Breaking Through the Plasma Wavelength Barrier to Extend the Transparency Range of Ultrathin Indium Tin Oxide Films into the Far Infrared

Abstract: Indium tin oxide (ITO) film, which is the most commonly used transparent conductive film (TCF), has traditionally been believed to be transparent in the visible spectrum but to reflect infrared (IR) light beyond the plasma wavelength (${\lambda}_p$). However, our theoretical analysis challenges this notion by demonstrating that an ultrathin ITO TCF that is thinner than the light's penetration depth, can overcome the transmission barrier at ${\lambda}_p$. To validate the theoretical modeling, we have successfully fabricated ITO films that, despite having ${\lambda}_p \approx$ 1 ${\mu}$m, remain transparent from 400 nm to 20 ${\mu}$m. This represents the broadest transparency range ever reported for any In$_2$O$_3$-based TCF. The 10-nm-thick ITO TCFs have high visible transmittance (91.0% at 550 nm), low resistivity (5 $\times$ 10$^{-4}$ ${\Omega}\cdot$ cm), and good IR transmittance (averaging 60% over 1.35 $\unicode{x2013}$ 18.35 ${\mu}$m). Their IR transparency facilitates radiative cooling of the underlying circuitry. When an operational resistor is enclosed by commercial ITO TCFs that are 140 nm thick, its temperature increases. However, using 10-nm-thick ITO TCFs instead of the commercial ones can completely avoid this temperature rise. Moreover, attaching a silver grid to a 10-nm-thick ITO TCF can reduce the effective sheet resistance to ~10 ${\Omega}/\square$ at the expense of only ~3% transmittance. This development paves the way for large-scale applications that require low sheet resistance and far-IR transparency.