Photoluminescence study of aligned ZnO nanorods grown using chemical bath deposition

The photoluminescence study of self-assembled ZnO nanorods grown on a pre-treated Si substrate by a simple chemical bath deposition method at a temperature of 80 °C is hereby reported. By annealing in O₂ environment the UV emission is enhanced with diminishing deep level emission suggesting that most of the deep level emission is due to oxygen vacancies. The photoluminescence was investigated from 10 K to room temperature. The low temperature photoluminescence spectrum is dominated by donor-bound exciton. The activation energy and binding energy of shallow donors giving rise to bound exciton emission were calculated to be around 13.2 meV, 46 meV, respectively. Depending on these energy values and nature of growth environment, hydrogen is suggested to be the possible contaminating element acting as a donor.     

Oxygen vacancies effects on phase diagram of epitaxial La<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> thin films

We investigated the effects of oxygen vacancies on the structural, magnetic, and transport properties of La_1–x Sr _x MnO₃ (x=0.1, 0.2, 0.33, 0.4, and 0.5) grown around a critical point (without/with oxygen vacancies) under low oxygen pressure (10 Pa) and high oxygen pressure (40 Pa). We found that all films exhibit ferromagnetic behavior below the magnetic critical temperature, and that the films grown under low oxygen pressures have degraded magnetic properties with lower Curie temperatures and smaller magnetic moments. These results show that in epitaxial La_1–x Sr _x MnO₃ thin films, the magnetic and transport properties are very sensitive to doping concentration and oxygen vacancies. Phase diagrams of the films based on the doping concentration and oxygen vacancies were plotted and discussed.     

An alternative approach to investigate the origin of p-type conductivity in arsenic doped ZnO

P-type conductivity in MOCVD grown ZnO was obtained by directional thermal diffusion of arsenic from semi-insulating GaAs substrate. The films were single crystalline in nature and oriented along (002) direction. Ab initio calculations in the framework of density functional theory have been carried out with different chemical states of arsenic in ZnO. Present calculations suggested As_Zn–2V_Zn defect is a shallow acceptor and results in ferromagnetism in ZnO. The magnetic measurements of the samples indeed showed ferromagnetic ordering at room temperature. X-ray photoelectron spectra confirmed the presence of As_Zn and V_Zn. The core level chemical shift in binding energy of As_Zn indicated the formation of As_Zn–2V_Zn. Diffused arsenic substitutes zinc atom and creates additional zinc vacancies. The zinc vacancies, surrounding the oxygen atoms, result in unpaired O 2p electrons which in turn induce ferromagnetism in the samples.     

Study of anti-clockwise bipolar resistive switching in Ag/NiO/ITO heterojunction assembly

The anti-clockwise bipolar resistive switching in Ag/NiO/ITO (Indium–Tin–Oxide) heterojunctional thin film assembly is investigated. A sequential voltage sweep in 0 → V _max → 0 → ?V _min → 0 order shows intrinsic hysteresis behaviour and resistive switching in current density (J)–voltage (V) measurements at room temperature. Switching is induced by possible rupture and recovery of the conducting filaments in NiO layer mediated by oxygen ion migration and interfacial effects at NiO/ITO junction. In the high-resistance OFF-state space charge limited current passes through the filamentary path created by oxygen ion vacancies. In OFF-state, the resistive switching behaviour is attributed to trapping and detrapping processes in shallow trap states mostly consisting of oxygen vacancies. The slope of Log I vs Log V plots, in shallow trap region of space charge limited conduction is ~2 (I ∝ V ²) followed by trap-filled and trap-free conduction. In the low-resistance ON-state, the observed electrical features are governed by the ohmic conduction.     

Shallow Donors and Deep-Level Color Centers in Bulk AlN Crystals: EPR, ENDOR, ODMR and Optical Studies

The results of studies of shallow donors and deep-level color centers in bulk AlN crystals are presented. Two shallow donors (presumably oxygen located on the nitrogen site and carbon located on the aluminum site) are suggested to exhibit the DX-relaxation. Third shallow donor (presumably silicon on the Al site) shows the shallow donor behavior up to the room temperature and can be observed without light excitation at temperatures above 200 K. The values of the Bohr radius of the shallow donors are estimated. The structure of deep-level color centers (neutral nitrogen vacancy V _N) in bulk AlN crystals is determined and analyzed by electron paramagnetic resonance, electron-nuclear double resonance, optical absorption and thermoluminescence induced by X-ray irradiation. Spin-dependent recombination processes in AlN crystals are studied by means of optically detected magnetic resonance.     

