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Notes: The reflectivity spectra from single-crystal epilayers of the diluted magnetic semiconductor Zn1−x Cox Se(x=0.0076, x=0.0104) have been studied as functions of magnetic field and temperature. The excitonic spin splitting in these crystals saturates for magnetic fields B〉5 T, indicating that (Zn,Co)Se is a Brillouin paramagnet, in agreement with magnetization and electron-paramagnetic-resonance studies on the same samples. The magnetoreflectivity data yield a value for the exchange parameter difference N0 (α−β)=2420±40 meV, significantly larger than the values obtained for either Fe- or Mn-based diluted magnetic semiconductors studied to date.

Notes: We have examined the initial interface formation and subsequent film growth for Fe films on the As-terminated (2×4) reconstructed surface of GaAs(001) in an effort to correlate the initial adsorption sites and film growth with the magnetic anisotropy. The growth and surface studies were performed in a four chamber ultrahigh vacuum system which incorporates UHV sample transfer between the chambers for III–V semiconductor growth, metal growth, photoelectron diffraction (PED), and scanning tunneling microscopy (STM). After coating with a thin film of Au (40 A(ring)) at room temperature, the samples were removed from the system for postgrowth characterization, which included ferromagnetic resonance and magnetometry. The GaAs(001) surface was prepared via homoepitaxial growth by MBE, with the growth terminated in a manner which resulted in a well-ordered As-dimer terminated (2×4) reconstructed surface as revealed by reflection high energy electron diffraction and STM. Fe deposition was performed in a second MBE chamber for coverages which ranged from 0.1 ML to several tens of monolayers at a substrate temperature of 175 °C.The sample was then transferred under UHV to either the PED or STM chamber to determine the initial adsorption sites, growth mode, and evolution of film structure with increasing coverage. For the lowest coverages studied (0.1 ML), we obtain evidence for preferential adsorption at As-dimer sites. At a coverage of 1 ML, two-dimensional islands form approximately 20 A(ring) in size, but elongated along the missing dimer rows with an aspect ratio of 2:1. At high coverages ((approximately-greater-than)40 A(ring)), we observe three-dimensional mounds or clusters with an average in-plane diameter of approximately 100 A(ring) and a narrow size distribution. Even at this coverage, the monolayer terraces of the GaAs substrate are clearly visible in the STM image, with the Fe film uniformly growing on each terrace. Photoelectron diffraction in the forward scattering regime (high electron kinetic energy) is utilized to determined the growth mode. At low kinetic energy, information on the adsorption site is obtained from Auger electron diffraction and compared with the results of the real space STM images.

Notes: Recently, there has been increased research activity on Fe-, Mn-, and Co-based diluted magnetic semiconductors (DMS) because of their interesting magneto-optical properties. Potential applications of DMS usually require thin films with a high degree of structural perfection, and high quality single crystal epilayers of Zn1−xFexSe and Zn1−xCoxSe have been grown by molecular beam epitaxy (MBE). In this work, the microstructural properties of (ZnSe/FeSe) and (ZnSe/MnSe) DMS superlattices grown on (001) GaAs substrates by MBE have been investigated using transmission electron microscopy. High-quality (ZnSe/FeSe) superlattices were grown by introducing a ZnSe buffer layer on the GaAs substrate prior to the growth of the superlattice. In contrast, nominal (ZnSe/FeSe) superlattices grown directly without a buffer layer on the substrate showed evidence for interdiffusion between the constituent layers of the superlattice. Chemical ordering of the Zn and Fe atoms was also observed in the resultant Zn1−xFexSe alloys along the [001] and [110] directions. This ordered structure corresponds to the CuAu–I type structure. In contrast, the (ZnSe/MnSe) superlattices did not show interdiffusion, but contained many microtwins and 60°-type misfit dislocations. The MnSe layer in the (ZnSe/MnSe) superlattices existed as a zinc-blende structure. © 1996 American Institute of Physics.

Notes: Room-temperature spectroscopic ellipsometry measurements were carried out on ZnSe thin films grown on (001)GaAs substrates by molecular-beam epitaxy for the study of the lattice mismatch-induced strain at the interface. The magnitude of the absorption coefficient at the E0+Δ0 critical point is very sensitive to the strain in the film. The variation in the magnitude of the absorption coefficient is used to estimate the critical thickness for the onset of dislocation generation. Almost complete relaxation of the films was obtained for thicknesses higher than 500 nm. Also, the strain-induced coupling between the valence subbands was found to cause additional shifting of the light-hole subband.

