Terahertz intersubband emission in strong magnetic fields

Abstract

Electroluminescence in quantum-cascade structures based on vertical transitions is studied in a strong perpendicular magnetic field in the limit in which the cyclotron energy is larger than the intersubband transition energy. Cyclotron emission and a luminescence intensity enhancement up to a factor of 6 is observed in GaAs/AlGaAs and InGaAs/InAlAs vertical transition-based quantum-cascade structures.The physics of intersubband transitions in the farinfrared is very different from the mid-infrared, because the scattering mechanisms are not only controlled by optical phonons. At low temperature the main scattering mechanisms are electron-electron scattering 1-3 and interface roughness.4,5 A strong perpendicular magnetic field will break the in-plane dispersion of the subbands and create a ladder of Landau levels attached to each subband, which strongly modifies these scattering mechanisms by reducing the phase space.6,7 The electron-electron scattering has a maximum of efficiency for processes in which the energy of the individual electrons is conserved.8 Therefore antiresonant Landau level emission is an efficient way of quenching the nonradiative scattering. Due to this additional quantization, the system has strong analogies with quantum boxes. Depending on the magnetic field, intersubband Landau resonances arise when the Landau levels of each subband align with a Landau level of a different subband with a level index-change ␦nϭ1,2,3, . . . The condition for such a resonance is thenwhere E trans is the intersubband transition energy and c ϭeB/m* is the cyclotron energy. Coherent tunneling between intersubband Landau levels with different indices is forbidden in ideal cases, but these resonances are possible due to the scattering between the Landau levels. Such resonances have already been used to derive the energy spectrum. 9-11In this paper we explore the magnetic limit at which the cyclotron energy ប c is larger than the intersubband transition energy E trans ; for our samples, this is the case at about 9.3 T for GaAs material system and 5.9 T for InGaAs material system ͑for hϭ16 meV͒. The increase of the lifetime of the upper state of the transition in a structure based on a diagonal transition, 9 as well as the suppression of a sequential tunneling current in a three-barrier, two-well heterostructure, 12 and the enhancement of the electroluminescence ͑EL͒ in a vertical transition 13 have also been observed by applying such a perpendicular magnetic field.The structures were grown using molecular beam epitaxy and were designed to emit photons at energies of 15 and 16 meV. These vertical transition structures are based on either InGaAs/InAlAs lattice matched to an n ϩ -InP substrate or GaAs/AlGaAs lattice matched to a n-doped GaAs substrate and consist of 35 periods. The latter structure is in fact a copy of the first design which gave far-infrared EL, 14 but with a higher doping that enables large injection currents ͑the negative differential resistance appears here at j ϭ150 A/cm 2 at low temperature͒. The ...

In Ga As – Al In As ∕ In P terahertz quantum cascade laser

“…Electroluminescence of In 0.52 Al 0.48 As-In 0.53 Ga 0.47 As/ InP structures in the THz has already been demonstrated in structures with a vertical transition in a single quantum well, 11,12 as well as in a chirped superlattice structure. Electroluminescence of In 0.52 Al 0.48 As-In 0.53 Ga 0.47 As/ InP structures in the THz has already been demonstrated in structures with a vertical transition in a single quantum well, 11,12 as well as in a chirped superlattice structure.…”

THz and sub‐THz quantum cascade lasers

“…In contrast, when the Landau level ladders are off resonance, the scattering rate will be minimized. Such modulation of lifetimes by these resonances has been demonstrated experimentally in electroluminescence measurements in ISB THz emitters [62,63] and lasers [64,65]. Our design for exploiting the magnetically assisted gain is kept very simple: the active region is constituted by a wide quantum well and the optical transition (intra-well) takes place between the second |3 and first |2 excited states.…”

“…The second main effect of the magnetic confinement is on the intra-subband dephasing. This dephasing, which contributes substantially to the linewidth profile, is also efficiently quenched by the magnetic field: electroluminescence measurements in magnetic field [63,68] report a strong narrowing of the THz emission with increasing field. The electron localization induced by the magnetic field and the disorder at the interfaces reduces the intra-Landau level scattering rate.…”

Rate of intersubband transitions of charge carriers in semiconductor quantum wells as a result of their Coulomb interaction

“…In a system in which all particles have equal spins, the Coulomb and exchange interactions compensate each other in the matrix element because of the interference and the rate of scattering is close to zero. Such a situation is realized when a magnetic field is applied to a heterostructure, which makes it possible to sharply increase the quantum yield of luminescence [8].…”