V. Muñoz

SUBSTRATE EFFECT ON CDTE LAYERS GROWN BY METALORGANIC VAPOR PHASE EPITAXY

CdTe layers were grown by metalorganic vapor phase epitaxy (MOVPE) on different substrates like sapphire, GaAs, and CdTe wafers. The growth was carried out at the temperature 340 °C and time in the range of 2–4 h using dimethyl-cadmium and diisopropil-tellurium as precursors. The layers were studied by scanning electron microscopy, Rutherford backscattering spectroscopy, and high resolution low-temperature photoluminescence spectroscopy. The surface morphology and RBS and PL spectra of CdTe MOVPE layers are reported and the substrate effect on the layer properties is demonstrated.

CdTe crystal growth process by the Bridgman method: numerical simulation

Abstract Numerical simulation of the CdTe crystal growth process by the Bridgman method is made by using the commercial computational code FLUENT for the mathematical solution of the governing equations. To reduce computational effort, we have made use of a two level strategy. In the first level we have considered the whole system formed by the ampoule with the liquid–solid charge, the furnace, and the air between them. The heat transfer is assumed to occur by conduction, convection and radiation between the furnace and the ampoule, and only by conduction through the ampoule wall and the solid and liquid CdTe. In the second level we focus on the ampoule and its content, using the values of the temperature field that were calculated in the first level as thermal boundary conditions at the ampoule wall. Heat transfer through CdTe is described at this level by conduction and convection. The phase change has been modeled using an enthalpy-porosity formulation. The calculation of the growth rate at the beginning of the growth shows that this growth rate can be very different from the ampoule translation rate, depending on geometrical and thermal conditions. We analyse the growth rate, axial temperature gradient, concavity of liquid/solid interface and convective flow field in the melt corresponding to the use of ampoules with two different tip geometries (conical and flat) and three graphite cover thicknesses.

Growth and characterisation of MnTe crystals

We report on the low temperature growth of MnTe crystals by means of travelling solution methods. Two different processes are considered; a classical THM process using a low temperature presynthesised MnTe ingot, and a modified THM process, in which an increasing length of solvent zone collects the tellurium that was added to the stoichiometric charge to decrease the reaction temperature. Ingots from the two methods are analysed by means of scanning electron microscopy, X-ray diffractometry, resistivity, susceptibility and optical absorption measurements.

Neutron irradiation defects in gallium sulfide: Optical absorption measurements

Gallium sulfide single crystals have been irradiated with different thermal neutron doses. Defects introduced by neutron irradiation turn out to be optically active, giving rise to absorption bands with energies ranging from 1.2 to 3.2 eV. Bands lying in the band-gap exhibit Gaussian shape. Their energies and widths are independent of the irradiation dose, but their intensities are proportional to it. Thermal annealing is completed in two stages, ending at around 500 and 720 K, respectively. Centers responsible for the absorption bands are proposed to be gallium-vacancy-galliuminterstitial complexes in which the distance between the vacancy (acceptor) and the interstitial (donor) determines the energy and intensity of the absorption band, as well as the annealing temperature.

Heat transfer simulation in a vertical Bridgman CdTe growth configuration

Modelling and numerical simulation of crystal growth processes have been shown to be powerful tools in order to understand the physical effects of different parameters on the growth conditions. In this study a finite difference/control volume technique for the study of heat transfer has been employed. This model takes into account the whole system: furnace temperature profile, air gap between furnace walls and ampoule, ampoule geometry, crucible coating if any, solid and liquid CdTe thermal properties, conduction, convection and radiation of heat and phase change. We have used the commercial code FLUENT for the numerical resolution that can be running on a personal computer. Results show that the temperature field is very sensitive to the charge and ampoule peculiarities. As a consequence, significant differences between the velocity of the ampoule and that of the isotherm determining the solid/liquid interface have been found at the onset of the growth.

