Charged Zinc Vacancies Research Articles

Impact of Cr doping on the structure, optical and magnetic properties of nanocrystalline ZnO particles

The doping of Cr ions on the structure, optical and magnetic properties of ZnO have been investigated. Zn1−xCrxO (0 ≤ x ≤ 0.05) samples were prepared by the solution combustion method. Rietveld refined x-ray diffraction results reveal that all the samples are in a hexagonal structure. Transmission electron microscopy (TEM) revealed that the particle size was 48.2 ±2.1 nm, 29.4 ± 2.2 nm and 25.3 ± 2.1 nm for the Zn1−xCrxO with x = 0, 0.03 and 0.05, respectively. The band-gap decreases slightly from 3.308 ± 0.003 eV to 3.291 ± 0.003 eV with increase of Cr content from x = 0 to 0.05, respectively. The field-dependent magnetization M(μοH) measurements of ZnO and Cr-doped ZnO exhibit room temperature ferromagnetism (RTFM). Photoluminescence and electron paramagnetic resonance exhibits negatively charged zinc vacancies (VZn−) and singly ionized oxygen vacancies (Vo+) in ZnO and these defects are responsible for RTFM. And the VZn− accountable for RTFM in Cr-doped ZnO. EPR results confirmed that Cr3+ ions replaces the Zn2+ sites in the hexagonal ZnO. The observed long-range ferromagnetism ordering is explained based on the bound magnetic polaron (BMP) mechanism. This work elucidates the origin of RTFM in synthesized ZnO and Cr-doped ZnO.

Особенности интерпретации спектров люминесценции пленок оксида цинка на сапфире

In the presented work features of interpretation of luminescent spectral dependence properties of ZnO films on sapphire are given. For complex analysis, films of ZnO of different thickness obtained in oxygen medium at different substrate temperatures are considered using the stage of recrystallysis annealing. It is shown that only a red (650 – 1000 nm) band ZnO sapphire substrate is observed in the spectrum of cathodoluminescence of thin films obtained at low temperature of the substrate, and luminescence of the film ZnO fed by excessive defect. Prolonged recrystallization annealing results in improved quality of thin ZnO films and a broad (430 – 740 nm) band in the ZnO. With an increase the film thickness, only bands associated with ZnO appear in the cathodoluminescence spectra: the edge luminescence band (maximum 390 nm) and the red band (500 – 950 nm with a maximum in the region of 710 nm) are associated with charged zinc vacancies. Focusing the beam leads to local heating of the sample and an increase in the concentration of interstitial zinc. This is due to the displacement of the edge light band into the 410 nm region, as well as the blue mixing of the defective luminescence band.

Diffusion of indium in single crystal zinc oxide: a comparison between group III donors

Dopant diffusion of indium (In) in single crystal zinc oxide is studied by secondary ion mass spectrometry and is interpreted using a reaction–diffusion model that invokes predictions from density functional theory (DFT). An apparent activation energy of 2.2 eV is obtained for the diffusion of In, when the local Fermi-level position is about 0.2 eV below the conduction band edge. The diffusion of In is found to be significantly faster that that reported for the other group III donors, aluminum and gallium, with several orders of magnitude higher effective diffusivities, that can be assigned to a lower migration barrier for the diffusion of In. Furthermore, our results reveal self-consistency in previous DFT results of defect formation- and migration energies. From this, the diffusion of In is suggested to occur through mobile charged zinc vacancies that form intermediate mobile ()− pairs. The pairs in turn dissociate rather readily at the studied temperatures (850 °C–1150 °C), which results in distinct and abrupt diffusion fronts for the In depth distribution profiles.

Oriented zinc oxide nanorods: A novel saturable absorber for lasers in the near-infrared.

Zinc oxide (ZnO) nanorods (NRs) oriented along the crystallographic [001] axis are grown by the hydrothermal method on glass substrates. The ZnO NRs exhibit a broadband (1–2 µm) near-IR absorption ascribed to the singly charged zinc vacancy VZn−1. The saturable absorption of the ZnO NRs is studied at ≈1 µm under picosecond excitation, revealing a low saturation intensity, ≈10 kW/cm2, and high fraction of the saturable losses. The ZnO NRs are applied as saturable absorbers in diode-pumped Yb (≈1.03 µm) and Tm (≈1.94 µm) lasers generating nanosecond pulses. The ZnO NRs grown on various optical surfaces are promising broadband saturable absorbers for nanosecond near-IR lasers in bulk and waveguide geometries.

