Growth Of CdTe Research Articles

The significance of bilayer window (CdS:O/CdS) on the performance of CdTe thin film solar cells

Ultrathin and compact CdS:O/CdS bilayer window in CdTe solar cells has higher potential for enhanced p-type doping, reduced Te diffusion, and improved power conversion efficiency (PCE) compared to traditional single-layer CdS or CdS:O windows. In this effort, we used a two-gun RF sputtering equipment to deposit compact CdS:O (∼50 nm)/CdS(∼50 nm) bilayers onto the borosilicate and ITO-coated glass substrates at low temperature (≤250 °C). Intriguingly CdS:O/CdS bilayers retain its hexagonal {0002} preferential orientation but with around 10.58 % optical bandgap (Eg) increment compared to CdS (Eg ≈ 2.32 eV) of equal thickness (∼100 nm). Close-spaced sublimation (CSS) technique was used to deposit the CdTe thin film onto the CdS and CdS:O/CdS layers at 650 °C. The effect of CdS and CdS:O/CdS layers on the microstructural and morphological features of CdCl2-treated CdTe films were critically analyzed in revealing a promising {0001}/{111}-like partial-epitaxy presumably due to the preferential cubic CdTe growth along the <111> orientation. The CdTe films grown over a CdS:O/CdS layer exhibited smaller average crystallite and grain sizes than those grown over a single CdS layer. An essential finding was the formation of an optimal intermix layer of CdSxTe1-x, particularly evident in the CdTe films grown on the CdS:O/CdS bilayer. To comprehensively explore the effect of CdS:O/CdS window layers on PCE, complete CdTe solar cells were fabricated using three different window layers: CdS, CdS:O, and CdS:O/CdS, which are all considered as rudimentary as no layer optimization was performed. Notably, CdTe solar cell that incorporates the CdS:O/CdS bilayer window exhibited the most promising performance among all the tested rudimentary cells and corroborated the numerically simulated findings conducted by the SCAPS-1D simulator.

Highly Crystalline and Stoichiometric Growth of CdTe by Cost-Effective Hydrothermal Technique

Highly Crystalline and Stoichiometric Growth of CdTe by Cost-Effective Hydrothermal Technique

Contributions to the development of crystal growth technologies

An overview of selected contributions to the development of crystal growth technology of the Laudise Prize awardee 2023 is presented. First some results on shaped crystal growth are given, such as layers with eutectic periodic structures, casting of high-resistivity CdTe sheets, growth of in situ core doped laser rods by double die EFG, and Czochralski growth of Si crystals with rectangular cross section. Then the correlation between melt structure and quality of II-VI crystals showing high bond ionicity is discussed. The importance of marked melt overheating for growth of high-quality CdTe and (Cd,Zn)Te crystals is derived. Also the in situ stoichiometry control for reducing intrinsic point defects, precipitates and inclusions is demonstrated by the examples of Bridgman and VGF CdTe growth with Cd extra source and vapor pressure controlled Czochralski method of 6 inch GaAs crystals. Next, the reason and general appearance of dislocation cell patterns in all growing thermomechanical stressed crystals and their possible prevention are explained. The contribution of point defect diffusion for dislocation climb at high temperatures to form globular cells is underlined. Finally, melt growth experiments under travelling magnetic field (TMF) generated in coil-shaped heater-magnet modules around the crucibles are discussed. The importance of global numeric modeling for optimization the TMF convection mode in the melt is accentuated. It is shown that the growing interface can be levelled, the solution flux at vertical liquid phase epitaxy can be homogenized, and the IR transmission and minority carrier live time in 640 kg mc-Si ingots for photovoltaics can be markedly enhanced and homogenize by such external field application. The close cooperation between academics, engineers and industry is particularly emphasized.

Structural properties of MBE-grown CdTe (133)B buffer layers on GaAs (211)B substrates with CdZnTe/CdTe superlattice-based dislocation filtering layers

