Chromium disilicide layers have been grown on Si(111) in a commercial molecular‐beam epitaxy machine. Thin layers (10 nm) exhibit two epitaxial relationships, which have been identified as CrSi_2(0001)//Si(111) with CrSi_2[1010]//Si[101], and CrSi_2(0001)//Si(111) with CrSi_2[1120]//Si[101]. The latter case represents a 30° rotation of the CrSi_2 layer about the Si surface normal relative to the former case. Thick (210 nm) layers were grown by four different techniques, and the best‐quality layer was obtained by codeposition of Cr and Si at an elevated temperature. These layers are not single crystal; the largest grains are observed in a layer grown at 825 °C and are 1–2 μm across

Chang, K. T.

Fathauer, R. W.

Grunthaner, P. J.

Jamieson, D. N.

Lin, T. L.

Mazur, J. H.

English

Caltech Authors - Main

Molecular-beam epitaxy of CrSi_2 on Si(111)

Molecular-beam epitaxy of CrSi2 on Si(111) 
R. W. Fathauer, P. J. Grunthaner, and T. L. lin 
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 
K. T. Chang and J. H. Mazur 
Materials Science Department, University ofSouthern California, Los Angeles, California 90089-0241 
D. N. Jamieson 
Division of Applied Physics, California Institute of Technology, Pasadena, California 91125 
(Received 9 September 1987; accepted 17 November 1987) 
Chromium disilicide layers have been grown on Si ( 111) in a commercial molecular-beam epitaxy 
machine. Thin layers ( l 0 nm) exhibit two epitaxial relationships, which have been identified as 
CrSi2 (0001)//Si(lll) with CrSi2 [10lO]//Si[10l], and CrSi2 (0001)//Si(lll) with 
CrSi2 [ 1120 ]I /Si [ 101]. The latter case represents a 30° rotation of the CrSi2 layer about the Si 
surface normal relative to the former case. Thick (210 nm) layers were grown by four different 
techniques, and the best-quality layer was obtained by codeposition of Cr and Si at an elevated 
temperature. These layers are not single crystal; the largest grains are observed in a layer grown at 
825 oc and are 1-2 11m across. 
I. INTRODUCTION 
Heteroepitaxy of a variety of materials on Si has been dem-
onstrated, including metals, insulators, and other semicon-
ductors. In the case of semiconductors, the growth of Ge, 1 
SiGe alloys, 2 and GaAs3 has been explored in detail. Both 
Ge and GaAs have lattice mismatches with Si of ~4%, and 
therefore are only suitable for devices localized near the sur-
face of a thick epitaxial semiconductor layer. For devices 
requiring semiconductor heterojunctions, SiGe alloys have 
been employed.2 The magnitude of the band discontinuities 
between these alloys and Si is limited by the lattice mismatch 
and relatively small difference in band gaps. 
Epitaxy of other metal silicides on Si has been reported, 
including CoSi2 , 4 NiSi2, 5 PdSi2, 6 PtSi,6 YSi2 _ x• 7 and numer-
ous refractory metal silicides. 8 High-quality epitaxial 
growth has only been demonstrated for materials with cubic 
structures though, and the best results have been obtained 
for CoSi2 and NiSi2 •4 ' 5 Both of these materials possess a close 
lattice match to Si and have the cubic-fluorite structure, 
which is very similar to the diamond structure of Si. 
Chromium disilicide is a semiconducting silicide with a 
band gap of0.3 eV.9 It possesses a hexagonal (space group 
P6222, or C40) structure with a0 = 0.4428 nm and 
c0 = 0.636 nm, and lattice matches Si(lll) to 0.1% [Fig 
1 (a)]. If CrSi2 layers with high-quality interfaces can be 
grown on Si, heterojunction devices with large band discon-
tinuities and small strain would be possible. In addition, the 
availability of a small-band-gap semiconductor on Si would 
allow the monolithic integration ofCrSi2-based devices such 
as medium-wavelength infrared detectors with signal-pro-
cessing circuitry based in the Si. 
Localized epitaxy of CrSi2 on Si( 111 ) has been reported 
by Shiau et al. 10 for layers formed by Cr deposition in a 
vacuum of 0.3-2.0 X 10-6 Torr. The samples were annealed 
after deposition in a separate vacuum furnace, and epitaxial 
regions of <211m extent were obtained. In addition, Shiau et 
