CdTe Crystals Research Articles

Characterization of Seamless CdTe Photon Counting X-Ray Detector

Spectrally selective X-ray imaging provides improved material and tissue discrimination in comparison with the state-of-the-art dual energy technologies that are commonly used in medical, industrial, and security applications. Cadmium telluride (CdTe)- and cadmium zinc telluride (CdZnTe)-based line scanners and small size two-dimensional X-ray sensors are emerging to the market, but the need for large-scale panels is axiomatic. In this study, a seamless CdTe tile was developed that enables the implementation of large-sized, energy selective X-ray detector panels. The developed tile consists of a $64\times64$ pixel array (with $150~\mu \text{m}$ pitch) with a necessary substrate, ASIC, and CdTe crystal. The performance of the constructed seamless tile was characterized by focusing on spectral resolution and stability. In addition, a simple pixel trimming method that automates the equalization of each energy selective pixel was developed and analyzed. The obtained results suggest that the proposed concept of seamless (tileable) detector structures is a feasible approach to scale up panel sizes. The seamless tile shows comparable spectral resolution and stability performance with commercial CdTe sensors. The effect of tile to tile variation, the realization of a large-scale panel, as well as the charge sharing performance were left out of the scope and are to be studied in the next phase.

Електричні та фотоелектричні властивості гетеропереходів MoN/p-CdTe та MoN/n-CdTe

Due to the physical properties of MoN and ITO thin films, it was decided to create MoN/p-CdTe and MoN/n-CdTe heterostructures and investigate their electrical and photoelectric properties. The method of reactive magnetron sputtering was used to create thin MoN and ITO films on single crystal CdTe substrates with different conductivity types. To manufacture test heterostructures, the following CdTe crystal substrates were used: 1) p-type conductivity, grown by Bridgman technique at low cadmium vapor pressures; 2) n-type conductivity, grown by Bridgman technique at high cadmium vapor pressures. During the deposition process, the argon pressure in the vacuum chamber was 0.4 Pa. The power of the magnetron was 30 W, the sputtering process continued 5 min at a substrate temperature of 150°C. I-V characteristics of the heterostructures at different temperatures were measured, the height of the potential barrier, the values of the series and shunt resistance were determined. Electrical and photoelectric properties of the heterostructures were studied, and the dominant mechanisms of current transfer at forward displacements was established. The tunnel-recombination mechanism was found to be the dominant mechanism of current transfer in the MoN/p-CdTe and MoN/n-CdTe heterostructures. It was shown that the photoelectric parameters for the MoN/p-CdTe heterostructure are higher than those for MoN/n-CdTe. MoN/p-CdTe heterojunctions have the following photoelectric parameters: open-circuit voltage Voc = 0.4 V, short-circuit current Isc = 24.6 mA/cm2 at an illumination intensity of 80 mW/cm2. This makes them a promising material for the manufacture of detectors of various radiation types.

Efficient terahertz generation and detection in cadmium telluride using ultrafast ytterbium laser

In this work, we study THz generation and detection using cadmium telluride (CdTe) crystals pumped by amplified (1.025 μm wavelength) and oscillator (1.045 μm wavelength) ytterbium (Yb) lasers. For each laser, we compare the performances of the CdTe THz emitter and detector to those of GaP crystals. Under optimum conditions, we demonstrate that the former shows 3 and 5 times better performances compared with the latter for detection and generation, respectively. When pumped by an amplified Yb laser, we find that the CdTe crystal is more efficient than the GaP crystal for emission at optical fluences lower than 250 μJ/cm2. Although CdTe has some limitations in comparison with GaP, such as high THz absorption above 1 THz and the appearance of two-photon absorption at relatively low optical intensity, our findings demonstrate the potential of this crystal to be used as the emitter and detector in combination with the Yb laser.

Effect of the Electric Field Strength on the Energy Resolution of Cr/CdTe/Pt Detectors

The electrical and spectrometric characteristics of the Cr/cadmium telluride (CdTe)/Pt structure with a metal–semiconductor type rectifying contact, used in X/ $\gamma $ -ray detectors, are investigated. For the detectors fabrication, chlorine-doped CdTe crystals with a resistivity of (3–6) $\times 10^{{9}}\,\,\Omega $ cm at 293 K were used. 137Cs spectra and their modification by applied voltage change were obtained. The reverse bias voltage corresponding to the best energy resolution of the Cr/CdTe/Pt structure is experimentally determined. The concentration of impurities N in the crystal, which determines the thickness of the space-charge region, is estimated. An energy resolution deterioration of the Cr/CdTe/Pt structure with increasing voltage is explained due to the Gunn effect.

