Structural properties of AlN films
deposited by plasma-enhanced atomic
layer deposition at different growth temperatures
Mustafa Alevli
*
, Cagla Ozgit, Inci Donmez, and Necmi Biyikli
**
UNAM, Institute of Material Science and Nanotechnology, Bilkent University, 06800 Ankara, Turkey
Received 21 July 2011, revised 26 October 2011, accepted 3 November 2011
Published online 29 November 2011
Keywords ALD, AlN, decomposition limited growth, self-limiting growth
*
Corresponding author: e-mail alevli@unam.bilkent.edu.tr, Phone: þ90-312-2903551, Fax: þ90-312-2664365
**
e-mail biyikli@unam.bilkent.edu.tr, Phone: þ90-312-2903556, Fax: þ90-312-2664365
Crystalline aluminum nitride (AlN) films have been prepared
by plasma-enhanced atomic layer deposition (PEALD) within
the temperature range from 100 to 500 8C. A self-limiting,
constant growth rate per cycle temperature window (100–
200 8C) was established which is the major characteristic of an
ALD process. At higher temperatures (>225 8C), deposition
rate increased with temperature. Chemical composition,
crystallinity, surface morphology, mass density, and spectral
refractive index were studied for AlN films. X-ray photo-
electron spectroscopy (XPS) analyses indicated that besides
main Al–N bond, the films contained Al–O–N, Al–O
complexes, and Al–Al metallic aluminum bonds as well.
Crystalline hexagonal AlN films were obtained at remarkably
low growth temperatures. The mass density increased from
2.65 to 2.96 g/cm
3
and refractive index of the films increased
from 1.88 to 2.08 at 533 nm for film growth temperatures of 100
and 500 8C, respectively.
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction The growth of III-nitride thin films
with well-controlled film thickness down to the sub-
nanometer scale, chemical stability, and suitable step
coverage is necessary for the integration of III-nitride
devices with the mature silicon CMOS technology.
Realization of III–V semiconductor on Si platform would
permit to create chip-to-chip and system-to-system optical
communications and enable the fabrication of electrically
pumped light sources. Aluminum nitride (AlN) has attracted
significant attention because of its high thermal conductivity
(320 W/mK at 300 K), high electrical resistance (10
13
V cm),
high thermal expansion coefficient, and wide and direct
optical band gap (6.2 eV) [1–3]. Furthermore, its compat-
ibility with III–V compounds makes AlN promising for the
fabrication of band-gap engineered Al
x
Ga
y
In
1xy
N-based
optoelectronic devices [4], high-frequency electro-acoustic
devices, and piezoelectric actuators and sensors [5]. As a
result of these material properties, a significant amount of
research has been devoted to the growth and characterization
of AlN thin films [6–9]. While high-temperature (typically
above 1100 8C) grown epitaxial AlN films are used in active
electronic and opto-electronic device layers, polycrystalline
and amorphous AlN films grown at CMOS-compatible
temperatures (lower than 300 8C) are widel y used as
dielectric and passivation layers for microelectronic devices
[10].
In recent years, atomic layer deposition (ALD) with
remote plasma capability is a promising growth technique
which not only reduces the film growth temperature, but
satisfies critical conformality and sub-monolayer thickness
control as well [11, 12]. In this work, we explored the
influence of the growth temperatures on the crystallinity,
composition, surface roughness, mass density, and refractive
index of AlN films grown by plasma-enhanced ALD
(PEALD). The growth of AlN films at different growth
temperatures (100–500 8C) have been studied by several
groups based on PEALD growth method [13–15]. However,
in those reports, either self-limiting growth with tempera-
ture-dependent growth rate per cycle was observed [13, 14]
or the resulting films were amorphous within the self-
limiting window [15] . In our work, we report crystalline AlN
thin films grown at temperatures as low as 100 8C. We carried
out systematic experiments to study the influence of growth
temperature on the crystalline quality of AlN within the
Phys. Status Solidi A 209, No. 2, 266–271 (2012) / DOI 10.1002/pssa.201127430
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range of 100–500 8C. Self-limiting growth with constant
growth rate per cycle and temperature-dependent growth rate
regimes as well as decomposition-limited growth regime
were characterized in detail.
2 Experimental Aluminum nitride films were depos-
ited on Si(100) substrates at growth temperatures ranging
from 100 to 500 8C by using a Fiji LL Cambridge Nanotech
ALD reactor equipped with a remote inductively coupled
rf-plasma source and base pressure of 0.2 Torr.
