Photonic integration in InGaAs/InGaAsP multiple-quantum well laser structures using quantum well intermixing
The damage introduced into an InGaAs/InGaAsP quantum well structure during CH4/H2 reactive ion etching (RIE) processes was measured, for plasma powers from 20 W to 100 W, using low temperature photoluminescence. The damage depth profile is estimated to be around 12 nm - 70 nm after annealing at 500 °C for 60 seconds using a rapid thermal annealer (RTA). A reduced damage RIE process has been developed to fabricate InGaAs/InGaAsP multi-quantum well ridge waveguide lasers. The performance of these lasers has been compared to that of lasers fabricated from the same epilayer using wet etching to form the ridge. The resultant threshold currents were essentially indistinguishable, being 44.5 mA and 43 mA respectively for dry and wet etched lasers with 500 μm long laser cavities. Quantum well intermixing in the InGaAs/InGaAsP material system was demonstrated using two techniques. The first was a laser irradiation process, which combines irradiation by continuous wave and Q-switched pulsed Nd: YAG lasers. Differential shifts up to 70 nm have been obtained. The second was a plasma process which involves sputtering a thin layer of Si02 and subsequent high temperature annealing using either a CW laser or rapid thermal annealer (RTA). Differential blue-shifts of the bandgap of up to 120 nm were obtained. The bandgap shift in the control regions is very insignificant. Measurement of the spatial selectivity of this technique shows that the spatial resolution is better than 50 μm. The design, fabrication and characterisation for 3-dB MMI couplers were carried out using both as-grown (peak emission wavelength of 1.48 μm) and bandgap widened material. The measured results show good agreement with the design. A splitting ratio of around 0.12 dB (51: 49) has been achieved for an MMI section length of 470 gm. Low loss waveguides have been fabricated using the laser process. A loss as low as 2.1 dB/cm was obtained for an operation wavelength of 1.556 um. Extended cavity ridge lasers (ECL) in InGaAs/InGaAsP multiple-quantum well structures have been successfully fabricated using the two QWI technique developed. The increases in threshold current were only 10 mA and 8 mA for cavity length of 800 4m active section and 1000 μm passive section, compared to the all active lasers with cavity length of 800 μm, and the losses in the passive sections of ECLs were calculated which were 2.4 cm-1 and 4.4 cm-1, for the two processes, respectively. Considerable theoretical work was carried out, which included the calculation of the optical confinement and gain in the InGaAs/InGaAsP MQW structure used throughout this thesis. Modelling of the intermixing of quantum wells was also performed and the results indicate that the changes of bulk bandgap energy are mainly responsible for the blue-shift of the photoluminescence.