratory scale [7], control of the buffer-mediated defect densities, and especially of threading
$3 1997 ElsevierScience B.V. All rights reserved
460
S. Zerlauth et al. /Journal
qf Cyvstal Growth I75:176 (1997) 459-464
dislocations, might become a problem for largescale integration. The substitutional incorporation of a small percentage of carbon has been proposed as an alternative means of strain adjustment [S, 91, which might allow entirely pseudomorphic layer sequences: Since the covalent radius of Si is larger than that of C, but smaller than the one of Ge, the coherent Sir _,_,Ge,C,/Si heterosystem allows the realization of both tensile and compressive in-plane strains. At a ratio x/y z 8.5 the Si lattice constant is preserved. If this ratio is maintained on the average, strain symmetrized Si, _,.C, /Si, _xGex superlattices (SL) with unlimited overall thickness become possible, provided the individual layer thicknesses remain below the critical thickness for the generation of misfit dislocations [lo]. This is an important feature especially for optical applications, where large enough active volumes are required. Despite the obvious advantages of ternary Si, _,_pGexCp compounds, the available information on structural and electronic properties [11] are quite rudimentary, as yet. Of special concern is the almost negligible solubility limit of C in Si and Ge, concomitant with the existence of stochiometric SIC. This necessitates non-equilibrium (low temperature) growth techniques for the realization of useful C concentrations in the low percent range [ 121. Interface morphology is another important topic for any kind of heterostructure, especially for lattice-mismatched heterosystems, where built-in strain can have a strong effect on the morphology of the growth front [13]. In the following we will restrict ourselves to an investigation of the interface roughness and replication of Sii _,C,/Si, _xGex superlattices grown at temperatures between 400 C and 500 C by solid-source MBE. Because of the possible interference with future modulation doping experiments, no surfactants [14] were employed in this study. Growth was performed in a two-chamber RiberSIVA45 MBE machine with a base pressure of lo- ‘” mbar. Three electron beam evaporators for Si, Ge and C are employed, which are feedbackcontrolled by a common quadrupole mass spectrometer. 4” Si(0 0 1) substrates with a miscut < 0.25 were suspended in all-silicon mounts underneath a radiation heater made of pyrolitic graphite. The preparation steps comprised thermal
Table I Growth and structural parameters of the ten-period Si, _,.C,iSi, _.Ge, superlattice samples: compositions and layer thicknesses are derived from X-ray rocking curves; T, is the substrate temperature during the SL growth Sample
)’(‘%)
x (‘Y0)
tls,c (A)
dSi& (A,
T,
SC14 SGCl8 SGC63 SGCZI SGC25
0 I 1.4 2 2.5
15 16 9.5 15 I5
I65 116 87 84 16
55 57 87 95 IO0
410 410 500 410 410
( C)
desorption of the naJura1 oxide at 900 C and the deposition of a 3000 A thick buffer layer at l-2 A/s, which was grown during the cool-down ramp to the growth temperature of the subsequent ten-period Sir _,C,/Si, _,Ge, superlattice (see Table 1). In a series of five samples (Table 1) the carbon content was systematically increased from 0% to 2.5%, whereas the Ge content was kept constant at 15% (except for sample SGC63 with x = 9.5%). With increasing carbon content the thickness of the Si, _$Z, layers was reduced in order to stay within the critical thickness limitation for the individual layers. The thickness of the Si, _,Ge, layers were adjusted properly in order to maintain strain symmetrization [lo]. The surface was continuously monitored by reflection high-energy electron diffraction (RHEED), which revealed a clear (2 x 1) reconstruction during buffer growth. In agreement with previous reports [15], deposition of the Sir _,C, layers led consistently to a spotty RHEED pattern, which recovered gradually during the growth of the subsequent Si, _xGex layer. Generally, the RHEED pattern became more spotty with increasing number of periods, and this effect was more pronounced for y > 1%. Under such conditions the RHEED patterns after the tenth period, although ending with a Sii _xGex layer, were indicative of a rough (3D) surface. The structural properties of our samples were determined by high-resolution X-ray diffraction (HRXRD) around the symmetric (0 0 4) reflex. In Fig. 1 we show rocking curves with increasing C concentration. The overlap of the zeroth order
S. Zerlauth et al. /Journal
ofCtystal Growth 175/I 7h (1997) 459-464
461
E”“““““‘I (lRR
ok
Si-buffer Si-substrate
1
Laue-Zunc
I
\ (0001
-1 5
-1 0
-0 5
0.0
0.5
IO
15
A w (de@
Fig. 1. HRXRD measurements of strain-compensated superlattices with different carbon contents and layer thicknesses, lattice matched to the silicon substrate. Also included is a simulation of the lowest curve.
