Optical sensor protector using wide-bandgap semiconductors

Optical power limiting is the increase of optical absorption with light intensity. Optical absorption in dependences of the light intensity and thickness of single crystals of wide-bandgap semiconductor cadmium iodide was studied using laser light. The transmission of the crystals was found to increase with increasing light intensity. However, at higher light intensity, an increase of optical absorption with intensity was observed. This power limiting characteristics was also found to depend on the thickness of the crystals. The findings resulting from this investigation might have potential applications in optical sensor protector using wide-bandgap semiconductors.     

Effect of tin-doping on polytypism of melt-grown crystals of cadmium iodide

The effect of doping of metallic tin in cadmium iodide crystals has been systematically studied. The method of zone melting has been employed both for purification and for the growth of single crystals of cadmium iodide. The doping has been carried out by zone levelling technique. The as-grown crystals have been characterized by X-ray diffraction and physical methods. All the crystals have been found to consist of the most common polytype 4H. Unlike the case of pure undoped crystals of cadmium iodide grown from melt, all the X-ray photographs showed the presence of arcing. None of the photographs showed the presence of streaking. Besides, the doped crystal were found to be harder than the crystals of undoped cadmium iodide. Unlike the undoped crystals, cleavage in the doped crystals was found to be difficult and highly localized. The results have been discussed.     

Phase transformations in PbI2 crystals with temperature

A systematic study of phase transformation with temperature has been undertaken in single crystals of lead iodide grown in gel. The crystals of the polytype 2H, which is known to be the common modification of PbI₂ at room temperature, have been finally found to transform into polytype 12R after heating at 150°C. During the intermediate period of heating the crystals show features of disorder, viz. streaking or arcing or both, on their X-ray diffraction photographs. The higher polytypes 12H and 16H do not show any change even after prolonged heating at 150°C. If silver iodide is added in a minute quantity during crystallization, the process of phase transformation is appreciably altered. The results have been discussed.     

Optical limiting and its mechanism in potash alum

Optical power limiting is the increase of optical absorption with light intensity. Pure crystalline potash alum was grown and optical absorption in dependences of the light intensity and thickness of the crystal was studied using laser light. The transmission of the crystals was found to increase with increasing light intensity. However, at higher light intensity, an increase of optical absorption with intensity was observed. The power limiting characteristics, e.g. linear transmission/dynamic range and saturation point, were found to be varied with thickness of the crystal. The findings resulting from this investigation might have potential applications in optical limiting.     

Low-temperature X-ray studies of TlInS2, TlGaS2, and TlGaSe2 single crystals

The results of X-ray diffraction studies of the unit-cell parameters and thermal-expansion coefficients of TlInS₂, TlGaS₂, and TlGaSe₂ crystals in the temperature range 100–300 K are described. It is shown that the unit-cell parameters of all the studied crystals gradually increase with increasing temperature. The temperature dependences of these parameters exhibit anomalies in the form of bends and kinks at temperatures corresponding to phase transitions in the crystals. The thermal-expansion coefficients along the [001] crystallographic direction of the crystals under study are determined. It is found that their values slightly change with increasing temperature.     

Single crystal elastic constants and magnetoelasticity of holmium from 4·2 to 300 K

The five independent elastic coefficients of holmium single crystals have been determined by means of an ultrasonic pulse technique at a frequency of 10 MHz, between 4·2 and 300 K. From the elastic constants the temperature variation of the directional adiabatic compressibilities, the limiting Debye temperature and the elastic anisotropy ratio were calculated. The elastic coefficients exhibit anomalies at the magnetic ordering transitions known to occur in holmium. Anomalous behavior in the elastic constants was also observed at about 80 K. The limiting value of the Debye temperature was found to be 191·5 K. The present measurements of the elastic constants, and the reported magnetostriction and thermal expansion data, enabled the calculation of the magnetoelastic contribution to the total Hamiltonian of holmium in the magnetically ordered states. A very small discontinuity in the temperature dependence of the magnetoelastic energy was observed at the Curie point of holmium. Below the Neel temperature, the magnetoelastic energy varies smoothly with decreasing temperature, attaining a value of ?2·13 J cm^?3 at liquid helium temperature. The temperature dependence of the magnetoelastic energy in the vicinity of the Curie point in holmium suggests that the magnetic transition from the antiferromagnetic arrangement into the ferromagnetic state is of second order.     

