Undoped CdTe Research Articles

Study of Near Band Edge Emission Spectra and Conductivity Transformation in Fe‐Doped Cadmium Telluride Nanoparticles for the Device Applications

AbstractThe research work discussed here explains the near band edge emission and conductivity transformation of the Fe‐doped CdTe nanoparticles synthesized by hydrothermal method by using an autoclave. The size of the undoped CdTe nanoparticles is found to be 5.48, 8.45, 9.40, and 14.79 nm, respectively for the nanoparticles collected at 15, 30, 45, and 60 minutes of duration of the synthesis. Whereas for the Fe‐doped CdTe nanoparticles, the size is found to be 12.81, 16.9, and 18.5 nm, respectively for the 10%, 20%, and 30% Fe concentrations as doping elements in CdTe nanoparticles. The XPS spectra clearly show the successful doping of Fe in the CdTe QDs and also the orbital state of Fe in CdTe. The EDAX spectra is taken to trace the elements present in the synthesized nanoparticles. From the UV–vis absorption spectrum, the band gap is found to decrease when doped with Fe in different concentrations. Also, an appreciable blue shift is found in the as synthesized undoped and Fe‐doped CdTe NPs. A near band edge emission property is observed for the Fe‐doped CdTe NPs when doped with different Fe concentrations. The van der Pauw Hall measurement technique is employed to study the electrical properties such as conductivity, carrier concentration, mobility, and Hall coefficient of the undoped and Fe‐doped CdTe nanoparticles. The consistent p‐type conductivity is observed for the undoped CdTe nanoparticles till the time duration of 45 minutes of synthesis. When doped with Fe in 30% concentration, the type of conductivity is found to transform from p‐type to n‐type. The luminescence property between 450 and 500 nm makes these materials suitable for device applications like light‐emitting diodes. Also, the study of Hall measurement here explains the application of these materials as photovoltaic cells.

Chemically processed CdTe thin films for potential applications in solar cells – Effect of Cu doping

Thin films of cadmium telluride (CdTe) have attained the attention of researchers due to the potential application in solar cells. However, cost-effective fabrication of solar cells based on thin films along with remarkable efficiency and control over optical properties is still a challenging task. This study presents an analysis of the structural, optical and electrical properties of undoped and Cu-doped CdTe thin films fabricated on ITO coated glass substrates using an electrodeposition process with a focus on practical applications. Electrolytes of cadmium (Cd), tellurium (Te) and copper (Cu) are prepared with a low molarity of 0.1 M. Thin films are deposited by keeping current density in the range of 0.12–0.3 mA/cm2. Copper doping is varied (2-10 wt%) for the optimized sample. X-ray diffraction crystallography indicates that both undoped CdTe and Cu-doped CdTe films crystallize into a dominant hexagonal lattice. Direct energy band gap is observed for both undoped and doped conditions. The study revealed a drop in the optical band gap energy to ∼1.46 eV with the increase in doping (Cu) concentration from 2 to 10 wt%. Increase in mobility and conductivity is observed with the increase in current density of the deposited undoped CdTe thin films. Whereas, Cu doping of 6 wt% produced thin films with acceptable mobility and conductivity for the doped samples. Furthermore, photoluminescence (PL) spectroscopy unveiled a multitude of emission peaks encompassing the visible spectrum, arising from the combination of electrons and holes through both direct and indirect recombination processes. Findings of this study suggest that chemically produced CdTe thin films would be suitable for use as low-cost applications pertaining to solar cells.

