Optical Guiding and Feedback Structures in GaN-Based Vertical-Cavity Surface-Emitting Lasers

Doktorsavhandling, 2016

The Vertical-Cavity Surface-Emitting Laser (VCSEL) made from GaAs-based materials is an established infrared light source, and is produced in large volumes for short-distance optical links, computer mice, and thermal processing systems. A wide range of applications, such as high-resolution printing, general lighting, and biomedical diagnosis and treatment could benefit from using a VCSEL with blue or ultraviolet emission. A promising solution is the use of GaN-based materials, here only a few research groups have so far demonstrated electrically pumped GaN VCSELs with optical output powers in the milliwatt-range and threshold current densities <10 kA/cm^2, unfortunately though the devices degrade after a few minutes. The major challenges are achieving broadband high-reflectivity mirrors, vertical and lateral carrier confinement, efficient lateral current spreading, accurate cavity length control, and lateral optical mode confinement. This thesis is focused on exploring the lateral optical Guiding and feedback structures for GaN VCSELs. By numerical simulations, the most commonly used GaN VCSEL cavities were investigated and it was shown that many structures used for lateral current confinement result in unintentionally anti-guided resonators with associated high optical losses. Such resonators have a built-in modal discrimination but the threshold gain is highly dependent on nanometer-sized changes in the structure. In addition, the anti-guided resonators have a strong Temperature dependent characteristic, since thermal lensing suppresses the lateral optical leakage. Both distributed Bragg reflectors (DBRs) and high contrast gratings (HCGs) were explored as optical feedback structures. The effect of compositional interlayers on the vertical electrical conductivity of Si-doped AlN/GaN DBRs was investigated. The DBR with no interlayers showed state-of-the-art resistivity for an AlN/GaN DBR as low as 0.044 Ohm-cm^2 for 8 mirror pairs. The DBRs with compositional interlayers had resistivities between 0.16–0.34 Ohm-cm2, indicating that the incorporation of interlayers can impair the vertical current transport. An alternative to a DBR, a high contrast grating (HCG) in TiO2/air was demonstrated for the first time. The fabricated HCG showed a high reflectivity of >95 % over a 25 nm wavelength span.