title: Etched facet photonic devices on InP/AlGaInAs using inductively coupled plasma etching
Creators: name: Fahim, Reasat Hasan
Publisher: name: University of Glasgow
Year: 2021
Subjects: subject: Photonic Integration, InP, EF-FP lasers EF-DFB lasers, ICP etching.

description: The work of this thesis is concerned with development of plasma etching on InP/AlGaInAs using an inductively coupled plasma in a chlorine-argon-hydrogen gas mixture. The achievements of this research are expected to contribute significantly toward EF-platform required for photonic integrations. Photonic devices such as EF-FP lasers and EF-DFB lasers are formed with optimised plasma etching, and resulting parametric performances are compared with conventional CF-FP lasers and CF-DFB lasers. The development of plasma etching has been carried out varying plasma parameters such as ICP source power, RF-bias power together with chamber pressure and relative gas flow ratio. The effects of these plasma parameters are methodically studied and characterised in terms of etch rates, selectivities, and etched profiles for deeply etched facets. The investigation shows that the etch rates of InP/AlGaInAs are increased with the increase of ICP source power, RF-bias power and chamber pressure, whereas etch rates of SiO2 layer are decreased only when ICP source power is increased. It is also evident that the etch rates of epitaxy are dominated by a chamber pressure and chlorine gas flow. The selectivity is found to depend strongly on ion flux and ion energy. A selectivity is decreased when both ICP source power and RF-bias power are increased. Meanwhile, it is evident that verticality of an etched profile strongly depends on RF-bias power. It is further evident that verticality of an etched profile develops a large degree of undercut when hydrogen and argon flow are changed. It is also observed that the SiO2 patterned mask layer is suffered from pyramid-like features causing roughness with a value of around 2.75 nm. And this sidewall roughness of the mask is thought to be the main contributor for the sidewall roughness of a deeply etched profile. It is evident that for an optimum deep etched profile including 90° vertical angle and approximately 3.65 nm of facet roughness, plasma etching condition should be set as an ICP power of 850/150 W including 25% higher hydrogen and 20% lower argon flow relative to chlorine flow in a total gas mixture and a chamber pressure of 2 mTorr. It is also evident that a selectivity as high as 27:1 with a moderate DC-bias voltage of 312 V are further requirements for an excellent etched profile. InP/AlGaInAs-based photonic devices such as FP lasers are formed in which optical cavity facets are formed with the developed plasma etching conditions stated above. The parametric performances of EF-FP lasers are compared with conventional CF-lasers with respect to SWRs of the facets. It is evident that the parametric performances of EF-Lasers are close to indistinguishable to that of CF-lasers when facets of EF-FP lasers are formed with 32% of chlorine and the relative plasma conditions, and associated roughness is about 3.65 nm. For EF-lasers a median Ith some of 11.80 mA is obtained which is 2% higher than that of CF-lasers. A negligible difference in emission spectra is observed for both laser types. Similarity of parametric performance is further supported by resluts of reliabilities where both laser types present similar degradation trend. End cavity facets of EF-DFB lasers are also formed with the developed plasma etching. For EF-DFB lasers, a median threshold current of around 18.70 mA is attained which is approximately 12% higher than that of CF-DFB lasers. For both device types, spectral emission is occurred at a wavelength of 1539 nm including sidemode suppression ratios of about 37 dBs.