SL signal with the substrate peak reveals almost perfect strain symmetrization for the samples with JJ > 0. The layer thicknesses and compositions in Table 1 were deduced from dynamical simulations that were fitted to the rocking curves. As an example, such a fit is included in Fig. 1 for sample SGC25. In addition, reciprocal space maps around the (0 0 4) and (2 2 4) reflex were recorded to make sure that the SLs are fully pseudomorphic. These experiments also showed that possible tilt angles of the epi-layer with respect to the substrate are very small (below l/1000 degree), and that no indication of mosaicity exists in the samples. To get further insight into the properties of the interfaces, XRR experiments were carried out at the OPTICS beamline at ESRF in Grenoble, and at the D4.1 beamline at Hasylab, Hamburg. For a concise characterization different geometries were utilized, which are schematically illustrated in Fig. 2. In a first configuration the intensity of the specular peak was recorded as a function of the incidence angle (w-28 scan). This scan is mainly used for deriving the overall root mean square (RMS)
Fig. 2. The different XRR scan modes employed for the extraction of the lateral and vertical correlation length are schematically depicted in reciprocal space as bold arrows. Black dots mark the SL peaks in (00 1) growth direction, and the elongated, shaded areas adjacent to each peak are due to diffuse scattering. The Laue zones are not accekble experimentally in this regime.
roughness gs of the SL interfaces [16. 173. In another mode, the diffusely scattered intensity was measured, which is sensitive to the vertical and in-plane correlation of the interface morphology. The vertical correlation length /1, represents the replication of the interface morphology along the growth direction and is derived from non-specular 20 scans, where the incidence angle w is kept constant. Pure w scans yield the in-plane correlation length il,, as well as an RMS roughness (TV.Generally, gs and o-~are not identical, because the overall interface roughness can have contributions from the pure morphology and from interdiffusion of the alloys across the interface. The specular reflex (cr,) is sensitive to both effects, whereas diffuse scattering is not affected by interdiffusion. Hence, cd is expected to be representative of the interface morphology, and should therefore be better suited for a comparison with atomic force micrographs. Fig. 3 shows XRR spectra and simulation curves of sample SGC25 in these three scan configurations. The upper panel depicts the specular (u--20 scan, which reveals several orders of SL reflexes. Their spacing depends on layer thicknesses and compositions similar to the diffraction curves in Fig. 1. The attenuation of these peaks with increasing order is
a measure of g,, which is deduced from the fits. In the center frame a diffuse 20 scan is shown, which consists of a sharp peak (specular reflex with tc) = U), and a number of smaller side maxima re-
10'1
h:'
lo-s""'L 0.0
11
0.5
”
”
”
1.0
w-scan, od=7A,
”
w(deg)
C ' 1.5
I
Ap=2000A
1
0.5
o(deg)
1.0
Fig. 3. (81 20 scan, 20 scan. and (r) scan of sample SGC25. Both measured (points) and simulated (lines) curves are shown, which are off-set for reasons of clarity.
Table 2 Interface parameters of two representative measurements at the SL surface
SGC63
SGC25
I .4 2.5
samples obtained
1200 f 500 300 * 100
sulting from resonant diffuse scattering (RDS). The latter is caused by the correlation of the roughness profiles of different interfaces. The replication length A, is deduced from fits to the shape of the RDS peaks, employing a distorted-wave Born approximation (DWBA) as described elsewhere [IS, 191. Since the height of the RDS peaks is also affected by other irregularities of the superlattice the fitted values listed in Table 2 should rather be read as a lower limit for /1,. Finally, the lowest frame in Fig. 3 shows (0 scans through the two labeled peaks seen in the (0-20 scan, and the fitted simulation curves for the extraction of nP and gd. Because of problems with the calibration of the primary intensity, the error margins of the in-plane correlation lengths are rather large. A compilation of the interface parameters for two representative samples with 2’= 1.4% and 2.5% are listed in Table 2. The most striking result of the XRR experiments is a systematic decrease of the replication length as ~1is increased. We get /1, values of 1200, 500 and 300 A for carbon concentrations of I 1.5%, 2% and 2.5%. respectively. Thus, while the /1, values at 4’I 1.5% are comparable to the thickness of the SL, a replication of the interface roughness is suppressed after less than two periods at y = 2.5%. In connection with the aforementioned RHEED observations these results lead to the conclusion that the pronounced vertical replication found by several groups in Si/Si, _xGe_YSLs [20, 211 is strongly disturbed in the presence of carbon-containing interlayers. Higher JJ, which enhance roughening of the Sii _,C,. layers according to RHEED, are obviously more effective in screening the replication process between subsequent Si i _ xGe,x layers. The reduction of il,, with increasing y is accompanied by an overall increase of the interface roughness. The respective RHEED results are quantitatively confirmed by the cr values derived
by lita to the XRR measurements.
