Implantation Erbium Doping In 6H SiC For Optimum
Optical Efficiency at 1.54 µm
A. Kozanecki*, V. Glukhanyuk*, W. Jantsch+, and B.J. Sealy#
*
Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland,
+
Department of the Semiconductor Physics, J. Kepler University, Linz A-4040, Austria,
#
Surrey Centre forResearch in Ion Beam Analysis, University of Surrey, Guildford, Surrey GU2 5XH, UK.
Abstract. Conditions for obtaining efficient near infrared luminescence at 1.54 µm of Er3+ ions in 6H SiC are studied. It
is shown that implantation of Er at elevated temperature is essential for the emission intensity. An evidence is presented
that N-donors act as luminescence sensitizers at low temperatures. Codoping with oxygen does not increase the Er
emission. Our data suggest that O atoms form all the emitting Er-related centres.
Recrystallization of the implanted layers in 6H SiC,
amorphized with ion beams requires very high
temperatures exceeding 1500°C [4], which are difficult
to handle and may lead to contamination and
decomposition of the surface. In our earlier papers
[5,6] we showed that this problem can be solved by
implanting MeV Er ions at elevated temperatures. We
have found that substrate temperature of ~ 350°C is
sufficient to avoid amorphization and to prevent
outdiffusion of Er atoms from the implanted layers. As
a result of elevated temperature implantation it was
also possible to reduce annealing temperatures to
1300-1350°C to obtain efficient PL of Er3+ ions.
INTRODUCTION
Wide bangap erbium-doped semiconductors are
very promising as materials for optoelectronic devices
operating at 1.54 µm based on the 4I13/2 – 4I15/2 intra-4fshell transitions of Er3+ ions. This is because of the
temperature quenching of luminescence, which is
directly related to width of the forbidden gap [1].
Among wide bandgap semiconductors GaN:Er and
SiC:Er are the most interesting because of their
technological importance. Choyke et al. [2,3] have
found that the integrated 4f-4f Er3+ photoluminescence
(PL) intensity near 1.54 µm in different SiC polytypes
is almost constant within a wide range of temperatures
up to 400 K. To date, however, little information has
been gained on the mechanism of energy transfer to
Er3+ ions, and on the role played by two native
impurities, such as nitrogen and oxygen. In particular,
it is still not clear whether they participate in the
formation of light emitting Er-centres or mediate the
energy transfer to Er3+ ions acting as luminescence
sensitizers.
This work presents the results of studies of Erimplanted 6H SiC aimed to determine optimum
implantation and doping conditions. The influence of
nitrogen and oxygen on the Er3+ emission is discussed.
EXPERIMENTAL
Ion implantation is the basic method of doping SiC
with Er, therefore perfect recrystallization of the
implanted layers is vital for the PL efficiency.
Location of Er ions in crystalline lattice is another key
issue, as it may determine the pathway of excitation
energy from electron-hole (e-h) pairs to the excited
states of Er3+ ions.
Samples of 6H SiC were doped with N-donors in a
wide range of concentrations (2 x 1014 – 2 x 1018 cm-3).
Most of samples were implanted with Er+ ions at
350°C at three energies of 850, 1300 and 2000 keV.
Two basic sets of samples differing in Er content by an
order of magnitude were prepared. Er fluences were
either 5x1013 or 5x1014 cm-2 for each ion energy.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
686
implantation. This is because samples implanted at RT
remain amorphous even after annealing at 1350°C [7].
Much higher temperatures are necessary for good
recrystallization in this case, however, it causes
outdiffusion of Er atoms from the layers.
Rutherford backscattering (RBS) of 1.5 MeV He+
ions in combination with channeling was used to study
damage in the layers. The scattering angle was 160°.
PL was excited with an Ar laser using the 365 nm UV
line at 30 mW excitation power. For high resolution
PL measurements a Bomem DA8 Fourier Transform
(FT) spectrometer equipped with a liquid nitrogen
cooled Ge p-i-n diode for detection of infrared
emission of Er3+ was used.