a/. 11 have reported localized epitaxy ofCrSi2 on Si( 100) and 
Si( 110), though the quality of the layers was poorer than on 
the (111) orientation. We report here the first growth of 
CrSi2 on Si(lll) in a commericial molecular-beam epitaxy 
(MBE) system, with a base pressure in the low 10- 11 Torr 
range. 
II. EXPERIMENTAL PROCEDURE 
Si ( 111) wafers, 2-in. diameter, were chemically cleaned 
by a process which leaves a thin, protective oxide on the 
wafers. 12 The wafers were then loaded in a RIBER EVA 32 
molecular-beam epitaxy machine, and the oxide was re-
moved by either a Si beam 13 or by thermal desorption. 12 This 
system is equipped with separate electron-beam evaporators 
which are used for Si and Cr. 
The CrSi2 layers were grown by four different techniques, 
which are described as follows. The first of these consists of 
the deposition ofCr on a room-temperature Si substrate fol-
lowed by annealing at elevated temperature. While this tech-
nique has been referred to as solid-phase epitaxy in work on 
CoSi2 and NiSi2, we will refer to it as solid-phase reactive 
epitaxy ( SRE) to signify the fact that considerable interdif-
fusion of the constituent species must occur. The second 
technique consists of the deposition of Cr on a hot Si sub-
strate, and is denoted reactive deposition epitaxy ( RDE), in 
keeping with earlier work on CoSi2 • In the third technique, 
Cr and Si are codeposited at room temperature in stoichio-
metric ratio and annealed at elevated temperature, which we 
will refer to as solid-phase epitaxy (SPE). The final tech-
nique consists of codeposition of Cr and Si in stoichiometric 
ratio on a hot Si substrate, and is referred to as MBE, also in 
keeping with earlier work on CoSi2• For all four techniques, 
the temperature reported is the maximum temperature to 
which the sample was subjected. 
The wafers were monitored in situ during growth using 
reflection high -energy electron diffraction ( RHEED) . 
After removal from the MBE system, samples were analyzed 
by scanning electron microscopy ( SEM), transmission elec-
tron microscopy (TEM), and Rutherford backscattering 
spectroscopy ( RBS). 
708 J. Vac. Sci. Techno!. B 6 (2), Mar/Apr 1988 0734-211X/88/020708-05$01.00 @ 1988 American Vacuum Society 708 
709 Fathauer et al.: MBE of CrSi2 on Si(111) 
0 + 0 
0 
+ 
~ 0 
+ 0 
® 0 
+ 
Ia! 
0 
+ 
0 
~· 0 
0 
+ 
0 
+ ®-r 
+ o. 
I 
~-~I" 
+ 
709 
0 0 0 
0 0 
®+ +® 
0 0 
0 0 0 
(b) 
FIG. 1. Schematic diagrams of a single basal plane of the CrSi2 lattice superimposed on a single plane of the Si lattice, where 8 represents Cr atoms, 0 
represents Si atoms in CrSi2, and + represents Si atoms in the Si(lll) substrate. (a) One possible alignment with 0.1% mismatch and (b) one possible 
alignment with 14% mismatch. In (b), the CrSi2 latticc is rotated by 30• with respect to the Si lattice relative to (a). 
Ill. RESULTS 
Four different 10-nm-thick CrSi2 layers have been grown, 
three by SRE at 580, 650, and 720 •c, and one by RDE at 
650 °C. SEM micrographs of these samples are shown in Fig. 
2. An increasing tendency to form islands is noted in the 
SRE samples at higher temperatures, while the RDE sample 
exhibits smaller islands. R.HEED patterns of the SRE layers 
show very diffuse streaks at 580 ·c and sharper streaks at 
higher temperatures (though the Si substrate may be con-
tributing in this case). The RHEED pattern for the RDE 
layer shows spots indicative of transmission of the electron 
(a I (b) 
ic) (d) 
J. Vac. Sci. Techno!. B, Vol. 6, No.2, Mar/ Apr 1988 
beam through islands. Planar TEM analysis of this sample 
reveals pronounced faceting of many of these islands. In ad-
dition, TEM analysis shows that all of these CrSi2 layers 
consist predominantly of grains with one of two orienta-
tions. In the first of these, the CrSi2 is oriented as expected 
(the orientation with a 0.1% mismatch) with respect to the 
Si substrate, i.e., with CrSi2 (0001)//Si(lll) and 
CrSi2 [10lO]I/Si[10l]. In the second, the CrSi2 is rotated 
30° with respect to the Si substrate about the surface normal 
