Atomic layer deposition of CdO and CdxZn1-xO films

Materials Chemistry and Physics 140 (2013) 465e471
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Materials Chemistry and Physics
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Atomic layer deposition of CdO and CdxZn1xO ﬁlms
Jonathan R. Bakke a, Carl Hägglund a, Hee Joon Jung b, Robert Sinclair b, Stacey F. Bent a, *
a
b
Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA
Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA
h i g h l i g h t s
Polycrystalline CdxZn1xO deposited via ALD at 150 C for the ﬁrst time.
CdO {111} planes are strongly oriented parallel to SiO2/Si(100) substrates.
Crystallinity shifts from cubic (CdO) to hexagonal (ZnO) without an observed amorphous region.
Bandgap bowing occurs due to the valence band offset and lattice mismatch of the binary materials.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 May 2012
Received in revised form
10 February 2013
Accepted 12 March 2013
Growth of CdxZn1xO by atomic layer deposition (ALD) is demonstrated at 150 C using diethylzinc
(DEZn), dimethylcadmium (DMCd), and water as the precursors. The relative ratio of the DMCd and DEZn
pulses is varied to achieve different compositions ranging from pure CdO to pure ZnO. The crystal
structure of CdO is rock salt cubic and that of ZnO is hexagonal, and the alloy from ZnO to at least
Cd0.56Zn0.44O has a hexagonal crystal structure. Transmission electron microscopy conﬁrms polycrystalline grain features and a growth rate of w2.0 Å cycle1, while selected area diffraction provides
crystallographic information indicating that {111} type planes of the pure CdO ALD ﬁlm are preferentially
oriented to the ﬁlm surface. Using spectroscopic ellipsometry, the ﬁlm’s optical constants are correlated
with elemental composition and crystal structure. Control of these properties allows for tuning of the
optical bandgap and index of refraction.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
A. Alloys
A. Thin ﬁlm
A. Semiconductors
A. Inorganic compounds
A. Oxides
D. Optical properties
D. Crystal structure
1. Introduction
Atomic layer deposition (ALD) is a vacuum processing technique for material deposition using thermal reactions of gas phase
precursors at a substrate, where the precursors are separated by
space or time. Since growth kinetics are surface reaction ratelimited, the deposition occurs with monolayer or submonolayer
growth per cycle [1e3]. The growth rate per ALD cycle typically
reaches a maximum value even when an excess of precursor is
dosed because of the self-limiting nature of the reaction. These
characteristics yield the ability to uniformly coat large areas and to
deposit materials on high aspect ratio substrates. The applications
of ALD are numerous and increasing, and they include microelectronics, photovoltaics, and catalysis [3e5]. The types of ﬁlms
* Corresponding author. Tel.: þ1 650 723 0385; fax: þ1 650 723 9780.
E-mail address: [email protected] (S.F. Bent).
0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.matchemphys.2013.03.038
deposited include metals, oxides, sulﬁdes, selenides, nitrides, alloys, and nanolaminates [1,2].
The IIeVI semiconductors e and related materials e have been
widely studied for various applications. Examples of IIeVI materials
grown by ALD include ZnO [6e8], ZnS [9e11], Zn(O,S) [12], Zn(S,Se)
[13], ZnxMg1xO [14], ZnO/Al2O3 [15], ZnO/SnOx [16], CdS [17,18],
and CdxZn1xS [19]. A key advantage enabled by varying the
composition of IIeVI materials is that properties such as bandgap,
resistivity, index of refraction, band positions, lattice constants, and
crystal structure may be tuned. Among the applications for these
materials are n-type semiconductor buffer layers [7,12,14,19,20] for
thin ﬁlm photovoltaics such as CuInxGa1xSySe2y and transparent
conducting oxides (TCO) [15,21] for use as electrodes in solar cells.
This report expands the existing IIeVI ALD systems by demonstrating for the ﬁrst time the deposition by ALD of CdO [22] and
CdxZn1xO.
Both CdO and CdxZn1xO are of interest for use in TCOs in the
blended CdO(ZnO)/SnOx system because of their low resistivity,
466
J.R. Bakke et al. / Materials Chemistry and Physics 140 (2013) 465e471
high mobility, and high transparency [23,24]. For example, CdO and
Cd2SnO4 [25] have speciﬁcally been used as TCOs for thin ﬁlm SnS
[26] and CdTe [23,27] solar cells. A theoretical study of the Cd/Zn/
Mg/O ternary alloy systems details how the properties change as a
function of composition [28]. Although not demonstrated previously by ALD, CdO has been deposited by metal organic chemical
vapor deposition (MOCVD) [29,30], sputtering [31,32], thermal
evaporation [33], and metal organic vapor phase epitaxy (MOVPE)
[34] while the CdxZn1xO alloy has also been grown by spray pyrolysis [35], thermal decomposition [36], molecular beam epitaxy
(MBE) [37], sputtering [38], and the solgel method [39].
