Doped CdTe Research Articles

Beyond thermodynamic defect models: A kinetic simulation of arsenic activation in CdTe

To overcome the limitations of thermodynamic defect models, we introduce a multilevel kinetic approach to simulate the activation of $p$ doping in CdTe by group V elements using arsenic as a ``model'' dopant. On the lowest level, we calculate thermodynamic and kinetic parameters of point defects, complexes, and reactions from first principles. On the intermediate level, we use these parameters to calculate the kinetic rates of defect reactions. Finally, we simulate the time evolution of defects and free carriers. Our results show the importance of kinetic factors in defect chemistry models. We reveal the primary arsenic activation pathway to be a fast reaction in which the tellurium atoms get kicked out and replaced on the regular anion sites by interstitial arsenic species. We discover the important role of ($\mathrm{A}{\mathrm{s}}_{\mathrm{i}}\mathrm{A}{\mathrm{s}}_{\mathrm{Te}}$) and ($\mathrm{A}{\mathrm{s}}_{\mathrm{i}}\mathrm{A}{\mathrm{s}}_{\mathrm{i}}$) complexes that arise during activation anneal to form kinetically stabilized transient states that not only compensate the doping but also can produce deep recombination levels. We expect that our modeling approach and the gained insight into the atomic processes behind the doping formation will advance the defect chemistry modeling of electronic properties of materials.

Rapid determination of the anti-cancer agent Gemcitabine in biological samples by fluorescence sensor based on Au-doped CdTe

Gemcitabine hydrochloride is an established chemotherapeutic agent in several solid tumors. Along with its outstanding effects, Gemcitabine has some side effects, which may cause suffering to patients. Herein we developed, a novel but simple method for the selective and sensitive quantitative determination of Gemcitabine drug based upon measuring the fluorescence quenching of functionalized Au doped quantum dots (QDs) in biological samples. The TGA capped Au doped CdTe quantum dots prepared through hydrothermal methods exhibit excellent photoluminescence intensity as well as good photostability. The QDs fluorescence can be quenched by Gemcitabine anticancer drugs via photoinduced charge transfer, which renders the system into fluorescence “OFF” status. Under the experimental conditions, the limits of detection were 0.1 μM for Gemcitabine in aqueous media, with a correlation coefficient of 0.9945 and a limit of detection of 0.3–100 μM. The relative standard deviation (RSD) for Gemcitabine was 3.4% (n = 3). To further investigate for perfect analysis performance, it was applied to the determination of Gemcitabine in human plasma and urine samples with satisfactory results.

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

Fundamental material doping challenges have limited CdTe electro-optical applications. In this work, the As atomistic diffusion mechanisms in CdTe are examined by spatially resolving dopant incorporation in both single-crystalline and polycrystalline CdTe over a range of experimental conditions. Density-functional theory calculations predict experimental activation energies and indicate that As diffuses slowly through the Te sublattice and quickly along GBs similar to Sb. Because of its atomic size and associated defect chemistry, As does not have a fast interstitial diffusion component similar to P. Experiments to incorporate and activate P, As, and Sb in polycrystalline CdTe are conducted to examine if ex situ Group V doping can overcome historic polycrystalline doping limits. The distinct P, As, and Sb diffusion characteristics create different strategies for increasing hole density. Because fast interstitial diffusion is prominent for P, less aggressive diffusion conditions followed by Cd overpressure to relocate the Group V element to the Te lattice site is effective. For larger atoms, slower diffusion through the Te sublattice requires more aggressive diffusion, however further activation is not always necessary. Based on the new physical understanding, we have obtained greater than 1016 cm−3 hole density in polycrystalline CdTe films by As and P diffusion.

