N. Hoffmann

Molecular beam epitaxial growth and characterization of ZnSe on GaAs

Abstract Systematic investigations have been performed concerning the effect of growth temperature and beam pressure ratio on the structural perfection and optical properties of molecular beam epitaxy (MBE) grown ZnSe on GaAs substrates. A surface phase diagram of ZnSe has been derived showing the different surface reconstructions as a function of these growth conditions. RHEED reflex profiling reveals more details about the surface morphology occurring in different stages of the growth process. The epitaxial layers are further characterized by means of X-ray diffraction and various optical techniques.

Surface engineering during molecular beam epitaxial growth of wide-gap II-VI structures

Abstract Systematic investigations have been carried out in order to control the surface morphology of ZnSe and related compounds. Rough surfaces can be smoothed by means of growth interruptions. The smoothing of the surface is reflected in the halfwidth of the intensity distribution along the 00 rod of the reflection high-energy electron diffraction (RHEED) pattern. Reduction of the halfwidth is observed to occur within times of less than a minute; a fast process removes short range disturbances of the surface morphology. A more detailed analysis of the intensity distribution reveals a fine structure. From this, it can be derived that the distance of surface terraces is made uniform during the growth interruption, but this takes several minutes; a slow process removes long range disturbances. Degree and time dependence of surface smoothing are influenced by the beam equivalent pressure ration during layer growth.

On the mechanism of reflection high-energy electron diffraction oscillations studied by phase-locked epitaxy of ZnSe

Abstract Molecular beam epitaxy (MBE) is one of the most promising methods for growth of sophisticated device structures. Starting the growth on a flat surface, reflection high-energy electron diffraction (RHEED) oscillations occur. The question which phase of the oscillations corresponds to lattice plane completion and the most flat surface morphology is not yet fully resolved. There is hardly a direct access to the answer. Phase-locked epitaxy (PLE), however, appears to be a tool for studying this phase problem. PLE permits the growth of layers without losing RHEED oscillations due to damping the thickness of which is great enough to become measurable by common techniques. This enables to compare the directly determined layer thickness with that obtained by counting the number of RHEED oscillation periods. Results are presented that show the phase relation between RHEED oscillations and lattice plane completion.

Bound- and bi-excitons in ZnCdSeZnSe multiple quantum wells

Abstract Using a set of high-quality MBE grown samples, a systematic study of bi- and bound-excitons in wide-gap II–VI quantum wells is carried out for the first time. The assignment of low-energy photoluminescence lines to these complexes is accomplished by their characteristic polarization properties in a magnetic field. The binding energy of the bi-exciton increases with growing band offset. A similar tendency is found for the bound-exciton, however, with generally smaller binding energies. The larger values found for the bi-exciton are attributed to localization on alloy fluctuations which is qualitatively confirmed by density functional calculations. Time-resolved photoluminescence is used to study the formation and decay kinetics of bi-excitons.

Magneto-optical study of ZnSe/(Zn,Mn)Se and ZnSe/(Zn,Cd,Mn)Se quantum well structures and superlattices

Abstract Quantum well structures with semimagnetic barriers, ZnSe /( Zn , Mn ) Se , or semimagnetic wells, ( Zn , Cd , Mn ) Se / ZnSe , are studied by luminescence and reflection spectroscopy in an external magnetic field up to 7.5 T.A. unique offset dependence on the manganese concentration and a type conversion of the ZnSe /( Zn , Mn ) Se heterostructures due to the giant Zeeman splitting of the barriers are found. In the ( Zn , Cd , Mn ) Se / ZnSe structures, a strong reduced energy transfer from the exciton states to the Mn internal transition was found, which is further suppressed by an increasing magnetic field. Therefore (Zn,Cd,Mn)Se quantum wells can be proposed to be suitable and effective for tuneable blue-green laser devices in an external magnetic field. The field dependent energy splitting both of well and barrier excitons are significantly anisotropic with respect to the orientation of the magnetic field B parallel or perpendicular to the growth axis. This magneto-optical anisotropy can be explained by the mixing of the heavy and light hole states in the framework of a multi-band envelope-function approximation.

MBE growth and characterization of (Zn, Mn)Se/ZnSe and MnSe/ZnSe structures on (001) GaAs

Abstract The MBE growth of ZnSe, (Zn,Mn)Se, MnSe, and related quantum well structures has been studied. Results are presented concerning the effect of growth interruptions and optical properties of SQW and MQW structures.

Growth and optical properties of ZnSe/ZnMnSe quantum structures

ZnSe/ZnMnSe MQW structures are grown by MBE. In situ RHEED control allows one to lock the growth cycle on the phase of the RHEED oscillations so that lattice plane completion is achieved independent of beam flux fluctuations and other irregularities. The band-edge resonant optical properties of the structures are dominated by sharp and pronounced excitonic features. The influence of strain and confinement on these excitons, their localization and interaction with phonons are discussed.

Dimensional dependence of magneto-optical properties of ZnMnSe MBE layers

A set of samples have been grown consisting of ZnMnSe MBE layers with x Mn = 0.12 and various layer thicknesses between 500 nm and 3 monolayers. Luminescence, excitation and reflection spectroscopy were performed in a magnetic field up to 7.5 T. Whereas for thick ZnMnSe layers of 500 and 50 nm direct excitons are to be seen, in the case of thin layers d < 10 nm only spatially indirect excitons are observable in luminescence with holes localized mainly in the ZnMnSe layers and the electrons localized in the adjacent ZnSe layers. These interface localized spatially indirect excitons exhibit a reduced Zeeman splitting compared to the direct excitons observable in the thick layers, due to the reduced number of Mn ions inside of the exciton volume. The corresponding ZnMnSe gap energy shift shows, however, a strong increase with decreasing layer thickness. The enhancement of paramagnetic response is only partly due to the interface. We suggest a strong increase of the exchange integrals for the dimensional crossover.

Growth and characterization of CdTe/ZnTe buffer layers on GaAs substrates

Abstract The MBE growth of ZnTe, CdTe, (Cd,Zn)Te and related superlattice structures on (001)GaAs has been investigated. Growth rate, structural perfection and mixed crystal composition have been determined as functions of different growth parameters. Optical properties and the ability of blocking substrate impurities with a superlattice structure have been studied.

Spin-flip Raman scattering studies of ZnSeZn1−xMnxSe multiple quantum well structures

Studies of electron spin-flip Raman scattering in a series of ZnSe/Zn 1-x Mn x Se heterostructures of different quantum well widths are reported. In all the specimens studied, a single spin-flip Raman band was observed with a Raman shift which showed the expected modified Brillouin function dependence on magnetic field and temperature. The dependence of the scattering intensity on the excitation energy shows strong resonant enhancement at energies corresponding to the quantum well exciton transitions, which, together with the selection rules, indicates that the spin-flip Raman signals arise from electrons localised in the ZnSe quantum wells. In our previous studies, the magnitude of the spin-flip Raman shift has been interpreted as a measure of the penetration of the electron envelope wavefunction into the barrier, leading to a simple means of determining the conduction band offset (including any contribution due to strain) of the system. However, in the present case, where the manganese concentration of the barrier is large and where the band offset is expected to be small, simple interpretation of the data in this way is not possible. Two effects in particular complicate the analysis of the data: the roughness of the heterointerface and the modified probability, compared to bulk material, of manganese antiferromagnetic spin-pairing at that interface. We use the results of recent theoretical work to show that the spin-flip Raman spectra are very sensitive to the state of the interface.