Towards the development of a new evaluation method of semiconductors by using terahertz (THz) waves
Wide-gap semiconductors such as gallium nitride (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.
As shown in Figure a, in the neighborhood of the surface of semiconductor of GaN, the energy band is bent due to the surface potential. It is thought that photoexcited carriers are accelerated by the band bending, thereby radiating THz. It is also thought that the change in THz wave emission intensity due to defects, as shown in Figure b, is because the energy band bent significantly due to electrons trapped by defects near the surface, which further accelerated carriers. This shows that LTEM is effective for measuring the defect density of the surface of wide-gap semiconductors and their distribution of the surface potential.
Credit: Osaka University
However, the quality of GaN crystals does not come up to that of conventional semiconductor materials such as silicon (Si) and this prevents GaN from being used for power devices.
For that reason, the establishment of technology for producing high-quality crystals with fewer defects and rearrangement is expected, and the development of a new evaluation technology is 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 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.
As a result, it was 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 (PL) 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.
This research was featured in the electronic version of Scientific Reports (UK) on September 9, 2015.
Iwao Kawayama | EurekAlert!
Nano-scale process may speed arrival of cheaper hi-tech products
09.11.2018 | University of Edinburgh
Nuclear fusion: wrestling with burning questions on the control of 'burning plasmas'
25.10.2018 | Lehigh University
Researchers at the University of New Hampshire have captured a difficult-to-view singular event involving "magnetic reconnection"--the process by which sparse particles and energy around Earth collide producing a quick but mighty explosion--in the Earth's magnetotail, the magnetic environment that trails behind the planet.
Magnetic reconnection has remained a bit of a mystery to scientists. They know it exists and have documented the effects that the energy explosions can...
Biochips have been developed at TU Wien (Vienna), on which tissue can be produced and examined. This allows supplying the tissue with different substances in a very controlled way.
Cultivating human cells in the Petri dish is not a big challenge today. Producing artificial tissue, however, permeated by fine blood vessels, is a much more...
Faster and secure data communication: This is the goal of a new joint project involving physicists from the University of Würzburg. The German Federal Ministry of Education and Research funds the project with 14.8 million euro.
In our digital world data security and secure communication are becoming more and more important. Quantum communication is a promising approach to achieve...
On Saturday, 10 November 2018, the research icebreaker Polarstern will leave its homeport of Bremerhaven, bound for Cape Town, South Africa.
When choosing materials to make something, trade-offs need to be made between a host of properties, such as thickness, stiffness and weight. Depending on the application in question, finding just the right balance is the difference between success and failure
Now, a team of Penn Engineers has demonstrated a new material they call "nanocardboard," an ultrathin equivalent of corrugated paper cardboard. A square...
09.11.2018 | Event News
06.11.2018 | Event News
23.10.2018 | Event News
16.11.2018 | Health and Medicine
16.11.2018 | Life Sciences
16.11.2018 | Life Sciences