Interfacial compound suppression and dielectric properties enhancement of F-N codoped Zr O2 thin films

Article abstract of DOI:10.1063/1.2709916

Fluorine and nitrogen codoped Zr O2 is produced on p -type Si (100) wafers by cathodic arc deposition and the interfacial and dielectric characteristics of the thin films are investigated. F-N codoping is found to effectively suppress the interfacial comp

Full text of DOI:10.1063/1.2709916

Interfacial compound suppression and dielectric properties enhancement of F–N
codoped Zr O 2 thin films
A. P. Huang and Paul K. Chu
Citation: Applied Physics Letters 90, 082906 (2007); doi: 10.1063/1.2709916
View online: http://dx.doi.org/10.1063/1.2709916
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APPLIED PHYSICS LETTERS 90, 082906 ͑2007͒
Interfacial compound suppression and dielectric properties enhancement
of F–N codoped ZrO2 thin films
A. P. Huang
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon,
Hong Kong and Department of Physics, Beihang University, Beijing 100083, China
Paul K. Chua͒
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon,
Hong Kong
͑Received 7 January 2007; accepted 25 January 2007; published online 21 February 2007͒
Fluorine and nitrogen codoped ZrO2 is produced on p-type Si ͑100͒ wafers by cathodic arc
deposition and the interfacial and dielectric characteristics of the thin films are investigated. F–N
codoping is found to effectively suppress the interfacial compounds between ZrO2 and silicon and
the dielectric properties are also improved. Negligible flatband shift and hysteresis are achieved,
implying that the fixed charge centers in the thin films and the interfacial states are obviously
reduced. The improvement can be attributed in part to the large electronegativity of F radicals that
are chemically more active. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2709916͔
Aggressive downscaling of complementary metal-oxide-
semiconductor transistors has led to silicon dioxide/nitride-
based gate dielectric layers that are so thin that the leakage
current can become very serious, possibly making these de-
vices inoperable. It is thus necessary to replace the SiO2 with
a physically thicker layer of oxide or one with a high dielec-
tric constant ͑k͒.1,2 Recently, various kinds of high-k materi-
als have been extensively studied to reduce the leakage cur-
rent. Among them, zirconia ͑ZrO2͒ is considered as one of
the most promising alternative materials due to its high per-
mittivity and good thermodynamic stability in contact with
silicon. The materials have been studied as storage capacitors
in dynamic random access memories and gate oxide layer in
field effect transistors.3,4 However, undesirable crystalliza-
tion of ZrO2 at temperatures below 400 °C and the high
defect density which can cause threshold voltage instability
and electron mobility degradation limit the applications of
the thin films in ultralarge-scale integrated circuits.5 More-
over, because of the higher coordination number compared to
SiO2, intrinsic defects in most high-k oxides can adversely
influence the dielectric properties particularly when the films
become ultrathin. It is known that oxygen vacancies and in-
terstitials are two major defects in high-k oxides.6 Therefore,
improvement of the thermal stability and removal of intrinsic
defects have become the major concerns for the use of ZrO2
as gate dielectrics. Nitrogen incorporation has been found to
be effective in improving the thermal stability and reducing
the gate leakage current. Unfortunately, N, when incorpo-
rated into high-k oxides, is generally present in various
chemical states such that N2 gas in the molecular state can be
generated in the thin films.7,8 Recently, the beneficial effects
introduced by fluorination on the dielectric properties have
been reported based on first principle calculation and the
underlying reason is believed to be the high electronegativity
of fluorine that can efficiently passivate the neutral oxygen
vacancy in threefold- and fourfold-coordinated sites in high-
k oxides.9 Furthermore, it has been reported that fluorination
can modify the interface energetics and charge reorganiza-
tion energy in pentacene/Au.10
In this work, F–N codoped ZrO2 films are fabricated on
p-type Si ͑100͒ wafers by cathodic arc deposition and the
interfacial and dielectric characteristics of the thin films are
evaluated. Our results reveal that although incorporation of a
small amount of N increases the thermal stability of ZrO2,
the interfacial states and defect density in the thin films can-
not be well suppressed. However, in the F–N codoped ZrO2,