In this study, ALD growth of pure CdO is demonstrated at optimized conditions. The bandgap value of w2.4 eV obtained for these
ﬁlms is within the range of reported literature values. The lattice
constants conﬁrm the preferential and oriented growth of cubic
crystals for pure CdO. The crystal structure is that of rock salt cubic
as opposed to the zincblende cubic which is observed for the sulﬁde
analogue CdxZn1xS [9,17,19]. Further, alloys of CdxZn1xO ranging
from pure CdO (x ¼ 1) to pure ZnO (x ¼ 0) are demonstrated with
bandgaps varying from w2.3 eV to w3.3 eV. Starting from pure
hexagonal ZnO, the crystal structure of the ALD alloy remains
hexagonal until at least 56% Cd/(Cd þ Zn) metal concentration, and
then goes through a transition to the rock salt cubic structure
before reaching a 70% Cd/(Cd þ Zn) metal concentration.
Science Instruments S-Probe monochromatized spectrometer using Al Ka 1486 eV radiation at a pressure of 6.7 1011 kPa
(5 1010 Torr). The argon sputter depth proﬁling was performed
at a pressure of 1.3 108 kPa (1 107 Torr) using 5 keV Arþ at
2 2 mm raster at 45 incidence to the sample. Survey scans were
performed with a step size of 1 eV. Crystal structure was determined with a PANalytical X’Pert PRO XRD system in parallel beam
mode using Cu Ka radiation. TEM samples with a thickness of
w80 nm were prepared using a focused ion beam (FIB, FEI Strata
235DB dual-beam FIB/SEM) lift-out Omniprobe technique with a
Ga ion beam at 30 keV. Cross-sectional bright ﬁeld (BFTEM) and
high-resolution transmission electron microscopy (HRTEM) images
and selected area diffraction (SAD) patterns were taken by an FEI
Tecnai G2 F20 X-TWIN operated at an accelerating voltage of
200 kV. The optical constants (refractive index n and extinction
coefﬁcient k) and bandgaps for ﬁlms deposited at 150 C were
determined using variable angle spectroscopic ellipsometry (VASE)
performed in reﬂection mode with a Woollam M2000 ellipsometer.
Ellipsometric and depolarization data were collected at 65, 70 and
75 angles of incidence, for a polarization of 45 and wavelengths in
the interval of 210e1700 nm. The data were ﬁtted using tabulated
optical constants for the Si substrate; the HerzingereJohs psemiM0 function [40] for representation of the CdxZn1xO layer and a
Bruggeman’s effective medium layer of air/ﬁlm to take surface
roughness into account.
2. Experimental details
ALD growth of CdxZn1xO was performed in a custom built
warm wall reactor that has been described in a previous publication
[9]. The precursors for this process were diethylzinc (DEZn, Sigmae
Aldrich), dimethylcadmium [17] (DMCd, Strem) and ultrapure
water (H2O). Nitrogen was used as the carrier and purge gas at a
constant ﬂow rate of 60 sccm. The ALD process in this study was
conducted at a substrate temperature of 150 C to study the growth
rates and the effects of composition on the material and optical
properties of the ﬁlms. The optimized cycle of ALD consisted of 0.4 s
DMCd/DEZn, 10 s purge, 0.4 s H2O, and a 10 s purge as determined
in previous reports [8,9,17]. The deposition of an alloy is deﬁned
here by the supercycle which describes the pulse sequence of the
deposition. An example of the notation used in this report is 40%
2 3 CdxZn1xO, which means that a x ¼ 0.4 ﬁlm was formed by 2
pulses of DMCd followed by 3 pulses of DEZn (the notation is
important because a 40% supercycle ﬁlm can also follow a
1 1 1 2 (i.e. DMCdeDEZneDMCdeDEZneDEZn) sequence, for
example) [19]. 20 supercycles in this case corresponds to 100 cycles
of ALD. All precursors were maintained at room temperature
(w22 C). Needle valves were used to control the rate of dosing of
each precursor.
