Interstitial Zn Research Articles

Metal-insulator transition in ZnO nanopowder during thermal cycling by impedance spectroscopy

We have investigated the electrical response on the pellet of ZnO nanopowder by impedance spectroscopy. Different trends of impedance plane plots have been discussed due to the modulations of relaxation time with temperatures. Thermal cycling induces irreversible changes in the resistance of the material which has been explained by incorporating the role of extended defects like oxidation of Zn interstitials at surfaces and intrinsic/extrinsic grain boundaries. Metal-insulator transition has been reported and discussed in term of combined effects of Zn interstitials and oxygen vacancies. Irreversible increase in the values of dielectric constant have been conferred with thermal cycling’s.

The role of Zn interstitials in cobalt-doped ZnO diluted magnetic semiconductors

Co-doped ZnO samples Zn1−xCoxO and CoyZn1−yO were prepared by the sol-gel and magnetron sputtering methods, respectively. Although the Co K-edge extended x-ray absorption fine structure spectra show that the doped Co ions are located at the Zn substitutional sites for both Zn1−xCoxO (x≤0.05) and CoyZn1−yO (y≤0.05) samples, magnetic measurements show paramagnetism in Zn1−xCoxO (x≤0.05) and high temperature ferromagnetism in CoyZn1−yO (y≤0.05). An experimental and numerical study of the O K-edge x-ray absorption near-edge structure spectra reveals that the incorporation of the Zn interstitials in the lattice is crucial to the appearance of high temperature ferromagnetism in CoyZn1−yO (y≤0.05) samples.

Optical properties of ZnO nanotubes

AbstractTubular ZnO nanostructures have been obtained via a hydrothermal method at low temperature (90 °C) without any catalysts or templates. The XRD measurement reveals that the tubes are single crystals with hexagonal wurtzite structure. SEM shows that the diameters of ZnO nanotubes ranged from 400 to 550 nm. The Raman and PL spectra indicate that oxygen vacancies or Zn interstitials are responsible for the green emission in the ZnO nanotubes. A possible growth mechanism on the formation of crystalline ZnO nanotubes has been presented. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of reducing anneal on the ferromagnetism in single crystalline Co-doped ZnO thin films

This paper reports that the high-quality Co-doped ZnO single crystalline films have been grown on a-plane sapphire substrates by using molecular-beam epitaxy. The as-grown films show high resistivity and non-ferromagnetism at room temperature, while they become more conductive and ferromagnetic after annealing in the reducing atmosphere either in the presence or absence of Zn vapour. The x-ray absorption studies indicate that all Co ions in these samples actually substituted into the ZnO lattice without formatting any detectable secondary phase. Compared with weak ferromagnetism (0.16 μB/Co2+) in the Zn0.95Co0.05O single crystalline film with reducing annealing in the absence of Zn vapour, the films annealed in the reducing atmosphere with Zn vapour are found to have much stronger ferromagnetism (0.65 μB/Co2+) at room temperature. This experimental studies clearly indicate that Zn interstitials are more effective than oxygen vacancies to activate the high-temperature ferromagnetism in Co-doped ZnO films, and the corresponding ferromagnetic mechanism is discussed.

Ferromagnetism in ZnO single crystal

Abstract Room temperature ferromagnetism (RTFM) with a saturation magnetization ( M s ) of 0.025 emu/cm 3 has been observed in single crystal of zinc oxide (ZnO). The presence of intrinsic defects like zinc (Zn) interstitials and oxygen (O) vacancies in single crystal ZnO is established through micro-Raman and photoluminescence studies. The role of intrinsic defects in introduction of RTFM in single crystal ZnO is discussed.

