Growth, characterization and design of indium phosphide-based strained-layer multiple quantum wells for optical modulator devices

Optical modulators based on the quantum-confined Stark effect (QCSE) were fabricated from strained InAsP/InP and strain-compensated InAsP/InGaP epitaxial layers grown by metal-organic vapor phase epitaxy. The series contained samples with both coherently strained, and partially relaxed multi-layers. Strain relaxation in the InAsP/InP series occurred exclusively via misfit dislocations localized to the outer interfaces of the multiple quantum well stack (MQWS). Strain relaxation in the InAsP/InGaP series occurred primarily via strain-induced growth instabilities. The heterojunction band discontinuities were determined by fitting the energy positions of the optical absorption peaks, measured at low and room temperature, with those computed using the Marzin-Bastard band structure model for strained-layer superlattices. The conduction band discontinuities were found to be linear in the As composition for the InAsP/InP samples.; The electric field-dependent redshift of the n = 1 electron-heavy hole transition was observed to be significantly enhanced in structures with lower valence band barrier heights. This led to a more general observation that strategies designed to optimize the performance of the MQWS in QCSE devices can be derived from the effective mass of the active quantum well material in III--V semiconductors is significantly smaller in the conduction band than that in the valence band. A detailed analysis and discussion of the device physics for the QCSE based upon barrier height and band alignment considerations was conducted. It concluded the following principles for designing the band structure of the MQWS.; A counterbalance relationship between maximum optical modulation depth and minimum drive field is implicit in the mechanics of the QCSE. As a result, it is not possible to optimize one independently of the other.; A strong asymmetry in the field response of the conduction and valence band eigenstates is due directly to the asymmetry of the conduction and valence band effective masses. Because of this, the maximum optical modulation depth per unit applied electric field is obtained by designing the multi-quantum well stack with a disproportionately large conduction band offset to match the effective mass asymmetry and balance the field-induced wave function leakage at the conduction band to that at the valence band.; When the conduction and valence band effective masses are respectively 0.055m0 and 0.5m0, we found that a balanced, optimal design requires a conduction band discontinuity 3--9 times larger than the valence band discontinuity. Applying the design principles to high-speed, low drive voltage optical modulators concludes that the overall performance may be improved by using designs that incorporate a combination of balanced band alignments and valence band barriers lower than 60--100 meV. Low valence band barriers reduce the drive field required to operate the devices, which can improve the overall performance by permitting a lower drive voltage and/or wider diode junction with larger active volume, lower capacitance, lower propagation losses and improved optical coupling.; A simple model was developed to conduct a brief investigation and review of theoretical and experimental results in the literature on the use of quantum wells to enhance the detailed balance efficiency limit for solar energy conversion. We conclude that practical efficiency enhancements, if they exist, are very small. (Abstract shortened by UMI.)...