Trimethylaluminum (TMA) and ammonia (NH
3
) enhanced
by rf-plasma were used as precursors while Ar was used as
the carrier gas. During all growth experiment s, NH
3
gas flow
rate was 50 sccm and exposure time was 40 s, TMA pulse
time was 0.1 s, purge time in between precursor pulses was
10 s, and plasma power was maintained at 300 W. To
investigate the effect of the growth temperature on the AlN
film properties 100 nm thick AlN layers were deposited at
100, 185, 400, and 500 8C.
Film thickness and refractive index were estimated by
using a variable angle spectroscopic ellipsom etry (SE) at
three angles of incidence (C (65, 70, 75), D). The index of
refraction and thickness of each AlN films were extracted
from SE data by using the Cauchy dispersion function. X-ray
reflectivity (XRR) and grazing incidence X-ray diffraction
(GIXRD) measurements with a Philips X‘pert PRO MRD
diffractometer using Cu K
a
radiation were carried out to
determine the crystallinity, mass density, and thickness of the
deposited films. The elemental profiles, chemical bonding
states, and impurity incorporation of the films were deter-
mined by X-ray photoelectron spectroscopy (XPS) operating
at monochromatized Al K
a
wavelength. Surface morphology
and roughness were determined by atomic force microscopy
(AFM). High-resolution transmission electron microscopy
(HR-TEM) imaging was performed by Tecnai G2 F30 TEM
(FEI). Cross-sectional TEM specimens were prepared by
focused ion beam (FIB Nova 600i Nanolab – FEI).
3 Results and discussion Initially, the self-limiting
growth window (100 8C < T
growth
< 225 8C) for AlN films
was obtained by precisely adjusting the deposition tempera-
ture, reactant dose, and length of the precursor pulses, in
which each surface reaction step is saturative. In this ideal
case, chemisorption of each reactant occurs on the surface of
the growing film where all the gas-phase reactions are
automatically eliminated. Within this window, overdosing
precursors and increasing the growth temperature does not
affect the growth rate and AlN material was produced with
sub-monolayer thickness control [11]. In the subsequent
experiments, the growth temperature was increased up to
500 8C in order to investigate the properties of AlN films in
the temperature dependent growth region (T
growth
> 225 8C).
The experimental results are summarized in Fig. 1, where
self-limited and temperature dependent growth regimes are
indicated.
We explored the influence of growth temperatures on the
properties of AlN films by using the aforementioned self-
limiting growth condition parameters. Figure 2a and b shows
the evolution of narrow scan XPS spectra of N1s and Al2p
spectra for different growth temperatures taken in the inner
layers of AlN, respectively. Three distinct chem ical states
were observed for AlN films; the Al2p#1 subpeak at
73.5 0.1 eV, Al 2p#2 at 72.4 0.1 eV and Al 2p#3 at
74.5 eV. According to the Al2p XPS results and dominat-
ing Al2p#1 subpeak, AlN was formed in all samples for all
growth temperatures including 100 8C. An Al2p#3 subpeak
is obtained for AlN films grown at 100 and 500 8C which is
correlated to Al–O bound states [16]. In the intermediate
growth temperatures, Al2p#2 subpeak is detected whose
binding energy (BE) is clearly related to the BE of metallic
Al–Al bond in AlN [1]. The intensity of Al 2p#1 subpeak was
always more than the Al2p#2 and Al2p#3 subpeaks at all
growth temperatures except 100 8C. However, the O1s XPS
analysis of AlN grown at 100 8C reveal a dominant subpeak
at 531.5 0.1 eV. Harris et al. [17] assigned the origin of this
oxygen defect to oxygen bound to aluminum (Al–O) with a
high coord ination number and would be associated with
oxygen defects within the AlN crystal columns. This can be
also explained with dominating characteristic of Al2p#2
subpeak and shifting of Al2p#1 subpeak to higher binding
energies. The O1s subpeak analysis for AlN films grown at
500 8C reveals a major subpeak at 531.8 0.2 eV, which is a
characteristic of Al–O bonding [16].
Additional information about the chemical structure of
the AlN films is provided by the N1s spectra analysi s. On the
basis of the deconvolution of N1s XPS peak, the strongest
peak at 396.3 0.3 eV which was detected for all AlN films,
is a typical N–Al bond [16]. The almost-negligible peak at
398 0.3 eV is attributed to AlO
x
N
y
[3, 16]. The Al–O–N
peak was observed to decrease in intensity as the growth
temperature increased, where the peak disappeared at higher
Phys. Status Solidi A 209, No. 2 (2012) 267
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Figure 1 (online color at: www.pss-a.com) Deposition rates of
AlN films at different temperatures. Depositions occur in two
different temperature windows: self-limited growth window and
temperature dependent growth window.
