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Photoluminescence spectroscopy and energy-level analysis of metal-organic-deposited Ga2O3:Cr3+ filmsYoshinori Tokida and Sadao Adachi Citation: J. Appl. Phys. 112, 063522 (2012); doi: 10.1063/1.4754517 View online: http://dx.doi.org/10.1063/1.4754517 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i6 Published by the American Institute of Physics. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors
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Photoluminescence spectroscopy and energy-level analysisof metal-organic-deposited Ga2O3:Cr31 films
Yoshinori Tokida and Sadao Adachia)
Graduate School of Engineering, Gunma University, Kiryu-shi, Gunma 376-8515, Japan
(Received 19 June 2012; accepted 24 August 2012; published online 26 September 2012)
The aims of this study are (i) to demonstrate the synthesis of Cr3þ-activated b-Ga2O3 films by
metal-organic deposition and (ii) to report the temperature-dependent photoluminescence (PL)
properties of such films from 20 to 300 K. An activation energy of �0.9 eV for the Cr3þ ions in
b-Ga2O3 is determined from a plot of PL intensity vs calcination temperature. The red-line
emission doublet R1 and R2 at �1.8 eV and the broad emission band with a peak at �1.7 eV are
ascribed to the Cr3þ ions in the b-Ga2O3 host. The energies of the excited states, i.e., 2E, 4T2, 2T2,4T1, and 4T1, in Cr3þ are determined from the experimental PL and PL excitation spectra using a
newly developed analysis model. The high-energy luminescence tail of the broad 4T2 ! 4A2
emission band can be explained by the hot-carrier effect of the photoexcited electrons in the 4T2
state. The relative intensities of the R-line emission doublet can also be explained very well by the
population and depopulation of the electron numbers in the �E (R1) and 2 �A (R2) states. PL
properties, such as the temperature-dependent PL intensity, peak energy, and spectral width, are
analyzed in detail. VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754517]
I. INTRODUCTION
The optical properties of Cr3þ ions in oxide materials
have been studied extensively in the past. However, to the
best of our knowledge, no good explanation has been
obtained for those of Cr3þ-doped Ga2O3.1–9 The crystal
structure of b-form Ga2O3 is monoclinic and comparable to
the a form, which is isostructural with a-Al2O3.10 Thus far,
only b-Ga2O3 has been grown in single-crystalline form,
since it is the only phase stable above �800 �C.
Tippins1 performed an optical absorption study of Cr3þ-
doped b-Ga2O3 single crystals from 300 K down to �4 K
and obtained their crystal-field and Racah parameters to be
Dq/B¼ 2.35 and B¼ 700 cm�1. Photoluminescence (PL)
studies were also performed on b-Ga2O3:Cr3þ samples2–4,7
and showed sharp red (R-) line emissions superimposed on a
broad luminescence band. The effects of Cr doping on the
optical properties of b-Ga2O3 were investigated using PL
and PL excitation (PLE) spectroscopies,9 and an intense and
broad PL peak at �400 nm in nominally pure b-Ga2O3 was
observed to be abruptly suppressed by Cr doping. Epitaxial
GaN contains a native Ga2O3 overlayer. Cr3þ luminescence
has, therefore, been observed from b-Ga2O3 overlayers pro-
duced on epitaxial GaN by post-growth thermal annealing in
dry O2 atmosphere.5 The luminescence properties of Cr3þ-
doped nanoparticles and nanowires have also been studied
using PL and PLE spectroscopies.6,8
Although the above-mentioned studies elucidated the
optical properties of b-Ga2O3:Cr3þ, there are still many fun-
damental properties of this material that remain insufficiently
evaluated or unknown. For example, no detailed temperature
dependence of the PL properties has been reported to date.
Also, no attempt to determine the zero-phonon line (ZPL)
energies of the intra-d-shell electronic states of Cr3þ in b-
Ga2O3 has been made yet.
The purpose of this study is twofold: (i) to demonstrate
the synthesis of Cr3þ-activated b-Ga2O3 by metal-organic
deposition (MOD) and (ii) to systematically investigate the
optical properties of MOD-synthesized b-Ga2O3:Cr3þ using
PL analysis and PLE spectroscopy. To the best of our knowl-
edge, MOD has not been used to synthesize a Cr3þ-activated
oxide phosphor. Temperature-dependent PL measurements
are performed from 20 to 300 K in 10 K increments. Various
analysis models are developed to elucidate the PL and PLE
features observed in this study.
