Electromagnetic absorption in transparent conducting films

In this article we calculate the skin depth of indium tin oxide (ITO) films (and related materials) as a result of free electron absorption within the visible spectrum using the simple Drude model. We also discuss the consequences of finite skin depth for the transparency of current spreading layers for light emitting diode (LED) applications. Low sheet resistances are highly desirable for these layers, but the free electron density n in ITO cannot be increased much beyond 2x10(21) cm(-3) without pulling the plasma frequency into the red end of the visible spectrum (thus making it highly reflective); furthermore, any increases in the film thickness cause reduced transparency due to the finite skin depth delta. However, above the plasma frequency for napproximate to10(21) cm(-3) we find that increases in the electron mobility mu(e) cause increases in delta, since then approximately deltaproportional tomu(e). Therefore, if mu(e) in ITO can be increased above present state-of-the-art values around 50 cm(2) V-1 s(-1) to intrinsic limiting values around 100 cm(2) V-1 s(-1) by improved film processing, then substantial increases in transparency are possible whilst not sacrificing the high conductivity. The output optical power of a LED using an ITO current spreading layer with high n is also approximately proportional to mu(e), so mobility increases also have a direct impact on the external power efficiency of these devices. (C) 2004 American Institute of Physics.