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growth temperatures. In the literature, possible H-related
defects were observed and reported using XPS. Formation of
possible –NH
x
residues such as AlN– NH
x
which have
binding energies of 398.5–399.9 eV were not observed in our
films [3]. A high resolution of very weak O1s core level peak
was analyzed as well and there was no OH peak from 532.2
to 532.3 eV [18]. Furthermore, we did not observe any H
related impurities based on structural experiments presented
in this paper . On the other hand, our samples include metallic
aluminum and oxygen impurities. Therefore, at the current
stage, discussing the possible effects of H in the films without
performing ERD-TOF and RBS experiments might result in
misleading conclusions.
In Fig. 3, the GIXRD profiles of the thin films grown at
different temperatures are presented. The thin films exhib-
ited prevalence of a highly polycrystalline hexagonal AlN
structure perfectly matching with the XRD Bragg diffraction
peak positions corresponding to the literature.
Grazing incidence X-ray diffraction patterns reveal no
change in phase and crystalline orientation with temperature.
Similar GIXRD patterns were obtained for all AlN films
deposited at different growth temperatures and no other
impurity related GIXRD pattern observed such as Al (111) at
2Q ¼ 38.58 and Al(200) at 2Q ¼ 44.78.Theh-AlN (100)
peak is dominant for the samples grown at low temperatures
in the self-limiting growth window. The dominance of (101)
and (002) becomes more significant in the GIXRD patterns
of AlN films grown at higher growth temperatures. The
GIXRD of AlN samples grown at 185, 400, and 500 8C
suggest a highe r degree of crystallization which is in good
agreement with mass density and refractive index values (see
Fig. 6). Upon decreasing the growth temperature down to
100 8C, AlN films possess their crystalline structure. GIXRD
results show that the onset temperature for crystalline growth
is below 100 8C.
Cross-sectional HR-TEM images of the grown AlN films
deposited by PEALD on Si substrates are shown in Fig. 4.
We observed that the AlN films consist of self-organized
nm-long hexagonal crystalline structures, i.e. (100), (111),
(002), etc. which confirms the GIXRD data.
Selected area electron diffraction pattern (SAED) of
these films contain continuous rings which are obtained from
the large number of diffracted spots (not included in this
paper). Continuous diffraction rings obtained from SAED
patterns also indicate a polycrystalline nature of AlN films.
Analysis of HR-TEM images show that the size of crystalline
ordering becomes longer in length and width as the
growth temperature incr eases and the degree of disordering
decreases accordingly.
268 M. Alevli et al.: Structural properties of deposited AlN films
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Figure 2 (online color at: www.pss-a.com) High-resolution XPS
spectra of (a) N1s and (b) Al2p peaks of PEALD grown AlN films
with different growth temperatures. AlO
x
N
y
impurities were
observed only in low temperatures.
Figure 3 GIXRD patterns for AlN films at different growth
temperatures on Si(100) with the growth parameters obtained in
the self-limiting growth window.
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To access surface morphological information, AFM
images were acquired for different size areas on the surfaces
of the AlN layers. In Fig. 5, the panels a, b, c, and d
demonstrate the AFM images of AlN films grown at growth
temperatures 100, 185, 400, and 500 8C, respectively. We
observed that the surfaces o f AlN films become rougher as
substrate temperature increases. This is attributed to the
larger crystal grains embedded in the films [19]. Root mean
square (rms) roughness values measur ed are relatively high
for the films grown at higher growth temperatures with 400
and 500 8C (2.54 nm for 400 8C and 2.16 nm for 500 8C). The
surface of the sample grown at low temperatures is smoother
(1.37 nm for 185 8 C and 0.93 nm for 100 8C). The surface
reaction kinetics and growth mechanisms for the films grown
in 100–200 8C is different compared to samples grown at
higher temperatures as aforementione d in Section 3.
Within the temperature-dependent growth regime,
precursor self-decomposition occurs for T > 300 8C
which destroys the self-limiting growth mechanism and
sub-monolayer thickness control, leading to films with
higher surface roughness. Film density, surface roughness,
and mean grain size results obtained for different growth
temperatures measured with XRR and AFM analysis are
shown in Fig. 6. The density, roughness, and mean grain size
increased with increas ing growth temperature. However,
rms roughness decreased for samples grown at 500 8C
compared to samples grown at 400 8C (Fig. 6b). This might
be due to the increased adatom mobility with increasing
growth temperature up to 500 8C which enables surface
diffusion and increases coalescence probability of nuclea-
tion centers (more CVD-like growth) [20]. XRR results
confirm the similar roughness values and similar behavior
with growth temperature (Fig. 6). On the other hand, the
grain size of AlN film grown at 100 8C is larger than the ones
grown at 185 8C (see Fig. 6c). Since AlN films grown at
100 8C contain Al–O and Al–O–N complexes according to
Phys. Status Solidi A 209, No. 2 (2012) 269
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Figure 4 Cross-sectional high-resolution TEM images of 100 nm
thickAlNfilms grown on Si (100) substrate at different temperatures.