The effect of defect emissions on enhancement photocatalytic performance of ZnSe QDs and ZnSe/rGO nanocomposites

A systematic study about the origin of defects emission of ZnSe structure was conducted by photoluminescence (PL) spectrometer at room temperature. It was observed that different intermediate energy levels in band-gap space of ZnSe structure were generated by different defects such as Se-, Zn-vacancies, Se-, Zn-interstitials, and surface states. Effects of these defects on the photocatalytic performance of ZnSe quantum dots (QDs) and ZnSe/graphene nanocomposites were investigated. The pristine ZnSe QDs and ZnSe/graphene nanocomposites were synthesized by a co-precipitation method. The PL spectra of the samples showed four emissions from four regions of the visible spectrum such as violet, green, orange, and red emissions. The violet emission was associated with the near-band-edge (NBE) of the ZnSe nanostructures, while, the other emissions were related to different defects of ZnSe structures. Annealing the samples in the H2 atmosphere caused to increase orange emission intensity and indicated that origin of orange emission was a donor-acceptor pair (DAPs) related to singly positively charged Se-vacancies (VSe) to singly negatively charged zinc vacancy (VZn−). Photocatalytic study of the samples to remove the methylene blue (MB) dye showed that the photocatalytic performance of the samples improved by graphene as an additive and increasing the orange emission intensity.

Effect of vacuumization on the photoluminescence and photoresponse decay of the zinc oxide nanostructures grown by different methods

Influence of vacuumization on the photoluminescence (PL) spectra and photoresponse decay of ZnO nanostructures fabricated by different methods was investigated. The visible band of photoluminescence and ultraviolet (UV) photosensitivity of the samples grown from a vapor phase was associated with the intrinsic defects such as doubly charged zinc vacancies, and for the samples grown by hydrothermal method – with presence of the oxygen vacancies. The experimental results show that ZnO nanostructures grown from the vapor phase would be promising for producing of the low cost and effective UV detecting devices.

Enhanced photoluminescence and Raman properties of Al-Doped ZnO nanostructures prepared using thermal chemical vapor deposition of methanol assisted with heated brass.

Vapor phase transport (VPT) assisted by mixture of methanol and acetone via thermal evaporation of brass (CuZn) was used to prepare un-doped and Al-doped zinc oxide (ZnO) nanostructures (NSs). The structure and morphology were characterized by field emission scanning electron microscopy (FESEM) and x-ray diffraction (XRD). Photoluminescence (PL) properties of un-doped and Al-doped ZnO showed significant changes in the optical properties providing evidence for several types of defects such as zinc interstitials (Zni), oxygen interstitials (Oi), zinc vacancy (Vzn), singly charged zinc vacancy (VZn -), oxygen vacancy (Vo), singly charged oxygen vacancy (Vo +) and oxygen anti-site defects (OZn) in the grown NSs. The Al-doped ZnO NSs have exhibited shifted PL peaks at near band edge (NBE) and red luminescence compared to the un-doped ZnO. The Raman scattering results provided evidence of Al doping into the ZnO NSs due to peak shift from 145 cm-1 to an anomalous peak at 138 cm-1. Presence of enhanced Raman signal at around 274 and 743 cm-1 further confirmed Al in ZnO NSs. The enhanced D and G band in all Al-doped ZnO NSs shows possible functionalization and doping process in ZnO NSs.

First principles study of defect formation in thermoelectric zinc antimonide, β-Zn4Sb3

Understanding the formation of various point defects in the promising thermoelectric material, β-Zn4Sb3, is crucial for theoretical determination of the origins of its p-type behavior and considerations of potential n-type dopability. While n-type conductivity has been fleetingly observed in Te:ZnSb, there have been no reports, to the best of our knowledge, of stable n-type behavior in β-Zn4Sb3. To understand the origin of this difficulty, we investigated the formation of intrinsic point defects in β-Zn4Sb3 density functional theory calculations. We found that a negatively charged zinc vacancy is the dominant defect in β-Zn4Sb3, as it is also in ZnSb. This explains the unintentional p-type behavior of the material and makes n-doping very difficult since the formation of the defect becomes more favorable at higher Fermi levels, near the conduction band minimum (CBM). We also calculated the formation energy of the cation dopants: Li, Na, B, Al, Ga, In, Tl; of these, only Li and Na are thermodynamically favorable compared to the acceptor Zn vacancy over a range of Fermi levels along the band gap. Further analysis of the band structure shows that Li:Zn4Sb3 has a partially occupied topmost valence band, making this defect an acceptor so that Li:Zn4Sb3 is indeed a p-type thermoelectric material. The introduction of Li, however, creates a more orderly and symmetric configuration, which stabilizes the host structure. Furthermore, Li reduces the concentration of holes and increases the Seebeck coefficient; hence, Li:Zn4Sb3 is more stable and better performing as a thermoelectric material than undoped β-Zn4Sb3.