The Al-doping and post-annealing treatment effects on the structural and optical properties of ZnO:Al thin films deposited on Si substrate

Al-doped ZnO (ZnO:Al) thin films with different Al contents were deposited on Si substrates using the radio frequency reactive magnetron sputtering technique. X-ray diffraction (XRD) measurements showed that the crystallinity of the films was promoted by appropriate Al content (0.75 wt.%). Then the ZnO:Al film with Al content of 0.75 wt.% was annealed in vacuum at different temperatures. XRD patterns revealed that the residual compressive stress decreased at higher annealing temperatures. While the surface roughness of the ZnO:Al film annealed at 300 °C became smoother, those of the ZnO:Al films annealed at 600 and 750 °C became rougher. The photoluminescence (PL) measurements at room temperature revealed a violet, two blue and a green emission. The origin of these emissions was discussed and the mechanism of violet and blue emission of ZnO:Al thin films were suggested. We concluded that the defect centers are mainly ascribed to antisite oxygen and interstitial Zn in annealed (in vacuum) ZnO:Al films.

Effect of Zn interstitials on the magnetic and transport properties of bulk Co-doped ZnO

We have demonstrated that the bound magnetic polaron model is responsible for ferromagnetism in Co–ZnO semiconductors, where the carriers are provided by the interstitial zinc (Zni). Our experiment is unique since by changing the temperature, we are able to cross the carrier concentration threshold above which a long-range ferromagnetic order is established. Consequently, the ferromagnetic order is observed at room temperature but is weakened at temperatures below 100 K. To support our conclusion we have performed a systematic investigation on the structural, magnetic and transport properties which all give consistent results in the context of our proposed two-region model, i.e. (a) a Zni layer where carriers are sufficient to couple Co ions ferromagnetically and (b) a region with little carriers that remain in a paramagnetic state.

Defects in Irradiated ZnO Thin Films Studied by Photoluminescence and Photoconductivity

AbstractPulsed-laser-deposited ZnO thin films were exposed to a 1.5 MeV helium ion beam to study the changes in radiative and non-radiative recombination. We first measured photoluminescence (PL) spectra at 4.2 K excited with the 325 nm line of a HeCd laser. The as-deposited films showed a donor-bound exciton peak at 3.3567 eV attributed to Zn interstitials. After irradiation the donor-bound-exciton dominated PL spectra shifted to acceptor-bound behaviour with a signal at 3.3519 eV, tentatively attributed to Li or Na acceptors. In contrast to the approximately 30 % decrease of the PL signal near the band edge, we observed a strong concomitant enhancement of the green/orange PL band, located between 2.1 eV and 2.8 eV, by a factor of over 4. Candidates for those transitions are Li impurities and/or O vacancies. For comparison, the steady-state photocurrent decreased strongly in the irradiated region, which can also be attributed to increased non-radiative recombination through oxygen-related defects.

Facile synthesis and photoluminescence of ZnSe nanowires

Well-defined ZnSe nanowires were successfully synthesized on the composite layer of gold and carbon coated Si substrates by using a facile thermal vaporation method. These nanowires are of well crystalline cubic structure and the average length is up to tens of micrometers. It is found that the growth process follows a typical vapor-liquid-solid (VLS) mechanism, and the carbon layer can enhance the VLS growth process of the ZnSe nanowires. Moreover, the photoluminescence (PL) properties of the nanowires were investigated. Only a broad and strong red emission band centered at 629 nm is observed. The main reason is ascribed to the donor-acceptor pair recombination induced by a large amount of Zn vacancies and Zn interstitials existing in ZnSe nanowires.

Defects in ZnO

Zinc oxide (ZnO) is a wide band gap semiconductor with potential applications in optoelectronics, transparent electronics, and spintronics. The high efficiency of UV emission in this material could be harnessed in solid-state white lighting devices. The problem of defects, in particular, acceptor dopants, remains a key challenge. In this review, defects in ZnO are discussed, with an emphasis on the physical properties of point defects in bulk crystals. As grown, ZnO is usually n-type, a property that was historically ascribed to native defects. However, experiments and theory have shown that O vacancies are deep donors, while Zn interstitials are too mobile to be stable at room temperature. Group-III (B, Al, Ga, and In) and H impurities account for most of the n-type conductivity in ZnO samples. Interstitial H donors have been observed with IR spectroscopy, while substitutional H donors have been predicted from first-principles calculations but not observed directly. Despite numerous reports, reliable p-type conductivity has not been achieved. Ferromagnetism is complicated by the presence of secondary phases, grain boundaries, and native defects. The famous green luminescence has several possible origins, including Cu impurities and Zn vacancies. The properties of group-I (Cu, Li, and Na) and group-V (N, P, As, and Sb) acceptors, and their complexes with H, are discussed. In the future, doping of ZnO nanocrystals will rely on an understanding of these fundamental properties.