1900 + 100 2000 t_ 1000
9+2 7*1
and RMS roughness
4il 712
obtained
4*2 IO + 2
by AFM
S. Zrrlauth
4
3
2
et al. /Journal
1
qf‘ctystal
0
Pm Fig. 4. AFM image of sample SGC63 features superimposed on a short-length
containing graininess.
hillock-like
from the XRR measurements, which yield a factor of two increase when y is increased from 1.4% to 2.5% (Table 2). But s$ll, the fitted RMS roughness values of less than 10 A appear rather small regarding the 3D appearance of the RHEED pattern at y = 2.5%. We therefore performed AFM measurements at the surfaces of the two SLs listed in Table 2. Since the morphology of the topmost Sir _xGex surface is probed, reasonable agreement between AFM and XRR RMS roughnesses is only to be expected for sample SGC63 (y = 1.4%) with a replication length comparable to the thickness of the SL. In the case of sample SGC25 (x = 2.5%), gAFMis expected to be larger than od, since the XRR measurements average over all interfaces, with the lower-lying periods being smoother according to RHEED observations. In fact, we find good agreement between oAi+, and (7d for SGC63. but a significantly (30%) higher AFM roughness for SGC25. Although restricted to the topmost layer, AFM yields additional information, because it provides a quantitative image of a several thousand urn2 large portion of the surface. Fig. 4 shows an AFM image of sample SGC63, which clearly reveals a hillock-like structure with lateral spacing on the
Growth 175,‘176 (1997) 459-464
463
order of l-urn, which is superimposed on a shortrange graininess with a length scale of a few hundred to 1000 A. In contrast, the XRR measurements yielded a /1, value of just about 2000 A for this sample. This means that the hillock feature is outside the sensitivity window of XRR, which is basically determined by the divergence of the beamline. In addition, the AFM line scans can also reconcile the 3D RHEED patterns with the surprisingly small RMS roughnesses: Due to the large average spacing of the hillock features, modulation heights are on the order of 20 A, which is sufficient to explain the RHEED observations, but still yields an RMS roughness of a mere 4 A. Even larger amplitudes are observed in the L’= 2.5% sample (SGC25), which. however, lacked the long-range hillock features. In this sample we just found a short-range graininess, the length scale of which might be just within the error margins given for nP. In either case, the RMS roughnesses determined by XRR have to be treated with care, since they are not easily comparable to each other as long as the lateral correlation of a surface or interface is not well characterized. Also, it became clear that the limited length scale on which meaningful lateral correlation lengths can be derived from XRR measurements may require additional techniques, such as AFM, in order to get a complete picture. In summary, we grew a series of strain-symmetrized, pseudomorphic Si 1 ,.C,/Si i JGex superlattices with varying C content.y, and characterized the interface properties by X-ray techniques. Upon increasing the carbon content from 0% to 2.5% the vertical correlation length .4, was found to decrease from a value comparable to the total thickness of the SL to a value comparable to a single-period length. Simultaneously, the RMS roughness increased by about a factor of two. According to AFM measurements the roughening is concomitant with a change of the interface morphology: With increasing carbon content long range features, which are tentatively attributed to strain induced growth phenomena at the Sil _xGex surfaces, are suppressed by the Si i _ J, interlayers. Financial support by the FWF, GMe, and BMWVK (Vienna), and the opportunity to use
464
S. Zdauth
the OPTICS beamline beamline at Hasylab, acknowledged.
