- Email: [email protected]
In situ RBS and channeling study of molecular beam epitaxial growth of metals and semiconductors on semiconductors
780
Nuclear Instruments and Methods in Physics Research B56/57 (1991) 780-784 North-Holland
In situ RIBS and channeling study of molecular beam epitaxial growth of metals and semiconductors on semiconductors Fulin Xiong, Eric Ganz, Jene A. Golovchenko
and Frans Spaepen
Division of Applied Sciences and Department of Physics, Harvard University, Cambridge, MA 02138, USA
High energy ion scattering (HEIS) has been used for in situ surface and interface studies of molecular beam epitaxial growth of metals and semiconductors on semiconductors. In the study of phase transitions during the initial growth and thermal desorption of Pb on Si(lll), HEIS measurement has directly and accurately determined the metal overlayer coverage associated with various surface structures and clearly revealed Si-Pb interface relaxation and Pb island formation. These results contribute to an unambiguous interpretation of LEED patterns and STM images. HEIS has been used to monitor the homoepitaxial growth of Ge on Ge(ll1) through liquid metal media, and to characterize in situ the grown thin films. This leads to a better understanding of the growth mechanism and optimal growth conditions.
1. Introdwtion
High energy ion scattering (HEIS), typically RBS and channeling analysis, has been established as a standard tool for material analysis as well as for surface and interface studies. Compared to most surface analysis methods such as LEED, AES, SIMS, and XPS, HEIS has great advantages in the ease of chemical identification, crystallinity assessment, and quantitative evaluation of absolute atomic concentrations and film thickness, as well as in determining atomic displacements in the crystal lattice. At the newly established accelerator laboratory at Harvard University, a UHVMBE system is directly connected to an ion accelerator, providing the opportunity to perform in situ HEIS surface and interface studies during material MBE growth and processing. In this paper we review our recent work done with this system; two examples will be presented. The first example is a surface study of the initial growth of a metal-on-~~~nductor system: Pb on Si(ll1). HEIS measurements have been used to accurately determine the metal overlayer coverage associated with the various surface structures and to observe the interface relaxation and surface Pb island formation. In the second example, HEIS has been used to monitor in situ the behavior of the metal transport medium at the metalsemiconductor interface in the process of MBE growth of Ge on Ge(ll1) with a liquid Au layer, and to characterize the grown crystalline films.
2. Experimental aspects The UHV system used in the experiments is outfitted with a load-lock system for quick sample exchange, molecular beam evaporators for metal or semiconductor deposition, a low energy ion beam source for ion etching and ion sputter deposition, as well as with a Cgrid LEED system for the analysis surface reconstructions. The UHV chamber is connected to a General Ionex 1.7 MV Tandetron accelerator through a differenEnergy 6
1.70
1.75
(MeV) 1.85
1.80
0
5 0 .!A?
as-deposited, JML Pb A annealed 39OC- 1Osec + annealed 39OC-35sec X annealed 39OC-8Osec
>4
0 740
760
780
800
820
Channel Fig. 1. RBS spectra of Pb from Pb/Si(lll) samples: 3 ML Pb as-deposited (0) and annealed at 390°C for 10 s at 1.5 ML plus islands (A),for 35 s at 1.3 ML plus small islands (+), and for 80 s at 1 ML with no islands (x).
0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
781
F. Xiong et al. / In situ RBS and channeling study
tially pumped beam line to perform HEIS measurements. The chamber has a base pressure of 1 X lo-” Torr and is maintained at a pressure below 5 X 10-t’ Torr during sample preparation and molecular beam deposition. Sample annealing is accomplished by resistive heating, i.e. by passing a regulated current through the sample. The sample temperature is measured with an optical pyrometer. HEIS analysis is performed with 2 MeV He + ions which are delivered to the UHV chamber through a 2 mm aperture. Scattered ions are detected by a water-cooled Si solid-state detector with energy resolution of 20 KeV (FWHM) and a detecting angle of 170 O. For random RBS measurements, the beam is projected on the target at 5-7’ off its normal direction. For channeling analysis, the beam is aligned along the sample surface normal axes.
