IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERINGIEEJ Trans 2016; 11: 5–9
Published online in Wiley Online Library (wileyonlinelibrary.com). DOI:10.1002/tee.22181

Paper
Structural Change Induced in LaAlO3 by Ion Implantation
Masayuki Harima∗ , Student member
Takaaki Morimoto∗ , Non-member
Yoshimichi Ohki∗,∗∗a , Fellow
Ion implantation is used for various purposes in manufacturing semiconductor devices such as MOS-FETs. In the present study,
the effects of implantation of P+ or B+ ions on the structural change of single-crystal LaAlO3 were examined. The opticalabsorption edge located at ∼5.6 eV, which seems to correspond to the bandgap energy of LaAlO3 , is hardly affected by the ion
implantation. The X-ray diffraction peak intensity at 2θχ = 23.5◦ is decreased by the ion implantation. The intensities of three
sharp photoluminescence (PL) peaks detected at 1.62, 1.65, and 1.69 eV, which appear only when the samples are crystalline,
become smaller by ion implantation. However, the intensity of a broad PL peak at ∼2.8 eV due to the oxygen vacancy, which
is detectable in both amorphous and crystalline samples, hardly changes after the ion implantation. These results indicate thatthe ion implantation degrades the crystallinity of LaAlO3 . © 2015 Institute of Electrical Engineers of Japan. Published by John
Wiley & Sons, Inc.
Keywords: LaAlO3 , ion implantation, photoluminescence, X-ray diffraction, impurity, defect, oxygen vacancy

Received 19 August 2014; Revised 9 December 2014

1. Introduction
Ion implantation is widely used in various industries as aversatile method to provide various functions to many solid
substances. For example, the authors have succeeded in giving
the birefringence to silica glasses by the implantation of lightions, namely H+ or He+ [1–5]. By utilizing the birefringence,
the authors have fabricated optical gratings, optical couplers, andpolarization control elements. For the semiconductor industry,
the most important use of ions is for the creation of donors
and acceptors. Ion implantation through a gate dielectric to a
Si substrate is also used for raising the threshold voltage ofa metal–oxide semiconductor (MOS) device [6]. However, ion
implantation inherently induces point defects, degradation of
crystallinity, and appearance of localized electronic states in solids.
On the other hand, with the continued down scaling of MOS
devices, SiO2 -based gate dielectrics have become even thinner,slated to reach >1 nm in the near future. It means that SiO2 ofthis thickness would not act as an insulating layer, since direct
tunneling dominates the leakage current. A possible solution is
the use of high-permittivity (high-k ) insulating layers [7]. For
this purpose, zirconium-based or hafnium-based oxides have been
investigated extensively [8–11]. Although hafnium-based oxides
are being already used practically, they form silicates when they
are on a Si substrate [12,13]. In this regard, lanthanum aluminate (LaAlO3 ) is attracting much attention as a next promisinggate insulator suitable for advanced MOS devices, because it
has a high dielectric constant (k ∼25), a wide bandgap energy(Eg = 5.6 eV), and an atomic diffusion rate lower than that ofhafnium-based oxides; also, it is thermally stable on Si [14].
Use of high-k dielectrics is also anticipated in SiC power transistors such as MOSFETs and insulated-gate bipolar transistorsaCorrespondence to: Yoshimichi Ohki. E-mail: yohki@waseda.jp* Department of Electrical Engineering and Bioscience, Waseda University,
Shinjuku, Tokyo 169-8555, Japan** Research Institute for Materials Science and Technology, Waseda
University, Shinjuku, Tokyo 169-8555, Japan(IGBTs) [15]. Since much higher voltage is applied to suchdevices than in the case of Si MOSFETs for large-scale integrations (LSIs), their gate dielectrics are thick, (e.g. 10–100 nm).
This needs higher ion acceleration energies to pass through thedielectrics.
In manufacturing an MOS device, the threshold voltage of adevice is often too low when it was manufactured. Therefore,
in order to raise the threshold voltage to the designed value,
ion implantation after the deposition of a gate dielectric is
required [6]. However, as mentioned above, ion implantation
may deform the crystal structure and induce defects in the
dielectric. In this context, in the present study, we implanted
ions into single-crystal LaAlO3 and examined its effects bymeasuring the photoluminescence (PL), which is an effective
tool for examining electronic localized states in various inorganic
insulating materials [16], optical absorption to measure Eg , and
X-ray diffraction (XRD) spectra to examine the crystal structure.