II. EXPERIMENTAL
The Ga2O3:Cr3þ films were prepared by thermal decom-
position, namely, MOD. A mixture of Ga(RCOO)n,
CH3OOC2H5, turpentine, and CxHyOz was used as the start-
ing solution. The solution was dropped on Si substrates,
spin-coated at 1000 rmp for 10 s, and dried in room ambient
at 120 �C for 10 min. Then, a metallic Cr film of about 1 nm
thickness was deposited on the sample surfaces by vacuum
evaporation. After vacuum evaporation, the samples were
again coated with the same solution and dried at 120 �C for
10 min. Such samples were finally calcined in dry O2 atmos-
phere between 200 and 1200 �C in 100 �C increments for
30 min to form Ga2O3 by thermolysis and to activate
vacuum-evaporated Cr impurities. The thickness of the
MOD-deposited samples was about 5 lm.
The structural properties of the MOD-synthesized
Ga2O3:Cr3þ films were determined by x-ray diffraction
(XRD) analysis using a RAD-IIC x-ray diffractometer
(Rigaku) with Cu Ka radiation. PL and PLE measurements
were carried out using a fluorescence spectrometer (Hitachi
F-4500) at room temperature. PL measurements were per-
formed using a single monochromator equipped with aa)Electronic mail: [email protected].
0021-8979/2012/112(6)/063522/9/$30.00 VC 2012 American Institute of Physics112, 063522-1
JOURNAL OF APPLIED PHYSICS 112, 063522 (2012)
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charge-coupled device (Princeton Instruments PIXIS 100)
between 20 and 300 K in steps of 10 K. The 325 nm line of a
He–Cd laser (Kimmon IK3302R-E) was used as the excita-
tion light source.
III. RESULTS
A. Calcination effects on XRD patternsand PL properties
The b-Ga2O3 crystal has a monoclinic structure with the
space group of C32h � C2=m.10 Figure 1 shows the XRD pat-
terns obtained from the MOD-synthesized Ga2O3:Cr3þ films
followed by calcination from Tc¼ 400 to 1200 �C. The larg-
est peak at 2h� 70� in Figs. 1(a)–1(e) originates from the
Si(100) substrate. The XRD patterns in Figs. 1(a) and 1(b)
reveal only a broad peak at 2h� 35�. This indicates that the
films calcined at Tc� 600 �C are somewhat amorphous. The
films calcined at T� 700 �C, on the other hand, show XRD
patterns consisting of many sharp peaks from various crystal-
lographic planes of b-Ga2O3. The peaks observed in
Figs. 1(c)–1(e) are in agreement with those reported in the
standard JCPDF card (monoclinic Ga2O3, #00-011-0370).
The diffraction intensity increases with increasing tempera-
ture (Tc� 700–1200 �C).
The PL spectra of the Ga2O3:Cr3þ films calcined at
Tc¼ 400–1200 �C are shown on the right-hand side of Fig. 1.
As shown in Figs. 1(a) and 1(b), the films without sufficient
calcination show a broad emission band with a peak at
�2.4 eV. After calcination at 700 �C, the film shows a rela-
tively narrow and asymmetric peak at �1.7 eV. The intensity
of this peak increases with increasing calcination tempera-
ture, revealing a fine structure at �1.8 eV. The PL spectrum
in Fig. 1(e) is characteristic of the emission of Cr3þ ions doped
into various oxides, such as a-Al2O3,11,12 spinels,13 Y3Ga5O12
(YGG),14 La3Ga5SiO14,15 topaz,16 and La3Ga5GeO14.
17
Figure 2 shows the integrated PL intensity IPL as a func-
tion of the calcination temperature Tc for the Ga2O3:Cr3þ
films. The IPL data were obtained by integrating the PL spec-
tra measured at 300 K (Fig. 1). A clear Arrhenius depend-
ence is inferred from Fig. 2, yielding �0.9 eV as the
activation energy of Cr3þ ions in b-Ga2O3. Because the
1200 �C-calcined sample yielded the strongest PL, we used
this sample in the following studies.