the interlayer can be controlled resulting in negligible flat-
band shift and hysteresis. It implies that fixed charge centers
in the thin films are significantly reduced.
ZrO2 thin films with thickness of 25–80 nm were fabri-
cated on p-type, 100 mm Si ͑100͒ wafers with resistivity
of 4–7 ⍀ cm using plasma immersion ion nitridation and
fluorination in conjunction with cathodic arc deposition.
The cathode was 99.9% pure Zr with a diameter of 1 cm and
the base pressure in the vacuum chamber was about 1
ϫ10−7 Torr. N2 and O2 with the ratio of 5:5 and HF vapor
were introduced into the vacuum chamber to maintain a
working pressure of 5ϫ10−4 Torr. Microstructural analyses
were carried out using a Perkin Elmer Fourier transform in-
frared ͑FTIR͒ spectrometer. The chemical composition and
FIG. 1. FTIR spectra of ZrO2 thin films prepared using different working
gases: ͑a͒ pure oxygen, ͑b͒ oxygen and nitrogen, and ͑c͒ oxygen, nitrogen,
and HF.
a͒
Author to whom correspondence should be addressed; FAX: 852-
27889549; electronic mail: paul.chu@cityu.edu.hk
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082906-2
A. P. Huang and P. K. Chu
Appl. Phys. Lett. 90, 082906 ͑2007͒
with the addition of nitrogen, the interfacial structure of the
ZrO2/Si thin films can be improved. The Si–O absorption
peaks almost disappear in Fig. 1͑c͒, obtained from sample
͑c͒. This desirable result means that plasma fluorination is an
effective method to control the interfacial compound. This
may be attributed to the fluorine atom with seven valence
electrons, thereby making it easy to gain an additional elec-
tron to form a closed shell. At the interface, the fluorine atom
can enhance the reactivity with silicon and further suppress
the formation of Si–O bonds. Furthermore, no band gap nar-
rowing and band offset reduction are observed after fluorine
incorporation into HfO2 because the fluorine 2p states are far
below the valence band edge.9 It also indicates that by con-
ducting plasma immersion ion nitridation in conjunction with
fluorination, the interfacial layer with low permittivity can be
effectively suppressed, improving the dielectric properties of
the thin films.
The composition of the thin films and the elemental
chemical bonding states are characterized by XPS. The
sample surfaces were precleaned by 4 keV Ar ion bombard-
ment for 5 nm to remove surface contaminants. The XPS
spectra obtained from the sample prepared using different
working gases show that only Zr and O are detected together
with a trace of N and/or F from the nitridated and/or fluori-
nated samples. The atomic concentration of N in sample ͑b͒
is about 0.5% and those of N and F in sample ͑c͒ are about
0.3% and 0.8%, respectively. RBS results show that the com-
position of the thin films produced using different working
gases is basically uniform with depth and that N and F–N
codoping has been successfully conducted. Figures 2͑a͒–2͑c͒
show the Zr binding energy XPS spectra of ZrO2 deposited
using different working gases. The Zr 3d core-level spectrum
of the plasma nitridated and/or fluorinated ZrO2 in Figs. 2͑b͒
and 2͑c͒ only shows a Zr 3d peak at a binding energy of
182.8 eV, corresponding to the Zr–O bond in bulk ZrO2,
FIG. 2. Zr XPS spectra of ZrO2 deposited using different working gases.
binding energy were determined by Rutherford backscatter-
ing spectrometry ͑RBS͒ and x-ray photoelectron spectros-
copy ͑XPS͒ employing monochromatic Al K␣ radiation. An
HP4284A precision LCR meter and HP4140B picoampere
meter were employed in the C-V and leakage current density
͑J-V͒ measurements, respectively.
Figure 1 shows the FTIR spectra of ZrO2 thin films de-
posited using different working gases. The thin film fabri-
cated with pure oxygen is denoted ͑a͒, that prepared using
oxygen plus nitrogen is designated ͑b͒, and that using oxy-
gen, nitrogen, and HF is labeled ͑c͒. Figure 1͑a͒ shows that
sample ͑a͒ exhibits strong absorption peaks located near 580,
510, and 420 cm−1 corresponding to the Zr–O vibrational
modes. Furthermore, the obvious absorption peak at around
1080 cm−1 of the Si–O vibration mode is found in addition to
the weak absorption bands at around 720 cm−1 of Zr–O–Si,
implying the formation of little SiOx and ZrSiOx at the
interface.11 As shown in Fig. 1͑b͒, acquired from sample ͑b͒,
the Si–O absorption peaks obviously weaken indicating that
FIG. 3. ͓͑a͒–͑c͔͒ Zr 3d core-level XPS depth profiles acquired from samples fabricated using different working gases shown in Fig. 1; ͓͑d͒ and ͑e͔͒ F 1s and
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N 1s core-level XPS depth profiles at higher resolution from sample ͑c͒.
On: Mon, 22 Dec 2014 17:28:20

