Abstract
The objectives of the research undertaken have been to investigate the properties of semiconductor lasers operating at around 1.3 mum. The aim of the investigation is to suggest modifications which give rise to improved operating characteristics especially in the high temperature (approaching 85 °C) range. The investigation can be divided into 2 sections: a theoretical approach and an experimental section. The theoretical study examined the performance of compressively strained InGaAsP/InP multiple quantum-well lasers emitting at 1.3 mum in order to investigate the important factors and trends in the threshold current density and differential gain with strain, well width and well number. Structures with a fixed compressive strain of 1% but variable well width, and also with a fixed well width but variable strain from 0% to 1.4% have been considered. It has been found that there is little benefit to having compressive strains greater than 1 %. For structures with a fixed 1% compressive strain and unstrained barriers, an optimum structure for lowest threshold current density and a high differential gain has been found to consist of six 35 A quantum-wells. In addition, compensated strain (CS) structures with compressive wells and tensile barriers have been examined. It is shown that the conduction band offset can be significantly increased and the valence band offset reduced in such structures, to give band-offset ratios comparable with aluminium based 1.3 mum devices. The gain calculations performed suggest that there is little degradation in the threshold carrier density or differential gain due to these alterations in the band offsets; and hence a better laser performance is expected due to a reduction in thermal leakage currents due to the improved electron confinement. The experimental study concentrates on looking at certain key design parameters to investigate their effect on the laser performance. These design parameters range from the number of quantum-wells to the device length. The experimental study confirms the conclusions drawn in the theoretical investigation that the optimum structure for a 1.3 mum InGaAsP laser for low threshold current, high efficiency and high characteristic temperature operation consists of six 1% compressively strained 50 A quantum-wells in a device of medium length (approx. 450 mum). The inclusion of a high reflection coating on one facet provides further improvement in the device performance, but increases the production cost dramatically. Also investigated in the experimental section is the effect of changing the device material from InGaAsP to InGaA1As. The results discussed do not offer firm evidence of any improvement in the device characteristics in switching from a P-based to an Al-based structure. This is mainly due to the added complication of switching to a RWG structure from a BH structure. Another explanation for the relatively poor performance of InGaAsP 1.3 mum lasers has been examined. That is leakage of the carriers out of the well region. Evidence of a leakage current has been seen primarily in devices with a low number of quantum-wells. A novel measurement technique has been demonstrated, which should prove useful for obtaining a numerical value for the leakage current in semiconductor lasers. The results presented suggest that leakage current is not significant for a 9 well device until operating at temperatures above around 373 K. This is supported by evidence supplied by the spontaneous emission spectra.