Under optical excitation above the fundamental energy of GaN QDs, the fundamental transition emission from the GaN QD host was not observed while bright emission from Tm3+ manifolds demo
Trang 1Optical study of excitation and deexcitation of Tm in GaN quantum dots
Thomas Andreev,1,*Nguyen Quang Liem,1,2Yuji Hori,1,3Mitsuhiro Tanaka,3Osamu Oda,3Daniel Le Si Dang,4
Bruno Daudin,1and Bruno Gayral1, †
1CEA/CNRS/UJF Research Group Nanophysique et Semiconducteurs, DRFMC/SP2M/PSC CEA-Grenoble, 17 rue des Martyrs,
38054-Grenoble cedex 9, France
2Institute of Materials Science (IMS), Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet-Cau Giay-Hanoi, Vietnam
and College of Technology, Hanoi National University, 144 Xuan Thuy-Cau Giay-Hanoi, Vietnam
3NGK Insulators, LTD 2-24 Sudacho, Mizuhoku, Nagoya, Japan
4CEA/CNRS/UJF Research Group Nanophysique et Semiconducteurs, Laboratoire Spectrométrie Physique (CNRS UMR5588),
Université J Fourier, Boîte Postale 87, 38402 Saint Martin d’Hères, France
共Received 16 May 2006; published 13 October 2006兲
We report on the optical properties of molecular beam epitaxy grown GaN quantum dots共QDs兲 doped with
Tm Under optical excitation above the fundamental energy of GaN QDs, the fundamental transition emission
from the GaN QD host was not observed while bright emission from Tm3+ manifolds demonstrated the
efficient energy transfer from the host to Tm3+ions The photoluminescence as a function of temperature was
fast quenched for transition from the high-lying manifolds state but very stable for the blue one resulting from
the 1D2→3F4transition Mechanisms for excitation to and deexcitation from Tm3+ions are discussed
DOI:10.1103/PhysRevB.74.155310 PACS number共s兲: 78.55.Cr, 68.55.Ln, 78.67.Hc, 85.60.Jb
I INTRODUCTION
Rare-earth trivalent ions共RE3+兲 exhibit well shielded intra
4f-transitions extending from the near infrared to the
ultra-violet, which are nearly independent from host materials For
semiconductor hosts the band gap has to be wide enough to
allow a good energy transfer probability from the
semicon-ductor host to the rare earth dopants in order to achieve light
emission in the whole visible range, for example red 共Eu,
Pr兲, green 共Tb, Ho, and Er兲 and blue 共Tm兲 Along this view
the combination of the RE luminescence with wide band-gap
nitride semiconductors has been found to be a promising
solution for light emitting devices.1 4
For strong blue Tm light emission from the1D2→3F4the
band-gap of the host has to be even wider than that of GaN,
which can be realized by either adding Al, e.g., in
AlGaN: Tm layers,5 , 6or by using GaN: Tm QDs.7In the case
of Tm doping of GaN QDs during MBE growth we have
shown by atomic force microscopy measurements that QDs
are typically small, with heights of about 1 nm and diameters
of about 20 nm.7 For such sizes, although QDs undergo a
quantum confined Stark effect due to a polarization induced
internal field, the fundamental transition of the QDs peaks
around 4 eV, at much higher energy than the band-gap of
GaN.7 GaN QDs have also other advantages compared to
thin共Al兲GaN films: they are defect free regions and they act
as efficient carrier confinement boxes Actually, we have
demonstrated Eu doping of共In兲GaN QDs for red emission,8 , 9
Tb for green10and Tm for blue emission For the main
emis-sion lines photoluminescence 共PL兲 has been found for all
GaN:RE QD systems to be thermally stable from liquid
he-lium to room temperature
For understanding the energy transfer mechanism a
con-siderable effort has been made in the case of infrared
emis-sion of InP: Yb共Refs.11–14兲 and Si:Er.15 – 18It is proposed
that after band to band excitation of the semiconductor host
the generated free carriers can be captured by RE-related traps located near-band edge The electron-hole
recombina-tion energy is then used to excite the 4f electrons in RE3+ion from the ground state to the excited states in a so called Auger process.19 , 20 Then excited RE3+ ions can relax by emitting light and/or transfer their energy back to the host material
Comparatively, the literature concerning RE-doped GaN thin films is rather scarce.21 Moreover, the energy transfer mechanism in rare earth doped GaN QDs has not been stud-ied yet although this is important for understanding of further device operations
