Band alignment of atomic layer deposited (ZrO2)x(SiO 2)1-x gate dielectrics on Si(100)

Article abstract of DOI:10.1063/1.3143223

The band alignment for atomic layer deposited (ZrO2) x (SiO2) 1-x (x=1.0, 0.75, 0.5, 0.25) gate dielectric thin films grown on Si (100) was obtained by using x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. The band gap,

Full text of DOI:10.1063/1.3143223

Band alignment of atomic layer deposited ( ZrO 2 ) x ( SiO 2 ) 1 − x gate dielectrics on
Si (100)
Dahlang Tahir, Eun Kyoung Lee, Suhk Kun Oh, Tran Thi Tham, Hee Jae Kang, Hua Jin, Sung Heo, Ju Chul
Park, Jae Gwan Chung, and Jae Cheol Lee
Citation: Applied Physics Letters 94, 212902 (2009); doi: 10.1063/1.3143223
View online: http://dx.doi.org/10.1063/1.3143223
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APPLIED PHYSICS LETTERS 94, 212902 ͑2009͒
Band alignment of atomic layer deposited „ZrO … „SiO …
gate
2
x
2 1−x
dielectrics on Si „100…
1
,a͒
1
1
1
1,b͒
Dahlang Tahir,
Eun Kyoung Lee, Suhk Kun Oh, Tran Thi Tham, Hee Jae Kang,
3 3 3 3
2
Hua Jin, Sung Heo, Ju Chul Park, Jae Gwan Chung, and Jae Cheol Lee
Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea
Advanced Surface Chemical Analysis Group, National Institute for Materials Science, 1-2-1 Sengen,
1
2
Tsukkuba, Ibaraki 305-0047, Japan
3
Analytical Engineering Center, Samsung Advanced Institute of Technology, Suwun 440-600,
Republic of Korea
͑
Received 8 February 2009; accepted 6 May 2009; published online 27 May 2009͒
The band alignment for atomic layer deposited ͑ZrO ͒ ͑SiO ͒
͑x=1.0, 0.75, 0.5, 0.25͒ gate
2 1−x
2
x
dielectric thin films grown on Si ͑100͒ was obtained by using x-ray photoelectron spectroscopy and
reflection electron energy loss spectroscopy. The band gap, conduction band offset, and the valence
band offset for ZrO thin film were 5.30, 1.83, and 2.35 eV, respectively. These values for Zr
2
silicates with x more than 0.5 are independent of SiO concentration but both holes and electrons
2
have more symmetric barrier heights when x=0.25. The Zr silicate dielectrics are strongly affected
by Zr–O nearest neighbor bonding. © 2009 American Institute of Physics.
͓
DOI: 10.1063/1.3143223͔
The rapid scaling down of silicon-based metal-oxide-
semiconductor field effect transistor ͑MOSFET͒ devices in-
volves a reduction of gate insulator thickness below 2 nm,
where quantum mechanical tunneling current significantly
reduces the insulating behavior of the gate dielectrics. In
order to avoid this undesirable physical phenomenon, several
alternative dielectrics have been proposed and investigated to
tunneling current through the gate oxide. There are two main
parameters for us to consider in reducing the tunneling cur-
rent. The first parameter is the physical thickness of dielec-
trics, and the second parameter is the proper band offset with
respect to Si. Hence, the band alignment is one of the most
fundamental physical properties needed in characterizing the
gate dielectrics. In addition to an adequate large band gap for
alternative high-k materials, asymmetry in barrier height be-
tween holes and electrons is a potentially significant limita-
tion for the operation and reliability of MOSFET devices and
circuits. To get a clear insight into the electrical properties
for thin high-k gate stack on Si, a better understanding for
the chemical and electronic structure of Zr silicates high-k
dielectrics is necessary, e.g., we need to characterize their
electronic structures such as band gap and band offsets on
nanometer-scale spatial resolution. In order to use Zr sili-
cates gate dielectrics materials in complimentary MOSFETs,
they should have sufficiently high tunneling barriers ͑at least
1
replace conventional SiO gate oxides. The basic criteria for