2. Samples and Experimental Procedure
Commercially available LaAlO3 (100) single crystals grown by
the Czochralski method were used as samples. They were of the
shape of a plate with a thickness of 0.5 mm. Positive monovalentions of phosphorus (P+ ) or boron (B+ ) were implanted into thesamples with an acceleration energy of 100 keV at a dose of
1.0 × 1015 cm−2 using an ion accelerator (Ulvac, UP-150) at
Waseda University. The reason for selecting 100 keV is that thepresent ion accelerator can work stably at this energy.
Before and after the ion implantation, PL measurements werecarried out using synchrotron radiation (SR) as a photon source at
the BL3B line of UVSOR Facility (Institute for Molecular Science,
Okazaki, Japan, beam energy: 750 MeV). The beam line has a
2.5-m off-plane Eagle-type normal-incidence monochromator todisperse the radiation to monochromatic light. The PL spectrum,
excited at each PL excitation peak energy, was measured with a
300-mm triple-grating monochromator (Acton, SpectraPro-300i)
and a CCD camera (Princeton Instruments) at 10 K. The PL
intensity was calibrated by taking into account the excitation SR
intensity at each photon energy.

© 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

M. HARIMA, T. MORIMOTO, AND Y. OHKI

Optical absorption spectra were also obtained at room temperature using a Shimadzu UV-3100PC spectrometer equippedwith deuterium and tungsten lamps as two photon sources. The
spectrometer was also equipped with an integrating sphere attachment (ISR3100) to integrate the light coming from almost all the
directions.
Furthermore, in-plane XRD spectra were obtained with Cu Kα
radiation at room temperature using a Rigaku Rint-Ultima III
diffractometer in order to examine the change in crystallinity ofthe sample induced by ion implantation.

2.5
BeforeP ion implanted
B ion implanted

Absorbance

2

1

0
1.5

3. Results

2

6 6.2

3
4
5
Photon energy (eV)Fig. 2. Optical absorption spectra observed for LaAlO3 samples
with a thickness of ∼0.5 mmat room temperature before and after
the implantation of P+ or B+ ions at a dose of 1015 cm−2

Figure 1(a) and (b) show PL spectra observed at 10 K in photon
energy ranges below and above 1.8 eV, respectively. Black solid
curves are for the unimplanted samples, and the orange (lightgray in b/w) ones are for the P+ -implanted samples, while theblue (dark gray in b/w) dashed curves are for the B+ -implanted
samples. The dose of implanted ions was 1015 cm−2 for eachion. From Fig. 1(a), the presence of three PLs excited by 6.0 eV
photons at 1.62, 1.65 and 1.69 eV is confirmed. The intensities of
the three PLs become weak by the ion implantation. On the other
hand, it is confirmed from Fig. 1(b) that there is a very broad PL
band spreading from ∼1.8 to ∼3.6 eV with a peak at ∼2.8 eV.
The intensity of this PL band shows almost no change by the ion
implantation. Although the PL excitation spectrum is not shown,
this PL showed the maximum intensity when it was excited byphotons with an energy of ∼5.0 eV.
Figure 2 shows the optical absorption spectra measured at room
temperature before and after the implantation of P+ and B+
ions at a dose of 1015 cm−2 . It is known that Eg of LaAlO3 is∼5.6 eV [17]. From the spectra shown in Fig. 2, Eg , which canbe estimated to have a similar value as the above, does not seem tobe sensitive to the ion implantation. However, it is clearly seen thation implantation decreases the absorption of photons with energies
from 5.8 to 6.2 eV, which are slightly higher than Eg . The reason
for this apparently strange phenomenon is unknown.