In previous work7 we have studied morphology of Tm doped GaN QDs embedded in AlN spacing layers and have primarily shown that GaN QDs can be efficiently doped with
Tm during molecular beam epitaxy共MBE兲 growth The aim
of this paper is to address in detail the excitation and deex-citation mechanism of Tm3+ ions in GaN QDs embedded in AlN spacing layers The related processes for high-lying ex-cited states of Tm3+ ion, i.e., 1I6 and 3P1, have been ad-dressed with a special care because the transition energies from these levels are similar and rather close to the funda-mental energy of GaN QDs that mediate possible excitation and deexcitation processes Furthermore, a particular atten-tion has been paid to the 1D2→3F4 transition which is the most thermally stable transition emitting blue light, and is therefore particularly important for applications Our discus-sion is based on the experimental data obtained from tem-perature dependent, excitation-wavelength dependent, and time-resolved PL measurements
II EXPERIMENTAL
All samples were grown on 1-m-thick AlN pseudosub-strates deposited by metal organic chemical vapor deposition
Trang 2on c sapphire.22After a standard chemical degreasing
proce-dure and acid cleaning, they were fixed onto a molybdenum
sample holder, and introduced in a molecular beam epitaxy
共MBE兲 chamber equipped with Al, Ga, and Tm effusion cells
and a radio-frequency plasma cell to produce monoatomic
nitrogen The GaN QDs were grown at a substrate
tempera-ture of 720 ° C following the Stranski-Krastanow growth
mode, i.e., the QDs appear after the deposition of about 2
GaN monolayers.23 , 24The growth conditions were controlled
with reflection high-energy electron diffraction 共RHEED兲,
which allows in situ monitoring of the QD formation During
the growth of GaN, the Tm shutter was opened in order to
dope the material Next, the QDs were capped by about
12 nm of undoped AlN This process was repeated to
achieve a superlattice of 100 QD planes From the chosen
growth conditions the Tm content inside the sample was
es-timated to be around 3%
PL measurements were carried out with a frequency
doubled Ar-ion laser emitting at 244 nm PL as a function of
excitation wavelength was carried out with a tunable
excita-tion source consisting of a 500 W high-pressure Xe lamp
equipped with a Jobin-Yvon high-resolution double-grating
monochromator共Gemini 180兲; the PL was analyzed by
an-other Jobin-Yvon grating monochromator 共Triax 550兲 and
detected by either a CCD camera operating at liquid nitrogen
temperature or a photomultiplier tube operating in the photon
counting mode 共Hamamatsu H8259兲 The excitation power
density was about 200W / cm2at 350 nm The sample was
mounted on the cold finger of a microcryostat which enabled
us to record PL and PLE spectra at various temperatures
between 5 K and 300 K
Time-resolved PL measurements were carried out using
the frequency tripled output of a pulsed Ti:sapphire laser
emitting at 250 nm with an average power of 0.6 mW The
base repetition rate was 54 MHz, which was then divided
using a cavity dumper, yielding a repetition frequency of
140 kHz Finally time-resolved PL was analyzed with a
Hamamatsu Streak camera in single sweep mode
III OPTICAL PROPERTIES OF GaN: Tm QDs
The PL spectra of GaN: Tm QDs excited at 244 nm, i.e.,
well above the absorption band gap of GaN QDs are
pre-sented in Fig 1 for various temperatures between 5 K and
300 K The lack of emission from fundamental transition in
GaN QD 共in UV region兲 even at 5 K indicates a high Tm
concentration and a high energy transfer probability from the
QD host to the Tm3+ ions Along with blue light, Tm3+ ions
emit sharp emission lines extending over the whole visible
range The identification of transitions resulting in these lines
is mainly based on the energetic position and is in agreement
with published data.5 , 6 , 25 , 26 In the present study, the
transi-tions from1I6,1D2,1G4, and3H4have been found While PL
quenching for the different lines has been previously
studied,7 we shall study here this behavior more in detail,
notably with time resolved PL data Actually PL from each
transition exhibits different thermal behavior: PL from the
