In a recent improvement upon InxGa1- xAs/InP semiconductor lasers of the bipolar cascade type, quantum wells are added to Esaki tunnel junctions, which are standard parts of such lasers. The energy depths and the geometric locations and thicknesses of the wells are tailored to exploit quantum tunneling such that, as described below, electrical resistances of junctions and concentrations of dopants can be reduced while laser performances can be improved.
InxGa1-xAs/InP bipolar cascade lasers have been investigated as sources of near-infrared radiation (specifically, at wavelengths of about 980 and 1,550 nm) for photonic communication systems. The Esaki tunnel junctions in these lasers have been used to connect adjacent cascade stages and to enable transport of charge carriers between them. Typically, large concentrations of both n (electron-donor) and p (electron-acceptor) dopants have been necessary to impart low electrical resistances to Esaki tunnel junctions. Unfortunately, high doping contributes free-carrier absorption, thereby contributing to optical loss and thereby, further, degrading laser performance.
In accordance with the present innovation, quantum wells are incorporated into the Esaki tunnel junctions so that the effective heights of barriers to quantum tunneling are reduced (see figure). Inasmuch as the tunneling current is approximately inversely proportional to the exponential of the barrier height, the introduction of quantum wells into the Esaki tunnel junction can significantly reduce the electrical resistance of the junction and thereby reduce the amounts of dopants needed. Furthermore, the numbers and shapes of the quantum wells constitute additional degrees of freedom in design that can be used to tailor carrier-transport and potential profiles to optimize laser performance.
Going beyond bipolar cascade lasers, the present innovation could also be beneficial in some light-emitting diodes and single-stage semiconductor lasers that contain Esaki tunnel junctions. For example, quantum wells could be incorporated into vertical-cavity surface-emitting lasers, wherein Esaki tunnel junctions are used to connect n-doped mirrors to avoid the use of p-doped resistive mirrors.
This work was done by Rui Q. Yang and Yueming Qiu of Caltech for NASA's Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-40195.