ZnTe Crystal Research Articles

Frequency resolved cross-correlation between optical and terahertz pulses: application to ultrashort laser pulse characterization.

We have analyzed both theoretically and experimentally the spectrum of the optical pulses produced by the interaction of optical and THz pulses in a ZnTe crystal. Recorded as a function of the delay between the two pulses, the resulting spectrogram can be viewed as a frequency resolved cross-correlation between the optical and THz pulses making it possible to characterize the optical pulse.

Photoluminescence of high‐resistivity ZnTe crystals doped with gallium and indium

AbstractWe have measured photoluminescence spectra of high‐resistivity ZnTe crystals doped with gallium and indium in the wide energy range from near‐band‐edge energy to deep energy region up to 0.78 eV. In the near‐band‐edge energy, we detected a characteristic excitonic peak at 2.363 eV in both gallium and indium doped high resistivity ZnTe. By comparison with a similar PL peak observed in high resistivity CdTe, we identified the characteristic peak at 2.363 eV as emission from excitons bound to defects complex arising from a Zn vacancy and gallium or indium atoms (VZn‐2D). In lower energy range, a newly strong deep emission band near 1.0 eV has been observed for the first time in high‐resistivity ZnTe crystals. The intensity of the deep emission is two order magnitude stronger than that in undoped one. Furthermore, the intensity and peak position of the deep emission band are almost unchanged by Zn‐saturated anneal at 600‐680 °C. We specualte that this deep emission band is responsible for the high resistivity of ZnTe and prevent conversion of n‐type low resistivity. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Scheme to extend the range of electro-optic sampling of an intense terahertz field

Generation of intense terahertz (THz) radiation requires an efficient method to determine the field intensity with a wide range of intensity response and accuracy. A new scheme is proposed to extend the limited response range of available time-domain spectroscopy measurement. The method applies two pieces of thin ZnTe crystals with a fixed angle between the (001) axes. At the optimized angle, the linear response range of the THz field can reach 31 MV/cm.

Emitting and Receiving Terahertz Vectorial Antennas Based on Cubic Electro-Optic Crystals

Based on optical rectification in a ZnTe crystal, a vectorial terahertz (THz) emitter is here theoretically and experimentally demonstrated. The use of the particular orientation $\langle 111\rangle$ leads to a polarization state angle tunable THz source. This configuration requires only an optical wave plate to manage the impinging optical polarization state, thus adjusting the transverse THz electric field orientation. Moreover, the optical rectification efficiency as well as the THz signal bandwidth remain constant from horizontal to vertical Terahertz polarization. The same crystal, acting then like a THz receiver, allows to perform the vectorial analysis of a THz pulse.

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.

Improving and Characterizing (Cd,Zn)Te Crystals for Detecting Gamma-Ray Radiation

${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te}$ ingots were synthesized from pure components (6N purity Cd, Zn, and Te with In as the dopant) and subsequently grown from the melt under an argon overpressure. Graphite crucibles (with and without an inner coating of pyrolytic BN) were used. The temperature gradient in the solidification zone was 7–30 K/cm, and the growth rate was 0.6-1.0 mm/hour. We investigated the chemical composition, structure, and electrical properties of the as-grown crystals, and established relationships between the crystal properties and the growth conditions. The bottom, middle, and top of the ingots had n-type conductivity, but slightly different properties. Resistivity reached a maximum in the middle of the ingots ( ${(2.5 - 5.0)} \times {10^{10}}~\hbox{Ohm-cm}$ ), and was less at the edges $\sim 0.8 \times {10^{10}}~\hbox{Ohm-cm}$ . The value of the bandgap was minimal in the middle of the ingots ( $\sim 1.5~\hbox{eV}$ ), and 1.53-1.55 eV at the edges. The compensation degree ( ${{\rm N}_{\rm d}}/{{\rm N}_{\rm a}}$ ) of the energy level responsible for the low dark conductivity showed a maximum value at the bottom of the ingots ( $\sim 60 - 90\% $ ), and a minimum in the middle part (1-2%). The crystals were then used to fabricate Cd(Zn)Te detectors for gamma-ray radiation.

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.

Investigation of Thermodynamic Properties in ZnTe, ZnSe, ZnS Crystals using ab initio Calculations

Using cluster models for cubic sphalerite for stoichiometric ZnX, X = S, Se, Te crystal was spent calculation of values of thermodynamic parameters under normal conditions, and was defined the analytical expressions of temperature dependences of energy ΔE and enthalpy ΔH, Gibbs free energy ΔG, entropy ΔS. It was calculated temperature dependences of the Debye temperature ΘD. The correlation beetwen thermodynamic (ΔE, ΔH, ΔG, ΔS), heat (Cv, Cp, ΘD) parameters of ideal crystals ZnTe, ZnSe, ZnS and their main fundamental characteristics (band gap Eg, the contribution of ionization δ, electronegativity ΔX, binding energy D, comprehensive compression module B) have been find.

