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Title: Transfer printing of nitride based light emitting diodes
Author: Trindade, Anto´nio Jose´ Marques
ISNI:       0000 0004 5357 3259
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2015
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The research presented in this thesis focuses on the implementation and development of transfer printing as a novel technique for heterogeneous integration of III-nitride based light emitting diodes (LEDs) onto a wide range of substrates, whether flexible or rigid in their nature. The initial steps towards a functioning prototype are described with the successful transfer printing of 2 µm-thick micron-size LEDs. The thin structures were assembled onto mechanically-flexible substrates in a representative 16x16 array format using a modifed dip-pen nano-patterning system. Two different methods of addressing the LEDs were studied by using conductive inks on the LED bonding pads or through the use of metal tracks. Both addressing schemes are compared and studied for reliability. Individual study of the printed array elements showed blue emission centred at 486 nm with a forward-directed optical output power up to 80 µW (355 mW/cm²) when operated at a current density of 20 A/cm². A relatively poor performance was observed with damage to the current spreading metal due to an alkaline wet etch step, needed during processing to yield suspended membranes. This issue was addressed by reversing the order of the major steps needed to fabricate the LEDs, which resulted in signifcantly improved LED performance. The capabilities of the nano-patterning system were demonstrated with successful LED placement as low as 150 nm (±14 nm) between dies. Further optimisation beyond the initial prototypes made use of heat-efficient substrates, namely fused silica and diamond, without the use of intermediary adhesion layers. Through the use of a liquid capillary bond, consistent van der Waals bonding was achieved, despite the curvature of the epitaxial layers that comprise an LED following their release from their native silicon growth substrates. The excellence of diamond as a heat-spreader allowed the printed membrane LEDs to achieve optical power output density of 10W/cm² when operated at a current density of 254 A/cm² - a signifcantly higher operational regime compared to the initial prototypes. To capitalise on the new achieved performance, demonstrations of data transmission and colour conversion are shown.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral