- Email: [email protected]
Microstructural characterization of iron ion implantation of silicon carbide
Nuclear
Instruments
and Methods
in Physics Research
B65
Nuclear lnstnments 8 Methods in Physics Research s~~C~lloil D
( 1902)345-351
N~~rth-H(~ll~lnd
Microstructural characterization of silicon carbide
In order to better use implantation species is needed. channeling “Fe
analytical
electron
have heen used to characterize
ions to fluences of I. 3, and hX IO’” ions/cm-‘.
even at the lowest fluence. AEM depth
to improve the properties
In this investigation,
(RIG-C)
of the peak
agreement
was performed
iron concentration
with the value calculated
ex~min~iti~~ns of the implanted
of iron ion implantation
measured by the TRIM
of ceramics, a detailed
microscopy
single crystal a-silicon RBS-ion
channeling
for both cross-sectional
carbide
implanted
demonstrated X-ray
of the nature of implanted
hackscattering
spectroscopy-ion
at room temperature
that the implanted
with IhO keV
region was amorphous
specimens with the highest fluence. The
spectroscopy
and with RBS were in ~{ppr(~xinl~t~
code assuming a specimen density equal to cryrtalline
silicon carhidc. The AEM
reyittns in this study did not reveal the presence of any pr~~ipit~~tes. ~ug&e~tin~ that the iron has not
into precipitates
implanted
specimens suggests that the iron is not metallically-banded;
(~)hse~~ti(~n
limit
= 2 nmt.
MGssbauer spectroscopy ICYEMS) that the iron is covalently
lnsp~~ti(~li of the electron
energy
loss fine structure
this result supports the conclusion
for iron
in the
from converGon electron
bonded.
1. Introduction High temperature environments arc becoming increasingly important for advanced technologies ranging from fusion reactors to advanced heat engines. Structural ceramics have excellent high temperature propcrtics, but suffer from poor mechanical properties for many of the anticipated applications. An cxtcnsivc effort is underway to improve the mechanical propertics of ceramic materials. One avenue to improve ccramic materials is the implantati~)n of other atomic species into the region within a few micrometers of the surface. Effcctivc utilization of this tcchniquc requires that an understanding bc developed of the fundamcntal materials science of the interactions of the implanted species with the host. Extcnsivc work has been done to characterize the nature (charge state, bonding, etc.) of the implanted species for Al,O,. (See, for example, the review by White ct al. [I]). The goal of
0
understanding
and Rutherford
and hack-thinned
with energy dispersive
eoalcsced
Ol~X-~X.3X/Y2/$OS.OO
(AEM)
the current investigation is to apply these same characturization tcchniqucs to the study of ion-implanted silicon carbide. one important non-oxide ccromic for high-tcmpcraturc structural applications. Earlier studies of the effect of ion implantation in SIC have demonstrated that it is easily amorphizcd for low ion fluenccs. The critical damage energy density for amorphization of ru-Sic at room tcmpcraturc has been found to lit hctwccn 0.02 and 0.05 keV/atom [Z-4]. Few ch~lracterization tcchniqucs can provide informati~~n about the structure of ~~rn(~rph(~usmaterials. In recent work reported by McHarguc ct al. [S], conversion clcctron Miissbaucr spectroscopy (GEMS) was used to determine the local cnvironmcnt of the “Fe implanted into single crystals of cu-Sic at room tcmpcraturc to flucnccs of 1. 3. and h x IO’” ions/cm”. The CEMS spectra were fit with a single component having a distribution of quadrupolc split values (see fig. I). The results suggcstcd a distribution of the iron ions
1992 - Elsevier Science Puhlishers B.V. All rights reserved
V. OXIDES/CERAMICS/CARBIDES
-0
0.5
1.0
OUADRUPOLE
1.5
2.0
SPLITTING
2.5
3.0
(mm/s)
Fig. I. Distribution of quadrupolt: splitting (QS) used to fit the CEMS data for Fc-implanted cr-Sic; (a) I x 10”‘. (b) 3 X 10”‘. and (c) 6 X IO’” ions/cm’.
among scvcral sites with slightly different local symmctries and the prcscncc of cctvalcnt F~-~(~nlp(~unds or low spin iron complcxcs with covalent bonding. One purpose of the current study was to further charactcrizc the same spccimcns used f&r the CEMS examination with analytical clcctron microscopy (AEM) and Kuthcrford backscattcring spectroscopy (RBS) to obtain additional information about the implanted iron and to ascertain whcthcr any secondary phases wcrc prcscnt in the implanted region.
