Structure of uniform and high-quality Al-doped ZnO films by magnetron sputter deposition at low temperatures

Accepted Manuscript Structure of uniform and high-quality Al-doped ZnO films by magnetron sputter deposition at low temperatures

Fanping Meng, Shou Peng, Genbao Xu, Yun Wang, Fangfang Ge, Feng Huang PII: DOI: Reference:

S0040-6090(18)30590-X doi:10.1016/j.tsf.2018.08.047 TSF 36861

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

22 April 2018 1 July 2018 7 August 2018

Please cite this article as: Fanping Meng, Shou Peng, Genbao Xu, Yun Wang, Fangfang Ge, Feng Huang , Structure of uniform and high-quality Al-doped ZnO films by magnetron sputter deposition at low temperatures. Tsf (2018), doi:10.1016/ j.tsf.2018.08.047

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ACCEPTED MANUSCRIPT Structure of uniform and high-quality Al-doped ZnO films by magnetron sputter deposition at low temperatures Fanping Meng,1

Shou Peng,2

Genbao Xu,2

Yun Wang,2

Fangfang Ge,1

Feng

Huang,1,* [email protected] 1

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences,

2

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Ningbo, Zhejiang 315201, China State Key Laboratory of Advanced Technology for Float Glass, Bengbu Design & Research

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Institute for Glass Industry, Bengbu, Anhui 233018, China *

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Corresponding author.

ACCEPTED MANUSCRIPT Abstract Spatial distribution of highly energetic negative ions inherent in magnetron sputtering of oxides has long made low temperature deposition unsuitable for high quality films uniform over relatively large areas.

Here we examine the distributions of structure as well as

physical properties of magnetron sputtered Al-doped ZnO (AZO) films deposited at low

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temperatures (≤393 K) in which the bombardment from the negative oxygen ions was systematically studied by changing the discharge voltage (i.e., ion energy) and the substrate

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position (i.e., ion flux). The film structure was characterized by X-ray diffraction, Raman

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spectroscopy, and transmission electron microscopy; and the electrical and optical properties were obtained by a Hall system and Spectroscopic Ellipsometry.

We found (i) that uniform

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yet high crystallite quality films can be obtained only when the energy of the negative ions

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was set below a threshold; (ii) that the ion flux exerted an ever-decreasing effect on modifying the film structure as the ion energy was reduced; and (iii) that a set of structural criteria,

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incorporating crystallite quality (orientations, size, lattice spacing) and point defects, were

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derived for low resistivity AZO films.

The benefit of lowering the ion energy is then

explained in terms of the favorable competition between radiation-induced defect generation and the subsequent dynamic annealing.

These findings may pave a way for large-area

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coating of high quality AZO films at low temperatures.

ACCEPTED MANUSCRIPT I.

INTRODUCTION Growth of thin films with a high structural order as well as tailor-made physical

properties is critical for many modern high technologies [1,2].

Vapor deposition of these

films typically involves various bombardment by energetic species.

In sputter deposition of

oxides, negative oxygen ions have been a significant source of bombardment in that they are

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accelerated to a high kinetic energy (typically >100 eV), depending on the discharge voltage

Vd , to impinge on the growing surface [3-7]. These highly energetic ions penetrate into the

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growing surface, and thus induce noticeable damage, for instance resputtering, compositional

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changes, and excessive structural imperfections (e.g., various point or line defects).

For

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magnetron discharges, a further complication comes into play because the inherent spatial distribution of these energetic negative ions, resulting from the nonuniform magnetic field, In

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manifests itself in a relatively higher ion flux from the erosion region of the target [4,5,7].

magnetron sputtered Al-doped ZnO (AZO) films, the poor uniformities in both the structure

For low-temperature (say <423 K) magnetron sputter deposition, these

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site [4,7-14].

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and the physical properties have been ascribed, in general, to varied ion fluxes from site to

distributions prevent the formation of uniform and high quality AZO films, especially when

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large-area coatings or temperature-sensitive substrates are needed. Microscopic structural heterogeneity in vapor deposited thin films reflects the spatial

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distribution of the amount of residual defects, which results from a site-specific balance between defect generation and their annihilation at the surface layers [14-16].

Under

kinetically constrained conditions in which thermally activated defect annihilation processes are limited, it is desirable to suppress the level of defect generation.

