Effects of ion implantation of 320 keV Si-28 at room temperature in pseudomorphic metastable GexSi1-x (x almost-equal-to 0.04, 0.09, 0.13) layers approximately 170 nm thick grown on Si(100) wafers were characterized by x-ray double-crystal diffractometry and MeV He-4 channeling spectrometry. The damage induced by implantation produces additional compressive strain in the GexSi1-x layers, superimposed on the intrinsic compressive strain of the heterostructures. This strain rises with the dose proportionally for doses below several times 10(14) Si-28/cm2. Furthermore, for a given dose, the strain increases with the Ge content in the layer. Upon thermal processing, the damage anneals out and the strain recovers to the value before implantation. Amorphized samples (doses of greater than 2 x 10(15) Si-28/cm2) regrow poorly

Bai, G.

Nicolet, M.-A.

English

Caltech Authors - Main

Generation and recovery of strain in 28Si-implanted pseudomorphic GeSi 
films on Si(lO0) 
G 13aia) and M.-A Nicolet 
C&ifrnia Institute o> Technology, Pasadena, California 91125 
(Received 28 October 1991; accepted for publication 29 January 1992) 
Effects of ion implantation of 320 keV “Si at room temperature in pseudomorphic metastable 
Ge$i,-, (x-0.04, 0.09, 0.13) layers - 170 nm thick grown on Si( 100) wafers were 
characterized by x-ray double-crystal diffractometry and MeV 4He channeling spectrometry. 
The damage induced by implantation produces additional compressive strain in the Ge$i, _ x 
layers, superimposed on the intrinsic compressive strain of the heterostructures. This strain rises 
with the dose proportionally for doses below several times lOI 28Si/cm2. Furthermore, for a 
given dose, the strain increases with the Ge content in the layer. Upon thermal processing, the 
damage anneals out and the strain recovers to the value before implantation. Amorphized 
samples (doses of greater than 2 x lOI 28Si/cm2) regrow poorly. 
INTRODUCTION RESULTS AND DISCUSSION 
Recent interest in GeSi-base heterojunction bipolar 
transistors has spurred numerous studies on various prop- 
erties of GeSi/Si structures, including the effect of ion im- 
plantation. The higher atomic number of Ge rather than Si 
results in heavier damage in the GeSi than in the Si layers, 
which can induce amorphous-crystalline superlattices. lr2 
Amorphized GeSi layers on crystalline Si recrystallize epi- 
taxially upon thermal processing.3*4 Defects produced in a 
metastable strained layer by ion implantation enhance 
strain relaxation.’ Strain modification was observed in ion- 
assisted deposition of GeSi films.6 We report here some 
results on the strain and damage induced by 320 keV 28Si 
implantation into pseudomorphic GeSi layers on Si( lOO), 
and the effects of thermal annealing. The stability of ion- 
implanted strained layers is also discussed. 
EXPERIMENTAL PROCEDURES 
Pseudomorphic metastable Ge,Si, --x (~~0.04, 0.09, 
0.13) layers - 170 nm thick were grown on Si( 100) wafers 
at - 600 “C by ultrahigh-vacuum chemical vapor deposi- 
tion at IBM.’ The samples were sent to Caltech where they 
were analyzed by backscattering spectrometry to deter- 
mine the composition and the thickness of the films, and by 
x-ray rocking curves to establish the strain state of the 
films. The samples were degreased before being loaded into 
the implanter. 320 keV 28Si ions were implanted into the 
samples at room temperature in vacuum ( - 10 - ’ Torr), 
The damage peaks at -200 nm beneath the interface in- 
side the Si substrate, according to TRIM88 simulation.* The 
28Si dose, 4, ranges from 1014/cm2 to 2x 101’/cm2, and the 
flux was kept below 10i2/cm2 s. Double-crystal diffracto- 
metry and MeV ion channeling were used to analyze the 
strain and damage induced by implantation. 
Samples were also annealed for 30 min at 300-700 “C 
