Interstitial Zn Research Articles

Preparation and optical properties of high-quality oriented of Al and Er co-doped ZnO thin films

High-quality c-axis oriented Al and Er co-doped ZnO films were prepared on the quartz glasses by sol–gel method. In order to obtain the optimal processing parameters for the growth of the oriented film, an L16 (45) orthogonal experimental design was chosen. The experimental results show the rank of 5-factors as follows: Er at.% > the number of coating layer > annealing temperature > Al at.% > the concentration of the sol. The Al and Er co-doped film prepared using the optimal parameters exhibits the preferential orientation along the c-axis perpendicular to the substrate surface. In addition, the structural, morphological and optical properties of the films were studied by X-ray diffraction, scanning electron microscopy, and UV–visible spectrophotometer, respectively. The photoluminescence spectra were also used to characterize the luminescence properties of the samples. It is found that when ZnO was co-doped with 7 % Al and 1.5 % Er, the blue emission centered at 465 nm disappears and the green emission centered at 547 nm increases with a blue shift, resulted from the rapid reducing of the interstitial Zn defect, and increasing of the oxygen defects and vacancies caused by Al3+ and Er3+ dopants.

The influence of Zn vacancy on thermal conductivity of β-Zn4Sb3: A molecular dynamics study

The influence of Zn vacancy on lattice thermal conductivity of β-Zn4Sb3 is studied by non-equilibrium molecular dynamics approach. The lattice thermal conductivity of single-crystal bulk β-Zn4Sb3 decreases rapidly when there is Zn vacancy, and then when the vacancy grows, the lattice thermal conductivity decreases further but rather slowly, which suggests a scaling law of kv∼nv−α of Zn atom vacancy (nv) to lattice thermal conductivity (kvac). This phenomenon is attributed to the fact that the existence of vacancy scattering can significantly decrease the mean free path. When the Zn atom vacant proportion reaches 10%, that is the vacancy model of β-Zn4Sb3, the lattice thermal conductivity is 1.32W/mk along the x-axis and 1.62W/mk along the z-axis, respectively, which drops by ∼90% that of its full occupancy model. Therefore, our calculations show that the 10% Zn atom vacancy in β-Zn4Sb3 is the main reason for its exceptionally low thermal conductivity, and the interstitial Zn atoms have little effect on the thermal conductivity of single-crystal β-Zn4Sb3.

Photoluminescence and field emission of 1D ZnO nanorods fabricated by thermal evaporation

Four kinds of new one-dimensional nanostructures, celery-shaped nanorods, needle-shaped nanorods, twist fold-shaped nanorods, and awl-shaped nanorods of ZnO, have been grown on single silicon substrates by an Au catalyst assisted thermal evaporation of ZnO and active carbon powders. The morphology and structure of the prepared nanorods are determined on the basis of field-emission scanning electron microscopy (FESEM) and x-ray diffraction (XRD). The photoluminescence spectra (PL) analysis noted that UV emission band is the band-to-band emission peak and the emission bands in the visible range are attributed to the oxygen vacancies, Zn interstitials, or impurities. The field-emission properties of four kinds of ZnO nanorods have been invested and the awl-shaped nanorods of ZnO have preferable characteristics due to the smallest emitter radius on the nanoscale in the tip in comparison with other nanorods. The growth mechanism of the ZnO nanorods can be explained on the basis of the vapor–liquid–solid (VLS) processes.

Enhanced Thermoelectric Performance and Thermal Stability in β-Zn4Sb3 by Slight Pb-Doping

We report a systematic study of the thermoelectric properties and thermal stability of Pb-doped (Zn1−x Pb x )4Sb3 (x = 0.0 to 0.02) bulk samples prepared by melting-quenching followed by subsequent spark plasma sintering, as a function of Pb-doping content and temperature. With introduction of Pb at Zn sites, the lattice parameters show an anomalous decrease which can be interpreted by preferential occupation of Pb at interstitial Zn sites. The exciting simultaneous enhancements of both electrical conductivity and Seebeck coefficient lead to a significant improvement in power factor for samples with low doping content. This improvement is mainly ascribed to the abnormal increase in hole mobility, which is realized by a decreased degree of disorder following introduction of heavier and larger Pb atoms, hampering the migration of “liquid” Zn in local disordered structure. Therefore, the improved power factors combined with intrinsic low thermal conductivity result in ZT >1.1 for lightly doped samples for T > 600 K, with peak ZT of 1.2 for the sample doped with 0.5 at.% Pb, representing an approximately 20% improvement compared with the undoped sample. What is more important here is the apparent improvement of high-temperature thermal stability by light Pb-doping, which is beneficial for commercial applications of this promising and cheap material for intermediate-temperature waste heat recovery.