The substrate for X-ray diffraction (XRD), X-ray photoelectron
spectroscopy (XPS), ellipsometry, and transmission electron microscopy (TEM) measurements was a Czochralski grown n-type
Si(100) wafer with a resistivity of 1e5 U-cm and a native silicon
oxide layer of typical thickness around 2 nm. The substrates were
cleaned via 5-min sonication steps each in acetone and ethanol
with deionized water rinses between each solvent bath. Residual
organics were then removed with a piranha clean (70% sulfuric acid
and 30% hydrogen peroxide) for 15 min followed by a deionized
water rinse and drying with pressurized air.
After ALD, resulting ﬁlm thicknesses and indices of refraction
were measured by a Gaertner L116C single-wavelength ellipsometer using 632.8 nm light at a 70 angle of incidence and with
the polarizer set to 45 . At least two 1 1 cm samples from each
run were used, and measurements were performed on three spots
for each sample to account for any non-uniformity. Elemental
composition of the ﬁlms was determined by XPS with a Surface
3. Results and discussion
3.1. ALD process characterization and alloy composition
The CdO ALD growth rate is determined at 150 C using optimized times for the precursor pulse and nitrogen purge [8,9,17,19].
The thickness is a linear function of the number of cycles, with a
growth rate of 2.0 Å cycle1 as shown in Fig. 1. When referenced to
the c(111) interplanar spacing of 2.72 Å, this growth of CdO corresponds to 0.75 monolayer cycle1. In comparison, the related IIeVI
chalcogenides CdS and ZnO have growth rates of 0.38 and 0.98
monolayer cycle1 when indexed to their major phases of c(111)
and h(002), respectively [8,17]. An XPS survey scan, as shown in
Fig. 2, substantiates that stoichiometric CdO without contamination is deposited under these conditions within the sensitivity of
w5% achievable by XPS [41e43].
ZnO ALD was previously studied by our group in the same
reactor, and was found to follow typical ALD behaviors [8]. The
knowledge gained from this work was used to form the CdxZn1xO
alloys studied here. For the mixed ﬁlms, the composition and the
growth rate are studied as a function of the cycle ratio of the alkyl
Fig. 1. The thickness of CdO at 150 C versus cycle number shows a constant growth
rate of 2.0 Å cycle1.
800
Cd(4s)
600
400
200
Binding Energy (eV)
Cd(4p)
Cd(4d)
Cd(3d5/2)
Cd(3d3/2)
467
Cd: 54%
O: 46%
O(1s)
Cd(3p1/2)
Cd(3p3/2)
Cd(3s)
Counts (a.u.)
J.R. Bakke et al. / Materials Chemistry and Physics 140 (2013) 465e471
0
Fig. 2. An XPS survey spectrum after 10 s Arþ ion sputter of CdO ﬁlm deposited at
150 C shows pure CdO without any detectable contaminations.
metal precursors. The growth of the CdxZn1xO ﬁlms is found to be
linear and to follow ALD saturation behavior characteristic of ZnO
[8], ZnS [9], ZnOyS1y [44], CdS [17], and CdxZn1xS [19] as detailed
in our previous studies. Fig. 3 summarizes the composition as
determined by XPS versus cycle ratio. The dashed line in the ﬁgure
corresponds to y ¼ x and is used to demonstrate that the ﬁlms are
Zn rich compared to the cycle ratio. This observation is not surprising given that the growth rates of ZnO and CdO are 2.54 [8] and
2.03 Å cycle1, respectively. Calculating an expected composition
for this alloy system is complicated by the fact that the crystal
phase changes with CdxZn1xO composition (vide infra) [19].
3.2. Material, electronic, and optical properties
The crystal phases, average grain size, and lattice constants of
ﬁlms of varying stoichiometry are studied using XRD. The diffraction pattern of pure CdO deposited at 150 C on Si(100) is shown in
Fig. 4. The three peaks correspond to cubic (111), (200), and (220)
orientations using the crystallographic notation (hkl), and the
orientation is preferentially c(111), which is similar to that observed
for ZnS [9,10], CdS [17], and CdxZn1xS [19] ﬁlms grown by ALD. The
location and full width half maximum (FWHM) of each peak are
determined by a pseudo-Voigt ﬁt of the patterns where the values
of 2q (where q is the angle between the incident X-ray and the
scattering plane), the height, FWHM, and Gaussian percentage are
allowed to vary. Using the Scherrer equation and the FWHM of the
(111) peak, the average grain size of CdO is determined to be at least
Fig. 4. XRD patterns of 500 cycle CdO (red) and ZnO ﬁlms (green) and 201 cycle
(67 supercycles) ﬁlms of CdxZn1xO (blue) on Si(100). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of
this article.)
