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Ion channeling analysis of Ge-Si superlattices
Solid State Communications, Vol. 67, No. 6, pp. 661-663, 1988. Printed in Great Britain.
0038-1098/88 $3.00 + .00 Pergamon Press plc
ION C H A N N E L I N G ANALYSIS OF Ge-Si S U P E R L A T T I C E S G.Q. Zhao, Y.H. Ren and Z.Y. Zhou Accelerator Laboratory, Fudan University, Shanghai, China and X.J. Zhang, G.L. Zhou and C. Sheng Surface Physics Laboratory, Fudan University, Shanghai, China
(Received 15 April 1988 by M. Balkanski) Multilayered structures periodically consisting of layers of pure Si and alternate layers of Ge and Si, grown on (100) Si substrates by molecular beam epitaxy, have been analyzed using 2 MeV 4He ion axial channeling technique combined with Rutherford backscattering. Layer thicknesses and compositions have been extracted from random Rutherford backscattering spectra measured at a glancing geometry. Channeled spectra and angular scans in <1 0 0> and (1 1 0> directions provide information on the crystalline quality of Ge-Si superlattices. The preliminary results indicate the 5125nm Si/10(0.4nm Ge/0.6nm Si)]/(I 00)Si superlattice can be successfully grown.
A R T I F I C I A L L Y structured semiconductor materials consisting of thin alternate layers of different compositions are being widely studied in recent years [1]. One of these materials is the Ge-Si heterostructure on the Si substrates, which exhibits some new physical properties that Si material does not have and offers a potential use in both fundamental studies and device fabrications. Two types of structures consisting of pure Si layers and Ge/Si alternate layers [2], or pure Si layers and Ge~Si~ x alloy layers [3] have been successfully grown on silicon without misfit dislocations by molecular beam epitaxy (MBE) if sufficiently small thickness of lattice mismatched layers are employed. In the studies of superlattices ion channeling and backscattering techniques have been used as p~werful tools to determine the crystalline quality, layer thickness, uniformity, composition and strain of multilayered structures [3-5], In this Communication we report a preliminary study, using ion axial channeling technique combined with Rutherford backscattering, for periodically muitilayered structures consisting of pure Si layers and Ge/Si alternate layers. Ge/Si strained-layer superlattices (SLS) were grown on (I 00)Si substrates by MBE in a UHV electron beam evaporation system made by Riber Company. The base pressure of the system was about 5 × 10 t~ torr. The substrate temperature for growing SLS was 550°C and the deposition rate was controlled to be within 0.02nm s ~. Prior to the
growth of SLS substrates were cleaned and annealed and then a 100nm epitaxial film of pure Si was deposited on the substrate as a buffer layer. A small part of the surface of Si substrate was masked using a Mo plate during depositions. This part of substrates would be used as a virgin Si crystal in channeling experiments. One pure Si layer of 25 nm in thickness was deposited by following an alternate Ge/Si deposition layer of 10nm, and each Ge/Si alternate layer consisted of 10 layers (10 "small period") of 0.4 nm Ge and 0.6 nm Si. This sequence was repeated five times (5 "large period"), resulting in a total film thickness of ! 75 nm. Backscattering and channeling measurements were performed using 2 MeV 4He ions from the Van de Graaff at Fudan University. Samples were mounted on a two-axis goniometer controlled by microprocessor and step motors. Back-scattered ions were detected using a surface barrier detector located at an angle of 160 ° with respect to the beam direction. In order to enhance the depth resolution of backscattering measurements, samples were tilted to 55 °. A random backscattering spectrum measured at the glancing angle is shown in Fig. 1 for one of 5[25 nm Si/10(0.4 nm Ge/0.6 nm Si)]/(1 0 0)Si superlattice samples. It can be seen that the Ge peaks are well resolved. These peaks are fitted by Gaussian functions in data processing. The thickness per pair of pure Si and Ge/Si alternate layer is determined by the position of the maxima of these oscillating Ge peaks using the stopping power for the average composition of the
661
ION C H A N N E L I N G ANALYSIS O F Ge-Si S U P E R L A T T I C E S
662 3000
55"
'W~,-~,¢,:d% '
( 1 0 0 ) ~ 2MeV ,*He+
2000
o
Ge
1
0
E
g
tO
..~
iooo
~ **
"V
140
I
j . . %J, ,p
;: .....; o.,°
•
.;v"
,,j
240
340
440
Chonnel