For magnetron

sputtered AZO films, this suppression can be realized through reducing the ion energy down to a still lower value.

Actually, it has been reported by Minami and Ellmer groups [7,10,17]

that the ion energy can be decreased — for example reducing the Vd

value by

superimposing a RF power onto a DC power — and thus the uniformity of the resistivity can

ACCEPTED MANUSCRIPT be improved.

However, the spatial distributions of the resistivity are still noticeable even at

Vd =110 V deposition conditions [10], which may be understood from the low defect formation energy (50~65 eV) for ZnO crystal [18,19]. sputtered AZO films at the still lower Vd

In our recent study of magnetron

value ~80 V, a pretty uniform and low resistivity

over the distance of the target diameter was readily obtained.

It is therefore of importance

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and of great interest in understanding the structural origin of this considerable improvement in

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electrical properties.

Here we report the structural evolution as well as the physical properties of magnetron

We show that, to achieve uniform films with high structural order, it is the

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the negative ions.

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sputtered AZO films low-temperature deposited under widely varied energies and fluxes of

ion energy, not the ion flux, that should be lowered down to a threshold.

The benefit of

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lowering the ion energy is rationalized in terms of favorable competition between radiation-induced defect generation and the subsequent dynamic annealing.

Our results

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suggest that the spatial distribution of the energetic negative ions inherent in magnetron

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sputter deposition does not pose a fundamental hurdle to its potential applications to

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large-area coating of high quality AZO films at low temperatures.

EXPERIMENTAL DETAILS

A.

Film deposition

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AZO films were deposited by magnetron sputtering a flat AZO ceramic target (2.0 wt.% Al2O3, 101 mm in diameter, and 4N in purity) in pure Ar discharges.

The energy of negative

ions was tuned by adjusting the discharge voltage ( Vd ) over the range of 220 V to 80 V through an 81 MHz RF-superimposed DC power supply, using the setup described in our previous work [14].

Here, the DC power was fixed at 50 W, while the RF power was fixed

at 50 W, 80 W, 120 W, and 200 W, respectively.

Corning EAGLE glass wafers (10×10 mm2

size) were used as substrates and placed from the position facing the center to the erosion of

ACCEPTED MANUSCRIPT the target, in order to investigate the effects of the flux of negative ions on our AZO films. During deposition the substrate temperature, measured by surface temperature indicating strips (Thermax®), was less than 393 K. Moreover, at the same Vd

value, an identical

temperature was measured for the samples located at different substrate positions.

In all runs,

the substrates were non-moving, positioned parallel to the target at a distance of ~50 mm, and All the deposition processes were carried out at a pressure of 0.5

profiles (see Table 1).

values, yielding the identical thickness

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Pa. Deposition time was kept constant for all Vd

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held at floating potential.

Prior to each deposition, the deposition chamber was evacuated to a

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vacuum better than 5.0×10-5 Pa, and the target was pre-cleaned for ~5 min at a 50 W DC power.

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Before being transferred into the process chamber, the substrates were sequentially cleaned by

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acetone, alcohol, and distilled water, and then blown dry with nitrogen flow.

Film characterization

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The film structure was characterized by X-ray diffraction (XRD), Raman spectroscopy,

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and high resolution transmission electron microscopy (HRTEM).

Specifically, XRD scans

were conducted on a Bruker D8 Advance diffractometer, equipped with a secondary

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monochromator selecting the Cu Kα radiation. All the symmetrical scans were recorded with a step of θ=0.01º and a speed of 0.6 s per step. The (002) peak profiles were fitted by the The lattice spacing of the (002) reflection d 002 was then

calculated using Bragg’s law.

The fraction of the (002)-oriented grains ( f 002 ) was calculated

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pseudo-Voigt shape function.

through the equation,

f 002 

s002 A002  shkl Ahkl

where shkl and Ahkl are the normalization parameter (JCPDS 36-1451) and the relative area, respectively, associated with the corresponding XRD reflections.

The instrumental

broadening for this configuration was determined to be 0.045º at 2θ=35.136º, a value close to

ACCEPTED MANUSCRIPT ~34.43º for the our AZO films, using a α-Al2O3 powder standard.