Photoluminescence properties of un-doped and Mn-doped ZnO nanostructures

We presented combinations of vapor phase transport (VPT) of mixture of methanol and acetone and thermal evaporation of brass (CuZn) which contain in-situ three stages in obtaining un-doped and Mn-doped zinc oxide (ZnO) nanostructures (NSs). The structure and morphology were characterized by energy dispersive X-ray analysis (EDAX), field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The role of Ar as feed gas and interaction between methanol and brass have been predicted based on work reported elsewhere. Photoluminescence (PL) properties of un-doped and Mn-doped ZnO showed significant changes in the optical properties providing evidence for several types of defects such as zinc interstitials (Zni), oxygen interstitials (Oi), zinc vacancy (VZn), singly charged zinc vacancy (V − ), oxygen vacancy (Vo), singly charged oxygen vacancy (V +) and oxygen antisite defects (OZn) in the grown NS. The Mn-doped ZnO NSs have exhibited shifted PL peaks compared to the un-doped ZnO.

Effect of Aluminum Doping on the Growth and Optical and Electrical Properties of ZnO Nanorods

AbstractAluminum‐doped zinc oxide (AZO) nanorods were successfully prepared by a convenient solvothermal route. The crystal structure and morphology of AZO were characterized by XRD, SEM, and high‐resolution TEM. The length and diameter of AZO nanorods decreased with increasing Al content. The optical and electrical properties of AZO were studied by UV/Vis spectroscopy and a four‐point probe. The optical band gap of AZO increased initially because of the Burstein–Moss effect and then decreased as the Al content increased owing to the defects of AZO. The electrical resistivity of AZO nanorods varied conversely because of the change of electron and defect concentration (native and impurity defects). The native defect types, which were singly charged zinc and oxygen vacancies, were confirmed by photoluminescence spectroscopy. Moreover, not only the properties but also the growth mechanisms of AZO nanorods were affected by the defect concentrations of singly charged zinc vacancies and substituted Al, which were caused by increasing Al content. Finally, the AZO exhibited the smallest electrical resistivity with 1.5 at. % Al doping content, which was four orders of magnitude smaller than that of ZnO.

Doping properties of hydrogen in ZnO

Abstract

The nature of the green luminescence band in zinc selenide crystals

This paper analyzes the features of the behavior of the green luminescence band of diffusion layers of ZnSe:Te, in which it is dominant. As a result of a detailed study of the emission, transmission, and reflection, it is established that deep-lying levels at 0.2 eV participate in its formation. It correlates with the ionization energy of acceptor centers of singly charged zinc vacancies, whose concentration in the objects of interest can reach 1021 cm−3. The experimental data are used as a basis for calculating the Franck-Condon shift, which equals 0.08 eV for the indicated centers.

The Influence of Processing Conditions on Point Defects and Luminescence Centers in ZnO

Positron lifetime spectroscopy and cathodoluminescence were employed to study luminescence centers in . The samples were high‐purity polycrystalline ceramics sintered at temperatures ranging from 800 to 1400°C for 2 to 40 h. Scanning electron microscopy shows that as annealing temperatures and/or times increase, the average grain size increases and can reach 30 μm for samples sintered at 1200°C. At the same time, the positron bulk lifetime approaches theoretically estimated single‐crystal values, while the integrated luminescence intensity increases significantly. A further increase of the sintering temperature beyond 1200°C results in a decrease in the luminescence intensity, in good agreement with the only weak luminescence observed in single‐crystalline material. The positron lifetime spectra clearly show the existence of one dominant vacancy‐type defect, most likely a complex involving , or the divacancy, , independent of sample thermal history. The concentration of this center steadily decreases with increasing sintering temperature. It is concluded that the yellow luminescence centers are related to charged zinc vacancies trapped in the grain boundary regions. We propose that the observed broadness of the spectra likely originates from the modification of the electronic configuration of the luminescence centers due to their complex environment. A direct connection between the positron and the luminescence results could not be established; instead, they appear to reflect two relatively independent aspects of the samples. It could be shown, however, that positron annihilation measurements can be used effectively to monitor the evolution of the microstructure of the samples, in good agreement with scanning electron micrographs.