Quantum transport and mobility spectrum of topological carriers in (001) SnTe/PbTe heterojunctions

Measurements of magnetotransport in SnTe/PbTe heterojunctions grown by the molecular beam epitaxy technique on (001) undoped CdTe substrates were performed. At low magnetic fields, quantum corrections to conductivity were observed that may be attributed to the presence of topological states at the junction interface. For a sample with a 5-nm-thick SnTe layer, the data analysis suggests that midgap states are actually gapped. However, the phase coherence effects in 10 and 20 nm SnTe/PbTe samples are fully explained assuming the existence of gapless Dirac cones. Magnetotransport at higher magnetic fields is described in the framework of mobility spectrum analysis (MSA). We demonstrate that the electron- and holelike peaks observed simultaneously for all SnTe/PbTe heterojunctions may originate from the concave and convex parts of the energy isosurface for topological states---and not from the existence of quasiparticles both carrying negative and positive charges. This interpretation is supported by numerical calculations of conductivity tensor components for gapless (100) Dirac cones, performed within a classical model and based on the solutions of the Boltzmann transport equation. Our approach shows the feasibility of MSA in application to magnetotransport measurements on topological matter.

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.

Influence of zinc doping on the molecular biocompatibility of cadmium-based quantum dots: Insights from the interaction with trypsin

Doping quantum dots (QDs) with extra element presents a promising future for their applications in the fields of environmental monitoring, commercial products and biomedical sciences. However, it remains unknown for the influence of doping on the molecular biocompatibility of QDs and the underlying mechanisms of the interaction between doped-QDs and protein molecules. Using the “one-pot” method, we synthesized N-acetyl-l-cysteine capped CdTe: Zn2+ QDs with higher fluorescence quantum yield, improved stability and better molecular biocompatibility compared with undoped CdTe QDs. Using digestive enzyme trypsin (TRY) as the protein model, the interactions of undoped QDs and Zn-doped QDs with TRY as well as the underlying mechanisms were investigated using multi-spectroscopy, isothermal titration calorimetry and dialysis techniques. Van der Waals forces and hydrogen bonds are the major driving forces in the interaction of both QDs with TRY, which leading to the loosening of protein skeleton and tertiary structural changes. Compared with undoped QDs, Zn-doped QDs bind less amount of TRY with a higher affinity and then release higher amount of Cd. Zn-doped QDs have a less stimulating impact on TRY activity by decreasing TRY binding and reducing Cd binding to TRY. Taken all together, Zn-doped QDs offer a safer alternative for the applications of QDs by reducing unwanted interactions with proteins and improving biocompatibility at the molecular level.

Influence of the surface roughness on electron-phonon interaction in an intrinsic CdTe single crystal

In this work, we found that the electron-phonon interaction (EPI) is dependent on the surface roughness. Simulations of Resonance Raman Spectroscopy (RRS) spectra, show that the Fano asymmetry parameter q, decreases as the Mean Surface Roughness (MSR) increases. They also make it clear, that the magnitude of the EPI strength increases with increasing roughness. Our results also show that the EPI along with the surface morphology in an undoped CdTe sample allow the manifestation of other phonons in the RRS spectra. This work contributes to understand and potentially control the EPI in semiconductors.

Influence of nanocellulose concentration on the tunability of energy bandgap of cadmium telluride thin films

Cadmium telluride (CdTe) doped nanocellulose thin films were successfully deposited on glass/fluorine-doped tin oxide substrates from an aqueous electrolyte bath containing hydrated cadmium sulfate (3CdSO4·8H2O) and tellurium dioxide using electrodeposition technique. Six samples of CdTe thin films were grown with nanocellulose concentrations of (0, 1, 3, 5, 7, and 9) ppm. The results obtained from electronic data of the deposited layers using UV–Vis spectrophotometer agree favourably with the theoretical formulation. The bandgap of the films ranges from 1.32 to 1.67 eV. The lowest bandgap of CdTe (1.32 eV) was achieved at a concentration of 9 ppm. This relatively low energy bandgap is remarkable in recent formation of CdTe. Photoelectrochemical cell measurement shows that the electrical conductivity type of undoped CdTe is n-type while the nanocellulose doped CdTe is p-type in electrical conduction. Hence, it is possible to effectively tune the conductive properties of CdTe by careful addition of nanocellulose concentration.

