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 ..deg..C. Here, we have investigated the combined materials phase space of oxygen stoichiometry and metals composition (In:Zn ratio) and made two key discoveries

Alleman, J.

Berry, J.

Dabney, M.

Gedvilas, L.

Ginley, D. S.

Leenheer, A.

O'Hayre, R.

Perkins, J.

van Hest, M.

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 A national laboratory of the U.S. Department of EnergyOffice of Energy Efficiency & Renewable EnergyNational Renewable Energy Laboratory Innovation for Our Energy Future 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 NREL is operated by Midwest Research Institute ● Battelle     Contract No. DE-AC36-99-GO10337  NOTICE The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes. This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Available electronically at http://www.osti.gov/bridge Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone:  865.576.8401 fax: 865.576.5728 email:  mailto:reports@adonis.osti.gov Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone:  800.553.6847 fax:  703.605.6900 email: orders@ntis.fedworld.gov online ordering:  http://www.ntis.gov/ordering.htm Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste 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 increas-ing 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 sub-stantially 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 coat-ings and chemical barriers [1].  An area of particular inter-est and active development is amorphous mixed metal oxide transparent conductors which can be grown by sputtering at low, or even ambient, temperature. For ex-ample, 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 experi-ments aimed at determining the relative roles of metals and oxygen stoichiometries on the opto-electronic proper-ties 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 sub-strates.  For each composition gradient sample, 11 spots were measured along the composition gradient to obtain data for different metals compositions.  The typical meas-urement spot size is ~ 1 mm.  X-ray diffraction was used to determine if the samples were amorphous or crystal-line.  A mapping 4-point probe system was used to deter-mine the sheet resistance.  Optical reflection and trans-mission spectra were measured from 300 – 1100 nm us-ing 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 con-ductivity of a-IZO as a function of both the oxygen content of the sputter gas (left axis) and the relative metals com-position (bottom axis).  Yellow spots indicate a high con-ductivity, 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 com-position 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).  1shows optical absorption spectra for several representa-tive 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 in-dium content, a nearly wavelength independent absorp-tion 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 varia-tion 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 con-centration (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, inde-pendent 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 con-duction for the highest mobility a-IZO materials.  At low carrier concentration, N < 1019/cm3, the mobility is ther-mally activated and at very high carrier concentration, N ≈ 5 x 1020/cm3, the mobility is nearly temperature independ-ent suggestive of screened ionized impurity scattering.   Fig. 2:  Optical absorption spectra of IZO samples 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 a-IZO 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 electri-cal, optical and structural properties of InxZn1-xOy (0 <= x <= 1) thin films by combinatorial techniques.", Measure-ment Science and Technology 16, 2005, pp. 90.  Fig. 3:  Mobility vs. carrier concentration for IZO sam-ples grown from single fixed composition sputter tar-gets 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. Gin-ley, Y. Lin and Y. Lu, "Optical analysis of thin film combi-natorial 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 car-rier concentration relationship in transparent amorphous indium zinc oxide films", Physical Review B 77, 2008, 115215  2F1147-E(09/2007) REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-0188 The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION. 1. REPORT DATE (DD-MM-YYYY) May 2008 2. REPORT TYPE Conference Paper 3. DATES COVERED (From - To) 11-16 May 2008 5a. CONTRACT NUMBER DE-AC36-99-GO10337 5b. GRANT NUMBER  4. TITLE AND SUBTITLE Optimization of Conductivity and Transparency in Amorphous  In-Zn-O Transparent Conductors: Preprint 5c. PROGRAM ELEMENT NUMBER  5d. PROJECT NUMBER NREL/CP-520-42546 5e. TASK NUMBER PVA74201 6. AUTHOR(S) J. Perkins, J.Berry, M. van Hest, J. Alleman, M. Dabney,  L. Gedvilas, D.S. Ginley, A. Leenheer, and R. O’Hayre 5f. WORK UNIT NUMBER  7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) National Renewable Energy Laboratory 1617 Cole Blvd. Golden, CO 80401-3393 8. PERFORMING ORGANIZATION REPORT NUMBER NREL/CP-520-42546 10. SPONSOR/MONITOR'S ACRONYM(S) NREL 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)  11. SPONSORING/MONITORING AGENCY REPORT NUMBER  12. DISTRIBUTION AVAILABILITY STATEMENT National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 13. SUPPLEMENTARY NOTES  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 16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON  a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 17. LIMITATION OF ABSTRACTUL 18. NUMBER OF PAGES  19b. TELEPHONE NUMBER (Include area code)  Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 

Optimization of Conductivity and Transparency in Amorphous In-Zn-O Transparent Conductors: Preprint

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Optimization of Conductivity and Transparency in Amorphous In-Zn-O Transparent Conductors: Preprint

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