Intensity (arb. unit)

9

BeforeP ion implanted
B ion implanted

6

3

0
22

22.5

23

23.5
24
2θχ (degree)

24.5

25

Fig. 3. In-plane XRD patterns observed before and after the
implantation of P+ or B+ ions at a dose of 1015 cm−2
Figure 3 shows the in-plane XRD spectra obtained from thesurface area through which the ions with a dose of 1015 cm−2 hadpassed. As indicated by the black solid curve, the unimplanted
sample exhibits a peak at 2θχ = 23.5◦ , which corresponds to thediffraction from the (012) planes of crystalline LaAlO3 [18]. The
height of this peak decreases when the ions were implanted.

PL intensity (arb. unit)

5
4

BeforeP ion implanted
B ion implanted

4. Discussion
First, the possibility of crystalline deformation is discussed
based on the XRD results shown in Fig. 3. When the samples are
hit by P+ or B+ ions, the ions seem to disrupt the crystal structure,since the XRD peak height decreases after ion implantation. This
seems enough to conclude that the LaAlO3 single crystal becomesat least partly amorphous when ions are implanted. However, sucha conclusion cannot be drawn until we compare the thickness ofthe crystal where the ions had passed through and the one that
could be properly analyzed by XRD.
When ions are implanted into a solid substance, they stop at a
certain distance, which is called the projected range of the ions.
Figure 4 shows projected ranges of P+ and B+ ions in LaAlO3 ,calculated numerically by SRIM2011 [19]. The ranges are ∼70 nm
for P+ ions and 160 nm for B+ ions with an energy of 100
keV. Next, the depth measurable by in-plane XRD was estimated.
Figure 5 shows the attenuation length of Cu Kα X-rays estimatednumerically [20]. When the grazing angle is <0.34◦ , X-rays arereflected totally, resulting in a very short attenuation length, while
the attenuation length increases rapidly when the angle exceeds
0.34◦ . This means that total reflection does not occur. Therefore,
the grazing angle must be smaller than 0.34◦ for in-plane XRD
measurements. Therefore, the grazing angle was set at 0.32◦ formeasuring the XRD spectra shown in Fig. 3. Whatever the angle,
the depth measurable by the in-plane XRD is less than 6 nm,
which is far less than the projected range of 70 nm for P+ ions or

3
2
1
0
1.5

1.6
1.7
Detected photon energy (eV)
(a)

1.8

PL intensity (arb. unit)

5
4

BeforeP ion implanted
B ion implanted

32
10
1.8 2.1 2.4 2.7 3 3.3 3.6 3.9 4.2
Detected photon energy (eV)(b)

Fig. 1. PL spectra induced by the irradiation of 6.0-eV (a) and
5.0-eV (b) photons observed before and after the implantation of
P+ or B+ ions at a dose of 1015 cm−2
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STRUCTURAL CHANGE INDUCED IN LaAlO3 BY ION IMPLANTATION
250
B ion
P ion

Ion range (nm)

200
150
100
50
0

0

50
100
Ion accelerating energy (keV)

150

Fig. 6. Schematic diagram of the unit cell of perovskite LaAlO3

Fig. 4. Projected ranges of P+ or B+ ions in LaAlO3

Attenuation length (nm)

20
16
12
8
4
0
0

0.1

0.2
0.3
0.4
Grazing angle (degree)

