Electrical resistivity measurements are used to study damage in CrSi_2 thin films induced by Ne, Ar, or Xe ion irradiation over a fluence range of 10^(10)–10^(15) ions cm^(−2). Irradiation produces a factor of 5–12 increase in film conductivity at the higher fluences. The influence of defect generation and recombination is evident. We speculate that formation of a compound defect is a dominant factor enhancing film conductivity. A temperature dependence at low fluences is reported and tentatively identified

Banwell, T. C.

Nicolet, M.-A.

Zhao, X.-A.

English

Caltech Authors - Main

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Effects of ion irradiation on conductijvity of CrSi2 thin films 
T. C. Banwell, X.-A. Zhao,al and M-A. Nicolet 
California Institute of Technology, Pasadena, California 91125 
(Received 21 October 1985; accepted for publication 14 January 1986) 
Electrical resistivity measurements are used to study damage in CrSi2 thin films induced by Ne, 
Ar, or Xe ion irradiation over a fluence range of 1010-1015 ions cm-2 • Irradiation produces a 
factor of 5-12 increase in film conductivity at the higher fluences. The influence of defect 
generation and recombination is evident. We speculate that formation of a compound defect is a 
dominant factor enhancing film conductivity. A temperature dependence at low ftuences is 
reported and tentatively identified. 
I. INTRODUCTION 
The use of metal silicides in contact and interconnect 
technologies has motivated investigation of irradiation dam-
age in silicides. 1- 3 Recent studies show that the electrical 
conductivity of CoSi2, NiSi2, and Pd2Si films are lowered by 
ion irradiation, 1•2 presumably due to the reduction in carrier 
mobility. In contrast, the conductivity of CrSi2 can be in-
creased by ion irradiation. 2•3 The details of the responsible 
processes are unknown. CrSi2 is particularly interesting 
since it is a narrow (0.27 eV) band-gap semiconductor.4•5 
Thermally grown CrSi2 films typicaHy show degenerate p-
type conductivity with carrier densities exceeding 1020 
em- 3• Electron and hole mobilities are 0.15 and 15 cm2 IV s, 
respectively.4 
We report here on our investigations of changes in elec-
trical transport in CrSi2 films associated with Xe, Ar, and Ne 
ion irradiation. Two models are proposed which offer some 
insight into the damage process. 
II. EXPERIMENT 
CrSi2 films were produced on several types of substrates. 
Approximately 1-pm-thick Si02 layers were thermally 
grown on 0.01 n em n-type < 111 > Si wafers by wet oxida-
tion. Following oxidation, 80-nm Si, 40-nm Cr, and 40-nm Si 
layers were sequentially deposited thereon bye-beam evapo-
ration; the background pressure remained below 10- 7 Torr. 
Also, 30-50 n em n-type < 100> Si and 5-30 n cmp-type 
< 111 > Si wafers were organically cleaned, etched in 10% 
HF, slightly oxidized in boiling [H20 2, NH3 (aq), H 20: 
1, 1,5] and finally dipped in 3% HF prior to loading for de-
position. Consecutive depositions of 40-nm Cr and 20-nm Si 
were made on these substrates. The Si cap was employed to 
minimize oxygen contamination. All samples were sequen-
tially annealed at 600 ·c, 30 s and 900 ·c, 60 s in vacuum at 
< 7 X 10- 7 Torr. 2-MeV He backscatter spectrometry and 
x-ray diffraction (Read camera) were used to characterize 
the stoichiometry and structure of sections of each wafer. 
This showed the formation of 120-nm CrSi2 in each case. 4 A 
slight ( < 10 nm) excess ofSi was present at the surface of the 
samples on Si02• The samples were cleaved into 8 X 8 mm 
squares and electrically characterized before further use by 
four-point probe measurements of sheet resistance and Hall 
•l Permanent address: Shanghai Institute of Metallurgy, Academy of Sci-
ences of China, Shanghai, China. 
coefficient at room temperature, using standard methods.6 
