Supplementary Online Material
Electronic transport in highly conducting Si-doped ZnO thin films
prepared by pulsed laser deposition
Vladimir L. Kuznetsov, Alex T. Vai, Malek Al-Mamouri, J.Stuart Abell,
Michael Pepper, and Peter P. Edwards
1. Experimental:
Sample Preparation
Deposition targets were prepared from phase-homogeneous powders of a (ZnO)1-x(SiO2)x solid
solution (0≤x≤0.05) synthesized by a sol-gel technique. Zinc oxide (Alfa Aesar, 99.99%) was added to a
solution of citric acid (Sigma-Aldrich, >99.5%) heated to 90 °C, then nitric acid (Fisher Scientific, 65%,
analytical reagent grade) was added. The mixture was stirred at 90 °C for two hours until all ZnO is dissolved.
Silicon tetraacetate (Sigma-Aldrich, 98%) was dissolved in hot ethylene glycol (Sigma-Aldrich, 99.8%,
anhydrous) and then added to zinc citrate solution. The temperature of the mixture was increased gradually
(over 14 hours) to 150 °C to evaporate water, then the solution was heated in air to dryness at 250 ºC for 7
hours. The resultant powder was ground and heated overnight in air in quartz crucibles at 350 °C, then it was
ground again and heated at 600 °C for 7 hours.
The powders were compacted with 12-14 tons of force into ceramic pellets, 25 mm in diameter and 2
mm thick, using a pellet press and then sintered at 700 ° C for 7 hours in air to increase their mechanical
strength. X-ray diffraction (XRD) analysis of the deposition targets using a PANalytical X’Pert pro
diffractometer did not show any presence of crystalline SiO2 or Zn2SiO4 phases. Based on an analysis of the
noise statistics for the spectrometer used, the estimated detection limit for a secondary phase is ~0.1 mol%.
Si-doped ZnO (SiZO) thin films were then deposited by pulsed laser deposition onto borosilicate glass
substrates held at a temperature of 350 °C. The substrates had been sequentially cleaned with ethanol and
acetone in an ultrasonic bath and then blown dry with purified nitrogen gas before use. Target ablation was
performed using a KrF excimer laser (Lambda Physik LPX 300, λ = 248 nm). For a thin film deposition,
5000 pulses with energy of 250-400 mJ per pulse were used at a frequency of 10 Hz.1 The deposition chamber
was initially evacuated to 5×10-6 Torr and then backfilled with pure oxygen gas to a dynamic partial pressure
of 2 mTorr which was held constant during deposition.
Sample Characterization
The crystalline structure of the thin films was studied by X-ray diffraction using a PANalytical X’Pert
Pro diffractometer operating in Bragg-Brentano geometry with monochromated Cu Kα1 radiation.
The electrical conductivity and Hall coefficient of the films were determined with the DC van der
Pauw technique using an Ecopia Hall Measurement System (HMS-3000) equipped with a 0.55 T permanent
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magnet and a custom-built cryogenic cooling and heating system. Small Ohmic electrical contacts were made
to the thin films were made using indium solder.
Optical transmission spectra were measured using a Cary 5000 UV-visible-near-IR spectrophotometer
over a wavelength range of 200 to 2500 nm. The optical data were used calculate the film thickness by
modeling the interference fringes.2 The thickness of some samples was also confirmed using a surface
profilometer (Dektak 3) after etching a step into film with hydrobromic acid. All films in this series were
found to be about 275 nm thick. The transmission spectra are also modeled to exclude the effect of
interference fringes.2 Reported transparency values are then an average over the visible range (400-750 nm) in
the absence of interference fringes and are uncorrected for the glass substrate, which is itself ~92% transparent
in this range.
X-ray photoemission spectra were recorded on thin film samples at the National EPSRC XPS User's
service (NEXUS) at Newcastle University (UK) using an AXIS NOVA (Kratos Analytical, Manchester, UK)
instrument with an Al Kα X-ray source (1486.6 eV).
A JEOL JSM-5510LV microscope with a tungsten filament and equipped with an X-Act EDX
(Oxford Instruments) spectrometer was used in high vacuum mode for elemental analysis of some samples.
The EDX instrument uses a silicon-drift detector for energy discrimination and uses the AZtec software
package for data acquisition and processing. A relatively low accelerating voltage of 6 kV was used to ensure
that all signal was from the film and not the substrate; this was verified by confirming that no silicon signal
was observed in the EDX spectrum collected from an undoped (silicon-free) sample. Analyses were carried
out on ~100 μm x 100 μm sections of film.