0.5

Fig. 5. Attenuation length of X-rays in LaAlO3

Fig. 7. Schematic diagrams of 3d orbitals in Cr3+

160 nm for B+ ions. On the other hand, the out-of-plane XRD is
unsuitable, since its measurable depth is ∼1–6 μm, which is toolarge to obtain reliable information on the ion implantation effect.
Therefore, we have to use an alternative suitable method.
From such a viewpoint, PL measurements were used as a methodthat could evaluate the crystallinity of the whole part in the sample
where ions had passed through. As mentioned above, the PL
spectra shown in Fig. 1(a) and 1(b) were, respectively, excited byphotons with energies of 6.0 and 5.0 eV, and their peak energies
range from about 1.6 to 3.6 eV. The optical absorption spectra
shown in Fig. 2 obtained using samples ∼0.5 mm thickness
strongly indicate that the incident photons with an energy of 5.0
or 6.0 eV can penetrate deep enough to cover the whole projected
ranges of ions and that the emitted photons can go out of thesample to reach the detector.
As for the origins of PLs, the following assignment has been
done. First, as for the three PL peaks at 1.62, 1.65 and 1.69 eV seenin Fig. 1(a), their spectral shapes are similar to those of the R-line
luminescence of Cr3+ in LaAlO3 [16,21,22]. When Al2 O3 contains
Cr, it is called ruby, and when Al2 O3 contains small amountsof Fe, it is sapphire. As indicated by these typical examples,
Cr is generally present in materials containing aluminum, such
as LaAlO3 and YAlO3 [23]. We have confirmed the presenceof Cr3+ in single crystals LaAlO3 and YAlO3 by electron spinresonance [23,24]. It is known that LaAlO3 is a perovskite crystal
with a structure shown in Fig. 6. Namely, La3+ and O2− form aface-centered cube. A face-centered cube has a tetrahedral vacancy
at its vertices and an octahedral vacancy at its center [18,22].
If Al3+ occupies the latter central vacancy, the lattice becomes
LaAlO3 . In the case of YAlO3 , Cr3+ is known to expel Al3+ andoccupy easily the central octahedral vacancy as an impurity [21].
Therefore, it seems that Cr3+ also occupies the octahedral vacancy
by replacing Al3+ in LaAlO3 [22].
It is known that the electronic transition between the energy levels associated with the R-line luminescence of Cr3+ is inherently
forbidden. Nevertheless, the luminescence is observable due tothe following mechanism. It has been reported that Cr3+ forms anoctahedral ligand field with O2− when the sample is crystalline andchanges the degeneracy of its 3d orbital as a result of the Stark
effect [22,25]. The 3d orbital generally shows fivefold degeneracy. However, in the octahedral ligand field shown in Fig. 7, twoof the five 3d orbitals are along the directions toward O2− ions,while the three others are on the three planes that are perpendicular
to the directions toward the O2− ions. Since electrons in the former
two orbitals receive Coulombic repulsive forces induced by negatively charged O2− , their energies increase. Compared with this,the energies of electrons in the latter three orbitals exhibit only asmall increase. Namely, the fivefold degeneracy is split into two
and three by the Stark effect. As a result, an excited level 2 Eg
and a ground level 4 A2g are generated. The R-line luminescence is
induced by the transition of electrons from 2 Eg to 4 A2g [25]. Therefore, the appearance of the three PLs at 1.62, 1.65 and 1.69 eV
attributable to Cr3+ indicates that the crystallinity of the sample ishigh.
We already confirmed that the above-mentioned three PLs
did not appear in thin amorphous LaAlO3 films made by spincoating an alkoxide solution containing La and Al and that they
appeared after the films had crystallized by thermal annealing athigh temperatures [16]. As shown in Fig. 1(a), ion implantation
decreases the intensities of the three PLs. This means that ion
implantation reduces the crystallinity of the sample.
Secondly, the very broad PL spreading widely around 2.8 eV
seen in Fig. 1(b) is due to oxygen vacancy [16]. This type of
PL due to oxygen vacancy is known to appear in many oxides
such as YAlO3 [24], yttria-stabilized zirconia [26], ZnO [27],
Ta2 O5 [28], and SiO2 [29,30], regardless of their crystallinity. For
example, the PL is clearly observed in crystalline LaAlO3 [16]and also in amorphous SiO2 glasses deposited by a soot remelting
method or by thermal oxidation [30]. Namely, the very littlechange in the intensity of the PL appearing in Fig. 1(b) and thesignificant decrease in the PL intensities shown in Fig. 1(a) are
not contradictory but reasonable if we take the PL’s origins into
account.
To summarize all the results, the PLs that are observable only
when the sample is crystalline become weak when P+ or B+ ions
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M. HARIMA, T. MORIMOTO, AND Y. OHKI

