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crystals
Article

Characterizations of Single-Crystal Lithium Niobate Thin Films
Qingyun Li, Honghu Zhang, Houbin Zhu and Hui Hu *
State Key Laboratory of Crystal Materials, School of Physics, Shandong University, Jinan 250100, China;
liqingyun@mail.sdu.edu.cn (Q.L.); hhzhang@mail.sdu.edu.cn (H.Z.); zhuhoubin@nanoln.com (H.Z.)
* Correspondence: hhu@sdu.edu.cn

Abstract: Single-crystal lithium niobate thin films (lithium niobate on insulator, LNOI) are becoming
a new material platform for integrating photonics. Investigation into the physical properties of LNOI
is important for the design and fabrication of photonic devices. Herein, LNOIs were prepared by two
methods: ion implantation and wafer bonding; and wafer bonding and grinding. High-resolution
X-ray diffraction (HRXRD) and confocal Raman spectroscopy were used to study the LNOI lattice
properties. The full-width at half-maximum (FWHM) of HRXRD and Raman spectra showed a regular
crystal lattice arrangement of the LNOIs. The domain inversion voltage and electro-optical coefficient
of the LNOIs were close to those of LN bulk material. This study provides useful information for
LNOI fabrication and for photonic devices in LNOI.
Keywords: lithium niobate thin films; crystal lattice; domain inversion voltage; electro-optical coefficient

1. Introduction

Citation: Li, Q.; Zhang, H.; Zhu, H.;
Hu, H. Characterizations of
Single-Crystal Lithium Niobate Thin
Films. Crystals 2022, 12, 667. https://
doi.org/10.3390/cryst12050667
Academic Editor: Shujun Zhang
Received: 5 April 2022
Accepted: 30 April 2022
Published: 6 May 2022
Corrected: 19 December 2022
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affiliations.

Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).

Lithium niobate (LN) is one of the most attractive materials in integrated photonics,
owing to its many excellent physical properties [1,2]. Classical LN photonic devices are
usually based on low refractive index waveguides formed by proton exchange and titanium
diffusion [3–5]. These devices have weak optical mode confinement, which greatly limits
their application in integrated photonics. Single-crystal lithium niobate thin film (lithium
niobate on insulator, LNOI, or thin film lithium niobate, TFLN) retain the excellent physical
properties of LN bulk material and exhibit a high refractive index contrast [6,7]. In the past
few years, with the development of LNOI preparation technology and the breakthrough of
micro-fabrication technology, high performance and highly integrated photonic devices
have been reported [8–10].
The study of LNOI physical properties is important for the design and fabrication
of integrated optical devices. There are few reports on the physical properties of LNOI
fabricated by ion implantation and wafer bonding (smart-cut or ion-cut). The refractive
index and crystal lattice of LNOI have been reported [11]. The modulation efficiency of
LN electro-optic devices depended strongly on the electro-optic (E-O) coefficient. The E-O
coefficient of the proton exchange waveguide in LNOI has been studied [12]. The proton
exchange process was able to change the E-O property, which added some uncertainty
to the E-O coefficient measurement. Domain polarization inversion is an important process for nonlinear optics employing quasi-phase-matching. Domain inversion voltage is
essential for fine, smooth, and perfect periodic polarization structure [13]. Various domain
inversion structures have been realized on LNOI, and the domain inversion voltage of
LNOI was found to be higher than that of LN bulk materials [14–16]. It is speculated
that the reasons were the Li+ out-diffusion during annealing processes and the interface
between the LN thin film and the SiO2 layer [15,16]. However, the relationship between
domain inversion voltage and annealing temperature has not been clarified. In addition,
LNOIs were able to be fabricated by wafer thinning technology (such as ion milling or
direct grinding/polishing) [17–21], which could avoid the crystal lattice damage caused
by the ion implantation process [22]. Micro-disk and micro-ring resonators have been