There was negligible substrate conduction in the samples on 
then-type < 100 > Si or Si02 substrates; these CrSi2 films 
were p-type with typical resistivities of -4400, - 5500 
f-lO em, carrier concentrations of - 1, -2 X Hf0 em- 3, and 
Hall mobilities of -13, -7 cm2/V s, respectively. The sub-
strate contributed -50% to the measured film conductivity 
in the samples on the p-type < lll > Si wafer. 
Two complementary irradiation experiments were per-
formed. In one, samples were irradiated at room tempera-
ture (RT) with Xe at energies of 300-600 keY to fluences of 
lOu-1 015 Xe em- 2 at particle current densities, correspond-
ing to a singly charged ion, of2-100nA/cm2• The irradiated 
films were then characterized as before. The second set of 
experiments employed four-point electrical resistivity mea-
surements ofCrSi2 films on Si02 made in situ with 300-500-
ke V Xe irradiation at room temperature and 77 K. Measure-
ments were made for accumulated doses of 2X 1010-1015 
ions cm-2 • Constant particle current density was main-
tained during irradiation, typically in the range 0.8-5 nAI 
cm2. The relative uncertainty in dosimetry was < 1% for a 
given run, and the absolute uncertainty was estimated to be 
< 10%. Similar implantations with 150-keV Ar and 90-keV 
Neat room temperature were also performed. 
Except for the 600-keV Xe irradiation, the (mean dam-
age depth)+ (damage straggling) 112 was less than the 120-
nm CrSi2 film thickness for all ions and energies used. 7 A 
thin film of thermally conductive paste (Dow Corning 340) 
was used to secure the samples to an implantation carousel of 
large thermal mass. We estimate the sample temperature to 
have risen less than 0.01 ·c;nA em -z. 
lll.RESULTS 
CrSi2 fi.lm conductance increases with incident ion 
fluence ¢J for room- temperature Xe and Ar irradiation. Sam-
ples irradiated with Xe at 77 K, or with Neat RT initially 
display a -2% decrease in conductance at fluences < 1011 
cm- 2, beyond which the conductivity also increases with 
ftuence. The film conductance g( ¢J) saturates at 5-12 X its 
initial value g0 for fluences of 6X 1013-1015 cm- 2• The ap-
parent saturation level increases when ]arger particle current 
densities are employed. Additional investigations indicate 
that this is not the result of Joule heating. 
Typical in situ results at fixed Xe current density for two 
irradiation temperatures are shown in Fig. 1. The ratio of 
3077 J. Appl. Phys. 59 (9). 1 May 1986 0021-8979/86/093077-04$02.40 © 1986 American Institute of Physics 3077 
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8 
77K 
6 
0.83 nA/cm2 y· 
500 keV Xe 
0 IRRADIATION (J) 
.......... 4 
(J) 1.25 
nA/cm2 
2 
II 12 13 14 
FIG. I. Change in relative CrSi2 film conductance g(¢J)/g0 vs ftuence ifJ, 
measured in situ, for 500-keV Xe irradiation at RT and 77 K. 
conductance after fl.uence ¢to initial conductanceg( ¢ )/g0 is 
plotted semilogarithmically versus fl.uence ¢ for 500-ke V Xe 
irradiations. The behavior is similar for other energies and 
ions. No significant substrate dependence was observed 
(correcting for background substrate conduction in CrSi2 
films grown on the p < 111 > Si substrates). 
The Hall effect measurements do not reveal a substan-
tial! change in the magnitude of the Hall mobility for Xe 
ftuences below 1014 em - 2, although there is an apparent con-
version to predominantly n-type conductivity at fluences ex-
ceeding 1012-1013 cm-2• 
It was observed in in situ experiments that at room tem-
perature, though not at 77 K, the film conductiviy decreases 
by 1-5% over a 20-60 s period after suspending irradiation. 
The temperature coefficient of resistivity is positive in this 
temperature range and cannot explain this change. Addi-
tionally, the magnitude and time scale are not consistent 
with thermal decay. Some self-annealing apparently occurs 
