Growth and annealing properties of Mg0.4Al2.4O4 crystal

Article abstract of DOI:10.1016/j.jallcom.2008.03.046

Mg_0.4Al_2.4O₄ single crystals with good optical quality were successfully grown by the Czochralski method. The transmission spectrum indicated that the absorption edge of the crystal was at 220 nm, while no apparent absorpt

Full text of DOI:10.1016/j.jallcom.2008.03.046

Journal of Alloys and Compounds 470 (2009) L29–L32
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
Growth and annealing properties of Mg Al O crystal
0
.4 2.4
4
Huili Tanga,b, Jun Xu , Yongjun Dong , Feng Wuc
a,∗
c
a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road,
Shanghai 200050, PR China
b
Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences,
c
P.O. Box 800-211, Shanghai 201800, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Mg0.4Al2.4O4 single crystals with good optical quality were successfully grown by the Czochralski method.
The transmission spectrum indicated that the absorption edge of the crystal was at 220 nm, while
no apparent absorption peaks were found. The X-ray diffraction and DSC curve analysis showed that
Mg0.4Al2.4O4 crystal was stable at room temperature. While after annealing in the air and hydrogen
Received 28 January 2008
Received in revised form 5 March 2008
Accepted 6 March 2008
Available online 22 April 2008
◦
atmosphere at about 1200 C, Mg0.4Al2.4O4 decomposed into Al2O3 and (MgO)0.4(Al2O3)x (0.4 < x < 1.2).
The reaction mainly occurred on the crystal surface, barely inside.
Keywords:
©
2008 Elsevier B.V. All rights reserved.
Oxide material
Crystal growth
X-ray diffraction
1. Introduction
based LEDs grown by MOCVD on MgAl O10-(1 1 1) exhibit a higher
6
EL-intensity and a narrower EL-linewidth than those grown on
sapphire [8]. Therefore, Mg0.4Al2.4O4 equivalent to MgAl O is a
promising substrate material for GaN epitaxial growth. Herein, we
report the Czochralski growth of Mg0.4Al2.4O4 crystal in detail, and
its annealing properties were studied.
Recently, GaN thin film has attracted considerable attention due
to their potential applications in optoelectronic devices such as
blue-, violet-, and ultraviolet (UV)-light emitting devices [1–3] as
well as high-temperature, high-frequency and high-power elec-
tronic devices [4]. One of the major difficulties hindering the
development of GaN-based devices is the lack of suitable substrate.
To date, sapphire has been the most widely used substrate for GaN
heteroepitaxy in industry. Although sapphire offers excellent crys-
tal quality, surface finish and stable physico-chemical performance,
6
10
2. Crystal growth
Single-crystal growth of Mg0.4Al2.4O4 was performed using a
typical Czochralski method. High-purity MgO (≥99.99%) and Al O
2
3
−
6
−1
and
its high thermal and lattice mismatch with GaN (3 × 10
K
(≥99.999%) compounds were weighted according to the following
13.9%, respectively) result in high-dislocation densities and residual
chemical reaction equation: MgO + 3Al O = 2.5Mg Al O . The
2
3
0.4 2.4
4
stress in the epitaxial layers, which affects both the electrical and
optical properties of the devices. In order to further improve the
quality of GaN epitaxial layers and therefore InGaN LEDs, SAINT-
GOBAIN Crystals, a major supplier of substrates, has developed
a new material, less mismatched with GaN but still chemically
close to sapphire: an alumina-rich cubic spinel, MgAl O [5], also
total weight of the initial charge was 220 g. After the starting mate-
rials were mixed, they were pressed into pieces and sintered in a
resistance-heated furnace at 1200 C for 10 h. Before putting the
◦
charge pieces into the iridium crucible, we adjusted the position
to make sure the radio-frequency coil, the crucible and the seed
rod were accurately concentric. Then the charge was loaded into
the iridium crucible (60 mm in diameter and 50 mm in height) and
6
10
denoted MgO nAl O with n = 3. This crystal has a lower thermal
2
−
6
−1
◦
expansion mismatch with GaN: ꢀ˛ ≈ 2.2 × 10
K
(measured at
was heated to approximately 2100 C to melt. The crystal growth
◦
8
00 C), together with a lower lattice mismatch of −11.5% [6]. Com-
was performed using a [1 1 1] MgAl O rod as a seed. The growth
2
◦
4
pared with stoichiometric spinel (MgAl O ), alumina-rich spinel
temperature was about 1950–2000 C. The pulling and rotational
speeds employed were 1–2 mm/h and approximately 20–25 rpm
to even out the thermal field of the melt in the crucible. During the
entire crystal-growing process, the growth boundary in solid-melt
was convex towards the melt so that dislocations, faults, bubbles
and impurities were reduced or eliminated from the crystal. Since
the spinel may dissolve alumina at lower temperature [9], the
2
4
benefits from compositional softening [7] and can easily be sliced
and polished into large diameter wafers [5]. Excitingly, the GaN-
∗ Corresponding author. Tel.: +86 21 69918484.
E-mail address: xujun@mail.shcnc.ac.cn (J. Xu).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.03.046