B. PL and PLE properties
Figure 3 shows the room-temperature PL and PLE spec-
tra of the 1200 �C-calcined b-Ga2O3:Cr3þ film. These spec-
tra were obtained by excitation at kex¼ 325 nm (PL) and by
monitoring at kem� 690 nm (PLE). As mentioned earlier
(Fig. 1), the PL spectrum of the well-calcined sample con-
sists of sharp R-line emissions at �1.8 eV, together with a
broad asymmetric emission band with a peak at �1.7 eV. The
sharp R-line emissions R1 and R2 are due to the 2E ! 4A2
transition of Cr3þ ions in b-Ga2O3. The PLE spectrum exhib-
its two small peaks at �2.0 and �2.8 eV and the largest peak
at �5.0 eV. The largest peak has a low-energy tail structure,
which may have a peak at �4.5 eV, as depicted by the
FIG. 1. Room-temperature XRD traces and PL spectra of Ga2O3:Cr3þ cal-
cined at (a) 400, (b) 600, (c) 700, (d) 900, and (e) 1200 �C.
FIG. 2. Integrated PL intensity (IPL) vs reciprocal calcination temperature
(1/Tc) for MOD-synthesized Ga2O3:Cr3þ films. An Arrhenius plot yields
�0.9 eV as the activation energy of Cr3þ ions in b-Ga2O3.
FIG. 3. PL and PLE spectra of b-Ga2O3:Cr3þ at 300 K. The vertical arrow at
�4.7 eV corresponds to the band-gap energy (Eg) of b-Ga2O3.
063522-2 Y. Tokida and S. Adachi J. Appl. Phys. 112, 063522 (2012)
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dashed line. The excitation peaks at �2.0, �2.8, and
�4.5 eV can be assigned to the 4A2 ! 4T2, 4A2 ! 4T1, and4A2! 4T1 transitions of Cr3þ ions in b-Ga2O3, respectively.
The largest excitation peak at �5.0 eV may correspond to
the intrinsic transition from the valence band to the conduc-
tion band in b-Ga2O3.
The PL spectra measured at several temperatures are
shown in Fig. 4. Above T� 100 K, a broad emission band
appears on the low-energy side of the R-line emissions and
its intensity gradually increases with increasing temperature.
This emission band is attributed to the 4T2 ! 4A2 transition
in Ga2O3:Cr3þ. As a result, the room-temperature PL spec-
trum consists of sharp R-line emissions superimposed on the
high-energy tail of the broad 4T2! 4A2 emission band.
In Fig. 5, we show in more detail the temperature de-
pendence of the PL spectrum in the R-line emission region
between T¼ 20 and 300 K, recorded in 10 K increments. At
T< 100 K, the PL spectrum consists of only the dominant R1
line; for example, at 20 K, the R1-line emission intensity is
about 100 times that of the R2-line emission intensity. No4T2 ! 4A2 emission is also observed below 100 K. With
increasing T, the R1-line emission intensity decreases but the
R2-line emission intensity gradually increases, peaks at
�150 K, and then decreases with further increase in T. As a
result, the R2 line becomes dominant above 200 K. Both the
R1- and R2-line emissions show a redshift with increasing T.
Figure 6 shows the expanded-scale PL spectra in the R-
line emission region of b-Ga2O3:Cr3þ. The R1- and R2-line
emissions at �1.78 and �1.80 eV are the dominant peaks in
these spectra. The energy separation of the R-line doublet is
determined to be �19 meV. Below 150 K, many sharp peaks
are clearly observed on the low-energy side of the R1-line
peak. An emission peak pairs with an energy separation of
�19 meV is also observed at �1.73 eV, but only in the lim-
ited temperature range of T� 150–250 K. As evidenced from
Figs. 4 to 6, each PL peak observed in b-Ga2O3:Cr3þ shows
a remarkable dependence on temperature.
IV. DISCUSSION
A. R1- and R2-line emissions
At high temperatures, alumina stably crystallizes in the
trigonal structure (a-Al2O3). Lightly Cr-doped alumina
(ruby) can emit red light at �1.78 eV (R1) and �1.80 eV
(R2). These R-line emission peaks R1 and R2, known as the
R-line doublet, are due to the 2E ! 4A2 transition of Cr3þ
ions in the a-Al2O3 host. The 2E state does not split by the
sole action of the trigonal field or by the spin–orbit interac-
tion, but only by the interplay of these two effects. The split-
ting energy of the four-fold degenerate state 2E can then be
written as18
2E
D2
ð2 �AÞ
� D2
ð �EÞ;
8>><>>: (1)
FIG. 4. PL spectra of b-Ga2O3:Cr3þ measured at 20, 100, 200, and 300 K. FIG. 5. Expanded-scale PL spectra of b-Ga2O3:Cr3þ measured between 20
and 300 K in 10 K increments.
