Growth Of Cd Research Articles

Growth Process of Cd0.8Zn0.2Te Crystal to Enhance its Performance as Detector for High-energy Radiation

Introduction: In the applications of room temperature detector for high-energy radiation, there are two critical requirements for the semiconducting material cadmium zinc telluride (CdZnTe): (1) high electrical resistivity to reduce the bulk leakage current and (2) low levels of structural defects which hinder the detectivity as trapping and recombination centers for the carriers. To enhance the performance of the detectors, an optimal single process has been developed in the melt growth of Cd0.8Zn0.2Te by directional solidification with controlled Cd overpressure to maximize the electrical resistivity as well as minimize the structural defects, including Te precipitates/inclusions of the grown CdZnTe crystals. Method: Using the phase diagram data of pressure-temperature-composition (P-T-X), melt growth of Cd0.8Zn0.2Te crystals by directional solidification from a starting melt at 1145oC has been performed with various Cd overpressures controlled by the temperature of a Cd reservoir. The grown crystals were sliced and were characterized by electrical resistivity measurements and chemical analysis of Glow Discharge Mass Spectroscopy (GDMS). The structural defects were studied by the infrared (IR) transmission images taken by an IR microscope. Result: By doping of In (4 – 6 ppm, atomic) and growing with a Cd reservoir in the range of 785 to 825oC, the electrical conductivity were consistently higher than 109cm and up to 2x1011cm. From the trend of the Te precipitates density observed by the IR micrographs, it was concluded that a Cd reservoir temperature of 820+10oC resulted in the lowest precipitate density. Conclusion: The employment of a Cd reservoir temperature of 820+5 oC during the growth process will provide the optimal Cd pressure over the melt at 1145oC to maximize the electrical resistivity as well as minimize the structural defects, including Te precipitates/inclusions of the grown Cd0.8Zn0.2Te crystals. Discussion: Since the solids of different compositions, x in the Cd1-xZnxTe system, have different liquidus/solidus temperatures as well as different homogeneity ranges, the procedure presented here for the Cd0.8Zn0.2Te solid may not applicable to other compositions.

Carbon dots as light converter for plant photosynthesis: Augmenting light coverage and quantum yield effect

Carbon dots (CDs) with gradient-changed quantum yield (QY) were prepared by regulating the graphitic N and hydroxyl group contents. Then, the QY effect of CDs on plant photosynthesis was studied using chloroplasts and rice plants. After incubation for 2 h in the dark, CDs entered into the chloroplasts and converted ultraviolet radiation to photosynthetically active radiation. By this mechanism, CD1:0.2 (300 μg·mL−1) with a moderate QY of 46.42% significantly increased the photosynthetic activity of chloroplast (200 μg·mL−1) to reduce DCPIP and ferricyanide by 43.77% and 25.45%, respectively. After spraying on rice seedlings, CD1:0.2 (300 μg·mL−1) was evenly distributed in the leaves and resulted in maximum increases in the electron transport rate and photosynthetic efficiency of photosystem II by 29.81% and 29.88%, respectively. Furthermore, CD1:0.2 significantly increased the chlorophyll content and RuBisCO carboxylase activity of rice by 64.53% and 23.39%, respectively. Consequently, significant increases were observed in the growth of CD1:0.2-treated rice, including 18.99%, 64.31%, and 61.79% increases in shoot length, dry weights of shoot and root. These findings contribute to the exploitation of solar energy and agricultural production using CDs in the future.