Comparative study of In/CdTe/Au Schottky- and p–n junction-diode detectors formed by backside laser irradiation doping

Two types of In/CdTe/Au diode structures were fabricated using detector-grade p-CdTe single crystals: (i) by vacuum evaporation of In and Au contacts on the chemically treated (111) surfaces; (ii) by backside laser doping when the In/CdTe structures were irradiated from the crystal side with a wavelength for which the semiconductor was transparent. Thus, diodes with a Schottky contact (In/CdTe) and a p–n junction were obtained, respectively. To prove the doping of the thin region near the In/CdTe interface by In (donor) and study thermal tolerance of both types of In/CdTe/Au diode detectors, room temperature I–V characteristics and 57Co isotope emission spectra were measured before and after annealing of the diodes at temperatures below and above the In melting point. Thermal annealing at lower temperatures led to a slight increase and decrease in reverse dark current of the unirradiated and laser-irradiated samples, respectively. Heating up to 200 °C resulted in a significant increase in reverse current and complete spectra degradation in the samples fabricated without laser processing. After such annealing, the electrical characteristics of the p–n junction diodes, formed by the backside laser doping technique, became optimized and 57Co spectra were almost unchanged. It was supposed that a Schottky barrier at the In/CdTe interface was degraded, while a p–n junction, created in a deeper region of the CdTe crystal, remained functional even after melting and solidifying the In contact.

Influence of phosphorus ion implantation on the optical properties of CdTe bulk crystal

Bulk CdTe crystals were implanted with 400 keV phosphorus ions to investigate the dopability of the host material. Induced devastation in crystal structure related to changes in optical constants of the material was examined by UV–Vis-IR transmittance measurements and spectrometric ellipsometry in a wide range of energies, corresponding to 200–2500 nm for transmittance and 191–2000 nm for ellipsometry. We used phosphorus particle fluences in a range of 5⋅1013 to 5⋅1016 P/cm2. Such fluences should provide p-type dopants concentration at a range of 1018 to 1021 P/cm3 assuming an average range of implanted ions (R) and longitudinal straggling (SR) as R ± SR = 403 ± 166 nm as estimated by SRIM software. The highest doping level was achieved with minor damages in the host lattice. Charged phosphorus particles implanted into bulk CdTe produce sublayer of CdTe:P, indicated by changes in the optical constants and luminescence. It was also observed that the doped layer of CdTe:P affects the transmittance of the crystals. Room temperature micro-Raman spectra of implanted samples, obtained with 532 nm and 785 nm lasers, do not differ from those for an undoped CdTe crystal. Luminescence measurements performed at temperature range down to 11 K show great improvement in luminescence emission, strong band-acceptor excitonic emission (A,X), exciton binding energy EXB = 8.7 meV and shallow acceptor level established by P-ion at about 19.1 meV above valence band maximum (VBM). We concluded that bulk crystals can be successfully doped with phosphorus ion of 400 keV energy up to ion fluences at the range of 5∙1016 P/cm2.

The optical performance of Cd1 − xTmxTe (0 ≤ x≤ 0.07) QDs synthesized via s,s′-phenylenebis-ethynediyl-phenylene-bisthioacetate assisted microwave approach for laser devices

Here, we have developed a new series of quantum dots based on the incorporation of thulium, Tm, atoms as efficient dopant in the cubic CdTe crystal structure for laser diode application potential. Therefore, Cd1 − xTmxTe (0 ≤ x ≤ 0.07) quantum dots were synthesized using s,s′-phenylenebis-ethynediyl-phenylene-bisthioacetate assisted microwave approach. The measurements of X-ray diffractions implied the preservation of the cubic phase of the CdTe crystals at all Tm dopant contents. The transmission electron microscopy inspected the formation of monodisperse and self-assemble nanoparticles with mean diameter from 1.8 ± 0.2 nm to 5.7 ± 0.3 nm, depending on the amount of Tm incorporated in the CdTe nanocrystals. A modulation of the optical bandgap was observed because of the doping of CdTe with Tm atoms. Highly luminescent emission spectra with tunable colors were obtained. The emission spectra showed sharp intense emitted light with an enhancement of its intensity reached to seven folds compared with the un-doped CdTe crystals. An increasing of quantum yields from 31% to 94% was realized. A decrease of the Stokes shift was observed which implies the synthesis of Cd1 − x TmxTe QDs free of defects for the first time. This unique performance renders the prepared Cd1 − x TmxTe to be potential candidate for the manufacturing of laser diodes.

