Spectroscopic ellipsometry study of thin NiO films grown on Si (100) by atomic layer deposition

Article abstract of DOI:10.1063/1.2938697

Thin NiO films are grown at 300 °C on Si (100) using atomic layer deposition. The dependence of annealing temperature on the optical properties of NiO films has been investigated using spectroscopic ellipsometry in the spectral region of 1.24-5.05 eV. It

Full text of DOI:10.1063/1.2938697

Spectroscopic ellipsometry study of thin NiO films grown on Si (100) by
atomic layer deposition
H. L. Lu, G. Scarel, M. Alia, M. Fanciulli, Shi-Jin Ding et al.
Citation: Appl. Phys. Lett. 92, 222907 (2008); doi: 10.1063/1.2938697
View online: http://dx.doi.org/10.1063/1.2938697
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APPLIED PHYSICS LETTERS 92, 222907 ͑2008͒
Spectroscopic ellipsometry study of thin NiO films grown
on Si „100… by atomic layer deposition
H. L. Lu,1,2,a͒ G. Scarel,2 M. Alia,2 M. Fanciulli,2 Shi-Jin Ding,1 and David Wei Zhang1,b͒
1State Key Laboratory of ASIC and System, Department of Microelectronics, Fudan University,
Shanghai 200433, People’s Republic of China
2Laboratorio Nazionale MDM-INFM, Via C. Olivetti 2, 20041 Agrate Brianza (MI), Italy
͑Received 6 April 2008; accepted 10 May 2008; published online 5 June 2008͒
Thin NiO films are grown at 300 °C on Si ͑100͒ using atomic layer deposition. The dependence of
annealing temperature on the optical properties of NiO films has been investigated using
spectroscopic ellipsometry in the spectral region of 1.24–5.05 eV. It is found that the refractive
index and thickness of NiO films are affected by high temperature annealing. The optical band gap
of the as-deposited thin NiO film is determined to be 3.8 eV, which is almost independent of the
annealing temperature. The indirect band gap of NiO film shifts toward lower photon energy with
an increase in annealing temperature. © 2008 American Institute of Physics.
͓DOI: 10.1063/1.2938697͔
NiO films have various interesting properties, such as
excellent chemical stability, good crystallinity, transparency
with low resistivity, and controllable transmittance for inci-
dent visible light.1,2 All these desirable properties have ren-
dered NiO film an attractive material for use as the antifer-
romagnetic layers of spin valve films, p-type transparent
conducting films, and electrochromic devices.3 In addition,
NiO films have also shown good resistance switching, such
as large resistance change, induced by external electric
fields.4,5 Therefore, the growth of nickel oxide films has re-
cently received great attention due to their potential applica-
tions for nonvolatile resistance random access memory de-
vices. In spite of these applications, very few reports have so
far been published on optical properties of NiO films. In this
letter, spectroscopic ellipsometry ͑SE͒ has been employed to
investigate the influence of thermal annealing on optical
properties of thin NiO films grown by atomic layer deposi-
tion ͑ALD͒.
range from 1.24 to 5.05 eV. The ellipsometric angle ␺ and
phase difference ⌬ were recorded at an incidence angle of
75°. The Tauc–Lorentz ͑TL͒ dispersion function6,7 is em-
ployed to characterize the dielectric function of the NiO
films, which is expressed as follows:
2
AE0C͑E − Eg͒
1
2 E
,
͑E Ͼ Eg͒
͑E ഛ Eg͒
2
͑E2 − E0͒2 + C2E
␧ ͑E͒ =
͑1͒
2
Ά
·
0,
and
ϱ
2
␰␧ ͑␰͒
2
␧ ͑E͒ = ␧ϱ +
1
P
d␰,
͑2͒
͵
␲
␰2 − E2
E
g
where A is the amplitude, E0 is the peak transition energy, C
is the broadening term, and Eg is the band gap. A simple
four-phase model consisting of substrate/SiO2 interfacial
layer/thin NiO film/surface rough layer has been used to rep-
resent the sample. The surface rough layer is modeled by the
Bruggeman effective medium approximation8 of 50% NiO
and 50% voids. The variables in the fitting procedure include
the layer thickness and all TL parameters. The fitting quality
Thin NiO films were deposited at 300 °C on n-type Si
͑100͒ by alternate pulsing Ni͑Cp͒ ͑Cp=cyclopentadienyl,
2
C5H5͒ and O3 in an ALD F-120 ͑ASM-Microchemistry,
Ltd.͒ reactor. Before being loaded into the ALD reactor,
the Si substrates were first cleaned using SC-2
͑HCl:H2O2:H2O=1:1:5, 85 °C, 10 min͒ solutions and then
dipped into a dilute HF solution ͑HF:H2O=1:50͒ to produce
H-terminated surfaces. Inside the reactor, Ni͑Cp͒ were
2
evaporated from an open boat at a source temperature 40 °C.