2. Experimental
procedure
The single crystal
dc Ciraaff accclcrator with a scattering angle of lh(t”. Channeling experiments were pcrformcd with the ion beam parallel to [OOt)l]. The ion irnpl~~~it~tt~~)~~s and ion spectroscopy cxpcriments were pcrformcd with instruments located at the Oak Ridge National Laboratory (ORNL) Surface Modification and Characterization Facility. Analytical electron microscopy was pcrformcd with ;I IO0 kV Philips EM 300T cquippcd with a field emission gun. a 6585 scanning transmission electron microscopy (STEM) unit, an EDAX PW 9101)77) encrgy dispersive X-ray spcctrometcr (EDS), and ;I Gatan hOA Parallel-dctcction Electron Energy l_oss Spcctrometcr (PEELS). X-ray microanalysis line-scans wcrc pcrformed in the STEM mode with a prohc of I, 3 nm diamctcr. with currents of > i nA, and pro~iucing X-ray count rates of’ 2000 to 3000 counts/s. An analoguc scan was employcd; ench data point in the EDS profile was obtained in 3 s. During this 3 s interval the probe advanced 7.4 nm. Sincc this distuncc is greater than the probe size and the cxpcctcd beam broaicning, it dcfincs the spatial resolution. Thu precision o! the peak composition is limited to < .7? by counting statistics. Intcgratcd X-ray intensities. Itc,S, wcrc convcrtcd to composition C’, c,s, (at.%) from the rclationship: C‘,.,/C,i = kb,,silb,/ls,, whcrc k ,,L.,s,= 0.707. as calculated with an updated version of the computer coclc (NEDQNTZ) hascd on the standardlcss approach of zaluzcc [h]. Spccimcnb wcrc prepared in both cross-section and plan-view gcomctries. The cross-sectional spccimcns wcrc prcparcd using the standard tcchniqucs cstablishcd for ceramics. The implanted surl’accs of two sections wcrc bonded with MBOND 610 adhcsivc epoxy; the resulting sandwich was ground and polished with diamond paste to a thickness of about 100 pm then dimpled with a Gatan Model 0% dimplor to a final thickness at the hottom of the dimple of about 20 pm. This specimen was mounted on a carbon ring and ion-milled at room temperature to perforation in ii C;atan Model hOODIS DuoMill ion mill opcrutcd with h kcV argon ions and a 15” milling angle. The plan-view specimens wcrc prcparcd with a special technique to allow examination near the peak iron region. In this tcchniquc. after the spccimcn was ground, polished, and dimpled from the tlriinipl~~nt~d surfact: to a thickness at the bottom of the dimple of about 30 1J-m; the impl~lnted surface of the specimen was sectioned with an ion mill to rcmovc ahout 80 nm. The milling conditions wcrc determined from trial runs in which half of the spccimcn surface was masked; thcsc conditions wcrc 1.7 min at 3 keV with a milling angle of 20”. After sectioning, the implanted surface was coated with salt to allow easy removal of any material deposited during the back-thinning operation of the ion mill [7]. Back-thinning operations were pcr-
formed at room temperature with 6 keV ions and a 20” milling angle. M~ssbau~r spectra were obtained with the tcchnique of conversion electron Miissbaucr spectroscopy (CEMS) [S]. The spectra were obtained at room tempcrature with a backscattercd geometry. The “7Co source was contained in a rhodium matrix and was mounted on a constant acceleration triangular-nl(~tion velocity transducer. The details of the data analysis tcchniqucs art’ reported clsewhcrc [S].
density for the amorphous SiC of 2.6 g/cm’. This density was selected based on earlier investigations of Cr-implanted SiC by McHarguc et al. [9]. In these studies, it was shown that single crystal Sic sweltcd by 20% after ion implantation. The relevant implantation parameters for the current cxperimcnt arc summarized in table I. Note that the lower specimen density changes the RBS results: thcrc is an increase in the ion range to YO nm, an increase in the F~~HM to 110 nm. and an incrcasc in the depth to which the specimen is amorphous to 250 nm.
3. Results
Channeling RBS experiments dcmonstratcd that the implanted spccimcns were amorphous even at the lowcst fluencc. Typical spectra are shown in fig. 2 for the specimen implanted to 3 x 10’” ions/cm’. The profiles in fig. 2 arc plotted assuming that the density of the Sic was equal to that of crystalline material, 3.2 g/cm”. The amorphous region extended from the surface to a depth of = 200 nm. Measurements of the implanted iron show an approximately Gaussian profile centered at a depth of = 75 nm. The full width at half of the maximunl yield (FWHM) is =X0 nm and the maxiIntegramum iron concentration is = 7 mot% Fe/Sic. tion of the iron peak yields a total of 2.8 x IO’” ions/cm’, in good agreement with the implantation dose. The damage and implanted iron profiles for this specimen were also calculated with the assumption of a
600 /I-_ 1600
500
a
DEPTH
(nm)
300
200
400
,
SIC density
I
=Fe
1160
=
3.2
A cross-sectional micrograph of the specimen implanted with 6 x to’” ions/cm’ is shown in fig. 3. The arrows on the micrograph indicate the implant surface (the lacy-appearing region to the left of the implant surface is the epoxy). The dark region extending to about 1% nm from the surface is the amorphous. implanted region. Note that a region of enhanced contrast indicated the expected transition layer at the end of the ion range. Convergent beam clcctron diffraction (CBEDI analyses (see insets in fig. 3) confirmed that the implanted region was amorphous and that the underlying substrate remained crystalline. The ~~staliine region in fig. 3 exhibits the 15nm-repeat lattice image of edge-on basal planes of the 6H silicon carbide. The profile of the implanted iron was measured with EDS line scans pcrpcndicular to the implant surface. The results of three of these scans are shown in fig. 4. The peak in the iron profile is found at a
DEF’TH (nm) 100
-100
0
,
/,
300
150
50
100
-___r_---r.-_
l
r
-50
3
g/cm3
keV.3elG.RTI
Sic blensity = 3 2 g/cm3 5’Fe(
200
0
random
160
keV,3elG,RT)
l
%?%
0 0.9
1.0
1.1 ENERGY
1.2
13
Fig. 2. RBS spectra for Sic implanted with 3 X
1.4
1.6
I.8
I7 ENERGY
(MeV)
(Me%‘)
ions/cm’ of iron assuming a density in the amtrrphous region equal to that of crystalline SK. (a) Silicon, (b) iron.