The ever-present demand for high-performance HgCdTe infrared detectors with larger array size and lower cost than currently available technologies based on lattice-matched CdZnTe (211)B substrates has fuelled research into heteroepitaxial growth of HgCdTe and CdTe buffer layers on lattice-mismatched alternative substrates with a (211)B orientation. Driven by the large lattice mismatch, the heteroepitaxial growth of (Hg)CdTe can result in (133)B-orientated material, which, however, has been less explored in comparison to (211)B-oriented growth. Herein, we report on the structural properties of heteroepitaxially grown single-crystal II–VI CdTe (133)B-oriented buffer layers on III–V GaAs (211)B substrates. Azimuthal-dependent x-ray double-crystal rocking curve measurements for the CdTe buffer layers show that the full-width at half-maximum value obtained along the GaAs [1¯11] direction is narrower than that obtained along the GaAs [011¯] direction, which is presumably related to the in-plane anisotropic structural characteristics of the grown CdTe layers. By incorporating strained CdZnTe/CdTe superlattice-based dislocation filtering layers (DFLs), a significant improvement in material quality has been achieved in (133)B-orientated CdTe buffer layers, including a reduced etch pit density in the low-105 cm−2 range and improved surface roughness. These results indicate that the CdTe (133)B DFL buffer layer process is a feasible approach for growing high-quality CdTe and HgCdTe materials on large-area, low-cost alternative substrates.

Impact of capacitive effect on the J-V hysteresis in MgZnO/CdTe solar cells

In this study, we demonstrate that the current–voltage (J-V) hysteresis of MgZnO (MZO)/CdTe solar cells mainly originates from the modification of J-V curves by nonsteady-state photocurrent, which is correlative with low-frequency capacitance corresponding to the MZO/CdTe interface. The hysteresis was significantly strengthened by introducing oxygen into the CdTe growth atmosphere, and became more pronounced as the MZO thickness increased. The electrical properties of MZO in CdTe solar cells were analyzed in detail by using thermomechanical lift-off technique. The results showed that oxygen during CdTe growth seriously deteriorated the conductivity of MZO by reducing the concentration of oxygen vacancy. Furthermore, a spike-like conduction band offset (CBO) greater than 0.25 eV was determined at MZO/CdTe interface due to the formation of Mg-rich surface layer. Thus, the low-frequency capacitance originating from electron accumulation at MZO/CdTe interface was increased due to the less probability of electron tunneling through the interface barrier. SCAPS simulations confirmed that the low-frequency capacitance was caused by substantially reduced conductivity of MZO and large CBO at MZO/CdTe interface. The experimental and simulation results indicate that the J-V hysteresis of MZO/CdTe cells can be suppressed by reducing low-frequency capacitance, which is achievable through enhanced electron transport at MZO/CdTe interface.

Interrelation of the CdTe Grain Size, Postgrowth Processing, and Window Layer Selection on Solar Cell Performance.

Recent improvements to the CdTe solar cell device structure have focused on replacing the CdS window layer with a more transparent material to reduce parasitic absorption and increase Jsc, as well as incorporating selenium into the absorber layer to achieve a graded band gap. However, altering the CdTe device structure is nontrivial due to the interdependent nature of device processing steps. The choice of the window layer influences the grain structure of the CdTe layer, which in turn can affect the chloride treatment, which itself may contribute to intermixing between the window and absorber layers. This work studies three different device architectures in parallel, allowing for an in-depth comparison of processing conditions for CdTe solar cells grown on CdS, SnO2, and CdSe. Direct replacement of the CdS window layer with a wider band gap SnO2 layer is hindered by poor growth of the absorber, producing highly strained CdTe films and a weak junction. This is alleviated by inserting a CdSe layer between the SnO2 and CdTe, which improves the growth of CdTe and results in a graded CdSexTe1–x absorber layer. For each substrate, the CdTe deposition rate and postgrowth chloride treatment are systematically varied, highlighting the distinct processing requirements of each device structure.

Indium doped CdTe colloidal quantum dots stabilised in aqueous medium for white light emission

Pristine and Indium (In) (1–5 at. %) doped Cadmium Telluride (CdTe:In) quantum dots (QDs) are synthesised through an aqueous colloidal route using 3-Mercaptoproponic acid (3- MPA) as capping agent. The structural and morphological details of these QDs are elucidated from XRD and HRTEM studies. The CdTe:In QDs are obtained by adding InCl3 precursor to reaction mixture. The increase in concentration of InCl3 precursor increases the In doping in CdTe QDs at the same time the particle size of CdTe:In QDs decreases from 3.5 nm to 2.3 nm. The chloride ions from InCl3 precursor control the growth of CdTe:In QDs and 3- MPA as capping agent stabilizes the QDs in aqueous medium. The size reduction of CdTe:In QDs results in blue shifted in absorbance and photoluminescence emission spectra compared to the pristine CdTe QDs with an estimated increase in the band gap from 2.13 to 2.3 eV. A white light emitting nanocomposite phosphour was prepared by mixing the blue emitting 1,4-bis-2-(5-phenyl oxazolyl)-benzene (POPOP) organic fluorophore with green emitting CdTe:In and red emitting CdTe QDs. White light emission was achieved by optimising the concentration of POPOP, In:CdTe and CdTe in the aqueous solution. This POPOP+In:CdTe + CdTe nanocomposite mixture shows the white light emission with CIE coordinates of (0.3321, 0.3391), CCT of 5561 K and a CRI of about 91. Hence, the present study reveals that POPOP + CdTe + CdTe:In (1%) hybrid nanocomposite has potential application in white LED and display technology.