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

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

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

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

Fano effect in resonant Raman spectrum of CdTe

In this work, we show a line-shape analysis of the resonant Raman scattering spectrum of un-doped CdTe single crystal. We found that the main peak correspondent to the longitudinal-optic (LO) phonon mode is asymmetrically broadened toward the high-frequency side. We attributed this asymmetry to an electron-phonon coupling effect, which was demonstrated using a Fano-type function to describe the spectrum line-shape. In our fitting we have additionally considered the weak contribution (in the low-frequency region) of two added phonons, whose presence is attributed to the rupture of the symmetry due to the surface. Results show an excellent agreement between the experimental data and the fitting model, where Fano asymmetry parameter was found to be q=4.5. The results obtained, are a tool that contributes to the understanding of the origin of LO phonon line-shape asymmetry of the Raman spectrum; and its use will be helpful to investigate the strength of electron-phonon coupling in semiconductors.

On the Characteristic Features of the Impurity Energy Spectrum in Arsenides

The impurity energy spectrum of undoped n-type GaAs, InAs, CdSnAs2, CdGeAs2, and CdTe, ZnO bulk crystals is studied based on a quantitative analysis of the baric and temperature dependences of the kinetic coefficients. It is found that the deep-level donor center corresponds to the intrinsic vacancy defect in the anion sublattice in the above-listed semiconductors. The conclusion on the nature of the donor center, i.e., the vacancy in the anion sublattice, is based on the fact that, in contrast to shallow impurity centers which trace the intrinsic band under uniform pressure, to which they are genetically related, the energy of deep impurity centers with respect to absolute vacuum remains constant under isotropic compression of the lattice. Therefore, it seems favorable to study the evolution of the carrier energy spectrum in semiconductors under uniform-pressure conditions. The energy-level positions with respect to the conduction-band edge and pressure coefficients of the energy gaps between them and the corresponding conduction-band bottom are determined. The shift of the energy level of the deep donor center to the conduction-band depth with decreasing band gap is observed.

Enhanced p-Type Doping in Polycrystalline CdTe Films: Deposition and Activation

An in situ nonequilibrium method to increase hole density in polycrystalline CdTe thin films to 1016 cm−3 using group V substitution on Te is presented. Single-phase CdTe films doped with P, As, and Sb were deposited at 550 °C at 100–200 nm/s onto moving cadmium sulfide/high resistance transparent buffer layer/transparent conductive oxide /glass superstrates by vapor transport deposition in Cd overpressure from high purity compound sources. Doping levels before and after activation were determined by capacitance-voltage analysis of diagnostic devices. Secondary ion mass spectrometry depth profiling confirmed dopant incorporation levels of 1017–1018 atoms/cm3 in as-deposited films. Electronic activation was carried out by post-deposition annealing in Cd or CdCl2 vapor with fast cooling, increasing acceptor concentrations to >1015 cm−3 for P and >1016 cm−3 for As and Sb, compared with mid −1014 cm−3 acceptor levels for undoped CdTe films. The activation methods are compatible with post-deposition processing presently used for high-efficiency CdTe solar cells. For the dopants As and Sb, the acceptor concentration increased by substitutional AsTe and SbTe formation, respectively, which was validated by cathodoluminescence spectroscopy.

Facile aqueous synthesis of Ni-doped CdTe quantum dots as fluorescent probes for detecting pyrazinamide in plasma

Doping with transition-metal ions is an intriguing way to tune the properties of quantum dots. In this study, water-soluble and stable Ni-doped CdTe quantum dots have been successfully synthesized via a facile, novel and green method. Compared with undoped CdTe quantum dots, the Ni-doped CdTe quantum dots exhibited improved photostability, and lower cytotoxicity and enhanced biocompatibility. A simple, affordable, selective, and sensitive method based on the fluorescence quenching of Ni-doped CdTe quantum dots for rapid determination of pyrazinamide (PZA) in plasma samples was developed. Fluorescence of Ni-doped CdTe quantum dots can be effectively quenched by PZA drug due to the binding of PZA drug on the surface of Ni-doped CdTe quantum. Under the optimized conditions, the response was directly proportional to the concentration of PZA in the range of 2 to 100 μM, with a detection limit of 0. 5 μM. The proposed method was applied to the determination of PZA in plasma samples with satisfactory results.

