phys. stat. sol. (c) 2, No. 7, 2765 – 2769 (2005) / DOI 10.1002/pssc.200461612
Cathodoluminescence and its temperature dependence
in Tm-doped AlxGa1–xN thin films
D. S. Lee1, A. J. Steckl*1, P. D. Rack2, and J. M. Fitz-Gerald3
1
2
3
Nanoelectronics Laboratory, University of Cincinnati, Cincinnati, Ohio 45221-0030, USA
University of Tennessee, Knoxville, TN 37996, USA
University of Virginia, Charlottesville VA, USA
Received 19 August 2004, accepted 22 December 2004
Published online 17 March 2005
PACS 68.55.Ln, 71.55.Eq, 78.60.Hf, 78.66.Fd
Cathodoluminescent (CL) emission from Tm-doped AlxGa1–xN (AlxGa1–xN:Tm) has been observed with
various Al compositions (0 ≤ x ≤ 1). According to CL spectrum from Al0.81Ga0.19N:Tm film 1D2 levelrelated CL emissions were observed to dominate: the strongest was blue at 464 nm and the next was UV
1
3
1
3
at 369 nm, corresponding to the Tm D2 → F4 and D2 → H6 transitions, respectively. CL luminance increases with Al composition up to x = 0.8, then decreases for AlN:Tm, which is the same as in EL and PL.
2
Luminance value was measured to be 19.4 cd/m . The corresponding efficiency was 0.012 lm/W under 5
kV bias and 10 µA current flow. Temperature dependent CL from the Al0.81Ga0.19N:Tm film shows that 1D2
emission increases with temperature up to room temperature (RT) but 1I6 emission becomes quenched
upon approaching RT.
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction A major challenge [1, 2] for full color displays and corresponding phosphors has been
bright blue emission. This is the motivation to seek wider bandgap hosts since blue emission carries the
highest energy per photon (2.6–2.7 eV) of the three primary colors. We have reported [3] dominant blue
electroluminescence (EL) emission from GaN:Tm. Moreover, we have investigated several methods for
enhancement of blue emission from GaN:Tm ELDs in both brightness and efficiency, including the
“photopumping” method [4] and optimization [5] of the growth temperature. We have recently reported
enhanced blue EL [6] emission and photoluminescence [7] (PL) emission from in-situ Tm-doped
AlxGa1–xN films. This was made possible by using the bandgap engineering method based on the fact that
III–V alloys of GaN-AlN span a large bandgap energy, ranging from 3.4 to 6.2 eV. Several groups have
been studying wider bandgap semiconductors than GaN, primarily AlN: AlN doped with Er, Eu and Tb
has resulted in photoluminescence [8] and cathodoluminescence (CL) [9, 10], and EL has been reported
for AlN:Er [11]. Very recently, Vetter et al. reported [12] CL from Tm-implanted AlN and Nakanishi et
al. reported [13] PL improvement from Tb-doped AlxGa1–xN (x ≤ 0.15). In this paper, we report on the CL
emission from in-situ Tm-doped AlxGa1–xN films (0 ≤ x ≤ 1) and its temperature dependence.
2 Experimental AlxGa1–xN:Tm films were grown on p-type (111) Si substrates by molecular beam
epitaxy (MBE) with a Ga elemental source and a nitrogen plasma source. Doping was performed in situ
during growth from a solid Tm source. The Tm cell temperature was fixed at 600 °C resulting in a concentration between ~0.2 and ~0.5 at.%. AlxGa1–xN:Tm layers were typically grown for one hour at 550 °C
with a growth rate of 0.5–0.6 µm/hr. Films were grown with various Al compositions (0 ≤ x ≤ 1). The
*
Corresponding author: e-mail: a.steckl@uc.edu
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2766
D. S. Lee et al.: CL and its temperature dependence in Tm-doped AlxGa1–xN thin films
growth was performed under slightly N-rich growth conditions: 1.5 sccm for nitrogen flow rate and 400
W for plasma power. Al composition was controlled by varying the Al cell temperature during growth.
The total flux of group III species (Ga and Al) was kept constant since EL properties are reported [14] to
be a strong function of V/III ratio during growth. Al composition was determined by the X-ray diffraction (XRD) method. According to Vegard’s law [15, 16], the (0002) XRD peak of AlxGa1–xN is a linear
function of Al composition between GaN and AlN. Films were grown with various Al compositions,
including x = 0 (GaN), 0.16, 0.21, 0.39, 0.62, 0.81 and 1 (AlN).