Raman spectra were

obtained in a backscattering geometry using a 325 nm laser as the excitation source, with a spectral resolution of 2 cm-1 (Renishaw plc, UK).

Cross-sectional TEM (XTEM) study was

performed on an FEI Tecnai F20 system. The optical properties in terms of transmittance and refractive index were analyzed by spectroscopic ellipsometry (SE) using an M-2000 DI For this study, three incidence angles (55º, 65º, and 75º)

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model tool (J. A. Woollam Co., Inc.).

of the measurements were used. Meanwhile, the Scotch tape was pasted onto the backside of To extract the

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the glass substrate to suppress the influence of the backside reflection.

previously addressed [20,21].

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refractive index ( n ), a Cauchy model combined with Urbach tail absorption term was used as Resistivity (  ), Hall mobility (  ), and carrier concentration

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( N ) of these AZO films were measured by a four-point probe, using a Hall system

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(Nanometrics HL5500PC) in the van de Pauw geometry at room temperature.

RESULTS

A.

Structural characterization

1.

X-ray diffraction

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III.

All reflections can be assigned to the

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Fig. 1 shows an overview of the XRD patterns.

hexagonal wurtzite-structured ZnO, and no other phases can be identified.

The predominant

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(002) reflection indicates a strong c-axis preferred orientation in these films, in excellent agreement with other magnetron sputtered AZO films [11,14,22].

Reflections other than

(002) were found in the films deposited at relatively strong bombardment characterized by a high ion energy (i.e., Vd 110 V) and high ion flux (i.e., the position facing erosion region). Bombarded by energetic negative ions, the grain growth toward close-packed direction (i.e., [002] for ZnO) can be suppressed since the close-packed (002) plane incurs more irradiation damage than other planes, which resulted in the development of non-(002) orientations [5]. This trend can be better appreciated from Fig. 2, in which the fraction of the (002) reflection,

ACCEPTED MANUSCRIPT f 002 , was summarized in terms of the ion flux and ion energy.

More importantly, the

dependence of the f 002 value on the ion flux became more significant as the ion energy was increased. Of particular interest is that the f 002 value exhibited a negligible dependence on the ion flux when the ion energy was decreased down to Vd =80 V. This negligible flux dependence also manifests itself in the full width at half

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maximum (FWHM) and the lattice spacing of the (002) reflections.

Fig. 3a gives some

A notable feature is that lower Vd

values typically led to smaller FWHM

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for all samples.

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representative (002) profiles, from which the FWHM value was derived and plotted in Fig. 3b

This trend

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values and a better distribution thereof (see, for example, the Vd =80 V case).

suggests that the uniformity of the crystallinity can be improved through suppressing the The corresponding lattice spacing, d 002 , is

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structural damage induced by ion bombardment.

shown in Fig. 3c, from which a similar trend to the FWHM can be readily identified.

Typically, a greater-than-bulk d 002 value, or c-axis expansion, is related to

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the bulk ZnO.

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Particular noteworthy is that the d 002 values for the Vd =80 V case are smaller than that for

the generation of various supersaturated point defects such as interstitials and vacancies,

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resulting from excessive ion bombardment [7,23].

These smaller-than-bulk d 002 values

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thus suggest a negligible amount of residual structural damage [14,24].

Raman spectroscopy Raman spectroscopy studies reveal that, when the energy of the negative ions was

reduced in our AZO films, the effect of bombarding ion flux on the structural order gradually become undetectable.

Fig. 4 shows the spectra taken from the films deposited at two

representative fluxes: (a) the lowest and (b) the highest.

Most films show two broad peaks,

centered at about 560 cm-1 and 500 cm-1, which represents longitudinal optical (LO) phonons and additional mode (AM) of wurtzite ZnO crystal, respectively [25-28].

The salient feature

ACCEPTED MANUSCRIPT of these spectra in Fig. 4 is a strong correlation between the relative intensity of LO band and the level of the bombardment.

Specifically, the LO band was pronounced when the ion

energy was high enough (for example Vd ≥110 V), especially when the films were grown under a higher ion flux (Fig. 4b). became weaker.

With the decreasing ion energy, the LO band gradually

For the Vd =80 V case, the two Raman spectra become indistinguishable,

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pointing to a high structural order almost independent of the flux of bombarding negative

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ions.