Fig. 1. Glancing angle backscattering spectrum of 2 MeV 4He ions for a 5125 nm Si/10(0.4 nm Ge/0.6 nm Si)]/(l 0 0)Si superlattice sample. sample and the thickness of Ge/Si alternate layers can be extracted from the peak width. However the thickness of Ge and Si in each Ge/Si alternate layer is only determined from a knowledge of the growth conditions or from an observation of cross-sectional transmission electron microscopy. The composition is determined by the Ge peak area and the Si backscattering yield. The obtained thicknesses of the pure Si layers and Ge/Si alternate layers and average composition of Ge in each Ge/Si alternate layer are in agreement with the values controlled during epitaxial growth. Figure 2 shows channeled spectra for the and ~1 1 0) axes of the same sample as in Fig. 1. Along the < 1 0 0 ) growth direction the aligned spectrum is nearly identical to that for a virgin Si substrate. The 1 0 0) channeling result indicates a good crystalline quality of the Ge-Si superlattice studied. However, along the ~1 1 0) inclined direction relative to the growth direction an increase of dechanneling for the superlattice sample over that for the Si substrate is observed. The dechanneling is linearly increasing with depth and at the depth corresponding to the interface between SLS and Si substrate the slope in dechanneling is decreased obviously. The high dechanneling rate along the ( l 1 0) axis is due to the presence of the strain which results in a tilt angle relative to the inclined crystal axis at the interface between layers in SLS. The channeling angular scans along the ( 1 0 0 ) and (1 1 0) axes also provide information on the crystalline quality of samples. The minima of normalized yields (Zmm, ratio of the channeling to random yields) in angular scans for whole superlattice layers of the same sample are 4.3% and 4.5% for Si and Ge, respec-
Vol. 67, No. 6
tively, along the < 1 0 0 ) direction. The Zmin value shows the crystalline quality for this SLS sample is quite good if compared with the value obtained for the 100nm thick GexSit x alloy layer with the same average composition of Ge [5]. In conclusion, ion channeling and backscattering are excellent tools for the studies of Ge-Si superlattices. Channeling data indicate a good epitaxial growth for multilayered Si/(Ge/Si) structures on the Si substrates has been achieved. The glancing backscattering measurements are sufficient to separate different Ge/Si alternate layers and to indicate the changes in composition, layer thickness and uniformity of SLS. Further studies on the strain measurements will be done for different compositions and thicknesses of layers.
I (a') < I 0 o>channelin(J
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Fig. 2. Random and channeled ion backscattering spectra for the sample as in Fig. I. Channeled spectra are shown for (a) normal < 1 0 0 ) axis and (b) inclined < 1 1 0 ) axis.
Vol. 67, No. 6
ION CHANNELING ANALYSIS OF Ge-Si SUPERLATTICES
Acknowledgements - - The authors are grateful to
2.
Prof. X.D. Xie, F.J. Yang and X. Wang for encouragement during this work. 3. 4. REFERENCES I.
L.L. Chang & K. Ploog, Molecular Beam Epitaxy and Heterostructures, Nijhoff, Boston (1985).
5.
663
J. Bevk, J.P. Mannaerts, L.C. Feldman, B.A. Davidson & A. Ourmasd, Appl. Phys. Lett. 49, 286 (1985). J.C. Bean, Science, 230, 306 (1986). S.T. Picraux, L.R. Dawson, G.C. Osbourn & W.K. Chu, Nucl. Instr. Meth. 218, 57 (1983). J.C. Bean, L.C. Feldman, A.T. Fiory, S. Nakahara & K. Robision, J. Vac. Sci. Technol. A2, 436 (1984).
0038-1098/88 $3.00 + .00 Pergamon Press plc
ION C H A N N E L I N G ANALYSIS OF Ge-Si S U P E R L A T T I C E S G.Q. Zhao, Y.H. Ren and Z.Y. Zhou Accelerator Laboratory, Fudan University, Shanghai, China and X.J. Zhang, G.L. Zhou and C. Sheng Surface Physics Laboratory, Fudan University, Shanghai, China
(Received 15 April 1988 by M. Balkanski) Multilayered structures periodically consisting of layers of pure Si and alternate layers of Ge and Si, grown on (100) Si substrates by molecular beam epitaxy, have been analyzed using 2 MeV 4He ion axial channeling technique combined with Rutherford backscattering. Layer thicknesses and compositions have been extracted from random Rutherford backscattering spectra measured at a glancing geometry. Channeled spectra and angular scans in <1 0 0> and (1 1 0> directions provide information on the crystalline quality of Ge-Si superlattices. The preliminary results indicate the 5125nm Si/10(0.4nm Ge/0.6nm Si)]/(I 00)Si superlattice can be successfully grown.
