Asymmetric quantum well structures for enhanced infrared photon absorption

Compared to inter-band transition for photon absorption in a quantum wells, intra-band (or inter-subband) transitions in heterojunction (GaAs/InP) quantum wells can provide access to a broader range of wavelengths for detector design, specifically detectors operating in the mid infrared region of spectrum (4-12 [micro]m) and beyond is possible. These quantum wells not only provide great flexibility in optimizing the Eigen energy levels or wavefunctions, and inter-subband optical matrix elements determining the corresponding transition probability, but also allow controlling electron-phonon scattering rates and thus electron lifetime. The research presented in this dissertation investigates asymmetric quantum well structures formed through III-V semiconductor material system such as AlGaAs/ InxGa(1-x)As/InyGa(1-y) As/AlGaAs that can further improve the responsivity through higher carrier mobility. Asymmetry is introduced by using multiple materials to form the well region. The advantage of exploring stepped quantum well structure stems from experimental evidence that such structures are capable of absorbing normal incidence and thus eliminates the requirement of incorporating additional optical coupling schemes such as grating structures. An important contribution of this research is the development of an analytical model to analyze single or multiple quantum well structures to quantify photon absorption. The physical model developed in this work is based on non-equilibrium Green's function (NEGF), Fermi's golden rule and quantum mechanical wave impedance concept. The approach has two distinct advantages. First, it is accurate, easily programmable and yet computationally efficient. Second, it facilitates quantifying the broadening of states resulting from both photon absorption and tunneling, which provides important insight for improving detection efficiency. Instead of being presented through calculations, such broadening has been simply assumed in previously reported works. The method developed in this researcIncludes bibliographical references (pages 112-113)