Impact of dopant-induced band tails on optical spectra, charge carrier transport, and dynamics in single-crystal CdTe

Cadmium telluride (CdTe) semiconductors are used in thin-film photovoltaics, detectors, and other optoelectronic applications. For all technologies, higher efficiency and sensitivity are achieved with reduced charge carrier recombination. In this study, we use state-of-the-art CdTe single crystals and electro-optical measurements to develop a detailed understanding of recombination rate dependence on excitation and temperature in CdTe. We study recombination and carrier dynamics in high-resistivity (undoped) and arsenic (As)-doped CdTe by employing absorption, the Hall effect, time-resolved photoluminescence, and pump-probe in the 80–600 K temperature range. We report extraordinarily long lifetimes (30 µs) at low temperatures in bulk undoped CdTe. Temperature dependencies of carrier density and mobility reveal ionization of the main acceptors and donors as well as dominant scattering by ionized impurities. We also distinguish different recombination defects. In particular, shallow AsTe and deep VCd−AsCd acceptors were responsible for p-type conductivity. AX donors were responsible for electron capture, while nonradiative recombination centers (VCd−AsTe, As2 precipitates), and native defects (VCd−TeCd) were found to be dominant in p-type and n-type CdTe, respectively. Bimolecular and surface recombination rate temperature dependencies were also revealed, with bimolecular coefficient T−3/2 temperature dependence and 170 meV effective surface barrier, leading to an increase in surface recombination velocity at high temperatures and excitations. The results of this study allowed us to conclude that enhanced crucible rotation growth of As-doped CdTe is advantageous to As activation, leading to longer lifetimes and larger mobilities and open-circuit voltages due to lower absorption and trapping.

Strained CdZnTe/CdTe Superlattices As Threading Dislocation Filters in Lattice Mismatched MBE Growth of CdTe on GaSb

In this work, multiple sets of CdZnTe/CdTe strained-layer superlattices have been used as dislocation filtering layers for reducing the threading dislocations and improving the material quality of CdTe buffer layers grown by molecular beam epitaxy (MBE) on GaSb (211)B substrates. By incorporating a CdZnTe/CdTe superlattice filtering structure, a significant improvement in material quality has been achieved, with a low etch pit density of ∼ 1 × 105 cm−2 demonstrated for CdTe grown on GaSb, which is two orders of magnitude lower than previously reported values for CdTe grown directly on lattice mismatched substrates, and is comparable to values for state-of-the-art CdTe grown on lattice matched CdZnTe substrates. The filtering efficiency for each set of dislocation filtering layers has been determined to be approximately 70%. This approach provides a promising pathway towards achieving hetero-epitaxy of high quality HgCdTe on large-area lattice-mismatched alternative substrates with a low dislocation density for the fabrication of next generation infrared detectors with features of lower cost and larger array format size.

Thin CdTe Layers Deposited by a Chamberless Inline Process using MOCVD, Simulation and Experiment

The deposition of thin Cadmium Telluride (CdTe) layers was performed by a chamberless metalorganic chemical vapour deposition process, and trends in growth rates were compared with computational fluid dynamics numerical modelling. Dimethylcadmium and diisopropyltelluride were used as the reactants, released from a recently developed coating head orientated above the glass substrate (of area 15 × 15 cm2). Depositions were performed in static mode and dynamic mode (i.e., over a moving substrate). The deposited CdTe film weights were compared against the calculated theoretical value of the molar supply of the precursors, in order to estimate material utilisation. The numerical simulation gave insight into the effect that the exhaust’s restricted flow orifice configuration had on the deposition uniformity observed in the static experiments. It was shown that &gt; 59% of material utilisation could be achieved under favourable deposition conditions. The activation energy determined from the Arrhenius plot of growth rate was ~ 60 kJ/mol and was in good agreement with previously reported CdTe growth using metalorganic chemical vapour deposition (MOCVD). Process requirements for using a chamberless environment for the inline deposition of compound semiconductor layers were presented.

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.