Annealing of Er+O implanted samples results in a
worse damage removal (Fig. 2) as compared to layers
implanted with Er only. Annealing of SiC samples
implanted with a single dose of 1013 cm-2 is almost
perfect as the channeling spectrum overlaps with that
of virgin SiC (therefore not shown in Fig. 2).
Backscattering yield /10
Thermal annealing was performed at temperatures up
to 1350°C in a nitrogen gas flow. Some samples were
also co-implanted with 125 keV O+ ions (to a dose of 3
x 1014 cm-2) to study the influence of oxygen on the PL
of erbium.
RESULTS
RBS/ Channeling
In Fig 1. we summarize RBS/channeling results
obtained on 6H SiC samples triply implanted with Er
ions at room temperature (RT) and at 350°C to
fluences of 5x1013 cm-2 for each energy.
as implanted, Er
o
T a =1300 C, Er+O impl.
80
o
T a =1300 C, Er
60
40
Si
O
C
20
0
100
150
200
250
300
Channel No.
15
Backscattering yield /10
3
random
as implanted at RT
o
as implanted at 350 C
o
o
impl. at 350 C, T a=1300 C
virgin
10
FIGURE 2. Channeled spectra for 6H SiC implanted
with Er, or Er+O and annealed at 1300°C.
Residual damage in Er+O implanted 6H SiC is
responsible for the lower PL intensity than in samples
implanted with Er only.
5
High Resolution FTIR Photoluminescence
Measurements
100
150
200
250
300
In a previous paper [6] we have shown that Er PL
intensity increases with N concentration. The highest
PL intensity was observed in samples containing Ndonors at a concentration of 6x1016 cm-3. In general,
the PL intensity by no means reflected the three orders
of magnitude difference in the concentration of donors.
In our low resolution PL measurements we could not
find differences in the spectra that would suggest the
existence of Er-centres related specifically to Nimpurity [6]. The high concentration of donors led
only to a broadening of the PL lines. As the resolution
of standard PL measurements was apparently too low
to see details in the spectra in this work we used high
resolution spectroscopy.
Channel No.
FIGURE 1. RBS/channeling spectra for 6H SiC implanted
with the total dose of Er+ ions of 1.5x1014 cm-2.
It is clearly seen that RT implantation results in
amorphization of the surface layer, whereas at 350°C
the layer is highly damaged, but still retains its
crystallinity. Thermal annealing at 1300°C results, in
the case of samples implanted at 350°C, in a very good
recovery of crystalline quality of the layers. Some
residual damage still remains (Fig. 1), nevertheless the
PL intensity is orders of magnitude higher than for RT
687
In Fig. 3 the high resolution FTIR PL spectra of
Er3+ in samples containing N-donors differing by three
orders of magnitude in concentrations are presented.
The PL intensity is higher in samples containing
2x1018 cm-2 of N donors, however, a comparison of the
spectra reveals no changes in their shapes that could be
tentatively assigned to some Er-N complexes. After
normalization the spectra overlap almost perfectly. We
take this observation as an evidence that N-donors do
not participate directly in the formation of optically
active Er-N centres.
hard to observe in samples not implanted with O. We
have found, however, that the intensity ratios of the
dominant lines are the same at all temperatures. We
think, therefore, that a weak luminescence, which
apparently depends on oxygen content (Fig. 4), is due
to the formation of Er-O complexes or, alternatively,
theses centres involve different number of O atoms
than the centres responsible for the dominant PL.
DISCUSSION
Among many factors influencing the PL intensity
of Er3+ at least three seem to be of primary importance:
(i) the high concentration of optically active centres,
which at optimum condition should be simply equal to
the total concentration of the introduced impurity, (ii)
efficient energy transfer from the photoexcited e-h
pairs to Er3+ ions, and (iii) in the case of ion
implantation doping perfect recrystallization is
necessary to reduce nonradiative recombination. The
last problem seems to have been solved at least in part
by raising the temperature during implantation. Since
defect removal is never complete, as shown using
RBS/channeling (Figs. 1 and 2), we think that
substrate temperatures above 500°C during
implantation should be used to reduce damage even
more [1] and to increase the PL intensity.
intensity, a.u.