3. Surface study of phase transitions of Pb on Si(ll1) An understanding of the microscopic formation of metal-on-semiconductor systems provides valuable insight into a variety of complex metal-semiconductor interface phenomena, such as Schottky barrier formation and Fermi level pinning [2]. The growth of Pb on Si(ll1) as a model system has been studied by several groups with a variety of surface analysis techniques, such as LEED, AES, X-ray diffraction, ellipsometry, and low energy ion scattering spectroscopy (LEISS) [3-71, as well as Schottky barrier measurements [2]. However, despite many advances made in this field, considerable confusion remains about the growth mechanism and dependence of phase transitions on coverage and temperature, due in part to the lack of a direct and
Fig. 2. LEED patterns from Pb/Si(lll) surfaces. (a) Si(lll)-(7 X 7)-Pb at a Pb coverage of 3 ML (the electron energy at 60 ev); (b) a 30” rotated incommensurate Pb phase at 1 ML (at 82.6 ev); (c) Si(lll)-(fi Xfi)R30°-Pb(a) at f ML (at 80.6 eV); (d) Si(lll)-(6
x &)R30
O-Pb(b) at k ML (at 47 eV). IX. MATERIALS
ANALYSIS
782
F. Xiong et al. / In situ RBS and channeling study
accurate measurement of the metal coverage. We carried out a detailed study of the initial growth and thermal desorption of Fb on Si(ll1) using HEiS combined with LEED and STM [8J. We started with deposition of Pb (98.999% purity) on Si(lIl)-7 X 7 surface r~ons~&t~ substrates held at room temperature fRT)_ Randum RBS spectra taken during this deposition process generally show a sharp Pb peak, with an intensity increasing linearly with the total deposition and a width limited by detector resolution. The curve labelled by 0 in fig. 1 shows a typical RBS spectrum of Pb at a coverage of 3 ML [l ML (monolayer) = 7.85 x 1Ol4 atoms/cm’ in substrate units]. LEED observation during the RT deposition promss reveals a gradual change from Si(llI)-7 x 7 to SifltI)-7 X 7-Fb (fig. 2a). When the sample is annealed above JoO0 C, the Si(lll)-7 x 7-Pb structure is de(RX) Pb phase stroyed and a rotated into ~sura~~ is observed [fig. 2b). The RBS spectra in fig. 1, takers during isothermal annealing at 330°C illustrate the evolution of the Pb overlayers wifh time. After the first step, a sharp Pb peak representing a flat overlayer changes into 1 ML plus a large diffuse tail (representing Pb islands), This indicates that a structure of a 2D layer plus JD islands in a size of about 15O-200 nm has been formed. Continued annealing causes desorption of Pb from both the overlayer and the islands, with Pb desorption from islands being much faster than from the flat layer. When the islands are completely gone, the average coverage reaches 1 ML where a well defined RXC phase is observed. At the average coverage of D.8 ML, a RT stable ~~~e~s~r~~e Si@llS_(l X 2)-m) plme appears, where Si adatoms are mixed with Pb at the surface. It is followed by iwo different fi X a-Pb phases at f ML and f ML. The latter has been found by STM to be a Pb-Si mixed phase and has higher intensity in the LEED pattern (figs. 2c and 2d). The behavior of isothermal desorption of Ph on Si(lll> at submonolayer coverage is shown in fig. 3. The Pb coverage was measured at RT using RBS. Four distinct transition stages between 0 and I ML were observed, LEED patterns taken at each turning point indicated in fig. 3 are shown in fig_ 2. We note that the isothermal deso~tio~ kinetics appears to be zero order for all transition stages, in contrast to the results of Saitoh et al. f4J and in agreement with the results of Le Lay et al f5,6]$ who, however, did not observe the transition at 6 ML. The zero-order kinetics may be explained by the 2D-gas model of Metois and Le Lay [PI, Channeling RBS (CRBS) measurements were used to study the relaxation and reordering of the Pb/Si(lll) interface We observed no change in the Si surface peak upon deposition of 3 ML at RT. This suggests that the initial Z&(111)-7x 7 reconstructed structure is kept intact. This is consistent with LEED observation where I./7-Pb spots surromd Si[lll) first-order sp~ts~ show-
