Quality CdTe Research Articles

Dislocation clustering and grain nucleation in THM growth of CdTe single crystals

We report a volume defect behavior in CdTe crystals grown by the travelling heater method (THM), involving dislocation clustering and grain nucleation. Spurious grain nucleation events occur within a specific radial zone in the crystal where the growth interface is planar. These grains were characterized by Laue x-ray diffraction and were found to have the same angle of misorientation to the bulk, with exception of an in-plane rotation. The grains were connected to columnar dislocation clusters that were aligned parallel to the growth axis of the crystal and were evidently linked to the planar region of the growth interface. Simulation of the growth process was used to investigate the effects of solvent zone convection and interface curvature with different rotational rates. The observed defect behavior was attributed to the effect of melt convection on the interface shape and instability. Evidence of faceting on the growth interface and its correlation with spurious grain nucleation is shown. These findings highlight the importance of interface engineering in THM growth of high quality CdTe single crystals.

Study of the Effect of Various Chemical Polishing Treatments on MBE-Grown CdTe/GaAs (211)B Heterostructures

A three-inch-diameter high quality CdTe thin film was grown on a GaAs (211)B substrate by molecular beam epitaxy (MBE) in ultra-high vacuum conditions. The CdTe/GaAs (211)B heterostructure was then cut into several sample pieces. A few as-grown sample pieces were subjected to chemical etching solutions which created etch pits on the surface. The scanning electron microscopy images of such samples were used to calculate the etch pit densities on the surface. In addition, several as-grown samples were subjected to chemical polishing treatments under different conditions to quantify the removal of O and Te-O structures from the surface. Atomic force microscopy was used to determine as-grown and polished surface morphology and the polish rate of chemical solutions. A study of the surface stoichiometry and the chemical composition of the as-grown and polished CdTe (211)B surfaces were carried out by using X-ray photoelectron spectroscopy. Bulk structural qualities of the as-grown and polished samples were studied in terms of the vibrational and phonon modes via confocal Raman spectroscopy. From a comparative analyses of the results, the best chemical polishing conditions for the MBE-grown CdTe (211)B heterostructure were determined.

Influence of Surface Structures on Quality of CdTe(100) Thin Films Grown on GaAs(100) Substrates**Supported by the National Key Research and Development Program of China under Grant Nos 2017YFB0405704 and 2017YFA0305400, the 1000-Young Talent Program of China, and the Shanghai Sailing Program under Grant No 17YF1429200.

We report the epitaxial growth of single-crystalline CdTe(100) thin films on GaAs(100) substrates using molecular beam epitaxy. By controlling the substrate pre-heated temperature with adjustable Te flux, three different reconstructed surfaces are realized, and their influence on the subsequent CdTe growth is investigated. More importantly, we find that both the presence of a thin native oxide layer and the formation of Ga-As-Te bonds at the interface enable the growth along the (100) orientation and help to reduce the threading dislocations and other defects. Our results provide new opportunities for compound semiconductor heterogeneous growth via interfacial engineering.

Influence of the type of conducting glass substrate on the properties of electrodeposited CdS and CdTe thin films

Owing to the greater efficiency in energy conversion by absorbing energy in a wider range of the solar spectrum, thin film CdS/CdTe solar cells have been popularized as a prominent application of photovoltaics and the technique of electrodeposition (ED) is an ideal method available for producing both CdS and CdTe materials upon its outrivaled simplicity, low cost, scalability and manufacturability. Typically the quality of these deposited thin films depends on several growth parameters and amid them, the type of conducting glass substrate plays a crucial role. This study is focused on the influence of conducting glass substrate on the properties of electrodeposited CdS and CdTe thin films. Two types of glass substrates coated with different transparent conducting oxide (TCO) layers namely; fluorine-doped tin oxide (FTO) and indium tin oxide (ITO) having different sheet resistances (FTO: 7 and 13 Ω/sq and ITO: 7 and 15 Ω/sq) were considered. CdS and CdTe materials were electrodeposited respectively on each of these types of TCO layers using a three electrode electrolytic system under a consistent set of growth parameters which has been pre-optimized with respect to FTO having sheet resistance of 7 Ω/sq. The deposited CdS and CdTe thin layers were subsequently heat treated and characterized to understand their optical, electrical, morphological and structural properties. Accordingly, CdS and CdTe semiconductor materials deposited on glass/FTO (7 Ω/sq) substrates have exhibited better optoelectronic qualities and hence, endorse the requirement of individual growth parameter optimization for each type of TCO glass substrate for the production of good quality CdS and CdTe thin films in photovoltaic device fabrications.