Pulsed laser deposition of high Mg-content MgZnO films: Effects of substrate temperature and oxygen pressure

High Mg-content, wurtzite MgZnO films were grown on sapphire (0001) substrates by pulsed laser deposition. It was found that the compositions, defect states, optical, and electrical properties of MgZnO films depend critically on the substrate temperature and oxygen pressure. As the substrate temperature increases, Mg content in single-phase wurtzite MgZnO films increases significantly from 27 close to 46 at. %, and corresponding band gap is widened from 3.49 to 3.88 eV. X-ray photoelectron spectroscopy studies show that plenty of Zn interstitials are present in MgZnO films grown at lower oxygen pressures, which lead to the lattice expansion and the high electron concentration as native donor defects; while the films grown at higher oxygen pressure exhibit high resistivity with indeterminate conductivity type and the relatively wide band gap.

Structural phase transition behaviour of Zn4Sb3 and its substitutional compounds (Zn0.98M0.02)4Sb3 (M = Al, Ga and In) at low temperatures

Structural phase transitions of Zn4Sb3 and its substitutional compounds (Zn0.98M0.02)4Sb3 (M = Al, Ga and In) are investigated by electrical transport measurement and differential scanning calorimetry below room temperature. The results indicate that both β → α and α → α′ phase transitions of Zn4Sb3 are reversible and exothermic processes, which may be explained as that both the transitions originate from the ordering of the disordered interstitial Zn and vacancies in regular sizes. The derived activation energies of β → α and α → α′ phase transition processes for Zn4Sb3 are E1 = 3.9 eV and E2 = 4.1 eV, respectively. Although no remarkable influence on activation energy E2 is observed after Al doping, Al substitution for Zn causes E1 to increase to 4.6 eV, implying its suppression of β → α transition to a great extent. Moreover, it is found that both β → α and α → α′ transitions are completely prohibited by substitution of either In or Ga for Zn in Zn4Sb3. The underlying mechanisms for these phenomena are discussed.

Photoluminescence of ZnO nanorods grown by hydrothermal method on Si substrate

ZnO nanorods were grown on Si substrate by hydrothermal method under various reaction time (12, 24, and 36 h), Zn2+ concentrations (0.01, 0.02, and 0.05 mol/L) and reaction temperatures (70–95 °C). Their photoluminescence (PL) tests were taken at room temperature. Nanorods grown under longer reaction times or higher temperatures can emit stronger UV light and depressed green light, suggesting better crystallization of ZnO nanorods. Higher Zn2+ concentration results in stronger green band emitting, whereas the UV peak is depressed with the Zn2+ concentration over 0.02 mol/L. This indicates that excessive interstitial Zn defects may be introduced into the nanorods in Zn-rich environment.

Thermal plasma synthesis and optical properties of Zn 2SnO 4 nanopowders

In this research, we used the DC thermal plasma technique to produce well-crystallized zinc stannate (Zn 2SnO 4) nanopowders from commercial Zn–Sn alloy wire. The as-synthesized Zn 2SnO 4 consisted of spherical, rhombohedral and hexagonal shapes with an average size of about 35 nm. The room-temperature photoluminescence spectroscopy of the Zn 2SnO 4 showed a UV emission at 390 nm and visible emissions at 577, 651 and 671 nm. The UV emission was assigned to the near band-edge emission while the visible emissions were believed to be attributed by the residual strain, the crystal defects, the oxygen vacancies and the interstitial Zn during the growth process of ZTO2. According to UV–vis spectrum, the absorption edge of Zn 2SnO 4 is 367 ± 5 nm, which corresponds to the band gap energy of 3.36 ± 0.06 eV.