15
6H SiC:Er,
PL spectra, 80K
15
-3
[N] = 2*10 cm
18
-3
[N] = 2*10 cm
10
5
0
6600
6580
6560
6540
wavenumber, cm
-1
6520
6500
FIGURE 3. High resolution FTIR PL spectra of 6H SiC
doped with different concentrations of N-donors.
As it was mentioned above, the most characteristic
feature of the PL spectra of Er3+ in 6H SiC is their
independence of the conditions of implantation,
annealing temperature and the content of N-donors
even for concentrations of N differing by three orders
of magnitude (Fig. 3). Apart from 6H SiC such an
independence of technological conditions was found
up to now only in InP:Yb [8]. In the case of Yb in InP
it reflects the existence of only one type of the emitting
centre – associated with substitutional Yb3+ ion
replacing In. It is a unique situation for rare earth
doped semiconductors. Taking into account the
number of PL lines observed at lowest temperatures in
6H SiC:Er we can say that there are at least 2-3
different low symmetry centres. Nevertheless, the
dominant PL seems to come from 1-2 centres only,
whose structure does not change as a result of different
technological steps during processing. However,
substitutional location of Er ions have not been
experimentally confirmed yet.
In Fig. 4 the high resolution FTIR PL spectra of 6H
SiC doped with Er and Er+O are compared. Some
differences in the intensities of minor lines can be
easily seen. In particular, a few low intensity lines are
6
intensity, a.u.
4
6 H S iC :E r,
P L sp e c tra , 8 0 K
15
-3
E r, [N ]= 2 * 1 0 c m
18
-3
E r+ O , [N ]= 2 * 1 0 c m (* 2 .4 )
2
0
6580
6560
6540
-1
w a v e n u m b e r, c m
6520
The high resolution FTIR PL measurements call in
question whether common impurities such as N or O
form optically active Er-related defects. At first sight,
taking into account all observations, it seems that there
are two possibilities – either all the emitting Er-centres
FIGURE 4. Comparison of the normalised FTIR PL spectra
of Er-doped and Er+O doped 6H SiC.
688
are complexes with O or/and N, or not. In our opinion
the influence of N donors on the Er emission seems to
be quite clear. At low temperatures the PL intensity
increases with the N-contents, at least up to some
optimum concentration (~1017 cm-2). On the other
hand, the PL intensity above 250 K shows that
differences in the PL intensities in samples containing
different concentrations of N-donors practically
disappear and the PL intensities in samples implanted
with the same dose of Er are almost equal [6].
Therefore, we believe that effect of N-doping on the
Er3+ PL is associated rather with energy transfer
mechanism than with direct involvement of N-atoms
into the structure of Er emitting centres. In our opinion
this result clearly shows that at low temperatures
excitation is mediated by N-donors binding excitons
which then recombine nonradiatively transfering
energy to Er3+ ions via an Auger-like process. It seems,
however that at RT N-donors do not participate in the
energy transfer, as they are too shallow to bind
carriers. Therefore, we think that at higher
temperatures some deep centres participate in the
excitation of erbium. At this stage, however, contrary
to the suggestions of Klettke et al [9], we do not
exclude that deep Er-related traps (i.e. isoelectronic)
mediate energy transfer process. This problem is being
studied now.
whereas at low temperatures the PL increases with Ncontents without altering to the spectra. It indicates
that N-donors act as sensitizers of Er emission.
Oxygen impurity seems to be directly involved in all
the emitting centres. Some decrease of the PL intensity
in O+Er doped samples is most probably due to poor
recrystalization of doubly implanted layer and the
resulting nonradiative recombination.
ACKNOWLEDGMENTS
This work was supported in part by the KBN grant
No.: 7T11B 007 21 in Poland and in Austria by FWF,
ÖeAD and Gme.
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CONCLUSIONS
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The results of our work show that the presence of
Er in the implanted layers makes good recrystallization
difficult. Elevated temperature implantation is very
helpful in improving the quality of the layers. The PL
efficiency of Er3+ ions at room temperature does not
seem to depend on the concentration of N-donors,
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