at room temperature. 
IV. DISCUSSION 
Our observations suggest that the enhancement of CrSi2 
conductivity results primarily from an increase in free-car-
rier concentration. We speculate that these carriers, possibly 
electrons, arise from completely ionized defects produced by 
the irradiation. The high mobility we observe ( - 10 
cm2 IV s) relative to lin = 0.15 cm2 IV s for electrons4 sug-
gests conduction via an impurity band. Figure 2 shows the 
variation in film resistivity with temperature before and. after 
the LNT irradiation. The cooling curve is consistent with 
previous reports:5 Carrier freeze-out is not an interference in 
this experiment. 
The curves of g/g0 vs ¢exhibit a simple structure at high 
fl.uences ( > 1013 cm-2 ), indicating that a first- or possibly 
second-order process may be responsible for saturation. 8 
This suggests plotting d(g/g0 )/d¢, which was calculated us-
ing three-point Lagrangian interpolation, versus g/g0 , as 
3078 J. Appl. Phys., Vol. 59, No.9, 1 May 1986 
worm- up 
0 after I014cm-2500keV Xe 
E 
u 
c; 
::t. -30 
~ 
<3 -so 
100 
cooling- down 
before irradiation 
200 
TEMPERATURE ( K) 
300 
FIG. 2. Graph of resistivity change relative toRT rxtlue vs temperature for 
CrSi2 film before and after irradiation at 77 K with 10 .. em - 2, 500-keV Xe. 
shown in Fig. 3 for samples implanted with 500 keV Xe at 
room temperature, 77 K, and with 150-keV Ar. The results 
for all in situ room-temperature implantations display simi-
tar behavior. All of the curves have a similar shape for 
g/g0 > -2, independent of temperature. One mechanism 
probably dominates this regime. Considering the behavior 
for g/g0 < -2, there appears to be an additional process 
present during RT irradiation which is suppressed at 77 K. It 
is evident from Fig. 3 that there is a nearly linear relationship 
0.4 
0.2 
N 
E 
(.) 
~ 
' 0 
0.4 
0.2 
~ 
0 
~ 
2 
"'~ 0 0.4 
0.2 
-
---
0 
0.83nA/cm2 
1,25nA/cm2 
tafurafion 
\ 
I 
4nA/cm2 
2 4 6 ( 9/Qo) 
(a) 
500keV Xe 
77K 
(b) 
500keV Xe 
RT 
plus 
12.5nA/cm2 
(c) 
150keV Ar 
RT 
8 10 
FIG. 3. Graph ofd(g/g0)/d;vs (g/g0 ) for 500-keVXei.rradiation at 77 K, 
RT and for 150-keV Ar at room tro~perature: (a), (b), and (c) respective-
ly. In (b), theXecurrentdensitywasincreased to 12.SnA/cm2 after satura-
tion bad occurred at 1.25 nA/cm2 for a dose of -101~ Xe cm-2• 
Banwell, Zhao, and N;colet 3078 
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0.6 
N 
E 
r.> 
~ 0.4 
0 
<!) 0.2 
Xe 
500ke/" 
2 30~v~77K 
2 
150keV / intercept G 
Ne Ar/ 
90ke~O 
/0 slope R 
6--
---
- - -
6--
077K 
6- -6- -0.0 1--..:::;....""-----'--------'-------J.--1 
0 100 200 300 
DAMAGE ENERGY V (keV) 
FIG. 4. Graph of intercept G and slope R vs damage energy v for various 
ion-energy combinations studied. 
between d(g/g0 )/drp and (g/g0 ) as they approach zero and 
saturation, respectively: 
.!!_(.!__) = G - R (.!__)· 
difJ go go 
(1) 
In Fig. 4 we plot the values of G and R derived from this 
asymptotic dependence (dashed lines in Fig. 3 ) versus the 
deposited damage energy v for the various ion-energy com-
binations studied. 7 The predominately linear dependence 
suggests that G corresponds to the generation of defects by 
the incident ion's nuclear energy loss, while existing defects 
recombine via radiation-enhanced diffusion.9•10 The typical 
influence of dose rate on damage in CrSi2 is demonstrated in 
Fig. 3(b). Further increases ing/g0 were produced after sat-
uration was reached (ifJ~l015 em - 2 ) at aXe current density 
of 1.25 nA/cm2 by raising the current to 12.5 nA/cm2• This 
effect requires further systematic investigation. 
Simple models of overlapping coverage10•11 are not ade-
quate to describe the saturation at high fluences. However, 
they may be pertinent to the room- temperature behavior in 
Figs. 3(b) and 3(c) for glg0 < 2. If an incident ion produces 
