GENERALIZED SUPERLATTICE K-CENTER-DOT-P THEORY AND INTERSUBBAND OPTICAL-TRANSITIONS

The eight-band superlattice crystal K . p formalism is extended to inc lude the higher-lying antibonding p states perturbatively. The initial 14X14 Hamiltonian matrix is block diagonalized (or folded down) so th at corrections due to these states appear in the modified 8X8 matrix. Their principal effect is to introduce a finite heavy-hole mass. The u se of the eight-band formalism for band-structure and interband optica l calculations is validated because other corrections are very small. The approximate wave functions associated with the original 14x14 Hami ltonian are used to calculate intersubband optical matrix elements. Th ese include new terms, present in crystals lacking inversion symmetry, which increase the magnitude of conduction intersubband absorption by about three orders of magnitude for in-plane polarized (TE) photons, while leaving absorption due to growth-axis polarized (TM) photons rel atively unaffected. The TE absorption in narrow GaAs/AlxGa1-xAs quantu m wells (QW's) is shown to be observable. However, use of the f-sum ru le and the cyclotron-resonance-determined conduction-band effective-ma ss anisotropy indicate an upper limit of about 20% relative to TM abso rption. Comparison with the measured TM absolute absorption coefficien t for a system used for quantum-well infrared photoconductors (QWIP's) yields excellent agreement. The conductivity sum rule involving these intersubband transitions is used to confirm the observed broadening a ssociated with very small QW width fluctuations for this system.