Horizontal Gradient Freeze Method Research Articles

SnS crystal grown using horizontal gradient freeze method and its electrical properties

Tin sulfide (SnS) single crystal was successfully grown using the horizontal gradient freeze method. Characterization of the grown crystal revealed a single SnS phase and an orthorhombic structure with lattice parameters a=4.327, b=11.18, and c=3.981Å. Composition analysis revealed a stoichiometric composition equivalent to a congruent melt composition. The grown SnS crystal was confirmed to be single crystal by the X-ray Laue pattern. The melting point of the SnS was measured by differential thermal analysis and found to be 878°C. The grown crystal had p-type conductivity, a carrier concentration of 8×1017cm−3, electrical resistivity of 0.08Ωcm, and hole mobility perpendicular to the (010) direction of 98cm2/Vs at room temperature.

Optical and EPR study of point defects in CdSiP 2 crystals

Optical absorption and electron paramagnetic resonance (EPR) are used to identify and characterize point defects in large single crystals of cadmium silicon phosphide (CdSiP 2) grown by the horizontal gradient freeze method. The dominant native defects in these crystals are cadmium vacancies and phosphorus vacancies, and the primary unintentional impurities are Fe +, Fe 2+, Fe 4+, and Mn 2+ ions. Optical absorption in the region between 1.5 and 2.5 μm was studied at temperatures from 10 to 300 K. At lower temperatures, zero-phonon lines and phonon sidebands observed near 1.87 μm are assigned to Fe 2+ ions (similar features observed at 10 K near 1.08 μm are assigned to Fe 4+ ions). At higher temperatures, an unidentified absorption band has a peak near 1.95 μm and a vibronically broadened absorption band due to the Fe 2+ ions has a peak near 1.75 μm. EPR lines due to Fe + and Mn 2+ ions appear near g=2. Illumination at 5 K with 632.8 nm laser light produces changes in the intensity of the Fe + EPR spectrum and introduces a three-line EPR spectrum caused by hyperfine interactions with two phosphorus nuclei. This latter spectrum is assigned to singly ionized cadmium vacancies ( V cd - ). Optical and EPR results from two CdSiP 2 samples exhibiting different defect behaviors are compared.

Improving optical transparency in CdGeAs 2 crystals by controlling crystalline defects

In this lecture, some interesting and important new results on the relationship between crystalline defects and optical absorption in CdGeAs 2 single crystals will be reviewed. Such crystals are very promising for infrared second harmonic generation, but their use has been limited by non-ideal transparency near 5.5 μm. Absorption and other crystal properties were found to vary from one laboratory to another, from boule to boule and also within each boule (the latter showing both axial and radial variations). Previous absorption studies were hindered by the lack of high-quality single crystals. Use of the horizontal gradient freeze method together with a low thermal gradient now makes it possible to prepare better quality material and perform more systematic property studies. The role played by point defects (arising from non-stoichiometry and impurities), dislocations, faceting and growth orientation on IR optical absorption will be discussed.

Correlation between dislocation etch pits and optical absorption in CdGeAs 2

While CdGeAs 2 single crystals are promising for infrared second harmonic generation, it has been difficult to reproducibly grow low loss optical material, particularly in the 5 μm region. In addition, a correlation between processing conditions and absorption has eluded various researchers studying this problem. This paper describes a series of etching experiments which were undertaken to see if there might be a relationship between dislocation density and optical absorption. In this study single crystals were grown from stoichiometric melts along the common [0 0 1] direction by the horizontal gradient freeze method. Etch pit densities (EPD) of >10 6 cm −2 were observed at the edges of the boules, while almost no etch pits were found at the center. Infrared transmission images indicated that the higher EPD regions had lower optical absorption. It was also found that the absorption coefficients ( α) varied from 2.1 to 18.2 cm −1 along the boule axis. Moreover, the etch pits, which formed near the edge of the crystal adjacent to the crucible walls, were created by slip along the {1 1 2} planes, which make up the growth interface. These results suggested that in order to obtain a more uniform absorption within each boule, dislocation formation should be minimized. Our first approach involved changing the orientation of the growing crystal to the [1 1 2] direction. The results of these experiments were very encouraging, showing that a considerably improved radial and axial absorption uniformity was achieved, as well as the lowest optical losses measured (minimum α=0.18 cm −1) compared with similar samples grown along the [0 0 1]. It can be concluded from this study, therefore, that the growth orientation plays an important role in the absorption behavior of CdGeAs 2.