Crystals 2022, 12, 667. https://doi.org/10.3390/cryst12050667

https://www.mdpi.com/journal/crystals

Crystals 2022, 12, 667

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demonstrated with Q factors beyond 108 , which approaches the intrinsic material absorption limit of LN [18,19]. Many frequency conversions have been observed in ultrahigh-Q
micro-resonators. For example, second harmonic generation (SHG), third harmonic generation (THG), fourth harmonic generation (FHG), and optical parametric oscillation (OPO).
The interface between LN thin film and substrate, surface roughness, and bonding strength
of LNOI with thicknesses of several micrometers have been investigated [20,21]. However,
the physical properties of submicron-thick LNOI fabricated by wafer thinning technology
have rarely been reported.
In this study, the lattice properties, domain inversion voltage, and E-O coefficient of
LNOIs were investigated. First, we characterized the LNOI fabricated by wafer bonding
and grinding. The FWHM of the (110) plane measured by HRXRD was 0.027◦ and the
FWHM of the Raman spectrum at 153 cm−1 was 9.41 cm−1 , which shows a regular lattice
arrangement. The domain inversion voltage of LNOIs measured by multiple electric pulses
was 20.4 ± 0.9 kV/mm, which is close to that of bulk material. The E-O coefficient of LNOI
was 27.3 ± 0.4 pm/V, which is also close to that of bulk material. Second, we characterized
the LNOIs fabricated by ion implantation and wafer bonding. After annealing at high
temperature, their properties were found to be similar to those of LNOIs fabricated by
wafer bonding and grinding. This study provides useful information for LNOI fabrication
and for photonic devices in LNOI.
2. Experiments
The LNOIs were prepared by two methods: ion implantation and wafer bonding,
and wafer bonding and grinding. The process with ion implantation and wafer bonding
is described in Ref [7]. The He ion energy was 250 keV with a dose of 4 × 1016 ions/cm2 ,
and an x-cut LNOI with a thickness of 700 nm was fabricated. The LNOI was annealed at
350 ◦ C (LNOI annealing at 350 ◦ C) or 500 ◦ C (LNOI annealing at 500 ◦ C) to repair lattice
damage caused by ion implantation. The process of fabricating LNOI by wafer bonding
and grinding (grinded LNOI) is shown in Figure 1. First, a layer of SiO2 was grown on
an Si substrate by thermal oxidation, and the SiO2 was polished to a suitable thickness
(2 microns) by chemical mechanical polishing (CMP). Then, an x-cut LN wafer was directly
bonded to the Si substrate. Next, the LN crystal was grinded to a suitable thickness and
CMP was used to reduce the surface roughness of the thin film to less than 0.5 nm. Finally,
an x-cut LNOI with a thickness of 700 nm was fabricated, which could avoid lattice damage
caused by the ion implantation process. High-resolution transmission electron microscope
(HRTEM) was used to investigate the interface and crystal lattice of the grinded LNOI.
Figure 2a shows a TEM image in which the LNOI structure is clearly seen. Figure 2b shows
a high-magnification TEM image, with a clear film interface between LN and SiO2 and an
ordered lattice arrangement of LN.
HRXRD with Cu Kα1 (λ = 1.54056 Å) as the X-ray radiation source was used to
investigate the lattice properties of the LNOI. First, ω, ψ and χ scans were used to find the
(110) crystal plane of the LNOI, and then an ω-2θ scan with a step of 0.0012◦ was used
to measure the diffraction peak of the LNOI. Confocal Raman spectroscopy was used to
investigate the LNOI as well. A 473 nm laser beam was focused onto the surface of the
LN thin film through a 100× objective lens (NA = 0.9), achieving a depth resolution of
approximately 320 nm. Raman probing was performed using an 1800 lines/mm grating,
achieving a spectral resolution of 0.69 cm−1 /pixel. Multiple electric pulses were used
to study the domain polarization inversion of the LNOI [14]. The domain polarization
inversion was achieved by applying high voltage pulses to the metal electrode deposited
on the LNOI. As shown in Figure 3a, a 100-nm-thick Cr electrode was deposited on an
x-cut LN thin film with a thickness of 700 nm. Figure 3b shows the scheme of the domain
polarization inversion configuration. The pulse period and duty cycle were 0.1 s and 50%,
respectively. The number of pulses was 20. The direction of the electric field was along
the z axis of the LN thin film. The sample was put in silicone oil to avoid air breakdown

Crystals 2022, 12, 667

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caused by the high electric field strength. Piezoresponse force microscopy (PFM) was used
to observe the inversion domains.

