Fabrication and characterisation of microcavity based III-nitride optoelectronics on a microscale

In this work, the design implementation, fabrication and characterisation on III-nitride microemitters with microcavity effects grown on c-plane sapphire are presented. Highly doped GaN layers have been effectively porosified by means of electrochemical etching (EC). Different porous morphology is established by varying the doping level of the GaN layer and the EC bias voltage, enabling a wide refractive index tunability of the porous layer. Through this thesis the potential of nearly-lattice matched nanoporous GaN as a cladding layer and as a Distributed Bragg reflector (DBR) has been fundamentally studied. There are three major parts that constitute the outcome of the work. The first part is on the development and characterisation of a novel microcavity based microemitter array, which partially mitigates one of the consequences of the Quantum Confined Stark Effect (QCSE). The so-called ‘blue shifting’ of the emission wavelength that is considered to be one of the long-standing problems of III-
nitrides has been reduced from 50 nm to 3 nm. The second part highlights a novel approach towards micro-Vertical Surface Emitting Lasers, which is achieved by developing a direct epitaxial method based on selective epitaxial growth. Nearly
lattice-matched porous DBRs have been achieved with reflectivities over 99%. Strong cavity effects have been demonstrated via electrical and optical pumping. Although stimulated emission is not fully confirmed, the full width at half maximum of the peak emission has been reduced by a factor of 5. Finally, the last part is an introduction on the potential enhancement effect of porous GaN as a cladding layer in III-nitride green edge emitting lasers, instead of conventional AlGaN. There is an enhancement in the optical confinement by a factor of 2.5, as verified using a Finite Difference Eigenmode solver. Porous cladding layers have
been effectively formed, achieving porosities of 50%. Effective current injection through the porous was demonstrated. However, the inherent challenges from the cleaving of sapphire substrates limited the facet formation. Thus, stimulated emission was not confirmed yet.