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Fabrication and characterisation of ridge waveguide InGaAs quantum well lasers

University of British Columbia

We have fabricated separate confinement heterostructure, graded index, indium gallium arsenide (InGaAs), strained layer, quantum well, ridge waveguide lasers. This thesis describes both the fabrication procedure we have used and the results of our characterisation of these devices. Our fabrication procedures differs from other common procedures in that we have used carbon for the p-type dopant and e-beam lithography to define the ridges. Carbon doping has been selected rather than beryllium because carbon does not diffuse rapidly in GaAs during the growth of the laser. Electron beam lithography has several advantages over conventional lithography including finer line widths and greater flexibility in pattern definition and pattern layout. We have demonstrated that ridge waveguides can be defined by electron beam lithography, and we have obtained good ridge profiles, etch resistance and write times. Characterisation of these devices used both electrical and optical techniques. DC and pulsed current-voltage characteristics have been measured for devices for different substrates. Substrate materials included two samples with doping pulled away from the quantum well, one doped with beryllium and the other with carbon, and one sample with the doping close to the quantum well doped with carbon. Diode ideality factors were determined and correlated to the doping profiles using a novel two diode model for the active region. In this model the active region is modelled as two separate diodes in series, separated by the quantum well, which acts as the source for the minority carriers. The series and shunt resistance were also determined from the current-voltage characteristics. Typical values for series resistance were about 10Ω. Shunt resistances cover a wide range of values from 100 kΩ to greater than 500 MΩ. We have also measured the specific contact resistance of an indium-silver n-type ohmic contact to be no more than 1.9 10⁻⁴ Ωcm2 using a coplanar dot resistance method described in this work. We have also measured the specific contact resistance of a chrome-gold p-type ohmic contact to be (12±5) 10⁻⁵ Ωcm² using a series resistance measurement. Threshold current densities of about 2.0 kA/cm² have been obtained for ridge waveguide lasers about 500 μm long and 3 μm wide on Be doped laser substrates. A similar laser was found to have a carrier lifetime at threshold of 11 ns from turn-on delay measurements. The value of T₀ for a laser 240 μm long and 6 μm wide has been measured to be 88 K below 10°C and 28 K above that temperature. This laser is also shown to lase in the second quantum subband at sufficiently high drive currents. Electroluminescence spectra have also been collected and compared to theoretical calculations.

Thesis/Dissertation

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2009-02-09

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http://hdl.handle.net/2429/4306

Engineering Physics

University of British Columbia

UBCV

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