Figure
Fabrication
process
grinded
LNOI.
(a)
layer
SiO
grown
on
an
substrate.
22 was
Figure1.1.1.Fabrication
Fabricationprocess
processofof
ofgrinded
grindedLNOI.
LNOI.(a)
(a)AA
Alayer
layerofof
ofSiO
SiO
wasgrown
grownon
onan
anSiSi
Sisubstrate.
substrate.
Figure
2 was
(b)
Si
substrate
and
LN
were
directly
bonded.
(c)
LN
crystal
was
grinded
and
polished.
(b)
Si
substrate
and
LN
were
directly
bonded.
(c)
LN
crystal
was
grinded
and
polished.
(b) Si substrate and LN were directly bonded. (c) LN crystal was grinded and polished.

Crystals 2022, 12, x FOR PEER REVIEW

4 of 9

Figure2.2.2.Cross
Crossection
sectionof
ofgrinded
grindedLNOI
LNOIobserved
observedby
byTEM
TEM(a)
(a)and
andhigh-magnification
high-magnificationTEM
TEM(b).
(b).
Figure
Figure
Cross
section
of
grinded
LNOI
observed
by
TEM
(a)
and
high-magnification
TEM
(b).

HRXRDwith
withCu
CuKK
1.54056Å)
Å)asasthe
theX-ray
X-rayradiation
radiationsource
sourcewas
wasused
usedtotoininHRXRD
α1α1
(λ(λ= =1.54056
vestigatethe
thelattice
latticeproperties
propertiesofofthe
theLNOI.
LNOI.First,
First,ω,ω,ψψand
andχχscans
scanswere
wereused
usedtotofind
findthe
the
vestigate
(110)crystal
crystalplane
planeofofthe
theLNOI,
LNOI,and
andthen
thenananω-2θ
ω-2θscan
scanwith
witha astep
stepofof0.0012°
0.0012°was
wasused
usedtoto
(110)
measurethe
thediffraction
diffractionpeak
peakofofthe
theLNOI.
LNOI.Confocal
ConfocalRaman
Ramanspectroscopy
spectroscopywas
wasused
usedtoto
measure
investigate
the
LNOI
as
well.
A
473
nm
laser
beam
was
focused
onto
the
surface
the
investigate the LNOI as well. A 473 nm laser beam was focused onto the surface ofofthe
LNthin
thinfilm
filmthrough
througha a100×
100×objective
objectivelens
lens(NA
(NA= =0.9),
0.9),achieving
achievinga adepth
depthresolution
resolutionofof
LN
approximately320
320nm.
nm.Raman
Ramanprobing
probingwas
wasperformed
performedusing
usinganan1800
1800lines/mm
lines/mmgrating,
grating,
approximately
−1−1
achievinga aspectral
spectralresolution
resolutionofof0.69
0.69cm
cm
/pixel.Multiple
Multipleelectric
electricpulses
pulseswere
wereused
usedtoto
achieving
/pixel.
study
the
domain
polarization
inversion
of
the
LNOI
[14].
The
domain
polarization
instudy the domain polarization inversion of the LNOI [14]. The domain polarization inversion
was
achieved
by
applying
high
voltage
pulses
to
the
metal
electrode
deposited
version was achieved by applying high voltage pulses to the metal electrode deposited
onthe
theLNOI.
LNOI.As
Asshown
shownininFigure
Figure3a,
3a,a a100-nm-thick
100-nm-thickCr
Crelectrode
electrodewas
wasdeposited
depositedon
onanan
on
x-cutLN
LNthin
thinfilm
filmwith
witha athickness
thicknessofof700
700nm.
nm.Figure
Figure3b3bshows
showsthe
thescheme
schemeofofthe
thedodox-cut
main
polarization
inversion
configuration.
The
pulse
period
and
duty
cycle
were
0.1
main polarization inversion configuration. The pulse period and duty cycle were 0.1 s s
and50%,
50%,respectively.
respectively.The
Thenumber
numberofofpulses
pulseswas
was20.
20.The
Thedirection
directionofofthe
theelectric
electricfield
field
and
was
along
the
−z
axis
of
the
LN
thin
film.
The
sample
was
put
in
silicone
oil
to
avoid
air
was
along
the
axis
ofof
the
thin
film.
sample
was
put in
silicone
oilinversion
to avoid
air
Figure
Cross−z
section
Cr
on
x-cut
LNOI
(a) and
scheme
of
domain
polarization
inversion
Figure
3.3.Cross
ection
of
Cr
onLN
x-cut
LNOI
(a)The
and
scheme
of domain
polarization
conbreakdown
causedby
bythe
thehigh
highelectric
electricfield
fieldstrength.
strength.Piezoresponse
Piezoresponseforce
forcemicroscopy
microscopy
breakdown
caused
configuration
figuration
(b). (b).
(PFM)was
wasused
usedtotoobserve
observethe
theinversion
inversiondomains.
domains.
(PFM)
A waveguide phase modulator was fabricated on the LNOI to measure the E-O coefficient r33. The LN thin film was x-cut and the TE light traveled along the y axis. The
two polished end faces of the waveguide formed a low-finesse Fabry–Perot cavity [23].

