Vapor Growth Technique Research Articles

Reduced optical losses in MOCVD grown lithium niobate thin films on sapphire by controlling nucleation density

Lithium niobate (LiNbO 3) epitaxial thin films of high crystalline quality have been grown on c-plane sapphire substrates by the solid-source MOCVD method. For waveguiding applications, low optical propagation losses (less than 2 dB/cm) are required, and this has been difficult to achieve in films grown by various vapor growth techniques. Since scattering from rough film surfaces is a major source of optical attenuation, a knowledge of the mechanisms involved in film roughening and their relationship with the initial stage of film formation is essential. In this study, it was observed that roughening and optical losses in lithium niobate films grown on sapphire substrates increased with grain size and the depth of grain grooves formed between them. Increasing the nucleation density during the initial stage of growth can inhibit grain growth, and thereby reduce surface roughness even at high growth temperatures. Based on these results, a two-step growth process was developed in which the nucleation and growth stages were controlled separately. This process involves (1) creating a high nucleation density by using either a low deposition temperature or high source partial pressure during the early stages of film growth, and (2) enhancing crystallinity by growing the balance of the film at a higher temperature or lower source partial pressure. Using this refined growth process, rms roughness was decreased to less than 2 nm, while the crystalline quality remained equal to the best films grown by the conventional one step growth method. Optical losses were reduced to below 1.8 dB/cm for the TE o mode at a wavelength of 632.8 nm.

Optimization on the crystal growth of fullerenes

Inspecting critically the former vapour growth technique of C 60 single crystals from the viewpoint of crystal growth dynamics, we conclude that the conditions should be improved. Both the temperature difference distribution throughout the growth process and the temperature distribution in the furnace are carefully selected. With this optimization, large C 60 single crystals of good quality with size more than 6 mm can be grown reproducibly. The Laue's analysis of the crystal show that the exposed surfaces of C 60 solids are of two types of crystalline faces, namely 1 1 1} and 1 0 0}.

Structural non-uniformities in mercuric iodide crystals grown by two different vapour growth techniques

The origin of extended structural defects and their distribution in vapour grown mercuric iodide crystals has been investigated. Starting from a highly purified and nearly stoichiometric source material, crystals were grown in different laboratories using different implementations of the vapour growth technique. The as-grown crystals were then investigated by means of γ-ray rocking curves (3D mapping) and synchrotron X-ray transmission topography. It has been found that not only the amount and distribution of mosaicity (tilt, twist), but also striations and grain boundaries depend primarily on the performance of the temperature control loops and on the 3D temperature fields during the growth in a particular apparatus. High radial or axial temperature gradients lead to the enhanced formation of grain boundaries and striations. Different growth mechanisms could be deduced from the different striation patterns observed.

Defects in CdTe single crystals grown by very fast vapor growth technique

Single crystals of CdTe were obtained by the very fast vapor growth technique. The four major defects, namely, multi-grains, lamellar, lateral, and micro-twins, which are fatal to the single crystallinity, were eliminated or limited by increasing the growth stability. This investigation indicates that the latent heat under high growth rate conditions should not be neglected. A model is developed to explain the effects of latent heat on the growth stability at the interface. A relationship between crystal morphologies and growth conditions was established. It strongly suggests that the above defects are growth stability related. The origin of twinning, dominated by growth stability, is discussed in this paper.

High J/sub c/ YBCO thick films prepared by LPE method

High-J/sub c/ YBa/sub 2/Cu/sub 3/O/sub y/ thick films were prepared by the liquid phase epitaxy (LPE) method. The growth rate was typically about 2 /spl mu/m/min, which was 10-10/sup 2/ times larger than that by conventional vapor growth techniques. The T/sub c/ of the best film exceeded 88 K after oxygen annealing, and the transport zero-field J/sub c/ was over 10/sup 5/ A/cm/sup 2/ at 77 K. The peak effect in J/sub c/-B characteristics was observed at several tesla by magnetic and transport measurements. Based on the micro-structure, the relevant peak effect is considered to be caused by stacking faults which act as a field induced pinning center.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Critical currents of YBa2Cu3Oy thick films prepared by liquid phase epitaxial growth

We have succeeded in preparing c-axis oriented high-Jc YBa2Cu3Oy films on MgO(100) substrate by the liquid phase epitaxy (LPE) technique. The growth rate was typically about 2 μm/min, which was 10–102 times larger than that by ordinary vapor growth techniques. The film thickness ranged 10–50 μm by choosing the dipping time. The Tc of the best film exceeded 88 K after oxygen annealing, and the transport Jc was 1.1×105 A/cm2 at 77 K and 0 T. In-field Jc’s at 77 K and 1.5 T were 2.8×104 A/cm2 and 2.0×104 A/cm2 for the B⊥ab plane and B∥ab plane, respectively. In addition, the peak effect of Jc was observed at several tesla for B∥ab plane geometry. Based on the microstructure observed by high resolution transmission electron microscopy, the relevant peak effect is considered to be caused by stacking faults which act as a field induced pinning center.