Doping properties of cadmium-rich arsenic-doped CdTe single crystals: Evidence of metastable AX behavior

Cd-rich composition and group-V element doping are of interest for simultaneously maximizing the hole concentration and minority carrier lifetime in CdTe, but the critical details concerning point defects are not yet fully established. Herein, we report on the properties of arsenic doped CdTe single crystals grown from Cd solvent by the travelling heater method. The photoluminescence spectra and activation energy of 74 ± 2 meV derived from the temperature-dependent Hall effect are consistent with AsTe as the dominant acceptor. Doping in the 1016 to 1017/cm3 range is achieved for measured As concentrations between 1016 and 1020/cm3 with the highest doping efficiency of 40% occurring near 1017 As/cm3. We observe persistent photoconductivity, a hallmark of light-induced metastable configuration changes consistent with AX behavior. Additionally, quenching experiments reveal at least two mechanisms of increased p-type doping in the dark, one decaying over 2–3 weeks and the other persisting for at least 2 months. These results provide essential insights for the application of As-doped CdTe in thin film solar cells.

Self-compensation in arsenic doping of CdTe

Efficient p-type doping in CdTe has remained a critical challenge for decades, limiting the performance of CdTe-based semiconductor devices. Arsenic is a promising p-type dopant; however, reproducible doping with high concentration is difficult and carrier lifetime is low. We systematically studied defect structures in As-doped CdTe using high-purity single crystal wafers to investigate the mechanisms that limit p-type doping. Two As-doped CdTe with varying acceptor density and two undoped CdTe were grown in Cd-rich and Te-rich environments. The defect structures were investigated by thermoelectric-effect spectroscopy (TEES), and first-principles calculations were used for identifying and assigning the experimentally observed defects. Measurements revealed activation of As is very low in both As-doped samples with very short lifetimes indicating strong compensation and the presence of significant carrier trapping defects. Defect studies suggest two acceptors and one donor level were introduced by As doping with activation energies at ~88 meV, ~293 meV and ~377 meV. In particular, the peak shown at ~162 K in the TEES spectra is very prominent in both As-doped samples, indicating a signature of AX-center donors. The AX-centers are believed to be responsible for most of the compensation because of their low formation energy and very prominent peak intensity in TEES spectra.

Iodine Doping of CdTe and CdMgTe for Photovoltaic Applications

Iodine-doped CdTe and Cd1−x Mg x Te layers were grown by molecular beam epitaxy. Secondary ion mass spectrometry characterization was used to measure dopant concentration, while Hall measurement was used for determining carrier concentration. Photoluminescence intensity and time-resolved photoluminescence techniques were used for optical characterization. Maximum n-type carrier concentrations of 7.4 × 1018 cm−3 for CdTe and 3 × 1017 cm−3 for Cd0.65Mg0.35Te were achieved. Studies suggest that electrically active doping with iodine is limited with dopant concentration much above these values. Dopant activation of about 80% was observed in most of the CdTe samples. The estimated activation energy is about 6 meV for CdTe and the value for Cd0.65Mg0.35Te is about 58 meV. Iodine-doped samples exhibit long lifetimes with no evidence of photoluminescence degradation with doping as high as 2 × 1018 cm−3, while indium shows substantial non-radiative recombination at carrier concentrations above 5 × 1016 cm−3. Iodine was shown to be thermally stable in CdTe at temperatures up to 600°C. Results suggest iodine may be a preferred n-type dopant compared to indium in achieving heavily doped n-type CdTe.

Growth and characterization of Arsenic doped CdTe single crystals grown by Cd-solvent traveling-heater method

We report the growth of p-type As-doped, Cd-rich CdTe single crystals using metallic Cd as the solvent in the traveling-heater method. We investigate the growth process from Cd solution in terms of the solid-liquid interface shape and the effects of As incorporation on p-type doping. The resulting CdTe crystals have Cd-rich composition which enhances p-type doping. The As doping efficacy was measured for As concentrations by the combination of inductively coupled plasma mass spectrometry, capacitance-voltage measurements. The p-type doping concentration varied from 6×1015 to 8×1016cm−3 with increasing As concentration, with an apparent doping limit just below 1017cm−3.