MBE-growth of CdTe on GaSb substrates: A case study on the influence of substrate quality

CdTe epitaxial layers were grown by MBE on GaSb (211)B substrates from two different suppliers in order to determine the influence of as-received substrate quality on the material quality of subsequently-grown CdTe epilayers. It is observed that GaSb substrates with smooth surface and lower degree of surface roughness can lead to CdTe epilayers with superior material quality, as evidenced by a lower dislocation density, lower etch pit density, lower degree of epilayer surface roughness, narrower XRD FWHM, and lower strain. It is concluded that in comparison to substrates with a relatively smooth surface (RMS surface roughness ∼ 0.62 nm), substrates with a high degree of surface roughness (RMS surface roughness ∼ 2.24 nm) lead to effects that generate misfit dislocations during the MBE growth process. This results in approximately an order of magnitude increase in measured etch pit density (from ∼9 × 105 to 7 × 106 cm−2) and the calculated dislocation density (from 6.51 × 105 to 6.87 × 106 cm−2, as determined from X-ray diffraction reciprocal space mapping), indicating that a smooth substrate surface is critical in achieving high quality CdTe epilayers on GaSb.

Molecular beam epitaxial growth, transmittance and photoluminescence spectra of zinc-blende CdTe thin films with high-quality on perovskite SrTiO3 (1 1 1) substrates

High-crystalline quality CdTe thin films are grown on the largely lattice-mismatched SrTiO3 (STO) (1 1 1) substrates by molecular beam epitaxy. A transformation from a three dimensional regime to a two dimensional one is observed by the reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM). The formation of an elastic deformation CdTe layer on STO (1 1 1), namely a pseudomorphic growth mode with a critical thickness of ∼40 nm, is supported by the RHEED, AFM and X-ray diffraction. Crystal structures and epitaxial relationships of CdTe epitaxial films on STO (1 1 1) are characterized by 2θ-ω scans and reciprocal space mapping. Two strong absorption peaks at the energies of ∼1.621 eV and ∼1.597 eV at 5 K are clearly observed for a ∼120 nm thick CdTe epitaxial film, which are proposed to be ascribed to the strained and unstrained epitaxial CdTe layers, respectively. Moreover, the presence of the exciton band while the absence of deep level defect states for the ∼120 nm thick CdTe film characterized by the temperature dependent photoluminescence spectra further supports the high-crystalline quality.

Embedded vertically aligned cadmium telluride nanorod arrays grown by one-step electrodeposition for enhanced energy conversion efficiency in three-dimensional nanostructured solar cells

Vertically aligned CdTe nanorods (NRs) arrays are successfully grown by a simple one-step and template-free electrodeposition method, and then embedded in the CdS window layer to form a novel three-dimensional (3D) heterostructure on flexible substrates. The parameters of electrodeposition such as deposition potential and pH of the solution are varied to analyze their important role in the formation of high quality CdTe NRs arrays. The photovoltaic conversion efficiency of the solar cell based on the 3D heterojunction structure is studied in detail. In comparison with the standard planar heterojunction solar cell, the 3D heterojunction solar cell exhibits better photovoltaic performance, which can be attributed to its enhanced optical absorption ability, increased heterojunction area and improved charge carrier transport. The better photoelectric property of the 3D heterojunction solar cell suggests great application potential in thin film solar cells, and the simple electrodeposition process represents a promising technique for large-scale fabrication of other nanostructured solar energy conversion devices.