Crystals 2022, 12, 667

4 of 9

A waveguide phase modulator was fabricated on the LNOI to measure the E-O coefficient r33 . The LN thin film was x-cut and the TE light traveled along the y axis. The two
polished end faces of the waveguide formed a low-finesse Fabry–Perot cavity [23]. By applying a continuously varying voltage to the electrodes on either side of the waveguide,
the output light intensity will oscillate. The relationship between the half-wave voltage
length product Vπ L and r33 is expressed as follows [24,25]:
Vπ L =

ne f f λG

(1)

n4 r33 Γ

where G and L are the gap and length of the electrodes, respectively. λ is the wavelength,
neff is the effective index of the waveguide, and n is the extraordinary refractive index of
LN. The Γ (overlapping factor) is defined as follows:
G
Γ=
V

s

2

Eele ( x, z) Eopt ( x, z) dxdz
s
Eopt ( x, z) 2 dxdz

(2)

where V is the voltage applied to the electrode, Eele is the electrostatic field, and Eopt is the optical field. The measurement setup is shown in Figure 4. Polarized light with a wavelength
of 1550 nm was emitted from a tunable semiconductor laser (Santec TSL-210). The laser
beam was coupled into the waveguide by a polarization-maintaining (PM) fiber, and the
output light was collected by a 40×/0.65 objective lens and displayed in an oscilloscope by
an InGaAs detector. The electrical signal from the waveform generator was applied to the
Crystals 2022, 12, x FOR PEER REVIEW
5 of 9
electrode to modulate the light signal in the waveguide. The electrostatic field (Eele ) and
optical field (Eopt ) were simulated by Charge Solutions and Mode Solutions, respectively.

Figure 4. Vπ measurement setup.
Figure 4. Vπ measurement setup.

3. Results and Discussion
3. Results
Discussion
Figureand
5a shows
the diffraction peaks of the (110) crystal planes of the LNOI as
Figure
shows the
peaksLNOI
of theannealing
(110) crystal
planes
of athe
LNOI as
measured
by5a
HRXRD.
The diffraction
black line from
at 350 ◦
C had
diffraction
◦
◦
◦
measured
by
HRXRD.
The
black
line
from
LNOI
annealing
at
350
°C
had
a
diffraction
peak of 34.809 and a FWHM of 0.026 . The red line from LNOI annealing at 500 C had a
◦ and aofFWHM
peak
of 34.809°
and a
FWHM
0.026°. of
The
red◦ .line
LNOI
annealing
500 °C
had
diffraction
peak of
34.815
0.026
Thefrom
blue line
from
grindedatLNOI
had
a
◦
a
diffraction
peak
34.815°
and a
FWHM
of ◦ .
0.026°.
bluewere
line from
diffraction
peak
of of
34.817
and a
FWHM
of 0.027
The The
FWHMs
small,grinded
showingLNOI
that
had a
diffraction peak
of 34.817°
and a
FWHM of
0.027°. The
FWHMs
small,
the
LNOI was
mono-crystalline
with an
ordered
crystal-lattice
arrangement.
Thewere
FWHM
of
showing
was differences.
mono-crystalline
withthe
anlattice
ordered
crystal-lattice
arrangeLNOIs
didthat
not the
showLNOI
significant
To study
structure
after undergoing
the
ion The
implantation
process,
5b show
shows the
polarized
Raman spectra
thethe
LNOI
at
ment.
FWHM of
LNOIsFigure
did not
significant
differences.
To of
study
lattice
153
cm− 1 with
X(YZ)X the
scattering
configuration. The
blue
line 5b
from
the the
grinded
LNOI
structure
after an
undergoing
ion implantation
process,
Figure
shows
polarized
had
a FWHM
of of
9.41the
cm−1 .
Theatblack
annealing
at 350 ◦ C had
a FWHM
Raman
spectra
LNOI
153 line
cm−1from
withthe
anLNOI
X(YZ)X
scattering
configuration.
The
−
1
of
10.75
cmfrom
, which
broadened,
potentially
thecm−1.
disorder
the line
crystal
lattice.
blue
line
the was
grinded
LNOI had a
FWHMdue
of to
9.41
The of
black
from
the
◦ C had −1
−1 . The FWHM
The
red line
fromat
the
LNOI
annealing
at of
50010.75
FWHM of
LNOI
annealing
350
°C had a
FWHM
cm a, which
was9.41 cm
broadened,
potentially
due to the disorder of the crystal lattice. The red line from the LNOI annealing at 500 °C
had a FWHM of 9.41 cm−1. The FWHM of LNOI annealing at 500 °C was consistent with
that of grinded LNOI, revealing that the crystal lattice of the LN thin film was well recovered without causing an expansion of the frequency range of the lattice vibration.

