Excitation Path of Rare Earth Ions in Nitride

International Conference on Electronics, Biomedical Engineering and its Applications (ICEBEA'2012) Jan. 7-8, 2012 Dubai
Excitation Path of Rare Earth Ions in Nitride
Semiconductors
A. Melouah, and M. Diaf

Questions still remain about the fundamental
understanding of the mechanisms underlying the excitation of
rare-earth ions in this host.
Abstract—In the last decade, great interest in rare-earth (RE)
doped wide band gap semiconductors, which combine the electronic
properties of semiconductors with the unique luminescence features
of RE ions[1]. Rare earth doped nitrides show light emission when
excited by photons with energy higher than their band gap (UV) or
lower than it (visible). Narrow emission lines associated to the weak
dependence of intensity on temperature put into evidence the
possibility of realizing full colour display devices based on the
nitrides using the intra-4f n transitions from the RE ions, Rare earth
doped GaN is an interesting system in which an energy transfer can
occur between the semiconductor and the internal 4f states of the rare
earth ions where Eu3+ stands for the red emission, Er3+ for the green
and Tm3+ for the blue one. GaN epilayers grown by MOCVD are
implanted with Tm, Er and Eu. Optical properties of RE-doped GaN
films are studied by using Er, Eu, and Tm-doped GaN in order to
maximize ELD brightness and efficiency as well as to apply the
results to real devices. Red emission at 621 nm from the 5D0→7F2
transition of Eu3+ has been obtained from GaN:Eu. Spectral
photoluminescence (PL) studies are performed on Eu-doped GaN
thin films.
II. SAMPLES AND EXPERIMENTS
Europium ions were implanted into nominally undoped
GaN films grown by metal organic chemical vapour
deposition (MOCVD) on c-plane sapphire. The implantation
energy was 300 keV with the ion beam perpendicular to the
surface channelled implantation. The implantation was
performed at room temperature for all samples. Four different
Eu implanted samples were investigated. The fluence was kept
fixed at 1·1015 Eu/cm2 for samples 23–25 and was slightly
lower at 7 · 1014 Eu/cm2 for sample 48. The main difference
between the samples comes from the annealing treatment.
Sample 48 was annealed for 120 s at 1000 °C in a rapid
thermal annealing apparatus between graphite strips under
flowing N2 gas using a piece of unimplanted GaN as a
proximity cap to inhibit the out-diffusion of nitrogen from the
surface. Samples 23–25 were prepared with a 10 nm thick
AlN capping layer prior to implantation allowing the use of a
high temperature annealing treatment without noticeable
deterioration of the GaN crystal quality. The annealing
temperatures were 1100 °C (sample 23), 1200 °C (sample 24)
and 1300 °C (sample 25). The Eu implanted samples were
mounted in an APD closed-cycle helium cryostat and cooled
down to 12 K. PL studies were performed by exciting the
Eu3+:GaN samples with a CW HeCd laser (λexc = 325 nm) and
a CW Ar+ ion laser (λexc = 514 nm). Visible luminescence was
recorded using a monochromator equipped with a PMT. The
monochromator resolution was kept below 0.1 nm for all
spectra.
We first investigated the photoluminescence (PL) spectra
of (Eu) transitions in Eu-implanted GaN samples at 12K to the
room temperature. Under below-band gap excitation, two
main Eu centers were identified. Some Eu centers are excited
via local defects or impurities forming therefore various Eudefect complexes while other Eu centers are isolated without
any defect in their vicinity. In case of above band gap
excitation the exact nature of the excitation path leading to the
excitation of rare-earth ions in GaN leads to think that Eu
takes the substitutional site of the Gallium. The most
commonly assumed mechanism involves Eu-defect complexes
where the defect captures an electron-hole pair before Eu
excitation. The exact nature of the defects mediating this Eu
Keywords— Rare earth, energy transfer, photoluminescence,
Gallium Nitride, Eu3+ ions.
R
I. INTRODUCTION
ARE earth doped III-Nitrides semiconductors have been
studied for the few last years because of the possibility to
develop compact and efficient electroluminescence devices [1,
2,3,4]
. Trivalent Europium ions are of special interest because
they exhibit an atomic-like transition at especially 621nm
which corresponds to the red luminescence of this rare earth.
Electroluminescence devices based on Eu doped were
reported however their efficiency was too low for practical
applications. It was observed that the room temperature Eu3+
photoluminescence (PL) intensity strongly depends on the
band gap of the host materials [5] . It was found that for larger
band gap there is less detrimental temperature quenching of
Eu 3+ PL occurring. Therefore, doping Eu3+ ions into wide gap
semiconductors is a promising approach to overcome the
thermal quenching of Eu PL.
Europium doped GaN is being widely studied from cryogenic
to elevated temperatures.
A. Melouah is in Laboratoire de physique des lasers, de spectroscopie
optique etd’optoélectronique (LAPLASO) Badji Mokhtar-Annaba University,
PoBox12, 23000 Annaba Algeria (E-mail : oussifamene@yahoo.fr)
M. Diaf is in Laboratoire de physique des lasers, de spectroscopie optique
etd’optoélectronique (LAPLASO) Badji Mokhtar-Annaba University,
PoBox12, 23000 Annaba Algeria (E-mail : oussifamene@yahoo.fr).
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International Conference on Electronics, Biomedical Engineering and its Applications (ICEBEA'2012) Jan. 7-8, 2012 Dubai
excitation needed to be clearly identified.
