Abstract

For electroabsorption modulators, the decrease in electroabsorption-oscillator strength accompanying the red shift with applied field is often an undesirable trait of type-I quantum wells. On the other hand, for type-II quantum wells the oscillator strength increases with applied field but the transition wavelength is blue shifted. Figure 1 shows the quasi-type-II (QT-II) quantum well, a new quantum structure that can be designed to balance out the characteristics of type-I and type-II quantum wells. The electrons and holes are confined in spatially offset regions, with a common overlap region in the center. The central narrow-band-gap overlap region contributes type-I characteristics, while the spatial offset contributes type II characteristics. The band structure shown in Fig, 1 can be obtained with strained InGaAsP materials.¹ Figure 2 shows the calculated electron and hole wave functions plotted against position for various applied biases, which have been offset vertically for clarity. For positive applied fields, the electron and hole wave functions are driven away from each other and so the overlap and absorption oscillator strength drops. For negative applied fields, the carriers are driven towards each other and the oscillator strength increases. A negative field also makes the wave functions taller and narrower by driving them against the confinement-region barriers, which also enhances the wave function overlap. The increase in wave function overlap will also increase the exciton binding energy.

© 1995 Optical Society of America