Properties of a Novel Amorphous Transparent Conductive Oxide, InGaO 3 (ZnO) m
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ABSTRACT Novel amorphous transparent conductive oxides, InGaO 3(ZnO),n, where m is an integer less than four, was developed. Optical transmittance in the visible region exceeded over 80 % and the electric conductivity at 300 K was as large as 400 S/cm. Both Seebeck and Hall coefficients exhibited negative values, indicating the conduction was n-type. It was suggested that 4s orbital of Zn 2t played a significant role for the formation of the extended state responsible for the conduction, while In3t acted as a modifier for the stabilization of amorphous state.
INTRODUCTION Beside well known amorphous transparent semiconductors, which could be grouped into two categories (tetrahedral and chalcogenide ones), amorphous transparent conductive oxides (TCO) such as ln203j() and Cd 2 SnO4(2)have been developed, in which 5s orbital of metal ions contributes to the conduction. Based on a working hypothesis that oxides composed of metal ions with electronic configuration (n-1)d'0 ns0 (n>5) might become conductive, novel amorphous TCOs such as a-AgSbO 3(3), a-Cd 2GeO 4(4), and a-Cd 2PbO 4(5) have been found.
In general, unoccupied state, or the conduction band, in oxides composed
of (n-i)dl'ns0 metal ions are mainly formed by the vacant ns orbital.
When the ns orbital
radii were large enough, the orbital could be delocalized around the neighboring metal ions by the overlap interaction. In addition, when the fracture of the metal ions was high enough, the delocalized state might extend all over the compound. The electron transfer through the extended state would lead to the good conductivity. It was reported that such extended state was formed by the 5s orbital of Cd ions in xCdO-(l-x)GeO2 system when the x values were
larger than 0.5.(6) The value was considered to be a threshold beyond which the delocalized state was spread all over the system. Since the 4s orbital of Ge was unlikely effective for the conductivity, the principal quantum number n should be larger than five to have sufficient overlap interactions. Thus, 3d' 0 4S0 cations such as Zn2t, Ga3+, Ge4+ have been excluded from constituents effective for the conductivity. Actually, no amorphous TCOs based on the 3d' 0 4s0 metal ions have been reported so far as far as we know. We have reported recently that a series of InGaOA(ZnO)m crystalline, where m value was an integer, became transparent conductors.(7) Particularly, good conductivity of 500 S/cm was obtained for the films with the m value of one.(t ) Although the molar fraction of In3+ ions in the system remained 0.33, which was smaller than the threshold value of 0.5, the 5s orbital of In3+ ions formed the extended states at the bottom of the conduction band. (9)This may be 291 Mat. Res. Soc. Symp. Proc. Vol. 623 © 2000 Materials Research Society
tentatively explained by the following consideration. The compound has layered structure composed of an alternative stacking of InO2 and GaZnO 2 layers. In ions locates two dimensionally on the InO 2 layer, which makes it possible for the In 5s orbital to form
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