Temperature Gradient Solution Research Articles

Luminescence and optical properties of Fe2+:ZnTe crystal grown by temperature gradient solution method

Iron-doped ZnTe (Fe2+:ZnTe) crystal can be used as promising gain media for mid-infrared laser. However, there are few studies in the optical properties of Fe2+:ZnTe crystal from mid-infrared to ultraviolet. In this work, large size Fe2+:ZnTe bulk crystal (the concentration of Fe ions is about 1019 cm−3) with a dimension of Φ30 mm × 50 mm was grown by the temperature gradient solution growth (TGSG) method. The luminescence and optical properties of Fe2+:ZnTe from terahertz wave to ultraviolet wave have been studied. The evaluated refractive index of Fe2+:ZnTe crystal is approximately 2.5–3.0 at the frequency range of 0.5–2.0 THz range. And its band gap is estimated to be about 2.24 eV. It is noted that strong absorptions at the wavelength of 2.8–4.7 μm and 705 nm were observed. Based on the experimental results, the energetic positions of Fe2+ center such as 3T1, 5T2, 5E in ZnTe crystal were proposed.

Optical and electrical properties of vanadium-doped ZnTe crystals grown by the temperature gradient solution method

Vanadium-doped ZnTe (ZnTe:V) crystals 30 mm in diameter and 45 mm in length were grown by the temperature gradient solution growth method. The band gap of as-grown ZnTe:V crystals was estimated to be about 2.22 eV. Infrared spectra exhibit a mean transmittance of 50%-60% in the wavenumber range from 500 cm−1 to 4000 cm−1. Compared with the intrinsic ZnTe crystal, the resistivity of ZnTe:V is increased 6-7 orders of magnitude up to 109 Ω·cm and the carrier concentration reduced from 1014 to 108 cm−3. Accordingly, the THz detection sensitivity is also enhanced by 20%-30%. The improvements on the optical and electrical properties were attributed to the compensation of Zn vacancies by the vanadium element.

The Effects of ACRT on the Growth of ZnTe Crystal by the Temperature Gradient Solution Growth Technique

A finite element method is used here to simulate the temperature field, the thermosolutal convection, the solute segregation, and the non-isothermal phase transformation during the growth of ZnTe crystal via the temperature gradient solution growth technique (TGSG) with an accelerated crucible rotation technique (ACRT). Three different trapezoid-wave ACRT sequences are proposed, and their effects are examined from the views of the constitutional supercooling and the mixing of the solution. The solution in front of the growth interface can be fully mixed only during the constant rotation stage of the ACRT when there is a clockwise Ekman flow. An inappropriate ACRT can produce excessively strong clockwise and counterclockwise Ekman flows, and this results in constitutional supercooling in front of the central part of the growth interface at the beginning of the stop stage and in front of the peripheral part at the end of the constant rotation stage. By adjusting the acceleration rate of the ampoule rotation, the appropriate Ekman flows can be obtained to well mix the solution and avoid the constitutional supercooling. An appropriate ACRT sequence is provided, which can facilitate the mixing of the solution, avoid constitutional supercooling, and improve the growth interface morphology.

Improvement of the THz response of Zn1−xMnxTe bulk crystals grown by a temperature gradient solution method

As a type of AII1−xMnxBVI alloy, Zn1−xMnxTe ingots with a diameter of 30 mm are grown as large single crystals by a temperature gradient solvent method under Te-rich conditions. The orange-red Zn1−xMnxTe crystals are cut and processed into size-appropriate wafers for fundamental studies, as well as for THz spectroscopy and magnetization response analyses. Mn segregation associated with the growth conditions is identified in the as-grown crystals, and an enriched higher concentration of Mn is observed inside the Te inclusion. To evaluate the effect of Mn on the Zn1−xMnxTe crystal, the site occupation is calculated via an ab initio study, and is further confirmed by electrical and optical property measurements. Mn tends to substitute Zn to form MnZn in Zn1−xMnxTe, which results in a low density of free charge carriers, through which the THz detection sensitivity is enhanced by 15–25% compared to the intrinsic ZnTe. Moreover, evident paramagnetic magnetization behavior is observed at variable temperatures due to the random distribution of an isolated Mn (Mnioct,Te6) spin with S = 5/2. We note that further optimization of the THz performance can be achieved by optimizing the growth process and tailoring the Mn content.

