NIM B
Beam Interactions
with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 257 (2007) 270–274
www.elsevier.com/locate/nimb
Amorphous–crystalline interface evolution during Solid Phase
Epitaxy Regrowth of SiGe films amorphized by ion implantation
D. D’Angelo a,*, A.M. Piro a, S. Mirabella a, C. Bongiorno b,
L. Romano a, A. Terrasi a, M.G. Grimaldi a
a
CNR-INFM Matis and Dipartimento di Fisica ed Astronomia, Università degli Studi di Catania, via Santa Sofia 64, I-95123 Catania, Italy
b
CNR-IMM, Sezione di Catania, Stradale Primosole 50, I-95121 Catania, Italy
Available online 8 January 2007
Abstract
Transmission Electron Microscopy was combined with Time Resolved Reflectivity to study the amorphous–crystalline (a–c) interface
evolution during Solid Phase Epitaxy Regrowth (SPER) of Si0.83Ge0.17 films deposited on Si by Molecular Beam Epitaxy and amorphized with Ge+ ion implantation. Starting from the Si/SiGe interface, a 20 nm thick layer regrows free of defects with the same SPER
rate of pure Si. The remaining SiGe regrows with planar defects and dislocations, accompanied by a decrease of the SPER velocity. The
sample was also studied after implantation with B or P. In these cases, the SPER rate raises following the doping concentration profile,
but no difference in the defect-free layer thickness was observed compared to the un-implanted sample. On the other hand, B or P introduction reduces the a–c interface roughness, while B–P co-implantation produces roughness comparable to the un-implanted sample.
2007 Elsevier B.V. All rights reserved.
PACS: 81.05.Cy; 61.50.
f; 64.70.KS; 81.10.Jt; 61.72.LK
Keywords: Silicon germanium alloys; Crystalline state; Solid Phase Epitaxy; Linear defects in crystals; Doping impurity
1. Introduction
The device scaling process in Si-based microelectronics
leads the research towards high performance materials to
increase physical properties such as charge carrier’s mobility. Strained Si shows this property and has been successfully used in a new generation of MOSFET by IBM [1].
Strained Si (e-Si) must be grown on a buffer layer, termed
virtual substrate, with higher lattice constant with respect
to relaxed Si, so that relaxed SiGe alloys are particularly
suitable to this end. To obtain e-Si or e-Si1 xGex alloys with
low density dislocations and smooth surface, graded relaxed
Si1 yGey (y < x) buffer layers are commonly realized by
Chemical Vapour Deposition (CVD) with intermediate step
of Chemical Mechanical Polishing (CMP) process [2,3].
*
Corresponding author. Tel.: +39 0 953785289; fax: + 39 0 953785243.
E-mail address: [email protected] (D. D’Angelo).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.nimb.2007.01.010
Due to the very large thickness (several microns) of the
graded relaxed buffer layers, other approaches have been
proposed to realize relaxed buffer layers for the fabrication
of e-Si, e-SiGe films [4–6]. The use of ion implantation and
subsequent Solid Phase Epitaxy Regrowth (SPER) to
realize e-SiGe or e-Si is very attractive due to its full integrability with standard industrial processes [7–9].
It has been already reported [9,10] that during SPER of
SiGe films on Si only a few nanometers of SiGe regrow
without defects. The small thickness of such defect-free
region is the main problem to realize high quality e-SiGe
by ion implantation. The formation of defects in SiGe during SPER seems to be determined by the competition
between regrowth and nucleation of dislocations; moreover, a delay time of nucleation can exist for the introduction of dislocation [10]. Another critical point, as far as the
formation of defects is concerned, is the sharpness of the
amorphous–crystalline (a–c) interface. During SPER along
the [0 0 1] direction, the interface loses its smoothness when
D. D’Angelo et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 270–274
2. Experimental
A e-Si0.83Ge0.17 layer 137 nm thick on top of a 230 nm Si
buffer layer was grown on (0 0 1) Si Cz substrate by MBE.
The growth rate was 0.1 nm/s, the substrate temperature
T = 550 C, with a base pressure of about 5 · 10 11 mbar.
