Intersubband transitions in GaP–AlP heterostructures

Intersubband optical transitions in doped AlP–GaP multiple quantum wells are investigated using midinfrared absorption. Strong p-polarized absorption corresponding to the transition from the first to the fourth electronic subband in the Xz valley is measured at wavelengths between 5 and 8 microns. Additional absorption peaks related to transitions between confined donor states are also observed. The measured intersubband transition energies are consistent with an X valley conduction band offset between AlP and GaP of 280 meV and a value of 1.1m0 for the longitudinal effective mass for the X valleys of AlP (where m0 is a free electron mass). These values, the role of donors and confined two-dimensional continuum states, and applications of this system for terahertz intersubband devices are discussed.