063522-3 Y. Tokida and S. Adachi J. Appl. Phys. 112, 063522 (2012)
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where D is in eV and can be determined experimentally
(Fig. 6, see also Fig. 9 below). Note that a-Al2O3 is optically
uniaxial, but b-Ga2O3 is biaxial. In the case of b-
Ga2O3:Cr3þ, therefore, the eigenvalue of the crystal-field
and spin–orbit interaction Hamiltonians should be different
from Eq. (1). In this study, we test the hypothesis that the
center of gravity of the 2E state is also at 6ðD=2Þ, as in
Eq. (1).
Figure 7 shows the R-line emission spectrum of b-
Ga2O3:Cr3þ measured at T¼ 20 K. Each R-line emission
peak can be decomposed into three Gaussian components:
�1.7767, �1.7798, and �1.7825 eV (R1); �1.7961,
�1.7992, and �1.8019 eV (R2). Similarly, Yamaga et al.14
observed a three-component R1 line in the T¼ 10 K PL spec-
trum of YGG:Cr3þ. They observed peaks at 691.5 nm
(�1.7925 eV), 690.2 nm (�1.7959 eV), and 689.8 nm
(�1.7969 eV) with the largest peak at 689.8 nm. The energy
separations between these three peaks were �1.0 to 4.4 meV,
while those for b-Ga2O3:Cr3þ are �2.7 to 5.8 meV. It was
concluded14 that the 689.8 nm peak was the same as that gen-
erally observed in YGG:Cr3þ. The weak peak at 690.2 nm
was the R1 line from the Cr3þ ion, but it was located at the
octahedral site in which one of the Ga3þ ions occupying
nearest-neighbor octahedral sites was replaced with a Y3þ
ion. The 691.5 nm peak was not an R1 line from the Cr3þ ion
in a different site, but was the one-phonon line of the
689.8 nm peak with a phonon energy of 37 cm�1. Yamaga
et al.14 did not determine whether the R2-line emission
exhibits such a multiple-peak structure. There is, therefore, a
possibility that the lower symmetry of host crystals results in
the splitting of the intra-d-shell transition energies of metal-
lic ions in host crystals such as b-Ga2O3.19 Further study is
needed to elucidate the multiple-peak structure, not only in
b-Ga2O3:Cr3þ but also in YGG:Cr3þ.
In Fig. 8, we plot the R1- and R2-line emission energies
vs temperature for b-Ga2O3:Cr3þ between 20 and 300 K.
The redshift in the R-line emission energies with increasing
T in b-Ga2O3:Cr3þ can be explained by the interaction
between the Cr3þ electronic levels and lattice vibrations. The
strength of this interaction is easily expected to be related to
the phonon population number
nqðTÞ ¼1
expð�h�xq=kTÞ � 1; (2)
where �h¼ h/2p (h is the Planck constant), k is the Boltzmann
constant, and �h�xq is the energy of the involved phonons with
mode q. The simplest assumption is that the energy shift is
proportional to nq(T),
FIG. 6. Expanded-scale PL spectra of b-Ga2O3:Cr3þ measured between 20
and 250 K in 10 K increments.
FIG. 7. PL spectrum in R1- and R2-line emission regions of b-Ga2O3:Cr3þ
at 20 K. The light solid lines represent the deconvolution of each spectrum
into three Gaussian peaks (see text).
FIG. 8. R-line emission energy and half-width (DEp) vs temperature in
b-Ga2O3:Cr3þ between 20 and 300 K. The solid lines show the results cal-
culated using Eqs. (3) and (4) for the R-line emission energy and DEp,
respectively.
063522-4 Y. Tokida and S. Adachi J. Appl. Phys. 112, 063522 (2012)
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ERðTÞ ¼ ERð0Þ � anq ¼ ERð0Þ �a
expð�h�xq=kTÞ � 1; (3)
where a is the proportionality factor and ERð0Þ is the T! 0 K
value of R-line emission energy.
The solid lines in Fig. 8 show the results calculated
using Eq. (3) with ERð0Þ¼ 1.7823 eV (R1) and 1.8017 eV (R2),
a¼ 9.0� 10�3 eV (R1 and R2), and �h�xq¼ 32 meV (R1 and
R2). The experimental R-line emission energies with respect to
T are found to be well explained by this simplified model.
