Estrella Torres, Avdelningen för fotonik

Opponent: Prof. Enrique Calleja, from Technical University of Madrid

Sammanfattning: 
The demand for compact, efficient, and low-cost ultraviolet (<400 nm) emitters has been increasing due to important applications in water, air and surface disinfection, sensing, and optical communication. However, the efficiency of light-emitting diodes (LEDs) emitting below 320 nm, i.e. in the UVB (280-320 nm) or the UVC (<280 nm) is still very low, typically below 15%. Moreover, not long ago the first laser diodes, an edge-emitting laser type, at these short wavelengths were demonstrated. These facts emphasize the attraction to exploring new types of device concepts in the UV. Microcavities are devices that can provide a more directional light emission pattern, higher spectral purity, and potentially higher extraction efficiency, which are interesting characteristics for many applications.
Microcavities are light-emitting devices embedded between two mirrors forming an optical cavity. The most suitable type of mirrors for microcavities are highly reflective distributed Bragg reflectors (DBRs) due to their potential reflectivities above 99%. However, unlike in the arsenide-system, epitaxial AlGaN-based DBRs are difficult to grow without relaxation due to their high lattice constant mismatch preventing them to reach the high reflectivities needed. Alternatively, dielectric SiO2/HfO2 DBRs have shown reflectivities above 99% in the UVB and UVC and have been used for the demonstration of optically pumped UV VCSELs. All-dielectric DBRs require substrate removal techniques to access both surfaces of the device for the deposition of the dielectric DBRs. This can be done by a process called electrochemical etching which is based on a tunnelling process carried out at the semiconductor/electrolyte junction, where a sacrificial layer is etched releasing the device from the substrate.
In this work, we employed electrochemical etching for the fabrication of microcavities leading to the first demonstration of UVB resonant-cavity LEDs (RCLEDs) and optically pumped UVC VCSELs with precise cavity length control. The UVB RCLEDs present a combination of a tunnel junction with a top current spreading layer which allows the ohmic contact to be placed far from the DBR which is not possible with a conventional LED design due to the low p-AlGaN hole conductivity that limits the transverse current spreading. The UVB RCLEDs show more directional emission patterns, and narrower emission spectrum. In addition, this device demonstration shows the possibility to electrochemically under-etch highly doped devices structures including tunnel junctions. On the other hand, the fabrication of the UVC VCSELs required underetching higher Al-containing sacrificial layer than the UVB RCLEDs, which was enabled by a new methodology we developed, referred to as photo-assisted electrochemical etching. The UVC VCSELs show a non-linear output power vs pump power with a threshold pump power density of around 5 MW/cm2. The angular-resolved far-field spectrum changes from dispersive to non-dispersive around threshold, showing a linewidth above threshold of about 29 pm. The lasing wavelength only varies 1.6 nm between different UVC VCSELs across a 1.3 mm x 0.4 mm area, indicating a physical length deviation of the cavity of less than 1%, thus demonstrating a very accurate cavity length control.