IOlh
V. OXIDES/CERAMICS/CARBIDES
Table
I
Comparison
of TRIM,
Technique
TRIM
RBS-C.
and AEM
Density
Iron
FWHM
Amorphous
Peak iron
range
[nml
concentration
[s/cm’1
[nml
depth [nml
XX
2.6
RBS
AEM
64
I IO
200 ‘I
= IO
3
”
= 20
”
= IO
6 3
= 20
6
”
=7
30
=7
7.x i 2.x L
X0
3.7
X5
X0
lY5
= I6
h.0
2.6
x5
x0
IV5
= I6
5.x
to the end of the deposited
?OO
from the TRIM
calculated
from RBS data for specimen
’ Integrated
profile
calculated
from AEM
data for specimen
implanted
nm from the implant surface. The iron at the peak is 16 moI% Fc/SiC. The of the profile is 80 nm. Assuming a Sic density
crystalline (inset CBED
of SiC implanted
to a depth
pattern
with 6 x IO “’ ions/cm’
of IYS nm (see insert
10’”
ions/cm’.
with 6x IO’” ions/cm’.
of 3.2 g/cm”, the intcgratcd iron profile is 6.0 x IO’” ions/cm’. in good agreement with the experimental value. If the lower Sic density is assumed. the intc-
85
micrograph
parameters. with 3~
implanted
concentration
extends
Cl <’
ion and collision profiles.
the peak iron concentration
profile
Fig. 3. Cross-sectional
ions/cm’]
”
I IO
’ Integrated
region
IO”
7s
corresponding
amorphous
[X
90
used to calculate
FWHM
iron profile SK]
2.n
” Value
of
[‘G Fe/mole
I50 I’
X0
Integrated
3.7
‘I Depth
depth
iron and damage profiles
SiC
3.2
TRIM
data for implanted
CBED
in upper right), note the IS nm-repeat
of iron. The arrows indicate
pattern
in upper
the implant
left); the underlying
surface; an
substrate
remains
lattice image from the basal planes: the inset EDS spectra is
typical of the implanted
region.
L.L. Horton et ul. / C‘huructerizutiorr 16
349
of Sic
the peak iron deposition. Mcasurcmcnt of the iron concentration with EDS confirmed that this was the region obtained with the specimen preparation technique described in section 2 of this paper. Fig. 5 shows high magnification, through-focus-series, micrographs taken with focus steps of 80 nm. With this technique, Fe-rich precipitates or clusters with diamcters of > 2 nm would have been easily observed. As shown in fig. 5, no precipitates were obscrvcd. Information about the bonding of the iron can bc obtained from examination of the electron cncrgy loss fine structure for the iron L,, cdgc. Fig. 6 shows a comparison of the fine structure for this edge in spcctra measured with PEELS for the plan-view Fc-implanted Sic specimen and for metallically-bonded-iron in type 304 stainless steel. From this comparison, subtic but significant differcnccs arc apparent. The ‘.whitc” lines in the spectrum from Fe-implanted Sic are much broader than those from the stainless steel. For this comparison, the height of the L, white lines have been normalized. Also, the intensity of the whim lines relative to the contribution from the continuum states is much higher in the stainless steel spectrum. These differences were confirmed with PEELS spectra taken from a number of regions of the Sic specimen. White lines arise from transitions to unfilled, bound states just above the Fermi level; the detailed fine structure in this region is thus a sensitive indicator of
with 160 keV "Fe 6 x 1Ol6
50
0
Fig. 4. EDS implanted
IO'"
are from three separate the specimen
lons/cm2. RT
200
(nm)
line scan of the implanted
with 6~
ions/cm’
iron profile
of iron. The
with a FWHM
in SiC
plotted
scans across the implanted
shown in fig. 3. Peak concentration Fe/Sic
--oSIC
150
100 DEPTH
data
region of
is 16 mol%
of X0 nm.
iron profile is 4.8 X 10’” ions/cm’. These values are also summarized in table 1. In order to bcttcr evaluate the microstructure of the implanted region, a plan-view (back-thinned) specimen was prepared. These specimens were sectioned to 80 nm to allow examination of the region of the specimen grated
- 160 nm Fig.
of Fe ion implantution
5. High
-8Onm
magnification
concentration.
images
FOCUS
of a plan-view
Through-focus-series
specimen
sectioned
+ 80 nm to allow
in steps of 80 nm; no precipitates
examination
+ 160 nm of region
are observed (observation
limit
with
the
= 2 nm).
