Band alignment engineering for high speed, low drive field quantum-confined Stark effect devices

An analysis and discussion of the device physics for the quantum-confined Stark effect based on barrier height and band alignment considerations is presented. It identifies two important design principles for band structure engineering of the multi-quantum well stack: (1) Due to the counterbalance relationship between field-induced redshift and field-induced polarization of the quantum well eigenstates, design strategies must look to attain an optimal balance or compromise between a minimum drive field and maximum absorption coefficient change. This can be achieved with an appropriate choice of the valence band discontinuity. (2) In III–V semiconductors, the 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. As a result, optimum device performance is obtained by using a heterostructure with a disproportionately large conduction band offset to compensate the effective mass asymmetry and balance the field-induced wave function leakage in the conduction band to that in the valence band. The relative wave function leakage between conduction and valence bands is compared by examining tunneling currents through the quantum well barriers as a function of the electric field and barrier height. For conduction and valence band effective masses of, respectively, 0.055 and 0.5 times the free electron mass, the optimal band alignment requires a conduction band discontinuity 3–9 times greater than the valence band discontinuity. Applying these design principles for high speed, low drive voltage optical modulators shows that the overall performance of these devices may be improved by using a combination of balanced band alignments and low valence band barriers. The low valence band barriers reduce the drive field required to operate the devices, which has direct effects upon the drive voltage, device capacitance, attenuation coefficient, and optical coupling and propagation losses. The analysis and discussion is supported by experimental modulation depth and drive field data obtained from strained-layer multiple quantum well InAsP/InP and strain-compensated InAsP/InGaP optical modulators fabricated with layers grown on InP(001) by metalorganic vapor phase epitaxy.