II-VI Semiconductor Quantum Research Articles

Atomic Ordering in Self-assembled Epitaxial II-VI and IV-VI Compound Semiconductor Quantum Dot Systems

ABSTRACTTransmission electron microcopy (TEM) in both the parallel illumination and scanning probe mode revealed atomically ordered entities within a 5 to 250 nm range in a IV-VI and a II-VI compound semiconductor quantum dot (QD) system. While the II-VI system was a nominal [001] CdSe/(Mn0.1Zn0.9)Se multilayer structure with the QDs embedded, the IV-VI system nominally consisted of [111] PbSe islands. The comparison of photoluminescence (PL) spectra from the CdSe/(Mn0.1Zn0.9)Se structure with those of a reference structure, that was grown to the same nominal specification under otherwise identical conditions except that no Mn was incorporated into the cladding layers, revealed for the former sample two peaks at approximately 2 and 2.1 eV. We tentatively attribute these two PL peaks to two groups of atomically ordered entities.

Theory of optical spectra of polar quantum wells: Temperature effects

Theoretical and numerical calculations of the optical absorption spectra of excitons interacting with longitudinal-optical phonons in quasi-2D polar semiconductors are presented. In II-VI semiconductor quantum wells, exciton binding energy can be tuned on- and off-resonance with the longitudinal-optical phonon energy by varying the quantum well width. A comprehensive picture of this tunning effect on the temperature-dependent exciton absorption spectrum is derived, using the exciton Green's function formalism at finite temperature. The effective exciton-phonon interaction is included in the Bethe-Salpeter equation. Numerical results are illustrated for ZnSe-based quantum wells. At low temperatures, both a single exciton peak as well as a continuum resonance state are found in the optical absorption spectra. By contrast, at high enough temperatures, a splitting of the exciton line due to the real phonon absorption processes is predicted. Possible previous experimental observations of this splitting are discussed.

Optically Detected Magnetic Resonance Studies of the Surface/Interface Properties of II−VI Semiconductor Quantum Dots

The described study concentrated on the investigation of II-VI semiconductor quantum dots, prepared as colloidal species imbedded in phosphate glass, or chemically deposited on a substrate. The ultimate goal of the present research was concerned with the examination of the influence of the surface/interface quality on the optical properties of those quantum dots. This was examined by the utilization of optically detected magnetic resonance spectroscopy. This method reflected knowledge of the chemical identity of the surface/interface trapping site, and trapped electron and trapped hole recombination mechanism. It also distinguished between radiative and nonradiative processes, determined the spin−lattice relaxation, estimated the trapped electron−hole exchange mechanism and the distribution of defects at the surface.

Mapping of electronic wave functions in II-VI semiconductor quantum wells viaMn2+electron paramagnetic resonance

Mapping of electronic wave functions in II-VI semiconductor quantum wells via<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Mn</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>electron paramagnetic resonance

Comment on “Dynamics of Ripening of Self-Assembled II-VI Semiconductor Quantum Dots”

A Comment on the Letter by S. Lee, et al., Phys. Rev. Lett. 81, 3479 (1998). The authors of the Letter offer a Reply.Received 20 January 1999DOI:https://doi.org/10.1103/PhysRevLett.83.239©1999 American Physical Society

Optical gain and excitonic processes in widegap semiconductor quantum wells

The dominant role of the strong electron-hole Coulomb correlations in widegap II-VI semiconductor quantum well heterostructures is illustrated by selectively highlighting key phenomena studied in experiments encompassing ZnSe-based QWs. These include gain and stimulated emission at low temperatures from excitonic molecules, excitonic enhancements to gain in room temperature green-blue diode lasers, and light-matter interaction in vertical microcavity lasers.

Dynamics of Ripening of Self-Assembled II-VI Semiconductor Quantum Dots

We report the systematic investigation of ripening of CdSe self-assembled quantum dots (QDs) on ZnSe. We investigate the size and density of the QDs as a function of time after deposition of CdSe has stopped. The dynamics of the ripening process is interpreted in terms of the theory of Ostwald ripening. Furthermore, the experimental results allow us to identify the growth mode of the QD formation process. [S0031-9007(98)07378-5]

Nature of the power-dependent ultrafast relaxation process of photoexcited charge carriers in II-VI semiconductor quantum dots: Effects of particle size, surface, and electronic structure