2
gate dielectrics include barrier heights that will effectively
block holes and electrons, chemical stability in contact with
both silicon substrate and gate materials, and a low density
interface electronic states. Among many possible candidates,
transition metal ͑TM͒ oxides Hafnium dioxide ͑HfO ͒ and
2
zirconium dioxide ͑ZrO ͒ with high dielectric constant of
2
about 25–30 are promising candidates for replacing the SiO
2
2
,3
gate dielectric. Even though they have higher dielectric
permittivity and good thermal stability with the Si at higher
temperatures, they tend to crystallize at relatively low tem-
peratures during postdeposition annealing, and hence there is
a definite need for further improvement. For this reason,
amorphous Zr and Hf based complex oxides have been in-
tensively studied because of their composition-tunable struc-
1
eV from silicon͒ of both electrons and holes to inhibit
conduction through Schottky emission of carriers into their
bands. These barriers correspond to conduction and valence
band offsets between the semiconducting silicon substrate
11
4
,5
and insulating dielectrics. The energy band alignment of Zr
compounds with Si is essential in understanding the transport
properties of MOSFET devices based on Zr compound di-
electrics. So far, band gap and band offset values of Zr sili-
cates have not been experimentally investigated adequately.
Hence, we would like to investigate the band gap, conduc-
tures and significant improvement in electrical properties.
Furthermore, the incorporation of SiO into TM oxides lends
2
help for the stabilization of an amorphous TM silicates struc-
ture and for avoiding the formation of low-k interfacial oxide
6
layers. In addition to the TM silicates, rare-earth ͑RE͒ sili-
cates, and aluminates, binary alloy between TM and RE ox-
ide or two different TM or RE oxides have been also inves-
tigated recently to improve properties since the
aforementioned oxides have high-k values and suitable wide
tion and valence band offsets of ͑ZrO ͒ ͑SiO ͒ thin films
2 x
2 1−x
by using reflection electron energy loss spectroscopy
REELS͒ and x-ray photoelectron spectroscopy ͑XPS͒ mea-
surements.
ZrO and ͑ZrO ͒ ͑SiO ͒ thin films were grown on
͑
7
–10
band gaps.
Zr silicates are other widely investigated ma-
terials because they have many attractive features. The pri-
2
2 1−x
2 x
mary goal of replacing SiO by a high-k material is to reduce
p-Si ͑100͒ substrate by atomic layer deposition method. Prior
to growing mixed oxide films, p-type Si substrates were
cleaned using the Radio Corporation of America method.
2
a͒Present address: Department of Physics, Hasanuddin University, Makassar,
Zr͓N͑CH ͒͑CH CH ͔͒ and SiH͓N͑CH3͒2͔3 were used as
90245, Indonesia.
3
2
3
4
b͒
Electronic mail: hjkang@cbu.ac.kr.
precursors for ZrO and SiO , respectively, and O vapor
2 2 3
0
003-6951/2009/94͑21͒/212902/3/$25.00
94, 212902-1
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© 2009 American Institute of Physics
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1

212902-2
Tahir et al.
Appl. Phys. Lett. 94, 212902 ͑2009͒
FIG. 1. ͑Color online͒ Zr 3d core level photoelectron spectra for
ZrO ͒ ͑SiO ͒ ͑x=1, 0.75, 0.5, 0.25͒ dielectrics thin films.
͑
2
x
2 1−x
FIG. 2. ͑Color online͒ Si 2p core level photoelectron spectra for
͑
ZrO ͒ ͑SiO ͒ ͑x=0.75, 0.5, 0.25͒ dielectrics thin films.
2 x 2 1−x
served as oxygen source. The films were grown in N ambi-
2
ance, which was supplied as the purge and carrier gas. The
growing temperature was below 300 °C. The physical thick-
ness of deposition was 7 nm. XPS and REELS spectra were
obtained by using the VG ESCALAB 210. Spectra were
measured using Mg source with the pass energy of 20 eV.