High-temperature electrical properties of undoped Cd1-xMnxTe (x = 0.05-0.55) crystals under cadmium vapor pressure

For the first time, the results of a study of the high-temperature electrical characteristics of undoped Cd1-xMnxTe single crystals in a wide composition range x = 0.05-0.55 are described. For this purpose, a study of Hall effect was made at the temperatures of 723–1073 K and isothermal and temperature dependences of electrical parameters were constructed. At 300 K, all the samples studied had a p-type conductivity, and Cd1-xMnxTe crystals with x = 0.02 and 0.15 had a low resistivity (102-103 Ohm×cm) while with x = 0.3 and 0.55 they had a relatively high ρ~ 107 Ohm×cm, which is due to an increase in the band gap with an increase in the MnTe content. The sample with the lowest Mn content (x = 0.02) showed the instability of the impurity-defect system during the first heating-cooling cycles: the conductivity type changed from p- to n- above 750 K and the mobility gradually increased as a result of the introduction of Cd atoms from the gas phase to the interstitial positions of the lattice. In this sample, during the first heating, the influence of donor impurities is noticeable, since the experimental line lg[e-] is significantly (~ 1-1.4 orders of magnitude) higher than that for model (undoped) CdTe, probably due to the donor behavior of Mn atoms. The crystal with the highest MnTe content (Cd0.45Mn0.55Te) also demonstrated the instability of the impurity-defect system, which was reflected in the decrease in the electrical conductivity after heat treatment at 773-873 K by almost one order of magnitude, which can be explained by a superposition of the results of the interaction of donor Cd atoms introduced from the gas phase with its native acceptor point defects (compensation) and the interaction of impurities between the matrix and Te inclusions. It has been established that a peculiarity of Cd1-xMnxTe crystals with a high content MnTe (x = 0.15, 0.55, and partially 0.3) is the inverse dependence of the carrier concentration on the cadmium vapor pressure caused by the onset of mixed conductivity of these crystals at high temperatures (T> 873 K). Accordingly, for these crystals, the value 1/RH at the moment of the onset of mixed conductivity characterizes the conditional rather than real mobility of the charge carriers. On the pressure dependences of the charge carrier concentration, the corresponding dependences are lower than the undoped CdTe (especially at low temperatures) for samples with x≥0.15 indicating that the introduction of a significant amount of MnTe (x≥0.15) leads to a decrease in the charge carrier concentration by 0.5-0.8 orders of magnitude (at ~ 773 K). This is due to the influence of Mn atoms, which form a stronger bond with Te than Cd, and therefore the generation of electrons requires more energy than in pure CdTe.

Spectroscopic and Microscopic Defect and Carrier-Lifetime Analysis in Cadmium Telluride

Using experiments and theory, we analyze “carrier-lifetime-killer” semiconductor bulk defects in CdTe. We can estimate the concentration of tellurium antisite (TeCd) recombination centers in undoped CdTe from the kinetic modeling of injection-dependent photoluminescence (PL) lifetimes. For the Cd-rich stoichiometry, the concentration of lifetime-limiting [TeCd] is ≥1013 cm–3. For the Te-rich stoichiometry, we find a smaller hole-capture rate constant than predicted by theory, which could be because of TeCd association with the Te interstitials. We also compare PL emission spectra, carrier lifetimes, and radiative efficiencies for Cu, P, and As doping and analyze the microscopic distribution of carrier lifetimes in As-doped single-crystal CdTe. In all the cases examined here, point defects are the dominant bulk recombination centers. Carrier lifetimes are the longest in undoped CdTe, but radiative efficiencies are the highest with group-V doping.