3 Results Fig. 1 shows the CL emission spectrum in log scale from an Al0.81Ga0.19N:Tm film at room
temperature (RT). The bias condition was 4 kV and 50 pA. Dominant CL emission peak was observed in
the blue region (464 nm) and secondarily in the UV region (369 nm). Both peaks have been reported
[6, 7, 12] to come from the same excited state of Tm3+, namely 1D2. 369 nm and 464 nm emissions are
due to 1D2 → 3H6 and 1D2 → 3F4 transitions, respectively. The 478 nm and 802 nm emissions, not as
strong as in EL or PL, are well-known and attributed to 1G4 → 3H6 and 3H4 → 3H6 transitions, respectively.
Note that 1I6–related emissions also observed in the spectrum: 1I6 → 3H6 (297 nm), 1I6 → 3F4 (357 nm) and
1
I6 → 3H5 (393 nm). Minor peaks were also observed at 527 nm and 684 nm due to 1D2 → 3H5 and 1D2 →
3
H4 transitions as in EL. Peaks at 594 nm, 738 nm and 928 nm (exactly doubles of 297 nm, 369 nm and
464 nm) are due the spectrometer grating. Note that 3F4 and 3H4 states are frequently reversed in the literatures, as are 1I6 and 3P0.
E (eV)
E (eV)
6.5
6.5
AlN
6.0
464
CL Intensity (a. u.)
105
Al0.81Ga0.19N:Tm
393
369
10
478
5.0
X=0.6
4.5
X=0.4
4.0
X=0.2
527
297
684
600
4.5
1I
802
(464×2)
1.5
800
1000
1200
wavelength (nm)
Fig. 1 CL emission spectrum in log scale from
Al0.81Ga0.19N:Tm under 3 kV bias. Strong CL emissions are observed at 369 and 465 nm. Note that weak
UV emissions are also seen at 297, 357 and 393 nm
which have not been previously observed from EL.
0.5
0
6(
1D
2
647
393
527
369
1G
4
684
802
0)
GaN
4.0
3.5
2.5
2.0
464
478
3P
3.0
297
2.5
1.0
400
5.0
357
2.0
6.0
5.5
3.0
357
200
X=0.8
3.5
(369×2)
(297×2)
4
5.5
3H
4
1.5
3H
5
1.0
3F
4
3H
6
0.5
0
Fig. 2 Diagram of energy levels and 4f–4f inner
shell transitions of Tm3+ in AlxGa1-xN:Tm. Strong
emission lines at 369 and 464 nm were attributed to
1
3
1
3
the transitions D2 → H6 and D2 → F4, respectively.
The energy levels of 4f–4f inner shell transitions of Tm3+ in AlxGa1–xN:Tm are shown in Fig. 2. The
bandgap energy in various AlxGa1–xN alloy compositions from 0.2 to 0.8 is shown according to Vegard’s
law, taking a bowing parameter (b) into account. We have used b = 1, which is close to the average of
many reported [15–17] values. We have attempted an attribution of the transitions for all the emissions
based on our current and previous results [6, 7] and other values reported in the literature. Note that 1I6
level-related emissions show very good agreement with those from PL results [7] from the same
AlxGa1–xN:Tm films. 1I6 level-related emissions were also reported [12] from AlN:Tm by Vetter et al.
They also reported dominant 1D2 → 3F4 emission at blue region, while Lozykowski et al. reported [18]
very weak 1D2 → 3H5 emission from GaN:Tm (no 1D2 → 3F4 emission and no 1I6 level-related emissions).
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
phys. stat. sol. (c) 2, No. 7 (2005) / www.pss-c.com
2767
The AlxGa1–xN:Tm CL emissions were found to change significantly with Al composition. Figure 3
shows a plot of CL luminance for various Al compositions with different current flows at constant bias
condition, 5 kV. Curves and dotted lines were drawn for guidance. Overall emission color was yellow
from films with lower Al compositions (x ≤ 0.21), which is believed to come from the host, due mainly
to defects or dislocations similar to the case of “yellow band” emission from GaN films. In contrast,
films with higher Al composition (x ≥ 0.39) showed blue CL emission and their luminance was proportional to current flow. It is interesting to note that the overall luminance increases with Al composition
up to x = 0.8, and then decreases for AlN:Tm. This is exactly the same trend as in EL and almost the
same as in PL. A luminance of 19.4 cd/m2 and corresponding efficiency of 0.012 lm/W were measured
for the sample with x = 0.81 under 5 kV bias and 10 µA current flow. This luminance value is comparable to (or better than) reported values from various other Tm host materials under the same bias condition (5 kV): 2.3 cd/m2 from Ba2B5O9Cl:Tm films [19], 9.1–12.9 cd/m2 from Y2O3:Tm on various glass
substrates [20] and 30.4 cd/m2 from Y2O3:Tm/aluminosilicate [20].