an initial change in conductivity which is large compared to 
that produced by subsequent overlapping cascades, then 
d(g/g0 )/difJ would decrease substantially when aifJ> 1, 
where (a) is the effective area of the initially modified re-
gion. The solid lines in Figs. 3(b) and 3(c) correspond to 
such a model, assuming that subsequent impacts produce no 
additional change (see Appendix). By fitting these lines to 
the data, we deduce effective areas of (8.7) 2, (10) 2 nm2 for 
150-ke V Ar, 500-ke V Xe irradiation, respectively. The areas 
corresponding to transverse damage straggling 1T <T > D 
are (33) 2, ( 44 )2 nm2, respectively, using Winterbon's calcu-
lations.7 Recent studies of ion irradiation damage in tung-
sten using field ion microscopy12 show that the average radi-
us of the damaged region is -0.25 <r > };'2• These results 
show that our estimates of (a) are quite reasonable. The fact 
that the initial transient vanishes at 77 K indicates that the 
initial process occuring at R T involves defect diffusion 
and/or complex formation. The defects may not be suffi-
ciently mobile at 77 K to produce this effect. This reduced 
3079 J. Appl. Phys., Vol. 59, No.9, 1 May 1986 
mobility of defects may also account for the reduced values 
of G and R at 77 K in Fig. 4. The slight initial reduction in 
conductivity for ftuences below 1011 em - 2 is accordant with 
this model of localized damage, since a reduction in carrier 
(hole) mobility due to scattering from these dispersed re-
gions may dominate at very low fluences. 
The 77 K result shown in Fig. 3(a) suggests that an 
electrically active compound defect is responsible for the in-
crease in film conductivity. A significant feature is that 
d(g/g0 )/drp initially starts near zero and increases very ra-
pidly with respect to (g/ g0 ). This indicates that other electri-
cally inactive defects are formed first and subsequently lead 
to formation of the active defect. The behavior at 77 K can be 
explained formally by the following model. We assume that 
the formation of the active defect A is limited by the forma-
tion of a simple inactive precursor defect I. All other inter-
mediaries are assumed to be steady state, and competing re-
actions are neglected. The model can be described by the 
coupled reactions 
(k,) 
0;;::=:{?} +I, (2a) 
(I<,) 
(k,) 
I+ {?'};;:=A, (2b) 
(k.) 
where{?} and{?'} refer to unspecified defects. In this case 
quasi-first- order chemical kinetics apply, and the corre-
sponding concentrations Ca (r/J), C1 (r/J) follow 13: 
d d 
-=-c; = k1 - k2C1 - -=-ca • ( 3) 
difJ d¢ 
d 
-=-ca = k3C;- k4Ca, (4) 
difJ 
where the empirical rate constants k1, k2, k3, k4 depend on 
ion flux difJ/dt, deposited energy v, steady-state densities of 
other defects, etc. g- go is assumed proportional to ca; 
(g/ g0 ) - l = PC a . The solution of Eqs. ( 3) and ( 4) for 
C1 (0) = Ca (0) = 0 gives 
(5) 
where 
- kz + k3 + k4 ± [(k2 + k3 + k4)2 k k ]1/2 p,q- - 2 4 • 2 2 
There are three relevant empirical parameters; (Pk 1 k3 ),p, 
and q. We assume p=f.q. The asymptotic behavior for this 
model is 
~ Ca -( k:3 )- qCa, as t/J-OC), (6a) 
and as <fJ---.0; 
~ - (2k k C ) 112[1- (p + q)( 2Ca )]J/2 (6b) 
df/J a I 3 a 3 k1k3 . ' 
which is clearly the desired behavior. Figure 5 shows the 
result of fitting this simple model (solid line: 
Pk1k3 = 9.01 X w-26 cm4, q = 0.064x w- 12 cm2, p = 
0.24 X 1 o- 12 cm2 ) to the results of Fig. 3 (a). The agreement 
is excellent, although only suggestive. Variations of this 
model may also fit equally as well. A physical model requires 
knowledge of CrSi2 defect chemistry which is lacking at 
Banwell, Zhao, and Nicolet 3079 
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N 
E 0.4 L> ~-co limit 
~ ...... 500 keV Xe 
' 
' 0 
' 
77K 
' 
' 0.83nA/cm2 
.... 
0 
00 
" 00 
-ol~ 
0 
2 4 6 
( g/go) 