The half-width vs temperature data for the R-line emis-
sions are plotted in Fig. 8. Like the R-line emission energies,
the half-width vs T data show a constant value up to �100 K
and then gradually increase with further increase in T. The
solid lines in Fig. 8 represent the results calculated using
essentially the same expression as Eq. (3),
DERðTÞ ¼ DERð0Þ þ bnq ¼ DERð0Þ þb
expð�h�xq=kTÞ � 1;
(4)
with DERð0Þ¼ 3.10� 10�6 eV (R1), DERð0Þ¼ 3.75� 10�6
eV (R2), b¼ 1.2� 10�5 eV (R1) and 1.5� 10�5 eV (R2), and
�h�xq¼ 32 meV (R1 and R2). Note that for �h�xq <<kT, Eqs.
(3) and (4) can be approximated as ERðTÞ / �ða0TÞ and
DERðTÞ / b0T with a0 ¼ ðk=�h�xqÞa and b0 ¼ ðk=�h�xqÞb,
respectively. The effective Debye temperature TD ¼ �h�xq=kobtained from this study is 375 K.
The low-temperature PL spectra in Fig. 6 reveal many
sharp peaks at energies below the R-line doublet. No reliable
explanation has been given for the origin of these peaks as
well as for that in ruby. However, some peaks in ruby have
been explained by the phonon sidebands together with the
characteristic R-line emissions from the Cr3þ ions with spe-
cific arrangements of Al neighbors labeled “N-lines” or
“Nebenlinien” (see the review article by Powell and
DiBartolo11).
In Fig. 6, we find at least only one phonon replica of the
R-line doublet with peaks at �1.724 and �1.744 eV. The
energy separation of these peaks (�19 meV) and their emis-
sion intensities as functions of temperature are the same as
those of the ZPL doublet at �1.78 eV (R1) and �1.80 eV
(R2). The emission intensity of the P0 peak in Fig. 6 gradu-
ally increases with increasing T, shows a maximum at
�150 K, and then decreases with further increase in T. This
temperature dependence is the same as that of the ZPL R2
emission at �1.80 eV. On the other hand, the multiple peaks
observed in the 1.685–1.765 eV region at low temperatures
show a gradual decrease in intensity with increasing T and
nearly completely disappeared above �150 K, similarly to
those in the case of the ZPL R1 emission at �1.78 eV. There-
fore, the origins of the P0 peak and the series of multiple peaks
in the 1.685–1.765 eV region seem to be the same as those of
the R2-line (2 �A) and R1-line ( �E) emissions, respectively.
B. 4T2fi 4A2 emission band
The configurational-coordinate (CC) model, schemati-
cally shown in Fig. 9, has often been used to explain the opti-
cal properties of solids.20 The ground state of the Cr3þ ion is4A2; the excited states are 2E, 4T2, 4T1, and 4T1. Assuming
that only one localized vibrational mode strongly interacts
with the absorption or emission band, we obtain the positions
of the optical absorption and emission lines using
�hxab ¼ E0 þ k �hxqe; (5a)
�hxem ¼ E0 � l �hxqg; (5b)
where �hxqe (�hxqg) is the energy of a vibronic quantum
coupled with electrons in the excited (ground) state, and kand l are positive integers. The observed absorption and
emission bands can be considered to be envelopes of numer-
ous lines, each of which is due to a transition between one
vibronic level, k, in the excited electronic state and one
vibronic level, l, in the ground state.
If some additional assumptions are made, the main one
being the use of a Gaussian function for each line shape, we
obtain the energy- and temperature-dependent spectral distri-
butions of the emitted light in the CC model as
IemðE; TÞ ¼ I0emðTÞ
X1n¼0
expð�SÞ ð S Þn
n!
� exp � ðE� EZPL � n �hxqeÞ 2
2rem2
" #nqe; (6)
where I0emðTÞ is the matrix element for the electric dipole
transition, S represents the average number of phonons
involved in a vibronic transition, EZPL is the ZPL energy,
rem is the broadening of each Gaussian emission line, and
nqe is given by Eq. (2). Similarly, we can express the energy-
and temperature-dependent spectral distributions of the
absorbed light by
IabðE; TÞ ¼ I0abðTÞ
X1n¼0
expð�SÞ ð S Þn
n!
� exp � ðE� EZPL þ n �hxqaÞ 2
2rab2
" #nqa: (7)
FIG. 9. Schematic energy diagram of Cr3þ ions in b-Ga2O3.
