Abstract
The work described in this thesis investigates the effect of modifying the band structure in order to reduce the threshold current and its temperature sensitivity in 1.55mum InGaAsP/InP lasers. The threshold current, amplified spontaneous emission spectrum and lasing wavelength of a bulk InGaAsP laser have been measured in strong magnetic fields up to 14 Tesla, over the temperature range 70 - 240 K. A study of the effect of a magnetic field, B, on the shape of the amplified spontaneous emission spectrum and the rate at which the amplified spontaneous and lasing spectra shift with B, confirms the formation of a quantum wire-like density of states distribution in the conduction band. At high temperature (T ~ 210 K), the value of the measured threshold current in the presence of an applied 14 T magnetic field, is 18% less than that measured with no applied magnetic field. However, at low temperature (T ~ 70 K), the value of Ith measured in the presence of 14 T is 20% greater than that measured with no applied magnetic field. As a result, the temperature sensitivity of In,, denoted by the characteristic temperature T0' is increased from 75 K to 97 K. To investigate the threshold current behaviour, theoretical calculations of gain and spontaneous emission spectra were carried out as a function of B and T. Results from these calculations provide, for the first time, a physical explanation for the surprising experimental observations, revealing that there are two competing effects caused by an increase in the band-edge density of states of a quantum wire laser, namely: (i) The differential gain is increased, which tends to decrease Ith. (ii) The transparency carrier density is increased, which tends to increase Ith. Thus, depending on the magnitude of the gain required to reach threshold, Ith either increases or decreases as a result of an increase in the DoS at the band edge. Hence, if the volume of the laser active region remains constant, introducing further degrees of carrier confinement (i.e. changing the DoS distribution from that of a bulk laser to that of a quantum wire) increases T0' however, it does not necessarily reduce Ith, as first anticipated. Conversely, it is found that modifying the band structure by introducing compressive strain into the active region results in a reduction of Ith but no significant improvement in To. Spontaneous emission spectra emitted from a window etched into the substrate of a compressively strained InGaAsP multi-quantum well laser have been measured as a function of temperature and analysed to reveal information on the temperature dependence of the gain and Auger recombination. In the compressively strained device measured, experimental results suggest that the temperature dependence of the gain is not a dominant factor in directly determining the measured To, but instead, affects the temperature sensitivity indirectly through the non-radiative Auger recombination current, (via the temperature dependence of the threshold carrier concentration, nth (T)). Theoretical calculations suggest that the temperature dependence of nth in this device, is close to that of the case of an ideal quantum well, i.e. nth, T; implying that the differential gain does not show an enhanced temperature dependence, as postulated by other authors. The measured To can be accounted for by the Auger current alone, with a contribution from the temperature dependence of the Auger coefficient, C(T), and also from nth(T). The Auger activation energy was determined from two different methods of analysis and found to be ~ 70 meV, which suggests band-to-band Auger recombination remains a significant loss process in the strained laser investigated.