New Developments in GaAs-based Quantum Cascade Lasers

This thesis presents a study of the design and optimisation of gallium-arsenide-based
quantum cascade lasers (QCLs). Traditionally, the optical and electrical performance of
these devices has been inferior in comparison to QCLs that are based on the InP
material system, due mainly to the limitations imposed on performance by the intrinsic
material properties of GaAs. In an attempt to improve the performance of GaAs QCLs,
indium-gallium-phosphide and indium-aluminium-phosphide have been used as the
waveguide cladding layers in several new QCL designs. These two materials combine
low waveguide losses with a high confinement of the laser optical mode, and are easily
integrated into typical GaAs QCL structures.
Devices containing a double-phonon relaxation active region design have been
combined with an InAlP waveguide, with the result being that the lowest threshold
currents yet observed for a GaAs-based QCL have been observed - 2.1kA/cm2
and
4.0kA/cm2
at 240K and 300K respectively. Accompanying these low threshold currents
however, were large operating voltages approaching 30V at room-temperature and 60V
at 80K. These voltages were responsible for a high rate of device failure due to
overheating. In an attempt to address this situation, two transitional layer (TL) designs
were applied at the QCL GaAs/InAlP interfaces in order to aid electron flow at these
points. The addition of the TLs resulted in a lowering of operating voltage by ~12V and
30V at 300K and 240K respectively, however threshold current density increased to
5.1kA/cm2
and 2.7kA/cm2
at the same temperatures.
By utilising a high-reflectivity coating and epi-layer down bonding process, a QCL
comprising an InGaP waveguide and double-phonon active region was observed to
operate in continuous-wave mode up to a temperature of 80K, with an optical output
power of 26mW.