Some fundamentals of the vapor and solution growth of ZnSe and ZnO

Abstract Some fundamentals of ZnSe and ZnO vapor and solution growth are investigated. Residual water present in gases or gaseous mixtures such as H 2 , Ar or H 2 +H 2 O is shown to act as a sublimation activator in the vapor-phase transport of both compounds. The processes involved in the growth by chemically activated sublimation with such gases and gas mixtures have been studied by close-spaced vapor transport (CSVT). The ZnSe growth rate is found to be constant, while in the ZnO case a high initial growth rate is followed by slower growth subsequently. Using a theoretical model, the thermodynamic constants of the transport – energies of activation, sublimation and condensation and enthalpy of formation – are determined independently from a fit to the variation of the growth rate as a function of the substrate temperature. No classical chemical transport reactions are shown to correspond to the sublimation mechanism, that is supposed to be chemically assisted by residual water. In the case of ZnO, the sublimation mechanism is found to be more complex than for ZnSe, as expressed by the variation of the growth rate with time. As an alternative to this chemically assisted sublimation, chemical vapor transport using chlorine as chemical agent is proposed for ZnO. Significant transport is found to occur at temperatures of about 1000°C. Concerning solution-growth, the pseudo-binary PbCl 2 –ZnO phase diagram established by differential thermal analysis (DTA) shows PbCl 2 to be a good solvent of ZnO, as for ZnSe. Zn–In alloys are proposed as good alternative solvents for ZnO not having the great reactivity of the PbCl 2 –ZnO mixture with silica.

Scanning electron microscopy characterization of ZnSe single crystals grown by solid-phase recrystallization

ZnSe single crystals were grown from n-type microcrystalline boules by a Solid Phase Recrystallization (SPR) method. The recrystallizations were performed under different atmospheres, Ar or Se, and pressures to investigate the influence of growth conditions on the structural features of the resulting crystals. The samples were mechanically and mechano-chemically polished in a bromine methanol solution and, then, etched in HCl for a short time, before characterization. The homogeneity and the nature of defects in the crystals were studied by Cathodoluminescence (CL) in the scanning electron microscope (SEM). CL measurements show the existence of slip bands in the recrystallized samples, likewise CL spectra show that on these samples the dislocation related Y band is enhanced and the emission bands appearing in the 2.0-2.5 eV region depend on the annealing conditions. In addition during SPR, twinned regions appear with different electronic recombination properties.

Low-pressure synthesis and Bridgman growth of Hg1−xMnxTe

To reduce Hg high pressure related to the high-temperature synthesis reaction between the components in elemental form, Hg 1-x Mn x Te bulk crystals were produced by a two-step procedure including (I) the alloy synthesis using HgTe crystals grown by the cold travelling heater method and elemental Mn and Te to complete the desired composition, followed by (II) the Bridgman growth. The growth was carried out at temperature in the range of 700-850°C and rate of 1 mm/h. The Hg 1-x Mn x Te crystals have been characterised by X-ray diffractometry, energy dispersive X-ray analysis, Fourier transformed infrared spectroscopy and Hall effect techniques. Although the distribution coefficient of Mn was found to be in the range of 3-3.5 in the first-to-freeze region, the crystals show a good axial composition uniformity throughout more than a half of the total length. The lattice parameter of Hg 1-x Mn x Te is reported for the studied composition range x = 0-0.15.

Cathodoluminescence microscopy and photoluminescence of defects in ZnTe

ZnTe single crystals grown by the cold travelling heater method have been investigated by means of photo- and cathodoluminescence. The spectral region covered in this work ranges from 2.48 eV (500 nm) which corresponds to band-edge emission to 0.62 eV (2000 nm). Visible and infrared cathodoluminescence images have been recorded, and the influence of extended defects on the observed luminescence has been studied.

Precursor effects of the Rhombohedral-to-Cubic Phase Transition in Indium Selenide

We report on the observation of precursor effects of the rhombohedral-to-cubic phase transition in Indium Selenide (InSe) with several experimental techniques. The pressure at which these precursor defects are first observed depends on the sensitivity of the experimental technique. In transport measurements, which are very sensitive to low defect concentrations, precursor effects are observed 5 to 6 GPa below the phase transition pressure whereas in X-ray diffraction measurements precursor effects are only observed 2 GPa below the phase transition pressure. We report optical absorption measurements, in which the precursor effects are shown by the growth and propagation of dark linear defects appearing 3 GPa below the phase transition pressure. On the base of a simple model of the stress field around edge dislocations, we attribute the darkening of the InSe samples to local phase transitions to a high-pressure modification along linear dislocations. These results agree with room-pressure and high-pressure Raman spectra of samples compressed up to 7-8 GPa, which show new phonon lines not corresponding to the low-pressure phase.