15e20 nm. The interplanar spacing d is determined by Bragg’s Law
(nl ¼ 2dsin (q)) for each phase seen in the pattern using the values
constant
a for the cubic CdO is
of 2q and l ¼ 1.5406 Å. Theplattice
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ﬃ
calculated from dhkl ¼ a= h2 þ k2 þ l2 for each peak, and the
average value of a is found to be 4.696 Å 0.004 Å. This value
agrees with the reported literature lattice constant for the rock salt
cubic structure of 4.69 Å [28,32,33,45].
Alloying CdO with ZnO yields a crystal structure that is cubic
from CdO to at least Cd0.70Zn0.30O as shown in the XRD patterns in
Fig. 4. The value of 2q shifts to higher values with the Zn content
due to smaller interplanar distances. The lattice constant using the
above equation is 4.596 Å 0.011 Å when averaged over the three
visible cubic peaks for Cd0.70Zn0.30O. The FWHM increases very
slightly for each peak. Furthermore, the average intensity of the
cubic peaks changes from a dominant presence of c(111) for CdO to
a more equally mixed ﬁlm of c(111) and c(200). The change in the
percent crystal phase orientation calculated from the integral of
each peak is given in Table 1.
Table 1
The fraction of total peak intensity for each crystal phase and orientation of the
various alloys is listed in the table. The second number in parenthesis is the position
of the peak, which is used for the calculations of Vegard’s law. The h(002) peak for
Cd0.56Zn0.44O is marked with an asterisk because it may be present in small quantities but the ﬁt is difﬁcult due to its low intensity and the increasing FWHM of the
peaks.
CdO
Fig. 3. Measured atomic percentage of CdxZn1xO ﬁlms vs. the cycle ratio of the DEZn/
DMCd precursors. The dashed line corresponds to y ¼ x.
c(111)
c(200)
c(202)
h(100)
h(002)
h(101)
h(110)
Cd0.70Zn0.30O Cd0.56Zn0.44O Cd0.12Zn0.88O ZnO
0.84 (33.01 ) 0.56 (33.66 ) 0
0
0.08 (38.28 ) 0.35 (39.24 ) 0
0
0
0.08 (55.34 ) 0.08 (56.64 ) 0
0
0
0.48 (30.03 ) 0.31
0
0
0*
0.43
0
0
0.48 (34.34 ) 0.18
0
0
0.04 (53.40 ) 0.08
(31.24 )
(33.90 )
(35.57 )
(55.71 )
0
0
0
0.18
0.77
0.02
0.04
(31.85 )
(34.40 )
(36.19 )
(56.70 )
468
J.R. Bakke et al. / Materials Chemistry and Physics 140 (2013) 465e471
Fig. 5. The linear shift in the lattice constant with composition signiﬁes that the
deposited CdxZn1xO ﬁlms are in the form of an alloy rather than a binary mixture of
oxides.
The diffraction patterns for the ﬁlms ranging from Cd0.56Zn0.44O
to ZnO are provided in Fig. 4. ZnO has been a well-studied system
by ALD and has been shown to exist in the hexagonal phase
[6,12,46] and to have a lattice constant a ¼ 3.249 Å (c/a ¼ 1.602)
[28]. The main visible peaks for ZnO in this study are h(100), h(002),
h(101), and h(110) with a preferential h(002) orientation. The
h(101) peak is only weakly present in the ﬁlm. The diffraction
patterns for the mixed ﬁlms show that a hexagonal alloy is formed
rather than co-existing phases of CdO and ZnO for concentrations
ranging from ZnO up to at least 56% Cd. The alloyed state is signiﬁed
by the shift in the peak positions, which occurs due to the larger
atomic radius of Cd compared to Zn. The value of 2q is given in
Table 1, and these values are used to calculate the lattice constants
which shift linearly with concentration, as shown in Fig. 5, in
agreement with Vegard’s law for alloys [19,38,47]. A stable alloy in
hexagonal form was observed in previous reports for the CdxZn1xO
system up to 62% Cd content e after which point the phase eventually shifted to the cubic phase seen for pure CdO [28,38]. Thermodynamically, it should be possible to have a two-phase region;
however, it is not observed here. Either, the two-phase region exists
in a very narrow compositional range, or the layer-by-layer growth
mechanism of ALD naturally yields a crystal phase that is dominated by one of the alloy constituents.
