Improvements Sought in Nitride-Based LEDs

Daniel S. Burgess

A team of engineers at National Cheng Kung University in Tainan, Taiwan, has reported the results of its investigations into the fabrication of InGaN/GaN multiple quantum-well green LEDs by metallorganic chemical vapor deposition. The group hopes that the work will enable the continued development of more efficient high-brightness green LEDs.

Kodigala Subba Ramaiah, a member of the team who currently is at Rensselaer Polytechnic Institute in Troy, N.Y., explained that nitride-based green LEDs tend to display low efficiencies. Two causes are lattice mismatch and the low miscibility between GaN and InN compounds, which result in indium segregation and phase separation in the active region of the devices. The incorporation of aluminum can mitigate these issues to some degree, but it is believed that alternative methods will be necessary to optimize performance, Ramaiah said.

Physical properties

To offer greater insight into the physics of these devices, the engineers used high-resolution transmission electron microscopy, high-resolution x-ray diffraction and low-temperature photoluminescence to characterize the structural and optical properties of the LEDs. The emitters were grown by metallorganic chemical vapor deposition and featured three quantum-well layers. Argon-ion milling thinned the back surface of the LEDs for the structural analysis, and a 325-nm HeCd laser served as the excitation source for the optical measurements.

The researchers found that the defect density decreased by a factor of 10 in the direction of growth and that indium segregation produced structures similar to quantum dots in the wells. The luminescence peak resulting from quantum-well resonance displayed an S-shape shift with decreasing temperature, showing a blueshift at 80 to 140 K and a redshift at 10 to 60 K, which the engineers attributed to inhomogeneity in the InGaN.

Ramaiah said that such work would enable the production of higher-quality devices as suitable growth parameters and methods to mitigate indium segregation become apparent.

Applied Physics Letters, July 19, 2004, pp. 401-403.