High-quality ZnS, ZnSe, and ZnTe epitaxial films were grown on (001)-GaAs-substrates by molecular beam epitaxy. The 1s-exciton peak energy positions have been determined by absorption measurements from 2 K up to about room temperature. For ZnS and ZnSe additional high-temperature 1s-exciton energy data were obtained by reflectance measurements performed from 300 up to about 550 K. These complete E1s(T) data sets are fitted using a recently developed analytical model. The high-temperature slopes of the individual E1s(T) curves and the effective phonon temperatures of ZnS, ZnSe, and ZnTe are found to scale almost linearly with the corresponding zero-temperature energy gaps and the Debye temperatures, respectively. Various ad hoc formulas of Varshni type, which have been invoked in recent articles for numerical simulations of restricted E1s(T) data sets for cubic ZnS, are discussed.

The transmission of ZnTe:Co was measured at 2 K under hydrostatic pressure up to 8 GPa. Two kinds of optical transitions were investigated: (i) Internal 3d7 transitions between crystal field levels of Co2+ in the tetrahedral field of the ZnTe lattice. The pressure dependence of the crystal field parameter Dq and of the Racah parameter B was determined. (ii) A charge transfer absorption edge, which is caused by an electron transfer from the valence band of ZnTe to the Co 3d7 states. The acceptor state of the impurity was found to be an absolute energy reference, which makes a determination of the hydrostatic deformation potential av of the valence band possible.

Photoluminescence and photoluminescence excitation spectra have been employed to study the phonon emission process in a ZnSe–Zn0.75Cd0.25Se double-superlattice at 1.4 K. Since the excitation energy is higher than the excitonic energy of the superlattice sample, excitons with a large kinetic energy will be generated. These excitons will then decay by emitting mainly longitudinal optical phonons and reach the bottom at K=0. The transitions of n=1 heavy- and light-hole hot excitons from a narrow well superlattice are observed through the LO-phonon energy mℏω. The results show that the n=2 heavy-hole excitonic band (E12hh) in the wide well superlattice (SL1) reabsorbs the radiation, generated by both, the n=1 light-hole (E21lh) and heavy-hole (E21hh) excitonic bands of an adjacent narrow well superlattice (SL2).

Under the excitation of a cw Ar+ laser with excitation density as low as 1 W/cm2, a biexcitonic PL peak is observed in the PL spectrum of a ZnMgSe/ZnSe single quantum well (QW). This peak shows strong superlinear increase with excitation density. The line shape analysis of this peak shows that it is in good agreement with the theoretical line shape for luminescence of biexcitons in QW. The binding energy is found to be about 10.7 meV. When the temperature increases from 3.4 K to 36.0 K, the PL intensity of biexcitons decreases much more rapidly than that of excitons.

We studied the low-temperature growth and doping of polycrystalline ZnSe by MOCVD using ditertiary-butylselenide (DtBSe) and dimethylzinc-triethylamine (DMZn-TEN) as precursors. With these alkyls a deposition of ZnSe at less than 400°C is possible without using the toxic H2Se. Polycrystalline ZnSe grows in [111] direction, therefore also the deposition on GaAs(111) substrates was studied. Polycrystalline ZnSe remained semi-insulating after doping with Ga and Cl and can therefore not replace ZnO as front electrode material. The final goal of this study is the application of ZnSe as buffer material in polycrystalline Cu(In,Ga)Se2 (CIGS) solar cells in order to substitute the usually used CdS buffer. By using MOCVD grown ZnSe as buffer layer Cd-free CIS-based solar cells with 11% efficiency have been fabricated.