The incident and take off angles of electrons were 55° and 0°
from the surface normal, respectively. XPS binding energies
were referenced to C 1s peak of hydrocarbon contamination
at 285 eV. REELS were measured with the primary electron
energy of 1.5 keV for excitation and with the constant ana-
lyzer pass energy of 20 eV. The full width at half maximum
of the elastic peak was 0.8 eV.
determined from the energy loss spectrum. The method was
described in our previous paper. The obtained band gap
values for these high-k gate dielectric films are shown in
1
4
Table I. In spite of the addition of Si elements in ZrO within
2
certain range, Zr silicate band gap values barely changed.
For ZrO gate oxide, plasmon peaks appear at 8, 15, 26, and
2
1
5
4
2 eV. A strong plasmon peak appears at 15 eV. Another
broad plasmon peak appears around 26 eV away from the
elastic peak center. For ͑ZrO ͒ ͑SiO2͒1−x, the intensity of the
2
x
plasmon peak at 15 eV decreases with reducing Zr content.
This implies that the plasmon peak at 15 eV should be at-
Figure 1 shows the Zr 3d photoelectron core-level spec-
tributed to ZrO , while another plasmon peak at about 23 eV
2
tra for ZrO and Zr silicate thin films. One of Zr 3d spectra is
2
mainly originates from the contribution of SiO . For the
2
4
+
characteristic of fully oxidized state of Zr for ZrO . The
2
SiO -rich Zr silicate, the plasmon peak at about 23 eV takes
2
binding energies of Zr 3d5/2 and Zr 3d3/2 peaks were mea-
advantage of the intensity relative to the contribution from
sured at 182.4 and 184.8 eV, respectively. Two peaks have
ZrO . The REELS spectrum for SiO dielectric shows a
1
2
2
2
the spin-orbital splitting of 2.4 eV. For ͑ZrO ͒ ͑SiO ͒
2
x
2 1−x
broad peak at 23 eV with a shoulder at 15 eV. The width at
thin films, the binding energy of Zr 3d shifts to higher bind-
ing energy with increasing Si contents in silicates. The shift
to higher binding energy of Zr 3d peak is suggestive of Zr
silicate formation. All the Zr 3d core levels show clear peak
the onset of the loss peak for SiO is the widest.
2
The band gap width is defined as the difference between
the bottom of the conduction band and the top of the valence
shift from pure ZrO . For silicates, the core level peaks shift
2
to higher binding energy is due to charge transfer. According
to Pauling’s theory, the charge transfer out of Zr in Zr silicate
is larger than that in ZrO because of the electronegativities
2
of both Si and O being larger than that of Zr.
Figure 2 shows the Si 2p core level spectra for Zr sili-
cate dielectrics. For ͑ZrO ͒ ͑SiO ͒ , the Si 2p peak lo-
2
0.5
2 0.5
cated at 101.9 eV is found to be reduced by 1.4 eV in com-
parison to that of SiO . The Si 2p spectra shift to lower
2
binding energy with increasing Zr content in silicates. The Si
2
p chemical shift depends on the number of second nearest
neighbor Zr atoms. The binding energy shift of these core
level spectra is consistent with an electronegativity equaliza-
tion model based on the relative atomic electronegativities of
1
3
Zr ͑1.33͒, O ͑3.44͒, and Si ͑1.90͒. By investigating the
chemical shifts of Zr 3d, Si 2p core level spectra using XPS,
the Zr compound formation was confirmed.
We made use of the REELS measurement to find the
FIG. 3. ͑Color online͒ Reflection electron energy loss spectra for
band gap. Figure 3 shows REELS spectra for ZrO , Zr sili-
2
͑ZrO ͒ ͑SiO ͒
͑x=1, 0.75, 0.5, 0.25, 0͒ dielectrics thin films with the
2
x
2
1−x
cate and SiO dielectric films. The band gap values were
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primary energies of 1500 eV.
