Ge heterostructures

Thin SolM Fihns, 222 (1992) 112 115

112

Effect of a surfactant on the growth of Si/Ge heterostructures Kunihiro Sakamoto, Kazushi Miki, Tsunenori Sakamoto, Hirotaka Yamaguchi, Hiroyuki Oyanagi, Hirohumi Matsuhata and Ken'ichi Kyoya Electrotechnical Laboratot3', 1-1-4 Umezono, Tsukuba 305 (Japan)

Abstract The effect of a surfactant on the growth of Si/Ge/Si heterostructures has been investigated by means of extended X-ray absorption fine structure and reflection high energy electron diffraction. Antimony as a surfactant partially prevents the mixing of silicon and germanium within 3.8 ML. It has been demonstrated that a major effect of a surfactant is to reduce the surface mobility of growing species which consequently restricts both islanding and segregation with the sacrifice of surface morphology.

1. Introduction Fabrication of an Si/Ge strained-layer superlattice (SLS) is an attractive challenge to tailoring the properties of silicon. The abruptness of the S i - G e interface in an SLS is of extreme importance. Investigations have shown that the real S i - G e interface undergoes intermixing of germanium and silicon caused by islanding and segregation of germanium [1-4]. A recent surfactant approach has succeeded in suppressing both islanding and segregation [ 5-11 ]. The effect of a surfactant has been examined by medium energy ion scattering [5-7], secondary ion mass spectroscopy [8, 9], X-ray photoemission spectroscopy [9], and surface X-ray diffraction [ 11]. The inhibition by a surfactant of surface transport has been proposed for the dynamics of growth [5, 6]. There has been, however, no experimental evidence for the surface dynamics in spite of these intensive structural studies. It is also uncertain whether a surfactant suppresses germanium segregation on the scale of a monolayer (ML) in an SimGe,, short period SLS. In this article, we report the experimental results of extended X-ray absorption fine structure (EXAFS) and reflection high energy electron diffraction (RHEED), and discuss the effect of a surfactant on the distribution of germanium atoms at the S i - G e interface and also on the kinetics of growth by molecular beam epitaxy.

2. Experimental details Samples were fabricated in an ion-pumped silicon MBE system with an electron beam evaporator for silicon and resistively heated effusion cells for germanium and dopants. The substrate was heated by radia-

0040-6090/92/$5.00

tion from a tantalum heater. An Si(001) substrate was subjected to a standard cleaning process [12]. After removal of the protective oxide an atomically clean starting surface was produced by the deposition of a silicon buffer layer at 700 °C and a subsequent 1000 °C anneal [13]. Antimony was selected as a surfactant. The growth rates of silicon and germanium were 0.3 ML s t and 0.1 ML s-J respectively. A 30 keV R H E E D system was used for surface analysis. Surface-sensitive EXAFS experiments were performed using synchrotron radiation at the Photon Factory in Tsukuba [14].

3. Extended X-ray absorption fine structure The profile of EXAFS oscillations on the germanium K-edge reflects the backscattering amplitude of chemical species around an excited germanium atom while the phase of oscillation contains information on the distance between the scatterer atom and a germanium atom. Since germanium and silicon bonds have different k dependences of the scattering amplitude, the fraction of G e - G e and G e - S i bonds and bond lengths around a germanium atom on average can be estimated by analysing the Ge-K EXAFS profile. Because interfacial mixing decreases the number of G e - G e bonds and increases the number of Si Ge bonds, their proportion is an atomic-scale measure of the germanium distribution. Two short period Si/Ge SLSs of(Si~2Ge4)5 were grown on Si(001) at 400 °C. One was fabricated by conventional MBE. The other was fabricated with antimony surfactant as follows: the first germanium layer was grown on a clean silicon surface, then 0.5 ML Sb was deposited. On completion of the antimony layer, the silicon layer and germanium layer were deposited alternately.