V. OXIDES/CERAMICS/CARBIDES
peak
iron
350
Type 304 Stainless
0
Steel
c 680
740
720
700
Energy
Loss
760
(eV)
Fig. 6. Comparison of the PEELS fine structure for the iron L zi edge of Fc-implanted SIC (top) md type 304 stainleas steel (hottom). L, lines normalized: note that the spectm do not match. suggesting the Fe in the SK in not metallicnll~-hondrd.
bonding. The strength of the signal rcprcsenting transitions to continuum states (the region above = 735 cV) provides a good measure of the quantity of the iron analyzed. The results suggest that the iron in the SiC is not primarily mctallic~iliy-bondcd~ this ohscrvation supports the conclusion that the iron is in covalcntly bonded sites reported for the earlier CEMS evaluations of this specimen [S]. chemical
support for the conclusion more appropriate can be between the experimental iron profile calculated from mental and TRIM values talline density.
that the higher density is seen from the comparison flucncc and the integrated the AEM data. The cxpcriagree for the higher. crys-
4. Conclusions .?,3. Comparison
of A EM,
RBS, and TRIM
calcth
tiorls
Table 1 summarizes TRIM calculati~~ns for the two densities of Sic, 3.2 g/cm” (crystalline) and 2.6 g/cm3 (“amorphous”), that were discussed above relative to the interpretation of the RBS and AEM data. As can be seen by inspection of table 1, comparisons of the TRIM calcuiations with the RBS and AEM data arc complex. For consistency, the TRIM and RBS values must be compared for the same density. The ion range is considcrcd for these comparisons since TRIM calculations of the ion range arc generally in better agrccmcnt with experimental values than some of the other parameters. However, no firm conclusions can be drawn from a comparison of the TRIM and RBS ion range values; the higher density values disagree by 15% difference and the values for the lower density disagree by 18%. A more definitive comparison is seen bctwccn the AEM and TRIM calculations. Again considering the ion range. the AEM measurement of 8.5 nm is in excellent agreement with the 88 nm calculated by TRIM for the higher density. The FWHM value for the RBS experiment, assuming a crystalline density, also agrees with the 80 nm value measured with AEM. Additional
In this investigation of single crystal Sic implanted with iron, the damaged region became amorph(~us for all of the fluences studied, 1, 3, and 6 X 10’” ions/cm’. The amorphous region extended from the implant surface to a depth of = 200 nm. For the highest fluence specimen, EDS measurements of cross-sectional spccimcns indicate that the peak in the implanted iron was at a depth of approximately = 85 nm. The peak iron concentration was = 7 mol% Fc/SiC for the spccimcn implanted with 3 x 10lh ions/cm’ (RBS measurement) and = 16 mol% Fc/SiC for the highest flucncc condition (EDS measurements). Comparisons among TRIM, RBS-C and AEM parameters for the deposited iron profiles suggest that the density of the implanted region is likely closer to that of crystalline SiC than earlier studies reported. High magnification AEM examinations of plan-view specimens from the highest iron flucncc failed to locate any precipitates within the zone with the peak iron concentration. Comparisons of the energy loss fine structure of the iron edge in the implanted SiC with that of mctallicaIly-bonded iron show substantial differences; this observation supports earlier CEMS reports that the iron was covalentiy bonded. Additional investigations are required to dc-
termine iron.
the bonding
and charge state of the implanted
Acknowledgements The authors would like to thank Drs. P. Thevenard for helpful discussions and review of this manuscript; and A.M. Williams and A.T. Fisher for preparation of the microscopy specimens. Research at Oak Ridge National Laboratory was sponsored by the Division of Materials Sciences, U.S. Dcpartmcnt of Energy under contract DE-ACOS-840R21400 with Martin Marietta Energy Systems, Inc. and J. Pawel
References [I] C.W White. C.J. McHargue, G.C. Farlow.
Mater.
P.S. Sklnd, L.A. Boatner Sci. Reports 4 (19X9) 3 I.
and
[2] J.M. Williams. C.J. McHargue and B.R. Appleton. Nucl. Instr. and Meth. 3OY/?lO (10x3) 317. [3] P.J. Burnett and T.F. Page, J. Mater. Sci. IY (1984) X45. [4] J.A. Spitznagel. S. Wood. W.J. Choyke. N.J. Doyle. J. Bradshaw and S.J. Fishman, Nucl. Instr. and Mcth. BI6 (IYX6) 237. [5] C-l. McHarguc. A. Perez and J.C. McCallum. Nucl. Instr. and Meth. BSY/hO (1991) 1.162. [6] N.J. Znluzec. in: Introduction to Analytical Electron Microscopy. eds. J.J. Hren, J.I. Goldstein and D.C. Joy (Plenum. New York. 1079) chap. 4. [7] A.T. Fisher and P. Angelini. Proc. 33rd Annual Meeting of the Electron Microscopy Society of America. ed. G.W. Bailey (San Franasco Press. San Francisco. CA. IYX5) p. I x2. [X] B.D. Sawicka and J.A. Sawicki. in: Topics in Current PhyGcs. vol. 25. ed. U. Gonser (Springer. Berlin. 1981 ) p. 14’). [Y] C.J. MclInrgue and J.M. Williams, in: Metastable Materials Formation by Ion Implantation, eds. ST. Picraux and W.J. Choyke (North-Holland. New York. IYX2) p, 303.
