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Optimization of Conductivity 

and Transparency in Amorphous 

In-Zn-O Transparent Conductors 

Preprint 

J. Perkins, J. Berry, M. van Hest, J. Alleman, 

M. Dabney, L. Gedvilas, and D.S. Ginley 

National Renewable Energy Laboratory 

A. Leenheer and R. O'Hayre 

Colorado School of Mines

Presented at the 33rd IEEE Photovoltaic Specialists Conference 

San Diego, California 

May 11–16, 2008 

Conference Paper 

NREL/CP-520-42546 

May 2008 

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OPTIMIZATION OF CONDUCTIVITY AND TRANSPARENCY 

IN AMORPHOUS IN-ZN-O TRANSPARENT CONDUCTORS

John Perkins, Joseph Berry, Maikel van Hest, Jeff Alleman, Matthew Dabney, Lynn Gedvilas, David S. Ginley 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 

Andrew Leenheer and Ryan O’Hayre 

Colorado School of Mines, Golden, CO 80401 

ABSTRACT 

Amorphous mixed metal oxide TCOs are of increasing interest due to the excellent opto-electronic properties 

and smoothness (RRMS < 0.5 nm) obtained for sputtered 

films deposited at less than 100 °C. In particular, for 

amorphous In-Zn-O (a-IZO) films grown from a ceramic 

target with 10 wt. % ZnO in In2O3, the current industry 

standard, conductivities σ ≥ 2500 S/cm are common. 

Here, we have investigated the combined materials phase 

space of oxygen stoichiometry and metals composition 

(In:Zn ratio) and made two key discoveries. First, that 

high conductivity a-IZO thin films can be made with substantially less indium provided that a corresponding 

change is also made in the oxygen content. And second, 

that for all compositions of a-IZO, the electron mobility (μ) 

and carrier concentration (N) fall on a single common 

curve when plotted as μ vs N. 

INTRODUCTION 

Transparent conducting oxides (TCOs) can serve a 

variety of important functions in thin film photovoltaics 

such as transparent electrical contacts, antireflection coatings and chemical barriers [1]. An area of particular interest and active development is amorphous mixed metal 

oxide transparent conductors which can be grown by 

sputtering at low, or even ambient, temperature. For example, amorphous In-Zn-O (a-IZO) films grown from a 

ceramic target with 10 wt. % ZnO in In2O3, the current 

industry standard, commonly have conductivities σ ≥ 2500 

S/cm and are extremely smooth as well with RRMS < 0.5 

nm [2,3]. Here, we report on two experiments two experiments aimed at determining the relative roles of metals 

and oxygen stoichiometries on the opto-electronic properties of a-IZO thin films. 

EXPERIMENTAL APPROACH

First, using co-sputtering from In2O3 and ZnO targets, 

compositional gradient samples spanning the amorphous 

composition range (50 – 85 at.% In) were deposited at a 

substrate heater temperature TS = 100 °C in Ar containing 

0, 2, 4 or 6 % O2 onto 2”x2” Corning 1737 glass substrates. For each composition gradient sample, 11 spots 

were measured along the composition gradient to obtain 

data for different metals compositions. The typical measurement spot size is ~ 1 mm. X-ray diffraction was used 

to determine if the samples were amorphous or crystalline. A mapping 4-point probe system was used to determine the sheet resistance. Optical reflection and transmission spectra were measured from 300 – 1100 nm using fiber optically coupled CCD array spectrometers. The 

sample thickness was determined on a spot by spot basis 

from the thin film interference oscillations in the reflection 

and transmission spectra [4]. 

RESULTS AND DISCUSSION 

Figure 1 uses a color intensity scale to show the conductivity of a-IZO as a function of both the oxygen content 

of the sputter gas (left axis) and the relative metals composition (bottom axis). Yellow spots indicate a high conductivity, σ > 2000 S/cm. Starting from the bottom of the 

graph, a band of high conductivity angles up and to the 

right. The solid black line shows the ridge of maximum 

conductivity. The fact that this line runs diagonally in this 

(O2, %In) plot shows that the optimization of conductivity 

in a-IZO is a coupled process with the best metals composition depending upon the oxygen content of the sputter 

gas. 

In addition to conductivity, transparency is a critical 

property for thin film transparent conductors. Figure 2 

Fig. 1: Conductivity of as-deposited IZO as a function 

of oxygen content in the sputter gas (left axis) and 

relative metals composition (bottom axis). 

 1
shows optical absorption spectra for several representative a-IZO with differing metals compositions but all grown 

in pure Ar (0% O2). The a-IZO samples with 39 cation% 

In (red line) and 61 cation% In (dashed blue line) show 

almost no absorption for wavelengths longer than about 

450 nm. However, for the a-IZO samples with higher indium content, a nearly wavelength independent absorption increases with increasing indium content. For the 

sample with 78 cation% indium, the visible wavelength 

absorption is nearly 50%. Examination of the absorption 

spectra for a-IZO samples grown in 2 to 6% O2 (not 

shown) show that this broad visible wavelength absorption 

occurs when the relative indium content is higher that the 

indium content to obtain the best conductivity for the given 

amount of oxygen used (see Figure 1). 