082906-3
A. P. Huang and P. K. Chu
Appl. Phys. Lett. 90, 082906 ͑2007͒
ϫ10−4 cm2 are fabricated with Au gate electrodes. Alumi-
num is deposited on the back side of the MOS capacitors for
better contact. The C-V characteristics of the Au
Al/ZrO2/p-Si MOS capacitors are displayed in Fig. 4 and
the inset in Fig. 4 is the corresponding J-V curve acquired by
scanning from a positive bias ͑+3 V͒ to a negative bias ͑
−3 V͒. It can be seen that the flatband in the C-V curves
shifts towards the negative voltage direction after plasma ni-
tridation. Negligible flatband voltage shift is observed from
the F–N codoped sample. Furthermore, the F–N codoped
ZrO2 shows no obvious hysteresis in the forward and reverse
C-V curves. It is well known that trapped charges at the
interface and fixed charges in the oxide can induce the flat
band voltage shift and loop hysteresis.15 Our results again
demonstrate that F–N codoping improves the interface prop-
erties and also reduces the fixed charge trapping centers in
the thin film in comparison with ZrO2 oxidized in pure oxy-
gen. This further suggests that intrinsic defects in the sample
can be removed by F–N codoping. As a result, the leakage
current density drastically diminishes, as shown by the leak-
age current density voltage ͑J-V͒ curve in Fig. 4.
In summary, the interfacial and dielectric properties be-
tween ZrO2 and silicon were investigated by F–N codoping.
XPS depth profiles indicate that F and N can be effectively
introduced into the ZrO2 sample using plasma nitridation and
fluorination in conjunction with cathodic arc deposition. For-
mation of the interfacial compound is well suppressed and
superior dielectric properties for F–N codoping sample such
as negligible hysteresis and flatband shift and low leakage
current density are observed. The improvement stems from
the reduction in the number of fixed charge centers and in-
trinsic defects in the thin films.
FIG. 4. C-V curves of Au/ZrO2 /Si MOS capacitors prepared using different
working gases measured at a frequency of 1 MHz. The leakage current
density as a function of voltage is shown in the insert.
whereas in pure oxidized ZrO2, there is a noticeable but
small shoulder ͑highlighted by the arrows͒ at the lower bind-
ing energy side of the main peak, as shown in Fig. 2͑a͒. It
can be attributed to Zr–Si bond.12 Our experimental results
illustrate unequivocally that plasma nitridation and/or fluori-
nation can significantly suppress the formation of silicates in
the thin films.
To further determine the compound composition near the
interface, Zr 3d core-level XPS depth profiles acquired from
samples produced employing different working gases are
shown in Figs. 3͑a͒–3͑c͒. Figure 3͑a͒ shows a sizable shift of
about 0.75 eV in the Zr 3d peak in the pure oxidized sample.
This shift can be assigned to the formation of Zr silicate.
Meanwhile, strong Zr–Si bonds are detected at even a lower
binding energy of about 178.5 eV at larger depths. This fur-
ther corroborates the formation of both Zr–silicate and Zr–
silicide near the interface after oxidation. Similar phenomena
were observed in our previous study on HfO2/Si.13 With
regard to the Zr 3d peak of the plasma nitridated ZrO2, al-
though a small shift can be observed and the Zr–Si bonds are
also found, the intensity obviously weakens from the bulk to
the interface as shown in Fig. 3͑b͒. This means that smaller
amounts of Zr silicides are formed at the interface by plasma
nitridation. In the nitridated and fluorinated sample ͑c͒, the
shift of the main peak and the Zr–Si bonds at lower binding
energy are almost not detectable, as shown in Fig. 3͑c͒. This
suggests that F–N codoping suppresses the formation of sili-
cate and silicide at the interface, which is in agreement with
the FTIR results in Fig. 1. This may be attributed in part to
the high electronegativity and small volume of F radicals
which are chemically more active leading to important inter-
molecular neighboring effects.14 The F 1s and N 1s core-
level XPS depth profiles obtained from sample ͑c͒ at higher
resolution are depicted in Figs. 3͑d͒ and 3͑e͒. It can be seen
from Fig. 3͑e͒ that only N 1s bond is found at binding ener-
gies of about 396 eV, implying chemical doping of N into
ZrO2 but no N2 gas in the molecular state. The F 1s depth
profile shows a similar phenomenon.
This work was financially supported by City University
of Hong Kong Strategic Research Grant ͑SRG͒ 7001981.
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