1I6 level is fast quenched and hardly observed at
tempera-tures higher than 100 K, whereas those starting from the1D2
are stable between liquid helium and room temperature An intermediate quenching is observed for the blue 1G4→3H6
and infrared3H4→3H6 transitions Detailed thermal behav-ior for the blue spectral range is displayed in the high reso-lution PL spectra in Fig.2 This spectral range is of particular interest as it involves transitions stemming from 1D2 and from1I6, and1G4mostly responsible for the blue light emis-sion
The1G4→3
H6transition is a main emission peak in bulk GaN doped with Tm,5 7however for Tm doped GaN QDs, it results only in weak luminescence This is due to the fact that for GaN QDs other transitions involving 1D2 and 1I6 are more efficiently excited than1G4→3
H6 共for bulk GaN,1
D2, and1I6transitions cannot be excited due to the too low band-gap of GaN兲 Therefore the1
G4→3
H6transition will not be further discussed in this paper
FIG 1 PL spectra from GaN: Tm QDs measured at various temperature between 5 K and 300 K as indicated on the curves The spectra are normalized by the excitation power density and the in-tegration time for detection The excitation source was a frequency doubled Ar-ion laser emitting at 244 nm Excitation power density was ⬃0.8 W/cm2 The three most intense transitions at 5 K are indicated in the figure The 共unstrained兲 GaN band-gap energy is indicated
FIG 2 High resolution PL spectra at various temperatures of
Tm doped GaN QDs for the blue spectral range The spectra are normalized to the accumulation time Excitation: 250 nm from the
Xe lamp Assigned transitions are indicated in the figure The ver-tical dotted lines are shown as a guide for the eyes
Trang 3Emissions in the spectral range of 463 nm to 471 nm
where both the1D2→3
F4and1I6→3
H4transitions can exist and give strong luminescence were analyzed in more detail
It is clearly seen that most of the emission lines in the
men-tioned range are stable with temperature because the 1D2
level is located at much lower energy than1I6, in agreement
with published data.26 We assign it to 1D2→3
F4 transition
By contrast the line at the longer wavelength side共469.5 nm兲
is fast quenched and hardly observed at temperatures higher
than 100 K Thus, this emission line is assigned as stemming
from the higher-lying1I6state The fast thermal quenching of
PL originating from 1I6 may result from various processes:
共i兲 From phonons emission to relax down to lower-lying
states of Tm3+ion This process is unlikely as many phonons
are needed to fill the energy gap between the starting and
final discrete levels.共ii兲 By phonon-assisted deexcitation to
the host material This is a most reasonable process because
transitions from high-lying 1I6 state can release an energy
comparable to the energy range of the fundamental energy of
GaN QDs Though we cannot determine the exact levels of
the RE3+ ion corresponding to the fundamental edge of the
semiconductor host, the near resonance energy of the 1I6
→3H6transition and fundamental gap of the host GaN QD at
least makes more likely Auger transfer with phonon
assis-tance Population of LO-phonons follows Bose-Einstein
dis-tribution, increasing fast at temperatures higher than 100 K
hence being responsible for thermal quenching of this
emis-sion Similar result has been reported for InP: Yb 共Refs
11–14兲 in which the temperature activated the energy back
transfer from the guest RE ions to the host semiconductor
Some other processes such as cross relaxation between
adja-cent Tm atoms for resonant 1I6→3
H4 and 3H6→1
G4 共this process takes place with increasing Tm concentration兲 or
emitting spontaneous IR to lower levels could be considered
but they are weakly dependent on temperature.27
Note that the phonon-assisted deexcitation from the 1D2
to the GaN QDs host is much less effective because any
transition from the1D2 to lower levels gives much less
en-ergy than the fundamental enen-ergy of the host On the other
hand, the1D2 excited state at lower energy can benefit from
repopulation from other higher-lying levels during their
quenching with temperature The above mentioned facts are
possible reasons for the temperature stability of emission
