Defects In CdTe Research Articles

The laser-induced rearrangement of extended defects in crystalline CdTe at a low temperature

The effect of subnanosecond laser pulses on the structure and electronic subsystem of extended defects in cubic semiconductors was studied using cadmium telluride (CdTe) as an example. A CdTe epitaxial film containing threading dislocations was exposed to pulsed laser emission at helium temperatures. This exposure led to a local rearrangement of dislocation, while the properties of the crystal lattice remained undisturbed. The rearrangement was visualized in situ via an observation of the single luminescent centers associated with the partial dislocation cores. The rearrangements in the center of the laser spot, as well as those far from this location, were detected, thus revealing the laser treatment’s non-thermal, relatively long-range influence. We associated the corresponding mechanism with Peierls’s gliding of dislocation under the impact of laser-induced hypersonic surface waves. The results we obtained are of interest in the development of all-optical methods for the local laser processing of extended defects in CdTe and subsequent expansion those methods to other A2B6 cubic semiconductors.

Vibrational Signatures of Substitutional Oxygen in CdTe and CdSe

Vibrational modes of substitutional oxygen on the anion site in cubic CdTe and wurtzite CdSe are identified from infrared (IR) absorption measurements on single crystals codoped with 18O and 16O. The 16O isotope variety of the defect in CdTe gives rise to a local vibrational mode at 349.8 (10 K), whereas the corresponding species in CdSe results in a doublet structure at 370.6 and 372.5 (10 K). The latter two modes are fully polarized for light parallel and perpendicular to the c axis of the CdSe crystal, respectively, and belong to the nondegenerate and the twofold degenerate E representations of the C3v point group.

Assessment of intrinsic and doped defects in Bridgman grown Cd1-xZnxTe alloys

A comprehensive experimental and theoretical study is presented to empathize the dynamical properties of Bridgman grown Cd1-xZnxTe samples. Low temperature Raman scattering study has offered an excellent testimony to the lattice phonons of the perfect alloys with a strong evidence of Te anti-site defect (TeCd) modes. An average transfer matrix (ATM) Green’s function (GF) method is employed to examine the isotopic shifts of localized vibrational modes (LVMs) of isolated oxygen OTe defects in CdTe and the low frequency A1 and E phonon modes of TeCd. For the nearest-neighbor (NN) OTe – VCd pair, while the ATM-GF’s study revealed quite different values of LVMs from the existing infrared (IR) absorption data – the results, however, provided good justification to a simple perturbation relationship of 16OTe isotopic mode. An excellent agreement between theory and IR data of impurity modes has validated the formation of NN donor–acceptor pairs and next-nearest-neighbor complex centers involving intrinsic defects. The study is important not only for empathizing the growth process of technologically important Cd1-xZnxTe alloys but also identifying the role of carrier compensation mechanism by various donors and acceptors.

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.

Optical and electrical characterization of Cadmium Telluride X-ray pad detectors

Tellurium defects in CdTe and CdZnTe detectors are known to degrade the detector performance by trapping passing charges. This causes losses in charge collection and ultimately degrades the energy resolution. The amount and size of the defects has previously been studied for small areas of these materials, and small defects (<5μ m) have been identified as the main cause for signal degradation. However, previous studies concentrated on the evaluation of a view regions of interest. In this study we present a system capable of scanning larger volumes of CdTe and CdZnTe and associate the locations of the found defects with the charge collection of detectors. Further, we show that the signal degradation is not uniform over the whole detector, but linked to local densities of defects. This results in variations of charge collection even within single detectors.

(Invited) Characterization of Phases and Defects in Compound Semiconductors

Defects in semiconductors can be difficult to characterize, especially in complex semiconductors such as the chalcogenides. This talk presents several different and relatively unusual methods for their study along with examples of results. The talk will focus on semiconductors used for photovoltaic applications but the talk should illustrate methods that would be of use with any semiconductor material. Specifically the talk will address the use of admittance spectroscopy, nuclear magnetic resonance (NMR), scanning microwave impedance microscopy (SMIM), scanning tunneling spectroscopy (STS) and others to study defects in CdTe, CuInSe2 (CIGS), and Cu2ZnSnS4 (CZTS), and related compounds. Examples of results include the identification of two defects in the energy gap of CIGS by admittance spectroscopy, band edge fluctuations in CIGS by STS and their impact on the ability to obtain atomic resolution images, organized point defect structures in CZTS by NMR, and grain and grain boundary behaviors in CdTe by SMIM. Other examples will be included as time permits.