Synthesis and characterization of polymethyl methacrylate with double electro-optic molecules thin film and its application

Polymethyl methacrylate (PMMA) with double electro-optic (EO) molecules was synthesized. One kind of chromophoric groups, hydroxyethyl carbazole, reacted with PMMA by a chemical method to form poly(methacrylic acid ethyl carbazole ester). Another small nonlinear optical molecule (NOL), 4–6-alkoxy-4′-cyano-biphenyl (6OCB), was mixed into the polymers by a physical method to further improve EO properties. The structure of the material was characterized and the effect on EO coefficient of different mixing proportions was investigated. Photoconductive properties and the orientation stability were studied. Experimental results showed that the enhanced EO coefficient was 44.8pm/V when the mixing proportion was 20wt%. Thermally stimulated discharge current (TSDC) testing curves were obtained, which confirmed the existence of two depolarization processes. Thin films formed by this polymer material were used to detect terahertz (THz) radiation. The detecting efficiency and the spectrum sensitivity were enhanced largely compared with ZnTe crystal. It can be concluded that this polymer material is a very good candidate for THz application.

Transient segregation behavior in Cd1−xZnxTe with low Zn content—A qualitative and quantitative analysis

The paper analyzes experimental compositional profiles in Vertical Bridgman (VB, VGF) grown (Cd,Zn)Te crystals, found in the literature. The origin of the observed axial ZnTe-distribution profiles is attributed to dendritic growth after initial nucleation from supercooled melts. The analysis was done by utilizing a boundary layer model providing a very good approximation of the experimental data. Besides the discussion of the qualitative results also a quantitative analysis of the fitted model parameters is presented as far as it is possible by the utilized model.

Selective etching of ZnTe in HF:H2O2:H2O solution: Interpretation of extended defect‐related etch figures

HF:H2O2:H2O solution (40%wt.HF: 30wt.%H2O2: H2O, 3:2:1 by volume) was used to reveal extended defects (line, face and volume defects) in bulk ZnTe crystals grown from Te solution. The etch patterns were analyzed based on their size, shape and distribution. The etch figures, both in the shape of pits and hillocks with high resolution, show forms controlled by the symmetries of the respective faces were produced. Two different sizes of pits were observed, the larger‐size pits correspond to dislocations penetrating the surface, however, the smaller‐size texture pits are produced on the defect‐free region, which serve as standard pits on respect faces. The face defects, such as grain boundaries, sub‐grain boundaries, dislocation walls, twins and stacking faults, can be all displayed clearly. Another essential feature of the etchant is that, it can effectively dissolve Te‐rich phase (Te inclusion/precipitates), which makes it promising to reveal the shape of this volume defect.

Temperature dependencies of the heat capacity of ZnS, ZnSe, and ZnTe crystals, obtained from first principles

Temperature dependencies of the heat capacity of ZnS, ZnSe, and ZnTe crystals, obtained from first principles

Detection Sensitivities of Photoluminescence Spectroscopy and Polariscopy to Residual Strains in a (110)-oriented ZnTe Single Crystal: Explore of Strain-sensitivity Inspection Techniques

We demonstrate that sensitivity of polariscopy to residual strains is higher than that of the photoluminescence spectroscopy in a (110)-oriented ZnTe single crystal. We carried out x-ray topography and micro θ-2θ x-ray diffraction measurements in order to thoroughly clarify the crystal quality connecting with the residual strains. The two x-ray analyses revealed that there is misalignment from the [110] direction in some regions of the present sample. We found the following main results: The polariscopic analysis detects the residual strain formed by the misalignment, whereas the photoluminescence measurement cannot detect the residual strain. Thus, we conclude that polariscopy has the sensitivity to the residual strain higher than that of the photoluminescence spectroscopy. We confirmed that the polariscopic analysis can detect the internal strain less than 0.02% in the ZnTe crystal. In addition, we discuss the applicability of polariscopic analysis to other compound semiconductors.

Homogeneity limits and nonstoichiometry of vapor grown ZnTe and CdTe crystals

A new understanding of the wurtzite-spalerite phase transition in CdTe and ZnTe compounds in connection with the vapor growth of single crystals.

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

X-ray diffraction imaging of ZnTe crystals grown by the multi-tube physical vapour transport technique

X-ray diffraction imaging (topography) has been used in monochromatic beam mode to demonstrate that 100mm diameter ZnTe crystals, several millimetres thick, and grown by the multi-tube physical vapour transport technique on (001) and (211B) GaAs substrates, have very high crystal perfection. Images taken in the Bragg geometry from planes containing the growth direction show evidence of cellular dislocation structure and give strong contrast from the whole, several mm2, sample area. Rocking curves taken from small areas show only moderate broadening from that expected from a perfect crystal, indicating dislocation density of typically 3.5×106cm−2 close to the seed and 3.3×105cm−2 at the top of the grown crystal. At the top surface, planes parallel to the seed show biaxial concavity, a feature attributed to the ZnTe boule bowing under tensile strain generated at the substrate-crystal interface due to the mismatch in thermal expansion coefficients of GaAs and ZnTe. Crystals grown on (211B) substrates are of {331} orientation, showing no evidence of twin boundaries, suggesting that {331} growth is initiated at, or very close to, nucleation.