The twins structure and electric properties of Cl doped CdTe film by magnetron sputtering

CdTe is a promising material for fabricating high-efficient and low-cost thin film solar cell. To achieve high energy conversion efficiency, polycrystalline CdTe films must go through an annealing process in an atmosphere containing chlorine. Numerous researches of the mechanisms of chlorine treatment have been conducted. It is generally believed that chlorine treatment can increase the quantum efficiency of CdTe, cause CdTe grain to recrystallize, and reduce the defect density. In 2014 a research discovered that after chlorine treatment, Cl atoms are segregated at grain boundaries of CdTe and form p-n-p junction, which can separate electrons and holes, thus inhibiting the carrier recombination at grain boundaries. Another first-principle calculation research claimed that Cl atoms form VCd-ClTe complex, which is also named A-center, and provide extra shallow p-energy level to improve shallow p-doping of CdTe. It seems that both segregation and doping of Cl atoms can enhance cell performance.To test whether chlorine doping can contribute to the enhancement of cell performance, in this work we study chlorine doping in CdTe absorption layer by experiment. We deposit chlorine doped CdTe (CdTe:Cl) film by well controlling the chlorine concentration ((1005) ppm) to investigate the effects of Cl doping on device performance. In this work, we also compare the lattice structure and electrical properties of CdTe:Cl films with those of conventional Cl treated CdTe films.The CdTe:Cl film deposited at low temperatures consists of both cubic and hexagonal phases. CdTe:Cl film deposited at high temperature consists of only cubic phase with (111) orientation. Phase structure remains stable after annealing. Serried twins can be observed in all CdTe:Cl rods and the twins each contain only several atom layers. The ultra-thin twins can be found in both as-deposited CdTe:Cl and post-annealing CdTe:Cl. There is neither separate conduction channel of electrons nor that of holes in CdTe:Cl. But for chlorine treated CdTe, grain boundaries are the conduction channels of electrons and holes traveling within grains. The resistivity of the CdTe:Cl film is found to increase drastically, and carrier density reduces to intrinsic state after annealing. The efficiency of CdTe:Cl cell is lower than that of chlorine treated CdTe cell. It seems that non-balanced heavy chlorine doping by magnetron sputtering is bad to CdTe absorption layer.

Theoretical Investigation of Electronic Properties of Undoped and Ag-Doped (CdTe)16×N Multi-cage Nanochains

The structural and electronic properties of highly stable structures, undoped and Ag substitutional doped (CdTe)16×N (N = 1–3) multi-cage nanochains, have been investigated by density functional theory. (CdTe)16 cage is chosen as the building unit to form (CdTe)16×N multi-cage nanochains. Two possible assembly modes, interconnecting two cages by their four-membered rings and interconnecting two cages by their six-membered rings, are considered. For Ag doping, two different substitutional sites have been studied. Our results show that the multi-cage nanochains have higher stability than that of the single cage, and the stability varies with the assembly modes and doping sites for both undoped and Ag-doped nanochains. After Ag doping, an acceptor level is observed, showing typical p-type properties. The properties of energy structure, charge density and density of states are analyzed in detail. Results show that the assembly modes and Ag substitutional doping site have a significant impact on the electronic properties of Ag-doped (CdTe)16×N multi-cage nanochains.