Evolutions of defects and blue–green emissions in ZnO microwhiskers fabricated by vapor-phase transport

The corresponding evolutions of morphologies, defect-states and the PL properties have been employed to explore the defect-origins of visible emissions and growth mechanisms of microstructured ZnO. ZnO microtube and microrod whiskers were fabricated by non-catalytic vapor-phase transport using ZnO and graphite powders in air. The microstructures of the samples were studied in detail based on scanning electron microscopy (SEM), x-ray diffraction (XRD), energy-dispersive x-ray spectroscopy (EDX), and electron paramagnetic resonance (EPR). The main PL emission bands evolve from blue to green, which corresponds to that the main defect-states change from Zn interstitials (Zni) to O vacancies (VO). The formation of Zni under the high temperature zinc-rich vapor environment is crucial both for the blue emission and for the formation of the microtubes.

Ga-catalyzed growth of ZnSe nanowires and the cathodoluminescence and electric transport properties of individual nanowire

We developed a facile thermal evaporation method to fabricate zinc-blende ZnSe nanowires on GaAs substrates by using Ga as catalyst. The Ga catalyst originates from the reduction of the amorphous GaOx layer pre-oxidized on surface of GaAs substrates. The cathodoluminescence (CL) spectra of an individual nanowire reveal a weak near-band-edge emission centered at 468nm and a strong deep-level emission at 593nm. The deep-level emission is assigned to self-activated luminescence induced by donor–acceptor pairs that are associated with intrinsic point defects: Zn vacancies and Zn interstitials. The electrical transport property of individual nanowire reveals that the current versus voltage curve remains linear relation even at high applied voltage, indicating that the conventional electric power dissipation cannot affect the electric transport properties of the nanowire.

Spectroscopy of Single Donors at ZnO(0001) Surfaces

Donors near the polar (0001) surface of nominally undoped ZnO were investigated with scanning tunneling microscopy at 5 K. Spatially resolved spectroscopy reveals single and double charging. Equidistant peaks in spectra of ionized donors are attributed to polaron excitation. The data are consistent with doping due to Zn interstitials or complexes.

Ti-incorporated ZnO films synthesized via magnetron sputtering and its optical properties

Undoped and Ti-doped ZnO films were deposited using radio frequency reactive magnetron sputtering at various sputtering powers. The crystal structures, surface morphology, chemical state and optical properties in Ti-doped ZnO films were systematically investigated via X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and ultraviolet visible (UV–Vis) spectrophotometer. Results indicated that titanium atoms may replace zinc atomic sites substitutionally or incorporate interstitially in the hexagonal lattices, and a moderate quantity of Ti atoms exist in the form of sharing the oxygen with Zn atoms and hence improve the (002) orientation. The photoluminescence (PL) spectra of the Ti-doped ZnO films contain one main blue peak, whose intensity increased with the increase of sputtering power. Our results indicated that a higher compressive stress in Ti-doped ZnO films results in a lower optical band gap and a lower transmittance, and various Ti impurities can affect the concentration of the interstitial Zn and O vacancies.

Anomalous Diffusion of Intrinsic Defects in K+ Implanted ZnO using Li as Tracer

ABSTRACTPotassium (K) ions have been implanted in hydrothermally grown ZnO to a dose of 1 × 1015 cm-2, followed by isochronal annealing in a tube furnace (30min) and by rapid thermal annealing (30s) on two separate samples. For annealing temperatures below 700°C, only a minor redistribution of Li is observed behind the projected range of the K+ ions. At temperatures between 700 and 750°C, however, both annealing treatments show a wide region behind the implantation peak which is depleted of Li, and this depletion is used as a tracer to monitor diffusion of intrinsic defects like the Zn interstitial. The results are interpreted as Zn interstitials being released from the implanted region in a burst at temperatures above ∼700°C, followed by rapid migration, replacement of Li on Zn site through the kick-out mechanism, and migration of Li away from the active region.