Crystals 2022, 12, 667

showing that the LNOI was mono-crystalline with an ordered crystal-lattice arrangement. The FWHM of LNOIs did not show significant differences. To study the lattice
structure after undergoing the ion implantation process, Figure 5b shows the polarized
Raman spectra of the LNOI at 153 cm−1 with an X(YZ)X scattering configuration. The
blue line from the grinded LNOI had a FWHM of 9.41 cm−1. The black line from5 the
of 9
LNOI annealing at 350 °C had a FWHM of 10.75 cm−1, which was broadened, potentially
due to the disorder of the crystal lattice. The red line from the LNOI annealing at 500 °C
had a FWHM of 9.41 cm−1◦. The FWHM of LNOI annealing at 500 °C was consistent with
of LNOI annealing at 500 C was consistent with that of grinded LNOI, revealing that the
that of grinded LNOI, revealing that the crystal lattice of the LN thin film was well recrystal lattice of the LN thin film was well recovered without causing an expansion of the
covered without causing an expansion of the frequency range of the lattice vibration.
frequency range of the lattice vibration.

Figure
Diffraction peak
peak of
ofLNOI
LNOImeasured
measuredby
byHRXRD.
HRXRD.(b)
(b)Polarized
PolarizedRaman
Raman
spectra
LNOI
Figure 5.
5. (a)
(a) Diffraction
spectra
of of
LNOI
at
−1.
at
153
cm
−
1
153 cm .
Crystals 2022, 12, x FOR PEER REVIEW

6 of 9
implantation process
process affects
affects the
the domain
domain inversion
inversion voltage
voltage of
of the
the LNOI.
LNOI.
The ion implantation
◦
LNOI
annealing
at at
350350C.°C.
TheThe
arFigure 6 shows
shows the
thePFM
PFMimages
imagesofofdomain
domaininversion
inversionofof
LNOI
annealing
eas inside the red frame are the electrodes. The gap between the metal electrodes was
11.2
µm. As
shown
Figure
6a–c, when
the to
voltage
was
260 V, there
waswas
no domain
inversion.
When
the in
pulse
voltage
increased
270 V,
domain
inversion
found.
inversion.
When
the
pulse
voltage
increased
to
270
V,
domain
inversion
was
found.
Upon
Upon increasing
voltage to 280 V, the domain inversion area increased. Figure
6d
increasing
voltage
to 280 V,
the when
domainthe
inversion
increased.
shows
shows
the the
electrical
pulse
waveform
voltage area
was 270
V. The Figure
voltage6dfluctuathe error
electrical
pulse
when the
voltage
was 270
V. of
TheLNOI
voltage
fluctuation
ertion
was 10
V. waveform
Therefore, the
domain
inversion
voltage
annealing
at 350
◦ was
rorwas
was270
10 ±
V. 10 V
Therefore,
inversion
voltage
of 1,
LNOI
annealing
at 350
°C
(24.1 ±the
0.9domain
kV/mm). As
shown in
Table
the same
method
wasCused
270 ±
10 V the
(24.1 ±
0.9 kV/mm).
Asvoltages
shown inofTable
1, the
same at
method
wasand
used to
meato
measure
domain
inversion
LNOI
annealing
500 °C
grinded
◦ C and grinded LNOI, which
sure
the
domain
inversion
voltages
of
LNOI
annealing
at
500
LNOI, which were 20.7 ± 0.9 and 20.4 ± 0.9 kV/mm, respectively, close to that of LN bulk
were 20.7(~21
± 0.9
and 20.4The
± 0.9
kV/mm,
respectively,
to that
of LN bulk
material
material
kV/mm).
domain
inversion
voltageclose
of LNOI
annealed
at 350
°C is
◦ C is higher than
(~21
kV/mm).
The
domain
inversion
voltage
of
LNOI
annealed
at
350
higher than that of LNOI annealed at 500 °C. A possible explanation is that the LN crys◦ C. A possible explanation is that the LN crystal-lattice
that
of LNOI
annealed
at by
500ion
tal-lattice
disorder
caused
implantation increased the domain inversion voltage.
disorder
implantation
increased
the crystal
domainlattice
inversion
voltage.
the
With
the caused
increaseby
in ion
annealing
temperature,
the LN
damage
was With
repaired,
increase
in
annealing
temperature,
the
LN
crystal
lattice
damage
was
repaired,
and
the
and the domain inversion voltage decreased.
domain inversion voltage decreased.