o.o”k--0
I
5
I
10
*
15
20
1
25
:
time (min) Fig. 3. Isothermal dcsorption spectra of I% on Si(lll) surfaces
annealed a6 440 and 46O”C, respectiv& showing four distinct Phase transition stages with zero-order desor@on kinetics at 1, 0.8,; and t &It covera@%.-I-he~~~~~on~jn~ LBED patterns are indicate&
ing that 7 X 7 long-range order is maintained. STM images [8J have comfirmed this behavior and have shown that the corner holes of the Si(lll)-7 X 7 structures persist during the initial growth at RT. After high temperature (HT) annealing to 400 o C, CRBS measurements show a IO% decrease in Si surface intensity, indicating that about 1 x lOi Si atoms/cm2 with a few layers at the interface are returned to their lattice sites as a result of interface relaxation. Continuous annealing causes no further change. This indicates that, under the influence of Pb atoms and heat treatment, the Si adatoms become mobile and the underlying Si-7 X 7 surface relaxes, At the Pb/Si interface, the remaining Si adatoms are intermixed with Pb atoms to form a regular well-ordered flat layer. Direct images of Pb-Si mixed fi x fi structure have been observed using STM in HT annealed samples, where a Pb-Si ratio of 1~1 is shown,
It has been known for a long time that the presence of metals can greatly enhance the crystal growth velocity of elemental semiconductors. Rapid crystal growth of semiconductors by the VLS mechanism, which involves vapor, liquid, and solid phases, was first demonstrated by Wagner and Ellis [lO,ll]. Crystals resulting from this process normally from long dislocation-free whisker-like or filamentary protrusions. Solid phase epitaxy (SPE) grawth of Si and Ge with metals as transport media has also been investigated in detail flZJ. in the SPE process, samples are prep& in a configura~ t&n of ~e~~o~d~c~~r (amo~hous)-mew-senicon-
783
F. Xiong et al. / In situ RBS and channeling study
ductor (crystal). Upon annealing at a temperature well below the eutectic melting temperature, the atoms in the amorphous layer diffuse across the metal layer and grow epitaxially onto the substrate. The epitaxial layers grown in this way generally lack lateral uniformity and good crystalliuity due to contamination and inhomogeneity at the interface [12]. The thrust of our investigation is to explore a novel method for large area devicequality thin film growth of semiconductors at low temperature by MBE with liquid metal media, and to study the surface and interface physics involved in the process. The in situ RBS and channeling are used to monitor the growth process and to optimize the growth conditions as well as to characterize the grown films. As our first demonstration [13], we have successfully produced homoepitaxial Ge film on Ge(ll1) at low temperature by the VLS mechanism. A thin Au layer was first deposited onto clean and c(2 x 8) reconstructed substrates at RT from an ion sputtering source. We found that the as-deposited Au stayed uniform and flat on the substrate. Upon thermal annealing at 400“ C, as in the Au/Si(lll) system [14], the Au reacts with the surface Ge atoms to form a eutectic alloy. Any extra Au migrates to form islands with an average size of 2000 A. RBS measurements shown in fig. 4 reveal a change from a high intensity sharp peak to a low-intensity one with a diffused tail. Ex situ SEM analysis cornfirmed the RBS results of island formation [13]. In order to produce flat thin film growth, we have reduced the thickness of the Au layer. We find that 30 A is the optimum thickness for the
Energy
(MeV)
~~~~1
-0 15 ,-El E E lo b =
5 0 650
760
7jo
800
Channel Fig. 4. Channeling RBS spectra taken in situ during Ge MBE crystal growth with a liquid gold alloy medium. 0: the initial Ge(ll1) single frystal at the beginning, a: 30 A Au as-deposited, + ; 30 A Au deposited and annealed during the grown process, and X : Ge/Ge(lll) grown sample (1.5 h deposition at a rate of 1000 A/h). During the growth process the substrate was maintained at 400 o C.