Raman spectra and room-temperature ferromagnetism of hydrogenated Zn0.95Mn0.05O nanopowders

Two additional vibration modes at 250 and 529 cm−1 were observed in hydrogenated Zn0.98Mn0.05O nanopowders. The number of oxygen vacancies was increased by hydrogenation, which induced the mode at 250 cm−1. The additional vibration mode at 529 cm−1 was due to the increase of the number of subsititutional Mn atoms occupying Zn sites, which resulted that some new lattice defects were introduced or intrinsic host-lattice defects became activated/intensified. The room-temperature ferromagnetism was observed in the hydrogenated Zn0.98Mn0.05O nanopowders prepared at high temperature (T=700 °C), which originated from the lattice defects such as oxygen vacancies and Zn interstitials.

Nonferromagnetic nanocrystalline ZnO:Co thin films doped with Zn interstitials

ZnCoO thin films were synthesized via a wet chemical route and subsequently annealed in Zn vapor to increase the conductivity by introducing Zn interstitials. All samples show small hysteresis loops close to the detection limit of the magnetometer. Thus the samples were thoroughly investigated to obtain evidence for further ferromagneticlike behavior. Optical and magneto-optical experiments show the crystal field transitions of Co2+ in the near infrared and visible spectral range. At energies above 2.8 eV a charge transfer transition of Co2+ is observed. The results of magnetotransport measurements are explained by the formation of an impurity band situated below the conduction band. No further evidence for ferromagnetism is obtained.

Comparable study on the effect of diluted NH 4Cl solution on the postdeposition surface texture of as-deposited aluminum-doped zinc oxide films by direct current pulse and direct current reactive magnetron sputtering

Effect of 5.0% diluted NH 4Cl aqueous solution was comparably investigated on the postdeposition surface texture of the as-deposited smooth aluminum-doped zinc oxide (AZO) films by direct current pulse reactive magnetron sputtering (DCP-sputtering) and direct current reactive magnetron sputtering (DC-sputtering). The as-deposited AZO films by DCP-sputtering showed an effective surface texture for light trapping upon the etching of 5.0% diluted NH 4Cl solution, while the as-deposited AZO films by DC-sputtering demonstrated an obscure surface texture upon the same etching treatment. The different result might be attributable to a big difference in film strain and film compactness. The formation of interstitial Zn, interstitial Al and grain boundary is the key to realize effective surface texture for the as-deposited AZO films.

Cu-doping induced ferromagnetism in ZnO nanowires

Cu-doped and undoped ZnO nanowires have been successfully fabricated at 600 degrees C using a vapor transport approach. Comprehensive structural analyses on as-fabricated nanowires reveal highly crystalline ZnO nanowires with 0.5 at. % of substitutional Cu doping. Ferromagnetism has been observed in Cu-doped ZnO nanowires but not in undoped ones, which is probably associated with defects involving both Cu dopants and Zn interstitials.

Phase stability of cubic Mg0.55Zn0.45O thin film studied by continuous thermal annealing method

The phase stability of cubic Mg0.55Zn0.45O thin film grown by metal-organic chemical vapor deposition was studied through continuous thermal annealing. The crystal quality and surface smoothness were greatly improved after a continuous thermal annealing at 750 °C. It is attributed to the reducing of interstitial Zn by thermal annealing. However, phase segregation occurred when the sample was annealed at a higher temperature (850 °C), which is identified from both x-ray diffraction patterns and optical transmission spectra.

Magnetism in wide band gap semiconductors implanted with non-magnetic ions

Single crystals of ZnO, TiO2 and LaAlO3 have been implanted with Ar with 100 keV and different fluencies. The Ar implanted crystals showed a week ferromagnetic-like signal between 10 K and 400 K. Hysteresis curves obtained at room temperature allowed confirming the ferromagnetic behaviour of the implanted samples. Spin polarised first principles density functional calculations were performed in the case of ZnO considering Zn interstitials and O vacancies. No net magnetic polarisation was found for O vacancies, but in the case of Zn vacancies a magnetic moment of 1μB was obtained.