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Solid State Electronics
journal homepage: www.elsevier.com/locate/sse
Improved dielectric properties of BeO thin films grown by plasma enhanced
atomic layer deposition
Yoonseo Janga,b, Seung Min Leea,b, Do Hwan Junga,b, Jung Hwan Yumc, Eric S. Larsenc,d,
Christopher W. Bielawskic,d,e, Jungwoo Oha,b,⁎
a School of Integrated Technology, Yonsei University, Incheon 21983, Republic of Korea
b Yonsei Institute of Convergence Technology, Incheon 21983, Republic of Korea
c Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
dDepartment of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
e Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
A R T I C L E I N F O
Keywords:
Beryllium oxide
Plasma enhanced atomic-layer deposition
Metal oxide semiconductor capacitors
Bandgap energy
Dielectric constant
A B S T R A C T
Beryllium oxide (BeO) thin films were grown on a p-type Si substrate by plasma enhanced atomic layer de-
position (PEALD) using diethylberyllium as a precursor and O2 plasma. The PEALD BeO exhibited self-saturation
and linear growth rates. The dielectric properties of PEALD were compared with those of thermal atomic layer
deposition (ThALD). X-ray photoelectron spectroscopy was performed to determine the bandgap energy of
PEALD BeO (8.0 eV) and ThALD BeO (7.9 eV). Capacitance–voltage curves revealed that PEALD BeO had low
hysteresis and frequency dispersion compared to ThALD BeO. In addition, PEALD showed a dielectric constant of
7.15 (at 1MHz) and low leakage current ( ×7.25 10 9 A/cm2 at −1MV/cm). These results indicate that the
highly activated radicals from oxygen plasma prompt the chemical reaction at the substrate, thus reducing
nucleation delay and interface trap density.
1. Introduction
The role of insulators is important because the size of the transistor
becomes smaller, thus reducing gate leakage current. Oxide materials
with a high dielectric constant (k) and wide bandgap energy have been
proposed for better electrical performance in the nano-scale region
[1–4]. Suitable materials that exhibit good matched band offset of the
conduction and valence bands and thermal stability with substrates are
uncommon. Beryllium oxide (BeO) has attractive properties with ex-
cellent thermal conductivity (300W/m-K) [5], wide bandgap (10.6 eV),
and high dielectric constant (~6.9) [6]. In addition, a previous study
reported that the high pressure phase of rocksalt-BeO can achieve a
dielectric constant of 275 by theoretical calculation [7]. Because of this
unique feature, BeO can be considered as a potential candidate to re-
place existing gate dielectrics.
Previously, BeO was synthesized in powder or particle forms by a
modified sol-gel process and sintering [8–10]. Recently, several re-
search groups have reported BexZn1-xO alloy films deposited by Mole-
cular Beam Epitaxy (MBE) [11–13]. Another group studied in situ oxi-
dation MBE BeO films as insulators and oxygen barriers for ZnO based
devices [14]. However, these methods have limited the use of BeO
because the processes are not compatible with current semiconductor
devices. Atomic layer deposition (ALD), which is based on self-limiting
reactions leading to layer-by-layer growth, exhibits a large-area uni-
formity and great conformability and offers atomic scale controllability.
Because of these advantages, several studies have recently demon-
strated thin films of BeO can be deposited on various semiconductor
substrates using ALD [15–19]. In addition, BeO growth behavior using
O3 as an oxygen source via ALD has been studied [20]. However, the
growth of BeO thin films by advanced deposition techniques such as
plasma exposure has not been elucidated yet.
In plasma-enhanced atomic layer deposition (PEALD), high quality
dielectric films are deposited by increasing the radical reactivity for
strong surface reactions. In general, ALD is a thermal process that uses
energy from the heat of a substrate to drive a surface reaction.
However, this process has limitations in material choices and tem-
perature windows. Unlike typical thermal ALD (ThALD), PEALD uses
plasma as a reactant to produce highly reactive radicals that enhance
https://doi.org/10.1016/j.sse.2019.107661
⁎ Corresponding author at: School of Integrated Technology, Yonsei University, Incheon 21983, Republic of Korea.
E-mail address: jungwoo.oh@yonsei.ac.kr (J. Oh).