Also of note, the prevalent h(002) peak seen in ZnO decreases
with increasing Cd content until it is almost completely absent by
56% Cd (Table 1). At this point, the intensity of the h(101) peak
increases to become the dominant peak. Another reported trend in
the literature was a broadening in the FWHM of the diffraction
peaks as Cd content increased [38], signifying either smaller crystals or more strain in the ﬁlm. In the present study, the FWHM of
the h(100) peak is similar for ZnO and Cd0.12Zn0.88O, yielding a
calculated average grain size of 15e20 nm. However, inline with
previous work [38], signiﬁcant broadening is observed for the more
Cd-rich Cd0.56Zn0.44O ﬁlm, and the corresponding calculated grain
size is w10 nm.
As for the CdO and Cd0.70Zn0.30O ﬁlms, the interplanar spacings
dhkl for the ZnO and CdxZn1xO ﬁlms are calculated from the values
in Table 1, and the lattice constants are then determined using the
2
l2/c
equation 1/d2 ¼ (4/3) * (h2 þ hk þ k2)/a2 þ p
ﬃﬃﬃﬃﬃﬃﬃﬃfor hexagonal
systems assuming the relationship c=a ¼ 8=3 [42,48e50].
Consistent with Vegard’s law, the lattice constants a and c vary
linearly with the composition as shown in Fig. 5. Each point is an
average value obtained from the peak position of the h(100),
h(101), and h(110) phases. The a-value for ZnO is 3.243
Å 0.002 Å
pﬃﬃﬃﬃﬃﬃﬃﬃ
and the c-value is 5.295 0.003 Å (using c=a ¼
8=3), which
corresponds well to the literature value of a ¼ 3.249 Å [28].
By examining the bright ﬁeld TEM image (Fig. 6a) of the 200
cycle CdO ﬁlm deposited at 150 C on Si (001), it is observed that
columnar grains of polycrystalline CdO extend from the silicon
substrate to the surface, and this conclusion is further conﬁrmed
with the analysis of the SAD pattern in Fig. 7. The grains are largely
aligned vertically to the sample. The high resolution TEM image in
Fig. 6b clearly shows that grain boundaries are perpendicular to the
surface. By measuring the ﬁlm in several places (total imaged region by TEM is around 200 200 nm), the average thickness is
determined to be 42.4 2.3 nm for the 200 cycles CdO ﬁlm. This
value corresponds to a growth rate of 2.1 Å cycle1 which is in good
agreement with the growth rate (2.0 Å cycle1) obtained by
ellipsometry in Fig. 1.
The diffraction pattern of the Si (001) substrate (Fig. 7a) is taken
as reference for orientation (the top direction of the CdO ﬁlm is the
Si [002] direction) with a 150 nm diameter SAD aperture. As shown
in Fig. 7b, the SAD pattern from both Si and CdO with the same
diffraction condition (same camera length and same Si [110] zone)
shows orientation information in that cubic {111} type planes of
CdO are preferentially aligned parallel to Si {002} planes. This
Fig. 6. BFTEM (left, a) and HRTEM (right, b) images of 200 cycles of CdO ﬁlm deposited at 150 C on Si(001) substrate. The average thickness and standard deviation are 42 nm and
2.3 nm, respectively, and grain boundaries are oriented in the vertical direction perpendicular to the substrate surface.
J.R. Bakke et al. / Materials Chemistry and Physics 140 (2013) 465e471
469
Fig. 7. Selected area diffraction (SAD) patterns of the 200 cycle CdO ﬁlm deposited at 150 C taken with a 150 nm diameter SAD aperture: The SAD pattern in (a) was taken from
only Si, the SAD pattern in (b) was from both Si and CdO. Both were taken from the same diffraction condition on Si [110]. The SAD pattern in (b) showed that strong reﬂection spots
of the CdO c{111} planes are nearly parallel to Si c{200} reﬂection spots, which demonstrates that CdO c{111} planes of the polycrystalline ﬁlm preferably grow parallel to the
surface.
observation indicates that CdO {111} planes of ALD polycrystalline
ﬁlm grains grow closely parallel to the surface, which supports
strong (111) peak of CdO in XRD. After direct comparison of the
reﬂection spots between Si (diamond structure, a ¼ 5.4309 Å) and
CdO in Fig. 7b by the camera length equation of a TEM diffraction
pattern, Rhkl dhkl ¼ L l ¼ constant, where R ¼ the distance from
the centerpspot
to a speciﬁc
spot corresponding to a speciﬁc plane,
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ﬃ
dhkl ¼ a= h2 þ k2 þ l2 , L ¼ the camera length, and l ¼ 2.51 pm
(electron wavelength at 200 kV acceleration), the CdO lattice
parameter is measured. When the same type of planes are
compared, the relationship Rhkl, Si aSi ¼ Rhkl, CdO aCdO applies as
shown in Table 2. The average CdO lattice parameter determined
using this method is 4.690 0.002 Å which supports the lattice
parameter extracted from the XRD data (vide supra).