A R T I F I C I A L L Y structured semiconductor materials consisting of thin alternate layers of different compositions are being widely studied in recent years [1]. One of these materials is the Ge-Si heterostructure on the Si substrates, which exhibits some new physical properties that Si material does not have and offers a potential use in both fundamental studies and device fabrications. Two types of structures consisting of pure Si layers and Ge/Si alternate layers [2], or pure Si layers and Ge~Si~ x alloy layers [3] have been successfully grown on silicon without misfit dislocations by molecular beam epitaxy (MBE) if sufficiently small thickness of lattice mismatched layers are employed. In the studies of superlattices ion channeling and backscattering techniques have been used as p~werful tools to determine the crystalline quality, layer thickness, uniformity, composition and strain of multilayered structures [3-5], In this Communication we report a preliminary study, using ion axial channeling technique combined with Rutherford backscattering, for periodically muitilayered structures consisting of pure Si layers and Ge/Si alternate layers. Ge/Si strained-layer superlattices (SLS) were grown on (I 00)Si substrates by MBE in a UHV electron beam evaporation system made by Riber Company. The base pressure of the system was about 5 × 10 t~ torr. The substrate temperature for growing SLS was 550°C and the deposition rate was controlled to be within 0.02nm s ~. Prior to the
growth of SLS substrates were cleaned and annealed and then a 100nm epitaxial film of pure Si was deposited on the substrate as a buffer layer. A small part of the surface of Si substrate was masked using a Mo plate during depositions. This part of substrates would be used as a virgin Si crystal in channeling experiments. One pure Si layer of 25 nm in thickness was deposited by following an alternate Ge/Si deposition layer of 10nm, and each Ge/Si alternate layer consisted of 10 layers (10 "small period") of 0.4 nm Ge and 0.6 nm Si. This sequence was repeated five times (5 "large period"), resulting in a total film thickness of ! 75 nm. Backscattering and channeling measurements were performed using 2 MeV 4He ions from the Van de Graaff at Fudan University. Samples were mounted on a two-axis goniometer controlled by microprocessor and step motors. Back-scattered ions were detected using a surface barrier detector located at an angle of 160 ° with respect to the beam direction. In order to enhance the depth resolution of backscattering measurements, samples were tilted to 55 °. A random backscattering spectrum measured at the glancing angle is shown in Fig. 1 for one of 5[25 nm Si/10(0.4 nm Ge/0.6 nm Si)]/(1 0 0)Si superlattice samples. It can be seen that the Ge peaks are well resolved. These peaks are fitted by Gaussian functions in data processing. The thickness per pair of pure Si and Ge/Si alternate layer is determined by the position of the maxima of these oscillating Ge peaks using the stopping power for the average composition of the
661
ION C H A N N E L I N G ANALYSIS O F Ge-Si S U P E R L A T T I C E S
662 3000
55"
'W~,-~,¢,:d% '
( 1 0 0 ) ~ 2MeV ,*He+
2000
o
Ge
1
0
E
g
tO
..~
iooo
~ **
"V
140
I
j . . %J, ,p
;: .....; o.,°
•
.;v"
,,j
240
340
440
Chonnel
Fig. 1. Glancing angle backscattering spectrum of 2 MeV 4He ions for a 5125 nm Si/10(0.4 nm Ge/0.6 nm Si)]/(l 0 0)Si superlattice sample. sample and the thickness of Ge/Si alternate layers can be extracted from the peak width. However the thickness of Ge and Si in each Ge/Si alternate layer is only determined from a knowledge of the growth conditions or from an observation of cross-sectional transmission electron microscopy. The composition is determined by the Ge peak area and the Si backscattering yield. The obtained thicknesses of the pure Si layers and Ge/Si alternate layers and average composition of Ge in each Ge/Si alternate layer are in agreement with the values controlled during epitaxial growth. Figure 2 shows channeled spectra for the
Vol. 67, No. 6
tively, along the < 1 0 0 ) direction. The Zmin value shows the crystalline quality for this SLS sample is quite good if compared with the value obtained for the 100nm thick GexSit x alloy layer with the same average composition of Ge [5]. In conclusion, ion channeling and backscattering are excellent tools for the studies of Ge-Si superlattices. Channeling data indicate a good epitaxial growth for multilayered Si/(Ge/Si) structures on the Si substrates has been achieved. The glancing backscattering measurements are sufficient to separate different Ge/Si alternate layers and to indicate the changes in composition, layer thickness and uniformity of SLS. Further studies on the strain measurements will be done for different compositions and thicknesses of layers.
I (a') < I 0 o>channelin(J
~
3000F
h£
Ron dora
-$
200
4
"
g g
Ge
l
~100(
(..,)
SLS
0 s i 140
.
,
~ ~ 240
. 340
440
crw:mnel (b)
3000
Rondom
"
6 2ooc *
E g
t~
Ge
SLS ~IIO> aligned
I000
I
,*
,
Io oE
140
/
'~'~"~-,,~"~;. , 240
340
**
440
Channel
Fig. 2. Random and channeled ion backscattering spectra for the sample as in Fig. I. Channeled spectra are shown for (a) normal < 1 0 0 ) axis and (b) inclined < 1 1 0 ) axis.
Vol. 67, No. 6
ION CHANNELING ANALYSIS OF Ge-Si SUPERLATTICES
Acknowledgements - - The authors are grateful to
2.
Prof. X.D. Xie, F.J. Yang and X. Wang for encouragement during this work. 3. 4. REFERENCES I.
L.L. Chang & K. Ploog, Molecular Beam Epitaxy and Heterostructures, Nijhoff, Boston (1985).
5.
663
J. Bevk, J.P. Mannaerts, L.C. Feldman, B.A. Davidson & A. Ourmasd, Appl. Phys. Lett. 49, 286 (1985). J.C. Bean, Science, 230, 306 (1986). S.T. Picraux, L.R. Dawson, G.C. Osbourn & W.K. Chu, Nucl. Instr. Meth. 218, 57 (1983). J.C. Bean, L.C. Feldman, A.T. Fiory, S. Nakahara & K. Robision, J. Vac. Sci. Technol. A2, 436 (1984).