Date of Award

2015

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Asif Khan

Abstract

III-Nitride based deep ultraviolet (UV) light emitting diodes (LEDs) emitting below 280nm has the potential to replace mercury lamps currently used in the systems for water/air purification, germicidal and medical instruments sterilization applications. However, this potential has not yet been fully realized as the output power, quantum efficiency and lifetime of deep UV-LEDs have been limited by the large number of dislocations in the active region of the devices, arising from the lattice mismatched sapphire substrate, which has been the substrate of choice due to its high transparency to deep UV radiation. To reduce dislocations in the active region of the DUV LEDs grown on sapphire, AlN/AlGaN short period superlattice is usually grown to manage strain and filter the dislocations. However, the growth of these thick superlattice is not only time consuming but it can also cause severe substrate bowing making device fabrication a challenge. An alternative method for reducing the dislocation density in deep UV-LEDs structure is the use of low defect density bulk AlN substrates, which has more than two orders of magnitude low defect density than sapphire/AlN template. In spite of such a lower density of defects, the quantum efficiency values for DUV LEDs on bulk AlN substrate are very similar to that of DUV LEDs on sapphire. This is due to the high absorption coefficient of AlN substrate in the deep UV region; also the high cost and limited availability are important issues. This dissertation focuses on the design, fabrication and characterization of Deep UV LEDs employing the pseudomorphic structure similar to those grown on bulk AlN, in order to keep the active layers crystalline quality very close to that of the underlying AlN/Sapphire template. Other LED designs, with different degree of relaxation of the n-AlGaN current spreading layer were also investigated with the main target of achieving a cost-effective design without compromising the resulting device performance, namely output power and reliability.

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