Energy 1.6
650
700
(MeV)
1.7
750 Channel
1.9
1.8
800
Fig. 5. RBS Spectra of Ge/Ge(lll) with Au medium from as-grown (0 random and A channeled) and Au-etched (+ random and x channeled) samples.
growth process, and direct deposition at high temperature would stimulate Au-Ge mixing with better film uniformity. Ge was then deposited on the Au/Ge(lll) structure from an effusion cell at a rate of 1000 A/h. During the deposition, the substrate was maintained at 400°C, slightly higher than the eutectic point. In the example shown below, Ge was deposited for 1.5 h to achieve a final thickness of 1600 A. Fig. 4 shows channeling spectra taken during and after the growth. From the spectrum of the as-grown sample, we find that the grown Ge layers has very good crystallinity with a dechanneling yield around 6%. Au still appears directly on the surface, after the growth, mainly as islands. To confirm this conclusion, we used a selective chemical etching to remove the surface Au, and then performed RBS measurement on the general-purpose RBS beamline (175 ’ detection) in our laboratory. The resulting spectra are presented in fig. 5. The Ge peak shows a dechanneling yield close to that of the original single crystal. Two small Au peaks are observed, one directly at the surface and the other at the interface. The gap between these two peaks marks the thickness of the Ge epitaxial layer (1600 A), consistent with the deposition. The RBS data also shown that Au at both the surface and the interface has mixed with Ge forming Ge/Au layers - 15 A thick in the ratio of Ge : Au = 4 : 1, corresponding to the eutectic concentration of 28 at.% Au. At this stage we may conclude that the mechanisms of our liquid metal mediated MBE crystal growth is similar to that of the old VLS process. Au first eutectically mixes with surface Ge atoms, and then forms a supersaturated liquid alloy at temperatures higher than the eutectic point. The in-coming molecular beam of Ge dissolves into the liquid, and subsequently precipitates as a crystal at the IX. MATERIALS ANALYSIS
784
F. Xiong et al. / In situ RBS and channeling study
liquid-substrate interface. We have also examined the grown samples by SEM and cross-sectional TEM, and found good crystal epitaxial growth of Ge, Au island formation on the surface, and Au trapping at the interface, in agreement with the RBS results. The detailed results will be presented elsewhere [13].
5. Summary remarks In summary, we have demonstrated in situ HEIS surface and interface studies of MBE growth of metal and semiconductor on semiconductors. In the Pb/ Si(ll1) system, HEIS measurements have directly and accurately determined the absolute Pb coverage associated with various surface structures. It has also revealed interface relaxation, Pb island formation and a 2D-3D reordering mechanism during high temperature annealing. Detailed isothermal desorption experiments reveal all phase transitions at submonolayer coverages. The results have provided conclusive information for an unambiguous interpretation of LEED and STM results on the surface structures of this system. This technical combination can be applied to solve problems in other similar metal-semiconductor systems. In situ HEIS monitoring of the MBE growth process has successfully assisted the control of the process and provided a very quick and nondestructive in situ characterization of the grown samples.
Acknowledgements This work is supported in part by the National Science Foundation (DMR-8920490) and the Office of
Naval Research (N00014-90-J-1234), and the Joint Services Electronics Program (N000014-89-J-1023). We would like to acknowledge Bill Fostor, Ing-Shouh Hwang, Anthony Leoser and Andrew Wagner for their technical assistance and helpful discussions, and Robert Martinez for his constructive comments on the manuscript.