lines from the1D2 excited state In Fig 2 one can also see
several emission lines between 465 nm and 468 nm, whose
intensity increase with temperature These are believed to be
Stark-split levels which are PL-observable depending on the
temperature-dependent population factor A similar feature
has been observed and discussed in the AlN: Tm system.26
The intermediate thermal quenching was observed for the
blue 1G4→3H6 transition and for the infrared 3H4→3H6
transition This can be explained assuming that transitions
from1G4 or 3H4 are not in resonance for deexcitation
pro-cesses and are not likely to benefit from repopulation with
temperature so that the excitation mechanism is temperature
independent In this case, the thermal quenching is likely due
to thermally activated nonradiative deexcitation only
Based on the energetic positions of emission lines and
their thermal quenching behaviors an energy diagram can be
established as shown in Fig.3.25As the blue and green
spec-tral regions are concerned, two sets of transitions result in emission lines at very similar energies in steady-state PL spectra, but exhibit distinct decay times, namely⬃0.5s for the emissions at 466.3 nm, 469.7 nm, 534.5 nm, 537.5 nm 关Figs.4共a兲and6共b兲兴 and ⬃1.2s for the emission at 532 nm 关Fig.6共b兲兴 The former emission group has been identified as resulting from the1I6level 共1I6→3H4 for the 466.3 nm and
FIG 3 Energy diagram of Tm3+ions and observed transitions
in GaN host The energy range of the QDs fundamental transition is indicated on the energy scale The indicated wavelengths present means values for transitions
FIG 4.共a兲 The open circles correspond to the PL from GaN:Tm QDs measured at 5 K with the time resolved setup共see experimen-tal part兲 The spectrum has been fitted with a multiple Lorentzian containing 5 peaks Measured decay times and assigned transitions are also indicated in the figure.共b兲 Time resolved PL signal of the
1D2-3F4transition measured at 468 nm at 5 K 共upper curve兲 and
300 K共lower curve兲 Fitting of the measurements with monoexpo-nential decay are plotted bold The inset shows decay times for the
1D2-3F4 transition at 468 nm and 465 nm as a function of temperatures
Trang 4469.7 nm lines, and1I6→3F3for the 531 nm, 534.5 nm, and
537.5 nm lines in agreement with the different thermal
quenching of the PL Further information can be gained
about the excitation and deexcitation of Tm3+ion in GaN QD
host based on time-resolved PL measurements As the
emis-sion lines in the 463–471 nm range are overlapped, before
analyzing the decay times the spectra have been
deconvo-luted with a multiple Lorentzian curve fit关Fig.4共a兲兴 For the
two strongest isolated lines共at 465.5 nm and 468.6 nm兲, the
PL decays could be well fitted with a monoexponential
func-tion Figure4共b兲presents the measured and fitted curves for
the PL decays of the 468.6 nm emission line at 5 K and
300 K using the typical equation: y = Ae −t/, where is the
decay time, t the elapsed time after the laser pulse, and A a
fitting parameter For lines that include a contribution from
nearby stronger lines共for which a monoexponential decay fit
could be done兲 the PL decay was fitted as the sum of a
known exponential decay 共stemming from the overlapping
stronger line兲 and another exponential 共that gives the decay
time of the line of interest兲 For the two main blue emission
lines at 468.6 nm and 465.5 nm and the one at the higher
energy side共464.5 nm兲 the decay times obtained at 5 K are
similar, i.e., respectively 3.2s, 2.6s, and 2.5s This is
to be expected since the three mentioned lines are just
origi-nating from the Stark splitting in C3 symmetry of the 1D2
→3F4transition as presented in the energy diagram共Fig.3兲
However, it is worth noticing that even if the three
men-tioned lines originate from the same 1D2→3F4 transition,
there is a trend to observe longer decay time for the
higher-energy Stark component This feature has also been observed
for the Stark-splitting components of the1I6→3H4transition
that result in the 466.2 nm共0.56s兲 and 470 nm 共0.45s兲
lines; and the1I6→3
F3transition that result in the 531.6 nm 共1.2s兲, 534.5 nm 共0.6s兲, and 537.5 nm 共0.5s兲 lines
This result is difficult to explain