A group of well-defined exciton transitions from the localized states were observed in ZnSe-Zn0.75Cd0.25Se double-superlattice structure. The photoluminescence and photoreflectance have been employed to study the subband transitions at low temperatures. At 1.4 K, except the two ground states and two higher subbands of n=1 light-hole and n=2 heavy-hole excitonic transitions, other four peaks (A, B, C, and D) also were observed in wider-well superlattice. Those peaks were attributed to the excitonic transitions from n=2 heavy-hole subband due to the fluctuation of well-barrier interface. Another localized excitonic transition from narrower-well superlattice appeared as increasing the modulated intensity in photoreflectance spectra.

Photoluminescence-excitation (PLE) measurements of wide ZnSe/Zn1−xMgxSe single quantum wells (SQW) are presented, performed under high hydrostatic pressure up to 4 GPa. The fully strained samples have been grown by MBE on GaAs(0 0 1) substrates or (1 1 0) substrates. The well transitions 1nH1s and 1mL1s with n = 1, 2, 3, 4, 5 and m = 1, 2 were recorded as well as excitonic signals from the barrier material at 2 K and their pressure dependence were determined. A cross-over between the 11L and 13H, as well as between the 12L and 14H transitions, was observed above 1.2 GPa. The transitions were assigned using a modified Kronig Penney model. The exciton binding energy was derived from the energy difference between the 11H1s and the 11H2s transitions.

We investigate II–VI optical waveguides consisting of ternary ZnSxSe1−x and Zn1−xMgxSe compounds. The refractive indices are determined experimentally as a function of their composition. A linear interpolation was found to be a good approximation in the case of ZnSxSe1−x. An empirical relation for Zn1−xMgxSe compounds was derived, giving the index of refraction from the band edge to the near infrared for magnesium contents up to 40%. Waveguide modelling is used to optimize waveguides according to the requirements of applications or epitaxial growth. Field distributions and propagation constants of multilayered guiding structures and losses caused by substrate leakage are calculated. Attenuation coefficients of 1–2 cm−1 are measured in waveguides containing ternary II–VI compounds.

We studied the low-temperature growth and doping of polycrystalline ZnSe by MOCVD using ditertiary-butylselenide (DtBSe) and dimethylzinc-triethylamine (DMZn-TEN) as precursors. With these alkyls a deposition of ZnSe at less than 400°C is possible without using the toxic H 2 Se. Polycrystalline ZnSe grows in [1 1 1] direction, therefore also the deposition on GaAs(1 1 1) substrates was studied. Polycrystalline ZnSe remained semi-insulating after doping with Ga and Cl and can therefore not replace ZnO as front electrode material. The final goal of this study is the application of ZnSe as buffer material in polycrystalline Cu(In,Ga)Se 2 (CIGS) solar cells in order to substitute the usually used CdS buffer. By using MOCVD grown ZnSe as buffer layer Cd-free CIS-based solar cells with 11% efficiency have been fabricated.

The temperature dependence of the Is exciton energy has been measured in Zn1-xMgxSe epitaxial films sat compositions x=0, 0.07, 0.12, and 0.19 from 2 K up to 280 K. Detailed numerical fits of the individual temperature dependences are provided on the basis of an analytical four-parameter representation developed recently by one of the authors. These are compared with previously used three-parameter models of Villa et al. and Varshni. The cc-dependence of the exciton energy, E-1s(T, x), and of the fundamental band gap energy, E-g(T, x), is given to very good approximation by linear functions of the composition x for any T from absolute zero up to room temperature. A comparison with recent room temperature band gap energy data by Jobst et al. shows that this linear dependence holds up to x approximate to 0.7. The magnitudes of the model-dependent empirical parameters, which control the temperature dependence of the band gap energy in different compounds, are found to change significantly with increasing magnesium content. From the magnitude of the effective phonon temperature, particularly in the case of ZnSe, we conclude that the main contributions to the band gap shrinkage effect are due to acoustic phonons.