Carbon and Metal Quantum Dots toxicity on the microalgae Chlorella pyrenoidosa

In this report, we investigated the cytotoxicity of two types of quantum dots(QDs) (carbon quantum dots(CQDs): N, S doped CQDs, N doped CQDs, no doped CQDs; metal QDs(MQDs): CdTe QDs, CdS QDs, CuInS2/ZnS QDs) on Chlorella pyrenoidosa(C. Pyrenoidosa) at different concentrations. We compared the toxicity of different QDs on C. Pyrenoidosa through determination of the algal growth inhibition, acute toxicity tests (EC50), Chlorophyll a(Chla) contents, protein contents, the activity of enzymatic and metabolites contents. When C. Pyrenoidosa was treated by various concentrations of QDs, the Chla contents were consistent to the number of algae cells, showing a good dose-response relationship. At the 96h, the EC50 of N, S doped CQDs, N doped CQDs, no doped CQDs and CdTe QDs, CdS QDs, CuInS2/ZnS QDs were 38.56, 185.83, 232.47, 0.015, 4.88, 459.5mg/l, respectively. The toxicity order of them was: CuInS2/ZnS QDs<no doped CQDs<N doped CQDs<N,S doped CQDs<CdS QDs<CdTe QDs. The activity of antioxidant enzymatic superoxide dismutase (SOD) was enhanced and the non-enzymatic antioxidant glutathion (GSH) level was decreased with the increasing of QDs concentration, respectively. The accumulation of Malondialdehyde (MDA), a product of lipid peroxidation caused by reactive oxygen species(ROS), was enhanced when algae were exposed to QDs. In conclusion, the toxicity of CQDs was smaller than MQDs, but the toxicity of CuInS2/ZnS QDs was the smallest.

Calculation of the High-Temperature Point Defects Structure in Te-Rich CdTe

A thermodynamic equilibrium model for CdTe annealed under Te vapor is established, in which possible point defects and a defect reaction existing in undoped and In-doped Te-rich CdTe crystals are taken into consideration. Independent point defects, such as VCd, Cdi, and Tei, as well as defect complexes, namely TeCd-VCd (B complex), \( {\hbox{Te}}_{\rm{Cd}}^{2 + } \)-\( {\hbox{V}}_{\rm{Cd}}^{2 - } \) (D complex), \( {\hbox{In}}_{\rm{Cd}}^{ + } \)-\( {\hbox{V}}_{\rm{Cd}}^{ - } \) (A-center) and Tei-VCd (TeCd), are discussed based on the defect chemistry theory. More specially, the mass action law and quasi-chemical equations are used to calculate defects concentration and Fermi level in undoped and doped CdTe crystals with different indium concentrations. It is found that the Fermi level is controlled by a \( {\hbox{V}}_{\rm{Cd}}^{2 - } \), TeCd, and B/D-complex in undoped crystal. The concentration of VCd drops down in an obvious manner and that of TeCd rises for doped crystal with increasing [In].

First-principlesDFT+GWstudy of oxygen-doped CdTe

The role of oxygen doping in CdTe is addressed by first-principles calculations. Formation energies, charge transition levels, and quasiparticle defect states are calculated within the $\text{DFT}+GW$ formalism. The formation of a new defect is identified, the $({\text{O}}_{\text{Te}}{\text{-Te}}_{\text{Cd}})$ complex. This complex is energetically favored over both isovalent $({\mathrm{O}}_{\text{Te}})$ and interstitial oxygen $({\mathrm{O}}_{\text{i}})$, in the Te-rich limit. We find that the incorporation of oxygen passivates the harmful deep energy levels associated with $({\mathrm{Te}}_{\text{Cd}})$, suggesting an improvement in the efficiency of CdTe based solar cells. Substitutional $({\text{O}}_{\text{Cd}})$ is only stable in the neutral charge state and undergoes a Jahn--Teller distortion. We also investigate the diffusion profiles of interstitial oxygen and find a low-energy diffusion barrier of only 0.14 eV between two structurally distinct interstitial sites.

Structural, optical and ferromagnetic properties of cobalt doped CdTe quantum dots

Cobalt doped cadmium telluride (CdTe) quantum dots (QDs) were synthesized with different Co concentrations by using non-aqueous method. CoxCd1−xTe QDs were characterized using optical absorption and photoluminescence spectroscopy, X-ray diffraction and high resolution transmission electron microscopy (HRTEM). It was noted that Co2+ was incorporated CdTe QDs without any shift in the diffraction peaks. HRTEM images revealed that the CdTe QDs were regular spherical particles with an average diameter of ~3 nm for undoped CdTe QDs and the average size of 2, 5 and 15 % Co2+ doped CdTe QDs were 4, 6.5 and 2.8 nm, respectively. Magnetization recognized for 0, 2, 5 and 15 % cobalt doped CdTe QDs revealed a ferromagnetic signal and ferromagnetic hysteresis loop. For pristine CdTe QDs a weak ferromagnetism was attributed to the charge transfer between capping agent and host CdTe QDs. Co2+ doped CdTe QDs exhibit stronger ferromagnetism and magnetic parameters such as saturation magnetization, MS, remanence, MR, and coercivity, HC, of CoxCd1−xTe QDs were obtained from the hysteresis loops. It was found that with increasing the doping concentration of Co2+ in CdTe QDs up to 5 %, Ms increased.