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H. Tang et al. / Journal of Alloys and Compounds 470 (2009) L29–L32
crystal-cooling period was set within 20 h to maintain Mg0.4Al2.4O4
single phase. The as-grown crystal was colorless and transparent.
The (1 1 1)-plane wafers were sliced perpendicular to the growth
direction and were polished with mechanical method. Then the
◦
polished wafers were annealed in the air at 1200 C for 15 h and
◦
in the hydrogen at 1250 C for 10 h, respectively.
The phase identification of the obtained crystals and the
annealed samples were investigated by X-ray diffraction, which
were carried out on D/MAX-2550 diffractometer, using Cu K␣ radi-
ation. Intensities for the diffraction peaks were recorded in the
◦ ◦
0–90 (2ꢁ) range, with a step of size of 0.02 and a scan speed
1
◦
−1
of 8 min . The optical transmission spectrum was measured via a
Model V-570 UV/VIS/NIR spectrophotometer (Japan JASCO) at room
temperature. Differential thermal and thermogravimetry curves
were measured by the differential scanning calorimeter (NETZSCH
STA 409 PG/PC) under nitrogen atmosphere. The temperature was
◦
◦
◦
−1
raised from 25 to 1400 at the heating rate of 100 min . The
sample was a thin wafer weighted 26.000 mg.
Fig. 2. X-ray powder diffraction pattern of as-grown Mg0.4Al2.4O4 crystal.
3
. Result and discussion
The transparent and integral Mg0.4Al2.4O4 single crystal with
the dimension of Ø26 mm × 55 mm by the Czochralski technique
is shown in Fig. 1. Green and red lasers were used to evaluate the
crystal’s overall quality. No light-scattering pellets were observed,
indicating nearly no inclusions in the entire crystal. Mg0.4Al2.4O4
¯
has the space group of Fd3m (2 2 7), classified as cubic. The X-ray
powder diffraction pattern of the as-grown crystal is presented in
Fig. 2. All the diffraction peaks can be indexed in cubic Mg0.4Al2.4O4
(
No. 87-0345). No other phase was found in the XRD patterns. It can
be calculated that the unit-cell parameters of Mg0.4Al2.4O4 crys-
tal were a = 7.995 A˚ , and the lattice mismatch with GaN is −11.3%.
The optical transmission spectrum of the Mg Al O crystal wafer
0
.4 2.4
4
with the thickness of 1.0 mm is given in Fig. 3. The crystal has an
absorption edge at about 220 nm and has no distinct absorption
peaks from 190 nm to 1900 nm. The transmittance is about 87%
in the wavelength range of 400–1900 nm, which demonstrates the
crystal has good optical quality.
Fig. 4 is the TG and DSC curves of Mg0.4Al2.4O4 crystal. In
the whole heating process, no mass change was observed. It
is noticeable that there are four overlapping exothermic peaks
Fig. 3. Transmission spectrum of the Mg0.4Al2.4O4 crystal wafer.
◦
mic peak at 1112 C. The corresponding reaction is given by
◦
in the temperature range of 25–1400 C. The baseline below
2
.5Mg0.4Al2.4O → MgAl O + 2Al O . The second one extends
4
2
4
2
3
◦
9
50 C is an increasing tendency, which means the sample is
◦
up to 1262 C, and Mg Al
O
decomposes into Al O3 and
0
.4 2.4
4
2
absorbing heat. According to the equilibrium phase diagram of
◦
MgO)0.4(Al O ) (0.4 < k < 1.2). The exothermic peak at 1182 C
2 3
k
(
the MgO–Al O system [10], Mg Al2.4O4 can decompose. The
2
3
0.4
◦
◦
first step is from 950 C to 1134 C and there is an exother-
Fig. 1. Photograph of as-grown Mg0.4Al2.4O4 crystal with the dimension of
Ø26 mm × 55 mm.
Fig. 4. TG and DSC curves of Mg0.4Al2.4O4 crystal.

