CdZnTe Single Crystals Research Articles

Enhanced performance CdZnTe single crystal with few surface damages via solution based annealing

In this study, heat treatments were employed using a CdCl2 solution as the annealing medium to investigate the evolution of internal defects and photoelectric properties of CdZnTe crystals. It was observed that CdCl2 crystals formed on the surface of the annealed crystals, and their size increased exponentially with prolonged annealing time. It could be a medium for Cl transport. The liquid-phase annealing process was found to be gentle, resulting in minimal damage and the absence of large defects migrating or accumulating dislocations and less nano-defects on the crystal surface than sample annealed in vapor condition. After a 30 hour annealing, the infrared (IR) transmittance reached 55%, and the resistivity increased to 1.36×1010 Ω·cm. The presence of Cd2+ facilitated the filling of Cd vacancies (VCd) defects through diffusion reduction, while the diffusion of Cl- introduced complex defect with extra VCd, causing the Fermi level to approach the center of the band gap. Consequently, a high-resistance CdZnTe material was achieved.

Development of travelling heater method for growth of detector grade CdZnTe single crystals

Development of travelling heater method for growth of detector grade CdZnTe single crystals

Evolution features and optimization of interface shape during CdZnTe crystal growth using Vertical Gradient Freeze (VGF) technique

The growth of CdZnTe crystal suffers from poor yield due to the low thermal conductivity, and a convex growth interface is usually essential to grow large-volume CdZnTe single crystal because it can lessen the adverse interactions between the growing crystal and crucible wall. The growth of 90 mm diameter CdZnTe crystal by VGF technique has been investigated using experimental and simulation methods. The influences of the growth parameters (such as temperature gradient and growth rate) and the heat transfer configurations on the growth interface shape have been inspected respectively by the unsteady simulation. The simulation results confirm that the interface shape exhibits the tendency to be “convex → flat → concave” due to the gradually deteriorated heat transfer environment with time during growth. In addition, a modified configuration has been designed and used in experiments based on the simulation results, with the aim of strengthening the conduction heat transfer of the center region of the configuration geometry. The modified configuration consists of the splicing tapered-contact quartz&graphite support with a shorter conical region and seed region, as well as the SiC holder. Experimental results demonstrate that the modified configuration would make the growth interface maintain a convex shape for a longer duration. The CdZnTe crystal grown by using the modified configuration possesses fewer grains compared with that using the initial configuration.

Tailoring the defects and resistivity in CdZnTe single crystal via one-step annealing with CdTe compound

Defects deteriorate the optical and electronic properties of the cadmium zinc telluride (CZT) single crystal. However, the growth of defects, such as vacancies and precipitation, could not be avoided in the industrial-grade preparation of CZT single crystal. Therefore, it is crucial to explore ways to eliminate defects after growth. This paper employs one-step heat treatment with the CdTe compound as an annealing atmosphere source. After annealing, some large-size Te inclusions are eliminated in CZT crystal, but nano Te precipitates still exist. At the same time, the crystal surface becomes rougher, caused by the thermal migration of Te droplets, dislocation proliferation, and aggregation. Increasing annealing time, dislocations migrate, decompose, and interact with each other to form dislocation walls, stacking faults or terminating at the surface, reducing dislocation etch pit density. In addition, a large number of ordered phases are still found in the crystal. Single annealing with CdTe significantly improved the crystal's optical and electrical properties. After 180 h annealing, the infrared (IR) transmittance of the as-grown wafer increased from 40% to 65%, and resistivity increased from 2.2 × 109 Ω cm to 1.7 × 1011 Ω cm. Compared to reducing the Te precipitation phase, these increasements were mainly dominated by replacing point defects, like Cd vacancies.

Investigation of seeded vertical gradient freeze (VGF) growth of CdZnTe bulk crystals

Due to the requirement for superheating above the melting point in the homogenization process of CdZnTe material, complete seed melting remains as a significant problem for the seeded vertical gradient freeze (VGF) growth of CdZnTe. In addition, due to the typical low single crystal yields in VGF growth of CdZnTe, obtaining single crystal seed material with desired geometries could be considered as a challenge. In this study, single crystal CdZnTe seeds of cylindrical geometry with 10 mm diameter and up to 30 mm length were successfully obtained using a Computer Numerical Control (CNC) cutting system. Moreover, to avoid complete seed melting, a novel design of the growth crucible and support system along with radiation shielding around seed region is employed. Temperature profiling and growth experiments were conducted to observe seed melting and seeded crystal growth of CdZnTe. Initial results of a seeded VGF growth using a single crystal CdZnTe seed is reported.

