APPLIED PHYSICS LETTERS 93, 122907 共2008兲
Role of fluorine in plasma nitridated ZrO2 thin films under irradiation
A. P. Huang,1,2,a兲 Z. S. Xiao,1 X. Y. Liu,2,3 L. Wang,2 and Paul K. Chu2,b兲
1
Department of Physics, School of Science, Beihang University, Beijing 100083, China
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon,
Hong Kong, China
3
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China
2
共Received 1 August 2008; accepted 5 September 2008; published online 26 September 2008兲
The role of fluorine in plasma-nitridated ZrO2 thin films under electron irradiation is investigated in
situ by real-time high-resolution transmission electron microscopy. Fluorine and nitrogen codoping
can suppress the microstructure evolution during electron beam bombardment and the
corresponding origin is probed and verified. The results obtained by irradiation with an ultraviolet
laser show that plasma fluorination can effectively remove the dissociative N or O particles in the
ZrO2 thin films which can escape from the interstitial sites under electron irradiation. The
mechanism of the irradiation stability of the F and N codoped ZrO2 thin film is also discussed.
© 2008 American Institute of Physics. 关DOI: 10.1063/1.2991445兴
Various kinds of high-k materials such as Zr-based and
Hf-based gate dielectrics are intensively studied in order to
reduce the leakage current because of their high dielectric
constant and wide band gap.1–3 Recent studies indicate that
the O vacancy 共VO兲 in high-k gate dielectrics plays a crucial
role in the electron leakage current and much attention has
been paid to the suppression of VO formation.4 Nitrogen incorporation is a widely accepted technique to reduce the
leakage current by deactivating the VO related gap states.5,6
In addition, the beneficial effects introduced by plasma nitridation and fluorination on interfacial compound suppression
and dielectric properties have been reported.7 The thermodynamic stability after high temperature postdeposition annealing can be characterized by high-resolution transmission
electron microscopy 共HRTEM兲. Nevertheless, the microstructure stability of nitrogen-doped ZrO2 or HfO2 gate dielectrics during electron beam bombardment or irradiation
has seldom been studied although the microstructure evolution as well as polycrystalline nanoarray formation in
nitrogen-doped ZrO2 or HfO2 gate dielectrics under electron
irradiation have been observed.8,9 Existence of suboxides introduced by nitrogen and/or fluorine doping tends to enhance
oxygen deficiency in the films which may change the states
of the band gap. It is thus of both theoretical and practical
interests to investigate the role of fluorine in plasmanitridated ZrO2 thin films under irradiation.10,11 The underlying mechanism by which the presence of fluorine affects the
microstructure stability during electron beam irradiation is of
practical interest but has not yet been investigated in detail.
In this study, the stability of F and N codoped ZrO2 thin
films under electron bombardment was observed in situ by
real-time HRTEM. It was found that incorporation of a small
amount of fluorine in plasma-nitridated ZrO2 could suppress
the microstructure evolution during electron beam irradiation. Irradiation with an ultraviolet laser at room temperature
gives rise to intense visible photoluminescence 共PL兲 spectra
from the F and N codoped ZrO2 thin film. It means that
a兲
FAX: 86-10-82317935. Tel.: 86-10-82317935. Electronic mail:
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Author to whom correspondence should be addressed. FAX: 85227887830, Tel: 852-27887724. Electronic mail: [email protected].
b兲
0003-6951/2008/93共12兲/122907/3/$23.00
plasma fluorination can change the states of band gap which
can result from effectively removing of the dissociative N or
O particles in the ZrO2 thin films.
ZrO2 thin films with a thickness of about 30 nm were
fabricated on p-type Si 共100兲 wafers using plasma immersion
ion nitridation and fluorination and the substrate temperature
was fixed at 450 ° C.7 The microstructure evolution of the
plasma-nitridated ZrO2 and F and N codoped ZrO2 samples
was assessed in real time in a HRTEM 共Tecnai 20 ST FEG
200 keV system兲 with an energy resolution of 0.7 eV. The
electron beam flux through the sample was about 4
⫻ 1026 electrons/ m2 s. The chemical composition and binding energy of the nitridated and F and N codoped samples
were determined by x-ray photoelectron spectroscopy 共XPS兲
employing monochromatic Al K␣ radiation. Prior to the XPS
analysis, the sample surface was cleaned by 4 kV Ar ion
bombardment for 1 min to remove surface contaminants.
The PL spectra were recorded at room temperature by a
spectrofluorophotometer of Shimadzu RF-5301 and the
250 nm line of the Xe lamp was used as the excitation wavelength.
