I. Broser

Gain studies of (Cd, Zn)Se quantum islands in a ZnSe matrix

By inserting stacked sheets of nominally 0.7 monolayer CdSe into a ZnSe matrix we create a region with strong resonant excitonic absorption. This leads to an enhancement of the refractive index on the low-energy side of the absorption peak. Efficient waveguiding can thus be achieved without increasing the average refractive index of the active layer with respect to the cladding. Processed high-resolution transmission electron microscopy images show that the CdSe insertions form Cd-rich two-dimensional (Cd, Zn)Se islands with lateral sizes of about 5 nm. The islands act as quantum dots with a three-dimensional confinement for excitons. Zero-phonon gain is observed in the spectral range of excitonic and biexcitonic waveguiding. At high excitation densities excitonic gain is suppressed due to the population of the quantum dots with biexcitons.

Properties of the intermediately bound and -excitons in ZnO:Cu

A microscopical model is proposed, describing the origin and properties of three closely spaced zero-phonon lines observed in the green Cu band in ZnO:Cu crystals labelled and . These excitations are known to be formed by a charge-transfer reaction with hole bound states. These lines are shown to originate from an intermediately bound exciton of acceptor type, . This sort of exciton, in which both carriers are captured at intermediate-radius orbitals, results from the wurzite-type symmetry of the ZnO:Cu system. The electronic structure obtained for these three intermediately bound excitons enables us to explain their magneto-optic behaviour and to calculate their g-values. Additionally, we determined the quantum efficiency of both intracentre and exciton transitions by using time-resolved and calorimetric absorption spectroscopy. While no luminescence is observed in ZnS, the exciton states in ZnO are purely radiative only to the ground state, . The picture of an intermediately bound exciton explains the recombination channels and also makes clear the difference between copper states in the ZnS and ZnO systems.

Mechanisms of optical gain in cubic gallium nitrite

We report on the mechanisms of optical gain in cubic GaN. Intensity-dependent gain spectra allow a distinction of the processes involved in providing optical amplification. For moderate excitation levels, the biexciton decay is responsible for a gain structure at 3.265 eV. With increasing excitation densities, gain is observed on the high energy side of the cubic band gap due to band filling processes. For the highest pump intensities, the electron-hole plasma is the dominant gain process. Gain values up to 210 cm−1 were obtained, indicating the high potential of cubic GaN for device applications. The observed gain mechanisms are similar to those of hexagonal GaN.

Fine structure and Zeeman effect of the excited state of the green emitting copper centre in zinc oxide

Abstract In the excitation spectrum of the green emission band of ZnO:Cu crystals, zero phonon lines have been detected for the first time. In the liquid-helium temperature range, a line with Δ ν = 2 cm−1 at λ = 433.5 nm coincides with the well-known zero-phonon line of the emission. Additional lines are located at 432.2 and 431.4 nm. In a distance of 525 cm−1 from these zero-phonon lines, phonon satellites superpose on the low-energy part of the broad excitation band. On its high-energy slope pronounced structures appear in the excitonic region. With selected highly-doped crystals, the new zero-phonon lines could also be detected in absorption. Zeeman experiments produce anisotropic splittings of the common ground state of all three lines with g∥ = 0.7 ± 0.1 and g⊥ = 1.5 ± 0.1 which are approximately the values known from emission. Different anisotropic g-factors around g = 2 are derived for the terminal states of the absorption lines. The experimental data are compatible with a model of the investigated centre in which the ground state is Cu2+ and the excited state is a Cu+ with a loosely bound hole: (Cu+)+ [3]. As to the position of the impurity levels within the forbidden gap, still two possibilities exist: the excited hole could either be localized near the oxygen neighbours so that a depth of some tenth of an eV results, or effective-mass-like states could be involved in a distance of only a few meV above the top of the valence band.

Zeeman spectroscopy of the Fe3+ center in GaN

We report an optical investigation of the Zeeman behavior of the deep iron acceptor in GaN grown on 6H–SiC. The characteristic ground state splitting of the near‐infrared luminescence transition at 1.2988 eV allows for an unambiguous assignment to Fe3+previously proposed on the basis of ODMR results. The observed luminescence lifetime of 8 ms as well as the fine structure of the excited state are consistent with a 4T1(G)–6A1(S) transition. The 4T1(G) state is found to couple only weakly to e‐type phonon modes.

Zeeman-effect of Cu2+-centers in II-VI-compounds

Abstract The fine structure of energy levels, responsible for infrared luminescence of Cu2+-ions in II-VI-compounds, and the corresponding g-values are calculated on the basis of the most general effective Hamiltonian. This theory is applied to measurements of Zeeman-splittings, on cubic and polytypic ZnS-Cu2+ and to ZnO-Cu2+single crystals. A comparison with some earlier measurements of other authors is made. New information on parameters of the energy level diagram is obtained.

Optical nonlinearity and fast switching due to birefringence and mode coupling in CdS crystals

Abstract 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 Δ n = 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.

The copper centre: a transient shallow acceptor in ZnS and CdS

Excitation measurements of the 2E(D)-2T2(D)Cu2+ luminescences in ZnS and CdS crystals are presented, revealing new excitation processes of the centres. In ZnS sharp resonances observed on the low energy onset of the charge transfer band at 1.17 eV enable one to identify a transient shallow acceptor state of the Cu2+ centre in ZnS. An energy transfer process by photoexcited holes between Cu2+ and Fe2+ centres yields an accurate value for the deep Cu2+-acceptor position in ZnS of (1.293+or-0.005) eV. In CdS the deep Cu2+-acceptor position is (1.20+or-0.02) eV above the valence band. Additional excitation bands just below the excitonic bandgap are observed and interpreted as transitions between the conduction band and the transient shallow acceptor state. The binding energies of these states are determined to be 119 and 94 meV for ZnS and CdS, respectively. The recombination energy of the transient shallow acceptor state is transferred efficiently to the excited 2E(D) state, leading to the internal Cu2+ luminescence.

Thin prism refraction. A new direct method of polariton spectroscopy

Abstract Refraction of light by thin prisms has been used for the first time to obtain dispersion curves of excitonic polaritons. This most direct method demonstrated here for the A- and B-exciton in thin prismatic CdS crystals is shown to be extremely precise in spite of its simplicity. Improved values of polariton parameters are reported. In the mixed mode orientation we observed birefringence due to spatial dispersion. This opens a way to study the problem of additional boundary conditions.

Influence of compensation on the luminescence of nitrogen-doped ZnSe epilayers grown by MOVPE

Abstract 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.