Indirect barrier electron-hole gas transitions in mixed type-I–type-II GaAs/AlAs multiple quantum wells

We studied the photoluminescence (PL) spectrum resulting of the indirect recombination of barrier electrons and the two-dimensional hole gas (2DHG) that is excited in a structure of mixed type-I–type-II GaAs/AlAs quantum wells. This structure consists of alternating narrow and wide GaAs quantum wells (QW), and is distinguished by a staggered conduction-band alignment that leads to a fast electron transfer from the narrow to the wide QW’s and a very slow hole transfer. Consequently, a 2DHG and a two-dimensional electron gas (2DEG) are formed in the narrow and wide QW’s, respectively. Their density is controlled by the photoexcitation intensity and is experimentally determined by fitting the band shape of the wide-well direct-recombination PL spectra (in the range of 10¹⁰<n_e<5×10¹¹cm^-2). A small fraction of the electrons recombine radiatively with the 2DHG while they are in the lowest X subband of the AlAs barrier, and the resulting spectrum is investigated at T=2K and for various excitation intensities. The indirect transitions consist of a no-phonon band and momentum conserving (zone-edge) phonon sidebands. All these bands are blueshifted with increasing photoexcitation intensity. This shift is well explained by calculating the lowest X subband energy in the electrostatic potential, generated by the separate 2DEG and 2DHG charges as a function of their density.