To better visualize the difference quantitatively in our Raman spectra, we have

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curve-fitted each spectrum using three Lorentzian peaks, as shown in our previous report [29].

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The intensity of the LO band and that of the AM peak (ILO/IAM) was plotted in Fig. 5.

Here,

lower Vd values typically led to smaller ILO/IAM ratios and a better distribution thereof.

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Again, flux-independent ILO/IAM ratios were found for the Vd =80 V case.

This observation

provides further support to the importance of lowering the ion energy in suppressing defect

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generation and thereby the microscopic structural inhomogeneity.

Transmission electron microscopy

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We performed XTEM studies on the sample with the least radiation-induced structural

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damage, that is, the film prepared at the location facing the target center and Vd =80 V. Fig. 6a is the low-magnification bright-field (BF) image, in which densely packed and well-aligned columns were observed.

The columns were in the range of 30 to 70 nm in diameter and

extended throughout the thickness of the film. A closer analysis reveals the competitive grain growth at the zone of ≤50 nm adjacent to the substrate.

The HRTEM image (Fig. 6b) sheds

more details on the structure of inner-columnar and inter-columnar boundaries in the film. The adjacent columns were both (002)-oriented with respect to the growth direction, in agreement with the mentioned XRD results (Fig. 1), and with a misorientation of <2º between

ACCEPTED MANUSCRIPT them.

No obvious grain boundaries or disturbed region between the grains can be found,

repeatedly pointing to a higher crystallization quality of the AZO film prepared at less radiation-damage processes.

In stark contrast, lots of defects are found in the films by

reactive DC magnetron sputtering at a low deposition temperature [30].

Optical properties

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B.

The effect of the ion bombardment on the films are also revealed by examining the

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optical properties such as refractive index ( n ) and transmittance.

Fig. 7(a) gives two The good

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representative ellipsometric spectra and their corresponding fitting curves.

agreement indicates that the used optical model, consisting of a glass substrate, a AZO film,

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and a roughness layer, is adequate to describe the ellipsometric data of these films.

The refractive index derived

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thickness of roughness layer was about 2~3 nm for each film.

The

from this fitting analysis is given in Fig. 7b, which shows a noticeable correlation between n A higher refractive index is observed when the film was

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values and the ion bombardment.

For each Vd

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deposited under a stronger bombardment, in terms of either the ion energy or the ion flux. value, the refractive index ( n ) reached a maximum ( nmax ) at the highest ion

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flux, followed by a steady decrease to a minimum ( nmin ) with the ion flux. This trend is expected since a stronger bombardment typically leads to more effective densification [14,31].

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Another point is that the range of the refractive index, nmax  nmin , at each Vd decreased with the ion energy.

value

Actually, this range turned into negligible when the ion

energy was lowered down to an ultralow level of Vd =80 V. The ion bombardment had a wavelength-dependent effect on the optical transmittance, as illustrated in Fig. 8, in which a higher transmittance was found in the near-infrared (NIR) range for the films with a stronger bombardment, while a similarly high visible (400-800 nm range) transmittance, >81% on average, was observed for all samples regardless of the bombardment levels.

A similar behavior over the visible spectral range has been previously

ACCEPTED MANUSCRIPT reported by others [10]. The wavelength-dependent transmittance over the NIR range, we believe, can be explained by the absorption from the free carriers [20].

C.

Electrical properties The spatial distribution of electrical properties of our AZO films also exhibited a Fig. 9

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noticeable dependence on both the energy and the flux of bombarding negative ions.

shows a well-informed behavior of the resistivity (  ) with the level of the bombardment,

Specifically, the films irradiated by a higher bombarding energy (i.e.,

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fluxes dependence.

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within which reducing the bombarding energy leads to a decrease of both  value and its

strongly depends on the bombarding flux.

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Vd ≥110 V) exhibited the deteriorated electrical properties.

The degree of the damage

As an illustration at Vd =220 V, the maximal 

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was over two times of the minimum (i.e., 20×10-4 vs. 9×10-4 Ω cm).

The nonuniformity r is

estimated to be 1.2 by (max-min)/min, where max and min are maximum and minimum In contrast, when the ion energy was reduced to an ultralow level

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resistivities, respectively.