4, 1992

Elsevier Sequoia. All rights reserved

K. Sakamoto et al./ Effect of surfactant on Si/Ge heterostructures TABLE 1. The G e - G e bond fraction determined by extended X-ray absorption fine structure on the germanium K-edge (Sil2Ge4)5/Si(001), 400 °C, without Sb

(Si12Ge4)s/Si(001), 400 °C, with Sb

Si/Ge4/Si(001), room temperature, without Sb

0.28 + 0.1

0.40 ± 0.1

0.44 ± 0.1

The G e - G e bond fractions determined by analysing the Ge-K EXAFS oscillations are summarized in Table 1. The fraction was improved from 0.28_0.1 to 0.40 ___0.1 by covering the surface with antimony. This value is comparable with that of the sample grown at room temperature, indicating that the Si-Ge interface grown at 400 °C with antimony is as abrupt as that grown at the room temperature. Although the antimony surfactant restricts intermixing of silicon and germanium, the fraction obtained experimentally is still smaller than the calculated value of 0.75 expected for the ideal interface. This difference between the calculation and experiment implies that degradation of the real Si-Ge interface caused by the intermixing still remains. It is instructive to estimate the germanium distribution at the interface from the fraction obtained experimentally. To this end, we assumed an asymmetric germanium distribution: the interface of germanium on silicon is ideal and, at the interface of silicon on germanium, the germanium concentration exponentially decays towards the surface. This model attributes the intermixing to the germanium segregation and gives the worst estimate for it. The decay length of germanium in which the germanium content decays to 1/e is thus estimated to be 5.6 ML without antimony and 3.8 ML with antimony. Our EXAFS results show that the antimony surfactant partially restricts the intermixing of silicon and germanium within a few monolayers.

4. Reflection high energy electron diffraction The growth dynamics in the antimony-covered surface was characterized using a RHEED analysis on the surface. An Si/Sb/Ge4/Si(001) structure was fabricated at 400 °C and the surface structure was monitored by RHEED. RHEED intensity evolutions of the specular beam are shown in Fig. 1. Without the antimony layer, an intensity oscillation with a large amplitude modulation was observed during silicon overlayer growth. This means that the growth proceeds layer by layer with two-dimensional (2D) nucleation of migrating silicon adatoms. The large amplitude modulation implies that a silicon adatom migrates on the surface for a distance as long as the length of electron beam coherency of about 0.1 Ixm. Another important point is the recovery

I

I

I

I

1

113

I

I

I

I

I

SilSb/F_.~Si(O011, [1001 azimuth,/,O0"C "~ -~-

Ge

a

Si

"1

g

'/

b

A

o.s

¢Y

"

I

I

los

I

I

I

,

I

,

'

i

i

a

Time

Fig. 1. RHEED intensity evolutions of the specular beam in the [ 100] azimuth during growth of the Si/Sb/Ge4/Si(001) structure at 400 °C: trace a, without antimony deposition; trace b, 0.5 ML Sb was deposited on the germanium surface before silicon overgrowth.

of peak intensity at the initial stage of silicon growth, for which the interpretation is that the germanium surface, which had been roughened atomically by strain, is smoothed out by impinging silicon atoms which migrate around the surface [1, 15]. On the antimony-covered surface, in contrast, rapid damping of the RHEED intensity and its oscillation amplitude was observed during silicon growth. This behaviour can be explained in two ways: (1) change of growth from 2D nucleation to step flow by enhanced migration; (2) shrinkage in size of 2D islands induced by restricted migration. The former possibility should be excluded because evidence of migration suppression by antimony surfactant was found in an experiment at 700 °C which indicated the change in growth from step flow to 2D nucleation. The surface morphology discussed below also supports the reduction in surface mobility of silicon adatoms caused by the antimony surfactant. Figure 2 shows RHEED patterns after the growth of a silicon overlayer for 50 ML. RHEED from the starting surface is also shown (Fig. 2(a)). With antimony surfactant (Fig. 2(c)), a diffuse RHEED pattern with broad streaks was observed. The existence of highdensity atomic layer steps caused by the poor surface mobility is implied. Degradation of surface morphology is apparent compared with the surface grown without antimony (Fig. 2(c)) or with the starting surface (Fig. 2(a)) in which a bright specular spot and narrow streaks are seen. Another apparent feature of a surfactant is prevention of the three-dimensional (3D) islanding of germanium. Figure 3 shows RHEED intensity evolutions

114

K. Sakamoto et al. /Effect of surfactant on Si/Ge heterostructures

,.•

i

i

a

i

i

i

i

i

i

5e/Sb/SilO011, [1001 azimuth, 400"C

C ¢._

~b a/ r-

~ 1ML

5e

n

ILl '-P

I

I

2Os

I

I

I

I

I

I

-I

Time

Fig. 3. RHEED intensity evolutions of the specular beam in the [ 100] azimuth during growth of the Ge/Si(001) structure at 400 °C: trace a, without antimony deposition; trace b, 1 ML Sb was deposited on the silicon surface before germanium growth.

growth of germanium on Si(001) at room temperature [1, 16]. These experimental results imply that the 3D islanding of germanium is suppressed when the surface transport is sufficiently inhibited.