◦ at 260 V (a), 270 V (b) and 280 V (c).
Figure 6.
6. Domain
Domain inversion images
Figure
images of
of LNOI
LNOIannealing
annealingatat350
350 C
°C at 260 V (a), 270 V (b) and 280 V
Electrical
pulse
waveform
when
the the
voltage
waswas
270 V
(c).
Electrical
pulse
waveform
when
voltage
270(d).
V (d).

Table 1. The domain inversion voltage of LNOI.

Sample
LNOI annealing at 350 °C

Domain Inversion Voltage (kV/mm)
24.1 ± 0.9

Crystals 2022, 12, 667

6 of 9

Table 1. The domain inversion voltage of LNOI.
Sample

Domain Inversion Voltage (kV/mm)

LNOI annealing at 350 ◦ C

24.1 ± 0.9
20.7 ± 0.9
20.4 ± 0.9

LNOI annealing at 500 ◦ C
grinded LNOI

Figure 7a shows a scanning electron microscope (SEM) image of the phase modulator
in grinded LNOI. The gap and length of the electrodes were 4.3 µm and 5 mm, respectively.
Figure 7b shows an atomic force microscope (AFM) image of the LNOI waveguide crosssection. The rib had a top width of 0.9 µm and a bottom width of 1.4 µm. The LN etch
depth was 350 nm, leaving a 370 nm slab (measured by the white light interferometry
method). Figure 7c,d show the profiles of the TE00 and TE10 mode at 1550 nm. When the
gap of the electrodes was 4.3 µm, the transmission losses of the TE00 and TE10 mode were
simulated to be 0.5 and 520 dB/cm, respectively. Owing to the high transmission loss of the
TE10 mode, the waveguide could be considered to be single-mode transmission. Figure 8a,c
show the electric fields Ez and Ex , respectively, of the TE00 mode. Figure 8b,d show the
electric fields Ez and Ex , respectively, of the electrostatic field after a voltage of 1 V was
applied to the electrodes. Because the overlapping factors Γ of the Ex of the TE00 mode and
Crystals 2022, 12, x FOR PEER REVIEW
9
the Ex of the electrostatic field were almost 0, their contribution to the phase change7ofofthe
TE00 mode can be ignored. The overlapping factors Γ of the Ez of the TE00 mode and the Ez
of the electrostatic field were 0.53.

Figure 7. SEM image of phase modulator in grinded LNOI (a) and cross-section AFM image of
Figure 7. SEM image of phase modulator in grinded LNOI (a) and cross-section AFM image of
LNOI waveguide (b). Simulated electric field distributions of the TE00 (c) and TE10 mode (d) in
LNOI waveguide (b). Simulated electric field distributions of the TE00 (c)
and TE10 mode (d) in LN
LN waveguide.
waveguide.