Formation and characterization of a CuPt-A type ordered structure in cadmium zinc telluride single crystals

The CuPt-A type ordered CdZnTe phases have first been characterized comprehensively in CdZnTe single crystals. The possible crystal structure and formation mechanism of such ordered phase have been proposed based on the TEM results.

Bragg Diffraction Imaging of CdZnTe Single Crystals

A high-quality CdZnTe single crystal has been imaged with micrometer resolution using monochromatic x-ray Bragg diffraction at the bending magnet (BM)-05 beamline of the European Synchrotron Radiation Facility. Since this material is used to produce substrates for the epitaxial growth of infrared sensitive HgCdTe photon detectors, imaging the defects is of strong interest to visualize their arrangement within the bulk. We show that large field-of-view rocking curve projection maps can be acquired in transmission geometry using samples as thick as 0.5 mm thanks to the Borrmann effect. Furthermore, section topography can be applied to provide information on the depth of the observed defects allowing for a complete 3D localization of lattice distortions. Examples of 1.5 mm × 1.5 mm full width at half-maximum maps are given to illustrate the spatial arrangement of dislocations in a number of samples cut with respect to different crystallographic planes in a particular CdZnTe ingot.

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.

Deep Radiation-Induced Defect Centers Created by a Fast Neutron Flux in CdZnTe Single Crystals

The effect of a fast neutron flux (Φ = 1014–1015 cm–2) on the electrical and photoluminescence properties of p-CdZnTe single crystals is studied. Isothermal annealing is performed (T = 400–500 K), and the activation energy of the dissociation of radiation-induced defects is determined at ED ≈ 0.75 eV.

Глубокие центры радиационных дефектов в монокристаллах CdZnTe, созданные потоком быстрых нейтронов

AbstractThe effect of a fast neutron flux (Φ = 10^14–10^15 cm^–2) on the electrical and photoluminescence properties of p -CdZnTe single crystals is studied. Isothermal annealing is performed ( T = 400–500 K), and the activation energy of the dissociation of radiation-induced defects is determined at E _D ≈ 0.75 eV.

HRTEM Analysis of Crystallographic Defects in CdZnTe Single Crystal

In recent years, CdZnTe has attracted much attention due to its superior electrical and structural properties for room-temperature operable gamma and x-ray detectors. However, CdZnTe (CZT) material has often suffered from crystallographic defects encountered during the growth and post-growth processes. The identification and structural characterization of these defects is crucial to synthesize defect-free CdZnTe single crystals. In this study, Cd0.95 Zn0.05 Te single crystals were grown using a three-zone vertical Bridgman system. The single crystallinity of the material was ensured by using x-ray diffraction measurements. High-resolution electron microscopy (HRTEM) was used to characterize the nano-scale defects on the CdZnTe matrix. The linear defects oriented along the ⟨211⟩ direction were examined by transmission electron microscopy (TEM) and the corresponding HRTEM image simulations were performed by using a quantitative scanning TEM simulation package.

Hypersound waves at the surface of CdZnTe crystals: The role of surface orientation and scattering at twin boundaries

The patterns of propagation of gigahertz surface acoustic waves (SAWs) are obtained for cuts of CdZnTe single crystals with different crystallographic orientations. By comparing the experimental patterns with the calculated ones, the existence of at least two SAW modes, one of which is a Rayleigh mode, is demonstrated. It is shown that the anisotropy of propagation of different SAW modes makes it possible to determine the local crystallographic orientation of the CdZnTe surface and detect local imperfections of the crystal structure. Strong anisotropic scattering of the Rayleigh wave by coherent twin boundaries is found.