The oxygen source O3 was generated from O2 ͑99.999%͒ in
an ozone generator and fed into the reactor at a mass flow
flux of 400 SCCM ͑SCCM denotes cubic centimeter per
minute at STP͒. After deposition, some prepared NiO
samples were then subjected to a rapid thermal annealing
͑RTA͒ at temperatures ranging from 500 to 700 °C for 60 s
in nitrogen ambient.
Optical properties of thin NiO films were characterized
by SE method. A spectroscopic ellipsometer ͑J. A. Woollam,
Co., M-2000͒ was used to acquire spectra in the visible-UV
a͒
Author to whom correspondence should be addressed. Tel./FAX:
FIG. 1. ͑Color online͒ Experimental ͑solid curves͒ and fitted ͑symbols͒
ellipsometric data, ⌬ and ␺, of thin NiO films: ͑open͒ as-deposited and
͑filled͒ 700 °C annealed.
ϩ862165642389. Electronic mail: honglianglu@fudan.edu.cn.
b͒
Electronic mail: dwzhang@fudan.edu.cn
0003-6951/2008/92͑22͒/222907/3/$23.00
92, 222907-1
© 2008 American Institute of Physics
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222907-2
Lu et al.
Appl. Phys. Lett. 92, 222907 ͑2008͒
TABLE I. Compilation of the fitted results for all the samples using double Tauc–Lorentz dispersion function. tf, ts, and tIL are the thicknesses of NiO film,
surface rough layer, and SiO2 interfacial layer with units of nanometers, respectively. ␴ is an unbiased estimator.
Sample
tf
ts
tIL
␧
A1
Eg1
E01
C1
A2
Eg2
E02
C2
␴
ϱ
As-deposited
500 °C
27.08
26.86
26.65
25.99
2.08
1.56
1.02
0.70
2.68
2.73
2.92
3.25
2.50
2.07
1.24
0.84
111.89
117.09
121.79
141.88
3.30
3.31
3.33
3.36
3.96
3.97
3.96
3.95
0.82
0.82
0.79
0.79
59.10
77.47
70.41
66.89
2.53
2.58
2.29
2.05
5.09
5.27
6.70
8.17
5.83
7.46
1.32
1.61
1.48
2.25
600 °C
9.47
700 °C
11.56
is poor when simple TL dispersion function is used, indicat-
ing that more than one single type of electronic transition
dominates the optical absorption in the ultraviolet region for
NiO films. Double TL dispersion function is then applied to
fit the measured ellipsometric data in the present work.
Figure 1 shows the experimental ͑solid curves͒ and fitted
͑symbols͒ spectroscopic spectra of ␺ and ⌬ as a function of
the photon energy for as-deposited and 700 °C annealed NiO
films. The similar spectra for the samples annealed at 500
and 600 °C are not shown here. It can be seen that a good
agreement between the experimental and fitted spectra is ob-
tained except in the range of 4.75–5.05 eV. This means that
the double TL dispersion function works well and the physi-
cal and optical properties of the NiO films can be exactly
determined by the best-fitted results. Table I summarizes all
of the layer thicknesses and the TL parameter values ob-
tained from data fitting with a 90% confidence limit.
