F. Firszt

Growth and characterisation of Zn1−xBexSe mixed crystals

Abstract Beryllium containing, wide-gap II–VI semiconducting compounds may offer a possibility of a significant reduction of the defect propagation in the active region and therefore increasing of II–VI lasers lifetimes due to more covalent bonding and lattice hardening [1] [Ch. Verie, in: B. Gil, R.L. Aulombard (Eds.), Semiconductor Heteroepitaxy, Growth, Characterization and Device Applications, World Scientific, Singapore, 1995, p. 73]. Until now, the published papers were concerned thin films [2,3] [F. Fischer, G. Landwehr, Th. Litz, H.J. Lugauer, U. Zehnder, Th. Gerhard, W. Ossau, A. Waag, J. Crystal Growth 175–176 (1997) 532; V. Bousquet, E. Tournie, M. Laugt, P. Vennegues, J.P. Faurie, Appl. Phys. Lett.70 (1997) 3564] but the fundamental properties of Zn 1− x Be x Se bulk crystals have not been investigated as yet. This work deals with an experimental study of structural and photoluminescence properties of Zn 1− x Be x Se mixed crystals as a function of composition.

A linear relationship between the Hall carrier concentration and the effective absorption coefficient measured by means of photothermal radiometry in IR semi-transparent n-type CdMgSe mixed crystals

In this work we demonstrate the ability to measure the effective infrared absorption coefficient in semiconductors by a photothermal infrared radiometry (PTR) experiment, and its correlation with the Hall carrier concentration. The amplitude and phase of the PTR signal were measured for Cd1?xMgxSe mixed crystals, with the magnesium content varying from x = 0 to x = 0.15. The PTR experiments were performed at room temperature in thermal reflection and transmission configurations using a mercury cadmium telluride infrared detector. The PTR data were analyzed in the frame of the one-dimensional heat transport model for infrared semi-transparent crystals. Based on the variation of the normalized PTR phase and amplitude on the modulation frequency, the thermal diffusivity and the effective infrared absorption coefficient were obtained by fitting the theoretical expression to experimental data and compared with the Hall carrier concentration determined by supplementary Hall experiments. A linear relationship between the effective infrared absorption coefficient and the Hall carrier concentration was found which is explained in the frame of the Drude theory. The uncertainty of the measured slope was 6%. The value of the slope depends on (1) the sample IR absorption spectrum and (2) the spectral range of the infrared detector. It has to be pointed out that this method is suitable for use in an industrial environment for a fast and contactless carrier concentration measurement. This method can be used for the characterization of other semiconductors after a calibration procedure is carried out. In addition, the PTR technique yields information on the thermal properties in the same experiment.

Growth and characterisation of Cd1−xMgxSe mixed crystals

Cd 1-x Mg x Se mixed crystals for 0 < x < 0.55 have been grown by the high-pressure Bridgman method under an argon overpressure. The dependence of the energy gap, the luminescence and the electrical properties on the Mg concentration has been investigated. Luminescence and transmission spectra show that the band-gap energy is considerably larger than that of pure CdSe and for Cd 0.45 Mg 0.55 Se we measured about 2.82 eV at T = 40 K. As grown Cd 1-x Mg x Se solid solutions with x < 0.4 exhibit n-type conductivity and low electrical resistivity (ρ < 1 Ω cm at room temperature). The maximum of electron concentration 1.3 x 10 18 cm -3 was obtained for Cd 0.85 Mg 0.15 Se. An attempt to explain the dependence of free-carrier concentration on composition by a model of Fermi-level pinning is presented.

Study of the third order nonlinear optical properties of Zn1−xMgxSe and Cd1−xMgxSe crystals

Third order nonlinear optical susceptibilities χ<3> of ternary Zn1−xMgxSe and Cd1−xMgxSe crystals have been measured using standard degenerate four-wave mixing (DFWM) method at 532 nm. The nonlinear transmission technique has been applied to check if our crystals exhibit two-photon absorption. The studied Zn1−xMgxSe and Cd1−xMgxSe solid solutions were grown from the melt by the modified high-pressure Bridgman method. For both crystals the energy gap increases with increasing Mg content.In the case of Zn1−xMgxSe, it was found that the value of third order nonlinear optical susceptibility χ<3> decreases with increasing Mg content. An explanation of this behaviour results from the dependence of optical nonlinearities on the energy band gap Eg of the studied crystals. In the case of Cd1txMgxSe with low content of Mg, no response was observed for the studied wavelength since the energy gap in such crystals is smaller than the photon energy of the used laser radiation.It was also found that the value of third order nonlinear optical susceptibility χ<3> for Cd0.70Mg0.30Se is higher than for Zn0.67Mg0.33Se. This behaviour can be understood if one take into consideration that the free carrier concentration in Cd1−xMgxSe samples is about four orders of magnitude higher than that in Zn1txMgxSe ones with comparable Mg content respectively. It is commonly known that when the electric conductivity increases, the values of nonlinear optical properties increase.From the performed measurements one can conclude that the incorporation of Mg as constituent into ZnSe and CdSe crystals leads to a change of the third order nonlinear optical susceptibilities.