X-TEM analysis, performed with a JEOL 2010 Microscopy
at 200 kV and RBS measurements, made using a 2 MeV
4
He+ beam produced by a High Voltage Singletron accelerator, confirmed the SiGe thickness and composition, as
well as the absence of any extended defect in the as-grown
sample.
Ge+ ion implantation at liquid nitrogen temperature,
270 keV and fluence of 3 · 1015 at./cm2, was used to amorphize the samples down to 360 nm from surface (the whole
surface SiGe and a part of underlying Si). Some of the
amorphized samples were also implanted with 7.5 · 1014
B+/cm2 at 36 keV and/or 7.5 · 1014 P+/cm2 at 100 keV.
The different implantation energies were chosen to have
the same projected range for both dopants at a depth of
about 135 nm, near to the SiGe/Si interface.
In order to induce SPER, all samples were annealed in a
vacuum chamber (about 10 7 mbar) at a temperature of
541 C. The sample temperature stabilization occurs during
the regrowth of few nanometers of the Si buffer underlying
the SiGe layers, afterwards the temperature is kept con-
stant within ±0.5 C. TRR measurements were performed
with a He–Ne laser beam (k = 623.8 nm) to extract the
SPER rate as a function of a–c interface depth [16] using
the refraction index of amorphous and crystalline Si and
SiGe [17–19].
3. Results and discussion
In Fig. 1 we report the TRR signal of intrinsic (a),
B-implanted (b) and B–P co-implanted (c) samples
acquired during annealing at 541 C. The modulation is
due to interference between the light reflected from the surface and the one reflected by the moving a–c interface. The
time in Fig. 1 is set zero at the fifth TRR peak from the surface for all samples and the time difference between two
consecutive maximum and minimum is inversely proportional to the SPER rate. The un-implanted (line a) and
co-implanted (line c) samples show a very similar signal,
with a reduction in the SPER rate with respect to the Bimplanted one (line b) as indicate by the higher time needed
to complete the recrystallization process. The P-implanted
sample (not shown here) shows the same behaviour of the
B-implanted one.
The interference distance d (i.e. the peak to valley distance in the spectrum) at this temperature is about 34 nm
for Si and SiGe [16,15], so we can convert the raw signal
of TRR into average SPER rate, as shown in Fig. 2. We
first observe that the un-implanted sample (open squares
in Fig. 2) shows the typical Si/SiGe SPER rate behaviour,
with constant velocity in the Si buffer layer, followed by a
decrease when the SiGe starts to recrystallize (vertical line
in Fig. 2 indicates the Si/SiGe interface position). After a
minimum at an average depth of about 80 nm, the SPER
rate shows a small increase that cannot be followed up to
the surface because, as later shown, the a–c interface
roughness is of the order of 40 nm. The decreasing SPER
a
Reflectivity (u.a.)
the a–c interface moves from Si to SiGe [11] and the
regrowth can also occur along different crystalline directions causing strain accumulation and planar defect formation [12,13]. Nevertheless, the actual mechanism of defect
introduction in SiGe during SPER as well as the thickness
of the defect free region is not clearly understood.
As the SPER velocities are concerned, many authors
report lower values for e-SiGe alloys with respect to pure
Si and higher rates for the relaxed SiGe [9,10,14]. In particular, Lee et al. used Time Resolved Reflectivity (TRR) to
study the SPER rate in situ during annealing of a
Si0.88Ge0.12 film at temperatures in between 503 C and
603 C. They confirmed that the average SPER velocity
in e-SiGe is lower than in pure Si. Furthermore, these real
time measurements demonstrated that the SPER rate for
e-SiGe is not constant at a fixed temperature but varies
as a function of the position of the a–c interface [15], even
if a relation between the SPER kinetics and defects has not
been evidenced.