in vacuum ( - 5 X 10 - ’ Torr). The recovery of damage 
and the change of strain in the GeSi layers were monitored. 
“Present address: Intel Corp., 3065 Bowers Ave., SCl-03, Santa Clara, 
CA 95052-8126. 
The perpendicular and parallel strain, er and et’, in 
the GeSi/Si( 100) heterostructures were extracted from Fe 
&i x-ray (wavelength = 0.1936 nm) rocking curves of 
both symmetrical (400) and asymmetrical (3 11) diffrac- 
tions. All the as-grown samples are pseudomorphic with- 
in experimental sensitivity (et’ <O.Ol%), in agreement 
with the experimental results of Ref. 9. These samples are 
metastable, meaning that they are all thicker than the equi- 
librium critical thickness predicted by Matthews- 
Blakeslee’s model. lo Figure 1 shows the (400) rocking 
curves of the as-grown Geo,osSio.s,/Si( 100) (solid line) 
and of the samples implanted to doses of 2 X (0) and 
5 x lot4 28 Si/cm’ ( 0 ) . The rocking curve of the as-grown 
sample shows clearly visible small-amplitude oscillations, 
indicating a high crystalline perfection of the Geo,osSio,sl 
layer. The angular width of the diffraction peak from the 
layer is due entirely to the finite thickness of the layer. The 
perpendicular strain of the as-grown sample is er = 0.69%. 
After implantation to 2~ lOI 28Si/cm2, the diffraction 
peak from the GeoesSio.sl layer shifts farther away from the 
substrate Si peak, meaning that the layer develops an ad- 
ditional positive strain (see 0 in Fig. 1). The peak intensity 
decreases, while the angular width remains about the same. 
These facts suggest that defects are produced in the layer, 
that the layer still diffracts x-ray coherently, and that the 
defects are laterally uniformly distributed in the layer and 
induce uniform lattice expansion. The perpendicular strain 
of the implanted sample is 2 = 0.97%, while the parallel 
strain remains zero. The sample is thus still pseudomor- 
phic and becomes more metastable than it was before. The 
additional perpendicular strain induced by the implanta- 
tion damage, A&, is 0.28%. One can also see the diffrac- 
tion peak (at - - 0.05” near the Si reference peak) from 
the damaged substrate Si (0 in Fig. 1) . The strain induced 
(-0.06%) is about the same as that in the implanted bulk 
Si samples,” suggesting that the Geo,osSio,s, overlayer has 
no influence on the implantation damage in the Si sub- 
strate. As the dose rises to 5 X 1014 28Si/cm2, the diffraction 
peak from the GeSi layer is buried in the background and 
4227 J. Appl. Phys. 71 (9), 1 May 1992 0021-8979/92/094227-03$04.00 @  1992 American Institute of Physics 4227 
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170 nm GeO~osSio.sl on Si(100) 
10’ (400) diffraction rocking curve 
h L RI 
w 100 V 
h 
$ 
g 10-l 
2 
10-2 
-0.6 -0.4 -0.2 0 
AQ ( ’ > 
320 keV 28Si into Ge,Sit-,/Si(lOO) 
0.4 
0 1.2x 10”/cm2 ,- 
FIG. 1. Fe K,, x-ray rocking curves for symmetrical (400) diffractions of 
Gq,,,&,&Si( 100) samples (err = 45.5’): as-grown (solid line) and im- 
planted at room temperature with 320 keV 28Si to 2X (O), 5X 10” 
**Si/cm2 ( 0 ) . 
FIG. 2. The strain induced by 320 keV 28Si implantation into pseudomor- 
phic GeSi layers vs the Ge composition. The dashed lines are the strains 
predicted from a linear interpolation model of Eq. ( 1) . 
becomes undetectable, while that from the damaged Si sub- 
strate is still measurable ( 0 in Fig. 1). These findings 
indicate that the Gee,$i,9, layer is more severely dam- 
aged than the Si substrate. In other words, the Ge,-,e9Sie9, 
alloy is more susceptible to radiation damage than Si is, in 
agreement with others’ results.“’ Although the Gee,Sic9, 
layer doesn’t produce a measurable x-ray diffraction peak, 
it is not yet amorphized by this dose of 5 x lOI 28Si/cm2, 