063522-5 Y. Tokida and S. Adachi J. Appl. Phys. 112, 063522 (2012)
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It is understood from Eqs. (6) and (7) that the emission or
absorption spectrum can be mainly characterized by the
Poisson distribution function
S0ðnÞ � expð�SÞ S n
n !: (8)
In Fig. 10, we replot the PL and PLE spectra of the
b-Ga2O3:Cr3þ film measured at 300 K (see Fig. 3). As men-
tioned before, the 300 K PL spectrum consists of sharp R-
line emissions superimposed on the high-energy tail of the
broad 4T2 ! 4A2 emission band. The vertical bars in Fig. 10
show the fitted result of the broad 4T2 ! 4A2 emission band
with Eq. (8). The fit-determined Poisson parameters are
EZPL¼ 1.7979 eV, S¼ 2, and �xq¼ 65 meV. It is seen in
Fig. 10 that the broad asymmetric emission band with a peak
at �1.7 eV can be well explained by Eq. (6) [Eq. (8)].
In Fig. 10, we can see a considerably high PL intensity
above the ZPL energy of the 4T2 state (1.7979 eV). This lu-
minescence component completely disappears at low tem-
peratures, as seen in Fig. 4. It is thus natural to consider that
the high-energy-tail luminescence component is due to the
anti-Stokes sideband of the 4T2 ! 4A2 emission. The anti-
Stokes sideband component can be easily formulated by
changing the sign of n �hxqe in Eq. (6) in the manner: E�EZPL � n �hxqe !E� EZPL þ n �hxqe. We found, however,
that the anti-Stokes expression does not explain the experi-
mental high-energy-tail luminescence component. Therefore,
we consider that this component is due to a hot-electron
effect in the 4T2 state, not arising from the anti-Stokes pho-
non sideband.
Figure 11 reproduces the PL spectra of the b-
Ga2O3:Cr3þ film measured at T¼ 20, 100, 200, and 300 K.
Note that the vertical axis is plotted on a logarithmic scale. It
is clearly understood from Fig. 11 that no strong emission is
observed in the PL spectrum above EZPL� 1.8016 eV at
20 K; however, its emission intensity becomes remarkable
with increasing temperature.
In semiconductors, luminescence caused by the recom-
bination of free electrons with bound holes directly reveals
the energy distribution of the electrons in the conduction
band. If the density of the electrons is not too low, they
are in thermal equilibrium, and their distribution can be char-
acterized by the Maxwell–Boltzmann tail of a Fermi distri-
bution with an effective electron temperature Te. The
hot-electron effect in the luminescence process can now be
considered using
IhotPL ðE; TÞ ¼ Ihot
PL ð0Þ exp � aE� EZPL
kTL
� �; (9)
where a is a constant and TL represents the lattice tempera-
ture. The electron temperature Te can be given by Te¼ TL/awith a� 1.0.
The dashed lines in Fig. 11 show the fitted results calcu-
lated using Eq. (9). It is found that the present model of Eq.
(9) adequately explains the high-energy-tail luminescence of
the 4T2 ! 4A2 emission band. The hot-electron parameter adetermined here is 0.6, independent of the lattice temperature
TL. The lack of a clear TL dependence of a observed in b-
Ga2O3:Cr3þ suggests the intra-d-shell nature of photoexcited
electrons in the 4T2 state.
C. PLE band
In Fig. 3, the excitation peaks observed at �2.0, �2.8,
and �4.5 eV are assigned to the 4A2 ! 4T2, 4A2 ! 4T1, and4A2! 4T1 transitions of Cr3þ ions in b-Ga2O3, respectively.
Not only the emission spectrum but also the absorptionFIG. 10. PL and PLE spectra of b-Ga2O3:Cr3þ at 300 K. The vertical bars
show the results calculated using Eq. (8) (see text).
FIG. 11. Log-scale PL spectra of b-Ga2O3:Cr3þ measured at 20, 100, 200,
and 300 K. Successive spectra are properly vertically shifted. The dashed
lines show the results calculated using Eq. (9) with a¼0.6.
063522-6 Y. Tokida and S. Adachi J. Appl. Phys. 112, 063522 (2012)
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spectrum can be explained by the CC model. We therefore
fitted our measured PLE spectrum using Eq. (7) [Eq. (8)].
The fitted results for the 4A2! 4T2 and 4A2! 4T1 excitation
transitions are shown in Fig. 10. The fit-determined Poisson
parameters are as follows: EZPL¼ 1.7979 eV, S¼ 4, and
�xq¼ 65 meV for the 4A2 ! 4T2 transition; EZPL¼ 2.40 eV,
S¼ 7, and �xq¼ 65 meV for the 4A2 ! 4T1 transition. It is
concluded that both the PL and PLE spectra observed in b-
Ga2O3:Cr3þ can be well explained by the CC model.