Change of the stoichiometry and electrocoloration due to the injection of Li and O ions into lithium niobate crystals

Congruently grown LiNbO₃ single crystals show both high oxygen and lithium ion conductivity at temperatures above 500 °C. The high oxygen ion conductivity can be understood in terms of a certain amount of oxygen vacancies already present in congruently grown LiNbO₃ single crystals. Thermal treatment of LiNbO₃ produces additional oxygen vacancies. The absorption bands introduced by this procedure are investigated. It is found that the electrons which are generated during the reduction process are homogeneously distributed among all oxygen vacancies in the LiNbO₃ single crystals. The electrocoloration phenomenon in LiNbO₃ single crystals is due to the process of injection of lithium ions and electrons into LiNbO₃ by a double charge mechanism. Investigations of the optical and electrical properties of electrocolored LiNbO₃ crystals are reported. It is shown that the absorption spectra of thermally and electrochemically reduced samples are identical and that the origin of the absorption processes has to be therefore the same in both cases. That means, additional electrons produced by the double charge injection of lithium ions and electrons are also homogeneously distributed among the oxygen vacancies. This supports our hypothesis that a certain amount of oxygen vacancies has to be present already in as-grown LiNbO₃ single crystals.     

The investigation of room temperature ferromagnetism in (1 0 0) oriented BaNb2O6 PLD films on LaAlO3 (1 0 0) substrate

(1 0 0) oriented BaNb₂O₆ films have been successfully grown on LaAlO₃ (1 0 0) substrate at 750 °C or 450 °C in vacuum by pulsed laser deposition. The deposited BaNb₂O₆ PLD films exhibit room-temperature ferromagnetism. Ab initio calculations demonstrate that stoichiometric BaNb₂O₆ and that with barium vacancy are nonmagnetic, while oxygen and niobium vacancy can induce magnetism due to the spin-polarization of Nb s electrons and O p electrons respectively. Moreover, ferromagnetic coupling is energetically more favorable when two Nb/O vacancies are located third-nearest-neighbored. The observed room temperature ferromagnetism in BaNb₂O₆ films should be mainly induced by oxygen vacancies introduced during vacuum deposition, with certain contribution by Nb vacancies.     

NMR investigation of defect properties in single crystal SrTiO3

Abstract

⁴⁹Ti and ⁸⁷Sr nuclear magnetic resonance measurements were performed in single crystalline SrTiO₃ between 90 K and 1800 K at various oxygen partical pressures. The NMR lines are found to be shifted with rising temperature due to oxygen vacancies acting as donors. Furthermore, three distinct motion-induced nuclear spin relaxation rates could be observed. The corresponding jump rates are attributed to the following processes: translational jumps of charged oxygen vacancies between oxygen sites, localized motion of oxygen vacancies in Fe-vacancy complexes, self diffusion of Sr²⁺.     

The coexistence of ferroelectricity and ferromagnetism in Mn-doped BaTiO3 thin films

Due to the fault of the first author, this article entitled “The coexistence of ferroelectricity and ferromagnetism in Mn-doped BaTiO3 thin films”, published in “Chinese Physics B”, 2011,Vol.20, Issue 12, Article No. 127701, has been found to copy from the article entitled“Decisive role of oxygen vacancy in ferroelectric versus ferromagnetic Mn-doped BaTiO3 thin films”, published in “Journal of Applied Physics”, 2011,Vol.109, Issue 8, article No. 084105. So the above article in “Chinese Physics B” has been withdrawn from the publication.<     

Multiferroic and resistive switching behaviors in BiFe0.95Cr0.05O3 thin films deposited on Pt/Ti/SiO2/Si substrates

Multiferroic and resistive switching properties of single-phase polycrystalline perovskite BiFe_0.95Cr_0.05O₃ (BFCO) thin films grown on Pt/Ti/SiO₂/Si substrates by radio-frequency magnetron sputtering were investigated. The BFCO film shows ferroelectric and magnetic properties simultaneously at room temperature, and also exhibits a good piezoelectric property with remanent effective piezoelectric coefficient d _33,f ～55±4 pm/V. An obviously resistive switching behavior was observed in the BFCO thin film at room temperature, which was discussed by the filamentary conduction mechanism associated with the redistribution of oxygen vacancies.     