V. OXIDES/CERAMICS/CARBIDES
Instruments
and Methods
in Physics Research
B65
Nuclear lnstnments 8 Methods in Physics Research s~~C~lloil D
( 1902)345-351
N~~rth-H(~ll~lnd
Microstructural characterization of silicon carbide
In order to better use implantation species is needed. channeling “Fe
analytical
electron
have heen used to characterize
ions to fluences of I. 3, and hX IO’” ions/cm-‘.
even at the lowest fluence. AEM depth
to improve the properties
In this investigation,
(RIG-C)
of the peak
agreement
was performed
iron concentration
with the value calculated
ex~min~iti~~ns of the implanted
of iron ion implantation
measured by the TRIM
of ceramics, a detailed
microscopy
single crystal a-silicon RBS-ion
channeling
for both cross-sectional
carbide
implanted
demonstrated X-ray
of the nature of implanted
hackscattering
spectroscopy-ion
at room temperature
that the implanted
with IhO keV
region was amorphous
specimens with the highest fluence. The
spectroscopy
and with RBS were in ~{ppr(~xinl~t~
code assuming a specimen density equal to cryrtalline
silicon carhidc. The AEM
reyittns in this study did not reveal the presence of any pr~~ipit~~tes. ~ug&e~tin~ that the iron has not
into precipitates
implanted
specimens suggests that the iron is not metallically-banded;
(~)hse~~ti(~n
limit
= 2 nmt.
MGssbauer spectroscopy ICYEMS) that the iron is covalently
lnsp~~ti(~li of the electron
energy
loss fine structure
this result supports the conclusion
for iron
in the
from converGon electron
bonded.
1. Introduction High temperature environments arc becoming increasingly important for advanced technologies ranging from fusion reactors to advanced heat engines. Structural ceramics have excellent high temperature propcrtics, but suffer from poor mechanical properties for many of the anticipated applications. An cxtcnsivc effort is underway to improve the mechanical propertics of ceramic materials. One avenue to improve ccramic materials is the implantati~)n of other atomic species into the region within a few micrometers of the surface. Effcctivc utilization of this tcchniquc requires that an understanding bc developed of the fundamcntal materials science of the interactions of the implanted species with the host. Extcnsivc work has been done to characterize the nature (charge state, bonding, etc.) of the implanted species for Al,O,. (See, for example, the review by White ct al. [I]). The goal of
0
understanding
and Rutherford
and hack-thinned
with energy dispersive
eoalcsced
Ol~X-~X.3X/Y2/$OS.OO
(AEM)
the current investigation is to apply these same characturization tcchniqucs to the study of ion-implanted silicon carbide. one important non-oxide ccromic for high-tcmpcraturc structural applications. Earlier studies of the effect of ion implantation in SIC have demonstrated that it is easily amorphizcd for low ion fluenccs. The critical damage energy density for amorphization of ru-Sic at room tcmpcraturc has been found to lit hctwccn 0.02 and 0.05 keV/atom [Z-4]. Few ch~lracterization tcchniqucs can provide informati~~n about the structure of ~~rn(~rph(~usmaterials. In recent work reported by McHarguc ct al. [S], conversion clcctron Miissbaucr spectroscopy (GEMS) was used to determine the local cnvironmcnt of the “Fe implanted into single crystals of cu-Sic at room tcmpcraturc to flucnccs of 1. 3. and h x IO’” ions/cm”. The CEMS spectra were fit with a single component having a distribution of quadrupolc split values (see fig. I). The results suggcstcd a distribution of the iron ions
1992 - Elsevier Science Puhlishers B.V. All rights reserved
V. OXIDES/CERAMICS/CARBIDES
-0
0.5
1.0
OUADRUPOLE
1.5
2.0
SPLITTING
2.5
3.0
(mm/s)
Fig. I. Distribution of quadrupolt: splitting (QS) used to fit the CEMS data for Fc-implanted cr-Sic; (a) I x 10”‘. (b) 3 X 10”‘. and (c) 6 X IO’” ions/cm’.
among scvcral sites with slightly different local symmctries and the prcscncc of cctvalcnt F~-~(~nlp(~unds or low spin iron complcxcs with covalent bonding. One purpose of the current study was to further charactcrizc the same spccimcns used f&r the CEMS examination with analytical clcctron microscopy (AEM) and Kuthcrford backscattcring spectroscopy (RBS) to obtain additional information about the implanted iron and to ascertain whcthcr any secondary phases wcrc prcscnt in the implanted region.