Linear and nonlinear temperature-dependent transmission/absorption characteristics of cadmium telluride crystal for terahertz generation.

This paper reports on the temperature-dependent investigation of linear and nonlinear transmission/absorption characteristics of CdTe crystal in the 300-408 K range using 780-970 nm tunable wavelengths of 140 fs pulses obtained from a Ti:Sapphire laser at 80 MHz repetition rate. The same pulses were also used for terahertz generation. The linear transmission/absorption properties were measured using a specially improvised temperature-tuned spectrophotometer in the 500-1500 nm wavelength range. The linear absorption of 750 nm wavelength gradually increases with respect to a rise in the temperature, and transmission becomes zero at 408 K. Nonlinear absorption induced by femtosecond pulses shows a sudden drop of 18% in transmission above the 800 nm range, due to electron-phonon interaction, which affects the strength of the terahertz signal. It is also responsible for change in the temperature along with the linear shift in the refractive index of the crystal.

DETERMINATION OF THE STRUCTURAL STATE AND STABILITY OF THE LASER CRYSTALLIZED Cd1-xМnxTe CRYSTAL SURFACE

The modified surface layers of the Cd1-xMnxTe crystals were obtained by the laser recrystallization of the crystal surface with the use of millisecond and nanosecond ruby ​​lasers. The determination and diagnostics of the layer structural state were performed by the study the electron channeling patterns in the SEM. The AFM studies showed that mechanically stable contact regions in the CdTe crystal – Cu film system can be formed, depending on the laser energy density and beam defocusing. On the base of the ellipsometric studies, it was found that while irradiating the Cd1-xMnxTe crystal surface, the refractive index of the oxide film on the modified surface changes depending on the laser beam energy density, which can be interpreted as the formation of the oxides of the different chemical composition.

Influence of laser radiation on optical properties of high resistivity crystals CdTe and solid solutions Cd1-xZnxTe

In this paper, the transmission and reflection spectra of single crystals р-CdTe(111) are measured; solid solutions of Cd1-хZnхTe (х=0.1) in the range (0.2 – 1.7)·10-6 m before and after laser irradiation at the wavelength λ = 532 nm in the energy range 66 - 164 mJ/cm2 for CdTe(111) and in the energy range 46.6 mJ/cm2 - 163.5 mJ/cm2 for Cd1-хZnxTe (x = 0.1). It is established that the main mechanism of influence of pulsed laser irradiation on the optical properties of thin surface layers of the investigated crystals is structural gettering, that is, the absorption due to the presence of sections of semiconductors that have a defective structure and have the ability to actively absorb defects and points.

Simulation of anti-reflecting dielectric films by the interference matrix method

In this work, the reflection coefficients of multilayer and gradient dielectric films are simulated. The basis of the conducted researches is the theoretical analysis of reflection coefficients for single and double layer films, which use the criterion of zero reflection coefficients for electromagnetic wave with a given wavelength. The specified length is selected based on the maximum spectral density data of the solar radiation. Numerical calculations were performed on the basis of the interference matrix method. Using this method, the reflection coefficients were calculated for the spectral range of electromagnetic radiation, in which photoelectric conversion of solar cells made of silicon occurs. The dependences of the reflection coefficients on the angle of incidence of the sun's rays are also calculated. Numerical methods were used to determine the average values of the reflection coefficients, taking into account the spectral density of solar radiation. Optimal values of the thicknesses of single and double layer films were found as well as gradient films with the linear law of change of the refractive index of the film material. The optimum values found were compared with the characteristics of the dielectric coatings used in solar power. Both traditional technologies based on CdTe and Cu(In,Ga)Se2 crystals and relatively new technologies based on the use of nanostructured films and porous silicon films have been considered. For these materials, the characteristics of the films are calculated, which provide values of reflection coefficients that are close to optimal values.

