Chemical and Thin-Film Strategies for New Transparent Conducting Oxides
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Film Strategies for New Transparent Conducting Oxides
A.J. Freeman, K.R. Poeppelmeier, T.O. Mason, R.P.H. Chang, and T.J. Marks Introduction Transparent conducting oxides (TCOs) have been known and employed technologically for more than 50 years, primarily in the form of doped single-cation oxides such as In2O3 and SnO2. Beginning in the 1990s, however, multi-cation oxide TCOs began to be developed in Japan (see the article by Minami in this issue and the references therein) and at the former Bell Laboratories.1,2 Since then, new TCO phases are being reported with increasing frequency as technological interest in this area heightens. At the same time, our fundamental understanding of the chemical and structural origins of transparent conductivity continues to expand and promises a pathway to dramatically improved materials for a host of applications. This article describes a collaborative, multi-investigator bulk and thin-film research effort at Northwestern University aimed at the synthesis, characterization, and enhanced understanding of multi-cation (compound and solidsolution) TCOs, and provides a brief account of what we are discovering about this important class of materials. In particular, our research to date indicates that Many new and novel compound and solid-solution bulk TCOs remain to be discovered within TCO “phase space.” Extended solid-solution phases afford tailorable TCO properties, such as optical gap and/or work function, in addition to electronic conductivity. The most promising multi-cation TCO materials share common chemical and structural features and possess similar carrier-generation mechanisms.
MRS BULLETIN/AUGUST 2000
Metastable TCO phases and solution ranges not available in bulk form can be obtained as thin films. In many instances, these complex new materials have properties comparable or superior to those of conventional TCOs, and their characterization should therefore be highly instructive.
Combinatorics and New Bulk Phases We begin by considering oxides of five closely grouped d10 cations known to be constituents of transparent conductors (Zn2, Cd2, In3, Ga3, and Sn4). Four of these form TCOs as single-cation oxides (ZnO, CdO, In2O3, and SnO2) when appropriately doped, whereas Ga2O3 is a widebandgap insulator. There are 10 binary, 10 ternary, five quaternary, and one quintinary, or a total of 26, combinations of these five oxides, most of which have not been fully explored insofar as stable phases, solid-solution ranges, and novel TCO phases are concerned. The ternaries in this “phase space” can be further divided into four categories based upon cation charge, namely, divalent (Zn and Cd), trivalent (In and Ga), and tetravalent (Sn), including (2-2-3), (2-2-4), (3-3-4), and (2-3-4) systems. Figure 1 displays the (2-3-3), the (3-3-4), and the two (2-3-4) systems highlighting TCO phases discovered and/or investigated at Northwestern. Non-TCO phases also exist, but have been omitted from the diagrams for clarity; complete or nearly complete subsolidus phase diagram
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