Figure 9a shows the variation of output light power with time after applying volt-ages
of 5 and 0 V on the electrodes. When the voltage was 0 V, the output light power was
stable with time; when the voltage was 5 V, the output light power varied significantly
with time. The light power variation was related to the DC bias drift, which resulted from
the flow and redistribution of electrical charge in the LN region under DC voltage [26].
This phenomenon would cause uncertainties in the measurement of Vπ . Triangular waves
(200 kHz with an amplitude of 18 V) were used to minimize the influence of the DC bias
drift. Figure 9b shows the measured light transmission at 1550 nm. The red and black
lines represent the triangular wave electrical and optical signals, respectively, indicating a
Vπ of 8.68 V. The uncertainty of the E-O coefficient mainly results from two aspects. One
was the overlapping factor calculation error caused by the uneven morphologies of the

Crystals 2022, 12, 667

7 of 9

waveguide and electrodes. The other was Vπ measurement error. The uncertainty of the
E-O coefficient was estimated to be within 3%. The E-O coefficient of ground LNOI was
27.3 ± 0.4 pm/V, which was close to that of LN bulk materials (28.6 pm/V [27]). The stress
at
the7. SEM
interface
between
LN thin
film and
cause
coefficient of
LNOI
to of
Figure
image
of phase
modulator
in SiO
grinded
LNOI
(a) the
andE-O
cross-section
AFM
image
2 might
LNOI
waveguide
Simulated
electric
field[28]. As
distributions
TE002,(c)
and TE
mode (d)
in LN
be smaller
than(b).
that of
LN bulk
material
shownofinthe
Table
using
the10same
method,
waveguide.
the
E-O coefficient of LNOI annealing at 350 ◦ C was 26.1 ± 0.4 pm/V, which was lower
than grinded LNOI. This might be due to lattice damage caused by ion implantation.

Crystals 2022, 12, x FOR PEER REVIEW

8 of 9

respectively, indicating a Vπ of 8.68 V. The uncertainty of the E-O coefficient mainly results from two aspects. One was the overlapping factor calculation error caused by the
uneven morphologies of the waveguide and electrodes. The other was Vπ measurement
error. The uncertainty of the E-O coefficient was estimated to be within 3%. The E-O coefficient of ground LNOI was 27.3 ± 0.4 pm/V, which was close to that of LN bulk materials (28.6 pm/V [27]). The stress at the interface between LN thin film and SiO2 might
cause the E-O coefficient of LNOI to be smaller than that of LN bulk material [28]. As
shown in Table 2, using the same method, the E-O coefficient of LNOI annealing at 350
Figure
Electric
fieldEEz
(a) and
and
E
distributions
ofof
the
TE00
mode.
Electric
field
E
(b)
electric
°C
was
± 0.4
pm/V,
which
was lower
than
grinded
LNOI.
This
might
due
latz(a)
z be
Figure
8. 8. 26.1
Electric
field
Exx (c)
(c)
distributions
the
TE00
mode.
Electric
field
Ez and
(b) to
and
elecfield
E
(d)
distributions
of
the
electrostatic
field
after
1
V
voltage
was
applied
to
the
electrodes.
tice
damage
caused
by
ion
implantation.
x
tric field Ex (d) distributions of the electrostatic field after 1 V voltage was applied to the electrodes.

Figure 9a shows the variation of output light power with time after applying
volt-ages of 5 and 0 V on the electrodes. When the voltage was 0 V, the output light
power was stable with time; when the voltage was 5 V, the output light power varied
significantly with time. The light power variation was related to the DC bias drift, which
resulted from the flow and redistribution of electrical charge in the LN region under DC
voltage [26]. This phenomenon would cause uncertainties in the measurement of Vπ.
Triangular waves (200 kHz with an amplitude of 18 V) were used to minimize the influence of the DC bias drift. Figure 9b shows the measured light transmission at 1550
nm. The red and black lines represent the triangular wave electrical and optical signals,

Figure 9.
9. (a)
(a)Variation
Variationofofoutput
outputlight
light
power
with
time
after
applying
voltages
5 0
and
on
Figure
power
with
time
after
applying
voltages
of 5of
and
V 0
onVelecelectrodes.
(b)
200
kHz
triangular
wave
sweep
with
amplitude
of
18
V
for
5-mm-long
phase
modtrodes. (b) 200 kHz triangular wave sweep with amplitude of 18 V for 5-mm-long phase modulator.
ulator. Black line, triangular wave signal; red line, detector signal.
Black line, triangular wave signal; red line, detector signal.
Table 2. The E-O coefficients of LN bulk and LNOI (λ = 1550 nm).
Table 2. The E-O coefficients of LN bulk and LNOI (λ = 1550 nm).