References PI L.C. Feldman and J.W. Mayer, Fundamentals
of Surface and Thin Film Analysis (North-Holland, New York, 1986). PI D.R. Heslinga, H.H. Weitering, D.P. van der Werf, T.M. Klapwijk and T. Hibma, Phys. Rev. Lett. 64 (1990) 1589. [31 P.J. Estrup and J. Morrison, Surf. Sci. 2 (1964) 465. [41 M. Saitoh, K. Oura, K. Asano, F. Shoji and T. Hanawa, Surf. Sci. 154 (1985) 394. [51 G. Le Lay, J.S. Peretti, M. Hahnbuecken and W.S. Yang, Surf. Sci. 204 (1988) 57. 161 G. Quentel, M. Gauch and A. Degiovanni, Surf. Sci. 154 (1985) 394. 171 F. Grey, R. Feidenhans’l, M. Nielsen and R.L. Johnson, Col. Phys. C7 (1989) 181. 181 E. Ganz. F. Xiong, I-S. Hwang and J.A. Golovchenko, Phys. Rev. B43 (1991) in press. [91 J.J. Metois and G. Le Lay, Surf. Sci. 133 (1983) 422. WI R.S. Wagner and W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. ill1 R.S. Wagner and W.C. Ellis, Trans. Metallug. Sot. of AIMS 233 (1965) 1053. WI S.S. Lau, J.W. Mayer and W. Tseng, in: Handbook on Semiconductors, vol. 3: Materials. Properties and Preparation, ed. S. Keller (North-Holland, Amsterdam, 1980) p. 531 and references therein. iI31 F. Xiong, F. Spaepen and J. Golovchenko, Mater. Res. Sot. Symp. Proc. (1991) in press. K. Kinoshita P41 T. Narusawa, Technol. 18 (1991) 872.
and W.M. Gibson,
J. Vat. Sci.
Nuclear Instruments and Methods in Physics Research B56/57 (1991) 780-784 North-Holland
In situ RIBS and channeling study of molecular beam epitaxial growth of metals and semiconductors on semiconductors Fulin Xiong, Eric Ganz, Jene A. Golovchenko
and Frans Spaepen
Division of Applied Sciences and Department of Physics, Harvard University, Cambridge, MA 02138, USA
High energy ion scattering (HEIS) has been used for in situ surface and interface studies of molecular beam epitaxial growth of metals and semiconductors on semiconductors. In the study of phase transitions during the initial growth and thermal desorption of Pb on Si(lll), HEIS measurement has directly and accurately determined the metal overlayer coverage associated with various surface structures and clearly revealed Si-Pb interface relaxation and Pb island formation. These results contribute to an unambiguous interpretation of LEED patterns and STM images. HEIS has been used to monitor the homoepitaxial growth of Ge on Ge(ll1) through liquid metal media, and to characterize in situ the grown thin films. This leads to a better understanding of the growth mechanism and optimal growth conditions.
1. Introdwtion
High energy ion scattering (HEIS), typically RBS and channeling analysis, has been established as a standard tool for material analysis as well as for surface and interface studies. Compared to most surface analysis methods such as LEED, AES, SIMS, and XPS, HEIS has great advantages in the ease of chemical identification, crystallinity assessment, and quantitative evaluation of absolute atomic concentrations and film thickness, as well as in determining atomic displacements in the crystal lattice. At the newly established accelerator laboratory at Harvard University, a UHVMBE system is directly connected to an ion accelerator, providing the opportunity to perform in situ HEIS surface and interface studies during material MBE growth and processing. In this paper we review our recent work done with this system; two examples will be presented. The first example is a surface study of the initial growth of a metal-on-~~~nductor system: Pb on Si(ll1). HEIS measurements have been used to accurately determine the metal overlayer coverage associated with the various surface structures and to observe the interface relaxation and surface Pb island formation. In the second example, HEIS has been used to monitor in situ the behavior of the metal transport medium at the metalsemiconductor interface in the process of MBE growth of Ge on Ge(ll1) with a liquid Au layer, and to characterize the grown crystalline films.
2. Experimental aspects The UHV system used in the experiments is outfitted with a load-lock system for quick sample exchange, molecular beam evaporators for metal or semiconductor deposition, a low energy ion beam source for ion etching and ion sputter deposition, as well as with a Cgrid LEED system for the analysis surface reconstructions. The UHV chamber is connected to a General Ionex 1.7 MV Tandetron accelerator through a differenEnergy 6
1.70
1.75
(MeV) 1.85
1.80
0
5 0 .!A?
as-deposited, JML Pb A annealed 39OC- 1Osec + annealed 39OC-35sec X annealed 39OC-8Osec
>4
0 740
760
780
800
820
Channel Fig. 1. RBS spectra of Pb from Pb/Si(lll) samples: 3 ML Pb as-deposited (0) and annealed at 390°C for 10 s at 1.5 ML plus islands (A),for 35 s at 1.3 ML plus small islands (+), and for 80 s at 1 ML with no islands (x).