Electroreflectance measurements performed in conjunction with absorption and photovoltage spectroscopy have allowed a determination of the modulation mechanisms that are responsible for the quantum confined Stark effect in a ZnSe-Zn0.75Cd0.25Se double superlattice. This study indicates that the modulation of the excitonic transitions in the two superlattices (E-1hh(1), E-2hh(1), and E-1hh(2)) are observed in the electroreflectance spectra. Due to the thermal effects of the photoexcited carriers the higher subband with a smaller exciton binding energy is more sensitive to the ac electric field.

Measurements of optical and photoelectron spectra are presented to determine the band offsets of ZnSe/Zn1-xMgxSe heterostructures. The samples are grown on GaAs(001) by MBE and investigated by absorption, photoreflection, photoluminescence, photoluminescence-excitation and photo-electron spectroscopy using a synchrotron light source. Ln the visible spectra discrete excitonic states are observed at the E-0 and E-0 + Delta(0) gap up to a composition of x less than or equal to 0.36. The optical transitions of ZnSe/Zn1-xMgxSe single quantum wells are fitted to a model. It contains the conduction band offset as the only free parameter, which is then determined by a least-squares fit of the data. Photoelectron spectra (UPS) are measured on ZnSe which was in situ deposited on MgSe in several monolayer thickness. The valence band edges of ZnSe and MgSe are simultaneously observable and thus a direct determination of the valence band offset was possible. The values of the band offsets from the two different methods agree reasonably well within the errors of the measurements. Different from other systems ZnSe/Zn1-xMgxSe does not fulfil the common anion rule, which may be due to the filled d-shell of Zn which is lacking in Mg.

Under an excitation density of 0.4 W/cm2, a strong biexcitonic peak was observed in the photoluminescence (PL) spectrum of a single quantum well which was grown in submonolayer deposition mode. Its intensity was found to increase superlinearly with excitation density. The shape of the biexcitonic peak was found to be similar to that of biexcitons in CuCl and CdS crystal and a binding energy of 9.8 meV was obtained. When the temperature increased from 10 to 31 K, the biexcitonic peak decreased considerably whereas the excitonic peak increased slightly.

We present absorption measurements of a pseudomorphic Zn0.75 Cd0.25 Se/ZnSe superlattice and a Zn 0 . 91 Cd0 .09 Se/ZnSe single quantum well under high hydrostatic pressure applied by a diamond anvil cell. Excitonic transitions as 1s-heavy hole and 1s-light hole between the first bound states or minibands as well as transitions in the buffer material are visible. Transition energies are well understood by calculations of the band structure near Γ. The observed energy distance between photoluminescence and hh-absorption signal gives information on exciton localization. Whereas this distance is nearly pressure independent in the buffer, there is a significant increase at pressures P > 5 GPa for quantum structures.

We present optical measurements of freestanding thin Zn1−xMgxSe films and of a ZnSe/Zn0.93Mg0.07Se single quantum well (SQW) under hydrostatic pressure applied by a diamond anvil cell. For the Zn1−xMgxSe compounds the pressure shift of the fundamental gap was determined from absorption measurements at room temperature for X ⩽ 0.37. A hydrostatic deformation potential a(x) = −4.85 + 0.67x eV is derived. The transition pressures Ptr to high pressure phases were determined by visual observation. The pressure dependent absorption and photoluminescence of the ZnSe SQW was investigated at 2 K. The energy shift of two SQW-related transitions was recorded and is reproduced by a simple theoretical model. LO-phonon replica were visible in photoluminescence at the sharp 11hh transition.

We present absorption measurements on free-standing ZnSe, ZnS and ZnSxSe1-x films (d = 0.4...2 mu m) under hydrostatic pressure up to 15 GPa. The refraction index n(lambda, P) of ZnS and ZnSe in the transparent region up to 800 nm and the pressure shift of the E0 absorption edge of ZnSe and some ZnSxSe1-x-compositions was investigated at 293 K. At 2 K free exciton states near the E(0)- and E(0) + Delta(0)-gap are visible in absorption. Increase in the Rydberg energy under pressure was found, which is explained with k . p-theory in the framework of the hydrogen model.