The transport phenomena during the growth of ZnTe crystal by the temperature gradient solution growth technique

A numerical model is developed to simulate the temperature field, the thermosolutal convection, the solute segregation and the growth interface morphology during the growth of ZnTe crystal from Te rich solution by the temperature gradient solution growth (TGSG) technique. Effects of the temperature gradient on the transport phenomena, the growth interface morphology and the growth rate are examined. The influences of the latent heat and the thermal conductivity of ZnTe crystal on the transport phenomena and the growth interface are also discussed. We find that the mass transfer of ZnTe in the solution is very slow because of the low diffusion coefficient and the lack of mixing in the lower part of the solution. During the growth, dilute solution with high density and low growth temperature accumulates in the central region of the growth interface, making the growth interface change into two distinct parts. The inner part is very concave, while the outer part is relatively flat. Growth conditions in front of the two parts of the growth interface are different. The crystalline quality of the inner part of the ingot is predicted to be worse than that of the outer part. High temperature gradient can significantly increase the growth rate, and avoid the diffusion controlled growth to some extent.

Effect of Cr/In-doping on the crystalline quality of bulk ZnTe crystals grown from Te solution by temperature gradient solution growth (TGSG) method**Project supported by the National Basic Research Program of China (No. 2011CB610406), the National Natural Science Foundation of China (No. 51372205), the 111 Project of China (No. B08040), the Specialized Research Fund for the Doctoral Program of Higher Education

The properties of undoped, Cr-doped, and In-doped bulk ZnTe crystals grown by the TGSG method were compared. Cr/In-doping leads to a slight red-shift of the absorption edge. Cr-doping also creates two characteristic absorption bands, centered at about 1750 nm and beneath the fundamental absorption edge. However, the fundamental reflectance spectra are not sensitive to the dopants. The resistivity of undoped, Cr-doped, and In-doped ZnTe is about 102 Ω·cm, 103 Ω·cm, and 108 Ω·cm, respectively. Only In-doped ZnTe has an IR transmittance higher than 60% in the range of 500 to 4000 cm−1. However, the IR transmittance of Cr-doped ZnTe is very low and decreases greatly as the wavenumber increases, which is mainly attributed to the scattering effects caused by some defects generated by Cr-doping.

Characterization and chemical surface texturization of bulk ZnTe crystals grown by temperature gradient solution growth

Using tellurium as a solvent, we grew ZnTe ingots of 30 mm in diameter and 70 mm in length by a temperature gradient solution growth method. Hall tests conducted at 300 K indicated that the as-grown ZnTe exhibits p-type conductivity, with a carrier concentration of approximately 1014 cm−3, a mobility of approximately 300 cm2·V−1·s−1, and a resistivity of approximately 102 Ω·cm. A simple and effective method was proposed for chemical surface texturization of ZnTe using an HF:H2O2:H2O etchant. Textures with the sizes of approximately 1 µm were produced on {100}, {110}, and {111}Zn surfaces after etching. The etchant is also very promising in crystal characterization because of its strong anisotropic character and Te-phase selectivity.

Characterization of Cr-doped ZnTe Crystals Grown by Temperature Gradient Solution Growth (TGSG)

Cr-doped ZnTe ingot was grown by temperature gradient solution growth (TGSG), using CrTe and Te as dopant source and solvent, respectively. Single crystals in the head of the ingot have larger size of more than 10 mm× 10 mm and less Te inclusions. The size, shape and distribution of Te inclusions well reflect the thermal field distribu- tion of the ingot. Radial asymmetric thermal field of the ingot leads to radial asymmetric distribution of Te inclusions. The thermal migration of Te inclusions under thermal gradient results in their merging, enlargement and enlongation. Defects such as cracks and voids can be introduced into ZnTe matrix by Te inclusions. Some excess amount of CrTe is rejected by the advancing interface, which indicates that the present growth method has self purification effect. The re- sistivity of Cr-doped ZnTe ( about 1000 Ω·cm ) is slightly higher than that of ZnTe (about 300 Ω·cm). Cr-doped ZnTe exhibits an absorption centered at about 1750 nm, which means Cr 2+ ions are successfully incorporated into ZnTe. IR

Solid evidence for magnetic moment enhancement in micro‐particles of Mn11Si19

AbstractWe study magnetic properties of Mn11Si19 both in a bulk and in a powder (the averaged diameter of particles was about 5 µm) states. The bulk sample was grown using the temperature gradient solution method, and the powder sample was obtained through pounding the bulk sample in a mortar. We measured magnetisation as a function of temperature using a SQUID magnetometer.We found that the bulk sample was paramagnetic while the power sample was soft ferromagnetic. The magni‐ tude of the magnetisation of the powder sample was al‐ most unchanged between5K to 300 K. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Solution growth and optical characterization of Mn11Si19