Fermi-liquid-type spectral function and angle resolved photoelectron spectra of the Ti-3d z2-band of TiTe 2

Angle and temperature dependent photoelectron spectra with high energy and momentum resolution (Δ E = 300 meV, Δθ< ±0.5°) were taken near the Fermi energy ( E b < 1 eV) of the quasi-two-dimensional, metallic layer compound TiTe 2 along the direction ΓM . The aim was to test the range of the validity of the Fermi liquid model using a Taylor expansion for the self-energy used by Claessen et al. [1] for a system whose low energy properties are those of a normal metal. Single crystalline samples have been prepared by vapor growth technique using different amounts of Te in order to influence the intercalation of surplus Ti. We found a dependence of the dispersion of the Ti-3d z 2 -derived band on the stoichiometry of the samples. For an exact line shape analysis the spectra were fitted by a least-squares-method to a Fermi-liquid-type spectral function taking into account the crucial effects due to the apparatus, as there are the energy resolution and finite angle resolution which leads to a limited integration in k-space. In the region where the Ti-3d z 2 -derived band crosses the Fermi level we found excellent agreement between the experimental line shape and the Fermi liquid model. On the other hand there are distinct deviations between the theoretical and the experimental line shape at the Brillouin zone boundary even for a self-energy proposed by Matho [2] without Taylor expansion.

Fast growth of Cd1−xZnxTe single crystals by physical vapor transport

Good quality single crystals of Cd1−xZnxTe (x=0.04, 0.08, and 0.115) were obtained by the very fast vapor growth technique developed in this laboratory. The abundance of lamellar twins seems to be less than that in CdTe grown under the same conditions. Chemical etching results indicate a similar quality of the vapor grown crystals to those obtained from melt growth. The overall compositions of the crystals are uniform and approach those of the initial source material with increasing amount transported.

Progress in crystal growth and purification of α-HgI2, a highly anisotropic detector crystal

Abstract The recent improvement in the purification of the starting material and in the vapour growth technique as well as the progress in understanding the kinetics of the vapour growth resulted in the achievement of large (0.5 kg) inclusion-free mercuric iodide crystals in our laboratory. The highlights of the investigations over the last three years are summarized.

Fundamental aspects of vapor growth and epitaxy

Vapor phase growth processes have become standard for many applications. This paper will deal specifically with the vapor phase growth (mainly epitaxial) of III/V semiconductor materials. This process has increased in importance in direct proportion to the need for elaborately structured materials such as heterojunctions, quantum wells, and superlattices for the fabrication of advanced electronic and photonic devices. As the vapor growth techniques become more advanced, and more stringent requirements on uniformity and reproductivity are imposed by the move to the production environment, it has become evident that only the techniques giving sufficient control of growth will be useful. This demand for increased control can be met only if we thoroughly understand the fundamental aspects of the growth processes. Thus, the emphasis of this paper will be a review of our current understanding of the fundamental aspects of vapor phase growth. Emphasis is on the design and selection of precursor materials and the thermodynamics and the kinetics of growth. The latter topic includes both the reaction mechanisms and rates of the pyrolysis of the precursor molecules as well as the physical surface processes, including the attachment of atoms at steps on the surface.

Selective vapor-phase deposition on patterned substrates

Abstract Vapor growth techniques such as evaporation, sputtering, molecular beam epitaxy (MBE), and chemical vapor deposition (CVD) are usually used for large-area thin film deposition. However, local, or so-called selective, deposition onto desired regions of patterned substrates can be attained. Principally, there exist several categories of selective growth systems. In area-selective growth, the deposition takes place only on one substrate material, and no deposition is obtained on the other. This selectivity is based on a difference in the interfacial reactions between the different substrate materials and the vapor. The interfacial reactions on one substrate material should be inhibited completely to avoid nucleation, whereas the deposition reactions should be stimulated on those substrate areas where the depositon will occur. In phase-selective deposition, different phases are deposited simultaneously and selectively on the different substrate materials. In analogy to the phase-selective deposition,...

Structural, electrical and optical properties of highly conductive FeCl3-intercalated graphitised poly(p-phenylene vinylene) films prepared by the solution method

A stage-3 FeCl3 graphite intercalation compound has been successfully synthesised by the solution method from graphitised poly(p-phenylene vinylene) film. Graphitised film intercalated by the solution method is a highly conductive material with a metallic conductivity as high as 2*105 S cm-1, and this is higher than the values for FeCl3-intercalated films prepared by vapour-growth technique. Metallic behaviour of the temperature dependence of the electrical conductivity and of the optical reflectance have been observed.