in Grenoble Hamburg,
et al. i Journal
r$Cgbstal
and the D4.1 are gratefully
References [I] [2] [3]
[4] [S] [6] [7]
[8]
Review: A. Schuppen and H. Dietrich, J. Crystal Growth 157 (1995) 207. Review: F. Schaffler. Solid State Electron, 37 (1994) 765. Review: H. Presting. H. Kibbel, M. Jaros, R.M. Turton, U. Menczigar. G. Abatreiter and H.G. Grimmeiss, J. Vat. Sci. Technol. 7 (1992)I 127. K. Ismail. M. Arafa, F. Stern, J.O. Chu and B.S Meyerson, Appl. Phys. Lett. 66 (1995) 842. U. Konig and F. Schiffler, IEEE Electron. Dev. Lett. EDL-14 (1993) 205. G. Abstreiter. H. Brugger, T. Wolf, H. Jorke and H.-J. Herzog, Phys. Rev. Lctt. 54 (1985) 2441. Y.J. Mii, Y.H. Xie. E.A. Fitzgerald, D. Monroe, F.A. Thiel, B.E. Weir and L.C. Feldman. Appl. Phys. Lett. 59 (1991) 1611. K. Eberl. S.S. Iyer and F.K. LeGoues. Appl. Phys. Lett. 64 (1994) 739.
Growth
175/l 76 (1997) 459-464
[9] W. Faschinger, S. Zerlauth, G. Bauer and L. Palmetshofer, Appl. Phys. Lett. 67 (1995) 3933. [lo] E. Kasper and F. Schaffler, in: Semiconductors and Semimetals, Vol. 33, Ed. T.P. Pearsall (Academic Press. Boston, 1990) p. 241. [1 I] SC. Jain, H.J. Osten, B. Dietrich and H. Rucker, Semicond. Sci. Technol. 10 (1995) 1289. [12] A.R. Powell, K. Eberl, F.E. LeGoues, B.A. Ek and S.S. Iyer, J. Vat. Sci. Technol. Bll (1993) 1064. 1131 A.J. Pidduck, D.J. Robbins, A.G. Cullis, W.Y. Leong and A.M. Pitt, Thin Solid Films 222 (1992) 78. [14] H.J. Osten, J. Klatt, G. Lippert and E. Bugiel, J. Appl. Phys. 74 (1993) 2507. [15] P.O. Pettersson, C.C. Ahn, T.C. McGill, E.T. Croke and A.T. Hunter, Appl. Phys. Lett. 67 (1995) 2530. [16] G. Parrat, Phys. Rev. 95 (1964) 359. [17] L. Nevot and P. Croce, Rev. Phys. Appl. 15 (1980) 761. [l S] V. Holy and T. Baumbach, Phys. Rev. B 49 (1994) 10688. 1191 SK. Sinha, E.B. Sirota. S. Garoff and H.B. Stanley, Phys. Rev. B 38 (1988) 2297. [20] R.L. Headrick. J.-M. Baribeau and Y.E. Strausser, Appl. Phys. Lett. 66 (1995) 96. 1211 J. Tersoff, C. Teichert and M.G. Lagally, Phys. Rev. Lett. 76 (1996) 1675.
