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-confi ned Stark effect based on barrier height and band alignment considerat ions is presented. It identifies two important design principles for b and structure engineering of the multi-quantum well stack: (1) Due to the counterbalance relationship between field-induced redshift and fie ld-induced polarization of the quantum well eigenstates, design strate gies must look to attain an optimal balance or compromise between a mi nimum drive field and maximum absorption coefficient change. This can be achieved with an appropriate choice of the valence band discontinui ty. (2) In III-V semiconductors, the strong asymmetry in the field res ponse of the conduction and valence band eigenstates is due directly t o the asymmetry of the conduction and valence band effective masses. A s a result, optimum device performance is obtained by using a heterost ructure with a disproportionately large conduction band offset to comp ensate 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 band s 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 a nd 0.5 times the free electron mass, the optimal band alignment requir es a conduction band discontinuity 3-9 times greater than the valence band discontinuity. Applying these design principles for high speed, l ow drive voltage optical modulators shows that the overall performance of these devices may be improved by using a combination of balanced b and alignments and low valence hand barriers. The low valence band bar riers reduce the drive field required to operate the devices, which ha s direct effects upon the drive voltage, device capacitance, attenuati on coefficient, and optical coupling and propagation losses. The analy sis and discussion is supported by experimental modulation depth and d rive field data obtained from strained-layer multiple quantum well InA sP/InP and strain-compensated InAsP/InGaP optical modulators fabricate d with layers grown on InP(001) by metalorganic vapor phase epitaxy. ( C) 1998 American Institute of Physics. [S0021-8979(98)07203-X].