relative to the :first case, i.e., with CrSi2 (0001)//Si(lll) 
and CrSi2 [ 1120 J/ /Si [ 10 I] . The mismatch for this rotated 
orientation is 14%. Electron diffraction patterns taken from 
Fro. 2. SEM micrographs of 10-
nm-thick CrSi2 layers grown on 
Si (Ill) using SRE with annealing 
at (a) 5BO, (b) 650, and (c) 
720 •c, and (d) using RDE at 
650 "C. The microscope stage was 
tilted 70• with respect to the elec-
tron beam for these layers. 
710 Fathauer eta!.: MBE of CrSi2 on Si(111) 
regions of planar samples with these two orientations are 
shown in Fig. 3. [Note that the extra spots in Fig. 3(b) are 
due to double diffraction.] Bright- and dark-field TEM mi-
crographs taken in cross section from the 720 oc SRE sam-
ple are shown in Fig. 4, which shows a region of the sample 
having an island with each of these orientations. 
Thicker layers ( 210 nm) have also been examined, and in 
this case all four growth techniques discussed above were 
employed. Figure 5 shows SEM micrographs ofCrSi2 layers 
grown with these four techniques, where the maximum tem-
perature is 650 •c for all layers. RHEED patterns for these 
samples show a streak pattern for the MBE layer, a more 
diffuse streak pattern for the RDE layer, and almost feature-
less patterns for the SRE and SPE layers. Very faint rings 
and spots are also seen in the RHEED pattern for the MBE 
layer, indicating the presense of some misoriented grains. 
Analysis of the RDE, SPE, and MBE layers with RBS shows 
them to be stoichiometric CrSi2 throughout the depth of the 
layer, and there is no evidence of tailing of Cr into the sub-
strate. However, no channeling was observed, which sug-
gests that there is no single epitaxial orientation occurring 
over a large fraction of the layer. On the other hand, the 
streak pattern observed for the MBE layer by RHEED sug-
gests that much of the layer has a single orientation. Prelimi-
nary TEM results show some grains with the expected orien-
tational relationship, but additional grains of other 
orientations. 
While the surface morphology of the SPE sample is much 
better than that of the other samples, the RHEED pattern 
suggests that this layer did not fully recrystallize. As the 
RHEED pattern of the MBE layer exhibited the sharpest 
streaks, and SEM revealed the largest single-crystal grains 
for this sample, higher temperature MBE growth was exam-
ined. The SEM micrographs of Fig. 6 show MBE layers 
grown at 650, 750, and 825 oc. The grain size is seen to in-
crease significantly with temperature, but RBS analysis of 
the 825 •c sample still failed to reveal channeling. 
J. Vac. Sci. Techno!. B, Vol. 6, No.2, Mar/Apr 1988 
IV. DISCUSSION 
710 
FIG. 3. Transmission electron dif-
fraction (TED) patterns taken from 
a 20-nm-thick CrSi2 layer grown by 
SRE at 720 'C. (a) A TED pattern 
from an area exhibiting the expected 
epitaxial relationship [Fig. 1 (a)], 
namely, CrSi2 (0001 )//Si(lll) 
with CrSi2 [10lO]//Si[!Ol]. (b) A 
TED pattern from an area exhibiting 
a 30' rotation about the Si surface 
normal [Fig. l(a}], CrSi2 (0001 }// 
Si(lll) with CrSi2 [11ZO]// 
Si[ lOT]. 
The initial attempts reported herein at growing CrSi2 epi-
taxiallayers in an MBE machine have not been successful in 
producing single-crystal layers. This is due at least in part to 
{a} 
{b) 
(cl 
FIG. 4. TEM micrographs of a 10-nm-thick CrSi2 layer grown on Si( Ill) 
using the SRE technique with an annealing temperature of720"C. (a) A 
bright-field micrograph showing two CrSi, islands (marked by arrows) on 
the Si substrate. (b) A dark-field micrograph formed with a diffraction spot 
(see inset TED pattern) from the unrotated material, showing that the 
island on the right-hand side is of this orientation. (c) A dark-field micro-
graph formed with a diffraction spot (see inset TED pattern) from the 30·-
rotated material, showing that the island on the left-hand side is of this 
orientation. (All three micrographs have the same scale, and the layer on 
top of the CrSi, is epoxy.) 
711 Fathauer et al.: MBE of CrSi2 on Si(111) 