Quality improvement of CdZnTe single crystal by ultrasound processing

Intrinsic defects and contaminations removal from the undoped p-type Cd0.96Zn0.04Te single crystals has been achieved by the ultrasound vibration processing at the room temperature. Surface analysis based on Auger Electron Spectroscopy, Energy Dispersive Spectroscopy, and Scanning Electron Spectroscopy shows a significant reconstruction of the crystal surface after processing, namely, the appearance of numerous “volcano craters” and triangle-shaped defects with a typical size of 0.2–5.0µm. Elemental analysis of these defects shows that they are Te inclusions emerged on the surface. The regular crystal surface outside the defects also displays a considerable enrichment by Te. Distinct presence of copper is found in both the thin surface layer and in the defects emerged on the surface. The surface reconstruction is associated with a remarkable change in the bulk material properties, electrical (an increase in the resistivity by a factor of ∼6) and optical (an IR transmittance increase). A post-polishing following the ultrasound processing makes the CdZnTe material more stable and reliable for a wide range of device applications.

Deep levels in high resistive CdTe and CdZnTe explored by photo-Hall effect and photoluminescence spectroscopy

High resistive CdTe and CdZnTe single crystals were measured by photo-Hall effect spectroscopy (PHES) and photoluminescence spectroscopy (PL) with the aim of discovering the position of deep levels (DLs) in the band gap. Illumination in the range of 0.65–1.77 eV, room temperature, and DC electrical measurements were used in the case of PHES. Low temperature (4 K) photoluminescence spectra were recorded in the spectral range above 0.47 eV. Eight samples, both n-type and p-type, were studied and typical shapes of spectra were collected, compared and interpreted for both spectroscopy methods. It was shown that a simple single-level model of PHES often fails in the interpretation of DLs distant from the midgap. Eight DLs with the energy Ec − 0.65 eV, Ec − 0.8 eV, Ec − 0.9 eV, Ec − (1.10–1.15) eV, Ev + 0.70 eV, Ev + 0.85 eV, Ev + 1.0 eV, and Ec − 1.25 eV were interpreted. A memory effect characterized by a relaxation time of about 60 s was observed at the 0.8 eV level and allowed us to determine the 1.7 × 10−17 cm2 capture cross-section of electrons on this level. It is argued that PHES is a convenient complementary method to identify and characterize DLs, including DLs inaccessible by thermal emission techniques. DLs observed by PHES were consistently verified by PL.

Infrared Attenuation Spectrum of Bulk High-Resistivity CdZnTe Single Crystal in Transparent Wavelength Region Between Electronic and Lattice Absorptions

We report measurement of the internal attenuation coefficient, $$\alpha _{att}$$ , of a bulk high-resistivity cadmium zinc telluride (CdZnTe) single crystal at wavelength, $$\lambda$$ = 0.84–26 $$\mu$$ m, to the unprecedentedly low level of $$\alpha _{att}$$ $$\sim$$ 0.001 cm $$^{-1}$$ . This measurement reveals the spectral behavior for small attenuation in the infrared transparent region between the electronic and lattice absorptions. This result is essential for application of CdZnTe as an infrared transmitting material. Comparing the attenuation spectrum with model spectra obtained on the basis of Mie theory, we find that sub-micrometer-sized Te particles (inclusions) with a number density of approximately 10 $$^{7.5-9}$$ cm $$^{-3}$$ are the principal source of the small attenuation observed at $$\lambda$$ = 0.9–13 $$\mu$$ m. In addition, we determine $$\alpha _{att}$$ = $$(7.7 \pm 1.9) \times 10^{-4}$$ cm $$^{-1}$$ at $$\lambda$$ = 10.6 $$\mu$$ m, which is valuable for CO $$_2$$ laser applications. Higher transparency can be achieved by reducing the number of inclusions rather than the number of precipitates. This study also demonstrates that high-accuracy measurement of CdZnTe infrared transmittance is a useful approach to investigating the number density of sub-micrometer-sized Te particles that cannot be identified via infrared microscopy.