The extracted optical constants n and k for as-deposited
and annealed NiO films are presented in Fig. 2. The energy
dependence of n and k is similar to those reported for NiO
single crystal1 and films.9,10 The value of n is 2.37 at the
photon energy of 2 eV for as-deposited sample, which is in
good agreement with the value reported by Powell and
Spicer.1 The n values of NiO film annealed at 500 °C are
similar to those of as-deposited sample. For the samples an-
nealed at temperature above 500 °C, it is found that n values
decrease by around 0.2 in the photon energy region from
1.2 to 3.3 eV. The refractive index of a thin film is propor-
tional to the film density. In general, the film density ͑and
thus the refractive index͒ increases with increasing annealing
temperature. This is contrary to the behavior shown in our
case. On the other hand, it is observed that the thickness of
thin NiO film decreases from 27.08 to 25.99 nm with the
increase in annealing temperature, as listed in Table I. The
decrease in n values and film thickness with increasing RTA
temperature may be attributed to the change in film compo-
sition when thin NiO films are annealed at and above
600 °C. X-ray photoelectron spectroscopy ͑XPS͒ is then em-
ployed to determine the film stoichiometry and chemical
state of samples under RTA processes. It has been demon-
strated by XPS ͑not shown here͒ that the formation of Ni
cluster and decrease in oxygen contents occur slightly in the
600 and 700 °C annealed NiO samples. The outdiffusion of
oxygen in the annealed sample increases the porosity of the
film, resulting in the decrease in refractive index and thin
film thickness. A similar behavior was reported by Sasi and
Gopchandran.11 In addition, the surface roughness of as-
deposited NiO film is determined to be 2.08 nm and it de-
creases as the annealing temperature increases from
500 to 700 °C. The decrease in NiO film surface roughness
is ascribed to the increase in mobility of film surface atoms
or molecules at high annealing temperature.
A slight variation of extinction coefficient induced by
RTA is observed in the annealed NiO films ͑see Fig. 2͒. The
absorption coefficients ␣ of sample can be obtained via
␣=4␲k/␭, where ␭ is wavelength of the incident light. The
spectral variation of ␣ as a function of the photon energy for
as-deposited and annealed NiO films is shown in Fig. 3. It is
observed that ␣ abruptly rises, reaching values of 105 cm−1
in the UV region, which is attributed to band gap absorption
in NiO sample. Based on the optical absorption spectra, the
nature of optical transition can be determined. The optical
band gap Eg can be calculated using the following:12
FIG. 2. ͑Color online͒ Spectra of refractive index ͑n͒ and extinction coef-
ficient ͑k͒ for thin NiO films: ͑᭿͒ as-deposited, ͑Ǣ͒ 500 °C annealed, ͑b͒
600 °C annealed, and ͑᭺͒ 700 °C annealed.
FIG. 3. ͑Color online͒ The absorption coefficient ͑␣͒ vs photon energy for
as-deposited and annealed thin NiO films.
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222907-3
Lu et al.
Appl. Phys. Lett. 92, 222907 ͑2008͒
is the energy of the absorbed ͑ϩ͒ or emitted ͑Ϫ͒ phonons.
1/2
Figure 4͑b͒ shows the plots of ͑␣h ͒ versus h for as-
v
v
deposited and annealed NiO films. Each plot can be resolved
into two distinct straight line portions. One is in the lower
photon energy region corresponding to phonon absorption
and has an intercept with the photon energy axis at ͑Egind
−Ep͒. The other straight line portion corresponds to a phonon
energy emission process and has an intercept ͑Egind+Ep͒.
From the energy intercepts, the values of Egind and Ep for all
NiO samples have been calculated. The Egind is found to be
about 2.92 eV for the as-deposited NiO film, and moves to
2.91, 2.80, and 2.69 eV for films annealed at 500, 600, and
700 °C, respectively. It is evident that the Egind obtained for
the indirect allowed transition shifts toward lower energy
when the annealing temperature increases. The variation in
Egind with annealing temperature may be attributed to the
changes in homogeneity, crystallinity, and composition of
NiO samples treated by RTA.