In this paper we report our studies on the crystalline–
amorphous (a–c) interface evolution during SPER of
Molecular Beam Epitaxy (MBE) grown e-Si0.83Ge0.17/Si
heterostructures, amorphized by Ge+ implantation. TRR,
cross-section Transmission Electron Microscopy (X-TEM)
and Rutherford Backscattering Spectrometry (RBS) have
been used as characterization techniques. The sample was
also implanted with B and/or P ions under their solid solubility limits, with similar concentration profile, in order to
investigate the role of doping on the defect introduction,
SPER rate and roughness of a–c interface.
271
b
c
0
1000
2000
3 000
4000
5000
6000
Time [s]
Fig. 1. TRR signal versus annealing time of un-implanted (a),
B-implanted (b) and P–B (c) co-implanted samples at 541 C. B-implant
was done at 36 keV and fluence of 7.5 · 1014 cm 2, P-implant was done at
100 keV and fluence of 7.5 · 1014 cm 2.
D. D’Angelo et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 270–274
19
Concentration [at./cm3]
7x10
Si SiGe
B profile
19
6x10
un-implanted
B implanted
B-P co-implanted
0.24
0.20
19
5x10
0.16
19
4x10
0.12
3x1019
19
0.08
19
0.04
2x10
1x10
0
Average SPER rate [nm/s]
272
0.00
240
200
160
120
80
40
0
Depth [nm]
Fig. 2. Average SPER rates of un-implanted (open squares), or Bimplanted (open circles) and B–P co-implanted (closed squares) sample
versus a–c interface depth. Data have been obtained by the analysis of the
TRR measurements shown in Fig. 1. The B profile obtained from SRIM
simulation is also reported (dotted line). The vertical line marks the
Si/SiGe interface position.
velocity between 140 nm and 80 nm depths is attributable
to the formation of defects, as also suggested by the
X-TEM analysis later shown and discussed. Thus, in agreement with the literature, our data indicate that during the
SPER of SiGe, after about 20 nm free of defects, a wide
band of dislocations and stacking faults is formed in the
SiGe layer with a simultaneous delay in the SPER rate.
At depth of 60 nm, the defects concentration decreases
and a small increase in the SPER rate is observed. The
same behaviour has been found for other temperatures in
the range 505–595 C (not shown here). Incorporation of
B causes an increase in the SPER velocity which seems to
follow the dopant concentration profile. This effects is
clearly visible in Fig. 2 (open circles), where the SPER rate
increases up to a factor of 2 for the B-implanted sample.
SPER rate follows the B profile calculated by SRIM simulation [20] (dotted line in Fig. 2) except for a depth shift of
about 30 nm towards the bulk. Indeed, the SPER velocity
reaches the maximum value at depth of 155 nm while the
calculated projected range is 120 nm. The same effect is
present also in the P-implanted sample (not shown). Concerning the SPER rate increase, we can attribute it to the
dopant presence. Still, the SPER rate decreases when the
a–c interface enters in the SiGe layer, even if the dopant
maximum concentration has to be reached (about 20 nm
inside the SiGe layer). This could be thought as the result
of two competing effects: the dopant enhancement and
the defect formation. In fact, the SPER rate reaches its
minimum at the same depth for all the samples (unimplanted or implanted). Finally, the P–B co-doped sample
(closed squares in Fig. 2) shows the same SPER rate as the
undoped film, in agreement with Suni et al. [21] for As-B or
P–B co-doping.
A second point to be addressed concerns the roughness
of the a–c interface and its dependence on the doping level.
When the a–c interface moves toward the surface, the laser
light absorption is reduced and the TRR reflectivity signal
increases (see Fig. 1). Nevertheless, the interface and surface roughnesses can strongly suppress the intensity of
the TRR signal [22], as observed in Fig. 1 for the undoped
sample (a). The effect of dopants in this case is also evident,
since B-implanted sample shows a TRR signal in Fig. 1
(line b) whose intensity increases with time, indicating a
smoother interface with respect to the un-implanted and
co-implanted samples (Fig. 1, lines ‘‘a’’ and ‘‘c’’). The same
effect of B-implanted sample in the TRR signal is present
also in the P-implanted sample (not shown).
