ITO is a heavily doped semiconductor. Essentially it is In2O3, containing a high density of tin donors (10^20 to 10^21cm-3). Undoped In2O3 is an insulator with a direct optical gap of about 3.5 to 3.7eV. This determines the optical properties of the ITO at high photon energies, i.e. in the UV. The conductivity of the ITO is due to the electrons donated to the conduction band. Because of the high carrier density, the conductivity approaches the level of metallic conduction. The free carriers in the conduction band can be excited to higher-energy levels inside the conduction band from where they rapidly fall back to the bottom of the conduction band. This free carrier absorption leads to a high level of optical absorption in the infrared. With the high carrier density in the conduction band, it can extend from very long wavelengths up to about 2 micrometers. ITO therefore reflects IR radiation very well. Application: coated window panes. These let short-wavelength visible light into the room but keep the warmth inside the room.

Hello, I apologise about the lack of clarity in my previous question. Let me rephrase, is ITO a semiconductor or a metallic conductor? I always thought it was the latter, that was why I was confused to see it has a band gap

ITO is a heavily doped semiconductor. Essentially it is In2O3, containing a high density of tin donors (10^20 to 10^21cm-3). Undoped In2O3 is an insulator with a direct optical gap of about 3.5 to 3.7eV. This determines the optical properties of the ITO at high photon energies, i.e. in the UV. The conductivity of the ITO is due to the electrons donated to the conduction band. Because of the high carrier density, the conductivity approaches the level of metallic conduction. The free carriers in the conduction band can be excited to higher-energy levels inside the conduction band from where they rapidly fall back to the bottom of the conduction band. This free carrier absorption leads to a high level of optical absorption in the infrared. With the high carrier density in the conduction band, it can extend from very long wavelengths up to about 2 micrometers. ITO therefore reflects IR radiation very well. Application: coated window panes. These let short-wavelength visible light into the room but keep the warmth inside the room.

The perfect answer from Gerhard covers it all!

Omotola, BTW ITO is not the only one who has both the band gap and conductivity! You may want to take a look, for example at the following reference which discusses the family of conducting oxides:

Huiyong Liu, V. Avrutin, N. Izyumskaya, Ü. Özgür, H. Morkoç "Transparent conducting oxides for electrode applications in light emitting and absorbing devices", Superlattices and Microstructures, v.48, Issue 5, November 2010, Pages 458–484

I wish to add little to Muller

Heavily doped semiconductors (like ITO) are better called degenerate semiconductors, not just semiconductors. Degenerate semiconductors resemble metal conductors with less conductivity. In degenerate SC conductivity may differ from in SC. In SC conductivity increases with higher temperature, whereas in deg. SC, conductivity does not change same way. This means that thermal excitation in deg. SC does not precisely describe conduction. Please recheck how conductivity in ITO varies with temperature.

I think that deg. SC differs from ordinary SC in that the former has Fermi level lying in the bend itself not in the forbidden band gap, and the latter has Fermi level in the gap.

If you look at powder or bulk ITO it has yellowish color, but in thin films it is colorless/transparent.

I am waiting for your confirming how temperature affects conduction in deg. semicondutors, please

to Hikmat

Actually, the post of Gerhard is correct from a semantics prospective since a degenarate semiconductor is a heavily-doped semiconductor that has a high level of doping which affects a position the Fermi level. ...and Gerhard calls it "a heavily doped semiconductor" not just "a semiconductor".

Although Gerhard's answer practically covered the question entirely, this short addition will help the inquirer to have a clear picture:

As per definition conductors do not have have a band gap, or in other terms the band gap is zero. Since a material can absorb photons with an energy higher than its band gap (in our case zero) conductors practically absorb incoming light at every wavelength. So as a rule of thumb we can state that conductors can not be transparent (for sure there are exceptions, e. g. very thin metal layers of only a few ten nm). Thus practically every material, that is transparent needs to have a band gap. And as Gerhard already said before, the quite high conductivity has to be reached by heavy doping (the conductivity is still more than one order of magnitudes lower than in good metallic conductors, if you compare ITO: below 10e4 to copper: 6 x 10e5, both in S/cm).

Hope this helped.

Tola,

I think your question is about the concept. You are asking about the difference between OPTICAL band gap and ELECTRONIC band gap? ITO doesn't have an OPTICAL band gap. It is the large ELECTRONIC band gap and not so high carrier density makes it transparent to visible light. OPTICAL band gap would only exist with photonic structures. This requires some understanding of wave-particle duality and band theory. Please refer to some solid-state physics textbook for detailed discussion.

Conduction is a phenomenon of charge flow. It exists even in some insulators, depending on the speed.

As Gerhard commented on - free-carrier behavior of metals makes them reflect light, so they look opaque. ITO is not as conductive as metals, they do reflect light to, but this reflection only kicks in at longer wavelength (beyond visible light wavelength), therefore, they are transparent in the visible region.

This will be counter-intuitive but interesting: If our eyes are only sensitive to ultra-violet light, you will find that silver is transparent, yet very conductive, as silver is transparent to UV light!

Hope this helps.

May be helpful ..!!!

http://www.materialsnet.com.tw/eng/TCO.html

What is the nature f ITO in NIR range. It has plasma frequency at 1550nm. what are its conductivity and transparency characteristics around this wavelength region?