Van der Waals epitaxy of CdTe thin film on graphene

van der Waals epitaxy (vdWE) facilitates the epitaxial growth of materials having a large lattice mismatch with the substrate. Although vdWE of two-dimensional (2D) materials on 2D materials have been extensively studied, the vdWE for three-dimensional (3D) materials on 2D substrates remains a challenge. It is perceived that a 2D substrate passes little information to dictate the 3D growth. In this article, we demonstrated the vdWE growth of the CdTe(111) thin film on a graphene buffered SiO2/Si substrate using metalorganic chemical vapor deposition technique, despite a 46% large lattice mismatch between CdTe and graphene and a symmetry change from cubic to hexagonal. Our CdTe films produce a very narrow X-ray rocking curve, and the X-ray pole figure analysis showed 12 CdTe (111) peaks at a chi angle of 70°. This was attributed to two sets of parallel epitaxy of CdTe on graphene with a 30° relative orientation giving rise to a 12-fold symmetry in the pole figure. First-principles calculations reveal that, despite the relatively small energy differences, the graphene buffer layer does pass epitaxial information to CdTe as the parallel epitaxy, obtained in the experiment, is energetically favored. The work paves a way for the growth of high quality CdTe film on a large area as well as on the amorphous substrates.

Towards defect-free epitaxial CdTe and MgCdTe layers grown on InSb (001) substrates

A series of three CdTe/MgxCd1−xTe (x~0.24) double heterostructures grown by molecular beam epitaxy on InSb (001) substrates at temperatures in the range of 235–295°C have been studied using conventional and advanced electron microscopy techniques. Defect analysis based on bright-field electron micrographs indicates that the structure grown at 265°C has the best structural quality of the series, while structures grown at 30°C lower or higher temperature show highly defective morphology. Geometric phase analysis of the CdTe/InSb interface for the sample grown at 265°C reveals minimal interfacial elastic strain, and there is no visible evidence of interfacial defect formation in aberration-corrected electron micrographs of this particular sample. Such high quality CdTe epitaxial layers should provide the basis for applications such as photo-detectors and multi-junction solar cells.

Growth and characterization of CdTe absorbers on GaAs by MBE for high concentration PV solar cells

AbstractCdTe based II‐VI absorbers are promising candidates for high concentration PV solar cells with an ideal band gap for AM1.5 solar radiation. In this study, we propose single crystal CdTe absorbers grown on GaAs substrates with a molecular beam epitaxy (MBE) which is a clean deposition technology. We show that high quality CdTe absorber layers can be grown with full width half maximum of X‐ray diffraction rocking curves (XRD RC) as low as 227 arc‐seconds with 0.5% thickness uniformity that a 2 µm layer is capable of absorbing 99% of AM1.5 solar radiation. Bandgap of the CdTe absorber is found as 1.483 eV from spetroscopic ellipsometry (SE) measurements. Also, high absorption coefficient is calculated from the results, which is ∼5 x 105cm‐1 in solar radiation spectrum. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis of High Quality CdTe Quantum Dots in Aqueous Solution Using Multidentate Polymer Ligands under Microwave Irradiation

Synthesis of High Quality CdTe Quantum Dots in Aqueous Solution Using Multidentate Polymer Ligands under Microwave Irradiation

Solution-processed efficient CdTe nanocrystal/CBD-CdS hetero-junction solar cells with ZnO interlayer

CdTe nanocrystal (NC)/CdS p–n hetero-junction solar cells with an ITO/ZnO-In/CdS/CdTe/MoO x /Ag-inverted structure were prepared by using a layer-by-layer solution process. The CdS thin films were prepared by chemical bath deposition on top of ITO/ZnO-In and were found to be very compact and pin-hole free in a large area, which insured high quality CdTe NCs thin-film formation upon it. The device performance was strongly related to the CdCl2 annealing temperature and annealing time. Devices exhibited power conversion efficiency (PCE) of 3.08 % following 400 °C CdCl2 annealing for 5 min, which was a good efficiency for solution processed CdTe/CdS NC-inverted solar cells. By carefully designing and optimizing the CdCl2-annealing conditions (370 °C CdCl2 annealing for about 15 min), the PCE of such devices showed a 21 % increase, in comparison to 400 °C CdCl2-annealing conditions, and reached a better PCE of 3.73 % while keeping a relatively high V OC of 0.49 V. This PCE value, to the best of our knowledge, is the highest PCE reported for solution processed CdTe–CdS NC solar cells. Moreover, the inverted solar cell device was very stable when kept under ambient conditions, less than 4 % degradation was observed in PCE after 40 days storage.