Excess noise peaks in porous silicon-based diode structures

Results of an experimental observation of the voltage oscillations associated with a discrete tunneling of holes in porous silicon at room temperature are presented. The noise characteristics of diode structures with a porous silicon interlayer formed on heavily boron-doped silicon single crystals are studied. Peaks of excessive noise are observed at frequencies of ～1 MHz, at which single-electron oscillations should be expected. The peak noise power is found to increase with current according to the ～2.5 power law and, at a current density of 0.15 A/cm², to exceed the noise power of the receiver by three to four orders of magnitude. The complex shape of the noise spectrum and its extension to the higher frequency region with increasing current are explained by the three-dimensionality of the system of nanometer-sized silicon grains embedded in insulating silicon dioxide of porous silicon.     

Crystal growth and luminescence properties of CuI single crystals

Recently, much attention has been attached to the material of cuprous iodide (CuI) single crystals. In this paper, a detailed study of the variation in the nucleation density and the growth of CuI crystals in silica gel as a function of the concentration of feed solution, pH of gel, gel aging time, growth temperature and volume of the feed solution is presented. The optimum conditions for growth of large size CuI single crystals in gel are: pH 5, concentration of complex 0.244 M, gel aging 72 h, temperature 45 °C. A simple procedure for increasing the size of the crystals is adopted and the CuI single crystal with the size of 4 mm³ is obtained. Also, the photoluminescence (PL) spectrum of as-grown CuI crystals in silica gel is evaluated by comparing its PL spectra with that of CuI crystals grown by solvent evaporation method. The results could provide a useful clue to further improve the properties of CuI single crystals.     

Luminescence and generation of laser radiation in zinc selenide and cadmium sulfide single crystals exposed to high-voltage subnanosecond pulses

The action of high-voltage subnanosecond pulses on A^IIB^VI semiconductor compounds is studied. A negative pulse with a duration of up to 500 ps is applied to electrodes of a special shape. The pulse amplitude can be varied from 20 to 250 kV. Plane-parallel plates with a thickness of 1–2 mm made of bulk zinc selenide or cadmium sulfide single crystals are placed between the electrodes. Experiments are carried out in air without submerging single crystal plates in a liquid dielectric medium. As soon as a voltage pulse is applied, diverging discharges propagate from sharp edges of the negative electrode along electric field lines. With increasing voltage, generation of laser radiation is observed in the bulk of the semiconductor, displaying all its characteristic features, such as a sharp increase in the radiation power, narrowing the spectrum, and the radiation directionality. For zinc selenide at room temperature, the radiation characteristics are as follows: the wavelength is 480 nm, the radiation divergence is about 3°, and the peak pulse power is 600 W.     

Band gap determination in lead iodide polytypes

The band gaps of the polytypic crystals of lead iodide have been determined by studying the temperature variation of conductivity. Results obtained on several polytypic crystals show that the band gaps of the higher polytypes are lower than that of the parent 2H structure (2.56 eV). As for example the band gaps for 12H, 14H, 18H and 20H (disordered) were found to be 1.65, 1.44, 1.07 and 1.01 eV. The significance of the band gap determination in regard to the recent anomalous results on the physical properties of lead iodide crystals, has been indicated.     

High field magneto-optical study of yttrium iron garnet (YIG)

The temperature (5–300 K) and field (0–100 kG) variation of the Faraday rotation of yttrium iron garnet single crystals has been measured at selected wavelengths in the range 0.8–1.6 μm. Above the magnetic saturation field of a few kG the Faraday rotation is found to decrease with increasing field for all temperatures and wavelengths. It is shown that a field dependence of the magneto-optical coefficients causes this surprizing effect.     