2. Experimental
procedure
The single crystal
dc Ciraaff accclcrator with a scattering angle of lh(t”. Channeling experiments were pcrformcd with the ion beam parallel to [OOt)l]. The ion irnpl~~~it~tt~~)~~s and ion spectroscopy cxpcriments were pcrformcd with instruments located at the Oak Ridge National Laboratory (ORNL) Surface Modification and Characterization Facility. Analytical electron microscopy was pcrformcd with ;I IO0 kV Philips EM 300T cquippcd with a field emission gun. a 6585 scanning transmission electron microscopy (STEM) unit, an EDAX PW 9101)77) encrgy dispersive X-ray spcctrometcr (EDS), and ;I Gatan hOA Parallel-dctcction Electron Energy l_oss Spcctrometcr (PEELS). X-ray microanalysis line-scans wcrc pcrformed in the STEM mode with a prohc of I, 3 nm diamctcr. with currents of > i nA, and pro~iucing X-ray count rates of’ 2000 to 3000 counts/s. An analoguc scan was employcd; ench data point in the EDS profile was obtained in 3 s. During this 3 s interval the probe advanced 7.4 nm. Sincc this distuncc is greater than the probe size and the cxpcctcd beam broaicning, it dcfincs the spatial resolution. Thu precision o! the peak composition is limited to < .7? by counting statistics. Intcgratcd X-ray intensities. Itc,S, wcrc convcrtcd to composition C’, c,s, (at.%) from the rclationship: C‘,.,/C,i = kb,,silb,/ls,, whcrc k ,,L.,s,= 0.707. as calculated with an updated version of the computer coclc (NEDQNTZ) hascd on the standardlcss approach of zaluzcc [h]. Spccimcnb wcrc prepared in both cross-section and plan-view gcomctries. The cross-sectional spccimcns wcrc prcparcd using the standard tcchniqucs cstablishcd for ceramics. The implanted surl’accs of two sections wcrc bonded with MBOND 610 adhcsivc epoxy; the resulting sandwich was ground and polished with diamond paste to a thickness of about 100 pm then dimpled with a Gatan Model 0% dimplor to a final thickness at the hottom of the dimple of about 20 pm. This specimen was mounted on a carbon ring and ion-milled at room temperature to perforation in ii C;atan Model hOODIS DuoMill ion mill opcrutcd with h kcV argon ions and a 15” milling angle. The plan-view specimens wcrc prcparcd with a special technique to allow examination near the peak iron region. In this tcchniquc. after the spccimcn was ground, polished, and dimpled from the tlriinipl~~nt~d surfact: to a thickness at the bottom of the dimple of about 30 1J-m; the impl~lnted surface of the specimen was sectioned with an ion mill to rcmovc ahout 80 nm. The milling conditions wcrc determined from trial runs in which half of the spccimcn surface was masked; thcsc conditions wcrc 1.7 min at 3 keV with a milling angle of 20”. After sectioning, the implanted surface was coated with salt to allow easy removal of any material deposited during the back-thinning operation of the ion mill [7]. Back-thinning operations were pcr-
formed at room temperature with 6 keV ions and a 20” milling angle. M~ssbau~r spectra were obtained with the tcchnique of conversion electron Miissbaucr spectroscopy (CEMS) [S]. The spectra were obtained at room tempcrature with a backscattercd geometry. The “7Co source was contained in a rhodium matrix and was mounted on a constant acceleration triangular-nl(~tion velocity transducer. The details of the data analysis tcchniqucs art’ reported clsewhcrc [S].
density for the amorphous SiC of 2.6 g/cm’. This density was selected based on earlier investigations of Cr-implanted SiC by McHarguc et al. [9]. In these studies, it was shown that single crystal Sic sweltcd by 20% after ion implantation. The relevant implantation parameters for the current cxperimcnt arc summarized in table I. Note that the lower specimen density changes the RBS results: thcrc is an increase in the ion range to YO nm, an increase in the F~~HM to 110 nm. and an incrcasc in the depth to which the specimen is amorphous to 250 nm.
3. Results
Channeling RBS experiments dcmonstratcd that the implanted spccimcns were amorphous even at the lowcst fluencc. Typical spectra are shown in fig. 2 for the specimen implanted to 3 x 10’” ions/cm’. The profiles in fig. 2 arc plotted assuming that the density of the Sic was equal to that of crystalline material, 3.2 g/cm”. The amorphous region extended from the surface to a depth of = 200 nm. Measurements of the implanted iron show an approximately Gaussian profile centered at a depth of = 75 nm. The full width at half of the maximunl yield (FWHM) is =X0 nm and the maxiIntegramum iron concentration is = 7 mot% Fe/Sic. tion of the iron peak yields a total of 2.8 x IO’” ions/cm’, in good agreement with the implantation dose. The damage and implanted iron profiles for this specimen were also calculated with the assumption of a
600 /I-_ 1600
500
a
DEPTH
(nm)
300
200
400
,
SIC density
I
=Fe
1160
=
3.2
A cross-sectional micrograph of the specimen implanted with 6 x to’” ions/cm’ is shown in fig. 3. The arrows on the micrograph indicate the implant surface (the lacy-appearing region to the left of the implant surface is the epoxy). The dark region extending to about 1% nm from the surface is the amorphous. implanted region. Note that a region of enhanced contrast indicated the expected transition layer at the end of the ion range. Convergent beam clcctron diffraction (CBEDI analyses (see insets in fig. 3) confirmed that the implanted region was amorphous and that the underlying substrate remained crystalline. The ~~staliine region in fig. 3 exhibits the 15nm-repeat lattice image of edge-on basal planes of the 6H silicon carbide. The profile of the implanted iron was measured with EDS line scans pcrpcndicular to the implant surface. The results of three of these scans are shown in fig. 4. The peak in the iron profile is found at a
DEF’TH (nm) 100
-100
0
,
/,
300
150
50
100
-___r_---r.-_
l
r
-50
3
g/cm3
keV.3elG.RTI
Sic blensity = 3 2 g/cm3 5’Fe(
200
0
random
160
keV,3elG,RT)
l
%?%
0 0.9
1.0
1.1 ENERGY
1.2
13
Fig. 2. RBS spectra for Sic implanted with 3 X
1.4
1.6
I.8
I7 ENERGY
(MeV)
(Me%‘)
ions/cm’ of iron assuming a density in the amtrrphous region equal to that of crystalline SK. (a) Silicon, (b) iron.