The power-dependent relaxation dynamics of photoexcited charge carriers in a number of II-VI semiconductor quantum dots have been studied using femtosecond laser spectroscopy. The dynamics are obtained via excitation of the quantum dots with high power 390 nm pulses of 150 fs duration, and probing of the photoexcited species by monitoring the change in absorption at 790 nm as a function of time. Particles with vastly differing surfaces, sizes, electronic structures, and solvents all show a fast 1.5–4 picosecond decay component which grows in with power, a 17 ps (CdSe) or 50 ps (CdS and Cd0.5Zn0.5S) decay component, and some transient absorption persisting beyond 600 ps. The power-dependent component for CdSe quantum dots in glass has a 1.5 ps decay time constant, while for the liquid dispersed CdS and Cd0.5Zn0.5S quantum dots it has 2–4 ps decay time constants. This variation in the time constant is due to its power dependence, the time constant decreases with increasing power. It is also shown that the power-dependent decay is only weakly dependent on surface, size, and electronic structure. With the assistance of a power-dependent nanosecond fluorescence study, we have assigned the power-dependent decay primarily to exciton–exciton annihilation. This decay mechanism becomes dominant under high intensity excitation where multiple photoexcited charge carriers are created in each quantum dot, leading to trap state saturation and an accumulation of band edge excitons. Auger recombination may also play an important role at very high excitation intensities.

Accumulated photon echo in semiconductor microcrystalline quantum dots

The nonlinear coherent emission of excitons in CuBr quantum dots was investigated using a phase-stabilized photon-echo technique. The echo signal was observed to be quite strong, with the emission efficiency being three orders of magnitude larger than that of II-VI semiconductor quantum wells. Time resolving of the echo signal confirmed that the strong nonlinear emission originates from the accumulation effect of the population grating due to the presence of the bottleneck state. Formation and decay kinetics of the population grating was analyzed by taking into account the light-induced deexcitation effect.

Multi-Band Bloch Equations and Gain Spectra of Highly Excited II-VI Semiconductor Quantum Wells

Quasi-equilibrium excitation dependent optical probe spectra of II–VI semiconductor quantum wells at room temperature are investigated within the framework of multi-band semiconductor Bloch equations. The calculations include correlation effects beyond the Hartree-Fock level which describe dephasing, interband Coulomb correlations and band-gap renormalization in second Born approximation. In addition to the carrier–Coulomb interaction, the influence of carrier–phonon scattering and inhomogeneous broadening is considered. The explicit calculation of single particle properties like band structure and dipole matrix elements using k · p theory makes it possible to investigate various II–VI material combinations. Numerical results are presented for CdZnSe/ZnSe and CdZnSe/MgZnSSe semiconductor quantum-well systems.

Excitonic properties of II-VI semiconductor quantum well structures and their lasing mechanism

Excitonic properties of II-VI semiconductor quantum well structures and their lasing mechanism

Low-temperature anti-Stokes luminescence mediated by disorder in semiconductor quantum-well structures.

In low-temperature photoluminescence experiments on II-VI semiconductor quantum wells we find an energy transfer from confined quantum-well states to above-barrier states. The observed anti-Stokes barrier luminescence exhibits a characteristic intensity dependence showing that this transfer is caused by a two-step absorption process involving localized or impurity bound-exciton states in the quantum well. Time-resolved photoluminescence experiments show that the photon for the second intraband absorption step can be provided by the quantum-well luminescence, i.e., the anti-Stokes barrier luminescence is a direct consequence of photon recycling.

Engineering the exciton linewidth in II-VI quantum well structures

The excitonic transition in II-VI quantum well materials has recently been used as the basis for optical modulators and also as the lasing transition at low temperatures. The central aspect in the use of the exciton resonance in optical devices is the understanding of the exciton linewidth. We present a detailed study of the interactions that affect the linewidth in II-VI semiconductor quantum well materials. The broadening of the resonance with increasing temperature can be controlled by altering the material parameters of the II-VI structures. In so doing the exciton binding energy can exceed the LO-phonon energy and thereby reduce the homogeneous contribution to the measured linewidth. Efforts to reduce well-width fluctuations in the growth of the II-VI quantum well structures, which are responsible for the inhomogeneous linewidth, must also be made to a limit where the room temperature Iinewidth is narrow and homogeneously broadened.

Simple formula for exciton binding energy in quantum wells with zero band offsets.

Band offsets in II-VI semiconductor quantum wells can be such as to show small or zero confinement in the conduction band while having considerable confinement in the valence band, or vice versa. By assuming purely two-dimensional motion and a simple variational exciton function, we calculate an explicit analytical expression for the 1s-exciton binding energy in this case and evaluate it for a range of electron- to hole-mass ratios.

Room-temperature excitonic saturation in CdZnTe/ZnTe quantum wells

We present the first measurements of room-temperature excitonic absorption saturation in a II-VI semiconductor quantum well. Strong room-temperature excitonic absorption in CdZnTe/ZnTe quantum wells is found to saturate at an incident optical intensity that is considerably higher than that for III-V quantum wells. We show that this phenomenon can be interpreted in terms of the smaller excitonic Bohr radius characteristic of wide-gap II-VI compounds.