as the [lOO] axial channeling spectrum of 2 MeV 4He in- 
dicates (minimum yield - 80%). As the dose increases 
further to 2~ 1Or5 28 Si/cm2, the rocking curve from the 
sample becomes featureless (not shown) because both the 
layer and the substrate are amorphized (minimum yield 
- lOO%J.t’J’ 
X-ray rocking curve measurements of other implanted 
samples with different Ge composition give results similar 
to those described above. First, the strain in the damaged 
substrate Si is that expected in the implanted bulk Si.” 
Second, the implantation damage induces additional strain 
in the GeSi layer besides the intrinsic strain of the hetero- 
structure. The layers remain pseudomorphic. At low dam- 
age level (f#K2 X 10 I4 28Si/cm2) where the diffraction peak 
from the GeSi layers is measurable, the additional strain 
induced by damage, A$, increases with the implantation 
dose (Fig. 2). We know that the strain increases linearly 
with the Si dose at low damage levels in implanted bulk 
Si” and Gel3 crystals. The lim ited data in Fig. 2 and the 
analogy with Si and Ge lead us to propose that for any Ge 
composition, A& is proportional to the dose at low damage 
levels. For a given dose, the induced strain in the GeSi 
alloy increases linearly with the Ge composition (Fig. 2). 
The dashed line is that obtained by interpolation between 
the corresponding strain of implanted Si and Ge. For that 
interpolation, the strain in implanted Si can be found di- 
rectly from the measurements. l1 The strain for implanted 
Ge was computed by multiplying the slope of strain versus 
dose by the corresponding dose (1.2X or 2X lOi4 
28Si/cm2), because this strain cannot be realized physically 
since the Ge is amorphized beyond -7X 1013 28Si/cm2. 
Figure 2 shows that the interpolation approximates the 
data well. This result suggests that the slope, SoeSil --x, of 
the implantation-induced strain A& versus the dose 4 for 
Ge,Si, _ x alloy at low damage level (linear damage re- 
gime), may be predicted from those of Ge and Si, So, and 
S’si according to 
s Ge;Si, -x = XSGe + ( 1 - X)Ssi* (1) 
Additional experiments are desirable to test this relation- 
ship over the entire Ge compositions (O<x< 1) . 
To investigate the stability of these implanted layers 
upon thermal processing, we annealed the samples at 300- 
700 “C in vacuum for 30 m in and monitored the change of 
strain at room temperature after the heat treatment. First, 
we noticed that the annealing characteristics of the dam- 
aged Si substrate are similar to those of implanted bulk Si 
samples.” The annealing behavior of the GeO,esSiO.s, layers 
can be categorized into three types according to the initial 
damage level. For the lightly damaged samples 
(@ax 10 I4 28Si/cm2), the annealing shifts the GeaosSiesl 
diffraction peak towards the main peak, and the peak in- 
tensity increases. The peak width remains the same. The 
rocking curve eventually reverses back to that of the as- 
grown sample after 700 “C. For the heavily damaged sam- 
ples (4 = 5X 10 I4 28Si/cm2, 0 in Fig. 3) where the peak 
from the implanted Ge,,esSio,9, layer becomes undetect- 
able, that peak remains undetectable after 300 “C (filled 
inverse triangle in Fig. 3), but becomes measurable after 
400 “C annealing (0)) and the samples completely recover 
after 700 “C (solid line in Fig. 3). Figure 4 plots the per- 
pendicular strain of the three samples studied here as a 
function of temperature for 30 m in annealing. The filled 
symbols represent the as-grown samples. Two conclusions 
are evident: (a) the major annealing stage occurs below 
300 ‘C, and (b) the strain and damage induced by the 
implantation heals out completely upon heating at 700 “C. 
Furthermore, no relaxation of the intrinsic strain is ob- 
4228 J. Appl. Phys., Vol. 71, No. 9, 1 May 1992 G. Bai and M.-A. Nicolet 4228 
