We have explored the dielectric functions of ternary Zn1−xMgxTe thin films using a variable angle spectroscopic ellipsometer for samples between x = 0 and x = 0.52. We obtained values for the complex dielectric function for Zn1−xMgxTe in both the transparent and absorption regions by incorporating a threelayer model to simulate the experimental data. To this end, we also used the previously published relations of the dispersion of the indices of refraction (in the transparent region) of Zn1−xMgxTe measured using a combination of prism coupler and reflectivity. We have fitted the second derivatives of both the real and the imaginary parts of the dielectric function to obtain the critical point parameters corresponding to the higher order electronic transitions in the lattice. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We provide experimental evidence that the upper limit of \ensuremath{\sim}110 K commonly observed for the Curie temperature ${T}_{C}$ of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ thin films (thickness >50 nm) is caused by Fermi-level-induced hole saturation. Ion channeling, electrical, and magnetization measurements on a series of ${\mathrm{Ga}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Mn}}_{x}{\mathrm{Be}}_{y}\mathrm{As}$ layers show a dramatic increase of the concentration of Mn interstitials accompanied by a reduction of ${T}_{C}$ with increasing Be concentration, while the free hole concentration remains relatively constant at $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{20}{\mathrm{cm}}^{\ensuremath{-}3}.$ These results indicate that the concentrations of free holes and ferromagnetically active Mn spins are governed by the position of the Fermi level, which controls the formation energy of compensating interstitial Mn donors.

We provide experimental evidence that the electrical and magnetic characteristics of Ga 1− x Mn x As for a given x depend primarily on the distribution of Mn atoms over their different possible locations in the crystal lattice. Using combined channeling Rutherford backscattering and particle-induced X-ray emission, we show that optimal postgrowth annealing—which leads to an increase of the Curie temperature T C and is accompanied by an increase of free hole concentration and saturation magnetization—is caused by the reduction in the number of Mn atoms occupying interstitial positions. On the other hand, when Ga 1− x Mn x Asis additionally doped with Be, we observe that—while the hole concentration remains nearly constant—there occurs a strong decrease of T C together with a dramatic increase in the concentration of Mn interstitials. These results indicate that there is a thermodynamic limit imposed on the maximum Curie temperature in Ga 1− x Mn x As.

We provide experimental evidence that the upper limit of similar to110 K commonly observed for the Curie temperature T-C of Ga1-xMnxAs thin films (thickness >50 nm) is caused by Fermi-level-induced hole saturation. Ion channeling, electrical, and magnetization measurements on a series of Ga1-x-yMnxBeyAs layers show a dramatic increase of the concentration of Mn interstitials accompanied by a reduction of T-C with increasing Be concentration, while the free hole concentration remains relatively constant at similar to5 x10(20) cm(-3). These results indicate that the concentrations of free holes and ferromagnetically active Mn spins are governed by the position of the Fermi level, which controls the formation energy of compensating interstitial Mn donors.

We report the observation of excitons localized in an asymmetric GaAs quantum well (QW) embedded between AlAs and ZnSe barriers. Samples with different QW thicknesses (10, 15, and 18 monolayers) have been studied, showing clear confining effects in the photoluminescence emission. To achieve confinement in the GaAs QWs, it was necessary to design the band alignment across the AlAs/GaAs/ZnSe layer sequence, and to carefully tailor the growth conditions in order to achieve the required band alignment profile.

By using a combination of prism coupler, reflectivity, and spectroscopic ellipsometry techniques, we have determined the dielectric function of a series of molecular beam epitaxy-grown Zn1−xMnxTe thin films. These results have enabled us to determine the critical point parameters that correspond to the electronic transitions in the Brillouin zone for this particular alloy family. We find that, although the fundamental band gap E0 increases with x for this alloy, E1 and E1+Δ1 transition energies decrease with x for the Zn1−xMnxTe system, in contrast to most other zinc-blende semiconductor alloys.

Zn 1−x Mn x Te and Zn1−xMgxTe ternary wide-gap semiconductor alloys were grown by molecular beam epitaxy on (100) GaAs substrates over a wide range of compositions (0⩽x⩽0.75 and 0⩽x⩽0.67, respectively). Values of the band gap were measured by photoluminescence at 12 K, and by optical reflectivity at room temperature. The wavelength dependence of the indices of refraction n of these ternary systems was also measured for these alloys at wavelengths below their respective energy gaps. The measurements were performed using a combination of the prism coupler method and reflectivity. Compilation of these results allows us to establish a set of empirical parameters for the two alloy families, that can be used to calculate the index of refraction for an arbitrary alloy composition and arbitrary wavelength. It is interesting that the values of n show a surprisingly linear dependence on the corresponding energy gaps for both these alloy systems.