Investigation of dislocation migration in substrate-grade CdZnTe crystals during post-annealing

The migration of dislocations in substrate-grade CdZnTe (CZT) single crystals during temperature gradient annealing under Cd/Zn vapor has been investigated. The etch pit density (EPD) and configuration of dislocations have been evaluated before and after annealing in CZT crystals with and without Cd-rich second phase (Cd-SP) particles, respectively. After Cd/Zn overpressure annealing, dislocation reduction in CZT crystals was observed. However, dislocation walls with 120° intervals along <211> crystalline direction were observed in the both types of CZT crystals. The formation of these dislocation walls can be attributed to the reaction of <110> dislocations. Moreover, it is considered that the release of the restored stress during annealing act as the domain driving force for dislocation migration, by comparing the variation of dislocation configuration in CZT crystals with and without Cd-SP particles.

Te inclusion-induced electrical field perturbation in CdZnTe single crystals revealed by Kelvin probe force microscopy

To understand the effects of tellurium (Te) inclusions on the device performance of CdZnTe radiation detectors, the perturbation of the electrical field in and around Te inclusions was studied in CdZnTe single crystals via Kelvin probe force microscopy (KPFM). Te inclusions were proved to act as lower potential centers with respect to surrounding CdZnTe matrix. Based on the KPFM results, the energy band diagram at the Te/CdZnTe interface was established, and the bias-dependent effects of Te inclusion on carrier transportation is discussed.

CdZnTe Detectors Operating at X-ray Fluxes of 100 Million &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\mathrm {Photons}/(\mathrm {mm}^{2.}\mathrm {sec})$ &lt;/tex-math&gt; &lt;/inline-formula&gt;

CdZnTe single crystals grown with the traveling heater method have been specifically engineered to support high-flux x-ray photon counting applications. Pixelated detector devices absorb hardened $120~\mathrm {kV}_{\mathrm {p}}$ fluxes up to $10^{8}~\mathrm {photons}/(\mathrm {mm}^{2.}\mathrm {sec})$ without any sign of electric field polarization at room temperature (23-28) °C. It is shown that detector functionality can be verified using photon counting electronics even under conditions of strong pulse pile-up. A non-paralyzable counting model satisfactory describes the average output count rate of the detector-electronics system as a function of the absorbed flux. Good pixel-to-pixel count rate uniformity can be achieved.

VGF bulk growth, crystalline perfection and mechanical studies of CdZnTe single crystal: A detector grade materials

Bulk size (∼80 mm long and 20 mm dia.) with high quality single crystal of Cadmium Zinc Telluride (CZT) has been successfully grown by Vertical Gradient Freeze (VGF) method using five zone furnace. The powder X-ray diffraction analysis confirms the phase of CZT crystal and growth direction was found to be (111). The grown crystal was cut and polished into different pieces and crystalline perfection was studied by high-resolution X-ray diffraction (HRXRD) by employing a multicrystal X-ray diffractometer. The HRXRD study shows that the crystalline perfection of the grown crystals is quite good. The mechanical properties were carried out on the (111) face of different portions (2–1, 2-2-, 6–2, 6–4) of CZT single crystal at room temperature in the load ranging from 0.1 to 5.0 N. The load independent hardness values for 2–1, 2-2, 6–2 and 6–4 are found to be 0.62, 0.65, 0.61 and 0.55 GPa, respectively. In addition, the observed crack profile of indentation mark has been recorded and various other parameters such as dependence of crack length, fracture toughness, brittle index number, yield strength were also calculated and discussed.

Investigation of the effect of argon ion beam on CdZnTe single crystals surface structural properties

Abstract Scanning Electron microscopy and Atomic Force Microscopy and X-ray photoemission spectroscopy were used to investigate the effect of argon ion bombardment on the surface of CdZnTe crystal structural morphology. The sample was irradiated by defined doses of 5 keV argon ions. We observed sudden increase of the surface roughness at low doses. After irradiation by higher doses up to 1.4 × 1016 ions/cm2, the surface showed smoothening. This value proved to be optimal for oxidized damaged layer removal and surface smoothening. AFM analysis confirmed lowest roughness of the surface. Higher values of ion fluences caused increasing roughness and waviness of the surface and creation of craters on the surface. The XPS analysis showed no preferential removal of Cd, Zn or Te by used ion irradiation doses. The sputter rate stabilized from ion fluences higher than 7 × 1015 Ar+ ions/cm2. The possible origin of this craters and their cross-like alignment is explained.