ELSEVIER
MBE growth and structural characterization of Si 1 _ ,C,/Sil _ xGex superlattices S. Zerlauth”, Institut
J. Stangl, A.A. Darhuber,
ftir Halbleiterphysik,
V. Holfl,
G. Bauer, F. Schiiffler
Johannes Kepler University Linz, Altenbergerstrasse
69, A-4040 Linz, Austria
Abstract
We report on the MBE growth and X-ray characterization of Sil _,C,/Si, _xGe, superlattices (SLs). The concentrations and thicknesses of the layers were chosen such as to get strain-symmetrized superlattices, lattice-matched to the Si (0 0 1) substrates. In-situ RHEED investigations showed increasing roughness during the growth of the Sii _YC, layers and a smoothing effect of the subsequent Sir _xGex layers. Further characterizations comprised double and triple axis X-ray diffraction, X-ray refraction (XRR), and atomic force microscopy (AFM). Dynamical simulations of the various X-ray configurations yielded the structural parameters of the SLs as well as information on the morphological and replication properties of the interfaces. With increasing carbon content we found an overall interface roughening concomitant with a significant decrease of the replication length. Lateral correlation length fits were compared with AFM-measurements, which give a more detailed picture of the interface morphology as long as the replication lengths are large. PAC.9
68.55.Bd; 68.65. + g; 68.35.B~; 61.1O.L~
Keywords;
MBE; Silicon; Germanium;
Carbon; Superlattice; Interface roughness; X-ray defraction; AFM
The search for higher performance levels within a silicon-dominated world of electronic circuits has led to widespread interest in heterostructures based on group-IV semiconductors. Besides the Si/SiGe heterobipolar transistor, which is at the very brink of commercial availability [l], mobility enhancement by modulation doping [2], and band-struc*Corresponding author. Fax: + 43 732 2468 650; e-mail: [email protected]. 1Permanent address: Faculty of Science, Masaryk University, Kotlarski 2, 61137 Brno. 0022-0248/97/$17.00 Copyright PII SOO22-0248(96)00931-l
ture engineering for optical applications [3] are major research targets. Modulation doped n- and p-type Si/SiGe and Si/SiGe/Ge heterostructures with drastically increased carrier mobilities have been reported [4, 51. For proper adjustment of the strain-dependent band offsets [6] concomitant with the use of Si substrates, strain-relaxed Sii _xGex buffer layers are usually introduced between the substrate and the active layers. Although very successful on a laboratory scale [7], control of the buffer-mediated defect densities, and especially of threading
$3 1997 ElsevierScience B.V. All rights reserved
460
S. Zerlauth et al. /Journal
qf Cyvstal Growth I75:176 (1997) 459-464
dislocations, might become a problem for largescale integration. The substitutional incorporation of a small percentage of carbon has been proposed as an alternative means of strain adjustment [S, 91, which might allow entirely pseudomorphic layer sequences: Since the covalent radius of Si is larger than that of C, but smaller than the one of Ge, the coherent Sir _,_,Ge,C,/Si heterosystem allows the realization of both tensile and compressive in-plane strains. At a ratio x/y z 8.5 the Si lattice constant is preserved. If this ratio is maintained on the average, strain symmetrized Si, _,.C, /Si, _xGex superlattices (SL) with unlimited overall thickness become possible, provided the individual layer thicknesses remain below the critical thickness for the generation of misfit dislocations [lo]. This is an important feature especially for optical applications, where large enough active volumes are required. Despite the obvious advantages of ternary Si, _,_pGexCp compounds, the available information on structural and electronic properties [11] are quite rudimentary, as yet. Of special concern is the almost negligible solubility limit of C in Si and Ge, concomitant with the existence of stochiometric SIC. This necessitates non-equilibrium (low temperature) growth techniques for the realization of useful C concentrations in the low percent range [ 121. Interface morphology is another important topic for any kind of heterostructure, especially for lattice-mismatched heterosystems, where built-in strain can have a strong effect on the morphology of the growth front [13]. In the following we will restrict ourselves to an investigation of the interface roughness and replication of Sii _,C,/Si, _xGex superlattices grown at temperatures between 400 C and 500 C by solid-source MBE. Because of the possible interference with future modulation doping experiments, no surfactants [14] were employed in this study. Growth was performed in a two-chamber RiberSIVA45 MBE machine with a base pressure of lo- ‘” mbar. Three electron beam evaporators for Si, Ge and C are employed, which are feedbackcontrolled by a common quadrupole mass spectrometer. 4” Si(0 0 1) substrates with a miscut < 0.25 were suspended in all-silicon mounts underneath a radiation heater made of pyrolitic graphite. The preparation steps comprised thermal
Table I Growth and structural parameters of the ten-period Si, _,.C,iSi, _.Ge, superlattice samples: compositions and layer thicknesses are derived from X-ray rocking curves; T, is the substrate temperature during the SL growth Sample
)’(‘%)
x (‘Y0)
tls,c (A)
dSi& (A,
T,
SC14 SGCl8 SGC63 SGCZI SGC25