Characterization of defects and research on impulse aging in ZnO varistor ceramics

The electrical and dielectric properties and the microstructures of a polynary ZnO-based varistor ceramics with 14000 times impulse current aging test are measured. The relationship between defect structure and impulse current aging is mainly investigated. It is found that the electrical properties decrease rapidly with impulse aging and the dimensional effect of ZnO varistor ceramics is dominated not only by grain but also by grain boundary. Additionally, four defect relaxations are found at different temperatures by using dielectric spectra. Two defect relaxations appearing below -60 ℃ with activation energies about 0.24 eV and 0.35 eV are identified to be intrinsic defects originating from interstitial Zn L(Zni··) and vacancy oxygen L(VO·), which are not affected by impulse current aging. Other two relaxations appearing above 80 °C are suggested to be extrinsic defects originating from trap levels L(ingr) at intergranular phase and trap levels L(gb) at grain-boundary interfaces, respectively. Only L(gb) decreases from 0.84 eV to 0.76 eV due to impulse current aging while other trap levels keep unchanged. It is further proposed that L(gb) is responsible mainly for the electrical property and stability of ZnO ceramics.

Effects of Annealing Environments on the Solution‐Grown, Aligned Aluminium‐Doped Zinc Oxide Nanorod‐Array‐Based Ultraviolet Photoconductive Sensor

We have fabricated metal‐semiconductor‐metal‐ (MSM‐) type ultraviolet (UV) photoconductive sensors using aluminium‐ (Al‐) doped zinc oxide (ZnO) nanorod arrays that were annealed in different environments: air, oxygen, or a vacuum. The Al‐doped ZnO nanorods had an average diameter of 60 nm with a thickness of approximately 600 nm that included the seed layer (with thickness ~200 nm). Our results show that the vacuum‐annealed nanorod‐array‐based UV photoconductive sensor has the highest photocurrent value of 2.43 × 10−4 A. The high photocurrent is due to the high concentration of zinc (Zn) interstitials in the vacuum‐annealed nanorod arrays. In contrast, the oxygen‐annealing process applied to the Al‐doped ZnO nanorod arrays produced highly sensitive UV photoconductive sensors, in which the sensitivity reached 55.6, due to the surface properties of the oxygen‐annealed nanorods, which have a higher affinity for oxygen adsorption than the other samples and were thereby capable of reducing the sensor’s dark current. In addition, the sensor fabricated using the oxygen‐annealed nanorod arrays had the lowest rise and decay time constants. Our result shows that the annealing environment greatly affects the surface condition and properties of the Al‐doped ZnO nanorod arrays, which influences the performance of the UV photoconductive sensors.

8 MeV Proton Irradiation Damage and Its Recovery by Annealing on Single-Crystalline Zinc Oxide Crystals

ABSTRACTII-VI compound semiconductor ZnO has a potential for high radiation hardness since large threshold displacement energy of constituent atoms can be expected due to the small unit-cell volume and large bandgap energy of 3.37 eV. In order to study the radiation hardness, singlecrystalline c-axis-oriented O-polar ZnO films with and without two-dimensional electron gas, a Zn-polar ZnO bulk crystal, and a Ga-polar GaN bulk crystal for comparison, were irradiated by an 8 MeV proton beam using a tandem-type accelerator. The radiation damage increased the electrical resistance and decreased the photoluminescence (PL) intensity of these samples with the increase of proton fluence over specific threshold values. In agreement with the expectation, ZnO samples were revealed to have superior radiation hardness; the threshold fluences for the deterioration of PL intensity were 3×1013 p/cm2 for the GaN bulk crystal, 2×1014 p/cm2 for the ZnO bulk crystal, and 5×1014 p/cm2 for the two ZnO films, in accordance with the order of the threshold fluences for the electrical resistance increase. The effect of post-irradiation annealing was also studied for these damaged bulk crystals; both electrical and optical properties of the ZnO bulk crystal were almost recovered to the pre-irradiation values, however, only the electrical properties of the GaN bulk crystal were recovered, by the annealing up to 700°C. Such a rapid recovery of the ZnO bulk crystal indicates the easy annihilation of Zn vacancy complexes acting as non-radiative centers by the recombination with interstitial Zn atoms. Since the migration barrier height energy of interstitial Zn atoms is known to be so small that it might occur even at room temperature, we ascribed the superior radiation hardness of ZnO crystals to the restoration of damage-induced defects by a self-annealing effect during irradiation.

