Transparent p- and n-Type Conductive Oxides With Delafossite Structure
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Mat. Res. Soc. Symp. Proc. Vol. 623 ©2000 Materials Research Society
hypotheses for exploring both p- and n-type TCOs. It turned out that CuAlO 2 and CuGaO 2 delafossites are p-type TCOs, while Sn doped AgInO 2 delafossite is n-type TCO.5 Our guideline to find p- and n- type TCOs are summarized in as follows:" 6 The essence of guideline to find p-type TCOs is to have monovalent copper as the major constituents. The selection of Cu' is based on its electronic configuration and energy levels of 3d orbitals. Cuprous ion has the electronic configuration of (Ar) 3d'0 4s° (closed shell), which is free from visible coloration arising from a d-d transition commonly seen in transition metal ions. The energy level of a 3d'° state is close to that of an 0 2p 6 state. As a consequence, covalent bonding formation or hybridization of orbitals is expected between Cu 3d'0 and 0 2p 6. The hybridization of the orbitals will bring large dispersion to the valence band or reduction of the localization of positive holes. Formation of a covalent bonding between Cu÷ and 02 ions demands an appropriate crystal structure. The crystal structure is required to meet two requirements, one is that Cu+-O 2- bonds with a moderated covalency exist, and another is to retain a wide energy gap for optical transparency. This argument can be valid also to Ag÷ instead of Cu'. For the n-type TCO, high mobility of carrier electron in the conduction band is required. A liner chain of edge-sharing octahedra, in which the p-block heavy cations (M'+) with ns° electric configuration (n: the principal quantum number) occupy the central position, is preferred to satisfy this requirement. Since there is no intervening oxygen between the two neighboring M cations in the chain of edge-sharing octahedra, direct overlap between ns atomic orbitals of the neighboring M cations is possible for the p-block heavy cations. In this case, the bottom edge of conduction band is mainly composed of ns atomic orbitals of M cations. As a consequent, a large dispersion of the conduction band, which is appropriate for high mobility of electron carrier, may be expected. Taking the above stated requirements into consideration, we selected as the candidate materials ternary noble metal oxide with delafossite structure (ABO 2). Figure 1 illustrates delafossite-type crystal structure. The delafossite is composed of an alternate stacking O-A-O
"J:O Cu, Ag
FIG. 1.Crystal structure of delafossite (ABO 2). This material has layer structure composed of A
(A = Cu, Ag) plane and : Al, Ga, In
B0
2
(B = Al, Ga, In)
layer alternately stacked along c-axis.
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dumbbells and BO 6 edge-sharing octahedral layer. According to our hypothesis, delafossite oxides satisfy the requirements for p- and n-type TCOs, when Cu and/or Ag as A cation and pblock heavy cations, such as Ga3 ÷ and In 3", as B cation are chosen. It is considered that BC 6 octahedral layers and O-A-C layers work as conduction paths for electrons and positive holes, respectively. That is to say, double oxides with delafossi
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