The luminescence of ZnSe grown by metalorganic vapour phase epitaxy and doped by a DC nitrogen plasma is investigated. With increasing N-2 flux the donor-acceptor pair (DAP) band continuously develops into a structureless band peaking at 2.62 eV for highest doping levels. This broad band evolves back into a structured DAP band peaking at 2.698 eV with increasing excitation density. At high N concentrations and at large degree of compensation potential fluctuations become important for the spatially indirect DAP recombination. These fluctuations can easily be screened by optically excited carriers making the experimental conditions decisive for luminescence spectra of strongly doped ZnSe:N samples.

Higher orders of diffraction observed in a transient four-wave mixing experiment with acceptor-bound exciton complexes in CdS are analyzed in terms of pulse propagation. The description is based on a mean-field correction to the driving electric field in the optical Bloch equations, which are shown to generally lack higher-order diffraction for two-pulse self-diffraction. A propagation-induced sequence of stimulated echoes diffracted at the population grating dominates at least the second order of diffraction. @S0163-1829~96!07147-0#

We report on degenerate-four-wave-mixing experiments mainly based on biexcitonic processes near the bandgap of CdS and ZnSe. Broadband femtosecond-excitation is applied in order to coherently create free and bound excitons as well as free and bound biexcitons in the sample. Using the results of nonlinear quantum beat spectroscopy the binding energies of the free and acceptor bound biexciton in CdS and ZnSe are determined.

Degenerate four-wave-mixing experiments (DFWM) at the neutral acceptor bound exciton complex in CdS and ZnSe are reported. ps- as well as fs-spectroscopy are carried out to investigate the dephasing processes at this complex. The results are discussed concerning scattering of free excitons, phonons, and interactions with impurity centers. Higher orders of diffraction in the DFWM process are modeled within the density matrix equations for noninteracting two-level systems including propagation effects. From nonlinear quantum beat spectroscopy the binding energies of the acceptor bound biexciton in CdS and ZnSe are estimated.

We present a comprehensive study of the excitonic recombination lifetime in strained and unstrained In1-xGaxAs/InP-quantum well structures as a function of well width and Ga mole fraction. In the lattice-matched case a minimum lifetime of 650ps is observed for a well thickness of 2nm. For widths larger as well as smaller than 2nm the lifetime increases. In strained In1-xGaxAs/InP quantum wells the recombination lifetime shows a strong dependence on the Ga content. While the lifetime is nearly constant in the compressively strained case (X(Ga) < 0.47), we observe a drastic increase with rising Ga content for tensile strain (X(Ga) > 0.47).

The incorporation processes and efficiencies of nitrogen doping for p-type conductivity in metalorganic vapour phase epitaxy (MOVPE) grown ZnSe/GaAs epilayers are investigated by means of time-integrated and time resolved photoluminescence (PL) spectroscopy. Two nitrogen-doping methods are compared, plasma-enhanced doping during growth, and ion implantation of nitrogen with annealing after growth. Both types of doped layers exhibit the IN1 transition from a neutral acceptor bound exciton complex (A0N, X), indicating an effective nitrogen embedding on selenium sites. With increasing nitrogen doping rates, a deeper bound exciton line IC1 appears, lowering the intensity of the IN1. An observed reduction of the IN1 and IC1 lifetimes for higher nitrogen doping concentrations results from an enhanced overlap of the bound exciton wave functions with those of other impurity centres.

Time resolved luminescence measurements of the donor-acceptor pair (DAP) luminescence of n-doped CdS:In bulk crystals, undoped bulk ZnSe crystals grown with various VI/II ratios, and p-doped ZnSe:N epilayers in the near band gap region are presented. The decay of the donor-acceptor pair recombination luminescence is investigated for dopant concentrations below the Mott density. A discussion on the basis of a statistical theory by Thomas and Hopfield for DAP recombination yields, by knowledge of the Bohr radii of the impurities, the number of electronically active impurity centres. The reliability of this method to determine impurity concentrations is discussed.

The coherent and incoherent dynamics of the neutral acceptor bound exciton complex in CdS is investigated. A dephasing time T2 of 700 ps and an energy relaxation time T1 of 980 ps are found indicating pure dephasing processes due to impurity interactions. The dependence of the dephasing time T2 on temperature and excitation density is discussed.

The coherent and incoherent dynamics of bound exciton complexes in CdS are investigated by means of degenerate four-wave mixing and differential transmission spectroscopy. We observe dephasing times T2 up to 800 ps for the (A0, X) complex and up to 150 ps for the forbidden A(F) exciton. The dephasing times T2 are found to be generally shorter than expected from the energy relaxation times T1. Pure dephasing processes not connected with exciton or phonon scattering are found and discussed in view of impurity interactions and nuclear spin-flip processes.

Thick poly-ZnSe layers used for optical components in infrared laser systems show strongly varying laser damage thresholds. This is due to differences in kind and concentration of carrier trapping defects in the material grown under various Se/Zn partial pressure ratios. The VI/II ratio determines a non-stoichiometric growth and determine an upper limit for the concentration of native defects. The optical properties of poly-ZnSe are studied by means of time-integrated and time-resolved photoluminescence (PL) as well as spatially resolved cathodoluminescence (CL). It is shown that samples grown in Se excess are of good quality and have the lowest defect concentrations. The changing of properties of non-stoichiometric grown samples by doping with Li and In in a post-grown diffusion process was also studied.

Interface and mode coupling between ordinary and extraordinary waves lead in non-isotropic media to a rotation of the plane of polarization of the transmitted light. Placing the specimen between two polarizers, pronounced interference patterns are observed near the isotropic point. Pump and probe experiments with donor doped and undoped CdS crystals demostrate for the first time a fast blue shift of the interference spectra by the action of intense laser light pulses. This gives rise to a nonlinear optical switching in the ns region. The induced change of the transmission due to saturation effects of the donors leads to a change of the birefringence Δn = n| - n⊥. Under ps excitation, very fast rotations of the plane of polarization have been observed. Coherent phenomena like two photon absorption or the optical induced Stark effect are apparently responsible for relaxation processes with decay times in the sub-ps region.

Time-resolved luminescence and “pump-and-probe” measurements of the 3T1(P)-3T1(F) Ni2+ transitions in Cds and ZnS are presented. The lifetimes of the 3T1(P) states in ZnS and CdS are determined to be 60 and 400 Ps, respectively. Saturation experiments additionally show that the 3T2(F) Ni2+ term in ZnS and the 3A2(F) Ni2+ term in CdS serve as storage states. This enables us to determine the radiative quantum efficiencies whuch are in agreement with those measured by calorimetric absorption spectroscopy. Some unexpected structures in the time resolved pump-and-probe spectra of ZnS can be explained by an optical Stark shift of the cubic Ni2+ line. Furthermore, the Ni2+ line broadening mechanisms are discussed.