0 I 1.4 2 2.5
15 16 9.5 15 I5
I65 116 87 84 16
55 57 87 95 IO0
410 410 500 410 410
( C)
desorption of the naJura1 oxide at 900 C and the deposition of a 3000 A thick buffer layer at l-2 A/s, which was grown during the cool-down ramp to the growth temperature of the subsequent ten-period Sir _,C,/Si, _,Ge, superlattice (see Table 1). In a series of five samples (Table 1) the carbon content was systematically increased from 0% to 2.5%, whereas the Ge content was kept constant at 15% (except for sample SGC63 with x = 9.5%). With increasing carbon content the thickness of the Si, _$Z, layers was reduced in order to stay within the critical thickness limitation for the individual layers. The thickness of the Si, _,Ge, layers were adjusted properly in order to maintain strain symmetrization [lo]. The surface was continuously monitored by reflection high-energy electron diffraction (RHEED), which revealed a clear (2 x 1) reconstruction during buffer growth. In agreement with previous reports [15], deposition of the Sir _,C, layers led consistently to a spotty RHEED pattern, which recovered gradually during the growth of the subsequent Si, _xGex layer. Generally, the RHEED pattern became more spotty with increasing number of periods, and this effect was more pronounced for y > 1%. Under such conditions the RHEED patterns after the tenth period, although ending with a Sii _xGex layer, were indicative of a rough (3D) surface. The structural properties of our samples were determined by high-resolution X-ray diffraction (HRXRD) around the symmetric (0 0 4) reflex. In Fig. 1 we show rocking curves with increasing C concentration. The overlap of the zeroth order
S. Zerlauth et al. /Journal
ofCtystal Growth 175/I 7h (1997) 459-464
461
E”“““““‘I (lRR
ok
Si-buffer Si-substrate
1
Laue-Zunc
I
\ (0001
-1 5
-1 0
-0 5
0.0
0.5
IO
15
A w (de@
Fig. 1. HRXRD measurements of strain-compensated superlattices with different carbon contents and layer thicknesses, lattice matched to the silicon substrate. Also included is a simulation of the lowest curve.
SL signal with the substrate peak reveals almost perfect strain symmetrization for the samples with JJ > 0. The layer thicknesses and compositions in Table 1 were deduced from dynamical simulations that were fitted to the rocking curves. As an example, such a fit is included in Fig. 1 for sample SGC25. In addition, reciprocal space maps around the (0 0 4) and (2 2 4) reflex were recorded to make sure that the SLs are fully pseudomorphic. These experiments also showed that possible tilt angles of the epi-layer with respect to the substrate are very small (below l/1000 degree), and that no indication of mosaicity exists in the samples. To get further insight into the properties of the interfaces, XRR experiments were carried out at the OPTICS beamline at ESRF in Grenoble, and at the D4.1 beamline at Hasylab, Hamburg. For a concise characterization different geometries were utilized, which are schematically illustrated in Fig. 2. In a first configuration the intensity of the specular peak was recorded as a function of the incidence angle (w-28 scan). This scan is mainly used for deriving the overall root mean square (RMS)
Fig. 2. The different XRR scan modes employed for the extraction of the lateral and vertical correlation length are schematically depicted in reciprocal space as bold arrows. Black dots mark the SL peaks in (00 1) growth direction, and the elongated, shaded areas adjacent to each peak are due to diffuse scattering. The Laue zones are not accekble experimentally in this regime.
roughness gs of the SL interfaces [16. 173. In another mode, the diffusely scattered intensity was measured, which is sensitive to the vertical and in-plane correlation of the interface morphology. The vertical correlation length /1, represents the replication of the interface morphology along the growth direction and is derived from non-specular 20 scans, where the incidence angle w is kept constant. Pure w scans yield the in-plane correlation length il,, as well as an RMS roughness (TV.Generally, gs and o-~are not identical, because the overall interface roughness can have contributions from the pure morphology and from interdiffusion of the alloys across the interface. The specular reflex (cr,) is sensitive to both effects, whereas diffuse scattering is not affected by interdiffusion. Hence, cd is expected to be representative of the interface morphology, and should therefore be better suited for a comparison with atomic force micrographs. Fig. 3 shows XRR spectra and simulation curves of sample SGC25 in these three scan configurations. The upper panel depicts the specular (u--20 scan, which reveals several orders of SL reflexes. Their spacing depends on layer thicknesses and compositions similar to the diffraction curves in Fig. 1. The attenuation of these peaks with increasing order is
a measure of g,, which is deduced from the fits. In the center frame a diffuse 20 scan is shown, which consists of a sharp peak (specular reflex with tc) = U), and a number of smaller side maxima re-
10'1
h:'
lo-s""'L 0.0
11
0.5
”
”
”
1.0
w-scan, od=7A,
”
w(deg)
C ' 1.5
I
Ap=2000A
1
0.5
o(deg)
1.0
Fig. 3. (81 20 scan, 20 scan. and (r) scan of sample SGC25. Both measured (points) and simulated (lines) curves are shown, which are off-set for reasons of clarity.