These considerations are better clarified by X-TEM
analyses (Fig. 3) of partially regrowth samples annealed
at 541 C. SPER of un-implanted and B-implanted films
were stopped approximately in correspondence of the last
and most intense, TRR peak of Fig. 1 (corresponding to
an interface depth of about 34 nm). In the X-TEM
images of Fig. 3 the defect-free regrown SiGe regions
are visible above the horizontal white line (Si/SiGe interface). As previously discussed, the doping process does
not influence the thickness of this defect-free layer (about
20 nm in both cases), which is always followed by a
region with a high density of dislocations and stacking
faults where the SPER rate was found to decrease (depth
of about 80 nm in Fig. 2). What is important to notice
is the lower average roughness in Fig. 3b for the
B-implanted sample with respect to Fig. 3a (un-implanted
sample) as already suggested by the different intensities of
the TRR signal of Fig. 1. A systematic study by X-TEM
of partially regrowth samples annealed at 541 C has been
performed in order to measure the average position of the
interface with time by sampling several regions of each
sample. Although the original Si/SiGe interface was quite
smooth (about 3 nm wide) the introduction of defects
induces the formation of different a–c fronts, at different
depths and with different regrowth rates. This causes a
rapid increase of the interface roughness and allows us
to characterize a faster and a slower front in the a–c
interface coinciding, respectively, with the highest peak
and deepest valley in the X-TEM images. It has been then
possible to plot the position of the faster and slower
fronts versus time. The results are reported in Fig. 4 for
un-implanted (open symbols) and B-implanted (closed
symbols) samples. This plot summarizes many of the features already discussed. First of all, it is evident that Bimplanted sample has always a higher SPER rate with
respect to the un-implanted one. Extrapolating the Bimplanted data, the slower front reaches the surface
before than the faster front of the un-implanted film.
Moreover, once the interface sharpness is broken after
about 300 s, the two fronts in the B-implanted sample
show approximately the same velocity (same slope in
between two consecutive points in Fig. 4). This means
that the average roughness does not increase continuously
with time. On the contrary, the un-implanted sample
shows that the two fronts, once formed, move always
with different velocities, this increasing the roughness of
the a–c interface during SPER.
D. D’Angelo et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 270–274
273
Fig. 3. Cross-section TEM images of un-implanted (a) and B-implanted (b) partially regrown samples annealed at 541 C. The white horizontal lines mark
the Si/SiGe interface. The crystalline and amorphous SiGe layers are also indicated.
140
observed and related to the implantation process, the final
structure of the regrown samples is very similar, in all
cases, showing a 20 nm defect free layer followed by dislocations and stacking faults. This study shows that such
defect formation occurs at the same depth, independently
from SPER kinetics. Moreover, the a–c interface roughness
is reduced in B or P single element implanted materials,
while B–P co-implantation annihilates the benefits of a single element implant.
un-implanted fast front
un-implanted slow front
B -implanted fast front
B -implanted slow front
120
Depth [nm]
100
80
60
40
Acknowledgments
20
0
0
5 00
1000
1500
2000
2500
Time [s]
Fig. 4. Depths of the faster and slower fronts of the a–c interface in SiGe
for partially regrown un-implanted (open triangles) and B-implanted
(closed triangles) samples at different annealing times at 541 C. Depths
and error bars have been evaluated by X-TEM analysis performed on
several views of each sample.
We strongly acknowledge the technical support of Salvatore Tatı̀ and Carmelo Percolla of the CNR-INFM Matis research laboratory in Catania for ion implantation and
TRR measurements and Dr. Isodiana Crupi, researcher of
the CNR-INFM Matis in Catania, for the development of
the TRR data acquisition code. This work has been
granted by MIUR within the PRIN 2004 project.
References
4. Conclusions
In this paper, we reported some of our experimental
studies concerning SPER of SiGe/Si heterostructures
grown by MBE and amorphized by ion implantation. We
also made some comparison among SPER of unimplanted, B- or P-implanted and B–P co-implanted materials. All data indicate that SPER of ion implanted SiGe
does produce highly defective materials independently
from the implanted chemical species. Although differences
in the a–c interface speed and roughness have been
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