In Fig. 10, however, we can still find poor agreement
between the experimental and calculated PLE spectra in the
2.3–2.6 eV region. This may be because we did not consider
the spin-forbidden 4A2! 2T2 transition in the present analy-
sis. The weak intensity of the 4A2 ! 2T2 transition (Fig. 10)
is in agreement with its spin-forbidden nature. The weak 4A2
! 2T2 absorption peak has also been observed in various
Cr3þ-activated phosphors, such as MgAl2O4,13 ZnAl2O4,13
La3Ga5SiO14,15 and a-Al2O3.21
Table I shows a summary of the ZPL energies of the
Cr3þ ions in b-Ga2O3, together with those of the Mn4þ ions
in K2SiF6.22 The Mn4þ ion has the same electronic configu-
ration 3d3 as Cr3þ. The energetic structure of these ions can
be well described by Tanabe–Sugano’s crystal-field
theory,23,24 as shown in Fig. 12. The sequence of the energy
levels is determined by the symmetry of ion sites, by the
crystal-field strength 10Dq, and by interelectronic interac-
tions determined by the Racah parameters B and C. From
Table I, we estimate the crystal-field strength to
be 10Dq� 1.80 eV for b-Ga2O3:Cr3þ and �2.52 eV for
K2SiF6:Mn4þ. The Racah parameter B is not largely depend-
ent on the type of host material. Thus, the horizontal axis
Dq/B in Fig. 12 is nearly proportional to the crystal-field
strength, 10Dq. For intermediate crystal fields, as is the case
in the present host (b-Ga2O3), the 4T2–2E energy separation
is nearly zero. In contrast, its value in K2SiF6:Mn4þ is con-
siderably large, i.e., �0.53 eV (see Table I).
An excitation band caused by the 4A2 ! 4T1 transition
was clearly observed in K2SiF6:Mn4þ at �4.38 eV (Table I).
In b-Ga2O3:Cr3þ, this excitation band is observed as a
shoulder of the strong band-to-band transition (Fig. 3). In
nominally pure b-Ga2O3,25–28 no strong absorption peak or a
shoulder structure appears between 800 and 280 nm, and at
approximately 270 nm, there is a steep absorption edge yield-
ing the band-gap energy of �4.7 eV for this material. We
can thus conclude that our observed absorption shoulder at
�3.5 to 4.5 eV is certainly caused by the Cr3þ activator tran-
sition (4A2! 4T1, see Figs. 9 and 12).
D. Temperature dependence of PL intensity
Figure 13 shows the temperature dependences of the PL
intensity for the (a) R1-line emission, (b) R2-line emission,
and (c) 4T2! 4A2 emission bands, together with their sum at
the bottom [(d)]. These data were obtained by integrating the
PL spectra in Fig. 6. As expected, the R1- and R2-line emis-
sions exhibit strong dependences on T. At T¼ 20 K, the
lower-energy R1-line emission intensity is about 50 times
stronger than the higher-energy R2-line one, but becomes
TABLE I. ZPL energies of Cr3þ ion in b-Ga2O3 and Mn4þ ion in K2SiF6 at
300 K. The fit-determined S values in Eq. (8) are given in the parentheses.
Transition b-Ga2O3:Cr3þ K2SiF6:Mn4þ (Ref. 22)
2E!4A2 1.7884 1.99344T2!4A2 (10Dq) 1.7979 (2)4T2!4A2 (10Dq) 1.7979 (4) 2.52 (4)4A2!2T2 2.34A2!4T1 2.40 (7) 3.09 (7)4A2!4T1 3.35 (13) 4.38 (7)
FIG. 12. Tanabe–Sugano energy diagram for a 3d3 system. The vertical bars
correspond to the ZPL energies in b-Ga2O3:Cr3þ and K2SiF6:Mn4þ listed in
Table I.
FIG. 13. Integrated PL intensity IPL vs 1/T for (a) R1-line emission, (b) R2-
line emission, and (c) 4T2 ! 4A2 emission band. The sum of these compo-
nents is shown in (d). The theoretical curves in (a)–(d) are obtained using
Eqs. (10)–(12) and (15).