Solid State Electronics 163 (2020) 107661
Available online 18 September 2019
0038-1101/ © 2019 Elsevier Ltd. All rights reserved.
T
chemisorption. Energetic radicals promote film deposition to improve
growth continuity and inhibit impurities from residual precursors and
by-products [21,22]. Previous studies have reported the benefits of
PEALD films including improved growth rate, reduced process tem-
peratures, less oxygen and carbon contamination, and enhanced step
coverages [23–26].
In this study, BeO films as prepared by PEALD using O2 plasma,
which is widely used in PEALD oxide films [27,28], were investigated
for the first time. Radicals activated by plasma energy increase
breakage of ligands and lead to absorption to improve the film quality
of the dielectric. Growth characteristics and plasma conditions of the
PEALD BeO films were investigated by changing the parameters of the
plasma reactant. Next, the dielectric properties of PEALD and ThALD
BeO were examined physically and electrically. The chemical states of
films were analyzed by X-ray photoelectron spectroscopy (XPS) to
calculate the bandgaps. The electrical properties of the PEALD and
ThALD BeO/Si MOS capacitor, including capacitance–voltage (C-V) and
current–voltage (I-V) characteristics, were also evaluated at room
temperature and compared.
2. Experiment
2.1. Preparation of PEALD BeO films
P-type silicon wafers (1 0 0) of resistivity 1–10 Ω∙cm were used as
the substrate. The wafers were cleaned with acetone, isopropanol, and
deionized water and then immersed in buffered oxide etchant (BOE)
prior to loading into the chamber to remove the native oxide layer. BeO
thin films were deposited by an atomic layer deposition system (Lucida
M100-PL, NCD Technology) which is capable of PEALD and ThALD.
The BeO films were deposited by PEALD at a substrate temperature of
287 °C. The Be precursor, diethylberyllium (DEB, Be(C2H5)2), was
maintained at 65 °C, and oxygen (O2) was used as the reactant. Ar gas
was maintained at the working pressure, 0.2 Torr, using the total ALD
process, and the by-products, excess precursors, and reactants were
purged at a flow of 20 sccm. Fig. 1 depicts the process of PEALD BeO
growth, and the sequence of one cycle is as follows: (1) DEB exposure,
(2) DEB purge, (3) O2 plasma exposure, and (4) O2 purge.
2.2. Fabrication of the MOS capacitor
To investigate the electric properties, a BeO MOS capacitor was
fabricated with cleaned p-type Si, and the schematic is shown in Fig. 2.
To compare the PEALD and ThALD methods, 200 cycles of BeO thin
film deposition were performed using each method. Next, to form the
top electrode, a 10-nm adhesion layer of Ni was evaporated by an
electron beam and then a 100-nm Au layer was deposited with a
thermal evaporator at a pressure of 10−6 Torr. The area of the electrode
was determined to be 1.25× 10−3 cm2 by shadow mask. Finally,
100 nmW was deposited by sputtering to form the backside ohmic
contact.
Fig. 1. Schematic of the PEALD BeO growth process: (a) diethylberyllium (DEB, Be(C2H5)2) exposure, (b) DEB purge, (c) O2 plasma exposure, and (d) O2 purge.
Fig. 2. Schematic of the PEALD BeO/p-Si MOS capacitor.
Y. Jang, et al. Solid State Electronics 163 (2020) 107661
2
2.3. Film analyses
Film thickness and refractive index were measured by ellipsometry
(Elli‐SE, Ellipso Technology Co.). XPS analysis was performed using
Thermo Fisher Scientific K-ALPHA system. The energy of Al Kα was
1486.6 eV, which was used for X-ray. The spot size was 30–400 μm and
the energy resolution was 0.085 eV. C-V and I-V characteristics were
evaluated using a power device analyzer/curve tracker Agilent B1505A
and high-power probe station.
3. Results and discussion
To investigate the optimized plasma conditions, BeO films were
deposited by changing parameters of the plasma exposure conditions
with a fixed DEB exposure time of 3.0 sec and a purge time of 20 s.