The optical constants (the refractive index n and extinction coefﬁcient k) are obtained from spectroscopic ellipsometry as
described in Experimental details and are shown in Fig. 8 for the
different CdxZn1xO compositions. The sharp features of the optical
constants seen for ZnO near the bandgap gradually broaden as the
Cd content increases. Bandgap values corresponding to direct
allowed transitions in the CdxZn1xO ﬁlms are derived by extrapolating the linear portion of (ahn)2 to zero (see Fig. 9a) following
Tauc plot analysis, where hn is the photon energy and a is the absorption coefﬁcient deﬁned by a ¼ 4pk/l for wavelength l. It is
found that these values compare very well with the center energy
parameter of the psemi-M0 parametric oscillator [40] used to ﬁt the
ellipsometric data, as seen in Fig. 9b. The values obtained are
EG, CdO ¼ 2.47 0.02 eV for CdO and EG, ZnO ¼ 3.29 0.03 eV for ZnO,
respectively, which is in good agreement with the previously reported bandgap values for CdO (2.2e2.7 eV) and ZnO (3.2e3.3 eV)
ﬁlms grown by other methods [28,31,32,34,35,51]. Fig. 9b also
shows a clear bandgap bowing as a function of composition
[28,52,53], such that the bandgaps can be ﬁtted by a quadratic
function E(x) ¼ xECdO þ (1 x)(EZnO ECdO) x(1 x)b, where the
bowing parameter b is found to be 2.0 eV (Fig. 9b). This strong
bowing is expected due to the valence band offset and lattice
mismatch of the binary materials [28]. The quadratic ﬁt results in a
minimum bandgap of 2.3 eV at a composition of x ¼ 0.7.
Table 2
CdO lattice parameter using direct comparison of Si and CdO reﬂection spots on SAD
pattern.
Rhkl, Si/Rhkl,
Si {200} &
CdO {200}
Si {111} &
CdO {111}
R200, Si/R200,
CdO
¼ 0.8635 0.0002
4.689 0.001
R111, Si/R111,
CdO
¼ 0.8642 0.0004
4.692 0.002
CdO
(Å)
aCdO ¼ aSi (Rhkl, Si/Rhkl, CdO) (Å)
Comparison
planes
Fig. 8. Optical constants derived from spectroscopic ellipsometry for different ﬁlm
compositions as indicated.
470
J.R. Bakke et al. / Materials Chemistry and Physics 140 (2013) 465e471
demonstrated in this work should allow for advances in transparent
conducting oxides for applications in electronics and photovoltaics.
Acknowledgments
This material is based upon work supported as part of the Center
on Nanostructuring for Efﬁcient Energy Conversion (CNEEC) at
Stanford University, an Energy Frontier Research Center funded by
the U.S. Department of Energy, Ofﬁce of Science, Ofﬁce of Basic
Energy Sciences under Award Number DE-SC0001060. J.R.B.
acknowledges funding from the Department of Defense (DoD)
through the National Defense Science and Engineering Graduate
(NDSEG) Fellowship and from the National Science Foundation
(NSF) Graduate Fellowship under Grant No. DGE-0645962. C.H.
acknowledges the Marcus and Amalia Wallenberg Foundation for
ﬁnancial support. We also recognize use of the Stanford Nanocharacterization Laboratory (SNL) and of the Center for Polymer
Interfaces and Macromolecular Assemblies (CPIMA).
References
Fig. 9. (a) Determination of direct allowed bandgap values from linear extrapolation to
zero of the function (ahn) [2]. (b) Direct bandgap dependence on composition for the
CdxZn1xO system from the extrapolation in (a) and from the bandgap energy
parameter of the psemi-M0 function. The quadratic ﬁts to each of these data sets gave
an average bowing parameter of 2.0 eV.