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approximately half at 200 K. This can be explained by the
thermal depopulation effect of the R1 ( �E) state, which occurs
with decreasing T. The thermal depopulation and population
in the R1 ( �E) and R2 (2 �A) states can be expressed by the
Boltzmann factor exp(�D/kT) with D in Eq. (1) (see also
Fig. 9). The effect of thermal depopulation in the R1 ( �E) state
on its luminescence intensity can be written as
IR1ðTÞ ¼I0R1
1þ a exp ð�D=kTÞ : (10)
Similarly, the thermal population-induced PL intensity in the
R2 (2 �A) state is given by
IR2ðTÞ ¼I0R2
1þ b exp ðD=kTÞ : (11)
The light solid lines in Figs. 13(a) and 13(b) show the
results calculated using Eqs. (10) and (11), respectively. The
fitting-determined parameters are I0R1¼ 0.31 and a¼ 35 [Eq.
(10), Fig. 13(a)]; and I0R2¼ 0.40 and b¼ 0.35 [Eq. (11), Fig.
13(b)]. The splitting energy D used is 19 meV (see Figs. 6
and 9). Our proposed models of Eqs. (10) and (11) can rea-
sonably explain the experimental R1- and R2-line emission
intensities.
Above T� 200 K, the R1- and R2-line emission inten-
sities exhibit a strong thermal quenching and can therefore
be fitted with
IPLðTÞ ¼IPLð0Þ
1þ expð�Eq=kTÞ : (12)
The heavy solid lines in Figs. 13(a) and 13(b) show the
results calculated using Eq. (12). The quenching energy
determined here is Eq¼ 0.2 eV for both the R1- and R2-line
emissions.
The temperature dependence of the phonon-activated
PL intensity can be expressed as29
IphPLðTÞ ¼ cjhijM0jf ij2ðEZPL7�h�xqÞ 4 nqðTÞ þ
1
27
1
2
� �; (13)
where c is a proportionality constant, jhijM0jf ij is an optical
transition operator, EZPL is the ZPL energy, �h�xq is the energy
of the involved phonons with mode q, and nq(T) is given by
Eq. (2). The upper and lower signs in Eq. (13) correspond to
the Stokes and anti-Stokes processes, respectively.
From Eq. (13), IPL(T) is written as
IphPLðTÞ ¼ I0
PL þ I1PL 1þ 2
expð�h�xq=kTÞ � 1
� �: (14)
If we neglect the anti-Stokes sideband, Eq. (14) becomes
IphPLðTÞ ¼ I0
PL þ I1PL
1
expð�h�xq=kTÞ � 1
� �: (15)
The light solid line in Fig. 13(c) shows the result calculated
using Eq. (15) with I0PL¼ 0.28, I1
PL¼ 40, and �h�xq¼ 55 meV.
The �h�xq of 55 meV is considerably larger than that obtained
in Fig. 8 (�h�xq¼ 32 meV), but is slightly smaller than
�xq¼ 65 meV in Fig. 10.
Above T� 200 K, the broad 4T2 ! 4A2 emission inten-
sity exhibits a strong thermal quenching. The heavy solid
line in Fig. 13(c) shows the result calculated using Eq. (12)
with Eq¼ 0.3 eV.
The sum of the R1-line, R2-line, and broad 4T2 ! 4A2
emission band intensities is plotted in Fig. 13(d). As
expected, the integrated PL intensity can be explained in the
T¼ 20–60 K region by Eq. (10) (I0R¼ 0.75 and a¼ 30), in the
T¼ 60–200 K region by Eq. (15) (I0PL¼ 0.48, I1
PL¼ 64, and
�h�xq¼ 55 meV), and in the T> 200 K region by Eq. (12)
(Eq¼ 0.3 eV).
V. CONCLUSIONS
In summary, we synthesized b-Ga2O3:Cr3þ phosphor by
MOD. The R1- and R2-line emissions and broad asymmetric
emission band were ascribed to the intra-d-shell electronic
transitions of Cr3þ in b-Ga2O3. The ZPL energies of the
Cr3þ states, 2E, 4T2, 2T2, 4T1, and 4T1, were determined to be
1.7884, 1.7979, 2.3, 2.40, and 3.35 eV at 300 K, respectively.
The high-energy-tail luminescence of the broad 4T2 ! 4A2
emission band was interpreted by the effect of the photoex-
cited hot electrons in the 4T2 state. Detailed analyses of the
temperature-dependent PL intensity, peak energy, and spec-
tral width were carried out, and our proposed models showed
good agreement with the experimental data.
ACKNOWLEDGMENTS
This work was supported by a Grant-in-Aid for Scien-
tific Research (B) (23360133) from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology, Japan. The
authors would like to thank T. Miyazaki, T. Nakamura, H.
Oike, Y. Kato, and J. Nara for their experimental supports
and useful discussion.
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