High pressure x-ray diffraction and extended x-ray absorption fine structure studies on ternary alloy Zn1−xBexSe

The ternary semiconductor alloy Zn1−xBexSe has been studied under high pressure by x-ray diffraction and extended x-ray absorption fine structure (EXAFS) at the Zn and Se K-edge in order to determine the bulk and bond-specific elastic properties. Our measurements on samples with x=0.06–0.55 show pressure induced phase transformation from zinc blende to NaCl. The phase transformation pressure increases linearly with x. Murnaghan equation of state fitting to the data yields the unit cell volume at ambient pressure and bulk modulus, both of which follow the Vegard’s law. Nearest neighbor bond distances derived from EXAFS do not show sharp phase transition except for x=0.06. Bond modulus derived for the Zn–Se bonds shows them to be apparently stiffer than the bulk alloy, which is nontrivial. This tendency increases with increasing x and a strong positive bowing from the Vegard’s law is observed. We attribute the observed anomalies to the contrastingly different properties of the two components ZnSe and BeSe.

Lattice relaxation in the highly-contrasted Zn1−xBexSe alloy: An extended x-ray absorption fine structure study

We have performed extended x-ray absorption fine structure (EXAFS) measurements at the Zn and Se K edges of (Zn,Be)Se alloy. This alloy exhibits an unusually large contrast in the physical properties (bond stiffness and bond lengths) of its constituting bonds, leading to a uniquely well-resolved 1-bond→2-mode behavior in the already reported Raman spectra of this material system. The nearest and next nearest neighbor distances of Zn and Se atoms, the pseudo Debye–Waller factors (σ2) for these distances, as well as various bond angles, are determined as a function of the alloy composition after fitting the EXAFS equations. The lattice relaxation is discussed in terms of the distorted tetrahedral structure of the ternaries. We conclude that the accommodation of the local strain due to the difference in Zn–Se and Be–Se bond lengths is predominantly achieved by a displacement of the Se sublattice toward Be, the (Be,Zn)-mixed cation sublattice remaining quasirigid. Besides, we find that the 1-bond→2-mode behav...

Near-forward Raman scattering by bulk and surface phonon-polaritons in the model percolation-type ZnBeSe alloy

We study the bulk and surface phonon-polaritons of the Zn0.67Be0.33Se zincblende alloy by near-forward Raman scattering. The short (Be-Se) bond exhibits a distinct percolation doublet in the conventional backscattering Raman spectra, corresponding to a three-mode behavior in total [1×(Zn-Se), 2×(Be-Se)] for Zn0.67Be0.33Se. This offers an opportunity to achieve a refined understanding of the phonon-polariton modes of a zincblende alloy beyond the current two-mode approximation, corresponding to a [1×(Zn-Se), 1×(Be-Se)] description in the present case. The discussion is supported by contour modeling of the Raman signals of the multi-mode bulk and surface phonon-polaritons within the formalism of the linear dielectric response.

Growth and characterisation of bulk Zn1−xBexSe, Zn1−x−yMgxBeySe and Zn1−xBexTe crystals

Abstract In this paper we report an extension of our earlier results for Zn 1− x Mg x Se and low-Be Zn 1− x Be x Se to related ternary (high-Be Zn 1− x Be x Se and Zn 1− x Be x Te) and quaternary (Zn 1− x − y Mg x Be y Se) alloys. A dependence of defect structure on crystal composition, growth and thermal treatment is observed. X-ray diffraction was used for phase analysis and determination of lattice parameters. High-resolution transmission electron microscopy was used to compare kind and density of defect. Among the studied crystals, the lowest defect density was found for Zn 1− x − y Mg x Be y Se.

Formation of 4H and 8H polytypes in bulk Zn1−xMgxSe crystals

Abstract X-ray diffraction, photoluminescence and high-resolution transmission electron microscopy (HRTEM) were used to show that several as-grown Zn 1− x Mg x Se crystals prepared by the high-pressure Bridgman method exhibit polytypism for x in the range from 0.17 to 0.21. The largest contribution of polytypes is found in two unannealed crystal plates characterised by x =0.17 (polytype 8H) and 0.18 (polytype 4H). Crystal domains with disordered stacking were also found. A small quantity of 4H or 8H is detected in other sphalerite type crystals of various x values lower than 0.17 and for wurtzite type crystals for x from 0.18 up to about 0.25. Exciton transition energy for the given Mg content depends on the polytype. With Mg content increasing along the crystal, the polytype sequence observed at diffractograms (in agreement with HRTEM observations and with luminescence spectra) is 3C→8H→4H→2H. The presence of higher polytypes (4H and 8H) depends on thermal history of the crystal growth. Annealing of the as-grown crystals containing these two polytypes results in full or almost full transformation to 3C, 2H or their mixture, depending on the Mg concentration..

Near-forward Raman study of a phonon-polariton reinforcement regime in the Zn(Se,S) alloy

We investigate by near-forward Raman scattering a presumed reinforcement of the (A-C,B-C)-mixed phonon-polariton of a A1-xBxC zincblende alloy when entering its longitudinal optical-like (LO-like) regime near the Brillouin zone centre Γ, as predicted within the formalism of the linear dielectric response. A choice system to address such issue is ZnSe0.68S0.32 due to the moderate dispersion of its refractive index in the visible range, a sine qua non condition to bring the phonon-polariton insight near Γ. The LO-like reinforcement regime is actually accessed by using the 633.0 nm laser excitation, testified by the strong emergence of the (Zn-Se,Zn-S)-mixed phonon-polariton at ultimately small scattering angles.