title: MOVPE growth for GaAs-Based Photonic Crystal Surface Emitting Lasers
Creators: name: McKenzie, Adam F.
Publisher: name: University of Glasgow
Year: 2023

description: Photonic crystal surface emitting lasers (PCSELs) have emerged recently as class of laser diode that are of great commercial interest, offering high-power, single-mode surface emission across a wide range of wavelengths. The optical and electrical characteristics of PCSELs are intrinsically linked to the nanoscale structure of the two-dimensional photonic crystal (PC) grating layer that is embedded within the device structure by metalorganic vapour phase epitaxy (MOVPE) regrowth. In this regard, an understanding of, and the ability to influence, the formation of crystallographic voids within the PC during epitaxial regrowth is one of the key aspects for achieving optimised designs and device performance. This thesis presents a number of studies related to the application of MOVPE in the fabrication of GaAs-based PCSELs, with a particular focus on epitaxial regrowth and the engineering of voids. The mechanism of void formation and the factors dictating the extent of grating infill are investigated by electron microscopy-based structural analysis of regrown PC structures and correlated with device results. Initially, AlAs/GaAs PCSELs are presented for which the morphology of the initial PC grating pits has been modified by controlling the degree of mass-transport of material during the pre growth temperature ramp within the MOVPE reactor. It is shown that void formation is the result of low adatom surface mobility and selfshadowing effects which drive rapid lateral growth of the upper pit surface. The growth kinetics associated with the crystal planes of the underlying grating pits are shown to greatly impact the size and shape of the voids, with vastly different geometries observed in each of the devices. Following this, the role of adatom mobility in driving grating infill is demonstrated in PC structures for which the composition of the regrowth material is varied. It is shown that void formation is favoured when aluminium-containing layers are used, for which the inherent adatom mobility is low, and that complete grating infill is promoted in the case of higher mobility GaAs. Analysis of the threedimensional void shape in AlAs- and AlGaAs-regrown structures reveals that natural asymmetries emerge in the shape of voids even when symmetric, circular grating pits are used, owing to the differing polarities and growth kinetics of high-Miller index crystal planes in III-V materials. It is shown that the in-plane asymmetry of void shape can be greatly enhance by the use of (311)B orientated substrates, and that voids in these PCs display an additional out-of-plane asymmetry compared to those in the conventional (100) orientated structures. In addition to the PC regrowth studies described above, an initial investigation into the use selective area growth (SAG) as a method for realising multicolour PCSEL arrays is presented. It is shown that the growth rate enhancement and wavelength tuning associated with SAG is effective for InGaAs/GaAs multi-quantum well structures deposited in masked growth windows with dimensions up to 300 x 300 μm2. For selected features, it is shown that large areas of material with uniform emission wavelengths up to 100 x 100 μm2 in size can be achieved, being appropriate for the fabrication of monolithically integrated devices in the future.