are implanted. On the other hand, the PL due to oxygen vacancy,
whose appearance is independent of the crystallinity, is notaffected by the ion implantation. Therefore, the PL measurements
consistently indicate that the crystallinity of single-crystal LaAlO3
is degraded by the ion implantation.
Similar experiments have already been carried out for YAlO3[24,31], which is also a perovskite high-k dielectric crystal similar
to LaAlO3 [21]. In contrast to the result of LaAlO3 shown in
Fig. 2, the optical band edge shifted toward a lower energy inthe case of YAlO3 [24,31], although the data is not shown here.
This indicates that new localized electronic states were formed in
the bandgap of YAlO3 by the ion implantation. Namely, the change
appearing in optical absorption near the absorption edge or at thephoton energies close to Eg is much more significant in YAlO3
than in LaAlO3 . However, Eg of YAlO3 is ∼7.9 eV and is muchhigher than that of LaAlO3 . Regarding this, the absorption increaseinduced in YAlO3 is significant only at energies higher than ∼6 eV.
That is, if we pay attention to the same energy, e.g. 5 eV, theincrease in optical absorption induced by the ion implantation
is similar in YAlO3 and LaAlO3 . Therefore, from an industrial
viewpoint, the two high-k dielectrics, i.e. YAlO3 and LaAlO3 , have
similar resistance against structural or crystallographic changes
induced by the ion implantation.

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5. Conclusion
Ions of P+ or B+ were implanted into single-crystal LaAlO3 ,and their PL, optical absorption, and in-plane XRD spectra were
measured. As a result, the following important results wereobtained.
1. By the ion implantation, the optical absorption at energies
beyond Eg is decreased.
2. By the ion implantation, the intensity of the XRD (012)
peak is decreased.
3. While the intensity of the PL at ∼2.8 eV due to oxygen
vacancy does not change by the implantation, the intensitiesof three PL bands due to Cr3+ at ∼1.65 eV become smaller
by the ion implantation.
From points 2 and 3 above, it is clear that ion implantation
degrades the crystallinity of LaAlO3 . This should be a matter ofconcern when LaAlO3 is used as a high-k gate dielectric of adevice in which ions are implanted.

Acknowledgments
Part of this research was supported by Power Academy and JSPS
Grant 25 3090 for JSPS Fellows. It was also supported by ResearchInstitute for Science and Engineering, Waseda University, througha grant from Mitsubishi Materials Corporation, ‘Early Bird’ grant
for young researchers, and the ENEOS Research Encouragement
Prize.
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STRUCTURAL CHANGE INDUCED IN LaAlO3 BY ION IMPLANTATION

Takaaki Morimoto (Non-member) was born on July 27, 1982,
in Tokyo, Japan. He received the M.E. degreein 2008 from Waseda University, Japan, andis currently pursuing the Ph.D. degree there.
From 2008 to 2012, he was with Hitachi Central Research Laboratory, engaged in research on
analog electronic circuits. His current researchinterests include high-permittivity insulating
materials for semiconductors.

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Yoshimichi Ohki (Fellow) is a Professor with the Department
of Electrical Engineering and Bioscience andResearch Institute for Materials Science andTechnology, Waseda University, Japan. He is
also an Honorary Professor with Xian Jiaotong
University, China. He was a Visiting Scientist
at the Massachusetts Institute of Technology,
USA, from 1982 to 1984. He is a recipientof the Forster, Whitehead, and Ieda Awards from IEEE-DEIS,
the Outstanding Achievement Award, the Technical Development
Award, and two Best Paper Awards from IEE Japan, among others.
His major research interests include various organic and inorganic
dielectrics, e.g. those used in optical fibers and power cables.

Masayuki Harima (Student member) was born on July 19, 1991,
in Saitama, Japan. He received the B.E. degreefrom Waseda University, Japan, in 2014, and iscurrently pursuing graduate studies there.

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