Quantum well devices are being developed in which the quantum well region includes an asymmetric characteristic. Improvements such as lower operating energy and reduced tolerances on the operating wavelength of incident light have been attributed to the incorporation of the asymmetric wells. In the related application cited above and as described in Appl. Phys. Lett., 54 (3), pp. 202-4 (1989), an improved self electrooptic device resulted by including within the device structure an intrinsic quantum well region having an asymmetric electronic characteristic across as narrow bandgap subregion between the two wide bandgap layers defining the quantum well region. As a result of the asymmetry, the quantum well region appeared to polarize electrons and holes within the subregion in an opposite direction relative to a direction for an electric field applied to the self electrooptic effect device. The asymmetric electronic characteristic was realized as a compositionally graded, narrow bandgap layer or as a pair of coupled narrow bandgap layers of differing thicknesses separated by a thin wide bandgap layer.
While the asymmetry may be produced by compositional grading of the energy bandgap for the quantum well materials or by coupling quantum wells of differing thicknesses, it has been shown that a particular class of strained-layer semiconductor structures grown along the [111] axis provide asymmetry and electrooptic effects which arise from large, piezoelectrically generated internal electric fields. The class of strained-layer semiconductor structure is one in which the layers of a superlattice comprise constituent materials which provide alternate regions of biaxial compression and biaxial tension via lattice mismatch to the substrate. Because the signas of the electric polarization vectors are opposite from one region to the next, there is a non-zero divergence of polarization (a polarization charge) at the superlattice interface and a resulting internal electric field directed along the growth axis having opposite polarities from one layer to the next. See, for example, B. Laurich et al., Phys. Rev. Lett., Vol. 62, No. 6, pp. 649-52 (1989); J. Beery et al., Appl. Phys. Lett., 54 (3), pp. 233-5 (1989); C. Mailhoit et al., Physical Review B, Vol. 37, No. 7, pp. 10415-8 (1988); D. Smith et al., Phys. Rev. Lett., Vol. 58, No. 12, pp. 1264-7 (1987); and D. Smith, Solid State Communications, Vol. 57, No. 12, pp. 919-21 (1986). Although device operaton has been postulated for these particular strained-layer structures, no actual device incorporating these structures has been reported.
USPTO Backgrounds
USPTO Backgrounds