Towards Novel AlGaN-Based Light Emitters

Licentiatavhandling, 2011

The unique properties of the AlGaN semiconductors make them a suitable choice for optoelectronic devices at both short and long wavelengths, based on interband and intersubband transitions, respectively. What sets AlGaN apart from other wide-bandgap semiconductors is the possibility to obtain both p- and n-type conductivity as well as the direct bandgap. However, effective p-type doping of AlGaN alloys with high Al mole fractions is still problematic, limiting the efficiency of AlGaN based light emitters. Measurement results of a comparatively large conductivity of 7 kΩ cm in a Mg-doped Al0.85Ga0.15N layer grown using hot-wall metal-organic chemical vapor deposition are presented. Light emitting diodes (LEDs) operating in the UVC spectrum (200-280 nm) have potential to improve current Hg based water purification systems. Improvements for these systems such as reduced power consumption, reduced size, increased mechanical robustness, reduction of hazardous waste and reduced turn on time are made possible by a switch to LED technology. The quantum cascade laser (QCL) is a fairly new type of device for infra-red emission that in recent years has emerged as a suitable light source for spectroscopic analysis of trace gases. Although an AlGaN based QCL has not been demonstrated yet, the material properties should make operation at wavelengths that have previously been unaccessible for room temperature semiconductor lasers possible. This can be attributed mainly to the large conduction band offset and large remote-valley separation in AlGaN. Operation at telecom wavelengths around 1.55 µm may even be possible. For telecom applications intersubband devices make chirp free modulation at very high speeds possible. It is shown that intersubband transitions in AlGaN can have a stable transition energy with a small temperature drift of merely 15 µeV K-1, potentially reducing the need for active thermal management in telecom systems. One problem hindering the demonstration of AlGaN based QCLs is the design and fabrication of effective waveguides for wavelengths far below the GaN bandgap. A waveguide design optimized for 1.55 µm QCLs, with a simulated modal-loss of 6.1 cm-1 due to interaction with the metal contacts, is presented.