Control of photoexcited-carrier distribution through intersubband resonances in semiconductor superlattices

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The purpose of this study is to clarify the mechanism which controls photoexcited-carrier distribution of high-order subbands and to pursue the possibility of intersubband light emission (mid-infrared region) in semiconductor superlattices (SLs). The samples used are GaAs/AlAs and GaAs/InAlAs SLs grown by molecular-beam epitaxy. The SL layers are embedded in p-i-n or n-i-n structure in order to apply a bias voltage (an electric field). The summary of the research results is as follows.(1) In the conduction band of a GaAs/AlAs SL, the GaAs and AlAs layers are the quantum wells for the Γ and X valleys, respectively ; therefore, we have focused on the resonance and scattering between the GaAs-Γ electron subband and AlAs-X one. We measured the interband and intersubband emissions under various bias voltages (electric fields) which control the energy spacing between the Γ and X subbands, and analyzed the experimental results. In a GaAs/AlAs double-QW SL [AlAs (5.7 nm)/GaAs (4.5 nm)/AlAs (1.1 nm)/GaAs (2.7 nm)] and a GaAs (15.3 nm)/AlAs (4.5 nm) SL, it is found that the Γ-X subband resonance and scattering causes the carrier distribution of the high-order Γ subbands. We have succeeded to detect the intersubband emission (electroluminescence) in a mid-infrared region under the above carrier-distribution condition.(2) In GaAs/InAlAs SLs, we have focused on the resonance and scattering between the heavy-hole (HH) and light-hole (LH) subbands, where the LH-subband energy is higher than the HH-subband one. The HH-LH resonance was confirmed by photocurrent-voltage characteristics in some samples. In a GaAs (3.7nm)/In_<0.3>Al_<0.7>As (0.85nm) SL, it is found from the interband emission properties under various bias voltages that the HH-LH resonance and scattering causes the carrier distribution of the LH subband, which results in appearance of the LH-related emission associated with disappearance of the HH-related emission.