Figure 5 (online color at: www.pss-a.com) Surface morphology
of AlN films grown at (a) 100 8C(1mm 1 mm), (b) 185 8C
(1 mm 1 mm), (c) 400 8C(1mm 1 mm), and (d) 500 8C
(1 mm 1 mm).
Figure 6 (online color at: www.pss-a.com) (a) Density derived
from XRR, (b) roughness measured by AFM and derived from
XRR, and (c) mean grain size derived by analyzing AFM images
of the AlN films.
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the XPS analysis, grain size and roughness values might have
changed not accordingly with AlN film s grown at higher
temperatures that have less or no Al
x
O
y
N
1xy
related
impurities.
Figure 7 shows the index of refraction for AlN films
grown at different growth temperatures obtaine d from
Cauchy dispersion model (300–1000 nm). Refractive index
n increased with growth temperature from 1.94 to 2.08 at
533 nm wavelength when the growth temperature increased
from 185 to 500 8C. A lower refractive index value 1.87 at
533 nm is observed for AlN film grown at 100 8C. The
extracted refractive index values are in good agreement with
polycrystalline AlN reported in literature [21] and confirms
our GIXRD and HR-TEM results. The most probable reason
for the lower refractive index value might be Al–O
complexes and higher oxygen concentrations which were
also obtained from XPS analysis. The o rigin of oxygen
incorporation is still under investigation. The gases used in
the experiments (ammonia and argon) are of ultra-high
purity (99.999%). There is no leak within the reactor
chamber, as confirmed by standard ALD null-tests.
However, the oxygen contamination might originate from
the Ar carrier gas or NH
3
plasma/group-V precursor gas
themselves, which contain certain trace amounts of humidity
and oxygen impurities.
The refractive index of the AlN films grown with higher
temperatures (185 8C) are in the range of 1.9–2.1 for
observed wavelength range. The extinction coefficient k
increases significantly when the wavelength is less than
400 nm for samples grown at higher temperatures (400 and
500 8C) but does not increase even at 300 nm for samples
grown at lower temper atures. The spectral behavior of k
extinction coefficient showed a weak dependence for AlN
films grown at low temperatures but reasonable dependence
for samples at higher temperatures.
4 Conclusions In summa ry, we have studied PEALD
grown crystalline AlN films in two main growth regimes:
self-limited growth regime where growth rate per cycle stays
constant with temperature and temperature-dependent
growth regime where growth rate increases with deposition
temperature. The structural properties were significa ntly
affected by the growth temperature. Compared to previously
published results, AlN films grown within low-temperature
self-limited regime showed significantly improved crystal-
line quality, higher refractive index and film density, and
lower surface roughness. We owe these improv ed results to
the truly self-limiting conditions achieved which eliminate
gas-phase reactions and provides ultimate sub-monolayer
thickness control. Both structural and optical analyses
showed that hexagonal crystalline AlN thin films were
obtained at growth temperatures as low as 100 8 Cby
PEALD. AlN films were composed of nm-sized crystallites
of wurtzite planes. Crystalline quality was further improved
with higher growth temperatures, where decomposition
limited grow th occurred and CVD-like growth kinetics
dominated. Oxygen was detected as a major impurity in the
films. With increasing temperature metallic Al and Al–N–O
bonding disappeared and only Al–O peak remained at the
highest growth temperature. With increased growth
temperature, the refractive index increased, which was
attributed to the densification of the films. The mass density
increased from 2.65 to 2.96 g/cm
3
and refractive index of the
films increased from 1.88 to 2.08 at 533 nm for film growth
temperatures of 100 and 500 8C, respectively.
Acknowledgements This work was performed at UNAM
supported by the State Planning Organization (DPT) of Turkey
through the National Nanotechnology Research Center Project.
N.B. acknowledges support from Marie Curie International Re-
integration Grant (grant # PIRG05-GA-2009-249196). M.A.
gratefully acknowledges the financial support from TUBITAK
(project No.: 232.01-660/4835).
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270 M. Alevli et al.: Structural properties of deposited AlN films
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Figure 7 (online color at: www.pss-a.com) Refractive index and
extinction coefficient of AlN filmsgrown at different growthtemper-
atures as a function of wavelength.
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