IOlh
V. OXIDES/CERAMICS/CARBIDES
Table
I
Comparison
of TRIM,
Technique
TRIM
RBS-C.
and AEM
Density
Iron
FWHM
Amorphous
Peak iron
range
[nml
concentration
[s/cm’1
[nml
depth [nml
XX
2.6
RBS
AEM
64
I IO
200 ‘I
= IO
3
”
= 20
”
= IO
6 3
= 20
6
”
=7
30
=7
7.x i 2.x L
X0
3.7
X5
X0
lY5
= I6
h.0
2.6
x5
x0
IV5
= I6
5.x
to the end of the deposited
?OO
from the TRIM
calculated
from RBS data for specimen
’ Integrated
profile
calculated
from AEM
data for specimen
implanted
nm from the implant surface. The iron at the peak is 16 moI% Fc/SiC. The of the profile is 80 nm. Assuming a Sic density
crystalline (inset CBED
of SiC implanted
to a depth
pattern
with 6 x IO “’ ions/cm’
of IYS nm (see insert
10’”
ions/cm’.
with 6x IO’” ions/cm’.
of 3.2 g/cm”, the intcgratcd iron profile is 6.0 x IO’” ions/cm’. in good agreement with the experimental value. If the lower Sic density is assumed. the intc-
85
micrograph
parameters. with 3~
implanted
concentration
extends
Cl <’
ion and collision profiles.
the peak iron concentration
profile
Fig. 3. Cross-sectional
ions/cm’]
”
I IO
’ Integrated
region
IO”
7s
corresponding
amorphous
[X
90
used to calculate
FWHM
iron profile SK]
2.n
” Value
of
[‘G Fe/mole
I50 I’
X0
Integrated
3.7
‘I Depth
depth
iron and damage profiles
SiC
3.2
TRIM
data for implanted
CBED
in upper right), note the IS nm-repeat
of iron. The arrows indicate
pattern
in upper
the implant
left); the underlying
surface; an
substrate
remains
lattice image from the basal planes: the inset EDS spectra is
typical of the implanted
region.
L.L. Horton et ul. / C‘huructerizutiorr 16
349
of Sic
the peak iron deposition. Mcasurcmcnt of the iron concentration with EDS confirmed that this was the region obtained with the specimen preparation technique described in section 2 of this paper. Fig. 5 shows high magnification, through-focus-series, micrographs taken with focus steps of 80 nm. With this technique, Fe-rich precipitates or clusters with diamcters of > 2 nm would have been easily observed. As shown in fig. 5, no precipitates were obscrvcd. Information about the bonding of the iron can bc obtained from examination of the electron cncrgy loss fine structure for the iron L,, cdgc. Fig. 6 shows a comparison of the fine structure for this edge in spcctra measured with PEELS for the plan-view Fc-implanted Sic specimen and for metallically-bonded-iron in type 304 stainless steel. From this comparison, subtic but significant differcnccs arc apparent. The ‘.whitc” lines in the spectrum from Fe-implanted Sic are much broader than those from the stainless steel. For this comparison, the height of the L, white lines have been normalized. Also, the intensity of the whim lines relative to the contribution from the continuum states is much higher in the stainless steel spectrum. These differences were confirmed with PEELS spectra taken from a number of regions of the Sic specimen. White lines arise from transitions to unfilled, bound states just above the Fermi level; the detailed fine structure in this region is thus a sensitive indicator of
with 160 keV "Fe 6 x 1Ol6
50
0
Fig. 4. EDS implanted
IO'"
are from three separate the specimen
lons/cm2. RT
200
(nm)
line scan of the implanted
with 6~
ions/cm’
iron profile
of iron. The
with a FWHM
in SiC
plotted
scans across the implanted
shown in fig. 3. Peak concentration Fe/Sic
--oSIC
150
100 DEPTH
data
region of
is 16 mol%
of X0 nm.
iron profile is 4.8 X 10’” ions/cm’. These values are also summarized in table 1. In order to bcttcr evaluate the microstructure of the implanted region, a plan-view (back-thinned) specimen was prepared. These specimens were sectioned to 80 nm to allow examination of the region of the specimen grated
- 160 nm Fig.
of Fe ion implantution
5. High
-8Onm
magnification
concentration.
images
FOCUS
of a plan-view
Through-focus-series
specimen
sectioned
+ 80 nm to allow
in steps of 80 nm; no precipitates
examination
+ 160 nm of region
are observed (observation
limit
with
the
= 2 nm).