Influence of rotating magnetic fields on THM growth of CdZnTe crystals under microgravity and ground conditions

The influence of rotating magnetic fields (RMF) on species transport and interface stability during the growth of Cd0.96Zn0.04Te:In crystals by using the traveling heater method (THM), under microgravity and terrestrial conditions, is numerically investigated. The numerical results are compared to ground and space experiments. The modeling of THM under ground conditions shows very deleterious effects of the natural convection on the morphological stability of the growth interface. The vertical flow transports the liquid of low Te concentration from the dissolution interface to the growth interface, which is consequently destabilized. The suppression of this flow, in low-gravity conditions, results in higher morphological stability of the growth interface. Application of RMF induces a two flow cell pattern, which has a destabilizing effect on the growth interface. Simulations performed by varying the magnetic field induction in the range of 1−3mT show optimal conditions for the growth with a stable interface at low strength of the magnetic field (B=1mT). Computations of indium distribution show a better homogeneity of crystals grown under purely diffusive conditions. Rotating magnetic fields of B=1mT induce low intensity convection, which generates concentration gradients near the growth interface. These numerical results are in agreement with experiments performed during the FOTON M4 space mission, showing good structural quality of Cd0.96Zn0.04Te crystals grown at very low gravity level. Applying low intensity rotating magnetic fields in ground experiments has no significant influence on the flow pattern and solute distribution. At high intensity of RMF (B=50mT), the buoyancy convection is damped near the growth front, resulting in a more stable advancing interface. However, convection is strengthening in the upper part of the liquid zone, where the flow becomes unsteady. The multi-cellular unsteady flow generates temperature oscillations, having deleterious effects on the growth process.

Growth and characterization of Cd1−xZnxTe/ZnO heterostructures from furnace-annealed CdTe/Zn multi-stacks

Cd1−xZnxTe/ZnO hetero-structures were grown on glass substrates by furnace annealing of stack-deposited CdTe/Zn/CdTe/Zn thin film multi-layers in air. The multilayered structures were annealed in furnace at various temperatures in between 350 and 600°C. XRD results revealed that while growth of Cd1−xZnxTe compound reached saturation by 500°C, ZnO growth geared up in between 550 and 600°C. The band-gap values of the composite structures were less than either standard Cd1−xZnxTe or ZnO. It was because of large presence of free Zn and Cd and staggered type-II band alignment. An exponential growth was observed for the Cd1−xZnxTe particle sizes against annealing temperature. Next SEM images showed reduced surface granularity with increasing Zn percentage in Cd1−xZnxTe. SEM micrographs further revealed growth of fiber like formations of ZnO on film surface with increasing annealing temperature. ImageJ software was further used to analyse the SEM micrographs and compositional characteristics from EDX results were co-related with structural, optical and morphological results.

Growth and defect structure of CdF2 and nonstoichiometric Cd1−xR x F2 + x phases (R = rare earth elements and In). I. Growth of Cd1−xR x F2 + x single crystals (R = La-Lu, Y)

This article begins a series of publications on growth of single crystals of nonstoichiometric Cd1− xR x F2 + x phases (R = La-Lu, Y, In) with the defect CaF2-type structure, their crystal structures, and some properties. The present article is dedicated to the phase diagrams of the CdF2-RF3 systems in the region of Cd1−xR x F2 + x formation. Their analysis shows that it is possible to synthesize homogeneous Cd0.9R0.1F2.1 crystals. The dependence of the defect structure of the crystals on the type and concentration of rare earth elements is studied on specially grown Cd0.9R0.1F2.1 (R = La-Lu) and Cd1−xYxF2 + x (x = 0.1, 0.15, 0.20) crystals. It is shown that, despite the fact that all the Cd0.9R0.1F2.1 crystals melted congruently, irrespectively of the rare earth elements used, they were rather homogeneous. The chemical compositions of the Cd1−xR x F2 + x phases (R = Sm, Gd, Tb, Ho, and Lu) determined by the method of inductively coupled plasma atomic emission spectroscopy (ICP-AES) turned out to be close to Cd0.9R0.1F2.1.

Growth of Cd1−xZnxTe crystals with different x values and their qualities comparison

Three Cd1−xZnxTe ingots with x values of 0.10, 0.15, and 0.20 were, respectively, grown by vertical Bridgman method (VBM). Characterization indicated that Cd0.85Zn0.15Te possessed preferable qualities. It had low radial Zn concentration deviation, low densities of dislocation and Te precipitate/inclusion, and high infrared transmission. What is more, Cd0.85Zn0.15Te obtained the highest resistivity, which was about one order higher than both Cd0.9Zn0.1Te and Cd0.8Zn0.2Te. From the point of the defects and impurities existing in the crystals, as well as the characteristics of the crystal growth by VBM, the opportunity for growth of high-qualities Cd1−xZnxTe was given in this paper.

