Comparison of hole and electron intersubband absorption strengths for quantum well infrared photodetectors

It is well known that the hole intersubband absorption of normally incident (TE polarized) radiation is nonzero for p-doped quantum well infrared photodetectors (p-QWIP´s) which have been fabricated without an optical grating. This present paper shows from k&oarr;·p&oarr; theory that, for typical p-QWIP designs, this hole intersubband absorption of TE polarized radiation (without the help of an optical grating) is significantly smaller than the electron intersubband absorption of TE polarized radiation in those n-doped QWIP´s (n-QWIP´s) fabricated with an optical grating. A second result of this present work is that, even when there is significant mixing of the light and heavy hole states, the p-QWIP absorption of TE polarized radiation (without the help of an optical grating) is still much smaller than the n-QWIP absorption of TE polarized radiation (with the help of an optical grating). The reason is that the mixing of light and heavy hole states never increases the amount of |S⟩-symmetry in the mixed hole wave function beyond the amount of |S⟩-symmetry which was present in the unmixed, purely light hole state. Finally, this present paper shows from k&oarr;·p&oarr; theory that strained layer growth on an (001) substrate does not significantly affect the strength of the hole intersubband absorption. The reason is that the Hamiltonian describing uniaxially strained quantum wells has precisely the same (tetragonal) symmetry as the Hamiltonian describing carrier confinement in unstrained quantum wells. All of these results are important in choosing a QWIP device design