A series of HRTEM micrographs taken from the same
region at 10 s intervals is displayed in Figs. 1共a兲–1共d兲 to
illustrate the self-crystallization and regrowth of the nanoparticles in the plasma-nitridated sample. It has been observed
that N doping affects the structure of Zr-based and Hf-based
amorphous gate dielectrics.8 However, the underlying
mechanism is not yet clear. In order to further fathom the
mechanism of the nanoparticle regrowth under electron beam
irradiation, F and N codoped ZrO2 films are also prepared on
Si wafers with a 5 nm SiO2 buffer layer by cathodic arc
deposition. The buffer layer enables the formation of the
amorphous ZrO2 structure. The microstructure of the F and
N codoped ZrO2 thin film is observed by HRTEM under the
same conditions and the results are shown in Figs. 1共e兲 and
1共f兲 at t0 and 共t0 + 20 s兲, respectively. In comparison, realtime evolution of the microstructure is not observed from the
amorphous F and N codoped ZrO2 sample. This phenomenon is the same as that observed from the amorphous HfO2
sample without nitridation.8 It means that incorporation of a
small amount of fluorine in plasma-nitridated ZrO2 improves
the stability under electron beam irradiation. According to
93, 122907-1
© 2008 American Institute of Physics
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Appl. Phys. Lett. 93, 122907 共2008兲
Huang et al.
(a)
0.3 nm
(b)
A
B
Zr-O-Si
Si(100)
(c)
(d)
Intensity (a. u.)
Zr 3d
(a)
(b)
(c)
192
B
188
184
180
176
Binding Energy (eV)
(e)
(f)
SiO2 buffer layer
N1s
Intensity (a.u.)
A
(b)
FIG. 1. Real-time HRTEM micrographs taken from the plasma-nitridated
ZrO2 sample after different periods of electron bombardment: 共a兲 t0, 共b兲 t0
+ 10 s, 共c兲 t0 + 20 s, and 共d兲 t0 + 30 s. 共e兲 and 共f兲 are that from the F and N
codoped ZrO2 sample at t0 and t0 + 20 s. The t0 represents the time the
HRTEM observation begins after electron beam focusing that takes less than
3 s.
404
402
400
398
396
394
Binding Energy (eV)
F 1s
Intensity (a.u.)
previous reports, nitrogen doping in high-k gate dielectrics
can greatly suppress film crystallization.12,13 However,
plasma nitridation alone cannot deter the microstructure alteration under electron beam bombardment which can influence the thermal stability and dielectric properties of high-k
gate oxides. Thus, F and N codoping can be an effective way
to improve the microstructure thereby boding well for applications of high-k gate oxides.
The XPS spectra obtained from the samples prepared
using different working gases show only Zr and O together
with a trace of N and/or F from the nitridated and/or fluorinated samples. The atomic concentration of N in sample b is
about 0.5% and those of N and F in sample c are about 0.3%
and 0.8%, respectively. Figure 2 depicts the Zr3d, N1s, and
F1s core-level XPS spectra of the ZrO2 samples prepared at
different working gases, respectively. A noticeable but small
shoulder 共indicated by the arrows兲 at the lower binding energy side of the main peak from the Zr 3d core-level spectra
in Fig. 2 shows the existence of Zr–Si bond in the pure ZrO2
oxidized under oxygen. This phenomenon is not observed
from the spectra of plasma nitridated or F and N codoped
samples, illustrating that plasma nitridation and/or fluorination can significantly suppress the formation of silicates in
the thin films. Comparing the N1s core-level spectra acquired
from the nitridated and F and N codoped samples, the
401 eV peak is obviously more intense than the 397 eV one,
suggesting that the nitrogen atoms are mostly located at interstitial sites in the nitridated samples. With regard to the F
and N codoped sample, the 401 eV peak almost disappears
and only a peak located at 397 eV is observed implying that
no or very few molecular nitrogen exists in the sample induced by plasma fluorination. Under electron beam irradiation, the atoms can obtain the energy from greater activation
of pre-existing embryos leading to faster nucleation due to
(a)
(c)
690
688
686
684
682
Binding Energy (eV)
FIG. 2. Zr 3d, N 1s, and F 1s core-level XPS spectra acquired from samples
fabricated using different working gases: 共a兲 pure oxygen, 共b兲 plasmanitridated sample, and 共c兲 F and N codoped.
higher atomic mobility. Meanwhile, since the electron beam
can penetrate the sample, it can induce the escape of dissociated N in the interstitial sites thereby reducing the inhibiting effects on the periodic arrangement of the crystal growth.