Table 2 Interface parameters of two representative measurements at the SL surface
SGC63
SGC25
I .4 2.5
samples obtained
1200 f 500 300 * 100
sulting from resonant diffuse scattering (RDS). The latter is caused by the correlation of the roughness profiles of different interfaces. The replication length A, is deduced from fits to the shape of the RDS peaks, employing a distorted-wave Born approximation (DWBA) as described elsewhere [IS, 191. Since the height of the RDS peaks is also affected by other irregularities of the superlattice the fitted values listed in Table 2 should rather be read as a lower limit for /1,. Finally, the lowest frame in Fig. 3 shows (0 scans through the two labeled peaks seen in the (0-20 scan, and the fitted simulation curves for the extraction of nP and gd. Because of problems with the calibration of the primary intensity, the error margins of the in-plane correlation lengths are rather large. A compilation of the interface parameters for two representative samples with 2’= 1.4% and 2.5% are listed in Table 2. The most striking result of the XRR experiments is a systematic decrease of the replication length as ~1is increased. We get /1, values of 1200, 500 and 300 A for carbon concentrations of I 1.5%, 2% and 2.5%. respectively. Thus, while the /1, values at 4’I 1.5% are comparable to the thickness of the SL, a replication of the interface roughness is suppressed after less than two periods at y = 2.5%. In connection with the aforementioned RHEED observations these results lead to the conclusion that the pronounced vertical replication found by several groups in Si/Si, _xGe_YSLs [20, 211 is strongly disturbed in the presence of carbon-containing interlayers. Higher JJ, which enhance roughening of the Sii _,C,. layers according to RHEED, are obviously more effective in screening the replication process between subsequent Si i _ xGe,x layers. The reduction of il,, with increasing y is accompanied by an overall increase of the interface roughness. The respective RHEED results are quantitatively confirmed by the cr values derived
by lita to the XRR measurements.
1900 + 100 2000 t_ 1000
9+2 7*1
and RMS roughness
4il 712
obtained
4*2 IO + 2
by AFM
S. Zrrlauth
4
3
2
et al. /Journal
1
qf‘ctystal
0
Pm Fig. 4. AFM image of sample SGC63 features superimposed on a short-length
containing graininess.
hillock-like
from the XRR measurements, which yield a factor of two increase when y is increased from 1.4% to 2.5% (Table 2). But s$ll, the fitted RMS roughness values of less than 10 A appear rather small regarding the 3D appearance of the RHEED pattern at y = 2.5%. We therefore performed AFM measurements at the surfaces of the two SLs listed in Table 2. Since the morphology of the topmost Sir _xGex surface is probed, reasonable agreement between AFM and XRR RMS roughnesses is only to be expected for sample SGC63 (y = 1.4%) with a replication length comparable to the thickness of the SL. In the case of sample SGC25 (x = 2.5%), gAFMis expected to be larger than od, since the XRR measurements average over all interfaces, with the lower-lying periods being smoother according to RHEED observations. In fact, we find good agreement between oAi+, and (7d for SGC63. but a significantly (30%) higher AFM roughness for SGC25. Although restricted to the topmost layer, AFM yields additional information, because it provides a quantitative image of a several thousand urn2 large portion of the surface. Fig. 4 shows an AFM image of sample SGC63, which clearly reveals a hillock-like structure with lateral spacing on the
Growth 175,‘176 (1997) 459-464
463
order of l-urn, which is superimposed on a shortrange graininess with a length scale of a few hundred to 1000 A. In contrast, the XRR measurements yielded a /1, value of just about 2000 A for this sample. This means that the hillock feature is outside the sensitivity window of XRR, which is basically determined by the divergence of the beamline. In addition, the AFM line scans can also reconcile the 3D RHEED patterns with the surprisingly small RMS roughnesses: Due to the large average spacing of the hillock features, modulation heights are on the order of 20 A, which is sufficient to explain the RHEED observations, but still yields an RMS roughness of a mere 4 A. Even larger amplitudes are observed in the L’= 2.5% sample (SGC25), which. however, lacked the long-range hillock features. In this sample we just found a short-range graininess, the length scale of which might be just within the error margins given for nP. In either case, the RMS roughnesses determined by XRR have to be treated with care, since they are not easily comparable to each other as long as the lateral correlation of a surface or interface is not well characterized. Also, it became clear that the limited length scale on which meaningful lateral correlation lengths can be derived from XRR measurements may require additional techniques, such as AFM, in order to get a complete picture. In summary, we grew a series of strain-symmetrized, pseudomorphic Si 1 ,.C,/Si i JGex superlattices with varying C content.y, and characterized the interface properties by X-ray techniques. Upon increasing the carbon content from 0% to 2.5% the vertical correlation length .4, was found to decrease from a value comparable to the total thickness of the SL to a value comparable to a single-period length. Simultaneously, the RMS roughness increased by about a factor of two. According to AFM measurements the roughening is concomitant with a change of the interface morphology: With increasing carbon content long range features, which are tentatively attributed to strain induced growth phenomena at the Sil _xGex surfaces, are suppressed by the Si i _ J, interlayers. Financial support by the FWF, GMe, and BMWVK (Vienna), and the opportunity to use
464
S. Zdauth
the OPTICS beamline beamline at Hasylab, acknowledged.