CdTe microwire-based ultraviolet photodetectors aligned by a non-uniform electric field

We report on ultraviolet (UV) photodetectors fabricated by positioning Cadmium Telluride (CdTe) microwires (μWs) precisely by dielectrophoretic (DEP) force, where CdTe μWs were grown using an Au-catalyst-assisted closed-space-sublimation (CSS) method. The optical properties of CSS-grown CdTe μWs were characterized by micro-photoluminescence and micro-Raman spectroscopies. Optoelectronic characteristics were obtained after CdTe μWs were aligned on a pre-patterned SiO2/Si substrate by a non-uniform electric field. Photocurrents were increased with increasing the light intensities. Fast and reliable photoresponse and recovery were observed when CdTe μWs were exposed to UV illuminations. We demonstrated that high quality CdTe μWs grown by the CSS method have significant potentials as optoelectronic devices.

Influence of electrodeposition conditions on the properties of CdTe films

The results of the influence of electrodeposition conditions on the structural, compositional, optical, and photoelectrochemical properties of CdTe thin films deposited in one-step electrochemical method are presented. The CdTe films were prepared electrochemically from aqueous acidic solution with low ratios of Cd2+ ions to Te(IV) ions concentration. Instead of commonly used TeO2, water-soluble Na2TeO3 was used as a source of tellurium ions. The cathodic deposition of CdTe was performed at different constant potentials from solutions containing different cadmium and tellurium ions concentration. As-deposited CdTe thin films were studied by different analytical techniques. The X-ray photoelectron spectroscopy spectra exhibited CdTe formation on the electrode with some amount of tellurium oxides and cadmium oxides. The best quality CdTe deposits, free of TeO2, were formed in bath containing excess of Cd2+ ions and at the potential of −0.65 V vs. saturated calomel electrode, slightly more positive than Eeq of Cd/Cd2+ system. Structural X-ray diffraction studies revealed polycrystallinity of deposits with the highest content of the (111)-oriented cubic (111) form. Optical band gap energy values were found in the range from 1.36 to 1.6 eV for CdTe films prepared at various synthesis conditions. The preliminary photoelectrochemical studies have shown that the variation of the deposition potential as well as bath composition leads to the formation of p- or n-type CdTe films. As-deposited CdTe films were not stable in polysulfide solution under illumination.

Hydrothermal synthesis of thiol-capped CdTe nanoparticles and their optical properties

Water soluble nanoparticles (NPs) with a high emission property were synthesized via hydrothermal routes. In this report, we chose thiol ligand N-acetyl-L-cysteine as the ideal stabilizer and have successfully employed it to synthesize readily size-controllable CdTe NPs in a reaction of only one step. Hydrothermal synthesis of CdTe NPs has been carried out in neutral or basic conditions so far. We found out that the pH value of precursor solutions plays an important role in the uniformity of the particle size. Actually, high quality CdTe NPs were synthesized under mild acidic conditions of pH 5. The resultant NPs indicated good visible light-emitting properties and stability. Further, the experimental results showed that the reaction temperature influenced significantly the growth rate and the maximum size of the NPs. The CdTe NPs with a high photoluminescence quantum yield (the highest value: 57%) and narrower half width at half maximum (the narrowest value: 33 nm) were attained in very short time, within 40 minutes, reaching diameters of 2.3 to 4.3 nm. The PL intensity was increased with an increase in the reaction time, reflecting the suppression of nonradiative recombination processes. Furthermore, the formation of CdTe/CdS core-shell structures was discussed from the viewpoint of PL dynamics and X-ray diffraction studies.

