Photoabsorption induced disordering of InGaAs-InGaAsP multiple quantum well structures for optoelectronic integration
This thesis is concerned with the intermixing of indium phosphide based multiple quantum well structures using a high power continuous wave (CW) operated Nd:YAG laser which shall be referred to as Photoabsorption Induced Disordering (PAID). A wide range of discrete devices have been fabricated from intermixed material such as low-loss single mode waveguides, broad area oxide strip lasers, strip loaded ridge waveguide lasers and electroabsorption ridge waveguide modulators. These devices have been characterised which gives an indication as to how the optical and electrical properties of the multiple quantum well material changes as it is intermixed. Single mode ridge waveguides have exhibited losses of around 5dBcm at 1550 nm while losses as low as 1.6 cm have been observed at 1570 nm. Broad area and ridge waveguide lasers have been fabricated from material which has been bandgap widened by up to 160 nm. There is little deterioration in the lasing performance as the material is intermixed with only a 20% increase in the threshold current density due to the alteration of the 2-D density of states towards bulk material. Electroabsorption in disordered material is investigated using photocurrent spectroscopy and also by fabricating ridge waveguide modulators. The ON/OFF ratios are 20 dB in 500 long devices fabricated from material which has been bandgap shifted by 120 nm while material shifted by only 80 nm yielded devices which extinction ratios as high as 27 dB. The thermodynamics of the PAID process have been modelled using a commercially available finite element package (ABAQUS). This modelling has yielded useful results which gives an insight into the resolution capabilities of the process, and the physical parameters (such as spot size and heat sink temperature) which affect it.