This document studies the thermally induced recrystallization of an amorphous layer of gallium arsenide (GaAs) created by bombarding a single crystal GaAs sample with 100 keV argon ions at a fluence of 5 × 1016 ions/cm2. Surface Brillouin scattering and Raman spectroscopy were used to investigate the structural changes during isochronal annealing. The Surface Brillouin scattering showed continuous stiffening of the layer beginning above 200°C, reaching a maximum value above 500°C. The Raman studies showed evidence of full recrystallization above 500°C, indicating the reformed layer was polycrystalline.

1. Thermally induced amorphous to crystalline transformation of argon ion
bombarded GaAs studied with surface Brillouin and Raman scattering
K. Jakata, D.M. Wamwangi, C. Sumanya, B.A. Mathe, R.M. Erasmus, S.R. Naidoo, J.D. Comins ⇑
Materials Physics Research Institute, School of Physics, University of the Witwatersrand, Johannesburg, South Africa
DST/NRF Centre of Excellence in Strong Materials, Physics Building, University of the Witwatersrand, Johannesburg, South Africa
a r t i c l e i n f o
Article history:
Received 31 August 2011
Received in revised form 18 December 2011
Available online 9 January 2012
Keywords:
GaAs
Ion implantation
Amorphisation
Recrystallisation
Surface Brillouin scattering
Raman scattering
a b s t r a c t
Surface Brillouin scattering (SBS) and Raman spectroscopy have been used to investigate the recrystalli-
sation of an amorphous layer of GaAs created on single crystal (001) GaAs by ion bombardment with
100 keV argon ions with a fluence of 5 Â 1016
ions/cm2
at a temperature of $65 °C. Samples were isochro-
nally annealed and the light scattering measurements were performed after each annealing step. The SBS
studies confirm structural changes resulting in continuous stiffening of the layer beginning above 200 °C
and finally attaining a maximum value above 500 °C. The Raman studies show evidence of full recrystal-
lisation above 500 °C, with the appearance of both LO and TO peaks indicating that the reformed layer is
polycrystalline.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Many studies have been performed on the removal of ion
implantation damage in semiconductors by high temperature
annealing. In Si and Ge the re-crystallisation of implanted amor-
phous layers basically arises from epitaxial regrowth on the under-
lying bulk substrate in a single step process [1]. However in GaAs,
the re-crystallisation process is considerably more complex as
shown in relevant publications [2–8]. Furthermore the process of
amorphisation is shown to depend critically on several variables,
namely ion type, fluence, ion flux and substrate temperature. This
is especially the case at elevated temperatures in which the resid-
ual ion damage is particularly sensitive to both of the last two vari-
ables [7]. For room temperature (RT) bombardments it was found
that partial re-growth can occur below 350 °C, but higher temper-
atures (>600 °C) are required to remove residual damage [4]. Using
ion channelling studies [5] a comparison of the amorphous damage
profile created with an argon ion fluence of 1 Â 1014
ions/cm2
at RT
(22 °C) and that for an elevated temperature estimated to be <70 °C
shows that the damage in the latter case is confined to a signifi-
cantly narrower near surface region. However, an increased argon
ion fluence (from 1 Â 1014
to 2 Â 1015
ions/cm2
) in RT bombard-
ments show a substantially increased amorphous damage profile
width.
Previous studies have used Rutherford backscattering, ion chan-
nelling, electron microscopy and Raman spectroscopy [2–8] in
order to achieve the current state of knowledge regarding amor-
phisation and recrystallisation of GaAs. The present work examines
these processes subject to a higher fluence argon ion bombard-
ment using a combination of optical techniques, namely surface
Brillouin scattering (SBS) [9,10] and Raman spectroscopy for the
first time. It is noted that the single-step annealing stage corre-
sponding to the re-crystallisation of ion-bombarded silicon has
been previously studied using SBS [11,12].
SBS is a non-contact technique that investigates thermally in-
duced surface acoustic waves in the GHz frequency range. In SBS
applications involving opaque solids and thin (<1000 nm) sup-
ported layers, light scattering takes place primarily by the surface
ripple mechanism and the frequency spectrum of the scattered
light provides information on the surface dynamics. The latter
are related to the elastic stiffnesses of the materials, these being
fundamental quantities, associated with the inter-atomic poten-
tials. They depend on variables such as composition, microstruc-
ture and temperature and are sensitive to certain solid state
phase transitions. Accordingly SBS studies are useful in the study
of ion beam modified surface layers. In the present case, the com-
bination of the amorphised over-layer and the stiffer crystalline
substrate determines the nature of the observed acoustic excita-
tions, namely the Rayleigh and Sezawa waves [9,10].
0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.nimb.2011.12.067
⇑ Corresponding author at: Materials Physics Research Institute, School of Physics,
University of the Witwatersrand, Johannesburg, South Africa.
E-mail address: Darrell.Comins@wits.ac.za (J.D. Comins).