129.21.35.191 On: Mon, 22 Dec 2014 00:11:09

212902-3
Tahir et al.
TABLE I. Band gap and band offsets values for gate oxide films
Appl. Phys. Lett. 94, 212902 ͑2009͒
Energy
eV͒
͑
ZrO2
͑ZrO2͒0.75͑SiO2͒0.25
͑ZrO2͒0.5͑SiO2͒0.5
͑ZrO2͒0.25͑SiO2͒0.75
SiO2
Eg
5.30
2.35
1.83
5.35
2.39
1.84
5.55
2.49
1.94
5.95
2.57
2.26
9.00
4.28
3.60
⌬
⌬
Ev
Ec
band. It is well known that in SiO the lowest conduction
⌬E of ZrO is 1.83 eV and those of Zr silicates are 1.84,
c 2
2
band state is formed from the Si s state, while the highest
valence band state is formed from the O p states. For TM
oxides, the unoccupied orbital is related to the localized d
1.94, and 2.26 eV, respectively, with ZrO /SiO ratio of 3:1,
2
2
1:1, and 1:3. The band alignment parameters for these dielec-
trics are listed in Table I. The conduction band offset for Zr
silicate also has an adequate value to satisfy the requirement
on band alignment.
states. In contrast to SiO , the band gap of Zr silicate is
2
reduced. The obtained band gap values suggest that the Zr-
introduced localized d electron state is located below the
For Zr silicates, when x is close to 0.5, the bottom of the
conduction band is mainly composed of Zr 4d orbitals. In Zr
silicates with the incorporation of SiO2 within a certain
range, the band gap does not change with the variation in Si
composition. The conduction band offset slightly increased
with increasing the Si contents, which is desirable for an
ideal gate dielectric.
In summary, we investigated the electronic structures
and chemical states for Zr silicates to find suitable dielectrics
with optimal band alignments via REELS and XPS analysis.
The investigation provided a useful guideline for the selec-
original conduction band edge of SiO . The band gap values
2
are almost independent of SiO concentration in Zr silicates
2
with x more than 0.5, and they are close to the theoretical
1
6
results obtained by others.
To obtain the valence band offset at the dielectric/Si in-
terface, the valence band spectra were measured to determine
the valence band maximum ͑VBM͒. The spectra are shown
in Fig. 4. The VBM is determined from the intersection of
two straight lines, where one line fits the valence band lead-
ing edge while the other line fits the background as shown in
Fig. 4. For ZrO film, the spectral intensity is spread over the
2
tion of an alternative replacement for SiO . The results
2
binding energies of 5–8 eV with the introduction of SiO2,
and the spectral intensity shifts to a higher binding energy.
From the energy difference between the VBM of gate dielec-
tric and that of Si, the valence band offset was obtained. The
VBM of Si is positioned at 0.24 eV. The valence band offset
increased with the introduction of SiO in ZrO , and became
showed that the band gap barely changes but the conduction
band barrier heights were adjusted by the variation of Zr
contents in Zr silicates.
This work was supported by the Korea Research Foun-
dation grant funded by the Korean Government ͑MOEHRD,
Basic Research Promotion Fund͒ ͑Grant No. KRF-2008-313-
C00225͒.
2
2
larger with more SiO . The valence band offsets ͑⌬E ͒ are
2
v
2
.35, 2.39, 2.49, and 2.57 eV for ZrO , ͑ZrO ͒ ͑SiO ͒
,
2
2 0.75
2 0.25
͑
ZrO ͒ ͑SiO ͒ , and ͑ZrO ͒ ͑SiO2͒0.75, respectively.
2 0.5 2 0.5 2 0.25
1
The band gap and valence band offset allow us to deter-
mine the conduction band offset ͑⌬E ͒ through the
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FIG. 4. ͑Color online͒ Valence band spectra for ͑ZrO ͒ ͑SiO ͒ ͑x=1,
2 1−x
2
x
0.75, 0.5, 0.25, 0͒ dielectrics thin films.
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