Investigation of AlBGaN Structures for UV-Lighting

Highly efficient blue light-emitting diodes (LED) already exist on the basis of Ga(In)N [1] enabling the generation of white light. Also LEDs in the UV range, especially the UV-C range (∼ 270nm), have many applications since they can be used for sterilization and disinfection of water and air [2, 3], phototherapy [2], flourescence analytical systems [3], bio-agent detection [4], spectrometry [5] and currency validation [5]. However, LEDs in the UV spectral range are typically much less efficient [2] due to the challenges of growing high quality AlN layers with low threading dislocation density (TDD). Such dislocations penetrating through the optically active layers, drastically decrease the efficiency of the device. Due to lattice mismatch between the sapphire substrate used for overgrowth and AlN, defects are generated. To minimize lattice mismatch between the substrate and the AlGaN with very high Al-content as well as reducing the absorption of light in the substrate, UV-LEDs are not grown on GaNbut on AlN substrates [6]. Moreover, lattice mismatch between layers of different Al content lead to further problems, e.g., further defect generation or even cracking. Due to the quantum confined Stark effect (QCSE), piezoelectric fields reduce the overlap of the electron and hole wave functions and, hence, the recombination probability in the active quantum wells decreases.