Nuclear Instruments and Methods in Physics Research B 286 (2012) 25–28
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2. 2. Experimental techniques
Single crystal samples of (001) GaAs of dimensions 4 Â 3 mm2
were bombarded at by argon ions of energy 100 keV using an angle
of 7o
to reduce the effects of ion channelling. The ion beam diam-
eter was considerably larger that the surface dimensions of the
GaAs samples, thus ensuring that the full area of the samples were
uniformly treated. The pressure in the irradiation chamber was
5 Â 10À4
Pa. The fluence of the beam was 5 Â 1016
ions/cm2
with
a beam current density of 2.5 lA/cm2
. Drawing from past experi-
ence, the temperature of the samples would be $65 °C under these
conditions.
Using SRIM2011 calculations [13] for the vacancy distribution
as a function of depth in GaAs created under the present condi-
tions, and with the arguments presented in Ref. [10], the peak
vacancy concentration would be $1025
cmÀ3
or $100 times the
atomic density; i.e. every atom at the peak of the distribution
is displaced on average about 100 times, suggesting that the en-
tire region would be uniformly amorphous up to the point
where the deep-edge tail has fallen to about 0.01 of the peak
height. These arguments would indicate that a uniform amor-
phous layer of $140 nm is formed on the crystalline substrate
of the GaAs samples. However for a temperature $65 °C, the
samples would be subject to the defect mobility and partial
annealing processes leading to the amorphous damage profile
narrowing effects discussed above [5,7]. Since the present flu-
ence used is substantially higher (5 Â 1016
ions/cm2
), this is ex-
pected to increase the width of the damage profile [5]. It
would appear that the actual profile width will be determined
by these opposing effects, but being narrower than the
$140 nm width predicted by SRIM2011 calculations.
In the surface Brillouin scattering (SBS) experiments laser
light of wavelength 514.5 nm from a frequency stabilised ar-
gon-ion laser was incident on the sample in a backscattering
configuration using an incident angle of 60°. The incident laser
light was scattered with a frequency shift determined by the
acoustic phonons propagating in the near surface region of the
sample. As discussed above, the surface ripple mechanism of
light scattering is dominant in the case of near opaque materials
such as GaAs. A JRS Instruments Sandercock-type (3 + 3)-pass
Fabry–Pérot interferometer was used to measure the fre-
quency-shifted scattered light. This instrument provides the nec-
essary high contrast and resolution.
The Raman studies were carried out using a Jobin–Yvon T64000
spectrograph fitted with a Raman confocal microscope and oper-
ated in single spectrograph mode using a 1800 lines/mm grating.
An argon-ion laser provided the exciting light of wavelength
514.5 nm.
Isochronal annealing was performed using a Linkam TS1500 cell
under argon gas for both the SBS and the Raman scattering measure-
ments. The sample was raised to each annealing temperature at a
rate of 10 °C per min and then annealed for 20 °min. After annealing,
cooling was done at the same rate to a chosen lower base tempera-
ture for the respective SBS and Raman measurements. As the sample
needed to be removed from the Linkam cell for the SBS measure-
ments, room temperature (22 °C) was the base temperature in this
case. A base temperature of 400 °C was used for the detailed Raman
studies of the anneals at and above this temperature as no spectral
changes were observed on initial annealing to 400 °C. Since the sam-
ple remained in the Linkam cell this procedure was much less time
consuming than cooling to room temperature. An independent
experimental check was carried out on a different ion-bombarded
sample using 22 °C as the base temperature for a few selected
annealing temperatures, showing consistent agreement with the
original Raman annealing experiment.
3. Results
The Raman spectrum of the virgin GaAs single crystal measured
at RT (22 °C) is shown in Fig. 1 The LO phonon mode at 292.8°cmÀ1
is
of narrow width typical of a high quality crystal with minimal defect
concentration and strain. It is noted that for reasons of symmetry
only the LO mode is observed for the Raman spectrum of the
(001) crystal face. Also shown in Fig. 1 is the Raman spectrum of
the argon-ion implanted GaAs crystal also measured at RT. The spec-
trum is relatively featureless with a broad peak near 250 cmÀ1
being
characteristic of amorphous GaAs as previously determined [14,15].
Surface Brillouin scattering (SBS) results are shown in Fig. 2. The
value for the frequency shown for the Rayleigh mode of the virgin
crystal is 9.78 GHz. However owing to the frequency dispersion of
the mode resulting from the crystalline structure the measured fre-
quency varies between 9.78 and 10.26 GHz. The instrumental
Fig. 1. Raman spectra of virgin and argon ion bombarded GaAs. The lower spectrum
shows the sharp LO mode of crystalline (001) GaAs with a peak at 292.8 cmÀ1
. The
upper spectrum shows a broad peak for the layer of amorphous GaAs on a
crystalline GaAs substrate (a-GaAs/c-GaAs) after bombardment with argon ions
with a fluence of 5 Â 1016
ions/cm2
. The measurements were made at 22 °C.
Fig. 2. SBS spectra of virgin and argon ion bombarded GaAs. The lower spectrum
shows the results for the (001) face of the virgin crystal showing the sharp Rayleigh
wave peak. The upper spectrum shows the spectrum for a-GaAs/c-GaAs after
bombardment with argon ions with a fluence of 5 Â 1016
ions/cm2
. The Rayleigh
wave peak has a reduced frequency. The measurements were made at 22 °C.