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170 nm Ge6,$it7g~ on Si(100) 
320 keV 5x10” 28Si/cm2 
30 min isochronal annealing 
10- 
.0.6 -0.4 -0.2 0 
AQ ( o ) 
FIG. 3. (400) x-ray rocking curves of a pseudomorphic 
Geees.Q9,/Si( 100) implanted at room temperature (RT, 0) by 320 keV 
5X lOI ‘8Si/cm2 and annealed for 30 min at 300 “C (V), 400°C (O), and 
700 T (solid line). The spectrum of the sample annealed at 700 “C (solid 
line) is indistinguishable from that of the as-grown sample (solid line of 
Fig. 1). 
served for any sample after 700 “C annealing, meaning that 
the presence of defects does not significantly enhance the 
relaxation of the metastable strain. Hull et al5 did observe 
some enhancement of strain relaxation in implanted GeSi/ 
Si structures by transmission electron microscopy. These 
authors suggested that the defects promote the nucleation 
of dislocations, but impede their propagation. The x-ray 
rocking curve technique is not sensitive enough to explore 
that regime of initial dislocation nucleations because the 
dislocation density is below the x-ray detection limit which 
lies at a total length of misfit dislocations per unit area of 
about 104/cm. 
The third type of annealing behavior is found for sam- 
ples irradiated by 2X 10 I5 28Si/cm2 in which both the 
320 keV 28Si into Ge,Sii-,/Si(lOO) 
T ( =‘C ) 
FIG. 4. The strain in the Ge$i, --x layer vs the annealing temperature. 
The square, triangle, and circle are for the samples with x = 0.04, 0.09, 
0.13, respectively. The filled symbols represent the unimplanted samples. 
The small (big) open symbols represent the samples implanted by 320 
keV 1.2~ (2x) lOI 28Si/cm2. 
Ge,Si, --x layer and the Si substrate were amorphized. We 
used x-ray diffraction and MeV ion channeling to study the 
solid-phase epitaxial regrowth of these three samples with 
composition ~~0.04, 0.09, 0.13. The regrown layers have 
larger channeling yields, and much weaker and broader 
x-ray diffraction peaks than the as-grown layers, indicating 
the presence of extended defects in the layers. To maintain 
the crystalline perfection of epitaxial GeSi layers, amor- 
phization transformation of the GeSi layers should thus be 
avoided. 
CONCLUSION 
In summary, we find that the damage produced by 28Si 
implantation in pseudomorphic GeSi layers on Si( 100) in- 
duces additional compressive strain in the layers. At a low 
damage level, the induced strain increases linearly with the 
dose. The slope of the induced strain versus the dose rises 
linearly with the Ge composition, and can be predicted by 
interpolation of the slopes for bulk Ge and Si crystals. 
Thermal annealing removes damage and eliminates the im- 
plantation-induced strain. Damaged, but not amorphized, 
samples fully recover after 700 “C annealing for 30 min; 
amorphized samples recover only partially. In Si, im- 
planted dopants are activated most effectively when Si is 
fully amorphized prior to thermal annealing. For epitaxial 
GeSi layers on Si, that procedure would appear problem- 
atic in the light of the present results. 
ACKNOWLEDGMENTS 
We would like to thank Dr. Akbar at IBM for provid- 
ing the as-grown GeSi/Si( 100) samples. This paper is 
based upon the work supported in part by the Semicon- 
ductor Research Corporation under contract No. 91-SJ- 
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4229 J. Appl. Phys., Vol. 71, No. 9, 1 May 1992 G. Bai and M.-A. Nicolet 4229 
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Generation and recovery of strain in (28)Si-implanted pseudomorphic GeSi films on Si(100)

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Generation and recovery of strain in (28)Si-implanted pseudomorphic GeSi films on Si(100)

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