Excitons In CdTe Quantum Wells Research Articles

Oscillator Strengths for Neutral, Charged, and Scattered Excitons in CdTe Quantum Wells Containing a 2D Electron Gas

Absorption spectra have been measured for a 10 nm CdTe(0.3%Mn) quantum well with electron concentration ne variable up to 1.5×10 cm−2. Following recent theory appropriate to low ne, here ≈ 0.1/πaB, the spectra are interpreted in a “strong exciton” model, where the initial oscillator strength of the excitonic resonance (X) is conserved, with screening and phase-space filling effects negligible. As ne increases in zero-field and as the filling factor ν increases in magnetic field, the intensity of X is transferred to: (i) trion processes, namely exciton-one electron scattering and the trion resonance (T ), and (ii) quatron processes, namely exciton-two electron scattering and trion-one electron scattering. In magnetic field, the threeand four-body scattering processes become discrete, combined “exciton and cyclotron” and combined “trion and cyclotron” excitations that take all the intensity of X and T for ν >∼ 1 and ν > 2, respectively.

Trions and Excitons in CdTe Quantum Wells: Lifetimes, Coherence, Diffusion and Localization

We present the results of a study of the dynamics of trions and excitons in a CdTe modulation-doped quantum well as a function of temperature in the low-density regime. Using three-pulse four-wave mixing technique, we show that, at low temperature, these quasi-particles are inhomogeneously broadened and localized due to disorder effects. At higher temperatures, trions and excitons are mobile and their homogeneous broadening is largely increased as a result of acoustic phonon scattering. Consistently, time-resolved luminescence measurements also reveal a free-particle character for trions at high temperatures.

Diffusion, localization, and dephasing of trions and excitons in CdTe quantum wells

Excitons and trions in a CdTe modulation-doped quantum well are investigated by picosecond three-pulse four-wave mixing experiments as a function of temperature in the low-density regime. We show that trions are mobile for high enough temperatures. The diffusivity of trions and excitons increases with temperature but, like their mobilities, is limited by acoustic-phonon scattering. Consistently, studying the decay of the interband coherence reveals an increase of the homogeneous broadening as the temperature is raised. At low temperatures, excitonic complexes are localized by both interface roughness scattering and by the electrostatic potential due to remote ionized donors.

Spin Relaxation of Negatively Charged Excitons in CdTe Quantum Wells

We investigate the spin relaxation of negatively charged excitons in modulation-doped CdTe quantum wells by time-resolved photoluminescence. The charged exciton spin flip process is found to be very sensitive to the excitation energy but not to a change of electron concentration. We exclude the Bir-Aronov-Pikus mechanism, which is based on the exchange interaction, as responsible for the X- spin flip.

Spin Relaxation of Negatively Charged Excitons in CdTe Quantum Wells

Spin Relaxation of Negatively Charged Excitons in CdTe Quantum Wells

Relative absorption strengths of neutral and negatively charged excitons in CdTe quantum wells

Excitonic absorption is studied in CdTe/Cd 1− x Zn x Te multiple quantum well samples that were double-doped in the barriers with indium donors and nitrogen acceptors. Under optical pumping, holes are trapped by acceptors and electrons form a 2D electron gas in the wells with power-dependent concentration n e. The absorption intensities of the neutral exciton X − and the charged exciton X − are measured in an 8 T field. As n e. varies from 0 upto about 10 11 cm −2, there is both a sharing of intensity between X and X − and an overall decrease of their summed absorption.

Relative absorption strengths of neutral and negatively charged excitons in CdTe quantum wells

Relative absorption strengths of neutral and negatively charged excitons in CdTe quantum wells

Empirical pseudo-potential calculation of the in-plane effective masses of electron and holes of two-dimensional excitons in CdTe quantum wells

The electron and hole effective masses associated with the in-plane motion of an exciton in a quantum well are often derived from the Luttinger parameters associated with the bulk band structure. In this work empirical pseudo-potential calculations of the bulk band structure of CdTe including spin-orbit coupling are presented. For an exciton, mapping the real space in-plane motion of the electron and hole on to k-space allows the effective masses to be calculated from the pseudo-potential band structure at all points around the exciton orbit. The calculations reveal that for CdTe the in-plane masses of the electron, light- and heavy-hole are similar to those commonly accepted for motion along the growth or z-axis, i.e. 0.11, 0.15 and 0.45, respectively. The implications of these effective masses for exciton binding energies are discussed.

Exciton Dynamics in CdTe/CdMnTe Multiquantum Well Structures Grown by Molecular Beam. Epitaxy on GaAs Substrate

The results of picosecond photoluminescence kinetics of four dicherent CdTe/CdMnTe multiquantum well structures grown by MBE on GaAs sub- strates are presented. The experimental results show that excitons in CdTe quantum wells are strongly localized by potential fluctuations. Photolumi- nescence decay times of the localized excitons are considerably shorter (about 120 ps) than those reported for free or quasi-free excitons. An influence of Mn in the barriers on exciton properties is demonstrated. For narrow quan- tum wells as well as for the multiquantum well structure with the highest Mn mole fractioii the excitons migrate during their decay to the states with a lower potential energy. Longer decay times are observed for quasi-localized excitons. We show also that for strongly localized excitons the energy transfer between localized and donor bound excitons is less efficient.