Dependence of surface morphology at initial growth of CdTe on the II/VI on (2 1 1) Si substrates by vapor phase epitaxy using metallic Cd source

The initial growth of CdTe having a thickness of 20–500 nm on (2 1 1) Si substrates was investigated to observe the growth of CdTe at the interface for hetero-epitaxial growth. The dependence of surface morphologies at the initial growth on the II/VI and the growth time was evaluated. The (1 3 3) surface orientation at the interface was shown in CdTe films grown on (2 1 1) Si substrates, which was irrespective of the II/VI. For a relatively high II/VI of 36, hillocks having a (2 1 1) orientation were observed on the underlayer having a (1 3 3) orientation. CdTe having a (2 1 1) orientation was dominant during the growth, due to its lateral coalescence on the underlayer. The dependence of the orientation between (1 3 3) and (2 1 1) CdTe films on (2 1 1) Si substrates on the II/VI was explained by the difference between the step-flow growth on the step and the spontaneous nucleation on the terrace.

Vapor Phase Epitaxy of (133) and (211) CdTe on (211) Si Substrates Using Metallic Cd Source

Single-crystalline CdTe films were grown in both (133) and (211) surface orientations on (211) Si substrates by vapor-phase epitaxy using metallic Cd source as a group-II precursor. The orientation of epitaxial films depended on the ratio of group-II and -VI precursors, i.e., II/VI. The orientation of epitaxial films was changed from (133) to (211) by increasing the II/VI under the CdTe growth condition. The surface morphology for (133) CdTe was smooth, whereas the surface for (211) CdTe was composed of hillocks with (111), (110), (101), and (100) facets. The full width at half maximum (FWHM) of the epitaxial films with the same thickness showed that the crystalline quality of (133) CdTe was better than that of (211) CdTe. The dependence of the orientation between (133) and (211) CdTe films on (211) Si substrates on the II/VI was explained by the difference between the step-flow growth on the step and the spontaneous nucleation on the terrace.

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.

Growth and characterization of Cd1-xZnxTe (0 \leq x \leq 1) nanolayers grown by isothermal closed space atomic layer deposition on GaSb and GaAs

In this work are presented the results obtained from the deposition of Cd1-xZnxTe nanolayers using as precursor the vapours of the elements Zn, Te, and a mixture of Cd and Zn on GaAs and GaSb (001) substrates by Atomic Layer Deposition technique (ALD), which allows the deposition of layers of nanometric dimensions. At each exposure of the growth surface to the of cation or anion precursors vapours, this surface is saturated. Therefore, it is considered that the process is self-regulated. The ZnTe layers were grown in a wide range of temperatures; however, ZnTe nanolayers with a shiny mirror-like surface could be grown at temperatures between 370 and 410oC. Temperatures higher than 400oC were necessary for the CdTe growth. The layers of the Cd1-xZnxTe ternary alloy were deposited at temperature range of 400 and 425oC. The grown nanofilms were characterized by Raman spectroscopy and high-resolution X-ray diffraction. The Raman spectrum shows the peak corresponding to LO-ZnTe at 208 cm-1, which is weak and is slightly redshifted in comparison with the reported for the bulk ZnTe. For the case of the CdTe nanolayers, Raman spectrum presents the LO-CdTe peak, which is indicative of the successfully growth of the nanolayers, its weakness and its slight redshifted in comparison with the reported for the bulk CdTe can be related with the nanometric characteristic of this layer. The performed high resolution X-ray diffraction measurements allowed to study some important characteristics, as the crystallinity of the grown layers. Additionally, the performed HR-XRD measurements suggest that the crystalline quality have dependence with the growth temperature.

CdTe, ZnTe and Cd1-XZnXTe Nanolayers Grown by Atomic Layer Deposition on GaSb and GaAs (001) Oriented Substrates

In this work are presented the results obtained from ZnTe, CdTe and Cd1-xZnxTe growth on GaSb and GaAs (001) oriented substrates by the regimen of Atomic Layer Deposition (ALD). The growths were performed by exposing the substrates alternatively to the elemental vapor sources for the case of ZnTe and CdTe and to the vapors of a solid solution in the case of Cd1-xZnxTe. Ellipsometric measurements showed that the value of the thickness per cycle was around 0.3 nm. That correspond to the value corresponding to a monolayer of ZnTe. As a result of High-resolution X-ray diffraction measurements it can be identified the mechanism of interface dislocation generation in the growth interface as the relaxation of the layers, additional the value of the full width at half maximum (FWHM) result about 300 to 600 arcsec, indicating a large density of dislocation.