4. Conclusions
Deposition of the CdxZn1xO alloy via ALD is demonstrated at a
substrate temperature of 150 C using the precursors DMCd, DEZn,
and H2O. The ﬁlm thickness as a function of cycle number is linear
as expected for an ALD system and no noticeable impurities are
detected via XPS. The measured Zn content is greater than the cycle
ratio of deposition due to the higher growth rate of ZnO compared
to CdO. The ﬁlms are of hexagonal structure from ZnO through
Cd0.56Zn0.44O, whereas CdO has a rock salt cubic structure, in
agreement with previous studies. The crystallographic lattice constants match reported data. Analysis of transmission electron microscopy (TEM) images conﬁrms the phases present in XRD
patterns and the growth rate obtained by ellipsometry. Selected
area diffraction (SAD) crystallographic information indicates that
{111} planes of pure CdO ALD ﬁlm are preferentially oriented to the
ﬁlm surface. Spectroscopic ellipsometric analysis determines a
bandgap for CdO that matches literature values for the material
deposited by other methods, and the optical constants show trends
that reﬂect the changes in composition and crystal structure of the
alloys. A large bandgap bowing parameter of b ¼ 2.0 eV suggests
increased disorder in the ﬁlms near the hexagonal to cubic phase
transition point. The ability to deposit CdxZn1xO via ALD
[1] R.L. Puurunen, Journal of Applied Physics 97 (2005) 121301.
[2] S.M. George, Chemical Reviews 110 (2009) 111e131.
[3] L. Niinistö, M. Nieminen, J. Päiväsaari, J. Niinistö, M. Putkonen, Physica Status
Solidi (a) 201 (2004) 1443e1452.
[4] J.R. Bakke, K.L. Pickrahn, T.P. Brennan, S.F. Bent, Nanoscale 3 (2011) 3482e3508.
[5] H. Kim, H.-B.-R. Lee, W.J. Maeng, Thin Solid Films 517 (2009) 2563e2580.
[6] H. Makino, A. Miyake, T. Yamada, N. Yamamoto, T. Yamamoto, Thin Solid Films
517 (2009) 3138e3142.
[7] U. Malm, J. Malmstrom, C. Platzer-Bjorkman, L. Stolt, Thin Solid Films 480
(2005) 208e212.
[8] J.T. Tanskanen, J.R. Bakke, T.A. Pakkanen, S.F. Bent, Journal of Vacuum Science
& Technology A: Vacuum, Surfaces, and Films 29 (2011) 31507e31516.
[9] J.R. Bakke, J.S. King, H.J. Jung, R. Sinclair, S.F. Bent, Thin Solid Films 518 (2010)
5400e5408.
[10] Y.S. Kim, S.J. Yun, Applied Surface Science 229 (2004) 105e111.
[11] J.T. Tanskanen, J.R. Bakke, S.F. Bent, T.A. Pakkanen, Langmuir 26 (2010)
11899e11906.
[12] C. Platzer-Bjorkman, T. Torndahl, D. Abou-Ras, J. Malmstrom, J. Kessler,
L. Stolt, Journal of Applied Physics 100 (2006) 044506.
[13] C.T. Hsu, Thin Solid Films 335 (1998) 284e291.
[14] T. Torndahl, C. Platzer-Bjorkman, J. Kessler, M. Edoff, Progress in Photovoltaics
15 (2007) 225e235.
[15] J.W. Elam, S.M. George, Chemistry of Materials 15 (2003) 1020e1028.
[16] A. Hultqvist, M. Edoff, T. Törndahl, Progress in Photovoltaics: Research and
Applications 19 (2011) 478e481.
[17] J.R. Bakke, H.J. Jung, J.T. Tanskanen, R. Sinclair, S.F. Bent, Chemistry of Materials
22 (2010) 4669e4678.
[18] J.T. Tanskanen, J.R. Bakke, S.F. Bent, T.A. Pakkanen, The Journal of Physical
Chemistry C 114 (2010) 16618e16624.
[19] J.R. Bakke, J.T. Tanskanen, H.J. Jung, R. Sinclair, S.F. Bent, Journal of Materials
Chemistry 21 (2011) 743e751.
[20] D. Hariskos, S. Spiering, M. Powalla, Thin Solid Films 480e481 (2005) 99e109.
[21] C.A. Hoel, T.O. Mason, J.-F.O. Gaillard, K.R. Poeppelmeier, Chemistry of Materials 22 (2010) 3569e3579.
