Evaluating Semiconductor Quality With Femtosecond Lasers

Ultrashort laser pulses could help identify high quality wide-gap semiconductor materials that can be used as power devices.

http://www.asianscientist.com/2015/11/in-the-lab/osaka-gallium-nitride-next-terahertz-laser-emission-microscope/

 AsianScientist (Nov. 9, 2015) - In a study published in Scientific Reports, researchers have developed a laser-based method to visualize defects in semiconductor materials such as gallium nitride (GaN). 

Wide-gap semiconductors such as GaN are widely used for optical devices such as blue LED and are also anticipated as materials for next-generation energy saving power devices and solar cells. 

However, the quality of GaN crystals does not match the standards of conventional semiconductor materials such as silicon, preventing GaN from being used for power devices. For that reason, new technology for producing high-quality crystals with fewer defects and rearrangement is expected, and the development of evaluation technologies are crucial. 

A group of researchers led by Iwao Kawayama, an associate professor of the Institute of Laser Engineering at Osaka University, in cooperation with Screen Holdings Co., Ltd., succeeded in visualizing changes in defect density on the surface of GaN through the laser terahertz emission microscope (LTEM) which measures terahertz (THz) waves generated by laser emission. 

This group's discovery shows that LTEM is useful as a new method for evaluating the quality of wide-gap semiconductors and it is also expected that LTEM will bring a breakthrough in the development of next-generation optical devices, super high frequency devices, and energy devices. 

The group examined the intensity distribution of THz generated by radiating ultraviolet femtosecond laser pulses on the surface of GaN crystal through LTEM. They found that there were regions with high intensity of THz emission and ones with low intensity of THz emission. 

Additionally, when the LTEM image was compared with the image obtained through photoluminescence using a conventional method, it was found that there was a strong correlation between the distribution of emission intensity due to lattice defects and the intensity distribution of THz wave emission. 

Furthermore, from results measurement through modification of excited lasers, it was confirmed that THz emission needs excitation light with larger energy than the band gap energy. 

The article can be found at: Sakai et al. (2015) Visualization of GaN Surface Potential Using Terahertz Emission Enhanced by Local Defects. ——— Source: Osaka University. Read more from Asian Scientist Magazine at: http://www.asianscientist.com/2015/11/in-the-lab/osaka-gallium-nitride-next-terahertz-laser-emission-microscope/