Conclusion

Fig. 2. RHEED patterns in the [100] azimuth: (a) starting surface of Si(001); (b) after the growth of the Siso/Ge4/Si(001 ) structure; (c) after the growth of the Siso/Sbo.5/Ge4/Si(001) structure. All patterns were taken at a sample temperature of 400 °C.

during germanium growth on Si(001). The difference in the oscillation amplitude between growth with and without antimony indicates that antimony surfactant also suppresses surface migration of germanium adatoms. The apparent damping of intensity after the fifth peak in growth without antimony represents the change in growth from 2D layer-by-layer to 3D islanding [1, 16]. With antimony surfactant, in contrast, the intensity oscillation lasts more than 12 periods, indicating that the germanium layer grows in a layer-by-layer mode up to 12 ML. The intensity oscillation lasting for more than 20 periods has been observed during MBE

We have examined the effect of a surfactant on the growth of the Si/Ge/Si heterostructure. An antimony layer deposited on the surface partially prevents the mixing of silicon and germanium within 3.8 ML and preserves 2D layer-by-layer germanium growth up to 12 ML. It was found that a major effect of a surfactant is to reduce the surface mobility of the growing species which consequently restricts both islanding and segregation with the sacrifice of surface morphology. In other words, a surfactant improves the abruptness of the Si-Ge interface but degrades its flatness at the same time.

Acknowledgments The authors would like to thank Professor K. Tomizawa and Dr. M. Ono for valuable discussion and continuous encouragement.

References 1 K. Sakamoto, T. Sakamoto, S. Nagao, G. Hashiguchi, K. Kuniyoshi and Y. Bando, Jpn. J. Appl. Sci., 26 (1987) 666. 2 S. S. lyer, J. C. Tsang, M. W. Copel, P. R. Pukite and R. M. Tromp, Appl. Phys. Lett., 54 (1989) 219. 3 K. Nakagawa and M. Miyao, J. Appl. Phys., 69 (1991) 3058. 4 S. Fukatsu, K. Fujita, H. Yaguchi, Y. Shiraki and R. Ito, Appl. Phys. Lett., 59 (1991) 2103.

K. Sakamoto et al. / Effect of surfactant on Si/Ge heterostructures 5 M. Copel, M. C. Reuter, E. Kaxiras and R. M. Tromp, Phys. Rev. Lett., 63 (1989) 632. 6 M. Copel, M. C. Reuter, M. Horn von Hoegen and R. M. Tromp, Phys. Rev. B, 42 (1990) 11682. 7 M. Copel and R. M. Tromp, Appl. Phys. Lett., 58 (1991) 2648. 8 P. C. Zalm, G. F. A. van de Walle, D. J. Gravesteijin and A. A. van Gorkum, Appl. Phys. Lett., 55 (1989) 2520. 9 K. Fujita, S. Fukatsu, H. Yaguchi, T. Igarashi, Y. Shiraki and R. Ito, Jpn. J. Appl. Phys., 29 (1990) L1981. 10 S. Higuchi and Y. Nakanishi, Surf Sci. Lett., 254 (1991) L465. 11 J. M. C. Thornton, A. A. Williams, J. E. Macdonard, R. G. van

12 13 14 15 16

115

Silfhout, J. F. van der Veen, M. Finney and C. Norris, J. Vac. Sci. Technol. B, 9 (1991) 2146. A. Ishizaka and Y. Shiraki, J. Electrochem. Soc., 133 (1986) 666. T. Sakamoto, T. Kawamura, S. Nagao, G. Hashiguchi, K. Sakamoto and K. Kuniyoshi, J. Cryst. Growth, 81 (1987) 59. H. Oyanagi, T. Sakamoto, K. Sakamoto, T. Matsushita, T. Yao and T. Ishiguro, J. Phys. Soc. Jpn., 57 (1988) 2086. K. Miki, K. Sakamoto, T. Sakamoto, H. Okumura, N. Takahashi and S. Yoshida, J. Cryst. Growth, 95 (1989) 444. K. Miki, K. Sakamoto and T. Sakamoto, Mater. Res. Soc. Syrup. Proc., 148 (1989) 323.