ZnO-Based Blue-Violet LEDs Fabricated on Sapphire
Anne L. Fischer
inc oxide has been investigated as a green fluorescent material for many years. Now researchers are looking at it for use in UV and blue emitters. It offers thermal stability and resists chemical attack and oxidation, and its exciton binding energy of 60 meV enables it to work well at room temperature.
After scientists at Changchun Institute of Optics and at Hong Kong University of Science & Technology, both in China, demonstrated UV lasing in ZnO thin films at room temperature, they realized that the material also could be used for LEDs. They fabricated a blue-violet ZnO PN-junction LED on an a-plane Al2O3 substrate by plasma-assisted molecular-beam epitaxy.
The ZnO LED fabricated on sapphire operates with an injection current of 130 μA at 80 K.
The electrically driven device features a 200-nm-thick layer of nitrogen-doped P-type ZnO grown directly on the substrate. The thickness of the N-type top layer also is 200 nm. The electrodes for the P- and N-type layers are produced in Ni/Au and In, respectively.
With an increase in temperature, unidentified donors in the P-type layer can be ionized, which leads to instabilities in P-type conduction and degradation of device performance. Recently, the researchers improved the stability of the P-type carriers, even at room temperature.
Another effect of higher temperature is that it significantly weak the electroluminescence band of the ZnO PN-junction LED in the blueviolet region. Zikang Tang, a professor at the Hong Kong university, explained that this occurs because heavy doping in the P-type layer causes the luminescence to be dominated by defect-related emission in the P-type side.
The scientists found that electroluminescence decreased by more than two orders of magnitude as the temperature was increased from 80 to 200 K. In contrast, Tang noted, the intensity of the photoluminescence decreased by less than one order of magnitude. They concluded that it is not thermal quenching but rather degradation of the diode junction that plays an important role in quenching the luminescence.
The investigators plan to focus on improving the quality of the nitrogen- doped P-type ZnO layer to get excitonlike electroluminescence, with the goal of realizing stable ZnO-based UV LEDs and laser diodes. Tang said that they also may replace the Al2O3 with bulk single-crystal ZnO.
He believes that once the fundamental issues are resolved, ZnO-based LEDs will be commercialized and widely used.
Applied Physics Letters, Jan. 16, 2006, 031911.
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