Epitaxy of gallium-antimonide-based short-wave infrared quantum well emitters

Abstract

This thesis investigates the molecular beam epitaxy (MBE) growth of GaSb-based edge-emitting laser (EEL) diodes designed for operation in the short-wave infrared regime around 2.7 μm. Such light sources are ideal for tunable diode laser absorption spectroscopy, which enables highly sensitive real time environmental gas monitoring. However, many challenges are faced in fabricating such diodes arising from the fundamental material properties to complexity in MBE growth. In this thesis, a focus is put on the MBE growth of the laser’s active region and specifically their tendency to deviate significantly from the targeted structure upon thermal annealing. This is investigated by MBE growth of samples containing the active region structure which are then rapid thermal annealed at different temperatures and their optical and structural properties are studied. Two sets of such samples study in particular the photoluminescence (PL) emission wavelength shift with two different approaches. While both of the PL sample sets provided insight into the effects of interdiffusion inside the active region, ultimately there was no clear reduction in the PL shift in any of the investigated structures. Additionally for the second sample set, the structural properties were characterized with high-resolution X-ray diffraction (HRXRD) which indicated that the overall crystal quality and the quantum well structure in the active region are likely maintained even with the highest temperatures used for annealing.

Based on the information gained of from the PL sample sets, three varying EEL-structures were grown and processed into working laser diodes. First, a reference EEL-structure using a typical higher growth temperature active region and top-cladding region was grown. And second, two alternative EEL-structures with lower growth temperatures of the active region and top-cladding were grown with the aim of minimizing the annealing effects during growth. The as-grown laser structures were characterized again with PL and HRXRD measurements, indicating good structural quality with similar PL emission wavelengths.

The EEL-structures were then processed into laser bars from which laser light-current (L-I) characteristics and emission spectra were measured. For the L-I characterization, all laser structures indicated lasing action, however with significantly different performance characteristics. For the reference structure, the lasing current threshold was significantly higher (by a factor of two) with poor slope-efficiency in comparison to the two low temperature structures. Furthermore, comparison between the two low temperature structures also indicated improved performance with reduced growth temperature. Overall, the best performance laser diode structure showed current threshold comparable to values found in literature.

In conclusion, the results in this thesis indicate that the active region of these EEL-structures are sensitive to thermal annealing effects that can already occur during the MBE growth of the structure and can severely impact the device performance.