Subsequently, constant composition IZO films were 

sputter deposited onto ambient temperature fused silica 

substrates from single composition targets with x = 0.6, 

0.7, 0.8 and 0.84 (InxZn1-xOy). The oxygen content of the 

Ar sputter gas was varied from 0 to 10 %, yielding a variation in the conductivity from σ ≈ 2 x103

 S/cm down to σ ≈

10-6 S/cm. For these samples, additional Hall effect 

measurements were done to determine the carrier concentration (N) and electron mobility (μ). Figure 3 shows 

the mobility as a function of the carrier concentration for 

the a-IZO samples with N ≥ 1018/cm3

. Surprisingly, for all 

these a-IZO samples, the mobility as a function of the 

carrier concentration is given by a common curve, independent of the differing metals compositions [5]. This 

supports the results shown in Figure 1. At N ≈ 1020/cm-3, 

the mobility maximum with μ ≈ 50 cm2

/V-s, the mobility 

increased upon cooling to 100 K indicating metallic conduction for the highest mobility a-IZO materials. At low 

carrier concentration, N < 1019/cm3

, the mobility is thermally activated and at very high carrier concentration, N ≈

5 x 1020/cm3

, the mobility is nearly temperature independFig. 2: Optical absorption spectra of IZO samples ent suggestive of screened ionized impurity scattering. 

with varying In content. All samples grown in pure Ar 

(0% O2) SUMMARY 

In summary, amorphous In-Zn-O (a-IZO) thin film 

transparent conductors with good TCO properties can be 

easily deposited at near-ambient temperatures. The aIZO thin films are also extremely flat. Taken together, 

these attributes make sputtered a-IZO thin film TCOs an 

attractive choice for many PV applications. 

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-99GO10337 with 

the National Renewable Energy Laboratory. 

REFERENCES 

 [1] D.S. Ginley and C. Bright, "Transparent Conducting 

Oxides", MRS Bulletin 25, 2000, pp. 15. 

[2] M.P. Taylor, D.W. Readey, C.W. Teplin, M.F.A.M. 

Van Hest, J.L. Alleman, M.S. Dabney, L.M. Gedvilas, B.M. 

Keyes, B. To, J.D. Perkins and D.S. Ginley, "The electrical, optical and structural properties of InxZn1-xOy (0 <= x 

<= 1) thin films by combinatorial techniques.", Measurement Science and Technology 16, 2005, pp. 90. 

Fig. 3: Mobility vs. carrier concentration for IZO samples grown from single fixed composition sputter targets as indicated in the legend. 

[3] T. Minami, T. Kakumu, Y. Takeda and S. Takata, 

"Highly transparent and conductive ZnO-In2O3 thin films 

prepared by dc magnetron sputtering", Thin Solid Films

291, 1996, pp. 1-5. 

[4] J.D. Perkins, C.W. Teplin, M. Van Hest, J.L. Alleman, 

X. Li, M.S. Dabney, B.M. Keyes, L.M. Gedvilas, D.S. Ginley, Y. Lin and Y. Lu, "Optical analysis of thin film combinatorial libraries", Applied Surface Science 223, 2004, pp. 

124-132. 

[5] A.J. Leenheer, J.D. Perkins, M. Van Hest, J.J. Berry, 

R.P. O'hayre and D.S. Ginley, "General mobility and carrier concentration relationship in transparent amorphous 

indium zinc oxide films", Physical Review B 77, 2008, 

115215

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Optimization of Conductivity and Transparency in Amorphous 

In-Zn-O Transparent Conductors: Preprint 

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14. ABSTRACT (Maximum 200 Words)

Amorphous mixed metal oxide TCOs are of increasing interest due to the excellent opto-electronic properties and 

smoothness (RRMS < 0.5 nm) obtained for sputtered films deposited at less than 100 °C. In particular, for 

amorphous In-Zn-O (a-IZO) films grown from a ceramic target with 10 wt. % ZnO in In2O3, the current industry 

standard, conductivities σ ≥ 2500 S/cm are common. Here, we have investigated the combined materials phase 

space of oxygen stoichiometry and metals composition (In:Zn ratio) and made two key discoveries. First, that high 

conductivity a-IZO thin films can be made with substantially less indium provided that a corresponding change is also 

made in the oxygen content. And second, that for all compositions of a-IZO, the electron mobility (μ) and carrier 

concentration (N) fall on a single common curve when plotted as μ vs N. 

15. SUBJECT TERMS

PV; amorphous silicon; transparent conducting oxides; electrical contacts; antireflecftion coatings; chemical barriers; 

X-ray diffraction; thin film 

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