Small polaron of β-FeSi 2 obtained from optical measurements

Beta-iron disilicide β-FeSi 2 polycrystals were synthesized by the horizontal gradient freeze method, and β-FeSi 2 single crystals were grown by the chemical vapor transport method using polycrystals as the starting material and iodine as a transport agent. The β-FeSi 2 polycrystals and single crystals were characterized by optical measurements. The Raman and infrared active optical phonons were observed by Raman scattering and optical reflection measurements, respectively. The dielectric constants were calculated by Kramers–Kronig transformation of the optical reflection spectrum. The small polaron reflecting LO phonon of β-FeSi 2 was discussed by Fröhlich's model using the obtained dielectric constants.

Synthesis and Properties of Semiconducting Iron Disilicide β-FeSi2

Beta-iron disilicide β-FeSi2 bulk crystals were prepared by a horizontal gradient freeze method. The grown β-FeSi2 bulk crystals were characterized by structural, electrical and optical measurements. The β-FeSi2 single phase was determined by X-ray diffraction measurements. The electrical resistivity of β-FeSi2 increased exponentially with decreasing temperature. From optical absorption spectra that were obtained from transmittance measurements, the energy band gap was determined to be 0.85 eV. The refractive index, extinction coefficient and dielectric constants were calculated using optical reflectance spectra by Kramers-Kronig analysis. In addition to β-FeSi2 bulk materials, β-FeSi2 films were prepared on Si(100) substrates by a laser ablation method using the above β-FeSi2 bulk crystals as target materials. Strong Raman signals from the β-FeSi2 films were observed at 171 cm-1, 190 cm-1, 199 cm-1 and 247 cm-1, indicating deposition of well-oriented high-quality films. The optical absorption coefficient at 1.0 eV and energy band gap were determined to be 1.99× 105 cm-1 and 0.85 eV, respectively.

Structural and Optical Properties of β-FeSi2/Si(100) Prepared by Laser Ablation Method

Abstractβ-FeSi2 is a promising material for the application of various electronic, optoelectronic and energy devices. We present here the semiconducting properties of β-FeSi2 films on Si(100) substrate prepared by laser ablation method. Samples were grown using poly-crystalline bulk β-FeSi2 prepared by horizontal gradient freeze method. For the monitoring of growth, in-situ observation of ablation plume was made through fluorescence spectroscopy. Reflection of highenergy electron beam diffraction (RHEED) was also made in-situ to see the surface morphology. Characterization of the films by X-ray diffraction presented purely P3(220) orientation. Raman scattering measurements at room temperature also indicated that the grown films are semiconducting β-FeSi2. Optical absorption spectra at room temperature showed absorption coefficient higher than 105 cm−1 above the band-gap (˜1.2 eV). It was revealed that high quality semiconducting β-FeSi2 films can be fabricated by laser ablation method without post-annealing.

High quality Si-InP bulk crystal growth by horizontal gradient freeze method under controlled phosphorus vapor pressure

High purity InP was grown by horizontal gradient freeze (HGF) method under controlled phosphorus vapor pressure. The ingot was free from residual In and In inclusion. The carrier concentration of undoped InP ingot was almost in the range of (1.1–4.0)×10 15 cm -3 at 77 K even though a quartz boat was used. Using this technique, Si-InP wafers with a high resistivity over 1.0×10 7 Ω cm were obtained by lower Fe doping (4.0×10 15 cm -3). The growth of single crystals using the seeding technique was also tried by direct synthesis. The strict control of the temperature gradient with the CPU system enabled the seeding to be performed reproducibly. A single crystal, 6 cm long, was obtained from the seed end of the ingot, but twin boundaries were generated from the middle of the ingot.

Synthesis and crystal growth of CdGeP2

A technique has been developed for growing single crystals of the chalcopyrite nonlinear optical material CdGeP2 that are of sufficient size (centimeter dimensions), electrical resistivity and optical quality to permit measuring the electrooptic coefficients. The horizontal gradient-freeze method is used for directional solidification of near- stoichiometric melts synthesized in a two-zone furnace by direct reaction of the elements sealed in fused-silica ampoules. The ingots (∼ 120 g) prepared in this manner are polycrystalline and therefore severely cracked due to anisotropic thermal contraction, but these ingots have yielded uncracked single crystals up to 6 cm3 in volume. Samples from these crystals have been used for the first measurements of the electrooptic coefficients of CdGeP2, as well as for the measurement of the thermal expansion coefficients and infrared transmission.

X‐Ray Diffraction Topographs of Imperfections in Gallium Arsenide by Anomalous Transmission of X‐Rays

Imperfections in single crystals of grown by the Czochralski and horizontal gradient freeze method have been observed by the anomalous transmission of x‐rays. Slip occurs equally on the three octahedral (111) planes in the Czochralski pulled ingot. Radial slip predominates in crystals grown by the gradient freeze technique. The variation in the distribution of dislocations in crystals grown by both techniques is observed and compared. Large inclusions of tin and tin arsenide have been detected in heavily doped . The formation of diffusion masks on single crystal causes large strains when the sample is heat‐treated under diffusion conditions.