Details

Autor(en) / Beteiligte

• Green, Jeremy

Titel

Design and fabrication of quantum cascade lasers for gas sensing

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2004

Link zum Volltext

• ProQuest

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• In this thesis, two software systems have been used for the modelling of the electronic structure of quantum cascade laser (QCL) gain media. The first system, portent, was written by the author, and uses a single-band tight- binding model to find the electronic structures. The second system, NEMO, was initially developed by a team at Raytheon TI systems, and offers multiband tight-binding models. The development of portent and the choice of the methods used with NEMO were guided by the desire to design, grow and fabricate a QC laser with an emission wavelength tailored for the detection of ethene gas and capable of operation on a thermoelectric cooling system. A consideration of the thermal tuning range of such a device, and of the optical absorption spectrum for ethene, lead to the conclusion that such a laser would require a gain spectrum with a peak gain centred at a wavelength of 10.5μm ±1% at 0 °C. A new design for a QCL wafer was designed using portent to this specification and was subsequently grown. A fabrication process was developed for the new wafer and used to produce a functioning laser. This laser was then characterized and found to emit TM polarized radiation at a wavelength of 11.0?m at a temperature of 77 K, which was not within the specified range required for the detection of ethene. The device was characterized up to a temperature of 160 K and a characteristic temperature of 124.7 K was found. The threshold current density at a temperature of 100 K was 6.4 kA cm-2. NEMO was then used to analyse the design and gave predictions for the emission wavelength that agreed to within a few percent of the experimental emission wavelength. This showed that the process used to produce the new design was in need of improvement if the target ±1% tolerance on the peak gain wavelength was to be met. In particular, the results showed that a second-nearest-neighbour ten-band sp3s* tight-binding model with spin-orbit coupling and the explicit inclusion of up and down spin states gave improved predictions compared with a single-band model. Various X-valley-induced features were found to be present in the calculated electronic structures, even for AlxGa1-xAs compositions below the T-X crossover composition. NEMO was also shown to be capable of modelling QCLs in the AlSb / GaSb / InAs material system and preliminary work towards the calculation of QCL gain spectra using tight-binding models was made. The modelling work performed using NEMO showed that, in order to achieve the desired tolerance on the emission wavelength, it would be necessary to extend the model to include the calculation of intersubband scattering rates using NEMO`s multi-band tight-binding models, a six-state set of rate equations for the gain coefficient, a model for the injection and extraction of electrons into and out of the active region of a stage and a model for the integration of stimulated emission scattering rates over in-plane momenta. Finally, a programme for the production of a state-of-the-art QCL modelling system was outlined.