Characterization of ZnO varistor degradation using lifetime positron-annihilation spectroscopy

Positron-annihilation spectroscopy (PAS) was used in the lifetime mode to study changes in the grain-boundary defect equilibrium associated with dc-bias-induced degradation of a ZnO varistor. The PAS lifetime spectra were collected while the sample was under an applied bias ranging from 100 V ( ∼ 400 V/cm) to 500 V ( ∼ 2000 V/cm). The current through the sample was continuously monitored. The simple trapping model was used to interpret the lifetime PAS results and to obtain an estimate for the positron-capture rate. The experimental results show that an increase in the bias voltage results in a decrease in the positron-trap density and an increase in the positron lifetime associated with the dominant positron trap. These results are explained on the basis of a decrease in the concentration of negatively charged zinc vacancies at the grain boundary. The PAS results support the ion-migration model for degradation, which suggests that the bias-induced migration of positively charged zinc interstitials to the grain boundary reduces the concentration of negatively charged zinc vacancies at the boundary. This results in a reduction in the barrier height (degradation) and is consistent with the PAS data.

Effect of annealing on the ac leakage components of the ZnO varistor. I. Resistive current

The ZnO varistor is known to require a postsintering heat treatment (annealing) to impart stability to an otherwise unstable device, when subjected to an applied voltage stress. A large body of experimental data in the literature indicate that the most desirable temperature of annealing is between 600 and 700 °C at which the varistor becomes highly stable. This paper is aimed at understanding the significance of this annealing temperature from the analysis of the activation energies, estimated from the Arrhenius plots of the resistive current versus temperature in the prebreakdown region. It is found that the activation energy in the annealing temperature range of 600–700 °C is significantly higher than at any other annealing temperature between 500 and 800 °C. This conclusion is borne out by the statistical evaluation of the experimental data. When this result is combined with the equally notable observations in the literature that the interface state density [J. Mater. Sci. 20, 3487 (1985)], the grain-boundary trap density [J. Appl. Phys. 63, 5375 (1988)], and the density of the negatively charged zinc vacancy at the interface [J. Appl. Phys. 66, 6132 (1989)] are all reduced, also at the same annealing temperature range, it becomes clear that the annealing temperature of 600–700 °C is very unique to varistor stability. The significance of these observations is that the annealing and accompanying stability should be recognized as a very complex process that brings fundamental changes in the grain-boundary junction characteristic of the polycrystalline ceramic.

Ion implantation of zinc sulphide

The elaboration of semiconductor devices suitable for short waves spectral region, is one of the tasks of modern optoelectronics. Green, blue and ultraviolet light emitting diodes are needed. The only way to solve this problem is to use direct band 11-VI and 111-V compounds with energy gap Eg 2 2.5 eV. The well-known luminescent material zinc sulphide is very interesting from this point of view. The quantum efficiency of ZnS luminophors is about 100%. Still, it is necessary to overcome tendencies which are peculiar to these compounds in order to use ZnS in light emitting diodes. First of all, the compounds tend to selfcompensation of the defects. The lattice vacancies compensate donor or acceptor impurities implanted. Hence, the conductivity would be low. Besides, it is difficult to change the initial type of conductivity of 11-VI compounds (usually n-type) and to obtain p-n junctions. The aim of our report is to show a possibility of governing the ZnS defects structure and suppressing the tendencies mentioned with the help of ion implantation. Let us consider briefly the main features of thermodynamics of defects creation in ZnS. The dependences of atomic and electronic defects concentration on the pressure of the compound components vapour at 900K, which have been calculated by the authors are shown in Figure 1. It can be seen, that the selfcompensation takes place in the wide middle range of pressures. The concentration of negatively charged zinc vacancies [ &,I' and positively charged sulphur vacancies [V,]' are approximately equal. Let us denote free electron and hole concentrations as n and p . Transition to p-type of conductivity [ p ] > [V,]' is possible when the pressure of sulphur vapour Ps2 2 100 atm. On the other hand, the electronic conductivity [n] > [V,,]' takes place at the zinc vapour pressure P,, atm already. Calculations show, that to obtain p-type material with the help of impurity implantation Ps2 2 50 atm is needed also. However, the pressure of saturated sulphur vapour Pgt Z 3 atm at T = 900°K. One could regard, that it is possible to increase the temperature in order to rise Pgt and to suppress the compensation, but the calculations show, that it is impossible. Let us consider the influence of temperature change on the impurity's compensation rate under the condition of thermodynamical equilibrium between the crystal and the vapour phase. To reduce the compensation we should increase the pressure of the correspondent compound component as much as it is possible, i.e. up to the saturated vapour pressure. If the impurity occurs to be a donor this pressure should be Pgi and PZ' otherwise. In the first case the compensation rate

