Ch. Fricke

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.

Influence of growth non-stoichiometry on optical properties of doped and non-doped ZnSe grown by chemical vapour deposition

Abstract 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.

Incorporation of nitrogen in ZnSe grown by metalorganic vapour phase epitaxy

Abstract 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 I N 1 transition from a neutral acceptor bound exciton complex (A 0 N , X), indicating an effective nitrogen embedding on selenium sites. With increasing nitrogen doping rates, a deeper bound exciton line I C 1 appears, lowering the intensity of the I N 1 . An observed reduction of the I N 1 and I C 1 lifetimes for higher nitrogen doping concentrations results from an enhanced overlap of the bound exciton wave functions with those of other impurity centres.

Low temperature growth and plasma enhanced nitrogen doping of ZnSe by metalorganic vapour phase epitaxy

Abstract The combination of diisopropylselenide (DIPSe) and diethylzinc (DEZn) was used to grow ZnSe by metalorganic vapour phase epitaxy (MOVPE). The results are compared with those obtained from samples grown with diethylselenide (DESe) and DEZn. Furthermore, we analysed the influence of plasma stimulation of DIPSe and DESe, as well as plasma enhanced nitrogen doping. Plasma stimulated growth with DESe at 480°C results in ZnSe which shows resolved free and bound excitons and negligible deep centre emissions in 11 K PL spectra. By using DIPSe without plasma stimulation, the kinetically controlled growth regime was found in the temperature range from 380 to 480°C. In PL only broad distributed emissions from 2.4 to 2.78 eV were observed, arising from impurities in the Se precursor. With plasma enhancement the diffusion controlled growth regime was extended down to 330°C. In the PL spectra, excitonic emissions were found beside the emission band (2.4–2.78 eV). Deep centre emission (2.0 eV) appeared only for T D 2 flux causes an increasing nitrogen concentration. Samples grown in N 2 carrier gas at 380°C show a maximum measured nitrogen concentration of 1.7 × 10 18 cm -3 . The layers were highly compensated, due to the impurities in the Se precursor and to additional hydrogen and carbon incorporation.

Electronic structure and dynamical behaviour of different bound-exciton complexes in ZnSe bulk crystals

Abstract High-quality ZnSe crystals grown by the high-pressure Bridgman method show emissions of the free exciton (X) in the excitonic energy range as well as bound exciton lines I1 and I2. The I2i-lines, which are observed in excitation spectra of the (D0, X)-complex, are explained taking into account a three-particle model with excited |n, l〉 hole states for the (D0, X)-complex. Otherwise the (A0, X) ground state is split by the (j, j)-coupling of the two Γ8-holes into two separate levels. From time-resolved luminescence the specified rise and decay times of different bound excitons complexes are determined.

Recombination kinetics of CdS: In

The luminescence of CdS: In crystals is investigated in the near band gap region by means of time resolved measurements in the ps regime. The changes of the electronic structure induced by the doping give rise to a pronounced decrease of the (D0, X) complex lifetime down to 30 ps at some 1017 cm−3. At higher doping levels, the degeneracy of the conduction band results in radiative band-to-band transitions. The corresponding broad luminescence band decays exponentially within 300 ps. Under high excitation conditions, the recombination rate of band-to-band transitions in heavily doped samples increases strongly. For medium and low dopant concentrations, an increase of the excitation density results in longer decay times due to saturation.

Energy transfer processes via the interface of ZnSe/GaAs epilayers

Abstract In the present paper, energy transfer processes via the interface of molecular beam epitaxy (MBE) grown ZnSe/GaAs epilayers are investigated by means of time-resolved photoluminescence spectroscopy. High excitation experiments show that an electron-hole plasma exists in free-standing ZnSe layers only. By comparing the time-resolved luminescence properties of ZnSe/GaAs heterostructures and of free-standing ZnSe layers, we demonstrate which kind of energy transfer mechanism is predominant in heterostructures: At low excitation densities energy is transfered by re-absorption of the ZnSe luminescence. At higher excitation densities, an ambipolar carrier diffusion via the ZnSe/GaAs interface is the dominating process.

Fast relaxation phenomena in Ni-doped II–VI compounds

Abstract Time-resolved luminescence and “pump-and-probe” measurements of the 3 T 1 (P)- 3 T 1 (F) Ni 2+ transitions in Cds and ZnS are presented. The lifetimes of the 3 T 1 (P) states in ZnS and CdS are determined to be 60 and 400 Ps, respectively. Saturation experiments additionally show that the 3 T 2 (F) Ni 2+ term in ZnS and the 3 A 2 (F) Ni 2+ 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 Ni 2+ line. Furthermore, the Ni 2+ line broadening mechanisms are discussed.