(a) (b) 
(c) ldl 
FIG. 6. SEM micrographs of 210-nm-thick CrSi2 layers grown on Si( 111) 
using the MilE growth technique at (a) 650, (b) 750, and (c) 825 °C. In 
contrast to the micrographs of Figs. 2 and 5, the stage is not tilted in this 
case. 
J. Vac. Sci. Techno!" B, Vol. 6, No.2, Mar/Apr Hl88 
711 
FIG. 5. SEM micrographsof210-
nm-thick CrSi2 layers grown on 
Si(lll) using (a) SRE, (b) 
RDE, (c) SPE. and (d) MBE 
growth techniques. The maxi-
mum temperature in all four 
cases is 650 T, and the micro-
scope stage was tilted 70° with re-
spect to the electron beam for 
these images. 
the tendency of CrSi2 layers on Si( 111) to form islands 
which assume more than one orientation with respect to the 
Si substrate. In addition to the expected epitaxial relation-
ship, a 30"-rotated relationship is commonly observed. 
While this orientation possesses a high degree of symmetry 
with respect to the Si substrate, the mismatch is 14% (as 
compared to 0.1% in the unrotated case). As seen in Fig. 
1 (a), the epitaxial relationship in the 0.1 %-mismatch case 
has a relatively large unit cell, so that a variety of bonding 
arrangements must occur. In contrast, were it not for the 
large mismatch in the rotated case, a one-to-one correlation 
of Cr atoms and substrate Si atoms would be found" Epitax-
ial nucleation of the rotation orientation does not therefore 
seem unlikely, but the large mismatch should prevent grains 
of appreciable size from developing. The fact that they are 
observed may indicate weak interfacial bonding between 
CrSi2 and Si. We note that this 30c-rotated orientation is not 
reported by Shiau et al. 10 This may be due to the difference in 
cleanliness between the two studies. 
It appears as if there is a greater variety of orientations 
present in the thicker layers than in the thin layers, though 
TEM analysis of these layers is incomplete at this time. The 
increasing grain size noted with increasing temperature for 
MBE layers implies that higher temperatures may aid in 
improving film quality. However, better understanding of 
the nucleation of islands is needed; in particular, a growth 
technique which results in the nucleation of islands predomi-
nantly of a single type is desired. 
V. SUMMARY AND CONCLUSIONS 
CrSi2 layers grown on Si ( 111) exhibit a strong tendency 
to form islands. Two particular epitaxial relationships are 
commonly observed, but other orientations are observed as 
well. The commonly occurring orientations are 
712 Fathauer et al: MBE of CrSi2 on Si( 111) 
CrSi2 (0001 )/ /Si(lll) with CrSi2 [ lOlO]/ /Si[ lOl] and 
CrSi2 (0001 )/ /Si(lll) with CrSi2 [ llZO]/ /Si[ lOT]. The 
latter case represents a 30° rotation of the CrSi2 layer about 
the surface normal as compared to the former case. For thick 
layers, the largest grains and best-quality RHEED pattern 
are obtained for layers grown by MBE ( codeposition of Cr 
and Si at elevated temperature). Furthermore, the grain size 
is observed to increase with increasing substrate tempera-
ture, with grains of 1-2 t-tm extent observed in a layer depos-
ited at 825 °C. 
ACKNOWLEDGMENTS 
We would like to thank R. Ruiz for SEM analysis and B. 
Pate for technical assistance. The research described in this 
paper was carried out by the Jet Propulsion Laboratory 
(JPL), California Institute of Technology, and was sup-
ported by the Strategic Defense Initiative Organization, In-
novative Science and Technology Office and the National 
Aeronautics and Space Administration. The work was per-
formed as part of the JPL's Center for Space Microelectron-
ics Technology. TEM work was performed at the University 
of Southern California, Center for Microscopy and Microa-
nalysis, and was supported by a University of Southern Cali-
J. Vac. Sci. Techno!. B, Vol. 6, No.2, Mar/Apr 1988 
712 
fornia Fac:;ulty Research and Innovation Fund award to Dr. 
J. H. Mazur. 
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Molecular-beam epitaxy of CrSi_2 on Si(111)

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