Suppression of LO phonon scattering in Landau quantized quantum dots

Abstract

Picosecond time-resolved far-infrared measurements are presented of the scattering between conductionband states in a doped quasi quantum dot. These states are created by the application of a magnetic field along the growth direction of an InAs/AlSb quantum well. A clear suppression of the cooling rate is seen, from 10 12 s Ϫ1 when the level spacing is equal to the phonon energy, to 10 10 s Ϫ1 away from this resonance, and thus the results provide unambiguous evidence for the phonon bottleneck. Furthermore, the lifetimes had only weak dependence on temperature between 4 and 80 K. ͓S0163-1829͑99͒50612-7͔Electronic lifetimes of two-dimensional ͑2D͒ systems in magnetic fields are of fundamental interest in part because the quantization effect of the magnetic field mimics the effect of a quantum dot potential with an easily variable degree of confinement. 1 The magnetic field perpendicular to the layers forces the electrons into confined orbits and the density of states becomes a ladder of broadened ␦ functions similar to that of a quantum dot. Recently much work has been carried out on the so-called ''phonon bottleneck'' that has been claimed to inhibit the cooling of carriers in quantum dots when the level separation is not equal to the phonon energy. [2][3][4][5][6] However, partly as a result of different groups using different growth techniques for interband photoluminescence samples and partly on fundamental grounds, this is controversial and is the subject of much debate. 7-12 Indeed several mechanisms have been proposed that may bypass the bottleneck, such as multiphonon scattering, 7 Auger processes, 8 excitonic effects, 9 and defect related processes. 10 In the present work we observe clear phonon suppression in n-type quasi 0D dots ͑i.e., Landau quantized rather than spatially quantized͒ by a time-resolved intraband absorption measurement. This provides unambiguous evidence for the phonon bottleneck effect independently of arguments concerning which processes dominate in the interband photoluminescence measurements in dots 3-6,9-12 and quasi dots 13 such as electron-hole scattering. Further, because of the very clean model system ͑much sharper interfaces and no wetting layer, etc.͒, the interpretation is not complicated by detailed questions about different growth techniques and the quality of different dot sample structures. The results should assist in the understanding of which aspect of these processes is fundamental and which is dependent on sample structure.Resonant absorption of longitudinal optic ͑LO͒ phonons causes a variety of transport properties to oscillate with applied magnetic field, such as the magnetoresistance. 14 Resonant phonon scattering occurs whenwhere ប LO is the LO phonon energy, ប c ϭបeB/m* is the cyclotron energy, and ⌬l is an integer. At these resonances the LO phonon absorption/emission probability is strongly enhanced giving rise to large changes in the electron energy relaxation lifetime. 15,16 In the present work we have used the pump-probe technique, with cyclotron resonan...

Effects of nonequilibrium electron distribution and electron-electron interaction observed in spin-split cyclotron resonance of InAs/AlSb single quantum wells at high magnetic fields

“…With nϭ5ϫ10 11 cm Ϫ2 , the plus and minus-spin states of the Nϭ0 Landau level are expected to be completely occupied in thermal equilibrium by electrons at 12.5 T, because a short inter-Landau-level relaxation time of 0.1 ns has been measured with the same sample at this field. 5 As the field is increased, the degeneracy of the plusspin increases and the increased levels should be filled by electrons relaxing from the minus-spin states. However, when the field increases faster than the relaxation time, some electrons will remain in the minus-spin states.…”

“…Far-infrared pump-probe measurements of CR in InAs/AlSb quantum well in magnetic fields determined the relaxation time between Landau levels and provided unambiguous evidence for the LO-phonon bottleneck effect. 5 In the present work, we introduce another method to create nonequilibrium states, which has never been reported in the past to the best of our knowledge. If a relaxation process is longer than the sweep speed of a magnetic field, a nonequilibrium electron distribution should be realized.…”

Far-infrared free-electron lasers and their applications

“…The density of states is very similar to that of a quantum dot (see below), but with controllable separation, so resonant relaxation when the level spacing is equal to the LO phonon energy can be explored as shown in Fig 5. The level spacing is determined by the effective mass, and, for typical III-V semiconductors and fields of order 1T, is in the far-infrared. Landau-level lifetimes have been determined from absorption saturation of the cyclotron resonance [12], [13] and picosecond farinfrared pump-probe measurements [14], [ 15], and demonstrate the ability to combine several techniques (high field, low temperature, FEL),…”

Energy levels of a parabolically confined quantum dot in the presence of spin-orbit interaction

“…Normally, optical measurements, such as optical absorption and transmission [16], cyclotron-resonance [17], etc., can be used to determine the energy spectrum of a quantum dot. Although recently there are some theoretical work published regarding the Rashba effect in quantum dots in the presence of magnetic fields [18,19,20], the effect of the SOI on energy spectrum of a quantum dot has not yet been fully analyzed.…”