Laser makes transparent conductive thin film

Tokyo--Researchers at Matsushita Re search Institute Tokyo (MRIT) have developed a novel laser-processing technology for making transparent conductive thin films. The process can be applied to substrates that are not heat resistant or that are easily oxidized, and it can be used to make crystalline-oxide conductive thin films that transmit 90% of visible light.

Laser makes transparent conductive thin film

Courtesy of O plus E magazine, Tokyo

Tokyo--Researchers at Matsushita Re search Institute Tokyo (MRIT) have developed a novel laser-processing technology for making transparent conductive thin films. The process can be applied to substrates that are not heat resistant or that are easily oxidized, and it can be used to make crystalline-oxide conductive thin films that transmit 90% of visible light.

Transparent conductive thin films are commonly used as light-transmitting electrodes. Those made with oxides are especially transparent at visible wavelengths, and the films are very strong. One such material is indium oxide (In2O3) with the addition of tin (ITO), which is widely used for electrodes in visual elements of liquid-crystal and plasma displays.

Until now, such films were created by shining ion or electron beams on the raw material, letting it vaporize, and depositing the gas onto a substrate placed ac ross the material. However, be cause the material is made of elements that have different melting points and vapor pressures, films created using this method have been prone to structural disintegration. More specifically, the films have tended to lack oxygen because oxygen, with its high vapor pressure, tends to escape as gas. Hence, transparency is sacrificed and ideal conditions cannot be achieved.

To prevent this loss, it has been customary to pipe oxygen into the reaction chamber. Also, the substrates for the film have been heated to more than 300°C. Under these conditions, however, the substrate could not be made of a material that is easily oxidized or that is not resistant to heat.

New fabrication method

The new method enables fabrication of a fully oxidized thin film at room temperature, without having to pipe in extra oxygen. Specifically, it is created by directing a pulsed excimer laser onto a block of In2O3 in a reduced-pressure helium atmosphere and then accumulating the crystalline film on the substrate facing the indium oxide (see figure). When the laser beam shines on the material, some of the material flies off because of ablation. This characteristic does not depend on the melting point or vapor pressure of the material.

The separated particles are cooled by repeated collision with helium atoms and are deposited onto the substrate without excessive dispersion. Hence, loss of oxygen is prevented. By changing the pressure of the helium gas, oxygen loss and the condition of the crystal can be controlled. The re searchers believe it is likely that thin films of ITO can be made in the same way.

The MRIT has been developing high-resolution light-emitting-diode displays using silicon microparticles. The researchers hope to use the new technology to make the conductive material that disperses silicon microparticles or to use it for electrodes in visual elements. For these purposes, In2O3 is a better material than ITO.

Ordinarily, silicon does not emit visible light. However, when electricity passes through nanometer-sized particles, visible light is emitted because of quantum-mechanical effects. The researchers have succeeded in confirming the emission of light related to these particles, but because silicon is easily oxidized the challenge has been to find a method for creating transparent conductive thin films without having to inject additional oxygen.

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