Synthesis and Optical Properties of Water-Soluble CdTe:Zn2+ Quantum Dots Prepared by the One-Pot Approach

This paper reports the synthesis and characterization of ternary CdTe:Zn2+ quantum dots (QDs) passivated with reduced L-glutathione (GSH) dispersed in water. The synthesis was performed through a one-pot approach and their size-dependent optical properties were investigated through steady-state absorption and emission spectroscopies along with the evolving factor analysis/multivariate curve resolution alternating least squares (EFA/MCR-ALS) method. Our results show that the incorporation of Zn2+ ions in CdTe:Zn2+ QDs, during the synthesis time, decreases the CdTe:Zn2+ QDs predominant diameter in the ensemble and increases their size dispersion. In addition, fluorescence quantum yield measurements suggest a reduction in the number of surface defects in CdTe:Zn2+ QDs as compared with CdTe QDs, when both are produced from the same synthesis route.

Carrier providers or killers: The case of Cu defects in CdTe

Defects play important roles in semiconductors for optoelectronic applications. Common intuition is that defects with shallow levels act as carrier providers and defects with deep levels are carrier killers. Here, taking the Cu defects in CdTe as an example, we show that relatively shallow defects can play both roles. Using first-principles calculation methods combined with thermodynamic simulations, we study the dialectic effects of Cu-related defects on hole density and lifetime in bulk CdTe. Because CuCd can form a relatively shallow acceptor, we find that increased Cu incorporation into CdTe indeed can help achieve high hole density; however, too much Cu can cause significant non-radiative recombination. We discuss strategies to balance the contradictory effects of Cu defects based on the calculated impact of Cd chemical potential, copper defect concentrations, and annealing temperature on lifetime and hole density. These findings advance the understanding of the potential complex defect behaviors of relatively shallow defect states in semiconductors.

Spatial luminescence imaging of dopant incorporation in CdTe Films

State-of-the-art cathodoluminescence (CL) spectrum imaging with spectrum-per-pixel CL emission mapping is applied to spatially profile how dopant elements are incorporated into Cadmium telluride (CdTe). Emission spectra and intensity monitor the spatial distribution of additional charge carriers through characteristic variations in the CL emission based on computational modeling. Our results show that grain boundaries play a role in incorporating dopants in CdTe exposed to copper, phosphorus, and intrinsic point defects in CdTe. The image analysis provides critical, unique feedback to understand dopant incorporation and activation in the inhomogeneous CdTe material, which has struggled to reach high levels of hole density.

Characterization of point defects in CdTe by positron annihilation spectroscopy

Positron lifetime measurements on CdTe 0.15% Zn-doped by weight are presented, trapping to monovacancy defects is observed. At low temperatures, localization at shallow binding energy positron traps dominates. To aid defect identification density functional theory, calculated positron lifetimes and momentum distributions are obtained using relaxed geometry configurations of the monovacancy defects and the Te antisite. These calculations provide evidence that combined positron lifetime and coincidence Doppler spectroscopy measurements have the capability to identify neutral or negative charge states of the monovacancies, the Te antisite, A-centers, and divacancy defects in CdTe.

Theoretical study of intrinsic defects in CdTe

The quantum states and thermodynamical properties of the Cd and Te vacancies in CdTe are studied by first principles calculations. It is shown that the band structure of a cubic 64-atoms supercell with a Te vacancy is dramatically different from the band structure of the perfect crystal, suggesting that it cannot be used as model to calculate isolated defects. This flaw is solved modeling the Te vacancy within a cubic 216-atoms supercell. However, even with this large supercell, the 2— charge state relaxes to an incorrect distorted structure. This distortion is driven by partial filling of the conduction band induced by the k-point sampling. The correct structures and formation energies are obtained by relaxation with restriction of system symmetry, followed by band-filling correction to the energy, or by using a larger supercell that allows sampling the Brillouin zone with a single k-point.