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.

Growth of MnS, Cd1-xMnxSe, and MnS1-xSex Films by Molecular Beam Epitaxy (MBE) and Elemental Vapor Transport Epitaxy (EVTE) Techniques

This work addressed the MBE and EVTE growth of wide band gap semiconductor materials such as MnSe, Cd1−xMnxSe, and MnS1−xSex which have potential application for blue-green light emitters. It is the first time that EVTE was applied for high (Se, S, Cd) and low (Mn) vapor pressure materials, which required modification of the conventional design. The films were grown on GaAs 20 off (100) substrates and characterized using optical microscopy, SEM, EDS, X-ray, SIMS, Hall, and sheet resistance measurements. The deposition process parameters will be reviewed and related to results such as film composition, surface morphology, uniformity, and crystallinity. We report on growth of Cd1−xMnxSe with 0.42<X<0.99 and MnS1−xSex with 0.58<X<0.67. Different methods of controling elemental composition will be discussed. Grown materials show good surface morphlogy and uniformity. SIMS analysis demonstrated stable elemental composition throughout the film thickness and a sharp interface with the substrate.

Growth of Cd1−xMnxTe films by pulsed laser evaporation and epitaxy

Epitaxial layers of (111) Cd1−xMnxTe (x=0.05) were grown on (111) GaAs substrates by pulsed laser evaporation and epitaxy. A XeCl excimer laser beam was directed at a Cd0.95Mn0.05Te target to produce the vapors necessary for deposition. A simultaneously operating pulse Nd:YAG laser was used to create the overpressure of Cd needed in the growth of stoichiometric films. In situ reflection high-energy electron diffraction, as well as scanning electron microscopy, energy-dispersive x-ray analysis, and photoluminescence study showed that the films had characteristics comparable to the best CdMnTe epilayers grown so far by molecular beam epitaxy or metalorganic chemical vapor deposition.

Growth of Cd1−xZnxTe by molecular beam epitaxy

Cd1−xZnxTe has been grown on GaAs substrates for compositions from x=0 to x=1. Binaries are shown to be of high quality, but x-ray rocking curve half-widths are extremely broad for most ternary compositions. Attempts to modify the interface yield only modest and uneven improvement in rocking curve half-widths. The poor quality appears to be due to a phase separation.

Molecular beam epitaxial growth and characterization of Cd1−xZnxTe, Hg1−xCdxTe, Hg1−xMnxTe, and Hg1−xZnxTe on GaAs(100)

The growth of Cd1−xZnxTe on GaAs (100) is reported here for the first time. In contrast to CdTe, when x is higher than 0.15 only the Cd1−xZnxTe(100)∥GaAs(100) orientation is observed. We also present the growth and characterization of n-type and p-type layers of Hg1−xCdxTe, Hg1−xMnxTe, and Hg1−xZnxTe for the first time, in most of the cases. These layers exhibit high structural and good electrical properties. Several different characterizations carried out on Hg1−xCdxTe layers grown by MBE on GaAs substrates show that this material is at least as good as the best material grown by any other technique and confirm that GaAs is emerging as the candidate for replacement of CdTe as the substrate for growth of Hg1−xCdxTe by MBE. Hg1−xMnxTe and mainly Hg1−xZnxTe, which have electrical characteristics similar to high quality Hg1−xCdxTe, appear as potential candidates for replacement of Hg1−xCdxTe.

Growth of Cd1−xMnxTe films with 0&amp;lt;x&amp;lt;0.9 by atomic layer epitaxy

Thin Cd1−xMnxTe semiconductor films with x ranging from 0 to 0.9 have been grown on CdTe (111) substrates using the atomic layer epitaxy method. The films grow epitaxially at all concentrations and show no manganese interdiffusion. The films are characterized by low-energy electron diffraction, Auger electron spectroscopy and angle-resolved UV photoemission.