CdSe–CdTe Heterojunction Nanorods: Role of CdTe Segment in Modulating the Charge Transfer Processes

Heterojunction nanorods having dissimilar semiconductors possess charge transfer (CT) properties and are proposed as active elements in optoelectronic systems. Herein, we describe the synthetic methodologies for controlling the charge carrier recombination dynamics in CdSe–CdTe heterojunction nanorods through the precise growth of CdTe segment from one of the tips of CdSe nanorods. The location of heterojunction was established through a point-by-point collection of the energy-dispersive X-ray spectra using scanning transmission electron microscopy. The possibilities of the growth of CdTe from both the tips of CdSe nanorods and the overcoating of CdTe over CdSe segment were also ruled out. The CT emission in the heterojunction nanorods originates through an interfacial excitonic recombination and was further tuned to the near-infrared region by varying the two parameters: the aspect ratio of CdSe and the length of CdTe segment. These aspects are evidenced from the emission lifetime and the femtosecond transient absorption studies.

Fabrication of high efficiency sputtered CdS:O/CdTe thin film solar cells from window/absorber layer growth optimization in magnetron sputtering

In this study, CdTe (up to 2.0µm thick) and oxygenated CdS (CdS:O, up to 100nm thick) films were deposited by magnetron sputtering and optimum conditions of film growth were investigated for CdS:O/CdTe solar cells. Favourable TeO2 has been confirmed in XRD after the CdCl2 heat treatment of the CdTe films. Moreover, improved structural, optical and electrical properties are observed in the CdCl2 heat treated films. A detailed quantitative study has also been executed using XPS that finds sulfide, sulfate and an intermediate oxide as a function of oxygen content. In many cases, CdS contribution remains predominant, however, the CdSOx contribution increases with the increase of oxygen's partial pressure and decrease of growth rate. The complete solar cell device was fabricated of various CdTe thin films with different growth rates in sputtering. A highly resistive transparent (HRT) buffer layer ZnO:Sn was placed in between the FTO and CdS:O to avoid the forward leakage problem and screen printed C:Cu/Ag is used as the back contact for low cost fabrication. The J-V characteristics and external quantum efficiency (EQE) were measured for the solar cells under the illumination of AM 1.5G and the highest efficiency of 10.3% was achieved for the optimized CdTe growth rate of 5.4Å/s, while CdS:O growth rate was 0.25Å/s.

Ultrathin Single-Crystalline CdTe Nanosheets Realized via Van der Waals Epitaxy.

Due to the novel physical properties, high flexibility, and strong compatibility with Si-based electronic techniques, 2D nonlayered structures have become one of the hottest topics. However, the realization of 2D structures from nonlayered crystals is still a critical challenge, which requires breaking the bulk crystal symmetry and guaranteeing the highly anisotropic crystal growth. CdTe owns a typical wurtzite crystal structure, which hinders the 2D anisotropic growth of hexagonal-symmetry CdTe. Here, for the first time, the 2D anisotropic growth of ultrathin nonlayered CdTe as thin as 4.8 nm via an effective van der Waals epitaxy method is demonstrated. The anisotropic ratio exceeds 103 . Highly crystalline nanosheets with uniform thickness and large lateral dimensions are obtained. The in situ fabricated ultrathin 2D CdTe photodetector shows ultralow dark current (≈100 fA), as well as high detectivity, stable photoswitching, and fast photoresponse speed (τrising = 18.4 ms, τdecay = 14.7 ms). Besides, benefitting from its 2D planar geometry, CdTe nanosheet exhibits high compatibility with flexible substrates and traditional microfabrication techniques, indicating its significant potential in the applications of flexible electronic and optoelectronic devices.

Substrate preparation effects on defect density in molecular beam epitaxial growth of CdTe on CdTe (100) and (211)B

Recent studies have demonstrated that growth of CdTe on CdTe (100) and (211)B substrates via molecular beam epitaxy (MBE) results in planar defect densities 2 and 3 orders of magnitude higher than growth on InSb (100) substrates, respectively. To understand this shortcoming, MBE growth on CdTe substrates with a variety of substrate preparation methods is studied by scanning electron microscopy, secondary ion mass spectrometry, x-ray photoelectron spectroscopy, cross sectional transmission electron microscopy, and atom probe tomography (APT). Prior to growth, carbon is shown to remain on substrate surfaces even after atomic hydrogen cleaning. APT revealed that following the growth of films, trace amounts of carbon remained at the substrate/film interface. This residual carbon may lead to structural degradation, which was determined as the main cause of higher defect density.