[22] C.X. Kronawitter, M. Kapilashrami, J.R. Bakke, S.F. Bent, C.H. Chuang,
W.F. Pong, J. Guo, L. Vayssieres, S.S. Mao, Physical Review B 85 (2012) 125109.
[23] X. Wu, Solar Energy 77 (2004) 803e814.
[24] A. Morales-Acevedo, Solar Energy 80 (2006) 675e681.
[25] X. Wu, T. Coutts, W. Mulligan, Journal of Vacuum Science & Technology A:
Vacuum, Surfaces, and Films 15 (1997) 1057e1062.
[26] M. Ristov, G. Sinadinovski, M. Mitreski, M. Ristova, Solar Energy Materials and
Solar Cells 69 (2001) 17e24.
[27] N. Romeo, A. Bosio, V. Canevari, A. Podestà, Solar Energy 77 (2004) 795e801.
[28] Y.Z. Zhu, G.D. Chen, H. Ye, A. Walsh, C.Y. Moon, S.-H. Wei, Physical Review B 77
(2008) 245209.
[29] D. Lamb, S. Irvine, Thin Solid Films 518 (2009) 1222e1224.
[30] D. Ellis, S. Irvine, Journal of Materials Science: Materials in Electronics 15
(2004) 369e372.
[31] N. Ueda, H. Maeda, H. Hosono, H. Kawazoe, Journal of Applied Physics 84
(1998) 6174.
[32] D. Ma, Z. Ye, L. Wang, J. Huang, B. Zhao, Materials Letters 58 (2004) 128e131.
[33] A. Dakhel, F. Henari, Crystal Research and Technology 38 (2003) 979e985.
[34] P. Jefferson, S. Hatﬁeld, T. Veal, P. King, C. McConville, J. Zúñiga-Pérez,
V. Munoz-Sanjose, Applied Physics Letters 92 (2009) 022101.
[35] G. Santana, A. Morales-Acevedo, O. Vigil, L. Vaillant, Thin Solid Films 373
(2000) 235e238.
[36] R. Saravanan, H. Shankar, T. Prakash, V. Narayanan, A. Stephen, Materials
Chemistry and Physics 125 (2011) 277e280.
J.R. Bakke et al. / Materials Chemistry and Physics 140 (2013) 465e471
[37] L. Li, Z. Yang, Z. Zuo, J.H. Lim, J.L. Liu, Applied Surface Science 256 (2010)
4734e4737.
[38] Z. Ye, D. Ma, J. He, J. Huang, B. Zhao, X. Luo, Z. Xu, Journal of Crystal Growth
256 (2003) 78e82.
[39] C. Yasemin, et al., Journal of Physics D: Applied Physics 42 (2009) 065421.
[40] B. Johs, C.M. Herzinger, J.H. Dinan, A. Cornfeld, J.D. Benson, Thin Solid Films
313 (1998) 137e142.
[41] Analytical Instrumentation Handbook, Marcel Denker, Las Vegas, New
Mexico, 1990.
[42] M. Ohring, The Materials Science of Thin Films, Academic Press Limited, 1992.
[43] C.D. Wagner, Analytical Chemistry 49 (1977) 1282e1290.
[44] J.R. Bakke, J.T. Tanskanen, C. Hagglund, T.A. Pakkanen, S.F. Bent, Journal of
Vacuum Science & Technology A: Vacuum, Surfaces, and Films 30 (2012),
01A135e01A138.
471
[45] A.B.M.A. Ashraﬁ, H. Kumano, I. Suemune, Y.W. Ok, T.Y. Seong, Applied Physics
Letters 79 (2001) 470.
[46] B. Sang, M. Konagai, Japanese Journal of Applied Physics 35 (1996) L602e
L605.
[47] A.R. Denton, N.W. Ashcroft, Physical Review A 43 (1991) 3161e3164.
[48] E.H. Kisi, M.M. Elcombe, Acta Crystallographica C 45 (1989) 1867e1870.
[49] Y. Raviprakash, K.V. Bangera, G.K. Shivakumar, Solar Energy 83 (2009) 1645e
1651.
[50] L.W. Yin, S.T. Lee, Nano Letters 9 (2009) 957e963.
[51] B. Cockayne, P.J. Wright, Journal of Crystal Growth 68 (1984) 223e230.
[52] S.V. Borse, S.D. Chavhan, R. Sharma, Journal of Alloys and Compounds 436
(2007) 407e414.
[53] N. Tit, I.M. Obaidat, H. Alawadhi, Journal of Physical: Condensed Matter 21
(2009) 6.

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