Theoretical analysis of non-radiative multiphonon recombination activity of intrinsic defects in CdTe

We present an analysis of recombination activity of intrinsic defects (VCd, TeCd, VTe, and Tei) in CdTe based on the multiphonon single-mode carrier-capture model, with vibronic parameters obtained using hybrid density functional theory. This analysis allows us to determine the defects and the corresponding electronic processes that have high trapping rates for electrons, for holes, or for both. The latter, being potentially the most active recombination centers, decreases the carrier lifetime in the absorber layer of a CdTe solar cell. Taking into account the relatively high calculated capture cross-sections of the TeCd antisite defect (σ = 8.7× 10−15 cm2 for electron capture on TeCd+2 defect, σ = 6.8 × 10−14 cm2 for hole capture on TeCd+1 defect at room temperature) and its deep trapping level (0.41 eV for +2/+1 level), we conclude that this defect is the most active recombination center among the intrinsic defects in p-type CdTe. Other processes that do not lead to effective recombination are: (i) fast hole capture on Tei+1 defect (σ = 1.1 × 10−13 cm−2), (ii) electron capture on TeCd+1 defect (σ = 2.9 × 10−15 cm−2), (iii) somewhat slower hole capture on TeCd0 defect (σ = 9.4 × 10−20 cm−2), (iv) hole capture on VCd−1 defect (σ = 7 × 10−19 cm2), and (v) electron capture on Tei+1 defect (σ = 4.4 × 10−19 cm−2). The cross-sections are found to be negligibly small for the remaining capture processes.

Positron annihilation spectroscopy of vacancy-related defects in CdTe:Cl and CdZnTe:Ge at different stoichiometry deviations.

Positron annihilation spectroscopy (PAS) was used to examine the effect of defined Cd-rich and Te-rich annealing on point defects in Cl-doped CdTe and Ge-doped CdZnTe semi-insulating single crystals. The as-grown crystals contain open-volume defects connected with Cd vacancies . It was found that the Cd vacancies agglomerate into clusters coupled with Cl in CdTe:Cl, and in CdZnTe:Ge they are coupled with Ge donors. While annealing in Cd pressure reduces of the density, subsequent annealing in Te pressure restores . The CdTe:Cl contains negatively-charged shallow traps interpreted as Rydberg states of A-centres and representing the major positron trapping sites at low temperature. Positrons confined in the shallow traps exhibit lifetime, which is shorter than the CdTe bulk lifetime. Interpretation of the PAS data was successfully combined with electrical resistivity, Hall effect measurements and chemical analysis, and allowed us to determine the principal point defect densities.

Numerical Simulation of Copper Migration in Single Crystal CdTe

With better CdTe material properties and more effective contacts, the compensated Cu doping is crucial for enhancing the performance of CdTe solar cells. Moreover, the metastable behavior of CdTe devices is associated with Cu migration. To understand the formation of Cu acceptors in CdTe and Cu-related metastabilities, we developed a one-dimensional diffusion-reaction simulator to study the kinetics of Cu-related defects in CdTe material. A detailed kinetic model that explicitly describes the evolutions of point defects was developed and applied for the first time. Migration of intrinsic and Cu-related extrinsic defects in single crystal CdTe (sx-CdTe) bulk has been investigated in time-space domain self-consistently with free carrier transport. Cu profiles achieved from simulations match with the experimental profiles quantitatively. The simulation results provide explanations of limited Cu incorporation (also known as the Cu solubility limits) and the development of Cu compensation in course of annealing and cooling processes. Although the one-dimensional simulation has intrinsic limitations when applied to poly-crystalline CdTe solar cells, the results suggest that the approach still has strong potential in better understanding the performance and the metastabilities of poly-crystalline CdTe photovoltaic devices.

Shockley-Read-Hall lifetimes in CdTe

A combination of first principles electronic structure calculations, Green's function method, and empirical tight-binding Hamiltonian method is used to evaluate the minority carrier lifetimes of CdTe due to recombination via native point defects in CdTe. For defect energy levels near mid-gap, our calculated value of the Shockley-Read-Hall capture cross section for both electrons and holes is ∼10−13 cm2, which is considerably different from the most commonly employed values. We further find that minority carrier lifetimes in doped CdTe are affected more by defect levels closer to the Fermi level than those in the mid-gap.

