The authors studied the effects of changing the barrier design of GaAs-Al/sub x/Ga/sub 1-x/As quantum wells on the electroabsorption, exciton saturation, and carrier sweep-out times. Five samples with x values ranging from 0.2 to 0.4 and barrier thicknesses from 35 to 95 AA were studied. Within this range, the authors find that the electroabsorption is not very sensitive to the barrier thickness, but that the ionization field of the excitons approximately doubles for an increase of x from 0.2 to 0.4. The samples with high, thick barriers have lower internal quantum efficiencies than those with low, thin barriers. It was found that the exciton saturation intensity increases with increasing applied field, and decreasing barrier thickness or height. Time-resolved electroabsorption measurements confirm the variation in sweep-out rates between samples, and indicate that the escape mechanism at low field is probably a thermally-assisted tunneling process. >

Quantum well carrier sweep out: relation to electroabsorption and exciton saturation

Ultrafast laser spectroscopy with time resolution down to 6 femtoseconds has been applied to study optical dephasing, relaxation, and recombination in various semiconductors and semiconductor microstructures. We report some applications of ultrafast laser spectroscopy to bulk and quantum well III–V and II–VI semiconductors in order to illustrate the potential. The characteristics time scales for dephasing, relaxation and recombination are discussed and determined experimentally.

Ultrafast spectroscopy of semiconductors

Double quantum wells are among the simplest semiconductor heterostructures exhibiting tunnel coupling. The existence of a quantum confinement effect for the energy levels of a narrow single quantum well has been largely studied. In double quantum wells, in addition to these confinement effects which characterize the levels of the isolated wells, one faces the problem of describing the eigenstates of systems interacting weakly through a potential barrier. In addition, the actual structures differ from the ideal systems studied in the quantum mechanics textbooks in many aspects. The presence of defects leads, for instance, to an irreversible time evolution for a population of photocreated carriers. This irreversible transfer is now clearly established experimentally. The resonant behaviour of the transfer has also been evidenced, from the study of biased structures. If the existence of an interwell transfer is now clearly established from the experimental point of view, its theoretical description, however, is not fully satisfactory. This review focuses on the theoretical description of the energy levels and of the interwell assisted transfer in double quantum wells. We shall firstly outline the problem of tunnel coupling in semiconductor heterostructures and then discuss the single particle and exciton eigenstates in double quantum wells. In the remaining part of the review we shall present and critically review a few theoretical models used to describe the assisted interwell transfer in these structures.

http://iopscience.iop.org/article/10.1088/0034-4885/60/3/002/pdf

Tunnelling and relaxation in semiconductor double quantum wells

Publisher Summary This chapter discusses the optical properties of semiconductors under pressure. The effect of a large hydrostatic pressure on semiconductor optical properties can be best illustrated with a simple experiment—namely, the optical inspection with a microscope of light transmitted through a piece of gallium arsenide (GaAs) as it is being pressurized in a diamond anvil cell (DAC). Absorption (transmittance) measurements often require high resolution and the coverage of a wide spectral range. These measurements are therefore done with a conventional scanning-type spectrometer. The chapter discusses the effects of high pressure on fundamental semiconductor band-structure parameters as they are largely determined by the magnitude of band gaps and the characteristics of the electronic states at valence- and conduction-band extrema.

Chapter 4 Optical Properties of Semiconductors under Pressure

Fabrication and optical properties of semiconductor quantum wells and superlattices

Nonresonant carrier tunneling is investigated by time‐resolved and time‐averaged optical methods for a series of samples with various barrier thicknesses. The electron tunneling times decrease exponentially with the decrease of barrier thickness from 8 to 3 nm, and the trend is well described by a semiclassical model. Additional efficient hole tunneling is observed in the 3 nm barrier sample, and the time constant is of the order of 50 ps.

Nonresonant electron and hole tunneling times in GaAs/Al0.35Ga0.65As asymmetric double quantum wells

Time‐resolved photoluminescence in the picosecond regime is performed on an asymmetric GaAs/Al0.35Ga0.65As double quantum well structure with a barrier thickness of 6 nm to obtain the Γ‐ and X‐point barrier contributions to nonresonant tunneling. Application of hydrostatic pressure up to 37 kbar at 5 K reveals that tunneling via virtual X states is at least 800 times less efficient than via virtual Γ states. Above 24.5 kbar an extremely fast scattering of electrons out of the n=1 quantized level of the narrower quantum well is observed.

Γ‐ and X‐band contributions to nonresonant tunneling in GaAs/Al0.35Ga0.65As double quantum wells

Prethermalization phenomena in weakly interacting Hubbard systems after electric‐field pump pulses with a finite duration are studied. The Hubbard interaction U is taken into account up to second order, by applying the prethermalization paradigm for time‐dependent interaction protocols, and the electric field strength beyond linear order. A scaling behavior with pulse duration is observed for the absorbed energy as well as individual prethermalized momentum occupation numbers, which are attributed to the leading quadratic orders in interaction and electric field. It is shown that a pronounced nonthermal momentum distribution can be created with pump pulses of suitable resonance frequencies, and how to distinguish them from thermal states is discussed.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/pssb.202100280

M. G. W. Alexander

Papers

Nonresonant electron and hole tunneling times in GaAs/Al0.35Ga0.65As asymmetric double quantum wells

Γ‐ and X‐band contributions to nonresonant tunneling in GaAs/Al0.35Ga0.65As double quantum wells

Photoinduced Prethermalization Phenomena in Correlated Metals