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(such as Vd =80 V conditions), a pretty constant (r=0.3) and lower resistivity (3-4×10-4 Ω cm) was identified than the values reported in previous works [7,9,10,17], supporting the

Although the film thickness varied from ~380 nm to ~550 nm at different substrate

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damage.

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dominant role of lowering the ion energy in reduction of the radiation-induced structural

locations (Table 1), for Vd =80 V film for instance, the resistivity has no strong film-thickness dependence. Hall mobility (  ) and carrier concentration ( N ) of the AZO films (Fig. 10) were then explored according to the relationship   1/(eN  ) . An ever-increasing  and N with the reducing energy of the bombarding ions was identified. This finding is in accordance with previous reports in the film prepared by comparable methods [7,10,22].

Moreover,

both  and N of these films obtained their maximal values at the position facing the

ACCEPTED MANUSCRIPT target center, where the flux of negative ions is the least.

Particularly important is the

negligible spatial distribution of both N (~61020 cm-3) and μ (~30 cm2/Vs) with the flux of bombarding negative ions, when the ion energy was further reduced to an ultralow level (i.e., Vd =80 V in our case).

DISCUSSION

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IV.

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The results presented in Sec. III show that, for the bombarding negative ions, there exists an energy threshold, below which these ions produced negligible residual structural

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damage and spatial distribution thereof, as illustrated in Fig. 11 (Zone I).

Above the energy

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threshold (Zone II in Fig. 11), the bombarding negative ions generated various kinds of structural defects, such as supersaturated point defects, reduced crystallite size, and mixed The

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crystalline orientation, at such a high level that their full annealing cannot be achieved.

nature of the residual defects depends on the level of the ion bombardment in terms of the

This energy threshold should be related to the defect formation

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labeled by IIa, IIb, and IIc.

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energy and the flux; Zone II in Fig. 11 can thus be further sub-divided into three parts as

energy and would be influenced by sputtering processes such as substrate temperature and A similar trend also manifests itself in the evolution of the corresponding

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deposition rate.

physical properties, in terms of the refractive index, the NIR transmittance, and the resistivity.

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Our results suggest that, when tuning the structural order as well as the physical properties of AZO films at low temperatures, the energy of the bombarding negative ions is more important.

In the case of magnetron sputter deposition, the ion flux is significant only

when the ion energy is too excessive.

In other words, the ion energy and the ion flux have

played different roles in the defect generation in our magnetron sputtered AZO films.

A.

Key structural features There have been some reports on magnetron sputtered AZO films deposited at

ACCEPTED MANUSCRIPT relatively low temperatures (such as <423 K).

These studies, typically performed under Vd

90 V conditions, have also identified various bombardment-induced structural features (such as c-axis expansion, reduced crystallite size, etc.), as a function of either the ion energy or the ion flux.

For example, Bikowski et al. showed that a lower ion energy (i.e., Vd = 450 V vs.

Vd =150 V) constantly leads to a smaller c-axis expansion as well as a larger crystallite size, In other instances, increasing the flux of highly energetic

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irrespective of the flux [7].

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negative ions alone, for example, by subjecting the substrates to different target areas while

non-(002) orientations, respectively [8,9,32].

These structural modifications are expected

zones (i.e., Zone IIa, IIb, or IIc) in Fig. 11.

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from the high Vd

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keeping Vd >300 V, results in a higher c-axis expansion, a reduced crystallite size, or

If the ion energy can be

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further reduced, however, not only the c-axis expansion is replaced by a slight contraction, but also a complete (002) orientation is obtained even at the highest ion flux, as shown in the Vd In addition, we emphasize

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=80 V case here (Zone I in Fig. 11) and our previous study [14].

the highest flux.

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here that, at Vd =80 V, increasing the ion flux leads to negligible structural change, even at

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Based on these considerations, we could identify some key structural features that are desirable for high quality AZO films with a low electrical resistivity (~310-4 Ω cm) and a

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high optical transmittance (>81% in visible range).

These features reflect contributions from

both constituent crystallites (orientation, size, and lattice spacing) and point defects. Specifically, they include (i) a complete (002) orientation, (ii) a <0.25 FWHM (2θ) for the (002) reflection, (iii) a c-axis contraction along the out-of-plane direction, and (iv) absence of the Raman LO mode (~560 cm-1). Previously, either a complete (002) orientation or a smaller FWHM value has been identified necessary for a low resistivity (~510-4 Ω cm) in magnetron sputtered AZO films [33-35].

