Abstract
The work described in this thesis investigates the effects of elastic strain on the performance of InP-based semiconductor lasers operating at wavelengths around 1.5mum. The results presented range from theoretical calculations of the strain relaxation observed at the edge of strained-layer structures to the experimental application of uniaxial stress to quantum well lasers. Using an atomistic approach, the strain distributions at the edge of a single 1% compressively-strained layer are calculated for layers of differing thickness. These results are compared with a continuum Fourier series method, to assess whether the continuum method becomes inappropriate at small layer thicknesses (< 30A.). Good agreement is observed in all results, even in layers with thicknesses of 5 monolayers (~15A), indicating that continuum calculations are still relevant for predicting the strain distributions of thin strained layers. The change in the performance of 1.55mum lasers, due to the introduction of tensile or compressive strain into the active region is studied theoretically using a bulk analysis. The study concentrates on the behaviour of the valence bands with strain. It is observed that the removal of the valence band degeneracy and subsequent change in the character and dispersion of the upper valence band states have a profound effect on the laser's performance. The work is quantified by investigating the threshold carrier and radiative current densities up to 2% tensile and compressive strain, and comparing with experimental quantum well laser results. The study of lasers using a bulk-like analysis can elucidate some of the important mechanisms responsible for the behaviour of strained quantum well lasers. However, it does not account for many other factors that influence quantum well devices. A qualitative and quantitative study of the threshold current in quantum well lasers operating around 1.5mum is considered. Sophisticated quantum- well modelling programs are used to calculate the band structure and gain in strained laser devices, and to compare directly with the experimental variation of the threshold current density with cavity length for lasers with unstrained, compressively-strained and tensile-strained active regions. Results indicate the persistence of Auger recombination as the dominant mechanism contributing to the threshold current, in all devices. The observed change in the threshold current density, and its cavity length dependence, with strain is attributed to several key device parameters; namely the differential gain, optical confinement factor, transparency carrier density and Auger recombination coefficient. The two-dimensional Auger coefficient is estimated numerically and compared with experimental and theoretical values in the literature. It is found to be in reasonable agreement with some quoted results. Poor electron confinement in the quantum wells and the presence of intervalence band absorption is also discussed and related to seemingly anomalous results. The design of quantum well lasers at differing strains, requires that the lasers be grown with active layers of differing material composition. This change in composition can provide uncertainties in a theoretical analysis. Thus, in order to evaluate the role of strain alone on a quantum well laser, uniaxial stress measurements are performed to apply elastic strain externally to a laser device without changing the composition. The design and implementation of a new uniaxial stress system using opposing metallised diamonds is described. The experimental technique is covered along with suggestions for improvements to the present system. Preliminary results on unstrained ridge-waveguide and tensile-strained oxide-defined stripe devices are shown, and discussed. Compressive uniaxial stresses, up to 6kbar, are applied along the growth direction of the laser, giving rise to an increase in the in-plane strain of the devices equivalent to approximately 0.3% tensile lattice-mismatch.