Magnetic and Optical Properties of Mn Doped ZnO Nanocrystalline Films

We have investigated the properties of Mn-doped ZnO nanocrystalline film growing on zinc foil by the hydrothermal method. X-ray photoelectron spectroscopy shows that the manganese ions exist as Mn2+ in the film. From UV-vis spectra, we observe a red shift in wavelength of absorption and greater reflectivity due to the Mn ion incorporation in ZnO lattices. The photoluminescence spectrum of the Mn-doped ZnO film shows two strong new blue peaks centered at 424 nm and 443 nm, besides the UV emission peak owing to the band gap of ZnO semiconductor. The magnetic property of the Mn-doped ZnO exhibits a room temperature ferromagnetic characteristic with a saturation magnetization (Ms) of 0.3902 x 10(-3) emu/cm3 and a coercive field of 47 Oe. We suggest that the blue emission of the Mn-doped ZnO film corresponds to the electron transition from the level of interstitial Zn and Mn to the valence band. The defects brought about by Mn ion incorporation are the main cause of the room temperature ferromagnetic property.

The field emission of Cu‐doped ZnO

AbstractCu‐doped ZnO films were deposited by direct current magnetron sputtering at room temperature in an atmosphere of argon and oxygen. The properties of Cu‐doped ZnO films were characterized and their field emission was studied. The field emission is related to the defect energy levels of Zn interstitials. Cu doping can weaken the field emission by reducing the number of Zn interstitials, deteriorating the crystalline quality and forming electron traps. The content of Cu has to be carefully controlled to achieve a field emission with a large current density and a low threshold voltage.

Photoluminescence and refractive index dispersion properties of ZnO nanofibers grown by sol–gel method

Zinc oxide thin film was grown on glass substrate using sol–gel spin coating method. The optical constants and dispersion energy parameters of the zinc oxide thin film were determined using optical characterization method. The optical band gap of the n-type ZnO semiconductor was found to be 3.24 eV. The refractive index dispersion of the ZnO obeys the single oscillator model. The dispersion energy and oscillator energy values of the ZnO film were found to be 13.74 eV and 6.61 eV, respectively. The real and imaginary parts of the dielectric constant of the ZnO film were determined. The photoluminescence spectra of the ZnO film indicate a peak around 401 nm (3.10 eV) due to the electron transition from the energy level of interstitial Zn to valance band and the another peak is observed around 530 nm (2.34 eV) which is attributed to the oxygen vacancies in ZnO.

Study of ferromagnetism in Bi 2S 3 and ZnS nanocrystalline powders

Results of magnetic measurements suggested that Bi 2S 3 and ZnS nanocrystalline powders prepared by hydrothermal method could possibly exhibit room temperature ferromagnetism. The measured saturation magnetization of the powders increases with an increase of annealing temperature from 300 to 500 °C. Ab initio calculations suggested that the cation vacancies on the surface of Bi 2S 3 and ZnS nanograins could be responsible for the observed magnetic moments. Heat-treatment of Bi 2S 3 or ZnS nanocrystalline powders in Bi or Zn vapor could bring about an enhancement of ferromagnetism. The calculation results indicated that the interstitial Bi or Zn atoms in Bi 2S 3 (0 0 1) or ZnS (0 0 1) surface could induce magnetic moments.