AbstractWe have investigated the optical property of single‐phase Mn11Si19 bulk crystals grown by the temperature gradient solution growth (TGSG) method using Ga solvent. The optical absorption spectrum near the fundamental absorption edge was evaluated by transmission and reflectance measurements between 5 K and 300 K. Optical constants of refractive index n and extinction coefficient k were determined from normal reflectance spectrum by Kramers‐Kronig analysis. The value of n was 5‐6 near the band edge energy. From the relation of the optical absorption spectra to photon energy, we found that Mn11Si19 has indirect band gap structure with the absorption edge energy of approximately 0.64 eV at 5 K. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study on Cd vacancy in CdZnTe Crystal by Positron Annihilation Technology

Cd vacancies in cadmium zinc telluride(CdZnTe) crystals have an important effect on the crystal properties. In this paper, position distribution and concentration change of Cd vacancy in CdZnTe crystal grown by the temperature gradient solution growth (TGSG) were investigated by positron annihilation technology (PAT), which was based on the potential energy distribution and probability density of the positron in the crystal. The results showed that, the density of Cd vacancy increased obviously from the first-to-freeze to stable growth of the ingots, while decreased along the radial direction of the ingots.

Growth and Characterization of CdZnTe Bulk Crystals Grown by Temperature Gradient Solution Growth

CdZnTe bulk crystals were grown by temperature gradient solution growth (TGSG) method with the lower starting growth temperature of 1223K, temperature gradient of 20-30 K/cm and a crucible dropping rate varied from 1 to 0.6 mm/h. Optical microscope, IR microscope, I-V characteristics and FTIR transmission spectroscopy were employed to analyze the growth interfaces, Te inclusions, resistivity and IR transmittance respectively. The results indicate that a smooth and uniform growth interface has been achieved. The density of Te inclusions is about 7.9-9.5×103cm-2, while the resistivity is higher than 6.87×109Ωcm and the IR transmittance is about 55-60%.

Synthesis and growth of nonlinear infrared crystal material CdSe via seeded oriented temperature gradient solution zoning method

Single crystals of CdSe were grown by using seeded oriented temperature gradient solution zoning (S-TGSZ) method with the sizes of 20 mm in diameter and 80 mm in length. The crystals were characterized with X-ray diffraction, transmission spectrophotometer and infrared microscope. The transmission spectra showed that the infrared transmission is above 65% and the mean absorption was 0.01–0.04 cm−1 in the range of 2.5–20 μm. With 2.797 μm Cr,Er:YSGG laser as pumping source, experiments of optical parametric oscillator (OPO) were performed by using fabricated 5 mm×5mm×30 mm device crystal. The signal and idler wavelengths and the output average power were respectively 4.3 μm, 8 μm and 400 μJ. Optical-to-optical conversion efficiency was obtained by 12%.

Magnetisation of bulk Mn 11Si 19 and Mn 4Si 7

The purpose of this paper is to determine by experiment whether Mn 11Si 19 and Mn 4Si 7 in their bulk states have a finite magnetic moment or not. Magnetisation measurements were carried out on these materials using both SQUID system and Kerr rotation system. The high quality samples were grown using the temperature gradient solution growth method. SQUID measurements revealed that Mn 11Si 19 has finite magnetism while Mn 4Si 7 does not in their bulk states. It was also confirmed that Mn 4Si 7 became magnetic and Mn 11Si 19 got to exhibit a distinctive hysteresis in their powdery state. The enhancement of magnetism implied that the surface of the samples was to a great extent linked to its magnetism.

Solution Growth and Thermoelectric Properties of Single-Phase MnSi1.75−x

We have investigated the crystal growth of single-phase MnSi1.75−x by a temperature gradient solution growth (TGSG) method using Ga and Sn as solvents and MnSi1.7 alloy as the solute, and measured the thermoelectric properties of the resulting crystals. Single-phase Mn11Si19 and Mn4Si7 crystals were grown successfully using Ga and Sn as solvents, respectively. The typical size of a grown ingot of Mn11Si19 was 2 mm to 4 mm in thickness and 12 mm in diameter, whereas Mn4Si7 had polyhedral shape with dimensions in the range of several millimeters. The single-phase Mn11Si19 has good electrical conduction (ρ = 0.89 × 10−3 Ω cm to 1.09 × 10−3 Ω cm) compared with melt-grown multiphase higher-manganese silicide (HMS) crystals. The Seebeck coefficient, power factor, and thermal conductivity were 77 μV K−1 to 85 μV K−1, 6.7 μW cm−1 K−2 to 7.2 μW cm−1 K−2, and 0.032 W cm−1 K−1, respectively, at 300 K.

