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Photodetector Operates to ~200 nm

Photonics Spectra
Feb 2008
Aluminum nitride builds bridge to extreme-UV lithography.

Hank Hogan

In semiconductor manufacturing, what is small today will seem large tomorrow, which is true as much for the wavelengths required to make the chips as it is for the devices themselves. Chip makers currently need photodetectors that sense radiation at ∼200 nm — the wavelength of light sources that are used in state-of-the-art photolithography techniques. However, they soon will require photodetectors for extreme-UV (∼13 nm).

Now, investigators from Kansas State University in Manhattan have demonstrated AlN-based avalanche photodetectors that have a cutoff and a peak responsivity of 210 and 200 nm, respectively. Their goal was to develop detectors for wavelengths less than 280 nm, said team co-leader and physics professor Hongxing Jiang. Jingyu Lin, a physics professor, was the other leader.

TWDetector.jpg

A cross section (left) and top view (right) of an AlN avalanche photodiode are shown. The device has a low dark current, a cutoff wavelength in the deep-UV and a response in the extreme-UV. It also is a vertical device, which makes fabrication simpler. Courtesy of Hongxing Jiang, Kansas State University.


They had to work with either AlN or AlGaN. There were sound reasons for selecting the first, Jiang explained. “AlN has the advantage of offering the shortest wavelength and better material quality compared to AlGaN alloys.”

This is not to say that AlN is a perfect material. Indeed, Jiang noted that there are many defects and dislocations in bulk AlN. He predicted that highly conductive AlN substrates would not be available for years.

To get around this, the researchers have been perfecting the epitaxial growth of AlN. In epitaxy, molecules of a crystalline material, such as AlN, are deposited on a substrate. The deposited layer aligns to the substrate, which helps the layer achieve a higher crystalline quality than it would otherwise.

In the case of their photodiode work, they used a conductive SiC substrate — partly because of a good match between the crystal lattices of the two materials — to create epitaxial layers of good enough quality that vertical conducting avalanche photodiodes could be constructed.

They grew a 0.9-μm-thick layer of AlN on top of the substrate, followed by the creation of a metallic contact to the device and by photolithographic steps to define various areas. They manufactured instruments with diameters of 30, 50 and 100 μm. Jiang noted that the fabrication was straightforward, with no etching involved. This simplicity, he added, resulted from the conducting substrate and the vertical design.

They measured the current-versus-voltage characteristics of the devices using a deep-UV source from Acton Research Corp. of Acton, Mass. These tests showed that the 100-μm instrument had a gain of 1200 at a reverse bias of –250 V. According to the investigators, the devices had the highest optical gain and the shortest cutoff wavelength of any deep-UV avalanche photodiode based on periodic table group III-nitride material.

Jiang said that work in this area continues, including ongoing efforts to improve the quality of the deposited layers and to reduce the dislocation density. There also have been recent measurements of AlN metal-semiconductor-metal detectors in the extreme-UV realm.

“It was found that the responsivity is very good down to 30 nm,” Jiang noted.

Applied Physics Letters, Dec. 10, 2007, Vol. 91, 243503.


GLOSSARY
radiation
The emission and/or propagation of energy through space or through a medium in the form of either waves or corpuscular emission.
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