0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
781
F. Xiong et al. / In situ RBS and channeling study
tially pumped beam line to perform HEIS measurements. The chamber has a base pressure of 1 X lo-” Torr and is maintained at a pressure below 5 X 10-t’ Torr during sample preparation and molecular beam deposition. Sample annealing is accomplished by resistive heating, i.e. by passing a regulated current through the sample. The sample temperature is measured with an optical pyrometer. HEIS analysis is performed with 2 MeV He + ions which are delivered to the UHV chamber through a 2 mm aperture. Scattered ions are detected by a water-cooled Si solid-state detector with energy resolution of 20 KeV (FWHM) and a detecting angle of 170 O. For random RBS measurements, the beam is projected on the target at 5-7’ off its normal direction. For channeling analysis, the beam is aligned along the sample surface normal axes.
3. Surface study of phase transitions of Pb on Si(ll1) An understanding of the microscopic formation of metal-on-semiconductor systems provides valuable insight into a variety of complex metal-semiconductor interface phenomena, such as Schottky barrier formation and Fermi level pinning [2]. The growth of Pb on Si(ll1) as a model system has been studied by several groups with a variety of surface analysis techniques, such as LEED, AES, X-ray diffraction, ellipsometry, and low energy ion scattering spectroscopy (LEISS) [3-71, as well as Schottky barrier measurements [2]. However, despite many advances made in this field, considerable confusion remains about the growth mechanism and dependence of phase transitions on coverage and temperature, due in part to the lack of a direct and
Fig. 2. LEED patterns from Pb/Si(lll) surfaces. (a) Si(lll)-(7 X 7)-Pb at a Pb coverage of 3 ML (the electron energy at 60 ev); (b) a 30” rotated incommensurate Pb phase at 1 ML (at 82.6 ev); (c) Si(lll)-(fi Xfi)R30°-Pb(a) at f ML (at 80.6 eV); (d) Si(lll)-(6
x &)R30
O-Pb(b) at k ML (at 47 eV). IX. MATERIALS
ANALYSIS
782
F. Xiong et al. / In situ RBS and channeling study
accurate measurement of the metal coverage. We carried out a detailed study of the initial growth and thermal desorption of Fb on Si(ll1) using HEiS combined with LEED and STM [8J. We started with deposition of Pb (98.999% purity) on Si(lIl)-7 X 7 surface r~ons~&t~ substrates held at room temperature fRT)_ Randum RBS spectra taken during this deposition process generally show a sharp Pb peak, with an intensity increasing linearly with the total deposition and a width limited by detector resolution. The curve labelled by 0 in fig. 1 shows a typical RBS spectrum of Pb at a coverage of 3 ML [l ML (monolayer) = 7.85 x 1Ol4 atoms/cm’ in substrate units]. LEED observation during the RT deposition promss reveals a gradual change from Si(llI)-7 x 7 to SifltI)-7 X 7-Fb (fig. 2a). When the sample is annealed above JoO0 C, the Si(lll)-7 x 7-Pb structure is de(RX) Pb phase stroyed and a rotated into ~sura~~ is observed [fig. 2b). The RBS spectra in fig. 1, takers during isothermal annealing at 330°C illustrate the evolution of the Pb overlayers wifh time. After the first step, a sharp Pb peak representing a flat overlayer changes into 1 ML plus a large diffuse tail (representing Pb islands), This indicates that a structure of a 2D layer plus JD islands in a size of about 15O-200 nm has been formed. Continued annealing causes desorption of Pb from both the overlayer and the islands, with Pb desorption from islands being much faster than from the flat layer. When the islands are completely gone, the average coverage reaches 1 ML where a well defined RXC phase is observed. At the average coverage of D.8 ML, a RT stable ~~~e~s~r~~e Si@llS_(l X 2)-m) plme appears, where Si adatoms are mixed with Pb at the surface. It is followed by iwo different fi X a-Pb phases at f ML and f ML. The latter has been found by STM to be a Pb-Si mixed phase and has higher intensity in the LEED pattern (figs. 2c and 2d). The behavior of isothermal desorption of Ph on Si(lll> at submonolayer coverage is shown in fig. 3. The Pb coverage was measured at RT using RBS. Four distinct transition stages between 0 and I ML were observed, LEED patterns taken at each turning point indicated in fig. 3 are shown in fig_ 2. We note that the isothermal deso~tio~ kinetics appears to be zero order for all transition stages, in contrast to the results of Saitoh et al. f4J and in agreement with the results of Le Lay et al f5,6]$ who, however, did not observe the transition at 6 ML. The zero-order kinetics may be explained by the 2D-gas model of Metois and Le Lay [PI, Channeling RBS (CRBS) measurements were used to study the relaxation and reordering of the Pb/Si(lll) interface We observed no change in the Si surface peak upon deposition of 3 ML at RT. This suggests that the initial Z&(111)-7x 7 reconstructed structure is kept intact. This is consistent with LEED observation where I./7-Pb spots surromd Si[lll) first-order sp~ts~ show-