The growth of CdS and Cd1−xZnxS

CdS and Cd1−xZnxS single crystals have been grown by a modified vapor growth technique. They have high transmission and high resistivity (&amp;gt;1010 Ω cm). The results of transmission measurements are reported.

Electrical properties of CdTe crystals grown by vuvg from nonstoichiometric charges

High purity CdTe crystals were grown by the vertical unseeded vapor growth technique. The growth charge composition varied between CdTe +0.1 mol% Cd and CdTe + 0.1 mol% Te. At a deviation from stoichiometry of δTe > 0.05 mol% in the charge p-type crystals with an electrical resistivity of 106 Ωcm were obtained, n-type crystals were obtained with charge compositions of δTe < 0.02 mol%, with the resistivity ρ increasing with δTe from 103 to < 109 Ωcm. Maximum resistivity ∿2.109 Ωcm was found when δTe ∿0.02 mol%. In all cases, the crystal composition shows a higher Cd concentration of ∿0.02 mol% relative to the charge, suggesting noncongruent sublimation of CdTe.

Vapour Growth of Nb3Se4 Crystals

Single crystals of Nb3Se4 were grown by vapour growth technique with chemical transport in the temperature region of about 1000°C, with iodine transporter. A batch of tiny needle-like or fibre crystals about 5 mm in length was found to grow after three to four weeks. Some of the crystals grew to a size of 0.3×0.3×5 mm3 and had a pseudo-hexagonal crystal habit. Observation by optical microscope revealed fine step structures on the surfaces of the crystals, suggesting that the crystals had grown by the surface diffusion process characteristic of the vapour transport growth.

Vapor transport of nonstoichiometric CdTe in closed ampoules

CdTe crystals were grown by the vertial unseeded vapor growth technique. Charge compositions varied in the range of CdTe1±§, where 0≤§<0.001. Mass transport was found to be strongly dependent on §, the behavior being different for §≤-0.01 mol%, 0<§≤0.04 mol% or §≥0.05 mol%. Partial pressures of Te2 and Cd were calculated for the growth conditions. Correlation between growth rate measurements and calculated vapor pressures suggests a noncongruent sublimation of CdTe.

Cadmium Selenide: A Promising Novel Room Temperature Radiation Detector

Large single crystals of CdSe weighing about 30g were grown by the vertical unseeded vapor growth technique at a linear growth rate of 5mm/day and a temperature gradient of 10°C/cm. Crystal perfection and homogeneity were evaluated by Laue X-ray diffraction, etch pit density, SEM and microprobe analysis methods. The dark resistivity of the as-grown and the heat treated crystal was about 1?cm and 1012?cm respectively. Slices were used to fabricate room temperature detectors for nuclear radiation energy. The detectors showed high efficiency and stability as a function of time for radiation sources from 1OKeV to 66OKeV.

Vertical unseeded vapor growth of large CdTe crystals

Single crystals of CdTe were grown by the vertical, unseeded vapor growth technique. The faceted, high quality crystals showed higher purity than the ∼6N source materials. The growth rate was found to be strongly dependent on the excess Te present in the growth charge. A maximum growth rate of about 2 g/day was observed at about 0.001–0.02 mol% excess Te at charge preparation. X-ray analysis and metallography of the crystals and mechano-chemical polished slices revealed high quality monocrystallinity. The crystals showed high resistivity (⩾ 10 8 ω cm) and did have nuclear radiation detection properties.

58. Neutron scattering experiments on well-staged graphite-FeCl 3

This paper reports results of a neutron diffraction study on both the magnetic and the lattice dynamic structure of graphite-FeCl/sub 3/. The FeCl/sub 3/ system was selected for this study because of the interesting magnetic properties previously reported for this system. Of particular interest is the low temperature magnetic phase transition, which has been shown to be stage dependent. Special emphasis was given to the study of the low temperature magnetic state of the Fe/sup 3 +/ spins and to the longitudinal phonon modes for k/sub z/ axis acoustic branches. All experiments reported here were carried out on a stage 2 sample of graphite-FeCl/sub 3/. The sample was prepared by a two-zone vapor growth technique and was characterized by (00l) x-ray and neutron diffraction patterns to be > 95% stage 2, with only minor inclusions of other stages.

Vertical unseeded vapor growth of large α-GeTe crystals

Well faceted, large, single crystals of α-GeTe were grown by the vertical, unseeded vapor growth technique. The growth rate was 2 mm/day. X-rays analysis and metallography of the crystal as well as mechano-chemical polished slices revealed good monocrystallinity and homogeneity. Ge crystallites separated out of the GeTe crystallizing material due to the noncongruent vaporization of GeTe.