Fig. 3 shows the variations in growth per cycle (GPC) and refractive
index (RI) of the PEALD BeO with respect to O2 exposure time, O2 flow,
and plasma power. When these three parameters were varied, the GPC
remained constant at about 1.1 Å/cycle, which was a higher growth
rate than that from previous studies [17,29,30]. The results show that
the oxygen reactants are activated enough to saturate the growth at O2
plasma exposure times, flow rates, and power of 2 s, 20 sccm, and 30W,
respectively. From this experiment, the conditions under which oxygen
plasma is reacted to oxidize the ligand were established, and the self-
limiting reaction of the typical ALD process was confirmed.
In general, refractive index is proportional to the density of the film.
The refractive index of the thin film BeO was found to be in the range of
1.59–1.64, reflecting high density characteristics as compared to bulk
BeO (1.71). As the O2 flow increases, the refractive index tends to de-
crease, which denotes reduced density. This tendency may be due to the
increased oxygen flow reducing the residence time in the surface region
and reducing the possibility of the reaction [31]. In addition, as the
plasma power increased, the same tendency was exhibited due to
plasma damage, which can reduce interface quality [32]. As reported in
previous studies, PEALD techniques require adequate time, O2 exposure
time, O2 flow, and plasma power [33,34]. Based on the results of
confirming the conditions under which the oxygen reactants were ac-
tivated, an appropriate plasma condition with O2 plasma exposure
times, flow rates, and power of 5 s, 50 sccm and 100W, respectively,
was fixed in subsequent experiments.
Fig. 4 shows the change in BeO thickness with respect to the number
of cycles of the PEALD and ThALD methods showing the initial growth
behavior. In all cycles, PEALD BeO films had a higher growth rate than
ThALD. Unlike ThALD (0.6 Å/cycle), PEALD BeO films maintained a
growth rate of 1.1 Å/cycle under 100 cycles because the plasma energy
during film formation contributes precursor absorption to the surface
and suppresses nucleation delay [35]. The linear growth behavior of
PEALD had been confirmed in previous studies [36]. This result shows
Fig. 3. Growth per cycle (GPC) and refractive index (RI) of BeO grown by
PEALD as a function of (a) O2 exposure time, (b) O2 flow, and (c) plasma power.
Fig. 4. Comparison of thicknesses of BeO films prepared by PEALD and ThALD
as a function of number of deposition cycles.
Y. Jang, et al. Solid State Electronics 163 (2020) 107661
3
that PEALD can precisely control the thickness, especially in sub 5 nm
films, with excellent conformity.
X-ray photoelectron spectroscopy (XPS) analysis was performed to
determine the bandgap of BeO thin films. Fig. 5 shows the O1s energy
loss spectra of BeO thin films deposited at a substrate temperature of
250 °C with PEALD and ThALD. The peak energy of BeO O1s (531.0 eV)
corresponds with the XPS database (531.6 eV). Generally, inelastic loses
occur in oxide films by band-to-band excitation. The bandgap (Eg) of
the oxide can be obtained by measuring the energy of electrons excited
from the valence band to the conduction band. The energy loss spectra
can be used to calculate the bandgap energy by obtaining the difference
between the core-level peak (EO1s) and the approximate onset of the
inelastic losses (Eloss) [4]. Eloss is the intersection of the zero level and
the onset of inelastic losses obtained by linear fitting [37]. The O1s
core-level peak of BeO is 531.0 eV and can be calculated as follows.=E E Eg loss O s1 (1)
According to the above equation, PEALD and ThALD BeO films have a
bandgap energy of 8.0 eV and 7.9 eV, respectively, which is similar to
the bandgap energy of ALD BeO films in a previous report [20].
BeO/p-type Si MOS capacitors were prepared by both PEALD and
ThALD methods to examine electric properties. Fig. 6(a) and (b) show
the C-V measurements at a frequency of 100 kHz and 1MHz. Obvious
differences in accumulation and depletion regions and common C-V
curves with hysteresis were observed. From the C-V curves, dielectric
constants (k) of PEALD BeO at 100 kHz and 1MHz were found to be 8
and 7.15. These results are close to the reported results that dielectric
constants of wurzite BeO are 6.9 (a-axis) and 7.74 (c-axis) at 1MHz [6].