FIG. 5. Result of fitting simple coupled reaction model Eq. 5, (solid line) to 
77 K Xe irradiation data (open circles) from Fig. 2(a). Asymptotic behav-
ior is shown by dashed lines [from Eqs. 6(a), 6(b) ]. 
present. It is likely that k1 follows a Kinchen-Pease depen-
dence (e.g., nearly linear in damage energy v). 14 General 
statements about the other rate constants are not so obvious. 
V. CONCLUSIONS 
Our preliminary investigations have revealed a substan-
tial amount of structure in the fluence dependence of inert 
ion implantation on CrSi2 conductivity. Several processes 
seen in other systems have been tentatively identified. Elec-
trical measurements provide a good qualitative characteri-
zation, but are relatively nonspecific and non unique in their 
interpretation. Structural analysis from TEM, etc., is essen-
tial. The defect chemistry of CrSi2 needs to be considered. 
The thermal annealing behavior of implanted CrSi2 may of-
fer some insight. Further investigation of irradiation damage 
in CrSi2 at very Iow fluences ( -1010 cm- 2 ) shows promise 
in elucidating the damage process, especially with light ions 
(e.g., Ne). CrSi2 may be a good material to use in the study 
of dose rate effects. 
ACKNOWLEDGMENTS 
We thank A. Ghatfari for assistance with the evapora-
tions and A. Coll.inwood for manuscript preparation. We 
acknowledge the partial financial support by the Office of 
3080 J. Appl. Phys., Vol. 59, No.9, 1 May 1986 
Naval Research under Contract no. N00014-84-K-0275 (D. 
Polk). T. Banwell thanks IBM for a fell.owship during this 
work. 
APPENDIX 
The effective conductivity of a two-dimensional medi-
um composed of a random arrangement of regions with con-
ductivities u0 and bu0 with fractional coverage 1 - p, p, re-
spectively can be approximated by15 : 
y 2 - y ( 2p - 1 )( b - 1) - b = 0, 
wherey = u/u0 . In our problemp = 1- exp(- a¢>) is the 
fraction of area involved in at lease one cascade and possess-
ing conductivity bu0 . Differentiating and rearranging gives 
_!_y = a(b- y)(y2 + Y). 
d¢> y2 + b 
The values of bused in Figs. 3 (b) and 3 (c) were determined 
by trial and error, with b = 1. 6 7 and 1. 71, respectively, pro-
ducing reasonable agreement with the experiment. 
1 J. C. Hensel, R. T. Tung, J. M. Poate, and F. C. Unterwald, Nucl. lnstrum. 
Methods (in press). 
2C. A. Hewett, I. Suni, S. S. Lau, L. S. Hung, and D. M. Scott, in Jon Im-
plantation and Ion Beam Processing of Materials, Vol. 27, edited by G. K. 
Hubler, 0. W. Holland, C. R. Clayton, and C. W. White (North-Holland, 
New York, 1984), p. 145. 
3U. Shreter and J. Nuchols (unpublished). 
4M-A. Nicolet and S. S. Lau, in VLSI Electronics: Microstructure Science, 
Vol. 6, edited by N. Einspruch and G. Larrabee (Academic, New York, 
1983), p. 391. 
'F. Nava, T. Tien, and K. N. Tu, J. Appl. Phys. 57, 2018 ( 1985). 
6L. J. Vander Pauw, Phillips, Res. Rep. 13, I ( 1958). 
7K. B. Winterbon, Ion Implantation Range and Energy Deposition Distri-
butions (IFJ Plenum, New York, 1975). 
8R. C. Birtcher, R. S. Averback, and T. H. Blewitt, J. Nucl. Mater. 75, 167 
(1978). 
9G. J. Dienes and A. C. Damask, J. Appl. Phys. 29, 1713 (1958). 
·~.Webb and G. Carter, Radiat. Effects 42, 159 ( 1979). 
11R. P. Webb and G. Carter, Radiat. Effects 59,69 (1981). 
12C-Y. Wei, M. I. Current, and D. N. Seidman, Philos. Mag. A 44, 459 
(1981). 
13R. Sizmann, J. Nucl. Mater. 67-70, 386 (1980). 
14B. M. Paine and B. X. Liu, in Materials Surfaces, Part A: Modification, 
edited by C. R. Clayton and J. B. Lumsden (Academic, New York), (in 
press). 
15S. Kirkpatrick, Phys. Rev. Lett. 1:7, 1722 (1971). 
Banwell, Zhao, and Nicolet 3080 


Effects of ion irradiation on conductivity of CrSi_2 thin films

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Effects of ion irradiation on conductivity of CrSi_2 thin films

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