Influence of surface defects in ZnO thin films on its biosensing response characteristic

Highly c-axis oriented zinc oxide (ZnO) thin films deposited by rf magnetron sputtering under varying processing pressure (20–50 mT) in a reactive gas mixture of argon and oxygen were studied for biosensing application. The as-deposited ZnO thin films were in a state of compressive stress having defects related to interstitial Zn and antisite oxygen. Glucose oxidase has been chosen as the model enzyme in the present study and was immobilized on the surface of ZnO thin films deposited on indium tin oxide coated Corning Glass substrate. The studies reveal a correlation between the biosensing characteristic and the presence of defects in the ZnO films. The ZnO films deposited under high pressure (50 mT) are found to be more sensitive for biosensing application due to availability of more surface area for effective immobilization of biomolecules and exhibits a suitable microenvironment with good electron transfer characteristic. The obtained results highlight the importance of desired microstate besides availability of suitable native defects in the ZnO thin film for exhibiting enhanced biosensing response.

Optimizing thermoelectric performance of Cd-doped β-Zn4Sb3 through self-adjusting carrier concentration

Crack-free Zn3.96+xCd0.04Sb3 (x = −0.05, 0.0, 0.05 and 0.1) ingots were successfully synthesized by a melting followed by a precisely controlled slow cooling process. The facile control of Zn content realizes the effective self-adjustment of carrier concentration, as well as the optimization of the thermoelectric figure of merit. The Zn-deficiency and stoichiometric samples are single phase, whereas a slight metal Zn phase can be detected in other two Zn-rich samples existing as forms of numerous evenly distributed nano-clusters with size of 20–50 nm and a spot of micro-scale precipitations embedded in the matrix. In particular, these multi-scale microstructures combined with the subtle variation of interstitial Zn apparently intensify phonon scattering and give rise to a “phonon-glass” feature of Zn-rich samples. However, Zn-deficiency sample benefiting from high Seebeck coefficient, shows a high power factor (>1.0 mW m−1 K−1) in the entire temperature range and a maximum value of 1.26 mW m−1 K−1 at 660 K. As a result, the enhanced effective hole mass by a slight Cd-doping coupled with the extremely low lattice thermal conductivity originated from crystalline complexities lead to a high figure of merit of 1.23 at 660 K for Zn3.91Cd0.04Sb3 sample, which is comparable with the highest value reported by T. Caillat et al. [T. Caillat et al. J Phys Chem Solids 1997; 58: 1119−25]. Furthermore, this study demonstrates a simple and easily-industrialized melting combined with slow cooling technique making the high performance β-Zn4Sb3 a promising candidate for low-grade waste heat recovery.

Structural and Optical Properties of Zn1-XCoXO Thin Films Fabricated by Laser Molecular Beam Epitaxy

Zn1-xCoxO thin films on sapphire (0001) substrates were synthesized by laser molecular beam epitaxy (LMBE) method at various temperatures under a work ambient pressure of 5.0 x 10-5Pa condition. X-ray diffraction (XRD) spectra, UV–visible transmission spectra and X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra were employed to characterize the properties of samples. All samples were of wurtzite hexagonal structure with the preferential c-axis-orientation. Co2+ions incorporated into ZnO lattice and substituted for Zn2+ions. ZnLMM Auger spectrum implied Zn interstitials existed in sample. The optical transmission of all samples was relatively high in visible region. Two PL emission peaks located at 418 nm and 490 nm were assigned to the electron transition from the Zn interstitials to the top of the valence band and from the Zn interstitials to the Zn vacancies, respectively.

Sb complexes and Zn interstitials in Sb-implanted ZnO epitaxial films

In the present work, post-annealing is adopted to investigate the formation and the correlation of Sb complexes and Zn interstitials in Sb-ion implanted ZnO films, by using Raman scattering technique and electrical characterizations. The damage of Zn sublattice, produced by ion bombardment process is discerned from the unrecovered E2 (L) peak in annealed high Sb+ dose implanted samples. It is suggested that the Zn sublattice may be strongly affected by the introduction of Sb dopant because of the formation of SbZn−2VZn complex acceptor. The appearance of a new peak at 510 cm−1 in the annealed high dose Sb+ implanted samples is speculated to result from (Zn interstitials-O interstitials) Zni−Oi complex, which is in a good accordance with the electrical measurement. The p-type ZnO is difficult to obtain from the Sb+ implantation, however, which can be realized by in-situ Sb doping with proper growth conditions instead.