Non-organometallic phosphine-free synthesis of high quality CdTe nanocrystals

Many efforts have been made recently to synthesize high-quality metal telluride nanocrystals in organic media due to their wide range of applications. However, hazardous, air/moisture-sensitive and expensive alkylphosphines were inevitably used in all of these works to prepare the tellurium precursors. In this paper, a novel phosphine-free tellurium precursor is reported for the first time, which was prepared at room temperature by using some common and cheap chemicals. High-quality CdTe nanocrystals were synthesized in octadecene using the phosphine-free tellurium precursor. Meantime, the synthesis temperature was significantly reduced. Our result presents a new strategy to synthesize various metal telluride nanocrystals in a non-organometallic phosphine-free way.

以巯基丙酸为稳定剂的水溶性CdTe量子点的水热合成及表征

Mercaptopropionic acid(MPA) modified high quality CdTe quantum dots(QDs) were prepared by hydrothermal synthesis method in a high temperature.The influences of different reaction time and different pH value of the reactive precursor solution on the optical properties of CdTe QDs were studied.The results indicate that the fluorescence emission wavelength of the as-prepared CdTe QDs can be adjusted continuously at a range of 510~661 nm,and the optical properties of CdTe QDs strongly depend on the pH value of the reactive precursor solution(the optimum pH value is 9).The samples were characterized by transmission electron microscopy and X-ray diffraction.The results show that the shape of the as-prepared CdTe QDs is closed to sphere,and the particle size distribution is uniform.Comparing with the reflux method for preparing water-soluble QDs,the hydrothermal synthesis method at a high temperature is relatively simple,and the reaction time is shorter,the growth of CdTe QDs is faster,the particle size can be grown to 3.5 nm within 100 min.The as-prepared CdTe QDs possess higher fluorescence intensity,good stability and higher fluorescence quantum yield(up to 44.6%).

Aqueous phase synthesis of CdTe quantum dots for biophotonics

Over the past few years, CdTe quantum dots have been demonstrated as powerful probes for biophotonics applications. The aqueous phase synthesis technique remains the best approach to make high quality CdTe QDs in a single-pot process. CdTe QDs prepared directly in the aqueous phase can have quantum yield as high as 80%. In addition, the surface of CdTe QDs prepared using the aqueous phase technique is functionalized with reactive groups that enable them to be directly conjugated with specific ligands for targeted delivery and sensing. In this contribution, we review recent progress in fabricating aqueous CdTe QDs and exploiting their optical properties in novel approaches to biomedical imaging and sensing applications.

Synthesis and spectrum stability of high quality CdTe quantum dots capped with stearate groups in N-oleoylmorpholine solvent

The stearate-capped CdTe quantum dots (QDs) have been first prepared via direct reaction of cadmium stearate with Te powder in N-oleoylmorpholine solvent, which was a kind of clean, air-stable and conveniently synthesized acylamide, and can readily dissolve precursors cadmium stearate and Te powder at a relative low temperature. The as-prepared CdTe QDs exhibited size-dependent optical properties, steep absorbance edge and narrow photoluminescence full width at half maximum. The high-resolution transmission electron microscopy images and X-ray diffraction revealed that the highly monodisperse CdTe QDs were of regular spherical morphology with zinc blende crystal structure displaying mean sizes of about 4 nm. The energy dispersed spectrometry measurement indicated the presence of Cd and Te, with the Cd:Te ratio being close to 1:1. Fourier transform infrared transmission spectra confirmed the existence of stearate on the CdTe QDs surfaces. The experimental results also demonstrated that the stearate-capped CdTe QDs had an unexpected good stability.

Microwave-assisted synthesis of monodispersed CdTe nanocrystals

High quality and monodispersed CdTe nanocrystals with tunable emission spectra ranging from 516 nm to 650 nm were synthesized by a highly reproducible microwave method.