PhD defence of Xue (Silvana) Yin – Molecular Beam Epitaxy of AlGaN Epilayers on Foreign Substrates for Semiconductor Deep Ultraviolet Lasers

Abstract

Aluminum gallium nitride (AlGaN) ultraviolet (UV) laser diodes (LDs) are of great research interest due to their wide range of applications and promising future to replace today’s dominant UV lasing technologies, such as gas lasers and solid-state lasers. Nonetheless, the majority studies on AlGaN deep UV lasers are still on optical pumping and the development of electrically driven counterparts is relatively slow, mainly rooted in the difficulties in achieving high quality AlGaN epilayers and insufficient p-type conduction. In addition, the AlGaN UV laser research predominantly relies on the structures grown by metalorganic chemical vapor deposition (MOCVD). In contrast, the research of AlGaN UV lasers by molecular beam epitaxy (MBE) is rare. It is also noted that the recent progress on electrically injected AlGaN deep UV lasers relies on expensive AlN substrates.

This thesis follows the track of MBE-grown AlGaN on cost-effective foreign substrates, such as Si and sapphire, to address the current impediments. This thesis first demonstrates a novel buffer layer technology, which enables low-cost, highly reproducible, high quality, and nearly strain-free AlN thin film through exploiting the low Al adatom migration during the coalescence process on a GaN nanowire template on Si. This thesis further demonstrates high quality AlGaN epilayers on such nanowire-assisted AlN templates by MBE. Peak internal quantum efficiency (IQE) of ~50% is derived, which is significantly improved compared to the previously reported bulk AlGaN epilayers on sapphire.

In parallel, this thesis also, for the first time, reports deep UV lasing from MBE-grown AlGaN on sapphire by optical pumping, with clear lasing evidence. Lasing at 298 nm from the AlGaN/AlN double heterostructure (DH) is first obtained, with a lasing threshold of 950 kW/cm². This thesis further studies the correlation between point defects and lasing threshold, reduces the lasing threshold to 530 kW/cm² at 287 nm, together with an increase in IQE of ~16%. Towards the electrically injected AlGaN UV LDs, this thesis designs and numerically investigates a unique Al-content engineered superlattice electron blocking layer (AESL-EBL), which not only improves charge carrier transport with the assistance of hot hole effects but also minimizes optical loss due to doping.