26 K. Jakata et al. / Nuclear Instruments and Methods in Physics Research B 286 (2012) 25–28

3. peaks indicated in the spectra result from the shutter system used
to block the strong elastically scattered light from the sample.
The SBS spectrum of a sample ion bombarded with an argon ion
fluence of 5 Â 1016
ions/cm2
is also shown. It is clear that there is a
significant reduction in the Rayleigh mode frequency that is con-
sistent with the presence of an amorphous layer of reduced elastic
stiffness. It is noted that the Rayleigh mode amplitude penetrates
throughout the amorphised layer and thus samples the layer prop-
erties. Furthermore there is the presence of a Sezawa mode in the
spectrum resulting from the presence of the elastically softened
amorphous layer and meeting the conditions for such a mode of
a ‘‘slow on fast’’ system [10].
The results for the isochronal annealing of the amorphised GaAs
layer are shown in Fig. 3(a) and (b). The annealing steps were of
20 min duration and the SBS measurements were each made at
22 °C. It is noted that the frequency of the Rayleigh wave increases
during the anneal, beginning above 200 °C and finally attaining a
maximum value of 8.79 GHz above 500 °C, but which is lower than
the range of values for single crystal (001) GaAs discussed above.
Thus the originally amorphised layer is undergoing structural
modifications at relatively low temperatures while it attains a
maximum stiffness less than that of the original single crystal at
the highest temperatures.
The Raman measurements shown in Fig. 4(a) and (b) for the iso-
chronal anneals reveal that full recrystallisation of the original
amorphous GaAs layer initiates above 500 °C as confirmed by the
relatively sharp LO and TO peaks that intensify and sharpen at
the higher measured temperatures. It is noted that this tempera-
ture range is in agreement with the SBS study in which the Ray-
leigh mode frequency attains its maximum value (Fig. 3). The
presence of both Raman modes indicates that the layer is polycrys-
talline. The reduced frequency shift of the LO mode as compared
with that of Fig. 1 arises from the 400 °C measurement tempera-
ture used for the annealing experiments.
4. Discussion
The recrystallisation of amorphous GaAs produced by ion bom-
bardment has been the subject of several investigations using
channelling experiments [4–7], Raman [8] and EXAFS studies
[16]. It is generally agreed that the process is unlike the simple epi-
taxial single stage recrystallisation observed in Si and Ge. As dis-
cussed in the Introduction, ion type, fluence, ion flux and
substrate temperature are shown to determine the nature of the
amorphisation and hence the recrystallisation. In the case of Ar+
-
ion bombardment at low temperatures ($77 K) with fluences in
Fig. 3. (a) SBS spectra measured during the isochronal annealing of the amorphised
GaAs layer. Each annealing step was of duration 20°min at the indicated
temperatures followed by cooling to 22 °C for the SBS measurements. (b) The
Rayleigh wave frequency shift is shown as a function of the isochronal annealing
temperature. Error bars are within the size of the symbols.
Fig. 4. (a) Raman spectra measured during the isochronal annealing of the
amorphised GaAs layer. Each annealing step was of duration 20 min at the
indicated temperatures followed by cooling to the measurement temperature. The
LO (283.7 cmÀ1
) and TO (258.8 cmÀ1
) peaks develop in the range above 500 °C. (b)
The Raman peak intensity of the amorphised GaAs in the lower temperature range
(annealing temperatures from 100 to 400 °C) followed by the intensity of the LO
and TO peaks at higher temperatures (500–700 °C). In both (a) and (b) the RT to
300 °C anneals have a measurement temperature of 22 °C, while it was 400 °C for
the anneals at 400 to 600 °C.
K. Jakata et al. / Nuclear Instruments and Methods in Physics Research B 286 (2012) 25–28 27

4. the range 1013
–1014
ions/cm2
, channelling experiments show that
amorphous GaAs layers can be recrystallised epitaxially at temper-
atures below 250 °C. However the regrowth process is shown to be
complex with the crystalline quality and regrowth rate being very
dependent on implant fluence. In particular room temperature and
higher implants resulted in partial but poor quality recrystallisa-
tion and annealing temperatures of >600 °C were required to re-
move residual damage.
The present work describes high fluence bombardment (5 Â
1016
ions/cm2
) of GaAs at substrate temperature of $65 °C studied
using light scattering techniques. The surface Brillouin scattering
experiments demonstrate continuous stiffening of the layer begin-
ning above 200 °C and finally attaining a maximum value above
500 °C. The Raman studies show evidence of full recrystallisation
above 500 °C, with the appearance of both LO and TO peaks indicat-
ing that the reformed layer is polycrystalline. The lower temperature
process extending over the temperature range 200–500 °C is consid-
ered to arise from structural re-arrangements of disordered regions
within the amorphous layer and, at sufficiently high temperatures,
incorporation into the advancing crystalline/amorphous interface
to produce a defective polycrystalline layer.
Acknowledgments
The authors wish to thank iThemba Labs (North) for use of the
ion implantation equipment, and the DST/NRF Centre of Excellence
in Strong Materials and the National Research Foundation for
financial support.
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