Conductivity of Oxide Layers

Transparent conducting oxides (TCO) have been widely used in different areas due to their high optical transparency, low resistivity and wide energy band gap and hence there has been great deal of work on investigating their preparation processes and optimizing their properties. Various ternary and compound oxide materials have been developed and obtained good opto-electronic transport properties. But the preparation of doped ternary and multi-component oxide layers with suitable composition and understanding of the chemistry involved are rather difficult when compared to undoped TCO materials. In general, undoped binary oxide films are insulators in its stoichiometric condition. But the conductivity of pure oxide layers can be improved to the level of doped layers by suitably controlling the density of oxygen vacancies, each of which donate two electrons to the conduction band. This deficiency determines the conductivity of undoped oxide layers. Moreover, the ultimate attainable properties are material dependent. Among the various TCO materials available, In2O3 is one of the potential candidates for solar cell and sensor applications. It is an n-type semiconductor that has high electrical conductivity. This material has an optical energy band gap of 3.6 eV with good adherence to the substrate surface and high chemical inertness.

....The analysis of the results in the present study suggested that there is a decrease of structural disorder in the films with the increase of substrate temperature from 250 C to 400 C, probably due to a reduction of defects at the grain boundaries of the layers that increased the band gap. Moreover, the decrease of lattice strain in the films with growth temperature might also have contributed to the increase of energy band gap in the present study.

Also the films prepared at substrate temperatures above 400 C, showed an increased resistivity with a reduced mobility and carrier concentration. This might be due to higher thermal energy provided to the substrate that caused to disturb the crystallinity of the films grown at such temperatures, as supported by the XRD analysis, which led to the formation of defects and scattering centers. The decrease in the carrier concentration at higher temperatures (>400 C) resulted from the depletion of oxygen vacancies arising from the crystallization effect. However, the electrical resistivity of pure oxide films mainly depends on the number of oxygen vacancies present in the films. The resistivity and mobility of the films prepared in the present study is close to the values observed for indium oxide films grown by pulsed laser deposition.

The crystallographic structure of the films was highly influenced by the deposition temperature. The layers deposited at 400 C showed a strong (222) orientation and exhibited cubic structure. The layers grown at lower temperature were either less crystalline or randomly oriented. The structural parameters such as the lattice strain and texture coefficient were evaluated. The analysis showed that higher substrate temperature enhances the degree of crystallinity of In2O3 layers. The SEM and AFM analysis of the films indicated that the surface topology was better for the films deposited at a temperature, 400 C with an average grain size and roughness values of 87 nm and 12.8 nm, respectively. The layers prepared at 400 C showed a transmittance of >85% in the visible region with a band gap of 3.6 eV. The evaluated electrical resistivity of the layers was 6.07 x 10-3 Ω cm with a charge carrier concentration, 3.44 x 1019 cm-3. The grain boundary potential evaluated from the temperature dependent carrier mobility for the as-grown and annealed layers was 0.068 eV and 0.044 eV, respectively. The figure of merit was evaluated as 1.09 x 10-3 Ω -1 for films deposited at 400 C. In summary, the studies revealed that the strain in the films could restrict the growth of the grains and influence the optoelectronic properties of sprayed In2O3 films.


C. Barret, T.B. Massalaski, Structure of Metals, Pergmon, Oxford, 1980

Growth and characterization of indium oxide films, Elsevier, 2007