Indium gallium zinc oxide
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Indium gallium zinc oxide (IGZO) is a semiconducting material, consisting of indium (In), gallium (Ga), zinc (Zn) and oxygen (O). IGZO thin-film transistor (TFT) is used in the TFT backplane of flat-panel displays (FPDs). IGZO-TFT was developed by Professor Hideo Hosono's group at Tokyo Institute of Technology and Japan Science and Technology Agency (JST) in 2003 (crystalline IGZO-TFT) and in 2004 (amorphous IGZO-TFT). IGZO-TFT has 20–50 times the electron mobility of amorphous silicon, which has often been used in liquid-crystal displays (LCDs) and e-papers. As a result, IGZO-TFT can improve the speed, resolution and size of flat-panel displays. It is thus considered to be one of the most promising thin-film transistors for use in organic light-emitting diode (OLED) displays.
In 2012, Sharp was first to start production of LCD panels incorporating IGZO-TFT. Sharp employs IGZO-TFT for smartphones, tablets, 32" LCD, etc. In these, the aperture ratio of the LCD is improved by up to 20%. Power consumption is improved by LCD idling stop technology, which is possible due to the high mobility and low off current of IGZO-TFT. Sharp has started to release high pixel-density panels for notebook applications. IGZO-TFT is also employed in the 14" 3,200x1,800 LCD of an ultrabook PC supplied by Fujitsu, also used in the Razer Blade 14" (Touchscreen Variant) Gaming Laptop and a 55" OLED TV supplied by LG Electronics.
IGZO's advantage over zinc oxide is that it can be deposited as a uniform amorphous phase while retaining the high carrier mobility common to oxide semiconductors. The transistors are slightly photo-sensitive, but the effect becomes significant only in the deep violet to ultra-violet (photon energy above 3 eV) range, offering the possibility of a fully transparent transistor.
The current roadblock to large-scale IGZO manufacturing is the synthesis method. The most widely used technique for Transparent Conducting Oxide (TCO) synthesis is Pulsed Laser Deposition (PLD) In PLD, a laser is used to focus on nano-sized spots on solid elemental targets. Laser pulse frequencies are varied between the targets in ratios to control the composition of the film. IGZO can be deposited onto substrates such as quartz, single-crystal silicon, or even plastic due to its ability for low-temperature deposition. The substrates are placed in a PLD vacuum chamber, which controls oxygen pressure in order to ensure favorable electrical properties. After synthesis, the film is annealed, or gradually exposed to air to adjust to the atmosphere.
While PLD is a useful and versatile synthesis technique, it requires expensive equipment and plenty of time for each sample to adjust to regular atmospheric conditions. This is not ideal for industrial manufacturing, where large corporations would aim to produce thousands of TFTs each day.
Solution Processing is a more cost effective alternative. Specifically, combustion synthesis techniques can be used. Kim et al used a metal nitrate solution with an oxidizer to create an exothermic reaction. One common type of combustion synthesis is spin coating, which involves depositing In and Ga solution layers onto a hot plate and annealing at temperatures roughly between 200 and 400 degrees C, depending on your target composition. The films can be annealed in air, which is a large advantage over PLD.
Combustion processing has potential as a new synthesis method, but more research is required to assess its viability. The Bedzyk Group at Northwestern University is currently comparing properties such as the crystallization temperature and electron mobility between IGO films made by PLD and combustion processing.
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