PhD defence of Qihua Zhang – "Molecular Beam Epitaxial Growth and Characterization of AlGaN Epilayers for Vertical Deep Ultraviolet LEDs on Silicon"

Abstract

Light emitting didoes (LEDs) through vertical current injection offer numerous advantages including excellent heat dissipation and size scalability. Yet it remains difficult to realize AlGaN deep-ultraviolet (deep-UV, wavelength shorter than 300 nm) LEDs through vertical injection. Silicon (Si) substrates, owing to their excellent conductivity, easy to removal by chemical wet etching, and mature manufacturing process, are a promising platform for developing vertical AlGaN deep-UV LEDs. Nevertheless, obtaining high quality AlGaN epilayers on Si substrate has remained a challenge due to the large lattice and thermal mismatch between AlN and Si. In this thesis, we demonstrate a nanowire template assisted buffer layer technology on Si that enables high-quality AlGaN epilayers on Si, which further enables vertical AlGaN deep-UV LEDs.

We first perform the molecular beam epitaxial growth and characterization of AlN epilayers on Si substrates using the nanowire template. Highly smooth AlN epilayers with root-mean-square (rms) roughness of less than 0.4 nm are obtained. Our detailed comparison between the AlN epilayers grown on the nanowire template and the AlN epilayers directly on Si confirms that the use of the nanowire template can improve the crystalline quality and relax the tensile stress from Si. Using such AlN buffer layer, AlGaN epilayers with Al content varying from 35% to 70% are developed. The internal quantum efficiency (IQE) for such AlGaN epilayers are in the range of 30 - 40% under low excitations.

With these material developments, vertically injected, surface emitting AlGaN deep-UV LEDs down to 247 nm are demonstrated, by far the shortest wavelength for deep UV LEDs on Si with AlGaN epilayers. To further improve the device electrical performance, we have explored the polarization doped AlGaN epilayers on Si by grading the Al content along the growth direction. Vertical LEDs emitting around 278 nm are realized, and with using the polarization enhanced doping, the device series resistance is reduced by a factor of 5. This thesis work provides a viable path not only for vertical semiconductor deep UV LEDs, but also for low-cost ultrawide bandgap semiconductor template, potentially impact both photonics and electronic devices.