The change in the local chemical states such as a few nearest
neighbor exchanges among equivalent sites can cause chemical bond breakage and valence state change of the substitutional N under electron irradiation altering their mean atomic
coordination and eventually leading to the microstructure
transformation. On the other hand, the F 1s core-level XPS
spectrum obtained from F and N codoped sample in Fig. 2
shows that a small amount of fluorine has been incorporated
into the plasma-nitridated sample and chemically reacts with
ZrO2 further influencing the states of its band gap. The neutral oxygen or partly dissociated nitrogen can be replaced by
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Appl. Phys. Lett. 93, 122907 共2008兲
Huang et al.
FIG. 3. PL spectra acquired from ZrO2 thin films prepared by different
working gases: 共a兲 pure oxygen, 共b兲 N2 + O2, and 共c兲, HF vapor+ N2 + O2.
fluorine. This may be attributed in part to the high electronegativity and small volume of F radicals which are chemically
more active leading to important intermolecular neighboring
effects.14 It can mitigate escape of the particles with a negative charge such as N induced by electron beam irradiation
and suppress the microstructural transformation. It enhances
the microstructure stability under electron beam irradiation
as corroborated by the HRTEM results. Our results demonstrate that the stability of high-k oxides under electron irradiation can be achieved by F and N codoping.
F and N codoping enhances oxygen deficiency in the
thin film and may change the states of band gap. Roomtemperature PL spectra are acquired from the ZrO2 samples
deposited using different working gases using a spectrofluorophotometer of Shimadzu RF-5301 with the 250 nm line as
the excitation wavelength. The PL spectra of the ZrO2 thin
film, plasma nitridated, and F and N codoped samples are
displayed in Figs. 3共a兲–3共c兲, respectively. As shown in Fig.
3共a兲, only a strong PL band at 370 nm can be observed
which can be attributed to the intrinsic emission from ZrO2
thin film due to defects such as oxygen vacancies.15 After a
small amount of nitrogen has been introduced, a broad PL
band centered at about 410 nm appears in the visible range,
as shown in Fig. 3共b兲. This shows that the plasma-nitridated
ZrO2 thin film can emit visible lights but with relatively low
intensity. This is consistent with the report by Jeon et al.,16
who found that incorporation of nitrogen into high-k gate
oxides leads to band gap narrowing. It is known that the
conduction and valence bands of ZrO2 are attributed to the
Zr 4d and O 2p orbitals, respectively. In pure ZrO2, the
valence band is mainly formed by the O 2p states. After N
incorporation, the N atoms in the ZrO2 thin films replace
some of the O atoms forming Zr–N bonds and subsequently,
the highest valence band is formed mainly by the N 2p states
which are above the O 2p states. That is, the N 2p orbital has
higher potential energies than O 2p. Therefore, increased N
incorporation should result in a higher negative potential of
the valence band and a smaller band gap compared to that in
pure ZrO2. When the plasma-nitridated sample is further flu-
orinated, the visible PL intensity increases significantly and
the position of the PL peak shifts to a longer wavelength of
430 nm which can result from the higher negative potential
of F 2p instead of N 2p. According to the asymmetric characteristics of the visible PL peak, the best Gaussian fit of the
PL band yields two peaks situated at 430 and 470 nm which
are denoted by the arrows in Fig. 3共c兲. Furthermore, another
PL peak located at short wavelength of 320 nm is observed
from Fig. 3共c兲. It can result from the effects of plasma fluorination on the replacement of neutral oxygen or partly dissociated nitrogen which can induce the impurity energy level
in the band gap of F and N codoped ZrO2. This further
indicates that plasma fluorination can effectively remove the
dissociative N or O particles in the ZrO2 thin films which can
escape from the interstitial sites under electron beam irradiation.
In conclusion, the role of fluorine in plasma-nitridated
ZrO2 thin films under irradiation was investigated. Our results show that fluorine and nitrogen codoping can suppress
the microstructure evolution during electron beam irradiation
and the dissociative N or O particles in the ZrO2 thin films
can be effectively replaced. It indicates that incorporation of
a small amount of fluorine improves the electron beam irradiation stability and reduces the impurity energy level from
dissociative N or O particles in plasma-nitridated ZrO2.
Our work was financially supported by Hong Kong Research Grants Council 共RGC兲 General Research Funds
共GRF兲 No. CityU 112307, National Natural Science Foundation of China 共Grant Nos. 50802005 and 10604003兲, and
Beijing Nova Program 共Grant No. 2006B15兲 from Beijing
Municipal Science and Technology Commission, and Program for New Century Excellent Talents in University
共NCET-07-0045兲.
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