Novel Superlattice and Quantum Well Structures: Physics and Device Applications.

The physics and device applications of new superlattice and quantum well structures are studied in this dissertation. A formalism based on the envelope function approximations and the transfer matrix method is developed to analyze the electrical and optical properties of general quantum well structures. This formalism allows the exact and analytical formulation of the eigenenergies and wave functions for an arbitrary one-dimensional potential profile. It takes into account the effective mass difference and incorporates the coupling of up to three bands (the conduction, light hole, and split-off hole bands). The transport characteristics (transmission coefficient, tunneling current, and carrier density) and optical properties (absorption coefficient, polarization dependent selection rule, and electro-optical coefficient) can all be modeled by this formalism as described in Chapters 2 and 3. A self-consistent program is also developed to simultaneously solve the Schrodinger and Poisson equations. The physics and applications of several novel device structures are also introduced and analyzed. First, different mini-band structures have been predicted by changing the potential profile in one period of a superlattice. The properties and applications of a superlattice with variable basis are discussed in Chapter 4. Second, stacking superlattices of different mini-band structures forms a new type of multiple superlattice structure, the band-aligned superlattice. Analogous to the heterostructure in bulk semiconductor material, the band-aligned superlattice affords a new class of superlattice devices as described in Chapter 5. Finally, to improve the quantum confined Stark effect, a thorough investigation of optical transitions in quantum wells under bias is given. A very large Stark shift with reduced oscillator strength is predicted for the asymmetric coupled-wells in Chapter 6. An enhanced Stark effect (a large energy shift with large oscillator strength) is found in a step-well structure in Chapter 7. Possible intersubband light emission and second harmonic generation for a step well are also discussed. In Chapter 8, a new type of optical transition mechanism in semiconductor quantum wells is introduced to produce an enhanced Stark effect.