20
Blue
5 kV Bias
Luminance (cd/m2)
I = 10 µA
15
I = 6.4 µA
10
Yellow
5
0
Fig. 3
Luminance from AlxGa1-xN:Tm films with
various Al compositions. Curves and lines are shown
for guidance. CL emission looked yellow at lower Al
compositions (x ≤ 0.21) and blue at higher Al compositions (x ≥ 0.39). Note that luminance intensity
increases with Al composition up to x = 0.8 and
decreases for AlN:Tm, which is the same trend as in
EL and PL.
I = 2.5 µA
0
0.2
0.4
0.6
0.8
1
Al Composition
The CL temperature dependence from Al0.81Ga0.19N:Tm films was investigated from 79 K to 373 K. CL
intensity spectra of UV and blue region are shown in Fig. 4. Combined 1D2-related emission (464 nm +
369 nm) and combined 1I6-related emission (357 nm + 393 nm) are plotted versus temperature in Fig. 5
Temperature (K)
300
400
357
369
393
464
478
500
Wavelength (nm)
Fig. 4 CL spectra in UV and blue region from an
Al0.81Ga0.19N:Tm film at various temperatures ranging
from 79 K to 373 K.
200
1D -related
2
150
100
1000
Al0.81Ga0.19N:Tm
Emission
EA=336 meV
104
100
1I
10
3
10
2
6-related
Emission
10
Intensity Ratio (1D2 / 1I6)
373K
323K
273K
223K
173K
123K
79K
10
Combined CL Intensity (a. u.)
CL Intensity (a. u.)
Al0.81Ga0.19N:Tm
300
5
EA=6.24 meV
2
4
6
8
10
12
14
1
Temperature (1000/K)
Fig. 5 Temperature dependence of combined 1D2related emission, combined 1I6-related emission and
their ratio.
Their intensity ratio, 1D2 / 1I6, is also plotted. The 1D2-related emission experiences an overall increase
with temperature up to near RT and then a slight decrease after that. However, over a wide temperature
range, from ~150 K to ~350 K, the intensity is essentially constant. The 1I6-related emission is more tem© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2768
D. S. Lee et al.: CL and its temperature dependence in Tm-doped AlxGa1–xN thin films
perature-sensitive: it increases with temperature up to 150 K and then decreases drastically reaching a
value smaller by almost 2 orders of magnitude at RT. The temperature dependence of the intensity ratio
shows two clearly separated regimes with a transition point at ~200 K. While the ratio increases slowly
with temperature below 200 K, above 200 K it increases very rapidly. In other words, the 1I6-related
emission is becoming quenched upon approaching RT, resulting in a big increase in the ratio.
Arrhenius-like thermal activation energies estimated from the fitting were 6.24 meV below ~200 K
and 336 meV above ~200K. Vetter et al. reported [12] a temperature trend for the 1D2 → 3F4 CL emission
similar to which we observed in CL intensity ratio. They concluded, mainly from lifetime measurements,
that the increase of the intensity of transitions starting from the 1D2 level are due to a population of this
level by the upper 1I6 level through cross-relaxation. We have reached a similar conclusion. At higher
temperatures, the 1I6 level could populate the 1D2 level by losing energy non-radiatively, either through
phonon-assisted transition or cross-relaxation. This is less likely to happen at lower temperatures since
the energy difference between two levels seems too large (~0.8 eV) to be thermally overcome in that
temperature range. However, we believe that more complicated mechanisms are probably involved in the
overall process. Additional study is now underway on the CL temperature-dependence from films with
various Al compositions.
4 Summary We have obtained dominant blue and UV CL emission from Tm-doped AlGaN. Various
CL emission lines from Tm3+ transitions show very good agreement with those from EL and PL results
from the same AlxGa1-xN:Tm films. CL luminance increases with Al composition up to x = 0.8, but decreases for AlN:Tm, which is the same trend as in EL and PL. Luminance value has been measured to be
19.4 cd/m2 and corresponding efficiency 0.012 lm/W. Temperature dependence of CL from the
Al0.81Ga0.19N:Tm film shows very interesting behaviour of 1D2 and 1I6 levels: 1D2 emission increases with
temperature up to RT, while 1I6 emission decreases rapidly upon approaching RT. The quenching of the
1
I6 emission near RT is believed to be due in non-radiative phonon-assisted or cross-relaxation process.
Acknowledgements This work at Cincinnati was supported by ARO and ARL grants. The authors are pleased to
acknowledge the support of E. Forsythe, D. Morton and J. Zavada.
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