in Grenoble Hamburg,
et al. i Journal
r$Cgbstal
and the D4.1 are gratefully
References [I] [2] [3]
[4] [S] [6] [7]
[8]
Review: A. Schuppen and H. Dietrich, J. Crystal Growth 157 (1995) 207. Review: F. Schaffler. Solid State Electron, 37 (1994) 765. Review: H. Presting. H. Kibbel, M. Jaros, R.M. Turton, U. Menczigar. G. Abatreiter and H.G. Grimmeiss, J. Vat. Sci. Technol. 7 (1992)I 127. K. Ismail. M. Arafa, F. Stern, J.O. Chu and B.S Meyerson, Appl. Phys. Lett. 66 (1995) 842. U. Konig and F. Schiffler, IEEE Electron. Dev. Lett. EDL-14 (1993) 205. G. Abstreiter. H. Brugger, T. Wolf, H. Jorke and H.-J. Herzog, Phys. Rev. Lctt. 54 (1985) 2441. Y.J. Mii, Y.H. Xie. E.A. Fitzgerald, D. Monroe, F.A. Thiel, B.E. Weir and L.C. Feldman. Appl. Phys. Lett. 59 (1991) 1611. K. Eberl. S.S. Iyer and F.K. LeGoues. Appl. Phys. Lett. 64 (1994) 739.
Growth
175/l 76 (1997) 459-464
[9] W. Faschinger, S. Zerlauth, G. Bauer and L. Palmetshofer, Appl. Phys. Lett. 67 (1995) 3933. [lo] E. Kasper and F. Schaffler, in: Semiconductors and Semimetals, Vol. 33, Ed. T.P. Pearsall (Academic Press. Boston, 1990) p. 241. [1 I] SC. Jain, H.J. Osten, B. Dietrich and H. Rucker, Semicond. Sci. Technol. 10 (1995) 1289. [12] A.R. Powell, K. Eberl, F.E. LeGoues, B.A. Ek and S.S. Iyer, J. Vat. Sci. Technol. Bll (1993) 1064. 1131 A.J. Pidduck, D.J. Robbins, A.G. Cullis, W.Y. Leong and A.M. Pitt, Thin Solid Films 222 (1992) 78. [14] H.J. Osten, J. Klatt, G. Lippert and E. Bugiel, J. Appl. Phys. 74 (1993) 2507. [15] P.O. Pettersson, C.C. Ahn, T.C. McGill, E.T. Croke and A.T. Hunter, Appl. Phys. Lett. 67 (1995) 2530. [16] G. Parrat, Phys. Rev. 95 (1964) 359. [17] L. Nevot and P. Croce, Rev. Phys. Appl. 15 (1980) 761. [l S] V. Holy and T. Baumbach, Phys. Rev. B 49 (1994) 10688. 1191 SK. Sinha, E.B. Sirota. S. Garoff and H.B. Stanley, Phys. Rev. B 38 (1988) 2297. [20] R.L. Headrick. J.-M. Baribeau and Y.E. Strausser, Appl. Phys. Lett. 66 (1995) 96. 1211 J. Tersoff, C. Teichert and M.G. Lagally, Phys. Rev. Lett. 76 (1996) 1675.