The prism coupler technique, together with reflectivity channeled spectra, were used to determine the dispersion of the indices of refraction for a series of ternary alloys of Zn1−xBexSe grown by molecular-beam epitaxy on GaAs substrates. The measurements covered the wavelength range of 400–1300 nm, and the entire Be concentration range, from x=0 to 1.00. The availability of accurate values of the index-of-refraction then enabled us to determine the elastic moduli c11 for the Zn1−xBexSe and its dependence on the Be concentration x from frequency shifts in Brillouin scattering spectra observed on the epilayers of these ternary alloys. The c11 results clearly indicate that the bonding in Zn1−xBexSe becomes more robust as the Be concentration increases.

We have fabricated a series of ZnSe/ZnCdSe multiple-quantum-well (MQW) structures in order to test the dependence of the resultant index of refraction n of the composite system on the n values of the constituents. The ZnSe/ZnCdSe MQWs were grown by molecular-beam epitaxy on GaAs substrates. We used photoluminescence and x-ray diffraction experiments to initially characterize the specimens. Using a prism coupler system—which measures n with high precision—we obtained n at three discrete laser wavelengths. We find that the effective values of n of these MQWs differ from their “Vegard-law-like” values; that is, the n of the composite structure is not a linear function of the n values of the constituents. This departure results from the energy-level structure of the MQW system associated with confinement of electrons and holes in the well layers.

A series of ZnSexTe1−x alloys with varying compositions that cover the entire range between the two binaries ZnSe and ZnTe were grown to determine the index of refraction, n, as a function of both wavelength and the alloy concentration. The ZnSexTe1−x alloys were all grown directly on GaAs (100) substrates using molecular beam epitaxy. X-ray diffraction experiments were performed to determine the quality and the compositions of each of the ZnSexTe1−x specimens. The n was then measured with an accuracy of at least 0.1% at four discrete wavelengths using a prism coupler method. Since these data points are inadequate to ascertain, an accurate dispersion of the index of refraction, we next performed reflectivity measurements on each of the samples to complement the prism coupler data. The reflectivity and the prism coupler data allow us to arrive at the dispersion relations of the indices of refraction of ZnSexTe1−x ternary alloys very accurately. We also find that, unlike most semiconductors, the values of n of ZnSexTe1−x alloys do not follow an inverse relationship with the energy gap.

Significant enhancement of the magnetic correlation range in [ZnTe[MnTe] magnetic semiconductor superlattices is produced by introduction of CI during growth. Results of Cl added to the ZnTe layers alone, to both layers, and to a partial MnTe layer suggests that the magnetism arises from an exchange mechanism dependent on the electronic states in addition to conventional superexchange. (C) 2001 Elsevier Science B.V. All rights reserved.

The effect of adding Cl impurity on the long-range antiferromagnetic coupling in [ZnTe|MnTe] magnetic semiconductor superlattices has been investigated by neutron diffraction. Superlattices consisting of five atomic planes of ZnTe alternated with ten atomic planes of MnTe[(ZnTe)5|(MnTe)10] have been prepared by molecular-beam epitaxy both with and without Cl added to the MnTe and/or ZnTe layers during deposition. This was motivated by the fact that Cl can form either shallow or deep donor levels in II–VI semiconductors, and that such electronic states can serve to modify magnetic interlayer exchange. Unchlorinated samples showed magnetic correlation lengths of around 225 Å (≈5 bilayers) at low temperatures, and the addition of Cl increased the correlation length at 15 K by nearly a factor of 2 to approximately 450 Å.

By using a prism coupler technique in conjunction with reflectivity measurements, we have obtained highly accurate relations for the dispersion of the indices of refraction n for a series of MBE-grown Cd 1−x Zn x Te alloys. Initially, the prism coupler technique was used to determine n at discrete wavelengths with an accuracy of at least 0.1%, and also to concurrently determine the epilayer thicknesses with an uncertainty of less than 0.5%. Having obtained precise values for both n (at discrete wavelengths) and the thicknesses of the Cd 1−x Zn x Te epilayers, we were then able to correctly decipher the values for n at the maxima and minima of the reflectivity spectra observed on the above epilayers, and thereby generate a continuous variation of the indices of refraction as a function of wavelength. Fitting the dispersion of n in each alloy to a Sellmeier-type dispersion relation, we have obtained the dependence of the constants appearing in this relation on the alloy concentration. This enables one to predict n not only as a function of wavelength, but also as a function of alloy composition.