Comparison of an electro-optical system and photo-conducting antenna employed as detectors of pulsed terahertz radiation by means of a new method for measuring spectral width

Two detection systems, electro-optical system and photo-conducting system, are tested by the method suggested previously for determining the boundaries of broadband terahertz radiation in time-domain spectroscopy. From a series of measurements the error in determining the operation ranges is calculated. The terahertz spectrometer with an electro-optical detector based on a ZnTe (110) crystal of thickness has the operation spectral range of . The detector utilizing an iPCA-21-05-1000-800-h photo-conducting antenna with the same source of signal demonstrates a wider operation band ranging from 0.017 to . The method developed makes it possible to experimentally compare the parameters of the considered terahertz spectrometers obtained under the same quality of adjustment.

Synthesis and properties of ZnTe and Eu3+ ion co-doped glass nanocomposites

In this study, ZnTe (II-VI) semiconductor and Eu+3-ion co-doped borosilicate glass has been prepared in the SiO2-K2O-CaO-BaO-B2O3 glass system followed by controlled heat-treatment to produce glass nanocomposites. Glass transition temperature and crystallization peak temperature have been evaluated using DSC analysis. Dilatometric studies were carried out to evaluate thermal expansion co-efficient, glass transition temperature, and dilatometric softening temperature and found to be 10.7 × 10−6/K, 580° C and 628° C, respectively. TEM micrographs demonstrate formation of nano sized crystallites of less than 50 nm. The ZnTe crystal formation also established through selected area electron diffraction (SAED) analysis and high resolution images obtained through TEM studies. With increasing heat treatment time, optical transmission cut-off wavelength (λcut-off) shifted towards higher wavelength. Excitation spectra were recorded by monitoring emission at 613 nm corresponding to the 5D0 → 7F2 transition. An intense 394 nm excitation band corresponding to the 7F0 → 5L6 transition was observed. Emission spectra were then recorded by exciting the glass samples at 394 nm. When the glass is heat-treated for 30 min at 610° C, a 6-fold increase in the intensity of the red emission at 612 nm has been observed, which is attributed to the segregation of Eu3+ ions into the low phonon energy ZnTe crystallites and as the size of the nanocrystals is smaller than the size of the exciton, quantum confinement effect is visible. Further increase in heat-treatment duration led to decrease in luminescence intensity due to the growth of larger size crystals. 5D1 → 7F0 transition is visible only in the samples heat-treated for 30 min and 1 h, which is a characteristic of presence of Eu3+ ions in the low phonon energy ZnTe crystal sites. The micro hardness of the precursor glass and glass nanocomposites was evaluated; base glass shows hardness of 6.7 GPa and hardness of heat-treated glass nanocomposites has been found to decrease with increase in heat-treatment duration (5.5-5.3 GPa). However, mechanical properties are found to be suitable for device applications.

Video-rate terahertz electric-field vector imaging

We present an experimental setup to dramatically reduce a measurement time for obtaining spatial distributions of terahertz electric-field (E-field) vectors. The method utilizes the electro-optic sampling, and we use a charge-coupled device to detect a spatial distribution of the probe beam polarization rotation by the E-field-induced Pockels effect in a ⟨110⟩-oriented ZnTe crystal. A quick rotation of the ZnTe crystal allows analyzing the terahertz E-field direction at each image position, and the terahertz E-field vector mapping at a fixed position of an optical delay line is achieved within 21 ms. Video-rate mapping of terahertz E-field vectors is likely to be useful for achieving real-time sensing of terahertz vector beams, vector vortices, and surface topography. The method is also useful for a fast polarization analysis of terahertz beams.

Indication of Te segregation in laser-irradiated ZnTe observed by in situ coherent-phonon spectroscopy

We irradiate a ZnTe single crystal with 10-fs laser pulses at a repetition rate of 80 MHz and investigate its resulting gradual modification by means of coherent-phonon spectroscopy. We observe the emergence of a phonon mode at about 3.6 THz whose amplitude and lifetime grow monotonously with irradiation time. The speed of this process depends sensitively on the pump-pulse duration. Our observations strongly indicate that the emerging phonon mode arises from a Te phase induced by multiphoton absorption of incident laser pulses. A potential application of our findings is laser-machining of microstructures in the bulk of a ZnTe crystal, a highly relevant electrooptic material.