Shallow levels in virgin hydrothermally grown n-type ZnO studied by thermal admittance spectroscopy

Three n-type single crystal hydrothermally grown ZnO samples with resistivities of 5.1±0.6, 15±2 and 220±20 Ω cm, respectively, have been electrically characterized using thermal admittance spectroscopy (TAS). The presence of three main donors: two shallow ones D₁ and D₂ and a deeper one D₃ with activation energies of ∼30, ∼50 and ∼290 meV, respectively, are detected. In addition, the TAS spectra reveal the presence of a fourth level, D_X, with a peak amplitude in the conductance spectra that decreases with the temperature occurrence. It is shown that this anomalous behavior is consistent with D_X being a negative-U defect of donor-type. An activation energy of ∼80 meV for the ++/+ transition, a capture cross section equal to ∼3×10⁻¹⁷ cm² and an energy barrier for atomic reconfiguration of ∼0.25 eV, respectively, deduced according to the assignment of D_X to a negative-U defect. A tentative assignment of the D_X defect with oxygen vacancies is discussed.     

Electrical conductivity and electric polarization in In<Subscript>2</Subscript>O<Subscript>3 − δ</Subscript>: Sr at different oxygen contents

The electrical characteristics of strontium-doped indium oxide have been experimentally investigated in the case of deviations from the stoichiometric composition with respect to oxygen. Doping considerably decreases the electrical conductivity of In₂O₃ and makes it extremely highly sensitive to variations in the oxygen content. A small oxygen deficiency of the samples corresponds to the high-resistance state, which is formed as a result of the decrease in the concentration of charge carriers and the transparency of potential barriers at the grain boundaries. It has been found that the oxygen deficiency in In₂O_{3 − δ}: Sr restores a high n-type electrical conductivity, which is consistent with the concepts of oxygen vacancies as shallow donors. For a sufficient oxygen deficiency, the potential barriers do not manifest themselves; however, as the oxygen content increases, there arises a potential relief that leads to a set of remarkable effects: the specific nonlinearity and hysteresis of the current-voltage characteristic, which are caused by the tunneling transparent potential barriers; an increase in the permittivity to 13.3 at a frequency of 10⁵ Hz and at a temperature T = 77 K; and the temperature anomalies of the measured parameters, which are associated with the topological phase transition to a spatially inhomogeneous conductance of the samples. It has been revealed that, for small deviations from the stoichiometric composition with respect to oxygen, not only the active component but also the capacitive component of the conductance of In₂O_{3 − δ}: Sr exhibit a sensitivity to the pressure and composition of the gas medium.     

Photoluminescence study of hydrogen donors in ZnO

A photoluminescence study of hydrogenated ZnO bulk crystals is presented. Two excitonic recombination lines at 3362.8 and 3360.1 meV are assigned to hydrogen shallow donors. Experimental evidence is presented that the corresponding donor to the line at 3362.8 meV, previously labeled I₄, originates from hydrogen trapped within the oxygen vacancy, H_O. The line at 3360.1 meV was found to be due to hydrogen located at the bond-centered lattice site, H_BC. The corresponding shallow donor has an ionization energy of 53 meV.     

Contribution of oxygen vacancies to the giant dielectric response in Sm1.5Sr0.5NiO4−δ ceramics

The dielectric properties of Sm_1.5Sr_0.5NiO_4?δ ceramics with different concentrations of oxygen vacancies were characterized. The ceramics with lower concentration of oxygen vacancies were prepared by directly sintering the sol–gel derived powders in air, while the higher one could be obtained by annealing the as-sintered ceramics in the flow of nitrogen. The post-densification annealing in the flow of nitrogen decreased the dielectric constant at low temperature and increased it at high temperature, while the dielectric loss increased in overall temperature range. The activation energy of low-temperature dielectric relaxation decreased with increasing the concentration of oxygen vacancies, and so did that of bulk electrical resistances although the values of resistances increased, while the activation energy of electrical resistances for grain boundary increased though the values of resistances decreased. The giant dielectric response in the as-sintered Sm_1.5Sr_0.5NiO_4?δ ceramics should be mainly attributed to the small polaronic hopping process, while that of annealed ceramics should be directly linked to the oxygen vacancies.     

Bipolar switching properties of amorphous TiO2 thin film grown on TiN/Si substrate

Pt/TiO₂/TiN device with the amorphous TiO₂ film grown at room temperature under an oxygen partial pressure of 1.0 mTorr showed reliable bipolar switching behavior. During the electroforming process, a large number of oxygen vacancies formed in the TiO₂ film and accumulated at the Pt/TiO₂ interface. The barrier height of the Schottky contact of the Pt/TiO₂ interface was reduced owing to the presence of these oxygen vacancies, resulting in the low-resistance state (LRS). Moreover, oxygen ions diffused into the TiN electrode during the electroforming and set processes. On the other hand, the oxygen ions in the TiN electrode diffused out and reacted with oxygen vacancies in the TiO₂ film during the reset process, and the device changed from the LRS to the high-resistance state (HRS). Conduction in the LRS and HRS can be attributed to Ohmic conduction and the trap controlled space charged limited mechanism, respectively.     