Notes: Spectroscopic ellipsometry was used to determine the real and imaginary parts of the dielectric function of ZnSe thin films grown on (001) GaAs substrates by molecular-beam epitaxy, for energies between 1.5 and 5.0 eV. A sum of harmonic oscillators is used to fit the dielectric function in order to determine the values of the threshold energies at the critical points. The fundamental energy gap was determined to be at 2.68 eV. The E0+Δ0 and E1 points were found to be equal to 3.126 and 4.75 eV, respectively. Below the fundamental absorption edge, a Sellmeir-type function was used to represent the refractive index. At the critical points, E0 and E0+Δ0, the fitting was improved by using an explicit function combining the contributions of these two points to the dielectric function.

Notes: Line-shape analysis of magnetoreflectance spectra from ZnSe/Zn0.99Fe0.01Se quantum well structures was performed using a classical dielectric function model. This model explains the spectral dependence on the sample geometry, as well as providing additional evidence of spin superlattice formation in ZnSe/ZnFeSe heterostructures.

Notes: We describe the results of a magneto-optical study of a class of semiconductor heterostructures in which one of the constituent layers is a diluted magnetic semiconductor (DMS). The large magnetic band splittings of DMS materials result in spin-dependent confining potentials for electrons and holes which can be changed externally by varying the applied magnetic field. The modifications in the band alignment result in changes, sometimes dramatic, of the optical properties in the vicinity of the band gap.

Notes: We have studied a new type of semiconductor quantum well system in which the carrier-ion exchange interactions are determined by the spin of the carrier. The samples consist of alternating layers of Zn1−xMnxSe and Zn1−yFeySe grown by molecular-beam epitaxy (MBE). At zero field, the carriers initially interact randomly with both transition metal species. When a magnetic field is applied, the excitonic wave functions are increasingly localized in one or the other of the magnetic layers according to their spin state as the competing spin exchange interactions define the confining potential. The spin components of the heavy hole exciton are subsequently dominated by different exchange interactions as revealed by their temperature and field dependence: the behavior of the spin-down component (−3/2,−1/2) is described by exchange interactions of the carriers with the Mn2+ ions and exhibits Brillouin paramagnetic behavior, while the spin-up component (+3/2,+1/2) is dominated by interactions with Fe2+ ions and exhibits Van Vleck paramagnetism. These structures are thus characterized by an initial competition and eventual coexistence of Brillouin- and Van Vleck-like paramagnetic behavior for the exciton.

Notes: The growth and initial characterization of a new II-VI-based diluted magnetic semiconductor compound that incorporates a rare-earth element, Zn1−xEuxSe, is reported. The samples were grown by molecular-beam epitaxy on GaAs(001) substrates using elemental source ovens. Reflection high-energy electron diffraction confirms single-crystal growth with an in-plane symmetry like that of the GaAs substrate, although the crystalline quality deteriorates with increasing epilayer thickness. Growth on a ZnSe buffer layer substantially improves the crystalline quality, suggesting that the lattice constant of the Zn1−xEuxSe is larger than that of GaAs. This is confirmed by x-ray θ−2θ scans, which further show that the epilayers grow in a (001) orientation and retain the zinc-blende structure of the host lattice. Eu is expected to substitute for Zn as Eu2+ (4f7, 8S7/2 free ion) with an 8A1 ground state, an orbital singlet with a spin S=7/2. Superconducting quantum interference device (SQUID) magnetometry data (2–150 K, 0–5 T) exhibit the temperature and field dependence characteristic of Brillouin paramagnetic behavior, consistent with substitutional incorporation.

Notes: Single-crystal films of (001)Zn1−xFexSe (x=0.017, 0.027, 0.043) grown by molecular beam epitaxy on (001)GaAs have been studied using reflectivity and photoluminescence spectroscopies. Data have been obtained over the temperature range 4–77 K in magnetic fields up to 8 T in the spectral region near the band gap of 2.8 eV. The luminescence spectra show the two lowest interband excitonic transitions. From these spectra, the Zeeman splitting of the bands and the corresponding Fe2+-band electron exchange integrals were determined. The temperature dependence of the band splitting was determined through reflectivity measurements. These studies confirm the fact that Zn1−xFexSe exhibits Van Vleck paramagnetism and yield the spin-orbit splitting of the lowest Fe2+ crystal field states.