Delta-doping superlattices in multiple quantum wells

The quantum confined Stark effect has been extensively used for amplitude modulation. One way of improving the performance of multiple quantum well structures to be used in light modulation at high bit rates is by increasing the Stark shift for a given externally applied voltage. GaAs/AlGaAs multiple quantum well structures containing an nipi delta-doping superlattice, where the n-type doping is inserted in the quantum wells and the p-type in the barriers, are expected to double the Stark shift, according to Batty and Alsopp (Electron. Lett. 29 (1993) 2066). Such structures have been studied in detail to evaluate their potential for use in the fabrication of optical modulators. It has been observed that the required balance between n- and p-type doping levels is not trivial to achieve due to the presence of interface hole traps whose population depends on the quantum well doping concentration. It is estimated that for undoped quantum wells around 15% of the holes provided by the p-doping are trapped at the interfaces. Photoluminescence measurements, supported by calculations, point out that even though an indirect transition between electrons in the quantum wells and holes in the barriers is present at low temperatures at energies below the quantum well fundamental transition energy, at room temperature such a transition is absent and the observed optical emission occurs at essentially the same energy as that of an equivalent undoped structure.