Exciton Formation Time Research Articles

Magnetophotoluminescence measurement of the formation time of an exciton in AlxGa1-xAs/GaAs quantum-well structures.

A magnetophotoluminescence technique is adopted to study the exciton formation processes in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs multi-quantum-well structures, with the cyclotron periods of carriers in the quantum wells as measures of time. It is found that exciton-related photoluminescence intensities decrease with increasing external magnetic field along the growth direction. Detailed experimental and theoretical analyses revealed that the formation time of an exciton and hence the amount of carriers taking part in radiative recombination decreases with increasing magnetic field. It is found that the exciton formation time is dominated by the time during which the electron and hole move toward each other under the Coulomb force between them. Through the fitting of experimental and theoretical results, exciton formation time is directly obtained. In addition, it is found that the formation time of an exciton is a function of carrier density, and that excitons are formed after the carrier system is cooled down.

Dynamical equilibrium between excitons and free carriers in quantum wells

We have investigated the exciton and free carrier populations dynamics in quantum wells by low temperature time-resolved photoluminescence spectroscopy. When the excitation energy is set above the free carrier quantum well gap, excitons are formed from random binding of electrons and holes. On the basis of this bimolecular formation process, we show that a 2D mass action law, describing the coexistence of free carriers and excitons, explains the time evolution of the exciton photoluminescence linewidth. This demonstrates the existence of a thermodynamic equilibrium between excitons and free carriers at each time delay following the excitation. The consequence is a very short exciton formation time (≲ 10 ps) in the density range investigated (10 9 – 10 10 cm −2), implying a bimolecular formation coefficient γ higher than 14 cm 2s −1.

Free exciton versus free carrier luminescence in a quantum well

Radiative recombination of an exciton in a quantum well is highly probable due to the removal of the k selection rule in the z-direction. As a consequence, this radiation path dominates over the free carrier recombination which is difficult to observe. We succeed in obtaining separately the free carrier luminescence and the exciton luminesecnce and deduce an exciton formation time of the order of 200 ps, in agreement with theoretical expectations

Picosecond transient photoluminescence spectra of ZnSe-ZnS strained-layer superlattices grown on GaAs(001) by molecular beam epitaxy

Picosecond transient photoluminescence spectra were carried out on ZnSe-ZnS strained-layer superlattices at 77 K. The typical exciton formation time and exciton lifetime are 40 and 100 ps, respectively. The relationship between exciton lifetime and the well width is also given.

Optical properties of [formula omitted] strained-layer superlattices

We present a study of the optical properties of ZnSe ZnS strained-layer superlattices (SLSs) by absorption, transient luminescence and Raman scattering measurements. In the absorption measurements, three absorption peaks corresponding to 1E-1HH, 1E-1LH and 1E-3HH transitions were observed up to room temperature. From the transient luminescence measurements, the exciton lifetime and exciton formation time were determined. We have also observed folded longitudinal acoustic (LA) phonon modes in the SLSs by room temperature Raman scattering measurements.

Time decays of donor-bound excitons in GaAs under pressure-induced Gamma -X crossover.

We report on photoluminescence time decays of excitons bound to donors in high-purity, n-type GaAs at He temperatures and high hydrostatic pressures (up to 80 kbar), for both direct (P\ensuremath{\lesssim}41.3 kbar) and indirect band gaps. Measured lifetimes for direct donor-exciton states (${D}_{\ensuremath{\Gamma}}^{+}$,X) are uniformly fast, being \ensuremath{\lesssim}1 nsec, and correspond to either the exciton formation or radiative decay times. For indirect donor-exciton states (${D}_{X}^{0}$,X), the observed lifetimes are much longer (20--100 nsec), increase with pressure, and reflect a combination of nonradiative Auger decay and band-structure-dependent natural radiative decay. The considerable difference observed between the donor lifetimes over the range of pressures inducing the \ensuremath{\Gamma}-X transition illustrates the importance of band structure for shallow, bound states in multivalley semiconductors.