Microscopy of porous nitride materials for long wavelength light emitting diodes

III-nitride based-RGB micro-LEDs and colour-converted (CC) micro-LEDs are two major contenders for achieving full-colour monolithic integration for micro-display applications. Each approach presents challenges, calling for innovative ways to improve the device performance from the point of view of material quality. To alleviate strain-related issues that persist in long-wavelength InGaN micro-LEDs, a porosified InGaN superlattice (SL) template obtained by electrochemical etching has been adopted as a pseudo-substrate for the overgrowth of InGaN multiple quantum wells (MQWs). Partial strain relaxation with respect to the GaN buffer layer has been achieved in the InGaN SL template via porosification, and this strain relaxation also results in relaxation of the overgrown InGaN MQWs. The porosity of the SL and the density of small V-pits in the MQWs are found to increase with the etching voltage, which together contribute to a maximum observed strain relaxation of ~45% for the most etched sample. The additional small V-pits that appear at the surface of the InGaN MQWs on the porous template are found to be either linked to dislocation loops or trench defects or to be unconnected to an extended defect, serving as a route for strain relaxation by themselves. The strain-related electric field variation in the vicinity of small V-pits has been mapped by a differential phase contrast (DPC) technique using scanning transmission electron microscopy. DPC is demonstrated to be capable of mapping the local electric fields induced by either change in the electrostatic potential or piezoelectric polarization in other III-nitride heterostructures without porosity, showing a good match with simulations in most cases. Infiltration of core-shell quantum dots (QDs) into porous GaN for potential CC micro- LEDs has also been demonstrated. The infiltration depth is often found to be only a few 100 nm due to the combined effects of the solvents used, the infiltration approaches applied and the pore structure. Inkjet printing of QD suspensions produces similar infiltration depths while maintaining a defined surface pattern. Enlarging the size of surface pores is found to facilitate the QD infiltration in the near-surface region, but further infiltration remains limited by the branched porous structure at deeper positions.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Ji, Yihong

Advisor dc:contributor.advisor

• Oliver, Rachel

• porous nitrides
• TEM
• long wavelength micro LED
• microscopy characterization
• DPC

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/dca220ca-16d0-4971-bac8-4eae597c6dfd/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.115771

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/379801