5
D2
5
D1
5
D0
621nm
7
F2
Fig3 :The experimental Setup
Fig 1 a) :Eu Spin Orbit Splitting
III. RESULTS AND DISCUSSION:
e
Europium ions excited resonantly within the 4f
configuration exhibit a specific emission spectrum composed
of sharp lines which are characteristic of Stark to Stark
sublevel transitions as already described in the literature [6].
Figure 4 shows that thus, the excitation path for above band
gap radiation appears to be involving defects with
characteristics similar to the Eu-defect complexes excited
below band gap. This also suggests that Eu centers excited by
above band gap radiation or directly within defect-complexes
are affected the same way by luminescence quenching
processes. The evolution of Eu luminescence as a function of
the photon flow for the He-Cd excitation wavelength
(λ=325nm) is displayed in Figure 4. The figure 5 exhibits a
saturation behavior, which is very pronounced for this above
band gap excitation. From this saturation curve it is possible
to derive an absorption cross-section for the excitation within
Eu-defect complexes or above band gap. It is to be noted that
this excitation cross-section is not a true cross-section which
by definition refers to an instantaneous process. Indeed, in
case of indirect excitation the Eu excitation path may be very
complicated and require several steps involving different
species.
CB
HeCd Laser
Trap
level
Excitation
Of
Europium
Eu3
+
5
D0
621nm
7
h
F2
VB
Fig1b): Illustration of the proposed sequential mechanism for Eu
excitation see text for the full description.
A. Experimental procedure:
The films Grown by atmospheric pressure metalorganic
chemical deposition (MOCVD)were mounted on a two axis
goniometer. Eu ions were implanted with an energy of 300
Kev into as-grown films at room temperature. The
implantation dosage was either. 1015 or 7.5.1014 ions/cm2.
During this operation, the GaN <0001> axis was either
aligned with the ion beam (‘channeled’ implantation) or tilted
by 10° (‘random’ implantation). Subsequently, the samples
were annealed in a tube furnace at 1000 to 1300° C for 20mn
under flowing N2.
The Eu-implanted wafers were mounted in APD Liquid
cryostat and cooled down to12K.
Photoluminescence
spectroscopy was performed at this and room temperatures by
exciting GaN samples with the 325 nm line from a HeCd laser
and an Argon laser. Infrared luminescence was recorded
using a monochromator equipped with a thermo-electric
cooled InGaAs photodiode. The monochromator resolution
was kept below 0,6nm for all spectra.
High photon flow
Medium photon flow
low photon flow
2,0
1,8
Intensity (a.u.)
1,6
TAnneal.=1300°C
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
6200
6205
6210
6215
6220
6225
6230
6235
Wavelength (A)
Fig4:Evolution of Eu luminescence as a function of the photon
flow
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International Conference on Electronics, Biomedical Engineering and its Applications (ICEBEA'2012) Jan. 7-8, 2012 Dubai
REFERENCES
Normalized PL intensity
1,0
Tanneal.=1200°C
exc=325nm
12K
0,8
[1]
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Semiconductors, Material Research Society Symposium Proceedings,
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[4] S. Coffa, A. Polman, and R.N. Schwartz, , Rare earth Doped
Semiconductors II, Material Research Society Symposium Proceedings,
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[5] P. N. Favennec, H.L. Haridon, D. Moutonnet, and Y. Le Guillou, Electr.
Lett. 25, 718 (1989).
[6] M.Pan, A.J Steckl Appl. Phys.Lett (83) N1 (2003) 9.
[7] A.Oussif, M. Diaf, ABraud, J.L.Doualand, R. Moncorgé, CISGM3_3rd
ICMSE ; Jijel 25-27 Mai 2004.
[8] A.Braud, J.L. Doualan, R. Moncorge, B. Pipeleers, A.Vantomme, Mat.
Sc. Eng. B 105 (2003).
[9] S.Kim, S.J.Rhee, D.A.Turnbull, E.E. Reuter, X.Li, J.J.Coleman and
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[10] S.Kim, S.J.Rhee, X.Li, J.J.Coleman and S.G.Bishop, , Appl. Phys. Lett.,
Vol.76 (17) (2000) 2403
0,6
0,4
det.=620,8nm
det.=622,6nm
0,2
0,0
0
1x10
20
2x10
20
-1
3x10
20
4x10
-2
20
Photon flow (s cm )
Fig5: Saturation of theEu2 faster than Eu1.
This question is addressed by means of a new setup
combining two lasers. Free electrons are first created by a HeCd laser at 325nm while the second laser corresponds to a
below band gap excitation(fig6) . The second laser has a
dramatic influence on Eu PL intensity related to the above
band gap excitation. The effect of this second laser on the
excitation path is investigated. For the first time to our
knowledge, we have clear evidence that the above band gap
excitation of Eu ions involves carrier traps which act as
mediators for the excitation towards the rare-earth ions. The
relevant trap is also clearly identified.
intensité PL(UA)
0.0008
E
E
E
0.0007
0.0006
0.0005
0.0004
0.0003
0.0002
0.0001
0.0000
6180
6190
6200
6210
6220
6230
6240
6250
6260
Longueur d'onde (A)
Fig 6: Quenching PL with a dual excitation
IV. CONCLUSION
Infrared PL spectra recorded in Eu-implanted GaN at 12K
to Room temperature enable us to identify two different Eucenters on one hand Eu centers excited by above band gap
radiation and on the other hand Eu ions excited via local
defects are undoubtedly very different. It shows that the
excitation path from above band gap to Eu ions involves
nearby defects which trap electron-hole pairs before Eu
excitation. The exact nature of the defects mediating this Eu
excitation still remains to be clearly identified. This issue
represents the key question of our future work.
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