Also in tendency the measured decay time of the 1D2
→3
F4transition at 5 K has been found to be⬃2.6s, which
is longer than those for the1I6→3F3共1.2s兲 This is likely
due to the fact that the1D2→3F4transition is spin forbidden
as in the case of the1I6→3F3 transition In addition the1D2
state is located much below the 1I6 and3P1 so that energy
released from the1D2transitions to any manifolds is far from
resonances with fundamental energy of the host material or
near-fundamental-edge traps to favor the back-energy
trans-fer which generally contributes to the reduction of the
mea-sured decay time Note that a similar trend, i.e., a longer
decay time for the 1D2 excited state compared to 1I6 was
observed in Tm doped AlGaN.5
Although the longest decay time has been found for
emis-sions resulting from the1D2→3F4 transition, as a result of
the reduced energy back transfer to the host material, a
re-duction of the decay time has been found when increasing
temperature, from⬃2.6s at 5 K to⬃1s at 300 K This
reduction in decay time can be attributed to an increase in
nonradiative recombination with increasing temperature due
to an increased back transfer to the host.11This increase in
the nonradiative recombination rate is however not
corre-lated with a corresponding decrease in luminescence
inten-sity Indeed, from 4 K to 300 K, the luminescence intensity
for the1D2→3
F4transition is constant within ±20% As we are in the low excitation regime共well below saturation of the transitions兲, this suggests that the excitation efficiency of the 1
D2level increases with increasing temperature and that this effect compensates for the nonradiative losses In particular,
1D2could benefit from depopulation of1I6with temperature,
as already discussed above
The excitation scheme proposed for the 468 nm emission
is as follows: 共i兲 photoexcited and/or generated carriers 共in host GaN QDs兲 → 共ii兲 traps 共isoelectronic Tm3+ions兲 → 共iii兲
4f-electrons of Tm3+ ions共by near resonant energy transfer
to high-lying states such as1I6 and 3P1 and/or nonresonant process to 1D2兲 → 共iv兲 transitions 共e.g., 1D2→3F2兲 to emit light From the time-resolved spectra taken at 5 K and 300 K 关Fig.4共b兲兴, it appears that the rise time is shorter than 50 ns This value can be accounted for by the energy transfer rate
from GaN QDs to the 4f electrons of Tm3+ ions through processes 共i兲 to 共iii兲 As the energy transfer processes from the QDs to the RE3+ions take place faster than other nonra-diative processes, one can practically expect high efficiency luminescence from the RE-doped GaN QDs This is also consistent with the fact that we did not observe the funda-mental level emission for GaN QDs doped with Tm because energy transfer processes to Tm3+ions are much faster than other possible processes共including radiative recombination兲 for electron-hole pairs trapped in QDs
As already observed for InGaN QDs doped with Europium,8 , 9 the photoluminescence excitation spectra for
Tm doped GaN QDs at 300 K display a gradual absorption edge related to the QD absorption共Fig.5兲 This confirms that the transfer to the Tm states is indeed mediated by the QD states, and that in these samples the QDs have rather high energy levels共absorption below 330 nm=3.75 eV兲
To gain more insight in the excitation mechanism, we checked how the different emissions behave in PL as a func-tion of the excitafunc-tion wavelength in a range extending from
230 nm to 330 nm to cover the whole band gap energy val-ues corresponding to the QDs size distribution The results are shown in Figs.6共a兲– 共c兲for the most intense emissions
In the blue spectral range 关Fig 6共a兲兴 the emission lines at
FIG 5 PLE spectra at 5 K and 300 K for the 468 nm emission line of Tm3+ion in GaN QDs Spectra have been normalized to the excitation intensity and presented in logarithmic scale The horizon-tally dotted line is a guide to the eye The共unstrained兲 GaN band-gap energy is indicated
Trang 5464.5 nm, 465.5 nm, and 468.6 nm have been assigned to
the 1D2→3
F4 transition Also in agreement with the above
discussion of the thermal quenching 共Fig 2兲 is the 1I6
→3H4transition emitting around 466.2 nm and 470 nm, and
the 1G4→3H6 emission at 479 nm Emissions from the
higher excited states共1I6兲 tend to be weakened when using