A comparison of the FWHM values [Fig. 3(b)]

with the resistivities (Fig. 9), however, immediately reveals that the FWHM criterion alone is

ACCEPTED MANUSCRIPT not sufficient for our low resistivities.

The inadequacy of criterion (i) alone, i.e., a complete

(002) orientation, can also be identified by comparing Fig. 2 with Fig. 9.

To further improve

the electrical properties, the film structure should be examined in more detail to incorporate more features such as (iii) and (iv). independent of each other.

These four structural features, of course, might not be

More investigations are thus needed to clarify their relationships.

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We believe, however, that they could be useful guidelines for further process development and

Origin of structural evolution

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B.

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microstructure optimization.

For thermodynamically far-from-equilibrium processes such as vapor deposition at

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relatively low temperatures (<0.3Tm), the amount of residual structural defects reflects the

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competition between defect generation and annihilation (i.e., dynamical annealing) [14-16]. In the growth of our AZO films, the defect generation processes depend on both the energy

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and the flux of the impinging ions, while the defect annihilation is controlled by the

annealing).

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thermally-activated atomic rearrangements occurring at the growing surface (i.e., dynamic The dynamical annealing under each Vd

value should be equivalent, because

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the films were grown under an identical temperature (as confirmed by our measurements by Actually, the dynamic annealing under the different Vd

the Thermax® strips).

values in

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current investigation should be comparable, given their relatively low homologous temperatures (≤0.17Tm). rate-limiting process.

The thermally activated defect annihilation stage, we believe, is the

In other words, the amount of residual structural damage in our AZO

films depends primarily on the defect generation. The improved uniformity in structural order with a reduced Vd

value, therefore, can

be ascribed to lowered dependence of the defect generation on the ion flux.

A higher flux

typically generates more defects because more atoms will be displaced, in principle, from their lattice sites.

The degree to which these defects are increased, however, depends on the

ACCEPTED MANUSCRIPT ion penetration into the subsurface layers, and thus the ion energy [16,31].

Highly energetic

negative ions render the AZO films structurally nonuniform, because only a small percentage of the defects generated can be dynamically annealed out.

The large difference in resistivity

shown in Fig. 9 (for example, by up to ~120% for the Vd =220 V case) may be seen as an Such flux-dependent distribution is alleviated when the ion energy is

reduced — as the Vd

value is lowered, but still persist at a non-negligible level, as If the ion energy is further reduced down

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evidenced by the Vd =110 V case (Fig. 2Fig. 5).

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indicator of this fact.

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to such an ultralow level that most, if not all, of the radiation-induced defects — even at the highest flux — can now be dynamically annealed over the relatively shallower ion penetration, This is

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the structural distribution should be reduced to negligible (i.e., good uniformity).

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exactly the case for the Vd =80 V case (Fig. 3 and Fig. 5).

Implications for engineering applications

be readily derived.

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From the results and discussion presented above, some technological significance can Specifically, the excellent structural uniformity shown in the Vd =80 V

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case, and the corresponding uniform electrical properties, suggest that large-area low-temperature magnetron sputter deposition of high quality AZO films can be achieved by

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reducing the discharge voltage to such an ultralow level that various structural defects generated by the bombarding negative ions, even at the highest flux, can be effectively removed by dynamic annealing. impact on the deposition rate.

Additionally, the reduction of Vd

has no significant

This finding was in accordance with the observation reported

by Ito et al., who also deposited AZO films using the similar approach [22]. deposition rate with reducing Vd

The constant

can be attributed to the compensation for the lowered

sputtering yield by the increased ion current.

Our results thus provide an alternative to

uniform film structure, in addition to the conventional method of movable substrates [13,36],

ACCEPTED MANUSCRIPT which just average the radiation-induced defects over the whole substrate rather than eliminate them.

V.

Conclusions The present results show that the structural features of low-temperature (<393 K)

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magnetron sputter deposition of AZO films, as well as the corresponding optical and electrical properties, depend sensitively on both the energy and the flux of the bombarding negative

We identify a threshold for the ion energy — obtained at Vd ~80 V in

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into indiscernible.

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ions; with a reduce ion energy, the effect of ion flux on the structural variation gradually turns

can be achieved.

