Master's thesis presentation Nelson Rebelo

Examiner: Åsa Haglund
Supervisor: Joachim Ciers and Estrella Torres
Opponent: Theresa Fuchs

Abstract: Among the various types of semiconductor lasers, vertical-cavity surface-emitting lasers (VCSELs) stand out for their small footprints, low threshold currents, and excellent beam characteristics.

The Earth's atmospheric ozone layer strongly attenuates ultraviolet-c (UVC). Hence most organisms have not developed a defence mechanism against it. This radiation becomes useful for its germicidal effect.

AlGaN-based VCSELs meet the requirements for these applications regarding optimal wavelength emission and output power level.

Fabricating UVC VCSELs poses several challenges that must be addressed. Most significantly, a flip-chip approach due to the all-dielectric distributed Bragg reflectors (DBR) that demands a new approach given the need of substrate removal combined with precise control of cavity length.

In this thesis, the fabrication and characterization of an optically pumped AlGaN-based UVC-emitting VCSEL with all-dielectric DBRs, with the shortest wavelength emission to date is realized. A novel technique is employed, photo-assisted electrochemical etching process (PECE), resulting in a smooth and uniformly well controlled 10λ cavity length. For some of the etched surfaces the RMS value was as low as 2.702 nm. All-dielectric DBRs with a stop-band of around 40 nm centered at 275 nm were successfully fabricated with a reflectance as high as 98.07%. Two batches of devices were produced. The device with the shortest emission wavelength emitted at 275.016 nm with a full-width half-maximum (FWHM) of 0.0562 nm at a threshold power density of 10.2MW/cm2. The lowest threshold power density was observed to be 9MW/cm2 for a device emitting at 276.00 nm with a FWHM of 0.0481 nm.