2. Thin Film X-ray Diffraction:
Figure S1 displays representative X-ray diffraction patterns of SiZO thin films with different nominal
doping levels grown by PLD at a substrate temperature of 350 °C and a fixed oxygen partial pressure of 2
mTorr. Only diffraction peaks corresponding to the (002) and (004) planes of ZnO in the hexagonal wurtzite
structure are present, indicating that crystallites in these films have grown with a strong preference for c-axis
orientation perpendicular to the substrate plane. There is no evidence for SiO2, Zn2SiO4, or any other
secondary phases in the XRD patterns.
There is also no evidence for changes in crystallite size, crystallinity, or parameter across this range of
compositions, as no significant line-broadening or peak shifting is observed with increasing silicon doping
level.
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Figure S1. X-ray diffraction patterns of polycrystalline SiZO films with nominal silicon content of 0 at%
(black), 1 at% (red), 2 at% (green), and 3 at% (blue). Note that the signal intensity is plotted on a logarithmic
scale.
3. X-ray Photoelectron Spectroscopy:
The fact that silicon is being incorporated into the thin films samples can be easily observed using Xray photoelectron spectroscopy (XPS). The Si(2p) core level XPS spectra of the as-deposited surface of a 3 at%
SiZO film and the same surface after 40 seconds of Ar ion etching are shown in Fig. S2. The Si(2p) peaks can
be fitted well with one component centered at binding energies of ~102.5 eV for the as-deposited surface and
~102.8 eV for Ar-etched surface (Figure S3). These binding energies are consistent with those expected for
oxidized forms of silicon and are intermediate between tabulated values for SiO (101.4 eV) and SiO2 (103.6
eV).3, 4 However, a definitive determination of the nature and oxidation state of the Si species present in the
films is not possible from these data alone.
Figure S2.Silicon 2p XPS spectra of the as deposited (filled symbols) and Ar plasma etched (open symbols)
of a 3 at% Si-doped ZnO film.
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4. Measurements of Film Composition:
As with the targets from which the thin films are deposited, all doping levels in deposited thin films
are expressed as a molar fraction of total metals (silicon and zinc) in the sample. For instance, a 2 at% silicondoped ZnO film would have a Si-to-Zn ratio of 2-to-98.
The composition of some films in this study were measured using EDX and XPS and the results for
the samples examined are tabulated as follows:
Nominal / at% Si
0
0.2
0.5
2
3
Analyzed / at% Si
0 ± 0.2
0 ± 0.2
0.5 ± 0.2
1.3 ± 0.3
3.5
Method
EDX
EDX
EDX
EDX
XPS
This clearly demonstrates that the concentration of silicon in the deposited thin films is close to the
concentration of silicon in the corresponding target. Furthermore, it is clear from these data that the
concentration of silicon in the films increases monotonically with the amount of silicon in the targets. In the
text, samples are consistently referred to using their nominal doping values.
The silicon content was below detection limits by EDX for the samples with 0 and 0.2 at% nominal
silicon concentrations (as would be expected given the known sensitivity of the spectrometer used). Fig. S3
shows EDX spectra collected for samples with nominal concentrations of 0.5 and 2 at%. The found values
based on these spectra were 0.5 ± 0.2 at% and 1.3 ± 0.3 at%, respectively.
Figure S3. EDX spectra of polycrystalline SiZO films with nominal silicon content of 0.5 at% (left) and 2 at%
(right). Note that the inset is on the same energy scale but offset and shown with 500x vertical exaggeration.
A quantitative analysis of XPS data was performed by comparison of the integrated areas of the Si(2p)
and Zn(2p) peaks (after scaling by the tabulated Scofield relative scattering factors) and indicates that the
surface of SiZO samples is richer in silicon that the interior. This could result from the impurity segregation of
Si to the film surface because of its limited solubility in ZnO lattice. The analyzed bulk silicon concentration
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of the nominally 3 at% SiZO film after the surface had been removed by argon plasma etching was
approximately 3.5 at%.
References
1. V. L. Kuznetsov, D. H. O'Neil, M. Pepper and P. P. Edwards, J. Appl. Phys. 110, 033702 (2011).
2. R. Swanepoel, J. Phys. E: Sci. Instrum. 16, 1214 (1983).
3. A. Thøgersen, J. H. Selj and E. S. Marstein, J. Electrochem. Soc. 159, D276 (2012).
4. B. V. Crist, Handbook of monochromatic XPS spectra: The elements and native Oxides (XPS
International, Inc., 1999).
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