Electro-absorption in InGaAs-GaAs strained-layer multiple quantum well waveguide modulators

Abstract

<p>Electric field induced changes in the excitonic band-edge absorption spectra of Multiple Quantum-Well (MQW) structures were investigated theoretically and experimentally. Three In(x)Ga(1-x)-GaAs quantum-well p-i-n waveguide structures were designed, and were then grown by the National Research Council of Canada. Band edge absorption changes with field were then investigated using light incident perpendicular to the sample surface. Theoretically, a comparison was made of three different exactly solvable methods for calculating quantum-well energies in an electric field, both with and without conduction-band non-parabolicity. An exactly solvable method was used to calculate the hole dispersion and thereby determine the heavy-hole parallel effective mass. By using calculated wavefunctions, and these mass values, a numerical method was used (Wu, 1989) to solve for the exciton binding energies. After determining broadening factors, the absorption edge was calculated at low and moderate fields for three In(x)Ga(1-x)As-GaAs structures for various fields. At high fields, a different Franz-Keldysh type of absorption calculation was developed. Comparisons vs. experiment were favourable, but some discrepancy was noted. Slab and rib waveguide modulators, with operating wavelengths from about 970nm to 1035nm, were processed from the three material growths. Waveguiding transmission spectra again agreed favourably with theory, although some differences were seen. The modulators typically exhibited greater than 20 dB of switching over a 40nm range with less than 2 dB of low-bias absorption loss. Switching speed was not examined. Theoretical analysis of different structures revealed many trade-offs, in design, with 3% to 17% indium, and 6.5nm to 13nm well-widths being acceptable.</p>