Identification of oxygen defects in CdTe revisited: First-principles study

By using first-principles calculations, several SO2 complexes in CdTe were studied. Based on experimental observation, SO2 complexes have been recently proposed by Lavrov et al. [Phys. Rev. B. 84, 233201 (2011)] to be the cause of the observed IR absorption peaks at 1096.8 and 1108.4 cm−1 in O-doped CdTe. Chen et al. [Phys. Rev. Lett. 96, 035508 (2006)] were originally proposed that the peaks come from OTe-VCd complex. Our calculations indicate that the SO2 molecule on the Te site [(SO2)Te] has a low formation energy but its calculated vibration frequencies (∼900 cm−1) are lower than the observed IR modes. However, (SO2)Te can form a complex with VCd with two possible configurations that give the vibration frequencies in a good agreement with the two observed IR peaks. The binding energies of the complex in these two configurations are about 1 eV under p-type conditions; indicating that the complex is quite stable. The two configurations are related to each other by a rotation of the SO2 molecule with an energy barrier of ∼0.4 eV. Therefore, the two configurations can co-exist at a low temperature and the high energy one gradually transforms to the low energy one as temperature increases. This agrees with the experimental observation that, at a high temperature, the two IR modes merged into one.

Dislocation emission caused by different types of nanoscale deformation defects in CdTe

Dislocation emission caused by different types of nanoscale deformation defects in CdTe

Effect of features of the technology of polycrystalline CdTe growth on the conductivity and deep level spectrum after annealing

The conductivity, morphology, and deep levels in polycrystalline CdTe are studied. Undoped p-CdTe is grown from the vapor phase by low-temperature methods of direct Cd and Te chemical reaction and CdTe vacuum sublimation at Pmin. Chlorine-doped CdTe is also grown. The resistivity of the grown samples is ∼105–109 Ω cm. After annealing in liquid cadmium or in cadmium vapor at ∼500°C, the conductivity type changes, the free-carrier concentration in the undoped and doped samples increases to 4 × 1015 and ∼2 × 1016 cm−3, respectively. For all samples, a defect ground level of ∼0.84 eV and continuous background are observed in DLTS spectra after annealing. A correlation between the primary-defect and free-carrier concentrations in undoped and doped CdTe is observed. Chlorine is a main residual impurity in the undoped samples. It is assumed that the defect is a complex including chlorine and observed structural defects in CdTe.

Search for the Major Chlorine-Related Defects in CdTe:Cl

ABSTRACTUnderstanding the effect of chlorine-related defects on the CdTe electric properties is important both for obtaining high resistivity CdTe-based detectors and for high efficiency CdTe-based thin-film solar cells. The actual mechanism of the effect of Cl on electric properties of CdTe is not clear and different sometimes contradictory hypotheses appear. For example ClTeVCd shallow acceptor complex defect was proposed both as a reason of increased carrier concentration in CdTe thin film and also as a reason of high resistivity of CdTe:Cl thin films. In the present work we are trying to clarify the effect of Cl on CdTe electric properties and to find the reason of high resistivity of CdTe:Cl crystals using first principles calculations and defect chemistry modeling. For the first time we are trying to develop a model capable to describe experimental data on both high temperature and room temperature conductivity of CdTe:Cl.

What causes high resistivity in CdTe

Shallow donors are often introduced into semiconductor materials to enhance n-type conductivity. However, they can sometimes also be used to obtain compensation between donors and acceptors, resulting in high resistivity in semiconductors. For example, CdTe can be made semi-insulating by shallow donor doping. This is routinely done to obtain high resistivity in CdTe-based radiation detectors. However, it is widely believed that the shallow donor alone cannot be responsible for the high resistivity in CdTe. This is based on the argument that it is practically impossible to control the shallow donor doping level so precisely that the free carrier density can be brought below the desired value suitable for radiation detection applications. Therefore, a deep native donor is usually assumed to exist in CdTe and pin the Fermi level near midgap. In this paper, we present our calculations on carrier statistics and energetics of shallow donors and native defects in CdTe and illustrate different donor-specific mechanisms for achieving carrier compensation. Our results show that the shallow donor can be used to reliably obtain high resistivity in CdTe without requiring additional deep donors. Since radiation detection applications require both high resistivity and good carrier transport, one should generally use shallow donors and shallow acceptors for carrier compensation and avoid deep centers that are effective carrier traps. This study highlights how the interaction between impurities and native defects intricately affects the Fermi level pinning in the semiconductor band gap and the associated resistivity of the material.