Material
Material
LN Bulk
LN Bulk at 350 °C
LNOI annealing
LNOI annealing at 350 ◦ C
grinded LNOI
grinded LNOI

E-O Coefficient (pm/V)
E-O Coefficient (pm/V)
28.6
28.6
26.1
± 0.4
26.1 ± 0.4
27.3 ± 0.4
27.3 ± 0.4

4. Conclusions
In conclusion, the lattice properties, domain inversion voltage, and E-O coefficient of
LNOIs were studied. For grinded LNOI, the FWHM of the (110) plane of HRXRD was
0.027°, and the FWHM of Raman spectrum at 153 cm−1 was 9.41 cm−1, revealing a regular
lattice arrangement; the domain inversion voltage was 20.4 ± 0.9 kV/mm, which was close

Crystals 2022, 12, 667

8 of 9

4. Conclusions
In conclusion, the lattice properties, domain inversion voltage, and E-O coefficient
of LNOIs were studied. For grinded LNOI, the FWHM of the (110) plane of HRXRD
was 0.027◦ , and the FWHM of Raman spectrum at 153 cm−1 was 9.41 cm−1 , revealing a
regular lattice arrangement; the domain inversion voltage was 20.4 ± 0.9 kV/mm, which
was close to that of bulk material. The E-O coefficient of LNOI was 27.3 ± 0.4 pm/V,
which was close to that of bulk material. After annealing at high temperature, the LNOI
fabricated by ion implantation and wafer bonding exhibited properties similar to those of
LNOI fabricated by wafer bonding and grinding, which indicated that both methods could
produce high-quality LNOIs.
Author Contributions: Conceptualization, Q.L. and H.H.; methodology, Q.L.; validation, Q.L.; formal
analysis, Q.L. and H.H.; investigation, Q.L.; resources, H.H.; writing-original draft preparation, Q.L.;
writing-review and editing, Q.L., H.Z. (Honghu Zhang), H.Z. (Houbin Zhu) and H.H.; data analysis,
Q.L.; supervision, H.H.; funding acquisition, H.H. All authors have read and agreed to the published
version of the manuscript.
Funding: This research is supported by the Natural Science Foundation of Shandong Province, grant
number ZR2020LLZ007. Additionally, it is supported by the National Key Research and Development
Program of China, grant number 2018YFB2201700 and 2019YFA0705000.
Data Availability Statement: Not applicable.
Conflicts of Interest: Houbin Zhu is an employee of Jinan Jingzheng Electronics Co., Ltd. (NanoLN,
Jinan, China) and involved in developing lithium niobate technologies at NanoLN. Hui Hu is one of
the founders of NanoLN and involved in developing lithium niobate technologies at NanoLN.

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</reference>

<statements>
1. High-Q microresonators beyond 10^8 have been demonstrated on LNOI, approaching intrinsic absorption limits
2. Structural studies of ion-implanted LNOI show that high-temperature annealing restores Raman linewidth and brings domain-inversion voltage and electro-optic coefficient closer to grinded LNOI, while lower-temperature annealing leaves measurable disorder
3. For LNOI structural anneal, 500 °C restored Raman FWHM to 9.41 cm⁻¹ and domain-inversion voltage to 20.7 kV/mm; 350 °C left FWHM 10.75 cm⁻¹, voltage 24.1 kV/mm, and E-O coefficient 26.1 pm/V versus 27.3 pm/V for grinded LNOI
4. For LNOI structural anneal, this characterizes ion-implantation/slicing damage, not direct plasma-etch damage
5. Grinding/CMP wafer preparation can avoid ion-implantation lattice damage, and CMP can reduce surface roughness below 0.5 nm, but these are wafer-level routes rather than nanophotonic pattern transfer
6. Annealing also changes stoichiometry and poling behavior: ion-implanted LNOI annealed at lower temperature has higher domain-inversion voltage, and Li+ out-diffusion during annealing has been proposed as a cause
7. Interface stress may also make LNOI electro-optic coefficients smaller than bulk
8. The strongest structural recovery evidence comes from He-implanted LNOI, where Raman probing had about 320 nm depth resolution and therefore characterizes slicing/implantation damage rather than direct plasma-etch damage
9. For LN nonlinear photonics, the decisive uncertainty is whether they preserve the domain pattern and stoichiometry required for quasi-phase-matching
</statements>

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