One pot synthesis of highly luminescent Cd1-xOsxTe (0 ≤ x ≤ 0.1) nanocrystals synthesized via poly(9,9-dioctylfluorene-alt-benzothiadiazole) assisted polyol approach

Here, we report for the first time the creation of Cd1-xOsxTe (0 ≤ x ≤ 0.1) quantum dot using poly(9,9-dioctylfluorene-alt-benzothiadiazole) assisted polyol approach. The stimulus of osmium, Os, on the structure and optical emission behavior of CdTe quantum dots were examined. The X-ray diffraction measurement revealed the preservation of the cubic structures of CdTe crystals at all Os atoms concentration, implies the successful partial replacement of the Os atoms in the Cd cites. The transmission electron microscopy inspected that the doping of CdTe with Os atoms caused an increase of its particle diameter from 1.9 ± 0.2 nm to 7.1 ± 0.3 nm. The inclusion of Os atoms in the Cd atom cites instigated a shift to the red spectrum for both of the optical absorption/emission spectra. Astonishing upgrading of the luminescent intensities and a reduction of the width of emission spectra was achieved. An enlargement of the luminescent quantum yield from 31 % to 92 % was perfectly accomplished. A decreasing of the Stokes shift owing to the inclusion of Os atoms in the Cd cites, thereby a successful constraining of the crystal defects was realized. These outstanding demeanors may concrete an avenue to utilizing the developed Cd1-xOsxTe semiconductor quantum dots for the fabrication of competent laser diodes.

Triple-sensitivity high-spatial-resolution X-ray computed tomography using a cadmium-telluride detector and its beam-hardening effect

To observe blood vessels at high contrasts, we constructed a first-generation triple-sensitivity X-ray computed tomography (TS-CT) scanner using a cadmium-telluride (CdTe) detector and a triple-amplifying system. X-ray photons are absorbed by the CdTe crystal, and the electric charges produced by photons are converted into voltages using a current-to-voltage (I–V) amplifier, and the I–V output is amplified by a voltage-to-voltage (V–V) amplifier. The V–V output 1 is sent to a dual V–V amplifier through a 5.0-m-length coaxial cable and amplified to two-different outputs of 2 and 3. The three outputs 1–3 are sent to a personal computer through an analog-to-digital converter to reconstruct three tomograms simultaneously. In the TS-CT, the scattering photons from the object are extremely reduced using a 0.5-mm-diameter lead pinhole behind the object. The translation and rotation steps were 0.1 mm and 0.5°, respectively, and the spatial resolutions were 0.25 × 0.25 mm2. The scanning time was 19.6 min, and blood vessels were visible using gadolinium contrast media. In particular, the effective photon energy increased with increasing amplification factor of the amplifier caused by beam hardening of the object.

Real-Space Mapping of Surface-Oxygen Defect States in Photovoltaic Materials Using Low-Voltage Scanning Ultrafast Electron Microscopy.

Ultrathin layers of native oxides on the surface of photovoltaic materials may act as very efficient carrier trapping/recombination centers, thus significantly affecting their interfacial photophysical properties. How ultrathin oxide layers affect the surface and interface carrier dynamics cannot be selectively accessed by conventional ultrafast transient spectroscopic methods due to the deep penetration depth (tens to thousands of nanometers) of the pump/probe laser pulses. Herein, scanning ultrafast electron microscopy (S-UEM) at a low voltage of 1 keV electrons was recently developed at KAUST to selectively map the ultrafast charge carrier dynamics of a few layers on the top surfaces of photovoltaic materials. Unlike high-voltage 30 keV experiments, at 1 keV, the depth of detected secondary electrons (SEs) underneath the surface is significantly reduced 5 times, thus making it possible to visualize the dynamics of charge carrier from the uppermost surface of photoactive layers. More specifically, this new approach has been employed to explore and decipher the tremendous impact of native oxide layers and oxygen defect states on charge carrier dynamics in space and time simultaneously at sub-nanometer levels on several photovoltaic material surfaces, including Si, GaAs, CdTe, and CdZnTe single crystals. Interestingly, the contrast in the SEs time-resolved difference images switched from a dark "energy-loss mechanism" to a bright "energy-gain mechanism" only by removing the layers of surface oxides. Moreover, the charge carrier recombination was estimated and found to be dramatically affected by the native oxide layers. The density functional theory (DFT) calculations demonstrate that the work function of oxygen-terminated Si surface also increases slightly upon optical excitation and makes for less SE intensity, providing another piece of evidence that the origin of the dark contrast observed on these material surfaces should be assigned to the surface oxide formation, mainly oxygen defect states in the band gap and/or work function increment. Our findings provide a new method and pave the way for evaluating the effect of surface morphology and defects, including but not limited to native oxide, on charge carrier dynamics, and interfacial properties of photovoltaic absorber layers.

