Subpicosecond luminescence study of tunneling and relaxation in coupled quantum wells

Abstract

We study, by time-resolved luminescence with subpicosecond resolution, the tunneling of electrons out of a narrow well (NW) coupled to a wider well (WW} by a thin barrier. The relative energy positions of the NW ground state and of the WW first excited level are changed by adjusting the WW width. Near resonance, and for narrow enough barriers, the transfer time of the electrons to the WW ground state is close to 2 ps and is analogous to LO-phonon-assisted intersubband relaxa-0 tion. When the barrier thickness increases above 40 A, this transfer time increases exponentially with the barrier thickness. The barrier thickness above which the variation becomes exponential strongly depends on the energy mismatch of the levels in the isolated wells and is indicative of interface roughness. The dependence of the decay times on barrier thickness for purposely misaligned samples is always exponential. 42 7021 1990 The American Physical Society 7022 B. DEVEAUD et al. 42 wells are selectively doped p type to a level of about 10" cm, the barriers are undoped. In the first series, designed to be "on resonance, " the well widths are kept at 60 and 140 A, the barrier being changed from 75 to 30 A (the well widths of 60 and 140 A correspond approximately to the resonant case when taking into account the p-type doping and thus the transfer of roughly half of the holes from the NW to the WW). In the second series, designed to be "off resonance, " the well widths are kept to 60 and 120 A, the barrier being changed from 80 to 15 A [for this series, the samples are undoped: this increases the decay times by about a factor of 2 (Ref. 26)]. In the third series, the barrier is kept at about 40 A, the NW at 60 A, and the WW width is changed from 120 to 190 A, thus changing the resonance conditions. In the following, the samples will be labeled according to the NW width, barrier width, and WW width in A.

Photoluminescence up-conversion induced by intersubband absorption in asymmetric coupled quantum wells

“…However, the role of interface phonon modes in the relaxation process may be enhanced in the ACQW structure because of the presence of four interfaces. Our T12 value is in close agreement with previously reported measurements on ACQW structures using subpicosecond time-resolved luminescence techniques [21].…”

“…An increase of the efficiency by more than 1 order of magnitude is then expected if the intersubband relaxation only involves acoustic phonons [3,21]. This condition could be fulfilled by a proper design of the ACQW structure in order to obtain an intersubband resonance at an energy inferior to the LO-phonon energy.…”

Carrier Dynamics Investigated by Time-Resolved Optical Spectroscopy

“…However, in this sample most of the photocarriers are created relatively close to the QW interface and the motion of the electrons through the GaAs barrier occurs within a time too short relative to the PL transient. The relatively fast rise time ($55 ps) for the QW emission of this sample agrees with the electron capture by QWs estimated from previous experimental results, which varies from 1 to 100 ps [5]. Since the electron generation at the QW is relatively fast for this sample, its decay time ($530 ps) represents the effective carrier lifetime at the QW.…”

<title>Electroabsorption and carrier dynamics in (Ga,In)As/(Al,In)As asymmetric double quantum wells</title>

“…The calculated tunneling times fit the measured data best using a carrier temperature of 150 K, indicating that the electron distributions are still hot during the tunneling process. [ 12] The tunneling times increase as the carrier density is increased, consistent with bandfihling in the WW acting as a blocking mechanism to tunneling electrons. …”