Effect of temperature modulation during temperature gradient solution growth of [formula omitted]- FeSi2

We have grown β- FeSi2 single crystals by the temperature modulated TGSG method using Ga solvent and Fe0.98Co0.02Si2 solute to observe the solution growth process. Growth striations corresponded to the temperature modulation cycles were clearly observed in the β- FeSi2 crystals grown using Fe0.98Co0.02Si2 solute, whereas they were not observed in the crystals grown using FeSi2 solute. SEM-EDX measurements revealed the striation was caused by the fluctuation of Co impurity. From the observation of striations, we found that the growth mechanism of β- FeSi2 would be explained by the constant growth rate model and also the growth rates along 〈011〉 and 〈010〉 directions were larger than that of 〈001〉 direction.

Solution growth of n-type β-FeSi 2 single crystals using Ni-doped Zn solvent

We have grown low-resistive and n-type β-FeSi 2 single crystals by the temperature gradient solution growth (TGSG) method using Ni-doped Zn solvent. These crystals were obtained at the Ni concentrations between 0.05 and 0.5 wt%. Above the Ni concentration of 1.0 wt%, nickel silicides were preferably grown. The resisitivity of the Ni-doped crystals was 0.2–0.5 Ω cm at room temperature (RT). The electron concentration and Hall mobility at RT were (5–6)×10 18 cm −3 and 3–6 cm 2/V s, respectively. We also determined the ionization energy of 26–60 meV for the Ni impurity in β-FeSi 2.

Thermoelectric Properties of Solution Grown β-FeSi2 Single Crystals

We have measured the Seebeck coefficient of single crystalline β-FeSi 2 grown by a temperature gradient solution growth (TGSG) method using Ga solvent. Rectangular-like β-FeSi 2 plates with the size of (3-6) x (1-2) x 0.3 mm 3 . where the longitudinal axis was [011], were prepared from the grown ingots. Typical resistivity and hole concentration of the crystals were 4 x 10 -4 Qm and 2 x 10 25 m -3 at room temperature (RT), respectively. The Seebeck coefficient measured along the [011] direction was approximately 350μV/K at RT and showed the maximum value of 500 μV/K between 20 and 25 K. We also found that the solution grown single crystals had large power factors below RT. The value was 3.4 x 10 -4 Wm -1 K -2 at RT. which was about three times larger than that of sintered poly-crystals and CVT-grown single crystals. The maximum power factor was 4.5 x 10 -4 Wm -1 K -7 around 150 K. The value was more than one order of magnitude larger than reported values.

Solution growth of n-type [formula omitted]- [formula omitted] single crystals using Sn solvent

We have grown n-type β - FeSi 2 single crystals by a temperature gradient solution growth method using Sn solvent. The typical size of crystals was (3–5) × 0.5 × 0.3 mm 3 for a growth period of 336 h. We found that there were two types of crystals in reference to the preferable growth direction. One type was grown toward the 〈 0 1 0 〉 direction. These crystals had good crystalline quality without twins. The other type was grown toward the 〈 0 1 1 〉 direction. In this case, most of the crystals had twins. Hall effect measurements revealed that the crystals showed n-type conductivity. The electron concentration and Hall mobility at room temperature were (0.3–5) × 10 18 cm - 3 and 2 – 4 cm 2 V - 1 s - 1 , respectively.

Solution growth of high quality P-Type β-FeSi 2 single crystals using Zn-solvent

We have grown high-quality β-FeSi 2 single crystals by the temperature gradient solution growth (TGSG) method using Fe 2Si 5 solute and Zn solvent. The grown crystals were polyhedral and typical size was (2–4)×(1–3)×(1–2) mm 3 for the growth period of 336 h. The FWHM of the X-ray rocking curve at β-FeSi 2 800 diffraction was 63 arcsec. Hall measurements revealed that the crystals showed a p-type conductivity. The hole concentration and Hall mobility at room temperature were (2–3)×10 17 cm −3 and 20–50 cm 2/Vs, respectively. We also measured the solubility of Fe 2Si 5 solute in Zn solvent. The solubility was about 2 wt% at 880 °C, which is about two times higher than that of FeSi 2 in Zn solvent.