Improving cytocompatibility of CdTe quantum dots by Schiff-base-coordinated lanthanides surface doping

BackgroundSuitable fluorophores are the core of fluorescence imaging. Among the most exciting, yet controversial, labels are quantum dots (QDs) with their unique optical and chemical properties, but also considerable toxicity. This hinders QDs applicability in living systems. Surface chemistry has a profound impact on biological behavior of QDs. This study describes a two-step synthesis of QDs formed by CdTe core doped with Schiff base ligand for lanthanides [Ln (Yb3+, Tb3+ and Gd3+)] as novel cytocompatible fluorophores.ResultsMicrowave-assisted synthesis resulted in water-soluble nanocrystals with high colloidal and fluorescence stability with quantum yields of 40.9–58.0%. Despite induction of endocytosis and cytoplasm accumulation of Yb- and TbQDs, surface doping resulted in significant enhancement in cytocompatibility when compared to the un-doped CdTe QDs. Furthermore, only negligible antimigratory properties without triggering formation of reactive oxygen species were found, particularly for TbQDs. Ln-doped QDs did not cause observable hemolysis, adsorbed only a low degree of plasma proteins onto their surface and did not possess significant genotoxicity. To validate the applicability of Ln-doped QDs for in vitro visualization of receptor status of living cells, we performed a site-directed conjugation of antibodies towards immuno-labeling of clinically relevant target—human norepinephrine transporter (hNET), over-expressed in neuroendocrine tumors like neuroblastoma. Immuno-performance of modified TbQDs was successfully tested in distinct types of cells varying in hNET expression and also in neuroblastoma cells with hNET expression up-regulated by vorinostat.ConclusionFor the first time we show that Ln-doping of CdTe QDs can significantly alleviate their cytotoxic effects. The obtained results imply great potential of Ln-doped QDs as cytocompatible and stable fluorophores for various bio-labeling applications.

High frequency noise calculation in Schottky metal-semiconductor-metal structure and parameter retrieval of nanometric CdTe structure

An analytical model to calculate the noise spectral density in Metal-Semiconductor-Metal (M-S-M) structure of Schottky contacts has been developed. The model based on the linearization of Langevin approach of carrier motion inside a bulk semiconductor, and taking into account the fluctuations in the leakage current through the structure due to the barrier lowering by the image force effect. Particularly, the calculations describe two quantities of noise: electric field and total current spectral densities. The results obtained from the Au-CdTe-Au Schottky structure exhibit sharp resonances due to the effect of plasma frequency oscillations and the relative thickness of the depletion region below the anode. Moreover, the noise current spectrum exhibits Lorentzian behavior when the M-S-M has the form of a homogenous structure (thickness of depletion region≈0). These results are in agreement with that reported by Monte Carlo technique of metal Schottky structure. The discussion of the noise spectra as a function of structure parameters revealed that the nanometric M-S-M structures with undoped CdTe can be used as Schottky detectors/emitters in Terahertz frequency applications.

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.

Defects in Undoped p-type CdTe Single Crystals

Recently, the device performance of Cadmium Telluride (CdTe) solar cells have improved significantly. However, little progress has been made in understanding the defected structures responsible for carrier trapping, which have limited further cell efficiency improvements. In order to better understand the underlying electrical compensation mechanism, an undoped CdTe bulk crystal boule was grown from melt with Te-rich stoichiometry using the Vertical Bridgman technique, where the growth process was controlled to maximize net acceptor density. The samples prepared from the boule indicate low resistivity of 100–500 Ω·cm, and Hall measurements provided net acceptor density of (1–2) $\times \,{\text 10^{15}}\,{\text{cm}^{- 3}}$ , which are comparable to relevant values reported from the CdTe films after CdCl2 treatment. Infrared microscopy suggests there are significant amounts of secondary phases (SP's) present in the samples, where the SP's are believed to be Te inclusions and precipitates. The point defects are characterized by thermoelectric effect spectroscopy (TEES), where we observed six TEES current peaks associated with the defect levels ranging from 110 to 700 mV within the bandgap.

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].