Ultraviolet vertical-cavity surface-emitting lasers and vertical microcavities for blue lasers

Doktorsavhandling, 2021

III-nitride materials are used for ultraviolet (UV) and visible light emitters. One such light source is the vertical-cavity surface-emitting laser (VCSEL) that could find applications within areas ranging from sterilization and medical treatment to car headlights and augmented reality displays. These devices are not yet commercialized, because of challenging mirror formation and electrical injection. However, due to the recent substantial performance improvement of blue-emitting VCSELs, it might not be long before they are available. UV VCSELs, on the other hand, are far from ready. Until recently, there were no VCSELs emitting in the UVB (280-320 nm) or at shorter wavelengths.

In this thesis, the first UVB VCSELs are demonstrated. These optically pumped devices emitting at wavelengths around 310 nm were realized by removing the substrate from high Al-content AlGaN structures using electrochemical etching, which allowed for the deposition of a high-reflectivity dielectric distributed Bragg reflector (DBR) on each side of the cavity. Thresholds below 1 MW/cm^2 were achieved by optimizing the sacrificial layer to achieve smooth etched surfaces and by accurately setting the cavity length and thereby the detuning.

Furthermore, electrically conductive DBRs for blue VCSELs were investigated. Insertion of interlayers in AlN/GaN DBRs increased the vertical resistance while measurements and simulations of ZnO/GaN multilayers showed that the resistance is similar to, or lower than, the lowest reported for pure III-nitride DBRs, as a result of the partial cancellation of polarization fields.

Finally, vertical cavities based on dislocation-free GaN microprism are demonstrated. The quality factor is strongly dependent on the prism diameter and is around 500 at yellow wavelengths for prisms with a 1-μm diameter. Simulations show that the quality factor should be approximately four times larger in the targeted blue wavelength regime.

These results serve as building blocks for future dislocation-free small-footprint VCSELs grown on low-cost substrates as well as short-cavity blue VCSELs with electrically conductive DBRs. Additionally, the UV VCSEL demonstration is an important step towards a compact, energy-efficient light source with attractive beam characteristics using a technology with great potential for realizing VCSELs in almost the full UV spectrum.