o.o”k--0
I
5
I
10
*
15
20
1
25
:
time (min) Fig. 3. Isothermal dcsorption spectra of I% on Si(lll) surfaces
annealed a6 440 and 46O”C, respectiv& showing four distinct Phase transition stages with zero-order desor@on kinetics at 1, 0.8,; and t &It covera@%.-I-he~~~~~on~jn~ LBED patterns are indicate&
ing that 7 X 7 long-range order is maintained. STM images [8J have comfirmed this behavior and have shown that the corner holes of the Si(lll)-7 X 7 structures persist during the initial growth at RT. After high temperature (HT) annealing to 400 o C, CRBS measurements show a IO% decrease in Si surface intensity, indicating that about 1 x lOi Si atoms/cm2 with a few layers at the interface are returned to their lattice sites as a result of interface relaxation. Continuous annealing causes no further change. This indicates that, under the influence of Pb atoms and heat treatment, the Si adatoms become mobile and the underlying Si-7 X 7 surface relaxes, At the Pb/Si interface, the remaining Si adatoms are intermixed with Pb atoms to form a regular well-ordered flat layer. Direct images of Pb-Si mixed fi x fi structure have been observed using STM in HT annealed samples, where a Pb-Si ratio of 1~1 is shown,
It has been known for a long time that the presence of metals can greatly enhance the crystal growth velocity of elemental semiconductors. Rapid crystal growth of semiconductors by the VLS mechanism, which involves vapor, liquid, and solid phases, was first demonstrated by Wagner and Ellis [lO,ll]. Crystals resulting from this process normally from long dislocation-free whisker-like or filamentary protrusions. Solid phase epitaxy (SPE) grawth of Si and Ge with metals as transport media has also been investigated in detail flZJ. in the SPE process, samples are prep& in a configura~ t&n of ~e~~o~d~c~~r (amo~hous)-mew-senicon-
783
F. Xiong et al. / In situ RBS and channeling study
ductor (crystal). Upon annealing at a temperature well below the eutectic melting temperature, the atoms in the amorphous layer diffuse across the metal layer and grow epitaxially onto the substrate. The epitaxial layers grown in this way generally lack lateral uniformity and good crystalliuity due to contamination and inhomogeneity at the interface [12]. The thrust of our investigation is to explore a novel method for large area devicequality thin film growth of semiconductors at low temperature by MBE with liquid metal media, and to study the surface and interface physics involved in the process. The in situ RBS and channeling are used to monitor the growth process and to optimize the growth conditions as well as to characterize the grown films. As our first demonstration [13], we have successfully produced homoepitaxial Ge film on Ge(ll1) at low temperature by the VLS mechanism. A thin Au layer was first deposited onto clean and c(2 x 8) reconstructed substrates at RT from an ion sputtering source. We found that the as-deposited Au stayed uniform and flat on the substrate. Upon thermal annealing at 400“ C, as in the Au/Si(lll) system [14], the Au reacts with the surface Ge atoms to form a eutectic alloy. Any extra Au migrates to form islands with an average size of 2000 A. RBS measurements shown in fig. 4 reveal a change from a high intensity sharp peak to a low-intensity one with a diffused tail. Ex situ SEM analysis cornfirmed the RBS results of island formation [13]. In order to produce flat thin film growth, we have reduced the thickness of the Au layer. We find that 30 A is the optimum thickness for the
Energy
(MeV)
~~~~1
-0 15 ,-El E E lo b =
5 0 650
760
7jo
800
Channel Fig. 4. Channeling RBS spectra taken in situ during Ge MBE crystal growth with a liquid gold alloy medium. 0: the initial Ge(ll1) single frystal at the beginning, a: 30 A Au as-deposited, + ; 30 A Au deposited and annealed during the grown process, and X : Ge/Ge(lll) grown sample (1.5 h deposition at a rate of 1000 A/h). During the growth process the substrate was maintained at 400 o C.