V. OXIDES/CERAMICS/CARBIDES
peak
iron
350
Type 304 Stainless
0
Steel
c 680
740
720
700
Energy
Loss
760
(eV)
Fig. 6. Comparison of the PEELS fine structure for the iron L zi edge of Fc-implanted SIC (top) md type 304 stainleas steel (hottom). L, lines normalized: note that the spectm do not match. suggesting the Fe in the SK in not metallicnll~-hondrd.
bonding. The strength of the signal rcprcsenting transitions to continuum states (the region above = 735 cV) provides a good measure of the quantity of the iron analyzed. The results suggest that the iron in the SiC is not primarily mctallic~iliy-bondcd~ this ohscrvation supports the conclusion that the iron is in covalcntly bonded sites reported for the earlier CEMS evaluations of this specimen [S]. chemical
support for the conclusion more appropriate can be between the experimental iron profile calculated from mental and TRIM values talline density.
that the higher density is seen from the comparison flucncc and the integrated the AEM data. The cxpcriagree for the higher. crys-
4. Conclusions .?,3. Comparison
of A EM,
RBS, and TRIM
calcth
tiorls
Table 1 summarizes TRIM calculati~~ns for the two densities of Sic, 3.2 g/cm” (crystalline) and 2.6 g/cm3 (“amorphous”), that were discussed above relative to the interpretation of the RBS and AEM data. As can be seen by inspection of table 1, comparisons of the TRIM calcuiations with the RBS and AEM data arc complex. For consistency, the TRIM and RBS values must be compared for the same density. The ion range is considcrcd for these comparisons since TRIM calculations of the ion range arc generally in better agrccmcnt with experimental values than some of the other parameters. However, no firm conclusions can be drawn from a comparison of the TRIM and RBS ion range values; the higher density values disagree by 15% difference and the values for the lower density disagree by 18%. A more definitive comparison is seen bctwccn the AEM and TRIM calculations. Again considering the ion range. the AEM measurement of 8.5 nm is in excellent agreement with the 88 nm calculated by TRIM for the higher density. The FWHM value for the RBS experiment, assuming a crystalline density, also agrees with the 80 nm value measured with AEM. Additional
In this investigation of single crystal Sic implanted with iron, the damaged region became amorph(~us for all of the fluences studied, 1, 3, and 6 X 10’” ions/cm’. The amorphous region extended from the implant surface to a depth of = 200 nm. For the highest fluence specimen, EDS measurements of cross-sectional spccimcns indicate that the peak in the implanted iron was at a depth of approximately = 85 nm. The peak iron concentration was = 7 mol% Fc/SiC for the spccimcn implanted with 3 x 10lh ions/cm’ (RBS measurement) and = 16 mol% Fc/SiC for the highest flucncc condition (EDS measurements). Comparisons among TRIM, RBS-C and AEM parameters for the deposited iron profiles suggest that the density of the implanted region is likely closer to that of crystalline SiC than earlier studies reported. High magnification AEM examinations of plan-view specimens from the highest iron flucncc failed to locate any precipitates within the zone with the peak iron concentration. Comparisons of the energy loss fine structure of the iron edge in the implanted SiC with that of mctallicaIly-bonded iron show substantial differences; this observation supports earlier CEMS reports that the iron was covalentiy bonded. Additional investigations are required to dc-
termine iron.
the bonding
and charge state of the implanted
Acknowledgements The authors would like to thank Drs. P. Thevenard for helpful discussions and review of this manuscript; and A.M. Williams and A.T. Fisher for preparation of the microscopy specimens. Research at Oak Ridge National Laboratory was sponsored by the Division of Materials Sciences, U.S. Dcpartmcnt of Energy under contract DE-ACOS-840R21400 with Martin Marietta Energy Systems, Inc. and J. Pawel
References [I] C.W White. C.J. McHargue, G.C. Farlow.
Mater.
P.S. Sklnd, L.A. Boatner Sci. Reports 4 (19X9) 3 I.
and
[2] J.M. Williams. C.J. McHargue and B.R. Appleton. Nucl. Instr. and Meth. 3OY/?lO (10x3) 317. [3] P.J. Burnett and T.F. Page, J. Mater. Sci. IY (1984) X45. [4] J.A. Spitznagel. S. Wood. W.J. Choyke. N.J. Doyle. J. Bradshaw and S.J. Fishman, Nucl. Instr. and Mcth. BI6 (IYX6) 237. [5] C-l. McHarguc. A. Perez and J.C. McCallum. Nucl. Instr. and Meth. BSY/hO (1991) 1.162. [6] N.J. Znluzec. in: Introduction to Analytical Electron Microscopy. eds. J.J. Hren, J.I. Goldstein and D.C. Joy (Plenum. New York. 1079) chap. 4. [7] A.T. Fisher and P. Angelini. Proc. 33rd Annual Meeting of the Electron Microscopy Society of America. ed. G.W. Bailey (San Franasco Press. San Francisco. CA. IYX5) p. I x2. [X] B.D. Sawicka and J.A. Sawicki. in: Topics in Current PhyGcs. vol. 25. ed. U. Gonser (Springer. Berlin. 1981 ) p. 14’). [Y] C.J. MclInrgue and J.M. Williams, in: Metastable Materials Formation by Ion Implantation, eds. ST. Picraux and W.J. Choyke (North-Holland. New York. IYX2) p, 303.
V. OXIDES/CERAMICS/CARBIDES