longer excitation wavelengths As a consequence the
corre-sponding emissions from the 1I6→3
H4 transition and the
1I6→3F3 transition could not be detected by exciting the
sample at 310 nm 关see Figs 6共a兲 and 6共b兲兴 In agreement
with this behavior transitions from the 1D2 excited state
show very weak intensity for excitation wavelengths longer
than 310 nm, while the infrared 3H4→3H6 transition could
be well detected at an excitation wavelength of 380 nm It is possible that the excitation wavelength dependent effect be assigned to the GaN QDs size distribution because the exci-tation energy transfer to the high-lying levels of Tm3+ ions needs a relatively high energy from GaN QDs, which corre-sponds to the smaller sizes dots By exciting the sample with shorter wavelengths共240 nm兲, Tm3+ ion luminescence from the AlN spacer is getting visible 关Figs 6共a兲 and 6共b兲兴: in agreement with cathodoluminescence studies in Ref.7, the weak emissions at 462.5 nm and 467 nm are consistent with the presence of Tm3+ ions in the AlN spacing layer that contains some defect-related energy states where carriers can
be captured so that they then can transfer their energy to
Tm3+ions One point we would like to mention regarding the temperature-activated emission lines 共within 466.5 nm to 467.5 nm兲 is that they are observable only at short excitation wavelength 共230 nm to 270 nm兲 and high temperatures 共Figs.2 and 7兲 At long excitation wavelengths these lines did not appear This again supports the assumption that they come from the high-lying 1I6 state that is not likely to be excited with long excitation wavelength
It is interesting to note that emission from the1D2excited state can be detected, using excitation energy共380 nm兲 even below its energetic position 共370 nm兲 This could be ex-plained by two photon processes, however further enlighten-ment is required
IV CONCLUSIONS
In conclusion, we have studied in detail the optical prop-erties of GaN QDs doped with Tm by analyzing the PL char-acteristics as functions of sample temperatures, excitation wavelengths, and measuring the decay times for some tran-sitions from the high-lying Tm3+manifolds as a function of temperature Tm3+ ions are excited through the excitation of GaN QDs host with shallow traps which are gradually dis-tributed below their fundamental edge and observable in the
FIG 6 PL spectra at 5 K for GaN: Tm QDs measured with
different excitation wavelengths共indicated at the curves兲 in the 共a兲
blue,共b兲 green, and 共c兲 infrared spectral region The spectra are
normalized by the excitation power density and the integration time
for detection Spectra with low emission intensity are multiplied by
a factor for clarity The arrows indicate Tm3+ emission from the
AlN spacing layer共see text兲 Assigned transitions are shown in the
figure The decay times for the green spectral region are also
indi-cated in共b兲 with respect to its measured wavelength position
FIG 7 High resolution PL spectra in blue spectral range for Tm doped GaN QDs measured at 300 K with various excitation wave-lengths as indicated in the figure The spectra are normalized to the power density and the accumulation time The spectra under
290 nm and 310 nm excitation are multiplied with a factor 2 and 10 for clarity Assigned transitions are also indicated in the figure
Trang 6low-temperature PLE spectrum The near resonant Auger
process with LO-phonon assistance is proposed for the fast
thermal quenching transition stemming from the 1I6 level
The1D2→3F4transition results in stable blue emission with
increasing temperature The PL by selective excitation is in
good agreement with this thermal behavior as longer
wave-length excitation is only possible for lower-lying energy
states of Tm3+ ions
ACKNOWLEDGMENTS
We acknowledge Marlène Terrier, Yann Genuist, and Yoann Curé for their technical assistance Joël Bleuse is ac-knowledged for experimental contributions One of the au-thors共N.Q.L.兲 thanks the National Programme for Basic Re-search 共Vietnam兲, CNRS, and CEA 共France兲 for financial support
*Present address: EADS Astrium GmbH, 81663 Munich, Germany
†Corresponding author Electronic address: bruno.gayral@cea.fr
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