Bonding and Spectral Character of Cr Doped Cdte, Cdse and Cds Nano-Clusters

The geometrical structures, bonding characteristics, Raman spectra, and molecular orbitals of hexagonal and tetrahedral Cr-doped CdX molecules have been investigated using density functional theory calculations at the B3LYP/LANL2DZ level. The structures of Cr-doped CdX molecules had obvious distortions compared with pure CdX clusters, especially CdCr2Se3 and CdCr2S3 clusters. The Mulliken charges of Cr atoms were negative and were lower than those of Cd atoms in the corresponding structures. Raman spectra showed that there was the vibration of the Cr–Cr bond in Cr-doped CdX molecules. Wiberg bond indices (WBI) values show that there were Cr–Cr bonds in some clusters. WBI values have also revealed that bonds between Cr and Te, Se, S were stronger than those between Cd and X atoms. Finally, molecular orbital observations indicate that the transitions in doped structures were from d orbital to d orbital, which is different from undoped structures.

Long Lifetime Hole Traps at Grain Boundaries in CdTe Thin-Film Photovoltaics.

A novel time-resolved cathodoluminescence method, where a pulsed electron beam is generated via the photoelectric effect, is used to probe individual CdTe grain boundaries. Excitons have a short lifetime (≤100 ps) within the grains and are rapidly quenched at the grain boundary. However, a ~47 meV shallow acceptor, believed to be due to oxygen, can act as a long lifetime hole trap, even at the grain boundaries where their concentration is higher. This provides direct evidence supporting recent observations of hopping conduction across grain boundaries in highly doped CdTe at low temperature.

Effect of Annealing Temperature on the Structural and Optical Properties and Effect of Thickness on the Electrical Properties of Phosphorus Doped CdTe

Thin films were deposited at room temperature and annealed at 100 and 200 °C. The particle size and the microstrain dependence on the temperature were studied by using Winfit Program. It was found that the particle size and the microstrain decrease with increasing the annealing temperature. Reflection and transmittance were recorded in the wavelength range (500–2500 nm). The type of electronic transition was determined; it was found that these films have direct allowed transition with an optical energy 1.64, 1.69 and 1.74 eV before and after annealing respectively. The Absorption index (k) and the refractive index (n) were measured before and after annealing. The electrical conductivity increases with increasing temperature in the (288–423)K range. Moreover, the electrical conductivity increases with increasing thickness.

Influence of Zn2+ doping on the crystal structure and optical–electrical properties of CdTe thin films

The present study reports the synthesis of Cd1−xZnxTe (x=0,0.025,0.050,0.075 and 0.100) nanocrystalline thin film through a simple two step method. In the first step fine nanoparticles of Cd1−xZnxTe was prepared by solvothermal microwave irradiation (SMI) technique and then deposited as thin film using dip-coating technique. X-ray diffraction study showed that films are polycrystalline with cubic phase, which are preferentially oriented along the (111) direction. No impurity phase was observed in the XRD pattern even after higher concentration of doping (x=0.100) of Zn. FESEM study revealed that the films are homogeneous without cracks and pinholes. TEM micrographs revealed the particles are slightly agglomerated and lesser than 25nm. The optical absorption study revealed that pure and doped CdTe films possess a direct band gap material with bandgap values between 2.39 and 2.63eV (±0.02eV). The values of optical bandgap increase with an increase in dopant (Zn) concentration from x=0.025 to 0.10. The pure cadmium telluride (CdTe) nanocrystalline film shows a strong green emission peak centered at about 525nm. The emission peaks of Cd1−xZnxTe nanocrystalline films are red shifted from 525nm to 611nm according to the dopant (Zn2+) concentration. The grains in the prepared films are uniformly distributed, which was confirmed by narrow full width at half maximum (FWHM) of the emission peaks (40–65nm). The DC conductivity has increased by 1.25 and 4 orders as the concentration of dopant increases from x=0.025 to 0.10 at room temperature (30°C) and 150°C respectively. The higher conductivity value is underpinned by the smaller activation energy value and is explained by thermionic emission mechanism.