A study on the primary and secondary nucleation of ice by power ultrasound

Several different investigations have been carried out to study the primary and secondary nucleation of ice by sonocrystallisation. Firstly, the primary nucleation of discrete ice crystals in a supercooled sucrose solution has been observed. For increasing concentrations of sucrose solutions from 0 to 45 wt%, the nucleation temperature consistently occurs at a higher nucleation temperature in the presence of ultrasound. The nucleation temperature also increases as the power output and duty cycle of a commercial ultrasonic horn are increased. Snap shot images of the bubble clouds obtained from the ultrasonic horn also show that the number of bubbles appears to increase as the ultrasonic output is increased. This suggests that the nucleation of ice is related to the power output and number of cavitation bubbles. The effect of a single bubble on the sonocrystallisation of ice is discussed. High-speed movies (1120 fps) have shown that the crystallisation appears to occur in the immediate vicinity of the single bubble. In most cases, many crystals are observed and it is not known whether a single ice crystal is being fragmented by the bubble or whether many crystals are being initiated. The bubble appears to undergo a dancing regime, frequently splitting and rejoining and also emitting some small microbubbles. A study on the secondary nucleation of ice in sucrose solutions has been carried out using a unique ultrasonic cold stage device. Images taken using a microscope system show that the pre-existing ice dendrite crystals can be broken up into smaller fragments by an ultrasonic field. Cavitation bubbles appear to be important during the fragmentation process, possibly melting any ice crystals in their path. Flow patterns around cavitation bubbles have also been observed, and these may be responsible for the fragmentation of ice crystals.     

Dynamic behavior of indium contacts with single crystals of cadmium sulfide above room temperature

Dark-current relaxation is observed with single crystals of cadmium sulfide after voltage has been applied. The kinetics of this reaction have been measured with respect to voltage and temperature. The observations are explained from the viewpoint of filling and emptying of electron traps in the contact regions of the crystals.     

Ionic conductivity of mixed silver halide crystals

To evaluate the effects of mixed halides on the lattice defect parameters of the silver halides, we have measured the ionic conductivity both of the entire range of mixed AgBr-AgCl single crystals, aftd also of several iodide-doped crystals. For the AgBr-AgCl system, the intrinsic conductivity at a given temperature decreases monotonically from pure AgBr to pure AgCl. The deduced Frenkel defect formation energy varies only a little from 0 to 50 mole % AgCl, and then increases rapidly with further increase in AgCl content, closely paralleling the ratio of bulk modulus to dielectric constant. The defect formation energy in these crystals hence reflects the average macroscopic properties of the solid solution. For the iodide-doped crystals, however, the results are quite different. Small amounts-of iodide cause large increases in the conductivity of AgBr andiAgCl, especially in the latter. These results suggest that the elastic strain introduced by the oversized iodide ion exerts an appreciable local effect on the Frenkel defect formation, in contrast to the crystal-averaged response found for the AgBr-AgCl solid solutions. Furthermore, the Arrhenius plots for the conductivitiei of the AgBr: I specimens show curvature which suggests a temperature-dependent pairing of the solute.     

Characterization of solution-synthesized CdTe and HgTe

We report the characterization of solution-synthesized CdTe and HgTe nanocrystals by X-ray diffraction, transmission electron microscopy, and photoluminescence. Methanol solutions of sodium telluride and cadmium iodide or mercury iodide, respectively, are reacted to precipitate the nanocrystalline metal tellurides, while the sodium iodide byproduct remains in solution. The existence of crystalline CdTe, HgTe, and ternary HgCdTe compounds has been demonstrated by powder X-ray diffraction after a post-synthesis sintering process. Precipitated crystallites from this synthesis were analyzed by transmission electron microscopy, which revealed that crystal diameters can vary from approximately 1 nm to 100 nm and that crystals are stoichiometric within the detection limit of the electron microprobe technique. Narrow size ranges can be selected and investigated due to an in-situ separation process in the electron microscope. Photoluminescence is found at energies above the bulk exciton energy for CdTe and is attributed to near-band-gap recombination which is blue-shifted due to quantum confinement. Both low defect luminescence and dark field imaging suggest a high crystalline quality. A comparative characterization by photoluminescence, transmission electron microscopy, and X-ray diffraction evaluates the effects of heat treatments during and after synthesis.     