However, in the case of ThALD, the film had k values of 8.5 (100 kHz)
and 5.18 (1MHz), which highly differed from those of PEALD, in-
dicating high frequency dispersion. The low frequency dispersion of
PEALD means that the film has a high density and a good quality in-
terface. The C-V curve hysteresis is obtained from the flat band voltage
shift. The flat band capacitance is calculated by the following equation,
where, Cox, Ld, and s are oxide capacitance, extrinsic Debye length, and
permittivity of silicon, respectively.
= +C 1FB C L1ox Ds (2)
At 100 kHz, the flat band capacitance (CFB) and flat band voltage
(VFB) of PEALD BeO are 224 nF/cm2 and −0.4 V, respectively, and
those of ThALD are 244 nF/cm2 and −2.5 V, respectively. From this,
flat band voltage shifts were obtained which showed that the PEALD
curve had smaller values (0.3 V) than the ThALD curve (0.55 V). In
addition, we confirmed that the frequency dispersion of PEALD BeO
was significantly improved compared with that of the thermal type BeO
by the difference in capacitance between 100 kHz and 1MHz.
Hysteresis and frequency dispersion are affected by the films and Si
substrate interface [38], which means that the plasma reactants of
PEALD can reduce the interface trap density and improve dielectric
properties due to reduced nucleation delay [39].
The leakage current densities of PEALD and ThALD with respect to
applied voltage are shown in Fig. 6(c). At −1 MV/cm, the leakage
current densities of PEALD ( ×7.25 10 9 A/cm2) are slightly lower than
those of ThALD ( ×1.06 10 8A/cm2). The low leakage current of BeO is
due to the large bandgap, and the reason the values are similar is
consistent with previous results with similar bandgaps of PEALD and
ThALD. Overall, the electric properties indicated that the quality of
PEALD BeO thin films was improved because of plasma energy.
4. Conclusions
In this study, the growth behavior and properties of BeO thin films
using PEALD were investigated. The GPC of PEALD BeO becomes sa-
turated at O2 exposure times, O2 flow, and plasma power of 2 s,
20 sccm, and 30W, respectively. In addition, the growth rate of PEALD
BeO (1.1 Å/cycle) is higher, showing linear growth behavior, than
when using ThALD (0.8 Å/cycle). This result may be attributed to im-
proved chemisorption from energetic plasma radicals. XPS analysis
revealed that both PEALD and ThALD BeO films have similar bandgaps
of 8.0 eV and 7.9 eV, respectively. However, from the C-V curves, the
dielectric constant of PEALD BeO was 7.15 at 1MHz, which is close to
the bulk modulus (6.9) and that of ThALD BeO was 5.18. In addition,
the PEALD films show better dielectric properties of low hysteresis and
frequency dispersion and low leakage current compared with ThALD
films. Therefore, the overall results show that the PEALD technique is
beneficial for growing high quality BeO thin films.
Fig. 5. X-ray photoelectron spectroscopy (XPS) data. O 1s energy-loss spectrum of (a) PEALD BeO films and (b) ThALD BeO films on Si as grown at a substrate
temperature of 250 °C.
Y. Jang, et al. Solid State Electronics 163 (2020) 107661
4
Acknowledgments
This research was supported by Korea Electric Power Corporation
(Grant number 3): R18XA06-03 and by the Ministry of Science and ICT
(MSIT), Korea, under the ICT Consilience Creative Program (IITP-2019-
2017-0-01015), supervised by the Institute for Information &
Communications Technology Promotion (IITP). CWB, JHY, and ESL are
grateful to the Institute for Basic Science (IBS-R019) as well as the BK21
Plus Program funded by the Ministry of Education and the National
Research Foundation of Korea for their support.
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Jungwoo Oh received a Ph.D. from the University of Texas
at Austin in Microelectronic Research Center. He joined
global research Consortium SEMATECH, where he worked
on Non-Si technologies for low power and high perfor-
mance CMOS. Dr. Oh is currently with the college of en-
gineering at Yonsei University since 2012, as a faculty
member in School of Integration Technology (SIT) and
Yonsei Institute of Convergence Technology (YICT).
Yoonseo Jang currently pursuing the Combined MS/Ph.D
degree in School of Integrated Technology, Yonsei
University, Incheon, South Korea. Her research interests are
focused on atomic layer deposition technique and MOS
devices.
Y. Jang, et al. Solid State Electronics 163 (2020) 107661
6