Magnetic and magneto-optical properties of doped and co-doped CdTe with (Mn, Fe): Ab-initio study

On the basis of ab-initio calculations performed by the Akai-KKR-CPA method within the spin polarized density functional theory (DFT) and local density approximation (LDA). The magnetic and magneto-optical properties of CdTe doped with Mn and Fe, and co-doped with transitions metals (TM), have been investigated. Moreover, the density of state (DOS) have been calculated and plotted with the energy diagram, for different dopants concentrations. In this work we study, these compounds and compare our theoretical results with the experimental works concerning the doped CdTe. Then we determine which one, Mn or Fe, is responsible of the appearing magnetic and/or optical properties. We also investigate the effect of the co-doping with these elements: Mn and Fe. We show that the iron Fe does not contribute strongly in the magnetism, but it affects the optical properties of the co-doped materials. When comparing our results with the existing experimental works, we found that a low concentration of Fe improves well the magneto-optical properties such as the Faraday rotation. On the other hand, we have investigated the microscopic behavior of electrons by studying their electronic structure and density of states (DOS).

Synthesis and characterization of high-quality water-soluble CdMnTe quantum dots capped by N-acetyl-L-cysteine through hydrothermal method

High-quality water-soluble Mn2+ doped CdTe quantum dots (QDs) with N-acetyl-L-cysteine (NAC) as capping reagent have been synthesized through hydrothermal route, allowing a rapid preparation time (<1h), tunable emitting peaks (from 530 to 646nm) and excellent quantum yields (approximately 50%). The influences of various experimental variables, including Mn-to-Cd ratio, Te-to-Cd ratio, pH value, and reaction time on the growth rate and luminescent properties of the obtained QDs have been systematically investigated. And the optimum reaction conditions (Cd:Mn:NAC:Te=1.0:1.0:2.4:0.2, pH=9.5, 35min, 200°C) are found out. The optical features and structure of the obtained CdMnTe QDs have been characterized through fluorescence spectroscopy, UV absorption spectroscopy and TEM. In particular, we realized qualitative, semi-quantitative and quantitative studies on the doping of Mn to CdTe QDs through XPS, EDS, and AAS. The actual molar ratio of Mn to Cd in CdMnTe QDs (551nm) is 1.166:1.00, very close to the feed ratios (1:1).

Annealing effects on physical properties of doped CdTe thin films for photovoltaic applications

Polycrystalline Cadmium Telluride (CdTe) thin films were prepared on glass substrates by thermal evaporation at the chamber ambient temperature and then annealed for an hour in vacuum ~1×10−5mbar at 400°C. These annealed thin films were doped with copper (Cu) via ion exchange by immersing these films in Cu (NO3)2 solution (1g/1000ml) for 20min. Further these films were again annealed at different temperatures for better diffusion of dopant species. The physical properties of an as doped sample and samples annealed at different temperatures after doping were determined by using energy dispersive x-ray analysis (EDX), x-ray diffraction (XRD), Raman spectroscopy, transmission spectra analysis, photoconductivity response and hot probe for conductivity type. The optical band gap of these thermally evaporated Cu doped CdTe thin films was determined from the transmission spectra and was found to be in the range 1.42–1.75eV. The direct energy band gap was found annealing temperatures dependent. The absorption coefficient was >104cm−1 for incident photons having energy greater than the band gap energy. Optical density was observed also dependent on postdoping annealing temperature. All samples were found having p-type conductivity. These films are strong potential candidates for photovoltaic applications like solar cells.