H. Tang et al. / Journal of Alloys and Compounds 470 (2009) L29–L32
L31
Fig. 5. X-ray diffraction pattern of the as-grown (1 1 1)-Mg0.4Al2.4O4 wafer.
Fig. 7. X-ray diffraction pattern of the (1 1 1)-plane polished wafer after annealing
in the air.
◦
corresponds with the process. The third one is from 1262 C
to 1337 C, one exothermic peak at 1317 C. The decomposition
◦
◦
Mg0.4Al2.4O4 needs external heating to provide energy, and the
decomposition products are different at different temperatures. To
reduce and avoid these decomposition reactions mentioned above,
the Mg0.4Al2.4O4 crystals should be taken a quick cooling speed in
the later growth.
products are Al O and (MgO)0.4(Al O )m (k < m < 1.2). The last
2
3
2
3
◦
one is till 1400 C. The decomposition reaction is described as
Mg0.4Al2.4O → (MgO) (Al O )n + Al O , and n satisfies the rela-
4
0.4
2
3
2
3
tion m < n < 1.2. The DSC curve shows that the decomposition of
Besides, annealing experiments of the polished (1 1 1)-plane
Mg0.4Al2.4O4 wafers have been performed. Surface of the polished
wafer is smooth. After annealing in the air and hydrogen at approx-
◦
imately 1200 C, the wafers become milky and opaque. The surface
becomes quite rough and appears grooves. Figs. 5 and 6(a) and
(b) show the X-ray diffraction patterns of the as-grown, the air
annealed and the hydrogen annealed (1 1 1)-Mg0.4Al2.4O4 wafer,
respectively. From Fig. 6, it can be clearly seen that after annealing
in the air and hydrogen, the samples’ XRD patterns are almost the
same. Moreover, there are new diffraction peaks that can be indexed
in hexagonal Al O . The experimental results demonstrate that
2
3
the Mg0.4Al2.4O4 wafers by the annealing treatment in the air and
hydrogen are no longer Mg0.4Al2.4O single phase, but become poly-
4
crystalline structure of Mg0.4Al2.4O , Al O and (MgO)0.4(Al O )x
4
2
3
2
3
(
0.4 < x < 1.2). The amount of the (MgO)0.4(Al O )x compounds is
2 3
possibly too trace to be detected in the XRD patterns. The result is
in accordance with the DSC analysis. Then the annealed samples
were polished with mechanical method, and they became color-
less transparent again. In the X-ray diffraction pattern we can find
Mg0.4Al2.4O4 peaks as well as (1 1 3)-plane and (2 2 3)-plane Al O3
2
peaks (see Fig. 7). The quantity of Al O ’s diffraction peaks is less
2
3
than that in Fig. 5. This shows that Mg0.4Al2.4O4 crystal is stable
at room temperature. While after annealing in the air and hydro-
◦
gen at about 1200 C, Mg0.4Al2.4O crystal decomposes. The reaction
4
mainly occurs on the crystal surface, barely inside.
4. Conclusion
In conclusion, we have successfully grown Ø26 mm × 55 mm
cubicMg0 Al O crystalsbytheCzochralskimethod usinga [1 1 1]
.4
2.4
4
MgAl O4 seed. The as-grown crystals are colorless transparent,
2
and the transmittance is about 87% in the wavelength range of
4
00–1900 nm. The wafer surfaces by the annealing treatment in
the air and hydrogen become quite rough. X-ray diffraction patterns
have proved that Mg0.4Al2.4O crystal is stable at room temperature.
4
While after annealing at high temperature, Mg0.4Al2.4O4 crystals
decompose into Al O and (MgO) (Al O )x (0.4 < x < 1.2). The reac-
2
3
0.4
2
3
tion mainly occurs on the crystal surface, barely inside. In order to
attain single crystal, Mg0.4Al2.4O crystal should be cooled down to
Fig. 6. X-ray diffraction patterns of the annealed (1 1 1)-Mg0.4Al2.4O4 wafer: (a) in
4
the air atmosphere; (b) in the hydrogen atmosphere.
room temperature rapidly in the later growth.

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H. Tang et al. / Journal of Alloys and Compounds 470 (2009) L29–L32
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