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our case — below which excellent uniformity in film structure and hence electrical properties This beneficial effect is rationalized in terms of favorable competition

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between radiation-induced generation of various types of defects and their dynamic annealing (i.e., thermally activated defect diffusion processes during film growth).

Furthermore, we

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derive a set of structural criteria, incorporating both the crystallite quality (orientations, size, Our

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and lattice spacing) and point defects, that are necessary for low resistivity AZO films.

results suggest that the ion energy, not the ion flux, should be controlled in large-area coating

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of high quality AZO films by magnetron sputtering at low temperatures, which may be

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applicable to other TCO thin films.

ACKNOWLEDGEMENTS This research is supported by the Open Project of State Key Laboratory of Advanced Technology for Float Glass (NO. KF1601), Zhejiang Postdoctoral Preferential Foundation, and Zhejiang Key Research and Development Program (No. 2017C01001).

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Fig. 1 (Color online) XRD patterns as a function of Vd

value (i.e., ion energy): (a) 220 V,

(b) 150 V, (c) 110 V, and (d) 80 V for the AZO films located at different substrate positions (as labeled on the right).

The feature at 2~33.3 in some patterns is a satellite of (002)

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peak due to the spectral dispersion of our x-rays.

The relative radius “0” and “1” represent the

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of Vd values and substrate positions.

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Fig. 2 (Color online) The dependence of f 002 (as labeled in the figure) on the combination

substrate position facing the lowest (i.e., target center) and the highest ion flux (i.e., Lines A and B are guides to the eye and have no

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erosion region), respectively.

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physical signification.

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Fig. 3 (Color online) (a) Typical (002) reflections for the AZO films irradiated by the highest value. (b) the FWHM and (c) d 002 values

from (002) reflections at different Vd

values and substrate positions.

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level of bombardment under each Vd

The dotted

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line in (c) indicates the value for bulk ZnO (JCPDS 36-1451).

Fig. 4 (Color online) Raman spectra of the AZO films irradiated by (a) the lowest and (b) the highest ion flux under each ion energy (i.e., Vd

value).

Note that the intensity of

each Raman spectrum was normalized.

Fig. 5 (Color online) Intensity ratio between LO band and AM mode from the Raman spectra as a function of both the Vd value (i.e., ion energy) and substrate position (i.e., ion

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Fig. 6 Cross-sectional TEM image with (a) low-resolution and (b) high-resolution (HRTEM) of the AZO film irradiated by our lowest level of bombardment ( Vd =80 V and

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substrate location facing the target center).

Fig. 7 (Color online) (a) Representative measured (symbol) and fitted (line) ellipsometric (b) Refractive index (at 550 nm

value and substrate position.

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wavelength) as a function of Vd

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data for Vd =220 V sample at erosion region.

Fig. 8 (Color online) Optical transmittance spectra of the AZO films deposited at two values (i.e., 80 V and 220 V cases) and substrate positions (i.e., facing

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different Vd

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target center and erosion region).

Fig. 9 (Color online) Electrical resistivity of the AZO films prepared at various Vd

values

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and substrate positions.

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Fig. 10 (Color online) Hall mobility and carrier concentration of the AZO films prepared at different substrate positions and Vd

values.

Fig. 11 (Color online) A schematic diagram of the structural evolution in our AZO films as a function of the combination of energy and flux of the negative ions. in the figure is plotted based on the type of the detected defects.

The dotted line

Cross () symbols

represent the films with the structural feature of c-axis expansion; squares (□) represent the feature of >0.25 FWHM value; and triangles (▲) represent the feature

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of f 002 <97%.

ACCEPTED MANUSCRIPT Table 1 The thickness (nm) of our AZO films deposited at different discharge voltages ( Vd ) and different substrate locations.

Substrate location

Vd (V) 1/3

2/3

1 (erosion)

220

555

540

491

422

150

542

528

110

563

540

80

545

507

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0 (center)

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482

403

481

390

460

382

ACCEPTED MANUSCRIPT Highlights Magnetron sputtering of AZO films at ultra-low discharge voltages.

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Uniform and high-quality AZO films were achieved.

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Structures were characterized by XRD, Raman, and TEM methods.



A set of structural criteria were derived for low resistivity AZO films..

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11