In conclusion, the effect of annealing temperature on op-
tical properties of ALD grown thin NiO films has been in-
vestigated by SE. The results show that the refractive index
of NiO films decreases as the annealing temperature in-
creases. The thickness of as-deposited NiO film is 28.7 nm,
which decreases to 25.9 nm as the annealing temperature
increases to 700 °C. The estimated optical band gap is
3.8 eV for as-deposited NiO film and is not affected by high
temperature annealing. The indirect band gap is found to
decrease as the annealing temperature is increased from
500 to 700 °C.
The authors thank L. Lamagna for his valuable sugges-
tion and help in the measurements.
1R. J. Powell and W. E. Spicer, Phys. Rev. B 2, 2182 ͑1970͒.
2R. H. Kodama, S. A. Makhlouf, and A. E. Berkowitz, Phys. Rev. Lett. 79,
1393 ͑1997͒.
2
1/2
FIG. 4. ͑Color online͒ Plots of ͑␣h ͒ ͑a͒ and ͑␣h ͒ ͑b͒ vs hv for as-
v
v
deposited and annealed thin NiO films.
3C. M. Lampert, Mater. Today 7, 28 ͑2004͒.
1/2
4S. Seo, M. J. Lee, D. H. Seo, E. J. Jeoung, D.-S. Suh, Y. S. Joung, I. K.
Yoo, I. R. Hwang, S. H. Kim, I. S. Byun, J.-S. Kim, J. S. Choi, and B. H.
Park, Appl. Phys. Lett. 85, 5655 ͑2004͒.
␣h␯ = A͑h␯ − Eg͒
,
͑3͒
where A is a constant and h is the incident photon energy.
The variation of ͑␣hv͒ versus h for all the NiO samples is
shown in Fig. 4͑a͒. The nature of the plots indicates the
existence of direct interband transition in thin NiO films. The
Eg values are determined from the energy intercept by ex-
v
2
5Y.-H. You, B.-S. So, J.-H. Hwang, W. Cho, S. S. Lee, T.-M. Chung, C. G.
Kim, and K.-S. An, Appl. Phys. Lett. 89, 222105 ͑2006͒.
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v
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Appl. Phys. Lett. 86, 232901 ͑2005͒.
2
trapolating the linear portion of the plot of ͑␣h␯͒ versus h␯
8D. A. G. Bruggemann, Ann. Phys. 24, 636 ͑1935͒.
9A. M. Lopez-Beltran and A. Mendoza-Galvan, Thin Solid Films 54, 40
͑2006͒.
to ␣=0. The Eg for the as-deposited NiO film is estimated to
be 3.8 eV, which is in good agreement with the results
͑3.6–4.0 eV͒ reported by Adler and Feinlieb13 and Kamal
et al.,14 and is larger than the value ͑3.25 eV͒ for NiO films
deposited by solution growth technique.15 It is noted that the
Eg is almost independent of the annealing temperature.
The optical absorption data are also analyzed for evi-
dence of indirect allowed transitions by the following:16
10D. Franta, B. Negulescu, L. Thomas, P. R. Dahoo, M. Guyot, I. Ohlidal, J.
Mistrik, and T. Yamaguchi, Appl. Surf. Sci. 244, 426 ͑2005͒.
11B. Sasi and K. G. Gopchandran, Nanotechnology 18, 115613 ͑2007͒.
12J. C. Tauc, Optical Properties of Solids ͑North-Holland, Amsterdam,
1972͒.
13D. Adler and J. Feinlieb, Phys. Rev. B 2, 3112 ͑1970͒.
14H. Kamal, E. K. Elmaghraby, S. A. Ali, and K. Abdel-Hady, J. Cryst.
Growth 262, 424 ͑2004͒.
␣h␯ = B͑h␯ − Egind Ϯ Ep͒2,
͑4͒
15A. J. Varkey and A. F. Fort, Thin Solid Films 235, 47 ͑1993͒.
16J. George, C. K. Valsala Kumari, and K. S. Joseph, J. Appl. Phys. 54,
5347 ͑1983͒.
where B is a constant, Egind is the indirect energy gap ͑mini-
mum gap between conductor band and valance band͒, and Ep
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