Electrical properties of ZnP2 single crystals of tetragonal and monoclinic modifications

Measurements are made of the electrical properties of both modifications of ZnP2 crystals, which are doped, undoped, and annealed in vacuum and in the vapour of the components. The measurements are carried out in the temperature range from 190 to 380 K. It is found that in the crystals of tetragonal (α) modification there are two acceptors at (Ev + 0.47) eV and (Ev + 0.21) eV and a donor at (Ec - 0.36) eV. In the crystals of monoclinic modification an acceptor at (Ev + 0.30) eV is established. By means of thermal treatment of the crystals these acceptor centres are shown to be intrinsic defects of singly and doubly charged zinc vacancies. The donor centres are found to be vacancies of phosphor. The dominating scattering mechanism is the scattering on ionized defects in the temperature range under investigation. From the experiments the concentrations of donor and acceptor centres for initial α-crystals are found to be equal to ND = −4.9 × 1021 m−3, NA = 6.0 × 1021 m−3 and for initial β-crystals ND = 4.6 × 1022 m−3, NA = −6.2 × 1022 m−3. The investigation of electrical properties of the doped crystals shows that the mechanism of the impurity intrusion corresponds to the well known model. [Russian Text Ignored].

Calculation of the electronic structure of charged vacancies in semiconductors: the zinc vacancy in ZnSe

A calculation of the electronic structure of the charged zinc vacancy in ZnSe using a cluster model is reported. The additional charge introduced onto the vacancy is simulated in the cluster using covalent and ionic models for this defect. A highly localized singlet state emerges from the valence band which is accounted for by realizing that zinc selenide is of mixed covalent and ionic character.

Self-diffusion of zinc and tellurium in zinc telluride

The high temperature defect equilibria of ZnTe were investigated by measuring the tracer self-diffusion coefficients of Zn 65 and Te 123 as functions of temperature, component pressure, and added impurity content. The experimental results reveal that the self-diffusion coefficient of Zn in undoped ZnTe is independent of component partial pressure and is described as D = 14exp −2.69±0·08 eV kT cm 2 sec The self-diffusion coefficient of Te increases with increasing partial pressure of Te, and in ZnTe saturated with Te is described by D = 2×10 4exp −3·8±0·4 eV kT cm 2 sec The addition of 10 19/cm 3 Al to ZnTe enhances the self-diffusion coefficient of Zn at temperatures below about 950°C. The addition of 5 × 10 19/cm 3 Ag to ZnTe does not influence the self-diffusion coefficient of Zn at 782°C. The results may be explained assuming that the self-diffusion of Zn in undoped ZnTe is controlled by native defect concentrations which depend only on thermal disorder, rather than deviation from stoichiometry. Either Frenkel disorder on the Zn sublattice Or Schottky disorder dominates the charge neutrality condition at high temperatures. The enhanced Zn diffusion in the Al doped crystals below 950°C is interpreted as extrinsic diffusion with charged zinc vacancies compensating the impurity donor. The dominant mobile defect proposed for the Te sublattice is an interstitial Te species of undetermined electrical activity.

Self-Compensation-Limited Conductivity in Binary Semiconductors. II.n-ZnTe

Paramagnetic resonance has geen observed in Al-doped ZnTe grown in excess Te vapor. The resonance is due to a hole trapped at a defect consisting of a doubly negatively charged zinc vacancy ${{V}_{\mathrm{Zn}}}^{2\ensuremath{-}}$ and an ${\mathrm{Al}}^{3+}$ donor ion on one of the twelve zinc sites nearest the ${{V}_{\mathrm{Zn}}}^{2\ensuremath{-}}$. The hole is trapped at the defect after irradiating the sample at 77\ifmmode^\circ\else\textdegree\fi{}K with light of energy less than that of the band gap. The $g$ tensor is orthorhombic, as expected for such a defect. Compensation of donor or acceptor impurities by doubly as opposed to singly ionized defects has been shown to be important in II-VI compounds. The observed resonance shows that compensation of Al donors in ZnTe by doubly charged zinc vacancies does indeed occur. A consequence of the self-compensation of donor impurities in ZnTe is that there is a limit to how $n$-type the material can be made. Similar attempts to dope ZnTe $n$-type were carried out with Ga, In, Br, and I. Only insulating crystals were produced, even after subsequent firing in Zn vapor. No analogous resonance signals were observed in these sample, however.