Enhanced properties of cadmium mercury thiocyanate bis(N-methyl formamide): A promising non-linear optical crystal

The present study deals with the synthesis and characterization of cadmium mercury thiocyanate bis(N-methyl formamide) or CMTN crystals where they were grown in two critical steps. In the first step, cadmium mercury thiocyanate (CMTC) single crystals were grown by intriguing cadmium chloride, mercuric chloride, and ammonium thiocyanate in 1:1:4 ratio and mixed with solvent by a slow solvent evaporation technique. The second step involves the reaction between CMTC and N-methyl formamide (NMF) in a 1:2 ratio leading to the formation of CMTN crystals. The growth parameters of CMTN grown crystals were optimized at different pHs (1- 5) and the solubility curve has also been reported. On characterization, the orthorhombic crystallinity having Pna2₁ space group of as-grown CMTN crystals has been revealed by single X-ray diffraction analysis (XRD) and the lattice cell parameters are found to be a = 15.195 Å, b =  7.722Å, c = 16.162 Å, and α = β = γ = 90°. Single the phase crystallinity of CMTN is observed by powder XRD pattern and the increase in the intensity of index peaks shows that there exists good coordination between the CMTC and NMF compounds. The FTIR analysis supported the presence of surface ligands groups of thiocyanate, while the Raman spectroscopy confirmed for the coordination of thiocyanate ions in the CMTN compound and thus both established for the metal-ligand bonding. The UV-vis spectroscopy showed the optical transparency of CMTN to have the cutoff wavelength at 335 nm and the Kurtz powder method for studying the second harmonic generation (SHG) output power is 5 times higher than the reference. Further increase of dielectric constant and dielectric loss with respect to the changes in frequency makes it a suitable material for the construction of photonic and non-linear optical (NLO) devices.     

温度对Ib型和IIa型金刚石大单晶(100)表面特征的影响

本文在5.6 GPa, 1250–1340 ℃的条件下, 利用温度梯度法, 以FeNiMnCo 合金为触媒, 沿籽晶的(100)晶面成功合成了不同晶形的优质Ib型和IIa型金刚石大单晶. 利用激光拉曼附件显微镜, 分别对上述不同温度下合成的两类金刚石样品上表面(100)面的中心区域及棱角区域进行观察分析. 研究发现, Ib型和IIa型金刚石大单晶(100)晶面上从中心到棱角处黑色纹路的分布逐渐变黑变密集; 另外, 随着金刚石合成温度的升高, Ib型金刚石大单晶(100)面上黑色纹路由稀疏逐渐变稠密, 而IIa型金刚石大单晶的黑色纹路较为稀疏; Ib型金刚石大单晶的形貌特征表现为从低温晶体的不规则分布过渡到中温、高温晶体的典型树枝状分布. IIa型金刚石大单晶(100)面特征随温度变化规律与Ib型的类似. 这两类金刚石大单晶表面特征的差异可能是由于IIa 型金刚石具有比Ib型更小的生长速度和更少的氮含量. 最后, 对两类塔状金刚石大单晶进行拉曼光谱测试分析, 结果表明IIa型金刚石大单晶的品质较Ib型金刚石大单晶好.     

Examen de la loi d'arrhenius de la diffusion dans les solides

Validity of the Arrhenius law has been investigated by measuring the influence of temperature on the transport processes in very pure single crystals of sodium iodide. For the first time, due to the high experimental accuracy and the precision of the temperature control, diffusion coefficients could be measured in a solid in steps of a few degrees up to a few tenths of a degree of the temperature of fusion. We have studied: (a) ionic conductivity, (b) self-diffusion of the cation Na⁺ and (c) self-diffusion of the anion I^?. The three transport processes follow the same temperature dependence: (a) strict verification of the Arrhenius law over several orders of magnitude up to about thirty degrees below the melting point and (b) in the last thirty degrees a positive departure from the simple exponential law is observed which increases rapidly to reach 50% at fusion. The different possible reasons of this departure are discussed. The phenomenon seems to be connected with the lattice dynamics before melting.     

Superfine interactions in cadmium sulphide single crystals doped with Mössbauer nuclei

Influence of external fields on hyperfine interactions in cadmium sulphide single crystals doped with Mössbauer nuclei has been investigated. The dependence of hfs on temperature and external acoustic field is revealed. Modulation of Mössbauer radiation is observed in AIIBVI-group single crystals.     

The concentration-dependence of the resistivity anomaly in dilute magnetic alloys at low temperatures

Ice single crystals were produced, which were homogeneously doped by HF. Distribution coefficients were determined, and ageing experiments were done. It is assumed: for fluorine-concentrations less than 10^?6 m/l the whole solute is concentrated in the grainboundaries of the microstructure, but at higher concentrations there exists no microstructure and the solute is regularly distributed in the bulk. The ageing experiments showed that the fluorineconcentration decreases while stored until a limiting concentration of 10^?6 m/l is reached. Then the microstructure may be rebuilt.