Energy 1.6
650
700
(MeV)
1.7
750 Channel
1.9
1.8
800
Fig. 5. RBS Spectra of Ge/Ge(lll) with Au medium from as-grown (0 random and A channeled) and Au-etched (+ random and x channeled) samples.
growth process, and direct deposition at high temperature would stimulate Au-Ge mixing with better film uniformity. Ge was then deposited on the Au/Ge(lll) structure from an effusion cell at a rate of 1000 A/h. During the deposition, the substrate was maintained at 400°C, slightly higher than the eutectic point. In the example shown below, Ge was deposited for 1.5 h to achieve a final thickness of 1600 A. Fig. 4 shows channeling spectra taken during and after the growth. From the spectrum of the as-grown sample, we find that the grown Ge layers has very good crystallinity with a dechanneling yield around 6%. Au still appears directly on the surface, after the growth, mainly as islands. To confirm this conclusion, we used a selective chemical etching to remove the surface Au, and then performed RBS measurement on the general-purpose RBS beamline (175 ’ detection) in our laboratory. The resulting spectra are presented in fig. 5. The Ge peak shows a dechanneling yield close to that of the original single crystal. Two small Au peaks are observed, one directly at the surface and the other at the interface. The gap between these two peaks marks the thickness of the Ge epitaxial layer (1600 A), consistent with the deposition. The RBS data also shown that Au at both the surface and the interface has mixed with Ge forming Ge/Au layers - 15 A thick in the ratio of Ge : Au = 4 : 1, corresponding to the eutectic concentration of 28 at.% Au. At this stage we may conclude that the mechanisms of our liquid metal mediated MBE crystal growth is similar to that of the old VLS process. Au first eutectically mixes with surface Ge atoms, and then forms a supersaturated liquid alloy at temperatures higher than the eutectic point. The in-coming molecular beam of Ge dissolves into the liquid, and subsequently precipitates as a crystal at the IX. MATERIALS ANALYSIS
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liquid-substrate interface. We have also examined the grown samples by SEM and cross-sectional TEM, and found good crystal epitaxial growth of Ge, Au island formation on the surface, and Au trapping at the interface, in agreement with the RBS results. The detailed results will be presented elsewhere [13].
5. Summary remarks In summary, we have demonstrated in situ HEIS surface and interface studies of MBE growth of metal and semiconductor on semiconductors. In the Pb/ Si(ll1) system, HEIS measurements have directly and accurately determined the absolute Pb coverage associated with various surface structures. It has also revealed interface relaxation, Pb island formation and a 2D-3D reordering mechanism during high temperature annealing. Detailed isothermal desorption experiments reveal all phase transitions at submonolayer coverages. The results have provided conclusive information for an unambiguous interpretation of LEED and STM results on the surface structures of this system. This technical combination can be applied to solve problems in other similar metal-semiconductor systems. In situ HEIS monitoring of the MBE growth process has successfully assisted the control of the process and provided a very quick and nondestructive in situ characterization of the grown samples.
Acknowledgements This work is supported in part by the National Science Foundation (DMR-8920490) and the Office of
Naval Research (N00014-90-J-1234), and the Joint Services Electronics Program (N000014-89-J-1023). We would like to acknowledge Bill Fostor, Ing-Shouh Hwang, Anthony Leoser and Andrew Wagner for their technical assistance and helpful discussions, and Robert Martinez for his constructive comments on the manuscript.
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