CdTe synthesis and crystal growth using the high-pressure Bridgman technique

Abstract Efficient, safe and cost-effective synthesis of CdTe from elements is rather challenging in silica sealed ampoules due to the high vapor pressure of Cd. In this article, we report on the integrated synthesis and crystal growth of high-purity CdTe using the high pressure Bridgman (HPB) technique that is scalable to large volumes. The process lends itself for cost competitive industrial production of polycrystalline feedstock material for photovoltaics, sensors and electro-optic applications. Cadmium telluride (CdTe) crystals exceeding 1 kg in size were synthesized from elemental Cd and Te sources with purity comparable to state-of-the-art gamma ray detector crystals. In addition, synthesis of highly-doped CdTe feedstock for thin film photovoltaics applications demonstrating effective incorporation of group V (As, Sb) dopants was achieved at growth speeds of ~500 mm/hr. The technique may be applicable to produce other II-VI compounds with volatile components.

Femtosecond Pump Probe Reflectivity Spectra in CdTe and GaAs Crystals at Room Temperature.

Ultrafast pump probe reflectivity (PPR) signal near band edge is modeled by taking into account band filling (BF) and band gap renormalization (BGR) effects with the carrier density of ~1017/cm3 in GaAs crystal at room temperature. The calculated results indicate that the transient reflectivity ΔR/R is determined by BF and BGR effects. The most interesting feature is that ΔR/R signal experiences a sign change from photo-bleaching (PB) to photo-absorption (PA) due to the competition between BF and BGR effects. We experimentally measured ΔR as a function of photon energy across band edge with carrier density of ~1017/cm3 in GaAs and CdTe crystals, which has a similar trend as that calculated according to our model. In addition, the reflectivity is very sensitive to electron spin orientation, which is well confirmed by the corresponding experiments with 100 fs pump probe reflectivity spectroscopy in bulk CdTe. Our research in this work provides a method to study optoelectronic properties of conventional semiconductors at moderate carrier density excited by ultrafast laser pulse. Importantly, this model can be used for other novel semiconductor materials beyond GaAs and will provide new insights into the underlying spin dependent photophysics properties for new materials.

Polarisation detection sensor with EO polymer‐dispersed liquid crystals

This Letter proposed a polarisation detection sensor by utilising electro-optic (EO) crystals in purpose of tumour's boundary detection. In this work, efficiency of uniaxial (anisotropic) EO crystals like CdTe and isotropic EO crystals such as polymer-dispersed liquid crystal (PDLC) as the polarisation detection sensor is studied and compared with each other. Based on, derived equations for refractive index change as a function of electric field (Pockels effect) for these two types of EO crystals, capability of using PDLCs in polarisation detection sensors is confirmed. In addition, simulations are prepared to support this claim. To this end, PDLC probe is used to detect boundaries of a simulated tumour (target with the ability to change polarisation of backscattered signal).

Mathematical Simulation of the Process of Growing a CdTe Single Crystal by the Obreimov–Shubnikov Method

For the first time, the process of growing a CdTe crystal by the modified Obreimov–Shubnikov method using the technique of self-nucleation from the initial cooling temperature (1100°C) to the time of reaching the stationary growth mode is simulated. The motion of the crystallization front during crystal growth is calculated. The results are confirmed by the X-ray topography method with use of synchrotron rad-iation.

Influence of Laser Radiation on Optical Properties of High Resistivity CdTe Crystals and Cd1 – xZnxTe Solid Solutions in the Field of the Fundamental Optical Transition E0

Influence of Laser Radiation on Optical Properties of High Resistivity CdTe Crystals and Cd1 – xZnxTe Solid Solutions in the Field of the Fundamental Optical Transition E0

Математическое моделирование процесса выращивания монокристалла CdTe методом Обреимова--Шубникова

Modeling the growth of CdTe crystal modified method Obreimov Shubnikov-using the of self-seeding techniques from the start cooling temperature (1100 °C) until the output to the mode stationary growth was carried out in this paper for the first time. Figures are given of changes in the crystallization front during crystal growth, which are confirmed by experiment using synchrotron radiation.