脱氧La0.5Ca0.5MnO3样品的铁磁-反铁磁转变和电阻率变化

研究了氧空位对La_0.5Ca_0.5MnO₃ （LCMO）多晶块材的电输运和磁性质的影响. 随着氧空位的增加, 样品在高温段的电阻率一直增加, 并满足绝热小极化子模型, 而低温段的电阻率先下降后上升, 并出现明显的dR/dT>0的行为, 直至最后变为绝缘的. 氧空位的增加抑止了反铁磁相的出现, 使得脱氧的LCMO样品不发生反铁磁转变, 进一步增加氧空位则会抑制铁磁相. 关键词： 0.5Ca_0.5MnO₃')" href="#">La_0.5Ca_0.5MnO₃ 反铁磁相变 铁磁相变 脱氧     

EPR study of charge compensation of chromium centers in the strontium titanate crystal

Chromium centers and their charge compensation in two single strontium titanate crystals, i.e., SrTiO₃: Cr (0.05 at %) and Sr_0.9995TiO₃: Cr_0.0005 grown with strontium deficiency, have been studied by the electron paramagnetic resonance method. The crystals have been investigated both immediately after growth and after oxidation and reduction procedures. Oxidation and reduction are performed by crystal annealing in a corresponding gas atmosphere at high temperature. Chromium centers associated with oxygen vacancy (Cr³⁺-V _O) are detected in the reduced crystals. It is shown that strontium vacancies are formed in the crystal grown with strontium deficiency, which leads to a lowering of the tetragonal symmetry of (Cr³⁺-V _O) and Cr⁵⁺ centers to the orthorhombic symmetry. Possible compensation mechanisms for charges of various chromium centers are considered.     

Depth-resolved cathodoluminescence study of zinc oxide nanorods catalytically grown on p-type 4H-SiC

Optical properties of ZnO nanorods (NRs) grown by vapour-liquid-solid (VLS) technique on 4H-p-SiC substrates were probed by cathodoluminescence (CL) measurements at room temperature and at 5 K complemented with electroluminescence. At room temperature the CL spectra for defect related emission intensity was enhanced with the electron beam penetration depth. We observed a variation in defect related green emission along the nanorod axis. This indicates a relatively poor structural quality near the interface between ZnO NRs and p-SiC substrate. We associate the green emission with oxygen vacancies. Analysis of the low-temperature (5 K) emission spectra in the UV region suggests that the synthesized nanorods contain shallow donors and acceptors.     

A LEED-AES study of the initial stages of oxidation of Fe (001)

LEED and AES have been used to study the structural changes and kinetics of the initial interaction between Fe(001) and oxygen at room temperature. The AES oxygen signal was quantified by using a two-dimensional oxide layer as a calibration point. This reproducible oxide layer was prepared by the high temperature reaction of H₂O at 10^?6 torr with Fe(001). The initial oxygen sticking coefficient was observed to be close to unity, which suggests that the chemisorption is non-activated and involves a mobile adsorption step. The rate of chemisorption decreased as (1-Θ) and exhibited a minimum at Θ = 0.5. LEED data indicate that the minimum value of the sticking coefficient corresponded to the completion of a c (2 × 2) surface structure. Upon additional exposure to oxygen, an increase in the sticking coefficient was observed in conjunction with the disappearance of the c (2 × 2) and a gradual fade out of all diffraction features. After mild heating, epitaxial FeO (001) and FeO (111) structures were observed. The simultaneous appearance of a shifted M_2,3M_4,5M_4,5 iron Auger transition with the increase in the sticking coefficient and the disappearance of the c (2 × 2) indicated that oxide nucleated on the surface after the complete formation of the c (2 × 2) structure. The relatively high sticking coefficient during the initial oxidation indicates that formation of a mobile adsorbed oxygen state precedes the formation of oxide.     

Electron diffraction and electron microscopic study of BaYCuO superconducting materials

Electron microscopy and electron diffraction have been applied to show that the orthorhombic phase in the compound Ba₂YCu₃O_7−δ is responsible for the high superconducting transition temperature. A positive correlation is found between the volume fration of the orthorhombic phase and the superconducting transition temperature. By means of an “in-situ” heating experiment it is found that the orthorhombic phase is formed on cooling from a high temperature tetragonal phase with disordered vacancies. It is suggested that the low temperature tetragonal phase that occurs in the same specimens as the orthorhombic phase also contains an ordered arrangement of vacancies different from that present in the orthorhombic phase. The order-disorder transition associated with the structural vacancies is shown to be reversible, provided there has been no oxygen loss.